Nickel sulfide-based electrocatalysts for overall water splitting

Hydrogen is a green energy with sustainability and high energy density. Electrochemical water splitting (EWS) is a promising green strategy for hydrogen production. Noble metal electrocatalysts exhibit excellent electrocatalytic activity in EWS. However, the applications of noble metals in EWS are limited because of their scarcity and high price. Therefore, the research on non-noble metal electrocatalysts has attracted much attention. Among them, nickel sulfide electrocatalysts, with a unique 3D structure, pretty conductivity, and adjustable electronic structure, show significant electrocatalytic activity in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this review, the mechanism of the electrocatalytic reaction, electrochemical parameters, and preparation methods of nickel sulfide are introduced first. Then, the five methods including atomic doping (including cations, anions and diatoms), morphological control, hybridization, integration with nanocarbon, and high-index facets exposure to regulate the electronic structure and active sites of nickel sulfide were illustrated, so as to improve the electrocatalytic activity of nickel sulfide. The electrocatalytic properties of these nickel sulfides were reviewed. However, there are some problems in the research of electrocatalysis, such as how to further improve the conductivity of the electrocatalyst, and the calculation method of current density is not unified. Therefore, our future development direction is to prepare a stable nickel sulfide electrocatalyst, study relevant strategies to simultaneously increase active sites and improve conductivity, and effectively make nickel sulfide into an EWS catalyst with higher performance.     

Synthesis of Ni2P/Ni5P4 on porous N decorated rGO foam for efficiently electrocatalytic hydrogen evolution reaction

As a new generation of non-precious metal catalysts, nickel phosphide is regarded as an ideal substitute for precious metal platinum in electrochemical hydrogen evolution. Here, a hydrogen evolution reaction (HER) electrocatalyst is developed by in situ growth of Ni₂P/Ni₅P₄ heterostructures on porous N decorated rGO foam (named Ni₂P/Ni₅P₄/N-rGO). The porous rGO foam structure provides a larger surface area and abundant active sites. The Ni₂P/Ni₅P₄ nanoparticles with heterostructures are uniformly distributed on the rGO sheet, which enhance the charge transfer ability. The decorating of N element also correspondingly improves the HER performance. The as-prepared Ni₂P/Ni₅P₄/N-rGO exhibits excellent HER performance in alkaline medium. When the current density is 10 mA cm^?2, the overpotential is only 22 mV. No obvious loss of HER activity after 2000 cyclic voltammetry indicates that the composite has excellent stability. This work presents a valuable route for fabricating inexpensive and high-performance catalysts for electrocatalysis.     

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.     

Stainless steel mesh-based CoSe/Ni3Se4 heterostructure for efficient electrocatalytic overall water splitting

The construction of high-efficiency bifunctional electrocatalysts is still a main challenge for hydrogen production from water splitting, in which comprehensive structure regulation plays a key role for synergistically boosting the intrinsic activity and charge collection. Here, we used a two-step hydrothermal method for construction of an interjaculated CoSe/Ni₃Se₄ heterostructure from NiCo LDH nanosheets grown on stainless steel (SS) meshes as bifunctional electrocatalysts for overall water splitting. The SS meshes containing Fe and Ni act as an excellent 3D scaffold for catalyst growth and charge collection. The SS@CoSe/Ni₃Se₄ composite exhibits outstanding electrocatalytic performances with low overpotentials of 97 mV for hydrogen evolution and 230 mV for oxygen evolution to reach a current density of 10 mA cm⁻², respectively. Moreover, by using SS@CoSe/Ni₃Se₄ as both the cathode and anode, the assembled electrolyze only required 1.55 V to reach 10 mA cm⁻² for overall water splitting. The outstanding performance of SS@CoSe/Ni₃Se₄ benefits from the synergy between excellent charge collection capability of SS meshes and the abundant active sites at the CoSe/Ni₃Se₄ heterointerface formed with the in-situ conversion of NiCo LDH nanosheets. Electrochemical active surface area and impedance spectrum indicate that the CoSe/Ni₃Se₄ loaded on SS has the most abundant electrochemically active sites and the smallest electrochemical resistance, thereby exposing more active sites and enhancing the charge transfer to promote the catalytic activity. By integrating the delicate nanoscale heterostructure engineering with the microscale SS mesh scaffold, our work provides a new perspective for the preparation of high-performance and cheap electrocatalysts that are easy to be integrated with industrial applications.     

