Surface Tailoring-Modulated Bifunctional Oxygen Electrocatalysis with CoP for Rechargeable Zn-Air Battery and Water Splitting

The transition metal phosphide (TMP)-based functional electrocatalysts are very promising for the development of electrochemical energy conversion and storage devices including rechargeable metal-air batteries and water electrolyzer. Tuning the electrocatalytic activity of TMPs is one of the vital steps to achieve the desired performance of these energy devices. Herein, we demonstrate the modulation of the bifunctional oxygen electrocatalytic activity of nitrogen-doped carbon-encapsulated CoP (CoP@NC) nanostructures by surface tailoring with ultralow amount (0.56 atomic %) of Ru nanoparticles (2.5 nm). The CoP at the core and the Ru nanoparticles on the shell have a facile charge transfer interaction with the encapsulating NC. The strong coupling of Ru with CoP@NC boosts the electrocatalytic performance toward oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution (HER) reactions. The surface-tailored catalyst requires only 35 mV to deliver the benchmark current density of 10 mA·cm-2 for HER. A small potential gap of 620 mV between ORR and OER is achieved, making the catalyst highly suitable for the development of rechargeable zinc-air batteries (ZABs). The homemade ZAB delivers a specific capacity of 780 mA·hgZn-1 and peak power density of 175 mW·cm-2 with a very small voltaic efficiency loss (1.1%) after 300 cycles. The two-electrode water splitting cell (CoP@NC-Ru||CoP@NC-Ru) delivers remarkably low cell voltage of 1.47 V at the benchmark current density. Stable current density of 25 mA·cm-2 for 25 h without any significant change is achieved. Theoretical studies support the charge transfer interaction-induced enhanced electrocatalytic activity of the surface-tailored nanostructure.

Green fabrication of ultrafine N-MoxC/CoP hybrids for boosting electrocatalytic water reduction

Developing non-noble-metal electrocatalysts for hydrogen evolution reactions with high activity and stability is the key issue in green hydrogen generation based on electrolytic water splitting. It has been recognized that the stacking of large CoP particles limits the intrinsic activity of as-synthesized CoP catalyst for hydrogen evolution reaction. In the present study, N-MoxC/CoP-0.5 with excellent electrocatalytic activity for hydrogen evolution reaction was prepared using N-MoxC as decoration. A reasonable overpotential of 106 mV (at 10 mA cm-2) and a Tafel slope of 59 mV dec-1in 1.0 M KOH solution was achieved with N-MoxC/CoP-0.5 electrocatalyst, which exhibits superior activity even after working for 37 h. Uniformly distributed ultrafine nanoclusters of the N-MoxC/CoP-0.5 hybrids could provide sufficient interfaces for enhanced charge transfer. The effective capacity of the hydrogen evolution reaction could be preserved in the complex, and the enlarged electrocatalytic surface area could be expected to offer more active sites for the reaction.

Phosphine-Enhanced Semi-Hydrogenation of Phenylacetylene by Cobalt Phosphide Nano-Urchins

Transition metal phosphides are promising, selective, and air-stable nanocatalysts for hydrogenation reactions. However, they often require fairly high temperatures and H2 pressures to provide quantitative conversions. This work reports the positive effect of phosphine additives on the activity of cobalt phosphide nano-urchins for the semi-hydrogenation of phenylacetylene. While the nanocatalyst's activity was low under mild conditions (7 bar of H2 , 100 °C), the addition of a catalytic amount of phosphine remarkably increased the conversion, e. g., from 13 % to 98 % in the case of Pn Bu3 . The heterogeneous nature of the catalyst was confirmed by negative supernatant activity tests. The catalyst integrity was carefully verified by post-mortem analyses (TEM, XPS, and liquid 31 P NMR). A stereo-electronic map was proposed to rationalize the activity enhancement provided over a selection of nine phosphines: the strongest effect was observed for low to moderately hindered phosphines, associated with strong electron donor abilities. A threshold in phosphine stoichiometry was revealed for the enhancement of activity to occur, which was related to the ratio of phosphine to surface cobalt atoms.

Regulating Electrochemical Kinetics of CoP by Incorporating Oxygen on Surface for High-Performance Li-S Batteries

Lithium-sulfur (Li-S) batteries are widely studied because of their high theoretical specific capacity and environmental friendliness. However, the further development of Li-S batteries is hindered by the shuttle effect of lithium polysulfides (LiPSs) and the sluggish redox kinetics. Since the adsorption and catalytic conversion of LiPSs mainly occur on the surface of the electrocatalyst, regulating the surface structure of electrocatalysts is an advisable strategy to solve the obstacles in Li-S batteries. Herein, CoP nanoparticles with high oxygen content on surface embedded in hollow carbon nanocages (C/O-CoP) is employed to functionalize the separators and the effect of the surface oxygen content of CoP on the electrochemical performance is systematically explored. Increasing the oxygen content on CoP surface can enhance the chemical adsorption to lithium polysulfides and accelerate the redox conversions kinetics of polysulfides. The cell with C/O-CoP modified separator can achieve the capacity of 1033 mAh g-1 and maintain 749 mAh g-1 after 200 cycles at 2 C. Moreover, DFT calculations are used to reveal the enhancement mechanism of oxygen content on surface of CoP in Li-S chemistry. This work offers a new insight into developing high-performance Li-S batteries from the perspective of surface engineering.

A Green Synthesis Strategy for Cobalt Phosphide Deposited on N, P Co-Doped Graphene for Efficient Hydrogen Evolution

The exploitation of electrocatalysts with high activity and durability for the hydrogen evolution reaction is significant but also challenging for future energy systems. Transition metal phosphides (TMPs) have attracted a lot of attention due to their effective activity for the hydrogen evolution reaction, but the complicated preparation of metal phosphides remains a bottleneck. In this study, a green fabrication method is designed and proposed to construct N, P co-doped graphene (NPG)-supported cobalt phosphide (Co2P) nanoparticles by using DNA as both N and P sources. Thanks to the synergistic effect of NPG and Co2P, the Co2P/NPG shows effective activity with a small overpotential of 144 mV and a low Tafel slope of 72 mV dec-1 for the hydrogen evolution reaction. This study describes a successful green synthesis strategy for the preparation of high-performance TMPs.

Construction of Co2 P-Ni3 S2 /NF Heterogeneous Structural Hollow Nanowires as Bifunctional Electrocatalysts for Efficient Overall Water Splitting

Designing efficient and stable transition metal-based catalysts for electrocatalytic water splitting is vital for the development of hydrogen production. Herein, a facile synthetic strategy is developed to fabricate transition metal-based heterogeneous structural Co2 P-Ni3 S2 hollow nanowires supported on nickel foam (Co2 P-Ni3 S2 /NF). Owing to the multiple active sites provided by transition metal compounds, large surface area of the unique hollow nanowire morphology, and the synergistic effect of Co2 P-Ni3 S2 heterostructure interfaces, Co2 P-Ni3 S2 /NF requires ultralow overpotentials of 110, 164 mV for HER and 331.7, 358.3 mV for OER at large current densities of 100, 500 mA cm-2 in alkaline medium, respectively. Importantly, the two-electrode electrolyzer assembled by Co2 P-Ni3 S2 /NF displays a cell voltage of 1.54 V at 10 mA cm-2 and operates stably over 24 h at 100 mA cm-2 , which performs better than reported transition metal-based bifunctional electrocatalysts. This work presents a successful fabrication of transition metal-based bifunctional HER/OER electrocatalysts at large-current density and brings new inspiration for developing applicable energy conversion materials.

