Proton-Coupled Electron Transfer Underpins H 2 Evolution

Realization that hydrogen evolution efficiency and rate can be improved through intentional incorporation of proton relays has sharpened interest in how the movement of protons and electrons are coupled. Within the context of hydrogen evolution, the catalyst must orchestrate the union of two electrons and two protons. The elementary proton and electron transfer (PT and ET, respectively) steps in this reaction are frequently proton-coupled electron transfer (PCET) reactions wherein the movement of an electron induces movement of a proton or vice versa. PCET is traditionally viewed in terms of a square scheme (Figure 1.2) where concerted and stepwise routes are possible. Reactions that proceed via the stepwise routes necessarily proceed through high energy intermediates. Consequently, mechanistic study of PCET reactions involving transition metal complexes is an important step towards designing more efficient catalysts for hydrogen evolution reactions. Further, as PCET reactions are additionally relevant outside the subfield of fuel formation reactions, the quest to understand PCET in hydrogen evolution catalysts simultaneously contributes more broadly useful fundamental knowledge.

Figure 1.2. Square scheme representation of a homogeneous PCET reaction where species M

receives one electron and one proton to form MH. H-A represents an acid molecule, and ET, PT, and CPET indicate electron transfer, proton transfer, and concerted proton-electron transfer, respectively.

Our group has focused on understanding PCET processes in matters relevant to energy conversion and storage processes; specifically in the context of homogeneous electrochemical catalysts for the hydrogen evolution reaction (HER). As noted above, it has become increasingly clear that an understanding of electrochemical PCET processes are crucial for advancing HER and other catalytic processes. Studies of PCET reaction mechanisms to date have primarily involved organic substrates relevant to biological charge transport pathways, with only a few examples of transition metal-based systems.22–25 Further, while a rich literature exists for the study of homogeneous based PCET, where electron and proton transfers occur between discrete molecules (Figure 1.3A),23,26–30 there are fewer reports which detail individual electrochemical PCET steps (specifically where the electrode is a partner in the electron transfer reaction, Figure 1.3B).25,31–38 This is not due to a lack of methodology or theory – extensive information and experimental examples exist for the study of electrochemical mechanism.35,36,39–46 In addition, detailed theoretical analyses for mechanism-dependent electrochemical PCET rate expressions have been reported.47–49 However, only recently have these methods received more use in the study of PCET processes relevant to energy

storage.35,37,38,41,45,50

In the study of electrochemical mechanisms, elementary steps are generally divided into two categories: electrochemical (E) steps and chemical (C) steps. An E step involves the movement of an electron to/from the electrode whereas any other homogeneous chemical transformation corresponds to a C step. Additional subscripts may designate reversibility (subscript r) or irreversibility (subscript i) of individual steps, while the term (EC) indicates a concerted E and C step. This classic electrochemical nomenclature is readily paired with traditional homogeneous PCET terminology. For example, an electron transfer followed by a proton transfer, referred to as ET-PT in PCET literature, is designated more generally as an EC electrochemical mechanism. Meanwhile, the concerted-proton electron transfer – CPET – pathway would be designated (EC).

Proton transfer generally occurs between discrete molecular proton donors and proton acceptors; however, electron transfer may occur between solution species or, as considered more frequently, to and from the electrode directly. As electron transfer occurs between solution and an electrode, two interesting, and, as of yet, incompletely explored influences on the PCET mechanism arise.

First, the kinetic barrier of the CPET pathway is impacted. In purely homogeneous systems, PCET may involve three discrete molecules which must encounter one another in a ternary reaction for a CPET process (Figure 1.3A). While the formation of high-energy, charged intermediates are circumvented, this termolecular reaction can give rise to a high kinetic barrier. As such, stepwise ET-PT or PT-ET mechanisms are often favored kinetically.51,52 Conversely, for electrochemical PCET, the reaction between the PCET substrate and the proton

donor/acceptor occurs at an essentially infinite electrode surface (Figure 1.3B). As such, the kinetic barriers for an electrochemical CPET process will be different than the termolecular homogeneous example.

Figure 1.3. Cartoon depictions of A) homogeneous termolecular CPET (case for two donors and one acceptor shown; more partners are possible) and B) electrochemical PCET. Reprinted with permission from ACS Catalysis 2016, 6, 3644. Copyright (2016) American Chemical

Second, the electron transfer driving force changes during a potential-sweep

electrochemical experiment. It has been demonstrated that via careful selection of chemical oxidants, the PCET oxidation of a tungsten-hydride bond can be induced to undergo a CPET mechanism.24 In an electrochemical experiment such as cyclic voltammetry, the driving force for electron transfer changes as a function of time. This suggests that the PCET mechanism may change during a single experiment as the potential is swept (Figure 1.4). It has already been demonstrated that mechanisms for hydrogen evolution can in some cases switch from ECEC to EECE as the applied potential is scanned cathodically.38,53 This strongly suggests that a change of potential may promote a change in the mechanism of an individual PCET event.

Figure 1.4. As applied potential changes throughout a variable-potential experiment, the PCET mechanism accessed may change as the driving force changes. Reprinted with permission from ACS Catalysis 2016, 6, 3644. Copyright (2016) American Chemical Society.