Insights into Transition Metal Phosphate Materials for Efficient Electrocatalysis

Electrocatalytic biomass upgrading coupled with hydrogen evolution and CO2 reduction

Clean energy production and CO2 utilization have attracted increasing interest. Electrocatalysis represents an effective way to produce green hydrogen from water and reduce CO2 to valuable compounds. However, for either the hydrogen evolution reaction (HER) or the CO2 reduction reaction (CO2RR), the reaction efficiency is significantly limited by the slow kinetics of the oxygen evolution reaction (OER) at the anode, which consumes most of the input energy. Therefore, great efforts have been made to replace the OER with organic oxidation reactions at the anode to decrease the reaction energy barrier. Biomass has an advantage of broad source, and when it is employed as an OER alternative in the anode oxidation reactions, not only can the reduction reaction efficiency at the cathode including the HER and CO2RR be enhanced but high-value chemicals can also be obtained, representing an attractive OER alternative. This review comprehensively summarizes the recent achievements in electrocatalytic biomass upgrading coupled with the HER and CO2RR, cataloged based on the type of biomass. The design of electrocatalysts for such coupled reaction systems is discussed. Finally, the challenges and perspectives in the field of this energy-saving and value-added coupling system are provided to inspire more efforts in pushing forward the development of this field.

Water oxidation on a sustainable polymeric proton relay: the role of post-phosphating of an oxide sub-layer on PCET

Lagging proton transfer (PT) toward electron transfer (ET) during multi-step proton-coupled electron transfer (PCET) is one of the humble approaches toward complete biomimicking of artificial leaves. In this work, we synthesized a typical sustainable polymeric interface (poly L-lysine) to investigate the effect of post-phosphating of the metal oxide substrate (NF/CoMoO4) on the yield of PCET during the electrocatalytic water oxidation reaction (WOR) in alkaline medium. The details of the PCET mechanism are unraveled using the kinetic isotope effect (KIE), proton inventory, Tafel slopes, pH studies, and high-frequency transmission line (Gerischer) impedance. The appealing Gerischer feature unveils an electrochemical reaction preceded and followed by a chemical exchange reaction (CEC mechanism), indicating a proton hopping mechanism that practically blocks natural diffusion of the protonic species (H+ and H2O) and the role of the interfacial phosphate#p-Lys hydrogen bond network (HBN) in PCET. The interfacial HBN can serve as a flexible proton hook to delocalize protons and may polarize the O-O bond, thereby facilitating the overall PCET for the progress of O-O bond formation. Poly L-lysine improved the response stability of the PCET catalyst through corrosion protection of the metallic substrate and also prevention of the spatiotemporal accumulation of protons as a surface corrosion agent. The results show a non-concerted PCET mechanism with the first ET step as the RDS of the heterogeneous WOR.

The Role of Alkali Metal Ions in Cooperative Electrocatalysis by Bifunctional Co-Mn-Mixed Phosphates

Developing cost-effective, non-precious metal bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial for advancing sustainable energy storage and conversion technologies, including zinc-air batteries, fuel cells, and water electrolyzers. This study presents a one-pot synthesis of cobalt-manganese mixed phosphates as effective bifunctional electrocatalysts for both ORR and OER. Among the catalysts tested, Na-Co-Mn-P [NaCo1.5Mn1.5(HPO4)2(PO4)] exhibited the highest catalytic activity, with a minimal ΔE of 0.86 V, indicating superior performance. The incorporation of alkali metals and the synergistic effects of metal and phosphate components enhance conductivity, electrochemical surface area, and mixed valency of transition metals, contributing to improved electrocatalytic activity. This work highlights a straightforward synthesis method and the beneficial role of metal-phosphate synergy in oxygen electrocatalysis.

Efficient Electrosynthesis of Hydrogen Peroxide Enabled by a Hierarchical Hollow RE-P-O (RE = Sm, La, Gd) Architecture with Open Channels

The electrochemical two-electron oxygen reduction reaction (2e- ORR) offers a sustainable pathway for the production of H2O2; however, the development of electrocatalysts with exceptional activity, selectivity, and long-term stability remains a challenging task. Herein, a novel approach is presented to addressing this challenge by synthesizing hierarchical hollow SmPO4 nanospheres with open channels via a two-step hydrothermal treatment. The produced compound demonstrates remarkable 2e- selectivity, exceeding 93% across a wide potential range of 0.0-0.6 V in 0.1 m KOH, with a peak of 96% at 0.45 V. When employed as the cathode in a flow cell, the synthesized SmPO4 exhibits impressive stability at 100 mA cm-2 for 12 h, consistently achieving a Faradaic efficiency above 90%. Using X-ray absorption, in situ Raman and Fourier-transform infrared spectroscopies, theoretical calculations, and post-ORR assessments, it is found that this hollow compound possesses intrinsic open channels and is characterized by the optimal metal atomic spacing, and exceptional structural and compositional stabilities. These factors significantly enhance the thermodynamics, kinetics, and stability of the 2e- ORR process. Notably, the produced compound also exhibits outstanding 2e- ORR performance in neutral environments. Furthermore, this strategy can be extended to other hollow rare-earth-P-O compounds, demonstrating excellent 2e- ORR performance under both neutral and alkaline conditions.

