Highly active Ni/Co-metal organic framework bifunctional electrocatalyst for water splitting reaction

Rational design of electrocatalycally active materials with excellent performance for renewable energy conversion is of great interest. We have developed a nanosheet array of Ni/Co metal-organic framework (MOF) grown on CoO modified Ni foam (CoO/NF) substrate via the solvothermal process. The high surface area and low resistance of Ni/Co-MOF@CoO/NF (NC@CoO/NF) catalyst contribute to efficient water splitting. We have prepared a series of NC-n/CoO/NF (n = 1–4) catalysts to optimize the molar ratio of the Co atom in Ni MOF-74. Among them, NC-2@CoO/NF shows an excellent electrochemical performance in alkaline medium, i.e., low overpotential of 290 and 139 mV for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. For a two-electrode system with NC-2@CoO/NF, a low cell voltage of 1.54 V at 10 mA cm^?2 has been obtained for overall water splitting which is much smaller than that with commercial Ir/C– Pt/C pair. This excellent performance can be attributed to the synergistic effects of Ni/Co-MOF and CoO/NF. In addition, the as-prepared NC-2@CoO/NF exhibits excellent long-term stability. The computational simulation also supports experimental results.     

Bifunctional electrocatalysts for water splitting from a bimetallic (V doped-NixFey) Metal–Organic framework MOF@Graphene oxide composite

An ongoing challenge still lies in the exploration of proficient electrocatalysts from earth-abundant non-precious metals instead of noble metal-based catalysts for clean hydrogen energy through large-Scale electrochemical water splitting. However, developing a non-precious transition metals based, stable electrocatalyst for cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) is important challenge for modern energy conversion technology. In this report Vanadium doped bimetallic nickel-iron nanoarray, fabricated by carbon supported architecture through carbonization process for electrochemical water splitting. Three types of catalysts were prepared in different molar ratio of Ni/Fe. The electrocatalytic performance demonstrated that the catalyst with equal mole ratio (0.06:0.06) of Ni/Fe possess high catalytic activity for both OER and HER in alkaline and acidic medium. Besides, our findings revealed that the doping of vanadium could play a strong synergetic effect with Ni/Fe, which provide a small overpotential of 90 mV and 210 mV at 10 mA cm^?2 for HER and OER respectively compared to the other two catalyst counterparts. Also, the catalyst with 1:1 (Ni/Fe) molar ratio showed a high current density of 208 mA cm^?2 for HER at 0.5 M H₂SO₄ and 579 mA cm^?2 for OER at 1 M KOH solution, the both current densities are much higher than the other two catalysts (different Ni/Fe ratio). In addition, the presented catalysts showed extremely good durability, reflecting in more than 20 h of consistent Chronoamprometry study at fixed overpotential η = 250 mV without any visible voltage elevation. Similarly, the (Ni/Fe) equal ratio catalyst showed better corrosion potential 0.209 V vs Ag/AgCl and lower current density 0.594 × 10^?12 A cm^?2 in high alkaline medium. The V-doping, MOF/GO surface defects are significantly increased the corrosion potential of the V-Ni_xFe_y-MOF/GO electrocatalyst. Besides, the water electrolyzed products were analysed by gas chromatography to get clear insights on the formed H₂ and O₂ products.     

Strategies for improving Co/Ni-based bimetal-organic framework to water splitting

The preparation of high-efficiency, stable, and low-cost oxygen evolution reactions (OER) and hydrogen evolution reactions (HER) electrocatalysts remains a challenge for new energy systems. In this study, three-dimensional (3D) cobalt-nickel bimetal MOFs were used as precursors to synthesize catalysts through thermal decomposition, carbonization, nitriding, oxidation, phosphating, sulfurizing, and selenization, respectively. In 1.0 M KOH electrolyte, the overpotential of Co/Ni-MOFs@Se for OER was 238 mV and the that of Co/Ni-MOFs@P for HER was 194 mV at a current density of 10 mA cm⁻². Based on the excellent OER and HER performances of Co/Ni-MOFs@Se and Co/Ni-MOFs@P, these two materials were further assembled into electrodes for overall water splitting. Results showed that a potential of only 1.59 V was required to provide a current density of 10 mA cm⁻². The electrodes also exhibited long-term durability in a 2000 min stability test without significant changes in the catalytic performances. According to the difference in the doped non-metal elements, an electrode pair with a suitable matching degree was constructed, thereby improving the overall water splitting performance. Thus, the controllable modification of the metal-organic frameworks (MOFs)-derived carbon materials (CMs) effectively improved the materials’ catalytic water splitting performance. It was possible to further develop an efficient, inexpensive, and low-cost assembled electrode pair.     

