Vermicular Ni3S2–Ni(OH)2 heterostructure supported on nickel foam as efficient electrocatalyst for hydrogen evolution reaction in alkaline solution

Alkaline solution is considered to be more suitable for industrial application of hydrogen production by water electrolysis. However, most of the low-cost electrocatalyts such as Ni₃S₂ has poor ability to dissociate HO–H, resulting in unsatisfied hydrogen evolution performance in alkaline media. In this paper, a novel vermicular structure of Ni₃S₂–Ni(OH)₂ hybrid have been successfully prepared on nickel foam substrate (v-Ni₃S₂–Ni(OH)₂/NF) through a facile two-step containing hydrothermal and electrodeposition processes. The heterostructure consists of rod-like Ni₃S₂ and Ni(OH)₂ nanosheets, in which Ni(OH)₂ is coated on the surface of Ni₃S₂. This structure not only constructs a fast electron transfer channel but also possesses rich heterointerface, thus accelerating the Volmer step and allowing more active sites of Ni₃S₂ to functioning well. As a result, v-Ni₃S₂–Ni(OH)₂/NF exhibited excellent electrocatalytic activity toward HER in 1.0 M KOH solution. It only needs 78 mV and 137 mV to drive current density of 10 mA cm⁻² and 100 mA cm⁻². Moreover, the catalytic stability of this electrocatalyst in alkaline solution is also satisfactory.     

Potentiostatic electrodeposited of Ni–Fe–Sn on Ni foam served as an excellent electrocatalyst for hydrogen evolution reaction

Ni–Fe–Sn electrocatalyst supported on nickel foam (Ni–Fe–Sn/NF) with high efficiency of hydrogen evolution reaction (HER) has been successfully fabricated through one-step potentiostatic electrodeposition route. The optimized Ni–Fe–Sn/NF displayed an extremely low overpotential of, respectively, 144 and 180 mV at 50 and 100 mA cm^?2 for HER in alkaline condition. Moreover, it could retain its superior stability for at least 12 h. The remarkable electrocatalytic activity of our electrocatalyst is ascribed to the high conductivity originated from synergistic effects between Ni, Fe, and Sn during HER process.     

Carbon supported bifunctional Rh–Ni(OH)2/C nanocomposite catalysts with high electrocatalytic efficiency for alkaline hydrogen evolution reaction

Pt group metals display lower HER activity in alkaline solution than in acidic solution, because they are inefficient in the water dissociation step (Volmer step). Compared with Pt, the activity difference of Rh in alkaline and acidic media is much smaller. Meanwhile, Ni(OH)₂ is proved to be an effective catalyst for water dissociation. Therefore, Rh–Ni(OH)₂/C nanocomposites with different Rh:Ni(OH)₂ ratios were synthesized by a co-deposition/partial reduction method, and their microstructures as well as electrocatalytic properties were studied. The results show that Rh and Ni(OH)₂ display synergistic effect in Rh–Ni(OH)₂/C nanocomposites. The Rh–Ni(OH)₂/C nanocomposite with a molar ratio of Rh to Ni(OH)₂ of 1:1 exhibits the highest activity. It shows an overpotential of 36 mV at 10 mA cm^?2 and a Tafel slope of 32 mV dec^?1 for HER in alkaline media, which is superior to commercial Pt/C. In addition, the Rh–Ni(OH)₂/C (1:1) nanocomposite shows excellent durability in alkaline media as well.     

NiCoP–CoP heterostructural nanowires grown on hierarchical Ni foam as a novel electrocatalyst for efficient hydrogen evolution reaction

The design and manufacture of effective non-noble metal catalysts for the H₂ evolution reaction (HER) are urgent for realizing a cost-effective hydrogen production. We report herein on flower-like structures consisting of NiCoP–CoP heterostructural nanowires grown directly on the hierarchically porous nickel framework (NiCoP–CoP/Ni/NF) to achieve a highly efficient HER in alkaline solution (1.0 M KOH). The NiCoP–CoP/Ni/NF is synthesized by electrodeposition of porous Ni layers on Ni foam, followed by simple hydrothermal reaction and phosphorization. For HER, the binder-free NiCoP–CoP/Ni/NF electrode can reach 10 mA cm^?2 current density at a quite low overpotential of 49 mV, because of the combination of porous Ni layers and highly active NiCoP–CoP nanowires. In addition, the NiCoP/CoP heterostructures exhibited remarkable stability under the long-term durability test. This work provides a new strategy that combines electrodeposition and hydrothermal reaction to synthesize effective HER catalysts.     

