Status of the development of DAFC :

A focus on higher alcohols

N.R.BandyopadhyaA, J.DattaB*

A. Dr. M.N.Dastur School of Materials Science & Engineering

B. Department of Chemistry, B.E.College(D.U.), Howrah- 711 103

To avoid irreparable damage to the environment as a consequence of burning fossil fuels, energy production must become cleaner and the use of energy more effective.

Viable alternatives to fossil fuel include :

• Fuel cells are poised for a breakthrough into the mainstream, and offer an attractive combination of highly efficient fuel utilisation and environmentally-friendly operation.

Solar PV

Wind PowerFuel Cell

Conventional-Nonconventional Fuel: A comparison

Different kinds of fuel cells Solid oxide fuel cell (SOFC) working between 700 and 1000 0C with a

solid electrolyte such as Yttria Stabilized Zirconia (ZrO2- 8% Y2O3)

Molten carbonate fuel cell (MCFC) working at about650 0C with a mixture of molten carbonates (Li2CO3/ K2CO3) as electrolyte

Phosphoric acid fuel cell (PAFC) working at 180-200 0C with a porous matrix of PTFE-bonded SiC impregnated of phosphoric acid as electrolyte

Alkaline fuel cell (AFC) working at 80 0C with concentrated KOH as electrolyte

Proton exchange membrane fuel cell (PEMFC) working at around 70 0C with a polymer membrane, such as Nafion, as a solid protonic conductor

Besides H2 as a fuel, methanol can be directly converted into electricity in a Direct Methanol Fuel Cell (DMFC)

• Does not require infrastructure for H2 storage

• Less aggressive• Liquid fuel is compatible to existing infrastructure• No need of reformer• Higher energy density of the fuel

Why DAFC is advantageous?

Components of DAFC

1. Fuel(methanol, ethanol,….)

2. Electrocatalyst

3. Membrane

4. Bipolar plates

Choice of fuel : Higher alcohols (ethanol, propanol,...)Thermodynamic data associated with the electrochemical oxidation of some alcohols

(under standard conditions)

Fuel Go (kJ/mol) Ecell (V) We (kWh/kg)

CH3OH -702 1.213 6.09

C2H5OH -1325 1.145 8.00

C3H7OH -1853 1.067 8.58

Ethanol !• Mass production from agricultural products => cheaper fuel

• Relatively nontoxic

• Good energy density (8.00 kWh/kg) compared to that of hydrocarbon and gasoline (e.g., 10-11 kWh/kg).

Electrocatalysts• Only platinum seems to be able to

adsorb alcohols and to break the C__H bonds

Scheme of the consecutive dissociative electrosorption of methanol at a Pt electrode

Ru is generally regarded as the best promoter of Pt catalyst in the electrooxidation of methanol. The

optimum amount of Ru surface coverage for CH3OH oxidation is low, about 10-15%.

The promoting effect of these metals is attributed to either a bifunctional or a ligand

effect

• Some promising results have been reported for PtRuMOx systems (where MOx = transition metal oxides) as the

next evolutionary step for fuel cell catalyst development

Bipolar platesDual function 1. Distribution of the fuel and air to the anode and cathode

2. Providing the electrical contact between adjacent cells

With respect to corrosion resistance, graphite materials are preferred

Disadvantagesconductivity of graphite materials is much

less than that of metallic materials

fabrication costs of graphite plates incorporating gas-distribution channels are high, making such components too expensive

graphite materials are porous

For bipolar plates, polymer/graphite compounds are developed with at least 10 S cm-1 conductivity

Another strategy is to use metallic bipolar plates

The most promising materials are stainless steel, as the other candidate metals such as titanium, noibium, tantalum and gold (including gold-plated metals) are too expensive.

