Ni- and Mn-Promoted Mesoporous Co3O4: A Stable Bifunctional Catalyst with Surface-Structure-Dependent Activity for Oxygen Reduction Reaction and Oxygen Evolution Reaction

Interface Synergistic Effect of NiFe-LDH/3D GA Composites on Efficient Electrocatalytic Water Oxidation

Currently, NiFe-LDH exhibits an excellent oxygen evolution reaction (OER) due to the interaction of the two metal elements on the layered double hydroxide (LDH) platform. However, such interaction is still insufficient to compensate for its poor electrical conductivity, limited number of active sites and sluggish dynamics. Herein, a feasible two-step hydrothermal strategy that involves coupling low-conductivity NiFe-LDH with 3D porous graphene aerogel (GA) is proposed. The optimized NiFe-LDH/GA (1:1) produced possesses a 257 mV (10 mA cm-2) overpotential and could operate stably for 56 h in an OER. Our investigation demonstrates that the NiFe-LDH/GA has a three-dimensional mesoporous structure, and that there is synergistic interaction between LDH and GA and interfacial reconstruction of NiOOH. Such an interface synergistic coupling effect promotes fast mass transfer and facilitates OER kinetics, and this work offers new insights into designing efficient and stable GA-based electrocatalysts.

Facile synthesis of nanostructured Ni/NiO/N-doped graphene electrocatalysts for enhanced oxygen evolution reaction

Electrocatalysts containing a Ni/NiO/N-doped graphene interface have been synthesised using the ligand-assisted chemical vapor deposition technique. NiO nanoparticles were used as the substrate to grow N-doped graphene by decomposing vapours of benzene and N-containing ligands. The method was demonstrated with two nitrogen-containing ligands, namely dipyrazino[2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (L) and melamine (M). The structure and composition of the as-synthesized composites were characterized by XRD, Raman spectroscopy, SEM, TEM and XPS. The composite prepared using the ligand L had NiO sandwiched between Ni and N-doped graphene and showed an overpotential of 292 mV at 10 mA cm-2 and a Tafel slope of 45.41 mV dec-1 for the OER, which is comparable to the existing noble metal catalysts. The composite prepared using the ligand M had Ni encapsulated by N-doped graphene without NiO. It showed an overpotential of 390 mV at 10 mA cm-2 and a Tafel slope of 78.9 mV dec-1. The ligand-assisted CVD route demonstrates a facile route to control the microstructure of the electrocatalysts.

Excellent Bifunctional Oxygen Evolution and Reduction Electrocatalysts (5A1/5)Co2O4 and Their Tunability

Hastening the progress of rechargeable metal-air batteries and hydrogen fuel cells necessitates the advancement of economically feasible, earth-abundant, inexpensive, and efficient electrocatalysts facilitating both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, a recently reported family of nano (5A1/5)Co2O4 (A = combinations of transition metals, Mg, Mn, Fe, Ni, Cu, and Zn) compositionally complex oxides (CCOs) [Wang et al., Chemistry of Materials, 2023,35 (17), 7283-7291.] are studied as bifunctional OER and ORR electrocatalysts. Among the different low-temperature soft-templating samples, those subjected to 600 °C postannealing heat treatment exhibit superior performance in alkaline media. One specific composition (Mn0.2Fe0.2Ni0.2Cu0.2Zn0.2)Co2O4 exhibited an exceptional overpotential (260 mV at 10 mA cm-2) for the OER, a favorable Tafel slope of 68 mV dec-1, excellent onset potential (0.9 V) for the ORR, and lower than 6% H2O2 yields over a potential range of 0.2 to 0.8 V vs the reversible hydrogen electrode. Furthermore, this catalyst displayed stability over a 22 h chronoamperometry measurement, as confirmed by X-ray photoelectron spectroscopy analysis. Considering the outstanding performance, the low cost and scalability of the synthesis method, and the demonstrated tunability through chemical substitutions and processing variables, CCO ACo2O4 spinel oxides are highly promising candidates for future sustainable electrocatalytic applications.

