Co3O4-δ Quantum Dots As a Highly Efficient Oxygen Evolution Reaction Catalyst for Water Splitting

Bimetallic Ni-Co-MOF Nanostructures for Seawater Electrolysis: Unveiling the Mechanism of the Oxygen Evolution Reaction Using Impedance Spectroscopy

Designing anode electrodes with long-term stability and efficiency for seawater electrolysis is crucial for addressing key challenges in sustainable hydrogen production and clean energy systems. Here, we developed self-supporting bimetallic Ni-Co-MOF electrodes, demonstrating exceptional performance and durability in alkaline seawater electrolysis due to their high voltammetric charge density and increased electrochemically accessible active sites. The reaction kinetics of the water oxidation reaction in the presence of Cl- ions (at concentrations ranging from 0.5 M to 3.5 M) were investigated through electrochemical impedance spectroscopy (EIS) analysis, focusing on the kinetic parameters, suggesting that the rate-determining step (RDS) is the chemical process following the initial electron transfer. Notably, Cl- ions in the electrolyte medium do not alter the OER rate-limiting step, as indicated by negligible variations in the anodic transfer coefficient values. However, blocking active sites is evident from the decrease in interfacial chemical capacitance (Cchem) values with increasing Cl- concentration. These findings offer a deeper understanding of OER reaction kinetics in chloride-containing environments by correlating electrochemical kinetic parameters with active site availability. This work highlights critical considerations for designing efficient and durable anodes for seawater electrolysis.

Self-supporting hierarchical Co3O4-nanowires@NiO-nanosheets core-shell nanostructure on carbon foam to form efficient bifunctional electrocatalyst for overall water splitting

Designing cost-effective and robust bifunctional electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is highly desired in hydrogen production from overall water splitting, but still suffers great challenges due to the sluggish catalytic OER/HER kinetics. In this paper, a surface/defect engineering strategy is developed to synthesize three-dimensional (3D) carbon foam (CF)-supported unique hierarchical Co3O4 nanowires@NiO nanosheets core-shell nanostructured catalyst (NiO@Co3O4/CF) with rich oxygen-vacancies as a novel bifunctional catalyst for alkaline water splitting electrolysis. Benefited from the synergy of the 3D hierarchical core-shell structure and rich oxygen vacancies, the as-obtained NiO@Co3O4/CF shows both excellent OER (η200 = 325 mV, η500 = 374 mV) and HER (η10 = 104 mV) activities with low Tafel slopes (64.26 mV dec-1 for OER and 109.14 mV dec-1 for HER, respectively) and outstanding stability. A simple overall water splitting electrolysis cell assembled by this bifunctional NiO@Co3O4/CF as both OER and HER catalysts requires only a cell voltage of 1.53 V to obtain the current density of 10 mA cm-2 and displays a long-term stability. This work has successfully developed an approach for rational design and novel synthesis of metal oxide hybrids as bifunctional electrocatalysts with high activity and stability for overall water splitting.

2D nanocomposite materials for HER electrocatalysts - a review

Hydrogen energy has the potential to be a cost-effective and strong technology for brighter development. Hydrogen fuel production by water electrolyzers has attracted attention. 2D nanocomposites with distinctive properties have been extensively explored for various applications from hydrogen evolution reactions to improving the efficiency of water electrolyzer, which is the most eco-friendly, and high-performance for hydrogen production. Recently, typical 2D nanocomposites such as Metal-Free 2D, TMDs, Mxene, LDH, organic composites, and Heterostructure have recently been thoroughly researched for use in the HER. We discuss effective ways for increasing the HER efficiency of 2D catalysts in this paper, And the unique advantages and mechanisms for specific applications are highlighted. Several essential regulating strategies for developing 2D nanocomposite-based HER electrocatalysts are included such as interface engineering, defect engineering, heteroatom doping, strain & phase engineering, and hybridizing which improve HER kinetics, the electrical conductivity, accessibility to catalytic active sites, and reaction energy barrier can be optimized. Finally, the future prospects for 2D nanocomposites in HER are discussed, as well as a thorough overview of a variety of methodologies for designing 2D nanocomposites as HER electrocatalysts with excellent catalytic performance. We expect that this review will provide a thorough overview of 2D nanocatalysts for hydrogen production.

