Nickel Phosphide Electrocatalysts for Hydrogen Evolution Reaction

Rational design and construction of hierarchical porous quasi-hexagonal Co2P nanosheets/Co heterostructures as highly efficient bifunctional electrocatalysts for overall water splitting

Constructing heterostructured electrocatalysts has proven effective in enhancing intrinsic catalytic activity. Herein, under guidance of theoretical calculations, hierarchical porous quasi-hexagonal Co2P nanosheets/Co heterostructures supported on carbon cloth (Co2P/Co/CC) with a high surface area were rationally designed and elaborately constructed through electroless Co plating, electrochemical oxidation, and phosphidation process, which showed significant electrocatalytic performance toward water electrolysis. Specifically, theoretical calculations revealed that the Co2P/Co heterostructure adjusted the electronic structure of Co2P and Co, reducing the energy barrier for target reactions and thereby boosting electrocatalytic activities for the hydrogen evolution reaction (HER). Notably, the typical Co2P/Co/CC catalyst demonstrated impressive HER performance, with low overpotentials of only 52 and 48 mV to achieve a current density of 10 mA/cm2 in 0.5 M H2SO4 and 1.0 M KOH solutions, respectively. The remarkable electrocatalytic performance of the catalyst can be attributed to the improved intrinsic activity resulting from the Co2P/Co heterostructures and the highly exposed active sites provided by the hierarchical porous structures. Furthermore, the Co2P/Co/CC catalyst exhibited excellent oxygen evolution reaction (OER) performance in alkaline electrolyte, requiring a low overpotential of only 306 mV to achieve a current density of 100 mA/cm2. Additionally, a two-electrode electrolyzer assembled with the Co2P/Co/CC electrodes achieved a current density of 10 mA/cm2 at a low cell voltage of 1.54 V and demonstrated excellent long-term stability. This work presents a novel and feasible strategy for constructing hierarchical heterostructured electrocatalysts that enable efficient water electrolysis. By combining rational design and theoretical guidance, our approach offers promising prospects for advancing the field of electrocatalysis and facilitating sustainable energy conversion.

Synergistic Integration of Amorphous Cobalt Phosphide with a Conductive Channel for Highly Efficient Electrocatalytic Nitrate Reduction to Ammonia

Electrocatalytic nitrate reduction to ammonia (NO3RR) is regarded as a viable alternative reaction to "Haber Bosch" process. Nevertheless, it remains a major challenge to explore economical and efficient electrocatalysts that deliver high NH3 yield rates and Faraday efficiencies (FE). Here, it demonstrates the fabrication of a 3D core-shell structured Co-carbon nanofibers (CNF)/ZIF-CoP for NO3RR application. Benefitting from the distinct electron transport property of Co-CNF and desirable mass transfer ability from amorphous CoP framework, the as-prepared Co-CNF/ZIF-CoP exhibits large NH3 FE (96.8 ± 3.4% at -0.1 V vs reversible hydrogen electrode (RHE)) and high yield rate (38.44 ± 0.65 mg cm-2 h-1 at -0.6 V vs RHE), which are better than Co-CNF/ZIF-crystal CoP. Density functional theory (DFT) calculations further reveal that amorphous CoP presents a lower energy barrier in the rate determination step of the protonation of *NO to produce *NOH intermediates compared with crystal CoP, resulting in a superior NO3RR performance. Eventually, an aqueous galvanic Zn-NO3 - battery is assembled by using Co-CNF/ZIF-CoP as cathode material to achieve efficient production of NH3 whilst simultaneously supplying electrical power. This work offers a reliable strategy to construct amorphous metal phosphide framework on conducting CNF as efficient electrocatalyst and enriches its promising application for NO3RR.

Highly Porous Carbon Flakes Derived from Cellulose and Nickel Phosphide Heterostructure towards Efficient Electrocatalysis of Oxygen Evolution Reaction

