Influence of deposition conditions on performance of Ni3S2 as the bifunctional electrocatalyst in alkaline solutions by galvanostatic deposition

The electrodeposition method is a popular synthesis method due to its low cost, simplicity, and short synthesis time. In addition, this synthesis route results in the preparation of a self-supporting electrocatalyst, which eliminates the use of binders and ultimately facilitates the durability as well as the activity of the catalyst. In this work, a series of Ni3S2/Ni mesh electrodes are prepared by galvanostatic deposition at different deposition current densities and times. The morphology, microstructure, and elemental composition distribution of these obtained electrodes are characterized, and the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performance of the series of Ni3S2/Ni meshes are tested. The results show that the Ni3S2/Ni mesh electrodes electrodeposited at 30 mA cm-2 for 1200 s have superior electrochemical performance for HER and OER. The overpotentials of Ni3S2/Ni mesh 30 mA cm-2-1200 s are 236 and 244 mV for HER and OER, respectively, at a current density of 10 mA cm-2. In addition, the Tafel slopes for HER and OER are 113 mV dec-1 and 176 mV dec-1, respectively. This research provides some valuable insights into the use of the electrodeposition method for the fabrication of electrocatalysts.

Synergistic Effect of Allium-like Ni9S8 & Cu7S4 Electrodeposited on Nickel Foam for Enhanced Water Splitting Activity

This study explores a water-splitting activity using a biphasic electrodeposited electrode on nickel foam (NF). The *Ni9S8/Cu7S4/NF electrode with citric acid reduction exhibits superior OER (oxygen evolution reaction) and HER (hydrogen evolution reaction) performance with reduced overpotential and a steeper Tafel slope. The *Ni9S8/Cu7S4/NF electrode displays the ultra-low overpotential value of 212 mV for OER and 109 mV for HER at the current density of 10 mA cm-2. The Tafel slope of 25.4 mV dec-1 for OER and 108 mV dec-1 for HER was found from that electrode. The maximum electrochemical surface area (ECSA), lowest series resistance and lowest charge transfer resistance are found in citric acid reduced electrode, showing increased electrical conductivity and quick charge transfer kinetics. Remarkably, the *Ni9S8/Cu7S4/NF electrode demonstrated excellent stability for 80 hours in pure water splitting and 20 hours in seawater splitting. The synergistic effect of using bimetallic (Cu&Ni) sulfide and enhanced electrical conductivity of the electrode are caused by reduction of metal sulfide into metallic species resulting in improved water splitting performance.

Copper-based electro-catalytic nitrate reduction to ammonia from water: Mechanism, preparation, and research directions

Global water bodies are increasingly imperiled by nitrate pollution, primarily originating from industrial waste, agricultural runoffs, and urban sewage. This escalating environmental crisis challenges traditional water treatment paradigms and necessitates innovative solutions. Electro-catalysis, especially utilizing copper-based catalysts, known for their efficiency, cost-effectiveness, and eco-friendliness, offer a promising avenue for the electro-catalytic reduction of nitrate to ammonia. In this review, we systematically consolidate current research on diverse copper-based catalysts, including pure Cu, Cu alloys, oxides, single-atom entities, and composites. Furthermore, we assess their catalytic performance, operational mechanisms, and future research directions to find effective, long-term solutions to water purification and ammonia synthesis. Electro-catalysis technology shows the potential in mitigating nitrate pollution and has strategic importance in sustainable environmental management. As to the application, challenges regarding complexity of the real water, the scale-up of the commerical catalysts, and the efficient collection of produced NH3 are still exist. Following reseraches of catalyst specially on long term stability and in situ mechanisms are proposed.

Sulfur ion-exchange strategy to obtain Bi2S3 nanostructures from Bi2O3 for better water splitting performance

A two-step simple and efficient ion-exchange chemical strategy is proposed to obtain nanostructured Bi2S3 electrodes of different surface morphologies from the Bi2O3. In the first step, nanoplates of the Bi2O3 are obtained on nickel-foam using successive ionic layer adsorption and reaction method at room-temperature (25 °C). In the second phase, as-obtained nanoplates of the Bi2O3 are transferred to the Bi2S3 using four autoclaves containing different sulfur precursor solutions at 120 °C for 8 h for phase change, structural conversion and surface morphological modification (i.e., walnuts, network-type, nanowires, and nanoflowers). Due to higher surface area and conductivity, lower charge transfer resistance, and reduced band gap caused by ionic and phase conversion, the Bi2S3 surpasses the Bi2O3 in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities. The overpotential of 112-370 mV for the Bi2S3 network is much lower than that of the nanoplates of the Bi2O3 (275-543 mV), and walnuts (134-464 mV), nanowires (125-500 mV), and nanoflowers (194-520 mV) of the Bi2S3. The Bi2S3 network-type Bi2S3 electrode shows considerable chemical stability through cycling measurement, suggesting the importance of the present study in obtaining metal sulfides from metal oxide with better water splitting activities.