Organic-Inorganic Hybrid Crystal-Assisted Etching of Nickel Foam for the Collectively Exhaustive Electrochemical Performance of Oxygen Evolution Reaction

In this work, an organic-inorganic hybrid crystal, violet-crystal (VC), was used to etch the nickel foam (NF) to fabricate a self-standing electrode for the water oxidation reaction. The efficacy of VC-assisted etching manifests the promising electrochemical performance towards the oxygen evolution reaction (OER), requiring only ~356 and ~376 mV overpotentials to reach 50 and 100 mA cm-2 , respectively. The OER activity improvement is attributed to the collectively exhaustive effects arising from the incorporation of various elements in the NF, and the enhancement of active site density. Furthermore, the self-standing electrode is robust, exhibiting a stable OER activity after 4,000 cyclic voltammetry cycles, and ~50 h. The anodic transfer coefficients (αa ) show that the first electron transfer step is the rate-determining step on the surface of NF-VCs-1.0 (NF etched by 1 g of VCs) electrode, while the chemical step involving dissociation following the first electron transfer step is identified as the rate-limiting step in other electrodes. The lowest Tafel slope value observed in the NF-VCs-1.0 electrode indicates the high surface coverage of oxygen intermediates and more favorable OER reaction kinetics, as confirmed by high interfacial chemical capacitance and low charge transport/interfacial resistance. This work demonstrates the importance of VCs-assisted etching of NF to activate the OER, and the ability to predict reaction kinetics and rate-limiting step based on αa values, which will open new avenues to identify advanced electrocatalysts for the water oxidation reaction.

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.

Unveiling the Origin of Co3 O4 Quantum Dots for Photocatalytic Overall Water Splitting

Spinel cobalt oxide displays excellent photocatalytic performance, especially in solar driven water oxidation. However, the process of water reduction to hydrogen is considered as the Achilles' heel of solar water splitting over Co₃ O₄ owing to its low conduction band. Enhancement of the water splitting efficiency using Co₃ O₄ requires deeper insights of the carrier dynamics during water splitting process. Herein, the carrier dynamic kinetics of colloidal Co₃ O₄ quantum dots-Pt hetero-junctions is studied, which mimics the hydrogen reduction process during water splitting. It is showed that the quantum confinement effect induced by the small QD size raised the conduction band edge position of Co₃ O₄ QDs, so that the ligand-to-metal charge transfer from 2p state of oxygen to 3d state of Co²⁺ occurs, which is necessary for overall water splitting and cannot be achieved in Co₃ O₄ bulk crystals. The findings in this work provide insights of the photocatalytic mechanism of Co₃ O₄ catalysts and benefit rational design of Co₃ O₄ -based photocatalytic systems.

Two-step facile synthesis of Co3O4@C reinforced PbO2 coated electrode to promote efficient oxygen evolution reaction for zinc electrowinning

The conventional Pb-Ag alloy possesses a high oxygen evolution reaction overpotential, poor stability, and short service life in acidic solutions, making it an unsuitable sort of anode material for the zinc electrowinning process. Therefore, a layered carbon-covered cobalt tetroxide (Co3O4@C)-reinforced PbO2-coated electrode is fabricated via a facile two-step pyrolysis-oxidation and subsequent electrodeposition process. As a result, the reinforced PbO2-coated electrode exhibits a low OER overpotential of 517 mV at 500 A m-2 and a Tafel slope of 0.152 V per decade in a zinc electrowinning simulation solution (0.3 M ZnSO4 and 1.53 M H2SO4). The reduced overpotential of 431 mV at 500 A m-2 compared to traditional Pb-0.76%Ag alloy leads to improved energy savings, which is attributable to the presence of Co3O4@C to refine the grain size and thus increase the effective contact area. Moreover, the reinforced PbO2-coated electrode has a prolonged service life of 93 h at 20 000 A m-2 in 1.53 M H2SO4. Therefore, an accessible and efficient strategy for preparing a coated electrode to improve OER performance for zinc electrowinning is presented in this research.

