Dispersing transition metal vacancies in layered double hydroxides by ionic reductive complexation extraction for efficient water oxidation

Interfacial Super-Assembly of Vacancy Engineered Ultrathin-Nanosheets Toward Nanochannels for Smart Ion Transport and Salinity Gradient Power Conversion

Ion-selective nanochannel membranes assembled from two-dimensional (2D) nanosheets hold immense promise for power conversion using salinity gradient. However, they face challenges stemming from insufficient surface charge density, which impairs both permselectivity and durability. Herein, we present a novel vacancy-engineered, oxygen-deficient NiCo layered double hydroxide (NiCoLDH)/cellulose nanofibers-wrapped carbon nanotubes (VOLDH/CNF-CNT) composite membrane. This membrane, featuring abundant angstrom-scale, cation-selective nanochannels, is designed and fabricated through a synergistic combination of vacancy engineering and interfacial super-assembly. The composite membrane shows interlayer free-spacing of ~3.62 Å, which validates the membrane size exclusion selectivity. This strategy, validated by DFT calculations and experimental data, improves hydrophilicity and surface charge density, leading to the strong interaction with K+ ions to benefit the low ion transport resistance and exceptional charge selectivity. When employed in an artificial river water|seawater salinity gradient power generator, it delivers a high-power density of 5.35 W/m2 with long-term durability (20,000s), which is almost 400 % higher than that of the pristine NiCoLDH membrane. Furthermore, it displays both pH- and temperature-sensitive ion transport behavior, offering additional opportunities for optimization. This work establishes a basis for high-performance salinity gradient power conversion and underscores the potential of vacancy engineering and super-assembly in customizing 2D nanomaterials for diverse advanced nanofluidic energy devices.

Tuning the Activity and Stability of CoCr-LDH by Forming a Heterostructure on Surface-Oxidized Nickel Foam for Enhanced Water-Splitting Performance

The development of low-cost, efficient catalysts for electrocatalytic water splitting to generate green hydrogen is a hot topic among researchers. Herein, we have developed a highly efficient heterostructure of CoCr-LDH on NiO on nickel foam (NF) for the first time. The preparation strategy follows the simple annealing of a cleaned NF without using any Ni salt precursor, followed by the growth of CoCr-LDH nanosheets over the surface-oxidized NF. The CoCr-LDH/NiO/NF catalyst shows excellent electrocatalytic activity and stability toward oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in a 1 M KOH solution. For OER, only 253 mV and for HER, only 185 mV overpotentials are required to attain a 50 mA cm-2 current density. Also, the long-term stability of both the OER and HER for 60 h proves its robustness. The turnover frequency value for the OER increased 1.85 times after the heterostructure formation compared to bare CoCr-LDH. The calculated Faradaic efficiency values of 97.4 and 94.75% for the OER and HER revealed the high intrinsic activity of the heterostructure. Moreover, the heterostructure only needs 1.57 V of cell voltage when acting as both the anode and the cathode to achieve a 10 mA cm-2 current density. The long-term stability of 60 h for the total water-splitting process proves its excellent performance. Several systematic pre- and post-experiment characterizations prove its durable nature. These excellent OER and HER activities and stabilities are attributed to the surface-modified electronic structure and thin nanosheet-like surface morphology of the heterostructure. The thin, wide, and modified surface of the catalyst facilitates the diffusion of ions (reactants) and gas molecules (products) at the electrode/electrolyte interface. Furthermore, electron transfer from n-type CoCr-LDH to p-type NiO results in enhanced electronic conductivity. This study demonstates the effective design of a self-supported heterostructure with minimal synthetic steps to generate a bifunctional electrocatalyst for water splitting, contributing to the greater cause of green hydrogen economy.

An inclusive review and perspective on Cu-based materials for electrochemical water splitting

In recent years, there has been a resurgence of interest in developing green and renewable alternate energy sources as a solution to the energy and environmental problems produced by conventional fossil fuel use. As a very effective energy transporter, hydrogen (H₂) is a possible candidate for the future energy supply. Hydrogen production by water splitting is a promising new energy option. Strong, efficient, and abundant catalysts are required for increasing the efficiency of the water splitting process. Cu-based materials as an electrocatalyst have shown promising results for application in the Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER) in water splitting. In this review, our aim is to cover the latest developments in the synthesis, characterisation, and electrochemical behaviour of Cu-based materials as a HER, and OER electrocatalyst, highlighting the impact that these advances have had on the field. It is intended that this review article will serve as a roadmap for developing novel, cost-effective electrocatalysts for electrochemical water splitting based on nanostructured materials with particular emphasis on Cu-based materials for electrocatalytic water splitting.

