Nanostructured Metal Phosphide Based Catalysts for Electrochemical Water Splitting: A Review

Electron and surface engineering of Ni2P/MnP4 heterojunction as high performance bifunctional electrocatalyst for amperage-level overall water splitting

Producing hydrogen through electrocatalytic overall water splitting with ampere-level current density is still limited by the high cost and poor stability of electrocatalysts. In this work, a new type Ni2P/MnP4 heterojunction composite material was designed and prepared as bifunctional electrocatalyst. Based on XPS spectra and theoretical calculation, the formation of Ni2P/MnP4 heterojunction successfully modulates the local electronic structure of Ni2P and enhances the ionization of H and Ni by increasing the electron transfer rate. Moreover, the special nanovilli structure and superhydropholic/superaerophobic surface of Ni2P/MnP4 heterojunction accelerates the transfer of electrolyte and gaseous products. Benefiting from these advantages, the as-prepared Ni2P/MnP4/CF not only exhibits superior electrocatalytic performance, which can release 10 mA/cm2 current density with a low overpotential of 69 mV and 247 mV for HER and OER respectively, but also shows admirable stability of continuous overall water splitting to drive 1000 mA/cm2 for 180 h without notable activity degradation. We believe this material possesses outstanding potential for industrial applications, and our strategy may provide a new pathway to design relative materials.

Construction of unique NiCoP/FeNiCoP hollow heterostructured ellipsoids with modulated electronic structure for enhanced overall water splitting

Transition metal phosphides have been demonstrated to be promising non-noble catalysts for water splitting, yet their electrocatalytic performance is impeded by unfavorable free energies of adsorbed intermediates. The achievement of nanoscale modulation in morphology and electronic states is imperative for enhancing their intrinsic electrocatalytic activity. Herein, we propose a strategy to expedite the water splitting process over NiCoP/FeNiCoP hollow ellipsoids by modulating the electronic structure and d-band center. These unique phosphorus (P) vacancies-rich ellipsoids are synthesized through an ion-exchange reaction between uniform NiCo-nanoprisms and K3[Fe(CN)6], followed by NaH2PO2-assisted phosphorization under N2 atmosphere. Various characterizations reveals that the titled catalyst possesses high specific surface area, abundant porosity, and accessible inner surfaces, all of which are beneficial for efficient mass transfer and gas diffusion. Moreover, density functional theory (DFT) calculations further confirms that the NiCoP/FeNiCoP heterojunction associated with P vacancies regulate the electronic structures of d-electrons and p-electrons of Co and P atoms, respectively, resulting in a higher desorption efficiency of adsorbed H* intermediates with a lower energy barrier for water splitting. Due to the aforementioned advantages, the resultant NiCoP/FeNiCoP hollow ellipsoids exhibit remarkably low overpotentials of 45 and 266 mV for hydrogen and oxygen evolution reaction to achieve the current densities of 10 and 50 mA cm-2, respectively. This work not only reports the synthesis of a hollow double-shell structure of NiCoP/FeNiCoP but also introduces a novel strategy for constructing a multifunctional electrocatalyst for water splitting.

Material Engineering Strategies for Efficient Hydrogen Evolution Reaction Catalysts

Water electrolysis, a key enabler of hydrogen energy production, presents significant potential as a strategy for achieving net-zero emissions. However, the widespread deployment of water electrolysis is currently limited by the high-cost and scarce noble metal electrocatalysts in hydrogen evolution reaction (HER). Given this challenge, design and synthesis of cost-effective and high-performance alternative catalysts have become a research focus, which necessitates insightful understandings of HER fundamentals and material engineering strategies. Distinct from typical reviews that concentrate only on the summary of recent catalyst materials, this review article shifts focus to material engineering strategies for developing efficient HER catalysts. In-depth analysis of key material design approaches for HER catalysts, such as doping, vacancy defect creation, phase engineering, and metal-support engineering, are illustrated along with typical research cases. A special emphasis is placed on designing noble metal-free catalysts with a brief discussion on recent advancements in electrocatalytic water-splitting technology. The article also delves into important descriptors, reliable evaluation parameters and characterization techniques, aiming to link the fundamental mechanisms of HER with its catalytic performance. In conclusion, it explores future trends in HER catalysts by integrating theoretical, experimental and industrial perspectives, while acknowledging the challenges that remain.

Synergetic Catalytic Effect between Ni and Co in Bimetallic Phosphide Boosting Hydrogen Evolution Reaction

The application of electrochemical hydrogen evolution reaction (HER) for renewable energy conversion contributes to the ultimate goal of a zero-carbon emission society. Metal phosphides have been considered as promising HER catalysts in the alkaline environment, which, unfortunately, is still limited owing to the weak adsorption of H* and easy dissolution during operation. Herein, a bimetallic NiCoP-2/NF phosphide is constructed on nickel foam (NF), requiring rather low overpotentials of 150 mV and 169 mV to meet the current densities of 500 and 1000 mA cm-2, respectively, and able to operate stably for 100 h without detectable activity decay. The excellent HER performance is obtained thanks to the synergetic catalytic effect between Ni and Co, among which Ni is introduced to enhance the intrinsic activity and Co increases the electrochemically active area. Meanwhile, the protection of the externally generated amorphous phosphorus oxide layer improves the stability of NiCoP/NF. An electrolyser using NiCoP-2/NF as both cathode and anode catalysts in an alkaline solution can produce hydrogen with low electric consumption (overpotential of 270 mV at 500 mA cm-2).

Precious Metal Free Hydrogen Evolution Catalyst Design and Application

The quest to identify precious metal free hydrogen evolution reaction catalysts has received unprecedented attention in the past decade. In this Review, we focus our attention to recent developments in precious metal free hydrogen evolution reactions in acidic and alkaline electrolyte owing to their relevance to commercial and near-commercial low-temperature electrolyzers. We provide a detailed review and critical analysis of catalyst activity and stability performance measurements and metrics commonly deployed in the literature, as well as review best practices for experimental measurements (both in half-cell three-electrode configurations and in two-electrode device testing). In particular, we discuss the transition from laboratory-scale hydrogen evolution reaction (HER) catalyst measurements to those in single cells, which is a critical aspect crucial for scaling up from laboratory to industrial settings but often overlooked. Furthermore, we review the numerous catalyst design strategies deployed across the precious metal free HER literature. Subsequently, we showcase some of the most commonly investigated families of precious metal free HER catalysts; molybdenum disulfide-based, transition metal phosphides, and transition metal carbides for acidic electrolyte; nickel molybdenum and transition metal phosphides for alkaline. This includes a comprehensive analysis comparing the HER activity between several families of materials highlighting the recent stagnation with regards to enhancing the intrinsic activity of precious metal free hydrogen evolution reaction catalysts. Finally, we summarize future directions and provide recommendations for the field in this area of electrocatalysis.

