A 2D bimetallic Ni-Co hydroxide monolayer cocatalyst for boosting photocatalytic H2 evolution

Here, we report two-dimensional (2D) Ni-Co hydroxide monolayers (NiCo-HMs) as a highly active cocatalyst for enhancing the photocatalytic H₂ evolution of 2D g-C₃N₄ nanosheets. The NiCo-HMs can expand the interface of electron migration, promote the separation of photogenerated carriers, provide synergistic Co-Ni active sites, and optimize atomic hydrogen adsorption to improve the kinetics of the catalytic reaction. The resulting 2D/2D NiCo-HMs/g-C₃N₄ heterojunction displays a high photocatalytic H₂ evolution of 3.58 mmol g^-1 h^-1.

Co-Ni Layered Double Hydroxide for the Electrocatalytic Oxidation of Organic Molecules: An Approach to Lowering the Overall Cell Voltage for the Water Splitting Process

Electrocatalytic oxidation of simple organic molecules offers a promising strategy to combat the sluggish kinetics of the water oxidation reaction (WOR). The low potential requirement, inhibition of the crossover of gases, and formation of value-added products at the anode are benefits of the electrocatalytic oxidation of organic molecules. Herein, we developed cobalt-nickel-based layered double hydroxide (LDH) as a robust material for the electrocatalytic oxidation of alcohols and urea at the anode, replacing the WOR. A facile synthesis protocol to form LDHs with different ratios of Co and Ni is adapted. It demonstrates that the reactants could be efficiently oxidized to concomitant chemical products at the anode. The half-cell study shows an onset potential of 1.30 V for benzyl alcohol oxidation reaction (BAOR), 1.36 V for glycerol oxidation reaction (GOR), 1.33 V for ethanol oxidation reaction (EOR), and 1.32 V for urea oxidation reaction (UOR) compared with 1.53 V for WOR. Notably, the hybrid electrolyzer in a full-cell configuration significantly reduces the overall cell voltage at a 20 mA cm^-2 current density by ∼15% while coupling with the BAOR, EOR, and GOR and ∼12% with the UOR as the anodic half-cell reaction. Furthermore, the efficiency of hydrogen generation remains unhampered with the types of oxidation reactions (alcohols and urea) occurring at the anode. This work demonstrates the prospects of lowering the overall cell voltage in the case of a water electrolyzer by integrating the hydrogen evolution reaction with suitable organic molecule oxidation.

Influence of Co2+, Cu2+, Ni2+, Zn2+, and Ga3+ on the iron-based trimetallic layered double hydroxides for water oxidation

In this work, we synthesized five novel iron-based trimetallic layered double hydroxides (LDHs) by the urea-assisted co-precipitation method for the electrocatalytic water oxidation reaction (WOR).

Strategies to Develop Earth-Abundant Heterogeneous Oxygen Evolution Reaction Catalysts for pH-Neutral or pH-Near-Neutral Electrolytes

The anodic oxygen evolution reaction (OER) is the bottleneck of water splitting to produce hydrogen due to its sluggish kinetics. In order to lower the energy cost, highly active and cost-efficient OER catalysts need to be used to overcome the OER reaction barrier, especially in neutral pH. Compared to alkaline or acidic electrolytes, pH-neutral or pH-near-neutral electrolytes are considered to be cheaper and safer, and water from rivers and the sea could be used directly under such conditions. However, OER under neutral pH is challenging compared to the OER catalysts for alkaline conditions. Therefore, OER catalysts for neutral or near-neutral pH have not been pursued significantly and, hence, there are limited advances in this area. Here, the progress made in the research and development of earth-abundant heterogeneous catalysts for OER in three pH-neutral or pH-near-neutral systems, namely, the phosphate system, the carbonate system, and the borate system, are systematically reviewed and summarized for the first time. Strategies to develop high-performance OER catalysts for neutral pH are reviewed and summarized. In addition, future challenges and opportunities in this field are discussed, which may shed some light on the future developments of earth-abundant heterogeneous catalysts for OER in neutral or near-neutral pH.

