Robust NiCoP/CoP Heterostructures for Highly Efficient Hydrogen Evolution Electrocatalysis in Alkaline Solution

Optimized Transition Metal Phosphides for Direct Seawater Electrolysis: Current Trends

Seawater electrolysis presents a viable route for sustainable large-scale hydrogen production, yet its practical application is hindered by several technical challenges. These include the sluggish kinetics of hydrogen evolution, poor stability, cation deposition at the cathode, electrode corrosion, and competing chloride oxidation at the anode. To overcome these obstacles, the development of innovative electrocatalysts is crucial. Transition metal phosphides (TMPs) have emerged as promising candidates owing to their superior catalytic performance and tunable structural properties. This review provides a comprehensive analysis of recent progress in the structural engineering of TMPs tailored for efficient seawater electrolysis. We delve into the catalytic mechanisms underpinning hydrogen and oxygen evolution reactions in different pH conditions, along with the detrimental side reactions that impede hydrogen production efficiency. Several methods to prepare TMPs are then introduced. Additionally, detailed discussions on structural modifications and interface engineering tactics are presented, showcasing strategies to enhance the activity and durability of TMP electrocatalysts. By analyzing current research findings, our review aims to inform ongoing research endeavors and foster advancements in seawater electrolysis for practical and ecologically sound hydrogen generation.

Nitrogen-Tungsten Oxide Nanostructures on Nickel Foam as High Efficient Electrocatalysts for Benzyl Alcohol Oxidation

Electrocatalytic alcohol oxidation (EAO) is an attractive alternative to the sluggish oxygen evolution reaction in electrochemical hydrogen evolution cells. However, the development of high-performance bifunctional electrocatalysts is a major challenge. Herein, we developed a nitrogen-doped bimetallic oxide electrocatalyst (WO-N/NF) by a one-step hydrothermal method for the selective electrooxidation of benzyl alcohol to benzoic acid in alkaline electrolytes. The WO-N/NF electrode features block-shaped particles on a rough, inhomogeneous surface with cracks and lumpy nodules, increasing active sites and enhancing electrolyte diffusion. The electrode demonstrates exceptional activity, stability, and selectivity, achieving efficient benzoic acid production while reducing the electrolysis voltage. A low onset potential of 1.38 V (vs. RHE) is achieved to reach a current density of 100 mA cm-2 in 1.0 M KOH electrolyte with only 0.2 mmol of metal precursors, which is 396 mV lower than that of water oxidation. The analysis reveals a yield, conversion, and selectivity of 98.41%, 99.66%, and 99.74%, respectively, with a Faradaic efficiency of 98.77%. This work provides insight into the rational design of a highly active and selective catalyst for electrocatalytic alcohol oxidation.

Well-defined ternary metal phosphide nanowires with stabilized Pt nanoclusters to boost alkaline hydrogen evolution reaction

Designing low-content and high-activity Pt-based catalysts with the high durability for the electrochemical hydrogen production remains a challenge. In this study, a ternary metal phosphide (NiCoP) with 1D nanowire (NW) and 2D nanosheet (NS) morphologies incorporating Pt clusters (denoted as Ptcluster-NiCoP@NF NWs and Ptcluster-NiCoP@NF NSs, respectively) was prepared using a hydrothermal-phosphorization-electrodeposition method. Based on the "tip effect" of NWs and a high electrochemical surface area, the as-prepared Ptcluster-NiCoP@NF NWs display better hydrogen evolution reaction (HER) performance, with a low overpotential of 65 mV at a high current density of 100 mA cm-2 and a low Tafel slope of 38.86 mV dec-1, than the Ptcluster-NiCoP@NF NSs, with an overportential of 95 mV at 42.53 mV dec-1. This indicates that the NiCoP NW-based support exhibits faster HER kinetics. The mass activity (11.47 A mgPt-1) of the Ptcluster-NiCoP@NF NWs is higher than that of commercial Pt/C catalysts. Significantly, the Ptcluster-NiCoP@NF NWs display excellent cyclic stability with negligible losses for 5000 cycles and 30-h tests at a high current of 500 mA cm-2.

