“Development of Advanced Noble Metal‐Alloy Electrocatalysts for Phosphoric Acid Fuel Cells (PAFC)”

Abstract

Catalysis 1 Colloid Particles Electrocatalysis J Electrochemistry J Fuel CellsIn the preparation of highly dispersed noble metal electrocatalysts, the technology is such that crystallites now can be routinely prepared with small dimensions so that 50% of the atom content resides at the particle surface. The crystallite diameters are of the order of 12 Angstrmn (1.2 nanometers). Operation of these materials, when applied to phosphoric acid fuel cells as electrocatalysts is, of course, critical. What is important is the realization that due to the high surface energies of these crystallites, they have properties that are unlike the properties of the bulk metals. Although this manufacturing achievement is very great, more recently there has been increased emphasis on preparing alloys of metals with platinum in order to increase the kinetic rate of reaction for the oxygen reduction. -For oxygen reduction, binary alloys of platinum with refractory metals such as vanadium showed higher activity, but the vanadium was rapidly leached out. Alloys with chromium and subsequently with the addition of cobalt to form ternary alloys have now exhibited greater resistance to degradation. These alloys are hard and resistant to sintering, until the alloying element is leached out, then crystallite growth is obtained. -More recently, process modifications to produce ordered alloys have shown stability for over 9000 hours in the hot phosphoric acid fuel cell environment, as have alloys of platinum with cobalt and/or chromium containing gallium additions. It is one theory that there is no catalytic enhancement for oxygen reduction due to these alloying elements, but the increased reactivity is due solely to leaching out of the alloying elements from the alloy surface to produce a microroughness in the platinum crystallite. -In the case of hydrogen oxidation, especially in the presence of electrocatalyst poisons such as carbon monoxide and hydrogen sulphide, alloys of platinum with palladium show greater tolerance to poisoning and greater stability against crystallite growth. Other alloys are now under development to lower the catalyst cost and increase the electrocatalyst performance. This work is coupled with developments of advanced theoretical models that are amenable to computer solution for the operation of gas-diffusion electrodes to maximize both the catalyst utilization within the electrode structure, and within the cell stack.

Final Report - MEA and Stack Durability for PEM Fuel Cells

“…By contrast, the high surface area more convoluted carbon will tend to "lock" the Pt particles in place. A similar observation has been reported by Stonehart 89 and Watanabe et. al 90 , who measured the Pt particle sizes as a function of the carbon surface area using TEM, CO adsorption and H 2 adsorption.…”

“…In the case of Pt/C samples having same Pt mass ratio but varying carbon support surface area, we observe an increase in Pt active area with increase in carbon support area (Figure 254) Figure 256, depicting the inverse of Pt active area and mol % peroxide formed, as a function of the carbon support surface area also shows a similar trend in peroxide production with Pt active area. The original Pt particle sizes for all these Pt/C samples are almost equal (2.2-2.6 nm) 89 . This indicates that the net peroxide generation is less favorable on Pt, on which the ORR reaction proceeds mainly by the 4 electron peroxide production on carbon is negligible at potentials positive to 0.4 V. This suggests a greater decomposition of peroxide, (which is formed on the Pt particles due to possible end-on nondissociative oxygen adsorption on the edge atoms), when a greater platinum active area is available for the reaction 87,91 .…”

“…These radii were then compared with the sizes provided by the catalyst vendors (Table 36 and Ref. 89). The calculated sizes are in agreement with the vendor data for only very high surface area carbon support (800-950 m 2 /g) Pt/C catalysts.…”

“…Other related studies, which do not address peroxide formation, but discuss Pt agglomeration were presented by Stonehart 89 and Watanabe et. al 90 .…”

Catalytic Activities of Pt–Metal Alloys on Oxygen Reduction Reaction in Fluorohydrogenate Ionic Liquid

“…Various potential cathode catalysts such as Pt-M (M = Fe, Co, Ni, etc.) alloy catalysts, [12][13][14][15][16][17][18][19][20][21][22][23] Pt core-shell catalysts, 24,25 and Ptfree catalysts 26,27 have been investigated by many research groups. Among these, Pt alloy catalysts were found to exhibit better ORR activities than Pt in acidic solutions.…”