Ultrasonication construction of the nano-petal NiCoFe-layered double hydroxide: An excellent water oxidation electrocatalyst

Unlike other preparation methods of NiCoFe-layered double hydroxides, the present study provides a facile ultrasound method for synthesis of the nano-petal NiCoFe-layered double hydroxide (LDH) prepared under intensification frequency of 40 kHz and ultrasonic power of 305 W. The effect of time reaction on the morphology of NiCoFe-LDH was investigated using the Field Emission-Scanning Electron Microscopy images. The results show that time reaction can affect the morphology and it also showed that the optimal time for synthesis of nano-petal NiCoFe-LDH was 60 min. Then, the effect of nano-petal NiCoFe-LDH on oxygen evaluation reaction activity was studied and compared with NiCoFe-LDH-c nano paricles. Also, in order to study the effect of Co²⁺ of nano-petal NiCoFe-LDH at water oxidation, the activity of NiFe-LDH synthesized in the same conditions was investigated. The results show that nano-petal NiCoFe-LDH has low onset potential (0.46 V vs. SCE), overpotential (~227 mV) and Tafel slope (234 mV per decade) in comparison with other NiCoFe-LDH nanoparticles (synthesis using co-precipitation method and ultrasonication method within 30 and 120 min), and NiFe-LDH. Based on the obtained results, the nano-petal NiCoFe-LDH can be as a suitable electrocatalyst with good stability for water oxidation reaction in the present 0.1 M KOH media.

Effective removal of bisphenols from aqueous solution with magnetic hierarchical rattle-like Co/Ni-based LDH

A novel unique adsorbent (Fe₃O₄@Co/Ni-LDH) has been successfully synthesized and applied for removal of bisphenols (BPs) from aqueous solution. The prepared adsorbent was characterized to appear in a hierarchical rattle-like structure, and possesses high specific surface area, abundant pore system, and magnetic properties. Adsorption kinetics fitted well with the pseudo-second-order model. Adsorption isotherms abide by the Langmuir model, and the maximum adsorption capacity for bisphenol A (BPA), F (BPF), AF (BPAF) and S (BPS) on Fe₃O₄@Co/Ni-LDH at pH of 7.0 were 238.96, 177.09, 320.56 and 345.84 mg/g, respectively. Moreover, it was found that the high pH and NaCl concentration were not conducive to the removal of BPs. The humic acid and real waters had no significant effects on the removal of BPs on Fe₃O₄@Co/Ni-LDH. Furthermore, the FT-IR spectra indicated that the removal of four BPs were primarily Hydrogen bond interaction between BPs and Fe₃O₄@Co/Ni-LDH. The Fe₃O₄@Co/Ni-LDH was regenerated effectively by methanol and can be repeatedly used. This novel Fe₃O₄@Co/Ni-LDH can be applied as a promising adsorbent for removal of BPs from aqueous matrices.

Ceria/cobalt borate hybrids as efficient electrocatalysts for water oxidation under neutral conditions

Oxygen evolution reaction (OER) catalysts are of importance for electrochemical water splitting and fuel generation. Despite enormous efforts, the design and development of OER catalysts with high catalytic activities under neutral conditions are highly desired but still remain a great challenge. Herein, we report a room temperature chemical route to prepare xceria/cobalt borate (xCeO2/Co-Bi) hybrids as efficient OER catalysts by tuning the molar ratio of Ce/Co (x represents the amount of CeO2). The optimised catalyst (20CeO2/Co-Bi hybrid) was found to exhibit remarkable OER catalytic activity with an overpotential of 453 mV at a current density of 10 mA cm-2, Tafel slope of 120 mV dec-1 and long-term stability in neutral medium due to its good conductivity, mass transportation and strong synergetic coupling effects, showing the potential of Co-based electrochemical materials for practical application in energy storage devices.

Synthesis of ultrasmall and monodisperse sulfur nanoparticle intercalated CoAl layered double hydroxide and its electro-catalytic water oxidation reaction at neutral pH

Ultrasmall and monodisperse sulfur nanoparticle (S-NP) intercalated CoAl-layered double hydroxide (CoAl-LDH) electrocatalyst exhibits an excellent electrocatalytic activity towards water oxidation with a low overpotential of 250 mV at a high current density of 7.9 mA cm-2 and a Tafel slope of 61 mV dec-1 at the neutral pH condition. The fabrication strategy to achieve a high-performance, robust and durable electrocatalyst is a scale-up in next generation renewable energy fields.