In Situ Reconstruction NiO Octahedral Active Sites for Promoting Electrocatalytic Oxygen Evolution of Nickel Phosphate

Electrochemical activation strategy is very effective to improve the intrinsic catalytic activity of metal phosphate toward the sluggish oxygen evolution reaction (OER) for water electrolysis. However, it is still challenging to operando trace the activated reconstruction and corresponding electrocatalytic dynamic mechanisms. Herein, a constant voltage activation strategy is adopted to in situ activate Ni₂ P₄ O₁₂ , in which the break of NiONi bond and dissolution of PO₄ ^3- groups could optimize the lattice oxygen, thus reconstructing an irreversible amorphous Ni(OH)₂ layer with a thickness of 1.5-3.5 nm on the surface of Ni₂ P₄ O₁₂ . The heterostructure electrocatalyst can afford an excellent OER activity in alkaline media with an overpotential of 216.5 mV at 27.0 mA cm^-2 . Operando X-ray absorption fine structure spectroscopy analysis and density functional theory simulations indicate that the heterostructure follows a nonconcerted proton-electron transfer mechanism for OER. This activation strategy demonstrates universality and can be used to the surface reconstruction of other metal phosphates.

Mesoporous LDH Metastructure from Multiscale Assembly of Defective Nanodomains by Laser Shock for Oxygen Evolution Reaction

Laser is a powerful tool for the synthesis of nanomaterials. The intensive laser pulses delivered to materials within nanoseconds allow the formation of novel structures that are inaccessible for conventional methods. Layered double hydroxide (LDH) nanostructures with high porosity, suitable dopants, and rich defects are desirable for catalysts, however, tremendously difficult in a one-pot synthesis. Here it is found that confined laser shock in solvent leads to the formation of nanoreactors which guide the assembly of multiscale LDH building units, larger nanosheets as frame and smaller nanodomains as building blocks. These nanodomains have rich vacancy defects and are interlocked in a high packed density of 10¹³  cm^-2 , leaving rich mesopores across the nanosheets and coral-like morphology. Like the natural coral reef that has multiscale structure to accommodate different marine organisms, the coral-like LDH metastructure provides large surface area and rich active sites for the interaction with guest molecules. Benefiting from the multiscale porous structure and rational dopant, this LDH catalyst exhibits a low overpotential of 220 mV at 10 mA cm^-2 for oxygen evolution reaction (OER), standing as one of the best LDH catalysts to date.

Tuning the Electronic Structure of a Ni-Vacancy-Enriched AuNi Spherical Nanoalloy via Electrochemical Etching for Water Oxidation Studies in Alkaline and Neutral Media

Internal Ni-vacancy-enriched spherical AuNi nanoalloys (AuNi_1-2-T) have been prepared via a noble electrochemical etching method. AuNi_1.5-T showed the highest oxygen evolution reaction (OER) activity compared to bare AuNi_1.5, and it demands only 239 mV overpotential, which was 134 mV lesser than the overpotential required by commercial RuO₂ at 10 mA cm^-2 current density in a 1 M KOH solution (pH = 14). The calculated turnover frequency (TOF) value for AuNi_1.5-T (0.0229 s^-1) was 11.74 times higher than that of AuNi_1.5 (0.00195 s^-1). Also, the electrochemically activated AuNi_1.5-T showed superior neutral water oxidation activity by demanding only 335 mV overpotential with a TOF value of 0.000135 s^-1 in a 1 M Na₂SO₄ solution (pH = 7) at 10 mA cm^-2. The long-term stability studies (over 60 h) reveal the excellent robustness of an electrochemically treated alloy system. Density functional theory based electronic structure calculations showed that in the case of AuNi and AuNi_1.5, Au d, Au s, and Ni d orbitals have significant contributions, whereas in the Ni-vacant systems, the density of states is mainly governed by d orbitals of Au and Ni. Also, the Ni-vacant system possesses a work function value of 4.96 eV, which is lower than that of the pristine system (5.27 eV) and thereby favored OH^- binding with an optimum adsorption energy. This result is in reasonable agreement with the experimental outcome of an accelerated OER in a vacancy-enriched Ni-rich AuNi alloy system. Also, mechanistic analysis reveals that the creation of a Ni vacancy can effectively alter the overall mechanism of the OER and thereby facilitate the same with a lower applied energy.

