A new hyperbranched water-splitting technique based on Co3O4/MoS2 nano composite catalyst for High-Performance of hydrogen evolution reaction

Due to the extensive use of fossil fuels & their direct influence on the environment, new ways of producing energy sources are highly needed. Hydrogen is the perfect candidate for renewable energy; however, H₂ gas production is associated with disadvantages due to a lack of efficient and active catalysts that could be cost-effective and comparable to platinum performance. Active hydrogen evolution reaction catalysts are needed to advance the development of a cheaper generation of solar fuels. Thus, outperformance, and stable earth abundant. And inexpensive catalysts are highly demanded. That is H₂ gas production from the electrolysis of water through HER. In this work, we present different analytical techniques that characterize an efficient and highly stable catalyst based on transition metal oxide Co₃O₄/MoS₂ nanostructures. And their composites for water splitting in harsh acidic conditions time and material chemical composition as like SEM, EDS, XRD, HRTEM & XPS. The composite material is highly best to produce HER at 10 mA cm^?2 and obtained 268 mV overpotential of nano Co₃O₄/MoS₂ (S3) and Tafel slope of 56 mv/dec. Faraday efficiencies of the hydrogen gas production measured for the 60 min and catalyst is highly durable for the 20 h. The presented catalysts are up to the mark of platinum metal performance and superior to several transition metal oxides. This fabrication technology is a new roadmap for developing active and scalable hydrogen-evolving catalysts by overcoming the issues of fewer catalytic edges, low density, and poor conductivity.     

Three-dimensional ZnCo/MoS2–Co3S4/NF heterostructure supported on nickel foam as highly efficient catalyst for hydrogen evolution reaction

Exploring inexpensive and earth-abundant electrocatalysts for hydrogen evolution reactions is crucial in electrochemical sustainable chemistry field. In this work, a high-efficiency and inexpensive non-noble metal catalysts as alternatives to hydrogen evolution reaction (HER) was designed by one-step hydrothermal and two-step electrodeposition method. The as-prepared catalyst is composed of the synergistic MoS₂–Co₃S₄ layer decorated by ZnCo layered double hydroxides (ZnCo-LDH), which forms a multi-layer heterostructure (ZnCo/MoS₂–Co₃S₄/NF). The synthesized ZnCo/MoS₂–Co₃S₄/NF exhibits a small overpotential of 31 mV and a low Tafel plot of 53.13 mV dec^?1 at a current density of 10 mA cm^?2, which is close to the HER performance of the overpotential (26 mV) of Pt/C/NF. The synthesized ZnCo/MoS₂–Co₃S₄/NF also has good stability in alkaline solution. The excellent electrochemical performance of ZnCo/MoS₂–Co₃S₄/NF electrode originates from its abundant active sites and good electronic conductivity brought by the multilayer heterostructure. This work provides a simple and feasible way to design alkaline HER electrocatalysts by growing heterostructures on macroscopic substrates.     

Multi-shelled CoS2–MoS2 hollow spheres as efficient bifunctional electrocatalysts for overall water splitting

The exploration of highly efficient non-precious electrocatalysts is essential for water splitting devices. Herein, we synthesized CoS₂–MoS₂ multi-shelled hollow spheres (MSHSs) as efficient electrocatalysts both for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) using a Schiff base coordination polymer (CP). Co-CP solid spheres were converted to Co₃O₄ MSHSs by sintering in air. CoS₂–MoS₂ MSHSs were obtained by a solvothermal reaction of Co₃O₄ MSHSs and MoS₄²⁻ anions. CoS₂–MoS₂ MSHSs have a high specific surface area of 73.5 m²g^-1. Due to the synergistic effect between the CoS₂ and MoS₂, the electrode of CoS₂–MoS₂ MSHSs shows low overpotential of 109 mV with Tafel slope of 52.0 mV dec⁻¹ for HER, as well as a low overpotential of 288 mV with Tafel slope of 62.1 mV dec⁻¹ for OER at a current density of 10 mA cm⁻² in alkaline solution. The corresponding two-electrode system needs a potential of 1.61 V (vs. RHE) to obtain anodic current density of 10 mA cm⁻² for OER and maintains excellent stability for 10 h.     