N,S co-doped NiCo2O4@CoMoO4/NF hierarchical heterostructure as an efficient bifunctional electrocatalyst for overall water splitting

Electrocatalytic overall water splitting technology has received considerable attention in recent years. The fabrication of low-cost, earth-rich and potent bifunctional electrocatalysts is vital for hydrogen evolution (HER) and oxygen evolution reactions (OER). Herein, the N and S co-doped NiCo₂O₄@CoMoO₄ heterostructures (N, S–NCO@CMO₄₀₀) are fabricated by CVD and hydrothermal methods. N and S atoms as auxiliary active centers can increase the activity of Ni, Co and Mo atoms at the same time. Hierarchical heterostructures generate more interfaces to accelerate mass transfer and enlarge the electrochemical surface area, which greatly enhances the catalytic activity. The catalyst displays outstanding OER performance. The overpotentials of OER and HER are 165 and 100 mV at a current density of 10 mA cm^?2, respectively. More importantly, the N, S–NCO@CMO₄₀₀-based water splitting cell has a low voltage of 1.46 V at 10 mA cm^?2. Furthermore, the N, S–NCO@CMO₄₀₀ runs for 120 h in stable operation. This work provides new ideas for the design of hierarchical heterostructures with two-element incorporation.     

Bimetallic metal-organic framework derived electrocatalyst for efficient overall water splitting

The development of bifunctional catalysts that can be applied to both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is widely regarded as a key factor in the production of sustainable hydrogen fuel by electrochemical water splitting. In this work, we present a high-performance electrocatalyst based on nickel-cobalt metal-organic frameworks for overall water splitting. The as-obtained catalyst shows low overpotential to reaches the current density of 10 mA cm⁻² with 249 mV for OER and 143 mV for HER in alkaline media, respectively. More importantly, when the electrolyzer was assembled with the as-prepared catalyst as anode and cathode simultaneously, it demonstrates excellent activity just applies a potential of 1.68 V to achieve 10 mA cm⁻² current density for overall water splitting.     

Efficient suppression of surface charge recombination by CoP-Modified nanoporous BiVO4 for photoelectrochemical water splitting

BiVO₄ is a promising photoanode material for water splitting due to its substantial absorption of solar light as well as favorable band edge positions. However, the poor water oxidation kinetics of BiVO₄ results in its insufficient photocurrent density. Herein, we demonstrate the use of CoP nanoparticles for facile surface modification of nanoporous BiVO₄ photoanode in potassium borate buffer solution (pH 9.0), which can generate a tremendous cathodic shift of ~430 mV in the onset potential for photoelectrochemical water oxidation. In addition, a remarkable photocurrent density of 4.1 mA cm^?2 is achieved at 1.23 V vs. RHE under AM 1.5G illumination. The photoelectrochemical measurement using sodium sulfite as a hole scavenger clearly shows that the greatly improved performances are attributed to the efficient suppression of interfacial charge recombination through loading of CoP catalyst. Moreover, the maximum surface charge injection yield can reach >81% at 1.23 V vs. RHE and the maximum IPCE of CoP/BiVO₄ can reach 75.8% at 420 nm, suggesting the potential application of CoP-modified BiVO₄ photoanode for overall solar water splitting in cost-effective tandem photoelectrochemical cells.     

Niobium-doped cobalt phosphide nanowires realizing enhanced electrocatalytic activity for overall water splitting

Designing cost-effective bifunctional catalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in alkaline electrolyte remains a significant challenge. Herein, we report adding Nb to pristine CoP nanowires enhances the material's catalytic activities towards HER and OER. Density functional theory (DFT) calculation unravels that the Nb atoms not only optimize hydrogen binding abilities on CoP surface, but also modulate the surface electron densities of in situ formed β-CoOOH during anodic oxidation, thereby greatly accelerate both the HER and OER kinetics in alkaline solutions. In addition, an alkaline electrolyzer using Nb-doped CoP nanowires as cathode and anode for overall water splitting, delivers 100 mA cm^?2 at low cell voltage of 1.70 V, superior to Pt//RuO₂ couple. This doping strategy can be extended to other transition metal phosphides as multifunctional catalysts towards overall water splitting and beyond.     

