Hundred Farad-scale Supercapacitor with Monolithic Biochar ... · Hundred Farad-scale Supercapacitor with Monolithic Biochar Electrodes – a Feasibility Study Aldrich Ngan Master

Hundred Farad-scale Supercapacitor with Monolithic Biochar Electrodes – a Feasibility Study

by

Aldrich Ngan

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science

Department of Chemical Engineering and Applied Chemistry University of Toronto

© Copyright by Aldrich Ngan 2018

ii

Hundred Farad-scale Supercapacitor with Monolithic Biochar

Electrodes – a Feasibility Study

Aldrich Ngan

Master of Applied Science

Department of Chemical Engineering and Applied Chemistry

University of Toronto

2018

Abstract

Supercapacitors are high power density electrochemical storage devices that are hindered from

widespread adoption by prohibitive material costs, low energy densities for practical

applications, and poor scalability. This thesis seeks to solve all three issues by using large-scale

monolithic maple-derived biochar as an effective electrode material. Biomass-derived carbon

monoliths are pyrolyzed at 800oC and treated with 0.5M nitric acid at room temperature. Two

electrode pairs, measuring ~5mm x 5mm x 5mm and ~23mm x 13mm x 10mm respectively,

orders of magnitude thicker than typical supercapacitor electrodes, are tested using a suite of

electrochemical techniques in 4M KOH electrolyte. Additional testing is done on large

electrodes in 1M and 6M KOH, as well as in 1M TEA-TFB organic electrolyte. Capacitive

performance of 107 F or 38.2 F/g is observed in the large monolithic electrodes, along with an

exceptional energy density of 9.12 Wh/kg.

iii

Acknowledgments

To Professor Jia and Professor Kirk, it was a tremendous pleasure and a privilege to work with

the two of you. Thank you both so much for all the support and guidance, and for teaching me

how to truly learn.

My sincere appreciation and admiration goes to my friend and group member Johnathon Caguiat,

without whom the biochar group at the Green Technology Research Group could not exist. You

were truly the greatest mentor I could ask for. Thank you for your patience in teaching me so

much about our research, and for always being there when your help was needed both in and out

of the lab.

Last but not least, a big thank you goes to my mother and father for the never-ending,

unconditional support in all of my endeavors, and to my brother Amory, whose advice has

always proven to be infallible.

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Table of Contents

Acknowledgments.......................................................................................................................... iii

Table of Contents ........................................................................................................................... iv

List of Tables ................................................................................................................................. vi

List of Figures ............................................................................................................................... vii

Introduction .................................................................................................................................1

1.1 Research Motivation ............................................................................................................1

1.2 Thesis Objectives .................................................................................................................3

Literature Review ........................................................................................................................4

2.1 Energy Storage in Supercapacitors ......................................................................................4

2.2 Carbon Materials and Properties in Supercapacitors ...........................................................5

2.3 Biochar Production and Properties ......................................................................................6

Experimental ...............................................................................................................................8

3.1 Biochar Production ..............................................................................................................8

3.2 Monolithic Electrode Fabrication and Characterization ......................................................8

3.3 Supercapacitor Device Fabrication ....................................................................................10

3.4 Electrochemical Evaluation of Device Performance .........................................................10

3.4.1 Cyclic Voltammetry ...............................................................................................11

3.4.2 Galvanostatic Charge and Discharge .....................................................................12

3.4.3 Chronoamperometry ..............................................................................................16

Results and Discussion ..............................................................................................................20

4.1 Characteristics of Biochar ..................................................................................................20

4.2 Electrochemical Performance of Small Electrodes ............................................................23

4.3 Electrochemical Performance of Large Electrodes ............................................................28

v

4.4 Effects of Electrolyte on Large-scale Device Performance ...............................................33

4.4.1 Concentration .........................................................................................................33

4.4.2 Organic Electrolyte Performance – 1M TEA-TFB ................................................41

4.5 Summary of Electrode Size and Electrolyte Concentration and Species Performance

Studies ................................................................................................................................46

4.6 Device Resistance Analysis and Implications ...................................................................53

4.7 Analysis of Power and Energy Density of Monolithic Biochar Supercapacitors ..............56

Conclusions ...............................................................................................................................58

Recommendations for Future Work ..........................................................................................59

References ......................................................................................................................................61

Appendices ................................................................................................................................68

7.1 Appendix A: Galvanostatic Charge-Discharge Cycles ......................................................68

7.1.1 Small-scale electrodes in 4M KOH .......................................................................68

7.1.2 Large-scale electrodes in 4M KOH .......................................................................70



7.1.5 Large-scale electrodes in 1M TEA-TFB ................................................................76

7.2 Appendix B: Chronoamperometric Charge/Discharge Cycles ..........................................77

7.2.1 All cycles for small-scale electrodes in 4M KOH .................................................77

7.2.2 All cycles for large-scale electrodes in 4M KOH ..................................................77



7.2.5 All cycles for large-scale electrodes in 1M TEA-TFB ..........................................79

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List of Tables

Table 1: Thermal treatment schedule for crack-free carbonized wood monoliths from expired US

Patent by Byrne and Nagle ............................................................................................................. 7

Table 2: Pyrolysis schedule for wood-derived biochar monoliths at 800oC adapted from a

procedure by Byrne and Nagle ....................................................................................................... 8

Table 3: Dimensions and masses of maple-derived monoliths used small-scale and large-scale

supercapacitor electrodes ................................................................................................................ 9

Table 4: Various electrochemical techniques used to analyze supercapacitor performance ........ 11

Table 5: Ion diameters (nm) for KOH and TEA-TFB electrolyte ions [23,60,61] ....................... 22

Table 6: Chronoamperometry results summary for small-scale electrodes in 4M KOH ............. 27

Table 7: Chronoamperometry results summary for small-scale electrodes in 4M KOH ............. 32

Table 8: Chronoamperometry results summary for large-scale electrodes in 1M KOH .............. 36

Table 9: Chronoamperometry results summary for large-scale electrodes in 6M KOH .............. 41

Table 10: Chronoamperometry results summary for large-scale electrodes in 1M TEA-TFB .... 45

Table 11: Conductivities and resistivities of carbon material and electrolytes. [48,64] ............... 54

Table 12: Calculated electrode bulk resistivities and resistances for large-scale electrodes in

various electrolytes ....................................................................................................................... 55

vii

List of Figures

Figure 1: Ragone Plot of Various Energy Storage Devices ............................................................ 1

Figure 2: Evolution of electric double layer models: (a) Helmholtz model (b) Gouy-Chapman

model (c) Stern model ..................................................................................................................... 4

Figure 3: Schematic of constructed supercapacitor cell with labelled components ..................... 10

Figure 4: Two-electrode setup of supercapacitor cell immersed in electrolyte in Teflon test jar. 11

Figure 5: Simple serial capacitor and resistor to model supercapacitor behaviour ...................... 13

Figure 6: Sedlakova equivalent circuit model introducing time dependency in diffuse capacitance

and resistance for charge diffusion [53]........................................................................................ 15

Figure 7: Universal equivalent circuit for porous electrodes using vertical ladder network in

series with an RC parallel network as proposed by Fletcher [54] ................................................ 16

Figure 8: Labelled axial-direction SEM image of maple-derived biochar featuring macroscopic

water-transport channels ............................................................................................................... 20

Figure 9: Angled side view of monolithic maple-derived biochar featuring exposed vessels ..... 21

Figure 10: Pore size distribution and cumulative specific surface area of maple-derived biochar

measured by CO2 physisorption and DFT data reduction ............................................................ 21

Figure 11: Cyclic Voltammetry of small-scale electrodes in 4M KOH (cycles 1 and 250) ......... 23

Figure 12: Cyclic voltammogram of small-scale electrodes in 4M KOH. Stable performance

shown between cycles 501 and 750. ............................................................................................. 24

Figure 13: GC specific capacitance vs current density for small-scale electrodes in 4M KOH ... 24

Figure 14: GC extrapolated device resistance vs current density in small-scale electrodes in 4M

KOH .............................................................................................................................................. 25

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Figure 15: Current response of sample charge-discharge cycle to a potential step from 0V to 1V

(charge) and from 1V to 0V (discharge) of small-scale electrodes in 4M KOH .......................... 26

Figure 16: Cyclic voltammogram of large-scale electrodes in 4M KOH (cycles 1 and 250) ...... 28

Figure 17: Cyclic voltammogram of large-scale electrodes in 4M KOH. Stable cycling shown

between cycles 750 and 1000. ...................................................................................................... 29

Figure 18: GC specific capacitance vs current density in large-scale electrodes in 4M KOH ..... 30

Figure 19: GC extrapolated device resistance vs current density in large-scale electrodes in 4M

KOH .............................................................................................................................................. 30


(charge) and from 1V to 0V (discharge) of large-scale electrodes in 4M KOH ........................... 31

Figure 21: Cyclic voltammogram for large-scale electrodes in 1M KOH (cycles 1 and 250) ..... 33

Figure 22: Cyclic voltammogram for large-scale electrodes in 1M KOH (cycles 251 and 500) . 34

Figure 23: GC specific capacitance vs current density for large-scale electrodes in 1M KOH ... 34

Figure 24: GC extrapolated device resistance vs current density for large-scale electrodes in 1M

KOH .............................................................................................................................................. 35



Figure 26: Cyclic voltammogram for large-scale electrodes in 6M KOH (cycles 1 and 250) ..... 37

Figure 27: Cyclic voltammogram for large-scale electrodes in 6M KOH (cycles 750-1000) ...... 38

Figure 28: GC specific capacitance vs current density for large-scale electrodes in 6M KOH ... 39


KOH .............................................................................................................................................. 39

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Figure 31: Cyclic voltammogram for large-scale electrodes in 1M TEA-TFB showing significant

growth between cycles 1 and 500 ................................................................................................. 42

Figure 32: Cyclic voltammogram for large-scale electrodes in 1M TEA-TFB showing stabilized

behavior between cycles 750 and 1000 ........................................................................................ 42

Figure 33: GC specific capacitance vs current density for large-scale electrodes in 1M TEA-TFB

....................................................................................................................................................... 43


TEA-TFB ...................................................................................................................................... 44


(charge) and from 2V to 0V (discharge) of large-scale electrodes in 1M TEA-TFB ................... 44

Figure 36: Complete set of specific capacitances vs current densities measured ......................... 46

Figure 37: Complete set of GC extrapolated device resistances vs current densities ................... 48

Figure 38: Bar graph comparing the complete set of device resistances calculated from

chronoamperometry ...................................................................................................................... 49

Figure 39: Bar graph comparing the energy densities of the small-scale device and large-scale

device measured across all electrolytes ........................................................................................ 50

Figure 40: Bar graph comparing the peak power density of small-scale and large-scale electrodes

measured across all electrolytes .................................................................................................... 51

Figure 41: Performance of small-scale and large-scale maple-derived biochar electrodes plotted

on a Ragone plot with typical supercapacitor and battery technology performances ................... 56

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Introduction

1.1 Research Motivation

With an ever-increasing global demand in energy, coupled with the risen awareness on climate

change and risks, renewable power has naturally been on the rise. However, renewable power

sources, such as wind turbine farms or photovoltaic power stations, cannot exist by themselves.

