Theory-driven design of high-valence metal sites for water oxidation confirmed using in situ soft X-ray absorption

Tailoring Second Coordination Sphere for Tunable Solid-Liquid Interfacial Charge Transfer toward Enhanced Photoelectrochemical H2 Production

The recombination of photogenerated charge carriers severely limits the performance of photoelectrochemical (PEC) H2 production. Here, we demonstrate that this limitation can be overcome by optimizing the charge transfer dynamics at the solid-liquid interface via molecular catalyst design. Specifically, the surface of a p-Si photocathode is modulated using molecular catalysts with different metal atoms and organic ligands to improve H2 production performance. Co(pda-SO3H)2 is identified as an efficient and durable catalyst for H2 production through the rational design of metal centers and first/second coordination spheres. The modulation with Co(pda-SO3H)2, which contains an electron-withdrawing -SO3H group in the second coordination sphere, elevates the flat-band potential of the polished p-Si photocathode and nanoporous p-Si photocathode by 81 mV and 124 mV, respectively, leading to the maximized energy band bending and the minimized interfacial carrier transport resistance. Consequently, both the two photocathodes achieve the Faradaic efficiency of more than 95 % for H2 production, which is well maintained during 18 h and 21 h reaction, respectively. This work highlights that the band-edge engineering by molecular catalysts could be an important design consideration for semiconductor-catalyst hybrids toward PEC H2 production.

Promoting Oxygen Evolution Electrocatalysis by Coordination Engineering in Cobalt Phosphate

Understanding the structure-activity correlation is an important prerequisite for the rational design of high-efficiency electrocatalysts at the atomic level. However, the effect of coordination environment on electrocatalytic oxygen evolution reaction (OER) remains enigmatic. In this work, the regulation of proton transfer involved in water oxidation by coordination engineering based on Co3(PO4)2 and CoHPO4 is reported. The HPO4 2- anion has intermediate pKa value between Co(II)-H2O and Co(III)-H2O to be served as an appealing proton-coupled electron transfer (PCET) induction group. From theoretical calculations, the pH-dependent OER properties, deuterium kinetic isotope effects, operando electrochemical impedance spectroscopy (EIS) and Raman studies, the CoHPO4 catalyst beneficially reduces the energy barrier of proton hopping and modulates the formation energy of high-valent Co species, thereby enhancing OER activity. This work demonstrates a promising strategy that involves tuning the local coordination environment to optimize PCET steps and electrocatalytic activities for electrochemical applications. In addition, the designed system offers a motif to understand the structure-efficiency relationship from those amino-acid residue with proton buffer ability in natural photosynthesis.

Highly efficient anion exchange membrane water electrolyzers via chromium-doped amorphous electrocatalysts

In-depth comprehension and modulation of the electronic structure of the active metal sites is crucial to enhance their intrinsic activity of electrocatalytic oxygen evolution reaction (OER) toward anion exchange membrane water electrolyzers (AEMWEs). Here, we elaborate a series of amorphous metal oxide catalysts (FeCrOx, CoCrOx and NiCrOx) with high performance AEMWEs by high-valent chromium dopant. We discover that the positive effect of the transition from low to high valence of the Co site on the adsorption energy of the intermediate and the lower oxidation barrier is the key factor for its increased activity by synchrotron radiation in-situ techniques. Particularly, the CoCrOx anode catalyst achieves the high current density of 1.5 A cm-2 at 2.1 V and maintains for over 120 h with attenuation less than 4.9 mV h-1 in AEMWE testing. Such exceptional performance demonstrates a promising prospect for industrial application and providing general guidelines for the design of high-efficiency AEMWEs systems.

Unveiling the Pivotal Role of dx2-y2 Electronic States in Nickel-Based Hydroxide Electrocatalysts for Methanol Oxidation

The anodic methanol oxidation reaction (MOR) plays a crucial role in coupling with the cathodic hydrogen evolution reaction (HER) and enables the sustainable production of the high-valued formate. Nickel-based hydroxide (Ni(OH)2) as MOR electrocatalyst has attracted enormous attention. However, the key factor determining the intrinsic catalytic activity remains unknown, which significantly hinders the further development of Ni(OH)2 electrocatalyst. Here, we found that thed x 2 - y 2 ${{d}_{{x}^{2}-{y}^{2}}}$ electronic state within antibonding bands plays a decisive role in the whole MOR process. The onset potential depends on the deprotonation ability (Ni2+ to Ni3+), which was closely related to the band center ofd x 2 - y 2 ${{d}_{{x}^{2}-{y}^{2}}}$ orbital. The closer ofd x 2 - y 2 ${{d}_{{x}^{2}-{y}^{2}}}$ orbital to the Fermi level showed the stronger the deprotonation ability. Meanwhile, in the high potential region, the broadening ofd x 2 - y 2 ${{d}_{{x}^{2}-{y}^{2}}}$ orbital would facilitate the electron transfer from methanol to catalysts (Ni3+ to Ni2+), further enhancing the catalytic properties. Our work for the first time clarifies the intrinsic relationship betweend x 2 - y 2 ${{d}_{{x}^{2}-{y}^{2}}}$ electronic state and the MOR activities, which adds a new layer of understanding to the methanol electrooxidation research scene.

Bifunctional Electrocatalysts for Overall and Hybrid Water Splitting

Electrocatalytic water splitting driven by renewable electricity has been recognized as a promising approach for green hydrogen production. Different from conventional strategies in developing electrocatalysts for the two half-reactions of water splitting (e.g., the hydrogen and oxygen evolution reactions, HER and OER) separately, there has been a growing interest in designing and developing bifunctional electrocatalysts, which are able to catalyze both the HER and OER. In addition, considering the high overpotentials required for OER while limited value of the produced oxygen, there is another rapidly growing interest in exploring alternative oxidation reactions to replace OER for hybrid water splitting toward energy-efficient hydrogen generation. This Review begins with an introduction on the fundamental aspects of water splitting, followed by a thorough discussion on various physicochemical characterization techniques that are frequently employed in probing the active sites, with an emphasis on the reconstruction of bifunctional electrocatalysts during redox electrolysis. The design, synthesis, and performance of diverse bifunctional electrocatalysts based on noble metals, nonprecious metals, and metal-free nanocarbons, for overall water splitting in acidic and alkaline electrolytes, are thoroughly summarized and compared. Next, their application toward hybrid water splitting is also presented, wherein the alternative anodic reactions include sacrificing agents oxidation, pollutants oxidative degradation, and organics oxidative upgrading. Finally, a concise statement on the current challenges and future opportunities of bifunctional electrocatalysts for both overall and hybrid water splitting is presented in the hope of guiding future endeavors in the quest for energy-efficient and sustainable green hydrogen production.

