In situ formation of NiCoP@phosphate nanocages as an efficient bifunctional electrocatalyst for overall water splitting

Porous Flower-Like Nanoarchitectures Derived from Nickel Phosphide Nanocrystals Anchored on Amorphous Vanadium Phosphate Nanosheet Nanohybrids for Superior Overall Water Splitting

Transition metal phosphides (TMPs) and phosphates (TM-Pis) nanostructures are promising functional materials for energy storage and conversion. Nonetheless, controllable synthesis of crystalline/amorphous heterogeneous TMPs/TM-Pis nanohybrids or related nanoarchitectures remains challenging, and their electrocatalytic applications toward overall water splitting (OWS) are not fully explored. Herein, the Ni2 P nanocrystals anchored on amorphous V-Pi nanosheet based porous flower-like nanohybrid architectures that are self-supported on carbon cloth (CC) substrate (Ni2 P/V-Pi/CC) are fabricated by conformal oxidation and phosphorization of pre-synthesized NiV-LDH/CC. Due to the unique microstructures and strong synergistic effects of crystalline Ni2 P and amorphous V-Pi components, the obtained Ni2 P/V-Pi/CC owns abundant active sites, suitable surface/interface electronic structure and optimized adsorption-desorption of reaction intermediates, resulting in outstanding electrocatalytic performances toward hydrogen and oxygen evolution reactions in alkaline media. Correspondingly, the assembled Ni2 P/V-Pi/CC||Ni2 P/V-Pi/CC electrolyzer only needs an ultralow cell voltage (1.44 V) to deliver 10 mA cm-2 water-splitting currents, exceeding its counterparts, recently reported bifunctional catalysts-based devices, and Pt/C/CC||IrO2 /CC pairs. Moreover, the Ni2 P/V-Pi/CC||Ni2 P/V-Pi/CC manifests remarkable stability. Also, such device shows a certain prospect for OWS in acidic media. This work may spur the development of TMPs/TMPis-based nanohybrid architectures by combining structure and phase engineering, and push their applications in OWS or other clean energy options.

Deep Reconstruction of the NiFeSP Nanosheet Catalyst for Large Current-Density Water Oxidation

Evidence shows that the crucial factor in achieving efficient water electrolysis for hydrogen production is the design and synthesis of electrocatalysts that exhibit both high performance and cost-effectiveness for the oxygen evolution reaction (OER). Herein, the NiFeSP nanosheets were facilely prepared on a Ni foam (NF) substrate through a cost-effective electrodeposition method. The electrode structure composed of nanosheets offers a high density of active sites and superior electrical conductivity, thereby enhancing the efficiency of the OER. In addition, the NiFeSP/NF-600 nanosheets exhibit superhydrophilic and superaerophobic characteristics, which effectively enhance the mass-transfer process by facilitating the penetration of electrolytes and enabling rapid release of gas bubbles. Consequently, NiFeSP/NF-600 demonstrates superior electrocatalytic efficacy for OER, exhibiting an overpotential of 292 mV at a high current density of 500 mA cm-2 as well as an exceptional long-term durability of 100 h. More importantly, the rapid reconstruction of FeOOH and NiOOH species from NiFeSP/NF-600 may be the true active species for OER, which is revealed utilizing in situ Raman spectroscopy in conjunction with ex situ characterization. This study not only offers an ideal "pre-catalyst" for an extremely effective OER but also offers a thorough understanding of the mechanism underlying the structural evolution of electrocatalysts and the identification of their actual active sites.

Designing N-doped graphene-like supported highly dispersed bimetallic NiCoP NPs as an efficient electrocatalyst for water oxidation

Electrocatalysts with a high oxygen evolution reaction (OER) activity are very important for electrochemical water oxidation, but they are also challenging. In this study, N-doped graphene-like supported highly dispersed bimetallic NiCoP NPs as an efficient electrocatalyst for water oxidation were prepared by using cation exchange resin as a carbon source and by loading cobalt and nickel on D001 by a high-temperature calcination method. The designed electrocatalyst with bimetallic phosphide as the active center shows excellent OER catalytic performance, with an overpotential of 324 mV at 10 mA cm-2 and a corresponding Tafel slope of 97.28 mV dec-1. The increase in NiCoP-3@GL activity may be due to the increase in surface area (933.49 m2 g-1) caused by the irregular morphology, rich interface contact, and porous structure. In addition, the strong combination of NiCoP and GL improves the structural stability and durability of the electrocatalyst. After 5000 cyclic voltammetry tests, the performance of the catalyst decreased by 16.9 %. This work provides a new idea for designing efficient bimetallic phosphide electrocatalysts.