Electrospun fibrous active bimetallic electrocatalyst for hydrogen evolution

Fabricating earth-abundant bifunctional water splitting electrocatalysts with high efficiencies to replace noble metal-based Pt and IrO₂ catalysts is in great demand for the development of clean energy conversion technologies. Molybdenum disulfide (MoS₂) nanostructures have attracted much attention as promising material for hydrogen evolution reaction (HER). The production of hydrogen gas by help of potential efficient earth abundant metal oxides, and stable electrolysis seems a promising for hydrogen evolution reaction pathway in 1 M potassium hydroxide electrolyte media is a hot research topic in the field for clean energy conversion, renewable energies and storage. Here we propose asystem composed NiO nanostructures and MoS₂ deposited on (MoS₂@NiO). Here, by hydrothermal method NiO prepared and MoS₂@NiO by an electrospinning technique complex, can be used as catalyst to produce a large amount of hydrogen gas bubbles. The NiO nanostructures composite having highest synergistic behavior fully and covered by the MoS₂. For the MoS₂@NiO nano composite catalyst, experiment applied in 1 M KOH for the production of hydrogen evolution reaction which exhibits distinct properties from the bulk material. Overpotential values recorded low 406 mV and current density 10 mA cm⁻² measured. Co-catalysts characterized by using different techniques for deep study as scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Owing to their unique structure, as-prepared nanocomposite exhibited enhanced catalytic performance for HER due to high electroactive surface area and swift electron transfer kinetics. Based on the HER polarization curves at low potential electrochemical to examine the effects of intercalants HER catalytic efficiency. Our findings establish low Tafel slope (44 mV/decade) and the catalyst stable for at least 13 h. This simple exploitation of MoS₂@NiO composite catalysts depending on the intended application of their electrochemistry.     

Efficient electrochemical hydrogen evolution reaction and solar activity via bi-functional GO/Co3O4–TiO2 nano hybrid structure

Highly proficient electro and solar catalyst of mixed metal oxides Co₃O₄–TiO₂ modified with graphene oxide (GO) have been synthesized by simple and cost-effective way using sol-gel methodology. This catalyst demonstrated versatile bi-functional features towards the hydrogen evolution reaction (HER) in catalytic water splitting along with solar photo catalytic activity in the degradation of Methyl Orange (MO). XRD profile confirmed that composite presented an anatase and cubic phase for TiO₂ and Co₃O₄, respectively, with the GO network. The morphological structures confirm flaky texture of Co₃O₄ with small irregular spheres of TiO₂ nanoparticles randomly dispersed on the broken sheets of GO. GO and clusters of Co²⁺/Co³⁺ in different regions of host TiO₂ are accountable for decreasing band gap in the composite samples. Co–O–Ti and Co–Ti–C linkages in the composite materials are confirmed by Raman and FTIR studies. In electro catalytic HER in alkaline medium GO/Co₃O₄–TiO₂ catalyst illustrated low onset potential ~343 mV vs. RHE, high current density ~43 mA cm⁻² corresponding small Tafel slope ~97 mV/dec and small R_ct as compared to other catalysts. For HER in GO/Co₃O₄–TiO₂, Co²⁺ sites are more catalytically active than Co³⁺ sites along with Ti⁴⁺ and GO provides the more active surface area by reducing the agglomeration between the mixed metal oxides. GO/Co₃O₄–TiO₂ shows the highest photo catalytic performance over MO as compared to binary and ternary composites. Pining of metal oxides with reactive oxygen functional moieties of GO considerably improve the photo catalytic degradation activity and helpful in the separation of charge carriers for HER.     