Partially crystallized Ni–Fe oxyhydroxides promotes oxygen evolution

The slow oxygen evolution reaction (OER) kinetics influences hydrogen production efficiency from water splitting. To break through the bottleneck of water splitting, it is urgent to develop efficient and economic electrocatalysts. Although NiFe-based catalysts exhibit outstanding OER activity, the complicated preparation process limits their large-scale synthesis and applications. Here, partially crystallized nickel-iron oxyhydroxides are synthesized by a facile sol-gel method. When the Fe/Ni mole ratio is 0.5:1, the NiFe_0.5(OH)_x catalyst shows superior OER performance with a low OER overpotential of 265 mV and good durability. Kinetic studies show that the energy barrier of NiFe_0.5(OH)_x is only 31.5 kJ mol^?1, much smaller than those of Ni(OH)_x (41.0 kJ mol^?1) and Fe(OH)_x (44.8 kJ mol^?1). The synergistic action between Ni and Fe sites not only facilitates mass and charge transfer, but also promotes the formation of 1OOH intermediate for the OER.     

Self-assembled nanocotton-like Co–B–P/bacterial cellulose based carbon nanofiber as highly efficient electrocatalyst for oxygen evolution reaction

It is urgent and challenging to exploit highly efficient oxygen evolution reaction (OER) catalysts for water splitting. Bacterial cellulose based carbon nanofiber (BCCNF) is highlighted for its 3D network structure, abundant surface functional groups and good conductivity. Herein, based on the different molar ratio of P/B precursor, a series of nanocotton-like Co–B–P/BCCNF electrocatalysts have been designed and prepared via a simple electroless deposition. When the molar ratio of P/B precursor is 3/7, nanocotton-like Co–B–P/BCCNF catalyst exhibits prominent catalytic performances, delivering the current density of 10 and 50 mA cm^?2 at low overpotentials of only 262 and 391 mV, respectively. Besides, it exhibits long-term stability for durative OER in alkaline solution. This result indicates that nanocotton-like Co–B–P/BCCNF catalyst has great potential as OER catalysts for large-scale hydrogen production via water splitting.     

Co-Ni based hybrid transition metal oxide nanostructures for cost-effective bi-functional electrocatalytic oxygen and hydrogen evolution reactions

Water splitting is widely regarded as one of the promising technologies for hydrogen fuel production and foreshadowed to assist in meeting the global energy demand as a sustainable and reliable energy technology. In this regard, we report on the facile chemical synthesis of hybrid Cobalt (Co) and Nickel (Ni) oxide nanostructure for low-cost bi-functional electrocatalytic water splitting applications. Their crystalline characteristics and chemical structure were studied using X-ray diffraction and Fourier-Transform infrared (FT-IR) spectrum. The nanostructure morphology was investigated by scanning and high-resolution transmission electron microscopy (SEM/HRTEM). The 2⁺ and 3⁺ valence state of Co and Ni metal ions was identified using X-ray photoelectron spectroscopy (XPS). The hybrid oxide electrocatalyst was found to display an excellent oxygen/hydrogen evolution reaction (OER/HER activity) in alkaline condition. The realization of random heterojunction configuration across the hybrid nanostructures was found to offer an improved conductivity and enhanced charge transfer capability to promote the gas evolution kinetics. Overpotential value of 203 and 378 mV was registered from the respective OER and HER polarization curves (for current density of ±10 mA cm⁻²). Tafel slope of 87 mV/dec for OER and 90 mV/dec for HER along with the long-term stability results authenticated the anodic/cathodic characteristics of hybrid oxides for overall water splitting applications.     