F-regulated Ni2P-F3 nanosheets as efficient electrocatalysts for full-water-splitting and urea oxidation

Heteroatomic anion doping represents a powerful approach for manipulating the electronic configuration of the active metal locus in electrocatalysts, resulting in enhanced multifunctional electrocatalytic properties in hydrogen/oxygen evolution reactions (HER/OER). Here, fluorine-tailored Ni2P-F3 nanosheets were synthesized and evaluated as a robust multifunctional electrocatalyst for HER, OER, and UOR. Our comprehensive experimental and theoretical investigations reveal that the anionic F effectively tailored the electronic states of the Ni2P-F3 nanosheets, resulting in an elevated d-band center and optimizing the sorption capacity of intermediates. In addition to thermodynamically and kinetically favoured redox reactions, F doping facilitates the reconstruction and generation of active γ-NiOOH. Resulting from the optimized electronic configuration and nanosheet architecture, outstanding catalytic activities are demonstrated by Ni2P-F3 with low overpotentials to reach 100 mA cm-2 for HER (177 mV) and OER (293 mV), surpassing Ni2P by 234 and 205 mV, respectively. Notably, 1.618 V is required for full-water-diversion to reach 10 mA cm-2, while 1.414 V is required with urea oxidation for 100 mA cm-2.

A pillar-layered Ni2P-Ni5P4-CoP array derived from a metal-organic framework as a bifunctional catalyst for efficient overall water splitting

Interfacial engineering emerges as a potent strategy for regulating the catalytic reactivity of metal phosphides. Developing a facile and cost-effective method to construct bifunctional metal phosphides for highly efficient electrochemical overall water splitting remains an essential and challenging issue. Here, a multiphase transition metal phosphide is constructed through the direct phosphorization of a Ni-Co metal-organic framework grown on nickel foam (Ni-Co-MOF/NF), which is prepared by utilizing nickel foam as conductive substrate and nickel source. The resulting transition metal phosphide manifests a pillar-layered morphology, wherein CoP, Ni2P, and Ni5P4 nanoparticles are embedded within each carbon sheet and these carbon sheets assemble into a pillar-shaped structure on the nickel foam (Ni2P-Ni5P4-CoP-C/NF). The heterogeneous Ni2P-Ni5P4-CoP-C/NF with multiple interfaces serves as a highly efficient bifunctional electrocatalyst with overpotentials of -100 mV and 293 mV in the hydrogen evolution reaction and oxygen evolution reaction, respectively, at 50 mA cm-2 in alkaline media. This superior catalytic performance should mainly be ascribed to its enriched active centers and multiphase synergy. When directly applied for alkaline overall water splitting, the Ni2P-Ni5P4-CoP-C/NF couple demonstrates satisfactory activity (1.55 V @10 mA cm-2) along with sustained durability over 18 hours. This method brings fresh enlightenment to the economical and controllable preparation of multi-metal phosphides for energy conversion.

Patterned Au@Ag nanoarrays with electrically stimulated laccase-mimicking activity for dual-mode detection of epinephrine

Epinephrine (EP) is a crucial neurotransmitter in the central nervous system. However, an abnormal level of EP in biological fluids can lead to various diseases. Therefore, it is essential to rapidly and accurately detect EP content. Herein, electrically stimulated patterned Au@Ag nanoarrays with laccase-mimicking activity were designed for the dual-mode detection of EP concentration. The patterned Au@Ag nanoarrays exhibit excellent electrochemical properties and electrically stimulated laccase-mimicking activity. They provide sensitive electrochemical responses for detecting EP content. Simultaneously, the Au@Ag nanoarrays can catalyze the oxidation of EP, enabling its detection through a colorimetric process. This dual-mode approach achieves the detection of EP content over a wide linear range of 0.5-200 μM, with a low detection limit of 0.152 μM. Furthermore, the utility of these nanoarrays for sensing EP in human serum was evaluated. This work provides a convenient method using patterned nanozyme array for the visible, rapid and accurate detection of EP content. It provides the important implication for the development of portable and reliable on-site analytical instruments.

Interfacial charge transfer in sheet Ni2P-FePx heterojunction to promote the study of electrocatalytic oxygen evolution

The substantial expense associated with catalysts significantly hampers the progress of electrolytic water-based hydrogen production technology. There is an urgent need to find non-precious metal catalysts that are both cost-effective and highly efficient. Here, the porous Ni2P-FePx nanomaterials were successfully prepared by hydrothermal method, nickel foam as the base, iron nitrate solution as the caustic agent and iron source, and finally phosphating at low temperature. The obtained porous Ni2P-FePx nanosheets showed excellent catalytic activity under alkaline PH = 14, and an overpotential of merely 241 mV was required to achieve a current density of 50 mA cm-2. The morphology of the nanosheet can still be flawlessly presented on the screen after 50 h of working at high current density.

Fe(OH)x modified ultra-small Ru nanoparticles for highly efficient hydrogen evolution reaction and its application in water splitting

Developing highly active electrocatalysts for overall water splitting is of remarkable significance for industrial production of H2. Herein, exceptionally active Fe(OH)x modified ultra-small Ru nanoparticles on Ni(OH)2 nanosheets array (Fe(OH)x-Ru/Ni(OH)2) for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are reported. The Fe(OH)x-Ru/Ni(OH)2 nanosheets array prepared with Fe/Ru molar ratio of 5 only requires extremely low overpotentials of 61, 127 and 170 mV to reach current densities of 100, 500 and 800 mA cm-2 in 1 M KOH, respectively, exceeding Pt/C catalyst (75, 160 and 177 mV). Meanwhile, the Fe(OH)x/Ni(OH)2 nanosheets array derived from Fe(OH)x-Ru/Ni(OH)2 exhibits excellent OER activity. It gains current densities of 100, 500 and 800 mA cm-2 at considerably low overpotentials of 265, 285 and 296 mV, respectively, much lower than those of RuO2 and most reported electrocatalysts. The introduction of Fe(OH)x significantly improves the HER activity of Ru nanoparticles by tunning the electronic structure and forming interfaces between Ru and Fe(OH)x. Dramatically, the integrated alkaline electrolyzer based on Fe(OH)x-Ru/Ni(OH)2 and Fe(OH)x/Ni(OH)2 nanosheets array pair just needs 1.649 V to yield a current density up to 500 mA cm-2, exceeding most reported water-splitting electrocatalysts. The strategy reported in this work can be facilely extended to prepare other similar Ru based materials and their derivatives with outstanding catalytic performance for water splitting.

