Recent Advancements in Ascribing Several Platinum Free Electrocatalysts Pertinent to Hydrogen Evolution from Water Reduction

The advancement of a sustainable and scalable catalyst for hydrogen production is crucial for the future of the hydrogen economy. Electrochemical water splitting stands out as a promising pathway for sustainable hydrogen production. However, the development of Pt-free electrocatalysts that match the energy efficiency of Pt while remaining economical poses a significant challenge. This review addresses this challenge by highlighting latest breakthroughs in Pt-free catalysts for the hydrogen evolution reaction (HER). Specifically, we delve into the catalytic performance of various transition metal phosphides, metal carbides, metal sulphides, and metal nitrides toward HER. Our discussion emphasizes strategies for enhancing catalytic performance and explores the relationship between structural composition and the performance of different electrocatalysts. Through this comprehensive review, we aim to provide insights into the ongoing efforts to overcome barriers to scalable hydrogen production and pave the way for a sustainable hydrogen economy.

Self-Supported Mn-Ni3Se2 Electrocatalysts for Water and Urea Electrolysis for Energy-Saving Hydrogen Production

Recently, there has been a huge research interest in developing robust, efficient, low-cost, and earth-abundant materials for water and urea electrolysis for hydrogen (H2) generation. Herein, we demonstrate the facile hydrothermal synthesis of self-supported Mn-Ni3Se2 on Ni foam for overall water splitting under wide pH conditions. With the optimized concentration of Mn in Ni3Se2, the overpotential for hydrogen evolution, oxygen evolution, and urea oxidation is significantly reduced by an enhanced electrochemical active surface area. Different electronic states of metal elements also produce a synergistic effect, which accelerates the rate of electrochemical reaction for water and urea electrolysis. Owing to the chemical robustness, Mn-doped Ni3Se2 shows excellent stability for long time duration, which is important for its practical applications. A two-electrode electrolyzer exhibits low cell voltages of 2.02 and 1.77 V for water and urea electrolysis, respectively, to generate a current density of 100 mA/cm2. Finally, the prepared nanostructured Mn-Ni3Se2@NF acts as an electrocatalyst for overall water splitting under wide pH conditions and urea electrolysis for energy-saving hydrogen production and wastewater treatment.

Transition Metals-Based Water Splitting Electrocatalysts on Copper-Based Substrates: The Integral Role of Morphological Properties

Electrocatalytic water splitting is a promising alternative to produce high purity hydrogen gas as the green substitute for renewable energy. Thus, development of electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are vital to improve the efficiency of the water splitting process particularly based on transition metals which has been explored extensively to replace the highly active electrocatalytic activity of the iridium and ruthenium metals-based electrocatalysts. In situ growth of the material on a conductive substrate has also been proven to have the capability to lower down the overpotential value significantly. On top of that, the presence of substrate has given a massive impact on the morphology of the electrocatalyst. Among the conductive substrates that have been widely explored in the field of electrochemistry are the copper based substrates mainly copper foam, copper foil and copper mesh. Copper-based substrates possess unique properties such as low in cost, high tensile strength, excellent conductor of heat and electricity, ultraporous with well-integrated hierarchical structure and non-corrosive in nature. In this review, the recent advancements of HER and OER electrocatalysts grown on copper-based substrates has been critically discussed, focusing on their morphology, design, and preparation methods of the nanoarrays.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Oxygen-doped NiCoP derived from Ni-MOFs for high performance asymmetric supercapacitor

Oxygen doping strategy is one of the most effective methods to improve the electrochemical properties of nickel-cobalt phosphide (NiCoP)-based capacitors by adjusting its inherent electronic structure. In this paper, O-doped NiCoP microspheres derived from porous nanostructured nickel metal-organic frameworks (Ni-MOFs) were constructed through solvothermal method followed by phosphorization treatment. The O-doping concentration has a siginificant influence on the rate performance and cycle stability. The optimized O-doped NiCoP electrode material shows a specific capacitance of 632.4 F-g-1at 1 A-g-1and a high retention rate of 56.9% at 20 A g-1. The corresponding NiCoP-based asymmetric supercapacitor exhibits a high energy density of 30.1 Wh kg-1when the power density is 800.9 W kg-1, and can still maintain 82.1% of the initial capacity after 10 000 cycles at 5 A g-1.