SOx Functionalized NiOOH Nanosheets Embedded in Ni(OH)2 Microarray for High-Efficiency Seawater Oxidation

A nano-micro heterostructure has been established to address the challenges of selectivity, stress, pitting corrosion, and long-term durability of anodes in unpurified seawater. The heterostructure comprised NiOOH nanosheets embedded within a high surface area Ni(OH)2 microarray, and the surface structure is further functionalized with sulfate (SOx). This cation-selective protective layer impedes chloride (Cl-) diffusion and abstracts H from reaction intermediates, leading to enhanced selectivity and corrosion resistance of the anode. The multilevel porosity within the randomly oriented nanosheets and the underlying support provide short diffusion channels for ions and mass migration, ensuring efficient ion transport and long-term structural and mechanical durability of the active sites, even at high current density. Remarkably, the catalyst requires a small input voltage of 400 mV to deliver a current density of 1 A cm-2 and maintains it for over 168 h without noticeable degradation or hypochlorite formation. Spectroscopic analysis and density functional theory (DFT) calculations reveal that the Ni electronic structure in the +3 valence state, its strong structural interaction with the underlying microarray, and the functionality of SOx significantly reduce the required potential for O-O coupling.

Manganese-Doped Bimetallic (Co,Ni)2P Integrated CoP in N,S Co-Doped Carbon: Unveiling a Compatible Hybrid Electrocatalyst for Overall Water Splitting

Rational design of highly efficient noble-metal-unbound electrodes for hydrogen and oxygen production at increased current density is crucial for robust water-splitting. A facile hydrothermal and room-temperature aging method is presented, followed by chemical vapor deposition (CVD), to create a self-sacrificed hybrid heterostructure electrocatalyst. This hybrid material, (Mn-(Co,Ni)2P/CoP/(N,S)-C), comprises manganese-doped cobalt nickel phosphide (Mn-(Co,Ni)2P) nanofeathers and cobalt phosphide (CoP) nanocubes embedded in a nitrogen and sulfur co-doped carbon matrix (N,S)-C on nickel foam. The catalyst exhibits excellent performance in both the hydrogen evolution reaction (HER; η10 = 61 mV) and oxygen evolution reaction (OER; η10 = 213 mV) due to abundant active sites, high porosity, and enhanced hetero-interface interaction between Mn-(Co2P-Ni2P) CoP, and (N,S)-C supported by significant synergistic effects observed among different phases through density functional theory (DFT) calculations. Impressively, (Mn-(Co,Ni)2P/CoP/(N,S)-C (+,-) shows an extra low cell voltage of 1.49 V@10 mA cm-2. Moreover, the catalyst exhibits remarkable stability at 100 and 300 mA cm-2 when operating as a single stack cell electrolyzer. The superior electrochemical activity is attributed to the enhanced electrode-electrolyte interface among the multiple phases of the hybrid structure.

Novel mixed heterovalent (Mo/Co)Ox-zerovalent Cu system as bi-functional electrocatalyst for overall water splitting

A novel hybrid ternary metallic electrocatalyst of amorphous Mo/Co oxides and crystallized Cu metal was deposited over Ni foam using a one-pot, simple, and scalable solvothermal technique. The chemical structure of the prepared ternary electrocatalyst was systematically characterized and confirmed via XRD, FTIR, EDS, and XPS analysis techniques. FESEM images of (Mo/Co)Ox-Cu@NF display the formation of 3D hierarchical structure with a particle size range of 3-5 µm. The developed (Mo/Co)Ox-Cu@NF ternary electrocatalyst exhibits the maximum activity with 188 mV and 410 mV overpotentials at 50 mA cm-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Electrochemical impedance spectroscopy (EIS) results for the (Mo/Co)Ox-Cu@NF sample demonstrate the minimum charge transfer resistance (Rct) and maximum constant phase element (CPE) values. A two-electrode cell based on the ternary electrocatalyst just needs a voltage of about 1.86 V at 50 mA cm-2 for overall water splitting (OWS). The electrocatalyst shows satisfactory durability during the OWS for 24 h at 10 mA cm-2 with an increase of only 33 mV in the cell potential.

Chemical catalytic upgrading of polyethylene terephthalate plastic waste into value-added materials, fuels and chemicals

The substantial production of polyethylene terephthalate (PET) products, coupled with high abandonment rates, results in significant environmental pollution and resource wastage. This has prompted global attention to the development of rational strategies for PET waste treatment. In the context of renewability and sustainability, catalytic chemical technology provides an effective means to recycle and upcycle PET waste into valuable resources. In this review, we initially provide an overview of strategies employed in the thermocatalytic process to recycle PET waste into valuable carbon materials, fuels and typical refined chemicals. The effect of catalysts on the quality and quantity of specific products is highlighted. Next, we introduce the development of renewable-energy-driven electrocatalytic and photocatalytic systems for sustainable PET waste upcycling, focusing on rational catalysts, innovative catalytic system design, and corresponding underlying catalytic mechanisms. Moreover, we discuss advantages and disadvantages of three chemical catalytic strategies. Finally, existing limitations and outlook toward controllable selectivity and yield enhancement of value-added products and PET upvaluing technology for scale-up applications are proposed. This review aims to inspire the exploration of waste-to-treasure technologies for renewable-energy-driven waste management toward a circular economy.