Selective electrooxidation of 5-hydroxymethylfurfural at low working potentials promoted by 3D hierarchical Cu(OH)2@Ni3Co1-layered double hydroxide architecture with oxygen vacancies

Selective electrooxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is of great significance in the manufacture of fine chemicals, liquid fuels, pharmaceuticals, plastics, etc., but still suffers from the high potential input, resulting in high electricity consumption. Developing active, low-cost and stable electrocatalysts is crucial for this electrochemical reaction at low working potentials. Herein, a three-dimensional (3D) hierarchical Cu(OH)2@Ni3Co1-layered double hydroxide architecture with abundant oxygen vacancies (Vo) was synthesized by facile electrodeposition of Ni3Co1-LDH nanosheets on copper foam (CF) supported-Cu(OH)2 nanorods (CF/Cu(OH)2@Ni3Co1-LDH) for the selective electrooxidation of HMF to FDCA. The 3D hierarchical architecture of the Cu(OH)2 nanorod core loaded with Ni3Co1-LDH nanosheet shell facilitates the rapid transfer of charges and exposes more active sites. The synergistic effect of the core-shell nanoarray structure, atomic level dispersion of Ni and Co on LDH laminates, and rich Vo gives 98.12% conversion of HMF, 98.64% yield and 91.71% selectivity for FDCA at a low working potential of 1.0 V vs. RHE. In addition, CF/Cu(OH)2@Ni3Co1-LDH exhibits superior stability by maintaining 93.26% conversion of HMF, 93.65% yield and 91.57% selectivity of FDCA after eight successive cycles, showing the immense potential of utilizing electrochemical conversion for biomass.

Synthesis of P-(NiCo)CO3 /TiO2 /Ti Self-Supported Electrode with High Catalytic Activity and Stability for Hydrogen Evolution

Hydrogen is considered an ideal clean energy due to its high mass-energy density, and only water is generated after combustion. Water electrolysis is a sustainable method of obtaining a usable amount of pure hydrogen among the various hydrogen production methods. However, its development is still limited by applying expensive noble metal catalysts. Here, the dissolution-recrystallization process of TiO2 nanotube arrays in water with the hydrothermal reaction of a typical nickel-cobalt hydroxide synthesis process followed by phosphating to prepare a self-supported electrode with (NiCo)CO3 /TiO2 heterostructure named P-(NiCo)CO3 /TiO2 /Ti electrode is combined. The electrode exhibits an ultra-low overpotential of 31 mV at 10 mA  cm-2 with a Tafel slope of 46.2 mV dec-1 in 1 m KOH and maintained its stability after running for 500 h in 1 m KOH. The excellent catalytic activity can be attributed to the structure of nanotube arrays with high specific surface area, superhydrophilicity, and super aerophobicity on the electrode surface. In addition, the uniform (NiCo)CO3 /TiO2 heterostructure also accelerates the electron transfer on the electrode surface. Finally, DFT calculations demonstrate that phosphating also improves the ΔGH* and ΔGH2O of the electrode. The synthesis strategy also promotes the exploration of catalysts for other necessary electrocatalytic fields.

ZIF template-based Fe-doped defect-rich hierarchical structure Co3S4/MoS2 as a bifunctional electrocatalyst for overall water splitting

To replace the current expensive precious metal catalysts for water electrolysis, it is important to develop inexpensive and powerful bifunctional catalysts for hydrogen production. It is an effective way to improve catalytic performance using excellent templates and elemental doping. Here, a hierarchical structure Fe-Co3S4/MoS2 was synthesized using an Fe-ZIF precursor prepared by ion exchange, followed by hydrothermal sulfuration and annealing. It required overpotentials of only 93 mV and 243 mV to achieve a current density of 10 mA cm-2 in the HER and OER, respectively. It also showed excellent catalytic performance for overall water splitting, requiring only 1.42 and 1.71 V to achieve current densities of 10 and 100 mA cm-2 in 1 M KOH. The catalyst also demonstrated excellent ultra-long-term stability. The superb catalytic performance and stability can be attributed to the Fe doping, exposing more active sites while retaining the highly stable framework of the ZIF. The component modulation of Co3S4 and MoS2 by Fe doping induced high intrinsic activity and excellent transfer coefficients. This work presents a novel approach to prepare noble metal-free catalysts with highly stable rich interfaces and defects for overall water splitting.