Ni─Co─O─S Derived Catalysts on Hierarchical N-doped Carbon Supports with Strong Interfacial Interactions for Improved Hybrid Water Splitting Performance

Simultaneously improving electrochemical activity and stability is a long-term goal for water splitting. Herein, hierarchical N-doped carbon nanotubes on carbon nanowires derived from PPy are grown on carbon cloth, serving as a support for NiCo oxides/sulfides. The hierarchical electrodes annealed in N2 or H2/N2 display improved intrinsic activity and stability for hydrogen evolution reaction (HER) and glucose oxidation reaction. Compared with Pt/C||Ir/C in alkaline media, the glucose electrolysis assembled with electrodes exhibits a cell voltage of 1.38 V at 10 mA cm-2, durability for >12 h at 50 mA cm-2, and resistance to glucose/gluconic acid poisoning. In addition, electrocatalysts can also be applied in ethanol oxidation reactions. Systematic characterizations reveal the strong interactions between NiCo and N-doped carbon support-induced partial charge transfer at the interface and regulate the local electronic structure of active sites. Density functional theory calculations demonstrate that the synergistic effect between N-doped carbon supports, metallic NiCo, and NiCo oxides/sulfides optimize the adsorption energy of H2O and the H* free energy for HER. The energy barrier of the dehydrogenation of glucose effectively decreased. This work will attract attention to the role of metal-support interactions in enhancing the intrinsic activity and stability of electrocatalysts.

Engineering High-Spin State Cobalt Cations in Sulfide Spinel for Enhancing Water Oxidation

The spin states of active sites have a significant impact on the adsorption/desorption ability of the reaction intermediates during the oxygen evolution reaction (OER). Sulfide spinel is not generally considered a highly efficient OER catalyst owing to the low spin state of Co3+ and the lack of unpaired electrons available for adsorption of reaction intermediates. Herein, it is proposed a novel Nd-evoked valence electronic adjustment strategy to engineer the spin state of Co ions. The unique f-p-d orbital electronic coupling effect stimulates the rearrangement of Co d orbital electrons and increases the eg electron filling to achieve high-spin state Co ions, which promotes charge transport by propagating a spin channel and generates a high number of active sites for intermediate adsorption. The optimized CuCo1.75 Nd0.25 S4 catalyst exhibits outstanding electrocatalytic properties with a low overpotential of 320 mV at 500 mA cm-2 and a 48 h stability at 300 mA cm-2 . In situ synchrotron radiation infrared spectra confirm the quick accumulation of key *OOH and *O intermediates. This work deepens the comprehensive understanding of the relationship between OER activity and spin configurations of Co ions and offers a new design strategy for spinel compound catalysts.

Two Birds with One Stone: Contemporaneously Enhancing OER Catalytic Activity and Stability for Dual-Phase Medium-Entropy Metal Sulfides

Transition metal-based sulfides exhibit remarkable potential as electrocatalysts for oxygen evolution reaction (OER) due to the unique intrinsic structure and physicochemical characteristics. Nevertheless, currently available sulfide catalysts based on transition metals face a bottleneck in large-scale commercial applications owing to their unsatisfactory stability. Here, the first fabrication of (FeCoNiMn2 )S2 dual-phase medium-entropy metal sulfide (dp-MEMS) is successfully achieved, which demonstrated the expected optimization of stability in the OER process. Benefiting from the "cell wall" -like structure and the synergistic effect in medium-entropy systems, (FeCoNiMn2 )S2 dp-MEMS delivers an exceptionally low overpotential of 169 and 232 mV at current densities of 10 and 100 mA cm-2 , respectively. The enhancement mechanism of catalytic activity and stability is further validated by density functional theory (DFT) calculations. Additionally, the rechargeable Zn-air batteries integrated with FeCoNiMn2 )S2 dp-MEMS exhibit remarkable performance outperforming the commercial catalyst (Pt/C+RuO2 ). This work demonstrates that the dual-phase medium-entropy metal sulfide-based catalysts have the potential to provide a greater application value for OER and related energy conversion systems.

High-energy all-solid-state lithium batteries enabled by Co-free LiNiO2 cathodes with robust outside-in structures

A critical current challenge in the development of all-solid-state lithium batteries (ASSLBs) is reducing the cost of fabrication without compromising the performance. Here we report a sulfide ASSLB based on a high-energy, Co-free LiNiO2 cathode with a robust outside-in structure. This promising cathode is enabled by the high-pressure O2 synthesis and subsequent atomic layer deposition of a unique ultrathin LixAlyZnzOδ protective layer comprising a LixAlyZnzOδ surface coating region and an Al and Zn near-surface doping region. This high-quality artificial interphase enhances the structural stability and interfacial dynamics of the cathode as it mitigates the contact loss and continuous side reactions at the cathode/solid electrolyte interface. As a result, our ASSLBs exhibit a high areal capacity (4.65 mAh cm-2), a high specific cathode capacity (203 mAh g-1), superior cycling stability (92% capacity retention after 200 cycles) and a good rate capability (93 mAh g-1 at 2C). This work also offers mechanistic insights into how to break through the limitation of using expensive cathodes (for example, Co-based) and coatings (for example, Nb-, Ta-, La- or Zr-based) while still achieving a high-energy ASSLB performance.

Catalytic Ozonation of Air toward Direct Nitric Acid Production Using Hierarchical Co3O4 with a Tunable Microstructure

The nitrogen oxidation reaction (NOR) to form nitric acid by applying natural air and H2O under ambient conditions is a sustainable approach to achieving efficient and selective N2 fixation for industrial applications. In this study, four kinds of Co3O4 catalysts with a controllable microstructure were prepared to catalyze the direct NOR of N2 in the air. At the same time, the reaction mechanism of the conversion of N2 to nitric acid under catalytic ozonation was explored through experimental research and density functional theory (DFT) calculation. The results showed that the prepared porous nanosheets self-assembled into microflower-structured samples. The HCOF showed extraordinary catalytic performance for direct NOR to produce a high concentration of nitric acid. The maximum rate of nitric acid formation could be as high as 6.67 mmol/(h·gcat), which was higher than those of most reported photocatalytic or electrocatalytic N2 fixation processes for direct NOR to produce NO3-. Furthermore, the 15N isotopic-labeling experiment confirmed that the produced NO3- originated from N2 in the air by the direct NOR process. In the direct NOR mechanism, inert N2 molecules were captured at the Co3+ active sites by the acceptance-donation electron conduction mode, and the oxygen vacancies boosted the chemical adsorption of N2 molecules and greatly reduced the activation energy barrier of N2 molecules. The active free radicals •OH and •O2- generated by the decomposition of O3 molecules oxidized N2 to the intermediate *NO, which was the rate-determining step, and it was then absorbed by water to form nitric acid. The air catalytic ozonation method in this study was proposed as a facile pathway for efficient nitrogen fixation. This research provides a new method for environmental protection and efficient production of nitric acid at distributed sources.