Developing porous electrocatalysts to minimize overpotential for the oxygen evolution reaction

The development of electrocatalysts for the oxygen evolution reaction (OER) is one of the most critical issues for improving the efficiency of electrochemical water-splitting, which can produce green hydrogen energy without CO2 emissions. This review outlines the advances in the precise design of inorganic- and organic-based porous electrocatalysts, which are designed by various strategies, to catalyze the OER in the electrolytic cycle for efficient water-splitting. For developing high-performance electrocatalysts with low overpotentials, it is important to design a chemical composition that optimizes binding energy for an intermediate in the OER and allows the easy access of reactants to active sites depending on the porosity of electrocatalysts. Porous structures give us the positive opportunity to increase the accessible surface of active sites and effective diffusion of targeting molecules, which is potentially one of the guidelines for developing active electrocatalysts. Further modification of the frameworks is also powerful for tailoring the function of pore surfaces and the environment of inner spaces. Designable organic molecules can also be embedded inside inorganic- and organic-based frameworks. According to chemical composition (inorganic and organic), nanostructure (crystalline and amorphous) and additional modification (metal doping and organic design) of porous electrocatalysts, the current status of resultant OER performance is surveyed with some problems that should be solved for improving the OER activity. The remarkable progress in OER electrocatalysts is also introduced for demonstrating the bifunctional hydrogen evolution reaction (HER) and for utilizing seawater.

Beyond conventional structures: emerging complex metal oxides for efficient oxygen and hydrogen electrocatalysis

The core of clean energy technologies such as fuel cells, water electrolyzers, and metal-air batteries depends on a series of oxygen and hydrogen-based electrocatalysis reactions, including the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), which necessitate cost-effective electrocatalysts to improve their energy efficiency. In the recent decade, complex metal oxides (beyond simple transition metal oxides, spinel oxides and ABO3 perovskite oxides) have emerged as promising candidate materials with unexpected electrocatalytic activities for oxygen and hydrogen electrocatalysis owing to their special crystal structures and unique physicochemical properties. In this review, the current progress in complex metal oxides for ORR, OER, and HER electrocatalysis is comprehensively presented. Initially, we present a brief description of some fundamental concepts of the ORR, OER, and HER and a detailed description of complex metal oxides, including their physicochemical characteristics, synthesis methods, and structural characterization. Subsequently, we present a thorough overview of various complex metal oxides reported for ORR, OER, and HER electrocatalysis thus far, such as double/triple/quadruple perovskites, perovskite hydroxides, brownmillerites, Ruddlesden-Popper oxides, Aurivillius oxides, lithium/sodium transition metal oxides, pyrochlores, metal phosphates, polyoxometalates and other specially structured oxides, with emphasis on the designed strategies for promoting their performance and structure-property-performance relationships. Moreover, the practical device applications of complex metal oxides in fuel cells, water electrolyzers, and metal-air batteries are discussed. Finally, some concluding remarks summarizing the challenges, perspectives, and research trends of this topic are presented. We hope that this review provides a clear overview of the current status of this emerging field and stimulate future efforts to design more advanced electrocatalysts.

Hydrogen Peroxide Electrosynthesis via Selective Oxygen Reduction Reactions Through Interfacial Reaction Microenvironment Engineering

The electrochemical two-electron oxygen reduction reaction (2e- ORR) offers a compelling alternative for decentralized and on-site H2O2 production compared to the conventional anthraquinone process. To advance this electrosynthesis system, there is growing interest in optimizing the interfacial reaction microenvironment to boost electrocatalytic performance. This review consolidates recent advancements in reaction microenvironment engineering for the selective electrocatalytic conversion of O2 to H2O2. Starting with fundamental insights into interfacial electrocatalytic mechanisms, an overview of various strategies for constructing the favorable local reaction environment, including adjusting electrode wettability, enhancing mesoscale mass transfer, elevating local pH, incorporating electrolyte additives, and employing pulsed electrocatalysis techniques is provided. Alongside these regulation strategies, the corresponding analyses and technical remarks are also presented. Finally, a summary and outlook on critical challenges, suggesting future research directions to inspire microenvironment engineering and accelerate the practical application of H2O2 electrosynthesis is delivered.

Rapid synthesis of cobalt manganese phosphate by microwave-assisted hydrothermal method and application as positrode material in supercapatteries

Electrochemical energy storage devices with high specific capacity are of utmost important for the next-generation electronic devices. Supercapatteries (SCs) are highly demanded energy storage devices nowadays as these bridge the low energy supercapacitors and low power batteries. Herein, we report a rapid synthesis of cobalt manganese phosphate (COMAP) by microwave-assisted hydrothermal method and facile fabrication of SCs using electrodes comprising of COMAP as positrode material. The effect of precursor concentration on the microstructure and surface morphology of the COMAP samples are examined initially. Further, the electrochemical performance of COMAP electrodes is studied systematically in 3 M KOH (aqueous) electrolyte. COMAP exhibits excellent charge storage capabilities where type of charge storage mechanism is found to be battery-type based on the calculation obtained from Dunn's method. The SC electrode fabricated with COMAP synthesized using cobalt: manganese precursor ratio as 80:20 exhibits a highest specific capacity of 191.4 C/g at a scan rate of 1 mV/s. An asymmetric SC (ASC) cell fabricated with COMAP as positrode and activated carbon (AC) as negatrode exhibits a specific capacity of 165.5 C/g at a current density of 1.8 A/g. The COMAP//AC ASC cell exhibits an energy density of 34.1 Wh/kg at a corresponding power density of 1875 W/kg at a current density of 1.8 A/g.