Progress in Manipulating Dynamic Surface Reconstruction via Anion Modulation for Electrocatalytic Water Oxidation

The development of efficient and economical electrocatalysts for oxygen evolution reaction (OER) is of paramount importance for the sustainable production of renewable fuels and energy storage systems; however, the sluggish OER kinetics involving multistep four proton-coupled electron transfer hampers progress in these systems. Fortunately, surface reconstruction offers promising potential to improve OER catalyst design. Anion modulation plays a crucial role in controlling the extent of surface reconstruction and positively persuading the reconstructed species' performances. This review starts by providing a general explanation of how various types of anions can trigger dynamic surface reconstruction and create different combinations with pre-catalysts. Next, the influences of anion modulation on manipulating the surface dynamic reconstruction process are discussed based on the in situ advanced characterization techniques. Furthermore, various effects of survived anionic groups in reconstructed species on water oxidation activity are further discussed. Finally, the challenges and prospects for the future development directions of anion modulation for redirecting dynamic surface reconstruction to construct highly efficient and practical catalysts for water oxidation are proposed.

A quaternary heterojunction nanoflower for significantly enhanced electrochemical water splitting

Designing highly-efficient, cost-effective, and stable electrocatalysts for water splitting is essential to producing green hydrogen. In this work, a nanoflower quaternary heterostructured Ni(NO3)2(OH)4/Ni(OH)2/Ni3S2/NiFe-LDH electrocatalyst is successfully synthesized by two-step hydrothermal reactions. The sulfur in the electrocatalyst induces higher valence state metal atoms as active sites to accelerate the formation of O2. As expected, benefiting from the unique structural features and solid electronic interactions, Ni(NO3)2(OH)4/Ni(OH)2/Ni3S2/NiFe-LDH exhibits remarkable oxygen evolution reaction performance with a low overpotential of 223 mV at a current density of 100 mA cm-2, a slight Tafel slope of 65.4 mV dec-1, and outstanding stability in alkaline media. Attractively, using Ni(NO3)2(OH)4/Ni(OH)2/Ni3S2/NiFe-LDH as both a cathode and an anode, the alkaline electrolyzer delivers a current density of 10 mA cm-2 only at a cell voltage of 1.67 V, accompanied by superior durability. This work provides a facile method for the rational design of high-performance quaternary electrocatalysts.

Hierarchical CoS2@NiFe-LDH as an efficient electrocatalyst for alkaline seawater oxidation

Developing earth-abundant non-noble electrocatalysts with high performance is significant but challenging for the oxygen evolution reaction (OER) in seawater. Herein, a hierarchical electrocatalyst, NiFe-layered double hydroxide (LDH) nanosheet anchored CoS2 nanowires supported on carbon cloth, is developed for efficient OER electrocatalysis in alkaline seawater, demanding a low overpotential of 256 mV to drive a current density of 100 mA cm-2, along with favorable catalytic durability for at least 48 h with negligible decay.

In Situ Formation of CoP/Co3 O4 Heterojunction for Efficient Overall Water Splitting

Electrochemical water splitting is an environmentally friendly and effective energy storage method. However, it is still a huge challenge to prepare non-noble metal based electrocatalysts that possess high activity and long-term durability to realize efficient water splitting. Here, we present a novel method of low-temperature phosphating for preparing CoP/Co3 O4 heterojunction nanowires catalyst on titanium mesh (TM) substrate that can be used for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water splitting. CoP/Co3 O4 @TM heterojunction showed an excellent catalytic performance and long-term durability in 1.0 M KOH electrolyte. The overpotential of CoP/Co3 O4 @TM heterojunction was only 257 mV at 20 mA cm-2 during the OER process, and it could work stably more than 40 h at 1.52 V versus reversible hydrogen electrode (vs. RHE). During the HER process, the overpotential of CoP/Co3 O4 @TM heterojunction was only 98 mV at -10 mA cm-2 . More importantly, when used as anodic and cathodic electrocatalyst, they achieved 10 mA cm-2 at 1.59 V. The Faradaic efficiencies of OER and HER were 98.4 % and 99.4 %, respectively, outperforming Ru/Ir-based noble metal electrocatalysts and other non-noble metal electrocatalysts for overall water splitting.

A Hierarchical CuO Nanowire@CoFe-Layered Double Hydroxide Nanosheet Array as a High-Efficiency Seawater Oxidation Electrocatalyst

Seawater electrolysis has great potential to generate clean hydrogen energy, but it is a formidable challenge. In this study, we report CoFe-LDH nanosheet uniformly decorated on a CuO nanowire array on Cu foam (CuO@CoFe-LDH/CF) for seawater oxidation. Such CuO@CoFe-LDH/CF exhibits high oxygen evolution reaction electrocatalytic activity, demanding only an overpotential of 336 mV to generate a current density of 100 mA cm-2 in alkaline seawater. Moreover, it can operate continuously for at least 50 h without obvious activity attenuation.

Layered Double Hydroxides for Regulating Phosphate in Water to Achieve Long-Term Nutritional Management

Hunger and undernourishment are increasing global challenges as the world's population continuously grows. Consequently, boosting productivity must be implemented to reach the global population's food demand and avoid deforestation. The current promising agricultural practice without herbicides and pesticides is fertilizer management, particularly that of phosphorus fertilizers. Layered double hydroxides (LDHs) have recently emerged as favorable materials in phosphate removal, with practical application possibilities in nanofertilizers. This review discusses the fundamental aspects of phosphate removal/recycling mechanisms and highlights the current endeavors on the development of phosphate-selective sorbents using LDH-based materials. Specific emphasis is provided on the progress in designing LDHs as the slow release of phosphate fertilizers reveals their relevance in making agro-practices more ecologically sound. Relevant pioneering efforts have been briefly reviewed, along with a discussion of perspectives on the potential of LDHs as green nanomaterials to improve food productivity with low eco-impacts.

