High throughput preparation of Ni–Mo alloy thin films as efficient bifunctional electrocatalysts for water splitting

The synthesis of cost-effective and high-performance electrocatalysts for water splitting is the main challenge in electrochemical hydrogen production. In this study, we adopted a high throughput method to prepare bi-metallic catalysts for oxygen/hydrogen evolution reactions (OER/HER). A series of Ni–Mo alloy electrocatalysts with tunable compositions were prepared by a simple co-sputtering method. Due to the synergistic effect between Ni and Mo, the intrinsic electrocatalytic activity of the Ni–Mo alloy electrocatalysts is improved, resulting in excellent HER and OER performances. The Ni₉₀Mo₁₀ electrocatalyst shows the best HER performance, with an extremely low overpotential of 58 mV at 10 mA cm^?2, while the Ni₄₀Mo₆₀ electrocatalyst shows an overpotential of 258 mV at 10 mA cm^?2 in OER. More significantly, the assembled Ni₄₀Mo₆₀//Ni₉₀Mo₁₀ electrolyzer only needs a cell voltage of 1.57 V to reach 10 mA cm^?2 for overall water splitting.     

A novel in-situ strategy develops for Mo2C nanoparticles incorporated on N,P co-doped stereotaxically carbon as efficient electrocatalyst for overall water splitting

Developing cost-effective and remarkable electrocatalysts toward oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) performs excelling role in boosting the hydrogen energy application. Herein, a novel in-situ one-pot strategy is developed for the first time to synthesize molybdenum carbide nanoparticles (Mo₂C NPs) incorporated on nitrogen (N) and phosphorous (P) co-doped stereotaxically carbon (SC). The optimized Mo₂C NPs/N, P–SC–800 electrocatalyst exhibits lower overpotentials of 131 and 287 mV for HER and OER to deliver a current density of 10 mA cm^?2 in 1.0 M KOH medium with smaller Tafel slopes of 58.9 and 74.4 mV/dec, respectively. In addition, an electrolyzer using Mo₂C NPs/N, P–SC–800 electrode as cathode and anode delivers a current density of 10 mA cm^?2 at a small voltage of 1.64 V for overall water splitting. The excellent water splitting performance could be ascribed to optimum Mo₂C NPs for more accessible active sites, highly active N, P-SC networks for accelerated electron transfers, and synergetic effect between Mo₂C NPs and N, P-SC networks. The N, P-SC network not only enhances the overall dispersion of Mo₂C NPs but also contributes numerous electroactive edges to enhance the performance of HER, OER, and overall water splitting activity. This research work explores the in-situ one-step strategies of advanced, cost-effective, and non-precious metal electrocatalysts for efficient water splitting and motivates the consideration of a novel class of heteroatom doped stereotaxically carbon nanocomposites for sustainable energy production.     

(Fe,Ni)S2@MoS2/NiS2 hollow heterostructure nanocubes for high-performance alkaline water electrolysis

Hollow hybrid heterostructures are regarded to be promising materials as bifunctional electrocatalysts for highly efficient water electrolysis due to their intriguing morphological features and remarkable electrochemical properties. Herein, with FeNi-PBA as both a precursor and morphological template, we demonstrate the rational construct of cost-effective (Fe,Ni)S₂@MoS₂/NiS₂ hollow hybrid heterostructures as bifunctional electrocatalysts for alkaline overall water splitting. Microstructural analysis shows that the hybrid is a kind of hierarchical heterostructure composed of MoS₂/NiS₂ nanosheets/nanoparticles in situ grown on hollow (Fe,Ni)S₂ nanocubes with abundant heterointerfaces, which effectively maximizes the electrochemical active sites to the accessible electrolyte ions, leading to the promoted charge transfer. As expected, the hybrid shows remarkable alkaline electrocatalytic performance, such as hydrogen evolution overpotential of 176 mV and oxygen evolution overpotential of 342 mV at 50 mA cm^?2, as well a cell voltage of 1.65 V at 20 mA cm^?2. Moreover, the stability and durability are greatly enhanced under harsh electrochemical conditions. This study opens a new venue for developing earth-abundant bifunctional electrocatalysts with hollow hybrid heterostructures for alkaline water electrolysis in the future.     

