Shinagawa Tatsuya, Garcia-Esparza Angel T, Takanabe Kazuhiro
Division of Physical Sciences and Engineering, KAUST Catalysis Center (KCC), King Abdullah University of Science and Technology (KAUST), 4700 KAUST, Thuwal, 23955-6900, Saudi Arabia.
Sci Rep. 2015 Sep 8;5:13801. doi: 10.1038/srep13801.
Microkinetic analyses of aqueous electrochemistry involving gaseous H2 or O2, i.e., hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), are revisited. The Tafel slopes used to evaluate the rate determining steps generally assume extreme coverage of the adsorbed species (θ≈0 or ≈1), although, in practice, the slopes are coverage-dependent. We conducted detailed kinetic analyses describing the coverage-dependent Tafel slopes for the aforementioned reactions. Our careful analyses provide a general benchmark for experimentally observed Tafel slopes that can be assigned to specific rate determining steps. The Tafel analysis is a powerful tool for discussing the rate determining steps involved in electrocatalysis, but our study also demonstrated that overly simplified assumptions led to an inaccurate description of the surface electrocatalysis. Additionally, in many studies, Tafel analyses have been performed in conjunction with the Butler-Volmer equation, where its applicability regarding only electron transfer kinetics is often overlooked. Based on the derived kinetic description of the HER/HOR as an example, the limitation of Butler-Volmer expression in electrocatalysis is also discussed in this report.
重新审视了涉及气态H₂或O₂的水电化学的微观动力学分析，即析氢反应（HER）、氢氧化反应（HOR）、氧还原反应（ORR）和析氧反应（OER）。用于评估速率决定步骤的塔菲尔斜率通常假定吸附物种的覆盖率极高（θ≈0或≈1），尽管在实际中，斜率是与覆盖率相关的。我们进行了详细的动力学分析，描述了上述反应中与覆盖率相关的塔菲尔斜率。我们的仔细分析为实验观察到的可归因于特定速率决定步骤的塔菲尔斜率提供了一个通用基准。塔菲尔分析是讨论电催化中涉及的速率决定步骤的有力工具，但我们的研究也表明，过于简化的假设会导致对表面电催化的描述不准确。此外，在许多研究中，塔菲尔分析是与巴特勒-沃尔默方程一起进行的，其仅适用于电子转移动力学这一点常常被忽视。以推导出的HER/HOR动力学描述为例，本报告还讨论了巴特勒-沃尔默表达式在电催化中的局限性。