Rapid High-Temperature Liquid Shock Synthesis of High-Entropy Alloys for Hydrogen Evolution Reaction

High-entropy-alloy nanoparticles (HEA-NPs) show great potential as electrocatalysts for water splitting, fuel cells, CO2 conversion, etc. However, fine-tuning the surface, morphology, structure, and crystal phase of HEA remains a great challenge. Here, the high-temperature liquid shock (HTLS) technique is applied to produce HEA-NPs, e.g., PtCoNiRuIr HEA-NPs, with tunable elemental components, ultrafine particle size, controlled crystal phases, and lattice strains. HTLS directly applied Joule heating on the liquid mixture of metal precursors, capping agents, and reducing agents, which is feasible for controlling the morphology and structure such as the atomic arrangement of the resulting products, thereby facilitating the rationally designed nanocatalysts. Impressively, the as-obtained PtCoNiRuIr HEA-NPs delivered superior activity and long-term stability for the hydrogen evolution reaction (HER), with low overpotentials at 10 mA cm-2 and 1 A cm-2 of only 18 and 408 mV, respectively, and 10000 CV stable cycles in 0.5 M H2SO4. Furthermore, in the near future, by combining the HTLS method with artificial intelligence (AI) and theoretical calculations, it is promising to provide an advanced platform for the high-throughput synthesis of HEA nanocatalysts with optimized performance for various energy applications, which is of great significance for achieving a carbon-neutral society with an effective and environmentally friendly energy system.

Self-Assembled Preparation of Porous Nickel Phosphide Superparticles with Tunable Phase and Porosity for Efficient Hydrogen Evolution

Self-assembly of colloidal nanoparticles enables the easy building of assembly units into higher-order structures and the bottom-up preparation of functional materials. Nickel phosphides represent an important group of catalysts for hydrogen evolution reaction (HER) from water splitting. In this paper, the preparation of porous nickel phosphide superparticles and their HER efficiencies are reported. Ni and Ni2 P nanoparticles are self-assembled into binary superparticles via an oil-in-water emulsion method. After annealing and acid etching, the as-prepared Ni-Ni2 P binary superparticles change into porous nickel phosphide superparticles. The porosity and crystalline phase of the superparticles can be tuned by adjusting the ratio of Ni and Ni2 P nanoparticles. The resulting porous superparticles are effective in driving HER under acidic conditions, and the modulation of porosity and phase further optimize the electrochemical performance. The prepared Ni3 P porous superparticles not only possess a significantly enhanced specific surface area compared to solid Ni-Ni2 P superparticles but also exhibit an excellent HER efficiency. The calculations based on the density functional theories show that the (110) crystal facet exhibits a relatively lower Gibbs free energy of hydrogen adsorption. This work provides a self-assembly approach for the construction of porous metal phosphide nanomaterials with tunable crystalline phase and porosity.

Controllable Electronic Transfer Tailoring d-band Center via Cobalt-Oxygen-Bridged Ru/Fe Dual-sites for Boosted Oxygen Evolution

Rational tailoring of the electronic structure at the defined active center of reconstructed metal (oxy)hydroxides (MOOH) during oxygen evolution reaction (OER) remains a challenge. With the guidance of density functional theory (DFT), herein a dual-site regulatory strategy is reported to tailor the d-band center of the Co site in CoOOH via the controlled electronic transfer at the Ru─O─Co─O─Fe bonding structure. Through the bridged O2- site, electrons are vastly flowed from the t2g -orbital of the Ru site to the low-spin orbital of the Co site in the Ru-O-Co coordination and further transfer from the strong electron-electron repulsion of the Co site to the Fe site by the Co-O-Fe coordination, which balancing the electronic configuration of Co sites to weaken the over-strong adsorption energy barrier of OH* and O* , respectively. Benefiting from the highly active of the Co site, the constructed (Ru2 Fe2 Co6 )OOH provide an extremely low overpotential of 248 mV and a Tafel slope of 32.5 mV dec-1 at 10 mA cm-2 accompanied by long durability in alkaline OER, far superior over the pristine and Co-O-Fe bridged CoOOH catalysts. This work provides guidance for the rational design and in-depth analysis of the optimized role of metal dual-sites.

An efficient cerium dioxide incorporated nickel cobalt phosphide complex as electrocatalyst for All-pH hydrogen evolution reaction and overall water splitting

Transition metal phosphides (TMPs) have been considered as potential electrocatalysts with adjustable valence states, metal characteristics, and phase diversity. However, it is necessary but remains a major challenge to obtain efficient and durable TMPs catalysts, which can realize efficiently for not only all-pH hydrogen evolution reaction (HER), but also oxygen evolution reaction (OER). Hence, cerium dioxide incorporated nickel cobalt phosphide growth on nickel foam (CeO2/NiCoP) is fabricated by hydrothermal and phosphating reaction. CeO2/NiCoP shows excellent activity for all-pH HER (overpotentials of 48, 58 and 72 mV in alkaline, neutral and acidic solution at the current density of 10 mA cm-2), and has a small OER overpotential (231 mV @ 10 mA cm-2). Moreover, the voltage of overall water splitting in alkaline solution and simulated seawater electrolyte is only 1.46 and 1.41 V (10 mA cm-2), respectively, coupled with outstanding operational stability and corrosion resistance. Further mechanism research shows that CeO2/NiCoP possesses rich heterointerfaces, which serves more exposed active sites and possesses a promising superhydrophilic and superaerophobic surface. Density functional theory calculations manifest that CeO2/NiCoP has appropriate energy for intermediates of reactions. This work provides a deep insight into the CeO2/NiCoP catalyst for high-performance water/seawater electrolysis.

Enhanced electrocatalytic efficiencies for water electrolysis and para-nitrophenol hydrogenation by self-supported nickel cobalt phosphide-nickel iron layered double hydroxide p-n junction

Charge redistribution across heterointerfaces is an important tactic to enhance the catalytic activities and bifunctionality of hybrid catalysts, especially for green hydrogen production from water electrolysis and harmless electrocatalytic valorization of organics. Herein, a self-supported p-n junction catalytic electrode was constructed by tandem electrodeposition of nickel cobalt phosphide (NiCoP) and nickel iron layered double hydroxide (NiFe LDH) onto Ni foam (NF) substrate, denoted as NiCoP@NiFe LDH/NF, to enhance the electrocatalytic capabilities for water electrolysis and hydrogenation of an organic, para-nitrophenol (4-NP). Benefitting from the charge redistribution across the p-n junction, high electrocatalytic efficiencies for oxygen evolution reaction (OER, overpotential of 388 mV at 100 mA cm-2) and hydrogen evolution reaction (HER, overpotential of 132 mV at 10 mA cm-2) could be achieved concurrently by the NiCoP@NiFe LDH/NF electrode, and both overpotentials were located within the mainstream levels in this domain. The bifunctional catalytic features enabled a full water electrolysis response of 10 mA cm-2 at 1.61 V. In addition, the p-n junction electrode catalyzed the hydrogenation of 4-NP at a conversion of 100%, para-aminophenol (4-AP) selectivity of 90% and faradaic efficiency (FE) of 88% at -0.18 V. The current work offers a feasible strategy for fulfilling electrochemical H2 production and hydrogenation valorization of 4-NP pollutant by constructing a self-supported p-n junction catalytic electrode.