Structural engineering of hierarchically hetestructured Mo2C/Co conformally embedded in carbon for efficient water splitting

Developing low-cost and high efficient electrocatalysts for both oxygen and hydrogen evolution reaction in an alkaline electrolyte toward overall water splitting is still a significant challenge. Here, a novel hierarchically heterostructured catalyst composed of ultrasmall Mo₂C and metallic Co nanoparticles confined within a carbon layer is produced by a facile phase separation strategy. During thermal reduction of CoMoO₄ nanosheets in CO ambient, in-situ generated nanoscale Co and ultrafine Mo₂C conformally encapsulated in a conductive carbon layer. In addition, some carbon nanotubes catalyzed by Co nanoparticles vertically grew on its surface, creating 3D interconnected electron channels. More importantly, the integrated C@Mo₂C/Co nanosheets assembled into the hierarchical architecture, providing abundant active surface and retaining the structural integrity. Benefiting from such unique structure, the constructed hierarchical heterostructure shows low overpotentials of 280 mV and 145 mV to reach a current density of 10 mA cm⁻² for OER and HER in an alkaline electrolyte. Furthermore, the symmetrical electrolyzer assembled with catalyst exhibits a small cell voltage of 1.67 V at 10 mA cm⁻² in addition to outstanding durability, demonstrating the great potential as a high efficient bifunctional electrocatalyst for overall water splitting.     

Heterogeneous layered multiple transition metal composite electrocatalysts with controlled composition for hydrogen production

The exploration of highly efficient and low-cost bifunctional electrocatalyst is essential for overall water splitting, especially for industrial application under alkaline conditions. Herein, we propose a controllable structural engineering strategy of constructing heterogeneous layered electrocatalyst with wetting surface for hydrogen evolution reaction and oxygen evolution reaction. Heterogeneous layered NiFe LDH (layered double hydroxide)/CoFeP/NF (Ni foam) with superhydrophilic surfaces is successfully fabricated by successive electrodeposition, phosphorization and solvothermal method. The NiFe LDH/CoFeP/NF for hydrogen evolution achieves a low overpotential of 198 mV at 50 mA cm^?2 in 1.0 M KOH. An overpotential of 269 mV is required at 50 mA cm^?2 for oxygen evolution. Meanwhile, the practical utilization of NiFe LDH/CoFeP/NF as bifunctional electrocatalysts for overall water splitting yields 1.73 V at 50 mA cm^?2 in the two-electrode cell. Moreover, NiFe LDH/CoFeP/NF can retain over 50 h without an obvious degradation at 10 mA cm^?2. The satisfactory operating stability and high activity of NiFe LDH/CoFeP/NF in alkaline solution can be attributed to the heterogeneous layered structure and excellent hydrophilic surface. The study provides a strategy to engineering heterogeneous layered structures with wetting surface for excellent electrocatalytic activities toward overall water splitting.     

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.     

Sulfuration of hierarchical Mn-doped NiCo LDH heterostructures as efficient electrocatalyst for overall water splitting

Constructing efficient bifunctional electrocatalysts for both cathode and anode is of great importance for obtaining green hydrogen by water splitting. Herein, sulfuration of hierarchical Mn-doped NiCo LDH heterostructures (Mn–NiCoS₂/NF) is constructed as a bifunctional electrocatalyst for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) via a facile hydrothermal-annealing strategy. Mn–NiCoS₂/NF shows an overpotential of 310 mV at 50 mA cm⁻² for OER and 100 mV at 10 mA cm⁻² for HER in 1.0 M KOH. Moreover, only 1.496 V@10 mA cm⁻² is required for overall water splitting by using Mn–NiCoS₂/NF as catalyst dual electrodes in a two-electrode system. The excellent performance of Mn–NiCoS₂/NF should be attributed to the ameliorative energy barriers of adsorption/desorption for HO⁻/H₂O through the modification of electronic structure of NiCo basal plane by Mn-doping and the acceleration of water dissociation steps via rich delocalized electron inside sulfur vacancies. The construction of hierarchical Mn–NiCoS₂/NF heterostructures provides new prospects and visions into developing efficient-advanced electrocatalysts for overall water splitting.     

