In a recent breakthrough in the field of electrochemical catalysis, researchers have unveiled a detailed mechanistic insight into the oxygen evolution reaction (OER) facilitated by nickel-iron (NiFe) sites within aza-conjugated microporous polymer (Aza-CMP) frameworks. The study focuses on the critical role of metal-hydroxyl species and their mediation of intramolecular proton transfer (IPT) during the formation of the oxygen-oxygen (O–O) bond, a pivotal step that governs the efficiency and kinetics of OER. This investigation not only elucidates the individual contributions of Ni and Fe centers but also pioneers the concept of a virtual transition state (TS_v) that captures the nuanced interplay of proton transfer dynamics during catalysis.
The authors report that the generation of high-valent iron-oxo species (Fe^4+=O) plays a central role in O–O bond formation, while the oxidation state of nickel remains predominantly unaltered, suggesting the O–O bond-forming event is centered on the iron site. This observation challenges previous assumptions where both Ni and Fe were considered equally active in the redox cycle, highlighting instead a site-specific mechanistic pathway mediated by water nucleophilic attack (WNA). The water solvent’s hydrogen bond network is implicated as a facilitator, enabling the O–O bond to form through proton transfer steps moderated by the surrounding medium.
To deconvolute the complex proton transfer mechanisms in this bimetallic system, the researchers employed electrochemical proton inventory (PI) techniques—a powerful method that probes proton dynamics by systematically varying isotopic substitution in the solvent. By analyzing the curvature of PI plots, which are sensitive to the fractionation factor (Φ) representing proton bonding characteristics at different steps, the study distinguishes between two key transition states: TS1, corresponding to the water nucleophilic attack step forming metal-hydroperoxide (M–OOH), and TS2, describing an intramolecular deprotonation relay converting M–OOH to metal-peroxo species (M–OO). The PI results notably reveal that these transition states contribute differently depending on environmental factors, underscoring a multi-step rate-determining step rather than a singular kinetic barrier.
Intriguingly, the concept of a virtual transition state (TS_v) is introduced to account for this multi-step behavior, modeling the observed kinetics as a weighted average of the intrinsic properties of TS1 and TS2. This framework offers a refined lens to interpret the proton transfer kinetics, where rate constants k_B (solution-mediated proton transfer) and k_R (relay site-mediated intramolecular deprotonation) dictate the observed reaction rate. The curvature and shape of the PI plots elegantly reflect the interplay between these two microscopic rate-determining steps.
The study further explores the influence of solution pH and buffer composition, correlating the base strength (pK_a) of different anions with their impact on proton transfer dynamics. In highly alkaline media such as NaOH, characterized by a high k_B, the PI plots exhibit pronounced nonlinear curvature indicative of TS2-dominated rate control, emphasizing intramolecular proton transfer as the bottleneck. In contrast, near-neutral borate buffers with lower k_B values induce near-linear PI behavior, signaling that the initial proton transfer step TS1 governs the reaction kinetics. In phosphate buffers of intermediate pK_a, the PI response is intermediate, consistent with comparable contributions from both TS1 and TS2, thereby reinforcing the significance of environment-modulated proton transfer dynamics.
CO poisoning experiments underscore the cooperative synergy between Ni and Fe sites in facilitating proton transfer and O–O bond formation. CO molecules preferentially bind to Ni centers, effectively blocking water and hydroxide adsorption and hindering proton transfer to the Ni site. This perturbation shifts the PI response from a curved, TS2-dominant behavior to a more linear TS1-characteristic curve, confirming the integral role of Ni sites in the intramolecular proton transfer relay mechanism. This finding highlights the critical cooperative function of both metal centers in orchestrating efficient OER catalysis.
In addition to mechanistic elucidation, the researchers investigate how the proton acceptor’s identity, whether a solution-phase buffer ion or a relay within the catalyst’s secondary coordination sphere, modulates catalytic rates. When external proton acceptors mediate proton transfer (TS1), the catalytic rate exhibits a first-order dependence on the buffer anion concentration, demonstrating solution concentration as a kinetic variable. Conversely, relay-mediated proton transfer within the catalytic site (TS2) shows negligible dependence on buffer concentration due to the intramolecular nature of the process, resulting in zero-order kinetics relative to proton acceptor concentration. The virtual transition state concept appropriately models these competing effects and complex dependencies.
