Sparkling Synergy: Enhancing Hydrogen Evolution with a Mesoporous CoP/FeP Interface

The reaction kinetics is predominantly determined by the surface and interface engineering of electrocatalysts. Herein, we demonstrate the growth of cobalt monophosphide and iron monophosphide (CoP/FeP) with an effective solid interface. The surface of CoP/FeP is mesoporous, which is obtained by phosphidizing mesoporous CoFe2O4. The CoP/FeP electrode exhibits substantially superior hydrogen evolution reaction (HER) performance compared to CoP and FeP. The overpotentials (η) required to generate 10 mA cm-2 are determined to be around 98 mVRHE (CoP/FeP), 220 mVRHE (FeP), and 265 mVRHE (CoP) in an acidic electrolyte. The exchange current density and Tafel slopes suggest that CoP/FeP has better redox properties and kinetic abilities compared to FeP and CoP. Furthermore, the CoP/FeP electrode exhibits reduced electrochemical impedance and superior surface charge transport characteristics in comparison to both the CoP and FeP electrodes. In addition to having a greater number of catalytically active sites, the turnover frequency of CoP/FeP is approximately 2 and 5 times higher than that of FeP and CoP, respectively. The CoP/FeP electrode maintains a consistent current density of around 25 mA cm-2 for a continuous period of 24 h during the HER, attesting to the excellent durability of the CoP/FeP electrode. In addition, a relationship between differential hydrogen adsorption energy (ΔEH), the corresponding Gibbs free energy change (ΔGH), and the hydrogen coverage on distinct surfaces, namely, CoP, FeP, and CoP/FeP, is established. The calculation findings show that the CoP/FeP surface, which is predominantly exposed with CoP, exhibits the highest catalytic potential for the HER. The estimation of the specific HER activity of the electrodes, normalized to the electrochemically active surface area, corroborates the calculation findings.

Substantial Impact of Built-in Electric Field and Electrode Potential on the Alkaline Hydrogen Evolution Reaction of Ru-CoP Urchin Arrays

Although great efforts on the delicate construction of a built-in electric field (BIEF) to modify the electronic properties of active sites have been conducted, the substantial impact of BIEF coupled with electrode potential on the electrochemical reactions has not been clearly investigated. Herein, we designed an alkaline hydrogen evolution reaction (HER) catalyst composed of heterogeneous Ru-CoP urchin arrays on carbon cloth (Ru-CoP/CC) with a strong BIEF with the guidance of density functional theory (DFT) calculations. Impressively, despite its unsatisfactory activity at 10 mA cm-2 (overpotential of 44 mV), Ru-CoP/CC exhibited better activity (357 mV) than the benchmark Pt/C catalyst (505 mV) at 1 A cm-2 . Experimental and theoretical studies revealed that strong hydrogen adsorption on the interfacial Ru atoms created a high energy barrier for hydrogen desorption and spillover, resulting in unsatisfactory activity at low current densities. However, as the electrode potential became more negative (i.e., the current density increased), the barrier for hydrogen spillover from the interfacial Ru to the Co site, which had near-zero hydrogen adsorption energy, significantly decreased, thus greatly accelerating the whole alkaline HER process. This explains why the activity of Ru-CoP is relatively susceptible to the electrode potential compared to Pt/C.

Recent Advances in Co-Based Electrocatalysts for Hydrogen Evolution Reaction

Water splitting is a promising technique in the sustainable "green hydrogen" generation to meet energy demands of modern society. Its industrial application is heavily dependent on the development of novel catalysts with high performance and low cost for hydrogen evolution reaction (HER). As a typical non-precious metal, cobalt-based catalysts have gained tremendous attention in recent years and shown a great prospect of commercialization. However, the complexity of the composition and structure of newly-developed Co-based catalysts make it urgent to comprehensively retrospect and summarize their advance and design strategies. Hence, in this review, the reaction mechanism of HER is first introduced and the possible role of the Co component during electrocatalysis is discussed. Then, various design strategies that could effectively enhance the intrinsic activity are summarized, including surface vacancy engineering, heteroatom doping, phase engineering, facet regulation, heterostructure construction, and the support effect. The recent progress of the advanced Co-based HER electrocatalysts is discussed, emphasizing that the application of the above design strategies can significantly improve performance by regulating the electronic structure and optimizing the binding energy to the crucial intermediates. At last, the prospects and challenges of Co-based catalysts are shown according to the viewpoint from fundamental explorations to industrial applications.

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.

Effect of Ni atomic fraction on active species of graphene growth on Cu–Ni alloy catalysts: a density functional theory study

The CH and C monomers on the surface are the active species on Cu–Ni catalysts with low Ni atomic fractions. In contrast, the C monomer species on the subsurface acts as an active species on a Cu–Ni catalyst with a high Ni atomic fraction.