Electrochemical water splitting enhancement by introducing mesoporous NiCoFe-trimetallic phosphide nanosheets as catalysts for the oxygen evolution reaction

Transition metal-based catalysts are widely used in electrocatalysis, especially in the field of water splitting, due to their excellent electrochemical performance, which focuses on improving the efficiency of the complex oxygen evolution reaction (OER) that occurs at the anode. Transition metal-based catalysts will undergo electrochemical surface reconstruction and form (oxy)hydroxide-based hybrids, which consider the actual active sites for OER. So many efforts have been made to know the origin of the effect of electrochemical surface reconstruction on the performance of the OER. Herein, NiCoFe-phosphide catalyst nanosheets were constructed by a simple one-step hydrothermal reaction by adding oleylamine and ethanol to water solvent during the preparation of the catalyst precursor and high-temperature gas-phase phosphating and significantly showed high effectiveness catalytic activity and conductivity in comparison to normal and traditional preparation methods. Electrochemical analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM) demonstrate that the surface was constructed during the electrochemical reaction and formed an amorphous layer of MOx(OH)y active sites, which increased the electrochemical surface area and promoted charge transfer. As well, the synthesized NiCoFePx-PNSs catalyst nanosheets exhibit excellent catalytic activity with a low overpotential equal to 259 mV to achieve the OER at a current density of 10 mA cm-2 and a low Tafel slope of 50.47 mV dec-1 which is better than for most reported transition metal-based electrocatalysts. This work provides a new design for a transition metal-based catalyst for OER as well as further insights into the effect of electrochemical surface reconstruction on intrinsic activity and OER performance.

Strategic Design and Insights into Lanthanum and Strontium Perovskite Oxides for Oxygen Reduction and Oxygen Evolution Reactions

Perovskite oxides exhibit bifunctional activity for both oxygen reduction (ORR) and oxygen evolution reactions (OER), making them prime candidates for energy conversion in applications like fuel cells and metal-air batteries. Their intrinsic catalytic prowess, combined with low-cost, abundance, and diversity, positions them as compelling alternatives to noble metal and metal oxides catalysts. This review encapsulates the nuances of perovskite oxide structures and synthesis techniques, providing insight into pivotal active sites that underscore their bifunctional behavior. The focus centers on the breakthroughs surrounding lanthanum (La) and strontium (Sr)-based perovskite oxides, specifically their roles in zinc-air batteries (ZABs). An introduction to the mechanisms of ORR and OER is provided. Moreover, the light is shed on strategies and determinants central to optimizing the bifunctional performance of La and Sr-based perovskite oxides.

Enhancing the Hydrogen Evolution Performance of Tungsten Diphosphide on Carbon Fiber through Ruthenium Modification

Hydrogen-based energy systems hold promise for sustainable development and carbon neutrality, minimizing environmental impact with electrolysis as the preferred fossil-fuel-free hydrogen generation method. Effective electrocatalysts are required to reduce energy consumption and improve kinetics, given the need for additional voltage (overpotential, η) despite the theoretical water splitting potential of 1.23 V. To date, platinum has been acknowledged as the most effective but expensive hydrogen evolution reaction (HER) catalyst. Hence, we introduce a cost-effective (∼2-fold cheaper) ruthenium-modified tungsten diphosphide (Ru/WP2) catalyst on carbon fiber for HER in ∼0.5 M H2SO4, with η ≈ 34 mV at -10 mA cm-2 which can be comparable (only ∼2-fold higher) to benchmark Pt/C (η ≈ 17 mV). The HER performance of WP2 can be enhanced through the modification of ruthenium, as indicated by the electrochemical characterizations. Considering the Tafel value of ∼40 ± 0.2 mV dec-1, it can be inferred that Ru/WP2 follows the Volmer-Heyrovsky reaction pathway for hydrogen generation. Furthermore, the Faradaic efficiency estimation indicates that Ru/WP2 demonstrates a minimal loss of electrons during the electrochemical reaction with an estimated value of ∼98.7 ± 1.4%. Therefore, this study could emphasize the potential of the Ru/WP2 electrode in advancing sustainable hydrogen production through water splitting.

