Electrochemically engineering defect-rich nickel-iron layered double hydroxides as a whole water splitting electrocatalyst

It is well proved that fabricating more defects on basal plane of layered double hydroxides (LDHs) is one of effective ways to boost the electrocatalytic performance for oxygen evolution reaction (OER). For the first time, the nickel iron LDHs (NiFe LDHs) with hierarchical morphology and abundant defects are simultaneously constructed by one-step electrodeposition (ED) strategy with easy operation, time-saving and green chemistry. Remarkably, the morphology is elaborately tailored by changing the species of doped anions which is unique. Also, the X-ray photoelectron spectroscopy (XPS) results elucidate that the Fe sites are in electron-rich state in LDHs which is revealed to enhance the catalytic activity strongly arising from the generation of oxygen vacancy. To deliver the current density of 10 mA cm⁻², the optimal NiFe LDHs require the overpotential of 128, 106 mV for OER and hydrogen evolution reaction (HER), and achieve 100 mA cm⁻² at the overpotential of 237, 242 mV, respectively. As a bifunctional electrocatalyst, the NiFe LDHs exhibit the current density of 10 mA cm⁻² at a cell voltage of 1.55 V and 100 mA cm⁻² at 1.76 V, which are lower than that of most of benchmarking materials reported previously.     

Defective layered double hydroxide formed by H2O2 treatment act as highly efficient electrocatalytic for oxygen evolution reaction

Electrocatalytic reaction is the important electrode reaction for many new generation electrochemistry energy and storage devices. However, the poor reaction kinetics of those electrode reaction severely restricts its application. Highly efficient electrocatalyst is essential to resolve the problem of commercial application of those electrochemistry energy and storage devices. Herein, by simple H₂O₂ treatment, the highly efficient CoFe-Layered Double Hydroxides (LDHs) electrocatalysts with multiple defects have been synthesized (noted as D-LDHs). The D-LDHs show a low overpotential of 283 mV at 10 mA cm⁻² and small Tafel slope of 39 mV dec⁻¹ for the oxygen evolution reaction (OER). The work offers a new strategy to create defects in LDHs as highly efficient electrocatalysts for OER.     

Highly active atomic Cu catalyst anchored on superlattice CoFe layered double hydroxide for efficient oxygen evolution electrocatalysis

Developing active, natural abundant and non-expensive electrocatalysts for large scale production and storage of clean hydrogen (H₂) fuel is a prerequisite to drive stable water splitting reaction. Herein, atomic Cu species loaded on hierarchical flower-like CoFe layered double hydroxides (LDHs) superlattice (denoted as Cu_x/CoFe LDHs) were firstly fabricated by a facile co-precipitation method followed by a room temperature treatment to load the atomic Cu species from alkaline copper salt solution. The superlattice structure was proved by the high resolution transmission electron microscopy (HRTEM). Remarkably, benefiting from high level long ordering associated with vacant cation sites and defects, the unique superlattice structural features and the atomic Cu % loading onto the LDHs matrix, the obtained Cu_x/CoFe LDHs electrocatalyst exhibited superior activity and stability for oxygen evolution reaction (OER). The loaded atomic Cu species improves the electronic structure and provides more exposed active sites due to synergetic electron coupling between Copper and the LDHs. These atomic species have outstanding potentials for achieving high selectivity and reactivity in electrocatalysis and heterocatalysis. Importantly, the efficient resulted Cu_4.76/CoFe LDHs electrode in which the atomic Cu % loading ratio is 4.76% showed the best electrocatalytic activity which only required the much lower overpotential of 253 mV to reach 10 mA/cm² and a small Tafel slope of 63 mV/decade in 1 M KOH. This electrocatalyst possessed unique superior features to many other state-of-the-art earth-abundant electrocatalysts. This work paves a facile and novel method for enhancing the catalytic activity of CoFe LDHs based electrocatalyst, which may be extended to the synthesis of future electrocatalysts having highly active OER performance.     