Boosting the kinetics of oxygen and hydrogen evolution in alkaline water splitting using nickel ferrite /N-graphene nanocomposite as a bifunctional electrocatalyst

Design, synthesize and application of metal-oxide based bifunctional electrocatalysts with sustainability and efficient activity in water splitting is significant among the wide spread researches in energy applications. Herein, bifunctional electrocatalysts composed of NiFe₂O₄ dispersed on N-doped graphene has been prepared by in-situ polymerization and characterized for further bifunctional catalytic performances. The electrocatalyst exhibited bespoken performances as cathode in HER as well as anode in OER at alkaline electrolyte. The nanocomposite N-doped graphene/NiFe₂O₄ (NGNF) exhibited low overpotential of 184 mV in HER and 340 mV in OER for attaining the current density of 10 mA/cm² which is far better than their pristine counterparts. Similarly its Tafel slopes were found to be 82.9 mV/dec and 93.2 mV/dec for HER and OER. As an electrocatalyst NGNF outperformed pure nickel ferrite and graphene/NiFe₂O₄ (GNF) as bifunctional electrocatalyst with low overpotential and Tafel slopes. This indicates the impact of graphene and N-doping on graphene in the activity of pure NF. The graphene in the composite and the N-dopants provoked the catalytic activity and tuned the electron transfer and interaction with the electrolyte. Thus, herein we endow with strategies of preparing highly efficient bifunctional electrocatalysts by coupling spinel oxides and N-doped graphene for HER and OER.     

MgO as promoter for electrocatalytic activities of Co3O4–MgO composite via abundant oxygen vacancies and Co2+ ions towards oxygen evolution reaction

Oxygen evolution reaction (OER) is known as bottleneck problem during the water splitting process due to high energy barrier and non-availability of efficient nonprecious electrocatalysts. The cobalt oxide (Co₃O₄) in the spinel phase has limited OER activity and stability in the alkaline media. For this purpose, we have carried out the synthesis of Co₃O₄–MgO (CM) composite by wet chemical method and it offers abundant oxygen vacancies and Co²⁺ concentration for the efficient OER reaction. The effect of different amounts of MgO on the OER activity of Co₃O₄ was also studied. Despite inactivity of MgO towards OER, it creates high density of oxygen vacancies and favored the formation Co²⁺ ions at the surface, thus accelerated the OER kinetics. The physical studies were performed to investigate the morphology, crystalline structure, surface information and chemical composition using several analytical techniques. The optimized CM-0.1 composite produced an overpotential of 274 mV at 10 mAcm⁻² which is lower in value than the pristine Co₃O₄. The significant enhancement in the OER activity was verified by the large value of electrochemical active surface area values 12.8 μFcm⁻² and the low charge transfer resistance of 45.96 Ω for the optimized CM-0.1 composite. The use of abundance materials for the synthesis of CM composite revealed an enhanced OER performance, suggesting the dynamic role of MgO, therefore it could be used for improving the electrochemical properties of extended range of metal oxides for specific application especially energy conversion and storage devices.     

Evidencing enhanced oxygen and hydrogen evolution reactions using In–Zn–Co ternary transition metal oxide nanostructures: A novel bifunctional electrocatalyst

Ternary transition metal oxides are gaining popularity for cost effective bifunctional electrocatalytic activities and to realization of novel water splitting devices. In this regard, In₂O₃/ZnO/Co₃O₄ based ternary oxide nanostructures were investigated in detail for their oxygen/hydrogen evolution reaction (OER/HER) in alkaline environment. The ternary oxides were at first processed through a simple chemical route involving hydrothermal treatment. The prepared nanostructures were then investigated by using high-resolution transmission electron microscopy (TEM/HRTEM) to ascertain their morphological traits. X-ray diffraction, Raman signals and photoluminescence data demonstrated the In₂O₃ phase to be prevalent in the ternary mixture on par with that of ZnO and Co₃O₄. The valence state of various metal ions and the In–O, Zn–O and Co–O bonding was verified using XPS. The ternary oxide coated electrodes exhibited excellent overall water splitting activity. Overpotential values of 398 and 510 mV were registered for OER and HER experiments under a current density of ±10 mA cm⁻², demonstrating the material to be an ideal OER/HER electrocatalyst at room temperature. The exceptional long-term stability in ternary oxides and their Tafel slope (88 mV/dec for OER and 60 mV/dec for HER) further affirmed their unique anodic/cathodic characteristics for water splitting applications.     