NiO nanobelts with exposed {110} crystal planes as an efficient electrocatalyst for the oxygen evolution reaction

The electrocatalytic oxygen evolution reaction (OER) is necessary and challenging for converting renewable electricity into clean fuels, because of its complex proton coupled multielectron transfer process. Herein, we investigated the crystal plane effects of NiO on the electrocatalytic OER activity through combining experimental studies and theoretical calculations. The experimental results reveal that NiO nanobelts with exposed {110} crystal planes show much higher OER activity than NiO nanoplates with exposed {111} planes. The efficient OER activity of the {110} crystal planes comes from their intrinsically high catalytic ability and fast charge transfer kinetics. Density functional theory (DFT) shows that the {110} crystal planes possess a lower theoretical overpotential value for the OER, leading to a high electrocatalytic performance. This research broadens our vision to design efficient OER electrocatalysts by the selective exposure of specific crystal planes.

Optimization of nickel selenide for hydrogen and oxygen evolution reactions by response surface methodology

In this study, the electrocatalytic activity of Ni-Se electrode synthesized on nickel foam by pulse electrodeposition was optimized through the design of experiments (DOE) approach using the response surface methodology (RSM) for both hydrogen and oxygen evolution reactions. The frequency (f), duty cycle (dc), current density (i), and electrodeposition time (sum of t_ons) were chosen as the parameters of the pulse electrodeposition method. The analyses of variance (ANOVA) were performed on the responses of the designed experiments that included the required overpotential at the current density of 10 mA/cm² for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) (η_10,HER and η_10,OER), active surface area (R_f) and intrinsic electrocatalytic activity (i/R_f). The results indicated that η_10,HER, η_10,OER, and R_f are mainly influenced by duty cycle and electrodeposition time, while i/R_f is affected by frequency and time. The optimized NiSe₂ electrode synthesized under optimal conditions of pulse electrodeposition (low duty cycle and prolonged electrodeposition time) showed the most desirable values for η_10,HER, η_10,OER, and R_f, equal to 44 mV (vs. RHE), 235 mV (vs. RHE) and 14700, respectively. The nanostructured NiSe₂ demonstrated the highest potential in the bifunctional application of OER and HER.

Construction of self-supporting, hierarchically structured caterpillar-like NiCo2S4 arrays as an efficient trifunctional electrocatalyst for water and urea electrolysis

In this study, we have developed intriguing self-supporting caterpillar-like spinel NiCo2S4 arrays with a hierarchical structure of nanowires on a nanosheet skeleton, which can be used as a self-supporting trifunctional electrocatalyst for the oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). The caterpillar-like NiCo precursor arrays are first in situ grown on carbon cloth (NiCo2O4/CC) by a facile hydrothermal reaction, which is followed by an anion exchange process (or sulfuration treatment) with Na2S to form self-supporting spinel NiCo2S4 arrays (NiCo2S4/CC) with a roughened nanostructure. Taking advantage of the bimetallic synergistic effect, the unique hierarchical nanostructure, and the self-supporting nature, the resultant NiCo2S4/CC electrode exhibits high activities toward the OER, HER and UOR, which are highly superior to the monometallic counterparts of NiS nanosheets and Co9S8 nanowires on a carbon cloth substrate. The comparison of the three electrodes also indicates that the hierarchically structured bimetallic electrode combines the morphological and structural characteristics of monometallic Ni-based nanosheets and Co-based nanowires. When assembling a two-electrode electrolytic cell with NiCo2S4/CC as both the anode and cathode, an applied cell voltage of only 1.66 V is required to deliver a current density of 10 mA cm-2 in water electrolysis. By using the same two-electrode setup, the applied voltage for urea electrolysis is further reduced to 1.45 V that produces hydrogen at the cathode with the same current density. This study paves the way for exploring the feasibility of future less energy-intensive and large-scale hydrogen production.

A Ni-MOF nanosheet array for efficient oxygen evolution electrocatalysis in alkaline media

Ni-MOF(bpdc) nanosheet array on nickel foam (Ni-MOF/NF) is a superior OER catalyst with a requirement of an overpotential of 350 mV to attain 20 mA cm⁻² in 1.0 M KOH.

