Modifying redox properties and local bonding of Co3O4 by CeO2 enhances oxygen evolution catalysis in acid

Ce4+/Ce3+ Redox Effect-Promoted CdS/CeO2 Heterojunction Photocatalyst for the Atom Economic Synthesis of Imines under Visible Light

The employment of stoichiometric alcohols and amines for imine synthesis under mild and green reaction conditions is still a challenge in the field. In this work, based on our research foundation in the thermocatalytic synthesis of imines over ceria, a CdS/CeO2 heterojunction photocatalyst was constructed and successfully realized the atom-economic synthesis of imines under visible light without additives at room temperature. Mechanistic experiments and corresponding characterizations indicated that the CdS/CeO2 heterojunction can improve the separation efficiency of photogenerated carriers, which can be further enhanced by the Ce4+/Ce3+ redox pair by rapidly combining photogenerated e-. The in situ-reduced Ce3+ can better activate O2 to form Ce-O-O·, which, together with h+, efficiently accelerates alcohol oxidation, which is the rate-determined step for the synthesis of imines via oxidative coupling reaction of alcohol and amine. In addition, our photocatalyst exhibited fairly decent reusability and substrate universality. This work solves problems of using base additives and excess amine or alcohol in the reported photocatalytic systems and provides new insight for designing CeO2-based photocatalytic oxidation catalysts.

Ir-Sn pair-site triggers key oxygen radical intermediate for efficient acidic water oxidation

The anode corrosion induced by the harsh acidic and oxidative environment greatly restricts the lifespan of catalysts. Here, we propose an antioxidation strategy to mitigate Ir dissolution by triggering strong electronic interaction via elaborately constructing a heterostructured Ir-Sn pair-site catalyst. The formation of Ir-Sn dual-site at the heterointerface and the resulting strong electronic interactions considerably reduce d-band holes of Ir species during both the synthesis and the oxygen evolution reaction processes and suppress their overoxidation, enabling the catalyst with substantially boosted corrosion resistance. Consequently, the optimized catalyst exhibits a high mass activity of 4.4 A mgIr-1 at an overpotential of 320 mV and outstanding long-term stability. A proton-exchange-membrane water electrolyzer using this catalyst delivers a current density of 2 A cm-2 at 1.711 V and low degradation in an accelerated aging test. Theoretical calculations unravel that the oxygen radicals induced by the π* interaction between Ir 5d-O 2p might be responsible for the boosted activity and durability.

Strain-Induced Electronic Structure Modulation on MnO2 Nanosheet by Ir Incorporation for Efficient Water Oxidation in Acid

Oxygen electrochemistry plays a key role in renewable energy technologies, such as fuel cells and electrolyzers, but its slow kinetics limits the performance and the commercialization of such devices. Here, a strained MnO2 nanosheet induced by Ir incorporation is developed with optimized electronic structure by a simple hydrothermal method. With the incorporation of Ir, the strain induces elongated Mn─O bond length, and thereby tuning the electronic structure to favor the oxygen evolution reaction (OER) performance. The obtained catalyst exhibits an excellent mass activity of 5681 A g-1 at an overpotential of 300 mV in 0.5 m H2 SO4 , and reaches 50 and 100 mA cm-2 at overpotentials of only 240 and 277 mV, respectively. The catalyst is also stable even at 300 mA cm-2 in 0.5 m H2 SO4 . Using the nanosheet as the OER catalyst and the Pt/C as the hydrogen evolution reaction catalyst, a two-electrode electrolyzer achieves 10 mA cm-2 with only a cell voltage of 1.453 V for overall water splitting in 0.5 m H2 SO4 . This strategy enables the material with high feasibility for practical applications on hydrogen production.

Unraveling and leveraging in situ surface amorphization for enhanced hydrogen evolution reaction in alkaline media

Surface amorphization provides electrocatalysts with more active sites and flexibility. However, there is still a lack of experimental observations and mechanistic explanations for the in situ amorphization process and its crucial role. Herein, we propose the concept that by in situ reconstructed amorphous surface, metal phosphorus trichalcogenides could intrinsically offer better catalytic performance for the alkaline hydrogen production. Trace Ru (0.81 wt.%) is doped into NiPS3 nanosheets for alkaline hydrogen production. Using in situ electrochemical transmission electron microscopy technique, we confirmed the amorphization process occurred on the edges of NiPS3 is critical for achieving superior activity. Comprehensive characterizations and theoretical calculations reveal Ru primarily stabilized at edges of NiPS3 through in situ formed amorphous layer containing bridging S22- species, which can effectively reduce the reaction energy barrier. This work emphasizes the critical role of in situ formed active layer and suggests its potential for optimizing catalytic activities of electrocatalysts.

Rare-Earth-Modified Metal-Organic Frameworks and Derivatives for Photo/Electrocatalysis

Metal-organic frameworks (MOFs) and their derivatives have attracted much attention in the field of photo/electrocatalysis owing to their ultrahigh porosity, tunable properties, and superior coordination ability. Regulating the valence electronic structure and coordination environment of MOFs is an effective way to enhance their intrinsic catalytic performance. Rare earth (RE) elements with 4f orbital occupancy provide an opportunity to evoke electron rearrangement, accelerate charged carrier transport, and synergize the surface adsorption of catalysts. Therefore, the integration of RE with MOFs makes it possible to optimize their electronic structure and coordination environment, resulting in enhanced catalytic performance. In this review, progress in current research on the use of RE-modified MOFs and their derivatives for photo/electrocatalysis is summarized and discussed. First, the theoretical advantages of RE in MOF modification are introduced, with a focus on the roles of 4f orbital occupancy and RE ion organic coordination ligands. Then, the application of RE-modified MOFs and their derivatives in photo/electrocatalysis is systematically discussed. Finally, research challenges, future opportunities, and prospects for RE-MOFs are also discussed.

