Spinel oxides have attracted growing interest over the years for catalysing the oxygen evolution reaction (OER) due to their efficiency and cost-effectiveness, but the fundamental understanding of the structure-property relationships remains elusive. Here we demonstrate that the OER activity on spinel oxides is intrinsically dominated by the covalency competition between tetrahedral and octahedral sites. The competition fabricates an asymmetric MT−O−MO backbone where the bond with weaker metal-oxygen covalency determines the exposure of cation sites and therefore the activity. Driven by this finding, a dataset with more than 300 spinel oxides is computed and used to train a machine learning model for screening the covalency competition in spinel oxides, with a mean absolute error of 0.05 eV. [Mn]T[Al0.5Mn1.5]OO4 is predicted to be a highly active OER catalyst and subsequent experimental results confirm its superior activity. This work sets mechanistic principles of spinel oxides for water oxidation, which may be extendable to other applications.

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.

expected to be a promising candidate for emerging smart grid technology in the near future. Nevertheless, the scarcity and uneven distribution of lithium resources hamper its further development. In pursuit of alternatives to LIBs for large-scale applications, sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) have great potential, owing to the low cost and high abundance of Na (2.36 wt%) and K (2.09 wt%) in the Earth's crust, as well as its similar chemical properties to those of lithium. [1][2][3][4][5] Much work so far has focused on SIBs, and significant progress has been achieved in the past few years. [6][7][8] On the contrary, the development of PIBs is still in its infancy, probably due to the larger ionic radius of K + (1.38 Å) than those of Na + (1.02 Å) and Li + (0.76 Å). [9] However, PIBs possess several advantages compared with SIBs, such as the more negative standard potential of K + /K (−2.93 V vs SHE, compared with −2.71 V for Na + / Na), reversible intercalation/deintercalation of K + in graphite (theoretical capacity of 279 mA h g −1 ), and fast ionic conductivity of K + in liquid electrolyte. [10][11][12] These properties of PIBs offer exciting opportunities to achieve low-cost batteries with high energy density and good rate performance. Nevertheless, it remains challenging to fabricate suitable electrode materials, The potassium-ion battery (PIB) represents a promising alternative to the lithium-ion battery for large-scale energy storage owing to the abundance and low cost of potassium. The lack of high performance anode materials is one of the bottlenecks for its success. The main challenge is the structural degradation caused by the huge volume expansion from insertion/extraction of potassium ions which are much larger than their lithium counterparts.Here, this challenge is tackled by in situ engineering of a yolk-shell FeS 2 @C structure on a graphene matrix. The yolk-shell structure provides interior void space for volume expansion and prevents the aggregation of FeS 2 . The conductive graphene matrix further enhances the charge transport within the composite. The PIB fabricated using this anode delivers high capacity, good rate capability (203 mA h g −1 at 10 A g −1 ), and remarkable long-term stability up to 1500 cycles at high rates. The performance is superior to most anode materials reported to date for PIBs. Further in-depth characterizations and density functional theory calculations reveal that the material displays reversible intercalation/deintercalation and conversion reactions during cycles, as well as the low diffusion energy barriers for the intercalation process. This work provides a new avenue to allow the proliferation of PIB anodes.