Paintbrush-like Co doped Cu3P grown on Cu foam as an efficient janus electrode for overall water splitting

In this work, we demonstrated a facile strategy to fabricate paintbrush-like Co Doped Cu₃P architecture grown on porous copper foam (Co-Cu₃P/CF), which was obtained from cation exchange reaction followed by a pyrolysis assisted phosphorization step. Co-Cu₃P/CF showed outstanding electrocatalytic performance for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in 1 M NaOH solution, affording low overpotential of 270 mV to reach the current density of 50 mA cm⁻² for OER. As for HER, a low overpotential of 200 mV is required to obtain the same catalytic current density. The overall water electrolyzer by using Co-Cu₃P/CF as both anode and cathode showed a low cell voltage of 1.55 V to deliver 10 mA cm⁻². The excellent electrocatalytic performance of Co-Cu₃P/CF could be ascribed to its paintbrush-like hierarchical architecture, offering plentiful of active sites and accelerating electrolyte penetration, the presence of Co dopant also could rationally modify its electronic properties, and thus lead to the synergetic effects.     

Mo/P doped NiFeSe as bifunctional electrocatalysts for overall water splitting

Designing high-efficiency catalysts for overall water splitting is critical to reduce the cost of hydrogen fuel as a clean and renewable energy source in future society. In this work, a Mo-, P-codoped NiFeSe was successfully synthesized on nickel foam (NF) by one-step electrodeposition. Through the doping strategy, the conductivity can be well promoted, and the production of nanosheets on the catalyst surface and active phases during hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) provided much more active sites, which leaded to efficient HER/OER performances of as-synthesized Mo-, P-codoped NiFeSe catalysts, i.e., a low overpotential of 100 mV/200 mV at current density of 10 mA cm⁻² in 1.0 M KOH with stability of 95 h/60 h, respectively. It only required 1.53 V to deliver a current density of 10 mA cm⁻² in overall water splitting and maintained outstanding durability for 100 h. This work is beneficial to future design of high efficient and low-cost bifunctional catalysts for overall water splitting.     

Construction of hierarchical Prussian Blue Analogue phosphide anchored on Ni2P@MoOx nanosheet spheres for efficient overall water splitting

In response to the energy crisis, molybdenum-based catalyst has been proposed as a high-performance electrocatalytic material due to its low price and excellent HER performance. However, in contrast with its excellent HER performance, its poor OER performance often limits practical application as a high-performance overall water splitting catalyst. In this study, Prussian blue analogue (PBA) is grown in-situ on molybdenum-based nanosheet spheres by a simple and ingenious method and then subjected to phosphorization. The resulting composite catalyst exhibits highly efficient overall water splitting performance, overpotentials at current densities of 10 mA cm⁻² and 100 mA cm⁻² for the HER and OER are −61 mV and 268 mV, respectively. Moreover, an alkaline electrolyzer makes up by the catalyst both as positive and negative can reach a cell voltage 1.494 V at 10 mA cm⁻² for the overall water splitting. This method has provided a new strategy to effective combine PBA and molybdenum-based catalyst.     

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.     

Fe7S8/FeS2/C as an efficient catalyst for electrocatalytic water splitting

Exploring effective bi-functional catalysts is of great significance to enhance the electrochemical activity for overall water splitting. To date, Fe₇S₈ has been rarely reported to realize electrochemical overall water splitting because of its intrinsic poor conductivity. In this paper, Fe₇S₈/FeS₂ heterostructured nanosheets with interface structures and defect sites are prepared via a facile hydrothermal method. Fe₇S₈/FeS₂/C electrocatalysts are constructed through the addition of carbon powder to weaken the electron transfer barrier. As expected, Fe₇S₈/FeS₂/C requires the overpotential of 262 mV and 198 mV to reach 10 mA/cm² toward oxygen evolution reaction and hydrogen evolution reaction, respectively. Moreover, Fe₇S₈/FeS₂/C attains a voltage of 1.67 V at 10 mA/cm² and maintains long-term stability for 24 h toward overall water splitting in a two-electrode system. The excellent activity can be related to interface structures and surface defect sites, which boost the charge transfer rate owing to the rich active sites.     