All-Gas-Phase Synthesis of Heterolayered Two-Dimensional Nanohybrids Decorated with Metallic Nanocatalysts for Water Splitting

Herein, a sequential gas-phase process involving air jet milling followed by chemical vapor deposition (CVD), is demonstrated to be an efficient strategy for the fabrication of heterolayered 2D nanohybrids (2DNHs) decorated with nanocatalysts. Tens of grams of the nanohybrids, which is a substantial quantity at the laboratory scale, are produced in the absence of solvents and water, and without the need for an extra purification procedure. Air jet milling enables the development of binary/ternary heterolayered structures consisting of graphene, WSe₂ , and/or MoS₂ via the gas-phase co-exfoliation of their bulk counterparts. Based on the X-ray photoelectron and Raman spectroscopy data, the heterolayers of the 2DNHs exert chemical and electronic effects on each other, while diminishing the interactions between same-component layers. Moreover, the electrochemically active surface area increases by >190% and the charge transfer resistance decreases by >35%. CVD is performed to introduce Pt and Ru nanoparticles with diameters of a few nanometers as additional electrocatalysts into the 2DNHs. The nanocatalyst-decorated 2DNHs show excellent performance for the production of hydrogen and oxygen gases in water-splitting cells. Notably, the proposed all-gas-phase processes allow for the large-scale production of functional 2DNHs with minimal negative environmental impact, which is crucial for the commercialization of nanomaterials.

Ru-doping modulated cobalt phosphide nanoarrays as efficient electrocatalyst for hydrogen evolution rection

Electrochemical water splitting is regarded as a prospective means for H₂ production. The lack of efficient active sites and the sluggish kinetics in alkaline media remain the major obstacles for hydrogen evolution reaction (HER). Herein, a rational construction of Ru-doped cobalt phosphide leaf-like nanoarrays supported on carbon cloth (Ru-CoP NAs) was designed via a MOF-derived route and subsequent phosphating treatment for accelerating HER in the alkaline. The unique hierarchical structure is conductive to exposing more active sites and accelerating the diffusion of electrolyte and the release of H₂ bubble. The optimized Ru-CoP-2.5 NAs exhibits a small overpotential of 52 mV to drive 10 mA cm^-2 for HER and a low Tafel slope of 39.7 mV dec^-1 in 1 M KOH, which outperforms most of other reported CoP-based electrocatalysts. Furthermore, density functional theory (DFT) calculations unveil that Ru dopants can modulate the electron environment around pure CoP and optimize the adsorption energy of H*, accelerating the reaction kinetics. This work provides an insight to promote the electrocatalytic activity of metal phosphide for hydrogen production.

Highly Effective Freshwater and Seawater Electrolysis Enabled by Atomic Rh-Modulated Co-CoO Lateral Heterostructures

Atomic metal-modulated heterostructures have been evidenced as an exciting solution to develop high-performance multifunctional electrocatalyst toward water splitting. In this research, a catalyst of continuous cobalt-cobalt oxide (Co-CoO) lateral heterostructures implanted with well-dispersed rhodium (Rh) atoms and shelled over conductive porous 1D copper (Cu) nano-supports for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in both freshwater and seawater under alkaline condition is proposed. It is found that synergistic effects coming from uniform Rh atoms at doping level and Co-CoO heterostructures afford rich multi-integrated active sites and excellent charge transfer, thereby effectively promoting both HER and OER activities. The material requires overpotentials of 107.3 and 137.7 mV for HER and 277.7 and 260 mV for OER to reach an output of 10 mA cm^-1 in freshwater and mimic seawater, respectively, surpassing earlier reported catalysts. Compared to a benchmark a Pt/C//RuO₂ -based two-electrode electrolyzer, a device derived from the 1D-Cu@Co-CoO/Rh on copper foam delivers comparable cell voltages of 1.62, 1.60, and 1.70 V at 10 mA cm^-2 in freshwater, mimic seawater, and natural seawater, respectively, together with robust stability. These results evidence that 1D-Cu@Co-CoO/Rh is a promising catalyst for green hydrogen generation via freshwater and seawater electrolysis applications.