Energy produced through renewable means is often intermittent, causing misalignments of time

between energy supply and energy demand. The practical implementation of renewable power

sources therefore relies on the ability to store electrical energy produced quickly and efficiently

to meet energy demands. As a result, energy storage as a field has received much attention in

both the academic and commercial setting.

Traditional energy storage devices, namely capacitors and batteries, fell short of the requirements

for using electrical energy efficiently in transportation, commercial and residential applications

[1]. As shown in Figure 1, capacitors did not offer acceptable energy densities despite their high

power densities, while traditional battery technologies suffered the opposite problem due to

limitations in their reaction kinetics [2].

Figure 1: Ragone Plot of Various Energy Storage Devices

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Supercapacitors bridge the gap between the two traditional technologies offering both high

power capabilities and acceptable energy densities. While new batteries in development also aim

to bridge the same gap, supercapacitors offer an additional benefit of having much longer

lifespans. Batteries store energy chemically through Faradaic oxidation and reduction reactions,

whereas supercapacitors store energy through the physical adsorption of electrolyte ions onto

charged surfaces – forming what is known as the electrical double layer [3].

The widespread adoption of supercapacitors is hindered by multiple aspects: their relatively low

energy densities in comparison to new battery technologies , high cost and difficulty in the

production of electrodes, and the inherently poor scalability of the electrodes [4,5]. As a result,

supercapacitor electrode materials are a popular research topic.

Supercapacitor electrodes are primarily made of highly conductive carbon materials with high

surface areas and porosity, such as graphene, carbon nanotubes, carbon aerogels, and activated

carbon [5]. The clear majority of supercapacitors in research, and all supercapacitors available

commercially, use electrodes made from carbon materials in powder form, which are held

together with non-conductive binder material. This composite material is then compressed into a

thin-film with thicknesses in the order of hundreds of microns. This setup is unable to scale past

this thickness due to the resistance of the composite material and difficulties in ion transport. As

a result, a large device would simply be an array of smaller devices connected in parallel in

series, leading to a high ancillary component to active material ratio, and poor energy densities.

Biochar, or biomass-derived carbon, is an increasingly popular carbon material in

supercapacitors [6]. As biochar is produced by the simple heat-treatment of biomass precursors

in the absence of oxygen, biochar is a readily available and inexpensive near-pure carbon

material. Supercapacitor electrodes have been made from waste biomass including cotton stalk,

palm tree waste and sunflower seed shells in hopes of reducing the production cost of high-

performance supercapacitors [7-9]. Other studies have explored biochar derived from woody

biomass, using binder-free monolithic electrodes [10-13].

The work presented in this thesis is one of multiple supercapacitor research projects at the Green

Technology Research Group, which has in recent years been dedicated to the development of

high performance supercapacitors from monolithic biochar electrodes derived from wood. It is

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proposed that the mass transport channels inherent in wood can be retained through pyrolysis,

and perhaps activation, to allow easy transport of electrolyte ions throughout the electrode. It is

also proposed that by eliminating the need for non-conductive binder materials, a relatively high

electrode conductivity may be maintained. The combination of the two propositions suggests that

monolithic electrodes made from wood-derived biochar may scale far beyond the hundreds-of-

microns thickness of thin-film electrodes while maintaining competitive performance. This will

therefore increase the energy density of devices by removing ancillary components, while

drastically decreasing the cost of electrodes by using inexpensive and readily available precursor

material.

1.2 Thesis Objectives

The main goal of the thesis is to determine the feasibility of using centimeter-scale thick

monolithic biochar derived from wood as biochar electrodes. By scaling monolithic electrodes

up to two orders of magnitude larger than the thin film counterparts, wood-derived biochar may

help in solving the main issues in supercapacitor research, namely cost, energy density and

scalability. The effect of electrolyte concentration and species is also investigated with the large-

scale electrodes. Important to note is that the electrodes referred to as ‘small-scale’ measure

approximately 5 mm x 5 mm x 5 mm, making them still orders of magnitude larger than

commercially available supercapacitor electrodes. The term ‘small-scale’ is used relative to the

electrodes referred to as ‘large-scale’ which measure approximately 23 mm x 13 mm x 10 mm.

Specific objectives of the thesis are as follows:

1. Compare electrochemical performances between small-scale electrodes and large-scale

electrodes in 4M KOH electrolyte.

2. Investigate electrolyte concentration effects on large-scale electrode performance through

electrochemical testing in 1M KOH, 4M KOH and 6M KOH.

3. Determine viability of common commercial organic electrolyte in large-scale biochar

electrodes by measuring electrochemical performance in 1M TEA-TFB in acetonitrile.

4

Literature Review

2.1 Energy Storage in Supercapacitors

Supercapacitors store charge through a phenomenon known as the electrical double-layer

capacitance (EDLC). This describes a particular arrangement of electrolyte ions at the electrode-

electrolyte surface interface when the electrode surface is charged [3]. The understanding of the

electric double layer started with the Helmholtz model, where a charged plate would result in the

formation of a layer immediately on the plate, made of oppositely charged ions from solution to

balance the charge of the plate. The Gouy-Chapman model sought to improve the understanding

of the double-layer and suggested a “diffuse layer”, which was a distribution of charged ions as a

function of distance from the surface, rather than a layer of adhered ions. The generally accepted

model is known as the Stern model and incorporates both the Helmholtz model, in the form of a

layer of solvated counter-ions in the Inner Helmholtz Plane, and the Gouy-Chapman model, in

the form of diffuse layer, where the concentration of counter ions decays until the charge-neutral

bulk electrolyte is reached [14]. These three models are depicted in Figure 2 below.

Figure 2: Evolution of electric double layer models: (a) Helmholtz model (b) Gouy-Chapman

model (c) Stern model

5

2.2 Carbon Materials and Properties in Supercapacitors

Various forms of carbon, such as activated carbons, carbon nanofibers, carbon nanotubes, and

graphene, have been the primary materials used in supercapacitors for their high porosity and

large surface areas, high electrical conductivities, and excellent chemical and thermal stabilities

[4,15]. Activated carbon refers to any carbon material that has undergone physical or chemical

activation to alter the porosity or surface chemistry in order to enhance performance [16,17].

While activation is often performed to increase surface area or add pseudocapacitive functional

groups to the carbon surface, it may also be used to increase wettability of the carbon surface

through surface oxygen groups [18]. By increasing the wettability, pore accessibility to

electrolyte is increased, leading to better EDL performance [19].

Porosity is a critical factor in supercapacitor applications. A large specific surface area (SSA) is

often seen as the most important parameter leading to large gravimetric capacitances and energy

densities, as the formation of the double-layer occurs at the electrode surfaces [20,21]. However,

a high surface area alone does not guarantee good EDLC behavior. The porosity, and more

specifically the pore size distribution, is an equally important electrode property.

Pores are classified into three categories: macropores (>50 nm), mesopores (2-50 nm), and

micropores (<2 nm). While micropores, with very high surface area to volume ratios, are

generally known to contribute most to gravimetric capacitance, a degree of mesoporosity and is

required for effective electrolyte mobility for micropore accessibility [22]. The ideal micropore

size is a function of electrolyte solution used, and more specifically the electrolyte ion sizes. In

2006, Chmiola et al. discovered that pores smaller than the solvated electrolyte ions actually

contributed to charge storage [23], suggesting that the solvation shells were able to distort in

order to fit into the carbon nanostructure’s pores. A follow-up study investigating ionic liquid

electrolyte, which do not have solvation shells, discovered that the pore size leading to the

maximum capacitance is very close to the size of the ion itself in the case of ionic liquids [24].

As such, the ideal micropore size for liquid and organic electrolytes with solvation shells should

be in between the bare ion size and the solvated ion size. Many researchers have experienced

poor EDLC performance in materials with small pores relative to the ion sizes, indicative of a

6

lower limit of ion accessibility [25-27]. However, the lower limit, and the extent to which

solvation shells may distort, has not yet been fully defined.

While work done by Chmiola et al. and Largeot et al. proved the strong participation of very

small pores in charge storage, smaller more hard-to-reach pores require more time for ion

migration during charging and discharging, due to a reduced ion mobility [23,28,29]. This

reinforces the need for materials with tailored pore size distributions with both small micropores

which contribute to large capacitance values, as well as mesopores which favor quick electrolyte

diffusion [30].

The pore size distribution also affects the types of electrolytes that are compatible with the

electrode material. Aqueous electrolytes, such as KOH and H2SO4 solutions, are known for their

strong ionic conductivities and relatively small ion sizes. However, their small voltage windows

(~1V) governed by the electrolysis of water at 1.23V acts as a limitation on energy density

[5,15]. Organic electrolytes, such as tetraethylammonium tetrafluoroborate (TEA-TFB)

dissolved in acetonitrile, offer higher voltage windows (~2.5V) and therefore higher energy

densities, but are around 10-25 times more resistive than their aqueous counterparts. Organic

electrolyte solvents are also dangerous and environmentally unfriendly chemicals [31]. Ionic

liquids offer even higher voltage windows (~4V) without being an environmental hazard.

Besides the manipulation of SSA and pore size distributions, other methods of enhancing

performance of supercapacitor electrodes include the incorporation of additional materials such

metal oxides and conductive polymers [32-35], or the doping of the carbon surface with

functional groups and heteroatoms with elements such as N, O, S, B and P [36-39]. Both

methods increase performance through pseudocapacitance, rapid faradaic reactions which

contribute to the capacitance and energy storage capacity [40].

2.3 Biochar Production and Properties

Biochar is a solid, carbon-rich product of heat-treated biomass. This heat treatment process,

known as pyrolysis, is performed in an oxygen-less environment at temperatures of several

hundred degrees Celsius or more [41]. The main process parameters of the pyrolysis process are

the heating rate, hold temperature, and hold time. Biochar is used across a wide variety of

7

applications, from an adsorbent for soil and water contaminants, to a means of energy storage in

supercapacitors [6].