Application of valence-variable transition-metal-oxide-based nanomaterials in electrochemical analysis: A review

The construction of materials with rapid electron transfer is considered an effective method for enhancing electrochemical activity in electroanalysis. It has been widely demonstrated that valence changes in transition metal ions can promote electron transfer and thus increase electrochemical activity. Recently, valence-variable transition metal oxides (TMOs) have shown popular application in electrochemical analysis by using their abundant valence state changes to accelerate electron transfer during electrochemical detection. In this review, we summarize recent research advances in valence changes of TMOs and their application in electrochemical analysis. This includes the definition and mechanism of valence change, the association of valence changes with electronic structure, and their applications in electrochemical detection, along with the use of density functional theory (DFT) to simulate the process of electron transfer during valence changes. Finally, the challenges and opportunities for developing and applying valence changes in electrochemical analysis are also identified.

Tuning electronic structure of hedgehog-like nickel cobaltite via molybdenum-doping for enhanced electrocatalytic oxygen evolution catalysis

As a typical spinel oxide, nickel cobaltite (NiCo2O4) is considered to be a promising and reliable oxygen evolution reaction (OER) catalyst due to its abundant oxidation states and the synergistic effect of multiple metal species. However, the electrocatalytic OER performance of NiCo2O4 has always been limited by the low specific surface area and poor intrinsic conductivity of spinels. Herein, the hedgehog-like molybdenum-doped NiCo2O4 (Mo-NiCo2O4) catalyst was prepared as an efficient OER electrocatalyst via a facile hydrothermal method followed with high-temperature annealing. The Mo-NiCo2O4-0.075 with Mo doping concentration of ∼ 1.95 wt% exhibits excellent OER performance with a low overpotential of 265 mV at a current density of 10 mA·cm-2and a Tafel slope of 126.63 mV·dec-1, as well as excellent cyclingstability.The results demonstrated that the hedgehog-like structure provides Mo-NiCo2O4 with the high surface area and mesopores that enhance electrolyte diffusion and optimal active site exposure. The in-situ Raman spectra and density functional theory calculations show that the Mo cations doping improve the intrinsic conductivity of the NiCo2O4 while modulating the chemisorption of intermediates. Meanwhile, the energy barriers of *OH and O* formation decrease significantly after Mo doping, effectively facilitating water dissociation and optimizing the reaction kinetics.

Comparative study of Co3O4(111), CoFe2O4(111), and Fe3O4(111) thin film electrocatalysts for the oxygen evolution reaction

Water electrolysis to produce 'green H2' with renewable energy is a promising option for the upcoming green economy. However, the slow and complex oxygen evolution reaction at the anode limits the efficiency. Co3O4 with added iron is a capable catalyst for this reaction, but the role of iron is presently unclear. To investigate this topic, we compare epitaxial Co3O4(111), CoFe2O4(111), and Fe3O4(111) thin film model electrocatalysts, combining quasi in-situ preparation and characterization in ultra-high vacuum with electrochemistry experiments. The well-defined composition and structure of the thin epitaxial films permits the obtention of quantitatively comparable results. CoFe2O4(111) is found to be up to about four times more active than Co3O4(111) and about nine times more than Fe3O4(111), with the activity depending acutely on the Co/Fe concentration ratio. Under reaction conditions, all three oxides are covered by oxyhydroxide. For CoFe2O4(111), the oxyhydroxide's Fe/Co concentration ratio is stabilized by partial iron dissolution.

Microwave-Assisted Rational Designed CNT-Mn3 O4 /CoWO4 Hybrid Nanocomposites for High Performance Battery-Supercapacitor Hybrid Device

Extensive research interest in hybrid battery-supercapacitor (BSH) devices have led to the development of cathode materials with excellent comprehensive electrochemical properties. In this work, carbon nanotube (CNT)-Mn3 O4 /CoWO4 triple-segment hybrid electrode is synthesized by using a two-step microwave-assisted hydrothermal route. Systematic physical characterization revealed that, with the assistance of microwave, granular Mn3 O4 and spheroid-like CoWO4 with preferred orientation, and oxygen vacancies are stacked or arranged on CNTs skeletons to construct a rational designed hybrid nanocomposite with abundant heterointerfaces and interfacial chemical bonds. Electrochemical evaluations show that the synergistic cooperation in CNT-Mn3 O4 /CoWO4 resulted in an ultra-high specific capacity (1907.5 C g-1 /529.8 mA h g-1 at 1 A g-1 ), a wide operating voltage window (1.15 V), the satisfactory rate capability (capacity maintained at 1016.5 C g-1 /282.3 mA h g-1 at 15 A g-1 ), and excellent cycling stability (117.2% initial capacity retention after 13000 cycles at 15 A g-1 ). In addition, the assembled CNT-Mn3 O4 /CoWO4 //N doped porous carbon (NC) BSH device delivered a stable working voltage of 2.05 V and superior energy density of 67.5 Wh kg-1 at power density of 1025 W kg-1 , as well as excellent stability (92.2% capacity retained at 5 A g-1 for 12600 cycles). This work provides a new and feasible tactic to develop high-performance transition metal oxide-based cathodes for advanced BSH devices.