Facile synthesis of three-dimensional spherical Ni(OH)2/NiCo2O4 heterojunctions as efficient bifunctional electrocatalysts for water splitting

Synthesis of highly efficient, non-noble and bi-functional electrocatalysts is exceedingly challenging and necessary for water splitting devices. In this work, three-dimensional spherical Ni(OH)₂/NiCo₂O₄ heterojunctions are prepared by a one-step hydrothermal method and the hybrids are explored as efficient electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in an alkaline electrolyte via tuning different Ni/Co atomic ratios of heterojunctions. The optimized Ni(OH)₂/NiCo₂O₄ (S (1:1)) exhibits high electrocatalytic activity with an ultralow over-potential of 189 mV at 10 mA cm⁻² for the HER. With regard to the OER, the over-potential of the as-synthesized S (1:1) heterojunction is only 224 mV at the current density of 10 mA cm⁻². The improved catalytic performance of the Ni(OH)₂/NiCo₂O₄ heterojunctions is attributed to the chemical synergic combining of Ni(OH)₂ and NiCo₂O₄, large specific surface area for exposing more accessible active sites, and heterointerface for activating the intermediates that facilitates electron/electrolyte transport. The prepared catalyst exhibits good durability and stability in HER and OER catalyzing conditions. This study provides a feasible approach for the building of highly efficient bifunctional water splitting electrocatalysts and stimulates the development of renewable energy conversion and storage devices.     

Pulse-electrodeposited Ni–Fe–Sn films supported on Ni foam as an excellent bifunctional electrocatalyst for overall water splitting

The development of extremely active bifunctional non-noble electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is pivotal for water splitting but remains challenging. Herein, self-supported Ni–Fe–Sn electrocatalysts were fabricated on nickel foam (NF) through a simple and facile pulse electrodeposition process. Under optimal conditions, the prepared Ni–Fe–Sn electrocatalysts exhibited excellent bifunctional properties in alkaline medium and required ultralow overpotentials of only 27 and 201 mV for HER and OER, respectively, to reach the current density of 10 mA cm^?2. Importantly, the same Ni–Fe–Sn electrocatalyst can be assembled as the anode and the cathode in a two-electrode system. It demanded a fairly low applied voltage of 1.55, 1.72, and 1.87 V to produce 10, 50, and 100 mA cm^?2, respectively, and exhibited excellent long-term stability. The excellent electrocatalytic water splitting performance of the Ni–Fe–Sn film was mainly associated with its intrinsic catalytic activity derived from the modulation of the electronic structures among Ni, Fe, and Sn by using the appropriate atomic ratio of Ni: Fe: Sn.     

Bimetallic NiCo–NiCoO2 nano-heterostructures embedded on copper foam as a self-supported bifunctional electrode for water oxidation and hydrogen production in alkaline media

Earth-abundant, non-precious metal-based bifunctional electrocatalysts with efficient water splitting activity are of valuable importance in the limitation of energy losses in an alkaline environment. Herein, we report NiCo–NiCoO₂ nano-heterostructures embedded on the oxidized surface of copper foam (NiCo–NiCoO₂@Cu₂O@CF) as an efficient bifunctional electrocatalyst for overall water splitting in 1 M KOH electrolyte solution. In this study, metallic Ni and Co interlinkage with NiCoO₂ nanoparticles (NPs) are suggested to form by thermal decomposition of nickel-cobalt hydroxide precursors embedding on copper foam under a nitrogen environment. Bimetallic thin layered nano-heterostructures of NiCo–NiCoO₂@Cu₂O@CF exhibits a synergic effect of doubly active metals Ni and Co to achieve remarkable small overpotentials of 133 and 327 mV to achieve a current density of 10 mA cm^?2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The influential synergetic and structural effects have been extensively discussed to understand the overall water splitting for designing an efficient electrocatalyst. Hence, this phenomenon for surface modification of conductive substrate (CF) with a suitable combination of metal/metal oxide alloying as catalytic material helps us to design and synthesize low cost, highly efficient, non-precious metal-based electrocatalysts for overall water splitting.     