Single Atom Ru Doped Ni2P/Fe3P Heterostructure for Boosting Hydrogen Evolution for Water Splitting

Modulating the chemical composition and structure has been considered as one of the most promising strategies for developing high-efficient water splitting catalysts. Here, a single-atom Ru doped Ni2P/Fe3P catalyst is synthesized by introducing the dispersed Ru atoms to adjust Ni2P/Fe3P heterostructure. Single atom Ru provides effective hydrogen evolution reaction (HER) active sites for boosting catalytic activities. The catalyst with only 0.2 wt.% content of Ru exhibits an overpotential of 19.3 mV at 10 mA cm-2, which is obviously lower than 146.1 mV of Ni2P/Fe3P. Notably, an alkaline overall water electrolyzer based on Ru-Ni2P/Fe3P catalysts achieves a cell voltage of 1.47 V and operates over 600 h at 10 mA cm-2, which is superior to that of benchmark RuO2//Pt/C (1.61 V). The theoretical calculations further confirm that Ru single atom doping can effectively optimize the hydrogen/water adsorption free energy of the active site and therefore improve the HER activity of heterostructure. This work provides a valuable reference to design high-activity and durability catalyst for water splitting through the double modulation of interface-effect and atomic doping.

Efficient electrocatalytic reduction of CO2 to CO enhanced by the synergistic effect of N,P on carbon aerogel

A metal-free catalyst, N,P-codoped carbon aerogel, was used to realize the high efficiency reduction of CO2 to CO. Therein, the pyridinic N acts as the active center to activate and reduce CO2 and the atom of P acts as the "transition atom" of the proton to reduce the free energy barrier from *CO2 to *COOH.

Designing a hybrid nanomaterial based on Cr-containing polyoxometalate and graphene oxide as an electrocatalyst for the hydrogen evolution reaction

A new polyoxometalate (POM)-based hybrid nanomaterial (denoted as PMo11-Cr-mGO) was designed via covalent interaction between the Cr(acac)3 complex and [PMo11O39]7- followed by immobilization on the surface of modified graphene oxide (mGO). The prepared nanomaterial was characterized using a series of physicochemical techniques. X-ray diffraction (XRD), Raman analysis, transmission electron microscopy (TEM), and FE-SEM-EDS revealed the preservation of layered GO during the formation of the desired hybrid nanomaterial. Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), and elemental analysis confirmed the immobilization of POM (PMo11-Cr) on the surface of mGO and the formation of PMo11-Cr-mGO. In order to evaluate the performance of PMo11-Cr-mGO in the hydrogen evolution reaction (HER), electrochemical measurements were also performed. The resulting PMo11-Cr-mGO exhibited excellent HER activities with a low overpotential of 153 mV at 10 mA cm-2 and good durability in acidic media, thus emerging as one of the most efficient POM-based electrocatalysts.

Regulation of Electronic Structures of the Urchin-Like NiCoP/CoP Nanocatalysts for Fast Hydrogen Evolution

The exploration of stable, efficient, and low-cost catalysts toward ammonia borane hydrolysis is of vital significance for the practical implementation of this hydrogen production technology. Integrating interface engineering and nano-architecture engineering is a favorable strategy to elevate catalytic performance, as it can modify the electronic structure and provide sufficient active sites simultaneously. In this work, urchin-like NiCoP/CoP heterostructures are prepared via a three-step hydrothermal-oxidation-phosphorization synthesis route. It is demonstrated that the original Ni/Co molar ratio and the amount of phosphorus are crucial for adjusting the morphology, enhancing the exposed surface area, facilitating charge transfer, and modulating the adsorption and activation of H2O molecules. Consequently, the optimal Ni1Co2P heterostructure displays remarkable catalytic properties in the hydrolysis of ammonia borane with a turnover frequency (TOF) value of 30.3 molH2 ⋅ min-1 ⋅ molmetal -1, a low apparent activation energy of 25.89 kJ ⋅ mol-1, and good stability. Furthermore, by combining infrared spectroscopy and isotope kinetics experiments, a possible mechanism for the hydrolysis of ammonia borane was proposed.

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.