Surface state engineering of carbon dot/carbon nanotube heterojunctions for boosting oxygen reduction performance

Platinum-based (Pt) catalysts are the most common commercial catalysts for oxygen reduction reactions (ORR). Unfortunately, their high price, scarcity and poor durability hinder their further development. Therefore, the development of effective and economical ORR electrocatalysts has received increasing attention. Here, carbon dots (CDs) enriched in amino functional groups were successfully loaded onto carbon nanotubes (CNTs) with a large surface area and helical structure through a surface state engineering strategy. The resulting composites (CD/CNTs) are 0D/1D nano heterojunction structures. The CD/CNTs showed superior ORR activity compared with CNTs and CDs (E_oneset = 0.95 V, E_1/2 = 0.81 V and limiting current density = 4.74 mA cm^-2). In addition, the stability of CD/CNTs in an alkaline medium was up to 30000 s. The excellent ORR performance of CD/CNTs can be attributed to the dominant role of amino-N, the synergistic effect of heterojunctions formed by CDs and CNTs, and the high Lewis basicity. The composite electrocatalyst synthesized by the CD-regulated CNT strategy is expected to be a reliable cathode candidate for future energy conversion devices.

Facile synthesis approach of bifunctional Co–Ni–Fe oxyhydroxide and spinel oxide composite electrocatalysts from hydroxide and layered double hydroxide composite precursors†

Zinc–air batteries (ZABs) are promising candidates for the next-generation energy storage systems, however, their further development is severely hindered by kinetically sluggish oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Facile synthesis approaches of highly active bifunctional electrocatalysts for OER and ORR are required for their practical applications. Herein, we develop a facile synthesis procedure for composite electrocatalysts composed of OER-active metal oxyhydroxide and ORR-active spinel oxide containing Co, Ni and Fe from composite precursors consisting of metal hydroxide and layered double hydroxide (LDH). Both hydroxide and LDH are simultaneously produced by a precipitation method with a controlled molar ratio of Co²⁺, Ni²⁺ and Fe³⁺ in the reaction solution, and calcination of the precursor at a moderate temperature provides composite catalysts of metal oxyhydroxides and spinel oxides. The composite catalyst shows superb bifunctional performances with a small potential difference of 0.64 V between a potential of 1.51 V vs. RHE at 10 mA cm⁻² for OER and a half-wave potential of 0.87 V vs. RHE for ORR. The rechargeable ZAB assembled with the composite catalyst as an air-electrode exhibits a power density of 195 mA cm⁻² and excellent durability of 430 hours (1270 cycles) of a charge–discharge cycle test.

Simple and durable: the multi-metal oxyhydroxide and spinal oxide composite catalyst containing Co, Fe and Ni are synthesized from hydroxide and layered double hydroxide composite precursors and shows excellent bifunctional ORR/OER activities.

Mn-doped Co3O4 for acid, neutral and alkaline electrocatalytic oxygen evolution reaction

This work reports the application of Mn-doped Co3O4 oxides in the electrocatalytic oxygen evolution reaction (OER). The materials were characterized by structural, morphological, and electrochemical techniques. The oxides with higher Co : Mn molar ratio presented a lower electron transfer resistance, and consequently the most promising OER activities. Pure Co3O4 shows an overpotential at j = 10 mA cm-2 of 761, 490, and 240 mV, at pH 1, 7, and 14, respectively, and a high TOF of 1.01 × 10-1 s-1 at pH 14. Tafel slopes around 120 mV dec-1 at acidic pH and around 60 mV dec-1 at alkaline pH indicate different OER mechanisms. High stability for Co3O4 was achieved for up to 15 h in all pHs, and no change in the structure and morphology after the electrocatalysis was observed. The reported excellent OER activity of the Mn-Co oxides in a wide pH range is important to broaden the practical applicability in different electrolyte solutions.