Nickel foam-supported Fe,Ni-Polyporphyrin microparticles: Efficient bifunctional catalysts for overall water splitting in alkaline media

Developing highly active, stable and sustainable electrocatalysts for overall water splitting is of great importance to generate renewable H₂ for fuel cells. Herein, we report the synthesis of electrocatalytically active, nickel foam-supported, spherical core-shell Fe-poly(tetraphenylporphyrin)/Ni-poly(tetraphenylporphyrin) microparticles (FeTPP@NiTPP/NF). We also show that FeTPP@NiTPP/NF exhibits efficient bifunctional electrocatalytic properties toward both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Electrochemical tests in KOH solution (1 M) reveal that FeTPP@NiTPP/NF electrocatalyzes the OER with 100 mA cm⁻² at an overpotential of 302 mV and the HER with 10 mA cm⁻² at an overpotential of 170 mV. Notably also, its catalytic performance for OER is better than that of RuO₂, the benchmark OER catalyst. Although its catalytic activity for HER is slightly lower than that of Pt/C (the benchmark HER electrocatalyst), it shows greater stability than the latter during the reaction. The material also exhibits electrocatalytic activity for overall water splitting reaction at a current density of 10 mA cm⁻² with a cell voltage of 1.58 V, along with a good recovery property. Additionally, the work demonstrates a new synthetic strategy to an efficient, noble metal-free-coordinated covalent organic framework (COF)-based, bifunctional electrocatalyst for water splitting.     

Iron-substituted Co-Ni phosphides immobilized on Ni foam as efficient self-supported 3D hierarchical electrocatalysts for oxygen evolution reaction

Electrocatalytic water splitting for hydrogen evolution is significantly impeded by the kinetically sluggish oxygen evolution reaction (OER). Thus, the development of highly efficient and durably stable non-noble-metal OER electrocatalyst is necessary and challenging for the large-scale electrocatalytic water splitting. Herein, a series of iron-substituted cobalt-nickel phosphides grown on Ni foam (FeCoNi-P/NFs) were easily prepared though successive hydrothermal and phosphorization treatments. The chemical compositions, crystalline and electronic structures as well as surface morphologies of these resulting electrocatalysts are strongly related with the iron substitution ratio. More interestingly, the FeCoNi-P/NF-2 nanosheet arrays prepared from equivalent molar ratio of iron and cobalt precursors exhibit the best OER performance with a low overpotential of 266 mV to produce a current density of 50 mA cm⁻² and a low Tafel slope of 61.2 mV dec⁻¹ in 1.0 M KOH condition, which is comparable to the reported state-of-the-art OER electrocatalysts. Additionally, the FeCoNi-P/NF-2 nanosheet arrays also show satisfactory long-term durability over 60 h. The superior OER activity of the electrocatalyst is essentially attributed to the heteroatomic substitution and the unique three-dimensional hierarchical morphology, which greatly increase the electrical conductivity, afford more active sites and facilitate the efficient charge transfer ability.     

Engineering FeNi-based electrocatalysts conjoined with Mo2C grown on carbon spheres toward efficient water oxidation via structure and electronic modulation

The development of active non-noble electrocatalysts for oxygen evolution reaction (OER) is urgently desired to accomplish high-performance electrocatalytic water splitting. Here, we report a unique structure electrocatalyst composed of FeNi and Mo₂C heterojunction as spherical shell supported on carbon sphere as core for efficient OER. With the aid of Mo₂C incorporation, FeNi–Mo₂C@C shows increased specific surface area, more electrocatalytic active sites, improved surface water adsorption, and reduced energy barrier. Benefiting from the synergy of shell-core and heterojunction sturcture, the optimized FeNi–Mo₂C@C exhibits superior activity for OER with an overpotential of 283 mV at 10 mA cm⁻² as well as Tafel slope of 29.2 mV dec⁻¹, which is comparable to that of the benchmark ruthenium oxide. The feasible bond between structural design and electronic alteration enhances the charge transfer efficiency, conductivity, and catalytic kinetics, thus intrinsically boost the electrocatalytic performance. This study hence supports a viable strategy to develop highly-efficient non-precious OER electrocatalysts through structural and electronic modification.     

