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.     

Race on engineering noble metal single-atom electrocatalysts for water splitting

Constructing noble metal single-atom catalysts (NMSACs) is essential to boosting the water splitting electrocatalysis by virtue of their maximum atom utilization and distinctive electronic structure, which can concurrently reduce the usage of noble metal and maintain high catalytic activity. In the past few years, great progress has been achieved in the field of NMSACs, including the structure and local coordination environment modifications, which significantly promote the electrochemical performance toward water splitting and deepen the understandings of the underlying mechanisms. Herein, in this review, the recent advances of the NMSACs in the field of water splitting applications have been comprehensively summarized, with a special emphasis on the advantages, synthetic strategies, and characterizations of the NMSACs. Moreover, some representative examples regarding the applications of NMSACs toward water splitting are also manifested according to the theoretical and experimental results. Furthermore, the challenges and future potentials of the NMSACs are also manifested to offer guidance for the development of more advanced NMSACs.     

Metal tungstate dominated NiCo2O4@NiWO4 nanorods arrays as an efficient electrocatalyst for water splitting

One of the most promising ways to produce high purity hydrogen is electrocatalytic water splitting, the slow rate of oxygen evolution reaction (OER) limited the whole water splitting process under alkaline conditions. Here, NiCo₂O₄@NiWO₄/NF is firstly acted as a metal tungstate dominated bifunctional water splitting catalyst, which a very low cell voltage of 1.59 V is obtained with 10 mA cm⁻² in alkaline media. Remarkably, the catalytic performance of NiCo₂O₄@NiWO₄/NF is also kept for at least 12 h, which shows long time electrochemical stability.     

Surface modification of a Co9S8 nanorods with Ni(OH)2 on nickel foam for high water splitting performance

Electrocatalytic water splitting to produce H₂ and O₂ has been considered as one of the most perfect solutions to resolve energy problems. Herein, we report the in situ deposit method on surface of Co₉S₈ by adjusting the number of sedimentary cycles to adjust Ni(OH)₂ loading capacity, and then firstly using the materials as water oxidation and water reduction catalysts. The one-dimensional Co₉S₈ nanorods was acted as an perfect support to enhance the uniform dispersion of amorphous Ni(OH)₂ nanoparticles, and the electrocatalytic deposited amorphous Ni(OH)₂ nanoparticles wrapped in the surface of Co₉S₈ nanorods highly improve the interfacial effect and afford more effective active centers for water splitting reaction. When Co₉S₈@Ni(OH)₂- amorphous/NF-2h-2 cycles, Co₉S₈@Ni(OH)₂ - amorphous/NF-10h-2 cycles and Ni(OH)₂- amorphous/NF is acted as a bifunctional catalyst, Co₉S₈@Ni(OH)₂- amorphous/NF-2h-2 cycles presents a lower cell voltage (1.55 V@10 mAcm⁻²) than Co₉S₈@Ni(OH)₂- amorphous/NF-10h-2 cycles (1.57 V@10 mAcm⁻²) and Ni(OH)₂- amorphous/NF (NA@10 mAcm⁻²). The activity of Co₉S₈@Ni(OH)₂- amorphous/NF-2h-2 cycles is compared to that of the ideal the benchmark 25 wt % RuO₂/NF−30 wt % Pt/NF (1.56 V@10 mAcm⁻²). Density functional theory (DFT) calculations were calculated to expound more mechanism on the interface effect of the sulfide/hydroxide-based catalysts for HER. For the two kinds of composite catalysts, both catalysts show good activity and stability in this work. Experiments show that Ni(OH)₂ does protect Co₉S₈ from corrosion under alkaline conditions. Our work paves the way for the development of environmentally friendly and relatively non-toxic water splitting electrocatalysts in terms of large-scale applications.     

Optimization of surface charge transfer processes on rutile TiO2 nanorods photoanodes for water splitting

Electrochemical impedance spectroscopy (EIS) has been performed to investigate the photocatalytic properties of titanium dioxide nanorods in a photoelectrochemical water splitting system. A two-channel transmission line model has been proposed to interpret the frequency response of the main charge transfer processes that occur at nanorod/electrolyte and platinum/electrolyte interfaces. EIS was then employed to determine that the dramatic effect of the annealing treatment on the photocurrent density had its origin on a poor charge transfer at the titania/electrolyte interface. X-ray photoelectron spectroscopy and thermogravimetry measurements have been used to prove the relevance of the presence of chlorine coming from the synthesis process of TiO₂ nanorods.     

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.     