Phosphorus-Modulated Generation of Defective Molybdenum Sites as Synergistic Active Centers for Durable Oxygen Evolution

It is well known that electrocatalytic oxygen evolution reaction (OER) activities primarily depend on the active centers of electrocatalysts. In some oxide electrocatalysts, high-valence metal sites (e.g., molybdenum oxide) are generally not the real active centers for electrocatalytic reactions, which is largely due to their undesired intermediate adsorption behaviors. As a proof-of-concept, molybdenum oxide catalysts are selected as a representative model, in which the intrinsic molybdenum sites are not the favorable active sites. Via phosphorus-modulated defective engineering, the inactive molybdenum sites can be regenerated as synergistic active centers for promoting OER. By virtue of comprehensive comparison , it is revealed that the OER performance of oxide catalysts is highly associated with the phosphorus sites and the molybdenum/oxygen defects. Specifically, the optimal catalyst delivers an overpotential of 287 mV to achieve the current density of 10 mA cm-2 , accompanied by only 2% performance decay for continuous operation up to 50 h. It is expected that this work sheds light on the enrichment of metal active sites via activating inert metal sites on oxide catalysts for boosting electrocatalytic properties.

Transition metal doped WSi2N4monolayer for water splitting electrocatalysts: a first-principles study

High-performance water splitting electrocatalysts are urgently needed in the face of the environmental degradation and energy crisis. The first principles method was used in this study to systematically examine the electronic characteristics of transition metal (Sc, Ti, V, Cr, Mn, Fe, and Ru) doped WSi2N4(TM@WSi2N4) and its potential as oxygen evolution reaction (OER) catalysts. Our study shows that the doping of TM atoms significantly improves the catalytic performance of TM@WSi2N4, especially Fe@WSi2N4shows a low overpotential (ηOER= 470 mV). Interestingly, we found that integrated-crystal orbital Hamilton population and d-band center can be used as descriptors to explain the high catalytic activity of Fe@WSi2N4. Subsequently, Fe@WSi2N4exhibits the best hydrogen evolution reaction (HER) activity with a universal overpotential of 47 mV on N1sites. According to our research, Fe@WSi2N4offers a promising substitute for precious metals as a catalyst for overall water splitting with low OER and HER overpotentials.

Electrocatalytic Mechanism of Defect in Spinels for Water and Organics Oxidation

Spinels display promising electrocatalytic ability for oxygen evolution reaction (OER) and organics oxidation reaction because of flexible structure, tunable component, and multifold valence. Unfortunately, limited exposure of active sites, poor electronic conductivity, and low intrinsic ability make the electrocatalytic performance of spinels unsatisfactory. Defect engineering is an effective method to enhance the intrinsic ability of electrocatalysts. Herein, the recent advances in defect spinels for OER and organics electrooxidation are reviewed. The defect types that exist in spinels are first introduced. Then the catalytic mechanism and dynamic evolution of defect spinels during the electrochemical process are summarized in detail. Finally, the challenges of defect spinel electrocatalysts are brought up. This review aims to deepen the understanding about the role and evolution of defects in spinel for electrochemical water/organics oxidation and provide a significant reference for the design of efficient defect spinel electrocatalysts.

Symmetrical-waveform alternating current-promoted NiOxHy electrocatalysis for the oxygen evolution reaction

Here we describe a symmetrical waveform alternating current strategy that provides a solution for obtaining gradient oxygen vacancies (VO) in situ. The unique gradient VO provides multiple stairs to reduce the reaction kinetics and thus contributes to a total increase of up to 84.7% in current density.

Dense Crystalline/Amorphous Phosphides/Oxides Interfacial Sites for Enhanced Industrial-Level Large Current Density Seawater Oxidation

Designing high-efficiency and low-cost catalysts with high current densities for the oxygen evolution reaction (OER) is critical for commercial seawater electrolysis. Here, we present a heterophase synthetic strategy for constructing an electrocatalyst with dense heterogeneous interfacial sites among crystalline Ni2P, Fe2P, CeO2, and amorphous NiFeCe oxides on nickel foam (NF). The synergistic effect of high-density crystalline and amorphous heterogeneous interfaces effectively promotes the redistribution of the charge density and optimizes the adsorbed oxygen intermediates, lowering the energy barrier and promoting the O2 desorption, thus enhancing the OER performance. The obtained NiFeO-CeO2/NF catalyst exhibited outstanding OER catalytic activity, with low overpotentials of 338 and 408 mV required to attain high current densities of 500 and 1000 mA cm-2, respectively, in alkaline natural seawater electrolytes. The solar-driven seawater electrolysis system presents a record-setting and stable solar-to-hydrogen conversion efficiency of 20.10%. This work provides directives for developing highly effective and stable catalysts for large-scale clean energy production.

Direct Dioxygen Radical Coupling Driven by Octahedral Ruthenium-Oxygen-Cobalt Collaborative Coordination for Acidic Oxygen Evolution Reaction

The acidic oxygen evolution reaction (OER) has long been the bottleneck of proton exchange membrane water electrolyzers given its harsh oxidative and corrosive environments. Herein, we suggest an effective strategy to greatly enhance both the acidic OER activity and stability of Co3O4 spinel by atomic Ru selective substitution on the octahedral Co sites. The resulting highly symmetrical octahedral Ru-O-Co collaborative coordination with strong electron coupling effect enables the direct dioxygen radical coupling OER pathway. Indeed, both experiments and theoretical calculations reveal a thermodynamically breakthrough heterogeneous diatomic oxygen mechanism. Additionally, the active Ru-O-Co units are well-maintained upon the acidic OER thanks to the electron transfer from surrounding electron-enriched tetrahedral Co atoms via bridging oxygen bonds that suppresses the overoxidation and thus dissolution of active Ru and Co species. Consequently, the prepared catalyst, even with a low Ru mass loading of ca. 42.8 μg cm-2, exhibits an attractive acidic OER performance with a low overpotential of 200 mV and a low potential decay rate of 0.45 mV h-1 at 10 mA cm-2. Our work suggests an effective strategy to significantly enhance both the acidic OER activity and stability of low-cost electrocatalysts.