Recent Advances in the Development of Water Oxidation Electrocatalysts at Mild pH

Developing anodic oxygen evolution reaction (OER) electrocatalysts with high catalytic activities is of great importance for effective water splitting. Compared with the water-oxidation electrocatalysts that are commonly utilized in alkaline conditions, the ones operating efficiently under neutral or near neutral conditions are more environmentally friendly with less corrosion issues. This review starts with a brief introduction of OER, the importance of OER in mild-pH media, as well as the fundamentals and performance parameters of OER electrocatalysts. Then, recent progress of the rational design of electrocatalysts for OER in mild-pH conditions is discussed. The chemical structures or components, synthetic approaches, and catalytic performances of the OER catalysts will be reviewed. Some interesting insights into the catalytic mechanism are also included and discussed. It concludes with a brief outlook on the possible remaining challenges and future trends of neutral or near-neutral OER electrocatalysts. It hopefully provides the readers with a distinct perspective of the history, present, and future of OER electrocatalysts at mild conditions.

Atomistic Investigation of Doping Effects on Electrocatalytic Properties of Cobalt Oxides for Water Oxidation

The development of high-performance oxygen evolution reaction (OER) catalysts is crucial to achieve the clean production of hydrogen via water splitting. Recently, Co-based oxides have been intensively investigated as some of the most efficient and cost-effective OER catalysts. In particular, compositional tuning of Co-based oxides via doping or substitution is shown to significantly affect their catalytic activity. Nevertheless, the origin of this enhanced catalytic activity and the reaction mechanism occurring at catalytic active sites remain controversial. Theoretical investigations are performed on the electrocatalytic properties of pristine and transition metal (Fe, Ni, and Mn)-substituted Co oxides using first-principle calculations. A comprehensive evaluation of the doping effects is conducted by considering various oxygen local environments in the crystal structure, which helps elucidate the mechanism behind the doping-induced enhancement of Co-based catalysts. It is demonstrated that the local distortion induced by dopant cations remarkably facilitates the catalysis at a specific site by modulating the hydrogen bonding. In particular, the presence of Jahn-Teller-active Fe(IV) is shown to result in a substantial reduction in the overpotential at the initially inactive catalysis site without compromising the activity of the pristine active sites, supporting previous experimental observations of exceptional OER performance for Fe-containing Co oxides.

Construction of Bimetallic ZIF-Derived Co-Ni LDHs on the Surfaces of GO or CNTs with a Recyclable Method: Toward Reduced Toxicity of Gaseous Thermal Decomposition Products of Unsaturated Polyester Resin

This work proposed an idea of recycling in preparing Co-Ni layered double hydroxide (LDH)-derived flame retardants. A novel and feasible method was developed to synthesize CO-Ni LDH-decorated graphene oxide (GO) and carbon nanotubes (CNTs), by sacrificing bimetal zeolitic imidazolate frameworks (ZIFs). Organic ligands that departed from ZIFs were recyclable and can be reused to synthesize ZIFs. ZIFs, as transitional objects, in situ synthesized on the surfaces of GO or CNTs directly suppressed the re-stacking of the carbides and facilitated the preparation of GO@LDHs and CNTs@LDHs. As-prepared hybrids catalytically reduced toxic CO yield during the thermal decomposition of unsaturated polyester resin (UPR). What is more, the release behaviors of aromatic compounds were also suppressed during the pyrolysis process of UPR composites. The addition of GO@LDHs and CNTs@LDHs obviously inhibited the heat release and smoke emission behaviors of the UPR matrix during combustion. Mechanical properties of the UPR matrix also improved by inclusion of the carbides derivatives. This work paved a feasible method to prepare well-dispersed carbides@Co-Ni LDH nanocomposites with a more environmentally friendly method.

Ultrasonication-assisted synthesis of ternary-component Ni3AlxFe1−x-layered double hydroxide nanoparticles for the oxygen evolution reaction in a neutral solution

In this study, ultrasonication, a facile and rapid process, was utilized for the preparation of ternary-component layered double hydroxides (LDHs), Ni₃Al_xFe_1−x-LDHs, as the electrocatalyst material for the oxygen evolution reaction (OER) in a neutral solution.