Electrocatalysis of Methanol Oxidation in Alkaline Electrolytes over Novel Amorphous Fe/Ni Biphosphate Material Prepared by Different Techniques

In this work, novel phosphate materials based on bimetallic character (Fe and Ni) were introduced by different chemical fabrication methods, the reflux method (FeNiP-R) and the sol-gel technique (FeNiP-S), and evaluated as non-precious electrodes for methanol electrooxidation in KOH electrolytes. The designed FeNiP-R and FeNiP-S samples were investigated using different characterization techniques, namely TEM, SEM, XPS, BET, DLS, and FT-IR, to describe the impact of the fabrication technique on the chemistry, morphology, and surface area. The characterization techniques indicate the successful fabrication of nanoscale-sized particles with higher agglomeration by the sol-gel technique compared with the reflux strategy. After that, the electrochemical efficiency of the fabricated FeNiP-R and FeNiP-S as electrodes for electrocatalytic methanol oxidation was studied through cyclic voltammetry (CV) at different methanol concentrations and scan rates in addition to impedance analysis and chronoamperometric techniques. From electrochemical analyses, a sharp improvement in the obtained current values was observed in both electrodes, FeNiP-R and FeNiP-S. During the MeOH electrooxidation over FeNiP-S, the current value was improved from 0.14 mA/cm² at 0.402 V to 2.67 mA/cm² at 0.619 V, which is around 109 times the current density value (0.0243 mA/cm² at 0.62 V) found in the absence of MeOH. The designed FeNiP-R electrode showed an improved electrocatalytic character compared with FeNiP-S at different methanol concentrations up to 80 mmol/L. The enhancement of the anodic current density and charge transfer resistance indicates the methanol electrooxidation over the designed bimetallic Fe/Ni-phosphates.

Ultralight, Safe, Economical, and Portable Oxygen Generators with Low Energy Consumption Prepared by Air-Breathing Electrochemical Extraction

Pure oxygen is vital in medical treatment, first aid, and chemical synthesis. Hypoxia can cause severe damage to the organ systems such as respiratory, digestive, and nervous systems and even directly cause death. Notably, the severe Coronavirus disease 2019 (COVID-19) pandemic has exacerbated the shortage of medical oxygen in the world. Hence, a safe, economical, and portable oxygen supply device is urgently needed. Here, we have successfully prepared a device with air-breathing electrochemical extraction of pure oxygen (ABEEPO) with light weight and high energy efficiency. By renovating the structure of the electrolytic cell, the components bipolar plate and end plate are replaced with a plastic membrane, and the component current collector is replaced with a highly conductive graphene composite membrane electrode. Due to the use of the plastic membrane and graphene composite membrane electrode, the weight of the electrolytic cell is reduced from 1319.4 to 1.6 g, and the flexibility of the electrolytic cell is successfully realized. Through optimizing anode catalysts, working area, and operating voltage, a high flow rate per mass (234 mL h^-1 g^-1) was achieved at a voltage of 1.2 V. The device exhibits high stability in 2 h. The new portable oxygen production device would be effective for hypoxia treatment.

A high-density nickel-cobalt alloy embedded in nitrogen-doped carbon nanosheets for the hydrogen evolution reaction

The development of novel non-noble electrocatalysts is critical for an efficient electrochemical hydrogen evolution reaction (HER). In this study, high-density nickel-cobalt alloy nanoparticles embedded in the bent nitrogen-doped carbon nanosheets are prepared as a high-performance catalyst. The optimized Ni₇Co₃/NC-500 catalyst displays quite a low overpotential of 90 mV at a current density of 10 mA cm^-2, and a small Tafel slope of 64 mV dec^-1 in alkaline medium, and even performs better than commercial 20% Pt/C at a high current density (η₁₅₀ = 233 mV for Ni₇Co₃/NC-500 and η₁₅₀ = 267 mV for 20% Pt/C). Specifically, the high-density nickel-cobalt alloy (with an average size of 6.2 nm and a distance of <3.0 nm) embedded in the bent carbon nanosheets provides plentiful active sites. Furthermore, in situ visualization of the produced hydrogen bubbles shows that the small size of hydrogen bubbles (d = 0.2 mm for Ni₇Co₃/NC-500 vs. d = 0.8 mm for 20% Pt/C) resulting from the small water contact angle and the bent nanosheet structure would inhibit the aggregation of H₂ bubbles on the surface to facilitate efficient mass diffusion. Density functional theory calculations reveal that the formation of the nickel-cobalt alloy can effectively lower water dissociation energy barriers and optimize hydrogen adsorption Gibbs free energy, manifesting a high HER activity.