Hierarchical 0D-2D Co/Mo Selenides as Superior Bifunctional Electrocatalysts for Overall Water Splitting

Development of efficient electrocatalysts combining the features of low cost and high performance for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) still remains a critical challenge. Here, we proposed a facile strategy to construct in situ a novel hierarchical heterostructure composed of 0D−2D CoSe₂/MoSe₂ by the selenization of CoMoO₄ nanosheets grafted on a carbon cloth (CC). In such integrated structure, CoSe₂ nanoparticles dispersed well and tightly bonded with MoSe₂ nanosheets, which can not only enhance kinetics due to the synergetic effects, thus promoting the electrocatalytic activity, but also effectively improve the structural stability. Benefiting from its unique architecture, the designed CoSe₂/MoSe₂ catalyst exhibits superior OER and HER performance. Specifically, a small overpotential of 280 mV is acquired at a current density of 10 mA·cm⁻² for OER with a small Tafel slope of 86.8 mV·dec⁻¹, and the overpotential is 90 mV at a current density of 10 mA·cm⁻² for HER with a Tafel slope of 84.8 mV·dec⁻¹ in 1 M KOH. Furthermore, the symmetrical electrolyzer assembled with the CoSe₂/MoSe₂ catalysts depicts a small cell voltage of 1.63 V at 10 mA·cm⁻² toward overall water splitting.

N, Ru Codoped Pellet Drum Bundle-Like Sb2S3: An Efficient Hydrogen Evolution Reaction and Hydrogen Oxidation Reaction Electrocatalyst in Alkaline Medium

Though investigations have been made on several metal chalcogenides in hydrogen evolution reactions (HERs) and hydrogen oxidation reactions (HORs), antimony sulfide (Sb₂S₃) has not generated much attention. In this direction, the present work reports on the synthesis of N, Ru codoped pellet drum bundle-like antimony sulfide (Sb₂S₃) via a simple reflux method. Subsequent HER and HOR electrocatalytic investigations in 1 M KOH revealed their suitability as an efficient and inexpensive alternative to platinum, as is evident from the overpotential (72 mV at a current density of 10 mA cm^-2), Tafel slope (193 mV/decade), exchange current density (1.42 mA/cm²), and breakdown potential at ∼0.6 V vs RHE, respectively. Such remarkable HER and HOR performance of N, Ru codoped Sb₂S₃ could be ascribed to the presence of relatively larger active sites compared to Sb₂S₃ and N-doped Sb₂S₃ individually due to synergistic effects arising from N and Ru dopants. Further, N, Ru codoped Sb₂S₃ demonstrated high intrinsic catalytic activity as indicated by its turnover frequency (2.03 s^-1) and current loss, corresponding to 35% after 10 h of continuous amperometric i-t operation. Alternatively, such excellent catalytic performance of N, Ru codoped Sb₂S₃ arises due to geometric lattice defects with surface oxygen vacancy, and the availability of abundant edges and its pellet drum-like morphology also cannot be overruled.

High-index faceted binary-metal selenide nanosheet arrays as efficient 3D electrodes for alkaline hydrogen evolution

Exploring highly active and durable Earth-abundant electrocatalysts to replace the precious noble metals holds great promise for the hydrogen evolution reaction (HER) from water splitting. Herein, a novel (110) high-index faceted binary-metal selenide (FeNiSe) nanosheet array grown on electrochemically exfoliated graphene foil (FeNiSe-NS/EG) is developed from its vertically-oriented NiFe-LDH nanosheet/EG precursor through a low-temperature selenization reaction. Benefiting from its unique 3D configuration and enhanced electrical conductivity, the obtained FeNiSe-NS/EG electrode exhibits excellent electrocatalytic activity toward the HER with small overpotentials of -187 and -222 mV at current densities of 10 and 20 mA cm-2, a low Tafel slope of 65 mV dec-1, and remarkable long term stability in alkaline media, outperforming the recently reported NiFe-based non-precious metal HER catalysts. Theoretical calculations and experimental results reveal that the synergistic effects of the exposed (110) high-index facets and Fe dopants give rise to a greatly enhanced HER performance.