Biochar can be made from a wide variety of biomass sources, such as crop residues, grasses and

even sewage sludge [42,43]. In this thesis, biochar derived from acer saccharum, or sugar maple,

is used as the precursor material.

The Green Technology Research Group views wood as an attractive precursor for biochar in

supercapacitor applications due to the inherent hierarchal structure of wood, providing structural

robustness as well as providing channels for the transport of water [44,45]. Under certain

conditions, entire pieces of wood can be pyrolyzed to create large crack-free, uniformly treated,

and highly carbonized monolithic structures [46]. These monolithic biochar pieces retain the

macrostructure of the precursor material, and with them, the ability to transport water throughout

the structure. This procedure was developed by Byrne and Nagle and is found in expired U.S.

Patent 6,124,028. The thermal treatment schedule used by Byrne and Nagle is shown in Table 1.

Table 1: Thermal treatment schedule for crack-free carbonized wood monoliths from expired

US Patent by Byrne and Nagle

Temperature Range (oC) Ramp Rate (oC/min) Dwell Time (min)

25-90 0.83 180

90-200 0.25 6

200-100 0.13 6

400-600 0.33 6

600-800 0.33 6

800-1000 0.33 6

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Experimental

3.1 Biochar Production

The precursor material used to produce the biochar used in the experiments was acer saccharum,

a hardwood species known commonly as sugar maple. This was bought locally and pyrolyzed in-

house in a quartz reactor under Grade 5.0 nitrogen gas. The pyrolysis procedure followed the

steps of the slow-pyrolysis process outlined by Byrne and Nagle [47], up to the peak temperature

of 800 oC. The procedure is summarized in tabular format in Table 2 below.

Table 2: Pyrolysis schedule for wood-derived biochar monoliths at 800oC adapted from a

procedure by Byrne and Nagle

All electrodes used in this study are made from the same piece of precursor wood, which

measured 19.0mm x 17.0mm x 40.0mm, where the 40.0mm length corresponded to the axial

direction of the wood structure.

3.2 Monolithic Electrode Fabrication and Characterization

Monolithic electrodes were hand cut from the larger pieces that underwent the pyrolysis

procedure up to 800oC. They were cut into approximate shape using aluminum oxide abrasive

cord, and then sanded down to precise desired shape and dimensions using 60, 100, 250, and 500

grit silicon carbide sandpaper. All sides were measured with a spirit level to ensure flat and

parallel surfaces. Dimensions were measured using Vernier calipers, and mass was measured

using a Sartorius balance. Dimensions and masses of the small-scale electrode pair and large-

scale electrode pair used in experiments are presented in Table 3 below. The Z-dimension

corresponded to the axial direction of wood structure.

Step Ramp rate

[°C/min]

Holding Temp.

[°C]

Dwell time

[min]

1 0.83 90 180

2 0.25 200 6

3 0.13 400 6

4 0.33 600 6

5 0.67 800 6

9

Table 3: Dimensions and masses of maple-derived monoliths used small-scale and large-scale

supercapacitor electrodes

Electrode X (height - mm) Y (width - mm) Z (thickness - mm) Mass (grams)

Small-scale

electrode A 4.9 5.0 4.7 0.05461

Small-scale

electrode B 4.9 5.0 4.9 0.05255

Large-scale

electrode A 22.8 12.7 9.7 1.4024

Large-scale

electrode B 22.6 12.7 9.9 1.4018

After electrode fabrication, all electrodes were rinsed in DI water to remove loose particulates

and dried in an oven at 120oC for 12 hours. The electrodes were then soaked in 0.5M nitric acid

for 48 hours as a post-treatment step for mild activation, increasing hydrophilicity and coverage

of surface oxygen groups [11]. After nitric acid treatment, electrodes were soaked in DI water for

one week, with the DI water being replaced daily to ensure removal of nitric acid from the

monoliths.

Biochar monolith structure was investigated through SEM imaging using a Hitachi SU3500 at

Ontario Centre for the Characterization of Advanced Materials (OCCAM) at the University of

Toronto. Imaging was done in the axial direction of the wood structure to observe tracheids,

vessels and rays. Electrode porosity and surface area was evaluated using physisorption on a

Quantachrome Autosorb-1 gas sorption analyzer. Unfortunately, due to technical problems with

the device, only the CO2 physisorption data could be collected for the small-scale sample.

However, as all electrodes originated from the same piece of pyrolyzed wood, this was assumed

to, at least approximately, represent the microporosity of both small-scale and large-scale

electrodes. N2 physisorption data was also unobtainable due to the same technical problems. N2

results can therefore only be inferred from the work done by other members of the Green

Technology Research Group using the same precursor material and pyrolysis procedure.

10

3.3 Supercapacitor Device Fabrication

Monolithic electrode pairs were assembled into supercapacitor test cells in an arrangement

shown in Figure 3.

Separator

Electrode

Bolts/Nuts

Current Collector

Housing Sheets

Graphite Adhesive

Figure 3: Schematic of constructed supercapacitor cell with labelled components

The symmetrical electrodes were kept apart with a polyphenylene sulfide (Ryton®) porous

polymer separator. Current collectors were made of nickel mesh (Nickel gauze, 40 mesh woven

from 0.13 mm wire – Wire Cloth) purchased from Alfa Aesar. The portion in contact with the

electrodes remained in mesh form, while the remainder of the nickel mesh was physically altered

into wire shape. To maximize area of contact and decrease contact resistance between current

collectors and biochar electrodes, the electrode surfaces facing the current collectors were coated

with an aqueous based graphite conductive adhesive, purchased from Alfa Aesar. The current

collectors, biochar electrodes and separator were held together between rectangular glass-fiber

reinforced polyphenylene sulfide housing sheets fastened with stainless steel nuts and bolts at

each corner.

3.4 Electrochemical Evaluation of Device Performance

Before any electrochemical testing, an assembled test cell was first dried overnight in an oven at

120oC to remove moisture. It was then left to soak in electrolyte for a minimum of one week to

ensure that the electrolyte was given sufficient time to permeate throughout the biochar

monoliths. The device and electrolyte were held within a sealed Teflon jar as shown in Figure 4.

The headspace was continuously purged with nitrogen gas to maintain an oxygen-less

environment.

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Current Collector

Separator

Electrode

Electrolyte

Teflon Jar

Nitrogen Purge Inlet

Nitrogen Purge Outlet

Figure 4: Two-electrode setup of supercapacitor cell immersed in electrolyte in Teflon test jar.

The current collectors were connected to a Metrohm Autolab PGSTAT302N potentiostat in a

two-electrode setup. Device performance was evaluated using various electrochemical

techniques summarized in Table 4 below.

Table 4: Various electrochemical techniques used to analyze supercapacitor performance

Electrochemical Technique Performance Parameters Studied

Cyclic Voltammetry (pre-treatment step) Electrode stability (qualitative)

Pseudocapacitance (qualitative)

Resistance (quantative)

Galvanostatic Charge and Discharge Capacitance

Device Resistance

Chronoamperometry Device Resistance

Energy Density

Power Density

3.4.1 Cyclic Voltammetry

Cyclic voltammetry was used as a pretreatment process as well as to judge overall electrode

stability. Supercapacitor test cells were cycled from -1.0V to +1.0V in the case of the aqueous

12

KOH electrolyte, and from -2.0V to +2.0V in the case of the organic TEA-TFB salt dissolved in

acetonitrile. These voltage windows are typical for the two electrolytes tested [15,48]. In both

cases, the sweep rate was 10 mV/s, and the cells were cycled for a minimum of 250 cycles, until

there was no discernable difference between successive cycles.

3.4.2 Galvanostatic Charge and Discharge

Galvanostatic Charge and Discharge cycling was performed to obtain Voltage vs. Time data used

for the calculation of specific capacitances of electrodes across various charge and discharge

rates, as well as the resistance across the cell. The GC experiments were conducted from -0.8V

to +0.8V in the case of aqueous KOH electrolyte, and from -2.0V to +2.0V in the case of organic

TEA-TFB electrolyte. The current densities used ranged from 5 mA per gram to 250 mA per

gram, normalized to the average mass of the two electrodes in a given cell. Five charge-

discharge cycles were performed at every current density for data replication, and the values for

capacitance and resistance were calculated for every charge and discharge process. Data points

were collected every 0.1 seconds.

3.4.2.1 Capacitance from GC Data

Capacitance was calculated from the linear regions of the GC data. The expression used to

calculate capacitance from GC originates from the expression for capacitance of a parallel plate

capacitor shown in Equation 1.

𝐶 =𝑞

𝑉

Equation 1: Capacitance in a parallel plate capacitor

Rather than using an instantaneous charge, q, the equation can be expressed in terms of current,

voltage and timeframe as shown in Equation 2 to give device capacitance in Farads (F).

𝑪𝒅𝒆𝒗𝒊𝒄𝒆 =𝑰𝒅𝒕

𝒅𝑽=

𝑰

𝒅𝑽/𝒅𝒕

Equation 2: Expression for device capacitance from GC data

13

To compare between different materials, the capacitance values can be normalized to mass of

active material, mT, to give gravimetric capacitance in Farads per gram (F/g). This is shown in

Equation 3.

𝐶𝑚,𝑑𝑒𝑣𝑖𝑐𝑒 =𝐼

𝑑𝑉𝑑𝑡

∗ 𝑚𝑇

Equation 3: Expression for gravimetric device capacitance from GC data

This gravimetric capacitance refers to the device capacitance normalized to the mass of active

material instead of an electrode capacitance, which would be four time larger. It is important to

differentiate the two as authors may use either device or electrode specific capacitances in

literature.

3.4.2.2 Resistance from GC Data

Device resistance was calculated from the ohmic drops, or IR drops, which appeared as

instantaneous voltage losses that occurred at the peaks (-0.8V and 0.8V for KOH, and -2.0V and

+2.0V for TEA-TFB). This device resistance encompassed all resistances across the cell,

including the resistance across the wires (deemed negligible), the contact resistance between the

current collectors and biochar surface, the resistance of the solid biochar monolith, and the

resistance of the electrolyte solution.

The expression used to calculate resistance in GCs is derived from the commonly used ideal

supercapacitor circuit model consisting of a capacitor and resistor arranged in series (Figure 5).

[49]

Figure 5: Simple serial capacitor and resistor to model supercapacitor behaviour

During a charging process, the voltage across the circuit model would be the sum of the voltage

across the capacitor, 𝑄/𝐶, and the voltage across the resistor, IR, as shown in Equation 4.

14

During a discharging process, the current changes from positive to negative, with the rest of the

expression remaining the same, as shown in Equation 5.