Understanding the Origin of Reconstruction in Transition Metal Oxide Oxygen Evolution Reaction Electrocatalysts

Electrochemical water splitting to generate hydrogen energy fills a gap in the intermittency issues for wind and sunlight power. Transition metal (TM) oxides have attracted significant interest in water oxidation due to their availability and excellent activity. Typically, the transitional metal oxyhydroxides species derived from these metal oxides are often acknowledged as the real catalytic species, due to the irreversible structural reconstruction. Hence, in order to innovatively design new catalyst, it is necessary to provide a comprehensive understanding for the origin of surface reconstruction. In this review, the most recent developments in the reconstruction of transition metal-based oxygen evolution reaction electrocatalysts were introduced, and various chemical driving forces behind the reconstruction mechanism were discussed. At the same time, specific strategies for modulating pre-catalysts to achieve controllable reconfiguration, such as metal substituting, increase of structural defect sites, were summarized. At last, the issues for the further understanding and optimization of transition metal oxides compositions based on structural reconstruction were provided.

Hierarchical Superhydrophilic/Superaerophobic Ni(OH)2@NiFe-PBA Nanoarray Supported on Nickel Foam for Boosting the Oxygen Evolution Reaction

The design of hierarchical electrocatalysts with plentiful active sites and high mass transfer efficiency is critical to efficiently and sustainably carrying out the oxygen evolution reaction (OER), which presents a challenging and pressing need. In this study, a hierarchical Ni(OH)2@NiFe-Prussian blue analogue nanoarray grown on nickel foam (NF) [labeled as Ni(OH)2@NiFe-PBA/NF] was synthesized by combining a mild electrodeposition method with an ion-exchange strategy. The resultant Ni(OH)2@NiFe-PBA/NF displays superhydrophilic/superaerophobic properties that optimize the contact with the electrolyte, improve mass transfer efficiency, and expedite detachment of O2 bubbles during the electrocatalytic OER. Specifically, Ni(OH)2@NiFe-PBA/NF exhibits exceptional capability in the OER with low overpotentials of 224 and 240 mV at the current densities of 50 and 100 mA cm-2, respectively, accompanied by a low Tafel slope of 37.1 mV dec-1 and outstanding stability over 100 h at a fixed potential of 1.78 V vs reversible hydrogen electrode (RHE). Furthermore, Ni(OH)2@NiFe-PBA/NF demonstrates remarkable OER performance even in alkaline simulated seawater. During the OER process, active metal-OOH intermediates were formed by the partial self-reconstruction of NiFe-PBA in the heterostructure, as revealed by in situ Raman spectroscopy.

Tandem Electro-Thermo-Catalysis for the Oxidative Aminocarbonylation of Arylboronic Acids to Amides from CO2 and Water

Direct CO2 electroreduction to valuable chemicals is critical for carbon neutrality, while its main products are limited to simple C1 /C2 compounds, and traditionally, the anodic O2 byproduct is not utilized. We herein report a tandem electrothermo-catalytic system that fully utilizes both cathodic (i.e., CO) and anodic (i.e., O2 ) products during overall CO2 electrolysis to produce valuable organic amides from arylboronic acids and amines in a separate chemical reactor, following the Pd(II)-catalyzed oxidative aminocarbonylation mechanism. Hexamethylenetetramine (HMT)-incorporated silver and nickel hydroxide carbonate electrocatalysts were prepared for efficient coproduction of CO and O2 with Faradaic efficiencies of 99.3 % and 100 %, respectively. Systematic experiments, operando attenuated total reflection surface-enhanced Fourier transform infrared spectroscopy characterizations and theoretical studies reveal that HMT promotes *CO2 hydrogenation/*CO desorption for accelerated CO2 -to-CO conversion, and O2 inhibits reductive deactivation of the Pd(II) catalyst for enhanced oxidative aminocarbonylation, collectively leading to efficient synthesis of 10 organic amides with high yields of above 81 %. This work demonstrates the effectiveness of a tandem electrothermo-catalytic strategy for economically attractive CO2 conversion and amide synthesis, representing a new avenue to explore the full potential of CO2 utilization.

Large electronegativity differences between adjacent atomic sites activate and stabilize ZnIn2S4 for efficient photocatalytic overall water splitting

Photocatalytic overall water splitting into hydrogen and oxygen is desirable for long-term renewable, sustainable and clean fuel production on earth. Metal sulfides are considered as ideal hydrogen-evolved photocatalysts, but their component homogeneity and typical sulfur instability cause an inert oxygen production, which remains a huge obstacle to overall water-splitting. Here, a distortion-evoked cation-site oxygen doping of ZnIn2S4 (D-O-ZIS) creates significant electronegativity differences between adjacent atomic sites, with S1 sites being electron-rich and S2 sites being electron-deficient in the local structure of S1-S2-O sites. The strong charge redistribution character activates stable oxygen reactions at S2 sites and avoids the common issue of sulfur instability in metal sulfide photocatalysis, while S1 sites favor the adsorption/desorption of hydrogen. Consequently, an overall water-splitting reaction has been realized in D-O-ZIS with a remarkable solar-to-hydrogen conversion efficiency of 0.57%, accompanying a ~ 91% retention rate after 120 h photocatalytic test. In this work, we inspire an universal design from electronegativity differences perspective to activate and stabilize metal sulfide photocatalysts for efficient overall water-splitting.