Facile Electrochemical Synthesis of Bifunctional Needle-like Co-P Nanoarray for Efficient Overall Water Splitting

The development of low-cost and high-performance bifunctional electrocatalysts for overall water splitting is still challenging. Herein, we employed a facile electrodeposition method to prepare bifunctional cobalt phosphide for overall water splitting. The needle-like cobalt phosphide (Co-P-1) nanoarray is uniformly distributed on nickel foam. Co-P-1 exhibits excellent electrocatalytic activity for hydrogen evolution reaction (HER, 85 mV at 10 mA/cm2, 60 mV/dec) and oxygen evolution reaction (OER, 294 mV at 50 mA/cm2, 60 mV/dec). The cell-voltage of 1.60 V is found to achieve the current density of 10 mA/cm2 for overall water splitting in the two-electrode system, comparable to that of previously reported Pt/C/NF||RuO2/NF. The excellent electrocatalytic performance can be attributed to the needle-like structure with more active sites, accelerated charge transfer and evolved bubbles' release. This work can provide new approach to the development of a bifunctional electrocatalyst for overall water splitting.

Optimizing the Electronic Structure of Atomically Dispersed Ru Sites with CoP for Highly Efficient Hydrogen Evolution in both Alkaline and Acidic Media

Developing efficient and stable electrocatalysts for hydrogen evolution reaction (HER) over a wide pH range and industrial large-scale hydrogen production is critical and challenging. Here, a tailoring strategy is developed to fabricate an outstanding HER catalyst in both acidic and alkaline electrolytes containing high-density atomically dispersed Ru sites anchored in the CoP nanoparticles supported on carbon spheres (NC@Ru_SA -CoP). The obtained NC@Ru_SA -CoP catalyst exhibits excellent HER performance with overpotentials of only 15 and 13 mV at 10 mA cm^-2 in 1 m KOH and 0.5 m H₂ SO₄ , respectively. The experimental results and theoretical calculations indicate that the strong interaction between the Ru site and the CoP can effectively optimize the electronic structure of Ru sites to reduce the hydrogen binding energy and the water dissociation energy barrier. The constructed alkaline anion exchange membrane water electrolyze (AAEMWE) demonstrates remarkable durability and an industrial-level current density of 1560 mA cm^-2 at 1.8 V. This strategy provides a new perspective on the design of Ru-based electrocatalysts with suitable intermediate adsorption strengths and paves the way for the development of highly active electrocatalysts for industrial-scale hydrogen production.

Tailored Heterojunction Active Sites for Oxygen Electrocatalyst Promotion in Zinc-Air Batteries

Rechargeable zinc-air batteries (ZABs) are promising energy storage systems due to their low-cost and safety. However, the working principle of ZABs is based on oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), which display sluggish kinetic and low stability. Herein, this work proposes a novel method to design a heterogeneous CoP/CoO electrocatalyst on mesopore nanobox carbon/carbon nanotube (CoP/CoO@MNC-CNT) that enriched active sites and synergistic effect. Moreover, the well-defined heterointerfaces could lower the energy barrier for intermediate species adsorption and promote OER and ORR electrochemical performances. The CoP/CoO@MNC-CNT electrocatalyst presents a high half-wave potential of 0.838 V for ORR and a small overpotential of 270 mV for OER. The ZABs-based CoP/CoO@MNC-CNT air-cathode shows an open-circuit voltage of 1.409 V, the long-term cycle life of 500 h with a small voltage difference change of 7.7%. Additionally, the flexible ZABs exhibit highly mechanical stability, demonstrating their application potential in wearable electronic devices.

Electrodeposition of Stable Noble-Metal-Free Co-P Electrocatalysts for Hydrogen Evolution Reaction

Hydrogen production via water splitting has been extensively explored over the past few decades, and considerable effort has been directed toward finding more reactive and cost-effective electrocatalysts by engineering their compositions, shapes, and crystal structures. In this study, we developed hierarchical cobalt phosphide (Co-P) nanosphere assemblies as non-noble metal electrocatalysts via one-step electrodeposition. The morphologies of the Co-P nanostructures and their electrocatalytic activities towards the hydrogen evolution reactions (HER) were controlled by the applied potentials during electrodeposition. The physicochemical properties of the as-prepared Co-P nanostructures in this study were characterized by field-emission scanning electron microscopy, X-ray photoemission spectroscopy and X-ray diffraction. Linear sweep voltammetry revealed that the Co-P grown at -0.9 V showed the best HER performance exhibiting the highest electrochemical active surface area and lowest interfacial charge transfer resistance. The Co-P electrocatalysts showed superior long-term stability to electrodeposited Pt, indicating their potential benefits.

In Situ Growth of CoP Nanosheet Arrays on Carbon Cloth as Binder-Free Electrode for High-Performance Flexible Lithium-Ion Batteries

Cobalt phosphide (CoP) is considered as one of the most promising candidates for anode in lithium-ion batteries (LIBs) owing to its low-cost, abundant availability, and high theoretical capacity. However, problems of low conductivity, heavy aggregation, and volume change of CoP, hinder its practical applicability. In this study, a binder-free electrode is successfully prepared by growing CoP nanosheets arrays directly on a carbon cloth (CC) via a facile one-step electrodeposition followed by an in situ phosphorization strategy. The CoP@CC anode exhibits good interfacial bonding between the CoP and CC, which can improve the conductivity of the integrated electrode. More importantly, the 3D network structure composed of CoP nanosheets and CC provides sufficient space to alleviate the volume expansion of CoP and shorten the electron/ion transport paths. Moreover, the support of CC effectively prevents the agglomeration of CoP. Based on these advantages, when CoP@CC is paired with the NCM523 cathode, the full cell delivers a high discharge capacity 919.6 mAh g^-1 (2.1 mAh cm^-2 ) after 200 cycles at 0.5 A g^-1 . The feasibility and safety of producing pouch cells are also explored, which show good flexibility and safety despite rigorous strikes (mechanical damage and severe deformations), implying a great potential for practical applications.

Unusual Activity of Rationally Designed Cobalt Phosphide/Oxide Heterostructure Composite for Hydrogen Production in Alkaline Medium

Design and development of an efficient, nonprecious catalyst with structural features and functionality necessary for driving the hydrogen evolution reaction (HER) in an alkaline medium remain a formidable challenge. At the root of the functional limitation is the inability to tune the active catalytic sites while overcoming the poor reaction kinetics observed under basic conditions. Herein, we report a facile approach to enable the selective design of an electrochemically efficient cobalt phosphide oxide composite catalyst on carbon cloth (CoP-Co_xO_y/CC), with good activity and durability toward HER in alkaline medium (η₁₀ = -43 mV). Theoretical studies revealed that the redistribution of electrons at laterally dispersed Co phosphide/oxide interfaces gives rise to a synergistic effect in the heterostructured composite, by which various Co oxide phases initiate the dissociation of the alkaline water molecule. Meanwhile, the highly active CoP further facilitates the adsorption-desorption process of water electrolysis, leading to extremely high HER activity.

Nano-dendrite structured cobalt phosphide based hybrid supercapacitor with high energy storage and cycling stability

The supercapacitors possessing high energy storage and long serving period have strategic significance to solve the energy crisis issues. Herein, fluffy nano-dendrite structured cobalt phosphide (CoP) is grown on carbon cloth through simple hydrothermal and electrodeposition treatments (CoP/C-HE). Benefit from its excellent electrical conductivity and special structure, CoP/C-HE manifests a high specific capacity of 461.4 C g^-1at 1 A g^-1. Meanwhile, the capacity retention remains 92.8% over 10 000 cycles at 5 A g^-1, proving the superior cycling stability. The phase conversion of Co₂P during the activation process also contributes to the improved performance. The assembled two-electrode asymmetric supercapacitor demonstrates excellent performance in terms of energy density (42.4 W h kg^-1at a power density of 800.0 W kg^-1) and cycling stability (86.3% retention over 5000 cycles at 5 A g^-1), which is superior to many reported cobalt-based supercapacitors. Our work promotes the potential of transition metal phosphides for the applications in supercapacitors.