Environmentally-friendly and ultrasonic-assisted preparation of two-dimensional ultrathin Ni/Co-NO3 layered double hydroxide nanosheet for micro solid-phase extraction of phenolic acids from fruit juices

In this paper, we report an environmentally-friendly and low cost synthetic approach for large-scale fabrication of 2-dimentional porous Ni/Co-NO₃-based layered double hydroxide (Ni/Co-NO₃-LDH) nanosheet through ultrasonic-assisted process. The synthesis procedure used ethylene glycol/water system as an eco-friendly solvent system. The synthesized LDH was characterized by FE-SEM, TEM, XRD, and FT-IR techniques. FE-SEM and TEM images showed porous structure surface morphology of the synthesized LDH. Also, For Ni/Co-NO₃-LDH, a hexagonal ultrathin layered was obtained owing to ultrasonic irradiation and applied processing conditions. The prepared LDH was used as sorbent in dispersive micro solid-phase extraction procedure. Two phenolic acids including p-hydroxybenzoic acid and p-coumaric acid were selected as model compounds. Some experimental factors affecting the extraction efficiency of the analytes were investigated and optimized. Finally, the sorbent was used for the extraction of model compound from fruit juice samples followed by high performance liquid chromatography. Linear dynamic range of 0.5-500µgL^-1 with a low detection limit (0.1µgL^-1) was obtained by the method. The relative standard deviations were 2.5 and 4.3% for p-hydroxybenzoic acid and p-coumaric acid, respectively. All recoveries were between 82 and 92%.

In situ electrochemical surface derivation of cobalt phosphate from a Co(CO3)0.5(OH)·0.11H2O nanoarray for efficient water oxidation in neutral aqueous solution

In this Communication, cobalt phosphate (Co-Pi) has been successfully developed on a Co(CO₃)_0.5(OH)·0.11H₂O nanoarray (CCH NA) on Ti mesh in neutral phosphate-buffered solution (PBS) via in situ electrochemical surface derivation. The resulting core@shell structured CCH@Co-Pi NA/Ti exhibits remarkable activity toward water oxidation with the need of an overpotential of 460 mV to achieve a geometrical catalytic current density of 10 mA cm^-2 in 0.1 M PBS, with a high turnover frequency of and long-term electrochemical stability.

Transition-Metal (Co, Ni, and Fe)-Based Electrocatalysts for the Water Oxidation Reaction

Increasing energy demands and environment awareness have promoted extensive research on the development of alternative energy conversion and storage technologies with high efficiency and environmental friendliness. Among them, water splitting is very appealing, and is receiving more and more attention. The critical challenge of this renewable-energy technology is to expedite the oxygen evolution reaction (OER) because of its slow kinetics and large overpotential. Therefore, developing efficient electrocatalysts with high catalytic activities is of great importance for high-performance water splitting. In the past few years, much effort has been devoted to the development of alternative OER electrocatalysts based on transition-metal elements that are low-cost, highly efficient, and have excellent stability. Here, recent progress on the design, synthesis, and application of OER electrocatalysts based on transition-metal elements, including Co, Ni, and Fe, is summarized, and some invigorating perspectives on the future developments are provided.

Self-Assembly of Single-Layer CoAl-Layered Double Hydroxide Nanosheets on 3D Graphene Network Used as Highly Efficient Electrocatalyst for Oxygen Evolution Reaction

A non-noble metal based 3D porous electrocatalyst is prepared by self-assembly of the liquid-exfoliated single-layer CoAl-layered double hydroxide nanosheets (CoAl-NSs) onto 3D graphene network, which exhibits higher catalytic activity and better stability for electrochemical oxygen evolution reaction compared to the commercial IrO2 nanoparticle-based 3D porous electrocatalyst.

Self-supported nanoporous NiCo2O4 nanowires with cobalt-nickel layered oxide nanosheets for overall water splitting

Water splitting via the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in producing H2 and O2 is a very important process in the energy field. Developing an efficient catalyst which can be applied to both HER and OER is crucial. Here, a bifunctional catalyst, CFP/NiCo2O4/Co0.57Ni0.43LMOs, has been successfully fabricated. It exhibits remarkable performance for OER in 0.1 M KOH producing a current density of 10 mA cm(-2) at an overpotential of 0.34 V (1.57 V vs. RHE), better than that of the commercial Ir/C (20%) catalyst. Simultaneously, it also exhibits good catalytic performance for HER in 0.5 M H2SO4 producing a current density of 10 mA cm(-2) at an overpotential of 52 mV and a Tafel slope of 34 mV dec(-1), approaching that of the commercial Pt/C (20%) nanocatalyst. Particularly, CFP/NiCo2O4/Co0.57Ni0.43LMOs present better durability under harsh OER and HER cycling conditions than commercial Ir/C and Pt/C. Furthermore, an H-type electrolyzer was fabricated by applying CFP/NiCo2O4/Co0.57Ni0.43LMOs as the cathode and anode electrocatalyst, which can be driven by a single-cell battery. This bifunctional catalyst will be very promising in overall water splitting.

