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It is well-known that Fe3+ impurities in the electrolyte can significantly promote the oxygen evolution reaction (OER) activity of Co-based and Ni-based electrocatalysts. However, it is not clear if they work for single-metal Fe-based catalysts. Here, we systematically investigate the influence of Fe3+ ions in the electrolyte on the OER activity of core-shell FeO x @C catalytic system. We find that the absorbed Fe3+ can induce the generation of surface oxygen vacancies and regulate the electronic structure of active sites, thus increasing the OER kinetics and decreasing the energy barrier. Combining with in-situ Raman and ex-situ X-ray photoelectron spectroscopy (XPS) analysis, we disclose that Fe4+ species is the active species for the OER and positively correlate the characteristic content of the high-valence Fe species and OER activity. Fe3+ impurities in the electrolyte can improve the OER activity of Fe-based catalysts, which can be oxidized into Fe4+ as the active species. [Display omitted] • FeO x @C-700 exhibits improved OER activity when dropping Fe3+ into the electrolyte. • Fe3+ impurities have synergistic effects with FeO x @C-700. • In-situ Raman and ex-situ XPS spectra reveal Fe4+ species during the OER. • Fe4+ favors the OER as the reactive species. [ABSTRACT FROM AUTHOR]

Searching for highly active and durable oxygen evolution reaction (OER) electrocatalysts is the key to break through the bottleneck of overall water splitting. Here, we prepare Ni x Fe-BDC (H 2 BDC = terephthalic acid) nanorods with different Ni/Fe ratios by a facile solvothermal method for the OER. The optimal Ni 3 Fe-BDC exhibits a low overpotential (η 10) of 265 mV and a Tafel slope of 90 mV·dec−1 in 1 M KOH. Moreover, it shows a low η 10 of 280 mV and excellent stability in the mixture of 1 M NaCl and 1 M KOH, which has strong corrosion resistance to Cl− anions. The role of Fe3+ not only increases the charge transfer rate of Ni 3 Fe-BDC, but also affects the specific surface area of the catalyst with high electrochemical activity. Kinetic studies show that both Fe and Ni sites act as active centers, which catalyze synergistically to reduce the reaction kinetic energy barrier. Characterization results of the used Ni 3 Fe-BDC reveal that the in situ formed rod-like Ni 3 FeOOH is the active site for the OER. Ni 3 Fe-BDC shows low η 10 of 265 mV and 280 mV for the OER in 1 M KOH and in 1 M NaCl + 1 M KOH, respectively. In situ transformation of nanorod-like Ni 3 Fe-BDC MOF to Ni 3 FeOOH can efficiently remain the advantages of MOF, e.g. porous structure, high specific surface area and uniformly dispersed metal sites. [Display omitted] • Ni 3 Fe-BDC shows low η 10 of 265 mV for the OER in 1 M KOH. • Ni 3 Fe-BDC shows η 10 of only 280 mV for the OER in 1 M NaCl +1 M KOH. • Ni 3 Fe-BDC exhibits a low OER energy barrier of 13.7 kJ/mol. • The in-situ formed rod-like Ni 3 FeOOH serves as efficient active site for the OER. [ABSTRACT FROM AUTHOR]

Compared with defect-free FeO x , the ultrafine FeO x in the FeO x (0.03)@HG-110 with abundant surface defects shows lower energy barrier and higher OER activity. [Display omitted] • The FeO x (0.03)@HG-110 exhibits an OER overpotential as low as 342 mV at 10 mA cm−2. • The average particle size of FeOx is ca. 1.6 nm. • FeO x (0.03)@HG-110 possesses a low E a of 35.9 kJ mol−1. • The rate-determining step is *O → *OOH with the energy barriers of 0.79 eV. Development of earth-abundant and robust oxygen evolution reaction (OER) catalysts is imperative for cost-effective hydrogen production via water electrolysis. Herein, we report ultrafine iron (oxy)hydroxide nanoparticles with average particle size of 2.6 nm and abundant surface defects homogeneously supported on oleum-treated graphite (FeO x (n)@HG-T), providing abundant active sites for the OER. The optimal FeO x (0.03)@HG-110 exhibits high electrocatalytic OER activity and excellent stability. Electrochemical testing results and theoretical calculations reveal that the outstanding OER activity of FeO x (0.03)@HG-110 is due to its stronger charge transfer ability and lower OER energy barrier than defect-free FeO x nanoparticles. This work demonstrates that the OER performance of oxyhydroxide-based electrocatalysts can be improved by surface defect engineering. [ABSTRACT FROM AUTHOR]

The slow oxygen evolution reaction (OER) kinetics influences hydrogen production efficiency from water splitting. To break through the bottleneck of water splitting, it is urgent to develop efficient and economic electrocatalysts. Although NiFe-based catalysts exhibit outstanding OER activity, the complicated preparation process limits their large-scale synthesis and applications. Here, partially crystallized nickel-iron oxyhydroxides are synthesized by a facile sol-gel method. When the Fe/Ni mole ratio is 0.5:1, the NiFe 0.5 (OH) x catalyst shows superior OER performance with a low OER overpotential of 265 mV and good durability. Kinetic studies show that the energy barrier of NiFe 0.5 (OH) x is only 31.5 kJ mol−1, much smaller than those of Ni(OH) x (41.0 kJ mol−1) and Fe(OH) x (44.8 kJ mol−1). The synergistic action between Ni and Fe sites not only facilitates mass and charge transfer, but also promotes the formation of ∗OOH intermediate for the OER. The OER energy barrier on NiFe 0.5 (OH) x is only 31.5 kJ mol−1, much lower than those on Ni(OH) x (41.0 kJ mol−1) and Fe(OH) x (44.8 kJ mol−1). [Display omitted] • Partially crystallized nickel-iron oxyhydroxides are synthesized by a facile sol-gel method. • The overpotential of NiFe 0.5 (OH) x is only 265 mV at 10 mA cm−2. • NiFe 0.5 (OH) x exhibits a low OER energy barrier of 31.5 kJ mol−1. • Synergy between Ni and Fe sites promotes charge transfer for the OER. [ABSTRACT FROM AUTHOR]