Autologous growth of Fe-doped Ni(OH)2 nanosheets with low overpotential for oxygen evolution reaction

Highly active and durable electrocatalysts for oxygen evolution reaction (OER) play a vital role in water splitting. Despite numerous efforts, the strategies to prepare durable and effective electrocatalysts via scalable methods still remain a great challenge. In this work, we fabricated Fe-doped Ni(OH)₂ ultrathin nanosheets (Fe–Ni–OH/Ni) via autologous growing of Ni(OH)₂ from Ni foam, and in situ electrochemical-assisted doping Fe into Ni(OH)₂. Benefiting from the unique structure with large surface areas and strong coupling effects between Fe and Ni, the optimal Fe–Ni–OH electrodes exhibit remarkable catalytic performance toward OER, which requires an overpotential of 220 mV to achieve a current density of 10 mA cm⁻² with a Tafel slope of 48.3 mV dec⁻¹. The Fe–Ni–OH electrodes also possess high stability even under a high current density of 500 mA cm⁻² for 600 h with an ultralow overpotential of 290 mV. Using Ni–Fe–OH electrodes as both anode and cathode for overall water splitting, only a small overpotential of 1.57 V is required to reach a current density of 10 mA cm⁻². Moreover, the high catalytic performance and scalable preparation method can meet the emergency needs for the practical application.     

Efficient MOF- derived V–Ni3S2 nanosheet arrays for electrocatalytic overall water splitting in alkali

The synergistic achievement of low-cost earth-abundant electrocatalysts and high efficiency to meet renewable energy need is highly desirable yet challenging. Here, we developed a simple Ni foam self -templating route for V-doped Ni₃S₂ nanosheet arrays through in situ formation of metal-organic frameworks (MOFs) combined with subsequent conversion. The as-prepared MOF-V-Ni₃S₂/NF catalyst delivers outstanding electrocatalytic performance in the alkaline solution, which requires low overpotentials of 118.1 mV @10 mA cm^?2 and 268 mV @10 mA cm^?2 for hydrogen evolution reaction and oxygen evolution reaction, respectively. The V-doping and MOF-derived 3D hieratical nanostructure play a vital role in the catalytic process, which provides efficient active sites and large surface areas. Furthermore, an alkaline electrolyzer was assembled with two pieces of MOF-V-Ni₃S₂/NF, which achieves efficient water splitting at 1.58 V @10 mA cm^?2. This strategy opens up new channels to synthesize MOF-based bifunctional electrocatalysts toward overall water spitting.     

Hierarchical flower-like CoS2-MoS2 heterostructure spheres as efficient bifunctional electrocatalyst for overall water splitting

Electrochemical water splitting technique requires high-efficient bifunctional electrocatalysts to obtain large-scale hydrogen production for resolving the impending energy and environmental crisis. Herein, hierarchical flower-like CoS₂-MoS₂ heterostructure hybrid spheres grown on carbon cloth (CoS₂-MoS₂/CC) were prepared by sulfuring wheel-shaped polyoxometalate {Co₂₀Mo₁₆}. The as-prepared CoS₂-MoS₂/CC as bifunctional electrocatalyst manifests excellent alkaline oxygen evolution and hydrogen evolution activities with low overpotentials of 240 mV for OER and 60 mV for HER at 10 mA cm^?2, respectively. When assembled as two-electrode cell, CoS₂-MoS₂/CC delivers an extremely low cell-voltage of 1.52 V at 10 mA cm^?2 accompanied with remarkable long-term durability. Additionally, CoS₂-MoS₂/CC exhibits favorable overall-water-splitting performance in simulated seawater. The superior performance of CoS₂-MoS₂/CC should be ascribed to the optimized intrinsic electron structure via electron transfer from MoS₂ to CoS₂ along with the synergistic effect of well-exposed heterostructure interfaces and favorable diffusion channels. This work offers a practical strategy for exploring high-efficient bifunctional electrocatalysts for overall water splitting.     

Electrodeposition of NiS/Ni2P nanoparticles embedded in amorphous Ni(OH)2 nanosheets as an efficient and durable dual-functional electrocatalyst for overall water splitting

To develop earth-abundant and cost-effective catalysts for overall water splitting is still a major challenge. Herein, a unique “raisins-on-bread” Ni–S–P electrocatalyst with NiS and Ni₂P nanoparticles embedded in amorphous Ni(OH)₂ nanosheets is fabricated on Ni foam by a facile and controllable electrodeposition approach. It only requires an overpotential of 120 mV for HER and 219 mV for OER to reach the current density of 10 mA cm⁻² in 1 M KOH solution. Employed as the anode and cathode, it demonstrates extraordinary electrocatalytic overall water splitting activity (cell voltage of only 1.58 V @ 10 mA cm⁻²) and ultra-stability (160 h @ 10 mA cm⁻² or 120 h @50 mA cm⁻²) in alkaline media. The synergetic electronic interactions, enhanced mass and charge transfers at the heterointerfaces facilitate HER and OER processes. Combined with a silicon PV cell, this Ni–S–P bifunctional catalyst also exhibits highly efficient solar-driven water splitting with a solar-to-hydrogen conversion efficiency of 12.5%.     