Interface Engineering a High Content of Co3+ Sites on Co3O4 Nanoparticles to Boost Acidic Oxygen Evolution

Non-noble metal oxides have emerged as potential candidate electrocatalysts for acidic oxygen evolution reactions (OERs) due to their earth abundance; however, improving their catalytic activity and stability simultaneously in strong acidic electrolytes is still a major challenge. In this work, we report Co3O4@carbon core-shell nanoparticles on 2D graphite sheets (Co3O4@C-GS) as mixed-dimensional hybrid electrocatalysts for acidic OER. The obtained Co3O4@C-GS catalyst exhibits a low overpotential of 350 mV and maintains stability for 20 h at a current density of 10 mA cm-2 in H2SO4 (pH = 1) electrolyte. X-ray photoelectron and X-ray absorption spectroscopies illustrate that the higher content of Co3+ sites boosts acidic OER. Operando Raman spectroscopy reveals that the catalytic stability of Co3O4@C nanoparticles during the acidic OER is enhanced by the introduction of graphite sheets. This interface engineering of non-noble metal sites with high valence states provides an efficient approach to boost the catalytic activity and enhance the stability of noble-metal-free electrocatalysts for acidic OER.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Reconstructing 3d-Metal Electrocatalysts through Anionic Evolution in Zinc-Air Batteries

As one of the promising sustainable energy storage systems, academic research on rechargeable Zn-air batteries has recently been rejuvenated following development of various 3d-metal electrocatalysts and identification of their dynamic reconstruction toward (oxy)hydroxide, but performance disparity among catalysts remains unexplained. Here, this uncertainty is addressed through investigating the anionic contribution to regulate dynamic reconstruction and battery behavior of 3d-metal selenides. Comparing with the alloy counterpart, anionic chemistry is identified as a performance promoter and further exploited to empower Zn-air batteries. Based on theoretical modeling, Se-resolved operando spectroscopy, and advanced electron microscopy, a three-step Se evolution is established, consisting of oxidation, leaching, and recoordination. The process generates an amorphous (oxy)hydroxide with O-sharing bonded Se motifs that triggers charge redistribution at metal sites and lowers the energetic barrier of their current-driven redox. A pervasive concept of Se back-feeding is then proposed to describe the underlying chemistry for 3d-metal selenides with diversity in crystals or compositions, and the feasibility to fine-tune their behavior is also presented.

Phase Engineering and Synchrotron-Based Study on Two-Dimensional Energy Nanomaterials

In recent years, there has been significant interest in the development of two-dimensional (2D) nanomaterials with unique physicochemical properties for various energy applications. These properties are often derived from the phase structures established through a range of physical and chemical design strategies. A concrete analysis of the phase structures and real reaction mechanisms of 2D energy nanomaterials requires advanced characterization methods that offer valuable information as much as possible. Here, we present a comprehensive review on the phase engineering of typical 2D nanomaterials with the focus of synchrotron radiation characterizations. In particular, the intrinsic defects, atomic doping, intercalation, and heterogeneous interfaces on 2D nanomaterials are introduced, together with their applications in energy-related fields. Among them, synchrotron-based multiple spectroscopic techniques are emphasized to reveal their intrinsic phases and structures. More importantly, various in situ methods are employed to provide deep insights into their structural evolutions under working conditions or reaction processes of 2D energy nanomaterials. Finally, conclusions and research perspectives on the future outlook for the further development of 2D energy nanomaterials and synchrotron radiation light sources and integrated techniques are discussed.

Lattice-Strain Engineering for Heterogenous Electrocatalytic Oxygen Evolution Reaction

The energy efficiency of metal-air batteries and water-splitting techniques is severely constrained by multiple electronic transfers in the heterogenous oxygen evolution reaction (OER), and the high overpotential induced by the sluggish kinetics has become an uppermost scientific challenge. Numerous attempts are devoted to enabling high activity, selectivity, and stability via tailoring the surface physicochemical properties of nanocatalysts. Lattice-strain engineering as a cutting-edge method for tuning the electronic and geometric configuration of metal sites plays a pivotal role in regulating the interaction of catalytic surfaces with adsorbate molecules. By defining the d-band center as a descriptor of the structure-activity relationship, the individual contribution of strain effects within state-of-the-art electrocatalysts can be systematically elucidated in the OER optimization mechanism. In this review, the fundamentals of the OER and the advancements of strain-catalysts are showcased and the innovative trigger strategies are enumerated, with particular emphasis on the feedback mechanism between the precise regulation of lattice-strain and optimal activity. Subsequently, the modulation of electrocatalysts with various attributes is categorized and the impediments encountered in the practicalization of strained effect are discussed, ending with an outlook on future research directions for this burgeoning field.

MOF Material-Derived Bimetallic Sulfide CoxNiyS for Electrocatalytic Oxidation of 5-Hydroxymethylfurfural

The electrocatalytic conversion of biomass into high-value-added chemicals is one of the effective methods of green chemistry. Conventional metal catalysts have disadvantages, such as low atomic utilization and small surface areas. Catalyst materials derived from metal-organic frameworks (MOFs) have received much attention due to their unique physicochemical properties. Here, an MOF-derived non-precious metal CoxNiyS electrocatalyst was applied to the oxidation of biomass-derivative 5-hydroxymethylfurfural (HMF). The HMF oxidation reaction activities were modulated by regulating the content of Co and Ni bimetals, showing a volcano curve with an increasing proportion of Co. When the Co:Ni ratio was 2:1, the HMF conversion rate reached 84.5%, and the yield of the main product, 2,5-furandicarboxylic acid (FDCA), was 54%. The XPS results showed that the presence of high-valent nickel species after electrolysis, which further proved the existence and reactivity of NiOOH, as well as the synergistic effect of Co and Ni promoted the conversion of HMF. Increasing the content of Ni could increase the activity of HMF electrochemical oxidation, and increasing the content of Co could reduce the increase in the anodic current. This study has important significance for designing better HMF electrochemical catalysts in the future.

Enhanced Oxygen Evolution Reaction Performance on NiSx@Co3O4/Nickel Foam Electrocatalysts with Their Photothermal Property

Based on the principle of heterogeneous catalysis for water electrolysis, electrocatalysts with appropriate electronic structure and photothermal property are expected to drive the oxygen evolution reaction effectively. Herein, amorphous NiS_x-coupled nanourchin-like Co₃O₄ was prepared on nickel foam (NiS_x@Co₃O₄/NF) and investigated as a electrocatalyst for photothermal-assisted oxygen evolution reaction. The experimental investigations and simulant calculations jointly revealed NiS_x@Co₃O₄/NF to be of suitable electronic structure and high near-infrared photothermal conversion capability to achieve the oxygen evolution reaction advantageously both in thermodynamics and in kinetics. Relative to Co₃O₄/NF and NiS_x/NF, better oxygen evolution reaction activity, kinetics, and stability were achieved on NiS_x@Co₃O₄/NF in 1.0 M KOH owing to the NiS_x/Co₃O₄ synergetic effect. In addition, the oxygen evolution reaction performance of NiS_x@Co₃O₄/NF can be obviously enhanced under near-infrared light irradiation, since NiS_x@Co₃O₄ can absorb the near-infrared light to produce electric and thermal field. For the photothermal-mediated oxygen evolution reaction, the overpotential and Tafel slope of NiS_x@Co₃O₄/NF at 50 mA cm^-2 were reduced by 23 mV and 13 mV/dec, respectively. The present work provides an inspiring reference to design and develop photothermal-assisted water electrolysis using abundant solar energy.