The 3D ultra-thin Cu1-xNixS/NF nanosheet as a highly efficient and stable electrocatalyst for overall water splitting

In recent years, the exploration of efficient and stable noble-metal-free electrocatalysts is becoming increasingly important, used mainly for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). In this work, a new ultrathin porous Cu_1-xNi_xS/NF nanosheets array was constructed on the 3D nickel skeleton by two-step method: hydrothermal method and vulcanization method. Through these two processes, Cu_1-xNi_xS/NF has a larger specific surface area than that of foamed nickel (NF) and Cu_1-xNi_xO/NF. The Cu_1-xNi_xS/NF materials show excellent catalytic activity by accelerating the electron transfer rate and increase the amount of H₂ and O₂ produced. The lower overpotential was obtained only 350 mV at 20 mA cm⁻² for OER, not only that, but also the same phenomenon is pointed out in HER, optimal Cu_1-xNi_xS/NF presents low overpotentials of 189 mV to reach a current density of 10 mA cm⁻² in 1.0 M KOH for HER. Both OER and HER shows a lower Tafel slope: 51.2 mV dec⁻¹ and 127.2 mV dec⁻¹, subsequently, the overall water splitting activity of Cu_1-xNi_xS/NF was investigated, and the low cell voltage was 1.64 V (current density 10 mA cm⁻²). It can be stable for 14 h during the overall water splitting reaction. These results fully demonstrate that Cu_1-xNi_xS/NF non-precious metal materials can be invoked become one of the effective catalysts for overall water splitting, providing a richer resource for energy storage.     

Multi-active site CoNi-MOFs as non-noble bifunctional electrocatalysts for highly efficient overall water splitting

The high-efficiency non-precious metal catalysts for oxygen evolution (OER) and hydrogen evolution (HER) are of great significance to the development of renewable energy technologies. Herein, a multiple active sites CoNi-MOFs-DBD electrocatalyst modified by low temperature plasma (DBD) was successfully synthesized by converting metal hydroxyfluoride on nickel foam into a well-arranged MOFs array using vapor deposition. The as-prepared CoNi-MOFs-DBD electrode showed better HER and OER catalytic activity, super hydrophilicity, and excellent stability. In an alkaline medium, the overpotential of HER is 203 mV at 10 mA cm^?2 and that of OER is 168 mV at 40 mA cm^?2. When CoNi-MOFs-DBD was used as a bifunctional electrocatalyst for overall water splitting in a two-electrode system, a current density of 10 mA cm^?2 can be achieved at a low voltage of 1.42 V, which shows great potential in electrocatalytic water splitting.     

Crossed FeCo2S4 nanosheet arrays grown on 3D nickel foam as high-efficient electrocatalyst for overall water splitting

Great efforts in developing low-cost, highly efficient and stable electrocatalysts are to tune the chemical compositions and morphological characteristics for enhancing efficiency of water splitting. In this communication, FeCo₂S₄ nanosheet was grown in situ on nickel foam (FeCo₂S₄/NF) via a facile hydrothermal sulfidization method and served as a high-efficient bifunctional electrocatalyst for overall water splitting. As-synthesized FeCo₂S₄/NF self-supported electrode delivers 20 mA cm^?2 at an overpotential of 259 mV toward OER and 10 mA cm^?2 at an overpotential of 131 mV toward HER in alkaline media. Moreover, when used as both anode and cathode in a two-electrode electrolyzer, only a small cell voltage of 1.541 V is needed to afford a current density of 10 mA cm^?2 for overall water splitting. Bifunctional electrode FeCo₂S₄/NF also revealed a distinguished electrochemical durability during a 12 h stability test at 1.63 V, which would provide a promising water splitting installation for commercial hydrogen production.     

Co(III) carboxamide complexes as electrocatalysts for water splitting

Water splitting is a promising approach for storing intermittent renewable energies, such as sunlight in the clean chemical bonds as a hydrogen fuel. Two water-soluble octahedral cobalt (III) complexes, [Co(bpb)(OAc)(H₂O)], 1, (bpb^2? = N,N′-bis[(2-pyridine carboxamide)-1,2-benzene] dianion) and [Co(cbpb)(OAc)(H₂O)], 2, (cbpb^2? = N,N′-bis[(2-pyridine carboxamide)-4-chloro-1,2-benzene] dianion) were synthesised and characterised by CHN elemental analysis, UV–Vis, FT-IR and single-crystal X-ray diffraction techniques. The two carboxamide ligands had been prepared in the ionic liquid TBAB as an environmentally benign reaction medium. The electrocatalytic water splitting activity of 1 and 2 showed that both complexes are highly active for the water splitting in aqueous solutions. Turn Over Frequency (TOF) values were, for 1 and 2 respectively, 527 and 490 mol of hydrogen in each mole of catalyst per hour at an overpotential of 738 mV (pH = 7.0). Such a performance can be ascribed to the flat ligands, the electroactivity of the metal centre and carboxamide ligands and the ability of losing the axial ligands around the metal-ion centre during the reduction process which provide different reduction pathways for an HER process.     