Breaking the Activity and Stability Bottlenecks of Electrocatalysts for Oxygen Evolution Reactions in Acids

Oxygen evolution reaction (OER) is a cornerstone reaction for a variety of electrochemical energy conversion and storage systems such as water splitting, CO2 /N2 reduction, reversible fuel cells, and metal-air batteries. However, OER catalysis in acids suffers from extra sluggish kinetics due to the additional step of water dissociation along with its multiple electron transfer processes. Furthermore, OER catalysts often suffer from poor stability in harsh acidic electrolytes due to the severe dissolution/corrosion processes. The development of active and stable OER catalysts in acids is highly demanded. Here,  the recent advances in OER electrocatalysis in acids are reviewed and the key strategies are summarized to overcome the bottlenecks of activity and stability for both noble-metal-based and noble metal-free catalysts, including i) morphology engineering, ii) composition engineering, and iii) defect engineering. Recent achievements in operando characterization and theoretical calculations are summarized which provide an unprecedented understanding of the OER mechanisms regarding active site identification, surface reconstruction, and degradation/dissolution pathways. Finally, views are offered on the current challenges and opportunities to break the activity-stability relationships for acidic OER in mechanism understanding, catalyst design, as well as standardized stability and activity evaluation for industrial applications such as proton exchange membrane water electrolyzers and beyond.

Reconstructed Ir‒O‒Mo species with strong Brønsted acidity for acidic water oxidation

Surface reconstruction generates real active species in electrochemical conditions; rational regulating reconstruction in a targeted manner is the key for constructing highly active catalyst. Herein, we use the high-valence Mo modulated orthorhombic Pr₃Ir_1-xMo_xO₇ as model to activate lattice oxygen and cations, achieving directional and accelerated surface reconstruction to produce self-terminated Ir‒O_bri‒Mo (O_bri represents the bridge oxygen) active species that is highly active for acidic water oxidation. The doped Mo not only contributes to accelerated surface reconstruction due to optimized Ir‒O covalency and more prone dissolution of Pr, but also affords the improved durability resulted from Mo-buffered charge compensation, thereby preventing fierce Ir dissolution and excessive lattice oxygen loss. As such, Ir‒O_bri‒Mo species could be directionally generated, in which the strong Brønsted acidity of O_bri induced by remaining Mo assists with the facilitated deprotonation of oxo intermediates, following bridging-oxygen-assisted deprotonation pathway. Consequently, the optimal catalyst exhibits the best activity with an overpotential of 259 mV to reach 10 mA cm_geo^-2, 50 mV lower than undoped counterpart, and shows improved stability for over 200 h. This work provides a strategy of directional surface reconstruction to constructing strong Brønsted acid sites in IrO_x species, demonstrating the perspective of targeted electrocatalyst fabrication under in situ realistic reaction conditions.

Unraveling the Electronic Structure and Dynamics of the Atomically Dispersed Iron Sites in Electrochemical CO2 Reduction

Single-atom catalysts with a well-defined metal center open unique opportunities for exploring the catalytically active site and reaction mechanism of chemical reactions. However, understanding of the electronic and structural dynamics of single-atom catalytic centers under reaction conditions is still limited due to the challenge of combining operando techniques that are sensitive to such sites and model single-atom systems. Herein, supported by state-of-the-art operando techniques, we provide an in-depth study of the dynamic structural and electronic evolution during the electrochemical CO₂ reduction reaction (CO₂RR) of a model catalyst comprising iron only as a high-spin (HS) Fe(III)N₄ center in its resting state. Operando ⁵⁷Fe Mössbauer and X-ray absorption spectroscopies clearly evidence the change from a HS Fe(III)N₄ to a HS Fe(II)N₄ center with decreasing potential, CO₂- or Ar-saturation of the electrolyte, leading to different adsorbates and stability of the HS Fe(II)N₄ center. With operando Raman spectroscopy and cyclic voltammetry, we identify that the phthalocyanine (Pc) ligand coordinating the iron cation center undergoes a redox process from Fe(II)Pc to Fe(II)Pc^-. Altogether, the HS Fe(II)Pc^- species is identified as the catalytic intermediate for CO₂RR. Furthermore, theoretical calculations reveal that the electroreduction of the Pc ligand modifies the d-band center of the in situ generated HS Fe(II)Pc^- species, resulting in an optimal binding strength to CO₂ and thus boosting the catalytic performance of CO₂RR. This work provides both experimental and theoretical evidence toward the electronic structural and dynamics of reactive sites in single-Fe-atom materials and shall guide the design of novel efficient catalysts for CO₂RR.

Recent Advances in Co3O4-Based Composites: Synthesis and Application in Combustion of Methane

In recent years, it has been found that adjusting the organizational structure of Co₃O₄ through solid solution and other methods can effectively improve its catalytic performance for the oxidation of low concentration methane. Its catalytic activity is close to that of metal Pd, which is expected to replace costly noble metal catalysts. Therefore, the in-depth research on the mechanism and methods of Co₃O₄ microstructure regulation has very important academic value and economic benefits. In this paper, we reviewed the catalytic oxidation mechanism, microstructure regulation mechanism, and methods of nano-Co₃O₄ on methane gas, which provides reference for the development of high-activity Co₃O₄-based methane combustion catalysts. Through literature investigation, it is found that the surface energy state of nano-Co₃O₄ can be adjusted by loading of noble metals, resulting in the reduction of Co-O bond strength, thus accelerating the formation of reactive oxygen species chemical bonds, and improving its catalytic effect. Secondly, the use of metal oxides and non-metallic oxide carriers helps to disperse and stabilize cobalt ions, improve the structural elasticity of Co₃O₄, and ultimately improve its catalytic performance. In addition, the performance of the catalyst can be improved by adjusting the microstructure of the composite catalyst and optimizing the preparation process. In this review, we summarize the catalytic mechanism and microstructure regulation of nano-Co₃O₄ and its composite catalysts (embedded with noble metals or combined with metallic and nonmetallic oxides) for methane combustion. Notably, this review delves into the substance of measures that can be used to improve the catalytic performance of Co₃O₄, highlighting the constructive role of components in composite catalysts that can improve the catalytic capacity of Co₃O₄. Firstly, the research status of Co₃O₄ composite catalyst is reviewed in this paper. It is hoped that relevant researchers can get inspiration from this paper and develop high-activity Co₃O₄-based methane combustion catalyst.