Electrodeposition of Ni-Co-Fe mixed sulfide ultrathin nanosheets on Ni nanocones: a low-cost, durable and high performance catalyst for electrochemical water splitting

The development of a bi-functional active and stable catalyst for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is an important challenge in overall electrochemical water splitting. In this study, firstly, nickel nanocones (NNCs) were formed using electrochemical deposition, and then Ni-Co-Fe based mixed sulfide ultrathin nanosheets were obtained by directly depositing on the surface of the nanocones using the CV method. With a hierarchical structure of Ni-Fe-Co-S nanosheets, not only was a high active surface area created, but also the electron transfer and mass transfer were enhanced. This structure also led to the faster release of hydrogen bubbles from the surface. An overpotential value of 106 mV was required on the surface of this electrode to generate a current density of 10 mA cm^-2 in the HER, whereas, for the OER, 207 mV overpotential was needed to generate a current density of 10 mA cm^-2. Furthermore, this electrode required 1.54 V potential to generate a current density of 10 mA cm^-2 in the total electrochemical water splitting. The resulting electrode also exhibited reasonable electrocatalytic stability, and after 10 hours of electrolysis in the overall water splitting reaction, the voltage change was negligible. This study introduces a simple, efficient, reasonable and cost-effective method of creating an effective catalyst for the overall water splitting process.

3D Metallic Ti@Ni0.85 Se with Triple Hierarchy as High-Efficiency Electrocatalyst for Overall Water Splitting

In this study, Ti@Ni_0.85 Se electrodes with a triple hierarchy architecture were designed, and their applications in electrocatalytic water splitting were studied. The 3D electrode is comprised of three types of structures including the bottom square Ti mesh structure as the conductive substrate, a vertical and uniform Ni_0.85 Se nanosheet arrays structure in the intermediate section, and the topmost Ni_0.85 Se flower structure. This triple hierarchy architecture is binder-free, conductive, and has a particular feature of enlarged surface areas, exposing more active sites, promoting mass- and charge-transfer, and accelerating dissipation of gases generated during water electrolysis. Moreover, DFT calculations confirmed that the Ni_0.85 Se possesses metallic character, which further promotes the charge transfer of the electrocatalyst. Benefiting from this special structure and metallic character, the electrode displays a superior activity of 10 mA cm^-2 at 120 mV hydrogen evolution reaction overpotential and 30 mA cm^-2 at 270 mV oxygen evolution reaction overpotential. By using this electrode as a bifunctional electrocatalyst, an alkali electrolyzer affords a water splitting current of 10 mA cm^-2 at a cell voltage of 1.66 V.

Prompt Electrodeposition of Ni Nanodots on Ni Foam to Construct a High-Performance Water-Splitting Electrode: Efficient, Scalable, and Recyclable

In past decades, Ni-based catalytic materials and electrodes have been intensively explored as low-cost hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalysts for water splitting. With increasing demands for Ni worldwide, simplifying the fabrication process, increasing Ni recycling, and reducing waste are tangible sustainability goals. Here, binder-free, heteroatom-free, and recyclable Ni-based bifunctional catalytic electrodes were fabricated via a one-step quick electrodeposition method. Typically, active Ni nanodot (NiND) clusters are electrodeposited on Ni foam (NF) in Ni(NO3)2 acetonitrile solution. After drying in air, NiO/NiND composites are obtained, leading to a binder-free and heteroatom-free NiO/NiNDs@NF catalytic electrode. The electrode shows high efficiency and long-term stability for catalyzing hydrogen and oxygen evolution reactions at low overpotentials (10ηHER = 119 mV and 50ηOER = 360 mV) and can promote water catalysis at 1.70 V@10 mA cm-2. More importantly, the recovery of raw materials (NF and Ni(NO3)2) is quite easy because of the solubility of NiO/NiNDs composites in acid solution for recycling the electrodes. Additionally, a large-sized (S ~ 70 cm2) NiO/NiNDs@NF catalytic electrode with high durability has also been constructed. This method provides a simple and fast technology to construct high-performance, low-cost, and environmentally friendly Ni-based bifunctional electrocatalytic electrodes for water splitting.

