Observations and theories of quantum effects in proton transfer electrode processes

Advances and challenges for experiment and theory for multi-electron multi-proton transfer at electrified solid–liquid interfaces

“…As indicated by theoretical analyses (see Sections 5-7), 81 the importance of quantum effects in electrode processes is valid, but confirming the microscopic mechanism and interpreting this phenomenon still presents huge challenges. Furthermore, previous reports showed the observation of a large difference in symmetry factors in H and D systems, 46,95 which indicates a strong quantum effect; 47 however, the reason for this observation still remains an open question. Several major challenges to understand the microscopic electrode mechanism are discussed in the next section.…”

“…4,44,45 Here we skip explaining the detailed history of these quantum electrochemical theories and their developments because one of the authors already summarized the aforementioned issues in recent reports. 46,47 It is clear that we have plenty of basic theories to consider microscopic electrode processes; however, the development of quantum mechanical descriptions of electrode processes had become almost stagnant after the late 1970s. One of the main reasons was the extraordinarily high complexity of surface electrochemical processes at solid-liquid interfaces, requiring advanced simulation methods, theoretical concepts, and wellestablished surface-sensitive spectrometry to observe reaction dynamics.…”

“…Starting from the most important point, based on previous investigations, we can conclude that quantum effects, for instance zero point energy (ZPE) and tunneling, are key in electrode processes at solid-liquid interfaces. 46,47,81,92 Therefore, quantum effects could be universally important for multi-electron multi-proton transfer electrode processes. For example, a strong indication of quantum proton tunneling was observed in a system composed of Pt electrode in an alkaline condition.…”

“…For these reasons, observing the signs of quantum electrode processes was enabled only quite recently. 46,47,81 To investigate NQEs in electrode processes, first, we should identify the RDS in the corresponding multi-electron multiproton transfer reaction. The first step is to classify a possible RDS as one of the following three cases: ET, PT, or CPET ( Fig.…”

“…46,47,92 These equations are easy to handle and were successfully applied to analyze a wide spectrum of electrocatalysis, 78,80 and also have been used to observe quantum electrode processes. 46,47,81 On the other hand, the pictures of reaction mechanisms obtained by these equations are relatively ambiguous or too simplified. The equations including additional effects for ORR (eqn (1)-(3)) were provided in the recent report to demonstrate more details of the microscopic mechanism of electrode processes.…”