𝑽𝑻,𝒄𝒉𝒂𝒓𝒈𝒆 =𝑸

𝑪+ |𝑰𝑹|

Equation 4: Voltage across ideal supercapacitor circuit during charge

𝑽𝑻,𝒅𝒊𝒔𝒄𝒉𝒂𝒓𝒈𝒆 =𝑸

𝑪− |𝑰𝑹|

Equation 5: Voltage across ideal supercapacitor circuit during discharge

By subtracting Equation 5 from Equation 4, we obtain the expression seen in Equation 6, where

∆𝑉𝑇 is the voltage drop which occurs at the switch from charge to discharge or from discharge to

charge. Rearranging the equation for R results in the expression shown in Equation 7 which is

used to calculate device resistance in GC data in Ohms (Ω).

∆𝑉𝑇 = 2𝐼𝑅

Equation 6: Ideal supercapacitor voltage drop from charge to discharge

𝑅 =∆𝑉𝑇

2𝐼

Equation 7: Expression for calculating ohmic resistance

Although straightforward at first glance, the calculation of device resistance remains an uncertain

topic due to both limitations with data collection, as well as the uncertainty in circuit modelling

of supercapacitors.

The ohmic drop expression relies on an instantaneous drop in voltage as there is no time

dependency in the circuit model. This becomes problematic when dealing with large amounts of

data when using software such as the Nova 2.1.2 provided by Metrohm in accompaniment with

the Metrohm Autolab PGSTAT302N potentiostat. A significant time delay was observed upon

switching current from positive to negative (or vice versa). The time delay ranged from 1s to 20s,

severely affected the apparent ohmic drop. The solution developed to analyze the data involved

using an 8th-order polynomial function regression on the data found after the voltage drop to

extrapolate backwards towards the expected time of the ohmic drop.

15

The high-order polynomial regression was done using a minimum of 100 data points, with

correlation factors (R-values) of greater than 0.9999. A sum of least squares method was used

through the Metrohm Nova 2.1.2 software. This method is purely empirical and holds no

fundamental basis.

3.4.2.3 Circuit Model Uncertainty

Observed curvature in GC data brings into question the validity of the ideal supercapacitor

model. According to the model, neither the capacitive component nor the resistive component is

dependent on time, suggesting that both values are constant. Many other models exist, such as

the de Levie model, which have been proposed for modeling supercapacitor behavior. [50-53]

New circuit models have been proposed with time dependent components, such as a diffuse

capacitance and diffusive resistance suggested in Sedlakova et al. [54] Their proposed circuit

model is shown in Figure 6, where C1 corresponds to the Helmholtz capacitance, R1 corresponds

to the equivalent series resistance, RL corresponds to the leakage resistance, C2 corresponds to a

time dependent diffuse capacitance, and R2 corresponds to a time dependent resistance to charge

diffusion.

Figure 6: Sedlakova equivalent circuit model introducing time dependency in diffuse

capacitance and resistance for charge diffusion [54]

Another group proposed the circuit model for symmetric carbon supercapacitors shown in Figure

7. This proposed model uses a vertical ladder network in series with an RC parallel network to

reflect a distribution of time constants in a real system [55].

16

Figure 7: Universal equivalent circuit for porous electrodes using vertical ladder network in

series with an RC parallel network as proposed by Fletcher [55]

This circuit model attempts to explain three phenomena observed in supercapacitors not covered

in previous models: open circuit voltage decay, high frequency capacitance losses, and

voltammetric distortion at higher scan rates.

While these newer and more complex models attempt to explain observable supercapacitor

behavior by assigning physical meanings to components within their circuit models, the

application of these circuit models to the data obtained from this study was problematic. Models

either provide very poor fits to the data obtained through GC experiments, or yield nonsensical

fitting parameters. In some cases, the complexity of the circuit model made it too difficult to fit

the model to the GC data. As such, this study used the high-order polynomial regression method

outlined in the previous section to quantify device resistance from GC data.

Given the uncertainty of GC data analysis at the fundamental level, another method of

quantifying device resistance and energy stored was desired.

3.4.3 Chronoamperometry

Chronoamperometry was used as supplemental method of quantifying device resistance, as well

as to determine the total amount of energy that can be discharged from a fully charged

supercapacitor test cell. In aqueous KOH, the voltage was stepped from 0V to 1V and held for

900s for small-scale electrodes, and 1200s for large-scale electrodes while the current response

was recorded. Following the 900s or 1200s, the voltage was stepped back down from 1V to 0V

17

and held for 900s or 1200s while observing the current response. This free-current charge and

discharge cycling was done five times for data replication. For organic TEA-TFB, a similar

procedure was applied, with the only change being the step voltage being from 0V to 2.0V and

from 2.0V back down to 0V. Data points were collected every 0.1 seconds.

The charging process occurred when the voltage was stepped from 0V to 1V in the case of

aqueous electrolyte, and from 0V to 2V in the case of organic electrolyte. A current response

which began at a high positive value and approached the zero in an exponential decay-like

fashion was observed for all charge processes. The discharge process occurred when the voltage

was stepped from 1V to 0V (KOH electrolyte) and from 2V to 0V (TEA-TFB electrolyte). The

current response was similar to that of the charge process, but in the negative region instead,

beginning with a high absolute value of current in the negative region, and decaying towards the

zero. Key features of the current response were the height of the current peaks, and the rate of

current decay towards zero.

3.4.3.1 Energy from Chronoamperometry

To calculate the amount of energy stored during charge or the amount of energy released during

discharge, the conservation of charge equation (Equation 8) can be re-arranged and integrated to

calculate the amount of charge stored or released between ti and tf (Equation 9).

𝑰 = −𝒅𝑸

𝒅𝒕

Equation 8: Current is equal to the charge leaving the system over time

𝑄 = ∫ 𝐼𝑑𝑡𝑡𝑓

𝑡𝑖

Equation 9: Charge transferred as a function of current

Once the amount of charge is known, the amount of energy stored or discharged is calculated

simply by multiplying the charge by the potential difference, or voltage step, that was used in the

experiment (Equation 10). This potential difference was 1V for aqueous electrolyte, and 2V for

organic electrolyte.

𝐸 = 𝑄 ∗ ∆𝑉

Equation 10: Energy stored/released as a function of charge

18

Equation 11 summarizes the calculation of energy from a current response (I vs t) plot obtained

from chronoamperometry.

𝐸 = ∆𝑉 ∗ ∫ 𝐼𝑑𝑡𝑡𝑓

𝑡𝑖

Equation 11: Expression for energy stored/released from current, time and voltage step

The integral is computed using Microsoft Excel by trapezoidal rule approximation to give energy

in units of Joules (J). This expression can then be normalized to mass of active material and

converted into the watt hour per kilogram (Wh/kg) unit that is more common in literature

3.4.3.2 Device Resistance from Chronoamperometry

Experimental data from chronoamperometry shows a difference between energy stored during

charge and energy released during discharge. This energy loss, captured in equation 12 below, is

attributed to Joule heating, or resistive heating described in the Joule-Lenz law in equation 13.

𝐸𝑙𝑜𝑠𝑠𝑒𝑠 = 𝐸𝑐ℎ𝑎𝑟𝑔𝑒 − 𝐸𝑑𝑖𝑠𝑐ℎ𝑎𝑟𝑔𝑒

Equation 12: Energy loss from difference in energy stored vs energy released

𝑃 = 𝐼2𝑅

Equation 13: Joule-Lenz Law (Joule's Law of Heating)

Integrating the power and current terms in the Joule-Lenz law with respect to time allows the

application of the equation over an entire discharge process rather than a single point in time.

Equations 14 shows the derivation of the ohmic resistance expression from integrating the Joule-

Lenz law with respect to time.

∫ 𝑃𝑑𝑡 = ∫ 𝐼2𝑑𝑡𝑡2

𝑡1

∗ 𝑅

𝑅 =∫ 𝑃𝑑𝑡

∫ 𝐼2𝑑𝑡𝑡2

𝑡1

=𝐸𝑙𝑜𝑠𝑠𝑒𝑠

∫ 𝐼2𝑑𝑡𝑡2

𝑡1

Equations 14: Resistance in terms of energy loss, current and time from Joule-Lenz law

Similar to the energy calculations in Section 3.4.3.1, the integrals were computed in Microsoft

Excel using the trapezoidal rule approximation to give values in Ohms (Ω).

19

While this method ignores the role of device resistance during the charging process, this is not an

issue since the time allotted for charging and discharging is greater than the actual time required

for a full charge or discharge. As such, the state of full charge is still achieved despite there being

losses due to resistive heating.

3.4.3.3 Power from Chronoamperometry

Power is calculated simply as the product of current and potential difference or voltage step

(Equation 15). These values are reported in units of Watts (W) and can be normalized to mass to

give the more common unit of Watts per kilogram (W/kg).

𝑃 = 𝑉 ∗ 𝐼

Equation 15: Power Expression

However, this expression only describes power at a point in time, and not an average power for a

given window of time. Given the nature of chronoamperometry, the current response is highest

immediately after the voltage step, and then follows an exponential decay behavior. This, paired

with the fact that the 15-minute window is in excess of the actual time it takes to fully charge and

discharge the cell, suggests that an average power value calculated from this experiment would

not be accurate or relevant. As such, a peak power value is calculated instead, using the peak

current from the current response to a voltage step.

20

Results and Discussion

4.1 Characteristics of Biochar

An SEM image of the maple-derived biochar material is shown in Figure 8. The image is

oriented in the axial direction of charred wood structure with macroscopic mass transport

channels of the wood structure indicated by arrows. Vessels, are on the order of 50-100 µm in

diameter and on the order of 1 centimeter in length and are the primary structures responsible for

water transport in hardwoods along the axis of a tree [44,45]. Rays serve as media for transport

in the radial direction and provide connections between adjacent vessels. Tracheids are on the

order of 2-10 µm in diameter and occupy a secondary role in the transport of water, with their

main role being structural reinforcement [56].

Tracheid

Ray

Vessel

Figure 8: Labelled axial-direction SEM image of maple-derived biochar featuring macroscopic

water-transport channels

Figure 9 shows an angled side-view image of the maple-derived biochar electrode. In terms of

wood structure, this angle represents the radial orientation and features exposed vessels running

along the electrodes.

21

Figure 9: Angled side view of monolithic maple-derived biochar featuring exposed vessels

The microporous pore size distribution obtained from DFT data reduction of a CO2 physisorption

isotherm is shown in Figure 10. Analysis was carried out using ASiQwin software provided by

Quantachrome using the NLDFT slit pore kernel.

