Molecular Engineering of Hydrogen-Bonded Organic Framework for Enhanced Nitrate Electroreduction to Ammonia.

Electrocatalytic nitrate reduction is an efficient way to produce ammonia sustainably. Herein, we rationally designed a copper metalloporphyrin-based hydrogen-bonded organic framework (HOF-Cu) through molecular engineering strategies for electrochemical nitrate reduction. As a result, the state-of-the-art HOF-Cu catalyst exhibits high NH3 Faradaic efficiency of 93.8%, and the NH3 production rate achieves a superior activity of 0.65 mmol h-1 cm-2. The in situ electrochemical spectroscopic combined with density functional theory calculations reveals that the dispersed Cu promotes the adsorption of NO3- and the mechanism is followed by deoxidation of NO3- to *NO and accompanied by deep hydrogenation. The generated *H participates in the deep hydrogenation of intermediate with fast kinetics as revealed by operando electrochemical impedance spectroscopy, and the competing hydrogen evolution reaction is suppressed. This research provides a promising approach to the conversion of nitrate to ammonia, maintaining the nitrogen balance in the atmosphere.

Oxygen-Bridged Indium-Nickel Atomic Pair as Dual-Metal Active Sites Enabling Synergistic Electrocatalytic CO2 Reduction.

Single-atom catalysts offer a promising pathway for electrochemical CO2 conversion. However, it is still a challenge to optimize the electrochemical performance of dual-atom catalysts. Here, an atomic indium-nickel dual-sites catalyst bridged by an axial oxygen atom (O-In-N6 -Ni moiety) was anchored on nitrogenated carbon (InNi DS/NC). InNi DS/NC exhibits superior CO selectivity with Faradaic efficiency higher than 90 % over a wide potential range from -0.5 to -0.8 V versus reversible hydrogen electrode (vs. RHE). Moreover, an industrial CO partial current density up to 317.2 mA cm-2 is achieved at -1.0 V vs. RHE in a flow cell. In situ ATR-SEIRAS combined with theory calculations reveal that the synergistic effect of In-Ni dual-sites and O atom bridge not only reduces the reaction barrier for the formation of *COOH, but also retards the undesired hydrogen evolution reaction. This work provides a feasible strategy to construct dual-site catalysts towards energy conversion.

Atomically Dispersed Indium-Copper Dual-Metal Active Sites Promoting C-C Coupling for CO2 Photoreduction to Ethanol.

Photoreduction of CO2 to C2+ solar fuel is a promising carbon-neutral technology for renewable energy. This strategy is challenged by its low productivity due to low efficiency in multielectron utilization and slow C-C coupling kinetics. This work reports a dual-metal photocatalyst consisting of atomically dispersed indium and copper anchored on polymeric carbon nitride (InCu/PCN), on which the photoreduction of CO2 delivered an excellent ethanol production rate of 28.5 µmol g-1 h-1 with a high selectivity of 92 %. Coupled experimental investigation and DFT calculations reveal the following mechanisms underpinning the high performance of this catalyst. Essentially, the In-Cu interaction enhances the charge separation by accelerating charge transfer from PCN to the metal sites. Indium also transfers electrons to neighboring copper via Cu-N-In bridges, increasing the electron density of copper active sites. Furthermore, In-Cu dual-metal sites promote the adsorption of *CO intermediates and lower the energy barrier of C-C coupling.

Engineering MoOx /MXene Hole Transfer Layers for Unexpected Boosting of Photoelectrochemical Water Oxidation.

The development of semiconductor photoanodes is of great practical interest for the realization of photoelectrochemical (PEC) water splitting. Herein, MXene quantum dots (MQD) were grafted on a BiVO4 substrate, then a MoOx layer by combining an ultrathin oxyhydroxide oxygen evolution cocatalyst (OEC) was constructed as an integrated photoanode. The OEC/MoOx /MQD/BiVO4 array not only achieves a current density of 5.85 mA cm-2 at 1.23 V versus a reversible hydrogen electrode (vs. RHE), but also enhances photostability. From electrochemical analysis and density functional theory calculations, high PEC performance is ascribed to the incorporation of MoOx /MQD as hole transfer layers, retarding charge recombination, promoting hole transfer and accelerating water splitting kinetics. This proof-of-principle work not only demonstrates the potential utilization of hole transfer layers, but also sheds light on rational design and fabrication of integrated photoanodes for feasible solar energy conversion.