Membranes• Properties of polymeric membranes to be

optimized for use in fuel cells :

1. high proton conduction, assured by acid ionic

groups (usually SO3H),

2. good mechanical, chemical and thermal strength

requiring the selection of a suitable polymer backbone,

3. low gas permeability,4. for DMFC applications low electro-osmotic drag

coefficient to reduce methanol crossover

Because of their PTFE-like backbone and relatively low equivalent weight, Nafion and related materials are commonly used in fuel-cell stacks

Disadvantages:1. Mmethanol crossover rate of ca. 100 mA cm-2 and the resulting cathode performance decay as well as the loss of fuel2. Operation beyond 100 oC is desired, but Nafion neither provides sufficient conductivity nor is there a comfortable thermal stability margin 3. In addition, Nafion is relatively expensive due to its fluorine-based synthesis• Development of cheaper membrane materials

One promising approach is to use basic polymers(polybenzimidazole & polyacrylamide) doped with inorganic acids

a ten-fold decrease in the methanol crossover rate as compared

to Nafion

satisfactory thermal stability

cheaper than Nafion

Our activities in Direct Alcohol Fuel Cell Research

Thrust areas : Development of potential electrocatalyst Fabrication of MEA Stack performance

Nanoscopic carbon-supported Pt electrocatalysts

• Size and distribution of Pt particles are important parameters that affect the reactivity of platinized electrodes of fuel cells

• Carbon supported Pt deposited at a controlled current density of 3 mA cm-2 yielded well-dispersed particles of 100-150 nm diameter, which translated to a pronounced increase in surface roughness compared to those platinized at higher current

densities

3mA cm-2 5 mA cm-2

SEM study of the catalyst surfaces revealed enhanced agglomeration of the Pt deposits as the cause of the loss in surface roughness on increasing the deposition current density.

10 mA cm-2

We were able to show that the variation of electrocatalytic activity with the amount of Pt incorporated in the catalyst layer is essentially guided by the difference in the roughness factor of the deposits.

A novel electrocatalyst on metallic support

• Polycrystalline deposits of platinum and platinum-ruthenium on CuNi (70:30) alloy support were investigated.

• CuNi alloy substrate can change the density of states of the d-band and hence the local electronic character of the active sites. Such changes in the local electronic structure may influence the electronic transfer between the adsorbate molecule and the catalyst layers.

CuNi/Pt CuNi/Pt(PTFE)

CuNi/PtRu (PTFE)

• For the CuNi/PtRu(PTFE) electrocatalyst, the SEM image show homogeneously distributed small dark particles of about 50nm in diameter which we attribute to Ru deposits on the platinum layers as confirmed by EDX.

Electroxidation current density achieved in the working potential range :

CuNi/PtRu(PTFE) > CuNi/Pt(PTFE) > CuNi/Pt

-700 -600 -500 -400 -300 -200 -100-1.6

-1.4

-1.2

-1.0

-0.8

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-0.4

-0.2

0.0

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log

(cu

rre

nt d

en

sity

/ m

A c

m-2 )

Potential / V

• There is a significant enhancement in the activity for ethanol electro-oxidation for the catalyst layers electro-deposited from PTFE suspension as compared to those prepared from HCl medium.

•This may in part be attributed to the better dispersion of the catalyst particles for the preparation technique involving PTFE as revealed in the SEM images.

Electrochemical Impedance Spectroscopy

• The charge transfer resistance Rct, is measured by the diameter of the semi-circle in the plot

• A significant decrease in the magnitude of Rct for PtRu codeposited surfaces indicating an increase in reaction kinetics

• The highest charge transfer resistance was observed for the Pt deposited electrode indicating the greater poisoning effect

on such surfaces

0 50 100 150 200

0

20

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Z''

/ oh

m

Z' / ohm

2 4 6 8 10 12 14 16 18 20 22-2

-1

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Z''

/ o

hm

Z' / ohm

Remarkable performance for electrocatalysts synthesized using

PTFE

• OCP generally increases with the rise in temperature

indicating an increase in reaction kinetics.

20 30 40 50 600.0

0.1

0.2

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CuNi

CuNi/PtRuCuNi/Pt

CuNi/PtRu(PTFE)

CuNi/Pt(PTFE)

OC

P/

V

Temperature/ 0C

Power density plots

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

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Cell

volta

ge /

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Current density / mA cm2

Pow

er d

ensity X

10

-3 / mW

cm2