A heterostructured catalyst composed of poly-2,6-diaminopyridine and TiO2 microspheres used as a photoanode for efficient water splitting

Photoelectrocatalytic (PEC) water splitting provides an alternative to direct solar-to-fuel production. In this study, a novel heterostructure formed between a conjugated polymer [poly-2,6-diaminopyridine (PDAP)] and three-dimensional TiO₂ microspheres was grown in situ on a Ti substrate (PDAP-3DTiO₂MSs/Ti) and used as photoanode for water oxidation in alkaline media under AM 1.5G illumination. The PDAP-3DTiO₂MSs/Ti can produce applied bias photon-to-current efficiency of 0.85% at 0.44 V vs. Pt and a photocurrent density of 1.56 mA cm⁻² at 1.23 V vs. RHE. Moreover, PDAP-3DTiO₂MSs/Ti displays impressive photoelectrochemical stability with 93% of its initial photocurrent being retained after 4 h of reaction. Based on physical-chemical characterization and photo-/electro-chemical measurements, the superior PEC water splitting performance of PDAP-3DTiO₂MSs/Ti should benefit from the coexistence of Ti³⁺ and Ti⁴⁺ in 3DTiO₂MSs, the light harvest capability of PDAP and the type II heterojunction formed between 3DTiO₂MSs and PDAP, which result in the enhanced generation and separation of photocarriers.     

Bifunctional electrocatalysts of CoFeP/rGO heterostructure for water splitting

Bifunctional non-precious electrocatalysts with high performance are highly desired for renewable energy but remain challenging. Herein, a CoFeP/rGO heterostructure was rational developed based on the synergistic effect, including superior conductivity, increased catalytic active sites of rGO support and the regulated electron distribution of bimetallic phosphide. At a current of 10 mA cm^?2, the CoFeP/rGO-2 composite exhibits excellent HER activity with low overpotentials of 101 mV and 76 mV in 1.0 M KOH and 0.5 M H₂SO₄ electrolyte, respectively. And highly active alkaline OER performance was provided with an overpotential of only 275 mV to reach a current density of 10 mA cm^?2. By the way, the CoFeP/rGO-2 electrode showed a pleasured working voltage of 1.58 V for overall water splitting in alkaline environment. More importantly, the long term durability and higher stability of the catalysts demonstrated their feasibility of bimetallic phosphide/rGO system as bifunctional electrocatalysts.     

Self-grown one-dimensional nickel sulfo-selenide nanostructured electrocatalysts for water splitting reactions

Herein, one-dimensional (1-D) self-grown nickel sulfide (NiS), and nickel sulfo-selenide (NiSSe) nanostructures on Ni-foam are successfully prepared via a simple and low-cost hydrothermal synthesis route. The S²⁻ and Se²⁻ ions are obtained after decomposition of sulfur and selenium precursors, which on reacting with oxidized Ni²⁺ ions, from the surface of pristine Ni-foam, produces NiS, and NiSSe superstructures. The crystal phase, surface morphology and chemical states of as-grown electrocatalysts are monitored through various measurment tools. Due to synergistic effects, the NiSSe nanostructured electrode provides a superior over potentials as low as 154 and 75 mV for OERs and HERs activity in electrochemical water splitting measurement. Moreover, the NiSSe electrode exhibits an excellent chemical stability compared to the NiS and NiSe electrodes during the electrolysis process. Such an outstanding catalytic performance makes the NiSSe electrode as a potential candidate in water splitting applications.     

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.     

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.     

Mo-doped Co9S8 nanorod array as a high performance electrochemical water splitting catalyst in alkaline solution

It is very important to exploit robust electrocatalysts for the water splitting in an alkaline medium. Hence, a series of Mo-doped Co₉S₈ nanorod array on Ni foam (Mo–Co₉S₈/NF) was successfully synthesized through hydrotherma and sulfuration processes for the first time and used as an efficient and stable difunctional electrocatalyst for the overall water splitting. Such Mo–Co₉S₈-3//Mo–Co₉S₈-2 electrodes couple display superior water splitting performance with the requirement of a cell voltage of 1.50 V to drive a catalytic current density of 10 mA cm⁻², which is lower than that of RuO₂//Pt/C (1.52 V). The activity of the catalyst is greatly enhanced by the molybdenum ion doping and the instability of the sulfide is resolved. The experiment result shows that the relationship between the current density and pH is different in neutral and alkaline media, which is most be likely assigned to the change of O–O formation by transforming the reactants from water molecule to the hydroxy ion.     