Heteroatom-Induced Accelerated Kinetics on Nickel Selenide for Highly Efficient Hydrazine-Assisted Water Splitting and Zn-Hydrazine Battery

Hydrazine-assisted water electrolysis is a promising energy conversion technology for highly efficient hydrogen production. Rational design of bifunctional electrocatalysts, which can simultaneously accelerate hydrogen evolution reaction (HER)/hydrazine oxidation reaction (HzOR) kinetics, is the key step. Herein, we demonstrate the development of ultrathin P/Fe co-doped NiSe2 nanosheets supported on modified Ni foam (P/Fe-NiSe2) synthesized through a facile electrodeposition process and subsequent heat treatment. Based on electrochemical measurements, characterizations, and density functional theory calculations, a favorable "2 + 2" reaction mechanism with a two-step HER process and a two-step HzOR step was fully proved and the specific effect of P doping on HzOR kinetics was investigated. P/Fe-NiSe2 thus yields an impressive electrocatalytic performance, delivering a high current density of 100 mA cm-2 with potentials of - 168 and 200 mV for HER and HzOR, respectively. Additionally, P/Fe-NiSe2 can work efficiently for hydrazine-assisted water electrolysis and Zn-Hydrazine (Zn-Hz) battery, making it promising for practical application.

MOF-derived CeO2/Co3O4-Fe2O3@CC nanocomposites as highly sensitive electrochemical sensor for bisphenol a detection

A novel CeO₂/Co₃O₄-Fe₂O₃@CC electrode derived from CeCo-MOFs was developed for detecting the endocrine disruptor bisphenol A (BPA). Firstly, bimetallic CeCo-MOFs were prepared by hydrothermal method, and obtained material was calcined to form metal oxides after doping Fe element. The results suggested that hydrophilic carbon cloth (CC) modified with CeO₂/Co₃O₄-Fe₂O₃ had good conductivity and high electrocatalytic activity. By the analyses of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), the introduction of Fe increased the current response and conductivity of the sensor, greatly increasing the effective active area of the electrode. Significantly, electrochemical test proves that the prepared CeO₂/Co₃O₄-Fe₂O₃@CC had excellent electrochemical response to BPA with a low detection limit of 8.7 nM, an excellent sensitivity of 20.489 μA/μM·cm², a linear range of 0.5-30 μM, and strong selectivity. In addition, the CeO₂/Co₃O₄-Fe₂O₃@CC sensor had a high recovery rate for the detection of BPA in real tap water, lake water, soil eluent, seawater, and PET bottle samples, which showed its potential in practical applications. To sum up, the CeO₂/Co₃O₄-Fe₂O₃@CC sensor prepared in this work had excellent sensing performance, good stability and selectivity for BPA, which can be well used for the detection of BPA.

CoMoO4-CoP/NC heterostructure anchored on hollow polyhedral N-doped carbon skeleton for efficient water splitting

We report the synthesis and electrocatalytic properties of a CoMoO₄-CoP heterostructure anchored on a hollow polyhedral N-doped carbon skeleton (CoMoO₄-CoP/NC) for water-splitting applications. The preparation involved the anion exchange of MoO₄^2- to the organic ligand of ZIF-67, the self-hydrolysis of MoO₄^2-, and NaH₂PO₂ phosphating annealing. CoMoO₄ was found to enhance thermal stability and prevent active site agglomeration during annealing, while the hollow structure of CoMoO₄-CoP/NC provided a large specific surface area and high porosity that facilitated mass transport and charge transfer. The interfacial electron transfer from Co to Mo and P sites promoted the generation of electron-deficient Co sites and electron-enriched P sites, which accelerated water dissociation. CoMoO₄-CoP/NC exhibited excellent electrocatalytic activity for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1.0 M KOH solution, with overpotentials of 122 mV and 280 mV at 10 mA cm^-2, respectively. The CoMoO₄-CoP/NC‖CoMoO₄-CoP/NC two-electrode system only required an overall water splitting (OWS) cell voltage of 1.62 V to achieve 10 mA cm^-2 in an alkaline electrolytic cell. In addition, the material showed comparable activity to 20% Pt/C‖RuO₂ in a pure water home-made membrane electrode device, demonstrating potential for practical applications in proton exchange membrane (PEM) electrolyzers. Our results suggest that CoMoO₄-CoP/NC is a promising electrocatalyst for efficient and cost-effective water splitting.

Activity-Stability Balance: The Role of Electron Supply Effect of Support in Acidic Oxygen Evolution

Developing efficient and durable electrocatalysts for the oxygen evolution reaction (OER) in proton exchange membrane (PEM) electrolyzers represents a significant challenge. Herein, the cobalt-ruthenium oxide nano-heterostructures are successfully synthesized on carbon cloth (CoO_x /RuO_x -CC) for acidic OER through a simple and fast solution combustion strategy. The rapid oxidation process endows CoO_x /RuO_x -CC with abundant interfacial sites and defect structures, which enhances the number of active sites and the charge transfer at the electrolyte-catalyst interface, promoting the OER kinetics. Moreover, the electron supply effect of the CoO_x support allows electrons to transfer from Co to Ru sites during the OER process, which is beneficial to alleviate the ion leaching and over-oxidation of Ru sites, improving the catalyst activity and stability. As a self-supported electrocatalyst, CoO_x /RuO_x -CC displays an ultralow overpotential of 180 mV at 10 mA cm^-2 for OER. Notably, the PEM electrolyzer using CoO_x /RuO_x -CC as the anode can be operated at 100 mA cm^-2 stably for 100 h. Mechanistic analysis shows that the strong catalyst-support interaction is beneficial to redistribute the electronic structure of RuO bond to weaken its covalency, thereby optimizing the binding energy of OER intermediates and lowering the reaction energy barrier.