Boosting Oxygen Evolution Reaction on Metallocene-based Transition Metal Sulfides Integrated with N-doped Carbon Nanostructures

In this study, utilizing metallocene and organosulfur chelating agent, an innovative synthetic route was developed towards electrochemically activated transition metal sulfides entrapped in pyridinic nitrogen-incorporated carbon nanostructures for superior oxygen evolution reaction (OER). Most importantly, the preferential electrochemical activation process, which consisted of both anodic and cathodic pre-treatment steps, strikingly enhanced OER and long-lasting cyclic stability. The substantial increase in OER electrocatalytic activity of Ni₉ S₈ /Ni₃ S₂ -NC and Co₉ S₈ -NC during the activation process was mainly attributed to the increase of faradaic active site density on the catalytic layer resulting from the reconstruction of catalytic interfaces. It was also found that Fe-based metallocene [ferrocene (Fc)]-incorporation in the Co₉ S₈ -NC and Ni₉ S₈ /Ni₃ S₂ -NC nanostructures significantly boosted the OER activity. Thus, the combined effects of Fc-incorporation and the electrochemical activation process reduced the overpotential to about 115 and 95 mV on the Ni₉ S₈ /Ni₃ S₂ -NC and Co₉ S₈ -NC nanostructures to derive a current density of 10 mA cm^-2 , respectively. Notably, Fc-Ni₉ S₈ /Ni₃ S₂ -NC electrocatalysts required very small overpotentials of around 222, 244, and 280 mV to acquire the current densities of 10, 20, and 50 mA cm^-2 , respectively. This work opens up a new avenue for superior OER electrocatalysts by the utilization of metallocene and the preferential electrochemical activation process.

A strategy for preparing high-efficiency and economical catalytic electrodes toward overall water splitting

Electrolyzing water technology to prepare high-purity hydrogen is currently an important field in energy development. However, the preparation of efficient, stable, and inexpensive hydrogen production technology from electrolyzed water is a major problem in hydrogen energy production. The key technology for hydrogen production from water electrolysis is to prepare highly efficient catalytic, stable and durable electrodes, which are used to reduce the overpotential of the hydrogen evolution reaction and the oxygen evolution reaction of electrolyzed water. The main strategies for preparing catalytic electrodes include: (i) choosing cheap, large specific surface area and stable base materials, (ii) modulating the intrinsic activity of the catalytic material through elemental doping and lattice changes, and (iii) adjusting the morphology and structure to increase the catalytic activity. Based on these findings, herein, we review the recent work in the field of hydrogen production by water electrolysis, introduce the preparation of catalytic electrodes based on nickel foam, carbon cloth and new flexible materials, and summarize the catalytic performance of metal oxides, phosphides, sulfides and nitrides in the hydrogen evolution and oxygen evolution reactions. Secondly, parameters such as the overpotential, Tafel slope, active site, turnover frequency, and stability are used as indicators to measure the performance of catalytic electrode materials. Finally, taking the material cost of the catalytic electrode as a reference, the successful preparations are comprehensively compared. The overall aim is to shed some light on the exploration of high-efficiency and economical electrodes in energy chemistry and also demonstrate that there is still room for discovering new combinations of electrodes including base materials, composition lattice changes and morphologies.

Structural dependence of electrosynthesized cobalt phosphide/black phosphorus pre-catalyst for oxygen evolution in alkaline media

The integration of black phosphorus (BP) with metal phosphides is known to produce high-performance electrocatalysts for oxygen evolution reduction (OER), although increased stability and prevention of the degradation of their lone pairs would be desirable improvements. In this work, cobalt phosphide (CoP)/BP heterostructures were electrochemically synthesized with a two-electrode system, where cobalt ions were generated in situ at a Co anode, and non-aggregated BP nanosheets (NSs) were exfoliated from the bulky BP cathode. With an electrolysis voltage of 30 V, the CoP/BP heterostructure exhibited a superior and stable OER performance (e.g., an overpotential of 300 mV at 10 mA cm^-2, which is 41 mV lower than that obtained with a RuO₂ catalyst). The CoO_x formed in situ during the OER catalysis and remaining CoP synergistically contributed to the enhanced OER performance. The present strategy provides a new electrosynthetic method to prepare stable BP electrocatalysts and also further expands their electrochemical applications.

