Chemical, Biological and Environmental Engineering Electrochemical Energy Systems (Fuel Cells and Batteries)

Chemical, Biological and Environmental Engineering

Electrochemical Energy Systems

(Fuel Cells and Batteries)

Advanced Materials and Sustainable Energy LabCBEE

Housekeeping• Final

– Final on Friday AM, here at 9:30


What is the major difference vs. combustion?

Electrochemical systems are not heat engines!– Therefore not Carnot limited!

Ree

,max

,max

Chemical Energy ( G) Electrical (W G)

Maximum electrical work is when T s=0 (no irreversible losses) and then

1

dox

oe

o oe

electrochem o o oe

W H

W G H T s T s

W H H H


Oxidation and Reduction• Oxidation occurs at anode

– Material gives up electrons– Ions dissolve into solution– E.g., Zn → Zn+2 + 2 e-

• Reduction at cathode– Material takes up electrons– Ions deposited from solution– E.g., Cu+2 + 2e- → Cu

• These are called Half-Cell reactions – Both need to happen!

(electron released at anode and consumed at the cathode so net charge is conserved)

(also need ion flow to maintain net charge conservation in electrolyte)


Each material has a reduction potential

DG=-nFE


Cell potentials• Ecell=Ecathode-Eanode

– Make sure you use the same reference.– Most tables give reference against “standard hydrogen electrode”

(H+ + e- -> ½ H2)

• DG=-nFE F=Faraday constant (96 458)•

0

If not at standard conditions,

ln

[ ]where Q=

[ ]

i

j

n

in

j

G G RT Q

reactants

products

0 ln

(known as the Nerst Equation)

RTE E Q

nF


Example

Half cell potential: • Anode: Zn2++2e- → Zn• Eo = -0.7628 V vs. SHE

Half cell potential: • Cathode: Cu2++2e- → Cu • Eo = +0.3402 V vs. SHE

Cell potential: • EC-EA=0.3402 V –(-0.7628V)=1.103V

• DGo=-nFEo=-213.8 kJ∙mol-1 • Since DGo<0 reaction proceeds spontaneously


Internal losses depend on current density


Battery termsPrimary battery: Non-rechargeable (e.g., Li / SOCl2)

Secondary battery: rechargeable (e.g., Li ion, we’ll talk about this)

Mechanically rechargeable: batteries are recharged by mechanical replacement of depleted electrode (e.g. metal anode in certain metal-air batteries)

Voltage: Potential difference between anode and cathode. (Related to energy of reactions)

Capacity: amount of charge stored in battery

(usually given as Coulombs per unit mass or volume)

(1A=1C.s-1↔1Ah=3600C)

(Question: how does capacity relate to energy?)


Rate effectsCurrent Drain: Different batteries respond differently

In general, as current is increased, the available voltage and capacity decrease

Charge rate and battery capacity usually specified

E.g., my laptop 5200mAh, 10.80V, 56Wh means C=5.2A


Discharge Rates


Important battery (and fuel cell) parametersBatteries (and Supercaps)• Specific Energy (Wh/kg) (gravimetric energy density) • Energy density (Wh/L) (volumetric energy density)• Specific power (W/kg) (gravimetric power density)• Power density (W/L) (volumetric power density)

Fuel cells also discuss:• Current density (mA/cm2) in electrode assembly• Power density (mW/cm2) in electrode assembly

These parameters are of primary concern for mobile systems (e.g., transportation or mobile electronics)

You want all these values to be as large as possible…


Some energy carrier comparisons (balance of plant efficiency and weight not included)

Material Energy Density (kWh/L) Specific Energy (kWh/kg)

Diesel 10.9 13.7

Gasoline 9.7 12.2

LNG 7.2 12.1

Biodiesel 9.9 12.2

EtOH 6.1 7.8

MeOH 4.6 6.4

NaBH4 7.3 7.1

NH3 4.3 4.3

LH2 2.6 39

Lead Acid 0.03 0.06

Nickel Cadmium 0.05 0.1

Li Ion 0.15 0.3



So what makes a battery rechargeable?

As long as the electrochemical reaction is reversible, the battery should be rechargeable

However, other effects are important– Decay of electrode surfaces

E.g., damage to electrode structural properties as ions move in and out of electrodes

– Decay/contamination of electrolyte– (Cost…)


The original rechargeable battery: Lead Acid

Anode/Oxidation: Lead grid packed with spongy lead.

Pb(s)+HSO4 –

(aq) → PbSO4(s)+H+(aq)+2e–

Cathode/Reduction: Lead grid packed with lead oxide.

PbO2(s)+3H+(aq)+HSO4

–(aq)+2e– → PbSO4(s)+2H2O(l)

Electrolyte: 38% Sulfuric Acid.

Cell Potential: 1.924V

A typical 12 volt lead storage battery

consists of six individual cells

connected in series.


