Bismuth vanadate (BiVO4) has been widely investigated as a photocatalyst or photoanode for solar water splitting, but its activity is hindered by inefficient cocatalysts and limited understanding of the underlying mechanism. Here we demonstrate significantly enhanced water oxidation on the particulate BiVO4 photocatalyst via in situ facet-selective photodeposition of dual-cocatalysts that exist separately as metallic Ir nanoparticles and nanocomposite of FeOOH and CoOOH (denoted as FeCoOx), as revealed by advanced techniques. The mechanism of water oxidation promoted by the dual-cocatalysts is experimentally and theoretically unraveled, and mainly ascribed to the synergistic effect of the spatially separated dual-cocatalysts (Ir, FeCoOx) on both interface charge separation and surface catalysis. Combined with the H2-evolving photocatalysts, we finally construct a Z-scheme overall water splitting system using [Fe(CN)6]3−/4− as the redox mediator, whose apparent quantum efficiency at 420 nm and solar-to-hydrogen conversion efficiency are optimized to be 12.3% and 0.6%, respectively.

For electrochemical energy conversion, highly efficient and inexpensive electrocatalysts are required, which are principally designed and synthesized by virtue of structural regulations. Herein, we propose a rational linker scission approach to induce lattice strain in metal–organic framework (MOF) catalysts by partially replacing multicoordinating linkers with nonbridging ligands. Strained NiFe-MOFs with 6% lattice expansion exhibit a superior catalytic performance for the oxygen evolution reaction (OER) under alkaline conditions; the overpotential is reduced to 230 mV (86.6 mV dec–1) from 320 mV (164.9 mV dec–1) for the unstrained NiFe-MOFs at a current density of 10 mA cm–2. Operando studies by using synchrotron radiation X-ray absorption and infrared spectroscopy identified the emergence of a key *OOH intermediate on Ni3+/4+ sites during OER, providing strong evidence that the Ni3+/4+ sites are the active sites and the formation of *OOH is the rate-limiting step. The first-principles calculations were performed to reveal the strain-induced electronic structure changes of the NiFe-MOFs and the Gibbs free energy profile during OER. It is found that the optimized Ni 3d eg-orbital facilitates the formation of *OOH, thus enhancing the OER performance of the strained MOFs.

highly dependent on the adsorption model of oxygen molecules (O 2 ) on the surface of catalysts. [2] The side-on adsorption with a "*O-O*" configuration (* presents the active site) is conducive to weakening the O-O bond for reduction of O 2 into H 2 O via a four-electron (4e) ORR pathway, [3] while the end-on configuration formed by a solo oxygen atom coordinated on a single active site ("*OOH" intermediate) facilitates to selectively catalyzing oxygen to generate H 2 O 2 via a two-electron (2e) ORR pathway. [4] To realize 2e oxygen electroreduction, various strategies including alloying, [5] chemical functionalization, [6] downsizing, [7] and single-atom engineering [8] have been developed to regulate the physicochemical properties of the catalysts. Though substantial progress has been made, the activity and durability of reported works still cannot compete with the demand of the practical application. [9] The cation vacancy engineering strategy could be an effective approach to develop high-performance catalysts for the electrocatalytic synthesis of H 2 O 2 owing to the following merits: creating cation vacancy on host materials can prolong the distance or spacing of the active sites, thereby leading to the formation of *OOH adsorption favorable; [4a] the charge density between active sites and adjacent coordination atoms will be redistributed, which optimizes the Electrocatalytic hydrogen peroxide (H 2 O 2 ) synthesis via the two-electron oxygen reduction reaction (2e ORR) pathway is becoming increasingly important due to the green production process. Here, cationic vacancies on nickel phosphide, as a proof-of-concept to regulate the catalyst's physicochemical properties, are introduced for efficient H 2 O 2 electrosynthesis. The as-fabricated Ni cationic vacancies (V Ni )-enriched Ni 2−x P-V Ni electrocatalyst exhibits remarkable 2e ORR performance with H 2 O 2 molar fraction of >95% and Faradaic efficiencies of >90% in all pH conditions under a wide range of applied potentials. Impressively, the as-created V Ni possesses superb longterm durability for over 50 h, suppassing all the recently reported catalysts for H 2 O 2 electrosynthesis. Operando X-ray absorption near-edge spectroscopy (XANES) and synchrotron Fourier transform infrared (SR-FTIR) combining theoretical calculations reveal that the excellent catalytic performance originates from the V Ni -induced geometric and electronic structural optimization, thus promoting oxygen adsorption to the 2e ORR favored "end-on" configuration. It is believed that the demonstrated cation vacancy engineering is an effective strategy toward creating active heterogeneous catalysts with atomic precision.