Figure 10: Pore size distribution and cumulative specific surface area of maple-derived biochar

measured by CO2 physisorption and DFT data reduction

22

The vast majority of pores measured across the 0.3 to 1.5 nm pore diameter range were found in

the 0.3 nm to 0.7 nm range, which contributes approximately 400 m2/g of the specific surface

area. The total specific surface area measured by CO2 was 454 m2/g.

This corresponds to the measurements made by Daniel Yanchus, a recent M.A.Sc. graduate from

the Green Technology Research Group, on biochar made from same precursor material and

pyrolysis procedure in 2017. Measured values included SSA values of 487 m2/g from CO2 using

NLDFT, and 172 m2/g from BET using N2 [12]. Although these values are low relative to typical

carbon materials in supercapacitors, there are many additional treatment processes, such as

activation, that could be applied to the biochar used in this study to increase the measured

specific surface area [57-59].

The pore size distribution in maple-derived biochar that is dominated by pores of less than 0.7

nm in diameter raises interesting issues of electrolyte-electrode compatibility. While studies in

the past have proven that electrolyte ions may have their solvation shells distorted to fit in pores

smaller than the solvated ion size, the extent to which solvation shells can distort in aqueous and

organic electrolytes is unclear [23,24]. The bare and solvated ion diameters for the two tested

electrolyte species are listed in Table 5.

Table 5: Ion diameters (nm) for KOH and TEA-TFB electrolyte ions [23,60,61]

Electrolyte Ion Bare Ion

Diameter (nm)

Solvated Ion

Diameter (nm)

Tetraethylammonium

tetrafluoroborate (TEA-

TFB) in acetonitrile

Tetraethylammonium (TEA+)

(C2H5)4N+

0.68 1.30

Tetrafluoroborate (TFB-)

BF4-

0.33 1.16

Potassium Hydroxide

(KOH) aqueous

Potassium

K+ 0.298 0.66

Hydroxide

OH- 0.266 0.60

In the case of K+ and OH- ions, bare ions would be able to fit all micropores available. While

fully solvated K+ ions, measuring 0.660 nm in diameter, and fully solvated OH- ions, measuring

0.60 nm in diameter, may have difficulty accessing the 0.3-0.7 nm pores in the maple-derived

23

biochar, some distortion of the solvation shell is expected, resulting in an effective diameter

between the bare and solvated ion diameter.

However, in the case of the organic electrolyte, even a fully desolvated TEA+ ion, measuring

0.68 nm in diamester, would be too large for the available micropores. As such, it was

hypothesized that the organic TEA-TFB electrolyte would not be compatible with the maple-

derived biochar, despite its prevalence in commercial supercapacitors. This was verified through

electrochemical performance testing of large-scale electrodes in TEA-TFB electrolyte, the results

of which are shown in Section 4.4.2.

4.2 Electrochemical Performance of Small Electrodes

Small-scale electrodes using 4M KOH electrolyte underwent 750 cycles of cyclic voltammetry

pretreatment. Significant changes in current response in the cyclic voltammogram (current versus

potential plot) was observed between cycles 1 and 250, shown in Figure 11.

Figure 11: Cyclic Voltammetry of small-scale electrodes in 4M KOH (cycles 1 and 250)

The increasing range of currents with cycle number suggested improvements in EDLC behavior.

This improvement continued with cycles until the 500th cycle, at which point the electrode

performances stabilized in a rectangle-like shape, indicative of ideal EDLC behavior. Another

250 cycles were performed to verify stability, shown in Figure 12. No significant change in

voltammogram shape was observed between cycles 500 and 750.

24

Figure 12: Cyclic voltammogram of small-scale electrodes in 4M KOH. Stable performance

shown between cycles 501 and 750.

Capacitance and device resistance values were obtained from galvanostatic charge and discharge

cycling. Specific capacitance normalized to the mass of maple-derived biochar in the device is

plotted versus current density in Figure 13. Peak capacitive performance was measured as 3.61 F

or 33.75 F/g at 5 mA/g. This value decreased to 1.89 F or 17.658 F/g at 200 mA/g. These values

were consistent with the values measured in the similar study of maple-derived biochar.

Figure 13: GC specific capacitance vs current density for small-scale electrodes in 4M KOH

Device resistance values are plotted against current density in Figure 14. Cell resistance was

found to be highest at the 5 mA/g current density, with a positive peak ohmic drop resistance of

25

1.31Ω and a negative peak ohmic drop resistance of 1.57Ω. The difference between ohmic drop

resistances measured at positive peaks and those measured at negative peaks decreased at higher

current densities, as did the measured resistance values. At 200 mA/g, the positive peak

resistance was 1.16Ω while the negative peak value was 1.22Ω.

Figure 14: GC extrapolated device resistance vs current density in small-scale electrodes in 4M

KOH

Neither the difference in positive and negative peak resistance values nor the decrease in

resistance with higher current densities are believed to be indicative of chemical or physical

phenomena within the device, but rather remnant effects of the software limitations described in

Section 3.4.2.2. Although polynomial regression is used to bypass the issue of software-induced

time delay in data acquisition, this solution may not be perfect.

The current response (current versus time) for a sample charge-discharge cycle from the

chronoamperometry cycling of small-scale electrodes in 4M KOH is presented in Figure 15. The

sudden increase in current to a peak of 0.4 Amperes corresponds to the potential step from 0V to

1V, which was the charging process. The sudden decrease of current to nearly -0.4 Amperes

corresponds to the potential step from 1V to 0V, which was the discharging process. The

apparent time to charge or time to discharge (the time after which the rate of change of current

with respect to time is near zero) was on the order of 30 to 50 seconds.

26


(charge) and from 1V to 0V (discharge) of small-scale electrodes in 4M KOH

The amount of energy stored and released per cycle is summarized in Table 6 below. The

average specific current or current density for each charge and discharge process is also indicated

as a measure of average power output. As mentioned previously in Section 3.4.3.3, the concept

of average power is less valuable in this experiment as the true average power would be

considerably underestimated. The underestimation comes from the fact that the 15 minutes

allotted for every charge or discharge process was far greater than the anticipated required time.

This ensured that the device was able to fully charge and discharge. The calculated resistance

values were calculated using the energy difference from charge and discharge as ohmic heat

losses due to device resistance.

27

Table 6: Chronoamperometry results summary for small-scale electrodes in 4M KOH

4M KOH

Energy

(J) Energy

Difference (J) Average Specific Current (mA/g)

Calculated Resistance (Ω)

Cycle 1 Charge 3.246

Discharge -2.935 0.311 -2.61 1.241


Discharge -2.946 0.333 -2.75 1.323


Discharge -2.899 0.287 -2.84 1.145


Discharge -2.917 0.281 -2.64 1.122


Discharge -2.928 0.286 -2.66 1.147

The energy values used to compare between devices in this study are the discharge values, as

those represent the amount of useable energy. The small-scale electrodes were able to store and

discharge an average of 2.925 J or 27.3 J/g. The peak power output was 0.376 W or 3.51 W/g,

calculated using the peak current values at discharge.

28

4.3 Electrochemical Performance of Large Electrodes

Large-scale electrodes used in 4M KOH performance testing underwent 1000 cycles of cyclic

voltammetry before they were deemed stable. Changes in the voltammograms occurred much

more slowly for the large-scale electrodes than their small-scale counterparts as seen in Figure

16.

Figure 16: Cyclic voltammogram of large-scale electrodes in 4M KOH (cycles 1 and 250)

The area within the cyclic voltammogram increased gradually over the course of repeated

cycling. In Figure 17, asymmetric behavior was observed as the current peak in the positive

region approached 0.3 A, while the current peak in the negative region approached -0.2A. With

cycling, the asymmetry was corrected, and the peak currents increased in absolute value until

stabilizing at approximately 0.4A and -0.4A after 750 cycles. Another 250 cycles were

performed after that to ensure stability, the results of which are shown in Figure 17.

29

Figure 17: Cyclic voltammogram of large-scale electrodes in 4M KOH. Stable cycling shown

between cycles 750 and 1000.

Key differences between the small-scale and large-scale electrode cyclic voltammograms are the

shapes of the voltammograms, as well as the amount of current entering and leaving the devices

during cycling. The small-scale electrodes exhibited very typical EDLC behavior in the

rectangular voltammograms but had relatively low peak currents in the 20-30 mA range. The

large-scale electrodes had voltammograms that were far less rectangular, indicative of a greater

resistance to charge and ion transport. This resistance is due to the sheer size of the electrodes,

which are twice as thick as the small-scale electrodes, and nearly 30 times larger by mass. A

larger resistance through the solid material was therefore expected, along with a longer time

needed for ion transport through the electrode. To compensate for this non-ideal behavior, the

large-scale electrodes exhibit far larger peak currents of -400 mA and 400 mA respectively.

Specific capacitances of the large-scale electrodes in 4M KOH electrolyte are plotted versus

current density in Figure 18. Behavior of the large-scale electrodes is very similar to that of the

small-scale electrodes, with a maximum capacitance value of 96.7 F or 34.5 F/g at 5 mA/g. This

capacitance decays with increasing current density to 65.7 F or 23.5 F/g at 250 mA/g.

30

Figure 18: GC specific capacitance vs current density in large-scale electrodes in 4M KOH

The resistance behavior was very similar to that of the small-scale electrodes, and the resistance

values calculated from polynomial extrapolation are plotted versus current density in Figure 19.

Figure 19: GC extrapolated device resistance vs current density in large-scale electrodes in 4M

KOH

The current response of a sample charge-discharge cycle from large-scale electrode

chronoamperometry is shown in Figure 20. Key differences between the small-scale current

response and large-scale current response were the peak currents, as well as apparent discharge

time (the time it takes for the rate of change of current with respect to time to be near zero).

31


(charge) and from 1V to 0V (discharge) of large-scale electrodes in 4M KOH

The peak currents for charge and discharge exhibited by the large-scale electrodes were 0.83 A

and -0.85 A respectively. Although still significantly larger than the small-scale electrodes’ peak

currents, an even larger peak current was expected from the large-scale electrodes. An order of

magnitude increase was observed in the apparent time to charge or discharge, as it took well over

500 seconds before the current response and rate of change of current would approach the zero.

The increases in peak current as well as time to charge/discharge suggested that the amount of

energy stored would be much higher in the large-scale electrodes, which was an expected

outcome. The results of the chronoamperometric charge-discharge cycles are summarized in

Table 7. The average amount of energy stored and discharged was 83.2 J or 29.7 J/g. The peak

power output measured with the peak current was 0.85 W or 0.30 W/g. An interesting result

from chronoamperometry was the very low resistance, calculated by taking the energy difference

between charge and discharge processes, and treating that value as heat loss due to ohmic

resistance. An average value of 0.25 Ω was obtained using chronoamperometry, while a value of

1.27 Ω was obtained using polynomial regression of GC data. While there is uncertainty in

performance modelling and GC data processing, it remains unclear which value is closer to the

true value of device resistance.

