The oxygen evolution reaction (OER) is the bottleneck that limits the energy efficiency of water-splitting. The process involves four electrons’ transfer and the generation of triplet state O2 from singlet state species (OH- or H2O). Recently, explicit spin selection was described as a possible way to promote OER in alkaline conditions, but the specific spin-polarized kinetics remains unclear. Here, we report that by using ferromagnetic ordered catalysts as the spin polarizer for spin selection under a constant magnetic field, the OER can be enhanced. However, it does not applicable to non-ferromagnetic catalysts. We found that the spin polarization occurs at the first electron transfer step in OER, where coherent spin exchange happens between the ferromagnetic catalyst and the adsorbed oxygen species with fast kinetics, under the principle of spin angular momentum conservation. In the next three electron transfer steps, as the adsorbed O species adopt fixed spin direction, the OER electrons need to follow the Hund rule and Pauling exclusion principle, thus to carry out spin polarization spontaneously and finally lead to the generation of triplet state O2. Here, we showcase spin-polarized kinetics of oxygen evolution reaction, which gives references in the understanding and design of spin-dependent catalysts.

Exploring robust catalysts for water oxidation in acidic electrolyte is challenging due to the limited material choice. Iridium (Ir) is the only active element with a high resistance to the acid corrosion during water electrolysis. However, Ir is rare, and its large-scale application could only be possible if the intrinsic activity of Ir could be greatly enhanced. Here, a pseudo-cubic SrCo0.9Ir0.1O3-δ perovskite, containing corner-shared IrO6 octahedrons, is designed. The Ir in the SrCo0.9Ir0.1O3-δ catalyst shows an extremely high intrinsic activity as reflected from its high turnover frequency, which is more than two orders of magnitude higher than that of IrO2. During the electrochemical cycling, a surface reconstruction, with Sr and Co leaching, over SrCo0.9Ir0.1O3-δ occurs. Such reconstructed surface region, likely contains a high amount of structural domains with corner-shared and under-coordinated IrOx octahedrons, is responsible for the observed high activity.

renewable and ecofriendly energy technologies, such as fuel cells and water splitting. Currently, theoretical insights of oxygen electrocatalysis mainly focus on the thermodynamic features of the adsorption/desorption of reactants and intermediates. Besides the thermodynamic features, the electron transfer between the adsorbed reactants/intermediates and the active sites, and the charge transport within the catalysts during the electrocatalysis, are also a crucial factor influencing the reaction kinetics. That is, a comprehensible understanding of oxygen electrocatalysis should consider both the orbital interactions and the electron transfer behavior. In the domain of oxygen electrocatalysis (at room temperature in water), the electron transfer exhibits highly spin-related character because the ground state of O 2 mole cule is a triplet state (↑OO↑) (Figure 1). That is why O 2 is paramagnetic. Therefore, either from H 2 O/OH − to O 2 (oxygen evolution reaction, OER) or from O 2 to H 2 O/OH − (oxygen reduction reaction, ORR), the involvement of the triplet O 2 requires spin-related electron transfer along these oxygen reactions, which plays a considerable role in the reaction kinetics. The aspect of spin-related electron transfer has been unintentionally neglected and only a few pioneer works have noticed and emphasized its role. An early attempt can be traced back Oxygen evolution and reduction reactions play a critical role in determining the efficiency of the water cycling (H 2 O ⇔ H 2 + 1 2 O 2), in which the hydrogen serves as the energy carrier. That calls for a comprehensive understanding of oxygen electrocatalysis for efficient catalyst design. Current opinions on oxygen electrocatalysis have been focused on the thermodynamics of the reactant/intermediate adsorption on the catalysts. Because the oxygen molecule is paramagnetic, its production from or its reduction to diamagnetic hydroxide/water involves spin-related electron transfer. Both electron transfer and orbital interactions between the catalyst and the reactant/intermediate show spin-dependent character, making the reaction kinetics and thermodynamics sensitive to the spin configurations. Herein, a brief introduction on the spintronic explanation of the catalytic phenomena on oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is given. The local spin configurations and orbital interactions in the benchmark transition-metalbased catalysts for OER and ORR are analyzed as examples. To further understand the spintronic oxygen electrocatalysis and to develop more efficient spintronic catalysts, the challenges are summarized and future opportunities proposed. Spin electrocatalysis may emerge as an important topic in the near future and help integrate a comprehensive understanding of oxygen electrocatalysis.

