The hydrogen oxidation and evolution reaction (HOR/HER) has been widely investigated due to its importance for a broad variety of applications especially in electrolysis and fuel cells. However, owing to its extremely fast kinetics, kinetic data can only be obtained with experimental setups that provide very fast mass transport, so that the effect of hydrogen partial pressure (pH2) and kinetic limitations at high overpotentials are not yet fully understood. Here we report detailed kinetic investigations on the temperature and pressure dependence of the HOR/HER kinetics on carbon supported platinum (Pt/C) using the PEMFC-based H2-pump approach. By using ultra-low platinum loadings, we could show that the apparent activation energy of the HOR/HER on platinum increases with increasing pH2, due to a diminishing effect of the hydrogen adsorption enthalpy with increasing coverage by adsorbed hydrogen atoms on the Pt surface. Consequently, the HOR/HER reaction order with respect to pH2 also depends on the temperature. We further observed that the HOR reaches a limiting current at high HOR overpotentials that showed a direct proportionality to pH2 and a pressure-independent activation energy. We ascribe this to a limitation of the hydrogen adsorption rate either by a rate limiting Tafel-reaction or mass transport limitations.
The hydrogen oxidation and evolution reaction (HOR/HER) has been widely investigated in electrocatalysis due to its importance for a broad variety of applications especially in electrolysis and fuel cells (FC). While the HOR/HER reaction rates on platinum and platinum alloy catalysts in alkaline environment can be determined precisely with rotating disk electrode (RDE) measurements,1-3 this is not possible at low pH due to the two orders of magnitude higher reaction rates that far exceed the slow mass transport limited rates afforded by the RDE configuration.4 As a result, meaningful measurements are limited to setups with much faster mass transport rates, such as floating electrodes or proton exchange membrane (PEM) fuel cell based electrochemical H2-pumps,4, 5 but the effect of hydrogen partial pressure and kinetic limitations at higher overpotentials are not yet fully understood. Here we report detailed kinetic investigations on the temperature and pressure dependence of the HOR/HER kinetics on carbon supported platinum (Pt/C) using the PEMFC based H2-pump approach. The measurements were performed on assymmetric membrane electrode assemblies (MEAs), where the working electrode has an ultra-low loading of only 1-2 µgPt cmMEA -2 (resulting in roughness factors of ≈1.5 cmPt 2 cmMEA -2), while the counter electrode has a much higher loading of 0.4 mgPt cmMEA -2. Using a fuel cell test station that allows for the feed of humidified hydrogen on both the anode and the cathode, we were able to show that the apparent activation energy of the HOR/HER on platinum increases with increasing partial pressure of hydrogen, which we ascribe to a decreasing effect of the hydrogen adsorption enthalpy with increasing coverage by adsorbed hydrogen atoms on the Pt surface. Consequently, the HOR/HER reaction order with respect to H2 partial pressure is also depending on the temperature. We further observed that the HOR quickly reaches a limiting current with increasing HOR overpotential, ηHOR (orange area in Fig. 1), which we ascribe to a change in the rate-determining step (rds) from the Volmer reaction (i.e., the electrochemical oxidation of adsorbed H) at low ηHOR to the Tafel reaction (i.e., the dissociative adsorption of H2 on the Pt surface) at higher ηHOR. The Tafel-limited current shows a pressure-independent activation energy and a direct proportionality to the partial pressure of hydrogen, which is exactly what one would expect for the dissociative Langmuir adsorption as the rds. Finally, the HOR/HER reaction rates on a Pt3Co alloy and a Ru@Pt core-shell catalyst will be compared to that on a Pt/C catalyst. References: G. Couturier, D. W. Kirk, P. J. Hyde, and S. Srinivasan, Electrochim. Acta, 32 (7), 995-1005 (1987). P. J. Rheinländer, J. Herranz, J. Durst, and H. A. Gasteiger, J. Electrochem. Soc., 161 (14), F1448-F1457 (2014). J. Zheng, W. Sheng, Z. Zhuang, B. Xu, and Y. Yan, Sci. Adv., 2 (3), e1501602 (2016). J. Durst, C. Simon, F. Hasché, and H. A. Gasteiger, J. Electrochem. Soc., 162 (1), F190-F203 (2015). A. R. Kucernak and C. Zalitis, J. Phys. Chem. C, 120 (20), 10721-10745 (2016). Figure 1. Temperature-dependent Butler-Volmer plots of the HOR/HER kinetic rates on Pt/C, measured in a PEMFC based H2-pump configuration (100 kPaH2, 90% RH, 2000/2000 nccm H2 flows, working/counter electrode loadings of 1.5/400 µgPt cm-2). The orange area indicates the limiting HOR current due to a rate-determining Tafel step. The Butler-Volmer fits were constrained by requiring the sum of the exchange coefficients to be 1 (Σα=1). Figure 1
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