Recent Progress on Bimetallic-Based Spinels as Electrocatalysts for the Oxygen Evolution Reaction

Iron-Nickel synergistic catalysis growth of (Fe,Ni)9S8/Ni3S2@N,S codoped carbon bridged nanowires enhanced oxygen evolution reaction performance

Improving the conductivity of the electrocatalyst itself is essential for enhancing its performance. In this work, N, S-rich 6-thioguanine (TG) is selected as the ligand to synthesize a Fe, Ni bimetallic porous coordination polymer (PCP), which is then derived to fabricate N,S codoped carbon (NSC)-coated (Fe,Ni)9S8/Ni3S2 bridged nanowires. The (Fe,Ni)9S8/Ni3S2@NSC bridged nanowires obtained through bimetallic synergistic catalysis and self-sulfurization processes not only introduced additional electrocatalytic active sites but also significantly enhance the overall conductivity of the catalyst due to the interconnected nanowire structure. The resulting (Fe,Ni)9S8/Ni3S2@NSC demonstrates remarkable oxygen evolution reaction (OER) performance, exhibiting an overpotential as low as 252 mV at a current density of 10 mA cm-2. This work proposes a novel strategy for enhancing the overall conductivity of catalysts by growing bridged nanowires, providing valuable insights and inspiration for the design and preparation of advanced transition metal sulfide electrocatalysts.

Electron-deficient Co7Fe3 induced by interfacial effect of molybdenum carbide boosting oxygen evolution reaction

Developing a high-activity and low-cost catalyst to reduce the anodic overpotential is essential for hydrogen production from water splitting. In this work, a hetero-structured Co7Fe3/Mo2C@C catalyst has been developed to efficiently catalyze oxygen evolution reaction (OER), the overpotential (ƞ10) of Co7Fe3/Mo2C@C-catalyzed OER with current density of 10 mA/cm2 is about 254 mV, substantially lower than the counterparts of Co7Fe3@C-catalyzed OER (ƞ10, 308 mV) and Mo2C@C-catalyzed OER (ƞ10, 439 mV), close to that of OER catalyzed by commercial RuO2. The mechanistic studies reveal that the distinct electron transfer across the Co7Fe3/Mo2C interface results in electron-deficient Co7Fe3, which has been identified as the highly active catalytic sites. Density functional theory (DFT) calculations manifest that Mo2C induces a distinct decrease in electron density on Co7Fe3 and upgrades the d-band centers of Co and Fe in Co7Fe3 towards Fermi energy level, thus substantially lowering the energy barrier of the rate-determining reaction step and conferring significantly improved OER activity on the Co7Fe3/Mo2C@C catalyst.

From computational screening to the synthesis of a promising OER catalyst

The search for new materials can be laborious and expensive. Given the challenges that mankind faces today concerning the climate change crisis, the need to accelerate materials discovery for applications like water-splitting could be very relevant for a renewable economy. In this work, we introduce a computational framework to predict the activity of oxygen evolution reaction (OER) catalysts, in order to accelerate the discovery of materials that can facilitate water splitting. We use this framework to screen 6155 ternary-phase spinel oxides and have isolated 33 candidates which are predicted to have potentially high OER activity. We have also trained a machine learning model to predict the binding energies of the *O, *OH and *OOH intermediates calculated within this workflow to gain a deeper understanding of the relationship between electronic structure descriptors and OER activity. Out of the 33 candidates predicted to have high OER activity, we have synthesized three compounds and characterized them using linear sweep voltammetry to gauge their performance in OER. From these three catalyst materials, we have identified a new material, Co2.5Ga0.5O4, that is competitive with benchmark OER catalysts in the literature with a low overpotential of 220 mV at 10 mA cm-2 and a Tafel slope at 56.0 mV dec-1. Given the vast size of chemical space as well as the success of this technique to date, we believe that further application of this computational framework based on the high-throughput virtual screening of materials can lead to the discovery of additional novel, high-performing OER catalysts.