Facile synthesis of molybdenum-based layered double hydroxide nanorods for boosting water oxidation reaction

Exploiting environmentally friendly and robust oxygen evolution reaction (OER) electrodes is still a great difficulty to promote the water oxidation for electrocatalytic water splitting. In the work, molybdenum-doped nickel copper hydrotalcite (NiCuMo_x LDH) nanoarrays have been firstly in situ grown on nickel foam (NF) via a typical hydrothermal process. When NiCuMo_0.2 LDH/NF was used as a OER electrocatalyst, it displays superior electrocatalytic performance with the need of small overpotentials of only 290 mV to drive 40 mA cm⁻² and a low Tafel slope of 39.8 mV dec⁻¹, which are almost one of the best water oxidation activities reported so far. The enhanced electrocatalytic performance is attributed to unique urchin-like structure, more exposure to active sites and enhanced charge transfer rate owing to the synergistic effect of Mo doping and NiCu LDH. The work put forward a new method for the development of efficient water oxidation electrocatalysts, which will fill the gap for the exploitation of trimetal LDH-based electrodes in large-scale water splitting applications in the future.     

Rapid synthesis of porous Ni/Co/Fe-LDHs nanosheets for effective electrochemical oxygen evolution reaction and zinc-air batteries

Transition metal compounds, especially layered double hydroxide materials (LDHs), show excellent catalytic activity in oxygen evolution reaction (OER). The ethanol oxidation reaction (EOR) is an innovative alternative anodic reaction to OER for improving the efficiency of water splitting to produce hydrogen. In order to improve the reactivity and explore the similarities and differences of active sites in the two reactions, three kinds of porous LDHs (NiFe, NiCo, CoFe LDHs) were synthesized and a series of tests were carried out. Among them, the best performing OER catalyst is NiFe-LDHs with a low overpotential of 1.44 V vs. RHE at 10 mA cm^?2 and a Tafel slope of 23.85 mV dec^?1. As for the EOR reaction, NiCo-LDHs is the best, with an overpotential of only 1.38 V vs. RHE at 10 mA cm^?2 and a Tafel slope of 71.58 mV dec-1. In addition, compared with OER, the LHDs material exhibited better stability in the EOR. This work provides a new direction for studying the electrocatalytic activity of LDHs materials in OER and EOR.     

Constructing microstructures in nickel-iron layered double hydroxide electrocatalysts by cobalt doping for efficient overall water splitting

The development of Ni–Fe layered double hydroxide (NiFe LDH) catalysts for overall water splitting (OWS) is urgently required. NiFe LDHs are promising catalysts for the oxygen evolution reaction (OER). However, their hydrogen evolution reaction (HER) performance is restricted by slow kinetics. The construction of multiple types of active sites to simultaneously optimise the OER and HER performance is significant for OWS using NiFe LDHs. Hence, a Co-doped NiFe LDH electrocatalyst with dislocations and stacking faults was designed to modulate the electronic structure and generate multiple types of activity sites. The Co_0.03-NiFe_0.97 LDH catalyst only required overpotentials of 280 (50 mA cm⁻², OER) and 170 mV (10 mA cm⁻², HER). However, it reached a current density of 50 mA cm⁻² at 1.53 V during OWS. Co_0.03-NiFe_0.97 LDHs could be stabilised for 140 h at 1.52 V. Furthermore, Co_0.03-NiFe_0.97 LDHs exhibited a higher electrocatalytic activity than commercial Raney nickel and Pt/C||IrO₂ under industrial conditions. The significant specific surface area, high conductivity, and unique microstructures are the major factors contributing to the excellent OWS performance. This study suggests an efficient strategy for introducing microstructures to fabricate catalysts with high activity for application in OWS.     

