Status of the development of DAFC : A focus on higher alcohols

N.R.Bandyopadhya

A

, J.Datta

B* 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 : Solar PV Fuel Cell Wind Power • Fuel cells are poised for a breakthrough into the mainstream, and offer an attractive combination of highly efficient fuel utilisation and environmentally-friendly operation.

Conventional-Nonconventional Fuel: A comparison

Different kinds of fuel cells

 Solid oxide fuel cell (SOFC) working between 700 and 1000 0 C with a solid electrolyte such as Yttria Stabilized Zirconia (ZrO 2 - 8% Y 2 O 3 )  Molten carbonate fuel cell (MCFC) working at about650 0 C with a mixture of molten carbonates (Li 2 CO 3 / K 2 CO 3 ) as electrolyte  Phosphoric acid fuel cell (PAFC) working at 180-200 electrolyte 0 C with a porous matrix of PTFE-bonded SiC impregnated of phosphoric acid as  Alkaline fuel cell (AFC) working at 80 0 C with concentrated KOH as electrolyte  Proton exchange membrane fuel cell (PEMFC) working at around 70 0 C with a polymer membrane, such as Nafion, as a solid protonic conductor

Besides H 2 converted into electricity in a Direct Methanol Fuel Cell (DMFC) as a fuel, methanol can be directly

Why DAFC is advantageous?

• Does not require infrastructure for H 2 storage • Less aggressive • Liquid fuel is compatible to existing infrastructure • No need of reformer • Higher energy density of the fuel

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  G o (kJ/mol) -702 E cell (V) 1.213

W e (kWh/kg) 6.09

CH 3 OH C 2 H 5 OH C 3 H 7 OH -1325 -1853 1.145

1.067

8.00

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 CH 3 OH 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 PtRuMO x systems (where MO x = transition metal oxides) as the next evolutionary step for fuel cell catalyst development

Bipolar plates

Dual 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

Disadvantages

 conductivity 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 SO 3 H), 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 cathode performance decay as well as the loss of fuel 2. Operation beyond 100 sufficient conductivity nor is there a comfortable thermal stability margin synthesis o -2 and the resulting C is desired, but Nafion neither provides 3. In addition, Nafion is relatively expensive due to its fluorine-based • 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

10 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.  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

0.6

0.4

0.2

0.0

-0.2

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

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

7 160 140 120 100 1 0 -1 -2 3 2 6 5 4 80 2 4 6 8 10 12 Z' / ohm 14 16 18 20 22 60 40 20 0 0 50 100 Z' / ohm 150 200 • The charge transfer resistance R ct , is measured by the diameter of the semi-circle in the plot • A significant decrease in the magnitude of R ct codeposited surfaces indicating an increase in reaction kinetics for PtRu • The highest charge transfer resistance was observed for the Pt deposited electrode indicating the greater poisoning effect on such surfaces

Remarkable performance for electrocatalysts synthesized using PTFE

0.7

CuNi/PtRu(PTFE) 0.6

CuNi/Pt(PTFE) 0.5

0.4

0.3

0.2

0.1

0.0

CuNi/PtRu CuNi/Pt CuNi 20 30 40 Temperature/ 0 C 50 60 • OCP generally increases with the rise in temperature indicating an increase in reaction kinetics.

Power density plots

800 700 600 500 400 300 200 CuNi/PtRu(PTFE) CuNi/Pt(PTFE) 320 280 240 200 160 120 0.0

0.1

0.2

0.3

0.4

0.5

0.6

Current density / mA cm 2 0.7

0.8

0.9