CoO supported NiFe layered double hydroxide sandwich-like nanosheets on hierarchical carbon framework for efficient electrocatalytic oxygen evolution

Exploration of greatly efficient and steady non-noble oxygen evolution reaction (OER) electrocatalysts is of great significance in improving the overall efficiency of energy density systems such as regenerative fuel cells, water electrolyzes, and metal-air batteries. Herein, inspired by hierarchical 3D porous structures with open microchannels of natural wood, CoO@NiFe LDH sandwich-like nanosheets were anchored on the carbonized wood (CW) via electrodeposition and calcination strategies. The strong interactions between CoO nanosheets and NiFe LDH nanosheets endow CoO@NiFe LDH/CW electrocatalyst with high catalytic properties toward the OER comparable to CoO/CW and NiFe LDH/CW. The optimized CoO@NiFe LDH/CW electrocatalyst demonstrates good OER catalytic performance with an overpotential of 230 mV at 100 mA cm-2. This work presents an innovative approach to utilize renewable resources for constructing advanced free-standing catalysts.

Nickel-Iron-Layered Double Hydroxide Electrocatalyst with Nanosheets Array for High Performance of Water Splitting

Developing high-performance and cost-competitive electrocatalysts have great significance for the massive commercial production of water-splitting hydrogen. Ni-based electrocatalysts display tremendous potential for electrocatalytic water splitting. Herein, we synthesize a novel NiFe-layered double hydroxide (LDH) electrocatalyst in nanosheets array on high-purity Ni foam. By adjusting the Ni/Fe ratio, the microstructure, and even the behavior of the electrocatalyst in the oxygen evolution reaction (OER), changes significantly. The as-obtained material shows a small overpotential of 223 mV at 10 mAcm-2 as well as a low Tafel slope of 48.9 mV·dec-1 in the 1 M KOH electrolyte. In addition, it can deliver good stability for at least 24 h of continuous working at 10 mAcm-2. This work proposes a strategy for engineering catalysts and provides a method for the development of other Ni-based catalysts with excellent performance.

Doping Ru on FeNi LDH/FeII/III-MOF heterogeneous core-shell structure for efficient oxygen evolution

Noble metal-based catalysts such as RuO2 and IrO2 are widely used in the catalysis of the OER. However, because of their high price and poor stability, it is urgent to develop transition metal-based electrocatalysts with low precious metal doping as an alternative. Layered double hydroxides (LDHs) grown on 3D metal-organic frameworks (MOFs) are ideal for doping precious metals owing to abundant defects at the heterointerface, large surface area, and intrinsic oxygen evolution activity. In this study, a novel FeNi LDH/MOF heterostructure was prepared via a two-step solvothermal method using Fe-soc-MOFs as the substrate. Subsequently, Ru was introduced through a hydrothermal process. The as-synthesized Ru@FeNi LDH/MOF has an overpotential of only 242 mV at a current density of 10 mA cm-2 and can be used in continuous electrolysis for 48 h. Its unique nanocubic core-shell structure and flower-like LDHs on its surface provide a large number of active sites, which become the key to ensuring high activity and stability. With the doping of Ru, the electronic structure was adjusted and electron transfer was accelerated, further improving electrochemical activity. This study provides a new idea for developing transition metal-based catalysts with low noble metal loading.

Boosting oxygen evolution reaction activity with Mo incorporated NiFe-LDH electrocatalyst for efficient water electrolysis

This work demonstrates a simple and scalable methodology for the binder-free direct growth of Mo-doped NiFe-layered double hydroxides on a nickel substrate via an electrodeposition route at room temperature. A three-dimensional (3D) nanosheet array morphology of the electrocatalyst provides immense electrochemical surface area as well as abundant catalytically active sites. Mo incorporation in the NiFe-LDH plays a crucial role in regulating the catalytic activity of oxygen evolution reaction (OER). The prepared electrocatalyst exhibited low overpotential (i.e., 230 mV) at 30 mA cm-2 for OER in an alkaline electrolyte (i.e., 1 M KOH). Furthermore, the optimized Mo-doped NiFe-LDH electrode was used as an anode in a laboratory-scale in situ single cell test system for alkaline water electrolysis at 80 °C with a continuous flow of 30 wt% KOH, and it shows the efficient electrochemical performance with a lower cell voltage of 1.80 V at a current density of 400 mA cm-2. In addition, an admirable long-term cell durability is also demonstrated by the cell for 24 h. This work encourages new designs and further development of electrode material for alkaline water electrolysis on a commercial scale.