Amorphous/crystalline heterostructure of NiFe (oxy)hydroxides for efficient oxygen evolution and urea oxidation

A V-doped amorphous/crystalline heterostructure of NiFe (oxy)hydroxide with nanoflower morphology is developed, which exhibits excellent OER and UOR catalytic activities. V doping changes the local charge density, lowers the reaction barrier, and optimizes the electron arrangement of the NiFe LDH catalyst.

Efficient and durable S-doped Ni/FeOOH electrocatalysts for oxygen evolution reactions

It is important to develop highly efficient and durable Earth-abundant oxygen evolution reaction (OER) electrocatalysts by an energy- and time-saving strategy. Herein, a facile strategy was used to synthesize S-doped nickel-iron oxyhydroxide (S-Ni/FeOOH) nanoparticles on nickel-iron foam (NFF) (S-Ni/FeOOH@NFF), which exhibits a striking enhancement of OER performance compared to Ni/FeOOH@NFF. The free-standing S-Ni/FeOOH@NFF electrode possesses a low overpotential of 229 mV at a current density of 10 mA cm^-2, which is 180 mV lower than that of Ni/FeOOH@NFF. In addition, the electrode was also remarkably stable. The current density still remains at 95% after 150 h at a high current density of 100 mA cm^-2.

Enhancing the catalytic OER performance of MoS2via Fe and Co doping

Enhancing the sluggish kinetics of the electrochemical oxygen evolution reaction (OER) is crucial for many clean-energy production technologies. Although much progress has been made in recent years, developing active, stable, and cost-effective OER electrocatalysts is still challenging. The layered MoS₂, based on Earth-abundant elements, is widely explored as a promising hydrogen evolution electrocatalyst but exhibits poor OER activity. Here, we report a facile strategy to improve the sluggish OER of MoS₂ through co-doping MoS₂ nanosheets with Fe and Co atoms. The synergistic effect obtained by adjusting the Co/Fe ratio in the Fe-Co doped MoS₂ induces electronic and structural modifications and a richer active surface area morphology resulting in a relatively low OER overpotential of 380 mV (at 10 mA cm^-2). The electronic modulation upon doping was further supported by DFT calculations that show favorable interaction with the OER intermediate species, thus reducing the energy barrier for the OER. This work paves the way for future strategies for tailoring the electronic properties of transition-metal dichalcogenides (TMDCs) to activate the structure for the sluggish OER with the assistance of non-noble-metal materials.

Facile fabrication of self-supporting porous CuMoO4@Co3O4 nanosheets as a bifunctional electrocatalyst for efficient overall water splitting

Research shows that redox complementarity and synergism among the ingredients of heterogeneous catalysts can enhance the performance of the catalyst. In this research, a porous CuMoO₄@Co₃O₄ nanosheet electrocatalyst is prepared, which is uniformly decorated on nickel foam (NF) by hydrothermal reactions and the impregnation method. The CuMoO₄@Co₃O₄ is an efficient bifunctional catalyst with prominent electrocatalytic activity and durability. It requires overpotentials of only 54 and 251 mV to obtain current densities of 10 and 50 mA cm^-2 for the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER) in 1.0 mol L^-1 KOH, corresponding to Tafel slope values of 98.8 and 87.4 mV dec^-1, respectively. Furthermore, the CuMoO₄@Co₃O₄ shows excellent stability of 120 h chronopotentiometry at a current density of 100 mA cm^-2 for the HER/OER. Notably, an alkaline electrolyzer (with CuMoO₄@Co₃O₄ as the HER and OER electrodes) can deliver a current density of 10 mA cm^-2 at a low voltage of 1.51 V. The catalytic activity of CuMoO₄@Co₃O₄ can be attributed to the structure of the porous nanosheets and the synergistic effect between CuMoO₄ and Co₃O₄.

Co, Mn co-doped Fe9S11@Ni9S8 supported on nickel foam as a high efficiency electrocatalyst for the oxygen evolution reaction and urea oxidation reaction

The Earth's fossil resources will be exhausted soon, so it is urgent to find clean and efficient new energy for replacing fossil resources. Hydrogen energy is gradually attracting the attention of the public and electrolysis of water is considered to be one of the important means of hydrogen production because of its simplicity and convenience. In this paper, a hydrothermal method for the synthesis of a Co and Mn co-doped bimetallic sulfide Fe₉S₁₁@Ni₉S₈ electrocatalyst is proposed for the first time. The prepared Co-Mn-Fe₉S₁₁@Ni₉S₈/NF electrocatalyst exhibits excellent electrocatalytic activity for the oxygen evolution reaction (OER) and urea oxidation reaction (UOR). It can provide a current density of 10 mA cm^-2 with only 193 mV overpotential for the OER and a current density of 10 mA cm^-2 with only 1.33 V potential for the UOR, which are far superior to those of most reported electrocatalysts. What is noteworthy is that the unique nanoflower structure of Co-Mn-Fe₉S₁₁@Ni₉S₈/NF increases the specific surface area of the material and the introduction of Co and Mn ions promotes the formation of high valence state Ni and Fe and enhances the charge transfer rate. The density functional theory (DFT) calculation shows that the in situ generated Co-Mn-Fe-NiOOH material derived from Co-Mn-Fe₉S₁₁@Ni₉S₈ exhibits the best water adsorption energy and the best electrical conductivity, thus improving the catalytic performance of the material. This work provided a new idea for the development of bimetallic cation doped electrocatalysts with high efficiency and low cost.