K+ Ion-Doped Mixed Carbon Nitride: A Daylight-Driven Photocatalyst and Luminophore for Enhanced Chemiluminescence

Photocatalytic production of reactive oxygen species from O₂ at the interface of the photocatalyst is significant to convert luminous energy like daylight into chemical energy and could be momentous for a reactive oxygen species-based chemiluminescence system. Herein, we synthesized a novel K⁺ ion-doped tri-s-triazine/triazine mixed carbon nitride (MCN), in which K⁺ ions were intercalated into the layers in a bridging manner. After a mild daylight treatment for 30 min, the MCN suspension could produce long-lifetime reactive oxygen species and further directly produce intense and stable chemiluminescence emission in the presence of luminol. In particular, the chemiluminescence intensity was 780 times that of H₂O₂-luminol, and MCN could be recycled several times in the chemiluminescence system. The mechanism results revealed a large number of reactive oxygen species that were generated from O₂ on the surface of MCN through a temperate photocatalytic process. In the theoretical calculation, the charge density of N interacting with K⁺ ions was significantly more negative than that at the corresponding position in graphitic carbon nitride, which was beneficial to the adsorption and activation of oxygen, and the narrower band gap suggested that the doping of K⁺ ions was conducive to the intramolecular charge transfer interaction. Then, the long-lifetime reactive oxygen species triggered the conversion of luminol into an excited-state intermediate, which further transferred energy to MCN, producing strong chemiluminescence emission. The K⁺ ion-doped MCN might conduct as an efficient photocatalyst for reactive oxygen species generation, recyclable catalysts, and luminophores in the photoinduced chemiluminescence system.

Structural Design Strategy and Active Site Regulation of High-Efficient Bifunctional Oxygen Reaction Electrocatalysts for Zn-Air Battery

Zinc-air batteries (ZABs) exhibit high energy density as well as flexibility, safety, and portability, thereby fulfilling the requirements of power batteries and consumer batteries. However, the limited efficiency and stability are still the significant challenge. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are two crucial cathode reactions in ZABs. Development of bifunctional ORR/OER catalysts with high efficiency and well stability is critical to improve the performance of ZABs. In this review, the ORR and OER mechanisms are first explained. Further, the design principles of ORR/OER electrocatalysts are discussed in terms of atomic adjustment mechanism and structural design in conjunction with the latest reported in situ characterization techniques, which provide useful insights on the ORR/OER mechanisms of the catalyst. The improvement in the energy efficiency, stability, and environmental adaptability of the new hybrid ZAB by the inclusion of additional reaction, including the introduction of transition-metal redox couples in the cathode and the addition of modifiers in the electrolyte to change the OER pathway, is also summarized. Finally, current challenges and future research directions are presented.

The impact of ultrasonic parameters on the exfoliation of NiFe LDH nanosheets as electrocatalysts for the oxygen evolution reaction in alkaline media

The ultrasonic process has been examined to exfoliate layered materials and upgrade their properties for a variety of applications in different media. Our previous studies have shown that the ultra-sonication treatment in water without chemicals has a positive influence on the physical and electrochemical performance of layered materials and nanoparticles. In this work, we have probed the impact of ultrasonication on the physical properties and the oxygen evolution reaction (OER) of the NiFe LDH materials under various conditions, including suspension concentration (2.5-12.5 mg mL-1), sonication times (3-20 min) and amplitudes (50-90%) in water, in particular, sonication times and amplitudes. We found that the concentration, amplitude and time play significant roles on the exfoliation of the NiFe LDH material. Firstly, the NiFe LDH nanosheets displayed the best OER performance under ultrasonic conditions with the concentration of 10 mg mL-1 (50% amplitude and 15 min). Secondly, it was revealed that the exfoliation of the NiFe LDH nanosheets in a short time (<10 min) or a higher amplitudes (≥80%) has left a cutdown on the OER activity. Comprehensively, the optimum OER activity was displayed on the exfoliated NiFe LDH materials under ultrasonic condition of 60% (amplitude), 10 mg mL-1 and 15 min. It demanded only 250 mV overpotentials to reach 10 mA cm-2 in 1 M KOH, which was 100 mV less than the starting NiFe LDH material. It was revealed from the mechanism of sonochemistry and the OER reaction that, after exfoliation, the promoted OER performance is ascribed to the enriched Fe3+ at the active sites, easier oxidation of Ni2+ to Ni3+, and the strong electrical coupling of the Ni2+ and Fe3+ during the OER process. This work provides a green strategy to improve the intrinsic activity of layered materials.