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.

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.

Tuning coordination microenvironment of V2CTx MXene for anchoring single-atom toward efficient multifunctional electrocatalysis

The rational design of low-cost and high-performance multifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution/reduction reaction (OER/ORR) is essential for efficient overall water splitting and rechargeable metal-air battery. Herein, through density functional theory calculations, we creatively regulate the coordination microenvironment of V₂CT_x MXene (M-v-V₂CT₂, T = O, Cl, F and S) as substrates of single-atom catalysts (SACs), and then systematically explore their HER, OER, and ORR electrocatalytic performance. Our results disclose that Rh-v-V₂CO₂ is a promising bifunctional catalyst for water splitting (overpotentials of 0.19 and 0.37 V for HER and OER). Besides, Pt-v-V₂CCl₂ and Pt-v-V₂CS₂ possess desirable bifunctional OER/ORR activity with overpotentials of 0.49/0.55 V and 0.58/0.40 V, respectively. More interestingly, Pt-v-V₂CO₂ is a promising trifunctional catalyst under vacuum, implicit and explicit solvation conditions, which transcends commercially used Pt and IrO₂ catalysts for HER/ORR and OER. The electronic structure analysis further demonstrates that surface functionalization can optimize the local microenvironment of the SACs and thus tune the interaction strength of intermediate adsorbates. This work provides a feasible strategy for developing advanced multifunctional electrocatalysts and enriches the application of MXene in energy conversion and storage.

One-Step High-Temperature Electrodeposition of Fe-Based Films as Efficient Water Oxidation Catalysts

Electrolysis of water to produce hydrogen requires an efficient catalyst preferably made of cheap and abundant metal ions for the improved water oxidation reaction. An Fe-based film has been deposited in a single step by electrochemical deposition at temperatures higher than the room temperature. Until now, the electrodeposition of iron oxide has been carried out at 298 K or at lower temperatures under a controlled atmosphere to prohibit atmospheric oxidation of Fe²⁺ of the iron precursor. A metal inorganic complex, ferrocene, and non-aqueous electrolyte medium propylene carbonate have been used to achieve electrodeposition of iron oxide without the need of any inert or controlled atmosphere. At 298 K, the amorphous film was formed, whereas at 313 K and at higher temperatures, the hematite film was grown, as confirmed by X-ray diffraction. The transformation of iron of the ferrocene into a higher oxidation state under the experimental conditions used was further confirmed by X-ray photoelectron spectroscopy, ultraviolet-visible, and electron paramagnetic resonance spectroscopic methods. The films deposited at 313 K showed the best performance for water oxidation with remarkable long-term electrocatalytic stability and an impressive turnover frequency of 0.028 s^-1 which was 4.5 times higher than that of films deposited at 298 K (0.006 s^-1). The observed overpotential to achieve a current density of 10 mA cm^-2 was found to be 100 mV less for the film deposited at 313 K compared to room-temperature-derived films under similar experimental conditions. Furthermore, electrochemical impedance data revealed that films obtained at 313 K have the least charge transfer resistance (114 Ω) among all, supporting the most efficient electron transport in the film. To the best of our knowledge, this is the first-ever report where the crystalline iron-based film has been shown to be electrodeposited without any post-deposition additional treatment for alkaline oxygen evolution reaction application.

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.

NiFe-based electrocatalyst anchored on nanocone with tip-field effect for improved oxygen evolution reaction

The development of high efficiency oxygen evolution electrocatalyst is of great significance for water splitting reaction. Herein, an efficient cone-structured NiFe-LDH/Nicone/Ti catalyst is fabricated by electrodeposition method towards enhanced oxygen evolution reaction (OER). The featured tip curvature of nanocone structure can accelerate the reaction kinetics of OER by offering a field-enhanced aggregation of local hydroxide ion reactant on the catalyst surface, and thus improves the performance of the NiFe catalyst. Accordingly, NiFe-LDH/Nicone/Ti requires only a low overpotential of 292 mV to achieve 50 mA cm^-2, and with high stability under continuous high-current operations. In addition, the alkali-electrolyzer using NiFe-LDH/Nicone/Ti electrode exhibits good performance with a voltage of 1.73 V at 50 mA cm^-2and displays excellent stability in long-term stability test. This cone-structured catalyst design with field-enhanced local hydroxide ion aggregation effect provides a promising method for the development of highly active OER electrocatalysts.

