It is well-known that Pt-Ru alloy electrocatalysts perform better than Pt catalyst for the anode reaction in alkaline membrane fuel cells.¹ However, it is unclear why Pt-Ru alloy catalyst performs better in alkaline membrane fuel cells. Strmcnik et al. suggested that the higher activity of Pt-Ru catalyst is due to increased oxophilicity of Pt catalyst by adding Ru component.² Others suggested that the Ru component weakens the Pt-H_ad binding energy to improve hydrogen oxidation reaction.^3,4 In this presentation, we demonstrate that the improved alkaline hydrogen oxidation reaction is due to the low benzene group adsorption on the catalyst surface when the Ru component is added to Pt.

In order to support our hypothesis, we first compared the voltammograms of hydrogen oxidation reaction of commercial Pt and Pt-Ru alloy catalysts in 0.1 M benzyl ammonium hydroxide and tetramethyl ammonium hydroxide solutions.⁵ These experiments showed that the hydrogen oxidation reaction of Pt in the benzyl ammonium hydroxide solution is much slower than that of Pt in the tetramethyl ammonium hydroxide solution. More importantly, the hydrogen oxidation reaction in the benzyl ammonium hydroxide solution can be substantially increased when Pt-Ru alloy catalyst was used. Density functional theory calculations indicate that the adsorption of benzyl ammonium on the bimetallic catalyst is endergonic for all configurations of the benzyl ammonium cation, which explains the significantly better hydrogen oxidation reaction activity observed for the bimetallic catalyst.

The adverse impact of benzene adsorption on electrocatalysts is further investigated in the fuel cell performance test. For this experiment, we decoupled the benzene adsorption effect from mechanistic alkaline HOR perspectives by measuring both acid and alkaline membrane fuel cell performance using polyaromatic and benzene-free perfluorinated ionomers. Both acid and alkaline fuel cells, we observed substantially better kinetic performance when Pt-Ru electrocatalyst was used with polyaromatic electrolytes. In stark contrast, Pt electrocatalyst showed better kinetic performance when benzene-free perfluorinated electrolyte was used. This indicates that the performance improvement using Pt-Ru catalyst in alkaline (and acid) conditions is not originated from the mechanistically slow hydrogen oxidation under alkaline environment but originated from the less benzene group adsorbing characteristics of Pt-Ru catalyst.

Acknowledgement

This work is supported by the US Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technology Office (Program Manager Dr. David Peterson).

References:

(1) Gottesfeld, S.; Dekel, D. R.; Page, M.; Bae, C.; Yan, Y.; Zelenay, P.; Kim, Y. S. J. Power Sources, 2017 doi:10.1016/j.jpowsour.2017.08.010.

(2) Strmcnik, D.; Uchimura, M.; Wang, C.; Subbaraman, R.; Danilovic, N.; van der Vliet, D.; Paulikas, A. P.; Stamnkovic, V. R.; Mackovic, N. M. Nat. Chem. 2013, 5, 300-306.

(3) Durst, J.; Siebel, A.; Simon, C.; Hasche, F.; Herranz, J.; Gasteiger, H. A. Energy Environ. Sci. 2014, 7, 2255-2260.

(4) Wang, Y.; Wang, G.; Li, G.; Huang, B.; Pan, J.; Liu, Q.; Han, J.; Xiao, L.; Lu, J.; Zhuang, L. Energy Environ. Sci. 2015, 8, 177-181.

(5) Matanovic, I.; Chung, H. T.; Kim, Y. S. J. Phys. Chem. Lett. 2017, 8 (19), 4918–4924.