Facile Construction of CuFe-Based Metal Phosphides for Synergistic NOx - Reduction to NH3 and Zn-Nitrite Batteries in Electrochemical Cell

The electrocatalytic nitrite/nitrate reduction reaction (eNO2 RR/eNO3 RR) offer a promising route for green ammonia production. The development of low cost, highly selective and long-lasting electrocatalysts for eNO2 RR/eNO3 RR is challenging. Herein, a method is presented for constructing Cu3 P-Fe2 P heterostructures on iron foam (CuFe-P/IF) that facilitates the effective conversion of NO2 - and NO3 - to NH3 . At -0.1 and -0.2 V versus RHE (reversible hydrogen electrode), CuFe-P/IF achieves a Faradaic efficiency (FE) for NH3 production of 98.36% for eNO2 RR and 72% for eNO3 RR, while also demonstrating considerable stability across numerous cycles. The superior performance of CuFe-P/IF catalyst is due tothe rich Cu3 P-Fe2 P heterstuctures. Density functional theory calculations have shed light on the distinct roles that Cu3 P and Fe2 P play at different stages of the eNO2 RR/eNO3 RR processes. Fe2 P is notably active in the early stages, engaging in the capture of NO2 - /NO3 - , O─H formation, and N─OH scission. Conversely, Cu3 P becomes more dominant in the subsequent steps, which involve the formation of N─H bonds, elimination of OH* species, and desorption of the final products. Finally, a primary Zn-NO2 - battery is assembled using CuFe-P/IF as the cathode catalyst, which exhibits a power density of 4.34 mW cm-2 and an impressive NH3 FE of 96.59%.

Cation-doped sea-urchin-like MnO2 for electrocatalytic overall water splitting

It is necessary to take full account of the activity, selectivity, dynamic performance, economic benefits, and environmental impact of the catalysts in the overall water splitting of electrocatalysis for the reasonable design of electrocatalysts. Designing nanostructures of catalysts and optimizing defect engineering are considered environmentally friendly and cost-effective electrocatalyst synthesis strategies. Herein, we report that metal cations regulate the microstructure of sea-urchin-like MnO2 and act as dopants to cause the lattice expansion of MnO2, resulting in crystal surface defects. The valence unsaturated Mn4+/Mn3+ greatly promotes the electrocatalytic oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The optimal Al-MnO2 showed that the overpotential is 390 and 170 mV in the process of catalyzing OER and HER, respectively, at a current density of 10 mA cm-2. It is exciting to note that after 5000 cycles of Al-MnO2 within the kinetic potential range of OER and HER, its performance remained almost unchanged. This work provides a simple, efficient, and environmentally friendly route for the design of efficient integrated water-splitting electrocatalysts.

Fe2O3/Ni Nanocomposite Electrocatalyst on Cellulose for Hydrogen Evolution Reaction and Oxygen Evolution Reaction

A key challenge in the development of sustainable water-splitting (WS) systems is the formulation of electrodes by efficient combinations of electrocatalyst and binder materials. Cellulose, a biopolymer, can be considered an excellent dispersing agent and binder that can replace high-cost synthetic polymers to construct low-cost electrodes. Herein, a novel electrocatalyst was fabricated by combining Fe2O3 and Ni on microcrystalline cellulose (MCC) without the use of any additional binder. Structural characterization techniques confirmed the formation of the Fe2O3-Ni nanocomposite. Microstructural studies confirmed the homogeneity of the ~50 nm-sized Fe2O3-Ni on MCC. The WS performance, which involves the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), was evaluated using a 1 M KOH electrolyte solution. The Fe2O3-Ni nanocomposite on MCC displayed an efficient performance toward lowering the overpotential in both the HER (163 mV @ 10 mA cm-2) and OER (360 mV @ 10 mA cm-2). These results demonstrate that MCC facilitated the cohesive binding of electrocatalyst materials and attachment to the substrate surface. In the future, modified cellulose-based structures (such as functionalized gels and those dissolved in various media) can be used as efficient binder materials and alternative options for preparing electrodes for WS applications.

Synergetic Effect and Phase Engineering by Formation of Ti3C2Tx Modified 2H/1T-MoSe2 Composites for Enhanced HER

The typical semi conductivity and few active sites of hydrogen evolution of 2H MoSe2 severely restrict its electrocatalytic hydrogen evolution performance. At the same time, the 1T MoSe2 has metal conductivity and plentiful hydrogen evolution sites, making it feasible to optimize the electrocatalytic hydrogen evolution behavior of MoSe2 using phase engineering. In this study, we, through a simple one-step hydrothermal method, composed 1T/2H MoSe2, and then used newly emerging transition metal carbides with several atomic-layer thicknesses Ti3C2Tx to improve the conductivity of a MoSe2-based electrocatalyst. Finally, MoSe2@Ti3C2Tx was successfully synthesized, according to the control of the additional amount of Ti3C2Tx, to form a proper MoSe2/ Ti3C2Tx heterostructure with a better electrochemical HER performance. As obtained MoSe2@4 mg-Ti3C2Tx achieved a low overpotential, a small Tafel slope and this work offers additional insight into broadened MoSe2 and MXenes-based catalyst's electrochemical application.

