Cost effective and facile low temperature hydrothermal fabrication of Cu2S thin films for hydrogen evolution reaction in seawater splitting

Electrolysis of seawater gets an attention to produce hydrogen for renewable energy technology. It significantly reduces the use of fresh water instead of seawater. Development of low temperature fabrication of electrocatalyst can explore seawater splitting by avoiding chloride reduction during the hydrogen production. In the present work, we fabricated low temperature hydrothermal growth of Cu₂S electrocatalyst on Ni foam at constant temperature of 80 °C at different growth times of 1–3 h. The prepared Cu₂S electrocatalyst grown for 1 h exhibited low overpotentials of 76 and 118 mV at 10 mA/cm² (289 and 358 mV overpotentials at 100 mA/cm²) in 1 M KOH deionized water and seawater, respectively for hydrogen evolution reaction (HER). The Tafel plot of Cu₂S catalyst grown for 1 h showed lesser Tafel slope value of 128 mVdec^?1 than that of other growth times 2 h (136 mVdec^?1) and 3 h (142 mV dec^?1) indicating elevated electrocatalytic behaviour of Cu₂S grown for 1 h. Electrochemical impedance spectroscopy (EIS) showed charge transfer resistance of 12.8Ω, 19.6 Ω and 25.7Ω, for Cu₂S grown for 1, 2 and 3 h, respectively, this lower charge transfer resistance indicated higher charge transfer properties. The Cu₂S electrocatalyst grown for 1 h sustained retention of 80% after 12 h continuous stability test. Therefore, the cost-effective and low temperature fabrication of Cu₂S electrocatalyst proceeds for development of largescale seawater splitting for hydrogen production.     

Electrochemically active and robust cobalt doped copper phosphosulfide electro-catalysts for hydrogen evolution reaction in electrolytic and photoelectrochemical water splitting

The area of non-noble metals based electro-catalysts with electrochemical activity and stability similar or superior to that of noble metal electro-catalyst for efficient hydrogen production from electrolytic and photoelectrochemical (PEC) water splitting is a subject of intense research. In the current study, exploiting theoretical first principles study involving determination of hydrogen binding energy to the surface of the electro-catalyst, we have identified the (Cu_0.83Co_0.17)₃P: x at. % S system displaying excellent electrochemical activity for hydrogen evolution reaction (HER). Accordingly, we have experimentally synthesized (Cu_0.83Co_0.17)₃P: x at. % S (x = 10, 20, 30) demonstrating excellent electrochemical activity with an onset overpotential for HER similar to Pt/C in acidic, neutral as well as basic media. The highest electrochemical activity is exhibited by (Cu_0.83Co_0.17)₃P:30 at. % S nanoparticles (NPs) displaying overpotential to reach 100 mA cm^?2 in acidic, neutral and basic media similar to Pt/C. The (Cu_0.83Co_0.17)₃P:30 at. % S NPs also display excellent electrochemical stability in acidic media for long term electrolytic and PEC water splitting process [using our previously reported (Sn_0.95Nb_0.05) O₂: N-600 nanotubes (NTs) as the photoanode]. The applied bias photon-to-current efficiency obtained using (Cu_0.83Co_0.17)₃P:30 at. % S NPs as the cathode electro-catalyst for HER in an H-type PEC water splitting cell (～4%) is similar to that obtained using Pt/C (～4.1%) attesting to the promise of this exciting non-noble metal containing system.     

Dysprosium doped copper oxide (Cu1-xDyxO) nanoparticles enabled bifunctional electrode for overall water splitting

The production of hydrogen, a favourable alternative to an unsustainable fossil fuel remains as a significant hurdle with the pertaining challenge in the design of proficient, highly productive and sustainable electrocatalyst for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Herein, the dysprosium (Dy) doped copper oxide (Cu_1-xDy_xO) nanoparticles were synthesized via solution combustion technique and utilized as a non-noble metal based bi-functional electrocatalyst for overall water splitting. Due to the improved surface to volume ratio and conductivity, the optimized Cu_1-xDy_xO (x = 0.01, 0.02) electrocatalysts exhibited impressive HER and OER performance respectively in 1 M KOH delivering a current density of 10 mAcm^?2 at a potential of ?0.18 V vs RHE for HER and 1.53 V vs RHE for OER. Moreover, the Dy doped CuO electrocatalyst used as a bi-functional catalyst for overall water splitting achieved a potential of 1.56 V at a current density 10 mAcm^?2 and relatively high current density of 66 mAcm^?2 at a peak potential of 2 V. A long term stability of 24 h was achieved for a cell voltage of 2.2 V at a constant current density of 30 mAcm^?2 with only 10% of the initial current loss. This showcases the accumulative opportunity of dysprosium as a dopant in CuO nanoparticles for fabricating a highly effective and low-cost bi-functional electrocatalyst for overall water splitting.     