Nanostructure Engineering of Metal-Organic Derived Frameworks: Cobalt Phosphide Embedded in Carbon Nanotubes as an Efficient ORR Catalyst

Heteroatom doping is considered an efficient strategy when tuning the electronic and structural modulation of catalysts to achieve improved performance towards renewable energy applications. Herein, we synthesized a series of carbon-based hierarchical nanostructures through the controlled pyrolysis of Co-MOF (metal organic framework) precursors followed by in situ phosphidation. Two kinds of catalysts were prepared: metal nanoparticles embedded in carbon nanotubes, and metal nanoparticles dispersed on the carbon surface. The results proved that the metal nanoparticles embedded in carbon nanotubes exhibit enhanced ORR electrocatalytic performance, owed to the enriched catalytic sites and the mass transfer facilitating channels provided by the hierarchical porous structure of the carbon nanotubes. Furthermore, the phosphidation of the metal nanoparticles embedded in carbon nanotubes (P-Co-CNTs) increases the surface area and porosity, resulting in faster electron transfer, greater conductivity, and lower charge transfer resistance towards ORR pathways. The P-Co-CNT catalyst shows a half-wave potential of 0.887 V, a Tafel slope of 67 mV dec⁻¹, and robust stability, which are comparatively better than the precious metal catalyst (Pt/C). Conclusively, this study delivers a novel path for designing multiple crystal phases with improved catalytic performance for energy devices.

Enhancing Polysulfide Confinement and Electrochemical Kinetics by Amorphous Cobalt Phosphide for Highly Efficient Lithium-Sulfur Batteries

The application of lithium-sulfur (Li-S) batteries is severely hampered by the shuttle effect and sluggish redox kinetics. Herein, amorphous cobalt phosphide grown on a reduced graphene oxide-multiwalled carbon nanotube (rGO-CNT-CoP(A)) is designed as the sulfur host to conquer the above bottlenecks. The differences between amorphous cobalt phosphide (CoP) and crystalline CoP on the surface adsorption as well as conversion of lithium polysulfides (LiPSs) are investigated by systematical experiments and density-functional theory (DFT) calculations. Specifically, the amorphous CoP not only strengthens the chemical adsorption to LiPSs but also greatly accelerates liquid-phase conversions of LiPSs as well as the nucleation and growth of Li₂S. DFT calculation reveals that the amorphous CoP possesses higher binding energies and lower diffusion energy barriers for LiPSs. In addition, the amorphous CoP features reduced energy gap and the increased electronic concentrations of adsorbed LiPSs near Fermi level. These characteristics contribute to the enhanced chemisorption ability and the accelerated redox kinetics. Simultaneously, the prepared S/rGO-CNT-CoP(A) electrode delivers an impressive initial capacity of 872 mAh g^-1 at 2 C and 617 mAh g^-1 can be obtained after 200 cycles, exhibiting excellent cycling stability. Especially, it achieves outstanding electrochemical performance even under high sulfur loading (5.3 mg cm^-2) and lean electrolyte (E/S = 7 μL_E mg^-1_S) conditions. This work exploits the application potential for amorphous materials and contributes to the development of highly efficient Li-S batteries.

Cobalt Phosphide Nanoflake-Induced Flower-like Sulfur for High Redox Kinetics and Fast Ion Transfer in Lithium-Sulfur Batteries

Sulfur reactivity in lithium-sulfur batteries highly depends on its distribution and morphology during cycling, which is of great significance to suppress the shuttle effect and promote conversion reaction. Herein, cobalt phosphide nanoflakes are prepared and used as a sulfur host. An improved redox kinetics from sulfur to lithium sulfide and the corresponding fast lithium-ion diffusion are observed to greatly promote the electrochemical performance of lithium-sulfur batteries. Meanwhile, for the first time, we propose "effective triple phase contact" and "insulated dead sulfur" to account for cycling performance differences of CoP@S and rGO@S batteries. The flower-like sulfur induced by CoP nanoflakes during cycling provides extra lithium-ion diffusion and electron transfer ways compared with agglomerated sulfur in the rGO@S cathode. The CoP@S battery shows good rate performance and delivers 520 mA h g^-1 after 1000 cycles with an excellent Coulombic efficiency of 99%. In contrast, no conversion reaction happens after 600 cycles in the rGO@S battery, implying no existence of reactive sulfur. This research reveals the effect of morphological evolution of sulfur on the cycling performance and affords an insight for developing high-performance lithium-sulfur batteries.

Controllable Heteroatom Doping Effects of CrxCo2-xP Nanoparticles: a Robust Electrocatalyst for Overall Water Splitting in Alkaline Solutions

The effect of doping Cr on the electrocatalytic activity of Co₂P supported on carbon black (Cr_xCo_2-xP/CB) for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline solution was investigated. A beneficial improvement in the performance of Co₂P toward HER and OER was discovered. For the HER at -200 mV overpotential, the turnover frequency (TOF) increases almost 6-fold from 0.26 to 1.52 electron site_Co^-1 s^-1 when Co₂P/CB has a small amount of Cr added to form Cr_0.2Co_1.8P/CB. Similarly, we estimate an increase from 0.205 to 0.585 electron site_Co^-1 s^-1 for the OER at 1.6 V for the same change in composition. With 10 atom % Cr doping, the Cr_0.2Co_1.8P/CB catalyst needed 226 mV overpotential to produce a cathodic current density of -100 A g_Co^-1 and 380 mV overpotential to produce an anodic current density of 100 A g_Co^-1. Based on both experimental results and theoretical calculations, the activity improvement results from optimization of the electronic properties of Co₂P after Cr doping.

The novel high-pressure/high-temperature compound Co12P7 determined from synchrotron data

Co₁₂P₇, synthesized at high pressure/temperature conditions, crystallizes isotypically with ordered Cr₁₂P₇ in space-group type P.

The structural properties of cobalt phosphides were investigated at high pressures and temperatures to better understand the behavior of metal-rich phosphides in Earth and planetary inter­iors. Using single-crystal X-ray diffraction synchrotron data and a laser-heated diamond anvil cell, we discovered a new high pressure–temperature (HP–HT) cobalt phosphide, Co₁₂P₇, dodeca­cobalt hepta­phosphide, synthesized at 27 GPa and 1740 K, and at 48 GPa and 1790 K. Co₁₂P₇ adopts a structure initially proposed for Cr₁₂P₇ (space-group type P, Z =1), consisting of chains of edge-sharing CoP₅ square pyramids and chains of corner-sharing CoP₄ tetra­hedra. This arrangement leaves space for trigonal–prismatic channels running parallel to the c axis. Coupled disordering of metal and phospho­rus atoms has been observed in this structure for related M₁₂P₇ (M = Cr, V) compounds, but all Co and P sites are ordered in Co₁₂P₇. All atomic sites in this crystal structure are situated on special positions. Upon decompression to ambient conditions, peak broadening and loss of reflections at high angles was observed, suggesting phase instability.

Metal-Organic-Framework-Derived Co2 P Nanoparticle/Multi-Doped Porous Carbon as a Trifunctional Electrocatalyst

Developing efficient and low-cost replacements for precious metals as electrocatalysts active in electrochemical reactions-the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and oxygen reduction reaction (ORR)-is a top priority in renewable energy technology. In this work a highly active and very stable trifunctional electrocatalyst composed of Co₂ P embedded in Co, N, and P multi-doped carbon has been synthesized using zeolitic imidazolate frameworks as precursors. The synergistic effects between Co₂ P and the multi-heteroatom-doped carbon substrates afford materials having electrocatalytic activities for HER, OER, and ORR, which are comparable-or even superior to-those of commercial RuO₂ or Pt/C catalysts. Density functional theory calculations show that Co₂ P has a higher density of states at the Fermi level than Co_n P (0 < n < 2), which promotes electron transfer and intermediates adsorption in the catalytic process. Zinc-air batteries and water splitting devices assembled using the materials as electrode electrocatalysts show good performance and outstanding stability. This work represents a breakthrough in improving the catalytic performance of non-precious metal electrocatalysts for OER, HER, and ORR, and opens new avenues for clean energy generation.