Mixed CoS2@Co3O4 composite material: An efficient nonprecious electrocatalyst for hydrogen evolution reaction

Hydrogen evolution reaction (HER) has been identified as a sustainable and environment friendly technology for a wide range of energy conversion and storage applications. The big barrier in realizing this green technology requires a highly efficient, earth-abundant, and low-cost electrocatalyst for HER. Various HER catalysts have been designed and reported, still, their performance is not up to the mark of Pt. Among them, cobalt-based, especially cobalt disulfide (CoS₂) has shown significant HER activity and found suitable candidature for HER due to its low cost, simple to prepare, and exhibits good stability. Herein, we synthesized various nanostructured materials including pure CoS₂, Co₃O₄ and their composites by wet chemical methods and found them active for HER. The scanning electron microscopy (SEM) has revealed a morphology of composite as a mixture of nanowires and round shape spherical nanoparticles with several microns in dimension. The X-ray diffraction (XRD) confirmed the cubic phase of CoS₂ and cubic phase of Co₃O₄ in the composite materials. The chemical deposition of CoS₂ onto Co₃O₄ has tailored the HER activity of CoS₂@Co₃O₄ composite material. Two CoS₂@Co₃O₄ composite materials were produced with varying amounts of Co₃O₄ and labeled as samples 1 and 2. The Co₃O₄ reduced the adsorption energy for hydrogen, decreased the aggregation of CoS₂ and uplifted the stability of CoS₂@Co₃O₄ a composite material in alkaline media. Sample 1 requires an overpotential of 320 mV to reach a current density of 10 mA/cm² and it exhibits a Tafel slope of 42 mVdec⁻¹which is the key indicator for the fast HER kinetics on sample 1. The sample 1 is highly durable for 50 h and also it has excellent stability. The electrochemical impedance spectroscopy (EIS) revealed a small charge transfer resistance of 28.81 Ohms for the sample 1 with high capacitance double-layer value of 0.81 mF. EIS has supported polarization and Tafel slope results. Based on the partial physical characterization and the electrochemical results, the as-obtained sample 1 (CoS₂@Co₃O₄ composite material) will find potential applications in an extended range of energy conversion and storage devices owing to its low cost, high abundance, and excellent efficiency.     

Electrospinning derivative fabrication of sandwich-structured CNF/Co3S4/MoS2 as self-supported electrodes to accelerate electron transport in HER

The substitution of noble metal catalysts with earth abundant TMs as electrocatalysts for hydrogen production is of great significance. One biggest bottleneck for high-efficiency water electrolysis in TM catalysts is the sluggish reaction kinetics or electron transport efficiency. The electrical coupling between the substrate and the catalytic material can accelerate the electron transport, enhancing the charge transfer kinetics, and thereby improve the catalytic performance of the catalyst. Herein, we report a sandwich-structured CNF/Co₃S₄/MoS₂, MoS₂ grown in-situ on N-doped nanofibers with Co₃S₄ nanoparticles via electrospinning, carbonization and hydrothermal process, as self-supported electrodes for hydrogen evolution reaction. The sandwich structure is comprised of CNFs/Co₃S₄/MoS₂ as substrate/accelerator/catalyst. Thereinto, the three-dimensional CNF framework, intrinsically doped by nitrogen, can open accessible channels for reactants and served as substrates for the in-situ growth of Co₃S₄ and MoS₂ nanocrystals with high conductivity and massive active sites. Hence, the CNF/Co₃S₄/MoS₂ shows outstanding catalytical performance in water electrospinning, only 80 mV required to drive 10 mA cm^?2 current density with the Tafel slope of 99.2 mV dec^?1 in alkaline media. Besides, the performance can be maintained for at least 40 h with negligible decline. This experiment can provide a new idea for the design of efficient and stable self-supporting electrodes.     

Zinc oxide functionalized molybdenum disulfide heterostructures as efficient electrocatalysts for hydrogen evolution reaction

Technology urges to replace the state-of-the-art catalysts such as platinum with low cost, earth abundant and durable electrocatalysts for efficient hydrogen evolution (HER) reaction which is going to become the major sustainable production of energy in future. Herein, we present the heterostructure based MoS₂.ZnO (MZO) heterostructures for successful electrochemical water splitting process. For HER, the prepared MoS₂.ZnO nanocomposites show the over potential as low as 239 mV at cathodic current density 10 mAcm⁻² with an exchange current density of 3.2 μAcm⁻². A Tafel slope of about 62 mV per decade suggested to have the Volmer-Heyrovsky mechanism for the HER process with MoS₂.ZnO nanocomposite as the catalyst. The small Tafel slope indicates a promising electrocatalyst for HER in practical application. The strong interface formation at the MoS₂.ZnO heterostructure facilitates higher catalytic activity and excellent cycling stability. The heterostructure formation based on semiconductor two dimensional (2D) transition metal dichalcogenides (TMDC) open up new avenues for effective manipulation of HER catalysts.     