RuSe2/CeO2heterostructure as a novel electrocatalyst for highly efficient alkaline hydrogen evolution

Constructing heterojunction to adjust the electronic structure of catalysts is a promising strategy for synergistically improving electrocatalytic activity. In addition, RuSe2is recognized as an effective alternative to Pt for boosting alkaline hydrogen evolution reaction (HER) on account of its outstanding catalytic properties. Herein, novel RuSe2/CeO2heterojunction electrocatalysts are fabricated through hydrothermal and thermal treatment methods. The optimal 50% RuSe2/CeO2heterojunction electrocatalyst exhibits a low HER overpotential of 16 mV to attain 10 mA cm-2current density and Tafel slope of 66.1 mV dec-1for hydrogen evolution in 1.0 M KOH. At the same time, the 50% RuSe2/CeO2heterojunction electrocatalyst also maintains a stable HER activity for 50 h or 3000 CV cycles. The experimental results show that formation of heterogeneous interface between RuSe2and CeO2results in the redistribution of electrons at the RuSe2/CeO2interface, thereby changing the electronic structure of RuSe2and enhancing the performance of the RuSe2/CeO2electrocatalyst. This work may provide a feasible way to design efficient hydrogen evolution heterojunction electrocatalysts by modulating the electronic structure in alkaline electrolytes.

Facile and Cost-effective Synthesis of CoP@N-doped Carbon with High Catalytic Performance for Electrochemical Hydrogen Evolution Reaction

The manufacture of efficient and low-cost hydrogen evolution reaction (HER) catalysts is regarded as a critical solution to achieve carbon neutrality. Herein, we developed an economical method to synthesize a CoP-anchored N-doped carbon catalyst via one-step pyrolysis using inexpensive starting materials (cobalt ion salt, phytic acid, and glycine). The size of the CoP nanoparticles was controlled by adjusting the Co/P ratio of the catalysts. Nanoscale CoP particles with adequate exposure to active sites were uniformly anchored on the surface of the conductive nitrogen-doped carbon substrate, ensuring the rapid transfer of electrons and species. When Co/P=0.89, the as-made catalyst exhibited outstanding HER activity, with an extraordinarily low overpotential of 202 mV at 10 mA cm-2 and long-term stability.

Coupling heterostructured CoP-NiCoP nanopin arrays with MXene (Ti3C2Tx) as an efficient bifunctional electrocatalyst for overall water splitting

Developing a highly effective bifunctional electrocatalyst for alkaline-condition electrochemical water splitting is both essential and challenging. The work presented here successfully synthesizes and employs a heterostructured CoP-NiCoP ultra-long nanopin array in situ growing on MXene (Ti₃C₂T_x) as a stable bifunctional electrocatalyst for electrochemical water-splitting. The heterogeneous structure formed by CoP nanoparticles and NiCoP nanopins provides extra active sites for water-splitting. Also, Ti₃C₂T_x works as a support substrate during electrochemical operations, accelerating mass transfer, ion transport, and rapid gas product diffusion. Meanwhile, throughout the catalytic process, the dense nanopin arrays shield Ti₃C₂T_x from further oxidation. At a result, the CoP-NiCoP-Ti₃C₂T_x (denoted as CP-NCP-T) demonstrated excellent catalytic activity, with overpotentials of just 46 mV for hydrogen evolution at 10 mA cm^-2 and 281 mV for oxygen evolution at 50 mA cm^-2. Furthermore, in 1.0 M KOH solution, the outstanding bifunctional electrode (CP-NCP-T || CP-NCP-T) exhibits efficient electrochemical water splitting activity (1.54 V@10 mA cm^-2) and outperforms the comparable device Pt/C || IrO₂ (1.62 V@10 mA cm^-2).

CoP@Ni core-shell heterostructure nanowire array: A highly efficient electrocatalyst for hydrogen evolution

The inferior performance of non-precious metals on electrocatalytic hydrogen evolution can be mainly attributed to the inappropriate adsorption strength of intermediates. A core-shell heterostructure of CoP@Ni is developed by hydrothermal reaction, thermal phosphorization and subsequent electrodeposition, with metallic Ni supported by CoP nanowire array. The as-prepared CoP@Ni core-shell heterostructure nanowire array has a superior activity on hydrogen evolution in alkaline electrolyte, with an overpotential of 71 mV to drive 10 mA cm^-2 and a Tafel slope of 66 mV dec^-1. The electronic structure of CoP@Ni is tuned for modulating the adsorption strength of intermediates. The theoretical calculations reveal that CoP@Ni has metallic characteristics with a zero-bandgap, which leads to the promoted charge transfer. More importantly, the intrinsic activity of CoP@Ni is greatly increased, with a lower energy barrier in the reaction pathway. This work points out the importance of constructing the heterostructure for improving the intrinsic activity, which can pave the way to the exploration of high-performance and cost-effective electrocatalysts for hydrogen production.