Controlled synthesis of Crx-FeCo2P nanoarrays on nickel foam for overall urea splitting

Urea splitting is a highly promising technology for hydrogen production to cope with the fossil energy crisis, which requires the development of catalysts with high electrocatalytic activity. In this article, Cr_x-FeCo₂P/NF catalysts were synthesized by hydrothermal and low-temperature phosphorylation and used in the overall urea splitting process. Cr_0.15-FeCo₂P/NF and Cr_0.1-FeCo₂P/NF exhibited excellent urea oxidation reaction (UOR) activity (potential of 1.355 V at 100 mA cm^-2) and hydrogen evolution reaction (HER) activity (overpotential of 173 mV at 10 mA cm^-2) in 0.5 M urea solution containing 1 M KOH. In the assembled Cr_0.15-FeCo₂P/NF//Cr_0.1-FeCo₂P/NF electrolytic cell, only a small voltage of 1.50 V is needed to reach 10 mA cm^-2. Density functional theory (DFT) calculation results demonstrate that an appropriate amount of Cr doping accelerates the kinetic performance of hydrogen production as well as improving the metallic properties of the electrode.

Crystalline-Dependent Discharge Process of Locally Enhanced Electrooxidation Activity on Ni2P

The state-of-the-art transition-based electrocatalysts in alkaline media generally suffer from unavoidable surface reconstruction during oxygen evolution reaction measurements, leading to the collapse and loss of the crystalline matrix. Low potential discharge offers a gentle way for surface reconstruction and thus realizes the manipulation of the real active site. Nevertheless, the absence of a fundamental understanding focus on this discharge region renders the functional phase, either the crystalline or amorphous matrix, for the controllable reconstruction still undecidable. Herein, we report a scenario to employ different crystalline matrices as electrocatalysts for discharge region reconstruction. The representative low crystalline Ni₂P (LC-Ni₂P) possesses a relatively weak surface structure compared with highly crystalline or amorphous Ni₂P (HC-Ni₂P or A-Ni₂P), which contributes abundant oxygen vacancies after the discharge process. The fast discharge behavior of LC-Ni₂P leads to the uniform distribution of these vacancies and thus endows the inner interface with reactant activating functionality. A high increase in current density of 36.7% is achieved at 2.32 V (vs RHE) for the LC-Ni₂P electrode. The understanding of the discharge behavior in this study, on different crystalline matrices, presents insights into the establishment of controllable surface reconstruction for an effective oxygen evolution reaction.

Recent Advances of Modified Ni (Co, Fe)-Based LDH 2D Materials for Water Splitting

Water splitting technology is an efficient approach to produce hydrogen (H₂) as an energy carrier, which can address the problems of environmental deterioration and energy shortage well, as well as establishment of a clean and sustainable hydrogen economy powered by renewable energy sources due to the green reaction of H₂ with O₂. The efficiency of H₂ production by water splitting technology is intimately related with the reactions on the electrode. Nowadays, the efficient electrocatalysts in water splitting reactions are the precious metal-based materials, i.e., Pt/C, RuO₂, and IrO₂. Ni (Co, Fe)-based layered double hydroxides (LDH) two-dimensional (2D) materials are the typical non-precious metal-based materials in water splitting with their advantages including low cost, excellent electrocatalytic performance, and simple preparation methods. They exhibit great potential for the substitution of precious metal-based materials. This review summarizes the recent progress of Ni (Co, Fe)-based LDH 2D materials for water splitting, and mainly focuses on discussing and analyzing the different strategies for modifying LDH materials towards high electrocatalytic performance. We also discuss recent achievements, including their electronic structure, electrocatalytic performance, catalytic center, preparation process, and catalytic mechanism. Furthermore, the characterization progress in revealing the electronic structure and catalytic mechanism of LDH is highlighted in this review. Finally, we put forward some future perspectives relating to design and explore advanced LDH catalysts in water splitting.

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.