High-Performance NixCo3-xO4/Ti3C2Tx-HT Interfacial Nanohybrid for Electrochemical Overall Water Splitting

This study highlights the facet structure control of regular Ni_xCo_3-xO₄ nanoplates and interfacial modulation through elemental doping and morphologically fitted assembly of Ti₃C₂T_x nanosheets for high performances in OER/HER and overall water splitting. Over the resulting Ni_0.09Co_2.91O₄/Ti₃C₂T_x-HT in a solution of 1 M KOH, the OER and HER overpotentials of 262 and 210 mV, respectively, are achievable at a current density of 10 mA cm^-2. In the case of the overall water splitting by using Ni_0.09Co_2.91O₄/Ti₃C₂T_x-HT as anode and cathode catalysts, only a potential of 1.66 V is needed to obtain a current density of 10 mA cm^-2, and the catalysts can stand for a period of 70 h, remarkably outperforming the RuO₂-Pt/C-based catalyst and benefiting from the intensive association and interfacial function between the Ti₃C₂T_x and Ni_xCo_3-xO₄ nanosheets. Interestingly, a surface reconstruction from the (112) to (111) facet structure occurred upon the fine-tuned Ni doping of regular Ni_xCo_3-xO₄ hexagonal nanoplates and led to a highly active catalyst surface. At x = 0.09, the amount of Ni³⁺ becomes the highest, which is favorable for the generation of the critical OH intermediates on Ni_xCo_3-xO₄/Ti₃C₂T_x-HT. The current study documented the significance of the well-controlled interfacial assembly of transition-metal oxide/MXenes as an effective electrocatalyst in the OER/HER and overall water splitting processes and provided the insights into the structure-performance correlation over such kinds of precious metal-free catalysts.

High-Efficiency of Bi-Functional-Based Perovskite Nanocomposite for Oxygen Evolution and Oxygen Reduction Reaction: An Overview

High efficient, low-cost and environmentally friendly-natured bi-functional-based perovskite electrode catalysts (BFPEC) are receiving increasing attention for oxygen reduction/oxygen evolution reaction (ORR/OER), playing an important role in the electrochemical energy conversion process using fuel cells and rechargeable batteries. Herein, we highlighted the different kinds of synthesis routes, morphological studies and electrode catalysts with A-site and B-site substitution co-substitution, generating oxygen vacancies studies for boosting ORR and OER activities. However, perovskite is a novel type of oxide family, which shows the state-of-art electrocatalytic performances in energy storage device applications. In this review article, we go through different types of BFPECs that have received massive appreciation and various strategies to promote their electrocatalytic activities (ORR/OER). Based on these various properties and their applications of BFPEC for ORR/OER, the general mechanism, catalytic performance and future outlook of these electrode catalysts have also been discussed.

Bimetallic Hollow Tubular NiCoOx as a Bifunctional Electrocatalyst for Enhanced Oxygen Reduction and Evolution Reaction

The development of high-efficiency oxygen electrocatalysts with earth-abundant transition metals rather than scarce noble metals has aroused growing interests due to their potential for energy storage and conversion applications. Herein, we developed a facile strategy to synthesize hollow tubular bimetallic Ni-Co oxide rooted with dense nanosheets for enhanced bifunctionality and facilitated redox reaction kinetics. Owing to the rational design of morphology and well-dispersed Ni and Co ions, the bimetallic samples exhibit admirable bifunctional electrocatalytic activities. This bimetallic Ni-Co oxide shows superior oxygen electrocatalytic performance in comparison with the monometallic Ni and Co oxides, according to the electrocatalytic synergistic effect from the bimetallic system. The optimized sample with the specific mass ratio of Ni and Co displays the oxygen reduction reaction (ORR) property comparable to commercial Pt/C and oxygen evolution reaction (OER) performance superior to commercial RuO₂. The electrochemical tests and structural characterizations offer in-depth dissection on the electrocatalytic behaviors, especially the superb stability in both ORR and OER tests, as well as the outstanding resistance to methanol poisoning, representing a promising candidate in the renewable energy field.