A semiconductor-electrocatalyst nano interface constructed for successive photoelectrochemical water oxidation

Photoelectrochemical water splitting has long been considered an ideal approach to producing green hydrogen by utilizing solar energy. However, the limited photocurrents and large overpotentials of the anodes seriously impede large-scale application of this technology. Here, we use an interfacial engineering strategy to construct a nanostructural photoelectrochemical catalyst by incorporating a semiconductor CdS/CdSe-MoS₂ and NiFe layered double hydroxide for the oxygen evolution reaction. Impressively, the as-prepared photoelectrode requires an low potential of 1.001 V vs. reversible hydrogen electrode for a photocurrent density of 10 mA cm^-2, and this is 228 mV lower than the theoretical water splitting potential (1.229 vs. reversible hydrogen electrode). Additionally, the generated current density (15 mA cm^-2) of the photoelectrode at a given overpotential of 0.2 V remains at 95% after long-term testing (100 h). Operando X-ray absorption spectroscopy revealed that the formation of highly oxidized Ni species under illumination provides large photocurrent gains. This finding opens an avenue for designing high-efficiency photoelectrochemical catalysts for successive water splitting.

Introducing High-Valence Iridium Single Atoms into Bimetal Phosphides toward High-Efficiency Oxygen Evolution and Overall Water Splitting

Single atoms are superior electrocatalysts having high atomic utilization and amazing activity for water oxidation and splitting. Herein, this work reports a thermal reduction method to introduce high-valence iridium (Ir) single atoms into bimetal phosphide (FeNiP) nanoparticles toward high-efficiency oxygen evolution reaction (OER) and overall water splitting. The presence of high-valence single Ir atoms (Ir⁴⁺ ) and their synergistic interaction with Ni³⁺ species as well as the disproportionation of Ni³⁺ assisted by Fe collectively contribute to the exceptional OER performance. In specific, at appropriate Ir/Ni and Fe/Ni ratios, the as-prepared Ir-doped FeNiP (Ir₂₅ -Fe₁₆ Ni₁₀₀ P₆₄ ) nanoparticles at a mass loading of only 35 µg cm^-2 show the overpotential as low as 232 mV at 10 mA cm^-2 and activity as high as 1.86 A mg^-1 at 1.5 V versus RHE for OER in 1.0 m KOH. Computational simulations confirm the vital role of high-valence Ir to weaken the adsorption of OER intermediates, favorable for accelerating OER kinetics. Impressively, a Pt/C||Ir₂₅ -Fe₁₆ Ni₁₀₀ P₆₄ two-electrode alkaline electrolyzer affords a current density of 10 mA cm^-2 at a low cell voltage of 1.42 V, along with satisfied stability. An AA battery with a nominal voltage of 1.5 V can drive overall water splitting with obvious bubbles released.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Recent Advances of Transition Metal Basic Salts for Electrocatalytic Oxygen Evolution Reaction and Overall Water Electrolysis

Electrocatalytic oxygen evolution reaction (OER) has been recognized as the bottleneck of overall water splitting, which is a promising approach for sustainable production of H2. Transition metal (TM) hydroxides are the most conventional and classical non-noble metal-based electrocatalysts for OER, while TM basic salts [M2+(OH)2-x(Am-)x/m, A = CO32-, NO3-, F-, Cl-] consisting of OH- and another anion have drawn extensive research interest due to its higher catalytic activity in the past decade. In this review, we summarize the recent advances of TM basic salts and their application in OER and further overall water splitting. We categorize TM basic salt-based OER pre-catalysts into four types (CO32-, NO3-, F-, Cl-) according to the anion, which is a key factor for their outstanding performance towards OER. We highlight experimental and theoretical methods for understanding the structure evolution during OER and the effect of anion on catalytic performance. To develop bifunctional TM basic salts as catalyst for the practical electrolysis application, we also review the present strategies for enhancing its hydrogen evolution reaction activity and thereby improving its overall water splitting performance. Finally, we conclude this review with a summary and perspective about the remaining challenges and future opportunities of TM basic salts as catalysts for water electrolysis.

Non-covalent ligand-oxide interaction promotes oxygen evolution

Strategies to generate high-valence metal species capable of oxidizing water often employ composition and coordination tuning of oxide-based catalysts, where strong covalent interactions with metal sites are crucial. However, it remains unexplored whether a relatively weak "non-bonding" interaction between ligands and oxides can mediate the electronic states of metal sites in oxides. Here we present an unusual non-covalent phenanthroline-CoO₂ interaction that substantially elevates the population of Co⁴⁺ sites for improved water oxidation. We find that phenanthroline only coordinates with Co²⁺ forming soluble Co(phenanthroline)₂(OH)₂ complex in alkaline electrolytes, which can be deposited as amorphous CoO_xH_y film containing non-bonding phenanthroline upon oxidation of Co²⁺ to Co^3+/4+. This in situ deposited catalyst demonstrates a low overpotential of 216 mV at 10 mA cm^-2 and sustainable activity over 1600 h with Faradaic efficiency above 97%. Density functional theory calculations reveal that the presence of phenanthroline can stabilize CoO₂ through the non-covalent interaction and generate polaron-like electronic states at the Co-Co center.

Zhang-Rice singlets state formed by two-step oxidation for triggering water oxidation under operando conditions

The production of ecologically compatible fuels by electrochemical water splitting is highly desirable for modern industry. The Zhang-Rice singlet is well known for the superconductivity of high-temperature superconductors cuprate, but is rarely known for an electrochemical catalyst. Herein, we observe two steps of surface reconstruction from initial catalytic inactive Cu¹⁺ in hydrogen treated Cu₂O to Cu²⁺ state and further to catalytic active Zhang-Rice singlet state during the oxygen evolution reaction for water splitting. The hydrogen treated Cu₂O catalyst exhibits a superior catalytic activity and stability for water splitting and is an efficient rival of other 3d-transition-metal catalysts. Multiple operando spectroscopies indicate that Zhang-Rice singlet is real active species, since it appears only under oxygen evolution reaction condition. This work provides an insight in developing an electrochemical catalyst from catalytically inactive materials and improves understanding of the mechanism of a Cu-based catalyst for water oxidation.

