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Abstract Cobalt oxyhydroxide (CoOOH) is a promising catalytic material for oxygen evolution reaction (OER). In the traditional CoOOH structure, Co3+ exhibits a low-spin state configuration ( $${t}_{2{{{{{\rm{g}}}}}}}^{6}{e}_{{{{{{\rm{g}}}}}}}^{0}$$ t 2 g 6 e g 0 ), with electron transfer occurring in face-to-face $${t}_{2{{{{{\rm{g}}}}}}}^{*}$$ t 2 g * orbitals. In this work, we report the successful synthesis of high-spin state Co3+ CoOOH structure, by introducing coordinatively unsaturated Co atoms. As compared to the low-spin state CoOOH, electron transfer in the high-spin state CoOOH occurs in apex-to-apex $${e}_{{{{{{\rm{g}}}}}}}^{*}$$ e g * orbitals, which exhibits faster electron transfer ability. As a result, the high-spin state CoOOH performs superior OER activity with an overpotential of 226 mV at 10 mA cm−2, which is 148 mV lower than that of the low-spin state CoOOH. This work emphasizes the effect of the spin state of Co3+ on OER activity of CoOOH based electrocatalysts for water splitting, and thus provides a new strategy for designing highly efficient electrocatalysts.

Abstract A coupled oxygen evolution mechanism (COM) during oxygen evolution reaction (OER) has been reported in nickel oxyhydroxides (NiOOH)-based materials by realizing e g * band (3d electron states with e g symmetry) broadening and light irradiation. However, the link between the e g * band broadening extent and COM-based OER activities remains unclear. Here, Ni1-xFexOOH (x = 0, 0.05, 0,2) are prepared to investigate the underlying mechanism governing COM-based activities. It is revealed that in low potential region, realizing stronger e g * band broadening could facilitate the *OH deprotonation. Meanwhile, in high potential region where the photon utilization is the rate-determining step, a stronger e g * band broadening would widen the non-overlapping region between d z 2 and a 1g * orbitals, thereby enhancing photon utilization efficiency. Consequently, a stronger e g * band broadening could effectuate more efficient OER activities. Moreover, we demonstrate the universality of this concept by extending it to reconstruction-derived X-NiOOH (X = NiS2, NiSe2, Ni4P5) with varying extent of e g * band broadening. Such an understanding of the COM would provide valuable guidance for the future development of highly efficient OER electrocatalysts.

Summary: Despite their widespread study, lithium metal anodes still face the bottleneck problem of low average Coulombic efficiency. Herein, we adopt an electroless plating method and introduce additive (vanillin) to develop nanoscale silver hosts. The uniform nanoscale silver layer is conducive to seamless Li/Ag alloying process, thereby enabling flat lithium deposition. Since the Li/Ag alloying process occurs after the formation of solid electrolyte interphase (SEI), the seamless alloying process stabilizes the SEI, which improves the reversibility of the lithium metal anode. On this basis, a 3D-printed copper host-coated dense nanoscale silver layer is constructed, which reduces the effective current density and hence lowers the polarization of lithium deposition or stripping. Moreover, the 3D structure induces the epitaxial growth of lithium and thus alleviates the volume change of lithium metal. As a result, an average Coulombic efficiency of 99.61% is achieved in the Li/3D Ag half-cell.