Functionalized 2D materials F–MoS2 and F-g-C3N4 with TiO2 as Composite Electrocatalysts for Electrochemical Hydrogen Evolution

Electrochemical hydrogen evolution is an important research field to produce renewable energy. Nanostructured two dimensional (2D) materials such as g-C₃N₄ and MoS₂ are potential electrocatalysts for hydrogen evolution reaction (HER). The incorporation of semiconducting material into 2D material enhances the hydrogen evolution. Here in, we have developed composite of acid functionalized MoS₂ and g-C₃N₄ with TiO₂ (F–MoS₂/TiO₂, F-g-C₃N₄/TiO₂). The F–MoS₂/TiO₂ composite exhibited excellent electrochemical HER activity with an overpotential of 103 mV Vs RHE at 20 mA/cm² compared to pristine F–MoS₂ of 232 mV, TiO₂ of 455 mV Vs RHE. In addition F-g-C₃N₄/TiO₂ showed high overpotential of 322 mV at 5 mA/cm² than pristine F-g-C₃N₄ and TiO₂ of 433 mV and 448 mV Vs RHE at 2.7 mA/cm² respectively.     

Flower-shaped and sea urchin-shaped structures coexisting in cobalt-based phosphate and oxides achieve efficient electrochemical overall water electrolysis

Active site engineering for electrocatalysts is an essential strategy to improve their intrinsic electrocatalytic capability for practical applications and it is of great significance to develop a new excellent electrocatalyst for overall water splitting. Here, Co₃O₄/nickel foam (NF) and Co₂(P₄O₁₂)/NF electrocatalysts with flower-shaped and sea urchin-shaped structures are synthesized by a simple hydrothermal process and followed by a post-treatment method. Among them, Co₂(P₄O₁₂)/NF shows good catalytic activity for hydrogen evolution reaction (HER), and at the current density of 10 mA cm^?2, the overpotential is only 113 mV Co₃O₄/NF exhibits good catalytic activity for oxygen evolution reaction (OER), and the overpotential is 327 mV at 20 mA cm^?2. An alkaline electrolyzer with Co₃O₄/NF and Co₂(P₄O₁₂)/NF catalysts respectively as anode and cathode displays a current density of 10 mA cm^?2 at a cell voltage of 1.59 V. This work provides a simple way to prepare high efficient, low cost and rich in content promising electrocatalysts for overall water splitting.     

Oxygen vacancy-based ultrathin Co3O4 nanosheets as a high-efficiency electrocatalyst for oxygen evolution reaction

Enhancing the catalytic activity of Co₃O₄ electrocatalysts featuring abundant oxygen vacancies is required to enable their application in oxygen evolution reaction (OER). However, developing a harmless defect engineering strategy based on mild conditions to realize such an enhancement remains a challenge. Here, ultrathin Co₃O₄ nanosheets with abundant oxygen vacancies were prepared through a simple two-step method comprising a hydrothermal process and pre-oxidation to study the catalytic activity of the nanosheets toward OER. The ultrathin sheet structure and the Co₃O₄ nanosheets surface provide abundant active sites. The oxygen vacancy not only improves the catalyst activity, but also improves the electron transfer efficiency. These advantages make ultrathin Co₃O₄ nanosheets with abundant oxygen vacancies an excellent electrocatalyst for oxygen evolution. In an alkaline medium, ultrathin Co₃O₄ nanosheets exhibited excellent OER catalytic activity, with a small overpotential (367 mV for 10 mA/cm²) and faster reaction kinetics (65 mV/dec).Moreover, the electrocatalyst still maintained 68% of its original catalytic activity after 24 h operation. This work provides an extensive and reliable method for the preparation of low-cost and highly efficient OER electrocatalysts.     