Liquid-Phase Deposition Synthesis of ZIF-67-Derived Synthesis of Co3O4@TiO2 Composite for Efficient Electrochemical Water Splitting

In this article, a novel Co3O4@TiO2 composite is synthesized by applying two-step methods. ZIF-67 is synthesized and used as a template for the synthesis of the composite. The composite is designed by using the effective photocatalytic properties of Co3O4 and TiO2. The resulting synthesized composite is supposed to offer superior properties compared to their counterparts. The synthesized Co3O4@TiO2 composite is characterized by powder X-ray diffraction (PXRD), Brunauer–Emmet–Teller (BET), atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Electrochemical water splitting, including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) studies on the Co3O4@TiO2 composite, is evaluated by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) analysis in a 2M aqueous KOH electrolyte. The current generation stability of these samples is deliberated by chronoamperometric measurements. It is observed, from LSV results at a 1 mV/s scan rate, that metal oxides incorporated on other metal oxides have a higher current density and lower onset potential as compared to pure metal oxides. From the obtained results, it has become evident that synthesized studies on the Co3O4@TiO2 composite possess significant potential for electrochemical water splitting with the lowest onset potential, highest current density, better OER, and HER activity.

Graphene-induced growth of Co3O4 nanoplates with modulable oxygen vacancies for improved OER properties

Graphene-induced growth of Co(OH)2 nanoplates from Co3O4 nanospheres was reported, showing an ultralow overpotential of 240 mV at 10 mA cm−2 and a Tafel slope of 107.8 mV dec−1.

Graphene oxide induced assembly and crumpling of Co3O4 nanoplates

Cobalt (II, III) oxide (Co₃O₄) has been widely studied and applied in various fields, however, it suffers from slow mass and electron transfer during applications. Herein, crumpled Co₃O₄ and Co₃O₄/reduced graphene oxide (rGO) with tunable 2D-in-3D structures were prepared by combining spray pyrolysis with a graphene oxide (GO) template. The 2D Co₃O₄ nanoplates were interconnected with each other to form a 3D ball with many wrinkles, resulting in defect enrichment on the abundant boundaries of the nanosheets, which provided more active sites for catalytic reactions. In addition, the unique 2D-in-3D structure allowed fast mass transfer and structural stability. Furthermore, the assembled structure could be understood as being composed of uniformly distributed oxygen-containing functional groups pinning metal cations on the GO surface through electrostatic interaction, and the 2D structure of the GO enabled the in situ converted Co₃O₄ to grow along the GO surface with excellent dispersion. Taking advantage of the above, the Co₃O₄/rGO balls demonstrated an excellent oxygen evolution reaction performance, an overpotential of 298 mV at a current density of 10.0 mA cm^-2 and a current density of 115.9 mA cm^-2 at the overpotential of η = 500 mV.

Microwave-Hydrothermal Tuning of Spinel-Type Co 3 O 4 Water Oxidation Catalysts

Water oxidation is the bottleneck reaction for overall water splitting as a direct and promising strategy toward clean fuels. However, the development of robust and affordable heterogeneous water oxidation catalysts remains challenging, especially with respect to the wide parameter space of synthesis and resulting material properties. Oxide catalysts performance in particular has been shown to depend on both synthetic routes and applied catalytic test methods. We here focus on spinel-type Co3O4 as a representative case for an in-depth study of the influence of rather subtle synthetic parameter variations on the catalytic performance. To this end, a series of Co3O4 samples was prepared via time-saving and tunable microwave-hydrothermal synthesis, while systematically varying a single parameter at a time. The resulting spinel-type catalysts were characterized with respect to key materials properties, including crystallinity, oxidation state and surface area using a wide range of analytical methods, such as PXRD, Raman/IR, XAS and XPS spectroscopy. Their water oxidation activity in electrocatalytic and chemical oxidation setups was then compared and correlated with the obtained catalyst properties. Both water oxidation methods displayed related trends concerning favorable synthetic parameters, namely higher activity for lower synthesis temperatures, lower precursor concentrations, addition of hydrogen peroxide and shorter ramping and reaction times, respectively. In addition to the surface area, structural features such as disorder were found to be influential for the water oxidation activity. The results prove that synthetic parameter screening is essential for optimal catalytic performance, given the complexity of the underlying performance-properties relationships.