CoP/RGO-Pd Hybrids with Heterointerfaces as Highly Active Catalysts for Ethanol Electrooxidation

The ethanol oxidation reaction is of critical importance to the commercial viability of direct ethanol fuel cell technology. However, owing to the poor C-C bond cleavage capability, almost all ethanol oxidation is incomplete and suffers from low selectivity toward the C1 pathway. Herein, under the support of theoretical calculations that the heterointerfaces between CoP and Pd can reduce the energy barrier of C-C bond cleavage, rich heterointerfaces in CoP/RGO-Pd hybrids were designed to improve ethanol electrooxidation performance through enhancing the selectivity toward the C1 pathway. The experimental results show that the faradaic efficiency of the C1 pathway of CoP/RGO-Pd hybrids is as high as 27.6%, surpassing most reported catalysts in the literature. As a result of this enhancement, CoP/RGO-Pd10 exhibits mass activity as high as 4597 mA·mg_Pd^-1 and specific activity as high as 10 mA·cm^-2, which are much higher than those of other Pd-based electrocatalysts.

Enhanced Oxygen Evolution Reaction Activity of a Co2P@NC-Fe2P Composite Boosted by Interfaces Between a N-Doped Carbon Matrix and Fe2P Microspheres

Constructing highly efficient and low-cost transition-metal-based electrocatalysts with a large number of interfaces to increase their active site densities constitutes a major advancement in the development of water-splitting technology. Herein, a bimetallic phosphide composite (Co₂P@NC-Fe₂P) is successfully synthesized from a ferric hydroxyphosphate-zeolitic imidazolate framework hybrid precursor (FeHP-ZIF-67). Benefitting from morphology and composition regulations, the FeHP-ZIF-67 precursor is prepared by a room-temperature solution synthesis method, which exhibits an optimal morphology, where FeHP microspheres are coated with excess ZIF-67 nanoparticles. During the annealing of FeHP-ZIF-67, FeHP serves as a source of phosphorus to form Fe₂P and Co₂P simultaneously, where Co₂P nanoparticles coated with an N-doped carbon (NC) matrix derived from ZIF-67 are partially adsorbed onto the surface of Fe₂P microspheres, thereby forming numerous NC-Fe₂P interfaces. The optimal Co₂P@NC-Fe₂P composite has an overpotential of 260 mV at a current density of 10 mA cm^-2, a small Tafel slope of 41 mV dec^-1, and long-term stability of over 35 h in an alkaline medium for oxygen evolution reactions (OERs). Such a superior OER performance is attributed to the active NC-Fe₂P interfaces in the Co₂P@NC-Fe₂P composite. This work provides a new strategy to optimize transition-metal phosphides with effective interfaces for OER electrocatalysis.

Gold-Incorporated Cobalt Phosphide Nanoparticles on Nitrogen-Doped Carbon for Enhanced Hydrogen Evolution Electrocatalysis

Transition metal phosphides (TMPs) demonstrate great potential for hydrogen evolution reaction (HER) electrocatalysis, but their activities need further improvement. Herein we report a novel Au incorporation strategy to boost the HER catalytic performance of CoP. As a proof of concept, heterostructured Au/CoP nanoparticles dispersed on nitrogen-doped carbon with unique porosity, denoted as Au/CoP@NC-3, are synthesized by thermal treatment of Au-nanoparticle-incorporated ZIF-67 precursor. It shows excellent HER activity as well as good durability in acidic and alkaline condition, respectively, greatly outperforming its Au-free analogue, namely, CoP@NC. In-depth analysis suggests that the improved HER activity of Au/CoP@NC-3 is attributed to the presence of Au nanoparticles which enlarge the electrochemical active surface areas and adjust the electronic structure of active CoP species to enhance the water adsorption and optimize H adsorption for the accelerated HER process.

An Efficient Cobalt Phosphide Electrocatalyst Derived from Cobalt Phosphonate Complex for All-pH Hydrogen Evolution Reaction and Overall Water Splitting in Alkaline Solution

The development of low-cost and highly efficient electrocatalysts via an eco-friendly synthetic method is of great significance for future renewable energy storage and conversion systems. Herein, cobalt phosphides confined in porous P-doped carbon materials (Co-P@PC) are fabricated by calcinating the cobalt-phosphonate complex formed between 1-hydroxyethylidenediphosphonic acid and Co(NO₃ )₂ in alkaline solution. The P-containing ligand in the complex acts as the carbon source as well as in situ phosphorizing agent for the formation of cobalt phosphides and doping P element into carbon material upon calcination. The Co-P@PC exhibits high activity for all-pH hydrogen evolution reaction (overpotentials of 72, 85, and 76 mV in acidic, neutral, and alkaline solutions at the current density of 10 mA cm^-2 ) and oxygen evolution reaction in alkaline solution (an overpotential of 280 mV at the current density of 10 mA cm^-2 ). The alkaline electrolyzer assembled from the Co-P@PC electrodes delivers the current density of 10 mA cm^-2 at the voltage of 1.60 V with a durability of 60 h. The excellent activity and long-term stability of the Co-P@PC derives from the synergistic effect between the active cobalt phosphides and the porous P-doped carbon matrix.

Stringing Bimetallic Metal-Organic Framework-Derived Cobalt Phosphide Composite for High-Efficiency Overall Water Splitting

Water electrolysis is an emerging energy conversion technology, which is significant for efficient hydrogen (H2) production. Based on the high-activity transition metal ions and metal alloys of ultrastable bifunctional catalyst, the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are the key to achieving the energy conversion method by overall water splitting (OWS). This study reports that the Co-based coordination polymer (ZIF-67) anchoring on an indium-organic framework (InOF-1) composite (InOF-1@ZIF-67) is treated followed by carbonization and phosphorization to successfully obtain CoP nanoparticles-embedded carbon nanotubes and nitrogen-doped carbon materials (CoP-InNC@CNT). As HER and OER electrocatalysts, it is demonstrated that CoP-InNC@CNT simultaneously exhibit high HER performance (overpotential of 153 mV in 0.5 m H2SO4 and 159 mV in 1.0 m KOH) and OER performance (overpotential of 270 mV in 1.0 m KOH) activities to reach the current density of 10 mA cm-2. In addition, these CoP-InNC@CNT rods, as a cathode and an anode, can display an excellent OWS performance with η10 = 1.58 V and better stability, which shows the satisfying electrocatalyst for the OWS compared to control materials. This method ensures the tight and uniform growth of the fast nucleating and stable materials on substrate and can be further applied for practical electrochemical reactions.

CoP/C Nanocubes-Modified Separator Suppressing Polysulfide Dissolution for High-Rate and Stable Lithium-Sulfur Batteries

A functioned PP was chosen as a separator to suppress the shuttling effect of soluble polysulfide in lithium-sulfur batteries (LSBs). Nanocubic cobalt phosphide/carbon (CoP/C) was modified on PP membrane through a simple vacuum filtration method. This CoP/C-modified PP separator not only efficiently captures polysulfides through strong chemical affinity but also facilitates the conversion of the soluble intermediates due to the fast transfer at the interface. In consequence, the cell with a CoP/C-modified separator exhibits a low-capacity decay of only 0.08% per cycle over 500 cycles at 1 C with an initial capacity of 938 mAh g^-1 and a superior rate performance of 594 mAh g^-1 at 4 C. Even with a high loading of 3.2 mg cm^-2, the cell still exhibits an excellent reversible capacity of 601.3 mAh g^-1 after 100 cycles at 0.5 C. This work provides a new strategy to effectively restrict the polysulfide shuttling.

Fabrication of Hollow CoP/TiOx Heterostructures for Enhanced Oxygen Evolution Reaction

Transition-metal phosphides have flourished as promising candidates for oxygen evolution reaction (OER) electrocatalysts. Herein, it is demonstrated that the electrocatalytic OER performance of CoP can be greatly improved by constructing a hybrid CoP/TiO_x heterostructure. The CoP/TiO_x heterostructure is fabricated using metal-organic framework nanocrystals as templates, which leads to unique hollow structures and uniformly distributed CoP nanoparticles on TiO_x . The strong interactions between CoP and TiO_x in the CoP/TiO_x heterostructure and the conductive nature of TiO_x with Ti³⁺ sites endow the CoP-TiO_x hybrid material with high OER activity comparable to the state-of-the-art IrO₂ or RuO₂ OER electrocatalysts. In combination with theoretical calculations, this work reveals that the formation of CoP/TiO_x heterostructure can generate a pathway for facile electron transport and optimize the water adsorption energy, thus promoting the OER electrocatalysis.