Co3(OH)2(HPO4)2 as a novel photocatalyst for O2 evolution under visible-light irradiation

Co₃(OH)₂(HPO₄)₂ was proved to be a novel visible-light-driven photocatalyst for O₂ evolution due to the unique characteristics of Co²⁺ octahedra.

Iron and nickel co-doped cobalt hydroxide nanosheets with enhanced activity for oxygen evolution reaction

Herein, we have successfully synthesized iron and nickel co-doped cobalt hydroxide (Co–Ni–Fe₅₁₁) nanosheets for the OER.

Fast electrosynthesis of Fe-containing layered double hydroxide arrays toward highly efficient electrocatalytic oxidation reactions

A new electrochemical synthesis route was developed for the fabrication of Fe-containing layered double hydroxide (MFe-LDHs, M = Ni, Co and Li) hierarchical nanoarrays, which exhibit highly-efficient electrocatalytic performances for the oxidation reactions of several small molecules (water, hydrazine, methanol and ethanol). Ultrathin MFe-LDH nanoplatelets (200-300 nm in lateral length; 8-12 nm in thickness) perpendicular to the substrate surface are directly prepared within hundreds of seconds (<300 s) under cathodic potential. The as-obtained NiFe-LDH nanoplatelet arrays display promising behavior in the oxygen evolution reaction (OER), giving rise to a rather low overpotential (0.224 V) at 10.0 mA cm-2 with largely enhanced stability, much superior to previously reported electro-oxidation catalysts as well as the state-of-the-art Ir/C catalyst. Furthermore, the MFe-LDH nanoplatelet arrays can also efficiently catalyze several other fuel molecules' oxidation (e.g., hydrazine, methanol and ethanol), delivering a satisfactory electrocatalytic activity and a high operation stability. In particular, this preparation method of Fe-containing LDHs is amenable to fast, effective and large-scale production, and shows promising applications in water splitting, fuel cells and other clean energy devices.

An Iron-based Film for Highly Efficient Electrocatalytic Oxygen Evolution from Neutral Aqueous Solution

An ultrathin Fe-based film was prepared by electrodeposition from an Fe(II) solution through a fast and simple cyclic voltammetry method. The extremely low Fe loading of 12.3 nmol cm(-2) on indium tin oxide electrodes is crucial for high atom efficiency and transparence of the resulted film. This Fe-based film was shown to be a very efficient electrocatalyst for oxygen evolution from neutral aqueous solution with remarkable activity and stability. In a 34 h controlled potential electrolysis at 1.45 V (vs NHE) and pH 7.0, impressive turnover number of 5.2 × 10(4) and turnover frequency of 1528 h(-1) were obtained. To the best of our knowledge, these values represent one of the highest among electrodeposited catalyst films for water oxidation under comparable conditions. The morphology and the composition of the catalyst film was determined by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray, and X-ray photoelectron spectroscopy, which all confirmed the deposition of Fe-based materials with Fe(III) oxidation state on the electrode. This study is significant because of the use of iron, the fast and simple cyclic voltammetry electrodeposition, the extremely low catalyst loading and thus the transparency of the catalyst film, the remarkable activity and stability, and the oxygen evolution in neutral aqueous media.

A high surface area flower-like Ni-Fe layered double hydroxide for electrocatalytic water oxidation reaction

Layered double hydroxide has been used in a variety of areas, including but not limited to catalysis, energy storage, drug or gene delivery, water treatment, etc. Herein, we report a new simple hydrothermal method to prepare a high surface area flower-like Ni-Fe layered double hydroxide (LDH) assembled by nanosheets by using nickel alkoxide and FeSO4 as the only starting materials. It is free of alkaline solution and other additives for directing or supporting in the synthesis procedure. The formation mechanism of this flower-like LDH formed by ultrathin nanosheets is also discussed. Moreover, the as-obtained LDH material shows increased electrocatalytic activity and stability toward WOR in alkaline media compared with the materials prepared without a Ni alkoxide precursor or Fe precursor, namely α-Fe2O3 and Ni(OH)2, respectively. In addition, the electrocatalytic activity is demonstrated to be related to the molar ratio of Fe and Ni in the final Ni-Fe material, and the best activity is achieved when the ratio reaches 0.52 : 1. The phase compositions of the resulting Ni-Fe(x) are discussed. Furthermore, the Ni-Fe LDH material reported herein might be employed as a promising noble-metal-free water oxidation catalyst to replace the IrOx material-the state-of-the-art water oxidation catalyst.