Three-dimensional Ni2P–MoP2 mesoporous nanorods array as self-standing electrocatalyst for highly efficient hydrogen evolution

Electrochemical hydrogen evolution reaction (HER) via the splitting of water has required electrocatalysts with cost-effectiveness, environmentally friendliness, high catalytic activity, and superior stability to meet the hydrogen economy in future. In this context, we report the successful synthesis of self-standing mesoporous Ni₂P–MoP₂ nanorod arrays on nickel foam (Ni₂P–MoP₂ NRs/3D-NF) through an effective phosphidization of the corresponding NiMoO₄ NRs/3D-NF. The as-synthesis Ni₂P–MoP₂ NRs/3D-NF, as an efficient HER electrocatalyst, exhibits small overpotential of 82.2 and 124.7 mV to reach current density of 10 and 50 mA cm⁻², a low Tafel slope of 52.9 mV dec⁻¹ and it retains its catalytic performance for at least 20 h in alkaline condition. Our work also offers a new strategy in designing and using transition metal phosphide-based 3D nanoarrays catalysts with enhanced catalytic efficiency for mass production of hydrogen fuels.     

Ni–MoS2 composite coatings as efficient hydrogen evolution reaction catalysts in alkaline solution

It remains an important project for the development of water splitting electrolyze to design and synthesis of more efficient non-noble metal catalyst. In this work, a structured Ni–MoS₂ composite coating has been synthesized under supergravity fields with nickel sulphamate bath containing suspended MoS₂ submicro-flakes. X-ray diffraction patterns indicate that the MoS₂ submicro-flakes have been successfully incorporated into the Ni matrix. Additionally, SEM shows that the prepared Ni–MoS₂ composite coatings display finer grain size than the pure Ni coatings, which can increase the electrochemistry surface area and the active site of hydrogen evolution reaction. Therefore, due to the synergistic effect of molybdenum disulfide and nickel, the Ni–MoS₂ composite coatings are directly used as binder-free electrode, which exhibits outstanding electrocatalytic activity for HER in 1.0 M NaOH solution at room temperature. The Ni–MoS₂ composite coatings demonstrated an outstanding performance toward the electrocatalytic hydrogen production with low overpotential (100 mA cm^?2 at η = 207 mV), a Tafel slope as small as 65 mV dec^?1, and stable cycling performance (1200 cycles). The preeminent HER performance of this catalyst suggests that it may hold great promise for practical applications.     

Synthesis and optimization of Mo–Ni–Se@NiSe core-shell nanostructures as efficient and durable electrocatalyst for hydrogen evolution reaction in alkaline media

Synthesis conditions are among the most influential factors in the electrocatalytic properties of the samples studied for the hydrogen evolution reaction (HER). In this study, conditions of NiSe synthesis over a Mo–Ni–Se layer were optimized to create core-shell nanostructures with excellent electrocatalytic properties. To optimize the synthesis conditions, first, two electrodeposition techniques in constant potential and pulse potential conditions were investigated and then the optimal temperature for electrodeposition between 5, 25, 40, and 60 °C was found. The electrocatalytic activity of the synthesized samples was investigated using linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chronopotentiometry tests in a 1 M KOH solution. Preliminary results showed that pulsed electrodeposition of NiSe could improve the electrocatalytic activity of Mo–Ni–Se by forming durable and suitable nanostructures, while electrodeposited NiSe at constant potential could reduce the electrocatalytic activity of the electrode by forming a dense structure. Then, to determine the appropriate temperature, electrodeposition at the optimal pulse potential at four temperatures of 5, 25, 40, and 60 °C was used to synthesize NiSe on Mo–Ni–Se. The final results showed that the sample synthesized at 60 °C with an electrochemically active surface area of 2870 cm² had the highest hydrogen production sites and required only an overpotential of 77 mV to achieve a current density of 10 mA cm^?2.     

Cu–Co–P electrodeposited on carbon paper as an efficient electrocatalyst for hydrogen evolution reaction in anion exchange membrane water electrolyzers

To solve the issues of energy shortage and environmental pollution, it is essential to develop highly effective catalysts for hydrogen evolution reaction (HER) in water electrolysis. Herein, we report a facile and rapid fabrication of a Cu–Co–P catalyst on a carbon paper (CP) substrate using electrodeposition. First, the deposition conditions for Co–P/CP were optimized. The prepared Co–P consisted of numerous spheres and exhibited acceptable catalytic activity towards HER in an alkaline medium with an overpotential of 72 mV at current density of ?10 mA/cm². Further performance enhancement was achieved by the incorporation of Cu to modify the electronic structure of the Co–P catalyst. In a half-cell test, the optimized Cu–Co–P/CP exhibits remarkable performance, achieving ?10 mA/cm² at an overpotential of 59 mV, and the Tafel slope is 38 mV/dec. In a single-cell test, an anion exchange membrane water electrolyzer with a Cu–Co–P/CP cathode and commercial IrO₂/CP anode exhibited high current density of 0.70 A/cm² at 1.9 V_cell.     

