Metal-hydroxyls mediate intramolecular proton transfer in heterogeneous O–O bond formation

nature chemistry Article https://doi.org/10.1038/s41557-025-01993-8 Metal-hydroxyls mediate intramolecular proton transfer in heterogeneous O–O bond formation Received: 14 September 2024 Accepted: 7 October 2025 Hao Yang 1,10, Fusheng Li 2,3,10 , Shaoqi Zhan4,10, Yawen Liu5, Tianqi Liu 1, Linqin Wang 6, Wenlong Li6, Mårten S. G. Ahlquist 7, Sumbal Farid8, Rile Ge8, Junhu Wang 8, Marc T. M. Koper9 & Licheng Sun 1,2,6 Published online: xx xx xxxx Check for updates Metal (hydro)oxides are among the most effective heterogeneous water oxidation catalysts. Elucidating the interactions between oxygen-bridged metal sites at a molecular level is essential for developing high-performing electrocatalysts. Here we demonstrate that adjacent metal-hydroxyl groups function as intramolecular proton–electron transfer relays to enhance water oxidation kinetics. We achieved this using a well-defined molecular platform with an aza-fused π-conjugated microporous polymer that coordinates molecular Ni or Ni–Fe sites that emulate the structure of the most active edge sites in Ni–Fe materials for studying the heterogeneous water oxidation mechanism. We combine experimental and computational results to reveal the origin of pH-dependent reaction kinetics for O–O bond formation. We find both the anions in solution and the adjacent Ni3+–OH site act as proton transfer relays, facilitating O–O bond formation and leading to pH-dependent water oxidation kinetics. This study provides significant insights into the critical role of electrolyte pH in water oxidation electrocatalysis and enhancement of water oxidation activity in Ni–Fe systems. The kinetics of the oxygen evolution reaction (OER) is the catalytic bottleneck in acidic and alkaline water electrolysis. The challenging requirement of bringing the two oxygen atoms in close proximity to each other to form the O–O bond often serves as the main obstacle of the OER1–3. The formation of metal-oxo (M=O) species is followed either by the interaction of two metal-oxos or by the water nucleophilic attack (WNA) pathway for the crucial O–O bond formation step. The two metal-oxos interaction mechanism necessitates an optimal spatial arrangement of the bimetallic centre during the coupling process. The corresponding oxyl and/or oxo coupling mechanisms have been proposed across biological systems4, molecular catalysis5,6 and materials-based catalysis7,8. In the WNA pathway, the pivotal step involves the nucleophilic attack of water molecules on electron-deficient M=O, with simultaneous proton transfer to external and/or internal acceptors, resulting in hydroperoxide intermediates (M–OOH). This solution-mediated oxygen atom–proton transfer (APT) Department of Chemistry, School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, Stockholm, Sweden. 2State Key Laboratory of Fine Chemicals, Frontier Science Center for Smart Materials, Dalian University of Technology, Dalian, China. 3SINOPEC (Dalian) Research Institute of Petroleum and Petrochemicals Co., Ltd, Dalian, China. 4Department of Chemistry-Ångström, Molecular Biomimetics, Uppsala University, Uppsala, Sweden. 5Department of Chemistry-Ångström, Physical Chemistry, Uppsala University, Uppsala, Sweden. 6Center of Artificial Photosynthesis for Solar Fuels and Department of Chemistry, School of Science, Westlake University, Hangzhou, China. 7Department of Theoretical Chemistry and Biology, School of Engineering Sciences in Chemistry Biotechnology and Health, KTH Royal Institute of Technology, Stockholm, Sweden. 8 CAS Key Laboratory of Science and Technology on Applied Catalysis, Mössbauer Effect Data Center, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China. 9Leiden Institute of Chemistry, Leiden University, Leiden, the Netherlands. 10These authors contributed equally: Hao Yang, Fusheng Li, Shaoqi Zhan. e-mail: ; 1 Nature Chemistry Article https://doi.org/10.1038/s41557-025-01993-8 a H H O O H O H Outer-sphere Mn B H O Mn B H H e O PCET O H B e O H O O O Mx Mn N N N O Ru N N OH OH N N S O O N N N N N N N N N N N N N N N N N 15 3+/4+ Ni Ni 5 0 –5 Electrochemical treatment 1.4 1.6 E (V versus RHE) N N N N N N N N N N N N N N N N N O2 Fe N N eN N N Ni N N Carbon N N H2 O Hydrogen Oxygen N N Mimic highly OER active Ni–Fe sites Aza-CMP–NiFe Iron site Nickel site 300 250 200 150 100 e Aza-CMP–NiFe RuO2 0.40 50 10 0.35 –1 66.6 mV dec 0.30 0.25 0.20 1.8 100 –1 TOFactive Ni–Fe 18.7 s @ 300 mV –1 1 s @ 261 mV TOFtotal Ni TOFtotal Fe OH NiO y Fe: eO xH NiF 1 –1 30.8 mV dec 0 0.5 1.0 1.5 2.0 0.1 2.5 –2 log[current density (mA cm )] 0 1.2 N N TOF (s–1) d 2+/3+ O Neighbouring metal site Aza-CMP–Ni Aza-CMP–NiFe 10 O Nitrogen Overpotential (V) 20 Metal active site N N N N N Fe N N Ni N N N N N N N N N N N Current density (mA cm–2) Current density (mA cm–2) c O Mx – 1 Mn – 1 N Redox active Ni sites Aza-CMP–Ni Base site Mn N N N N N Ru N O N OH O Ru N e N N N N O N N H b N Ru O S O 3 H e O O Mx PCET N O O Inner-sphere O WNA with neighbouring metal oxo/oxyl site (M): 3 2 N H H Mn – 1 WNA with dangling base site (B): 1, 2 1 H O 0 0.05 0.10 0.15 0.20 0.25 Overpotential (V) 0.30 0.35 0.01 0.24 0.26 0.28 0.30 Overpotential (V) Fig. 1 | Schematic structures of the Aza-CMP–Ni and Aza-CMP–NiFe electro catalysts and their electrochemical characterization. a, Representative O–O bond formation steps via the WNA pathway with proton relays and corresponding representative WOCs. b, Schematic diagram of molecular Ni sites in Aza-CMP–Ni and molecular Ni–Fe sites in Aza-CMP–NiFe. c, CV curves of Aza-CMP–Ni and Aza-CMP–NiFe in 1.0 M NaOH solution (scan rate: 50 mV s−1, the applied potential (E) is in RHE scale, without resistance compensation in solution (iR)). d, LSV curves of Aza-CMP–NiFe and reference RuO2 in 1.0 M KOH (scan rate: 1 mV s−1). Inset: corresponding Tafel plots. e, TOFs of Aza-CMP–NiFe based on the redox-active Ni–Fe sites, total Ni and Fe contents (1.0 M KOH) in comparison with selected state-of-the-art catalysts. The tabulated values of TOFs and overpotentials were obtained from ref. 34, with the complete dataset displayed in Supplementary Fig. 46. is strongly affected by the basicity of proton acceptors9. Rate enhancements with external proton buffers in solution facilitate water oxidation kinetics. In Mn4CaO5 systems, carboxylate side-chain-assisted deprotonation of an internal Mn–OH species is identified as the key step in forming a reactive Mn–O∙ radical10. Likewise, the strategic placement of intramolecular proton transfer (IPT) sites, often referred to as proton relays, near the metal centres in the secondary coordination sphere is proposed to markedly accelerate proton transfer and stabilize charged intermedi (...truncated)