On the evaluation of hydrogen evolution reaction performance of metal-nitrogen-doped carbon electrocatalysts using machine learning technique

Single-atom catalysts (SACs) introduce as a promising category of electrocatalysts, especially in the water-splitting process. Recent studies have exhibited that nitrogen-doped carbon-based SACs can act as a great HER electrocatalyst. In this regard, Adaptive Neuro-Fuzzy Inference optimized by Gray Wolf Optimization (GWO) method was used to predict hydrogen adsorption energy (ΔG) obtained from density functional theory (DFT) for single transition-metal atoms including Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Os, Ir, Pt, and Au embedded in N-doped carbon of different sizes. Various descriptors such as the covalent radius, Zunger radius of the atomic d-orbital, the formation energy of the single-atom site, ionization energy, electronegativity, the d-band center from - 6 to 6 eV, number of valence electrons, Bader charge, number of occupied d states from 0 to - 2 eV, and number of unoccupied d states from 0 to 2 eV were chosen as input parameters based on sensitivity analysis. The R-squared and MSE of the developed model were 0.967 and 0.029, respectively, confirming its great accuracy in determining hydrogen adsorption energy of metal/NC electrocatalysts.

A DFT study of the adsorption energy and electronic interactions of the SO2 molecule on a CoP hydrotreating catalyst

The adsorption energy and electronic properties of sulfur dioxide (SO2) adsorbed on different low-Miller index cobalt phosphide (CoP) surfaces were examined using density functional theory (DFT). Different surface atomic terminations and initial molecular orientations were systematically investigated in detail to determine the most active and stable surface for use as a hydrotreating catalyst. It was found that the surface catalytic reactivity of CoP and its performance were highly sensitive to the crystal plane, where the surface orientation/termination had a remarkable impact on the interfacial chemical bonding and electronic states toward the adsorption of the SO2 molecule. Specifically, analysis of the surface energy adsorption revealed that SO2 on Co-terminated surfaces, especially in (010), (101) and (110) facets, is energetically more favorable compared to other low index surfaces. Charge density difference, density of states (DOS) and Gibbs free energy studies were also carried out to further understand the bonding mechanism and the electronic interactions with the adsorbate. It is anticipated that the current findings will support experimental research towards the design of catalysts for SO2 hydrodesulfurization based on cobalt phosphide nanoparticles.

CoP QD anchored carbon skeleton modified CdS nanorods as a co-catalyst for photocatalytic hydrogen production

An important strategy to improve the performance of catalysts is loading nanoparticle co-catalysts of better dispersion and conductivity. In this work, the ZIF-67-derived CoP quantum dot (QD) anchored graphitized carbon skeleton as a co-catalyst is loaded on CdS nanorods (NRs), while the CoP QDs derived from ZIF-67 are anchored to the carbon skeleton under phosphation and carbonization simultaneously. The porous, graphitized carbon skeleton can not only disperse CoP QDs, increasing active sites for the hydrogen reduction reaction, but also provide electron transfer channels, promoting electron transfer and increasing conductivity. In addition, the metallicity of CoP QDs makes it possible to form Schottky junctions, which is beneficial to the electron transfer at the interface. The results show that the composite photocatalyst can extensively improve the photocatalytic activity and stability, the H2 production rate is 104 947 μmol h-1 g-1 under visible light irradiation (λ ≥ 400 nm), up to 55.2 times that of bare CdS NRs, the apparent quantum yield (AQY) reaches a high value of 32.16% at 420 nm, and the structure of the photocatalyst did not change after the reaction. This work provides an innovative method for the preparation of highly efficient noble metal-free photocatalysts for the conversion of solar energy into hydrogen energy, which has bright prospects in industrial application.

Hydrogen evolution reaction activity related to the facet-dependent electrocatalytic performance of NiCoP from first principles

Transition metal phosphides (TMPs) have been proven to act as highly active catalysts for the hydrogen evolution reaction (HER). Recently, single-phase ternary NiCoP electrocatalysts have been shown through experiments to display remarkable catalytic activity for the HER during water splitting. But, the inherent mechanism is not well understood. Herein, the HER activity of NiCoP with low-Miller-index facets, including (111), (100), (001)-NiP-t, and (001)-CoP-t, was systematically investigated using periodic density functional theory (DFT). The calculated Gibbs free energy of hydrogen adsorption (ΔG H) values reveal that all calculated facets have good catalytic activity for the HER. The (111) facet with the lowest surface energy in a vacuum has optimal ΔG H values close-to-zero for a range of hydrogen coverage. Ab initio thermodynamic analysis of hydrogen coverage was conducted to obtain the stabilities of surfaces, which follow the trend: (111) > (001)-CoP-t > (100) > (001)-NiP-t at 1 atm H2 and 298 K. We hope that this work can shed new light on further understanding the HER in relation to NiCoP and can give guidance for the design and synthesis of transition bimetal phosphide-based catalysts.