Conformal Macroporous Inverse Opal Oxynitride-Based Photoanode for Robust Photoelectrochemical Water Splitting.

Direct photoelectrochemical (PEC) water splitting is of prime importance in sustainable energy conversion systems; however, it is a big challenge to simultaneously control light harvesting and charge transport for the improvement of PEC performance. Herein, we report a three-dimensional ordered macroporous (3DOM) CsTaWO6-xNx inverse opal array as a promising candidate for the first time. To address the critical challenge, an ultrathin carbon-nitride-based layer-intercalated 3DOM CsTaWO6-xNx architecture as a conformal heterojunction photoanode was assembled. This state-of-the-art conformal heterojunction photoanode with carrier-separation efficiency up to 88% achieves a high current density of 4.59 mA cm-2 at 1.6 V versus a reversible hydrogen electrode (vs RHE) under simulated AM 1.5G illumination, which is approximately 3.4 and 17 times larger than that of pristine CsTaWO6-xNx inverse opals and powers photoelectrodes in alkaline media, corresponding to an incident photon-to-current efficiency of 32% at 400 nm and outstanding stability for PEC water splitting. Density functional theory calculations propose that the intimate interface of a conformal photoanode optimizes the charge separation and transfer, thus enhancing the intrinsic water oxidation performance. This work enables us to elucidate the pivotal importance of 3DOM architectures and conformal heterostructures and the promising contributions to excellent PEC water-splitting applications.

Engineering Single-Atomic Ni-N4-O Sites on Semiconductor Photoanodes for High-Performance Photoelectrochemical Water Splitting.

Direct photoelectrochemical (PEC) water splitting is a promising solution for solar energy conversion; however, there is a pressing bottleneck to address the intrinsic charge transport for the enhancement of PEC performance. Herein, a versatile coupling strategy was developed to engineer atomically dispersed Ni-N4 sites coordinated with an axial direction oxygen atom (Ni-N4-O) incorporated between oxygen evolution cocatalyst (OEC) and semiconductor photoanode, boosting the photogenerated electron-hole separation and thus improving PEC activity. This state-of-the-art OEC/Ni-N4-O/BiVO4 photoanode exhibits a record high photocurrent density of 6.0 mA cm-2 at 1.23 V versus reversible hydrogen electrode (vs RHE), over approximately 3.97 times larger than that of BiVO4, achieving outstanding long-term photostability. From X-ray absorption fine structure analysis and density functional theory calculations, the enhanced PEC performance is attributed to the construction of single-atomic Ni-N4-O moiety in OEC/BiVO4, facilitating the holes transfer, decreasing the free energy barriers, and accelerating the reaction kinetics. This work enables us to develop an effective pathway to design and fabricate efficient and stable photoanodes for feasible PEC water splitting application.

Engineering Lattice Oxygen Activation of Iridium Clusters Stabilized on Amorphous Bimetal Borides Array for Oxygen Evolution Reaction.

Developing robust oxygen evolution reaction (OER) catalysts requires significant advances in material design and in-depth understanding for water electrolysis. Herein, we report iridium clusters stabilized surface reconstructed oxyhydroxides on amorphous metal borides array, achieving an ultralow overpotential of 178 mV at 10 mA cm-2 for OER in alkaline medium. The coupling of iridium clusters induced the formation of high valence cobalt species and Ir-O-Co bridge between iridium and oxyhydroxides at the atomic scale, engineering lattice oxygen activation and non-concerted proton-electron transfer to trigger multiple active sites for intrinsic pH-dependent OER activity. The lattice oxygen oxidation mechanism (LOM) was confirmed by in situ 18 O isotope labeling mass spectrometry and chemical recognition of negative peroxo-like species. Theoretical simulations reveal that the OER performance on this catalyst is intrinsically dominated by LOM pathway, facilitating the reaction kinetics. This work not only paves an avenue for the rational design of electrocatalysts, but also serves the fundamental insights into the lattice oxygen participation for promising OER application.

Enriching Metal-Oxygen Species and Phosphate Modulating of Active Sites for Robust Electrocatalytical CO2 Reduction.