32

Table 7: Chronoamperometry results summary for small-scale electrodes in 4M KOH

4M KOH

Energy (J)

Energy Difference (J)

Average Specific Current (mA/g)



Discharge -83.108 6.177 -49.48 0.340


Discharge -83.808 4.565 -39.91 0.254


Discharge -83.366 3.711 -39.70 0.209


Discharge -82.958 3.906 -39.51 0.221


Discharge -82.549 4.236 -39.30 0.241

While the amount of energy stored increased proportionally with the mass increase (i.e. the

energy per mass remained virtually the same), the peak power output decreased dramatically on

a per mass basis. This indicates that by matching maple-derived biochar electrodes’ thicknesses

to the length of the mass transport channels in the precursor material, the amount of energy

stored can be increased proportionally with electrode weight. Even though the power density

decreased by an order of magnitude, the power density offered by the large-scale electrodes is

still competitive with other supercapacitor devices. With possibilities of further electrode

enhancement through activation and electrode-doping, the use of wood-derived biochar for large-

scale electrodes is promising.

33

4.4 Effects of Electrolyte on Large-scale Device Performance

4.4.1 Concentration

The effect of electrolyte concentration on large-scale device performance was explored by

testing device performance in 1M KOH and 6M KOH to supplement the data collected in the 4M

KOH tests used to compare small-scale electrodes to large-scale electrodes. All tests were

conducted using the same electrode pairs.

The changing of electrolyte was performed in a 3-step process. First, a weeklong DI-water

leaching process was used, where the electrodes were placed in a 1L glass beaker and

continuously flushed with DI water. Next, the electrodes were dried in an oven at 120oC for 12

hours. Finally, the electrodes were re-assembled into a supercapacitor test cell and left to soak in

new electrolyte for a week to ensure electrolyte permeation.

4.4.1.1 1M KOH Performance

Cyclic voltammograms for large-scale electrodes in 1M KOH are plotted in Figures 21 and 22.

The voltammogram shapes are similar to those observed in 4M KOH, however the peak currents

are significantly lower in value. Final values of peak currents observed in cycle 500 are 190 mA

and -190 mA, while values observed in 4M KOH were as high as 400 mA and -400 mA. Lower

performance was expected due to the decrease in electrolyte concentration.

Figure 21: Cyclic voltammogram for large-scale electrodes in 1M KOH (cycles 1 and 250)

34


The specific capacitance is plotted against current density in Figure 23. Capacitance values are

lower than those measured in 4M KOH, ranging from 80.55 F or 28.77 F/g at 5 mA/g to 39.58 F

or 14.134 F/g at 100 mA/g.

Figure 23: GC specific capacitance vs current density for large-scale electrodes in 1M KOH

Current densities beyond 100 mA/g were not able to be tested, as the ohmic drop would have

been in excess of the voltage window. Lower capacitance values were expected as double-layer

capacitance typically increases with increasing electrolyte concentration. This phenomenon

occurs due to the screening of charge for higher concentration electrolytes, which results in the

lowering of the Debye length, λD, and the increase in capacitance [62,63]. The significant drop

35

off in capacitance from 50 mA/g to 100 mA/g is believed to be due to the depletion of ions at

rapid current densities.

An increase in device resistance was also observed from GC results. The device resistances

extrapolated from galvanostatic cycling are plotted versus current density in Figure 24. This

increase in resistance relative to the resistance measured in 4M KOH is due to the relationship

between electrolyte conductivity and concentration. The concentration of 4M KOH at room

temperature is approximately 570 mS/cm, while the concentration of 1M KOH at room

temperature is less than half that, at approximately 215 mS/cm [64]. As the resistance of the

electrode remained the same (the same electrodes were used in both the 1M and 4M cases), and

the same procedure was used to minimize contact resistance, electrolyte resistance is the best

explanation for the difference in device resistances when changing electrolyte concentration.

Figure 24: GC extrapolated device resistance vs current density for large-scale electrodes in

1M KOH

The current response for a sample chronoamperometric charge-discharge cycle using 1M KOH is

shown in Figure 25. Relatively small peak current responses of 0.35 A and -0.35 A were

observed, along with a long time for charge and discharge, in the order of hundreds of seconds.

36



Results from the chronoamperometry tests in 1M KOH are displayed in Table 8. A significantly

lower amount of energy is stored and discharged in 1M KOH than in 4M KOH, with an average

discharge value of 53.45 J or 19.09 J/g. Lower values for energy were expected, as lower

electrolyte concentrations would result in less ions in the double layer at steady state.

Table 8: Chronoamperometry results summary for large-scale electrodes in 1M KOH

1M KOH

Energy (J)





Discharge -50.645 13.040 -30.15 3.030


Discharge -54.417 11.698 -25.91 2.604


Discharge -54.309 11.396 -25.86 2.558


Discharge -53.951 11.832 -25.69 2.664


Discharge -53.914 12.130 -25.72 2.726

37

The resistance values calculated from the difference in energy charged vs energy discharged are

similar, albeit slightly lower than the values obtained from the GCs.

Overall, the large-scale electrodes in 1M KOH behaved as expected, with lower specific

capacitance and energy storage values and higher device resistance than the large-scale

electrodes in 4M KOH.

4.4.1.2 6M KOH Performance

The cyclic voltammograms for the large-scale electrodes in 6M KOH are shown in Figures 26

and 27. While the 6M voltammograms exhibit the expected shapes, higher current values were

expected from the electrodes in 6M KOH. Higher current values than those observed in 4M

KOH CVs were expected from CV tests using 6M KOH. Instead, the 6M KOH stable peak

current values after 1000 cycles approached 300 mA and -300 mA, which are between the 1M

and 4M KOH CV peak current values.


38

Figure 27: Cyclic voltammogram for large-scale electrodes in 6M KOH (cycles 750-1000)

Specific capacitance values obtained from GCs are plotted against current density in Figure 28.

Higher capacitances values than those obtained from 4M KOH GCs are observed at slow current

densities. The capacitance values at these current densities range from 107.16 F or 38.27 F/g at 5

mA/g to 94.49F or 33.75 F/g at 100 mA/g. This increased capacitance was expected due to

higher electrolyte concentration. However, between 100 mA/g and 200 mA/g, a significant drop

in capacitance occurred. Measured capacitance at 200 mA/g was 36.62 F or 13.08 F/g. This drop

is believed to be due to ion mobility. At slower current densities, enough time is given for ion

migration to complete. However, past some critical point, it is hypothesized that the high

concentration of ions may hinder the effective movement of ions.

39

Figure 28: GC specific capacitance vs current density for large-scale electrodes in 6M KOH

Resistance values obtained from GCs were plotted against current density in Figure 29. The

resistance values were noticeably higher than those obtained from 4M KOH GCs, but lower than

resistance values obtained from 1M KOH GCs.


6M KOH

This was contrary to expectations, as the electrolyte resistance for more 6M KOH is lower than

that of 4M KOH. The conductivity of 6M KOH is approximately 627 mS/cm, whereas the value

for 4M KOH is 570 mS/cm [64]. This suggests that with all else constant, the device resistance

in 6M KOH should have been less than the device resistance in 4M KOH. As the resistance

40

across the solid-material within the electrodes remained the same, the only possible explanation

is an increased contact resistance. Although attempts were made to minimize contact resistance

by using a conductive graphite adhesive at the interface between current collector and electrode,

the method is likely imperfect.

The current response for a sample chronoamperometric charge-discharge cycle in 6M KOH is

shown in Figure 30. The 6M KOH current response is very similar to the current response

measured in 4M KOH, with a slightly higher peak current for charging processes, but a lower

peak current for discharging processes. Although a higher discharge peak current was expected,

this lower value is consistent with the GC results. A higher contact resistance resulting in a

higher device resistance would affect the current response in the way shown in

chronoamperometry. The time to charge/discharge remains similar, on the order of hundreds of

seconds.



Results for chronoamperometric cycling in 6M KOH are displayed in Table 9 below. Large-scale

electrodes in 6M KOH charged and discharged the highest amount of energy across all maple-

derived biochar chronoamperometry tests. On average, 91.96 J or 32.84 J/g were discharged per

cycle. As was in the case of large-scale electrodes in 4M KOH, the calculated resistances, with

an average value of 0.808 Ω, were much lower than the resistances obtained through GCs.

41

Table 9: Chronoamperometry results summary for large-scale electrodes in 6M KOH

6M KOH

Energy (J)





Discharge -91.872 9.523 -54.69 0.706


Discharge -94.524 8.038 -45.01 0.611


Discharge -92.778 9.190 -44.18 0.727


Discharge -90.960 10.981 -43.32 0.904


Discharge -89.659 12.773 -41.30 1.090

In summary, while the large-scale device performance in 1M KOH electrolyte matched

expectations derived from double-layer theory and relationships between electrolyte

concentration and conductivity, the device performance in 6M KOH deviated from expectations

due to the inability to fully control contact resistance. While contact resistance affected the

resistance and power capabilities of the large-scale electrodes in 6M KOH, the capacitance

values and the amounts of energy able to be stored and released were higher than those measured

in 4M KOH as expected.

4.4.2 Organic Electrolyte Performance – 1M TEA-TFB

The cyclic voltammograms for the large-scale electrodes in 1M TEA-TFB in acetonitrile are

shown in Figures 31 and 32. Substantial changes were observed within the first 500 cycles, as a

very flat voltammogram with current peaks of 6 mA and -7 mA grew with increasing cycles to a

more angular voltammogram, similar to the ones observed in the large-scale electrodes in 1M,

4M, and 6M KOH. After 500 cycles, larger current peaks of 28 mA and -37 mA were observed.

42

Figure 31: Cyclic voltammogram for large-scale electrodes in 1M TEA-TFB showing

significant growth between cycles 1 and 500

The asymmetry in current peak values observed at cycle 500 disappeared with additional cycling.

Figure 32 shows the final observed changes between cycle 750, where asymmetry has lessened

but is still present, and cycle 1000, where the asymmetry has been eliminated, and current peaks

of 28 mA and -28 mA are observed.

Figure 32: Cyclic voltammogram for large-scale electrodes in 1M TEA-TFB showing stabilized

behavior between cycles 750 and 1000

43

While the final shape of the cyclic voltammograms for the test cell in 1M TEA-TFB is similar to

the shapes observed in KOH studies, the order of magnitude decrease in current responses from

hundreds of mA to tens of mA suggests lower performance from 1M TEA-TFB than all of the

KOH concentrations.

