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.     

Designing of CoP4/FeP4 with hollow nanocages as bifunctional catalysts for overall water splitting

Efficient and stable electrocatalysts are essential for water splitting. Feasible structural design can facilitate electron transport and increase specific surface area. Herein, the porous CoP₄/FeP₄ hollow cubes are synthesized by two steps: synthesizing the Co–Fe prussian blue analogues via co-precipitation and phosphating it by calcination. The construction of heterojunction in CoP₄/FeP₄ not only accelerates electronic transmission but also provides active sites, which acts synergistically on the oxygen and hydrogen evolution reactions. Therefore, the CoP₄/FeP₄ hollow cubes with the exist of mesoporous exhibit the promising performance for water splitting. The enhanced performance that basically originates from bimetallic synergy and unique morphological structure is acquired with a low overpotential of 270 mV at 10 mA cm^?2 and the Tafel slope of 42.4 mV dec^?1 towards the oxygen evolution reaction (OER). Electrolyzer with two-electrode system assemble by utilizing the CoP₄/FeP₄ hybrid as anode and cathode exhibits a cell voltage of 1.74 V to achieve 10 mA cm^?2 for overall water splitting. This study that provides a simple strategy to design and construct the heterogeneous interface may promote the development of non-noble metal for HER and OER.     

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.     

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.     

High-performance bifunctional Fe-doped molybdenum oxide-based electrocatalysts with in situ grown epitaxial heterojunctions for overall water splitting

The design and development of low-cost, abundant reserves, high catalytic activity and durability bifunctional electrocatalysts for water splitting are of great significance. Here, simple hydrothermal and hydrogen reduction methods were used to fabricate a uniform distribution of Fe-doped MoO₂/MoO₃ sheets with abundant oxygen vacancies and heterojunctions on etched nickel foam (ENF). The Fe– MoO₂/MoO₃/ENF exhibited a small overpotential of 36 mV at 10 mA cm⁻² for hydrogen evolution reaction (HER), an excellent oxygen evolution reaction (OER) overpotential of 310 mV at 100 mA cm⁻² and outstanding stabilities of 95 h and 120 h for the HER and OER, respectively. As both cathode and anode catalysts, the heterogeneously structured Fe– MoO₂/MoO₃/ENF required a low cell voltage of 1.57 V at 10 mA cm⁻². Density functional theory (DFT) calculations show that Fe doping and MoO₂/MoO₃ heterojunctions can significantly reduce the band gap of the electrode, accelerate electron transport and reduce the potential barrier for water splitting. This work provides a new approach for designing metal ion doping and heterostructure formation that may be adapted to transition metal oxides for water splitting.     

ZnO/Cu2O-decorated rGO: Heterojunction photoelectrode with improved solar water splitting performance

In present work, we report a facile fabrication process to improve the photoelectrochemical (PEC) performance of ZnO-based photoelectrodes. In order to achieve that, the Cu₂O nanocubes are cathodic-deposited on the as-prepared ZnO nanorods. Then rGO nanosheets are electrodeposited on the ZnO/Cu₂O heterostructures. The fabricated photoelectrodes are systematically studied in detail by different characterization techniques such as powder X-ray diffraction, micro-Raman, X-ray photoelectron spectroscopy, ultraviolet diffused reflectance spectroscopy and photoluminescence spectroscopy analysis. Morphologies of the fabricated photoelectrodes are investigated through electron microscopy in scanning and transmission mode. To evaluate the PEC performance of the fabricated photoelectrodes, the line scan voltammetry (LSV) measurement is performed using a three-electrode system in 0.5-M Na₂SO₄ electrolyte solution under stimulated light illumination at 100 mW/cm² from a 300-W Xenon Arc lamp coupled with an AM 1.5G filter using a three-electrode system. The photocurrent measurement demonstrates that the photoelectrodes based on ZnO/Cu₂O/rGO possess enhanced PEC performance compared to the pristine ZnO and ZnO/Cu₂O photoelectrodes. The photocurrent density of ZnO/Cu₂O/rGO-15 photoelectrode (10.11 mA/cm²) is ∼9 and ∼3 times higher than the photoelectrodes based on pristine ZnO (1.06 mA/cm²) and ZnO/Cu₂O (3.22 mA/cm²). The enhanced PEC performance of ZnO/Cu₂O/rGO photoelectrode is attributed to the excellent light absorption properties of Cu₂O and excellent catalytic and charge transport properties of rGO. Experimental results reveal that the proposed functional nanomaterials have a great potential in water splitting applications.     