Adjusting the valence band center of Co-Ni-bimetallic sulfides through lattice expansion and stacking faults triggered by strain engineering to boost oxygen evolution reaction

Stress engineering can improve catalytic performance by straining the catalyst lattice. An electrocatalyst, Co₃S₄/Ni₃S₂-10%Mo@NC, was prepared with abundant lattice distortion to boost oxygen evolution reaction (OER). With the assistance of the intramolecular steric hindrance effect of metal-organic frameworks, slow dissolution by MoO₄^2- of the Ni substrate and recrystallization of Ni²⁺ was observed in the process of Co(OH)F crystal growth with mild temperature and short time reaction. The lattice expansion and stacking faults created defects inside the Co₃S₄ crystal, improved the material conductivity, optimized the valence band electron distribution of the material, and promoted the rapid conversion of the reaction intermediates. The presence of reactive intermediates of the OER under catalytic conditions was investigated using operando Raman spectroscopy. The electrocatalysts exhibited super high performance, a current density of 10 mA cm^-2 at an overpotential of 164 mV and 100 mA cm^-2 at 223 mV, which were comparable to those of integrated RuO₂. Our work for the first time demonstrates that the dissolution-recrystallization triggered by strain engineering is a good modulation approach to adjust the structure and surface activity of catalyst, suggesting promising industrial application.

Rhenium Suppresses Iridium (IV) Oxide Crystallization and Enables Efficient, Stable Electrochemical Water Oxidation

IrO₂ as benchmark electrocatalyst for acidic oxygen evolution reaction (OER) suffers from its low activity and poor stability. Modulating the coordination environment of IrO₂ by chemical doping is a methodology to suppress Ir dissolution and tailor adsorption behavior of active oxygen intermediates on interfacial Ir sites. Herein, the Re-doped IrO₂ with low crystallinity is rationally designed as highly active and robust electrocatalysts for acidic OER. Theoretical calculations suggest that the similar ionic sizes of Ir and Re impart large spontaneous substitution energy and successfully incorporate Re into the IrO₂ lattice. Re-doped IrO₂ exhibits a much larger migration energy from IrO₂ surface (0.96 eV) than other dopants (Ni, Cu, and Zn), indicating strong confinement of Re within the IrO₂ lattice for suppressing Ir dissolution. The optimal catalysts (Re: 10 at%) exhibit a low overpotential of 255 mV at 10 mA cm^-2 and a high stability of 170 h for acidic OER. The comprehensive mechanism investigations demonstrate that the unique structural arrangement of the Ir active sites with Re-dopant imparts high performance of catalysts by minimizing Ir dissolution, facilitating *OH adsorption and *OOH deprotonation, and lowering kinetic barrier during OER. This study provides a methodology for designing highly-performed catalysts for energy conversion.

NiMo solid-solution alloy porous nanofiber as outstanding hydrogen evolution electrocatalyst

Developing a high-efficiency hydrogen evolution reaction (HER) electrocatalyst for the large-scale production of hydrogen is essential but challenging. In this study, we used NiMo solid-solution alloy porous nanofibers to develop a robust HER electrocatalyst through electrospinning, oxidization, and high-temperature reduction treatment. In 1 M KOH electrolyte, the fabricated NiMo solid-solution alloy porous nanofibers exhibited higher HER activity than Ni nanofibers, which required a low overpotential of 69, 208, and 300 mV at 100, 500, and 1000 mA cm^-2, respectively, and had outstanding durability at 100 mA cm^-2 over 60 h. We developed a promising candidate for a high-efficiency HER electrocatalyst, and our findings provided valuable information for fabricating highly robust alloy-based electrocatalysts.

Efficient Electrocatalytic Nitrate Reduction to Ammonia Based on DNA-Templated Copper Nanoclusters

In alkaline solutions, the electrocatalytic conversion of nitrates to ammonia (NH₃) (NO₃RR) is hindered by the sluggish hydrogenation step due to the lack of protons on the electrode surface, making it a grand challenge to synthesize NH₃ at a high rate and selectivity. Herein, single-stranded deoxyribonucleic acid (ssDNA)-templated copper nanoclusters (CuNCs) were synthesized for the electrocatalytic production of NH₃. Because ssDNA was involved in the optimization of the interfacial water distribution and H-bond network connectivity, the water-electrolysis-induced proton generation was enhanced on the electrode surface, which facilitated the NO₃RR kinetics. The activation energy (E_a) and in situ spectroscopy studies adequately demonstrated that the NO₃RR was exothermic until NH₃ desorption, indicating that, in alkaline media, the NO₃RR catalyzed by ssDNA-templated CuNCs followed the same reaction path as the NO₃RR in acidic media. Electrocatalytic tests further verified the efficiency of ssDNA-templated CuNCs, which achieved a high NH₃ yield rate of 2.62 mg h^-1 cm^-2 and a Faraday efficiency of 96.8% at -0.6 V vs reversible hydrogen electrode. The results of this study lay the foundation for engineering catalyst surface ligands for the electrocatalytic NO₃RR.