Laser-induced immobilization of an amorphous iron-phosphate/Fe3O4 composite on nickel foam for efficient water oxidation

A laser-induced immobilization strategy is applied to prepare an amorphous iron-phosphate/Fe3O4 (L-FePO) composite on a nickel foam (NF) support. By laser-irradiating an iron hydrogen phosphate (FeHP) precursor, a melting and oxidation process leads to the generation of L-FePO with hierarchical pores and an amorphous structure. L-FePO shows exceptional electrocatalytic performance for the OER in an alkaline electrolyte, demonstrating an overpotential of 256 mV at 100 mA cm-2, a Tafel slope of 71 mV dec-1, and good stability over 100 h. The active Fe3O4, partially dissolved phosphate, and newly formed FeOOH species provide abundant active sites, contributing to the excellent OER performance.

A Membrane-Free Decoupled Water Electrolyzer Operating at Simulated Fluctuating Renewables with Tri-Functional NiCo-P Electrode

Water electrolysis has been considered a promising technology for the conversion of renewables to hydrogen. However, preventing mixing of products (H2 and O2 ) and exploring cost-efficient electrolysis components remains challenging for conventional water electrolyzers. Herein, we designed a membrane-free decoupled water electrolysis system by using graphite felt supported nickel-cobalt phosphate (GF@Nix Coy -P) material as a tri-functional (redox mediator, hydrogen evolution reaction (HER), oxygen evolution reaction (OER)) electrode. The versatile GF@Ni1 Co1 -P electrode obtained by a one-step electrodeposition not only exhibits high specific capacity (176 mAh g-1 at 0.5 A g-1 ) and long cycle life (80 % capacity retention after 3000 cycles) as a redox mediator, but also has relatively outstanding catalytic activities for HER and OER. The excellent properties of the GF@Nix Coy -P electrode endow this decoupled system with more flexibility for H2 production by fluctuating renewable energies. This work provides guidance for multifunctional applications of transition metal compounds between energy storage and electrocatalysis.

Surface Adsorption of Amorphous Phosphate on RuNi-Doped Molybdate for the Hydrogen Evolution Reaction

Developing highly active and cost-effective electrocatalysts is critical for enhancing the intrinsic performance of electrocatalytic water splitting. Oxoanion-based compounds, such as phosphates and molybdates, have emerged as promising electrocatalysts owing to their advantageous properties of nontoxicity, low price, and strong water adsorption ability. However, their relatively inferior activity has impeded extensive investigation into electrochemical applications. Herein, an amorphous phosphate-adsorbed and RuNi-doped molybdate (RuNiMo-P) composite is synthesized on nickel foam (NF) support by using a simple two-step method. Significantly, an acidic solution of phosphomolybdic acid (PMo12), containing a low concentration of Ru, can etch the NF, contributing to the in situ growth of the RuNi-doped molybdate precursor. Subsequent phosphating ensures the surface formation of the amorphous phosphate layer due to abundant oxygen in the precursor. The strong structural interaction between RuNi-doped molybdate and amorphous phosphate in RuNiMo-P prompts an enhanced hydrogen evolution reaction (HER) performance, delivering an overpotential of 38 mV at a current density of -10 mA cm-2, a Tafel slope of 53 mV dec-1, and good stability in an alkaline medium. Characterizations after HER reveal that RuNi doping, partial dissolution of phosphate and molybdate species, and newly formed NiOOH nanosheets can expose active sites, facilitate charge transfer, and modify electronic structures, thereby improving the HER performance effectively.

Enriched Fe Doped on Amorphous Shell Enable Crystalline@Amorphous Core-Shell Nanorod Highly Efficient Electrochemical Water Oxidation

The rational design of efficient and cost-effective electrocatalysts for oxygen evolution reaction (OER) with sluggish kinetics, is imperative to diverse clean energy technologies. The performance of electrocatalyst is usually governed by the number of active sites on the surface. Crystalline/amorphous heterostructure has exhibited unique properties and opens new paradigms toward designing electrocatalysts with abundant active sites for improved performance. Hence, Fe doped Ni-Co phosphite (Fe-NiCoHPi) electrocatalyst with cauliflower-like structure, comprising crystalline@amorphous core-shell nanorod, is reported. The experiments uncover that Fe is enriched in the amorphous shell due to the flexibility of the amorphous component. Further density functional theory calculations indicate that the strong electronic interaction between the enriched Fe in the amorphous shell and crystalline core host at the core-shell interface, leads to balanced binding energies of OER intermediates, which is the origin of the catalyst-activity. Eventually, the Fe-NiCoHPi exhibits remarkable activity, with low overpotentials of only 206 and 257 mV at current density of 15 and 100 mA cm-2 . Unceasing durability over 90 h is achieved, which is superior to the effective phosphate electrocatalysts. Although the applications at high current remain challenges , this work provides an approach for designing advanced OER electrocatalysts for sustainable energy devices.