Regulating Phase Conversion from Ni3 Se2 into NiSe in a Bifunctional Electrocatalyst for Overall Water-Splitting Enhancement

Phase engineering has been demonstrated as an efficient method for the enhancement of catalytic activity. This study concerns the phase and morphology modulation of Ni₃ Se₂ /NiSe nanorod arrays through a hydrothermal process. Partial phase conversion can effectively enhance the electrical conductivity and yield more active sites through atom rearrangement during phase transformation. Quite low optimal overpotentials of 166 mV for the hydrogen evolution reaction (HER) and 370 mV for the oxygen evolution reaction (OER) are obtained in a sample containing 32.4 % of NiSe phase and 67.6 % of Ni₃ Se₂ phase. The performance is superior to the samples with only one phase. Furthermore, a water electrolyzer was assembled by using two symmetrical NiSe/Ni foam electrodes as the anode and cathode, which can deliver 10 mA cm^-2 at a low voltage of 1.61 V. More significantly, the water electrolyzer can be operated at 10 mA cm^-2 over 10 h without noticeable degradation, showing extraordinary operational stability. This phase conversion control strategy provides a new way to improve the catalytic activity of NiSe and may have potential use in the design of other selenide electrocatalysts.

Combining Co3S4 and Ni:Co3S4 nanowires as efficient catalysts for overall water splitting: an experimental and theoretical study

In the quest for mass production of hydrogen from water electrolysis, to develop highly efficient, stable and low-cost catalysts is still the central challenge. When designing a novel catalyst, it is necessary to optimize the exposure and accessibility of its active sites as well as the reaction kinetics. This can be realized by combining an appropriate chemical composition of the material, including doping with metal elements, and a properly nanostructured morphology offering a high surface contact. We report here on the design and performances of cobalt-based oxide and sulfide nanowires as catalysts that can be used for both hydrogen and oxygen evolution reactions (denoted HER and OER respectively) in the same compatible electrolyte. Following a sulfuration process, Co3O4 nanowires are entirely converted into Co3S4 nanowires showing greatly improved OER catalytic performances with an overpotential of 283 mV (instead of 371 mV for Co3O4) to deliver a current density of 10 mA cm-2. Besides, when doping the surface of these Co3S4 nanowires with small amounts of nickel, the resulting Ni:Co3S4 nanowires exhibit an HER overpotential of 199 mV to reach 10 mA cm-2. But most importantly, two-electrode electrolyzer cells combining Co3S4 and Ni:Co3S4 electrodes show operating voltages as low as 1.70 V at 10 mA cm-2 over 40 hours, a value that competes advantageously with the best reported catalysts in 1.0 M KOH. Meanwhile, density functional theory (DFT) calculations show that the free energy of the intermediates has been reduced after the introduction of sulfur and nickel atoms, which have smaller overpotentials and corresponding enhanced electrocatalytic performance. Our results open a new avenue in the quest for overall water splitting using electrochemical systems.

A New Member of Electrocatalysts Based on Nickel Metaphosphate Nanocrystals for Efficient Water Oxidation

High-performance electrocatalysts are desired for electrochemical energy conversion, especially in the field of water splitting. Here, a new member of phosphate electrocatalysts, nickel metaphosphate (Ni₂ P₄ O₁₂ ) nanocrystals, is reported, exhibiting low overpotential of 270 mV to generate the current density of 10 mA cm^-2 and a superior catalytic durability of 100 h. It is worth noting that Ni₂ P₄ O₁₂ electrocatalyst has remarkable oxygen evolution performance operating in basic media. Further experimental and theoretical analyses demonstrate that N dopant boosts the catalytic performance of Ni₂ P₄ O₁₂ due to optimizing the surface electronic structure for better charge transfer and decreasing the adsorption energy for the oxygenic intermediates.

