Surface Poisoning Induced By PGM-Based Catalyst Degradation in Alkaline Media and Self-Recovery By "CO-like" Stripping

Abstract

Fuel cells are advanced CO2 emission-free generators that are currently used for automobiles. Since the cost of platinum group metal (PGM) catalysts for both hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) is too high, switching to alkaline fuel cells (AFCs) is necessary. However, the alkaline media is much more challenging for HOR since extra energy is required to generate proton from hydroxide anion [1]. In addition, the alkaline durability of PGM/C catalysts is not granted, because Pt assists the local carbon corrosion, by promoting oxidation of “CO-like species”, leading to CO2 and carbonates formation, and finally particles detachment [2]. Lower degradation rate for Pd/C was measured compared to Pt/C in neutral (Ar) or reducing (H2) environment [3-5]. This work is an in-depth investigation on the differences between Pt/C and Pd/C in alkaline during AST in H2 (0-0.5 VRHE at 100 mV/s, for 1000 CVs). Figure 1 shows that prolonged cycling in this low potential region in reducing atmosphere induces “poisoning” of the PGM NPs surface by “CO-like species”; one cleaning method to recover the ECSA of Pt and Pd, is to simply strip these “CO-like species” by cycling the potential at higher values, as shown by Castanheira, et al. on Pt/C [6]. Comparing electrochemistry results and IL-TEM images suggests that stripping the “CO-like species” is a reversible process for the remaining particles that are still intact on the carbon substrates. Another cleaning method is to apply 500 CVs, from 0-1 VRHE, at 100 mV/s in Ar inert gas (Ar cleaning method). This method gives higher recovering percentage comparing to the “CO-like species” stripping one and is considered to be a better cleaning method. The reversible ECSA percentage for the catalysts from the Ar cleaning methods were calculated to be 11 % wrt. the initial ECSA of Pt/C and 14% wrt. the initial ECSA of Pd/C. Finally, the overall ECSA loss of Pt/C after cleaning was 46% (wrt. initial ECSA) while Pd/C’s case was only 8%: so, Pt assists the C corrosion better than Pd. Moreover, Pt is easy to be poisoned and is not as reversible as Pd. References [1] Danilovic, N., Subbaraman, R., Strmcnik, D., Chang, K. C., Paulikas, A. P., Stamenkovic, V. R., Markovic, N. M. Enhancing the Alkaline Hydrogen Evolution Reaction Activity through the Bifunctionality of Ni(OH)2/Metal Catalysts. Angew. Chemie - Int. Ed. 2012, 51 (50), 12495–12498. [2] Zadick, A., Dubau, L., Sergent, N., Berthome, G., Chatenet, M. Huge instability of Pt/C catalysts in alkaline medium. Acs Catalysis, 2015, 5(8), 4819-4824. [3] Zadick, A., Dubau, L., Chatenet, M., Demirci, U., Serov, A., Atanassov, P. Instability of commercial Pt/C and Pd/C electrocatalysts in alkaline media. ECS Transactions, 2015, 69(17), 553-558. [4] Lafforgue, C., Zadick, A., Dubau, L., Maillard, F., Chatenet, M. Selected Review of the Degradation of Pt and Pd‐based Carbon‐supported Electrocatalysts for Alkaline Fuel Cells: Towards Mechanisms of Degradation. Fuel Cells, 2018, 18(3), 229-238. [5] Lafforgue, C., Maillard, F., Martin, V., Dubau, L., & Chatenet, M. Degradation of Carbon-Supported Platinum-Group-Metal Electrocatalysts in Alkaline Media Studied by in Situ Fourier Transform Infrared Spectroscopy and Identical-Location Transmission Electron Microscopy. ACS Catalysis, 2019, 9(6), 5613-5622. [6] Castanheira, L., Dubau, L., Mermoux, M., Berthomé, G., Caqué, N., Rossinot, E., ... & Maillard, F. Carbon corrosion in proton-exchange membrane fuel cells: from model experiments to real-life operation in membrane electrode assemblies. ACS Catalysis, 2014, 4(7), 2258-2267. Figure 1