Download No Slide Title

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 :
Solar PV
Wind Power
Fuel Cell
• 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 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)
Why DAFC is advantageous?
•
•
•
•
•
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
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 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 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 fuel
2. 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
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
0.4
-2
log (current density / mA cm )
0.6
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
6
160
5
4
Z'' / ohm
140
120
3
2
1
0
Z'' / ohm
100
-1
-2
2
80
4
6
8
10
12
14
16
18
20
22
Z' / ohm
60
40
20
0
0
50
100
150
200
Z' / ohm
• 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
Remarkable performance for
electrocatalysts synthesized using
PTFE
0.7
CuNi/PtRu(PTFE)
0.6
CuNi/Pt(PTFE)
OCP/ V
0.5
0.4
0.3
CuNi/PtRu
CuNi/Pt
CuNi
0.2
0.1
0.0
20
30
40
50
60
0
Temperature/ C
•
OCP generally increases with the rise in temperature indicating
an increase in reaction kinetics.
Power density plots
CuNi/PtRu(PTFE)
CuNi/Pt(PTFE)
800
600
240
500
200
400
-3
Cell voltage / mV
280
Power density X 10 / mW cm
700
320
160
300
200
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Current density / mA cm
0.7
2
0.8
0.9
2
120