Self-Supported Ternary Ni-S-Se Nanorod Arrays as Highly Active Electrocatalyst for Hydrogen Generation in Both Acidic and Basic Media: Experimental Investigation and DFT Calculation

In this study, a novel three-dimensional self-supported ternary NiS-Ni₉S₈-NiSe nanorod (NR) array cathode has been successfully in situ constructed by a two-step hydrothermal route. When applied to hydrogen evolution, the synthesized NiS-Ni₉S₈-NiSe-NR electrode demonstrates optimized electrocatalytic activity and long-term durability, only requiring overpotentials as low as 120 and 112 mV to drive 10 mA cm^-2 for hydrogen evolution reaction in 0.5 M H₂SO₄ and 1.0 M KOH, respectively. Density functional theory calculation reveals that after Se doping Se 3d orbitals are bonded to Ni 3d orbitals and S p orbitals near Fermi level, attesting a significant electron transfer between nickel and selenium atoms. The success of enhancing the electrocatalytic performance via introducing the Se dopant holds great promise for the potential optimization of other transition-metal compounds in highly efficient electrochemical water splitting for large-scale hydrogen production.

Surface engineering of FeCo-based electrocatalysts supported on carbon paper by incorporating non-noble metals for water oxidation

FeCoMo nanosheets, as a highly efficient, cost-effective and stable electrocatalyst for the oxygen evolution reaction, significantly lower the overpotential and cost.

Phase-controlled synthesis and the phase-dependent HER and OER performances of nickel selenide nanosheets prepared by an electrochemical deposition route

Phase-controlled synthesis of nickel selenide nanostructures was successfully realized via a facile electrodeposition route with the same electrolyte at room temperature.

Self-Supported Nickel Iron Layered Double Hydroxide-Nickel Selenide Electrocatalyst for Superior Water Splitting Activity

The design of efficient, low-cost, and stable electrocatalyst systems toward energy conversion is highly demanding for their practical use. Large scale electrolytic water splitting is considered as a promising strategy for clean and sustainable energy production. Herein, we report a self-supported NiFe layered double hydroxide (LDH)-NiSe electrocatalyst by stepwise surface-redox-etching of Ni foam (NF) through a hydrothermal process. The as-prepared NiFe LDH-NiSe/NF catalyst exhibits far better performance in alkaline water oxidation, proton reduction, and overall water splitting compared to NiSe_x/NF or NiFe LDH/NF. Only 240 mV overpotential is required to obtain a water oxidation current density of 100 mA cm^-2, whereas the same for the hydrogen evolution reaction is 276 mV in 1.0 M KOH. The synergistic effect from NiSe and NiFe LDH leads to the evolution of a highly efficient catalyst system for water splitting by achieving 10 mA cm^-2 current density at only 1.53 V in a two-electrode alkaline electrolyzer. In addition, the designed electrode produces stable performance for a long time even at higher current density to demonstrate its robustness and prospective as a real-life energy conversion system.

Facile and Scalable Synthesis of Robust Ni(OH)2 Nanoplate Arrays on NiAl Foil as Hierarchical Active Scaffold for Highly Efficient Overall Water Splitting

Developing highly efficient low-cost electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline electrolyte is essential to advance water electrolysis technology. Herein, Ni(OH)₂ nanoplates aligned on NiAl foil (Ni(OH)₂/NiAl) are developed by simply dealloying NiAl foil in KOH, which exhibits high electrocatalytic activity for OER with a small overpotential of 289 mV to achieve 10 mA cm^-2 and outstanding durability with no detectable degradation during long-term operation. Furthermore, such Ni(OH)₂/NiAl can effectively act as an active and robust hierarchical scaffold to simply electrodeposit other catalysts with intrinsically higher activity such as NiMo and NiFe nanoparticles for highly efficient HER and OER, respectively. The prepared NiFe/Ni(OH)₂/NiAl displays superior OER catalytic activity with overpotentials of 246, 315, and 374 mV at 10, 100, and 500 mA cm^-2, respectively. While NiMo/Ni(OH)₂/NiAl catalyst exhibits remarkable HER performance with a small overpotential of 78 mV to deliver 10 mA cm^-2. Consequently, the electrolysis device composed of the above two electrocatalysts demonstrates superb water splitting performance with a cell voltage of 1.59 V at 10 mA cm^-2. These results open up opportunities to explore and optimize low-cost advanced catalysts for energy applications.