Application of Oxygen-Group-Based Amorphous Nanomaterials in Electrocatalytic Water Splitting

Environmentally friendly energy sources (e.g., hydrogen) require an urgent development targeting to address the problem of energy scarcity. Electrocatalytic water splitting is being explored as a convenient catalytic reaction in this context, and promising amorphous nanomaterials (ANMs) are receiving increasing attention due to their excellent catalytic properties.Oxygen group-based amorphous nanomaterials (O-ANMs) are an important component of the broad family of ANMs due to their unique amorphous structure, large number of defects, and abundant randomly oriented bonds, O-ANMs induce the generation of a larger number of active sites, which favors a better catalytic activity. Meanwhile, amorphous materials can disrupt the inherent features of conventional crystalline materials regarding electron transfer paths, resulting in higher flexibility. O-ANMs mainly include VIA elements such as oxygen, sulfur, selenium, tellurium, and other transition metals, most of which are reported to be free of noble metals and have comparable performance to commercial catalysts Pt/C or IrO2 and RuO2 in electrocatalysis. This review covers the features and reaction mechanism of O-ANMs, the synthesis strategies to prepare O-ANMs, as well as the application of O-ANMs in electrocatalytic water splitting. Last, the challenges and prospective remarks for future development in O-ANMs for electrocatalytic water splitting are concluded.

Highly N-Doped Fe/Co Phosphide Superstructures for Efficient Water Splitting

Developing an inexpensive bifunctional electrocatalyst for overall water splitting is critical for acquiring scalable green hydrogen and thereby realizing carbon neutralization. Herein, an "all-in-one" method is developed for the fabrication of highly N-doped binary FeCo-phosphides (N-FeCoP) with hierarchical superstructure, this delicately designed synthesis route allows the following merits for benefiting water splitting electrocatalysis in alkaline, including high N/defect-doping for mediating the surface property of the as-made N-FeCoP, binary Fe and Co components exhibiting strong coupling interaction, and 3D hierarchical superstructure for shortening diffusion length and thereby improving reaction kinetics. Electrochemical measurements reveal that the N-FeCoP sample exhibits very low overpotentials for initiating the hydrogen and oxygen evolution reactions. Remarkably, overall water splitting can be promoted on N-FeCoP using a commercial primary Zn-MnO2 battery. The developed synthesis strategy may potentially inspire the preparation of other N-doped metal-based nanostructures for broad electrocatalysis.

A Facile Molecular Approach to Amorphous Nickel Pnictides and Their Reconstruction to Crystalline Potassium-Intercalated γ-NiOOHx Enabling High-Performance Electrocatalytic Water Oxidation and Selective Oxidation of 5-Hydroxymethylfurfural

The low-temperature molecular precursor approach can be beneficial to conventional solid-state methods, which require high temperatures and lead to relatively large crystalline particles. Herein, a novel, single-step, room-temperature preparation of amorphous nickel pnictide (NiE; EP, As) nanomaterials is reported, starting from NaOCE(dioxane)n and NiBr2 (thf)1.5 . During application for the oxygen evolution reaction (OER), the pnictide anions leach, and both materials fully reconstruct into nickel(III/IV) oxide phases (similar to γ-NiOOH) comprising edge-sharing (NiO6 ) layers with intercalated potassium ions and a d-spacing of 7.27 Å. Remarkably, the intercalated γ-NiOOHx phases are nanocrystalline, unlike the amorphous nickel pnictide precatalysts. This unconventional reconstruction is fast and complete, which is ascribed to the amorphous nature of the nanostructured NiE precatalysts. The obtained γ-NiOOHx can effectively catalyse the OER for 100 h at a high current density (400 mA cm-2 ) and achieves outstandingly high current densities (>600 mA cm-2 ) for the selective, value-added oxidation of 5-hydroxymethylfurfural (HMF). The NiP-derived γ-NiOOHx shows a higher activity for both processes due to more available active sites. It is anticipated that the herein developed, effective, room-temperature molecular synthesis of amorphous nickel pnictide nanomaterials can be applied to other functional transition-metal pnictides.

Iron-doped bimetallic boride Fe-Ni2B/NF-x nanoparticles toward efficient oxygen evolution reaction at a large current density

Transition metal borides are seen as potential candidates for oxygen evolution reaction (OER) electrocatalysts due to their superconductivity and rich surface-active sites, but monometallic borides only display generic OER catalytic performance. Hence, iron-doped bimetallic boride nanoparticles (Fe-Ni2B/NF-x) on Ni foam are reported and applied as superior OER electrocatalysts with high catalytic activities. Such bimetallic boride electrocatalysts require overpotentials of only 194 and 336 mV to afford current densities of 10 and 500 mA cm-2 toward the OER in 1 M KOH electrolyte, and Fe-Ni2B/NF-3 can retain this catalytic stability for at least 100 h at 1.456 V. The performance of the improved catalyst Fe-Ni2B/NF-3 matches the best nickel-based OER electrocatalysts reported so far. Analysis of X-ray photoelectron spectroscopy (XPS) and Gibbs free energy calculations show that Fe-doping essentially acts to modulate the electronic density of Ni2B and lower the free energy of O adsorption in the OER. The charge density differences and d-band theory proved that Fe sites have a high charge state and can be taken as catalytic sites for the OER. This proposed synthesis strategy provides a different view for preparing efficient bimetallic boride electrocatalysts.

Self-supported amorphous phosphide catalytic electrodes for electrochemical hydrogen production coupling with methanol upgrading

Efficient catalytic electrodes for cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) are pivotal for massive production of green hydrogen from water electrolysis, and the further replacement of kinetically sluggish OER by tailored elecrooxidation of certain organics is a promising way to co-produce hydrogen and value-added chemicals via a more energy-saving and safer manner. Herein, amorphous Ni-Co-Fe ternary phosphides (Ni_xCo_yFe_z-Ps) with different Ni:Co:Fe ratios electrodeposited onto Ni foam (NF) substrate were served as self-supported catalytic electrodes for alkaline HER and OER. The Ni₄Co₄Fe₁-P electrode deposited in solution at Ni:Co:Fe ratio of 4:4:1 displayed low overpotential (61 mV at -20 mA cm^-2) and acceptable durability for HER, while the Ni₂Co₂Fe₁-P electrode fabricated in deposition solution at Ni:Co:Fe ratio of 2:2:1 showed good OER efficiency (overpotential of 275 mV at 20 mA cm^-2) and robust durability, the further replacement of OER by anodic methanol oxidation reaction (MOR) enabled selective production of formate with 110 mV lower anodic potential at 20 mA cm^-2. The HER-MOR co-electrolysis system based on Ni₄Co₄Fe₁-P cathode and Ni₂Co₂Fe₁-P anode could save 1.4 kWh of electric energy per cubic meter of H₂ relative to mere water electrolysis. The current work offers a feasible approach to co-produce H₂ and value-upgraded formate via an energy-saving manner by rational design of catalytic electrodes and construction of co-electrolysis system, and paves the way for cost-effective co-preparation of more value-added organics and green hydrogen via electrolysis.