Synergistic effect of oxidation etching and phase transformation triggered by controllable ion-bath microenvironments toward constructing ultra-thin porous nanosheets for accelerated industrial water splitting at high current density

Precisely tailoring the structure of inorganic materials at the micron and nanometer scales, especially in collaboration with component customization to design efficient, stable and low-cost transition-metal-based catalysts for industrial electrocatalytic water splitting (EWS) is a key renewable energy technology, but still facing a daunting challenge. Here, the controllable escape of Ni atom is adopted to disturb the hydrothermal ion-bath environment, thereby resulting in the coexistence of high valence Ni and Fe ions. Combined with a one-step hydrothermal coordination strategy, the timeline-adjusted ion-bath microenvironment can effectively trigger the phase transformation of carbonate hydroxide hydrate nanosheets (NFCH) to nickel ferrite intercalated NFCH ultra-thin porous nanosheets (NF-CH-O). Thanks to the high-energy phase boundary synergistic effect and the rapid mass transfer advantages of ultra-thin porous nanostructures, the as-prepared NF-CH-O nanosheets exhibit remarkable oxygen and hydrogen evolution reaction (OER/HER) catalytic activity and stability, with low overpotentials of 207/191 mV at 50 mA cm^-2, respectively, as well as the activity retention for 100 h. The alkaline water electrolyzer set up with NF-CH-O as both anodic and cathodic electrodes only requires a cell potential of 1.688 V to reach 50 mA cm^-2 in a continuous operation of 100 h. More impressively, NF-CH-O only requires overpotentials of 266, 292 mV and 1.877 V to drive high current densities up to 500 mA cm^-2 for OER, HER and EWS, respectively, and exhibits excellent stability with a reduction in the activity of less than 10% over cycles of more than 65 h. This work highlights the room-temperature controllable ion-bath oxidative etching strategy to design efficient bifunctional catalysts with ultra-thin porous structure and high-current-density activity. Meanwhile, combined with the advantages of direct growth on the substrate for mass production, such meticulous consideration of nanostructured design will be more competitive in the H₂-production industry.

Construction of CoP/Co2P Coexisting Bifunctional Self-Supporting Electrocatalysts for High-Efficiency Oxygen Evolution and Hydrogen Evolution

Development of a low cost, high activity, and stable nonprecious metal bifunctional catalyst for electrocatalytic water cracking is a hot topic and big challenge. In this paper, we prepared a nitrogen-doped carbon nanotube (NCNT)-enhanced three-dimensional self-supported electrocatalyst with CoP and Co₂P coexistence by a two-step strategy of high-temperature carbonization and low-temperature phosphorylation. Furthermore, the induced three-dimensional carbon network skeleton facilitates rapid charge transfer. In addition, the active sites of the carbon foam (CF) are greatly increased by the construction of hollow structures. As a bifunctional electrocatalyst, CoP/Co₂P/NCNT@CF exhibited excellent catalytic activity for both hydrogen evolution reaction and oxygen evolution reaction in alkaline media, requiring low overpotentials of 133 and 289 mV to obtain a current density of 10 mA cm^-2, respectively. Additionally, the synthesized catalysts also exhibit good long-term stability, maintaining high catalytic activity after 20 h of continuous operation. We also confirmed the main driving force to improve the electron transfer between the heterostructures of Co and P by XPS spectra. The excellent electrocatalytic performance can be attributed to the close synergy between the highly active CoP/Co₂P/NCNT and CF. This study provides a new strategy for the design of highly active bifunctional self-supporting electrocatalysts.

Feasibility of Nickel–Aluminum Complex Hydroxides for Recovering Tungsten Ions from Aqueous Media

In this study, the adsorption and/or desorption capacity of tungsten ions using nickel–aluminum complex hydroxides was assessed. Nickel–aluminum complex hydroxides at various molar ratios, such as NA11 were prepared, and the adsorption capacity of tungsten ions was evaluated. Precisely, the effect of temperature, contact time, pH, and coexistence on the adsorption of tungsten ions in the water layer was demonstrated. Among the nickel–aluminum complex hydroxides at various molar ratios, the adsorption capacity onto NA11 was the highest of all adsorbents. The sulfate ions in the interlayer of NA11 was exchanged to tungsten ions, that is, the adsorption mechanism was ion exchange under our experimental conditions. Additionally, to elucidate the adsorption mechanism in detail, the elemental distribution and X-ray photoelectron spectroscopy of the NA11 surface were analyzed. Finally, the results indicated that the tungsten ions adsorbed using NA11 could be desorbed (recovered) from NA11 using sodium hydroxide solution. These results serve as useful information regarding the adsorption and recovery of tungsten ions using nickel–aluminum complex hydroxides from aqueous media.

