(6cu) New Materials for Reduced Cost, High Performance, Micro Direct Methanol Fuel Cells

Direct methanol fuel cells (DMFCs) have the potential to provide clean, efficient, sustainable power for a broad spectrum of applications. Heat engines are highly competitive in high power-density applications where device size and fuel infrastructure is not a primary concern. However, at smaller sizes, electrochemical devices become more competitive because they are simpler than heat engines, requiring no moving parts, while providing higher energy densities since they scale as function of their surface area, not volume. Thus, the natural advantages of electrochemical devices can be most easily used in portable and low-power devices where room temperature operation, limited support infrastructure, and simple designs can be exploited. Also, liquid-based fuel cells provide higher energy density compared to Li-ion batteries (approximately 10 X), lower cost, and are environmentally friendly. There are several factors that should be addressed before commercialization of these devices is seriously considered, including: low open circuit voltage and performance of passive cells, cost, fuel loss (mostly through crossover), fuel delivery and integration. Therefore, it is critical to develop new materials that alleviate these technical concerns and to target power levels and applications where fuel cells are truly competitive with established technologies and market insertion is facile. The presented work has been done with the intention of creating new fuel cells and designs which address some of the fundamental problems with state-of-the-art devices.

Thin-film catalyst/glass composite electrodes for methanol oxidation and oxygen reduction reactions

Carbon-supported platinum and platinum-ruthenium alloy nanoparticles were incorporated into a porous silicon dioxide glass matrix. The glass was prepared by the sol-gel hydrolysis reaction between TEOS and water in the presence of methanol. During the hydrolysis reaction, the nanoparticles are introduced to the system, which is then deposited on the cell substrate and imposed to several curing steps where the solvent is evaporated and the reaction completed. The resulting structure was porous, homogeneous and the catalyst was well dispersed. Finally, the catalyst islands were grown by electroless deposition until the propagation threshold was met and the resulting layer was used as both catalyst and current collector. The electrochemical performance of the resulting films for both methanol oxidation and oxygen reduction was studied by ex-situ cyclic voltammetry in sulfuric acid electrolyte, polarization measurements in a micro DMFC and ac impedance in a two-compartment glass cell. Experimental results indicate that the electrochemical performance of the DMFC is improved compared to cells prepared with Nafion®-based catalyst layers, where a 50 mV increase in the open circuit potential and higher currents are realized for fuel cells prepared with the composite catalyst.

High activity, methanol tolerant Co-Pd alloy electrocatalysts for the oxygen reduction reaction

Cobalt-palladium (CoPdx) bimetallic electrocatalysts of various compositions were prepared and their physical and electrochemical properties elucidated. The electrocatalytic activity electrodeposited polycrystalline CoPdx electrodes for the oxygen reduction reaction was compared using cyclic voltammograms obtained in oxygen saturated, 0.5 M H2SO4 at 25 oC. CoPd3 was identified as having the highest activity, showing peak currents comparable to a polycrystalline Pt electrode under identical experimental conditions. The rotating disk electrode technique was used to collect kinetic data for the oxygen reduction on CoPd3 and the activity of the binary electrocatalyst was evaluated. It was found that the ORR on CoPd3 has a modest activation energy, 52 kJ/mol, and a Tafel slope of approximately 60 mV/decade, indicating that the rate determining step is a chemical stop following the first electron transfer step and likely includes the breaking of the O-O bond. Also, the electrochemical activity of carbon supported cobalt-palladium alloy electrocatalysts of various compositions was investigated for the oxygen reduction reaction in a 5 cm2 single cell polymer electrolyte membrane fuel cell. Polarization experiments have been conducted at various temperatures between 30 and 60 oC and it appears that the catalyst with a nominal cobalt-palladium atomic ratio of 3:1, CoPd3, exhibits the best performance of all studied catalysts, exhibiting a catalytic activity comparable to a commercial Pt catalyst. The CoPd3 catalyst also exhibits excellent chemical stability, with the current decreasing by only 10 % at 0.8 V over 25 hours. The CoPd3 catalyst also exhibits superior tolerance to methanol crossover poisoning than Pt.

Novel passive fuel delivery and ion-transport mechanism for DMFCs

In this work, a new type of passive direct methanol fuel cell was developed to allow the fuel cell to operate with limited support infrastructure and no moving parts. In a departure from previous work, this passive DMFC utilizes a stationary fuel phase and allows the anode to ?float? in the fuel reservoir. During operation, ions are generated at the floating anode electrode (PtRu), conducted through the solution phase to the liquid solution saturated wicking material (cotton, cellulose, etc.). Also, a small amount of an earth metal salt (i.e. sodium bisulfate) was added to the aqueous liquid fuel (10-12 M methanol) in order to increase the ionic conductivity of the solution phase. The wicking material moves the ions to the solid proton exchange membrane where they are transported to the cathode and consumed. The current invention addresses one of the most significant issues associated with conventional passive DMFCs: consistent and predictable fuel delivery to the electrode surface. In providing an ion pathway, instead of a fuel delivery device, one intrinsic flaw of wicking a binary fuel mixture is addressed. All fibrous or porous wicking materials transport liquids selectively, dictated by the surface tension of the liquid, resulting in inconsistent concentration gradients and unpredictable performance for long-term operation. Both polarization and chronoamperometric results indicate that the device shows excellent performance stability and its operation is not affected by cell orientation.

Research Interests

Nafion®-free micro DMFCs; Anion-exchange membranes; Novel oxygen reduction and methanol oxidation catalysts in acid and alkaline media; Kinetic evaluation of carbonate formation and consumption in anionic fuel cells; Predictive modeling for electrochemical systems with a focus on the oxygen reduction reaction in acid media; Reaction pathway and kinetic decomposition for methanol oxidation.

References

1. W.E. Mustain, S. Prakash, H. Kim and P. Kohl, ?Fully Passive High Energy Density Hybrid Power Module?, Invention disclosure filed May 2007.

2. S. Prakash, W.E. Mustain and P. Kohl, ?Ionic Conductivity of Phosphorous-doped SiO2 Ultra Thin Films?, to be submitted May 2007.

3. W.E. Mustain, S. Prakash, H. Kim and P. Kohl, ?Thin-film Pt-Glass and PtRu-Glass Composite Catalyst Layer for Fuel Cells?, Invention disclosure filed May 2007.

4. W. Mustain and P. Kohl, ?Characterization of Thin-Film Electrodes on Proton-Conducting Glass Membranes for Micro DMFC Applications?, 211th Meeting of the Electrochemical Society, May 2007.

5. W.E. Mustain, K. Kepler and J. Prakash, ?Investigations of carbon-supported CoPd3 catalysts as oxygen cathodes in PEM fuel cells?, Electrochem. Comm., 8 (2006) 406.

6. W.E. Mustain, K. Kepler and J. Prakash, ?CoPdx oxygen reduction electrocatalysts for polymer electrolyte membrane and direct methanol fuel cells?, Electrochim. Acta, 52 (2007) 2102.

7. W.E Mustain and J. Prakash, ?Kinetics and Mechanism for the Oxygen Reduction Reaction on Polycrystalline Cobalt-Palladium Electrocatalysts in Acid Media?, J. Power Sources, In Press.

8. W. Mustain and J. Prakash, ?Cobalt-Palladium Electrocatalysts for Oxygen Reduction in Acid Media?, 211th Meeting of the Electrochemical Society, May 2007.

9. W.E. Mustain and P. Kohl, ?Floating Anode Ion Wicking Direct Methanol Fuel Cell?, Invention disclosure filed May 2007.