Magnesium-regulated oxygen vacancies of cobalt-nickel layered double hydroxide nanosheets for ultrahigh performance asymmetric supercapacitors

Rational design of layered double hydroxide (LDH) electrodes is of great significance for high-performance supercapacitors (SCs). Herein, ultrathin cobalt-nickel-magnesium layered double hydroxide (CoNiMg-LDH) nanosheets with plentiful oxygen vacancies are synthesized via sacrificial magnesium-based replacement reaction at room temperature. Self-doping and mild reduction of magnesium can significantly increase the concentration of oxygen vacancies in CoNiMg-LDH, which promotes the electrochemical charge transfer efficiency and enhances the adsorption ability of electrolytes. Density functional theory (DFT) calculations also indicate that Mg²⁺ doping can decrease the formation energy of oxygen vacancies in CoNiMg-LDH nanosheets, which increases the concentration of oxygen vacancies. Thus, the assembled asymmetric supercapacitor CoNiMg-LDH//Actived Carbon accomplishes a superior capacity of ∼ 333 C g^-1 (208 F g^-1) at 1 A g^-1 and presents a gravimetric energy density of 73.9 Wh kg^-1 at 0.8 kW kg^-1. It presents only 13% capacity loss at 20 A g^-1 after 5000 cycles. This discovery emphasizes the positive role of magnesium in regulating oxygen vacancies to improve the performance of supercapacitors, which should be beneficial for extending the scope of superior SCs active materials.

Fundamental understanding of electrocatalysis over layered double hydroxides from the aspects of crystal and electronic structures

Layered double hydroxides (LDHs) composed of octahedral ligand units centered with various transition metal atoms display unique electronic structures and thus attract significant attention in the field of electrocatalytic oxygen evolution reactions (OER). Intensive experimental explorations have therefore been carried out to investigate the LDHs synthesis, amorphous control, intrinsic material modifications, interfacing with other phases, strain, etc. There is still the need for a fundamental understanding of the structure-property relations, which could hinder the design of the next generation of the LDHs catalysts. In this review, we firstly provide the crystal structure information accompanied by the corresponding electronic structures. Then, we discuss the conflicts of the active sites on the NiFe LDHs and propose the synergistic cooperation among the ligand units during OER to deliver a different angle for understanding the current structure-property relations beyond the single-site-based catalysis process. In the next section of the OER process, the linear relationship-induced theoretical limit of the overpotential is further discussed based on the fundamental aspects. To break up the linear relations, we have summarized the current strategies for optimizing the OER performance. Lastly, based on the understanding gained above, the perspective of the research challenges and opportunities are proposed.

Recent Progress on Transition Metal Based Layered Double Hydroxides Tailored for Oxygen Electrode Reactions

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), namely, so-called oxygen electrode reactions, are two fundamental half-cell reactions in the energy storage and conversion devices, e.g., zinc–air batteries and fuel cells. However, the oxygen electrode reactions suffer from sluggish kinetics, large overpotential and complicated reaction paths, and thus require efficient and stable electrocatalysts. Transition-metal-based layered double hydroxides (LDHs) and their derivatives have displayed excellent catalytic performance, suggesting a major contribution to accelerate electrochemical reactions. The rational regulation of electronic structure, defects, and coordination environment of active sites via various functionalized strategies, including tuning the chemical composition, structural architecture, and topotactic transformation process of LDHs precursors, has a great influence on the resulting electrocatalytic behavior. In addition, an in-depth understanding of the structural performance and chemical-composition-performance relationships of LDHs-based electrocatalysts can promote further rational design and optimization of high-performance electrocatalysts. Finally, prospects for the design of efficient and stable LDHs-based materials, for mass-production and large-scale application in practice, are discussed.

Solid-state synthesis of single-phase nickel monophosphosulfide for the oxygen evolution reaction

High-performance and cost-effective nonprecious-metal catalysts are essential for the next-generation oxygen evolution reaction (OER). However, the electrocatalysis of the OER during water splitting is often carried out by using noble metal catalysts, such as RuO₂ or IrO₂ with high-cost and limited stability. Herein, we reported a successful synthesis of a ternary nickel monophosphosulfide (NiPS) compound via a simple solid-state route and further investigated its electrocatalytic performances for water oxidation. It is found that the NiPS electrocatalyst exhibits good OER performance in 1.0 M KOH solution, i.e., achieving a current density of 20 mA cm^-2 at an overpotential of 400 mV and a Tafel slope of 126 mV dec^-1, comparable to commercial benchmark RuO₂. The ternary NiPS electrocatalyst for the OER is superior to its binary counterparts, i.e., Ni₂P and NiS. Density functional theory (DFT) calculations combined with ex situ XPS were performed to obtain further insights into the intrinsic catalytic mechanism of NiPS, and their results clearly revealed that the instability of the NiO intermediate during the OH* → O* process and the easy oxidation of the (PS)^3- anion favoring the formation of hydroxyl-based species (i.e., Ni(OH)₂/NiOOH) on the surface of the catalyst, which plays a crucial role in facilitating the OER activity. Furthermore, we creatively extended this method to the fabrication of heteroatom substituted catalysts and a new quaternary CoNiP₂S₂ compound was successfully synthesized for the first time in the same way. The structural properties and electrocatalytic performance towards the OER for CoNiP₂S₂ (e.g., 20 mA cm^-2 at 376 mV) are also systematically investigated in this work.