This is confirmed by the specific capacitance values obtained from GC. The low specific

capacitance values are plotted versus current density in Figure 33. The largest capacitance value

obtained with TEA-TFB electrolyte, 1.68 F or 0.60 F/g was lower than even the lowest

capacitance values obtained using KOH electrolyte, including those measured with small-scale

electrodes.

Figure 33: GC specific capacitance vs current density for large-scale electrodes in 1M TEA-

TFB

Poor capacitive performance suggests that there is very little formation of a double-layer. As

mentioned in Section 4.1, the pore size distribution of maple-derived biochar showed the

majority pores measured by CO2 would be inaccessible to the large TEA+ cation, making double-

layer formation impossible in that pore range, even in the unlikely case of full ion desolvation.

The poor capacitive performance is accompanied by very high device resistance values, plotted

against current density in Figure 34. This resistance is believed to originate from the electrolyte

resistance. The electrolyte conductivity for TEA-TFB is on the order of 59.9 mS/cm [5,48],

while the electrolyte conductivities for KOH concentrations range from 215 mS/cm to 626

44

mS/cm [64]. This order of magnitude change in electrolyte conductivities matches the change

observed in device resistance.


1M TEA-TFB

The current response for a sample chronoamperometric charge-discharge cycle is plotted in

Figure 35. Large-scale electrodes in TEA-TFB exhibits an absolute performance similar to that

of the small-scale electrodes in 4M KOH. This similarity is observed through the peak current

values of 0.37 A and -0.35 A and an apparent time to full charge/discharge of nearly 30 seconds.


(charge) and from 2V to 0V (discharge) of large-scale electrodes in 1M TEA-TFB

45

The numerical results of the 1M TEA-TFB chronoamperometric charge and discharge cycles are

displayed in Table 10. The average amount of energy able to discharge after charging was 2.62 J

or 0.94 J/g. The value of absolute energy stored is comparable to the amount stored in the small-

scale electrodes, however due to the mass difference between small- and large-scale electrodes,

the energy difference in 1M TEA-TFB is very low. The resistance calculated from the energy

difference was similar to the values at low current densities measured in GCs.

Table 10: Chronoamperometry results summary for large-scale electrodes in 1M TEA-TFB

1M TEA-TFB

Energy (J)





Discharge -2.390 0.310 -0.73 11.612


Discharge -2.672 0.318 -0.61 11.601


Discharge -2.750 0.376 -0.60 13.900


Discharge -2.653 0.343 -0.60 12.799


Discharge -2.620 0.296 -0.64 11.096

46

4.5 Summary of Electrode Size and Electrolyte Concentration and Species Performance Studies

This section aims to summarize and explain the findings in the electrolyte concentration and

species experiments as well as the comparison between large- and small-scale electrodes in 4M

KOH. Starting with the GC results, the plot of specific capacitances versus current densities for

all electrolyte concentrations and species using large-scale electrodes, and the specific

capacitances versus current densities for small-scale electrodes in 4M KOH are shown in Figure

36.

Figure 36: Complete set of specific capacitances vs current densities measured

High capacitance exhibited by electrodes in 6M KOH from 5 mA/g to 100 mA/g was an

expected outcome of the study, as capacitance in the double-layer is inversely proportional to the

debye length, λD, which itself scales inversely with electrolyte concentration. Past 100 mA/g, the

high capacitance falls drastically below capacitances measured in other concentrations of KOH,

due to a high concentration causing hindered ion movement in and out of the small micropores

found in maple-derived biochar.

47

As double layer capacitance is theoretically proportional to electrolyte concentration, given that

all else remains constant, devices in 4M KOH should exhibit the next highest capacitive

performances after 6M KOH. This was observed in both large-scale and small-scale electrode

devices. At 5 mA/g, both small-scale and large-scale electrodes had capacitances of

approximately 34.5 F/g, however at higher current densities, large-scale electrodes had

significantly higher capacitances due to the decay of capacitance with increasing current density

occuring in a concave-down manner. Small-scale electrodes on the other hand experienced

capacitance decay with increasing current density in a convex-down manner. It remains

uncertain what the difference in curvature represents.

1M KOH electrolyte shows the poorest capacitive performance out of all KOH electrolyte tests,

due to the electrolyte concentration and double layer capacitance relationship.

The organic TEA-TFB electrolyte exhibited negligible capacitance values relative to those

measured in KOH electrolyte. TEA-TFB is incompatible with maple-derived biochar electrodes

used in this study, as the TEA+ ion, even as a bare ion without a solvation shell, is too large to fit

in the micropores available in the electrodes [23,61]. Thus, very little surface area is available for

double-layer formation.

The complete set of GC extrapolated device resistance values measured in the study are plotted

versus current densities in Figure 37.

48

Figure 37: Complete set of GC extrapolated device resistances vs current densities

The device using large-scale electrodes in 1M TEA-TFB resulted in the highest device resistance

values. Electrolyte resistance is considered the main contribution to this high resistance as the

conductivity of TEA-TFB at room temperature is on the order of 59.9 mS/cm [5,48]. This value

is an order of magnitude lower than room temperature conductivity values of 1M KOH, the least

conductive of the KOH set.

1M KOH has a conductivity of approximately 215 mS/cm at room temperature [64]. As the least

conductive KOH solution, device resistance was expected to be the highest in 1M KOH

compared to other KOH species. This was consistent with experimental results.

4M KOH has a conductivity of approximately 570 mS/cm and was expected to exhibit less

device resistance than both the 1M KOH device tests and 1M TEA-TFB device tests [64]. Small-

scale device resistance and large-scale device resistance measured in 4M KOH were essentially

the same.

6M KOH has a conductivity of approximately 627 mS/cm and was expected to have the least

device resistance of all electrolyte solutions tested [64]. However, resistance values measured

from GCs in 6M KOH were higher than the values measured in 4M KOH, and slightly lower

49

than those measured in 1M KOH. As contact resistance and electrolyte resistance were the only

two resistances that could change in the system, the only possible reasoning was that the contact

resistance increased, despite the steps taken to minimize contact resistance. This would have

occurred during the cell assembly process.

Cell resistance was also calculated from heat loss in the chronoamperometric charge and

discharge cycling. The difference in energy stored and energy discharged was assumed to be lost

as heat. The values obtained from this calculation are presented in Figure 38.

Figure 38: Bar graph comparing the complete set of device resistances calculated from

chronoamperometry

While the values themselves may differ from the resistance values obtained in GCs, the trends

between the different electrolyte concentrations and the 1M TEA-TFB remain the same. The

exception to this is the small-scale device tested in 4M KOH. While the small-scale device

exhibited the same resistance as the large-scale device in 4M KOH in GCs, it exhibited device

resistance values between the values measured in the large-scale device using 6M KOH and the

values measured using 1M KOH. However, as shown in Equation 16, resistance is a function of

contact area, and as the contact area in the small-scale electrode setup differs largely from the

contact area in the large-scale electrode setup, the two device resistances should in fact be

different.

50

𝑅 = 𝜌𝑙

𝐴

Equation 16: Pouillet's law: material resistance, R, increases with with length, l, but decreases

with cross-sectional area, A.

The ability to account for this difference in contact area, paired with the successful replication of

resistance trends across the different electrolyte solutions, suggests that this method of

calculating device resistance from ohmic heating in chronoamperometric charge and discharge

cycling may be more reliable than the GC method which uses ohmic drop. While the GC method

using ohmic drop data is affected by the experimental and theoretical uncertainty described in

Sections 3.4.2.2 and 3.4.2.3, the chronoamperometry method uses a simple heating method, with

the only assumption being that the energy difference between charge and discharge processes is

lost as heat. More rigorous testing is needed before definite conclusions are to be made.

The energy densities available for discharge calculated from chronoamperometric charge and

discharge cycling for all tested devices are plotted in Figure 39.

Figure 39: Bar graph comparing the energy densities of the small-scale device and large-scale

device measured across all electrolytes

51

Very similar energy densities were observed between the small-scale electrodes and large-scale

electrodes in 4M KOH. This suggests that monolithic wood-derived biochar electrodes can be

used in the scale-up of supercapacitor electrodes without the decrease in energy capacity.

The energy density of the large-scale device in 6M KOH was slightly higher than the energy

density of the large-scale device in 4M KOH due to electrolyte conductivity and the resultant

increase in double-layer capacitance. For similar reasons, the energy density for a large-scale

device in 1M KOH was significantly lower than the energy density measured in 4M KOH. This

difference. This is because the ratio of conductivity between 4M KOH and 1M KOH is much

greater than the ratio of conductivity between 6M KOH and 4M KOH.

As TEA-TFB could not form a double-layer in the micropores, the energy density was

negligible.

The peak power values calculated from peak currents in chronoamperometric charge and

discharge were normalized to the total electrode masses of each device. The peak power

densities for each device and electrolyte are presented in Figure 40.

Figure 40: Bar graph comparing the peak power density of small-scale and large-scale

electrodes measured across all electrolytes

52

Interestingly, the small-scale electrodes in 4M KOH had the highest peak power output of 3494

W/kg, due to comparable peak current to the large-scale electrodes (0.376 A of current

discharged), with a total electrode mass of 0.1076 g.

When scaled to large-scale electrodes in 4M KOH, the peak power output dropped to 304 W/kg

as the peak current output merely increased my slightly more than two-fold, while the mass

increased by more than 25 times.

The large-scale electrodes in 1M KOH had the lowest power density of the KOH group, 125

W/kg, as was expected due to the low electrolyte conductivity. However large-scale electrodes in

6M KOH did not have the expected high power density. Due to contact resistance, the 6M KOH

device’s power density was lower than the 4M KOH large-scale device.

While TEA-TFB electrolyte was not compatible with the maple-derived biochar’s micropores,

the electrolyte still exhibited good power density values, as the macropores and possible

mesopores could still provide small amounts of area for double-layer formation, while the wider

voltage window of acetonitrile solvent doubled the amount of power produced per unit of

current. These surfaces would also allow for far more ion movement as the volume to surface

area ratio in mesopores and macropores far exceeds the ratio in micropores. While the power

density of large-scale electrodes in 1M TEA-TFB may have been comparable to large-scale

electrodes in KOH electrolytes, the time to charge/discharge was a full order of magnitude less

(~50 seconds versus ~500 seconds). As a result, the total amount of charge stored in electrodes

using 1M TEA-TFB was miniscule.