Mo-Doped Zn, Co Zeolitic Imidazolate Framework-Derived Co9S8 Quantum Dots and MoS2 Embedded in Three-Dimensional Nitrogen-Doped Carbon Nanoflake Arrays as an Efficient Trifunctional Electrocatalysts for the Oxygen Reduction Reaction, Oxygen Evolution Reaction, and Hydrogen Evolution Reaction

Herein, we first propose a facile strategy to synthesize Co₉S₈ and MoS₂ nanocrystals embedded in porous carbon nanoflake arrays supported on carbon nanofibers (Co₉S₈-MoS₂/N-CNAs@CNFs) by the pyrolysis of Mo-doped Zn, Co zeolitic imidazolate framework grown on carbon nanofibers and subsequent sulfuration. The electrocatalyst shows high and stable electrocatalytic performance, with a half-wave potential of 0.82 V for oxygen reduction reaction and an overpotential at 10 mA cm^-2 for oxygen evolution reaction (0.34 V) and hydrogen evolution reaction (0.163 V), which outperform the metal-organic framework-derived transition metal sulfide catalysts reported so far. Furthermore, the Co₉S₈-MoS₂@N-CNAs@CNFs are employed as an air cathode in a liquid-state and all-solid-state zinc-air battery, presenting high power densities of 222 and 96 mW cm^-2, respectively. Such excellent catalytic activities are mainly owing to the unique three-dimensional structure and chemical compositions, optimal electronic conductivity, adequate surface area, and the abundance of active sites. Thus, this work provides an important method for designing other metal-organic framework-derived three-dimensional structural sulfide quantum dot multifunctional electrocatalysts for wider application in highly efficient catalysis and energy storage.

Multifunctional Electrocatalysis on a Porous N-Doped NiCo2O4@C Nanonetwork

Developing a multifunctional electrocatalyst with eminent activity, strong durability, and cheapness for the hydrogen/oxygen evolution reaction (HER/OER) and oxygen reduction reaction (ORR) is critical to overall water splitting and regenerative fuel cells. Herein, a nitrogen-doped nanonetwork assembled by porous and defective NiCo₂O₄@C nanowires grown on nickel foam (N-NiCo₂O₄@C@NF) is crafted via biomimetic mineralization and following carbonization of phase-transited lysozyme (PTL)-coupled NiCo₂O₄. The as-obtained N-NiCo₂O₄@C@NF electrocatalysts exhibit an exceptional catalytic activity with ultralow overpotentials for the HER (42 mV) and OER (242 mV) to afford 10 mA cm^-2 while maintaining good stability in alkaline media. Meanwhile, the N-NiCo2O4@C electrocatalysts presents a superior catalytic activity for ORR and a favorable four-electron pathway. The unprecedented catalytic performance arises from a highly porous structure and abundant defects and synergistic effects of components. This work may offer a new possibility in the exploration of multifunctional electrocatalysts for various energy-related electrocatalytic reactions.

MoOx Nanoparticle Catalysts for d-Glucose Epimerization and Their Electrical Immobilization in a Continuous Flow Reactor

Activity and immobilization of catalysts in liquid-phase reactions seem not to coexist. We report here the excellent activity of an MoO_x nanoparticle (NP) catalyst for d-glucose epimerization to d-mannose and the electrical immobilization of NPs in a flow reaction. Prior to that, a green and one-pot method to synthesize the MoO_x NPs (3.05 nm) via oxidizing metal Mo by hydrogen peroxide was presented. The NPs overwhelmed the reported catalysts including epimerase for d-glucose epimerization, originating from a strong interaction between the NPs and the reactant that was demonstrated by ex situ and in situ characterizations and theoretical calculations. The electrically charged feature of NPs inspired us to find a convenient way to "immobilize" them inside an activated carbon bed, and thereby, a flow reactor was assembled. The continuous epimerization was run under 24 V for 16 days with an almost unchanged activity, and only 3.2% of total Mo was lost.