Aligned Co3O4–CoOOH heterostructure nanosheet arrays grown on carbon paper with oxygen vacancies for enhanced and robust oxygen evolution

Developing efficient and stable non-noble metal oxygen evolution reaction (OER) electrocatalysts for sustainable overall water-splitting is extremely desirable but still a great challenge. Herein, we developed a facile strategy to fabricate Co₃O₄–CoOOH heterostructure nanosheet arrays with oxygen vacancies grown on carbon paper (Co₃O₄–CoOOH/CP). Benefiting from the unique 3D architecture, large surface area, synergistic effects between Co₃O₄, CoOOH and oxygen vacancies, the obtained self-supporting Co₃O₄–CoOOH/CP presents excellent electrocatalytic OER activity (low overpotentials of 245 and 390 mV at 10 and 100 mA cm⁻²) and robust long-term stability in alkaline condition. The present strategy provides the opportunities for the future rational design and discovery of high-performance non-noble metal based electrocatalysts for advanced water oxidation and beyond.     

MoS2/NiFeS2 heterostructure as a highly efficient electrocatalyst for overall water splitting at high current densities

Searching for efficient, stable and low-cost nonprecious catalysts for oxygen and hydrogen evolution reactions (OER and HER) is highly desired in overall water splitting (OWS). Herein, presented is a nickel foam (NF)-supported MoS₂/NiFeS₂ heterostructure, as an efficient electrocatalyst for OER, HER and OWS. The MoS₂/NiFeS₂/NF catalyst achieves a 500 mA cm⁻² current density at a small overpotential of 303 mV for OER, and 228 mV for HER. Assembled as an electrolyzer for OWS, such a MoS₂/NiFeS₂/NF heterostructure catalyst shows a quite low cell voltage (≈1.79 V) at 500 mA cm⁻², which is among the best values of current non-noble metal electrocatalysts. Even at the extremely large current density of 1000 mA cm⁻², the MoS₂/NiFeS₂/NF catalyst presents low overpotentials of 314 and 253 mV for OER and HER, respectively. Furthermore, MoS₂/NiFeS₂/NF shows a ceaseless durability over 25 h with almost no change in the cell voltage. The superior catalytic activity and stability at large current densities (>500 mA cm⁻²) far exceed the benchmark RuO₂ and Pt/C catalysts. This work sheds a new light on the development of highly active and stable nonprecious electrocatalysts for industrial water electrolysis.     

Three dimensional (3D) nanostructured assembly of MoS2-WS2/Graphene as high performance electrocatalysts

A promising electrocatalyst material composed of 2D layered MoS₂-WS₂ heterostructure hierarchically assembled into a 3D highly interconnected macroporous network of graphene was facilely fabricated. This in-situ synthesis method involves hydrothermal reaction followed by moderate thermal annealing which guarantees the uniform distribution of the MoS₂-WS₂ heterojunctions within graphene matrix. The presence of 3D conductive and porous graphene network and the combined merits of MoS₂ and WS₂ endow the resulting 3D MoS₂-WS₂/graphene nanohybrids with unique conductivity pathways and channels for electrons and with outstanding electrocatalytic performance towards enhanced hydrogen evolution reaction (HER). This 3D nanohybrid delivered the small overpotential of 110 mV, and the small Tafel slope of 41 mV per dec, demonstrating high HER activity. Furthermore, the resulting nanohybrids exhibit excellent stability as very trivial drop in the current density was noticed even after 2000 cycles. The superior electrocatalytic performance of 3D MoS₂-WS₂/graphene over other non-precious metal electrocatalysts is accredited to the robust synergism of 2D MoS₂-WS₂ with 3D graphene that offer ample active sites and improved conductivity for HER. The proposed approach can be further extended to modify other layered transition metal dichalcogenides with hierarchical 3D porous structure as a competent electrocatalysts for HER.     