Active site recovery and N-N bond breakage during hydrazine oxidation boosting the electrochemical hydrogen production

Substituting hydrazine oxidation reaction for oxygen evolution reaction can result in greatly reduced energy consumption for hydrogen production, however, the mechanism and the electrochemical utilization rate of hydrazine oxidation reaction remain ambiguous. Herein, a bimetallic and hetero-structured phosphide catalyst has been fabricated to catalyze both hydrazine oxidation and hydrogen evolution reactions, and a new reaction path of nitrogen-nitrogen single bond breakage has been proposed and confirmed in hydrazine oxidation reaction. The high electro-catalytic performance is attributed to the instantaneous recovery of metal phosphide active site by hydrazine and the lowered energy barrier, which enable the constructed electrolyzer using bimetallic phosphide catalyst at both sides to reach 500 mA cm^-2 for hydrogen production at 0.498 V, and offer an enhanced hydrazine electrochemical utilization rate of 93%. Such an electrolyzer can be powered by a bimetallic phosphide anode-equipped direct hydrazine fuel cell, achieving self-powered hydrogen production at a rate of 19.6 mol h^-1 m^-2.

Self-supporting NiMo-Fe-P nanowire arrays as bifunctional catalysts for efficient overall water splitting

Developing efficient bifunctional hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalysts is beneficial for simplifying the design of electrolytic cells and reducing the cost of device manufacturing. Herein, a metal phosphide nanoarray (NiMo-Fe-P) electrocatalyst was designed by in situ ion exchange and low-temperature phosphating to promote overall water splitting in 1 M KOH. NiMo-Fe-P demonstrates superb HER and OER activities as reflected by the low overpotentials of 73.1 mV and 215.2 mV, respectively, at a current density of 10 mA cm^-2. The addition of Fe changes the electronic structure of Ni, which is conducive to the chemisorption of oxygen-containing intermediates and reduces the energy barrier for water decomposition. Besides, the metal phosphide not only acts as the active site of the HER, but also improves the conductivity of the catalyst. Furthermore, nanowire arrays and the small particles generated on their surfaces provide a high electrochemical active surface area (ECSA), which was beneficial for the exposure of active sites. Attributed to these advantages, the cell voltage of the water electrolyzer constructed with NiMo-Fe-P as both the cathode and anode is only 1.526 V at 10 mA cm^-2, and it maintains excellent stability for 100 h with near negligible changes in potential.

Heterostructured Co2P Nanocomposite Embedded in a N, P Co-Doped Carbon Layer as a High Performance Electrocatalyst for Overall Water Splitting

Hydrogen is the mainstream future energy source because of its high energy density and environmentally-friendly properties. In this study, Fe-Co2P/NPC materials were prepared by the wet chemical synthesis method, in which Fe-Co2P nanowires were wrapped by the N, P co-doped carbon layers (NPC) under aging and phosphorylation strategies. When Fe-Co2P/NPC/NF was subjected to hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), the overpotential was only 73 mV and 217 mV to reach the current density of 10 mA cm−2, respectively. When the cathode and anode were both Fe-Co2P/NPC/NF, a current density of 10 mA cm−2 was achieved with only 1.56 V. This work provides a new idea for the design and preparation of non-precious metal-based transition metal phosphide catalysts.

Urea-oxidation-assisted electrochemical water splitting for hydrogen production on a bifunctional heterostructure transition metal phosphides combining metal-organic frameworks