Advanced Transition Metal-Based OER Electrocatalysts: Current Status, Opportunities, and Challenges

Oxygen evolution reaction (OER) is an important half-reaction involved in many electrochemical applications, such as water splitting and rechargeable metal-air batteries. However, the sluggish kinetics of its four-electron transfer process becomes a bottleneck to the performance enhancement. Thus, rational design of electrocatalysts for OER based on thorough understanding of mechanisms and structure-activity relationship is of vital significance. This review begins with the introduction of OER mechanisms which include conventional adsorbate evolution mechanism and lattice-oxygen-mediated mechanism. The reaction pathways and related intermediates are discussed in detail, and several descriptors which greatly assist in catalyst screen and optimization are summarized. Some important parameters suggested as measurement criteria for OER are also mentioned and discussed. Then, recent developments and breakthroughs in experimental achievements on transition metal-based OER electrocatalysts are reviewed to reveal the novel design principles. Finally, some perspectives and future directions are proposed for further catalytic performance enhancement and deeper understanding of catalyst design. It is believed that iterative improvements based on the understanding of mechanisms and fundamental design principles are essential to realize the applications of efficient transition metal-based OER electrocatalysts for electrochemical energy storage and conversion technologies.

Conductive Polymer Intercalation Tunes Charge Transfer and Sorption-Desorption Properties of LDH Enabling Efficient Alkaline Water Oxidation

Controlling and tuning surface properties of a catalyst have always been a prime challenge for efficient hydrogen production via water splitting. Here, we report a facile method for tuning both charger transfer and sorption-desorption properties of NiFe layered double hydroxide (LDH) by intercalating a conductive polymer of polypyrrole (ppy) via an interlayer confined polymerization synthesis (ICPS) process. Ex situ characterizations and in situ electrochemical quartz-crystal microbalance with dissipation (EQCM-D) tracking experiments showed that the intercalated ppy not only improved the charge transfer property of the resulting hybrid catalyst LDH-ppy but also made it more flexible and adaptive for quick and reversible sorption-desorption of reactants and intermediates during the oxygen evolution reaction (OER) process. Consequently, the as-prepared LDH-ppy exhibited a doubled catalytic current density over the bare LDH, as visualized by in situ scanning electrochemical microscopy (SECM) at the subnanometer scale. This work sheds light on orchestrating the charge and sorbate transfer abilities of catalysts for efficient water splitting by smartly combining inorganic and organic layers.

Fast microwave-assisted preparation of nickel-copper-chromium-layered double hydroxide as an excellent electrocatalyst for water oxidation

Herein, we describe a simple microwave method for the doping of Cu2+ into NiCr-LDH and the preparation of ternary Ni2.25Cu0.75Cr-LDH as a superior electrocatalyst for water oxidation in a neutral solution. The obtained Ni2.25Cu0.75Cr-LDH was characterized by XRD, DRS, TEM and FE-SEM techniques. The results showed that Ni2.25Cu0.75Cr-LDH was formed with a size of 30 nm. In order to examine the water oxidation activity of Ni2.25Cu0.75Cr-LDH, the as-prepared samples were used as an electrocatalyst-modified carbon paste electrode in neutral solution. The electrochemical results revealed that the optimized Ni2.25Cu0.75Cr-LDH presented extraordinary water oxidation activity with a low onset potential of 1.40 V (vs. RHE) and an overpotential of 170 mV compared to other molar ratios (Ni2.5Cu0. 5Cr-LDH), (Ni2CuCr-LDH), and bimetallic (CuCr-LDH), and even outperformed NiCr-LDH. Also, a small Tafel slope of 31 mV and high durability of 14 h could be obtained for Ni2.25Cu0.75Cr-LDH. The excellent OER could be assigned to the decreased band gap energy and increased charge transfer at Ni2.25Cu0.75Cr-LDH. Therefore, Ni2.25Cu0.75Cr-LDH is a promising water oxidation catalyst owing to its improved charge transfer ability.