Hybridization of bimetallic cobalt-molybdenum oxide multihole nanosheets with selenium adulteration as advanced bifunctional electrocatalysts for boosting overall water splitting

Electrocatalytic water splitting produces green and pollution-free hydrogen as a clean energy carrier, which can effectively alleviate energy crisis. In this paper, bimetallic and selenium doped cobalt molybdate (Se-CoMoO4) nanosheets with rough surface are resoundingly prepared. The multihole Se-CoMoO4 nanosheets display ultrathin and rectangular architecture with the dimensions of ∼ 3.5 μm and 700 nm for length and width, respectively. The Se-CoMoO4 electrocatalyst shows remarkable water electrolysis activity and stability. The overpotentials of oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are 270 and 63.3 mV at 10 mA cm-2, along with low Tafel slopes of 51.6 and 62.0 mV dec-1. Furthermore, the Se-CoMoO4 couple electrolyzer merely requires a cell voltage of 1.48 V to achieve 10 mA cm-2 current density and presents no apparent attenuation for 30 h. This investigation declares that the hybridization of transition bimetallic oxide with nonmetallic adulteration can afford a tactic for the preparation of bifunctional non-precious metal-based electrocatalysts.

Tuning Octahedron Sites of CoV2O4 via Cationic Competition for Efficient Oxygen Evolution Reaction

Doping transition metal oxide spinels with metal ions represents a significant strategy for optimizing the electronic structure of electrocatalysts. Herein, a bimetallic Fe and Ru doping strategy to fine-tune the crystal structure of CoV2O4 spinel for highly enhanced oxygen evolution reaction (OER) is presented performance. The incorporation of Fe and Ru is observed at octahedral sites within the CoV2O4 structure, effectively modulating the electronic configuration of Co. Density functional theory calculations have confirmed that Fe acts as a novel reactive site, replacing V. Additionally, the synergistic effect of Fe, Co, and Ru effectively optimizes the Gibbs free energy of the intermediate species, reduces the reaction energy barrier, and accelerates the kinetics toward OER. As expected, the best-performing CoVFe0.5Ru0.5O4 displays a low overpotential of 240 mV (@10 mA cm-2) and a remarkably low Tafel slope of 38.9 mV dec-1, surpassing that of commercial RuO2. Moreover, it demonstrates outstanding long-term durability lasting for 72 h. This study provides valuable insights for the design of highly active polymetallic spinel electrocatalysts for energy conversion applications.

Selenium-enriched hollow NiCo2O4/NiO heterostructured nanocages as an efficient electrocatalyst for oxygen evolution reaction

Finding clean, sustainable, and environmentally friendly technologies is especially crucial in addressing both energy and environmental challenges. To accelerate the oxygen evolution reaction (OER) and to overcome the obstacle of high energy consumption, exploring high-performance electrocatalysts is imperative to maximize the practical applicability of water splitting. Developing electrocatalyst through strategic surface modifications represents a significant approach for the construction of active catalytic centers. In the present work, we successfully synthesized selenium-incorporated hollow NiCo2O4/NiO heterostructured nanocages as electrocatalysts for the OER by precisely controlling the structure and composition of the material. The findings demonstrated that the surface-reconstructed hollow 5 wt% Se-NiCo2O4/NiO heterostructured nanocages resulted in an increased number of active sites through interfacial engineering. Benefiting from the structural control, mass transport was further expedited and due to increased conductivity, accelerated the charge transfer processes within the system. The electrocatalyst exhibited remarkable activity for the OER and displayed a low overpotential (η = 288 mV) at a current density (j) of 10 mA cm-2, small Tafel slope (66.7 mV dec-1) and better stability. This work offers a viable and adaptable method for fabricating a range of functional coordinated MOF compounds that are capable of utilization across diverse energy applications, including storage, conversion and environmental purposes.

Atomic/molecular layer deposition strategies for enhanced CO2 capture, utilisation and storage materials

Elevated levels of carbon dioxide (CO2) in the atmosphere and the diminishing reserves of fossil fuels have raised profound concerns regarding the resulting consequences of global climate change and the future supply of energy. Hence, the reduction and transformation of CO2 not only mitigates environmental pollution but also generates value-added chemicals, providing a dual remedy to address both energy and environmental challenges. Despite notable advancements, the low conversion efficiency of CO2 remains a major obstacle, largely attributed to its inert chemical nature. It is imperative to engineer catalysts/materials that exhibit high conversion efficiency, selectivity, and stability for CO2 transformation. With unparalleled precision at the atomic level, atomic layer deposition (ALD) and molecular layer deposition (MLD) methods utilize various strategies, including ultrathin modification, overcoating, interlayer coating, area-selective deposition, template-assisted deposition, and sacrificial-layer-assisted deposition, to synthesize numerous novel metal-based materials with diverse structures. These materials, functioning as active materials, passive materials or modifiers, have contributed to the enhancement of catalytic activity, selectivity, and stability, effectively addressing the challenges linked to CO2 transformation. Herein, this review focuses on ALD and MLD's role in fabricating materials for electro-, photo-, photoelectro-, and thermal catalytic CO2 reduction, CO2 capture and separation, and electrochemical CO2 sensing. Significant emphasis is dedicated to the ALD and MLD designed materials, their crucial role in enhancing performance, and exploring the relationship between their structures and catalytic activities for CO2 transformation. Finally, this comprehensive review presents the summary, challenges and prospects for ALD and MLD-designed materials for CO2 transformation.