The two-dimensional surface or one-dimensional interface of heterogeneous catalysts is essential to determine the adsorption strengths and configurations of the reaction intermediates for desired activities. Recently, the development of single-atom catalysts has enabled an atomic-level understanding of catalytic processes. However, it remains obscure whether the conventional concept and mechanism of one-dimensional interface are applicable to zero-dimensional single atoms. In this work, we arranged the locations of single atoms to explore their interfacial interactions for improved oxygen evolution. When iridium single atoms were confined into the lattice of CoOOH, efficient electron transfer between Ir and Co tuned the adsorption strength of oxygenated intermediates. In contrast, atomic iridium species anchored on the surface of CoOOH induced inappreciable modification in electronic structures, whereas steric interactions with key intermediates at its Ir−OH−Co interface played a primary role in reducing its energy barrier toward oxygen evolution.

The homogeneity of single-atom catalysts is only to the first-order approximation when all isolated metal centers interact identically with the support. Since the realistic support with various topologies or defects offers diverse coordination environments, realizing real homogeneity requires precise control over the anchoring sites. In this work, we selectively anchor Ir single atoms onto the three-fold hollow sites (Ir1/TO–CoOOH) and oxygen vacancies (Ir1/VO–CoOOH) on defective CoOOH surface to investigate how the anchoring sites modulate catalytic performance. The oxygen evolution activities of Ir1/TO–CoOOH and Ir1/VO–CoOOH are improved relative to CoOOH through different mechanisms. For Ir1/TO–CoOOH, the strong electronic interaction between single-atom Ir and the support modifies the electronic structure of the active center for stronger electronic affinity to intermediates. For Ir1/VO–CoOOH, a hydrogen bonding is formed between the coordinated oxygen of single-atom Ir center and the oxygenated intermediates, which stabilizes the intermediates and lowers the energy barrier of the rate-determining step.

Transition-metal sulfides are investigated as promising electrocatalysts for oxygen evolution reaction (OER) in alkaline media; however, the real active species remain elusive and the development of oxyhydroxides reconstructed from sulfides delivering stable large current density at low applied potentials is a great challenge. Here, we report a synergistic hybrid catalyst, composed of nanoscale heterostructures of Co9S8 and Fe3O4, that exhibits only a low potential of 350 mV and record stability of 120 h at the 500 mA cm–2 in 1.0 M KOH. Voltage-dependent soft X-ray absorption spectroscopy (XAS) and Operando Raman spectroscopy demonstrate that the initial Co9S8@Fe3O4 is reconstructed into CoOOH/CoO x @Fe3O4 and further to complete CoOOH@Fe3O4. Operando XAS and electron microscopy imaging analyses reveal that the completely reconstructed CoOOH acts as active species and Fe3O4 components prevent the aggregation of CoOOH. Operando infrared spectroscopy indicates cobalt superoxide species (*OOH) as the active intermediates during the OER process. Density functional theory calculations demonstrate the formation of *OOH as the rate-determining step of OER and CoOOH@Fe3O4 exhibits a lower energy barrier for OER. Our results provide an in-depth understanding of the dynamic surface structure evolutions of sulfide electrocatalysts for alkaline OER and insights into the design of excellent nanocatalysts for stable large current density.

Inheritance and transformation: an in situ topological transformed NiCoFe-MOF nanosheet electrocatalyst exhibits highly efficient activity for water oxidation in an anion exchange membrane water electrolyzer.

Metal ion substitution and anion exchange are two effective strategies for regulating the electronic and geometric structure of spinel. However, the optimal location of foreign metallic cations and the exact role of these metals and anions remain elusive. Herein, CoFe2O4-based hollow nanospheres with outstanding oxygen evolution reaction activity are prepared by Cr3+ substitution and S2– exchange. X-ray absorption spectra and theoretical calculations reveal that Cr3+ can be precisely doped into octahedral (Oh) Fe sites and simultaneously induce Co vacancy, which can activate adjacent tetrahedral (Td) Fe3+. Furthermore, S2– exchange results in structure distortion of Td-Fe due to compressive strain effect. The change in the local geometry of Td-Fe causes the *OOH intermediate to deviate from the y-axis plane, thus enhancing the adsorption of the *OOH. The Co vacancy and S2– exchange can adjust the geometric and electronic structure of Td-Fe, thus activating the inert Td-Fe and improving the electrochemical performance.