• Energy Research

Pure palladium thorn clusters were synthesized using the electrodeposition method. The clusters were composed of several thorns growing on one basis. Each thorn was composed of hexahedral units with decreasing sizes. The whole thorn was a single crystal along the direction. The cluster was synthesized by square wave electrodeposition. By changing the deposition factors, a mixture of thorns and particles was synthesized, where each thorn grew on one basis and the thorn was composed of dodecahedral bases. Compared with the mixture, the cluster had higher activity toward the electrooxidation of formic acid, and also much higher activity than Pd powder, which was evidenced by the improved current density and onset potential of formic acid oxidation. The fact that pure thorn clusters had a higher catalytic activity than the mixture of thorns and particles proved that the higher activity was ascribed to the single crystal property of the thorns.

Hydrogen produced from water and renewable energy could fuel a large fleet of proton-exchange-fuel-cell vehicles in the future. However, the dependence on expensive Pt-based electrocatalysts in such fuel cells remains a major obstacle for a widespread deployment of this technology. One solution to overcome this predicament is to reduce the Pt content by a factor of ten by replacing the Pt-based catalysts with non-precious metal catalysts at the oxygen-reducing cathode. Fe- and Co-based electrocatalysts for this reaction have been studied for over 50 years, but they were insufficiently active for the high efficiency and power density needed for transportation fuel cells. Recently, several breakthroughs occurred that have increased the activity and durability of non-precious metal catalysts (NPMCs), which can now be regarded as potential competitors to Pt-based catalysts. This review focuses on the new synthesis methods that have led to these breakthroughs. A modeling analysis is also conducted to analyze the improvements required from NPMC-based cathodes to match the performance of Pt-based cathodes, even at high current density. While no further breakthrough in volume-specific activity of NPMCs is required, incremental improvements of the volume-specific activity and effective protonic conductivity within the fuel-cell cathode are necessary. Regarding durability, NPMCs with the best combination of durability and activity result in ca. 3 times lower fuel cell performance than the most active NPMCs at 0.80 V. Thus, major tasks will be to combine durability with higher activity, and also improve durability at cell voltages greater than 0.60 V.