Understanding Electric and Non-Electric Field Effects on Electrochemical Double Layer Restructuring for the Model Platinum HOR

Abstract

Recent progresses in hydroxide exchange membrane fuel cells (HEMFCs) have led to renewed interest in the strong pH effect on hydrogen oxidation reaction (HOR) rate on precious metals: two orders of magnitude decrease when pH is changed from 0 to 14.1 Various theories have been proposed to explain this, ranging from adsorbed cation effects, changes in the electric field strength, to pH induced changes to the apparent hydrogen binding energy.2-4 One leading argument is the increase of pH leads to stronger electric field and thus more rigid ordering of water at the electrode/electrolyte interface, resulting in greater resistance to proton/hydroxide transfer through the double layer. This increased energetic barrier due to reorganization results in the decreased activity.5 In this work, we examine this theory by using a combination of traditional electochemical and spectroscopic techniques to highlight the importance of both electric field and non-electric field effects on reaction rates. Further we use this distinction to understand how each factor contributes to water structure re-arrangement on the surface and in the double layer, which we hypothesize to be the pivotal factor in the aforementioned activity decrease from acid to base.The electrochemical double layer is traditionally described by specifically adsorbed species (inner helmholtz plane, IHP), the next layer of hydrated ions (outer Helmholtz plane, OHP) and further layers (diffuse region) protruding and mixing into the bulk electrolyte. It has been shown that the stark tuning rate, which is a function of electric field strength, is dictated by hydrated cation size, and provides a metric to measure the thickness of the electrochemical double layer.6-8 By introducing crown ether to chelate cations, the stark tuning rate, and thus the electric field, can be decreased up to a 0.5:1 ratio of crown ether to cation. Beyond this point, no change in stark tuning rate and electric field were observed. However, further decreases in HOR activity are observed beyond this threshold. This decreasing activity at a constant field strength provides strong evidence for mixed electric and non-electric field effects. We hypothesize that cations and ether molecules impact the electric field strength to a degree, but also interact with surface water species. Spectroscopic and reactivity data demonstrates that both chelating and linear ethers, while having drastically different impacts on the strength of the electric field, result in similar activity decreases. Additionally, both types of ether demonstrate removal of the same types of interfacial water, suggesting that their influence on the surface water structure rather than their effect on the electric field strength determines the HOR activity.(1) Durst, J.; Siebel, A.; Simon, C.; Hasché, F.; Herranz, J.; Gasteiger, H. A. New Insights into the Electrochemical Hydrogen Oxidation and Evolution Reaction Mechanism. Energy Environ. Sci. 2014, 7 (7), 2255–2260. https://doi.org/10.1039/c4ee00440j.(2) Chen, X.; McCrum, I. T.; Schwarz, K. A.; Janik, M. J.; Koper, M. T. M. Co-Adsorption of Cations as the Cause of the Apparent PH Dependence of Hydrogen Adsorption on a Stepped Platinum Single-Crystal Electrode. Angew. Chemie - Int. Ed. 2017, 56 (47), 15025–15029. https://doi.org/10.1002/anie.201709455.(3) Yang, X.; Nash, J.; Oliveira, N. J.; Yan, Y.; Xu, B. Understanding the PH Dependence of Underpotential Deposited Hydrogen on Platinum. Angew. Chemie - Int. Ed. 2019, 58, 17718–17723. https://doi.org/10.1002/ange.201909697.(4) Zheng, J.; Nash, J.; Xu, B.; Yan, Y. Perspective—Towards Establishing Apparent Hydrogen Binding Energy as the Descriptor for Hydrogen Oxidation/Evolution Reactions. J. Electrochem. Soc. 2018, 165 (2), H27–H29. https://doi.org/10.1149/2.0881802jes.(5) Ledezma-Yanez, I.; Wallace, W. D. Z.; Sebastián-Pascual, P.; Climent, V.; Feliu, J. M.; Koper, M. T. M. Interfacial Water Reorganization as a PH-Dependent Descriptor of the Hydrogen Evolution Rate on Platinum Electrodes. Nat. Energy 2017, 2 (4), 1–7. https://doi.org/10.1038/nenergy.2017.31.(6) Ringe, S.; Clark, E. L.; Resasco, J.; Walton, A.; Seger, B.; Bell, A. T.; Chan, K. Understanding Cation Effects in Electrochemical CO 2 Reduction. Energy Environ. Sci. 2019. https://doi.org/10.1039/C9EE01341E.(7) Dunwell, M.; Wang, J.; Yan, Y.; Xu, B. Surface Enhanced Spectroscopic Investigations of Adsorption of Cations on Electrochemical Interfaces †. Phys. Chem. Chem. Phys 2017, 19, 971. https://doi.org/10.1039/c6cp07207k.(8) Waegele, M. M.; Gunathunge, C. M.; Li, J.; Li, X. How Cations Affect the Electric Double Layer and the Rates and Selectivity of Electrocatalytic Processes. J. Chem. Phys. 2019, 151 (16), 160902. https://doi.org/10.1063/1.5124878. Figure 1