Electrocatalyzed urea-assisted wastewater splitting is a promising approach for sustainable hydrogen production. However, the lack of cost-efficient electrocatalysts hinders its practical application. Herein, bimetal phosphide (NiCoP_x) nanowire arrays decorated with ultrathin NiFeCo metal-organic framework (NiFeCo-MOF) nanosheets on porous nickel foam (NF) were designed for urea-assisted wastewater splitting. The core-shell NiCoP_x@NiFeCo-MOF hybrids were prepared via successive hydrothermal, gas-phase phosphorization and hydrothermal strategies. Encouragingly, the novel NiCoP_x@NiFeCo-MOF/NF electrode served as an excellent bifunctional electrocatalyst for both the cathodic hydrogen evolution reaction (HER) and the anodic urea oxidation reaction (UOR) in urea-assisted water splitting, which merely required an overpotential of 44 mV to deliver a current density of 10 mA cm^-2 for HER and a voltage of 1.37 V to deliver a current density of 100 mA cm^-2 for UOR in 1.0 M KOH + 0.5 M urea. Benefiting from the highly exposed electroactive sites in exquisite three-dimensional (3D) hierarchical structure, multicomponent synergistic effect, accelerated electron transfer, easy electrolyte access and diffusion of released gas bubbles, the as-fabricated NiCoP_x@NiFeCo-MOF/NF exhibited outstanding electrocatalytic performance. The mechanism of water splitting was elucidated by density functional theory calculations. Interestingly, NiFeCo-MOF possessed optimized COO* adsorption ability on Ni sites that were beneficial to UOR intermediates. More significantly, this work paves the way for the design and fabrication of bifunctional electrocatalysts for urea-containing wastewater treatment and sustainable hydrogen production.

Tuning the Mn Dopant To Boost the Hydrogen Evolution Performance of CoP Nanowire Arrays

Because of its advantages such as abundant resources, low cost, simple synthesis, and high electrochemical stability, cobalt phosphide (CoP) is considered as a promising candidate for electrocatalytic hydrogen evolution reaction. Through element doping, the morphology and electronic structure of the catalyst can be tuned, resulting in both the increase of the active site number and the improvement of the intrinsic activity of each site. Herein, we designed and fabricated Mn-doped CoP nanowires with a length of 3 μm, a diameter of 50 nm, and the pores between the grains of 10 nm. As a highly efficient electrocatalyst for alkaline hydrogen evolution, the Mn₁₀-doped CoP/NF (doping amount is about 10 atom %) electrode presented overpotentials of 60 mV @ 10 mA cm^-2 and 112 mV @ 100 mA cm^-2, improved by 35 and 23%, respectively, compared with CoP/NF. Characterizations indicate that Mn doping increases the electrochemical active area, reduces the impedance, and tunes the electronic structure of the material. Density functional theory calculations also revealed that an appropriate amount of Mn dopant at a suitable location can both react as an active site itself and boost the activity of the surrounding Co sites, delivering favorable H* adsorption and rapid reaction kinetics. This result may not only promote the development of hydrogen evolution reaction catalysts but also encourage explorations of the relationship between the property and fine doping structure.

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

Amongst various futuristic renewable energy sources, hydrogen fuel is deemed to be clean and sustainable. Electrochemical water splitting (EWS) is an advanced technology to produce pure hydrogen in a cost-efficient manner. The electrocatalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are the vital steps of EWS and have been at the forefront of research over the past decades. The low-cost nanostructured metal phosphide (MP)-based electrocatalysts exhibit unconventional physicochemical properties and offer very high turnover frequency (TOF), low over potential, high mass activity with improved efficiency, and long-term stability. Therefore, they are deemed to be potential electrocatalysts to meet practical challenges for supporting the future hydrogen economy. This review discusses the recent research progress in nanostructured MP-based catalysts with an emphasis given on in-depth understanding of catalytic activity and innovative synthetic strategies for MP-based catalysts through combined experimental (in situ/operando techniques) and theoretical investigations. Finally, the challenges, critical issues, and future outlook in the field of MP-based catalysts for water electrolysis are addressed.

Sequential Phase Conversion-Induced Phosphides Heteronanorod Arrays for Superior Hydrogen Evolution Performance to Pt in Wide pH Media

Developing an efficient and non-precious pH-universal hydrogen evolution reaction electrocatalyst is highly desirable for hydrogen production by electrochemical water splitting but remains a significant challenge. Herein, a hierarchical structure composed of heterostructured Ni₂ P-Ni₁₂ P₅ nanorod arrays rooted on Ni₃ S₂ film (Ni₂ P-Ni₁₂ P₅ @Ni₃ S₂ ) via a simultaneous corrosion and sulfidation is built followed by a phosphidation treatment toward the metallic nickel foam. The combination of theoretical calculations with in/ex situ characterizations unveils that such a unique sequential phase conversion strategy ensures the strong interfacial coupling between Ni₂ P and Ni₁₂ P₅ as well as the robust stabilization of 1D heteronanorod arrays by Ni₃ S₂ film, resulting in the promoted water adsorption/dissociation energy, the optimized hydrogen adsorption energy, and the enhanced electron/proton transfer ability accompanied with an excellent stability. Consequently, Ni₂ P-Ni₁₂ P₅ @Ni₃ S₂ /NF requires only 32, 46, and 34 mV overpotentials to drive 10 mA cm^-2 in 1.0 m KOH, 0.5 m H₂ SO₄ , and 1.0 m phosphate-buffered saline electrolytes, respectively, exceeding almost all the previously reported non-noble metal-based electrocatalysts. This work may pave a new avenue for the rational design of non-precious electrocatalysts toward pH-universal hydrogen evolution catalysis.