Co-based Catalysts for Selective H2 O2 Electroproduction via 2-electron Oxygen Reduction Reaction

Electrochemical production of hydrogen peroxide (H₂ O₂ ) via two-electron oxygen reduction reaction (ORR) process is emerging as a promising alternative method to the conventional anthraquinone process. To realize high-efficiency H₂ O₂ electrosynthesis, robust and low cost electrocatalysts have been intensively pursued, among which Co-based catalysts attract particular research interests due to the earth-abundance and high selectivity. Here, we provide a comprehensive review on the advancement of Co-based electrocatalyst for H₂ O₂ electroproduction. The fundamental chemistry of 2-electron ORR is discussed firstly for guiding the rational design of electrocatalysts. Subsequently, the development of Co-based electrocatalysts involving nanoparticles, compounds and single atom catalysts is summarized with the focus on active site identification, structure regulation and mechanism understanding. Moreover, the current challenges and future directions of the Co-based electrocatalysts are briefly summarized in this review.

Template-free synthesis of mesoporous and amorphous transition metal phosphate materials

We present how mesoporosity can be engineered in transition metal phosphate (TMPs) materials in a template-free manner. The method involves the transformation of a precursor metal phosphate phase, called M-struvite (NH₄MPO₄·6H₂O, M = Mg²⁺, Ni²⁺, Co²⁺, Ni_xCo_1-x²⁺). It relies on the thermal decomposition of crystalline M-struvite precursors to an amorphous and simultaneously mesoporous phase, which forms during degassing of NH₃ and H₂O. The temporal evolution of mesoporous frameworks and the response of the metal coordination environment were followed by in situ and ex situ scattering and diffraction, as well as X-ray spectroscopy. Despite sharing the same precursor struvite structure, different amorphous and mesoporous structures were obtained depending on the involved transition metal. We highlight the systematic differences in absolute surface area, pore shape, pore size, and phase transitions depending on the metal cation present in the analogous M-struvites. The amorphous structures of thermally decomposed Mg-, Ni- and Ni_xCo_1-x-struvites exhibit high surface areas and pore volumes (240 m² g^-1 and 0.32 cm^-3 g^-1 for Mg and 90 m² g^-1 and 0.13 cm^-3 g^-1 for Ni). We propose that the low-cost, environmentally friendly M-struvites could be obtained as recycling products from industrial and agricultural wastewaters. These waste products could be then upcycled into mesoporous TMPs through a simple thermal treatment for further application, for instance in (electro)catalysis.

Cu-induced NiCu-P and NiCu-Pi with multilayered nanostructures as highly efficient electrodes for hydrogen production via urea electrolysis

Since urea is commonly present in domestic sewage and industrial wastewater, its use in hydrogen production by electrolysis can simultaneously help in water decontamination. To achieve this goal, the development of highly active and inexpensive urea electrolysis catalysts is necessary. This study deals with the preparation of multilayered nickel and copper phosphides/phosphates (NiCu-P/NF and NiCu-Pi/NF) supported on Ni foam (NF) and their application as new electrocatalyst types for the electrolysis of urea-containing wastewaters. In these materials, Cu atoms induce the formation of multilayer nanostructures and modulate electron distribution, allowing for the exposure of additional active sites and acceleration of the process kinetics. NiCu-P/NF is used as a cathode and NiCu-Pi/NF as an anode in an electrolysis cell and exhibits significant catalytic activity and stability in the urea oxidation reaction (UOR) and the hydrogen evolution reaction (HER). The NiCu-Pi/NF||NiCu-P/NF electrolysis cell, operating with an alkaline urea-containing aqueous electrolyte, achieves a current density of 10 mA cm^- at a potential of 1.41 V, which is less than required by the RuO₂||Pt/C cell utilizing commercial noble metal-based electrodes. The study provides a novel strategy for designing efficient catalysts to produce hydrogen by urea electrolysis.

High-Alkaline Water-Splitting Activity of Mesoporous 3D Heterostructures: An Amorphous-Shell@Crystalline-Core Nano-Assembly of Co-Ni-Phosphate Ultrathin-Nanosheets and V- Doped Cobalt-Nitride Nanowires

Introducing amorphous and ultrathin nanosheets of transition bimetal phosphate arrays that are highly active in the oxygen evolution reaction (OER) as shells over an electronically modulated crystalline core with low hydrogen absorption energy for an excellent hydrogen evolution reaction (HER) can boost the sluggish kinetics of the OER and HER in alkaline electrolytes. Therefore, in this study, ultrathin and amorphous cobalt-nickel-phosphate (CoNiPOx ) nanosheet arrays are deposited over vanadium (V)-doped cobalt-nitride (V3% -Co4 N) crystalline core nanowires to obtain amorphous-shell@crystalline-core mesoporous 3D-heterostructures (CoNiPOx @V-Co4 N/NF) as bifunctional electrocatalysts. The optimized electrocatalyst shows extremely low HER and OER overpotentials of 53 and 270 mV at 10 mA cm-2 , respectively. The CoNiPOx @V3% -Co4 N/NF (+/-) electrolyzer utilizing the electrocatalyst as both anode and cathode demonstrates remarkable overall water-splitting activity, requiring a cell potential of only 1.52 V at 10 mA cm-2 , 30 mV lower than that of the RuO2 /NF (+)/20%-Pt/C/NF (-) electrolyzer. Such impressive bifunctional activities can be attributed to abundant active sites, adjusted electronic structure, lower charge-transfer resistance, enhanced electrochemically active surface area (ECSA), and surface- and volume-confined electrocatalysis resulting from the synergistic effects of the crystalline V3% -Co4 N core and amorphous CoNiPOx shells boosting water splitting in alkaline media.

