NiFe oxyhydroxide is one of the most promising oxygen evolution reaction (OER) catalysts for renewable hydrogen production, and deciphering the identity and reactivity of the oxygen intermediates on its surface is a key challenge but is critical to the catalyst design for improving the energy efficiency. Here, we screened and utilized in situ reactive probes that can selectively target specific oxygen intermediates with high rates to investigate the OER intermediates and pathway on NiFe oxyhydroxide. Most importantly, the oxygen atom transfer (OAT) probes (e.g., 4-(diphenylphosphino) benzoic acid) could efficiently inhibit the OER kinetics by scavenging the OER intermediates, exhibiting lower OER currents, larger Tafel slopes, and larger kinetic isotope effect (KIE) values, while probes with other reactivities demonstrated much smaller effects. Combining the OAT reactivity with electrochemical kinetic and operando Raman spectroscopic techniques, we identified a resting Fe═O intermediate in the Ni–O scaffold and a rate-limiting O–O chemical coupling step between a Fe═O moiety and a vicinal bridging O. DFT calculation further revealed a longer Fe═O bond formed on the surface and a large kinetic energy barrier of the O–O chemical coupling step, corroborating the experimental results. These results point to a new direction of liberating lattice O and expediting O–O coupling for optimizing NiFe-based OER electrocatalyst.
Han Jiao,Chun Wang,Hao Tian,Zi-Yang Zhang,Yang Zhao,Ping Na,Yusuke Yamauchi,Zhong-Li Wang
NiFe oxyhydroxides (NiFeO x H y ) are the benchmark catalysts for alkaline oxygen evolution reaction (OER), however, it is still challenging for these electrocatalysts to achieve both high activity and long-term stability. Herein, we demonstrate that strong interaction heterointerface of NiFeO x H y /cerium oxide (CeO 2 ) can control the reconstruction process of NiFeO x H y and the reaction pathway of OER, which simultaneously improves the activity and stability of the catalyst. Interestingly, after electrochemical activation, with and without CeO 2 , NiFeO x H y oxyhydroxides exhibit completely different structures, with the former being uniformly distributed ultrafine nanosheets and the latter being spherical agglomerates of larger sized nanosheets. The strong heterointerface interaction induces the electrons of Ni and Fe to shift to Ce. The optimized catalyst exhibits a low overpotential of 240 mV at 100 mA cm −2 with the durability over 2000 h at 200 mA cm −2 . The OER mechanism studies indicate that NiFeO x H y /CeO 2 heterointerface follows lattice oxygen oxidation mechanism with accelerated kinetics.
Noble metal decoration is one of the most efficient strategies to boost the performance for water oxidation . However, identifying the role of noble metals and especially the active sites remain a persistent challenge. Here, we report a direct synthesis of Au nanoparticles (NPs) decorated NiFe oxyhydroxides via in-situ electrochemical conversion and a comprehensive study of the activity boost via a combination of systematic experimental and theoretical studies. We found that Fe doping greatly improved the electrical conductivity and the Fe sites were the dominant active centers, collectively responsible for the remarkably enhanced performance in NiFe oxyhydroxide. Upon Au NPs decoration, the content of high-valence Fe/Ni ions was substantially reduced and the formation of surface oxygen vacancy was greatly suppressed. As a result, the dominant active centers transfer to the interfacial Au sites that directly participate in catalyzing water oxidation. Our study presents a fundamental insight into the origin of activity enhancement and especially the active centers in noble metal decorated NiFe oxyhydroxide catalysts.
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