Ultrathin MoS2 nanosheets in situ grown on rich defective Ni0.96S as heterojunction bifunctional electrocatalysts for alkaline water electrolysis

Developing earth-abundant and highly active bifunctional electrocatalysts are critical to advance sustainable hydrogen production via alkaline water electrolysis but still challenging. Herein, heterojunction hybrid of ultrathin molybdenum disulfide (MoS₂) nanosheets and non-stoichiometric nickel sulfide (Ni_0.96S) is in situ prepared via a facile one-step hydrothermal strategy, followed by annealing at 400 °C for 1 h. Microstructural analysis shows that the hybrid is composed of intimate heterojunction interfaces between Ni_0.96S and MoS₂ with exposed active edges provided by ultrathin MoS₂ nanosheets and rich defects provided by non-stoichiometric Ni_0.96S nanocrystals. As expected, it is evaluated as bifunctional electrocatalysts to produce both hydrogen and oxygen via water electrolysis with a hydrogen evolution reaction (HER) overpotential of 104 mV at 10 mA cm⁻² and an oxygen evolution reaction (OER) overpotential of 266 mV at 20 mA cm⁻² under alkaline conditions, outperforming most current noble-metal-free electrocatalysts. This work provides a simple strategy toward the rational design of novel heterojunction electrocatalysts which would be a promising candidate for electrochemical overall water splitting.     

Facile construction of porous and heterostructured molybdenum nitride/carbide nanobelt arrays for large current hydrogen evolution reaction

The development of cost-effective, highly efficient and stable electrocatalysts for alkaline water electrolysis at a large current density has attracted considerable attention. Herein, we reported a one-dimensional (1D) porous Mo₂C/Mo₂N heterostructured electrocatalyst on carbon cloth as robust electrode for large current hydrogen evolution reaction (HER). The MoO₃ nanobelt arrays and urea were used as the metal and non-metal sources to fabricate the electrocatalyst by one-step thermal reaction. Due to the in-situ formed abundant high active interfaces and porous structure, the Mo₂C/Mo₂N electrocatalyst shows enhanced HER activity and kinetics, as exemplified by low overpotentials of 54, 73, and 96 mV at a current density of 10 mA cm^?2 and small Tafel slopes of 48, 59 and 60 mV dec^?1 in alkaline, neutral and acid media, respectively. Furthermore, the optimal Mo₂C/Mo₂N catalyst only requires a low overpotential of 290 mV to reach a large current density of 500 mA cm^?2 in alkaline media, which is superior to commercial Pt/C catalyst (368 mV) and better than those of recently reported Mo-based electrocatalysts. This work paves a facile strategy to construct highly efficient and low-cost electrocatalyst for water splitting, which could be extended to fabricate other heterostructured electrocatalyst for electrocatalysis and energy conversion.     

Nickel foam-supported Fe,Ni-Polyporphyrin microparticles: Efficient bifunctional catalysts for overall water splitting in alkaline media

Developing highly active, stable and sustainable electrocatalysts for overall water splitting is of great importance to generate renewable H₂ for fuel cells. Herein, we report the synthesis of electrocatalytically active, nickel foam-supported, spherical core-shell Fe-poly(tetraphenylporphyrin)/Ni-poly(tetraphenylporphyrin) microparticles (FeTPP@NiTPP/NF). We also show that FeTPP@NiTPP/NF exhibits efficient bifunctional electrocatalytic properties toward both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Electrochemical tests in KOH solution (1 M) reveal that FeTPP@NiTPP/NF electrocatalyzes the OER with 100 mA cm⁻² at an overpotential of 302 mV and the HER with 10 mA cm⁻² at an overpotential of 170 mV. Notably also, its catalytic performance for OER is better than that of RuO₂, the benchmark OER catalyst. Although its catalytic activity for HER is slightly lower than that of Pt/C (the benchmark HER electrocatalyst), it shows greater stability than the latter during the reaction. The material also exhibits electrocatalytic activity for overall water splitting reaction at a current density of 10 mA cm⁻² with a cell voltage of 1.58 V, along with a good recovery property. Additionally, the work demonstrates a new synthetic strategy to an efficient, noble metal-free-coordinated covalent organic framework (COF)-based, bifunctional electrocatalyst for water splitting.     