Direct electrochemical reduction of CO2 (CO2 RR) into value-added chemicals is a promising solution to reduce carbon emissions. The activity of CO2 RR is influenced deeply by the reaction microenvironment and electronic properties of the catalysts. Herein, the surface PO4 3- anions are tuned to modulate the local microenvironment and the electronic properties of the indium-based catalyst with abundant metal-oxygen species enabling efficient electrochemical conversion of CO2 to HCOO- . Indium nanoparticles coupled with PO4 3- anions (PO4 3- -In NPs) achieve a high selectivity of HCOO- up to 91.4% at a low potential of -0.98 V versus reversible hydrogen electrode (versus RHE) and a high HCOO- partial current density of 279.3 mA cm-2 at -1.1 V versus RHE in the electrochemical flow cell. In situ and ex situ characterizations confirm the PO4 3- anions keep stable on the surface of indium during CO2 RR, accelerating the generation of OCHO* intermediate. From density functional theory calculations, PO4 3- anions enrich the metal-oxygen species on the substrate to optimize the electronic structure of the catalysts and induce a local microenvironment with massive K+ ions on the interface, thus reducing the activation energy barrier of CO2 RR.

Regulating electronic states of nitride/hydroxide to accelerate kinetics for oxygen evolution at large current density.

Rational design efficient transition metal-based electrocatalysts for oxygen evolution reaction (OER) is critical for water splitting. However, industrial water-alkali electrolysis requires large current densities at low overpotentials, always limited by intrinsic activity. Herein, we report hierarchical bimetal nitride/hydroxide (NiMoN/NiFe LDH) array as model catalyst, regulating the electronic states and tracking the relationship of structure-activity. As-activated NiMoN/NiFe LDH exhibits the industrially required current density of 1000 mA cm-2 at overpotential of 266 mV with 250 h stability for OER. Especially, in-situ electrochemical spectroscopic reveals that heterointerface facilitates dynamic structure evolution to optimize electronic structure. Operando electrochemical impedance spectroscopy implies accelerated OER kinetics and intermediate evolution due to fast charge transport. The OER mechanism is revealed by the combination of theoretical and experimental studies, indicating as-activated NiMoN/NiFe LDH follows lattice oxygen oxidation mechanism with accelerated kinetics. This work paves an avenue to develop efficient catalysts for industrial water electrolysis via tuning electronic states.

Identification of the Origin for Reconstructed Active Sites on Oxyhydroxide for Oxygen Evolution Reaction.

The regulation of atomic and electronic structures of active sites plays an important role in the rational design of oxygen evolution reaction (OER) catalysts toward electrocatalytic hydrogen generation. However, the precise identification of the active sites for surface reconstruction behavior during OER remains elusive for water-alkali electrolysis. Herein, irreversible reconstruction behavior accompanied by copper dynamic evolution for cobalt iron layered double hydroxide (CoFe LDH) precatalyst to form CoFeCuOOH active species with high-valent Co species is reported, identifying the origin of reconstructed active sites through operando UV-Visible (UV-vis), in situ Raman, and X-ray absorption fine-structure (XAFS) spectroscopies. Density functional theory analysis rationalizes this typical electronic structure evolution causing the transfer of intramolecular electrons to form ligand holes, promoting the reconstruction of active sites. Specifically, unambiguous identification of active sites for CoFeCuOOH is explored by in situ 18 O isotope-labeling differential electrochemical mass spectrometry (DEMS) and supported by theoretical calculation, confirming mechanism switch to oxygen-vacancy-site mechanism (OVSM) pathway on lattice oxygen. This work enables to elucidate the vital role of dynamic active-site generation and the representative contribution of OVSM pathway for efficient OER performance.

Triggering Lattice Oxygen Activation of Single-Atomic Mo Sites Anchored on Ni-Fe Oxyhydroxides Nanoarrays for Electrochemical Water Oxidation.