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.     

rGO decorated semiconductor heterojunction of BiVO4/NiO to enhance PEC water splitting efficiency

Monoclinic phase of BiVO₄ is a promising photoanode material for photoelectrochemical (PEC) water splitting, but its sluggish water oxidation kinetics and frequent bulk charge recombination greatly reduce its efficiency of PEC water splitting. A novel BiVO₄/NiO/rGO photoanode was very simply prepared by electrodeposition, solution immersion and spin coating methods, in particular, the solution immersion method to loading NiO has never been reported in PEC research. Compared with BiVO₄, the photocurrent density of the ternary photoanode reaches 1.52 mA/cm² at 1.23 V vs RHE, which is 2.41 and 1.39 times higher than that of pure BiVO₄ and binary BiVO₄/NiO photoanode, respectively. The onset potential of the ternary photoanode shows a significant cathodic shift of 130 mV compared with the BiVO₄ photoanode. Moreover, the measured incident photon-to-current efficiency (IPCE) value reaches 50.52% at λ = 420 nm. The improvement is attributed to the type-II heterojunction formation that enhances the separation efficiency of electron/hole and the rGO decoration that accelerates the electron transfer and provides more active sites for gas adsorption.     

Heterostructural Co/CeO2/Co2P/CoP@NC dodecahedrons derived from CeO2-inserted zeolitic imidazolate framework-67 as efficient bifunctional electrocatalysts for overall water splitting

The design and fabrication of highly active, robust and cost-efficient electrocatalysts for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is of great significance towards overall water splitting, but remains challenging as well. Herein, we report, for the first time, heterostructural Co/CeO₂/Co₂P/CoP@NC dodecahedrons as bifunctional electrocatalyst, in which abundant interfaces are formed between different components. Typical ZIF-67 (ZIF = zeolitic imidazolate framework) dodecahedrons with pre-inserted CeO₂ nanowires were selected as precursors to synthesize Co/CeO₂/Co₂P/CoP@NC via a direct carbonization process followed by phosphidation, simultaneously generating the strong coupled heterojunction interfaces through interactions between CeO₂ and Co_xP species. Abundant porous structure leads to the exposure of more active sites and the carbon encapsulation of nanodomains sustains the high robustness and conductivity and the synergistic effect between the multi-components heterostructure. Benefiting from the above collective advantages, the Co/CeO₂/Co₂P/CoP@NC electrocatalysts exhibit small overpotentials of 307 and 195 mV to derive 10 mA cm⁻² for OER and HER, respectively. Furthermore, an alkaline electrolyzer assembled by using Co/CeO₂/Co₂P/CoP@NC as both cathode and anode can achieve a current density of 10 mA cm⁻² at a low voltage of 1.76 V and work continuously for over 15 h. This work would provide a rational protocol for fabrication multi-phase interface enriched electrocatalysts toward highly efficient energy conversion.     

Amorphous CoFeP/NC hybrids as highly efficient electrocatalysts for water oxidation

The rational design and regulate structure and composition are pivotal for the development of highly efficient oxygen evolution reaction (OER) catalysts for water splitting. In this study, amorphous CoFeP/NC hybrid electrocatalyst has been synthesized by a simple and effective phosphorization of a CoFe-based coordination polymer under N₂ atmosphere. The synergistic effects between the CoFeP and N-doped carbon has led to high electronic conductivity attributed to the optimal Fe contents with N-doped carbon and enlarged electrocatalytic active surface area aroused by the nanostructure of CoFeP/NC, as well as the surface structural evolution of oxyhydroxide/phosphate during OER process. The resulting Co_0.35Fe_0.17P_0.48/NC electrocatalyst can attain a current density of 10 mA/cm² at an overpotential of 275 mV with a Tafel slope of 31 mV/dec on glassy carbon electrode and 228 mV on Ni foam electrode in 1 M KOH solution, long-term OER stability of this Co_0.35Fe_0.17P_0.48/NC under the applied potential of 1.53 V vs. RHE demonstrates no obvious decline in current densities of 110 mA/cm² within 17 h, which outperforms those of the contrast electrocatalysts in this work and also comparable to that of many of the reported electrocatalysts in the literatures. This Co_0.35Fe_0.17P_0.48/NC electrocatalyst highlights the rational modulation of optimal composition and electronic structure with homogeneous incorporation of the foreign metal-doped and N-doped carbon for the synthesis of highly efficient electrocatalysts toward to the water oxidation reactions.     

