Lattice oxygen activation enabled by high-valence metal sites for enhanced water oxidation

ARTICLE https://doi.org/10.1038/s41467-020-17934-7 OPEN Lattice oxygen activation enabled by high-valence metal sites for enhanced water oxidation 1234567890():,; Ning Zhang 1,2,7, Xiaobin Feng3,4,7, Dewei Rao 5,7, Xi Deng6, Lejuan Cai1,2, Bocheng Qiu1,2, Ran Long Yujie Xiong6, Yang Lu 3,4 ✉ & Yang Chai 1,2 ✉ 6, Anodic oxygen evolution reaction (OER) is recognized as kinetic bottleneck in water electrolysis. Transition metal sites with high valence states can accelerate the reaction kinetics to offer highly intrinsic activity, but suffer from thermodynamic formation barrier. Here, we show subtle engineering of highly oxidized Ni4+ species in surface reconstructed (oxy) hydroxides on multicomponent FeCoCrNi alloy film through interatomically electronic interplay. Our spectroscopic investigations with theoretical studies uncover that Fe component enables the formation of Ni4+ species, which is energetically favored by the multistep evolution of Ni2+→Ni3+→Ni4+. The dynamically constructed Ni4+ species drives holes into oxygen ligands to facilitate intramolecular oxygen coupling, triggering lattice oxygen activation to form Fe-Ni dual-sites as ultimate catalytic center with highly intrinsic activity. As a result, the surface reconstructed FeCoCrNi OER catalyst delivers outstanding mass activity and turnover frequency of 3601 A gmetal−1 and 0.483 s−1 at an overpotential of 300 mV in alkaline electrolyte, respectively. 1 Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, P. R. China. 2 The Hong Kong Polytechnic University Shenzhen Research Institute, 518057 Shenzhen, P. R. China. 3 Department of Mechanical Engineering, City University of Hong Kong, Kowloon, Hong Kong, P. R. China. 4 Nano-Manufacturing Laboratory (NML), Shenzhen Research Institute of City University of Hong Kong, 518057 Shenzhen, P. R. China. 5 School of Materials Science and Engineering, Jiangsu University, 212013 Zhenjiang, Jiangsu, P. R. China. 6 Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), School of Chemistry and Materials Science, National Synchrotron Radiation Laboratory, University of Science and Technology of China, 230026 Hefei, Anhui, P. R. China. 7These authors contributed equally: Ning Zhang, Xiaobin Feng, Dewei Rao. ✉email: ; NATURE COMMUNICATIONS | (2020)11:4066 | https://doi.org/10.1038/s41467-020-17934-7 | www.nature.com/naturecommunications 1 ARTICLE I NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17934-7 n recent few decades, there have been continuous developments towards water electrolysis, as the cathodically electrolytic hydrogen is proposed as an ideal energy carrier for the storage of sustainable but intermittent energy, such as wind and solar energy1–3. Current bottleneck mainly originates from fourelectron process in anodic oxygen evolution reaction (OER), which requires large overpotential to surmount its sluggish reaction kinetics4,5. However, the high cost and instability of state-of-the-art iridium- and ruthenium-based electrocatalysts largely prevent their practical applications6,7. Earth-abundant catalysts based on 3d transition metals have been demonstrated as the promising alternatives for OER, especially in alkaline electrolyte8–13. Meanwhile, experimental and theoretical studies reach a consensus that late transition metals with high valence states exhibit superior activities2,14–16. The increased holes in dband of highly oxidized metal species can enhance the covalency of metal–oxygen (M–O) bonds to promote the charge transfer9,14. More importantly, high valency typically induces the downshift of metal d-band to penetrate p-band of oxygen ligands17. The redox electrochemistry of oxygen ligands will be triggered by driving holes into the related oxygen p-band, making lattice oxygen atoms electrophilic to participate in water oxidation, so called lattice oxygen activation mechanism (LOM)12,17,18. This alternative pathway facilitates the direct lattice oxygen coupling (LOC) by sharing the ligand holes, thereby lowering the limiting energy barrier. Thus, we can expect that rationalization of LOM pathway with highly oxidized metal species provides a promising avenue to maximize efficiency of OER electrocatalysts. However, according to the Pourbaix diagrams, it usually requires more elevated potential to realize deep oxidation of metal species15,16, causing the thermodynamically unfavorable formation of highly oxidized metal species. Those disadvantages make LOM pathway unpredictable and hinder the exploitation of efficient OER electrocatalysts. Therefore, it is highly desirable to steer the highly oxidized metal species with minimizing their formation energy. In general, the OER electrocatalysts undergo the surface reconstruction into (oxy)hydroxides, independent of initial composition and structure19–21, wherein the structural flexibility of (oxy)hydroxides enables the dynamic self-optimization of catalytically active sites. This dynamic reconstruction normally involves oxidation of metal sites, along with adsorption of oxygen species and/or deprotonation of hydroxyl. Therefore, the redox electrochemistry is directly related to the chemical affinity between metal sites and oxygen species, which can be subtly manipulated by the variation of electronic states11,21–23. Multimetal-based electrocatalysts typically endows more superior activities than single-metal catalysts, as the interatomically electronic interplay can efficiently modulate the electronic structure of metal sites that can hardly achieve for single-metal catalysts, offering an effective approach to tune the redox electrochemistry and engineer highly oxidized metal species24–26. Owing to their structural complicacy, it is still a challenging task to rationally design efficient multimetal OER catalysts with high valence metal sites and fundamentally understand their structure-activity correlation. Multicomponent alloy (MCA) materials have recently received extensive attention because of their unique intrinsic properties27,28. The adjustable components make them feasible to serve as templates for multimetal-based electrocatalysts. By rational engineering, it enables the formation of highly oxidized metal species and tune on LOM. Here, we demonstrate that an FeCoCrNi MCA film can dynamically form highly oxidized metal species during OER process with decreasing formation energy after undergoing irreversible surface reconstruction. Our spectroscopic investigations and theoretical simulations reveal that the interatomically electronic interplay in surface reconstructed 2 multimetal (oxy)hydroxide (MOxHy) plays a key role on subtly engineering highly oxidized Ni4+ species to favor LOM pathway. Fe component induces electron depletion in Ni species to ensure the dynamic formation of Ni4+ species. Meanwhile, a multistep evolution of Ni2+→Ni3+→Ni4+ lowers the ove (...truncated)