Rod-like nickel doped Co3Se4/reduced graphene oxide hybrids as efficient electrocatalysts for oxygen evolution reactions

In the oxygen evolution reaction (OER), highly active catalysts are essential for reducing the overpotential and improving the slow kinetics of the process. Cobalt selenide (Co₃Se₄) has always been considered as a promising electrocatalyst for the OER due to the well-suited electronic configuration of the Co ions in it. However, poor exposure of the active sites and low electron conductivity are still its biggest problems. In this study, we report an efficient Ni-doped rod-like Co₃Se₄ hybridized with reduced graphene oxide (Ni-Co₃Se₄/rGO) as an OER electrocatalyst. The Ni doping regulates the electronic structure of Co₃Se₄ and significantly reduces the overpotential of Co₃Se₄ toward the OER under alkaline conditions. Simultaneously, hybridization of the reduced graphene oxide (rGO) enhances the conductivity which leads to the improvement in OER activity. The Ni-Co₃Se₄/rGO catalyst shows a lower overpotential (284 mV at 10 mA cm^-2) as well as a Tafel slope (71 mV dec^-1), which outperformed the benchmark of commercial RuO₂. Moreover, Ni-Co₃Se₄/rGO also shows high stability and long-term durability.

Enabling and Inducing Oxygen Vacancies in Cobalt Iron Layer Double Hydroxide via Selenization as Precatalysts for Electrocatalytic Hydrogen and Oxygen Evolution Reactions

Production of hydrogen by water electrolysis is an environment-friendly method and comparatively greener than other methods of hydrogen production such as stream reforming carbon, hydrolysis of metal hydride, etc. However, sluggish kinetics of the individual half-cell reactions hinders the large-scale production of hydrogen. To minimize this disadvantage, finding an appropriate, competent, and low-cost catalyst has attracted attention worldwide. Layer double hydroxide (LDH)-based materials are promising candidates for oxygen evolution reaction (OER) but not fruitful and their hydrogen evolution reaction (HER) activity is very poor, due to the lack of ionic conductivity. The inclusion of chalcogenide and generation of inherent oxygen vacancies in the lattice of LDH lead to improvement of both OER and HER activities. The presence of rich oxygen vacancies was confirmed using both the Tauc plot (1.11 eV, vacancy induction) and the photoluminescence study (peak at 426 nm, photoregeneration of oxygen). In this work, we have developed vacancy-enriched, selenized CoFe-LDH by the consequent wet-chemical and hydrothermal routes, respectively, which was used for OER and HER applications in 1 M KOH and 0.5 M H₂SO₄ electrolytes, respectively. For OER, the catalyst required only 251 mV overpotential to reach a 50 mA/cm² current density with a Tafel slope value of 47 mV/dec. For HER, the catalyst demanded only 222 mV overpotential for reaching a 50 mA/cm² current density with a Tafel slope value of 126 mV/dec. Hence, generating oxygen vacancies leads to several advantages from enhancing the exposed active sites to high probability in obtaining electrocatalytically active species and subsequent assistance in oxygen and hydrogen molecule cleavage.

Activity manifestation via architectural manipulation by cubic silica-derived Co3O4 electrocatalysts towards bifunctional oxygen electrode performance

A KIT-6-derived Co3O4 material demonstrates superior bifunctional activity due to its higher densities of Co3+ and Co2+ sites.

Recent advances of noble-metal-free bifunctional oxygen reduction and evolution electrocatalysts

Bifunctional oxygen reduction and evolution constitute the core processes for sustainable energy storage. The advances on noble-metal-free bifunctional oxygen electrocatalysts are reviewed.

Comparison of the effects of cation and phosphorus doping in cobalt-based spinel oxides towards the oxygen evolution reaction

Phosphorus incorporation further boosted the OER activity of cation-doped Co-based spinel oxides via remarkably tuning the oxygen vacancies, crystallinity and electrochemically active surface area on the surface.