Critical Review, Recent Updates on Zeolitic Imidazolate Framework-67 (ZIF-67) and Its Derivatives for Electrochemical Water Splitting

Design and construction of low-cost electrocatalysts with high catalytic activity and long-term stability is a challenging task in the field of catalysis. Metal-organic frameworks (MOF) are promising candidates as precursor materials in the development of highly efficient electrocatalysts for energy conversion and storage applications. This review starts with a summary of basic concepts and key evaluation parameters involved in the electrochemical water-splitting reaction. Then, different synthesis approaches reported for the cobalt-based Zeolitic imidazolate framework (ZIF-67) and its derivatives are critically reviewed. Additionally, several strategies employed to enhance the electrocatalytic activity and stability of ZIF-67-based electrocatalysts are discussed in detail. The present review provides a succinct insight into the ZIF-67 and its derivatives (oxides, hydroxides, sulfides, selenides, phosphide, nitrides, telluride, heteroatom/metal-doped carbon, noble metal-supported ZIF-67 derivatives) reported for oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and overall water splitting applications. Finally, this review concludes with the associated challenges and the perspectives on developing the best economic, durable electrocatalytic materials.

yMoO42- modified amorphous Co(PO3)2 cubes as an efficient bifunctional electrocatalyst for alkaline overall water splitting

The exploration of efficient bifunctional electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) under alkaline conditions is an importantway to promote the development of electrolytic water technology. Herein, the reduced graphene oxide-supported MoO₄^2- modified amorphous cobalt metaphosphate cubes (a-Co(PO₃)₂/MoO₄/rGO) as bifunctional OER/HER catalyst is prepared by anion exchange and phosphating, using the Prussian blue analogue (PBA) as a precursor. The resulting composite exhibits the low overpotentials (η) that of 290 and 50 mV for OER and HER in 1.0 M KOH solution at 10 mA cm^-2, respectively. The electrochemical test and density functional theory (DFT) results reveal that the MoO₄^2--modified optimizes the adsorption/desorption energy of H* of Co(PO₃)₂, thus enhance the HER activity. Benefiting from efficient HER and OER performances, an efficient and stable alkaline water electrolysis operation using a-Co(PO₃)₂/MoO₄/rGO used as bifunctional catalyst can be carried out, which can deliver a current density (j) of 20 mA cm^-2 at 1.65 V cell voltage and work continuously for 24 h.

A needle-like cobalt-based bifunctional catalyst supported on carbon materials for effective overall water splitting

Nowadays, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) dual-functional electrocatalyst in the field of water electrolysis has great advantages in reducing costs and simplifying electrolytic cell installations. Herein, Co-Mo particles were electrodeposited on the carbon nanotubes (CNTs)/reduced graphene oxide (rGO)-modified copper foam to form the Co-Mo-CNTs/rGO-copper foam (CF), then it was subjected to a certain potential for alkaline etching, thus needle-like E-Co-Mo-CNTs/rGO-CF was synthesized. Results showed that the material surface mainly formed by the interlacing of Co oxide was more conducive to capturing the intermediates in the HER/OER reaction, while the CNTs/rGO-CF structure was closely connected to the metal layer, making excellent performance of total hydrolysis in KOH. The electrocatalyst exhibited remarkable electrocatalytic activity for HER and OER in 1 M KOH, requiring only 71 and 268 mV overpotential to drive 10 mA·cm^-2, respectively. Especially, only a battery voltage of 1.52 V was needed to drive 10 mA·cm^-2in two-electrode system for overall water splitting. This work provides a method for the construction of dual-functional electrocatalyst that combined carbon materials and metals.