Tuning the reactivity of lattice oxygen is of significance for lowering the energy barriers and accelerating the oxygen evolution reaction (OER). Herein, single-atomic Mo sites are anchored on Ni-Fe oxyhydroxide nanoarrays by a facile metal-organic-framework-derived strategy, exhibiting superior performance toward the OER in alkaline media. In situ electrochemical spectroscopy and isotope-labeling experiments reveal the involvement of lattice oxygen during OER cycles. Combining theoretical and experimental investigations of the electronic configuration, it is comprehensively confirmed that the incorporation of single-atomic Mo sites enables higher oxidation state of the metal and strengthened metal-oxygen hybridization, as well as the formation of oxidized ligand holes above the Fermi level. In a word, the considerable acceleration of water oxidation is achieved via enhancing the reactivity of lattice oxygen and triggering the lattice oxygen activation. This work may provide new insights for designing ideal electrocatalysts via tuning the chemical state and activating the anions ligands.

Engineering single-atomic ruthenium catalytic sites on defective nickel-iron layered double hydroxide for overall water splitting.

Rational design of single atom catalyst is critical for efficient sustainable energy conversion. However, the atomic-level control of active sites is essential for electrocatalytic materials in alkaline electrolyte. Moreover, well-defined surface structures lead to in-depth understanding of catalytic mechanisms. Herein, we report a single-atomic-site ruthenium stabilized on defective nickel-iron layered double hydroxide nanosheets (Ru1/D-NiFe LDH). Under precise regulation of local coordination environments of catalytically active sites and the existence of the defects, Ru1/D-NiFe LDH delivers an ultralow overpotential of 18 mV at 10 mA cm-2 for hydrogen evolution reaction, surpassing the commercial Pt/C catalyst. Density functional theory calculations reveal that Ru1/D-NiFe LDH optimizes the adsorption energies of intermediates for hydrogen evolution reaction and promotes the O-O coupling at a Ru-O active site for oxygen evolution reaction. The Ru1/D-NiFe LDH as an ideal model reveals superior water splitting performance with potential for the development of promising water-alkali electrocatalysts.

Two-Dimensional Defective Boron-Doped Niobic Acid Nanosheets for Robust Nitrogen Photofixation.

Direct nitrogen photofixation is a feasible solution toward sustainable production of ammonia under mild conditions. However, the generation of active sites for solar-dirven nitrogen fixation not only limits the fundamental understanding of the relationship among light absorption, charge transfer, and catalytic efficiency but also influences the photocatalytic activity. Herein, we report two-dimensional boron-doped niobic acid nanosheets with oxygen vacancies (B-Vo-HNbO3 NSs) for efficient N2 photofixation in the absence of any scavengers and cocatalysts. Impressively, B-Vo-HNbO3 NS as a model catalyst achieves the enhanced ammonia evolution rate of 170 µmol gcat-1 h-1 in pure water under visible-light irradiation. The doublet coupling representing 15NH4+ in an isotopic labeling experiment and in situ infrared spectra confirm the reliable ammonia generation. The experimental analysis and density functional theory (DFT) calculations indicate that the strong synergy of boron dopant and oxygen vacancy regulates band structure of niobic acid, facilitates photogenerated charge transfer, reduces free energy barriers, accelerates reaction kinetics, and promotes the high rates of ammonia evolution. This work provides a general strategy to design active photocatalysts toward solar N2 conversion.

Engineering active sites on hierarchical transition bimetal oxides/sulfides heterostructure array enabling robust overall water splitting.

Rational design of the catalysts is impressive for sustainable energy conversion. However, there is a grand challenge to engineer active sites at the interface. Herein, hierarchical transition bimetal oxides/sulfides heterostructure arrays interacting two-dimensional MoOx/MoS2 nanosheets attached to one-dimensional NiOx/Ni3S2 nanorods were fabricated by oxidation/hydrogenation-induced surface reconfiguration strategy. The NiMoOx/NiMoS heterostructure array exhibits the overpotentials of 38 mV for hydrogen evolution and 186 mV for oxygen evolution at 10 mA cm-2, even surviving at a large current density of 500 mA cm-2 with long-term stability. Due to optimized adsorption energies and accelerated water splitting kinetics by theory calculations, the assembled two-electrode cell delivers the industrially relevant current densities of 500 and 1000 mA cm-2 at record low cell voltages of 1.60 and 1.66 V with excellent durability. This research provides a promising avenue to enhance the electrocatalytic performance of the catalysts by engineering interfacial active sites toward large-scale water splitting.