Bundle-shaped cobalt–nickel selenides as advanced electrocatalysts for water oxidation

Searching for high-performance and earth-abundant electrocatalysts for oxygen evolution reaction (OER) is of paramount significance for overall water splitting to produce hydrogen. Herein, an advanced class of CoNi selenides containing rich oxygen vacancies, with a hierarchical bundle-like and holey nanosheets as noble-metal-free catalysts were first synthesized through a facile hydrothermal method. Benefitting from abundant oxygen vacancies, bundle-like nanostructure, as well as strong synergistic effects, such CoNi selenides demonstrate a greatly enhanced surface area to supply more electrocatalytic active sites to contact with electrolyte, accompanied by largely promoted reaction kinetics, which show outstanding electrocatalytic performances for OER. Remarkably, the optimal Co₁N_i0.5Se can display outstanding OER activity with the small Tafel slope of 48 mV dec⁻¹ and low overpotential of 250 mV (at 10 mA cm⁻²), which are much superior to those of Co/Ni-based catalysts. This electrocatalyst can also maintain high activity and structure stability during long-term electrolysis of 35 h, demonstrating a desirable electrocatalyst for OER. This work elucidates the sophisticated construction of well-defined non-noble metal catalysts for the practical applications in water oxidation.     

Efficient CoMoRu0.25Ox/NF nanoplate architectures for overall electrochemical water splitting

The development of inexpensive electrocatalysts with excellent electrocatalytic activity for the hydrogen and oxygen evolution reactions (HER and OER, respectively) has been challenging. In this study, we synthesized cobalt molybdenum ruthenium oxide with porous, loosely-assembled nanoplate morphology. The CoMoRu_0.25O_x/NF electrocatalyst exhibited the highest electrocatalytic activity, requiring overpotentials of 230 and 78 mV for the OER and HER, respectively, to attain a current density of 10 mA cm^?2; moreover, its long-term stability was outstanding. The electrocatalyst required a cell voltage of only 1.51 V for overall water splitting in an alkaline medium, which was lower than that required by many CoMo-based catalysts.     

Nitrogen and molybdenum co-doped CoP nanohoneycombs on 3D nitrogen-doped porous graphene as enhanced electrocatalyst for oxygen evolution reaction

The four-electron transfer process involved in anodic oxygen evolution reaction (OER) of electrocatalytic water splitting causes the sluggish kinetics and significantly limiting the efficiency of energy conversion. It's urgent to explore low-cost, efficient and stable electrocatalysts for OER. In the work, we design the nitrogen and molybdenum co-doped CoP with nanohoneycombs structure on three-dimensional (3D) nitrogen-doped porous graphene (N/Mo–CoP@NPG) as an efficient OER electrocatalyst. The N/Mo–CoP@NPG delivers the current density of 10 mA cm⁻² at a low overpotential value of 201 mV in 1.0 M KOH, meanwhile the electrocatalytic activity shows no obvious degradation after 50 h. The NPG substrate provides plentiful ligaments for growth of N/Mo–CoP nanohoneycombs and 3D network for rapid electronic transfer. Additionally, doping N and Mo atoms into CoP synergistically modifies the micromorphology and electronic structure, benefiting the electrocatalytic ability. This work offers a promising strategy to improve the electrocatalytic activity of transition metal phosphides.     

Hierarchically hollow interconnected rings of nickel substituted cobalt carbonate hydroxide hydrate as promising oxygen evolution electrocatalyst