Enhanced water splitting through different substituted cobalt-salophen electrocatalysts

Synthesis of stable catalysts for water splitting is important for the renewable and clean energy production. Here, water oxidation activities of cobalt (II) complexes CoL¹-CoL³ (1–3) with salophen type ligands (N,N′-bis(salicylidene)-4-chloro-1,2-phenylendiamine (H₂L¹), N,N′-bis(salicylidene)-4-bromo-1,2-phenylendiamine (H₂L²) and N,N′-bis(salicylidene)-4-nitro-1,2-phenylendiamine (H₂L³)) are studied by electrochemical techniques, FE-SEM images and XRD patterns. Linear sweep voltammetry studies indicate that 2 and 3 have superior activities and only require the overpotential of 316 and 247 mV vs. RHE at current density of 10 mA/cm² with Tafel slopes of 75 and 50 mVdec^?1 at pH = 11. Experiments show relationships between the stability of the complexes and their catalytic activity. It is revealed that substituents on ligands affect the catalytic behaviors. Experiments show that in the presence of 2 and 3, the complexed cobalt ions are likely candidates as molecular catalysts for water oxidation. It is speculated that the O–O bond formation occurs by oxidizing the active center of cobalt complexes.     

Plasmonic Ag nanoparticles decorated NaNbO3 nanorods for efficient photoelectrochemical water splitting

In the present work, photoanodes comprising of NaNbO₃ nanorods and Ag nanoparticles decorated NaNbO₃ nanorods have been fabricated by using hydrothermal and chemical solution method respectively. Photoelectrochemical water splitting performance of the fabricated photoanodes have been measured and it is found that Ag decorated NaNbO₃ nanorods exhibit ～4 fold enhancement in photocurrent as compared to bare NaNbO₃ nanorods. The enhancement in the photoelectrochemical water splitting activity of Ag decorated NaNbO₃ nanorods is attributed to efficient charge carrier separation and visible light sensitization due to Ag nanoparticles on NaNbO₃ nanorods.     

Junction energetics engineering using Ni/NiOx core-shell nanoparticle coating for efficient photoelectrochemical water splitting

We report a sparse Ni/NiOx core-shell nanoparticle coating on an n-GaN photoanode that yields a high photocurrent by engineering junction energetics. A conventional thin film coating of high work function NiOx induces a large band bending that helps generate high photovoltage. However, the high work function causes a Fermi level downshift and compromises photopotential. The discrete core-shell nanoparticle coating balances these two effects. The Ni core creates localized large band bending to produce a high photovoltage. Meanwhile, the reduced coating surface area decreases Fermi level downshift. The resulting higher photopotential together with the catalytic NiOx shell enables a photocurrent 50% higher than NiOx thin film coating and multiple times higher than Ni nanoparticle or film coating. The localized large band bending also forms a potential well to deplete holes from semiconductor, thereby providing full surface protection against corrosion. This core-shell nanoparticle coating demonstrates a new junction energetics engineering paradigm useful for photoelectrode optimization.     

A review of heteroatomic doped two-dimensional materials as electrocatalysts for hydrogen evolution reaction

Emerging two-dimensional (2D) materials, such as graphene, transition metal disulfide compounds (TMDCs), MXenes, layer double hydroxides (LDHs), black phosphorus (BP) and hexagonal boron nitride (h-BN), play an important role in speeding up hydrogen evolution reaction (HER) due to its large specific surface area as well as function of loading and efficient support. However, as an electrocatalyst, pure 2D materials cannot meet HER needs caused by their monotonous performance. Therefore, some nanoparticles are used to load and tune the 2D materials to develop efficient and inexpensive catalysts. Herein, we conduct a thorough analysis for materials based on heteroatoms, especially transition metal atoms and non-metal atoms (N, P, S, etc.) doped with graphene, TMDCs, MXenes, LDHs, BP and h-BN. It can be found that doping or coupling between 2D materials will affect the electronic structure, energy band, active area, conductivity and stability of the catalyst, which will induct a huge change in the catalytic performance. This review reveals the relationship between active centers, H₂O adsorption and chemical reaction processes. It also analyzes and summarizes the design principles and performance improvement mechanisms of hybrid catalysts. These discussions can provide references for other researchers to develop derivatives of related catalysts.