Crystalline nickel sulfide integrated with amorphous cobalt sulfide as an efficient bifunctional electrocatalyst for water splitting

The development of highly efficient and low-cost electrocatalysts is critical to the mass production of hydrogen from water splitting. Herein, a facile yet effective method was developed to synthesize bimetallic sulfides Ni₃S₂/CoS_x, which were aimed for use as the electrocatalysts in both HER and OER. Encouragingly, the Ni₃S₂/CoS_x demonstrated a low overpotential of 110 mV for HER at a current density of 10 mA·cm^?2. It was discovered that the surface of Ni₃S₂/CoS_x during OER process would undergo an in-situ oxidation to form MOOH (M = Co, Ni), that is, MOOH/Ni₃S₂/CoS_x were the real functioning species in catalysis, which had an excellent OER activity and a low overpotential of 226 mV. Additionally, the assembled electrolyzer required only a low cell voltage of 1.53 V to achieve a current density of 10 mA·cm^?2 in a 1 M KOH solution, and its performance was stable. Overall, this work provided a promising strategy for the facile fabrication of low-cost amorphous electrocatalysts, which is expected to promote the progress of overall water splitting.     

Ni,Zn Co-doping ZIF-67-derived electrocatalyst based on CNT film for efficient overall water splitting

Water electrolysis has been acknowledged as a renewable, scalable, and effective way of producing hydrogen. However, for water splitting, efficient and noble metal-free electrocatalysts were lacking. Here, a Co–Ni–Zn porous three-dimension N-doped carbonization structure on the carbon nanotube film (CNTF) was synthesized through a metal organic frameworks (MOFs) annealing procedure. The porous morphology, caused by the evaporation of zinc at high temperatures, enhances the interaction between the catalysts and electrolyte, and the self-supporting structure minimizes the contact resistance between the catalysts and the substrate, which reduces obstructions during current flow. More active sites, multiple mesopores, and high conductivity are all features of this composite structure. It can achieve a small overpotential of 112 mV and 270 mV at a current density of 10 mA cm⁻², respectively, for hydrogen and oxygen evolution reactions (HER and OER). At a current density of 10 mA cm⁻², the Co–Ni–Zn/NCNTF has an external voltage of 1.58 V and is very durable for overall water splitting.     

Facile one-step fabrication of bimetallic Co–Ni–P hollow nanospheres anchored on reduced graphene oxide as highly efficient electrocatalyst for hydrogen evolution reaction

It is significant but challenging to develop noble-metal-free electrocatalysts exhibiting high activity and long-term stability toward hydrogen evolution reaction (HER) to satisfy the ever-increasing demand for clean and renewable energy. Herein, an environment-friendly and low-temperature electroless deposition method is developed for the synthesis of Co–Ni–P hollow nanospheres anchored on reduced graphene oxide nanosheets (Co–Ni–P/RGO). By optimizing the molar ratio of Ni/Co precursor, composition dependent electrocatalytic performances toward HER of nanostructured Co–Ni–P/RGO electrocatalyst are investigated in 1.0 M KOH solution. The results suggest that when the molar ratio of Ni/Co precursor is 3/7, as-prepared ternary Co–Ni–P/RGO electrocatalyst exhibits a remarkably enhanced HER activity in comparison to binary Ni–P/RGO and Co–P/RGO electrocatalysts, delivering a current density of 10 mA cm⁻² at the overpotential of only 207 mV. The value of Tafel slope for nanostructured Co–Ni–P/RGO electrocatalyst reveals that HER process undergoes Volmer-Heyrovsky mechanism. Besides, nanostructured Co–Ni–P/RGO electrocatalyst features superior stability under alkaline condition. The results suggest that nanostructured composite of Co–Ni–P hollow nanospheres/RGO is a potential candidate for hydrogen production through water splitting.     