One-step electrodeposition of cauliflower-like Ni–Fe–Sn particles as a highly-efficient electrocatalyst for the hydrogen evolution reaction

Herein, a Ni–Fe–Sn coating was synthesized in-situ on Ni mesh by one-step electrodeposition at different durations. The Ni–Fe–Sn60 electrode obtained after 1 h deposition exhibits cauliflower-like morphology and the best electrocatalytic properties for the hydrogen evolution reaction (HER) compared to other electrodes. The electrode requires an overpotential of 43 mV at a current density of 10 mA cm⁻² and a small Tafel slope of 70 mV dec⁻¹ in a 1 M KOH solution. Moreover, the electrode shows outstanding stability in prolonged electrolysis and overall water splitting performance, generating a current density of 93 mA cm⁻² at 1.8 V, which is thrice that of an industry electrode. This electrocatalytic activity is ascribed to the high active surface area produced by the cauliflower-like Ni–Fe–Sn particles and the synergistic interaction of Ni, Fe and Sn. The simple synthesis method and excellent performance endow this electrode with great potential for large-scale applications.     

Novel 3-D urchin-like Ni– Co–W porous nanostructure as efficient bifunctional superhydrophilic electrocatalyst for both hydrogen and oxygen evolution reactions

Fabrication of an electrocatalyst with remarkable electrocatalytic activity for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is important for the production of hydrogen energy. In this study, Ni–Co–W alloy urchin-like nanostructures were fabricated by binder-free and cost-effective electrochemical deposition method at different applied current densities and HER and OER electrocatalytic activity was studied. The results of this study showed that the microstructure and morphology are strongly influenced by the electrochemical deposition parameters and the best electrocatalytic properties are obtained at the electrode created at the 20 mA.cm⁻²applied current density. The optimum electrode requires −66 mV and 264 mV, respectively, for OER and HER reactions for delivering the 10 mA cm⁻² current density. The optimum electrode also showed negligible potential change after 10 h electrolysis at 100 mA cm⁻², which means remarkable electrocatalytic stability. In addition, when this electrode used as a for full water splitting, it required only 1.58 V to create a current density of 10 mA cm⁻². Such excellent electrocatalytic activity and stability can be related to the high electrochemical active surface area, being binder-free, high intrinsic electrocatalytic activity and hydrophilicity. This study introduces a simple and cost-effective method for fabricating of effective electrodes with high electrocatalytic activity.     

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.     

Self-supported cobalt–molybdenum oxide nanosheet clusters as efficient electrocatalysts for hydrogen evolution reaction

In this paper, we report the three-dimensional self-supported CoMoO₄ nanosheet clusters on the nickel foam (denoted as CoMoO₄/NF) by a facile hydrothermal-calcination method for efficient hydrogen generation. As a result, the freestanding CoMoO₄ electrode exhibits an efficient electrochemical activity towards hydrogen evolution reaction, showing overpotentials as low as 68 and 178 mV at current densities of 10 and 100 mA cm⁻² in the alkaline condition (1 M KOH), respectively, a Tafel slope value of 82 mV per decade. Moreover, the electrode exhibits remarkable electrochemical durability for 1000 cycles. Significantly, the water splitting electrolyzer assembled with CoMoO₄/NF || NiFe LDH/NF (the nickel iron layered double hydroxide supported on the nickel foam) system achieved 20 mA cm⁻² at 1.63 V, showing the CoMoO₄/NF is promising for practical water splitting applications.     

Electrodeposition of Ni–Fe–Mn ternary nanosheets as affordable and efficient electrocatalyst for both hydrogen and oxygen evolution reactions

The synthesis of high performance and economical electrocatalysts in the process of overall water splitting is very important for the production of hydrogen energy and has become one of the most important challenges. Here, various Ni, Ni–Fe, Ni–Mn nanosheets and Ni–Fe–Mn ternary nanosheets were created using cost-effective, versatile and binder-free electrochemical deposition methods, and the electrocatalytic activity of various electrodes for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were investigated in an alkaline environment. Due to the high electrochemical active surface area due to the fabrication of nanosheets, the synergistic effect between different elements on the electronic structure, the high wettability due to the formation of nanosheets and the quick detachment of formed gasses from the electrode, the Ni–Fe–Mn nanosheets electrode showed excellent electrocatalytic activity. In order to deliver the 10 mA cm⁻² current density in HER and OER processes, this electrode required values of 64 mV and 230 mV overpotential, respectively. Also, the stability test showed that after 10 h of electrolysis at a current density of 100 mA cm⁻², the overpotential changes was very small (less than 4%), indicating that the electrode was excellent electrostatic stability. Also, when using as a bi-functional electrode in the full water splitting system, it only needed a cell voltage of 1528 V to deliver a current of 10 mA cm⁻². The results of this study indicate a new strategy for the synthesis of active and stable electrocatalysts.     