Fe and Cu dual-doped Ni3S4 nanoarrays with less low-valence Ni species for boosting water oxidation reaction

Transition metal materials with high efficiency and durable electrocatalytic water splitting activity have attracted widespread attention among scientists. In this work, two cation co-doped Ni₃S₄ nanoarrays grown on a Ni foam support were firstly synthesized through a typical two step hydrothermal process. Cu and Fe co-doping can regulate the internal electron configuration of the material, thus reducing the activation energy of the active species. Moreover, density functional theory calculations demonstrate that a low Ni²⁺ amount improves the adsorption energy of H₂O, which facilitates the formation and reaction of intermediate species in the water splitting process. The experimental results indicate that the Cu and Fe co-doped Ni₃S₄ material has superior electrochemical activity for water oxidation reaction to pure Ni₃S₄, Fe doped Ni₃S₄ and Cu doped Ni₃S₄. The Fe-Cu-Ni₃S₄ material displays a significantly enhanced electrocatalytic performance with low overpotentials of 230 mV at 50 mA cm^-2 and 260 mV at 100 mA cm^-2 for the oxygen evolution reaction under alkaline conditions. It's worth noting that when Fe-Cu-Ni₃S₄ was used as the anode and cathode, a small cell voltage of 1.59 V at 10 mA cm^-2 was obtained to achieve stable overall water splitting. Our work will afford a novel view and guidance for the preparation and application of efficient and environmentally friendly water splitting catalysts.

Role of Ce in enhanced performance of water oxidation reaction and urea oxidation reaction for NiFe Layered Double Hydroxide

The atomistic doping and surface engineering affords a promising method for improving their electrochemistry performance toward the water oxidation reaction and urea oxidation reaction (UOR) for the layered double hydroxides...

Rapid “self-healing” behavior induced by chloride anions to renew Fe-Ni(oxy)hydroxide surface for long-term alkaline seawater electrolysis

Due to the surface adsorption and interlayer insertion behavior of chloride anions, Fe-Ni(oxy)hydroxide catalytic surface is easily destroyed, making it difficult to be used for long-term seawater electrolysis. Here, we...

Amorphous-crystalline FeNi2S4@NiFe-LDH nanograsses by molten salt as an industrially promising electrocatalyst for oxygen evolution

Inexpensive and accessible NiFe-based oxygen evolution reaction (OER) electrocatalyst are limited for practical industrial applications by its activity and stability under industrial conditions. Herein, FeNi2S4@NiFe-LDH heterostructure is constructed by molten...

Synergistic coupling of FeOOH with Mo-incorporated NiCo LDH towards enhancing the oxygen evolution reaction

FeOOH-modified NiCoMo LDH/NF with excellent OER activity and stability was successfully prepared using a hydrothermal method combined with electrodeposition.

Hierarchical Structure of CuO Nanowires Decorated with Ni(OH)2 Supported on Cu Foam for Hydrogen Production via Urea Electrocatalysis

Owing to the low theoretical potential of the urea oxidation reaction (UOR), urea electrolysis is an energy-saving technique for the generation of hydrogen. Herein, a hierarchical structure of CuO nanowires decorated with nickel hydroxide supported on 3D Cu foam is constructed. Combined theoretical and experimental analyses demonstrate the high reactivity and selectivity of CuO and Ni(OH)₂ toward the UOR instead of the oxygen evolution reaction. The hierarchical structure creates a synergistic effect between the two highly active sites, enabling an exceptional UOR activity with a record low potential of 1.334 V (vs the reversible hydrogen electrode) to reach 100 mA cm^-2 and a low Tafel slope of 14 mV dec^-1 in 1 m KOH and 0.5 m urea electrolyte. Assembling full urea electrolysis driven by this developed UOR electrocatalyst as the anode and a commercial Pt/C electrocatalyst as the cathode provides a current density of 20 mA cm^-2 at a cell voltage of ≈1.36 V with promising operational stability for at least 150 h. This work not only enriches the UOR material family but also significantly advances energy-saving hydrogen production.

Ce-Substituted Spinel CuCo2O4 Quantum Dots with High Oxygen Vacancies and Greatly Improved Electrocatalytic Activity for Oxygen Evolution Reaction

Exploring effective electrocatalysts for oxygen evolution reaction (OER) is a crucial requirement of many energy storage and transformation systems, involving fuel cells, water electrolysis, and metal-air batteries. Transition-metal oxides (TMOs) have attracted much attention to OER catalysts because of their earth abundance, tunable electronic properties, and so forth. Defect engineering is a general and the most important strategy to tune the electronic structure and control size, and thus improve their intrinsic activities. Herein, OER performance on spinel CuCo₂O₄ was greatly enhanced through cation substitution and size reduction. Ce-substituted spinel CuCe_δCo_2-δO_x (δ = 0.45, 0.5 and 0.55) nanoparticles in the quantum dot scale (2-8 nm) were synthesized using a simple and facile phase-transfer coprecipitation strategy. The as-prepared samples were highly dispersed and have displayed a low overpotential of 294 mV at 10 mA·cm^-2 and a Tafel slope of 57.5 mV·dec^-1, which outperform commercial RuO₂ and the most high-performance analogous catalysts reported. The experimental and calculated results all confirm that Ce substitution with an appropriate content can produce rich oxygen vacancies, tune intermediate absorption, consequently lower the energy barrier of the determining step, and greatly enhance the OER activity of the catalysts. This work not only provides advanced OER catalysts but also opens a general avenue to understand the structure-activity relationship of pristine TMO catalysts deeply in the quantum dot scale and the rational design of more efficient OER catalysts.

Amorphous nickel tungstate films prepared by SILAR method for electrocatalytic oxygen evolution reaction

Development of electrocatalyst using facile way from non-noble metal compounds with high efficiency for effective water electrolysis is highly demanding for production of hydrogen energy. Nickel based electrocatalysts were currently developed for electrochemical water oxidation in alkaline pH. Herein, amorphous nickel tungstate (NiWO₄) was synthesized using the facile successive ionic layer adsorption and reaction method. The films were characterized by X-ray diffraction, Raman spectroscopy, Fourier transfer infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy techniques. The electrochemical analysis showed 315 mV of overpotential at 100 mA cm^-2 with lowest Tafel slope of 32 mV dec^-1 for oxygen evolution reaction (OER) making films of NiWO₄ compatible towards electrocatalysis of water in alkaline media. The chronopotentiometry measurements at 100 mA cm^-2 over 24 h showed 97% retention of OER activity. The electrochemical active surface area (ECSA) of NW120 film was 25.5 cm^-2.