One-Dimensional La0.2Sr0.8Cu0.4Co0.6O3-δ Nanostructures for Efficient Oxygen Evolution Reaction

Producing oxygen and hydrogen via the electrolysis of water has the advantages of a simple operation, high efficiency, and environmental friendliness, making it the most promising hydrogen production method. In this study, La0.2Sr0.8Cu0.4Co0.6O3-δ (LSCC) nanofibers were prepared by electrospinning to utilize non-noble perovskite oxides instead of noble metal catalysts for the oxygen evolution reaction, and the performance and electrochemical properties of LSCC nanofibers synthesized at different firing temperatures were evaluated. In an alkaline environment (pH = 14, 6 M KOH), the nanofibers calcined at 650 °C showed an overpotential of 209 mV at a current density of 10 mA cm-2 as well as good long-term stability. Therefore, the prepared LSCC-650 NF catalyst shows excellent potential for electrocatalytic oxygen evolution.

MOF-derived S-doped NiCo2O4 hollow cubic nanocage for highly efficient electrocatalytic oxygen evolution

Inducing the surface reconstruction of spinels is critical for improving the electrocatalytic oxygen evolution reaction (OER) activity. Herein, S-doped NiCo2O4 hollow cubic nanocage was synthesized by anion etching Metal-Organic Frameworks (MOFs) template and air annealing strategies. The hollow structure possesses a large specific surface area and pore size, facilitating active site exposure and mass transport. S2- doping regulates the electronic structure, reducing the oxidation potential of Ni sites during the OER process, thus promoting the surface reconstruction into γ-NiOOH active species. Meanwhile, S2- doping enhances conductivity, accelerating interfacial charge transfer. As a result, S-NiCo2O4-6 exhibits superior OER activity (262 mV overpotential @ 10 mA cm-2) and stability in 1.0 M KOH solution. Furthermore, 20 % Pt/C‖S-NiCo2O4-6 only needs 1.832 V to achieve 50 mA (the electrochemical active area is 4 cm2) in a homemade anion exchange membrane (AEM) electrolyzer. This work proposes a novel approach for preparing efficient anion-doped spinel-based OER electrocatalysts.

Electrochemical Etching Switches Electrocatalytic Oxygen Evolution Pathway of IrOx /Y2 O3 from Adsorbate Evolution Mechanism to Lattice-Oxygen-Mediated Mechanism

Oxygen evolution reaction (OER) plays key roles in electrochemical energy conversion devices. Recent advances have demonstrated that OER catalysts through lattice oxygen-mediated mechanism (LOM) can bypass the scaling relation-induced limitations on those catalysts through adsorbate evolution mechanism (AEM). Among various catalysts, IrOx , the most promising OER catalyst, suffers from low activities for its AEM pathway. Here, it is demonstrated that a pre-electrochemical acidic etching treatments on the hybrids of IrOx and Y2 O3 (IrOx /Y2 O3 ) switch the AEM-dominated OER pathway to LOM-dominated one in alkali electrolyte, delivering a high performance with a low overpotential of 223 mV at 10 mA cm-2 and a long-term stability. Mechanism investigations suggest that the pre-electrochemical etching treatments create more oxygen vacancies in catalysts due to the dissolution of yttrium and then provide highly active surface lattice oxygen for participating OER, thereby enabling the LOM-dominated pathway and resulting in a significantly increased OER activity in basic electrolyte.

Recent advances of bifunctional catalysts for zinc air batteries with stability considerations: from selecting materials to reconstruction

With the growing depletion of traditional fossil energy resources and ongoing enhanced awareness of environmental protection, research on electrochemical energy storage techniques like zinc-air batteries is receiving close attention. A significant amount of work on bifunctional catalysts is devoted to improving OER and ORR reaction performance to pave the way for the commercialization of new batteries. Although most traditional energy storage systems perform very well, their durability in practical applications is receiving less attention, with issues such as carbon corrosion, reconstruction during the OER process, and degradation, which can seriously impact long-term use. To be able to design bifunctional materials in a bottom-up approach, a summary of different kinds of carbon materials and transition metal-based materials will be of assistance in selecting a suitable and highly active catalyst from the extensive existing non-precious materials database. Also, the modulation of current carbon materials, aimed at increasing defects and vacancies in carbon and electron distribution in metal-N-C is introduced to attain improved ORR performance of porous materials with fast mass and air transfer. Finally, the reconstruction of catalysts is introduced. The review concludes with comprehensive recommendations for obtaining high-performance and highly-durable catalysts.

