Exner Kai S
Faculty of Chemistry, Theoretical Inorganic Chemistry, University Duisburg-Essen, Universitätsstraße 5, 45141 Essen, Germany.
Cluster of Excellence RESOLV, 44801 Bochum, Germany.
ACS Phys Chem Au. 2023 Jan 31;3(2):190-198. doi: 10.1021/acsphyschemau.2c00054. eCollection 2023 Mar 22.
In the last decade, trends for competing electrocatalytic processes have been largely captured by volcano plots, which can be constructed by the analysis of adsorption free energies as derived from electronic structure theory in the density functional theory approximation. One prototypical example refers to the four-electron and two-electron oxygen reduction reactions (ORRs), resulting in the formation of water and hydrogen peroxide, respectively. The conventional thermodynamic volcano curve illustrates that the four-electron and two-electron ORRs reveal the same slopes at the volcano legs. This finding is related to two facts, namely, that only a single mechanistic description is considered in the model, and electrocatalytic activity is assessed by the concept of the limiting potential, a simple thermodynamic descriptor evaluated at the equilibrium potential. In the present contribution, the selectivity challenge of the four-electron and two-electron ORRs is analyzed, thereby accounting for two major expansions. First, different reaction mechanisms are included into the analysis, and second, (), a potential-dependent activity measure that factors overpotential and kinetic effects into the evaluation of adsorption free energies, is applied for approximation of electrocatalytic activity. It is illustrated that the slope of the four-electron ORR is not constant at the volcano legs but rather is prone to change as soon as another mechanistic pathway is energetically preferred or another elementary step becomes the limiting one. Due to the varying slope of the four-electron ORR volcano, a trade-off between activity and selectivity for hydrogen peroxide formation is observed. It is demonstrated that the two-electron ORR is energetically preferred at the left and right volcano legs, thus opening a new strategy for the selective formation of HO by an environmentally benign route.
在过去十年中，竞争电催化过程的趋势在很大程度上已被火山图所体现，火山图可通过分析密度泛函理论近似下电子结构理论推导的吸附自由能来构建。一个典型的例子是四电子和两电子氧还原反应（ORR），分别导致水和过氧化氢的形成。传统的热力学火山曲线表明，四电子和两电子ORR在火山曲线的分支处具有相同的斜率。这一发现与两个事实有关，即模型中只考虑了单一的机理描述，并且电催化活性是通过极限电位的概念来评估的，极限电位是在平衡电位下评估的一个简单的热力学描述符。在本论文中，分析了四电子和两电子ORR的选择性挑战，从而考虑了两个主要扩展。首先，分析中纳入了不同的反应机理；其次，（此处原文缺失部分内容），一种将过电位和动力学效应纳入吸附自由能评估的电位依赖活性度量，被用于近似电催化活性。结果表明，四电子ORR在火山曲线分支处的斜率并非恒定不变，一旦另一种机理途径在能量上更有利或者另一个基元步骤成为限速步骤，斜率就容易发生变化。由于四电子ORR火山曲线斜率的变化，观察到了过氧化氢生成的活性和选择性之间的权衡。结果表明，在火山曲线的左右分支处，两电子ORR在能量上更有利，从而为通过环境友好途径选择性生成过氧化氢开辟了一条新策略。