P doped CoMoO4/RGO as an efficient hybrid catalyst for hydrogen evolution

Transition metal oxides, as newly earth-abundant and low-cost catalysts, have been regarded as promising materials for electrocatalytic oxygen evolution. However, they are rarely used as an electrocatalyst in hydrogen evolution reaction (HER) due to the poor HER activity. Herein, we present a facile two-step method to synthesize P doped CoMoO₄/RGO (P-CoMoO₄/RGO) with different atomic ratios of Co²⁺/Co³⁺ through a simple phosphorization strategy by changing the mass of NaH₂PO₂. The effective P-doping into CoMoO₄/RGO can modify the electronic properties and modulate the atomic ratio of Co²⁺/Co³⁺, which promotes the electron transfer and creates more activity sites. Therefore, the optimized P-CoMoO₄/RGO with a relatively larger atomic ratio of Co²⁺/Co³⁺ shows superior HER performances in alkaline media, which affords a current density of 10 mA cm⁻² at a small overpotential of 90 mV and a low Tafel slope of 62 mV dec⁻¹ along with having satisfactory long-term stability. This work provides a valuable route to enhance the HER activity of transition metal oxides.     

Facial synthesis of p-p heterojunction composites: Evaluation of their electrochemical properties with photovoltaics-electrolyzer water splitting using two-electrode system

Mixed transition metal oxides have garnered widespread interest as alternative electrocatalysts for the oxygen and hydrogen generation reactions; however, they tend to require extended synthetic routes, in addition to possessing limited electrocatalytic activities and stabilities. Herein, we report the observation of a synergistic effect between the non-precious metal oxides Mn₃O₄ and Co₃O₄ with CuO and NiO, wherein the resulting composites exhibit promising properties as catalysts for the alkaline water electrolysis process. The activities of these composites in both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) were improved compared to their counterparts, and the dynamic potentials of these processes were reduced. Importantly, low overpotentials of 202 and 380 mV were found for the CuO–Mn₃O₄ composite catalysts for the OER and the HER at 10 mA/cm², respectively. In addition, electrochemical impedance spectroscopy measurements showed a reduced impedance response for the composites, which was dominated by the relaxation of the intermediate frequency associated with the adsorption of the intermediate. Furthermore, the superior catalytic activities of the composites were attributed to their structural properties, high electroactive surface areas, fast electron transport kinetics, and good chemo-electrical bonding between Mn₃O₄ and CuO. Importantly, merging with a marketable silicon-based solar cells, the accumulated PV-EC water splitting device obtains greater hydrogen production under stimulated solar light irradiation. This work offers a typical demonstration and respected strategies for practical large-scale solar H₂ production via an economical PV-EC technology.     

Co2FeO4@rGO composite: Towards trifunctional water splitting in alkaline media

Development of efficient, low cost and multifunctional electrocatalysts for water splitting to harvest hydrogen fuels is a challenging task, but the combination of carbon materials with transition metal-based compounds is providing a unique and attractive strategy. Herein, composite systems based on cobalt ferrite oxide-reduced graphene oxide (Co₂FeO₄) @(rGO) using simultaneous hydrothermal and chemical reduction methods have been prepared. The proposed study eliminates one step associated with the conversion of GO into rGO as it uses direct GO during the synthesis of cobalt ferrite oxide, consequently rGO based hybrid system is achieved in-situ significantly, the optimized Co₂FeO₄@rGO composite has revealed an outstanding multifunctional applications related to both oxygen evolution reaction (OER) and hydrogen counterpart (HER). Various metal oxidation states and oxygen vacancies at the surface of Co₂FeO₄@rGO composites guided the multifunctional surface properties. The optimized Co₂FeO₄@rGO composite presents excellent multifunctional properties with onset potential of 0.60 V for ORR, an overpotential of 240 mV at a 20 mAcm^?2 for OER and 320 mV at a 10 mAcm^?2 for HER respectively. Results revealed that these multifunctional properties of the optimized Co₂FeO₄@ rGO composite are associated with high electrical conductivity, high density of active sites, crystal defects, oxygen vacancies, and favorable electronic structure arisinng from the substitution of Fe for Co atoms in binary spinel oxide phase. These surface features synergistically uplifted the electrocatalytic properties of Co₂FeO₄@rGO composites. The multifunctional properties of the Co₂FeO₄@ rGO composite could be of high interest for its use in a wide range of applications in sustainable and renewable energy fields.