High-Performance Flexible Solid-State Asymmetric Supercapacitors Based on Bimetallic Transition Metal Phosphide Nanocrystals

Transition metal phosphides (TMPs) have recently emerged as an important type of electrode material for use in supercapacitors thanks to their intrinsically outstanding specific capacity and high electrical conductivity. Herein, we report the synthesis of bimetallic Co_xNi_1-xP ultrafine nanocrystals supported on carbon nanofibers (Co_xNi_1-xP/CNF) and explore their use as positive electrode materials of asymmetric supercapacitors. We find that the Co:Ni ratio has a significant impact on the specific capacitance/capacity of Co_xNi_1-xP/CNF, and Co_xNi_1-xP/CNF with an optimal Co:Ni ratio exhibits an extraordinary specific capacitance/capacity of 3514 F g^-1/1405.6 C g^-1 at a charge/discharge current density of 5 A g^-1, which is the highest value for TMP-based electrode materials reported by far. Our density functional theory calculations demonstrate that the significant capacitance/capacity enhancement in Co_xNi_1-xP/CNF, compared to the monometallic NiP/CNF and CoP/CNF, originates from the enriched density of states near the Fermi level. We further fabricate a flexible solid-state asymmetric supercapacitor using Co_xNi_1-xP/CNF as positive electrode material, activated carbon as negative electrode material, and a polymer gel as the electrolyte. The supercapacitor shows a specific capacitance/capacity of 118.7 F g^-1/166.2 C g^-1 at 20 mV s^-1, delivers an energy density of 32.2 Wh kg^-1 at 3.5 kW kg^-1, and demonstrates good capacity retention after 10000 charge/discharge cycles, holding substantial promise for applications in flexible electronic devices.

Porous Organic Polymer-Driven Evolution of High-Performance Cobalt Phosphide Hybrid Nanosheets as Vanillin Hydrodeoxygenation Catalyst

Hydrodeoxygenation (HDO) is a promising route for the upgrading of bio-oils to eco-friendly biofuel produced from lignocellulose. Herein, we report the sequential synthesis of a hybrid nanocatalyst Co_xP@POP, where substoichiometric Co_xP nanoparticles are distributed in a porous organic polymer (POP) via solid-state phosphidation of the Co₃O₄@POP nanohybrid system. We also explored the catalytic activity of the above two nanohybrids toward the HDO of vanillin, a typical compound of lignin-derived bio-oil to 2-methoxy-4-methylphenol, which is a promising future biofuel. The Co_xP@POP exhibited superior catalytic activity and selectivity toward desired product with improved stability compared to the Co₃O₄@POP. Based on advanced sample characterization results, the extraordinary selectivity of Co_xP@POP is attributed to the strong interaction of the cation of the Co_xP nanoparticle with the POP matrix and the consequent modifications of the electronic states. Through attenuated total reflectance-infrared spectroscopy, we have also observed different interaction strengths between vanillin and the two catalysts. The decreased catalytic activity of Co₃O₄@POP compared to Co_xP@POP catalyst could be attributed to the stronger adsorption of vanillin over the Co₃O₄@POP catalyst. Also from kinetic investigation, it is clearly demonstrated that the Co₃O₄@POP has higher activation energy barrier than the Co_xP@POP, which also reflects to the reduction of the overall efficiency of the Co₃O₄@POP catalyst. To the best of our knowledge, this is the first approach in POP-encapsulated cobalt phosphide catalyst synthesis and comprehensive study in establishing the structure-activity relationship in significant step-forwarding in promoting biomass refining.

LiCl as Phase-Transfer Catalysts to Synthesize Thin Co2 P Nanosheets for Oxygen Evolution Reaction

Inorganic salts have been widely studied as templates for the synthesis of 2D layer structures. However, these salts normally can only serve as templates without any catalytic activity. Here, we report that LiCl used for the synthesis of ultrathin nanosheets not only serves as template for the synthesis of ultrathin Co₂ P nanosheets with a thickness of 0.7 nm but also acts as a catalyst that induces the phase-transfer from CoP to Co₂ P. The Co₂ P nanosheets have a high electrochemical performance for oxygen evolution reaction.

Co2P@N,P-Codoped Carbon Nanofiber as a Free-Standing Air Electrode for Zn-Air Batteries: Synergy Effects of CoNx Satellite Shells

Here, a free-standing electrode composed of cobalt phosphides (Co₂P) supported by cobalt nitride moieties (CoN_x) and an N,P-codoped porous carbon nanofiber (CNF) in one-step electrospinning of environmentally friendly benign phosphorous precursors is reported. Physiochemical characterization revealed the symbiotic relationship between a Co₂P crystal and surrounding nanometer-sized CoN_x moieties embedded in an N,P-codoped porous carbon matrix. Co₂P@CNF shows high oxygen reduction reaction and oxygen evolution reaction performance owing to the synergistic effect of Co₂P nanocrystals and the neighboring CoN_x moieties, which have the optimum binding strength of reactants and facilitate the mass transfer. The free-standing Co₂P@CNF air-cathode-based Zn-air batteries deliver a power density of 121 mW cm^-2 at a voltage of 0.76 V. The overall overpotential of Co₂P@CNF-based Zn-air batteries can be significantly reduced, with low discharge-charge voltage gap (0.81 V at 10 mA cm^-2) and high cycling stability, which outperform the benchmark Pt/C-based Zn-air batteries. The one-step electrospinning method can serve as a universal platform to develop other high-performance transition-metal phosphide catalysts benefitting from the synergy effect of transition nitride satellite shells. The free-standing and flexible properties of Co₂P@CNF make it a potential candidate for wearable electronic devices.

Atomic Heterointerface-Induced Local Charge Distribution and Enhanced Water Adsorption Behavior in a Cobalt Phosphide Electrocatalyst for Self-Powered Highly Efficient Overall Water Splitting

Developing economical and highly efficient noble metal-free electrocatalysts for overall water splitting is an essential precondition for renewable energy conversion. Herein, we highlight atomic heterointerface engineering in constructing highly efficient cobalt phosphide (CoP)/Co₉S₈ electrocatalysts for full water splitting. A CoP/Co₉S₈ hybrid was prepared for the first time by partial homogeneous transformation of in situ-formed Co₉S₈, in which the atomic heterointerface was formed between CoP and Co₉S₈. Systematic experiments and theoretical calculations confirm that the as-formed atomic heterointerface can induce local charge distribution in CoP/Co₉S₈, which can not only accelerate the charge transfer but also optimize the hydrogen adsorption energy of CoP in favor of the fast transformation of H_ads into H₂. Meanwhile, the Co₉S₈ component can also increase the water adsorption capability of CoP/Co₉S₈. Benefiting from these outstanding advantages, an alkaline electrolyzer based on CoP/Co₉S₈ as both electrodes achieves a low cell voltage of 1.6 V at an operating current density of 10 mA cm^-2, and at the same time, it can also be self-powered by a home-assembled Zn-air battery employing the same CoP/Co₉S₈ as the air electrode for prospectively achieving renewable energy conversion. This work demonstrates the importance of heterostructure engineering in developing noble metal-free catalysts for high-performance water electrolysis.

Three-Dimensional Porous Cobalt Phosphide Nanocubes Encapsulated in a Graphene Aerogel as an Advanced Anode with High Coulombic Efficiency for High-Energy Lithium-Ion Batteries

An ingeniously designed porous structure can synergistically optimize the desired properties and maximize the advantages of a material as an electrode for a high-performance energy storage system. The active material with a porous nanostructure could reduce the ion diffusion path and buffer the strain caused by the volume changes during cycling. Furthermore, combining the active material with a three-dimensional (3D) graphene aerogel (GA) matrix is an ideal way to maintain the structural integrity, improve the conductivity, and overcome the aggregation problem of the nanomaterials. Herein, we adopted a facile template-based strategy to derive a composite of 3D hierarchically porous cobalt phosphide nanocubes with a graphene aerogel (CoP@GA). The as-prepared CoP@GA features porous cobalt phosphide nanocubes that are firmly encapsulated and uniformly distributed in the well-defined graphene aerogel skeleton. Benefiting from the hierarchical porosity, structural integrity, and conductive network, the CoP@GA electrode manifests an ultrahigh initial Coulombic efficiency (88.6%), outstanding lithium storage performance in terms of excellent cycling performance (805.3 mAh·g^-1 after 200 cycles at 200 mA·g^-1), superior high-energy performance (351.8 mAh·g^-1 after 4000 cycles at 10 A·g^-1), and exceptional rate capability. Moreover, this synthesis protocol could be an instructive precedent for fabricating transition-metal-phosphide-based 3D porous composites with excellent electrochemical performances.