Cauliflower-like NiFe alloys anchored on a flake iron nickel carbonate hydroxide heterostructure towards superior overall water and urea electrolysis

Exploring efficient, stable and multifunctional Earth-rich electrocatalysts is vital for hydrogen generation. Hence, an efficient heterostructure consisting of cauliflower-like NiFe alloys anchored on flake iron nickel carbonate hydroxide which is supported on carbon cloth (NiFe/NiFeCH/CC) was synthesized as a trifunctional electrocatalyst for efficient hydrogen production by overall water and urea splitting. While optimizing and regulating the ratio of Ni to Fe, benefiting from the special morphology and synergistic effect between the NiFe alloy and NiFeCH, the NiFe/NiFeCH/CC heterostructure exhibits outstanding oxygen evolution reaction (OER) performance with a low overpotential of 190 mV at 10 mA cm^-2 after a stability test for 150 h. Notably, when the NiFe/NiFeCH/CC heterostructure is used as both the anode and cathode simultaneously, it merely requires a cell voltage of 1.49 V for the overall water splitting and 1.39 V for urea electrolysis at 10 mA cm^-2 with excellent durability. Thus, this work not just provides the application of NiFe-based catalysts in overall water splitting, but also offers a viable method for the treatment of urea-rich wastewater.

Plasma-assisted synthesis of hierarchical defect N-doped iron–cobalt sulfide@Co foam as an efficient bifunctional electrocatalyst for overall water splitting

N-doped CoFeS was synthesized via an ion exchange method to prepare a precursor, followed by sulphidation and plasma-assisted engraving in nitrogen gas.

Modulating the Surface Electronic Structure of Active Ni Sites by Engineering Hierarchical NiFe-LDH/CuS over Cu Foam as an Efficient Electrocatalyst for Water Splitting

Water electrolysis encounters a challenging problem in designing a highly efficient, long durable, non-noble metal-free electrocatalyst for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Here, in our work, a two-step hydrothermal reaction was performed to construct a hierarchal NiFe-layer double hydroxide (LDH)/CuS over copper foam for the overall water splitting reaction. While employed the same as an anode material, the designed heterostructure electrode NiFe-LDH/CuS/Cu exhibits excellent OER performance and it demands 249 mV overpotential to reach a current density of 50 mA cm^-2 with a lower Tafel slope value of 81.84 mV dec^-1. While as a cathode material, the NiFe-LDH/CuS/Cu shows superior HER performance and it demands just 28 mV of overpotential value to reach a current density of 10 mA cm^-2 and a lower Tafel slope value of 95.98 mV dec^-1. Hence, the NiFe-LDH/CuS/Cu outperforms the commercial Pt/C and RuO₂ in terms of activity in HER and OER, respectively. Moreover, when serving as both the cathode and anode catalysts in an electrolyzer for total water splitting, the synthesized electrode only needs a cell potential of 1.55 V versus RHE to reach a current density of 20 mA cm^-2 and long-term durability for 25 h in alkaline media. To study the interfacial electron transfer, Mott-Schottky experiments were performed, representing that the electron is transferred from n-type NiFe-LDH to p-type CuS as a result of creating the p-n junction in NiFe-LDH/CuS/Cu. The formation of this p-n junction allows the LDH layer to be more active toward the OH- adsorption and thereby could allow the OER or HER with a less energy input. This work affords another route to a cost effective, highly efficient catalyst toward producing clean energy across the globe.

Facile Synthesis and Origin of Enhanced Electrochemical Oxygen Evolution Reaction Performance of 2H-Hexagonal Ba2CoMnO6-δ as a New Member in Double Perovskite Oxides

Perovskite oxides have been considered promising oxygen evolution reaction (OER) electrocatalysts due to their high intrinsic activity. Yet, their poor long-term electrochemical and structural stability is still controversial. In this work, we apply an A-site management strategy to tune the activity and stability of a new hexagonal double perovskite oxide. We synthesized the previously inaccessible 2H-Ba₂CoMnO_6-δ (BCM) perovskite oxide via the universal sol-gel method followed by a novel air-quench method. The new 2H-BCM perovskite oxide exhibits outstanding OER activity with an overpotential of 288 mV at 10 mA cm^-2 and excellent long-term stability without segregation or structural change. To understand the origin of outstanding OER performance of BCM, we substitute divalent Ba with trivalent La at the A-site and investigate crystal and electronic structure change. Fermi level and valence band analysis presents a decline in the work function with the Ba amount, suggesting a structure-oxygen vacancy-work function-activity relationship for Ba _x La_2-x CoMnO_6-δ (x = 0, 0.5, 1, 1.5, 2) electrocatalysts. Our work suggests a novel production strategy to explore the single-phase new structures and develop enhanced OER catalysts.

