LaCoO 3 is an active, stable catalyst in alkaline solution for oxygen evolution reaction (OER). With lower cost, it is a potential alternative to precious metal oxides like IrO 2 and RuO 2 in water electrolysis. However, room still remains for improving its activity according to recent understandings of OER on perovskite oxides. In this work, Fe substitution has been introduced in LaCoO 3 to boost its OER performance. Density function theory (DFT) calculation verified that the enhanced performance originates from the enhanced Co 3d-O 2p covalency with 10 at% Fe substitution in LaCoO 3 . Both DFT calculations and Superconducting Quantum Design (SQUID) magnetometer (MPMS-XL) showed a Co 3+ spin state transition from generally low spin state (LS: t 2g 6 e g 0 , S = 0) to a higher spin state with the effect of 10 at% Fe substitution. X-ray absorption near-edge structure (XANES) supports DFT calculations on an insulator to half-metal transition with 10 at% Fe substitution, induced by spin state transition. The half-metallic LaCo 0.9 Fe 0.1 O 3 possesses increased overlap between Co 3d and O 2p states, which results in enhanced covalency and promoted OER performance. This finding enlightens a new way of tuning the metal−oxygen covalency in oxide catalysts for OER.
The development of efficient electrocatalysts that lower the overpotential of oxygen evolution reaction (OER) is of great importance in improving the overall efficiency of hydrogen fuel production by water electrolysis. [1] Commercially, precious metal oxides catalysts such as IrO 2 are used. [2] However, their elemental scarcity and high cost have triggered a search for cost-effective OER electrocatalysts such as 3d transition metal oxides. Among them, families such as the perovskite ABO 3 and the spinel AB 2 O 4 ones have attracted great attention due to their tunable structural/elemental properties allowed by A and B site cation substitution. [3,4] Perovskite ABO 3 oxides have a simple structure with rare-earth or alkaline earth element occupying cuboctahedral A-site while the B-site transition metal (TM) sites in an octahedral environment. [3] Spinel oxides, however, can be either normal or inverse structure depending on the relative occupancy of divalent and trivalent cations in the octahedral and Developing highly active electrocatalysts for oxygen evolution reaction (OER) is critical for the effectiveness of water splitting. Low-cost spinel oxides have attracted increasing interest as alternatives to noble metalbased OER catalysts. A rational design of spinel catalysts can be guided by studying the structural/elemental properties that determine the reaction mechanism and activity. Here, using density functional theory (DFT) calculations, it is found that the relative position of O p-band and M Oh (Co and Ni in octahedron) d-band center in ZnCo 2−x Ni x O 4 (x = 0-2) correlates with its stability as well as the possibility for lattice oxygen to participate in OER. Therefore, it is testified by synthesizing ZnCo 2−x Ni x O 4 spinel oxides, investigating their OER performance and surface evolution. Stable ZnCo 2−x Ni x O 4 (x = 0-0.4) follows adsorbate evolving mechanism under OER conditions. Lattice oxygen participates in the OER of metastable ZnCo 2−x Ni x O 4 (x = 0.6, 0.8) which gives rise to continuously formed oxyhydroxide as surface-active species and consequently enhances activity. ZnCo 1.2 Ni 0.8 O 4 exhibits performance superior to the benchmarked IrO 2 . This work illuminates the design of highly active metastable spinel electrocatalysts through the prediction of the reaction mechanism and OER activity by determining the relative positions of the O p-band and the M Oh d-band center.
Ar ational design for oxygen evolution reaction (OER) catalysts is pivotal to the overall efficiency of water electrolysis.M uchw ork has been devoted to understanding cation leaching and surface reconstruction of very active electrocatalysts,b ut little on intentionally promoting the surface in acontrolled fashion. We now report controllable anodic leaching of Cr in CoCr 2 O 4 by activating the pristine material at high potential, which enables the transformation of inactive spinel CoCr 2 O 4 into ah ighly active catalyst. The depletion of Cr and consumption of lattice oxygen facilitate surface defects and oxygen vacancies,exposing Co species to reconstruct into active Co oxyhydroxides differ from CoOOH. An ovel mechanism with the evolution of tetrahedrally coordinated surface cation into octahedral configuration via non-concerted proton-electron transfer is proposed. This work shows the importance of controlled anodic potential in modifying the surface chemistry of electrocatalysts.
We have developed a glucose-responsive metal-organic framework (MOF)-based insulin delivery nanosystem via a one-pot process. The system relies on the MOF response to glucose stimulation and this can promote insulin delivery. This nanosystem was successfully applied for glucose-responsive and self-regulated insulin release.
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