Hollow Structured Transition Metal Phosphates and Their Applications

Hollow nanostructures of transition metal phosphate are of immense interest in the existing and evolving areas of technology, due to their high surface area, presence of hollow void, and easy tuning of compositions and dimensions. Emerging synthesis methods such as template-free methods, hard-templating, and soft-templating are discussed in this review. Applications of these hollow metal phosphates dominate in energy storage and conversions, with specific advantages as supercapacitor materials. Other applications, including drug delivery, water splitting, catalysis, and adsorption, are reviewed. Finally, additional perspectives on the progress of these nanostructures, and their existing challenges related to the current synthesis routes are covered. Therefore, with the strategic modifications of the unique properties of these hollow metal phosphates, broader application requirements are fulfilled.

Cobalt and iron phosphates with modulated compositions and phases as efficient electrocatalysts for alkaline seawater oxidation

An electrocatalyst which is suitable for use in both fresh water and real seawater electrolysis is very uncommon. In this work, we have developed a series of iron-tuned cobalt phosphates and cobalt-tuned iron phosphate solid solutions as electrocatalysts exhibiting excellent OER activities not only in freshwater but also in alkaline real seawater with a faradaic efficiency of 95%.

Three-dimensional reduced graphene oxide decorated with cobalt metaphosphate as high cost-efficiency electrocatalysts for the hydrogen evolution reaction

The development of cost-effective non-noble metal electrocatalysts is critical for the research of renewable energy. Transition metal cobalt metaphosphate-based materials have the potential to replace the noble metal Pt. Hence, in this work, we synthesize three-dimensional graphene-supported cobalt metaphosphate (Co(PO₃)₂-3D RGO) for the first time through the one-step hydrothermal synthesis method at low temperature with the aid of PH₃ phosphating. In a 0.5 mol L⁻¹ H₂SO₄ solution, the obtained electrocatalyst exhibits excellent electrochemical activity for the hydrogen evolution reaction (HER) with a small overpotential of 176 mV at a current density of 10 mA cm⁻² and a Tafel slope of 63 mV dec⁻¹. Additionally, in a 1 mol L⁻¹ KOH solution, the electrocatalyst also shows outstanding HER activity with a small overpotential of 158 mV at a current density of 10 mA cm⁻² and a Tafel slope of 88 mV dec⁻¹. Co(PO₃)₂-3D RGO can maintain its catalytic activity for at least ten hours whether in acid or alkali. This work not only demonstrates an excellent electrocatalyst for the hydrogen evolution reaction, but also provides an extremely convenient preparation technology, which provides a new strategy for the development and utilization of high-performance electrocatalysts.

The development of cost-effective non-noble metal electrocatalysts is critical for the research of renewable energy.

Properties and Applications of Metal Phosphates and Pyrophosphates as Proton Conductors

We review the progress in metal phosphate structural chemistry focused on proton conductivity properties and applications. Attention is paid to structure-property relationships, which ultimately determine the potential use of metal phosphates and derivatives in devices relying on proton conduction. The origin of their conducting properties, including both intrinsic and extrinsic conductivity, is rationalized in terms of distinctive structural features and the presence of specific proton carriers or the factors involved in the formation of extended hydrogen-bond networks. To make the exposition of this large class of proton conductor materials more comprehensive, we group/combine metal phosphates by their metal oxidation state, starting with metal (IV) phosphates and pyrophosphates, considering historical rationales and taking into account the accumulated body of knowledge of these compounds. We highlight the main characteristics of super protonic CsH2PO4, its applicability, as well as the affordance of its composite derivatives. We finish by discussing relevant structure-conducting property correlations for divalent and trivalent metal phosphates. Overall, emphasis is placed on materials exhibiting outstanding properties for applications as electrolyte components or single electrolytes in Polymer Electrolyte Membrane Fuel Cells and Intermediate Temperature Fuel Cells.

Iron pyrophosphate doped carbon nanocomposite for tetracycline degradation by activation of peroxymonosulfate

An Fe2P2O7@C catalyst was synthesized by simple carbonization of complex precursors and showed strong resistance to interference.

Plasma Enhanced Atomic Layer Deposition of Nickel and Cobalt Phosphate for Lithium Ion Batteries

A plasma-enhanced ALD process has been developed to deposit nickel phosphate. The process combines a trimethylphosphate (TMP) plasma with an oxygen plasma and nickelocene at a substrate temperature of 300°C....