Interface engineering of Ni2P/MoOx decorated NiFeP nanosheets for enhanced alkaline hydrogen evolution reaction at high current densities

The rational design of high-performance non-noble metal electrocatalysts at large current densities is important for the development of sustainable energy conversion devices such as alkaline water electrolyzers. However, improving the intrinsic activity of those non-noble metal electrocatalysts remains a great challenge. Therefore, Ni₂P/MoO_x decorated three-dimensional (3D) NiFeP nanosheets (NiFeP@Ni₂P/MoO_x) with abundant interfaces were synthesized using facile hydrothermal and phosphorization methods. NiFeP@Ni₂P/MoO_x exhibits excellent electrocatalytic activity for hydrogen evolution reaction (HER) at a high current density of -1000 mA cm^-2 with a low overpotential of 390 mV. Surprisingly, it can operate steadily at a large current density of -500 mA cm^-2 for 300 h, indicating its long-term durability under high current densities. The boosted electrocatalytic activity and stability can be ascribed to the as-fabricated heterostructures via interface engineering, leading to modifying the electronic structure, improving the active area, and enhancing the stability. Besides, the 3D nanostructure is also beneficial for exposing abundant accessible active sites. Therefore, this research proposes a considerable route for fabricating non-noble metal electrocatalysts by interface engineering and 3D nanostructure applied in large-scale hydrogen production facilities.

Heterostructure Interface Engineering in CoP/FeP/CeOx with a Tailored d-Band Center for Promising Overall Water Splitting Electrocatalysis

Accomplishing a green hydrogen economy in reality through water spitting ultimately relies upon earth-abundant efficient electrocatalysts that can simultaneously accelerate the oxygen and hydrogen evolution reactions (OER and HER). The perspective of electronic structure modulation via interface engineering is of great significance to optimize electrocatalytic output but remains a tremendous challenge. Herein, an efficient tactic has been explored to prepare nanosheet-assembly tumbleweed-like CoFeCe-containing precursors with time-/energy-saving and easy-operating features. Subsequently, the final metal phosphide materials containing multiple interfaces, denoted CoP/FeP/CeO_x, have been synthesized via the phosphorization process. Through the optimization of the Co/Fe ratio and the content of the rare-earth Ce element, the electrocatalytic activity has been regulated. As a result, bifunctional Co3Fe/Ce0.025 reaches the top of the volcano for both OER and HER simultaneously, with the smallest overpotentials of 285 mV (OER) and 178 mV (HER) at 10 mA cm^-2 current density in an alkaline environment. Multicomponent heterostructure interface engineering would lead to more exposed active sites, feasible charge transport, and strong interfacial electronic interaction. More importantly, the appropriate Co/Fe ratio and Ce content can synergistically tailor the d-band center with a downshift to enhance the per-site intrinsic activity. This work would provide valuable insights to regulate the electronic structure of superior electrocatalysts toward water splitting by constructing rare-earth compounds containing multiple heterointerfaces.

Prussian blue analog-derived nickel iron phosphide-reduced graphene oxide hybrid as an efficient catalyst for overall water electrolysis

Efficient and bifunctional nonprecious catalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are essential for the production of green hydrogen via water electrolysis. Transition metal (Ni, Co, Fe, etc.) phosphides are frequently documented HER catalysts, whereas their bimetallic oxides are efficient OER catalysts, thus enabling bifunctional catalysis for water electrolysis via proper operation. Herein, phosphide-reduced graphene oxide (rGO) hybrids were prepared from graphene oxide (GO)-incorporated bimetal Prussian blue analog (PBA) precursors. The hybrids could experience partial surface oxidation to create oxide layers with OER activities, and the hybrids also possessed considerable HER properties, therefore enabling bifunctional catalytic features for water electrolysis. The typical NiFeP-rGO hybrid demonstrated an overpotential of 250 mV at 10 mA cm^-2 and good durability for OER, as well as moderate HER catalytic features (overpotential of 165 mV at -10 mA cm^-2 and acceptable catalytic stability). Due to the bifunctional catalytic features, the NiFeP-rGO-based symmetric water electrolyzer demonstrated a moderate input voltage and high faradaic efficiency (FE) for O₂ and H₂ production. The current work provides a feasible way to prepare OER and HER bifunctional catalysts by facile phosphorization of PBA-associated precursors and spontaneous surface oxidation. Given the oxidation/reduction bifunctional catalytic behaviors, phosphide-rGO hybrid catalysts have great potential for widespread application in fields beyond water electrolysis, such as electrochemical pollution abatement, sensors, energy devices and organic syntheses.

Recent advances in interface engineering of Fe/Co/Ni-based heterostructure electrocatalysts for water splitting

Among various methods of developing hydrogen energy, electrocatalytic water splitting for hydrogen production is one of the approaches to achieve the goal of zero carbon emissions. It is of great significance to develop highly active and stable catalysts to improve the efficiency of hydrogen production. In recent years, the construction of nanoscale heterostructure electrocatalysts through interface engineering can not only overcome the shortcomings of single-component materials to effectively improve their electrocatalytic efficiency and stability but also adjust the intrinsic activity or design synergistic interfaces to improve catalytic performance. Among them, some researchers proposed to replace the slow oxygen evolution reaction at the anode with the oxidation reaction of renewable resources such as biomass to improve the catalytic efficiency of the overall water splitting. The existing reviews in the field of electrocatalysis mainly focus on the relationship between the interface structure, principle, and principle of catalytic reaction, and some articles summarize the performance and improvement schemes of transition metal electrocatalysts. Among them, few studies are focusing on Fe/Co/Ni-based heterogeneous compounds, and there are fewer summaries on the oxidation reactions of organic compounds at the anode. To this end, this paper comprehensively describes the interface design and synthesis, interface classification, and application in the field of electrocatalysis of Fe/Co/Ni-based electrocatalysts. Based on the development and application of current interface engineering strategies, the experimental results of biomass electrooxidation reaction (BEOR) replacing anode oxygen evolution reaction (OER) are discussed, and it is feasible to improve the overall electrocatalytic reaction efficiency by coupling with hydrogen evolution reaction (HER). In the end, the challenges and prospects for the application of Fe/Co/Ni-based heterogeneous compounds in water splitting are briefly discussed.