Synthesis of amorphous trimetallic PdCuNiP nanoparticles for enhanced OER

Metal phosphides with multi-element components and amorphous structure represent a novel kind of electrocatalysts for promising activity and durability towards the oxygen evolution reaction (OER). In this work, a two-step strategy, including alloying and phosphating processes, is reported to synthesize trimetallic amorphous PdCuNiP phosphide nanoparticles for efficient OER under alkaline conditions. The synergistic effect between Pd, Cu, Ni, and P elements, as well as the amorphous structure of the obtained PdCuNiP phosphide nanoparticles, would boost the intrinsic catalytic activity of Pd nanoparticles towards a wide range of reactions. These obtained trimetallic amorphous PdCuNiP phosphide nanoparticles exhibit long-term stability, nearly a 20-fold increase in mass activity toward OER compared with the initial Pd nanoparticles, and 223 mV lower in overpotential at 10 mA cm^-2. This work not only provides a reliable synthetic strategy for multi-metallic phosphide nanoparticles, but also expands the potential applications of this promising class of multi-metallic amorphous phosphides.

Reversible and Irreversible Cation Intercalation in NiFeOx Oxygen Evolution Catalysts in Alkaline Media

For electrocatalysts with a layered structure, ion intercalation is a common phenomenon. Gaining reliable information about the intercalation of ions from the electrolyte is indispensable for a better understanding of the catalytic performance of these electrocatalysts. Here, we take a holistic approach for following intercalation processes by studying the dynamics of the catalyst, water molecules, and ions during intercalation using operando soft X-ray absorption spectroscopy (XAS). Sodium and oxygen K-edge and nickel L-edge spectra were used to investigate the Na⁺ intercalation in a Ni_0.8Fe_0.2O_x electrocatalyst during the oxygen evolution reaction (OER) in NaOH (0.1 M). The Na K-edge spectra show an irreversible intensity increase upon initial potential cycling and a reversible intensity increase at the intercalation potential, 1.45 V_RHE, coinciding with an increase in the Ni oxidation state. Simultaneously, the O K-edge spectra show that the Na⁺ intercalation does not significantly impact the hydration of the catalyst.

Etching-Induced Surface Reconstruction of NiMoO4 for Oxygen Evolution Reaction

Rational reconstruction of oxygen evolution reaction (OER) pre-catalysts and performance index of OER catalysts are crucial but still challenging for universal water electrolysis. Herein, we develop a double-cation etching strategy to tailor the electronic structure of NiMoO4, where the prepared NiMoO4 nanorods etched by H2O2 reconstruct their surface with abundant cation deficiencies and lattice distortion. Calculation results reveal that the double cation deficiencies can make the upshift of d-band center for Ni atoms and the active sites with better oxygen adsorption capacity. As a result, the optimized sample (NMO-30M) possesses an overpotential of 260 mV at 10 mA cm-2 and excellent long-term durability of 162 h. Importantly, in situ Raman test reveals the rapid formation of high-oxidation-state transition metal hydroxide species, which can further help to improve the catalytic activity of NiMoO4 in OER. This work highlights the influence of surface remodification and shed some light on activating catalysts.

Transformation mechanism of high-valence metal sites for the optimization of Co- and Ni-based OER catalysts in an alkaline environment: recent progress and perspectives

As an important semi-reaction process in electrocatalysis, oxygen evolution reaction (OER) is closely associated with electrochemical hydrogen production, CO₂ electroreduction, electrochemical ammonia synthesis and other reactions, which provide electrons and protons for the related applications. Considering their fundamental mechanism, metastable high-valence metal sites have been identified as real, efficient OER catalytic sites from the recent observation by in situ characterization technology. Herein, we review the transformation mechanism of high-valence metal sites in the OER process, particularly transition metal materials (Co- and Ni-based). In particular, research progress in the transformation process and role of high-valence metal sites to optimize OER performance is summarized. The key challenges and prospects of the design of high-efficiency OER catalysts based on the above-mentioned mechanism and some new in situ characterizations are also discussed.

A Template Editing Strategy to Create Interlayer-Confined Active Species for Efficient and Durable Oxygen Evolution Reaction

Substantial overpotentials and insufficient and unstable active sites of oxygen evolution reaction (OER) electrocatalysts limit their efficiency and stability in OER-related energy conversion and storage technologies. Here, a template editing strategy is proposed to graft highly active catalytic species onto highly conductive rigid frameworks to tackle this challenge. As a successful attempt, two types of NiO₆ units of layered Ni BDC (BDC stands for 1,4-benzenedicarboxylic acid) metal organic frameworks are selectively edited by chemical etching-assisted electroxidation to create layered γ-NiOOH with intercalated Ni-O species. In such an interlayer-confined intercalated architecture, the large interlayer space with high ion permeability offers an ideal reaction region to sufficiently expose the OER active sites comprising high-density intercalated Ni-O species, which also suppresses the undesirable γ to β phase transformation, thus exhibiting efficient and durable OER activity. As a result, water oxidation can occur at an extremely low overpotential of 130 mV and affords 1000 h stability at 100 mA cm^-2 . The strategy conceptually shows the possibility of achieving stable homogeneous-like catalysis in heterogeneous catalysis.

Surface Reconstruction of an FeNi Foam Substrate for Efficient Oxygen Evolution

Designing earth-abundant electrocatalysts that are highly active, low-cost, and stable for the oxygen evolution reaction (OER) is crucial for electrochemical water splitting. However, in conventional electrode fabrication strategies, NiFe layered double hydroxide (NiFe LDH) catalysts are usually coated onto substrates as external components, which suffers from poor conductivity, easily detaches from the substrate, and hinders their long-term utilization. Herein, the surface-reconstruction strategy is used to synthesize in situ autologous NiFe LDH to increase the surficial active sites numbers. The FeNi foam (FNF) serves as both the metal source and substrate, and the obtained NiFe LDH nanosheets (NSs) are firmly anchored in the monolithic FNF. What needs to be emphasized is that the strategy does not involve any high-temperature or high-pressure processes, apart from a cost-effective etching and a specified drying treatment. The nanostructure of NiFe LDH and the synergistic effect between Fe and Ni simultaneously lead to an enhanced catalytic effect for the OER. Remarkably, the sr-FNF46 requires only an ultralow overpotential of 283 mV to achieve a current density of 100 mA cm^-2 for the OER in 1 M KOH electrolyte, and exhibits excellent stability. Thus, the obtained electrode holds promise for electrocatalytic applications. Finally, the formation mechanism of NiFe LDH NSs due to surface reconstruction is investigated and discussed in detail.

