(Invited) Highly Active and Durable Ir-Pt Bifunctional Electrocatalyst for Oxygen Evolution and Hydrogen Oxidation Reactions

Abstract

Unitized regenerative fuel cells (URFCs) are single-unit electrochemical devices that operate in both electrolyzer and fuel-cell modes.1 Major barriers to widespread implementation of the traditional URFC device are high cost, low efficiency, and limited lifetime. These shortcomings are primarily imposed by the cell design, which utilizes one side of the cell for the oxygen reactions, oxygen evolution (OER) and oxygen reduction (ORR), and the other side for the hydrogen reactions, hydrogen evolution (HER) and hydrogen oxidation (HOR). One disadvantage of this design is that the kinetics of the OER and ORR are orders of magnitude slower than that of the HOR and HER reactions and the most active catalysts for these two reactions are Ir-based and Pt-based, respectively. Therefore, bifunctional OER/ORR catalysts suffer from poor kinetics for both reactions and the ORR component of the catalysts must be robust enough to survive transitions between ORR potentials (0.6 to 0.8 V) and OER potentials (>1.5 V). This requirement necessitates high loadings of unsupported catalyst and, due to durability concerns, excludes the most active ORR catalysts: high-surface-area carbon-supported PtCo and PtNi nanoparticles. An alternative cell design that combines the kinetically-challenging oxygen reactions with the facile hydrogen reactions would allow the bifunctional catalysts to be tailored to enhance the oxygen reaction kinetics while still maintaining high activity for the hydrogen reactions. Combining the OER and HOR functions on one electrode (the anode) and HER and ORR functions on the other electrode (the cathode) would enable an improved electrode layer design and novel catalysts both of which decrease precious metal loadings. Anode catalysts that couple the robust and facile HOR reaction on platinum2 with the most stable and active iridium OER catalyst 3-5 were selected for development. These include fabrication of a thin catalyst coating on high-surface area, electronically-conductive and stable non-carbon supports with the goal of maximizing the electrochemically-active surface area of the catalysts and decreasing the loading of precious metal.6 In this work a conformal and thin coating of Ir-Pt catalyst was deposited on different non-carbon supports using atomic layer deposition (ALD) and characterized using several techniques, including transmission electrons microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Thin-films of the catalysts deposited on glassy carbon electrodes were evaluated for both OER and HOR in 0.1 M HClO4 electrolyte using rotating disk electrode (RDE). The catalysts stability was tested by cycling the thin-film electrodes in the HOR−OER potential range. Electrochemical results show that a Ir-Pt/TiO2 material exhibits a balance between the OER and HOR mass activities and stability up to 5000 potential cycles of the aggressive accelerated stress test protocol. AcknowledgementsThis work was supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office. Argonne National Laboratory is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC under contract DE-AC-02-06CH11357. References J. Pettersson, B. Ramsey, D. Harrison, J. Power Sources 157, 28-34(2006)J. Stacy, Y. N. Regmi, B. Leonard, M. Fan, Renewable Sustainable Energy Rev 69, 401-414 (2017)C. S. Gabriel, R. F. Mauro, A. T. Edson, ACS Catal. 8, 208-2092 (2018),N. Danilovic, et al. J. Phys. Chem. Lett. 5, 2474-2478 (2014),Y. Zhang, H. Zhang, Y. Ma, J. Cheng, H. Zhong, et al. J. Power Sources 195, 142-145 (2010)Y. C. Kimmel, X. Xu, W. Yu, X. Yang, J. G. Chen, ACS Catal. 4, 1558-1562 (2014)