An efficient way to improve water splitting electrocatalysis by electrodepositing cobalt phosphide nanosheets onto copper nanowires

Herein, catalysts with improved electrocatalysis of water splitting in alkaline media are accomplished by constructing bushy Cu nanowires on Cu foam to support cobalt phosphide nanosheets (Co–P/Cu NWs/CF). Results demonstrate that the highly efficient electroactivity of Co–P/Cu NWs/CF originates from the high intrinsic activity of Co–P species, Cu nanowires which possess accessible active sites, plentiful bubbles releasing channels, and honored electric conductivity, as well as the interaction between Cu NWs and Co–P species. To achieve a current density of 200 mA cm^?2, Co–P/Cu NWs/CF require an overpotential of only 155 mV for hydrogen evolution and 349 mV for oxygen evolution, which are less than that of Co–P/Cu(OH)₂ NWs/CF and Co–P/CF. This work provides a simple and efficient approach to designing water splitting electrocatalysts by manufacturing nanostructure-rich conductive substrates to support active species.     

Study of cobalt boride-derived electrocatalysts for overall water splitting

The production of clean hydrogen fuel by the electrolysis of water requires highly active, low-cost and facilely prepared electrocatalyst that minimizes energy consumption. Here we report an active cobalt boride (CoB)-derived electrocatalyst for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The CoB catalyst can be readily deposed on 3D nickel foam (NF) using a simple electroless plating method. A comprehensive analysis of the CoB catalyst with scanning electron microscopy transmission (SEM), transmission electron microscopy (TEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) techniques revealed that CoOOH is formed on the surface of CoB catalyst during the OER process and Co(OH)₂ is formed in the HER process. The catalyst derived from CoB/NF exhibits low overpotentials towards both OER and HER in alkaline solution. The electrolysis cell using the CoB-derived catalyst couple requires a cell voltage of 1.69 V to afford a current density of 10 mA/cm², which compares favorably with most non-noble bifunctional electrocatalysts. The favorable combination of high-performance, low cost and facile preparation suggests that transition metal borides may act as promising electrocatalyst for water splitting.     

Nitrogen-doped cobalt sulfide as an efficient electrocatalyst for hydrogen evolution reaction in alkaline and acidic media

The enhancement in intrinsic catalytic activity and material conductivity of an electrocatalyst can leads to promoting HER activity. Herein, a successful nitrogenation of CoS₂ (N–CoS₂) catalyst has been investigated through the facile hydrothermal process followed by N₂ annealing treatment. An optimized N–CoS₂ catalyst reveals an outstanding hydrogen evolution reaction (HER) performance in alkaline as well as acidic electrolyte media, exhibiting an infinitesimal overpotential of ?0.137 and ?0.097 V at a current density of ?10 mA/cm² (?0.309 and ?0.275 V at ?300 mA/cm²), corresponding respectively, with a modest Tafel slope of 117 and 101 mV/dec. Moreover, a static voltage response was observed at low and high current rates (?10 to ?100 mA/cm²) along with an excellent endurance up to 50 h even at ?100 mA/cm². The excellent catalytic HER performance is ascribed to improved electronic conductivity and enhanced electrochemically active sites, which is aroused from the synergy and mutual interaction between heteroatoms that might have varied the surface chemistry of an active catalyst.     

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.     

Recent advances in electrocatalysts for seawater splitting in hydrogen evolution reaction

Electrocatalytic water splitting is an important method to produce green and renewable hydrogen (H₂). One of the hindrances for wide applications of electrocatalysis in H₂ production is the lack of freshwater resources. Comparatively, seawater splitting has become an effective approach for large-scale H₂ production due to its abundant reserves. However, the increased complexity of seawater content emerged more problems in electrocatalytic seawater splitting. Recently, various strategies have been reported on improving the performance of electrocatalysts applied in seawater. Herein, this review firstly analyzed the mechanisms and challenges of electrocatalytic seawater splitting to evolve H₂, and summarized the recent progress on H₂ production in electrocatalytic seawater splitting. Furthermore, suggestions for future work have been provided for guidance.     