Structural Variations of Metal Oxide-Based Electrocatalysts for Oxygen Evolution Reaction

Electrocatalytic oxygen evolution reaction (OER), an important electrode reaction in electrocatalytic and photoelectrochemical cells for a carbon-free energy cycle, has attracted considerable attention in the last few years. Metal oxides have been considered as good candidates for electrocatalytic OER because they can be easily synthesized and are relatively stable during the OER process. However, inevitable structural variations still occur to them due to the complex reaction steps and harsh working conditions of OER, thus impending the further insight into the catalytic mechanism and rational design of highly efficient electrocatalysts. The aim of this review is to disclose the current research progress toward the structural variations of metal oxide-based OER electrocatalysts. The origin of structural variations of metal oxides is discussed. Based on some typical oxides performing OER activity, the external and internal factors that influence the structural stability are summarized and then some general approaches to regulate the structural variation process are provided. Some operando methods are also concluded to monitor the structural variation processes and to identify the final active structure. Additionally, the unresolved problems and challenges are presented in an attempt to get further insight into the mechanism of structural variations and establish a rational structure-catalysis relationship.

Ethylene Glycol Electrochemical Reforming Using Ruthenium Nanoparticle-Decorated Nickel Phosphide Ultrathin Nanosheets

In this work, ruthenium nanoparticle-decorated ultrathin nickel phosphide nanosheets on nickel foam substrate (Ru/Ni₂P/NF) nanocomposites are synthesized conveniently by a cyanogel-NaBH₄ method and a subsequent phosphating process, which displays excellent electroactivity for both the hydrogen evolution reaction (HER) and ethylene glycol electro-oxidation reaction (EGEOR) in an alkaline solution. Concretely, at Ru/Ni₂P/NF nanocomposites, only 1.37 and -0.13 V potentials are required to obtain a current density of 100 mA cm^-2 for EGEOR and HER, respectively. Meanwhile, Ru/Ni₂P/NF nanocomposites also exhibit pre-eminent electrocatalytic performance of the long-running process for both EGEOR and HER. Density functional theory calculations demonstrate that the introduction of Ru nanoparticles results in an optimization of the surface adsorption energy and construction of a synergistic catalysis interface, which improve the electrocatalytic performance of nickel phosphide nanosheets. Notably, a symmetric Ru/Ni₂P/NF||Ru/Ni₂P/NF ethylene glycol electrolyzer needs only 1.14 V electrolysis voltage to obtain 10 mA cm^-2 for hydrogen production, which effectively eliminates the H₂/O₂ explosion risk and highlights an energy-saving mode for electrochemical hydrogen production.

Comprehensive Understandings into Complete Reconstruction of Precatalysts: Synthesis, Applications, and Characterizations

Reconstruction induced by external environment (such as applied voltage bias and test electrolytes) changes catalyst component and catalytic behaviors. Investigations of complete reconstruction in energy conversion recently receive intensive attention, which promote the targeted design of top-performance materials with maximum component utilization and good stability. However, the advantages of complete reconstruction, its design strategies, and extensive applications have not achieved the profound understandings and summaries it deserves. Here, this review systematically summarizes several important advances in complete reconstruction for the first time, which includes 1) fundamental understandings of complete reconstruction, the characteristics and advantages of completely reconstructed catalysts, and their design principles, 2) types of reconstruction-involved precatalysts for oxygen evolution reaction catalysis in wide pH solution, and origins of limited reconstruction degree as well as design strategies/principles toward complete reconstruction, 3) complete reconstruction for novel material synthesis and other electrocatalysis fields, and 4) advanced in situ/operando or multiangle/level characterization techniques to capture the dynamic reconstruction processes and real catalytic contributors. Finally, the existing major challenges and unexplored/unsolved issues on studying the reconstruction chemistry are summarized, and an outlook for the further development of complete reconstruction is briefly proposed. This review will arouse the attention on complete reconstruction materials and their applications in diverse fields.

An Orthorhombic Modification of KCoPO4 Stabilized under Hydrothermal Conditions: Crystal Chemistry and Magnetic Behavior

A novel modification of the KCoPO₄, δ-phase has been prepared by hydrothermal synthesis at 553 K. The compound crystallizes in the orthorhombic system with the unit-cell parameters a = 8.5031(8), b = 10.2830(5), c = 54.170(4) Å. The crystal structure was determined based on synchrotron low-temperature single-crystal X-ray diffraction data obtained from an inversion twin in the space group P2₁2₁2₁ and refined to R = 0.077 for 5156 reflections with I > 3σ(I). The δ-KCoPO₄ possesses a new structure type which is based on a framework built from sharing vertices Co- and P-centered tetrahedra. The {CoPO₄^-}_∞ construction of tetrahedra may be described as assembled from networks formed by two topologically diverse six-membered rings of tetrahedra stacked together through vertex-bridging contacts along the a axis. The ratio of the (UUUDDD) and (UUDUDD) rings, where (U) and (D) denote the orientation of the tetrahedra in the six-membered rings up and down relative to the plane grids, is equal to 5:1. The (UUDUDD) rings form bands parallel to the [010] direction each surrounded from both sides along the c axis by slabs of five ribbons width having alternative (UUUDDD) topology. Open in the [100] direction channels incorporate K⁺ ions; this structural feature permits to suppose ion-conductive and/or electrochemical properties of the title compound. The possible mechanism of the δ → γ phase transition is discussed on the basis of the crystal chemical analysis of the KCoPO₄ polymorphs. The title compound orders magnetically at T_N = 24.8 K.