Promoting electrocatalytic overall water splitting by sulfur incorporation into CoFe-(oxy)hydroxide

The design and fabrication of highly cost-effective electrocatalysts with high activity, and stability to enhance the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has been considered to be one of the most promising approaches toward overall water splitting. In this study, sulfur-incorporated cobalt-iron (oxy)hydroxide (S-(Co,Fe)OOH) nanosheets were directly grown on commercial iron foam via galvanic corrosion and hydrothermal methods. The incorporation of sulfur into (Co,Fe)OOH results in superior catalytic performance and high stability in both the HER and OER conducted in 1 M KOH. The incorporation of sulfur enhanced the electrocatalytic activity by modifying the electronic structure and chemical states of (Co,Fe)OOH. An alkaline water electrolyzer for overall water splitting was fabricated using a two-electrode configuration utilizing the S-(Co,Fe)OOH bifunctional electrocatalyst in both the HER and OER. The fabricated electrolyzer outperformed a precious metal-based electrolyzer using Pt/C as the HER electrocatalyst and IrO2 as the OER electrocatalyst, which are the benchmark catalysts. This electrolyzer provides a lower potential of 1.641 V at 10 mA cm-2 and maintains 98.4% of its performance after 50 h of durability testing. In addition, the S-(Co,Fe)OOH-based electrolyzer successfully generated hydrogen under natural illumination upon its combination with a commercial silicon solar cell and exhibited a solar to hydrogen (STH) efficiency of up to 13.0%. This study shows that S-(Co,Fe)OOH is a promising candidate for application in the future renewable energy industry due to its high cost-effectiveness, activity, and stability during overall water splitting. In addition, the combination of a commercial silicon solar cell with an alkaline water electrolyzer has great potential for the production of hydrogen.

Selenium-induced NiSe2@CuSe2 hierarchical heterostructure for efficient oxygen evolution reaction

Electrochemical water splitting is widely studied in the hope of solving environmental deterioration and energy shortage. The design of inexpensive metal catalysts exhibiting desired catalytic performance and durable stability for efficient oxygen evolution is the pursuit of sustainable and clean energy fields. Herein, a three-dimensional (3D) flower-like NiSe₂ primary structure, modified with highly dispersed CuSe₂ nanoclusters as the secondary structure, is obtained by regulating the growth trend of the nanosheets. Benefiting from the metallicity of selenides and the formation of a heterogeneous interface, NiSe₂@CuSe₂/NF shows comparable performance toward the oxygen evolution reaction (OER) in an alkaline environment. Upon regulating the synthesis conditions, the catalyst exhibits its optimal performance with ultralow overpotential for the OER when the Ni/Cu molar ratio is 1 : 0.2 and the hydrothermal temperature and hydrothermal time are 200 °C and 6 h, respectively. It provides a current density of 10 mA cm^-2 when a potential of 201 mV is applied without iR compensation. In this work, the hierarchical heterostructures of NiSe₂ and CuSe₂ are synthesized, which exhibit high electrocatalytic activity towards the oxygen evolution reaction and provides a new possibility for the extensive application of copper-based compounds in advanced energy fields.

"Pit-dot" ultrathin nanosheets of hydrated copper pyrophosphate as efficient pre-catalysts for robust water oxidation

Herein, hydrated copper pyrophosphate ultrathin nanosheets with a unique "pit-dot" nanostructure were fabricated as efficient pre-catalysts for the oxygen evolution reaction, and systematic post-catalytic characterization studies confirmed the important role of the boosted pre-oxidation reaction in promoting the OER catalysis.

Structural and Interfacial Engineering of Ni2P/Fe3O4 Porous Nanosheet Arrays for Efficient Oxygen Evolution Reaction

Rational design of transition-metal phosphide (TMPs)-based electrocatalysts can effectively promote oxygen evolution reaction (OER). Herein, the novel efficient Ni₂P/Fe₃O₄ porous nanosheets arrays supported on Ni foam (Ni₂P/Fe₃O₄/NF) as alkaline OER catalysts were synthesized using structural and interfacial engineering. The three-dimensional (3D) porous hierarchical structure of Ni₂P/Fe₃O₄/NF provides abundant active sites for OER and facilitates the electrolyte diffusion of ions and O₂ liberation. Furthermore, the strong interfacial coupling and synergistic effect between Ni₂P and Fe₃O₄ modify the electronic structure, resulting in the enhanced intrinsic activity. Consequently, the optimized Ni₂P/Fe₃O₄/NF exhibits excellent OER performance with low overpotentials of 213 and 240 mV at 60 and 100 mA cm^-2 in 1.0 M KOH, respectively, better than the RuO₂/NF and most Ni/Fe-based OER catalysts. Impressively, it can maintain its catalytic activity for at least 20 h at 60 mA cm^-2. In addition, the relationship between the structure and performance is fully elucidated by the experimental characterizations, indicating that the metal oxyhydroxides in situ generated on the surface of catalysts are responsible for the high OER activity.