Promoting Oxygen Evolution Reaction of Co-Based Catalysts (Co3 O4 , CoS, CoP, and CoN) through Photothermal Effect

The increase of reaction temperature of electrocatalysts is regarded as an efficient method to improve the oxygen evolution reaction (OER) activity. Herein, it is reported that the electrocatalytic performance of dual functional (i.e., electrocatalytic and photothermal functions) Co₃ O₄ can be dramatically improved via its photothermal effect. The operating temperature of the Co₃ O₄ electrode is elevated in situ under near infrared (NIR) light irradiation, resulting in enhanced oxygen evolution activity due to its accelerated electrical conductivity, reaction kinetics, and desorption rate of O₂ bubbles from the electrode. In addition, photothermal effect can also enhance the electrocatalytic reaction rates of metal-doped Co₃ O₄ electrodes, indicating that it is able to significantly improve the OER activities of electrodes together with other modification strategies. With the assistance of the photothermal effect, the obtained Ni-doped Co₃ O₄ catalyst requires an extremely low overpotential of 208 mV to achieve a benchmark of 10 mA cm^-2 with a small Tafel slope, superior to most reported Co-based catalysts. Significantly, the electrocatalytic performance of other electrodes with photothermal effect, such as CoN, CoP, and CoS, are also boosted under NIR light irradiation, indicating opportunities for implementing photothermal enhancement in electrocatalytic water splitting.

Preparative History vs Driving Force in Water Oxidation Catalysis: Parameter Space Studies of Cobalt Spinels

The development of efficient, stable, and economic water oxidation catalysts (WOCs) is a forefront topic of sustainable energy research. We newly present a comprehensive three-step approach to systematically investigate challenging relationships among preparative history, properties, and performance in heterogeneous WOCs. To this end, we studied (1) the influence of the preparative method on the material properties and (2) their correlation with the performance as (3) a function of the catalytic test method. Spinel-type Co3O4 was selected as a clear-cut model WOC and synthesized via nine different preparative routes. In search of the key material properties for high catalytic performance, these cobalt oxide samples were characterized with a wide range of analytical methods, including X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, powder X-ray diffraction, Raman spectroscopy, BET surface area analysis, and transmission electron microscopy. Next, the corresponding catalytic water oxidation activities were assessed with the three most widely applied protocols to date, namely, photocatalytic, electrocatalytic, and chemical oxidation. The activity of the Co3O4 samples was found to clearly depend on the applied test method. Increasing surface area and disorder as well as a decrease in oxidation states arising from low synthesis temperatures were identified as key parameters for high chemical oxidation activity. Surprisingly, no obvious property-performance correlations were found for photocatalytic water oxidation. In sharp contrast, all samples showed similar activity in electrochemical water oxidation. The substantial performance differences between the applied protocols demonstrate that control and comprehensive understanding of the preparative history are crucial for establishing reliable structure-performance relationships in WOC design.

Phase-Transited Lysozyme-Driven Formation of Self-Supported Co3O4@C Nanomeshes for Overall Water Splitting

The development of highly efficient catalysts for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is of paramount importance in water splitting. Herein, a phase-transited lysozyme (PTL) is employed as the platform to synthesize nitrogen-doped Co3O4@C nanomesh with rich oxygen vacancies supported on the nickel foam (N-Co3O4@C@NF). This PTL-driven N-Co3O4@C@NF integrates the advantages of porous structure, high exposure of surface atoms, strong synergetic effect between the components and unique 3D electrode configuration, imparting exceptional activity in catalyzing both HER and OER. Remarkably, an alkaline electrolyzer assembled by N-Co3O4@C@NF as both cathode and anode delivers a current density of 10 mA cm-2 at an ultralow cell voltage of 1.40 V, which is not only much lower than that of the commercially noble Pt/C and IrO2/C catalyst couple (≈1.61 V) but also a new record for the overall water splitting. The finding may open new possibilities for the design of bifunctional electrocatalysts for application in practical water electrolysis.

A, Paul A. Tuning water oxidation reactivity by employing surfactant directed synthesis of porous Co3O4 nanomaterials

We have explored Co₃O₄ based nanomaterials for the oxygen evolution reaction prepared via a surfactant directed soft-templating strategy.

Oxygen vacancies confined in Co3O4 quantum dots for promoting oxygen evolution electrocatalysis

Abundant oxygen vacancies confined in Co₃O₄ quantum dots provide more efficient Co(ii), more active sites and improved conductivity for superior OER performance.