In situ grown Cu-Based metal-organic framework on copper foam as high-performance electrocatalysts for oxygen evolution reaction

It is of great significance to develop highly efficient and robust oxygen evolution reaction (OER) electrocatalysts derived from earth-abundant and inexpensive elements for future hydrogen economy via electrochemical water splitting. Herein, Cu-based metal-organic framework (MOF) is directly supported on conductive Cu foam (CF) by a simply chemical oxidation of Cu substrate to grow Cu(OH)₂ nanowire arrays, followed by solvothermal treatment to obtain in situ grown Cu-based MOF electrode (MOF [Cu(OH)₂]/CF). The as-prepared 3D electrode shows superior OER activity with a low potential of 330 mV to deliver a current density of 10 mA cm⁻², a Tafel slope of 108 mV dec⁻¹, and excellent durability in alkaline media (1.0 M KOH). After electrolysis, XRD confirms that the initial MOFs have been transformed into CuO species, which are essentially active components for OER performance. This demonstrates that the MOFs can serve as efficient precursors for formation of highly active Cu oxide catalysts towards OER. This work provides a new strategy to develop MOFs-derived electrocatalysts for future clean energy conversion and storage systems.     

Effect of cation substitution on the water splitting performance of spinel cobaltite MCo2S4 (M = Ni,Cu and Co)

The introduction of different ions is an effective method for regulating electron distribution and increasing the electrocatalytic activity of spinel cobalt sulfide (Co₃S₄). However, the effect of doping different ions on water splitting performance has not been systematically clarified. Therefore, a detailed research is done to illuminate the doping of different ions on the water splitting performance of spinel cobalt sulfide MCo₂S₄ (M = Ni, Cu and Co) nanorods grown on Ni foam. To drive the electrocatalytic current of 50 mA/cm² and 10 mA/cm², the CuCo₂S₄/NF material only requires an overpotential of 240 mV for oxygen evolution reaction (OER) and an overpotential of 142 mV for hydrogen evolution reaction (HER). The results of density functional theory and experiment demonstrate that the strong water adsorption energy and the large electrochemical activity area make CuCo₂S₄/NF show good catalytic activity. The CuCo₂S₄/NF nanorods material presents superior electrochemistry performance with a small voltage 1.53 V. The water oxidation activity increases linearly before nonlinearly improving with the increasing of pH, indicating that the substrate changes from water to hydroxyl. It is noteworthy that CuCo₂S₄/NF will be transformed into amorphous oxide active species, which will act as a stable catalyst during the reaction.     

Black phosphorous dots phosphatized bio-based carbon nanofibers/bimetallic organic framework as catalysts for oxygen evolution reaction

As a multi-step and more complex half-cell reaction than the hydrogen reverse evolution reaction (HER), the oxygen evolution reaction (OER) always requires a higher overpotential than HER. In order to minimize the associated energy loss as an overpotential, these electrochemical half-reactions of water splitting should be catalyzed by suitable materials. Due to the abundant exposed surface area and extensive active edge sites, black phosphorous quantum dots (BP QDs) have shown great potential in OER. Here, BP QDs was introduced to incorporate with bio-based carbon nanofibers (CNF) and Co–Ni bimetallic organic framework (CoNiMOF), preparing a novel catalyst for oxygen evolution reaction (OER) by a facile one-pot reaction (Scheme 1). The unique structures and greater BET surface areas of CoNiMOF-BP QDs/CNF could possibly supply a larger electrocatalytic surface, expose further active sites. The obtained CoNiMOF-BP QDs/CNF possesses excellent electrocatalytic activity in alkaline electrolyte (1 M KOH) with a low overpotential of 281 mV at 10 mA cm^?2 and a low Tafel slope of 111.9 mV dec^?1. The CoNiMOF-BP QDs/CNF can remain stable for 25,000 s under alkaline electrolyte, showing excellent stability. The increase of electrocatalyst activity is mainly attributed to the synergistic effect of excellent conductivity and enriched active sites arising from BP QDs. This work not only provides an effective strategy for the development of bimetallic MOFs derived electrocatalysts, but also puts forward a new insight for the application of BP QDs in water splitting.     