The present work highlights fabrication of nanostructured nickel-substituted cobalt carbonate hydroxide hydrates (NCCHH) through one-step reflux method. It is noted that optimized 30 mol% nickel-substituted cobalt carbonate hydroxide hydrate (NCCHH-30) nanostructures show quite high specific surface area (～229.55 m² g^?1) owing to the formation of hierarchically hollow interconnected ring-type morphology facilitating the electrode-electrolyte interfacial interaction. As a result, NCCHH-30 showed significant amplification in electrocatalytic oxygen evolution reaction (OER) activity with ultralow overpotential (～141 mV @ 10 mA cm^?2), Tafel slope (～49 mV dec^?1), and excellent durability (12 h and 2000 cycles) in 1.0 M KOH. Notably, to the best of our knowledge, interconnected NCCHH-30 hierarchical hollow rings exhibited the best overpotential (η₁₀₀ ～198 mV) value reported for cobalt-based electrocatalysts in alkaline OER. In addition, this material exhibited exceptionally high oxygen evolution performance in comparison to the state-of-the-art commercial RuO₂ electrocatalyst in 1.0 M KOH. Such interconnected hierarchically hollow nickel (30 mol%)-substituted cobalt carbonate hydroxide hydrate nanostructured rings could act as an ultraefficient, cost-effective, and stable electrocatalyst for OER in alkaline medium.     

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

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

Three-dimensional Ni/Fe doped graphene oxide @ MXene architecture as an efficient water splitting electrocatalyst

Developing a highly efficient, stable, and earth-abundant electrocatalyst for both HER and OER is essential for water splitting. Herein, we report a bifunctional electrocatalyst in the form of iron/nickel doped graphene oxide @ MXene (GMX). The GMX-based electrocatalytic materials were prepared by annealing at different temperatures in an inert atmosphere. The GMX was characterized by various analytical tools such as PXRD, FE-SEM, HRTEM, Raman spectroscopy, and XPS. All the electrocatalysts exhibit high activity when acting as OER and HER catalyst in alkaline and acidic electrolytes. GMX-500 is considered an effective bifunctional electrocatalyst, and 10 mA cm^?2 current density is achieved at a low overpotential, i.e., 370 mV for OER and 470 mV for HER, respectively. This paper focuses on the progress of inexpensive and effective electrocatalysts with high activity and long-term durability for water splitting, which is relevant in power conservation.     

Vanadium-induced fragmentation of crystalline CoFe hydr(oxy)oxide electrocatalysts for enhanced oxygen evolution reaction

Transition metal hydr(oxy)oxides are one of the most efficient electrocatalysts towards oxygen evolution reaction (OER). However, structural-dependant and low stability severely hinder their overall performance and catalytic efficiency. Herein, we report the enhancement of OER activity via vanadium doping into the CoFe hydr(oxy)oxide electrocatalyst (denoted as CoFeV). Our results show that the incorporation of vanadium induces fragmentation of the polycrystalline structure of the CoFe hydr(oxy)oxide (CoFe) and transformation to a more active amorphous structure, greatly exposing the CoO and FeOOH active sites on the surface for OER. The optimised electrocatalyst exhibits an overpotential of 376 mV to reach a current density of 10 mA cm⁻² and requires a small Tafel slope of 26 mV dec⁻¹ for OER in alkaline media. These results illustrate that vanadium doping can efficiently improve the catalytic activity of CoFeV which can be extended to other vanadium-doped transition metal hydr(oxy)oxides as promising electrocatalysts for OER applications.     

Interfacial interaction between NiMoP and NiFe-LDH to regulate the electronic structure toward high-efficiency electrocatalytic oxygen evolution reaction

Development of low-cost, high-efficiency electrocatalysts for the oxygen evolution reaction (OER) is challenging, even though it is critical for the overall electrochemical splitting. Herein, we report a NiMoP@NiFe-LDH heterostructure electrode supported on nickel foam. The study shows that the electrocatalytic activity for the OER can be improved by coupling NiMoP and NiFe-LDH. The resulting NiMoP@NiFe-LDH heterostructure exhibited remarkable catalytic performance with an ultralow overpotential of only 299 mV at a current density of 150 mA cm^?2 and a Tafel slope of 23.3 mV dec^?1 in 1.0 M KOH solution. Electron transfer from NiFe-LDH to NiMoP at the nanointerface reduces the energy barrier of the catalytic process, thus improving the OER activity performance. Thus, high-efficiency electrocatalysts can be utilised by constructing heterojunctions to regulate the electronic structure at the interface of the electrocatalysts.     