Optimized Mo-doped IrOx anode for efficient degradation of refractory sulfadiazine

Electrochemical advanced oxidation processes (EAOPs) is considered to be an efficacious method to degrade antibiotics. However, the performance of the anode has become the main limiting factor of this technology. In this study, due to the electron-deficient characteristics and the improvement of OER performance of Mo, we chose to use thermal decomposition to incorporate Mo into IrO2 to prepare anodes with industrial applicability. Under the optimal ratio of Ir to Mo is 7:3, (Ir0.7Mo0.3)Ox electrode's particular pore structure can expose more active sites and create a channel for the transportation of electrons, thereby promoting the formation of free radicals and degrading pollutants more efficiently. (Ir0.7Mo0.3)Ox electrode also has a higher mass activity (6.332 A g-1, three times that of the IrO2 electrode) and a larger electrochemical active area (ECSA, 375.43 cm2, seven times that of the IrO2 electrode). In addition, the optimal conditions of (Ir0.7Mo0.3)Ox electrode for degrading sulfadiazine(SDZ) were explored, which achieved a higher removal than traditional electrodes (90% removal within 4 h) when the Ti plate was the substrate. Through the intermediate products of SDZ degradation and related literatures, two possible degradation pathways of SDZ were speculated. This research provides a new type of anode catalyst for the degradation of sulfonamide antibiotics, which is possible for industrial application.

Amorphous Co-Mo-B Film: A High-Active Electrocatalyst for Hydrogen Generation in Alkaline Seawater

The development of efficient electrochemical seawater splitting catalysts for large-scale hydrogen production is of great importance. In this work, we report an amorphous Co-Mo-B film on Ni foam (Co-Mo-B/NF) via a facile one-step electrodeposition process. Such amorphous Co-Mo-B/NF possesses superior activity with a small overpotential of 199 mV at 100 mA cm⁻² for a hydrogen evolution reaction in alkaline seawater. Notably, Co-Mo-B/NF also maintains excellent stability for at least 24 h under alkaline seawater electrolysis.

High-Performance Ternary NiCoMo Electrocatalyst with Three-Dimensional Nanosheets Array Structure

Oxygen evolution reaction is a key process in hydrogen production from water splitting. The development of non-noble metal electrode materials with high efficiency and low cost has become the key factor for large-scale hydrogen production. Binary NiCo-layered double hydroxide (LDH) has been used as a non-noble metal electrocatalyst for OER, but its overpotential is still large. The microstructure of the catalyst is tuned by doping Mo ions into the NiCo-LDH/NF nanowires to form ternary NiCoMo-LDH/NF nanosheet catalysts for the purpose of enhancing the active sites and reducing the initial overpotential. Only 1.5 V (vs. reversible hydrogen electrode (RHE), ≈270 mV overpotential) is required to achieve a catalytic current density of 10 mA cm^-2 and a small Tafel slope of 81.46 mV dec^-1 in 1 M KOH solution, which manifests the best performance of NiCo-based catalysts reported up to now. Electrochemical analysis and micro-morphology show that the high catalytic activity of NiCoMo-LDH/NF is attributable to the change of the microstructure. The interconnected nanosheet arrays have the obvious advantages of electrolyte diffusion and ion migration. Thus, the active sites of catalysts are significantly increased, which facilitates the adsorption and desorption of intermediates. We conclude that NiCoMo-LDH/NF is a promising electrode material for its low cost and excellent electrocatalytic properties.

Bifunctional keel flower-like Ni-Co-V multicomponent oxide catalyst with enhanced electron transport for accelerating overall water splitting

Heteroatom doping is considered a typical method for improving the electrochemical properties of composites. In this work, the multi-component oxide catalyst (Ni(VO₃)₂ and Co₂V₂O₇ on Ni foam, referred to as NiCoVO_x@NF) is formed by hydrothermal doping of V element into NiCo-based precursors followed by co-oxidation. In the catalyst NiCoVO_x@NF, all three components of Ni, Co and V are particularly tightly coordinated, exhibiting an integrated structure of keel flower-like arrays. The catalyst NiCoVO_x@NF's contact surface with water is increased thanks to this unusual structure, exposing a high number of active sites. Furthermore, NiCoVO_x@NF owns efficient electronic pathways, which greatly enhances the electron transport ability. To generate a current density of 10 mA cm^-2 for hydrogen evolution reaction, just a 107 mV overpotential is required. The electrode exhibits a low overpotential of 217 mV to deliver 50 mA cm^-2 for oxygen evolution reaction. In addition, the total water splitting performance of NiCoVO_x@NF is also excellent, which could be achieved by only one 1.5 V AA battery. This study provides a feasible heteroatom doping route to design bifunctional catalysts with improved performances.