Hierarchical NiCo2S4@NiFe LDH Heterostructures Supported on Nickel Foam for Enhanced Overall-Water-Splitting Activity

Low-cost and highly efficient bifunctional electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) are intensively investigated for overall water splitting. Herein, we combined experimental research with first-principles calculations based on density functional theory (DFT) to engineer the NiCo₂S₄@NiFe LDH heterostructure interface for enhancing overall water-splitting activity. The DFT calculations exhibit strong interaction and charge transfer between NiCo₂S₄ and NiFe LDH, which change the interfacial electronic structure and surface reactivity. The calculated chemisorption free energy of hydroxide (ΔE_OH) is reduced from 1.56 eV for pure NiFe LDH to 1.03 eV for the heterostructures, indicating a dramatic improvement in OER performance, while the chemisorption free energy of hydrogen (ΔE_H) maintains almost invariable. By the use of the facile hydrothermal method, NiCo₂S₄ nanotubes, NiFe LDH nanosheets, and NiCo₂S₄@NiFe LDH heterostructures are prepared on nickel foam, of which the corresponding experimental OER overpotentials are 306, 260, and 201 mV at 60 mA cm^-2, respectively. These results are good agreement with the theoretical predictions. Meanwhile, the HER performance has little improvement, with an overpotential of about 200 mV at 10 mA cm^-2. Due to the dramatic improvement in OER performance, there was an enhancement in the overall water-splitting activity of the NiCo₂S₄@NiFe LDH heterostructures, with a low voltage of 1.6 V.

Microwave-Initiated Facile Formation of Ni3Se4 Nanoassemblies for Enhanced and Stable Water Splitting in Neutral and Alkaline Media

Molecular hydrogen (H₂) generation through water splitting with minimum energy loss has become practically possible due to the recent evolution of high-performance electrocatalysts. In this study, we fabricated, evaluated, and presented such a high-performance catalyst which is the Ni₃Se₄ nanoassemblies that can efficiently catalyze water splitting in neutral and alkaline media. A hierarchical nanoassembly of Ni₃Se₄ was fabricated by functionalizing the surface-cleaned Ni foam using NaHSe solution as the Se source with the assistance of microwave irradiation (300 W) for 3 min followed by 5 h of aging at room temperature (RT). The fabricated Ni₃Se₄ nanoassemblies were subjected to catalyze water electrolysis in neutral and alkaline media. For a defined current density of 50 mA cm^-2, the Ni₃Se₄ nanoassemblies required very low overpotentials for the oxygen evolution reaction (OER), viz., 232, 244, and 321 mV at pH 14.5, 14.0, and 13.0 respectively. The associated lower Tafel slope values (33, 30, and 40 mV dec^-1) indicate the faster OER kinetics on Ni₃Se₄ surfaces in alkaline media. Similarly, in the hydrogen evolution reaction (HER), for a defined current density of 50 mA cm^-2, the Ni₃Se₄ nanoassemblies required low overpotentials of 211, 206, and 220 mV at pH 14.5, 14.0, and 13.0 respectively. The Tafel slopes for HER at pH 14.5, 14.0, and 13.0 are 165, 156, and 128 mV dec^-1, respectively. A comparative study on both OER and HER was carried out with the state-of-the-art RuO₂ and Pt under identical experimental conditions, the results of which revealed that our Ni₃Se₄ is a far better high-performance catalyst for water splitting. Besides, the efficiency of Ni₃Se₄ nanoassemblies in catalyzing water splitting in neutral solution was carried out, and the results are better than many previous reports. With these amazing advantages in fabrication method and in catalyzing water splitting at various pH, the Ni₃Se₄ nanoassemblies can be an efficient, cheaper, nonprecious, and high-performance electrode for water electrolysis with low overpotentials.

A Zn-doped Ni3S2nanosheet array as a high-performance electrochemical water oxidation catalyst in alkaline solution

A Zn-doped Ni₃S₂nanosheet array on Ni foam (Zn-Ni₃S₂/NF) acts as a high-performance and durable electrocatalyst for the oxygen evolution reaction in 1.0 M KOH, driving a catalytic current density of 100 mA cm⁻²at an overpotential of 330 mV, 90 mV less than that for Ni₃S₂/NF.