Structural Reconstruction of Catalysts in Electroreduction Reaction: Identifying, Understanding, and Manipulating

Electroreduction transformation of small molecules (CO₂ , N₂ , and H₂ O) into chemical feedstocks offers a promising approach to eliminate carbon emissions and harness renewable energy. Most cathodic catalysts often undergo structural transformation under operating electroreduction conditions. These structural reconstructions are reflected in changes in their catalytic activity. In-depth understanding of the change of active sites and influence parameters of reconstruction behaviors is an essential precondition for the design of highly efficient catalysts. Despite the previous achievements, comprehensive insight toward the structural evolution mechanism in cathodic catalysts, compared to anode ones, under reaction conditions is still lacking. Herein, an overview of structural reconstruction for cathodic catalysts in terms of fundamental mechanisms, reconstruction process, advanced characterizations, and influencing parameters is provided. On this basis, the typical strategies for manipulating the structural reconfiguration of catalysts are also explicitly discussed from the catalyst structure and working environment. By delivering the mechanism, strategies, insights, and techniques, this review will provide a comprehensive understanding of the structural reconstruction of cathodic catalysts in electroreduction reactions and future guidelines for their rational development.

Oxygen Evolution Activity of Amorphous Cobalt Oxyhydroxides: Interconnecting Precatalyst Reconstruction, Long-Range Order, Buffer-Binding, Morphology, Mass Transport, and Operation Temperature

Nanocrystalline or amorphous cobalt oxyhydroxides (CoCat) are promising electrocatalysts for the oxygen evolution reaction (OER). While having the same short-range order, CoCat phases possess different electrocatalytic properties. This phenomenon is not conclusively understood, as multiple interdependent parameters affect the OER activity simultaneously. Herein, a layered cobalt borophosphate precatalyst, Co(H₂ O)₂ [B₂ P₂ O₈ (OH)₂ ]·H₂ O, is fully reconstructed into two different CoCat phases. In contrast to previous reports, this reconstruction is not initiated at the surface but at the electrode substrate to catalyst interface. Ex situ and in situ investigations of the two borophosphate derived CoCats, as well as the prominent CoP_i and CoB_i identify differences in the Tafel slope/range, buffer binding and content, long-range order, number of accessible edge sites, redox activity, and morphology. Considering and interconnecting these aspects together with proton mass-transport limitations, a comprehensive picture is provided explaining the different OER activities. The most decisive factors are the buffers used for reconstruction, the number of edge sites that are not inhibited by irreversibly bonded buffers, and the morphology. With this acquired knowledge, an optimized OER system is realized operating in near-neutral potassium borate medium at 1.62 ± 0.03 V_RHE yielding 250 mA cm^-2 at 65 °C for 1 month without degrading performance.

Orbital Occupancy and Spin Polarization: From Mechanistic Study to Rational Design of Transition Metal-Based Electrocatalysts toward Energy Applications

Over the past few decades, development of electrocatalysts for energy applications has extensively transitioned from trial-and-error methodologies to more rational and directed designs at the atomic levels via either nanogeometric optimization or modulating electronic properties of active sites. Regarding the modulation of electronic properties, nonprecious transition metal-based materials have been attracting large interest due to the capability of versatile tuning d-electron configurations expressed through the flexible orbital occupancy and various possible degrees of spin polarization. Herein, recent advances in tailoring electronic properties of the transition-metal atoms for intrinsically enhanced electrocatalytic performances are reviewed. We start with discussions on how orbital occupancy and spin polarization can govern the essential atomic level processes, including the transport of electron charge and spin in bulk, reactive species adsorption on the catalytic surface, and the electron transfer between catalytic centers and adsorbed species as well as reaction mechanisms. Subsequently, different techniques currently adopted in tuning electronic structures are discussed with particular emphasis on theoretical rationale and recent practical achievements. We also highlight the promises of the recently established computational design approaches in developing electrocatalysts for energy applications. Lastly, the discussion is concluded with perspectives on current challenges and future opportunities. We hope this review will present the beauty of the structure-activity relationships in catalysis sciences and contribute to advance the rational development of electrocatalysts for energy conversion applications.

Doping Mo into NiFe LDH/NiSe Heterostructure to Enhance Oxygen Evolution Activity by Synergistically Facilitating Electronic Modulation and Surface Reconstruction

It is of great significance to design highly efficient electrocatalysts with abundant earth elements instead of precious metals for water splitting. Herein, Mo-doped NiFe-layered double hydroxides/NiSe heterostructure (Mo-NiFe LDH/NiSe) was fabricated by coupling Mo-doped NiFe LDH and NiSe on nickel foam (NF). The heterostructure electrocatalyst showed ultra-low overpotential (250 mV) and remarkable durability for oxygen evolution reaction (OER) at 150 mA cm^-2 . Both theoretical and experimental results confirmed that Mo doping and interfacial synergism induced the interfacial charge redistribution and the lifted d-band center to weaken the energy barrier (EB) of the formation of OOH^* . Mo doping also facilitated the surface reconstruction of NiFe LDH into Ni(Fe)OOH as the active sites under electro-oxidation process. This work provides a facile strategy for electronic modulation and surface reconstruction of OER electrocatalyst by transition metal doping and heterostructure generation.

Promoting nickel oxidation state transitions in single-layer NiFeB hydroxide nanosheets for efficient oxygen evolution

Promoting the formation of high-oxidation-state transition metal species in a hydroxide catalyst may improve its catalytic activity in the oxygen evolution reaction, which remains difficult to achieve with current synthetic strategies. Herein, we present a synthesis of single-layer NiFeB hydroxide nanosheets and demonstrate the efficacy of electron-deficient boron in promoting the formation of high-oxidation-state Ni for improved oxygen evolution activity. Raman spectroscopy, X-ray absorption spectroscopy, and electrochemical analyses show that incorporation of B into a NiFe hydroxide causes a cathodic shift of the Ni²⁺(OH)₂ → Ni^3+δOOH transition potential. Density functional theory calculations suggest an elevated oxidation state for Ni and decreased energy barriers for the reaction with the NiFeB hydroxide catalyst. Consequently, a current density of 100 mA cm^–2 was achieved in 1 M KOH at an overpotential of 252 mV, placing it among the best Ni-based catalysts for this reaction. This work opens new opportunities in electronic engineering of metal hydroxides (or oxides) for efficient oxygen evolution in water-splitting applications.