Needle-like CoP/rGO growth on nickel foam as an efficient electrocatalyst for hydrogen evolution reaction

In this work, CoP/NF is synthesized at different temperature (250 °C, 300 °C, 350 °C) (denoted as CoP/NF-T, T = 250, 300, 350). Then, CoP/NF-300 with the best performance towards hydrogen evolution reaction (HER), is used to synthesize compounds with different ratio of reduced graphene oxide (rGO) (CoP/rGO/NF-X, X (quality ratio of rGO/CoP) = 1,3,5). In terms of morphology, under the synergistic effect of rGO, uniform and dense CoP provides the possibility to increase the electrochemical area. While CoP/rGO/NF-3 shows the minimum overpotential of 136 mV to drive 50 mA/cm, and the smallest Tafel slope 135 mV/dec among as-synthesized materials. Furthermore, CoP/rGO/NF-3 has good stability during at least 25 h. These result can be construed as the large electrochemical active area, high conductivity and long-time stability.     

NiS2@V2O5/VS2 ternary heterojunction for a high-performance electrocatalyst in overall water splitting

Water splitting is regarded as an effective way to produce hydrogen energy to solve the energy crisis all over the world. However, the electrocatalysts suffer from expensive prices, high voltage, and sluggish kinetics. The heterojunction is composed of two semiconductors and can accelerate electron transfer by relying on interface engineering. Herein, we first prepare NiS₂@V₂O₅/VS₂ ternary heterojunction electrocatalyst, showing the low OER overpotential of 333 mV and HER overpotential of 216 mV at 10 mA cm^?2, as well as good stability. Meanwhile, the NiS₂@V₂O₅/VS₂ heterojunction is assembled to the two-electrode system for overall water splitting, exhibiting a very low voltage value of 1.49 V, which is much superior to that of the benchmark RuO₂//Pt/C system. The energy band calculation reveals the mechanism that the NiS₂ and VS₂ lower the Fermi level of V₂O₅, thus promoting the electrons transfer in the electrocatalytic reactions. Our work opens up a novel route for heterojunction application in the electrocatalytic field.     

N,P-co-doped carbon coupled with CoP as superior electrocatalysts for hydrogen evolution reaction and overall water splitting

To achieve high activity and stability for both hydrogen and oxygen evolution reactions through the non-precious-metal based electrocatalysts is still facing the great challenge. Herein, we demonstrate a facile strategy to prepare CoP nanoparticles (NPs) loaded on N, P dual-doped carbon (NPC) electrocatalysts with high concentration N and P dopants through a pyrolysis-deposition-phosphidation process. The great bifunctional electrocatalytic activity for both HER (the overpotential of 98 mV and 86 mV at 10 mA cm⁻² in both 0.5 M H₂SO₄ and 1 M KOH electrolytes, respectively) and OER (the overpotential of 300 mV at 10  mA cm⁻² in 1 M KOH electrolyte) were achieved. When CoP@NPC hybrid was used as two electrodes in the 1 M KOH electrolyte system for overall water splitting, the needed cell potential for achieving the current density of 10 mA cm⁻² is 1.6 V, and it also showed superior stability for HER and OER after 10 h’ test with almost negligible decay. Experimental results revealed that the P atoms in CoP were the active sites for HER and the CoP@NPC hybrid showed excellent bifunctional electrocatalytic properties due to the synergistic effects between the high catalytic activity of CoP NPs and NPC, in which the doping of N and P in carbon led to a stronger polarization between Co and P in CoP, promoting the charge transfer from Co to P in CoP, enhancing the catalytic activity of P sites and Co sites in CoP for HER and OER, respectively. Specifically, the improvements could result from the changed charge state, the increased active specific surface area, and the facilitated reaction kinetics by N, P co-doping and admixture. This work provides a high-efficient, low-cost and stable electrocatalyst for overall water splitting, and throws light on rational designing high performance electrocatalysts.     