Chemically prepared Polypyrrole/ZnWO4 nanocomposite electrodes for electrocatalytic water splitting

ZnWO₄, PPy, and PPy/ZnWO₄ nanoparticles were prepared using chemical synthesis. The structural, compositional and morphological properties of the prepared samples have been investigated using XRD, FTIR, SEM, and HRTEM respectively. The powder XRD reveals the monoclinic wolframite structure for both ZnWO₄ and PPy/ZnWO₄ nanocomposite. SEM confirms the wrapping of ZnWO₄ with PPy. The electrodes of ZnWO₄, PPy, and PPy/ZnWO₄ have been tested as bifunctional electrocatalyst towards HER and OER using constant current chronopotentiometry (CP) and Linear Sweep Voltammetry (LSV). The electrochemical surface area and the electrocatalytic activity PPy/ZnWO₄ nanocomposite towards HER and OER are greater than that of pure ZnWO₄ and PPy. The Tafel slope of PPy/ZnWO₄ nanocomposite is 76 and 84 mV dec⁻¹ in 0.5 M H₂SO₄ and 1 M KOH at room temperature for HER and OER respectively. The results suggest that PPy/ZnWO₄ nanocomposite is a good candidate for the bifunctional electrocatalyst for water splitting.     

A novel efficient dual-functional electrocatalyst for overall water splitting based on Ni0.85Se/RGO/CNTs nanocomposite synthesized via different nickel precursors

Synthesizing efficient and affordable electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) remains a crucial problem on the way to practical applications for producing clean H₂ fuel. Herein, high-efficiency and stable transition metal based electrocatalysts Ni_0.85Se-1, Ni_0.85Se-2 and Ni_0.85Se-3 materials with different morphological characteristics were derived via a one-step hydrothermal route using the Ni(OH)₂ and metal-organic framework (Ni-BDC and Ni-BTC) as precursors, respectively. The results showed that Ni_0.85Se-2 exhibited excellent electrocatalytic activity. Subsequently, introducing carbon nanomaterials (RGO and CNTs) to form Ni_0.85Se/RGO/CNTs nanocomposite material further improves the catalytic activity owing to high conductivity. The resulting Ni_0.85Se/RGO/CNTs nanocomposites electrocatalyst showed a low overpotential of 232 mV and 165 mV and a low Tafel slope of 64 mV dec^?1 and 98 mV dec^?1 when the current density was 10 mA cm^?2 for OER and HER, respectively. In addition, the Ni_0.85Se/RGO/CNTs nanocomposites were used as an anode and cathode of the water electrolysis device and the overall water splitting performance was investigated. The results show just a voltage of 1.59 V was required when the current density was 10 mA cm^?2 and good overall water splitting stability for 20 h. The outstanding electrocatalytic performance of Ni_0.85Se/RGO/CNTs is mostly due to its noticeable porous structure, the high conductivity and the large surface area that came from RGO and CNTs.     

Three-dimensional hierarchical nanoporous (Mn,Ni)-Doped Cu2S architecture towards high-efficiency overall water splitting

Dealloying technique is an important approach to design porous structures and highly active catalysts. In this work, monolithic nanoporous (Mn,Ni)-doped Cu₂S skeletons with controllable composition and tunable porosity are synthesized via dealloying and sulfuration technique. The as-prepared S-np-Mn₇₀Cu₂₉Ni₁ electrode exhibits outstanding catalytic performance toward HER and OER in 1.0 M KOH solution, which drives high current density of 50 mA cm^?2 at the overpotentials of 136 and 317 mV respectively. The excellent catalytic performance is attributed to the unique three-dimensional interconnected bicontinuous nanoporous architecture, which not only exposes high-density catalytic active sites, but also accelerates electron/mass transfer between catalyst surface and electrolyte. Density functional theory (DFT) calculations also reveal that (Mn,Ni)-doped Cu₂S matrix can accelerate water adsorption/dissociation and optimize adsorption-desorption energetics of H intermediates, thus improving the intrinsic HER activity of nanoporous Cu₂S electrocatalysts. Meanwhile, an alkaline water electrolyzer is constructed with the S-np-Mn₇₀Cu₂₉Ni₁ electrode as anode and cathode respectively, depicting remarkable performance in water electrolysis. In the light of advantages such as adjustable composition and tunable porosity in alloying-dealloying process, it offers a new vision for tuning the porosity and catalytic activities of transition metal sulfides and other active catalysts.     