FeNi LDH/V2CTx/NF as Self-Supported Bifunctional Electrocatalyst for Highly Effective Overall Water Splitting

The development of bifunctional electrocatalysts with efficient oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is still a key challenge at the current stage. Herein, FeNi LDH/V₂CT_x/nickel foam (NF) self-supported bifunctional electrode was prepared via deposition of FeNi LDH on V₂CT_x/NF substrate by hydrothermal method. Strong interfacial interaction between V₂CT_x/NF and FeNi LDH effectively prevented the aggregation of FeNi LDH, thus exposing more catalytic active sites, which improved electrical conductivity of the nanohybrids and structural stability. The results indicated that the prepared FeNi LDH/V₂CT_x/NF required 222 mV and 151 mV overpotential for OER and HER in 1 M KOH to provide 10 mA cm⁻², respectively. Besides, the FeNi LDH/V₂CT_x/NF electrocatalysts were applied to overall water splitting, which achieved a current density of 10 mA cm⁻² at 1.74 V. This work provides ideas for improving the electrocatalytic performance of electrocatalysts through simple synthesis strategies, structural adjustment, use of conductive substrates and formation of hierarchical structures.

Silver decorated nickel-cobalt (oxy)hydroxides fabricated via surface reconstruction engineering for boosted electrocatalytic oxygen evolution and urea oxidation

Electrochemical water splitting is considered to be a promising renewable hydrogen generation technology but is significantly limited by the kinetically sluggish oxygen evolution reaction (OER) at the anode. Herein, a silver nanoparticle decorated nickel-cobalt (oxy)hydroxide composite is fabricated on nickel foam (Ag@NiCo(OH)_x/NF) via electrodeposition followed by spontaneous redox reaction. Benefitting from the synergetic contributions of an amorphous/crystalline phase, abundant artificial heterointerfaces, and a 3D porous architecture, the as-designed Ag@NiCo(OH)_x/NF shows substantially enhanced electrocatalytic performance toward the OER and urea oxidation reaction. Impressively, in the urea-assisted alkaline electrolyzer (coupled with commercial Pt/C on NF as the cathode) for hydrogen production, a cell voltage of only 1.49 V is required to deliver a current density of 50 mA cm^-2, much lower than that of traditional water splitting (1.69 V). Importantly, this work represents a facile and feasible method to exploit efficient self-supported electrocatalysts toward overall water splitting and urea-rich wastewater purification.

Nanocrystals of CuCoO2 derived from MOFs and their catalytic performance for the oxygen evolution reaction

In this work, two different solvothermal synthesis routes were employed to prepare MOF-derived CuCoO₂ (CCO) nanocrystals for electrocatalytic oxygen evolution reaction (OER) application. The effects of the reductants (ethylene glycol, methanol, ethanol, and isopropanol), NaOH addition, the reactants, and the reaction temperature on the structure and morphology of the reaction product were investigated. In the first route, Cu-BTC derived CCO (CCO1) nanocrystals with a size of ∼214 nm and a specific surface area of 4.93 m² g^-1 were prepared by using Cu-BTC and Co(NO₃)₂·6H₂O as the Cu and Co source, respectively. In the second route, ZIF-67 derived CCO (CCO2) nanocrystals with a size of ∼146 nm and a specific surface area of 11.69 m² g^-1 were prepared by using ZIF-67 and Cu(NO₃)₂·3H₂O as the Co and Cu source, respectively. Moreover, the OER performances of Ni foam supported CCO1 (Ni@CCO1) and CCO2 (Ni@CCO2) electrodes were evaluated in 1.0 M KOH solution. Ni@CCO2 demonstrates a better OER catalytic performance with a lower overpotential of 394.5 mV at 10 mA cm^-2, a smaller Tafel slope of 82.6 mV dec^-1, and long-term durability, which are superior to those of some previously reported delafossite oxide or perovskite oxide catalysts. This work reveals the preparation method and application potential of CCO electrocatalysts by using Cu-BTC/ZIF-67 as the precursor, providing a new approach for the preparation of delafossite oxide CCO and the enhancement of their OER performances.