Effective surface roughening of three-dimensional copper foam via sulfurization treatment as a bifunctional electrocatalyst for water splitting

Cost effective electrocatalysts in water splitting reaction are critically important for the practical application of hydrogen fuel. The surface of three-dimensional copper foam is successfully roughened via one-step sulfurization reaction, and cuprous sulfide is formed on copper foam accordingly, which is denoted as Cu₂S@Cu. The as-prepared Cu₂S@Cu electrocatalyst exhibits remarkable performance on oxygen evolution reaction in basic solution, with a low overpotential of 345 mV to achieve 20 mA cm⁻². Cu₂S@Cu also shows enhanced performance on hydrogen production, compared to the original copper foam. Furthermore, Cu₂S@Cu can work as both cathode and anode in full water splitting, with superior activity to the noble metal-based electrocatalysts under large current densities. This study demonstrates that surface roughening technique on copper foam by sulfurization reaction can be valuable for developing novel copper-based electrocatalysts for water splitting.     

Template-free synthesis of 1D hollow Fe doped CoP nanoneedles as highly activity electrocatalysts for overall water splitting

Development of low cost and high efficiency electrocatalysts for water splitting systems to produce renewable hydrogen energy is still a significant requirement. The engineering of nanostructure and element doping are effective methods to further improve the performance of catalysts. Nonmetal (such as N, P, S) doping has been extensively investigated, while the report of metal doping is relatively few. Herein, Fe doped CoP 1D hollow nanoneedles on carbon cloth (CC) are designed and fabricated by a hydrothermal method and subsequent phosphorization procedure. The conversion of Fe doped Co-hydroxide@CC to Fe–CoP can produce large number of nanopores, which are closely connected to each other, and form hollow structures within the nanoneedles. Benefiting from the effective Fe doping and the particular hollow nanoneedle structure, the obtained Fe–CoP@CC demonstrates good electrocatalytic activity for hydrogen evolution reaction (HER) both in alkaline and acidic solution, affording a current density of 10 mA cm⁻² at overpotential of 49 mV and 80 mV, respectively. Moreover, the two-electrode electrolyzer with Fe–CoP@CC as both the cathode and anode catalyst achieve a current density of 10 mA cm⁻² at a cell voltage of 1.58 V in 1.0 M KOH solution. The results illustrate that the obtained hollow Fe–CoP@CC nanoneedles can serve as an efficient catalyst for overall water splitting.     

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.     

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.     

Electrodeposited nickel–iron–carbon–molybdenum film as efficient bifunctional electrocatalyst for overall water splitting in alkaline solution

Designing and synthesizing of efficient and inexpensive bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is one of the current research topics. In this study, NiFeCMo film in nickel mesh substrate is prepared by one-step direct-current electrodeposition method. The obtained NiFeCMo film shows the excellent electrocatalytic activity, which only requires overpotentials of 254 mV for HER and 256 mV for OER to drive current density of 10 mA cm⁻², with corresponding Tafel slopes of 163.9 and 60.3 mV·dec⁻¹ in 30% KOH medium, respectively. Moreover, NiFeCMo film only needs a low cell voltage of 1.61 V to drive current density of 10 mA cm⁻² in an alkaline electrolyzer. Such remarkably HER and OER properties of NiFeCMo alloy is attributed to the increased effective electrochemically active surface area and the synergy effect among Ni, Fe, C and Mo.     

Enhanced photoelectrochemical water splitting using gadolinium doped titanium dioxide nanorod array photoanodes

In this study highly oriented, rutile phase one dimensional Titania nanorod array (TiO₂ NRA) modified by gadolinium doping were synthesized on the conductive glass substrate (FTO) by the hydrothermal method. The effect of Gd doping on the photoelectrochemical performance of TiO₂ NRA was investigated. Crystal phase, structural, morphological and composition characteristics of these synthesized photoelectrodes were analyzed by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), and atomic force microscopy (AFM). FE-SEM images clearly show that some of the Gd dopant is uniformly distributed on the surface of TiO₂ NRA in the form Gadolinia (Gd₂O₃) microsphere. These gadolinia microsphere play an important role in reducing the surface recombination of electron and hole supported by photoluminescence's studies. Linear sweep voltammetry results show that Gd doping results in a two-fold increase in photocurrent density as compared to pristine TiO₂ NRA. UV–visible spectra, and Mott-Schotty measurements show that Gd doping shift the flat-band potential of TiO₂ NRA more toward negative potential that results in effective charge separation and transportation in the Gd doped TiO₂ NRA (Gd@TiO₂ NRA). Applied biased photon to current efficiency (ABPE) equation was used to find solar to hydrogen efficiency (STH). Gd@TiO₂ NRA show optimum conversion efficiency of ~0.64% at 0.03 V vs Ag/AgCl, while pristine TiO₂ NRA display ~0.33% at −0.21 V vs Ag/AgCl.     