Producing hydrogen by water electrolysis suffers from the kinetic barriers in the oxygen evolution reaction (OER) that limits the overall efficiency. With spin-dependent kinetics in OER, to manipulate the spin ordering of ferromagnetic OER catalysts (e.g., by magnetization) can reduce the kinetic barrier. However, most active OER catalysts are not ferromagnetic, which makes the spin manipulation challenging. In this work, we report a strategy with spin pinning effect to make the spins in paramagnetic oxyhydroxides more aligned for higher intrinsic OER activity. The spin pinning effect is established in oxideFM/oxyhydroxide interface which is realized by a controlled surface reconstruction of ferromagnetic oxides. Under spin pinning, simple magnetization further increases the spin alignment and thus the OER activity, which validates the spin effect in rate-limiting OER step. The spin polarization in OER highly relies on oxyl radicals (O∙) created by 1st dehydrogenation to reduce the barrier for subsequent O-O coupling.

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.

Surface composition dependent ligand effect in tuning the activity of nickel-copper bimetallic electrocatalysts toward hydrogen evolution in alkaline.

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.

Cobalt spinel oxides are ac lass of promising transition metal (TM) oxides for catalyzing oxygen evolution reaction (OER). Their catalytic activity depends on the electronic structure.I naspinel oxide lattice,e acho xygen anion is shared amongst its four nearest transition metal cations,ofwhich one is located within the tetrahedral interstices and the remaining three cations are in the octahedral interstices. This work uncovered the influence of oxygen anion charge distribution on the electronic structure of the redox-active building blockC o ÀO. The charge of oxygen anion tends to shift toward the octahedral-occupied Co instead of tetrahedraloccupied Co,w hich hence produces strong orbital interaction between octahedral Co and O. Thus,t he OER activity can be promoted by pushing more Co into the octahedral site or shifting the oxygen charge towards the redox-active metal center in CoO 6 octahedra.The clean-burning hydrogen fuel, if produced by electrochemical water splitting, would revolutionize the global energy infrastructure.T he major limitation of water splitting is the sluggish oxygen evolution reaction (OER) at the anode. [1] To date,the most efficient OER electrocatalysts are made from noble metal ruthenium or iridium. In order to meet the broader goal of sustainability,e xploring earthabundant transition metal (TM) oxide catalysts have been prioritized. [1b, 2] Better understanding of the OER reaction on TM oxides is necessary to this end. It has been found that the surface redox-active centers in TM oxides play ak ey role in oxygen electrocatalysis. [3] Thec onventional perception of oxygen evolution regards the redox-active metallic center as the active site and it is the redox ability of TM that mediates the transition of [M n+ ÀOH ad ]/[M n+1 ÀO ad ]d uring OER. [3,4] However,t he redox of late transition metal oxides (e.g., Coand Ni-based) could involve both the transition metal and oxygen ligand due to the increased orbital hybridization between TM 3d and O2 p. [2,5] Earlier reports had demonstrated that the energy in TM 3d orbital cannot be treated in isolation from O2pwhen there is significant overlap between TM 3d and O2 p. Recent studies on oxygen-deficient perovskite oxides reveal that the oxygen anion could also act as the redox partner in OER. [6] Direct evidence for lattice oxygen participated OER using in situ 18 Oi sotope labelling mass spectrometry has been given by Alexis et al. [6b] Thefact that the oxygen anion can also act as the redox-active center emphasizes the importance of considering TM À Obond as the redox-active building block. More recently,t he covalent character (covalency) of TM Bs ite ÀOb ond (TM B the TM in B site) has been proposed to be adominating factor in OER on perovskite oxides. [6b, 7] TheA -site rare-earth metals (low in electronegativity) tend to form an ionic bond with Oa nd weaken the influence of M A -O block on OER. Spinel oxides, ah uge crystal family for oxygen electrocatalysis, [2,4,8] require more complex analysis because the tetrahedral and octahe...