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.

Corrosion-Engineered Morphology and Crystal Structure Regulation toward Fe-Based Efficient Oxygen Evolution Electrodes

The rational regulation of catalysts with a well-controlled morphology and crystal structure has been demonstrated effective for optimizing the electrochemical performance. Herein, corrosion engineering was employed for the straightforward preparation of FeAl layered double hydroxide (LDH) nanosheets and Fe₃O₄ nanooctahedrons via the feasible modification of dealloying conditions. The FeAl-LDH nanosheets display an excellent catalytic performance for oxygen evolution reactions in 1 M KOH solution, such as low overpotentials (333 mV on glass carbon electrode and 284 mV on Ni foam at 10 mA cm⁻²), a small Tafel slope (36 mV dec⁻¹), and excellent durability (24 h endurance without deactivation). The distinguished catalytic features of the FeAl-LDH nanosheets comes from the Al and Fe synergies, oxygen vacancies, and well-defined two-dimensional (2D) layered LDH structure.

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.

Hydrothermal synthesized delafossite CuGaO2 as an electrocatalyst for water oxidation

Hydrogen production from water splitting provides an effective method to alleviate the ever-growing global energy crisis. In this work, delafossite CuGaO₂ (CGO) crystal was synthesized through hydrothermal routes with Cu(NO₃)₂·3H₂O and Ga(NO₃)₃·xH₂O used as reactants. The addition of cetyltrimethylammonium bromide (CTAB) was found to play an important role in modifying the morphology of CuGaO₂ (CGO-CTAB). With the addition of CTAB, the morphology of CGO-CTAB samples changed from irregular flake to typical hexagonal sheet microstructure, with an average size of 1-2 μm and a thickness of around 100 nm. Furthermore, the electrocatalytic activity of CGO-CTAB crystals for oxygen evolution reaction (OER) was also studied and compared with that of CGO crystals. CGO-CTAB samples exhibited better activity than CGO. An overpotential of 391.5 mV was shown to be able to generate a current density of 10 mA/cm². The as-prepared samples also demonstrate good stability for water oxidation and relatively fast OER kinetics with a Tafel slope of 56.4 mV/dec. This work highlights the significant role of modification of CTAB surfactants in preparing CGO related crystals, and the introduction of CTAB was found to help to improve their electrocatalytic activity for OER.

Metal-organic framework derived bimetal oxide CuCoO2 as efficient electrocatalyst for the oxygen evolution reaction

Metal-organic framework (MOF) materials with tunable porous morphology, controlled crystalline structure, various compositions, and high specific surface area are widely used as precursors to synthesize electrocatalysts for water splitting, which is beneficial for improving their oxygen evolution reaction (OER) performance. Using ZIF-67 as a Co source and Cu-BTC as a Cu source, hexagonal MOF-derived CuCoO₂ (MOF-CCO) nanocrystals with the size of ∼288 nm were prepared through a one-step solvothermal method. The influence of the content of the precursor solvents (absolute ethanol and deionized water), reaction temperature, mass ratio of reactants, NaOH addition, and reactant concentration of precursors on the structure and morphology of the products was investigated. The optimal CuCoO₂ nanocrystals (MOF-CCO1) around 288 nm present the highest OER activity, such as a low overpotential of 364.7 mV at 10 mA cm^-2, a small Tafel slope of 64.1 mV dec^-1, and attractive durability in 1.0 M KOH solution. The XPS results showed that the higher catalytic efficiency of MOF-CCO1 nanocrystals could be due to the oxygen vacancies caused by lattice oxygen loss, the increase of OH^- content on the surface, and the synergistic effect of Cu²⁺/Cu⁺ and Co²⁺/Co³⁺ redox pairs. Finally, a possible OER mechanism for MOF-CCO nanocrystals of water splitting was proposed. This study provides a new approach for the preparation of delafossite nanomaterials and for the improvement of their OER performances.

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.