Atomically Dispersed Silver Atoms Embedded in NiCo Layer Double Hydroxide Boost Oxygen Evolution Reaction

Bimetallic layered double hydroxides (LDHs) are promising catalysts for anodic oxygen evolution reaction (OER) in alkaline media. Despite good stability, NiCo LDH displays an unsatisfactory OER activity relative to the most robust NiFe LDH and CoFe LDH. Herein, a novel NiCo LDH electrocatalyst modified with single-atom silver grown on carbon cloth (AgSA -NiCo LDH/CC) that exhibits exceptional OER activity and stability in 1.0 m KOH is reported. The AgSA -NiCo LDH/CC catalyst only requires a low overpotential of 192 mV to reach a current density of 10 mA cm-2 , obviously boosting the OER activity of NiCo LDH/CC (410 mV@10 mA cm-2 ). Inspiringly, AgSA -NiCo LDH/CC can maintain its high activity for up to 500 h at a large current density of 100 mA cm-2 , exceeding most single-atom OER catalysts. In situ Raman spectroscopy studies uncover that the in situ formed NiCoOOH during OER is the real active species. Hard X-ray absorption spectrum (XAS) and density functional theory (DFT) calculations validate that single-atom Ag occupying Ni site increases the chemical valence of Ni elements, and then weakens the adsorption of oxygen-contained intermediates on Ni sites, fundamentally accounting for the enhanced OER performance.

Engineering Cu/NiCu LDH Heterostructure Nanosheet Arrays for Highly-Efficient Water Oxidation

The development of stable and efficient electrocatalysts for oxygen evolution reaction is of great significance for electro-catalytic water splitting. Bimetallic layered double hydroxides (LDHs) are promising OER catalysts, in which NiCu LDH has excellent stability compared with the most robust NiFe LDH, but the OER activity is not satisfactory. Here, we designed a NiCu LDH heterostructure electrocatalyst (Cu/NiCu LDH) modified by Cu nanoparticles which has excellent activity and stability. The Cu/NiCu LDH electrocatalyst only needs a low over-potential of 206 mV and a low Tafel slope of 86.9 mV dec^-1 at a current density of 10 mA cm^-2 and maintains for 70 h at a high current density of 100 mA cm^-2 in 1M KOH. X-ray photoelectron spectroscopy (XPS) showed that there was a strong electronic interaction between Cu nanoparticles and NiCu LDH. Density functional theory (DFT) calculations show that the electronic coupling between Cu nanoparticles and NiCu LDH can effectively improve the intrinsic OER activity by optimizing the conductivity and the adsorption energy of oxygen-containing intermediates.

Ultralow Charge Voltage Triggering Exceptional Post-Charging Antibacterial Capability of Co3 O4 /MnOOH Nanoneedles for Skin Infection Treatment

The post-charging antibacterial therapy is highly promising for treatment of Gram-negative bacterial wound infections. However, the therapeutic efficacy of the current electrode materials is yet unsatisfactory due to their low charge storage capacity and limited reactive oxygen species (ROS) yields. Herein, the design of MnOOH decorated Co₃ O₄ nanoneedles (MCO) with exceptional post-charging antibacterial effect against Gram-negative bacteria at a low charge voltage and their implementation as a robust antibacterial electrode for skin wound treatment are reported. Taking advantaging of the increased active sites and enhanced OH^- adsorption capability, the charge storage capacity and ROS production of the MCO electrode are remarkably boosted. As a result, the MCO electrode after charging at an ultralow voltage of 1.4 V gives a 5.49 log and 5.82 log bacterial reduction in Escherichia coli (E. coli) and Pseudomonas aeruginosa (P. aeruginosa) within an incubation time of only 5 min, respectively. More importantly, the antibacterial efficiency of the MCO electrode against multi-drug resistant (MDR) bacteria including Klebsiella pneumoniae (K. pneumoniae) and Acinetobacter baumannii (A. baumannii) also reaches 99.999%. In addition, the MCO electrode exhibits excellent reusability, and the role of extracellular ROS in enhancing post-charging antibacterial activity is also unraveled.