Phytic Acid-Based FeCo Bimetallic Metal-Organic Gels for Electrocatalytic Oxygen Evolution Reaction

Electrocatalysts have been developed to improve the efficiency of gas release for oxygen evolution reaction (OER), and finding a simple and efficient method for efficient electrocatalysts has inspired research enthusiasm. Herein, we report bimetallic metal-organic gels derived from phytic acid (PA) and mixed transition metal ions to explore their performance in electrocatalytic oxygen evolution reaction. PA is a natural phosphorus-rich organic compound, which can be obtained from plant seeds and grains. PA reacts with bimetallic ions (Fe³⁺ and Co²⁺ ) in a facile one-pot synthesis under mild conditions to form PA-FeCo bimetallic gels, and the corresponding aerogels are further partially reduced with NaBH₄ to improve the electrocatalytic activity. Mixed valence states of Fe(II)/Fe(III) and Co(III)/Co(II) are present in the materials. Excellent OER performance in terms of overpotential (257 mV at 20 mA cm^-2 ) and Tafel slope (36 mV dec^-1 ) is achieved in an alkaline electrolyte. This reduction method is superior to the pyrolysis method by well maintaining the gel morphology structure. This strategy is conducive to the further improvement of the performance of metal-organic electrocatalysts, and provides guidance for the subsequent application of metal-organic gel electrocatalysts.

Vertically-interlaced NiFeP/MXene electrocatalyst with tunable electronic structure for high-efficiency oxygen evolution reaction

Layered double hydroxides (LDHs) with decent oxygen evolution reaction (OER) activity have been extensively studied in the fields of energy storage and conversion. However, their poor conductivity, ease of agglomeration, and low intrinsic activity limit their practical application. To date, improvement of the intrinsic activity and stability of NiFe-LDHs through the introduction of heteroatoms or its combination with other conductive substrates to enhance their water-splitting performance has drawn increasing attention. In this study, vertically interlaced ternary phosphatised nickel/iron hybrids grown on the surface of titanium carbide flakes (NiFeP/MXene) were successfully synthesised through a hydrothermal reaction and phosphating calcination process. The optimised NiFeP/MXene exhibited a low overpotential of 286 mV at 10 mA cm^-2 and a Tafel slope of 35 mV dec^-1 for the OER, which exceeded the performance of several existing NiFe-based catalysts. NiFeP/MXene was further used as a water-splitting anode in an alkaline electrolyte, exhibiting a cell voltage of only 1.61 V to achieve a current density of 10 mA cm^-2. Density functional theory (DFT) calculations revealed that the combination of MXene acting as a conductive substrate and the phosphating process can effectively tune the electronic structure and density of the electrocatalyst surface to promote the energy level of the d-band centre, resulting in an enhanced OER performance. This study provides a valuable guideline for designing high-performance MXene-supported NiFe-based OER catalysts.

Self-Foaming Metal-Organic Gels Based on Phytic Acid and Their Mechanical, Moldable, and Load-Bearing Properties

A catalogue of metal-organic gels are synthesized from phytic acid (PA) and a diversity of metal ions (Fe³⁺ , Cr³⁺ , Al³⁺ , Ce³⁺ , Y³⁺ , Co²⁺ , Ni²⁺ , Mn²⁺ , Cu²⁺ , Zn²⁺ , Mg²⁺ ) upon heating at 80 °C. PA-M gels have various morphologies, including irregular granular (PA-Fe, PA-Al, PA-Ce, PA-Cr, PA-Ni, PA-Co), spongy (PA-Y), and hollow tremella-like (PA-Cu) morphologies. Interestingly for PA-Fe-1 : 4 (PA:Fe³⁺ =1 : 4) a large amount of gas is generated during the gelation process leading to a self-foaming gel. The PA-Fe-1 : 4 self-foaming gel shows reversible gel-sol phase transition. The gel is unusually weakened and transformed into a sol at room temperature, and the sol is reversed to gelation when heated again at 80 °C. PA-Fe-1 : 4 gel also shows shapeable and load-bearing properties, and it can bear up to 200 times of its weight, depending on the gas amount fixed in the foam gel and the aging time. This work provides a catalogue of self-foaming supramolecular gels with tunable properties based on naturally abundant resources.

Formation of graphene encapsulated cobalt–iron phosphide nanoneedles as an attractive electrocatalyst for overall water splitting

Hierarchical CoFe–P nanoneedles encapsulated in the graphene texture are fabricated for water splitting applications.

Bimetallic phosphide NixCo1−xP decorated flower-like ZnIn2S4 for enhanced photocatalytic hydrogen evolution

2%Ni_0.1Co_0.9P–ZIS exhibited the highest photocatalyst hydrogen production rate of 3839 μmol g⁻¹ h⁻¹ under visible light irradiation (λ > 420 nm), which is 5.4 times higher than pure ZIS (714 μmol g⁻¹ h⁻¹).