Role of flower-like ultrathin Co3O4 nanosheets in water splitting and non-aqueous Li-O2 batteries

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are both fundamental and essential processes for various energy conversion and storage systems. The kinetics of ORR and OER play a critical role in their energy efficiency and practicality. Here, flower-like ultrathin Co3O4 nanosheets synthesized through a facile solvothermal technique were studied as a bifunctional catalyst for both water splitting and non-aqueous Li-O2 batteries. Due to the novel structure and highly active {110} and {100} exposed facets, which can effectively facilitate mass transfer and enhance catalytic capability, Co3O4 nanosheets exhibit better stability and higher ORR/OER activity than Co3O4 nanoparticles, Co3O4 bulks, Pt/C, and RuO2 in alkaline solution. More importantly, Li-O2 batteries with ultrathin Co3O4 nanosheets catalyst can enhance the initial discharge capacity from 6400 to 8600 mA h g-1 and improve the cyclability up to 160 cycles at 500 mA g-1. Unexpectedly, XRD and UV/Vis techniques suggest that the main product in Co3O4 nanosheets based cathodes is LiOH, with resulting LiOH also demonstrating reversible formation/decomposition behavior, rather than Li2O2 in pure Super P based cathodes. Further investigation confirms that Co3O4 can also catalyze the electrolyte decomposition responsible for the formation of LiOH, and a reaction mechanism was illustrated. This work highlights that the traditional high-efficiency bifunctional catalyst in aqueous media may not be suitable for non-aqueous Li-O2 batteries, and the effect of catalyst on electrolyte besides the discharge product should also be carefully considered for the design of more stable and practical Li-O2 systems.

Noble Metal-Free Nanocatalysts with Vacancies for Electrochemical Water Splitting

The fast development of nanoscience and nanotechnology has significantly advanced the fabrication of nanocatalysts and the in-depth study of the structural-activity characteristics of materials at the atomic level. Vacancies, as typical atomic defects or imperfections that widely exist in solid materials, are demonstrated to effectively modulate the physicochemical, electronic, and catalytic properties of nanomaterials, which is a key concept and hot research topic in nanochemistry and nanocatalysis. The recent experimental and theoretical progresses achieved in the preparation and application of vacancy-rich nanocatalysts for electrochemical water splitting are explored. Engineering of vacancies has shown to open up a new avenue beyond the traditional morphology, size, and composition modifications for the development of nonprecious electrocatalysts toward efficient energy conversion. First, an introduction followed by discussions of different types of vacancies, the approaches to create vacancies, and the advanced techniques widely used to characterize these vacancies are presented. Importantly, the correlations between the vacancies and activities of the vacancy-rich electrocatalysts via tuning the electronic states, active sites, and kinetic energy barriers are reviewed. Finally, perspectives on the existing challenges along with some opportunities for the further development of vacancy-rich noble metal-free electrocatalysts with high performance are discussed.

Engineering NiS/Ni2P Heterostructures for Efficient Electrocatalytic Water Splitting

Developing high-active and low-cost bifunctional materials for catalyzing the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) holds a pivotal role in water splitting. Therefore, we present a new strategy to form NiS/Ni₂P heterostructures. The as-obtained NiS/Ni₂P/carbon cloth (CC) requires overpotentials of 111 mV for the HER and 265 mV for the OER to reach a current density of 20 mA cm^-2, outperforming their counterparts such as NiS and Ni₂P under the same conditions. Additionally, the NiS/Ni₂P/CC electrode requires a 1.67 V cell voltage to deliver 10 mA cm^-2 in a two-electrode electrolysis system, which is comparable to the cell using the benchmark Pt/C||RuO₂ electrode. Detailed characterizations reveal that strong electronic interactions between NiS and Ni₂P, abundant active sites, and smaller charge-transfer resistance contribute to the improved HER and OER activity.

A ratiometric photoelectrochemical immunosensor based on g-C3N4@TiO2 NTs amplified by signal antibodies–Co3O4 nanoparticle conjugates

Herein, we report a ratiometric photoelectrochemical (PEC) immunosensor coupled with secondary antibodies–Co₃O₄ nanoparticle conjugates (Ab₂–Co₃O₄ NPs) for signal amplification.

Supercritical fluid processing for the synthesis of NiS2 nanostructures as efficient electrocatalysts for electrochemical oxygen evolution reactions

A facile supercritical fluid process was demonstrated for the synthesis of cubic NiS₂ nanostructures for efficient electrochemical oxygen evolution reactions.