Nickel-doped MoSe2 nanosheets with Ni–Se bond for alkaline electrocatalytic hydrogen evolution

Molybdenum diselenide (MoSe₂) is a potential catalytic material for the electrocatalytic hydrogen evolution reaction (HER). However, due to the low density of its active sites, MoSe₂ nanosheets feature high overpotential in HER, which limits its practical application. This describes the method of doping the Ni in MoSe₂ nanosheets to increase active sites. The NiO₂ evenly dispersed on MoSe₂ by ethanol solution reduces to ~4 nm Ni nanoclusters under annealing process, which is firmly adhered to MoSe₂ nanosheets with Ni–Se bond. The electrochemical active surface area of Ni-doped MoSe₂ expands, proving that Ni dopants produce more activity sites in MoSe₂ nanosheets. The overpotential of MoSe₂ (at 10 mA cm⁻²) decreases from 335 mV to 181 mV with 4.5 at.% Ni doped in 1 M KOH. The Ni–MoSe₂ also characterizes excellent stability for 12 h with the formation of Ni–Se bond. The study of doping Ni in MoSe₂ nanosheets is of great guiding significance to the design and production of non-noble electrocatalysts for HER in alkaline media.     

Metal–organic framework-derived Co@NMPC as efficient electrocatalyst for hydrogen evolution reaction: Revealing the synergic effect of pyridinic N–Co

Developing hydrogen economy is one of the feasible routes to reduce carbon emission in response to the energy crisis and global warming. The hydrogen generation by electrochemical water splitting has received widespread attention, but it is still challenging to fabricate high-efficient electrocatalysts to decrease the kinetic energy barrier of hydrogen evolution reaction (HER). Loading transition metal (TM) nanoparticles (NPs) into heteroatom-doped carbon materials (HCM) has been reported as a capable scheme to increase the electrochemical activity and stability, but the synergic effect between TM surface and HCM is still worth exploring. Ascertaining that, we used metal-organic frameworks (MOFs) as the sacrificial precursor to synthesis a series of Co NPs encapsulated in N-doped microporous carbon (NMPC) nanocatalysts (denoted as Co@NMPC) with different N species (such as pyrrolic, pyridinic and graphitic N). The nanocatalyst prepared at an appropriate condition displayed an outstanding HER activity with an overpotential of 193 mV in 1 M KOH solution and 132 mV in 0.5 M H₂SO₄ solution to reach 10 mA cm^?2 current density. Furthermore, the results of in situ shielding tests indicate that the synergy of pyridinic N–Co site owing to the intimate contact between Co surface and NMPC play the pivotal role in boosting HER performance. Density functional theory (DFT) calculations were employed to obtain an in-depth mechanism of synergic effect between Co and NMPC.     

Hexagonal CoFe2O4/β-Ni(OH)2 heterojunction composite as an advanced electrocatalyst for the oxygen evolution reaction

Transition metal-based heterostructure materials are considered as promising alternatives to state-of-the-art noble metal-based catalysts toward the oxygen evolution reaction (OER). Herein, for the first time, a simple interface engineering strategy is presented to synthesize efficient electrocatalysts based on a novel CoFe₂O₄/β-Ni(OH)₂ heterogeneous structure for the electrochemical OER. Remarkably, the optimized CoFe₂O₄/β-Ni(OH)₂ electrocatalyst, benefiting from its hierarchical hexagonal heterostructure with strong electronic interaction, enhanced intrinsic activity, and electrochemically active sites, exhibits outstanding OER electrocatalytic performance with a low overpotential of 278 mV to reach a current density of 10 mA cm⁻², a small Tafel slope of 67 mV dec⁻¹, and long-standing durability for 30 h. Its exceptional OER performance makes the CoFe₂O₄/β-Ni(OH)₂ heterostructure a prospective candidate for water oxidation in alkaline solution. The proposed interface engineering provides new insights into the fabrication of high-performance electrocatalysts for energy-related applications.     