Constructing Ni-Modified Co-FcDA nanosheets for enhancing electrocatalytic oxygen evolution reaction

Electrocatalytic water splitting is an emerging technology for the development of maintainable hydrogen energy. It remains challenging to manufacture a stable, efficient, and cost-effective electrocatalyst that can conquer the slow reaction kinetics of water electrolysis. Herein, A metal-organic framework (MOF) based material is manufactured and productively catalyze the oxygen evolution reaction (OER). The introduction of elemental nickel enhances the catalytic activity of Co-FcDA. The results show that single Ni was well doped in the CoNi-FcDA catalysts and the doping of Ni has a great influence on the OER activity of CoNi-FcDA catalysts. CoNi-FcDA displayed a low overpotential of 241 mV to arrive at the benchmark current density (10 mA cm^?2) with a remarkably small Tafel slope of 78.63 mV dec^?1. It surpassed the state-of-the-art electrocatalyst for OER, that is, RuO₂ (260 mV and 97.26 mV dec^?1) in efficiency as well as instability. Density functional theory (DFT) calculations show that suitable Ni doping at the same time can increase the density of states of the Fermi level, resulting in excellent charge density and low intermediate adsorption energy. These discoveries provide a practical route for designing 2D polymetallic nanosheets to optimize catalytic OER performance.     

NiFe-LDH/MWCNTs/NF nanohybrids as a high-performance bifunctional electrocatalyst for overall urea electrolysis

It is crucial for the storage and conversion of hydrogen energy to substitute the low theoretical potential of urea oxidation reaction (UOR) for the high theoretical potential of anodic water electrolysis (oxygen evolution reaction (OER)). In this paper, it puts forward a brief and scalable strategy, so as to synthesize a novel bifunctional nickel-iron layered double hydroxide and multi-walled carbon nanotube composites supported on Ni foam, represented as NiFe-LDH/MWCNTs/NF. Electrochemical measurements demonstrate that NiFe-LDH/MWCNTs/NF is able to realize an efficient electrocatalysis for UOR. During this process, merely a potential of 1.335 V is needed at 10 mA cm⁻², which can be taken to replace OER, thus reducing overpotential in H₂-production as well as power consumption. In addition, NiFe-LDH/MWCNTs/NF also exhibits electro-defense electrocatalytic efficiency to achieve the reaction of hydrogen evolving process, which provides a low overpotential of 98 mV at 10 mA cm⁻². To further prove it, all-water-urea electrolysis measurement is carried out in 1 M KOH and 0.5 M Urea with NiFe-LDH/MWCNTs/NF as cathode and anode respectively. NiFe-LDH/MWCNTs/NF||NiFe-LDH/MWCNTs/NF electrode manages to provide 10 mA cm⁻² at a voltage of merely 1.507 V, 156 mV lower than that of water splitting, which proves its commercial viability in energy-saving hydrogen production.     

Cobalt-nickel bimetal carbon sphere catalysts for efficient hydrolysis of sodium borohydride: The role of synergy and confine effect