Nickel-Cadmium (NiCd or Nicad)Anode/Oxidation: Cadmium metal

Cd(s)+2OH–(aq) → Cd(OH)2(s)+2e–

Cathode/Reduction: NiO(OH) on nickel metal

NiO(OH)(s)+H2O(l)+e– → Ni(OH)2(s)+OH–(aq)


~ 50Wh/kg ~100Wh/L ~150W/kg

High current rates due to Grotthus transport of OH- in water


Li-ion

Li+ ions intercalate into the crystal structure of the electrode materials– Li metal is very reactive which causes side reactions– Recharging by growing Li metal doesn’t work well– Instead of using Li metal use LiC8


Electrochemical Potentials for Lithium Insertion(relative to Li, not SHE)


Li-ionAnode/Oxidation: LixGraphite (“LixC6”)

LixC6(s) → “C6”(s) +Li+(solv)+e–

Cathode/Reduction: Li Spinel or layered oxide

Li1-xMO2(s)+Li+(solv)+e– → LiMO2(s)


~160Wh/kg ~300Wh/L ~300W/kg

Electrolyte cannot have any water!

Li salt in organic ether (LiPF6 / LiBF4 / LiOTf)

Overcharge/Overdischarge significantly damages cells


Effect of Depth of Discharge on Lifetime


Parasitic Losses in BatteryMost batteries have a “self discharge” rate• Secondary chemical reactions

– E.g., Zn(s) + H2O(l) → ZnO(s) + H2(g)↑

• Particularly important problem for some systems – Like Zn/air cells (“use it or lose it”)– Li cells have least self discharge of rechargeable batteries

Self Discharge rate (%/month)

Lead Acid 5

NiCd 20

Li-ion 5


Metal-Air BatteriesMetal Air battery

M → Mn++ne–

½O2+H2O+2e– → 2OH- Eo=0.4V

Looks reasonable for specific energy

Specific power is horrible (slow reaction kinetics at oxygen electrode)

Atomic Mass

Eo(V) Density (g/cm3)

Capacity(Ah/g)

Sp Energy(Wh/g)

Li 6.94 3.05 0.54 3.86 13.3Zn 65.4 0.76 7.1 0.82 1.77Al 26.9 1.66 2.7 2.98 6.13Mg 24.3 2.37 1.74 2.20 6.09Na 23.0 2.71 0.97 1.16 3.61


Fuel CellsFuel Cells use externally fed fuel (H2 for now)

Fuel reacts with O2 to form water.

Anode/Oxidation: Carbon felt with catalyst

2H2(g) + 4OH–(aq) → 4H2O(l) + 4e–

Cathode/Reduction: Carbon felt with catalyst

O2(g) + 2H2O(l) + 4e– → 4OH–(aq)

Overall Reaction: 2H2(g) + O2(g) → 2H2O(l)

Various electrolytes (also reaction, etc)


Common Fuel CellsBy electrolyte:• Alkaline Fuel Cells (AFC)• Phosphoric Acid Fuel Cells (PAFC)• Polymer Electrolyte Membrane Fuel Cells (PEMFC)• Molten Carbonate Fuel Cells (MCFC)• Solid Oxide Fuel Cells (SOFC)

By fuel:• Hydrogen / Air(Oxygen); Reformate Gas• Direct Methanol Fuel Cell (DMFC)• Carbon (coal?)

By operating temperature:• High Temperature vs. Low Temperature Cells


Conceptual Fuel Cell Structure


Closer to real fuel cell structure


Fuel cell typesAFC PAFC PEMFC MCFC SOFC

Power range (kW)

2 - 100 100-400 0.1W - 250 250 – 10k 1 – 10k

Electrolyte Aq. KOH (30- 40%)

Aq H3PO4 (30-40%)

sulphonated organic polymer (Nafion)

Molten (Li/Na/K)2CO3

YSZ

Temp (°C) 50 - 250 150 -220 70 - 110 600 - 700 650 - 1000 C

Charge Carrier

OH- H+ H+ CO32- O2-

Anode Ni Pt Pt Ni/Cr2O3 nickel/YSZ Cathode Ni/Pt/Pd platinum Pt / Ru Ni/NiO SrxLa1-xMnO3 (h %) 50-60 40-45 40-50 50-60 50-75


Alkaline Fuel CellLots of experience

– UTC has been making AFCs for NASA since Apollo

Anode: Porous Ni

2H2 + 4OH– → 4H2O+4e–

Cathode: Porous NiO

O2+2H2O+4e– → 4OH–

Electrolyte is aq. KOH~35% for low temp (120oC)

~80% for high temp (250oC)