Binary spindle-like cobalt–iron layered-double hydroxide as an efficient electrocatalyst for oxygen evaluation reaction

In this research, inexpensive transition metals are used for electrocatalytic oxygen evolution reactions. Nickel foam was modified as a substrate using spindle-like Co–Fe LDH. XRD, FE-SEM, EDS, and elemental mapping techniques are used for characterization. Based on LSV results, overpotentials of 280 and 355 mV were obtained at 10 and 100 mA cm^?2, respectively. To evaluate the stability of electrocatalyst, cyclic voltammetry, chronoamperometry (at constant potentials of 1.55 and 1.65 V vs. RHE), and chronopotentiometry (at applied current densities of 10 and 100 mA cm^?2) methods are used. All stability tests were performed consecutively using one electrode. Current density increased by 0.05% after performing 500 CV cycles. The catalytic performance was enhanced during the chronoamperometry analysis. However, the potential decreased slightly by less than 0.44% after 7200 s at 100 mA cm^?2 in chronopotentiometry analysis. The high durability and current density ensure the efficient performance of the as-prepared electrocatalyst in water splitting process.     

Advanced Co3O4–CuO nano-composite based electrocatalyst for efficient hydrogen evolution reaction in alkaline media

In this study, we incorporate a copper impurity into (Co₃O₄) nanowires precursor that turn them into an active catalyst for the hydrogen evolution reaction in 1M KOH. The XRD and XPS results are in good agreement and confirmed the formation of Co₃O₄–CuO nano-composite by wet chemical method. To date, the performance of hydrogen evolution reaction in alkaline for the composite catalyst is comparable or superior to cobalt oxide based HER electro-catalysts. The HER catalyst exhibits the lowest Tafel slope of 65 mVdec⁻¹ for the cobalt-based catalysts in alkaline media. A current density of 10 mA/cm² is achieved at a potential of 0.288 V vs reversible hydrogen electrode (RHE). The mixed transition metal oxide Co₃O₄–CuO based HER electro-catalyst is highly stable and durable. The EIS results demonstrates that HER is highly favorable on the Co₃O₄–CuO due to the relatively small charge transfer resistance (173.20 Ohm) and higher capacitance values (1.97 mF).     

Ru–MoS2@PPy hollow nanowire as an ultra-stable catalyst for alkaline hydrogen evolution reaction

Electrocatalytic hydrogen evolution reaction (HER) is one of the green and effective method to produce clean hydrogen energy. However, the development of non-Pt HER catalysts with excellent catalytic activity and long-term stability still remains a great challenge. Herein, a vertically aligned core-shell structure material with hollow polypyrrole (PPy) nanowire as a core and Ru-doped MoS₂ (Ru–MoS₂) nanosheets as a shell is firstly reported as a highly efficient and ultra-stable catalyst for HER in alkaline solutions. Results indicate that Ru–MoS₂@PPy catalyst demands a low overpotential of 37 mV at 10 mA cm^?2. In addition, the overpotential at 100 mA cm^?2 is 157 mV and it is almost unchanged after 40,000 cyclic voltammetry cycles. The existence of PPy core not only ensures the vertical growth of MoS₂ nanosheets to expose more edge sites, but also promotes the rapid transfer of electrons, contributing to the improvement of catalytic activity. More importantly, the strong interface interaction between MoS₂ and PPy prevents the collapse of the vertical structure of MoS₂ sheets in the electrocatalytic process and greatly enhances the stability of catalysts, which offers an effective strategy to design and synthesize the HER catalysts with superior catalytic stability.     

Electroplated Zn–Ni nanocrystalline alloys as an efficient electrocatalyst cathode for the generation of hydrogen fuel in acid medium

Energy conversion and renewable energy are the valuable research fields for the future of the energy. Synthesis of electroplated thin film of low cost elements and their alloys is promising nanomaterials for energy conversion. Electroplating of Zn–Ni alloys were performed using natural products such as cysteine and gluconate under direct current and ultrasound waves. The morphological and crystalline structures of the electroplated Zn–Ni alloys were examined using scanning electron microscopy, SEM, and X-ray diffraction techniques, XRD. The chemical composition of the electroplated Zn–Ni alloys was determined using energy dispersive X-ray analysis, EDX. The morphological structures of electroplated Zn–Ni alloys changed from smooth to coral reef-like and granular structures with the increase of Zn wt%. Electrocatalysis of the hydrogen evolution reaction using the electroplated Zn–Ni alloys was studied in 0.5 M H₂SO₄ medium by the cathodic polarization and electrochemical impedance spectroscopy, EIS. The electroplated Zn–9.5Ni cathode of cubic γ-brass arrangement exhibits the highest rate of hydrogen evolution reaction.