Buffer-dependent reaction orders further emphasize the influence of proton acceptor identity and efficacy. In borate buffers, reaction orders for borate ions range from 0.34 to 0.63, aligning with dominance of solution-mediated proton transfer. In phosphate buffers, the reaction order significantly decreases to near zero, implying a shift towards intramolecular relay-dominated proton transfer. Under alkaline hydroxide-rich conditions, the reaction order approaches zero, robustly validating IPT as the rate-determining step due to superior relay effectiveness compared to external proton acceptors.
From a broader perspective, these findings have profound implications for the design of next-generation heterogeneous OER catalysts. By revealing the critical mechanistic role of metal-hydroxyl-mediated intramolecular proton transfer and clarifying the distinct yet interdependent functions of Ni and Fe sites, the study points toward catalyst architectures that optimize secondary coordination sphere interactions, proton relay pathways, and site cooperativity. Such design principles could drastically enhance catalytic turnover rates and durability under operational conditions.
Additional electrochemical analyses provide supplemental insights aligning with the proton inventory data. Notably, a pH-dependent Tafel slope shift from approximately 63 mV dec^-1 to 32 mV dec^-1 is observed, indicative of a transition from an apparent proton transfer-dominated rate-determining state at lower pH to one governed by intramolecular proton transfer at higher pH. This transition corroborates the mechanistic narrative of environment-tuned proton transfer pathways shaping overall OER kinetics and energetics.
The ability to dissect these proton transfer steps mechanistically using a combination of isotope effect studies, chemical poisoning, and buffer variation represents a significant advancement in heterogeneous catalyst research. The nuanced picture that emerges underscores the importance of molecular-level understanding in bridging the gap between catalyst structure, surface chemistry, and catalytic performance.
Overall, the elucidation of intramolecular proton transfer mediated by metal-hydroxyl groups within a dual-metal framework provides a blueprint for engineering catalysts with enhanced cooperative dynamics. The strategic positioning of proton relay sites and the fine-tuning of the catalyst’s protonation microenvironment emerge as potent tools to accelerate oxygen evolution, a reaction central to artificial photosynthesis, renewable energy conversion, and sustainable fuel production.
This work not only sharpens our mechanistic understanding of the OER but also paves the way for rational catalyst design targeting efficient, durable, and cost-effective electrochemical energy conversion technologies. The demonstration that subtle proton transfer mechanisms profoundly influence rate-determining processes calls for future explorations into tailored secondary coordination spheres and dynamic proton relays in other catalytic transformations.
In essence, this study redefines the conceptual framework for OER catalysis on mixed-metal sites by identifying intramolecular proton transfer as a dominant, tunable factor in reaction kinetics. Such insights hold promise for unlocking new levels of catalytic activity and selectivity previously obscured due to experimental complexity and mechanistic ambiguity.
Going forward, expanding these experimental strategies to other transition metal systems and incorporating operando spectroscopic methods could yield even deeper understanding of transient proton-coupled electron transfer events. The integration of computational modeling with proton inventory data will also be invaluable to extend these findings to a broader spectrum of catalytic materials and reactions.
Ultimately, the comprehensive characterization of metal-hydroxyl interactions and proton relays within this study spotlights key challenges and opportunities in the field of electrochemical conversion, energizing efforts to design catalysts with unprecedented control over proton dynamics and catalytic performance.
Subject of Research: Mechanistic investigation of proton transfer dynamics in heterogeneous oxygen evolution reaction catalyzed by Ni–Fe sites in aza-conjugated microporous polymers.
Article Title: Metal-hydroxyls mediate intramolecular proton transfer in heterogeneous O–O bond formation.
Article References:
Yang, H., Li, F., Zhan, S. et al. Metal-hydroxyls mediate intramolecular proton transfer in heterogeneous O–O bond formation. Nat. Chem. (2025). https://doi.org/10.1038/s41557-025-01993-8
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