Facilitating interface charge transfer via constructing NiO/NiCo2O4 heterostructure for oxygen evolution reaction under alkaline conditions

Designing high-activity electrocatalysts to enhance the slow multielectron-transfer process of the oxygen evolution reaction (OER) is of great importance for hydrogen generation. Here, we employ hydrothermal and subsequent heat-treatment strategies to acquire nanoarrays-structured NiO/NiCo₂O₄ heterojunction anchored Ni foam (NiO/NiCo₂O₄/NF) as efficient materials for catalyzing the OER in an alkaline electrolyte. Density functional theory (DFT) results demonstrate that NiO/NiCo₂O₄/NF exhibits a smaller overpotential than those of single NiO/NF and NiCo₂O₄/NF owing to interface-triggered numerous interface charge transfer. Moreover, the superior metallic characteristics of NiO/NiCo₂O₄/NF further enhance its electrochemical activity toward OER. Specifically, NiO/NiCo₂O₄/NF delivered a current density of 50 mA cm^-2 at an overpotential of 336 mV with a Tafel slope of 93.2 mV dec^-1 for the OER, which are comparable with those of commercial RuO₂ (310 mV and 68.8 mV dec^-1). Further, an overall water splitting system is preliminarily constructed via using a Pt net as cathode and NiO/NiCo₂O₄/NF as anode. The water electrolysis cell performs an operating voltage of 1.670 V at 20 mA cm^-2, which outperform the Pt net||IrO₂ couple assembled two-electrode electrolyzer (1.725 V at 20 mA cm^-2). This study proposes an efficient route to acquire multicomponent catalysts with rich interfaces for water electrolysis.

Dopant-Induced Surface Self-Etching of Cobalt Carbonate Hydroxide Boosts Efficient Water Splitting

Herein, vanadium-doped cobalt carbonate hydroxide, V-CoCH, was synthesized as efficient catalyst for water splitting. Vanadium species were partially dissolved in the early stages of the oxygen-evolution reaction (OER), inducing self-etching of the catalyst surface, which is helpful for catalyst surface reconstruction and resulted in a higher number of active sites and oxygen vacancies. The synergy between V-doping and oxygen vacancies improved the catalytic activity: V-CoCH showed an exceptional OER catalytic performance with an overpotential of 183 mV at 10 mA cm^-2 . The water-splitting cell consisting of V-CoCH only required 1.52 V to reach 10 mA cm^-2 . Theoretical calculations revealed that vanadium in V-CoCH played an important role in electron regulation of active sites. The oxygen vacancies had an important effect on improvement of the OER performance through not only the exposure of more active sites but also through modulation of the electronic structure. This work provides an effective strategy for constructing high-performance electrocatalysts.

Unveiling the Position Effect of Ce within Layered MnO2 to Prolong the Ambient Removal of Indoor HCHO

The position of Ce doping has a significant effect on ambient HCHO storage and catalytic oxidation on layered MnO₂. By associating structure and performance, it is unveiled that doping Ce into the in-layered lattice of MnO₂ is favorable to the generation of high-valence Mn cations, enhancing the oxidizing ability and capacity, but an opposite influence is displayed by interlayered Ce doping. From the aspect of energy minimization calculated by DFT, in-layered Ce doping is also recommended due to the decreased energies for molecule adsorption and oxygen vacancy formation. As a result, in-layered Ce-doped MnO₂ displays exceptional activity in catalyzing the deep oxidation of HCHO and a fourfold higher capacity of ambient HCHO storage than pristine MnO₂. The optimal oxide is combined with electromagnetic induction heating to complete the "storage-oxidation" cycle as a promising approach absolutely depending on non-noble oxides and household appliances to realize the long-acting removal of indoor HCHO at room temperature.

Enhancing the oxygen evolution reaction of cobalt hydroxide by fabricating nanocomposites with fluorine-doped graphene oxide

Fluorine and nitrogen codoped cobalt hydroxide-graphene oxide nanocomposites (N,F-Co(OH)₂/GO) were synthesized by a simple hydrothermal method and demonstrated highly enhanced oxygen evolution activity in an alkaline medium. N,F-Co(OH)₂/GO synthesized under optimized reaction conditions required an overpotential of 228 mV to produce the benchmark current density of 10 mA cm^-2 (scan rate 1 mV s^-1). In contrast, N,F-Co(OH)₂ without GO and Co(OH)₂/GO without fluorine required higher overpotentials (370 (N,F-Co(OH)₂) and 325 mV (Co(OH)₂/GO)) for producing the current density of 10 mA cm^-2. The low Tafel slope (52.6 mV dec^-1) and charge transfer resistance, and high electrochemical double layer capacitance of N,F-Co(OH)₂/GO compared to N,F-Co(OH)₂ indicate faster kinetics at the electrode-catalyst interface. The N,F-Co(OH)₂/GO catalyst showed good stability over 30 h. High-resolution transmission electron microscope (HR-TEM) images showed good dispersion of polycrystalline Co(OH)₂ nanoparticles in the GO matrix. X-ray photoelectron spectroscopic (XPS) analysis revealed the coexistence of Co²⁺/Co³⁺ and the doping of nitrogen and fluorine in N,F-Co(OH)₂/GO. XPS further revealed the presence of F in its ionic state and being covalently attached to GO. The integration of highly electronegative F with GO stabilizes the Co²⁺ active centre along with improving the charge transfer and adsorption process that contributes to improved OER. Thus, the present work reports a facile method for preparing F-doped GO-Co(OH)₂ electrocatalysts with enhanced OER activity under alkaline conditions.

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.

Cu2O/Co3O4 nanoarrays for rapid quantitative analysis of hydrogen sulfide in blood

2D heterostructure nanoarrays have emerged as a promising sensing material for rapid disease detection applications. In this study, a bio-H₂S sensor based on Cu₂O/Co₃O₄ nanoarrays was proposed, the controllable preparation of the nanoarrays being achieved by exploring the experimental parameters of the 2D electrodeposition in situ assembly process. The nanoarrays were designed as a multi-barrier system with strict periodicity and long-range order. Based on the interfacial conductance modulation and vulcanization reaction of Cu₂O and Co₃O₄, the sensor exhibited superior sensitivity, selectivity, and stability to H₂S in human blood. In addition, the sensor exhibited a reasonable response to 0.1 μmol L^-1 Na₂S solution, indicating that it had a low detection limit for practical applications. Moreover, first-principles calculations were performed to study changes in the heterointerface during the sensing process and the mechanism of rapid response of the sensor. This work demonstrated the reliability of Cu₂O/Co₃O₄ nanoarrays applied in portable sensors for the rapid detection of bio-H₂S.