Novel FeNi-Based Nanowires Network Catalyst Involving Hydrophilic Channel for Oxygen Evolution Reaction

Developing novel, efficient, and low-cost 3D FeNi-based oxygen evolution reaction (OER) catalysts with the special hydrophilic channel is still a challenge for improving hydrogen production efficiency. Herein, a novel 3D ethoxy substituted FeNi oxalate (ENWs-FeNi-C₂ O₄ ) nanowires network catalyst with hydrophilic channels is reported firstly, which shows an outstanding OER activity with a low overpotential (215 mV at 10 mA cm^-2 ) and small Tafel slope (54.5 mV dec^-1 ). OER catalytic mechanism indicates that the OH adsorption step and O₂ bubble diffusion step of OER reaction process can be significantly improved due to the special hydrophilic channels, and the ethoxy as an interlayer ligand not only expands the interlayer distance of layered FeNi (oxy) hydroxide true active species but modulates its electronic structure, promoting the *OOH formation step, and thus exhibiting the outstanding OER performance. This work provides a novel idea for the preparation of novel and efficient OER electro-catalysts with special 3D structures.

Enhanced electrocatalytic activity of FeNi alloy quantum dot-decorated cobalt carbonate hydroxide nanosword arrays for effective overall water splitting

The development of earth-abundant catalysts toward high-efficiency overall water splitting is of critical importance for electrochemical hydrogen production. Here, novel FeNi alloy quantum dot (QD)-decorated cobalt carbonate hydroxide (CoCH) nanosword arrays were successfully constructed on Ni foam (FeNi/CoCH/Ni foam) and used as an efficient bifunctional electrocatalyst for overall water splitting in alkaline media. Benefiting from the synergistic effect between the FeNi alloy QDs and CoCH, the FeNi/CoCH/Ni foam electrode delivers a current density of 20 mA cm^-2 at an overpotential of 240 mV and a small Tafel slope of 44.8 mV dec^-1 for the oxygen evolution reaction (OER). Further, it displays excellent performance for overall water splitting with a voltage of 1.49 V at 10 mA cm^-2 and maintains its activity for at least 23 h. In particular, it only needs low cell voltages of 1.54 and 1.6 V to drive high current densities of 100 and 400 mA cm^-2, respectively, which is much better than commercial Pt/C/Ni foam‖RuO₂/Ni foam, providing great potential for large-scale application.

A Superior and Stable Electrocatalytic Oxygen Evolution Reaction by One-Dimensional FeCoP Colloidal Nanostructures

Transition metal phosphides (TMPs) are expected to be excellent electrocatalysts for oxygen evolution reaction (OER) because of their high stability, highly conducting metalloid nature, highly abundant constituting elements, and the ability to act as a precatalyst due to in situ surface-formed oxy-hydroxide species. Herein, a "one-pot" colloidal approach has been used to develop a rod-shaped one-dimensional non-noble metal FeCoP electrocatalyst, which exhibits an excellent OER activity with an exceptionally high current density of 950 mA cm^-2, a turnover frequency value of 7.43 s^-1, and a low Tafel slope value of 54 mV dec^-1. The FeCoP electrocatalyst affords OER ultralow overpotentials of 230 and 260 mV at current densities of 50 and 100 mA cm^-2, respectively, in 1.0 M KOH, and demonstrates a superior catalytic stability of 10,000 cycles and durability up to 60 h at 50 mA cm^-2. An insight into the superior and stable electrocatalytic OER performance by the FeCoP nanorods is obtained by extensive X-ray photoelectron spectroscopy, X-ray diffraction, Raman and infrared spectroscopy, and cyclic voltammetry analyses for a mechanistic study. This reveals that a high number of electrocatalytically active sites enhance the oxygen evolution and kinetics by offering metal ion sites for utilitarian in situ surface formation and adsorption of *O, *OH, and *OOH reactive species for OER catalysis.

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...

Triple captured iron by defect abundant NiO for efficient water oxidation

Fe-doped NiO host in well-defined nanorod assembly (Fe-NiMoO4@NiO-30) with large surface area is designed to achieve the triple capture of Fe via adjustable surface reconstruction and impregnation to optimize OER...

A CoNi telluride heterostructure supported on Ni foam as an efficient electrocatalyst for the oxygen evolution reaction

The excellent oxygen evolution reaction performance of a CoNi telluride heterostructure (0.4CoNi LDH@Te-180C) can be attributed to the inherent layered structure, interconnected nanoarray structures and the synergistic effect of Co and Ni species.

High-efficiency NiCo layered double hydroxide electrocatalyst

We report several kinds of NiCo-LDH composites by a hydrothermal reaction and subsequent electrodeposition process. The prepared NiCo-LDH@PEDOT-200 sample shows an overpotential of 52 mV for the HER at 10 mA cm−2 in 1.0 M KOH.