Construction of FeCo2O4@N-Doped Carbon Dots Nanoflowers as Binder Free Electrode for Reduction and Oxidation of Water

Electrochemical water splitting is known as a potential approach for sustainable energy conversion; it produces H₂ fuel by utilizing transition metal-based catalysts. We report a facile synthesis of FeCo₂O₄@carbon dots (CDs) nanoflowers supported on nickel foam through a hydrothermal technique in the absence of organic solvents and an inert environment. The synthesized material with a judicious choice of CDs shows superior performance in hydrogen and oxygen evolution reactions (HER and OER) compared to the FeCo₂O₄ electrode alone in alkaline media. For HER, the overpotential of 205 mV was able to produce current densities of up to 10 mA cm⁻², whereas an overpotential of 393 mV was needed to obtain a current density of up to 50 mA cm⁻² for OER. The synergistic effect between CDs and FeCo₂O₄ accounts for the excellent electrocatalytic activity, since CDs offer exposed active sites and subsequently promote the electrochemical reaction by enhancing the electron transfer processes. Hence, this procedure offers an effective approach for constructing metal oxide-integrated CDs as a catalytic support system to improve the performance of electrochemical water splitting.

Oxygen Reduction and Evolution on Ni-modified Co3 O4 (1 1 1) Cathodes for Zn-Air Batteries: A Combined Surface Science and Electrochemical Model Study

The performance of structurally and chemically well-defined Ni-free and Ni-modified single-crystalline Co3 O4 (1 1 1) thin-film electrodes in the oxygen reduction and evolution reactions (ORR and OER) was investigated in a combined surface science and electrochemistry approach. Pure and Ni-modified Co3 O4 (1 1 1) film electrodes were prepared and characterized under ultrahigh-vacuum conditions by scanning tunneling microscopy and X-ray photoelectron spectroscopy. Both Ni decoration (by post-deposition of Ni) and Ni doping (by simultaneous vapor deposition of Ni, Co, and O2 ) induced distinct differences in the base cyclic voltammograms in 0.5 m KOH at potentials higher than 0.7 V compared with Co3 O4 (1 1 1) electrodes. Also, all oxide film electrodes showed a higher overpotential for the ORR but a lower one for the OER than polycrystalline Pt. Ni modification significantly improved the ORR current densities by increasing the electrical conductivity, whereas the OER onset of approximately 1.47 VRHE (RHE: reversible hydrogen electrode) at 0.1 mA cm-2 was almost unchanged.

Bifunctional Catalysts for Reversible Oxygen Evolution Reaction and Oxygen Reduction Reaction

Metal-air batteries (MABs) and reversible fuel cells (RFCs) rely on the bifunctional oxygen catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Finding efficient bifunctional oxygen catalysts is the ultimate goal and it has attracted a great deal of attention. The dilemma is that a good ORR catalyst is not necessarily efficient for OER, and vice versa. Thus, the development of a new type of bifunctional oxygen catalysts should ensure that the catalysts exhibit high activity for both OER and ORR. Composites with multicomponents for active centers supported on highly conductive matrices could be able to meet the challenges and offering new opportunities. In this Review, the evolution of bifunctional catalysts is summarized and discussed aiming to deliver high-performance bifunctional catalysts with low overpotentials.

Hierarchical Co3O4 nanorods anchored on nitrogen doped reduced graphene oxide: a highly efficient bifunctional electrocatalyst for rechargeable Zn–air batteries

The electrocatalytic activity of the N-rGO/Co₃O₄ nanocomposites was tuned towards highly efficient bifunctional air-cathodes for Zn–Air batteries.

The one-pot synthesis of porous Ni0.85Se nanospheres on graphene as an efficient and durable electrocatalyst for overall water splitting

Ni_0.85Se/RGO composite exhibits extraordinary water splitting.

Study of “Ni-doping” and “open-pore microstructure” as physico-electrochemical stimuli towards the electrocatalytic efficiency of Ni/NiO for the oxygen evolution reaction

Added “Ni-doping” and “open-pore microstructure” act as physico-electrochemical stimuli towards enhanced electrocatalytic efficiency and electromechanical stability of Ni/NiO for the low-overpotential oxygen evolution reaction in alkaline medium.