PO6 geometric configuration unit enhanced electrocatalytic performance of Co3O4 in acidic oxygen evolution

It is challenging to develop high-efficient and stable nonprecious metal-based electrocatalyst for oxygen evolution reaction (OER) in acid for proton exchange membrane (PEM) water splitting. Herein, P atoms were introduced into the lattice of spinel Co₃O₄ (P-Co₃O₄) to replace with octahedral coordinated Co³⁺ via a hydrothermal process following a thermal treatment. The formation of PO₆ geometric configuration unit in Co₃O₄ can trigger electron rearrangement around Co ions, which resulted in the high-active Co²⁺ site on the surface, significantly decreasing the energy barrier of rate-determining step for OER. Moreover, the weaker covalency of the Co 3d-O 2p bond and higher formation energy of oxygen vacancy around Co²⁺ in P-Co₃O₄ inhibited the participation of lattice oxygen during OER process, enabling that P-Co₃O₄ can work stably in acidic media. The obtained P-Co₃O₄ afforded satisfying stability over 30 h in a PEM electrolysis device with an overpotential of 400 mV@10 mA/cm² in 0.1 M HClO₄.

Apparent activity and specific activity of lanthanides (La, Ce, Nd) decorated Co-MOF derivatives for electrocatalytic water splitting

Lanthanide (Ln) rare Earth (RE) elements are often used to incorporate and regulate the local coordination environment and electronic configuration of transition metal based electrocatalysts for acquiring improved electrocatalytic performance. But for a given pristine electrode, is a Ln element concentrated more on promoting the apparent activity of original electrode or on enhancing its specific activity? To address this issue, Ln (La, Ce and Nd) decorated ZIF-67 derivative electrodes (Ln/Co/NC) were fabricated following with the detailed experimental testing of apparent activity and specific activity of assembled electrodes. X-ray photoelectron spectroscopy data confirmed that Ce, Nd and La have played their own role in regulating the coordination electronic structure of the surface atoms of the derived Co/NC by forming different types of chemical bonds. Electrochemical (EC) results confirmed that Ce is concentrated more on the apparent activity of derived Co/NC electrode with the smallest overpotential at 50 mA cm^-2(η₅₀), while Nd contributes more to its reaction kinetic property with the smallest value of Tafel slope in alkaline hydrogen evolution reaction process. But for oxygen evolution reaction, all of La, Ce and Nd deteriorate the apparent activity of the pristine Co/NC electrode. Comparatively, La shows a greater ability to modulate the specific activity of Co/NC with a larger electrochemical active surface area normalized current density, while Nd exhibits the best ability to re-establish the properties of reaction centers. This work illustrates the difference influence of La, Ce and Nd on the apparent activity and specific activity of the ZIF-67 derivative Co/NC electrode. It will do some favors in engineering RE elements modified composite electrodes for EC applications.

Crystalline-Dependent Discharge Process of Locally Enhanced Electrooxidation Activity on Ni2P

The state-of-the-art transition-based electrocatalysts in alkaline media generally suffer from unavoidable surface reconstruction during oxygen evolution reaction measurements, leading to the collapse and loss of the crystalline matrix. Low potential discharge offers a gentle way for surface reconstruction and thus realizes the manipulation of the real active site. Nevertheless, the absence of a fundamental understanding focus on this discharge region renders the functional phase, either the crystalline or amorphous matrix, for the controllable reconstruction still undecidable. Herein, we report a scenario to employ different crystalline matrices as electrocatalysts for discharge region reconstruction. The representative low crystalline Ni₂P (LC-Ni₂P) possesses a relatively weak surface structure compared with highly crystalline or amorphous Ni₂P (HC-Ni₂P or A-Ni₂P), which contributes abundant oxygen vacancies after the discharge process. The fast discharge behavior of LC-Ni₂P leads to the uniform distribution of these vacancies and thus endows the inner interface with reactant activating functionality. A high increase in current density of 36.7% is achieved at 2.32 V (vs RHE) for the LC-Ni₂P electrode. The understanding of the discharge behavior in this study, on different crystalline matrices, presents insights into the establishment of controllable surface reconstruction for an effective oxygen evolution reaction.

Oxygen Spillover Effect at Cu/Fe2O3 Heterointerfaces to Enhance Oxygen Electrocatalytic Reactions for Rechargeable Zn-Air Batteries

Rational design and synthesis of high-performance electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are critical for practical application of Zn-air batteries (ZABs). In this work, the bifunctional composite Cu-Fe₂O₃/PNC was prepared by a simple and effective wet-hydrothermal coupled dry-annealing synthesis strategy. The Cu-Fe₂O₃/PNC displayed excellent catalytic activity in ORR and OER with a potential difference of 0.63 V. More importantly, the ZAB assembled with Cu-Fe₂O₃/PNC exhibited a high-power density of 138.00 mW cm^-2 and an excellent long-term cyclability. X-ray photoelectron spectroscopy (XPS) demonstrated that the excellent performance is due to the strong electronic interaction between Cu and Fe₂O₃ that arises as a result of the fast electron transfer through the Cu-O-Fe bond and the higher concentration of surface oxygen vacancies. Meanwhile, the spillover factor Bsp/2zF of Cu/PNC and Cu-Fe₂O₃/PNC obtained by the rotating disk experiment was 1.00 × 10^-7 and 1.10 × 10^-7 cm²·s^-1, respectively, indicating that the oxygen spillover effect between Cu and Fe₂O₃ lowers the energy barrier, increases the number of active sites, and alters the rate-determining reaction step. This work demonstrated the significant potential of Cu-Fe₂O₃/PNC in energy conversion and storage applications, providing a new perspective for the rational design of bifunctional electrocatalysts.