One-Dimensional Porous Hybrid Structure of Mo2C-CoP Encapsulated in N-Doped Carbon Derived from MOF: An Efficient Electrocatalyst for Hydrogen Evolution Reaction over the Entire pH Range

The development of outstanding noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER) has attracted broad interest. Herein, a novel one-dimensional (1D) HER hybrid catalyst consisted of cobalt phosphide (CoP) and molybdenum carbide (Mo₂C) nanoparticles wrapped by nitrogen-doped graphitic carbon (called CoP/Mo₂C-NC) was successfully fabricated by a facile continuous-flow method and a simple two-step annealing process. During these processes, the successful synthesis of the MoO₃ nanorods coated with cobalt zeolitic imidazolate frameworks (Co-ZIF-67) (Co-ZIF-67@MoO₃) through the continuous-flow method plays a key role. The as-synthesized CoP/Mo₂C-NC hybrid electrocatalyst exhibits a significantly enhanced HER electrocatalytic activity over the entire pH range relative to that of the control materials CoP, Mo₂C-NC, and physically mixed CoP and Mo₂C-NC. The outstanding HER catalytic performance is mainly due to the fact that the electron cloud transfers from Co to Mo in CoP/Mo₂C-NC through the Co-P-Mo bond, resulting in the formation of a high valence state for Co (Co³⁺) species and lower valence states for Mo (i.e., Mo²⁺, Mo³⁺) species, providing the abundant HER active sites. Moreover, the Gibbs free energy (Δ G_H*) of CoP/Mo₂C-NC obtained by the density function theory calculations indicates a good balance between the Volmer and Heyrovsky/Tafel steps in HER kinetics. Such a cobalt zeolitic imidazolate framework-mediated strategy depicted in this work offers an interesting perspective for developing highly efficient noble-metal-free electrocatalysts for hydrogen production.

Nitrogen-Doped CoP Electrocatalysts for Coupled Hydrogen Evolution and Sulfur Generation with Low Energy Consumption

Hydrogen production is the key step for the future hydrogen economy. As a promising H₂ production route, electrolysis of water suffers from high overpotentials and high energy consumption. This study proposes an N-doped CoP as the novel and effective electrocatalyst for hydrogen evolution reaction (HER) and constructs a coupled system for simultaneous hydrogen and sulfur production. Nitrogen doping lowers the d-band of CoP and weakens the H adsorption on the surface of CoP because of the strong electronegativity of nitrogen as compared to phosphorus. The H adsorption that is close to thermos-neutral states enables the effective electrolysis of the HER. Only -42 mV is required to drive a current density of -10 mA cm^-2 for the HER. The oxygen evolution reaction in the anode is replaced by the oxidation reaction of Fe²⁺ , which is regenerated by a coupled H₂ S absorption reaction. The coupled system can significantly reduce the energy consumption of the HER and recover useful sulfur sources.

3D Architectures of Cox P Using Silk Fibroin Scaffolds: An Active and Stable Electrocatalyst for Hydrogen Generation in Acidic and Alkaline Media

Developing nonprecious, highly active, and stable catalysts is essential for efficient electrocatalytic hydrogen evolution reaction in water splitting. In this study, the facile synthesis of a 3D flower-like Co_x P/carbon architecture is proposed composed of an assembly of nanosheets interconnected by silk fibroin that acts as 3D scaffolds and a carbon source. This unique 3D architecture coupled with a carbon matrix enhances catalytic activity by exposing more active sites and increasing charge transport. The flower-like Co_x P/carbon can facilitate a lower overpotential, Tafel slope, charge transfer resistance, and a higher electrochemically active surface than carbon-free and silk-free Co_x P. The nanostructured architecture exhibits excellent catalytic performance with low overpotentials of 109 and 121 mV at 10 mA cm^-2 and Tafel slopes of 55 and 62 mV dec^-1 in acidic and alkaline media, respectively. Furthermore, it minimally degrades the overpotential and current density after long-term stability tests 10 000 cyclic voltammetry cycles and a chronoamperometric test over 40 h, respectively, in acidic media, which confirms the high durability and stability of the flower-like Co_x P/carbon.

One-Step Facile Synthesis of Cobalt Phosphides for Hydrogen Evolution Reaction Catalysts in Acidic and Alkaline Medium

Catalysts for hydrogen evolution reaction are in demand to realize the efficient conversion of hydrogen via water electrolysis. In this work, cobalt phosphides were prepared using a one-step, scalable, and direct gas-solid phosphidation of commercially available cobalt salts. It was found that the effectiveness of the phosphidation reaction was closely related to the state of cobalt precursors at the reaction temperature. For instance, a high yield of cobalt phosphides obtained from the phosphidation of cobalt(II) acetate was related to the good stability of cobalt salt at the phosphidation temperature. On the other hand, easily oxidizable salts (e.g., cobalt(II) acetylacetonate) tended to produce a low amount of cobalt phosphides and a large content of metallic cobalt. The as-synthesized cobalt phosphides were in nanostructures with large catalytic surface areas. The catalyst prepared from phosphidation of cobalt(II) acetate exhibited an improved catalytic activity as compared to its counterpart derived from phosphidation of cobalt(II) acetylacetonate, showing an overpotential of 160 and 175 mV in acidic and alkaline electrolytes, respectively. Both catalysts also displayed an enhanced long-term stability, especially in the alkaline electrolyte. This study illustrates the direct phosphidation behavior of cobalt salts, which serve as a good vantage point in realizing the large-scale synthesis of transition-metal phosphides for high-performance electrocatalysts.

Toward High-Performance and Low-Cost Hydrogen Evolution Reaction Electrocatalysts: Nanostructuring Cobalt Phosphide (CoP) Particles on Carbon Fiber Paper

In this communication, we facily fabricated nanostructured CoP particles (150 to 200 nm) on carbon fiber paper (CFP) for hydrogen evolution reaction (HER) by a simple two-step process via a green route. In the first step, crystalline Co₃O₄ nanocubes (150-200 nm) were loaded on CFP through a hydrothermal process at low temperature (120 °C). Interestingly, crystalline Co₃O₄ nanocubes with a size 150-200 nm exhibited different growth mechanisms in contrast to the crystalline Co₃O₄ nanocubes with a size <100 nm reported earlier. In the second step, these crystalline Co₃O₄ nanocubes were converted to catalytically active CoP particles through chemical vapor deposition (CVD) phosphorization (denoted as CoP/CFP-H). Remarkably, CoP/CFP-H exhibited a low Tafel slope of 49.7 mV/dec and only required overpotentials of 128.1, 144.4, and 190.8 mV to drive geometric current densities of -10, -20, and -100 mA cm^-2, respectively. Besides, the CoP/CFP-H also demonstrated an excellent durability in an acidic environment under 2000 sweeps at a high scan rate (100 mV s^-1) and a 24 h chronopotentiometry testing. For comparison, CoP was also fabricated through the electrodeposition method, followed by CVD phosphorization (denoted as CoP/CFP-E). It was found that the latter had exhibited inferior activity compared to CoP/CFP-H. The good performances of CoP/CFP-H are essentially due to the rational designs of electrode: (i) the applications of highly HER active CoP electrocatalyst, (ii) the intimate contact of nanostructured CoP on carbon fibers, and (iii) the large electrochemical surface area at electrocatalyst/electrolyte interface due to the large retaining of particles features after phosphorization. Notably, the intermediate Co₃O₄/CFP can serve as a platform to develop other cobalt-based functional materials.

