Journal
JOURNAL OF PHYSICAL CHEMISTRY C
Volume 120, Issue 50, Pages 28489-28496Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.jpcc.6b10602
Keywords
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Funding
- University of Southern California
- AFOSR YIP Award [FA9550-13-1-0128]
- NSF CAREER Award [1454467]
- Division Of Chemistry
- Direct For Mathematical & Physical Scien [1454467] Funding Source: National Science Foundation
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The empirical electron-transfer coefficient a is a valuable electrochemical observable that bridges the thermodynamics and kinetics of redox reactions. For reactions that involve protons, the value of a is expected to be pH dependent. However, even for the simplest redox processes, the nature of this dependency remains unclear. Toward clarifying this problem, we follow two goals. First, we calculate the electron-transfer coefficient a and its pH-dependence based on a model 2D potential energy surface that has been investigated by Koper and Schmickler for proton reduction. According to the model, a is pH-independent for high-pH values and pH-dependent for low-pH values, with a increasing as the pH is lowered. Second, we report our experimentally measured a for hydrogen evolution on several electrode materials over a wide pH range. We observe that several features of the data show similarities to the predictions of the model. The data. show different behavior in two distinct pH regions. In the acidic region, a linearly increasing a with decreasing pH and in the basic side a pH-independent a are observed for several electrodes. However, certain predictions of the model, in particular the transition pH between the two regions, do not seem consistent with the data, which we propose likely arises due to mass-transfer limitations of the rate. We hope that this work will help better understand the pH dependence of interfacial electron-proton transfer reactions. and, in particular, inspire further work to isolate mass-transfer limitations from interfacial chemistry effects in measuring and interpreting the electron-transfer coefficient.
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