Hierarchical CoP Nanostructures on Nickel Foam as Efficient Bifunctional Catalysts for Water Splitting

A highly active and cheap catalyst is also key to hydrogen production by water splitting. However, most of the high-efficiency catalysts reported to date only are catalytically active for either the hydrogen evolution reaction (HER) or the oxygen evolution reaction (OER), which makes the development of multifunctional catalysts more meaningful. Here, for the first time, Co(CO₃ )_0.5 OH^. 0.11 H₂ O (CHCH) as precursor with different microstructures on the surface of nickel foam (NF) was obtained using a facile hydrothermal method. The CoP/NF catalyst was obtained after thermal phosphating that retained the microhierarchical structure of the precursor and greatly improved the catalytic performance, with a highly efficiency performance as HER and OER dual-functional catalyst. Density functional theory (DFT) calculations showed that the possible reason for the excellent performance of the CoP/NF layered structure is an increase in the number of of surface defects and an increased active surface area. The results reported in this paper show that CoP/NF, a layered bifunctional electrocatalyst, is a cost-effective and efficient water-splitting electrode. This finding can offer the opportunity for the commercial use of excess electric energy for large-scale water splitting hydrogen production.

Investigation on nanostructured Cu-based electrocatalysts for improvising water splitting: a review

In this review, various forms of Cu based nanostructures have been explored in terms of improvise and enhancing their activity and durability with vast investigation for OER, HER and TWS applications.

Enhanced Electrochemical Properties and OER Performances by Cu Substitution in NiCo2O4 Spinel Structure

In order to improve the electrochemical performance of the NiCo2O4 material, Ni ions were partially substituted with Cu2+ ions having excellent reducing ability. All of the electrodes were fabricated by growing the Ni1-xCuxCo2O4 electrode spinel-structural active materials onto the graphite felt (GF). Five types of electrodes, NiCo2O4/GF, Ni0.875Cu0.125Co2O4/GF, Ni0.75Cu0.25Co2O4/GF, Ni0.625Cu0.375Co2O4/GF, and Ni0.5Cu0.5Co2O4/GF, were prepared for application to the oxygen evolution reaction (OER). As Cu2+ ions were substituted, the electrochemical performances of the NiCo2O4-based structures were improved, and eventually the OER activities were also greatly increased. In particular, the Ni0.75Cu0.25Co2O4/GF electrode exhibited the best OER activity in a 1.0 M KOH alkaline electrolyte: the cell voltage required to reach a current density of 10 mA cm-2 was only 1.74 V (η = 509 mV), and a low Tafel slope of 119 mV dec-1 was obtained. X-ray photoelectron spectroscopy (XPS) analysis of Ni1-xCuxCo2O4/GF before and after OER revealed that oxygen vacancies are formed around active metals by the insertion of Cu ions, which act as OH-adsorption sites, resulting in high OER activity. Additionally, the stability of the Ni0.75Cu0.25Co2O4/GF electrode was demonstrated through 1000th repeated OER acceleration stability tests with a high faradaic efficiency of 94.3%.

The Principle of Introducing Halogen Ions Into β-FeOOH: Controlling Electronic Structure and Electrochemical Performance

Coordination tuning electronic structure of host materials is a quite effective strategy for activating and improving the intrinsic properties. Herein, halogen anion (X^-)-incorporated β-FeOOH (β-FeOOH(X), X = F^-, Cl^-, and Br^-) was investigated with a spontaneous adsorption process, which realized a great improvement of supercapacitor performances by adjusting the coordination geometry. Experiments coupled with theoretical calculations demonstrated that the change of Fe-O bond length and structural distortion of β-FeOOH, which is rooted in halogen ions embedment, led to the relatively narrow band gap. Because of the strong electronegativity of X^-, the Fe element in β-FeOOH(X)s presented the unexpected high valence state (3 + δ), which is facilitating to adsorb SO₃^2- species. Consequently, the designed β-FeOOH(X)s exhibited the good electric conductivity and enhanced the contact between electrode and electrolyte. When used as a negative electrode, the β-FeOOH(F) showed the excellent specific capacity of 391.9 F g^-1 at 1 A g^-1 current density, almost tenfold improvement compared with initial β-FeOOH, with the superior rate capacity and cyclic stability. This combinational design principle of electronic structure and electrochemical performances provides a promising way to develop advanced electrode materials for supercapacitor.

Recent advances in transition metal based compound catalysts for water splitting from the perspective of crystal engineering

A review of the recent progress on the transition metal based catalysts for water splitting with emphasis on crystal engineering.