A Co2 P/WC Nano-Heterojunction Covered with N-Doped Carbon as Highly Efficient Electrocatalyst for Hydrogen Evolution Reaction

The hydrogen evolution reaction (HER) produces clean hydrogen through an electrochemical process. However, new nonprecious-metal electrocatalysts for the HER are required to reduce the consumption of energy. Herein, we report a new Co₂ P/WC nano-heterojunction that consists of Co₂ P and WC composite phases coated with a few-layer N-doped graphitic carbon shells (Co₂ P/WC@NC). The composite was prepared by a one-step annealing of the polyoxometalate Na₉ (NH₄ )₅ [{(B-α-PW₉ O₃₄ )Co₃ (OH)(H₂ O)₂ (Ale)}₂ Co]⋅35 H₂ O (Co₇ P₆ W₁₈ ) and dicyandiamide (DCA). The preparation method consisted of the simultaneous phosphorization of Co and carbonization of W in a confined space to isolate a Co₂ P/WC nano-heterojunction phase for the first time. Co₂ P/WC@NC facilitated the generation of hydrogen in the electrolysis process, which had an overpotential of only 91 mV at a current density of 10 mA cm^-2 in the acid solution; an excellent HER performance (2 H⁺ +2 e^- →H₂ ) and Tafel slope (40 mV dec^-1 ) as well as durability over a period of 50 h were achieved. Theoretical calculations showed that the Co₂ P, WC, and N_pyridinic C dopants in the material synergistically promoted the HER activity rather than the individual constituents. Furthermore, Co₂ P/WC@NC nano-heterojunctions showed good HER performance in the whole pH range of electrolytes and considerable durability in acidic media containing transition metal ions, which may attract more attention for the exploration and optimization of nano-heterojunction catalysts for the HER.

Colloidal Cobalt Phosphide Nanocrystals as Trifunctional Electrocatalysts for Overall Water Splitting Powered by a Zinc-Air Battery

Highly efficient and stable electrocatalysts, particularly those that are capable of multifunctionality in the same electrolyte, are in high demand for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). In this work, highly monodisperse CoP and Co₂ P nanocrystals (NCs) are synthesized using a robust solution-phase method. The highly exposed (211) crystal plane and abundant surface phosphide atoms make the CoP NCs efficient catalysts toward ORR and HER, while metal-rich Co₂ P NCs show higher OER performance owing to easier formation of plentiful Co₂ P@COOH heterojunctions. Density functional theory calculation results indicate that the desorption of OH* from cobalt sites is the rate-limiting step for both CoP and Co₂ P in ORR and that the high content of phosphide can lower the reaction barrier. A water electrolyzer constructed with a CoP NC cathode and a Co₂ P NC anode can achieve a current density of 10 mA cm^-2 at 1.56 V, comparable even to the noble metal-based Pt/C and RuO₂ /C pair. Furthermore, the CoP NCs are employed as an air cathode in a primary zinc-air battery, exhibiting a high power density of 62 mW cm^-2 and good stability.

Stability of CoP x Electrocatalysts in Continuous and Interrupted Acidic Electrolysis of Water

Cobalt phosphides are an emerging earth‐abundant alternative to platinum‐group‐metal‐based electrocatalysts for the hydrogen evolution reaction (HER). Yet, their stability is inferior to platinum and compromises the large‐scale applicability of CoP_x in water electrolyzers. In the present study, we employed flat, thin CoP_x electrodes prepared through the thermal phosphidation (PH₃) of Co₃O₄ films made by plasma‐enhanced atomic layer deposition to evaluate their stability in acidic water electrolysis by using a multi‐technique approach. The films were found to be composed of two phases: CoP in the bulk and a P‐rich surface CoP_x (P/Co>1). Their performance was evaluated in the HER and the exchange current density was determined to be j₀=−8.9 ⋅ 10⁻⁵ A/cm². The apparent activation energy of HER on CoP_x (E_a=81±15 kJ/mol) was determined for the first time. Dissolution of the material in 0.5 M H₂SO₄ was observed, regardless of the constantly applied cathodic potential, pointing towards a chemical instead of an electrochemical origin of the observed cathodic instability. The current density and HER faradaic efficiency (FE) were found to be stable during chronoamperometric treatment, as the chemical composition of the HER‐active phase remained unchanged. On the contrary, a dynamic potential change performed in a repeated way facilitated dissolution of the film, yielding its complete degradation within 5 h. There, the FE was also found to be changing. An oxidative route of CoP_x dissolution has also been proposed.

MOF-Derived Ultrathin Cobalt Phosphide Nanosheets as Efficient Bifunctional Hydrogen Evolution Reaction and Oxygen Evolution Reaction Electrocatalysts

The development of a highly efficient and stable bifunctional electrocatalyst for water splitting is still a challenging issue in obtaining clean and sustainable chemical fuels. Herein, a novel bifunctional catalyst consisting of 2D transition-metal phosphide nanosheets with abundant reactive sites templated by Co-centered metal-organic framework nanosheets, denoted as CoP-NS/C, has been developed through a facile one-step low-temperature phosphidation process. The as-prepared CoP-NS/C has large specific surface area and ultrathin nanosheets morphology providing rich catalytic active sites. It shows excellent electrocatalytic performances for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in acidic and alkaline media, with the Tafel slopes of 59 and 64 mV/dec and a current density of 10 mA/cm² at the overpotentials of 140 and 292 mV, respectively, which are remarkably superior to those of CoP/C, CoP particles, and comparable to those of commercial noble-metal catalysts. In addition, the CoP-NS/C also shows good durability after a long-term test.

Polyaniline Derived N-Doped Carbon-Coated Cobalt Phosphide Nanoparticles Deposited on N-Doped Graphene as an Efficient Electrocatalyst for Hydrogen Evolution Reaction

The development of highly efficient and durable non-noble metal electrocatalysts for the hydrogen evolution reaction (HER) is significant for clean and renewable energy research. This work reports the synthesis of N-doped graphene nanosheets supported N-doped carbon coated cobalt phosphide (CoP) nanoparticles via a pyrolysis and a subsequent phosphating process by using polyaniline. The obtained electrocatalyst exhibits excellent electrochemical activity for HER with a small overpotential of -135 mV at 10 mA cm^-2 and a low Tafel slope of 59.3 mV dec^-1 in 0.5 m H₂ SO₄ . Additionally, the encapsulation of N-doped carbon shell prevents CoP nanoparticles from corrosion, exhibiting good stability after 14 h operation. Moreover, the as-prepared electrocatalyst also shows outstanding activity and stability in basic and neutral electrolytes.

Transition-Metal Phosphide-Carbon Nanosheet Composites Derived from Two-Dimensional Metal-Organic Frameworks for Highly Efficient Electrocatalytic Water-Splitting

The preparation of highly active, sustainable, nonprecious metal materials as hydrogen evolution and oxygen evolution reaction (HER and OER) catalysts that can relieve the environmental pollution and energy shortage problems present a great challenge to chemists. We herein report the fabrication of a highly active metal phosphide-carbon composite catalyst for HER and OER in acid and basic solution, respectively. The catalyst is derived through carbonization and subsequent phosphorization of two-dimensional (2D) cobalt porphyrinic metal-organic framework nanosheets. It consists of cobalt phosphide nanoparticles embedded in mesoporous N-doped graphitic carbon materials. The catalyst shows good electrocatalytic activities for HER in 0.5 M H₂SO₄ and OER in 1 M KOH with overpotentials of 98 and 370 mV at a current density of 10 mA cm^-2 and the Tafel slopes of 74 and 79 mV dec^-1, respectively. In addition, the catalyst also shows good durability. The method used in this study could be applied to prepare new, highly efficient water-splitting catalysts by using diverse 2D metal-organic frameworks as templates.