High electrochemical stability and low-agglomeration of defective Co3O4 nanoparticles supported on N-doped graphitic carbon nano-spheres for oxygen evolution reaction

The design of hybrid electrocatalysts with abundant active sites and long term stability is crucial for efficient oxygen evolution reaction (OER) application. Cobalt oxide is considered as one of the most promising electrocatalysts to replace noble metal due to its low cost, availability, and electrocatalytic activity towards the oxygen evolution reaction in alkaline media. However, nano-scale cobalt oxide suffers from severe surface self-agglomeration during the OER process, so that leading to poor activity and durability. Herein, ultra-small cobalt oxide nanoparticles are anchored on the surface of nitrogen doped porous 3D graphitic carbon nano-spheres (N-ACS@Co₃O₄) to increase the amount of exposed active site and avoid the self-agglomeration. The obtained electrocatalyst (N-ACS@Co₃O₄) is enriched with abundant oxygen vacancies and exhibits a superior OER activity (Overpotential of 237 mV at 10 mA.cm⁻²) and exceptional stability for at least 30 h in alkaline electrolyte (1 M KOH). The DFT calculations demonstrate that the strong adsorption of Co₃O₄ on N-doped graphene can prevent its agglomeration, and therefore improves the stability of Co₃O₄ nanoparticles during OER process in line with the experimental results.     

Defect-induced FexNi1-xSe2 nanoparticles based electrocatalysts towards enhanced hydrogen and oxygen evolution reactions

Transition metal selenides are regarded as promising materials for the production of clean energy through electrocatalytic water splitting. Creation of defects in these metal selenides is one of the prudent strategies to enrich the active sites which in turn enhances the electrocatalytic activity of these materials and makes them viable for broader applications. Herein, defect-induced, iron-doped nickel selenide nanoparticles were prepared for the first time and their electrocatalytic efficacy towards hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has been demonstrated. Fe_xNi_1-xSe₂ nanoparticles (x = 0.25, 0.50, 0.75) were prepared using a facile hydrothermal method, in which defects were induced by annealing at 300 °C to obtain DI-Fe_xNi_1-xSe₂. The structural and morphological investigations confirmed the size reduction and creation of defects after annealing, without any significant change in the crystal structure, which in turn is expected to promote the electrocatalytic activity. Accordingly, among all the materials investigated, DI-Fe_0.25Ni_0.75Se₂ has shown the highest HER activity in 0.5 M H₂SO₄ at a lesser overpotential of 128 mV at 10 mA cm^?2 and the Tafel slope was calculated to be 37.9 mV dec^?1. Interestingly, the same material has displayed high performance towards OER in 1 M KOH with a lesser overpotential at 205 mV and a Tafel slope of 55.5 mV dec^?1. Thus obtained electrocatalytic activity was much better than the reported nickel selenide based electrocatalysts. Further, the DI-Fe_0.25Ni_0.75Se₂ electrocatalyst has demonstrated impressive stability in the acidic and alkaline medium during continuous electrolysis even up to 12 h.     

Flower-like HEA/MoS2/MoP heterostructure based on interface engineering for efficient overall water splitting

The development of cheap, efficient, and active non-noble metal electrocatalysts for total hydrolysis of water (oxygen evolution reaction (OER) and hydrogen evolution reaction (HER)) is of great significance to promote the application of water splitting. Herein, a heterogeneous structured electrode based on FeAlCrMoV high-entropy alloy (HEA) was synthesized as a cost-effective electrocatalyst for hydrogen and oxygen evolution reactions in alkaline media. In combination of the interfacial synergistic effect and the high-entropy coordination environment, flower-like HEA/MoS₂/MoP exhibited the excellent HER and OER electrocatalytic performance. It showed a low overpotential of 230 mV at the current density of 10 mA cm⁻² for OER and 148 mV for HER in alkaline electrolyte, respectively. Furthermore, HEA/MoS₂/MoP as both anode and cathode also exhibited an overpotential of 1.60 V for overall water splitting. This work provides a new strategy for heterogeneous structure construction and overall water splitting based on high-entropy alloys.