Abstract

The oxygen evolution reaction (OER) is the limiting factor in an electrolyzer and the oxygen reduction reaction (ORR) the limiting factor in a fuel cell. In OER, water is converted to O₂ and H⁺/e⁻ pairs, while in ORR the reverse process happens to form water. Both reactions and their efficiency are important enablers of a hydrogen economy where hydrogen will act as a fuel or energy storage medium. OER and ORR can both be described assuming a four-step electrochemical mechanism with coupled H⁺/e⁻ transfers between four intermediates (M-*, M-OH, M = O, M-OOH, M = active metal site). Previously, it was shown for mixed metal-oxyhydroxides that an unstable M-OOH species can equilibrate to an M-OO species and a hydrogenated acceptor site (M-OOH/eq), enabling a bifunctional mechanism. Within OER, the presence of Fe within a nickel-oxyhydroxide (NiOOH) acceptor site was found to be beneficial to lower the required overpotential (Vandichel et al. in Chemcatchem 12(5):1436–1442, 2020). In this work, we present the first proof-of-concept study of various possible mechanisms (standard and bifunctional ones) for OER and ORR, i.e. we include now the active edge sites and hydrogen acceptor sites in the same model system. Furthermore, we consider water as solvent to describe the equilibration of the M-OOH species to M-OOH/eq, a crucial step that enables a bifunctional route to be operative. Additionally, different single Fe-dopant positions in an exfoliated NiOOH model are considered and four different reaction schemes are studied for OER and the reverse ORR process. The results are relevant in alkaline conditions, where the studied model systems are stable. Certain Fe-dopant positions result in active Ni-edge sites with very low overpotentials provided water is present within the model system.

Graphic Abstract

中文翻译：

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掺杂Fe的NiOOH中的析氧和还原：溶剂，掺杂位置和反应机理的影响

摘要

在电解器中，析氧反应（OER）是限制因素，而在燃料电池中，氧还原反应（ORR）是限制因素。在OER，水被转换至O ₂和H ⁺ / E ^-对，而在ORR相反的过程会发生，以形成水。反应及其效率都是氢经济的重要推动力，其中氢将充当燃料或能量存储介质。OER和ORR都可以描述假定一个四步的电化学机构具有耦合ħ ⁺ / E ^-转移4中间体（间M- *，M-OH ，M = 0 ，M-OOH，中号 =活性金属位）。以前，对于混合金属羟基氧化物，已经证明不稳定的M-OOH物质可以平衡为M-OO物质和氢化受体位点（M-OOH / eq），从而实现双功能机理。在OER内，发现在羟基氧化镍（NiOOH）受体位点中存在Fe有助于降低所需的超电势（Vandichel等人，Chemcatchem 12（5）：1436–1442，2020）。在这项工作中，我们提出了OER和ORR各种可能机制（标准和双功能机制）的首次概念验证研究，即，我们现在在同一模型系统中包括了活性边缘位点和氢受体位点。此外，我们以水为溶剂来描述M-OOH的平衡物种达到M-OOH / eq，这是使双功能途径有效的关键步骤。此外，在剥落的NiOOH模型中考虑了不同的单个Fe掺杂位置，并针对OER和反向ORR过程研究了四种不同的反应方案。结果与碱性条件有关，在碱性条件下，所研究的模型系统稳定。如果模型系统中存在水，则某些铁掺杂位置会导致活性极低的Ni边缘位点。

图形摘要