Introducing High-Valence Iridium Single Atoms into Bimetal Phosphides toward High-Efficiency Oxygen Evolution and Overall Water Splitting

Single atoms are superior electrocatalysts having high atomic utilization and amazing activity for water oxidation and splitting. Herein, this work reports a thermal reduction method to introduce high-valence iridium (Ir) single atoms into bimetal phosphide (FeNiP) nanoparticles toward high-efficiency oxygen evolution reaction (OER) and overall water splitting. The presence of high-valence single Ir atoms (Ir⁴⁺ ) and their synergistic interaction with Ni³⁺ species as well as the disproportionation of Ni³⁺ assisted by Fe collectively contribute to the exceptional OER performance. In specific, at appropriate Ir/Ni and Fe/Ni ratios, the as-prepared Ir-doped FeNiP (Ir₂₅ -Fe₁₆ Ni₁₀₀ P₆₄ ) nanoparticles at a mass loading of only 35 µg cm^-2 show the overpotential as low as 232 mV at 10 mA cm^-2 and activity as high as 1.86 A mg^-1 at 1.5 V versus RHE for OER in 1.0 m KOH. Computational simulations confirm the vital role of high-valence Ir to weaken the adsorption of OER intermediates, favorable for accelerating OER kinetics. Impressively, a Pt/C||Ir₂₅ -Fe₁₆ Ni₁₀₀ P₆₄ two-electrode alkaline electrolyzer affords a current density of 10 mA cm^-2 at a low cell voltage of 1.42 V, along with satisfied stability. An AA battery with a nominal voltage of 1.5 V can drive overall water splitting with obvious bubbles released.

Transition Metal Phosphide Nanoarchitectonics for Versatile Organic Catalysis

Transition metal phosphides (TMP) posses unique physiochemical, geometrical, and electronic properties, which can be exploited for different catalytic applications, such as photocatalysis, electrocatalysis, organic catalysis, etc. Among others, the use of TMP for organic catalysis is less explored and still facing many complex challenges, which necessitate the development of sustainable catalytic reaction protocols demonstrating high selectivity and yield of the desired molecules of high significance. In this regard, the controlled synthesis of TMP-based catalysts and thorough investigations of underlying reaction mechanisms can provide deeper insights toward practical achievement of desired applications. This review aims at providing a comprehensive analysis on the recent advancements in the synthetic strategies for the tailored and tunable engineering of structural, geometrical, and electronic properties of TMP. In addition, their unprecedented catalytic potential toward different organic transformation reactions is succinctly summarized and critically analyzed. Finally, a rational perspective on future opportunities and challenges in the emerging field of organic catalysis is provided. On the account of the recent achievements accomplished in organic synthesis using TMP, it is highly anticipated that the use of TMP combined with advanced innovative technologies and methodologies can pave the way toward large scale realization of organic catalysis.

In-situ Surface Reconstruction of Single-crystal (NiFe)3Se4 Nano-pyramid Arrays for Efficient Oxygen Evolution

Transition metal-based selenides (TMSe) are considered as efficient pre-electrocatalysts towards oxygen evolution reaction (OER). However, the key factor in determining the surface reconstruction of TMSe under OER condition is not yet clear. Herein, we uncover that the crystallinity of TMSe will obviously impact the conversion degree from TMSe to transition metal oxyhydroxides (TMOOH) during OER. A novel single-crystal (NiFe)₃Se₄ nano-pyramid array grown on NiFe foam is fabricated by a facile one-step polyol process, which exhibits an excellent OER activity and stability, only requiring 170 mV to reach a current density of 10 mA cm^-2 and can sustain for more than 300 h. In situ Raman spectrum studies reveals that the single-crystal (NiFe)₃Se₄ is partially oxidized on its surface during OER, generating a dense heterostructure of (NiFe)OOH/(NiFe)₃Se₄. Benefiting from the in situ formed heterointerface, the adsorption of OER intermediates on Ni active sites calculated by density functional theory (DFT) analysis is optimized, leading to the reduced energy barrier, which accounts for the enhanced intrinsic activity. This work not only reports a novel single-crystal (NiFe)₃Se₄ nano-pyramid array electrocatalyst with high-efficient OER performance, but also gains a deep insight into the role of the crystallinity of TMSe on the surface reconstruction during OER.

A Rising 2D Star: Novel MBenes with Excellent Performance in Energy Conversion and Storage

As a flourishing member of the two-dimensional (2D) nanomaterial family, MXenes have shown great potential in various research areas. In recent years, the continued growth of interest in MXene derivatives, 2D transition metal borides (MBenes), has contributed to the emergence of this 2D material as a latecomer. Due to the excellent electrical conductivity, mechanical properties and electrical properties, thus MBenes attract more researchers' interest. Extensive experimental and theoretical studies have shown that they have exciting energy conversion and electrochemical storage potential. However, a comprehensive and systematic review of MBenes applications has not been available so far. For this reason, we present a comprehensive summary of recent advances in MBenes research. We started by summarizing the latest fabrication routes and excellent properties of MBenes. The focus will then turn to their exciting potential for energy storage and conversion. Finally, a brief summary of the challenges and opportunities for MBenes in future practical applications is presented.

Attractive Electron Delocalization Behavior of FeCoMoPB Amorphous Nanoplates for Highly Efficient Alkaline Water Oxidation

The rational design of high-performance and cost-effective electrocatalysts to overcome the kinetically sluggish water oxidation reaction is a grand challenge in water electrolysis. Transitional metals with incompletely filled d orbitals are expected to have intrinsic electronic interaction to promote the reaction kinetics, however, the construction of multiple active sites is still a bottleneck problem. Here, inspired by an amorphous alloy design strategy with chemical tunability, a noble-metal-free FeCoMoPB amorphous nanoplate for superior alkaline water oxidation is developed. The achieved overpotentials at current densities of 10, 100, and 500 mA cm^-2 are 239, 281, and 331 mV, respectively, while retaining a reliable stability of 48 h, outperforming most currently available electrocatalysts. Experimental and theoretical results reveal that the chemical complexity of the amorphous nanoplate leads to the formation of multiple active sites that is able to greatly lower the free energy of the rate-determining step during the water oxidation reaction. Moreover, the Mo element would result in an electron delocalization behavior to promote electron redistribution at its surrounding regions for readily donating and taking electrons. This amorphous alloy design strategy is expected to stimulate the development of more efficient electrocatalysts that is applicable in energy devices, such as metal-air batteries, fuel cells, and water electrolysis.