Ni3S2-MoSx nanorods grown on Ni foam as high-efficient electrocatalysts for overall water splitting

In order to solve the problem of large overpotential in water electrolysis for hydrogen production, transition metal sulfides are promising bifunctional electrocatalysts for hydrogen evolution reaction/oxygen evolution reaction that can significantly reduce overpotential. In this work, Ni₃S₂ and amorphous MoS_x nanorods directly grown on Ni foam (Ni₃S₂-MoS_x/NF) were prepared via one-step solvothermal process, which were used as a high-efficient electrocatalyst for overall water splitting. The Ni₃S₂-MoS_x/NF composite exhibits very low overpotentials of 65 and 312 mV to reach 10 mA cm⁻² and 50 mA cm⁻² in 1.0 M KOH for HER and OER, respectively. Besides, it exhibits a low Tafel slope (81 mV dec⁻¹ for HER, 103 mV dec⁻¹ for OER), high exchange current density (1.51 mA cm⁻² for HER, 0.26 mA cm⁻² for OER), and remarkable long-term cycle stability. This work provides new perspective for further the development of highly effective non-noble-metal materials in the energy field.     

Porous NiFe alloys synthesized via freeze casting as bifunctional electrocatalysts for oxygen and hydrogen evolution reaction

Binder-free NiFe-based electrocatalyst with aligned pore channels has been prepared by freeze casting and served as a bifunctional catalytic electrode for oxygen and hydrogen evolution reaction (OER and HER). The synergistic effects between Ni and Fe result in the high electrocatalytic performance of porous NiFe electrodes. In 1.0 M KOH, porous Ni₇Fe₃ attains 100 mA cm⁻² at an overpotential of 388 mV with a Tafel slope of 35.8 mV dec⁻¹ for OER, and porous Ni₉Fe₁ exhibits a low overpotential of 347 mV at 100 mA cm⁻² with a Tafel slope of 121.0 mV dec⁻¹ for HER. The Ni₉Fe₁//Ni₉Fe₁ requires a low cell voltage of 1.69 V to deliver 10 mA cm⁻² current density for overall water splitting. The excellent durability at a high current density of porous NiFe electrodes has been confirmed during OER, HER and overall water splitting. The fine electrocatalytic performances of the porous NiFe-based electrodes owing to the three-dimensionally well-connected scaffolds, aligned pore channels, and bimetallic synergy, offering excellent charge/ion transfer efficiency and sizeable active surface area. Freeze casting can be applied to design and synthesize various three-dimensionally porous non-precious metal-based electrocatalysts with controllable multiphase for energy conversion and storage.     

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.     

Co/Mo2C composites for efficient hydrogen and oxygen evolution reaction

Non-precious transition metal electrocatalysts with high catalytic performance and low cost enable the scalable and sustainable production of hydrogen energy through water splitting. In this work, based on the polymerization of CoMoO₄ nanorods and pyrrole monomer, a heterointerface of carbon-wrapped and Co/Mo₂C composites are obtained by thermal pyrolysis method. Co/Mo₂C composites show considerable performance for both hydrogen and oxygen evolution in alkaline media. In alkaline media, Co/Mo₂C composites show a small overpotential, low Tafel slope, and excellent stability for water splitting. Co/Mo₂C exhibits a small overpotential of 157 mV for hydrogen evolution reaction and 366 mV for oxygen evolution reaction at current density of 10 mA cm⁻², as well as a low Tafel slope of 109.2 mV dec⁻¹ and 59.1 mV dec⁻¹ for hydrogen evolution reaction and oxygen evolution reaction, respectively. Co/Mo₂C composites also exhibit an excellent stability, retaining 94% and 93% of initial current value for hydrogen evolution reaction and oxygen evolution reaction after 45,000 s, respectively. Overall water splitting via two-electrode water indicates Co/Mo₂C can hold 91% of its initial current after 40,000 s in 1 M KOH.     

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.