While water-splitting electrolysis offers a potential renewable means to store energy, the oxygen evolution half-reaction’s sluggish kinetics limits performances. Here, authors incorporation boron into nickel-iron hydroxide catalysts to promote electrocatalytic water oxidation activities

High-Performance RuOx Catalyst with Advanced Mesoporous Structure for Oxygen Evolution Reaction

Polymer electrolyte membrane water electrolysis (PEMWE) is regarded as one of the most important cornerstone technologies in the upcoming hydrogen society. However, one of the major problems it encounters is its slow oxygen evolution kinetics, which necessitates the use of large amounts of precious metal catalysts to ensure a satisfactory reaction rate. Herein, we have prepared a series of RuO_x with porous structures and ultrahigh Ru utilization toward the oxygen evolution reaction. All porous samples exhibit an enhanced catalytic performance compared with commercial RuO_x. Particularly, for the RuO_x-350 sample, the overpotential to reach 10 mA cm^-2 is as low as 225 mV. It has obvious advantages among all reported pure RuO₂-based catalysts. Here, a new strategy was raised to construct efficient RuO₂ electrocatalysts with outstanding activity and stability for water electrolysis technology.

Optimized NiFe-Based Coordination Polymer Catalysts: Sulfur-Tuning and Operando Monitoring of Water Oxidation

In-depth insights into the structure-activity relationships and complex reaction mechanisms of oxygen evolution reaction (OER) electrocatalysts are indispensable to efficiently generate clean hydrogen through water electrolysis. We introduce a convenient and effective sulfur heteroatom tuning strategy to optimize the performance of active Ni and Fe centers embedded into coordination polymer (CP) catalysts. Operando monitoring then provided the mechanistic understanding as to how exactly our facile sulfur engineering of Ni/Fe-CPs optimizes the local electronic structure of their active centers to facilitate dioxygen formation. The high OER activity of our optimized S-R-NiFe-CPs outperforms the most recent NiFe-based OER electrocatalysts. Specifically, we start from oxygen-deprived O_d-R-NiFe-CPs and transform them into highly active Ni/Fe-CPs with tailored sulfur coordination environments and anionic deficiencies. Our operando X-ray absorption spectroscopy analyses reveal that sulfur introduction into our designed S-R-NiFe-CPs facilitates the formation of crucial highly oxidized Ni⁴⁺ and Fe⁴⁺ species, which generate oxygen-bridged Ni^IV-O-Fe^IV moieties that act as the true OER active intermediates. The advantage of our sulfur-doping strategy for enhanced OER is evident from comparison with sulfur-free O_d-R-NiFe-CPs, where the formation of essential high-valent OER intermediates is hindered. Moreover, we propose a dual-site mechanism pathway, which is backed up with a combination of pH-dependent performance data and DFT calculations. Computational results support the benefits of sulfur modulation, where a lower energy barrier enables O-O bond formation atop the S-Ni^IV-O-Fe^IV-O moieties. Our convenient anionic tuning strategy facilitates the formation of active oxygen-bridged metal motifs and can thus promote the design of flexible and low-cost OER electrocatalysts.

Uncovering the Nature of Active Sites during Electrocatalytic Reactions by In Situ Synchrotron-Based Spectroscopic Techniques

Catalysts can effectively accelerate the reaction kinetics process and are recognized as the core to realize the conversion and supply of carbon-free energy. However, the active sites of catalysts, especially nanocatalysts, usually undergo dynamic structural evolution under realistic working conditions, which may be induced by various reaction effects such as the applied voltages, electrolytes, or adsorbed intermediates. Therefore, in-depth and systemic insights into the nature of the active sites involved under the working conditions are prerequisites for correlating structure-performance relationships. However, uncovering and identifying active sites under operation conditions are still formidable scientific and technical challenges, which are severely hindered by the complex physical and chemical processes occurring on the active sites. Meanwhile, complementary and important information could be missed by conducting only the conventionally employed ex situ microscopic and spectroscopic measurements. Accordingly, it is highly desirable for us to develop the ever-increasing in situ synchrotron-based techniques to identify the nature of active sites, which renders the rational design of functional catalysts achievable.In this Account, we elaborately highlight the substantial achievements in cutting-edge in situ X-ray spectroscopy (XAS) techniques by presenting several representative carbon-neutral electrocatalytic examples performed in our group to broadcast the principles and virtues of identifying the active sites and tracing intermediate species during electrocatalytic water splitting and electrocatalytic CO₂ reduction (ECR). Specifically, we believe that the interactions between the active sites and the support as well as the adsorption behaviors of intermediates are considered to be the important factors that govern the performance in the water splitting reaction. Meanwhile, the structural rearrangement of alloy catalysts driven by the cathodic potential significantly governs the activity and selectivity toward ECR. More importantly, the directions and suggestions for addressing the current limitations and pitfalls that we may encounter in the course of executing in situ experiments are also provided. Accordingly, it is necessary to use multiple in situ synchrotron-based techniques to obtain the comprehensive details. Furthermore, bridging the gap between the real energy devices and half-reactions could help us to approach the realistic mechanism. Beyond that, developing the rapid time resolution of in situ XAS will overcome the challenge of timescale mismatch to capture the faster structural kinetics of catalysts. Therefore, this Account is aimed to increase the awareness and appreciation of conducting in situ investigations on energy conversion reactions, which would be a guideline for us to explore catalytic scopes that remain challenging.

Boosting Oxygen-Evolving Activity via Atom-Stepped Interfaces Architected with Kinetic Frustration

A highly active interface is extremely critical for the catalytic efficiency of an electrocatalyst; however, facilely tailoring its atomic packing characteristics remains challenging. Herein, a simple yet effective strategy is reported to obtain copious high-energy atomic steps at the interface via controlling the solidification behavior of glass-forming metallic liquids. By adjusting the chemical composition and cooling rate, highly faceted FeNi₃ nanocrystals are in situ formed in an FeNiB metallic glass (MG) matrix, leading to the creation of order/disorder interfaces. Benefiting from the catalytically active and stable atomic steps at the jagged interfaces, the resultant free-standing FeNi₃ nanocrystal/MG composite exhibits a low oxygen-evolving overpotential of 214 mV at 10 mA cm^-2 , a small Tafel slope of 32.4 mV dec^-1 , and good stability in alkaline media, outperforming most state-of-the-art catalysts. This approach is based on the manipulation of nucleation and crystal growth of the solid-solution nanophases (e.g., FeNi₃ ) in glass-forming liquids, so that the highly stepped interface architecture can be obtained due to the kinetic frustration effect in MGs upon undercooling. It is envisaged that the atomic-level stepped interface engineering via the physical metallurgy method can be easily extended to other MG systems, providing a new and generic paradigm for designing efficient yet cost-effective electrocatalysts.