CO, CO2, H2S is harmful


Effect of gas pressure

Higher pressure leads to higher voltages: Higher Eo


Effect of gas composition

As expected 100% O2 is better than air…


Effect of temperature

Higher temperature leads to higher conductivity of electrolyteLower IR losses


Effect of contaminant (CO2 in AFC)

CO2 + 2OH– → CO32– + H2O

You trade 2 highly mobile OH– charge carriers for one low mobility CO3

2– ion…


Phosphoric Acid Fuel CellSeveral commercial designs

Anode: Pt on carbon black

2H2 + 4OH– → 4H2O+4e–

Cathode: Pt on carbon black

O2+2H2O+4e– → 4OH–

Electrolyte is aq. H3PO4

~95% for (200oC)

CO2 tolerant

CO, COS, H2S poison catalyst


Proton Exchange Membrane Fuel Cell

Widely viewed as best bet for vehicle applications– No liquid electrolyte means safer system (?)


H2 → 2H+ + 2e–


½O2+2H++4e– → H2O

Electrolyte is Sulfonated polymer“Nafion” – must keep wet!

Water management is important

Very sensitive to CO, COS, H2S


Direct Methanol Fuel CellBasically a PEM FC that uses MeOH instead of H2

– For transportation, methanol much easier to store than H2

– People also looking at direct hydrocarbon fuel cells


MeOH + H2O → CO2+ 6H+ + 6e–


3/2O2+6H++6e– → 3H2O

Some CO produced, poisons catalyst…

Fuel crossover through membrane lowers efficiencies

Also note that water is consumed at anode – more water management


Molten Carbonate Fuel CellHigh tolerance to CO (good to use reformate gas!)

– H2S is still harmful

– Inefficiencies as high quality heat (cogen possible)

Anode: Porous Ni/Cr

H2 + CO32– → H2O+ CO2+2e–

Cathode: Porous NiO

½O2+CO2+2e– → CO32–

Electrolyte is (Li/Na)2CO3 melt~50/50 and 650oC

Note CO2 consumed at cathode– CO2 management needed


Internal reformation possible(no need to separate CO2 and H2)


Solid Oxide Fuel CellHigh efficiency, especially if cogen used

– Inefficiencies as high quality heat (cogen possible)

Anode: Porous Ni/ZrO2 cermet

H2 + O2– → H2O+2e–

Cathode: SrxLa1-xMnO3

½O2+2e– → O2–

Electrolyte is Yttria stabilized ZrO2

~8% Y, T=950oC


An SOFC design


Where do fuel cells fit in?Electrical Efficiency

CHP efficiency Best for

AFC 60% 80(low qual. heat)

Special applications:Needs high purity H2

PAFC 40% 85(low qual. heat)

Distributed generation:Can use low sulfur reformate

PEMFC 55% (transp.)35 %(stationary)

80%(low qual. heat)

Transportation:Use DMFC instead of H2

MCFC 45% 80%(high qual. heat)

Power generation:Can use coal / NG reformate

SOFC 40% 90%(high qual. heat)

Power generation:Use Coal / NG reformate


Combined Brayton-Fuel Cell Power System

• Fuel cell efficiency about 55% (losses + wasted fuel)• Boosted to about 80% by using turbine

Chemical, Biological and Environmental Engineering

Summary


Why AY did the course like this• Various courses in higher education:

1. “teach you how to approach the problem like an engineer”

2. “show you how interrelated the system is”

3. “help you figure out your (social) responsibility”

4. “give you fundamental information about what is done now”

• Assumption is you got 1-3 elsewhere, what you need to help you improve the system is more of 4

• "The thinking it took to get us into this mess is not the same thinking that is going to get us out of it.“ (attributed to Albert Einstein)– AY hopes some of you will get us out of the mess


Course Learning Objectives1. Describe the dependence of our current industrial society on energy

2. Discuss the various approaches to conventional and alternative energy generation and describe the basic operational principles of each

3. Ability to analyze data pertaining to a certain situation and create/design an idealized energy conversion system

4. Solve quantitative, energy-related problems that use and reinforce engineering fundamentals

5. Formulate decisions on energy choices based upon consideration of the entire lifecycle of the energy source in question, socioeconomic trends, safety, and environmental impact

6. Describe and apply fundamental system calculations to predict expected system efficiency


We looked at• Basics:

– Why do we need energy (and how much?)– Power vs. Energy– Brief review of Thermo; Brayton and Rankine cycles– Fundamentals of electrochemistry

• What our energy system looks like right now– Coal, NG, Hydro, Nuclear, Wind– Generators, Transformers, the Grid

• How we may make out power in the future– Ocean, Solar, Fuel Cells


And so…

• Thanks for your help developing this course!

• Course evaluation follows– Note additional questions

• See you on Monday at 12:00 in Wilk108.

Documents

Chemical, Biological and Environmental Engineering Electrochemical Energy Systems (Fuel Cells and Batteries)