In-situ Surface Reconstruction of Single-crystal (NiFe)3Se4 Nano-pyramid Arrays for Efficient Oxygen Evolution

Transition metal-based selenides (TMSe) are considered as efficient pre-electrocatalysts towards oxygen evolution reaction (OER). However, the key factor in determining the surface reconstruction of TMSe under OER condition is not yet clear. Herein, we uncover that the crystallinity of TMSe will obviously impact the conversion degree from TMSe to transition metal oxyhydroxides (TMOOH) during OER. A novel single-crystal (NiFe)₃Se₄ nano-pyramid array grown on NiFe foam is fabricated by a facile one-step polyol process, which exhibits an excellent OER activity and stability, only requiring 170 mV to reach a current density of 10 mA cm^-2 and can sustain for more than 300 h. In situ Raman spectrum studies reveals that the single-crystal (NiFe)₃Se₄ is partially oxidized on its surface during OER, generating a dense heterostructure of (NiFe)OOH/(NiFe)₃Se₄. Benefiting from the in situ formed heterointerface, the adsorption of OER intermediates on Ni active sites calculated by density functional theory (DFT) analysis is optimized, leading to the reduced energy barrier, which accounts for the enhanced intrinsic activity. This work not only reports a novel single-crystal (NiFe)₃Se₄ nano-pyramid array electrocatalyst with high-efficient OER performance, but also gains a deep insight into the role of the crystallinity of TMSe on the surface reconstruction during OER.

Construction of 3D Hierarchical Co3O4@CoFe-LDH Heterostructures with Effective Interfacial Charge Redistribution for Rechargeable Liquid/Solid Zn-Air Batteries

Constructing three-dimensional (3D) hierarchical heterostructures is an appealing but challenging strategy to improve the performance of catalysts for electrical energy devices. Here, an efficient and robust flexible self-supporting catalyst, interface coupling of ultrathin CoFe-LDH nanosheets and Co₃O₄ nanowire arrays on the carbon cloth (CC/Co₃O₄@CoFe-LDH), was proposed for boosting oxygen evolution reaction (OER) in rechargeable liquid/solid Zn-air batteries (ZABs). The strong interfacial interaction between the CoFe-LDH and Co₃O₄ heterostructures stimulated the charge redistribution in their coupling regions, which improved the electron conductivity and optimized the adsorption free energy of OER intermediates, ultimately boosting the intrinsic OER performance. Besides, the 3D hierarchical nanoarray structure facilitated the exposure of catalytically active centers and rapid electron/mass transfer during the OER process. As such, the CC/Co₃O₄@CoFe-LDH catalyst achieved excellent OER catalytic activity in alkaline medium, with a small overpotential of 237 mV at 10 mA cm^-2, a low Tafel slope of 35.43 mV dec^-1, and long-term durability of up to 48 h, significantly outperforming the commercial RuO₂ catalyst. More impressively, the liquid and flexible solid-state ZABs assembled by the CC/Co₃O₄@CoFe-LDH hybrid catalyst as the OER catalyst presented a stable open circuit voltage, large power density, superb cycling life, and satisfactory flexibility, indicating great potential applications in energy technology. This work provides a good guidance for the development of advanced electrocatalysts with heterostructures and an in-depth understanding of electronic modulation at the heterogeneous interface.

Enhanced Electrocatalytic Water Oxidation of Ultrathin Porous Co3O4 Nanosheets by Physically Mixing with Au Nanoparticles

Ultrathin porous Co₃O₄ nanosheets are synthesized successfully, the thickness of which is about three unit-cell dimensions. The enhanced oxygen evolution reaction (OER) performance and electronic interaction between Co₃O₄ and Au is firstly reported in Co₃O₄ ultrathin porous nanosheets by physically mixing with Au nanoparticles. With the loading of the Au nanoparticles, the current density of ultrathin porous Co₃O₄ nanosheets is enhanced from 9.97 to 14.76 mA cm^-2 at an overpotential of 0.5 V, and the overpotential required for 10 mA cm^-2 decreases from 0.51 to 0.46 V, smaller than that of commercial IrO₂ (0.54 V). Furthermore, a smaller Tafel slope and excellent durability are also obtained. Raman spectra, XPS measurement, and X-ray absorption near edge structure spectra (XANES) show that the enhanced OER ascribed to a higher Co²⁺/Co³⁺ ratio and quicker charge transfer due to the electronic interaction between Au and ultrathin Co₃O₄ nanosheets with low-coordinated surface, and Co²⁺ ions are beneficial for the formation of CoOOH active sites.