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

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

Electrocatalyst based on Ni2P nanoparticles and NiCoP nanosheets for efficient hydrogen evolution from urea wastewater

Urea electrolysis is a promising approach to produce hydrogen while simultaneously purifying urea-rich wastewater. In practice, it is highly desirable but still challenging, through the structure construction strategy, to implement a method with controllable synthesis of ultra-thin nanosheet arrays with rich interfaces, and then apply them into the catalysis operations of hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). In this work, the bifunctional electrocatalyst Ni₂P/NiCoP nanosheets anchored nickel foam (NF) were prepared with ultra-thin rich interfaces by regulating the Co- and P-doping. The results showed that the elaborated Ni₂P/NiCoP/NF electrode delivered the excellent electrocatalytic activities for both UOR and HER operations. Particularly for UOR, it required only a cell voltage of 1.41 V at 100 mA cm^-2, which was 400 mV lower than that in the traditional overall water splitting operation.

Increasing Oxygen Vacancy of CeO2 Nanocrystals by Ni Doping and reduced Graphene Oxides Decoration towards the Electrocatalytic Hydrogen Evolution

The oxygen vacancy (VO) engineering is proved to be an effective approach for improving the hydrogen evolution reaction (HER) performance of low-cost metal oxides electrocatalysts. Cerium dioxide (CeO2), one of...

Tuning the Electronic Structure of the CoP/Ni2P Nanostructure by Nitrogen Doping for an Efficient Hydrogen Evolution Reaction in Alkaline Media

As one of the most sustainable, efficient, and cleanest ways for hydrogen production, electrochemical water splitting relies heavily on cost-efficient and stable electrocatalysts. Herein, a self-supported and nitrogen-doped hybrid CoP/Ni₂P was synthesized through a simple two-step hydrothermal process followed by low-temperature phosphorization and nitridation (N-CoP/Ni₂P@NF). Both experimental and density functional theory calculation results suggest that nitrogen doping can tune the electrical structure of the CoP/Ni₂P heterostructure and thus optimize the free energy of adsorbed H on the surface of N-CoP/Ni₂P@NF and accelerate the electronic transport activity. The prepared N-CoP/Ni₂P@NF exhibits excellent electrocatalytic hydrogen evolution reaction (HER) performance, which merely requires an overpotential of -46 mV at -10 mA cm^-2 and shows a negligible decay after a long durability test for 72 h in alkaline (1.0 M KOH) media. Consequently, this work supplies a novel strategy with great potential for designing transition metal phosphate-based catalysts with high HER performance.

Efficient Nitrate-to-Ammonia Electroreduction at Cobalt Phosphide Nanoshuttles

The nitrate electroreduction reaction (NO₃^--ERR) is an efficient and green approach for nitrate remediation, which requires a highly active and selective electrocatalyst. In this work, porous and amorphous cobalt phosphide nanoshuttles (CoP PANSs) are successfully synthesized by using Mg²⁺ ion-doped calcium carbonate nanoshuttles (Mg-CaCO₃ NSs) as the initial reaction precursor via precipitation transformation and a high-temperature phosphidation strategy. Various physical characterizations show that CoP PANSs have porous architecture, amorphous crystal structure, and big surface area. Electrochemical measurements reveal for the first time that CoP PANSs have outstanding electroactivity for NO₃^--ERR in a neutral electrolyte. At an applied potential of -0.5 V vs reversible hydrogen electrode, CoP PANSs can achieve a high Faraday efficiency (94.24 ± 2.8%) and high yield rate (19.28 ± 0.53 mg h^-1 mg_cat^-1) for ammonia production, which exceeds most reported values at various electrocatalysts for NO₃^--ERR. Thus, the present result indicates that cobalt phosphide nanomaterials have promising application for NO₃^--ERR.

