Understanding the Enhanced Activity of Pd/CeO₂ in Hydrogen Oxidation Reaction in Alkaline Media

Palladium supported on ceria (Pd/CeO₂) has recently raised strong interest as an alternative catalyst to platinum on the anode electrode in anion exchange membrane fuel cells^1,2. The enhanced activity of Pd/CeO₂catalysts for hydrogen oxidation reaction (HOR) activity in alkaline media has notably been attributed to strong metal-support interactions, but the exact mechanism is still under debate.<br/><br/>In this work, we investigate Pd/CeO₂ thin films with well-defined compositions and structures to gain fundamental understanding of the metal-support interface and its influence on HOR activity in alkaline media. We develop a novel approach combining <i>in situ </i>electrochemical quartz crystal microbalance (E- QCM), <i>ex situ </i>microscopy and spectroscopy, and theoretical calculations to describe the interface between Pd and CeO2.<br/><br/>Pd/CeO₂ leads to expected enhancement of HOR in alkaline media compared to pure Pd. Using E-QCM, we provide new fundamental understanding of the adsorption, absorption, physisorption and desorption phenomena occurring during HOR. We show that on pure Pd thin films, <i>absorption of hydrogen</i> dominates while on Pd/CeO₂ the oxidation/reduction of ceria and <i>adsorption of hydroxyl groups</i> are the dominating phenomena³.<br/><br/>One of the hypotheses for enhanced reactivity of Pd/CeO₂ in HOR is that CeO₂ – at the interface with Pd - serves as a source of OH groups which react with adsorbed H to form water⁴. With this work, we provide new quantitative evidence that this hypothesis is valid. Strong metal-support interactions are confirmed by TEM, XPS and DFT calculations. An important finding is that Pd atoms embedded in CeO₂ are present on the prepared thin films and could be active sites for hydrogen activation.<br/><br/>This work thus provides strong evidence that the interface between Pd and ceria must be carefully designed to provide synergies and to lead to enhanced reactivity in HOR. Finally, this study underlines the importance of model thin films as appropriate tools to study the interface between metal and supports <i>in situ, </i>during electrochemical reactions.<br/><br/>References:<br/>1. K. Singh, E. S. Davydova, J. Douglin, A. O. Godoy, H. Tan, M. Bellini, B. J. Allen, J. Jankovic, H. A. Miller, A. C. Alba-Rubio and D. R. Dekel, <i>Adv. Funct. Mater.</i>, 2020, <b>30</b>, 2002087<br/>2. Bellini, M. V. Pagliaro, A. Lenarda, P. Fornasiero, M. Marelli, C. Evangelisti, M. Innocenti, Q. Jia, S. Mukerjee, J. Jankovic, L. Wang, J. R. Varcoe, C. B. Krishnamurthy, I. Grinberg, E. Davydova, D. R. Dekel, H. A. Miller and F. Vizza, <i>ACS Appl. Energy Mater.</i>, 2019, <b>2</b>, 4999-5008<br/>3. Luneau, L. Strandberg, G. Montserrat-Siso, V. Shokhen. R. Mohan, H. Gronbeck, B. Wickman. <i>J. Mater. Chem. A</i>, 2023,11, 16370-16382<br/>4. D. Speck, F. S. M. Ali, M. T. Y. Paul, R. K. Singh, T. Bohm, A. Hofer, O. Kasian, S. Thiele, J. Bachmann, D. R. Dekel, T. Kallio and S. Cherevko, <i>Chem. Mater.</i>, 2020, 32, 7716-7724