Manganese oxide embedded graphene nanohybrid for pH-universal electrochemical overall water splitting

Efficient non-noble metal bifunctional electrocatalysts for overall water splitting in pH universal is highly desired in application. Herein, MnO₂/graphene composition are applied as efficient electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in pH-universal electrolytes with the help of plasma dc arc method. The couple of MnO₂ and graphene highly benefits to the H₂O, H⁺ and OH⁻ absorption respectively. The defects and stable Mn³⁺ contribute to the transfer of electron and charge. The low overpotentials and small Tafel slopes reveal attractive activities of HER and OER. The good electrocatalytic performances are attributed to the synergistic effect and abundant heterogeneous interfaces in MnO₂/graphene. These can offer rich electroactive sites and accelerate electron transfer. Thus, it may provide facile route for developing nonprecious electrocatalysts of water splitting.     

Electronically tuned defective Prussian-blue on graphene for electrochemical water splitting

Exploiting high-performance and stable bifunctional electrocatalysts is highly desirable for water splitting applications to obtain large-scale renewable and clean fuels. Herein, a defective Prussian-blue (δ-PB or Fe^III_1.23[Fe^II_1.12(CN)₆]_0.87·δ_0.13) on graphene composite electrocatalyst was fabricated through a facile hydrolytic precipitation process, followed by a single-step carbonization treatment at 600 °C (denoted as δ-PB/G-600). The resultant optimized δ-PB/G-600 exhibits remarkable electrocatalytic water splitting in alkaline media, producing lower overpotentials of 189 and 105 mV at a current density of 10 mA cm^?2 for the oxygen evolution reaction and hydrogen evolution reaction, respectively. Most importantly, the δ-PB/G-600 shows remarkable durability for both reactions. The remarkable electrocatalysis was attributed to the abundant active sites and high electrical conductivity with a defective nature, which not only facilitate the electrolyte flux but also maintain the structural stability of δ-PB/G-600. Additionally, the high surface area confirms the facile mass transport and prompts the gaseous release of the composite.     

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.     

CdS supported on electrochemically reduced rGO for photo reduction of water to hydrogen

The paper reports synthesis of CdS supported rGO where reduction of GO to rGO was carried out electrochemically in situ. This catalyst had a better activity for photo-electrochemical dissociation of water to hydrogen. Catalysts were characterized by XRD, FTIR, DRS, XPS, EIS and Mott-Schottky analysis. It is reported that this catalyst showed formation of heterojunction at the interface of CdS and rGO by chemical interaction between the two. This resulted in a greater band bending, a higher charge carrier density, low resistance for electronic mobility and low recombination rate. A better activity of the catalyst is due to the above mentioned attributes.     

Highly active Ni/Co-metal organic framework bifunctional electrocatalyst for water splitting reaction

Rational design of electrocatalycally active materials with excellent performance for renewable energy conversion is of great interest. We have developed a nanosheet array of Ni/Co metal-organic framework (MOF) grown on CoO modified Ni foam (CoO/NF) substrate via the solvothermal process. The high surface area and low resistance of Ni/Co-MOF@CoO/NF (NC@CoO/NF) catalyst contribute to efficient water splitting. We have prepared a series of NC-n/CoO/NF (n = 1–4) catalysts to optimize the molar ratio of the Co atom in Ni MOF-74. Among them, NC-2@CoO/NF shows an excellent electrochemical performance in alkaline medium, i.e., low overpotential of 290 and 139 mV for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. For a two-electrode system with NC-2@CoO/NF, a low cell voltage of 1.54 V at 10 mA cm^?2 has been obtained for overall water splitting which is much smaller than that with commercial Ir/C– Pt/C pair. This excellent performance can be attributed to the synergistic effects of Ni/Co-MOF and CoO/NF. In addition, the as-prepared NC-2@CoO/NF exhibits excellent long-term stability. The computational simulation also supports experimental results.     

A mini-review on transition metals-based 1D nanotubular bifunctional electrocatalysts for overall water splitting

Research and development on bifunctional electrocatalysts for water electrolysis are essential in achieving cost reduction, simplifying electrolyzer design, and a sustainable hydrogen economy. One widely investigated approach to boosting electrocatalyst's efficiency is morphology control via one-dimensional nanostructures. Nanotubes are attractive among the different one-dimensional nanostructures due to their high surface-to-volume ratio, internal void space, low density, and fast charge/mass transport. This mini-review provides a concise account of transition metals-based nanotubular bifunctional electrocatalysts reported for overall water splitting. Nanotubes of precious and non-precious metals/alloys/compounds, their preparation methods, hydrogen and oxygen evolution reaction kinetics, overall water splitting performance, and mechanism of action are deliberated.