Dual-functional Co3O4@Co2P4O12 nanoneedles supported on nickel foams with enhanced electrochemical performance and excellent stability for overall urea splitting

Urea splitting to produce hydrogen is one of the most promising solutions to the energy crisis in the future. A series of Co₃O₄ and cobalt phosphate composites on nickel foam were synthesized by hydrothermal and calcination process and firstly used as dual-functional electrode for the overall urea splitting. When the current density is 20 mA cm⁻², the required cell voltage is significantly lower than that of fully water splitting. The stability test results show that the composition and morphology of our catalyst do not change significantly before and after the reaction. By controlling the morphology under the same conditions, we concluded that the main factor affecting the activity of urea splitting was specific surface area and synergistic effect.     

Enhancing the electrocatalytic water splitting efficiency for amorphous MoSx

Amorphous molybdenum sulfide (MoS_x) materials have been considered as cheap and promising catalysts for hydrogen evolution reaction (HER). In this contribution, we report that the amorphous MoS_x catalysts prepared by the low temperature thermolysis of the (NH₄)₂MoS₄ precursors on carbon clothes (catalyst loading: 3.2 mg/cm²) exhibit a Tefal slope of 50.5 mV/dec and a high exchange current density of 1.5 × 10⁻³ mA/cm² in 0.5 M H₂SO₄ solutions. Spectroscopic studies of the amorphous MoS_x catalysts show that the increase of HER efficiency is positively correlated to the concentration of S₂²⁻ species, providing strong evidence to support the argument that S₂²⁻ is an active species for electrocatalytic HER. Additionally, the method for preparing catalysts is simple, scalable and applicable for large-scale production.     

Binder-free bifunctional SnFe sulfide/oxyhydroxide heterostructure electrocatalysts for overall water splitting

Developing robust non-noble catalysts towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is vital for large-scale hydrogen production from electrochemical water splitting. Here, we synthesize Sn- and Fe-containing sulfides and oxyhydroxides anchored on nickel foam (SnFeS_xO_y/NF) using a solvothermal method, in which a heterostructure is generated between the sulfides and oxyhydroxides. The SnFeS_xO_y/NF exhibits low overpotentials of 85, 167, 249, and 324 mV at 10, 100, 500 and 1000 mA cm^?2 for the HER, respectively, and a low overpotential of only 281 mV at 100 mA cm^?2 for the OER. When it serves as both anode and cathode to assemble an electrolyzer, the cell voltage is only 1.69 V at 50 mA cm^?2. The sulfides should be the efficient active species for the HER, while the oxyhydroxides are highly active for the OER. The unique sulfide/oxyhydroxide heterostructure facilitates charge transfer and lowers reaction barrier, thus promoting electrocatalytic processes.     

Comparative study on MoS2 and WS2 for electrocatalytic water splitting

Replacing Pt by earth abundant catalysts is one of the most important tasks toward potential large-scale HER applications. Among many potential candidates, low cost and earth abundant transition metal dichalcogenides such as MoS₂ and WS₂ have been promising as good H₂ evolution electrocatalysts when they are engineered into the structures with active sites. In this work, we have performed systematic studies on the catalytic reactivity of both MoS₂ and WS₂ materials produced by one-step and scalable thermolysis from (NH₄)₂WS₄ and (NH₄)₂MoS₄ precursors respectively. Structural analysis shows that these materials prepared at a higher thermolysis temperature exhibit higher crystallinity. The H₂ evolution electrocatalysts efficiency for the MoS₂ prepared at a lower temperature is higher than those at higher temperatures, where amorphous MoS₂ or S₂²⁻${{\text{S}}_2}^{2-}$ species instead of crystalline MoS₂ is the main active site. By contrast, crystalline WS₂ prepared at high temperature is identified to be the key reaction site. Both catalysts display excellent efficiency and durability as an electrocatalyst operating in acidic electrolytes. This work provides fundamental insights for further design and preparation of emergent metal dichalcogenide catalysts, beneficial for the development in clean energy.