Multi-Phase Heterostructure of CoNiP/Cox P for Enhanced Hydrogen Evolution Under Alkaline and Seawater Conditions by Promoting H2 O Dissociation

Hydrogen evolution reaction (HER) is a key step for electrochemical energy conversion and storage. Developing well defined nanostructures as noble-metal-free electrocatalysts for HER is promising for the application of hydrogen technology. Herein, it is reported that 3D porous hierarchical CoNiP/Co_x P multi-phase heterostructure on Ni foam via an electrodeposition method followed by phosphorization exhibits ultra-highly catalytic activity for HER. The optimized CoNiP/Co_x P multi-phase heterostructure achieves an excellent HER performance with an ultralow overpotential of 36 mV at 10 mA cm^-2 , superior to commercial Pt/C. Importantly, the multi-phase heterostructure shows exceptional stability as confirmed by the long-term potential cycles (30,000 cycles) and extended electrocatalysis (up to 500 h) in alkaline solution and natural seawater. Experimental characterizations and DFT calculations demonstrate that the strong electronic interaction at the heterointerface of CoNiP/CoP is achieved via the electron transfer from CoNiP to the heterointerface, which directly promotes the dissociation of water at heterointerface and desorption of hydrogen on CoNiP. These findings may provide deep understanding on the HER mechanism of heterostructure electrocatalysts and guidance on the design of earth-abundant, cost-effective electrocatalysts with superior HER activity for practical applications.

Interface Engineering of Binder-Free Earth-Abundant Electrocatalysts for Efficient Advanced Energy Conversion

Gas-involved electrocatalysts for the hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction are crucial for many clean and effective energy technologies. The interface chemistry of electrocatalysts plays an important role in the optimization of their catalytic activity and stability. However, these gas-involved reactions exhibit sluggish kinetics and complex reactions at triple-phase interfaces. Thus, interface engineering at multiscale levels plays a decisive role. Binder-free electrocatalysts have gained increasing popularity, owing to their enhanced electron transfer and improved mass diffusion. This Review summarizes the influence of binder-free electrocatalysts with optimized interfaces and emphasizes three key interfaces, including the electrocatalyst/substrate interface, the inner interface of the electrocatalyst, and the electrocatalyst/electrolyte/gas interface, which are integral to determining the properties of gas-involved electrocatalysts, including the electrical conductivity, intrinsic catalytic activity, and mass transfer behavior. Finally, prospects and future challenges for the further development of binder-free electrocatalysts are discussed.

Design of 3D Hollow Porous Heterogeneous Nickel-Cobalt Phosphides for Synergistically Enhancing Catalytic Performance for Electrooxidation of Methanol

The synergistic effect among different components and the structural and morphological control of catalytic nanomaterials have attracted considerable research interest in the field of electrocatalysis, as using a rational design of the catalytic nanomaterials with the desired structure, morphology, and chemical compositions is an effective strategy for enhancing catalytic performance. Here, by changing the Ni/Co atomic ratio of raw materials, a series of samples with a three-dimensional (3D) hollow porous ternary multicomponent heterostructure has been successfully synthesized via a facile template-free solvothermal approach and subsequently annealing and phosphating treatments, and its formation mechanism is also investigated. By virtue of compositional and structural advantages, the optimized Ni₁Co₂P_x (NiCoP/CoP/CoP₂) nanoparticles show very high mass activity (436.9 mA mg^-1) and area-specific activity (155 mA cm^-2), as well as remarkable durability toward the methanol electrooxidation reaction (MOR) in alkaline solution. This catalytic activity is better than those of most of reported Ni/Co-based nonprecious metal catalysts. Particularly, a multicomponent synergistic effect on the MOR was observed. The present study not only provides a simple method for the fabrication of 3D hollow porous multicomponent composite nanomaterials, but also gives insights into the synergistic effect among the porous structure, chemical compositions, and catalytic activity of nanomaterials in the electrocatalytic oxidation of methanol.