Morphology-Controlled Metal Sulfides and Phosphides for Electrochemical Water Splitting

Because H₂ is considered a promising clean energy source, water electrolysis has attracted great interest in related research and technology. Noble-metal-based catalysts are used as electrode materials in water electrolyzers, but their high cost and low abundance have impeded them from being used in practical areas. Recently, metal sulfides and phosphides based on earth-abundant transition metals have emerged as promising candidates for efficient water-splitting catalysts. Most studies have focused on adjusting the composition of the metal sulfides and phosphides to enhance the catalytic performance. However, morphology control of catalysts, including faceted and hollow structures, is much less explored for these systems because of difficulties in the synthesis, which requires a deep understanding of the nanocrystal growth process. Herein, representative synthetic methods for morphology-controlled metal sulfides and phosphides are introduced to provide insights into these methodologies. The electrolytic performance of morphology-controlled metal sulfide- and phosphide-based nanocatalysts with enhanced surface area and intrinsically high catalytic activity is also summarized and the future research directions for this promising catalyst group is discussed.

Enhancing electrocatalysis for hydrogen production over CoP catalyst by strain: a density functional theory study

The facet-dependent strain effects on the hydrogen evolution reaction catalyzed by CoP were studied using density functional theory methods.

Theoretical design of a series of 2D TM–C3N4 and TM–C3N4@graphene (TM = V, Nb and Ta) nanostructures with highly efficient catalytic activity for the hydrogen evolution reaction

Coupled with high structural stability and metallic conductivity, all of the new composite systems TM–C₃N₄ and TM–C₃N₄@graphene (TM = V, Nb and Ta) can possess considerably high catalytic activity for the hydrogen evolution reaction.

Potential-driven surface active structure rearrangement over FeP@NC towards efficient electrocatalytic hydrogen evolution

Understanding the variation of active structure during the hydrogen evolution reaction (HER) process is of great importance for aiding in the design of optimized electrocatalysts.

Surface reconstruction of cobalt phosphide nanosheets by electrochemical activation for enhanced hydrogen evolution in alkaline solution

The surface reconstruction of cobalt phosphide nanosheets is investigated by an in situ electrochemical strategy for enhanced hydrogen evolution.

Transition metal phosphides exhibit promising catalytic performance for the hydrogen evolution reaction (HER); however their surface structure evolution during electrochemical operation has rarely been studied. In this work, we investigate the surface reconstruction of CoP nanosheets by an in situ electrochemical activation method. After remodeling, CoP nanosheets experience an irreversible and significant evolution of the morphology and composition, and low-valence Co complexes consisting of Co(OH)_x species are formed on the surface of CoP nanosheets, and they largely accelerate the dissociation of water. Benefiting from the synergistic effect of CoP and Co(OH)_x, the working electrode shows a remarkably enhanced HER activity of 100 mV at 10 mA cm^–2 with a Tafel slope of 76 mV dec^–1, which is better than that of most transition metal phosphide catalysts. This work would provide a deep understanding of surface reconstruction and a novel perspective for rational design of high performance transition metal phosphide electrocatalysts for water related electrolysis.

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.

An efficient Co3S4/CoP hybrid catalyst for electrocatalytic hydrogen evolution

The development of efficient, universal and inexpensive electrocatalysts for hydrogen evolution reaction (HER) is central to the area of sustainable energy conversion. Considering the Co-based sulfides/phosphides have the same catalytic mechanism with the hydrogenases occurring in nature. Here, a new catalyst based on Co3S4/CoP hybrid that is comprised entirely cheap and earthabundant elements, was first synthesized via a two-step method, the Co(CO3)0.5(OH)·0.11H2O precursor was prepared by a hydrothermal method, followed by phosphidation and sulphidation under Ar atmosphere simultaneously. The resulting Co3S4/CoP hybrid material possessed porous core-shell structure with a homogeneous element distribution and large electroactive surface area (~21.04 mF cm-2). More importantly, the nanostructured Co3S4/CoP electrode exhibits excellent HER properties in acid medium with a low onset overpotential of 34 mV, a small Tafel slope of 45 mV dec-1, as well as a large exchange current density of 150 μA cm-2. These results obtained in this study indicate that the Co3S4/CoP hybrid nanorod is promising replacement to the Pt-based catalysts for H2 production. Moreover, the synthetic method presented in this work can provide an efficient way to synthesis other nanocomposites.

Three-dimensional metal–organic framework derived porous CoP3 concave polyhedrons as superior bifunctional electrocatalysts for the evolution of hydrogen and oxygen

Three-dimensional metal–organic framework-derived porous CoP₃ concave polyhedrons (CoP₃ CPs) were prepared and used as superior bifunctional electrocatalysts for the evolution of hydrogen and oxygen.