Bifunctional Electrocatalyst of Low-Symmetry Mesoporous Titanium Dioxide Modified with Cobalt Oxide for Oxygen Evolution and Reduction Reactions

Hybrids of low-symmetry (disordered) mesoporous titanium dioxide modified with different weight ratios of cobalt oxide nanoparticles (Co3O4(x)/lsm-TiO2) are prepared using a one-pot self-assembly surfactant template. The physicochemical characterization of Co3O4(x)/lsm-TiO2 hybrids by scanning and transmission electron microscopy, X-ray diffraction, N2 adsorption–desorption isotherms, and X-ray photoelectron spectroscopy confirm the successful incorporation of cobalt oxide nanoparticles (2–3 nm in diameter) with preservation of the highly mesoporous structure of titanium dioxide substrate. Among these mesoporous hybrids, the ~3.0 wt.% Co3O4/lsm-TiO2 exhibits the best performance toward both the oxygen evolution (OER) and reduction (ORR) reactions in alkaline solution. For the OER, the hybrid shows oxidation overpotential of 348 mV at 10 mA cm−2, a turnover frequency (TOF) of 0.034 s−1, a Tafel slope of 54 mV dec−1, and mass activity of 42.0 A g−1 at 370 mV. While for ORR, an onset potential of 0.84 V vs. RHE and OER/ORR overpotential gap (ΔE) of 0.92 V are achieved which is significantly lower than that of commercial Pt/C, hexagonal mesoporous, and bulk titanium dioxide analogous. The Co3O4/lsm-TiO2 hybrid demonstrates significantly higher long-term durability than IrO2. Apparently, such catalytic activity performance originates from the synergetic effect between Co3O4 and TiO2 substrate, in addition to higher charge carrier density and the presence of disordered mesopores which provide short ions diffusion path during the electrocatalytic process.

Preparation of Yolk-Shell-Structured Cox Fe1-x P with Enhanced OER Performance

The design and development of low-cost, highly efficient, and stable electrocatalysts to take the place of noble-metal catalysts for the oxygen evolution reaction (OER) remain a significant challenge. Herein, the synthesis of yolk-shell-structured binary transition metal phosphide Co_x Fe_1-x P with different Co/Fe ratios by phosphidation of a cobalt ferrite precursor is reported. The as-synthesized Co_x Fe_1-x P catalysts were used for the OER. All yolk-shell Co_x Fe_1-x P catalysts with different Co/Fe ratios showed much better performance than the corresponding solid catalyst. The formation of Co oxides on the catalyst surface during OER and the optimal Co/Fe ratio were found to be critical to their activity. Among the as-prepared Co_x Fe_1-x P catalysts, that with a Co/Fe ratio of 0.47/0.53 (Co_0.47 Fe_0.53 P) exhibited the best performance. Co_0.47 Fe_0.53 P has an overpotential of 277 mV at a current density of 10 mA cm^-2 , a Tafel slope of 37 mV dec^-1 , and superior stability in alkaline medium. The outstanding performance is partly ascribed to the transfer of valence electrons from Co to P and Fe. The Co_0.47 Fe_0.53 P matrix with excellent conductivity and Fe phosphate that is stable on the surface of the catalyst are also helpful for the OER performance. In addition, the yolk-shell structure of Co_0.47 Fe_0.53 P increases the contact area between electrolyte and catalyst. These characteristics of Co_0.47 Fe_0.53 P greatly improve its OER performance. This optimized binary transition metal phosphide provides a new approach for the design of nonprecious-metal electrocatalysts.

Rose-like Nanocomposite of Fe-Ni Phosphides/Iron Oxide as Efficient Catalyst for Oxygen Evolution Reaction

In order to accelerate the reaction rate of water splitting, it is of immense importance to develop low-cost, stable and efficient catalysts. In this study, the facile synthesis of a novel rose-like nanocomposite catalyst (Ni₂ P/Fe₂ P/Fe₃ O₄ ) is reported. The synthesis process includes a solvothermal step and a phosphatization step to combine iron oxides and iron-nickel phosphides. Ni₂ P/Fe₂ P/Fe₃ O₄ performs well in catalyzing oxygen evolution reaction, with a very low overpotential of 365 mV to reach 10 mA cm^-2 current density. The Tafel slope is as low as 59 mV dec^-1 . Ni₂ P/Fe₂ P/Fe₃ O₄ has a large double-layer capacitance that contributes to a high electrochemically active area. Moreover, this catalyst is very stable for long-term use. Therefore, the Ni₂ P/Fe₂ P/Fe₃ O₄ catalyst has a high potential for use in oxygen evolution reactions.