Design Strategies of Non-Noble Metal-Based Electrocatalysts for Two-Electron Oxygen Reduction to Hydrogen Peroxide

Hydrogen peroxide (H₂ O₂ ) is a highly value-added and environmentally friendly chemical with various applications. The production of H₂ O₂ by electrocatalytic 2e^- oxygen reduction reaction (ORR) has drawn considerable research attention, with a view to replacing the currently established anthraquinone process. Electrocatalysts with low cost, high activity, high selectivity, and superior stability are in high demand to realize precise control over electrochemical H₂ O₂ synthesis by 2e^- ORR and the feasible commercialization of this system. This Review introduces a comprehensive overview of non-noble metal-based catalysts for electrochemical oxygen reduction to afford H₂ O₂ , providing an insight into catalyst design and corresponding reaction mechanisms. It starts with an in-depth discussion on the origins of 2e^- /4e^- selectivity towards ORR for catalysts. Recent advances in design strategies for non-noble metal-based catalysts, including carbon nanomaterials and transition metal-based materials, for electrochemical oxygen reduction to H₂ O₂ are then discussed, with an emphasis on the effects of electronic structure, nanostructure, and surface properties on catalytic performance. Finally, future challenges and opportunities are proposed for the further development of H₂ O₂ electrogeneration through 2e^- ORR, from the standpoints of mechanistic studies and practical application.

Aqueous Rechargeable Zn-N2 Battery Assembled by Bifunctional Cobalt Phosphate Nanocrystals-Loaded Carbon Nanosheets for Simultaneous NH3 Production and Power Generation

Developing cost-effective and controllable technologies beyond traditional overall N₂ electrocatalysis is critical for the large-scale production of NH₃ through electrochemical N₂ reduction reaction (NRR) under ambient conditions. Herein, the aqueous rechargeable Zn-N₂ battery, assembled by coupling the bifunctional cobalt phosphate nanocrystals-loaded heteroatoms-doped carbon nanosheets (CoPi/NPCS) as cathode electrocatalyst and the commercial Zn plate as anode with KOH electrolyte, was fabricated for the sustainable reduction of N₂ to NH₃ and power generation during discharge process. Benefiting from the desirable active components of cobalt phosphate nanocrystals and the synergistic effect between nanocrystals and carbon substrates, the CoPi/NPCS catalyst exhibits the enhanced NRR and oxygen evolution reaction (OER) performance in alkaline electrolyte. And the cobalt phosphates are confirmed as active components through the associative pathway toward NRR. When measured in the flow battery configuration with gas diffusion electrode by flowing N₂ during discharge, this CoPi/NPCS-catalyzed Zn-N₂ battery enables the high N₂-to-NH₃ yield rate of 14.7 μg h^-1 mg_cat.^-1 and Faradaic efficiency of 16.35% at 0.6 V vs Zn²⁺/Zn, which can be able to maintain stable in discharge processes during cycling tests. Moreover, the impressive power output of the peak power density of 0.49 mW cm^-2 and the energy density of 147.6 mWh g_zn^-1 are still achieved by this Zn-N₂ battery, which are both higher than those of previously reported Zn-N₂ batteries. This work not only provides the guideline for the rational design of robust and active bifunctional NRR-OER catalysts but also develops a reasonable and promising technology for efficient electrochemical N₂-to-NH₃ and power generation.

Design Strategies of Transition-Metal Phosphate and Phosphonate Electrocatalysts for Energy-Related Reactions

The key challenge to developing renewable energy conversion and storage devices lies in the exploration and rational engineering of cost-effective and highly efficient electrocatalysts for various energy-related electrochemical reactions. Transition-metal phosphates and phosphonates have shown remarkable performances for these reactions based on their unique physicochemical properties. Compared with transition-metal oxides, phosphate groups in transition-metal phosphates and phosphonates show flexible coordination with diverse orientations, making them an ideal platform for designing active electrocatalysts. Although numerous efforts have been spent on the development of transition-metal phosphate and phosphonate electrocatalysts, some urgent issues, such as low intrinsic catalytic efficiency and low electronic conductivity, have to be resolved in accordance with their applications. In this Review, we focus on the design strategies of highly efficient transition-metal phosphate and phosphonate electrocatalysts, with special emphasis on the tuning of transition-metal-center coordination environment, optimization of electronic structures, increase of catalytically active site densities, and construction of heterostructures. Guided by these strategies, recently developed transition-metal phosphate and phosphonate materials have exhibited excellent activity, selectivity, and stability for various energy-related electrocatalytic reactions, showing great potential for replacing noble-metal-based catalysts in next-generation advanced energy techniques. The existing challenges and prospects regarding these materials are also presented.

A solution-assisted etching preparation of an MOF-derived NH4CoPO4·H2O/Ti3C2Tx MXene nanocomposite for high-performance hybrid supercapacitors

A novel synthetic is explored by introducing the Co-MOF(ZIF-67) into the layer space of Ti₃C₂T_x and subsequent in situ etching to transform into NH₄CoPO₄·H₂O/Ti₃C₂T_x nanocomposite and used for high-performance hybrid supercapcitor.