Plasma Transforming Ni(OH)2 Nanosheets into Porous Nickel Nitride Sheets for Alkaline Hydrogen Evolution

Nickel nitride (Ni₃N) is a superior hydrogen evolution reaction (HER) catalyst where the nitrogen source is usually ammonia and the reaction temperature is high during the synthesis process. Herein, we employed an innovative method to obtain three-dimensional porous nickel nitride nanosheets on Ni foam (Ni₃N/NF) by transforming Ni(OH)₂ nanosheets in N₂-H₂ glow discharge plasma. The obtained Ni₃N/NF displays a high HER activity with a small overpotential of 44 mV and a low Tafel slope of 46 mV dec^-1, which is competitive to a Pt/C catalyst. Both the test data and simulation results prove that active ions and radicals in plasma play essential roles in achieving the facile nitridation, as well as building a nanostructured morphology over the Ni₃N/NF surface. The unique synthesis method opens new avenues for metal nitrides of HER catalysts and beyond.

Interface engineering of oxygen-vacancy-rich NiCo2O4/NiCoP heterostructure as an efficient bifunctional electrocatalyst for overall water splitting

Inexpensive bifunctional electrocatalysts towards oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is highly desirable from the perspective of energy conversion.

Hierarchical Bimetallic Ni-Co-P Microflowers with Ultrathin Nanosheet Arrays for Efficient Hydrogen Evolution Reaction over All pH Values

Designing efficient nonprecious catalysts with pH-universal hydrogen evolution reaction (HER) performance is of importance for boosting water splitting. Herein, a self-template strategy based on Ni-Co-glycerates is developed to prepare bimetallic Ni-Co-P microflowers with ultrathin nanosheet arrays. The highly porous core-shell structure gives rise to affluent mass transfer channels and availably prevents the aggregation of nanosheets, while the ultrathin nanosheets are favorable for producing abundant active sites. Besides, the produced CoP/NiCoP heterostructure in the bimetallic Ni-Co-P catalyst has excellent HER performance in a wide pH range. The as-prepared catalyst shows low potentials of 90, 157, and 121 mV to deliver a current density of 10 mA cm^-2 in 0.5 M H₂SO₄, 0.5 M PBS, and 1 M KOH solution, respectively. Meanwhile, negligible overpotential decay is achieved in the polarization curves after a long-term stability determination. This work supplies a promising strategy for developing pH-universal HER electrocatalysts based on solid-state metal alkoxides.

Direct Growth of Highly Strained Pt Islands on Branched Ni Nanoparticles for Improved Hydrogen Evolution Reaction Activity

The direct growth of Pt islands on lattice mismatched Ni nanoparticles is a major synthetic challenge and a promising strategy to create highly strained Pt atoms for electrocatalysis. By using very mild reaction conditions, Pt islands with tunable strain were formed directly on Ni branched particles. The highly strained 1.9 nm Pt-island on branched Ni nanoparticles exhibited high specific activity and the highest mass activity for hydrogen evolution (HER) in a pH 13 electrolyte. These results show the ability to synthetically tune the size of the Pt islands to control the strain to give higher HER activity.

Enhanced Hydrogen Evolution Reaction Performance of NiCo2P by Filling Oxygen Vacancies by Phosphorus in Thin-Coating CeO2

A series of NiCo₂P-based electrocatalysts, which were wrapped by CeO₂ whose oxygen vacancies (V_O) are partially filled with phosphorus atoms (named as NiCo₂P^x/P^xFVo-CeO₂, where x refers to the consumption of NaH₂PO₂·H₂O), have been fabricated to improve the electrocatalytic reactivity of NiCo₂P toward hydrogen evolution in alkaline solution. In the novel catalysts, the P atoms fill the oxygen vacancies, elevate the chemical valence state of Ni²⁺ and Co³⁺, and increase the hydride acceptors, which reinforcing the promoting effect of CeO₂ in the hydrogen evolution reaction (HER). Moreover, the negatively charged P atoms capture the positively charged protons more easily, benefiting the Volmer step during HER. Furthermore, the synergistic effect between oxygen vacancies and the filled P atoms accelerates the migration rate of electrons/ions and increases the electrochemical active area. All of the above are advantageous to the hydrogen evolution of NiCo₂P^x/P^xFVo-CeO₂ in alkaline electrolyte. As a result, the overpotential as low as 33.6 mV is achieved for NiCo₂P^0.3/P^0.3FVo-CeO₂ in alkaline media to drive a current density of 10 mA cm^-2. The reactivity is superior to that of Pt/C at a large current density along with a Tafel slope of 61.24 mV dec^-1 and long-term durability, which giving a new technology for efficient transition-metal catalyst candidates toward HER in alkaline solution.