MOF-derived nanoarrays as advanced electrocatalysts for water splitting

Developing efficient, nanostructured electrocatalysts with the desired compositions and structures is of great significance for improving the efficiency of water splitting toward hydrogen production. In this regard, metal-organic framework (MOF) derived nanoarrays have attracted great attention as promising electrocatalysts because of their diverse compositions and adjustable structures. In this review, the recent progress in MOF-derived nanoarrays for electrochemical water splitting is summarized, highlighting the structural design of the MOF-derived nanoarrays and the electrocatalytic performance of the derived composite carbon materials, oxides, hydroxides, sulfides, and phosphides. In particular, the structure-performance relationships of the MOF-derived nanoarrays and the modulation strategies toward enhanced catalytic activity for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are discussed, providing insights into the development of advanced catalysts for the HER and OER. The challenges and prospects in this promising field for future industrial applications are also addressed.

Recent advances in hollow nanomaterials with multiple dimensions for electrocatalytic water splitting

Electrocatalytic water splitting has great research prospects in the production of green hydrogen energy, and electrocatalysts are the prerequisite. As widely employed efficient electrocatalysts, hollow nanostructures have attracted a lot of research attention due to their excellent catalytic activity and structural stability. Moreover, the abundant catalytically active sites and tunable morphology also make hollow nanomaterials promising electrocatalysts for water splitting. Despite these advantages, the industrial applications of these hollow nanocatalysts are impeded by limitations like the lack of effective synthesis methods and unclear formation mechanisms. Therefore, extensive efforts have been devoted to the development of efficient synthesis strategies to boost the development of more efficient hollow electrocatalysts, and great progress has been achieved in recent years. To gain a better understanding of the rapid development of hollow nanocatalysts for water splitting, we herein organize a review to summarize the recent synthetic methods and advantages of hollow materials with different dimensions. The specific advantages of hollow nanomaterials in electrocatalytic water splitting, such as abundant active sites, a stable structure, high mass transfer efficiency, and reduced aggregation of catalytic particles, are also summarized. Finally, the challenges and prospects of hollow nanostructures with multiple dimensions in electrocatalytic water splitting are further explored.

Synergistically Coupling of Manganese-Doped CoP Nanowires Arrays with Highly Dispersed Ni(PO3)2 Nanoclusters toward Efficient Overall Water Splitting

Co-based phosphides are considered to be highly promising electrocatalysts for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, their electrocatalytic efficiencies are greatly limited by the weak water dissociation process and unsatisfactory adsorption ability toward reaction intermediates. Herein, novel Mn-doped CoP/Ni(PO₃)₂ heterostructure array electrocatalysts which are composed of highly dispersed Ni(PO₃)₂ nanoclusters that are tightly wrapped on Mn-doped CoP nanowire arrays are designed. An electrocatalytic performance test suggested that the heterostructure arrays exhibited competitive electrocatalytic performance toward both HER and OER, which needed overpotentials of 116 and 245 mV to drive a current of 10 mA/cm², respectively. Encouragingly, a symmetric two electrode water splitting system constructed by the heterostructure arrays required an ultralow cell voltage, suggesting the potential in overall water splitting. First-principles calculations combined with experimental characterization were further performed to clarify the electrocatalytic mechanism. On the one hand, effective doping of Mn atoms could optimize the surface electronic structure of CoP and promote the intrinsic activity. On the other hand, the compact and abundant heterogeneous interface between Ni(PO₃)₂ and CoP not only made more active sites exposed but also promoted the effective adsorption of intermediate reaction species on the catalyst surface. This work provides a new strategy to improve electrocatalytic performance of Co-based phosphides through the synergistic coupling of metal-doping and phosphate surface decoration, which will greatly promote the development of highly efficient electrocatalysts for overall water splitting.

Boosting Hydrogen Evolution Electrocatalysis via Regulating the Electronic Structure in a Crystalline-Amorphous CoP/CeOx p-n Heterojunction

The modulation of the electronic structure is the effective access to achieve highly active electrocatalysts for the hydrogen evolution reaction (HER). Transition-metal phosphide-based heterostructures are very promising in enhancing HER performance but the facile fabrication and an in-depth study of the catalytic mechanisms still remain a challenge. In this work, the catalytically inactive n-type CeOx is successfully combined with p-type CoP to form the CoP/CeOx heterojunction. The crystalline-amorphous CoP/CeOx heterojunction is fabricated by the phosphorization of predesigned Co(OH)2/CeOx via the as-developed reduction-hydrolysis strategy. The p-n CoP/CeOx heterojunction with a strong built-in potential of 1.38 V enables the regulation of the electronic structure of active CoP within the space-charge region to enhance its intrinsic activity and facilitate the electron transfer. The functional CeOx entity and the negatively charged CoP can promote the water dissociation and optimize H adsorption, synergistically boosting the electrocatalytic HER output. As expected, the heterostructured CoP/CeOx-20:1 with the optimal ratio of Co/Ce shows significantly improved HER activity and favorable kinetics (overpotential of 118 mV at a current density of 10 mA cm-2 and Tafel slope of 77.26 mV dec-1). The present study may provide new insight into the integration of crystalline and amorphous entities into the p-n heterojunction as a highly efficient electrocatalyst for energy storage and conversion.

Plant polyphenol-involved coordination assembly-derived Mo3Co3C/Mo2C/Co@NC with phase regulation and interface engineering for efficient hydrogen evolution reaction electrocatalysis

Heterostructured Mo3Co3C/Mo2C/Co@NC catalysts were obtained by the calcination of tannic acid ligand-assembled precursors, enabling enhanced HER electrocatalysis through phase and interface regulation.