Cobalt Phosphide Coupled with Heteroatom-Doped Nanocarbon Hybrid Electroctalysts for Efficient, Long-Life Rechargeable Zinc-Air Batteries

Metal phosphides and heteroatom-doped carbons have been regarded as promising candidates as bifunctional catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). However, both have suffered from stability issues during repeated ORR and OER operations in zinc-air batteries (ZABs). Herein, this study reports a versatile cobalt-based hybrid catalyst with a 1D structure by integrating the metal-organic framework-derived conversion approach and an in situ crosslinking method. Among them, the 1D hybrid catalyst composed of ultrasmall cobalt phosphide nanoparticles supported by nitrogen-, sulfur-, phosphorus-doped carbon matrix shows remarkable bifunctional activity close to that of the benchmark precious-metal catalysts along with an excellent durability in the full potential range covering both the OER and ORR. The overall overpotential of the rechargeable ZABs can be greatly reduced with this bifunctional hybrid catalyst as an air-electrode, and the cycling stability outperforms the commercial Pt/C catalyst. It is revealed that the cobalt phosphide nanoparticles are in situ converted to cobalt oxide under the accelerated conditions during OER (and/or ORR) of the ZABs and reduces the anodic current applied to the carbon. This contributes to the stability of the carbon material and in maintaining the high initial catalytic properties of the hybrid catalyst.

Porous Cobalt Phosphide Polyhedrons with Iron Doping as an Efficient Bifunctional Electrocatalyst

Iron (Fe)-doped porous cobalt phosphide polyhedrons are designed and synthesized as an efficient bifunctional electrocatalyst for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The synthesis strategy involves one-step route for doping foreign metallic element and forming porous cobalt phosphide polyhedrons. With varying doping levels of Fe, the optimized Fe-doped porous cobalt phosphide polyhedron exhibits significantly enhanced HER and OER performances, including low onset overpotentials, large current densities, as well as small Tafel slopes and good electrochemical stability during HER and OER.

Sugar Blowing-Induced Porous Cobalt Phosphide/Nitrogen-Doped Carbon Nanostructures with Enhanced Electrochemical Oxidation Performance toward Water and Other Small Molecules

Rational design of high active and robust nonprecious metal catalysts with excellent catalytic efficiency in oxygen evolution reaction (OER) is extremely vital for making the water splitting process more energy efficient and economical. Among these noble metal-free catalysts, transition-metal-based nanomaterials are considered as one of the most promising OER catalysts due to their relatively low-cost intrinsic activities, high abundance, and diversity in terms of structure and morphology. Herein, a facile sugar-blowing technique and low-temperature phosphorization are reported to generate 3D self-supported metal involved carbon nanostructures, which are termed as Co₂ P@Co/nitrogen-doped carbon (Co₂ P@Co/N-C). By capitalizing on the 3D porous nanostructures with high surface area, homogeneously dispersed active sites, the intimate interaction between active sites, and 3D N-doped carbon, the resultant Co₂ P@Co/N-C exhibits satisfying OER performance superior to CoO@Co/N-C, delivering 10 mA cm^-2 at overpotential of 0.32 V. It is worth noting that in contrast to the substantial current density loss of RuO₂ , Co₂ P@Co/N-C shows much enhanced catalytic activity during the stability test and a 1.8-fold increase in current density is observed after stability test. Furthermore, the obtained Co₂ P@Co/N-C can also be served as an excellent nonprecious metal catalyst for methanol and glucose electrooxidation in alkaline media, further extending their potential applications.

Synthesis of Nickel Phosphide Electrocatalysts from Hybrid Metal Phosphonates

Transition-metal phosphides (TMPs) have recently emerged as efficient and inexpensive electrocatalysts for electrochemical water splitting. The synthesis of nanostructured phosphides often involves highly reactive and hazardous phosphorous-containing compounds. Herein, we report the synthesis of nickel phosphides through thermal treatment under H₂(5%)/Ar of layered nickel phenylphosphonate (NiPh) or methylphosphonate (NiMe) that act as single-source precursors. Ni₁₂P₅, Ni₁₂P₅-Ni₂P, and Ni₂P nanoparticles (NPs) with sizes of ca. 15-45 nm coated with a thin shell of carbonaceous material were produced. Thermogravimetric analysis coupled with mass spectrometry (TG-MS) showed that H₂, H₂O, P₂, and -C₆H₅ are the main compounds formed during the transformation of the precursor under argon and no hazard phosphorous-containing compounds are created, making this a simple and relatively safe route for fabricating nanostructured TMPs. The H₂ most likely reacts with the -PO₃ groups of the precursor to form H₂O and P₂, and the latter subsequently reacts with the metal to produce the phosphide. The Ni₁₂P₅-Ni₂P and Ni₂P NPs efficiently catalyze the hydrogen evolution reaction (HER), with Ni₂P showing the best performance and generating a current density of 10 mA cm^-2 at an overpotential of 87 mV and exhibiting long-term stability. Co₂P and CoP NPs were also synthesized following this method. This approach may be utilized to explore the rich metal phosphonate chemistry for fabricating phosphide-based materials for electrochemical energy conversion and storage applications.

Interconnected Network of Core-Shell CoP@CoBiPi for Efficient Water Oxidation Electrocatalysis under Near Neutral Conditions

Developing earth-abundant electrocatalysts for efficient and stable water oxidation under near neutral conditions is of great importance but still remains a key challenge. Herein, we demonstrate the development of an interconnected network of core-shell CoP@CoBiPi through anodic polarization of a CoP nanoarray in potassium borate aqueous electrolyte (KBi). This 3 D CoP@CoBiPi exhibits high catalytic activity for water oxidation at pH 9.2 and needs an overpotential (η) of only 410 mV to drive a geometrical catalytic current density of 10 mA cm^-2 , with a high turnover frequency of 819 h^-1 at an overpotential of 610 mV. Remarkably, this catalyst also demonstrates high long-term electrochemical stability with its activity being maintained for at least 27 h in KBi. This study provides us an attractive earth-abundant 3 D catalyst electrode for water-splitting devices toward efficient and stable water oxidation under benign conditions.

Cobalt Phosphide Modified Titanium Oxide Nanophotocatalysts with Significantly Enhanced Photocatalytic Hydrogen Evolution from Water Splitting

Production of hydrogen from photocatalytic water splitting holds promise as an alternative energy source with superiority of cleanliness, environment friendliness, low price, and sustainability. Perfectly constructing the noble-metal-free and stable hybrid structure photocatalyst is quite essential; herein, for the first time the authors aim to use cobalt phosphide as the cocatalyst on titanium oxide to form a novel hybrid structure to enhance the utilization of the photoexcited electrons in redox reactions for improved photocatalytic H₂ evolution activity. Thus, the achieved significantly increased photocatalytic H₂ -evolution rate on the optimized CoP/TiO₂ (8350 µmol h^-1 g^-1 ) is 11 times higher than that of the pristine TiO₂ . Moreover, this work is expected to spur more insight into synthesizing such novel photofunctional systems, achieving high photocatalytic H₂ evolution activity and sufficient stability for solar-to-chemical conversion and utilization.

Synthesis of Cobalt Phosphide Nanoparticles Supported on Pristine Graphene by Dynamically Self-Assembled Graphene Quantum Dots for Hydrogen Evolution

A highly active, durable, and low-cost hydrogen evolution reaction (HER) catalyst is desirable for energy storage through water splitting but its fabrication presents great challenges. Herein, mediated by dynamically self-assembled graphene quantum dots (GQDs), small, uniform, high-density, and well-dispersed CoP nanoparticles were grown in situ on pristine graphene for the first time. This hybrid nanostructure was then employed as HER electrocatalyst, showing an onset potential of 7 mV, an overpotential of 91.3 mV to achieve 10 mA cm^-2 , a Tafel slope of 42.6 mV dec^-1 , and an exchange current density of 0.1225 mA cm^-2 , all of which compare favorably to those of most reported non-noble-metal catalysts. The developed catalyst also exhibits excellent durability with negligible current loss after 2000 cyclic voltammetry cycles (+0.01 to -0.17 V vs. RHE) or 34 h of chronoamperometric measurement at an overpotential of 91.3 mV. This work not only develops a new strategy for the fabrication of high-performance and inexpensive electrocatalysts for HER but also provides scientific insight into the mechanism of the dynamically self-assembled GQDsmediated synthesis process.