Enhanced alkaline/seawater hydrogen evolution reaction performance of NiSe2 by ruthenium and tungsten bimetal doping

The exploration of high-efficiency and stable electrocatalysts for alkaline and seawater hydrogen evolution reaction (HER) is the key to realize energy conversion, but there is still a significant challenge owing to the slow HER kinetics in alkaline and seawater systems. In this study, we prepared nickel foamed-supported Ru, W co-doped NiSe₂ (Ru, W–NiSe₂/NF) by a brief two-step hydrothermal strategy and the prepared Ru, W–NiSe₂/NF displays exceptional HER property, requiring only a low overpotential of 100 and 353 mV to reach 10 mA cm⁻² in 1 M KOH and natural seawater, respectively, far superior to Ru–NiSe₂/NF, W–NiSe₂/NF and NiSe₂/NF. Electrochemical surface area (ECSA) and operando electrochemical impedance spectroscopy (EIS) verify the abundant active sites and superior electron transfer rate of Ru, W–NiSe₂, which optimized the HER kinetics in alkaline solution and natural seawater. The ECSA normalization and TOF results indicated that Ru, W co-doping increased the intrinsic activity of NiSe₂. This study revealed the impact of bimetallic doping on the intrinsic activity of NiSe₂, and provided a practical strategy for designing and developing the HER electrocatalysts with excellent performance.     

Synthesis of bristlegrass-like Co-doped Ni2P composites on Ni foam for overall seawater splitting

Seawater is the most abundant resource on earth, so developing cost-effective, highly durable corrosion resistance and efficient electrocatalysts are crucial to enhance seawater splitting. Herein, we prepared 3D bristlegrass-like Co-doped Ni₂P (Co-Ni₂P) composites supported on Ni foam (NF) through a facile solvothermal method combined and a subsequent phosphatization treatment. Benefiting from the unique structure, Co-Ni₂P shows excellent electrocatalytic activity as an electrode material for both the hydrogen evolution reaction (HER, low overpotential of 116 mV at 50 mA cm^?2) and oxygen evolution reaction (OER, low overpotential of 266 mV at 50 mA cm^?2). Moreover, the as-prepared Co-Ni₂P composites exhibit excellent stability and corrosion resistance in an alkaline medium. Density functional theory (DFT) calculations were employed to evaluate the H1 adsorption of Co-Ni₂P, and the results proved the high catalytic activity for the HER. This study provides new materials with a unique morphology for overall water splitting.     

Recent advances on electrocatalytic and photocatalytic seawater splitting for hydrogen evolution

Hydrogen is a clean and renewable energy source, which has aroused increasing attentions. Water splitting can effectively evolve hydrogen. Since freshwater is scarce, the direct exploitation of seawater as the feedstock to produce hydrogen has become a hotspot. Such direct exploitation can lower the cost of hydrogen produce and facilitate the rational exploitation of seawater resources. Over the past few years, advanced technologies (e.g. photocatalysis, electrocatalysis and photoelectrocatalysis) have been introduced into seawater splitting and have showed significant potentials. In this study, representative reports on photo-electro-catalytic seawater splitting to produce hydrogen were comprehensively reviewed. Besides, advancements and defects of each process were discussed. Furthermore, recommendations for subsequent study in this research field have been proposed.     

Crystal interpenetration featured NiWSe@NF acicular nanowires for performance enhanced water splitting

Synergistic composite catalysts have always been the research focus of water splitting. Whereas, amorphous phases usually occurred on grain boundaries, which greatly hinders electron transfer and reduces the catalytic performance. Herein, a NiWSe@NF acicular nanowire electrode was fabricated by a simple hydrothermal strategy, affording outstanding activity and durability, requiring overpotentials of only 103 mV (HER) and 203 mV (OER) under 10 mA cm^?2, respectively. Experimental studies and theoretical analysis demonstrate that the excellent catalytic activity is attributed to the interpenetrate structure, which eliminates the formation of amorphous phase and provides a high-speed channel for electron transmission. The crystal interpenetration on grain boundaries adjusts the electronic, promotes the intermediates adsorption and reduce reaction energy barrier. The super-hydrophilic arisen from crystal interpenetration would also reduce the adhesion of generated bubbles and avoid performance attenuation. This work provides a new perspective for the development of high-efficiency composite catalysts.     

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.     

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.