53

4.6 Device Resistance Analysis and Implications

In the previous sections, device resistances were compared, and their differences explained by

changes in electrolyte conductivity, electrode geometry, and in the case of large-scale electrodes

in 6M KOH electrolyte, contact resistance. This section attempts to explain the physical meaning

of device resistance, and the relative contributions of its constituent resistive components.

Taking into consideration the physical arrangement of a supercapacitor cell, the total device

resistance can be represented by the model shown in Figure 41, where Rcontact represents the

contact resistance between the nickel current collectors and the electrode surfaces, Rseparator

represents the resistance across the porous separator, Rcarbon represents the resistance of the solid

carbon material of the electrodes, and Relectrolyte represents the electrolyte contributions to

electrode resistance. Electrode resistance is modelled as carbon resistance and electrolyte

resistance arranged in parallel.

Figure 41: Proposed model of device resistance of supercapacitor test cells with contact

resistances, electrode resistances and separator resistance arranged in series.

From the model, an equation for device resistance as a function of the constituent resistances can

be derived. This equation is presented in Equation 17.

𝑅𝑑𝑒𝑣𝑖𝑐𝑒 = 𝑅𝑐𝑜𝑛𝑡𝑎𝑐𝑡,1 + 𝑅𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒,1 + 𝑅𝑠𝑒𝑝𝑎𝑟𝑎𝑡𝑜𝑟 + 𝑅𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒,2 + 𝑅𝑐𝑜𝑛𝑡𝑎𝑐𝑡,2

Equation 17: Device resistance is the sum of constituent resistances arranged in series

Rcarbon

Relectrolyte

Rcarbon

Relectrolyte

RseparatorRcontact Rcontact

Electrode 1 Electrode 2

54

The resistance of an electrode is determined by the relative amounts of carbon material and

electrolyte in each electrode, as well as the geometry of the electrode. As a result, electrode

resistivity can be expressed as:

𝜌𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑑𝑒−1 = (

𝑆𝑜𝑙𝑖𝑑 𝐹𝑟𝑎𝑐𝑡𝑖𝑜𝑛

𝜌𝑐𝑎𝑟𝑏𝑜𝑛+

𝑉𝑜𝑖𝑑 𝐹𝑟𝑎𝑐𝑡𝑖𝑜𝑛

𝜌𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑙𝑦𝑡𝑒)

Equation 18: Electrode resistance as a function of carbon resistivity, electrolyte resistivity

and relative amounts of solid material and void space.

Through the work done by Randeep Gabhi, Ph.D. Candidate at the Green Technology Research

Group, bulk and skeletal conductivities of maple-derived biochar pyrolyzed to 800oC were

obtained through a 4-point probe method, along with the solid and void fractions of the material.

The solid fraction was measured to be 21.8%, and the void fraction, 78.2%. The bulk and

skeletal conductivities for the biochar measured by Randeep Gabhi are presented in Table 11,

along with the conductivities and resistivities of the electrolytes (presented in earlier sections).

Table 11: Conductivities and resistivities of carbon material and electrolytes. [48,64]

Material Conductivity (S/m) Resistivity (mΩ/m)

Bulk Carbon Electrode 83 12

Skeletal Electrode (graphitic) 381 2.3

Skeletal Electrode

(amorphous) 302 3.3

1M TEA-TFB Electrolyte 6.0 167

1M KOH Electrolyte 21.5 47



With the values listed above, the bulk resistivities of the large-scale electrodes and the resulting

resistances were calculated. The results are shown in Table 12 below.

55

Table 12: Calculated electrode bulk resistivities and resistances for large-scale electrodes in

various electrolytes

Electrolyte Electrode Bulk Resistivity

(mΩ/m) Electrode Resistance (Ω)

1M TEA-TFB Electrolyte 11.40 0.38

1M KOH Electrolyte 10.02 0.34



From these theoretical calculations, we can see the expected resistances for the electrodes across

all electrolytes were below 0.4 Ω, lower than most of the resistances measured, except for large-

scale electrodes in 4M KOH. This would suggest that contact resistance and/or separator

resistance can play large roles in device resistance. The ability to control the contact resistance

and separator resistance would contribute largely to reducing device resistance, and in turn,

increase the power capabilities of biochar electrode supercapacitors.

On another note, the overall resistivity for the electrodes are affected by both the carbon

conductivity and the electrolyte conductivity. However, as the conductivity of the carbon

material increases, electrolyte conductivity becomes less and less relevant, as electrode

resistivity would be largely dominated by the carbon term. This is possible with higher pyrolysis

temperatures as shown by other studies from the Green Technology Research Group. When

pyrolysis temperature increases to 1000oC, the conductivities for maple-derived biochar increase

by over 10x from the values obtained at 800oC. It is therefore possible to remove the effect of

electrolyte on electrode resistance.

56

4.7 Analysis of Power and Energy Density of Monolithic Biochar Supercapacitors

While the previous section explored the performances of the small-scale and large-scale

electrodes relative to one another, as well as the performances of various electrolyte solutions

relative to one another, this section aims to compare the performances of the devices and

electrolytes used in this study to typical devices found in the real world.

This was achieved by plotting the values of energy density and power density directly onto a

Ragone plot [21]. This is shown in Figure 42.

Figure 42: Performance of small-scale and large-scale maple-derived biochar electrodes plotted

on a Ragone plot with typical supercapacitor and battery technology performances

57

When compared to the typical electrochemical devices, the small-scale electrode device in 4M

KOH showcases specific energy values far beyond the limits of the electrochemical capacitor

region to rival energy density values of lead acid and nickel-metal hydride batteries at 7.58

Wh/kg [65,66], while maintaining a very competitive specific power of nearly 3500 W/kg,

comparable to high-performance supercapacitors.

The large-scale electrodes in KOH electrolytes have similar energy densities of between 5.30-

9.12 Wh/kg, extending far beyond the limits of typical supercapacitor energy densities, however

the drop in power density from 3500 W/kg in small-scale electrodes to the order of a few

hundreds of W/kg yields performance that only matches that of the battery technologies

mentioned previously. This could be due to the resistance to ion transport across the separator, as

the larger electrodes (thicker electrodes) result in a higher number of ions required to move

across the separator. It is possible that ion transport is inhibited due to the separator used, akin to

traffic congestion on busy streets, resulting in a high separator resistance.

The poor performance of the large-scale electrodes in 1M TEA-TFB electrolyte is surprisingly

still found within the boundaries of the electrochemical capacitor technologies, thanks in part to

its still-relevant power density.

58

Conclusions

This study demonstrates the feasibility of using large monolithic wood-derived biochar as

supercapacitor electrodes in aqueous electrolyte. The successful scale-up of monolithic maple-

derived biochar to 22.7mm x 12.7mm x 9.8mm electrodes weighing 1.4 grams each was

completed without a drop of either capacitive performance or energy storage ability, and without

any increase in device resistance. This scale-up test produced a large-scale supercapacitor device

with an absolute capacitance of 107 Farads using 6M KOH electrolyte, and 97 Farads using 4M

KOH electrolyte. The competitive energy density of 9.12 Wh/kg was measured in 6M KOH

electrolyte, as was a similar energy density of 8.25 Wh/kg in 4M KOH. These values extend far

beyond far beyond typical energy densities of supercapacitor technologies.

While the power capability of small-scale maple-derived biochar electrodes was still much

greater than that observed in large-scale electrodes, the scaled-up devices could still provide a

competitive 304 W/kg power density value, comparable to many battery technologies. As such,

maple-derived biochar is proven to be an inexpensive, scalable and high energy density material

for supercapacitors that has potential for significant improvement.

While the scalability of the maple-derived carbon electrodes is remarkable, the ability to apply

additional processes to increase supercapacitor performance makes the material even more

appealing, and a lucrative opportunity for future work.

From this study, a new method for calculating device resistance using chronoamperometric

charge and discharge is proposed. This method uses the difference between energy stored during

charge and energy released during discharge as ohmic heating losses to calculate device

resistance.

59

Recommendations for Future Work

While both large-scale and small-scale electrodes showed promising results for supercapacitor

application, significant room for improvement exists for the monolithic wood-derived biochar

electrodes. Biochar activation should be investigated to alter the porosity of the carbon structure

while maintaining the macroscopic water-transport channels inherent to wood, as well as to

create surface functional groups for pseudocapacitive performance. The ability to tailor and

optimize pore size distribution while retaining a robust monolithic structure would be a

monumental discovery allowing allow for (a) increased pore accessibility and rate capability for

aqueous electrolytes [22,29], (b) the ability to effectively use organic and ionic liquid

electrolytes with larger ion sizes and the much larger voltage windows that such electrolytes

allow for, and (c) the ability to incorporate additive materials, such as metal oxides, conductive

polymers and heteroatoms, into the micropores for pseudocapacitive behavior [29-35].

While the micropores available in the current wood-derived biochar may be too small to be

compatible with the mentioned pseudocapacitive materials, the large macroporous channels

(vessels, rays) which serve a negligible role in double-layer formation could instead serve as

hosts for other carbon structures more active in capacitive behavior. Examples include the

chemical vapor deposition of carbon structures such as graphene [67] and carbon nanotubes [68-

70], which could form within the macropores to increase surface area available for double-layer

formation.

This study tested the feasibility of large-scale electrodes with thicknesses of approximately 10

millimeter, as this thickness corresponded to the length-scale of vessels, the primary water

transport channel in hardwoods [44,45]. Even thicker electrodes may be explored to determine

the upper limit of electrode thickness with respect to energy density, if one exists, or if only

power density is affected by electrode thickness. Performance of large-scale electrodes in series

and parallel arrangements should also be tested for higher device capacity and power

capabilities.

While the previous recommendations deal with physical and practical steps which could be taken

to enhance performance of wood-derived biochar monoliths in supercapacitor applications, it is

important to not neglect fundamental and theoretical understanding, particularly in the modelling

60

of capacitive and resistive behavior. Current equivalent circuit models fail to fully model the

complex and non-ideal behavior exhibited by the maple-derived biochar electrodes. As such,

additional work can be done in the field. With an accurate equivalent circuit model and logical

physical meanings for each component in the circuit, ways in which to improve electrodes and

devices would be clearer.

61

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Appendices

7.1 Appendix A: Galvanostatic Charge-Discharge Cycles

7.1.1 Small-scale electrodes in 4M KOH

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7.1.2 Large-scale electrodes in 4M KOH

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7.1.5 Large-scale electrodes in 1M TEA-TFB

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7.2 Appendix B: Chronoamperometric Charge/Discharge Cycles

7.2.1 All cycles for small-scale electrodes in 4M KOH

7.2.2 All cycles for large-scale electrodes in 4M KOH

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7.2.5 All cycles for large-scale electrodes in 1M TEA-TFB

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