Sodium borohydride exhibits great potential in the field of chemical hydrogen storage. A competent catalyst would accelerate its practical application for hydrogen utilization by enhance the efficiency of hydrogen generation from hydrolysis of sodium borohydride. Herein, a kind of highly efficient and durable synergistic Co–Ni bimetal inlaid carbon sphere catalyst (Co-NiΦC) was prepared by a co-pyrolysis method, of which the configuration of metal inlaid carbon sphere could effectively expose and anchor the active component by contrast with the capsule catalyst (Co–Ni@C) and supported catalyst (Co–Ni/C). Further, diverse cobalt-nickel contents of the Co-NiΦC catalysts were optimized to achieve the best hydrolysis performance of sodium borohydride. The structure-performance relationship of inlaid catalyst and the bimetallic synergistic mechanism were investigated by multiple characterization measurements and the density functional theory (DFT). As demonstrated, the inlaid Co-NiΦC-2 catalyst (Co/Ni molar ratio of 8/2) shows a promising catalytic activity of hydrogen generation rate up to 6364 mL_H2·min^?1·g_metal^?1, a relative low reaction activation energy of 30.3 kJ/mol as well as robust durability where it still remains about 83.4% of its initial reaction rate after the fifth cycle. The outstanding performance of the optimized catalyst may ascribe to the high dispersion, remarkable Co–Ni synergy and high stabilization of the Co–Ni nanoparticles under the confinement effect of the inlaid metal-carbon sphere configuration. This work provides an alternative avenue for the application of efficient carbon-supported bimetal catalysts in the future.     

Mixed metal oxides as efficient electrocatalysts for water oxidation

Water splitting is a promising reaction for storing sustainable but intermittent energies. The critical bottleneck for it is oxygen evolution reaction (OER) requiring insufficiently low overpotentials, η. Metal oxides are the group of high performance catalysts for water oxidation, so far. We report a facile synthesis of the mixed metal oxide composite (NiO/Mn-doped NiCo₂O₄) and an easy dip-coating method to create electrocatalysts on nickel foam as electrode substrates cause significant efficiency for OER. The mixed metal oxides catalyst was characterized by using electrochemical methods, high-resolution transmission electron microscopy (HR-TEM), field emission-scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), inductively coupled plasma (ICP) and fourier-transform infrared spectroscopy (FTIR). Electrocatalysts were shown a Tafel slope of 34 mV dec^?1 with overpotential η = 482 mV (for 100 mA cm^?2) and at least 20 h durable OER activity. The electrochemical data demonstrate the synergistic effect of the coupling between the three metal-centres of Ni, Co, and Mn to decline the overpotential value. The current of (OER) is related to the electrolyte pH, displaying a non-proton-concerted mechanism in an approach to identifying rate-determining steps for OER. This could be concluded by the direct neighbour lattice O^?– coupling to form an O–O bond. The simple and rapid fabrication method and the promising stability and high performance of the herein developed electrodes render them quite promising for technological water splitting systems.     

Structure-design and synthesis of nickel-cobalt oxide/sulfide/phosphide composite nanowire arrays for efficient overall water splitting

The construction of cost-effective bifunctional electrocatalysts with the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is significant for efficient overall water splitting. Herein, this work demonstrates a novel strategy for the synthesis of nickel-cobalt oxides/sulfides/phosphides composite (denoted as NiCoO–2P/S) nanoarrays on Ni foam. In this method, Ni–Co bimetallic oxide nanowires on Ni foam were partially phosphorized and sulfurized simultaneously in situ to yield Ni–Co oxide/sulfide/phosphide composite. The NiCoO–2P/S arrays have good interfacial effects and display many holes in the nanowires, giving it the advantage of large accessible surfaces on the nanowires and a beneficial for the release of gas bubbles, resulting in an excellent OER performance with a low overpotential (η) of 254 mV at 100 mA cm^?2 and good HER activity (η₁₀ = 143 mV at 10 mA cm^?2). The electrocatalytic test results demonstrate small Tafel slopes (82 mV dec^?1 for HER, 88 mV dec^?1 for OER) and the satisfying durability in an alkaline electrolyte, indicating that the HER and OER activity was enhanced by the introduction of the Ni/Co sulfides and phosphides into Ni–Co oxides composite nanowires. Furthermore, the as-prepared NiCoO–2P/S catalyst can be used as both the anode and the cathode simultaneously to realize overall water splitting in the two-electrode electrolyzer. This system can be driven at low cell voltages of 1.50 and 1.68 V to achieve current densities of 10 and 100 mA cm^?2, respectively. This work provides an alternative strategy to prepare high-performance bifunctional electrochemical materials and demonstrates the advantages of Ni–Co oxide/sulfide/phosphide composites for water splitting.