Accelerated water activation and stabilized metal-organic framework via constructing triangular active-regions for ampere-level current density hydrogen production

Two-dimensional metal-organic frameworks (MOFs) have been explored as effective electrocatalysts for hydrogen evolution reaction (HER). However, the sluggish water activation kinetics and structural instability under ultrahigh-current density hinder their large-scale industrial applications. Herein, we develop a universal ligand regulation strategy to build well-aligned Ni-benzenedicarboxylic acid (BDC)-based MOF nanosheet arrays with S introducing (S-NiBDC). Benefiting from the closer p-band center to the Fermi level with strong electron transferability, S-NiBDC array exhibits a low overpotential of 310 mV to attain 1.0 A cm^-2 with high stability in alkaline electrolyte. We speculate the newly-constructed triangular "Ni₂-S₁" motif as the improved HER active region based on detailed mechanism analysis and structural characterization, and the enhanced covalency of Ni-O bonds by S introducing stabilizes S-NiBDC structure. Experimental observations and theoretical calculations elucidate that such Ni sites in "Ni₂-S₁" center distinctly accelerate the water activation kinetics, while the S site readily captures the H atom as the optimal HER active site, boosting the whole HER activity.

Surface Boron Modulation on Cobalt Oxide Nanocrystals for Electrochemical Oxygen Evolution Reaction

Herein, we show that coupling boron with cobalt oxide tunes its structure and significantly boost its electrocatalytic performance for the oxygen evolution reaction (OER). Through a simple precipitation and thermal treatment process, a series of Co-B oxides with tunable morphologies and textural parameters were prepared. Detailed structural analysis supported first the formation of an disordered and partially amorphous material with nanosized Co₃ BO₅ and/or Co₂ B₂ O₆ being present on the local atomic scale. The boron modulation resulted in a superior OER reactivity by delivering a large current and an overpotential of 338 mV to reach a current density of 10 mA cm^-2 in 1 M KOH electrolyte. Identical location transmission electron microscopy and in situ electrochemical Raman spectroscopy studies revealed alteration and surface re-construction of materials, and formation of CoO₂ and (oxy)hydroxide intermediate, which were found to be highly dependent on crystallinity of the samples.

Tuning Electronic Structures of Transition Metal Carbides to Boost Oxygen Evolution Reactions in Acidic Medium

Developing low-cost, efficient, and robust nonprecious metal electrocatalysts for oxygen evolution reactions (OER) in acidic medium is the major challenge to realize the application of the proton exchange membrane water electrolyzer (PEM-WE). It is well-known that transition metal carbides (TMCs) have Pt-like electronic structures and catalytic behaviors. However, monometallic carbides in acidic medium show ignored OER activities. Herein, we reported that the catalytic activity of the TMCs can be enhanced by constructing bimetallic carbides (TiTaC₂) fabricated through hydrothermal treatment followed by an annealing process, and further by doping fluorine (F) into the bimetallic carbides (TiTaF_xC₂). The as-prepared reduced graphene oxide (rGO) supported TiTaF_xC₂ nanoparticles (TiTaF_xC₂ NP/rGO) show state-of-the-art OER catalytic activity, which is even superior to Ir/C catalyst (an onset potential of only 1.42 V vs RHE and the overpotential of 490 mV to reach 100 mA cm^-2), fast kinetics (Tafel slope of only 36 mV dec^-1), and high durability (maintaining the current density at 1.60 V vs RHE for 40 h). Detailed structural characterizations together with density functional theory (DFT) calculations reveal that the electronic structures of the bimetallic carbides have been tuned, and their possible mechanism is also discussed.

Direct growth of three-dimensional nanoflower-like structures from flat metal surfaces

Here we report on a facile top-down approach for the direct growth of Co₃O₄ hierarchical nanoflowers from a bulk Co surface via chemical etching and thermal annealing. The effect of the annealing temperature was investigated, showing that amorphous Co₃O₄ was formed at 250 °C, while crystalline Co₃O₄ with notable oxygen vacancies was created at 550 °C. The formed 3D nanostructures exhibited excellent oxygen evolution reaction (OER) activities with a low overpotential of 0.34 V at 10 mA cm^-2 and high durability. The proposed novel approach was further demonstrated by the direct growth of 3D NiO and CuO nanostructures on Ni and Cu substrates.

Electronegativity Enhanced Strong Metal-Support Interaction in Ru@F-Ni3N for Enhanced Alkaline Hydrogen Evolution

Precious metals (Pt, Ir, Ru, and so on) and related compounds usually demonstrate superb catalytic activity for electrochemical hydrogen production. However, scarcity and stability are still challenges for hydrogen evolution reaction, even for single-atomic-site electrocatalysts. Herein, a fluorine (F) doping strategy is proposed to enhance the strong metal-support interaction between the F-doped Ni₃N support and the loaded ruthenium (Ru) species. Via synergistically modulating both the Ru loading amount and F doping concentration, outstanding HER activity was achieved in Ru@F-Ni₃N with an overpotential (η) of 115 mV at 100 mA cm^-2, superior to the benchmark Pt/C (η = 201 mV). Density functional theory simulation in combination with X-ray photoelectron spectra and X-ray absorption spectroscopy characterizations convincingly demonstrate that, with the strongest electronegativity, F doping could effectively stabilize Ru atoms doped in the F-Ni₃N substrate and simultaneously reduce the H bonding strength, which accelerated the desorption of H₂. These findings provide a facile strategy to modulate both catalytic activities and stabilities of heteroatom-loaded catalytic materials.

Ultra-dispersed copper nanoparticles constructing crystalline-amorphous interface sites for alkaline water splitting

In literature, the creation of an interface between a highly conductive crystalline phase and an amorphous phase with unsaturated sites has been proven to be an effective strategy in the design of electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). However, the procedural complexity and limited formation of interfaces have compromised the envisioned effects. In this work, the dense crystalline Fe₂O₃/amorphous Cu interface was created simultaneously by the combination of solverthermal and annealing processes. The results showed that the ultra-dispersed Cu nanoparticles attributed to the formation of crystalline-amorphous (c-a) interface sites, which facilitated the electron transfer with the tuned electronic structures as well as the favorable adsorption of surface oxygen species. As a result, the developed Fe₂O₃/Cu-PNC catalyst outperformed most of the competing bifunctional catalysts reported for both OER and HER operations.