This study delves into the pressing challenges of climate change and the escalating carbon dioxide (CO2) emissions by exploring hydrogen technology as a sustainable alternative. In particular, there is focus on nickel phosphide-based electrocatalysts, known for their promising performance in hydrogen evolution reactions (HERs) and oxygen evolution reactions (OERs). Therefore, here we have designed a facile strategy to deliver highly porous carbon flakes derived from cellulose fibers via carbonization at 850 °C, yielding highly porous structures and outstanding specific surface area (SSAcel_carb_850_act = 3164 m2/g) after activation. As-fabricated carbon was utilized as a support for Ni12P5 with an optimized mass ratio. Electrochemical testing revealed that the composite of Ni12P5 and carbon flakes with a ratio of 100:1, respectively, exhibited the most favorable kinetics for the oxygen evolution reaction (OER). Importantly, the durability tests of this sample demonstrated the most stable behavior and lowest potential change under high current density among the studied samples, making it a promising candidate in practical applications. Moreover, the analysis of electrocatalysts after an OER does not show any changes, indicating that the sample does not undergo undesired intermediate reactions and that unwanted products are not released, explaining its stable behavior. This provides a straightforward approach for creating a cellulose-derived composite with enhanced electroactivity and durability.

Recent Tendency on Transition-Metal Phosphide Electrocatalysts for the Hydrogen Evolution Reaction in Alkaline Media

Hydrogen energy is regarded as an auspicious future substitute to replace fossil fuels, due to its environmentally friendly characteristics and high energy density. In the pursuit of clean hydrogen production, there has been a significant focus on the advancement of effective electrocatalysts for the process of water splitting. Although noble metals like Pt, Ru, Pd and Ir are superb electrocatalysts for the hydrogen evolution reaction (HER), they have limitations for large-scale applications, mainly high cost and low abundance. As a result, non-precious transition metals have emerged as promising candidates to replace their more expensive counterparts in various applications. This review focuses on recently developed transition metal phosphides (TMPs) electrocatalysts for the HER in alkaline media due to the cooperative effect between the phosphorus and transition metals. Finally, we discuss the challenges of TMPs for HER.

Insights into the Origin of High Activity of Ni5P4(0001) for Hydrogen Evolution Reaction

Hydrogen evolution reaction (HER) is directly relevant to green hydrogen production from water splitting. Recently, a low-cost Ni₅P₄ material has been demonstrated experimentally and theoretically to exhibit excellent electrocatalytic activity toward HER. However, a fundamental understanding of the origin of Ni₅P₄(0001) activity is still lacking. In this work, density functional theory (DFT) calculations were employed for a comprehensive investigation. The calculation results indicate that the Ni₅P₄(0001) surface exposing Ni₃P₄ termination gains the highest stability, on which a nearly thermoneutral hydrogen adsorption was found at the P₃-hollow sites, providing a high activity for HER. The activity was also observed to be maintained over a wide H-coverage. HER occurs via the Volmer-Heyrovsky mechanism as evidenced from the optimal hydrogen adsorption free energy, but unlikely through the Tafel reaction due to its large energy barrier. Furthermore, the P₃-hollow sites also exhibit a low kinetic barrier for water dissociation, promoting HER in alkaline media. A series of electronic structure analyses were performed in gaining insights into the origin of the HER activity. First, the density of states (DOS) and crystal orbital Hamilton population (COHP) analyses revealed a favorable interaction of electronic states between P and H atoms, leading to stable H adsorption at P₃-hollow sites. In addition, the Bader charge analysis demonstrates that the strength of H adsorption at P₃-hollow sites linearly increases with the electrons carried by the latter. The optimal net charge on the P₃-hollow sites leads to a desired ΔG _H that is close-to-zero. Finally, a highly efficient electron transfer was observed between the P₃-hollow sites and their neighboring atoms, facilitating the HER.

Tunable Synthesis of Metal-Rich and Phosphorus-Rich Nickel Phosphides and Their Comparative Evaluation as Hydrogen Evolution Electrocatalysts

Flexible synthetic routes to crystalline metal-rich to phosphorus-rich nickel phosphides are highly desired for comparable electrocatalytic HER studies. This report details solvent-free, direct, and tin-flux-assisted synthesis of five different nickel phosphides from NiCl₂ and phosphorus at moderate temperatures (500 °C). Direct reactions are thermodynamically driven via PCl₃ formation and tuned through reaction stoichiometry to produce crystalline Ni-P materials from metal-rich (Ni₂P, Ni₅P₄) to phosphorus-rich (cubic NiP₂) compositions. A tin flux in NiCl₂/P reactions allows access to monoclinic NiP₂ and NiP₃. Intermediates in tin flux reactions were isolated to help identify phosphorus-rich Ni-P formation mechanisms. These crystalline micrometer-sized nickel phosphide powders were affixed to carbon-wax electrodes and investigated as HER electrocatalysts in acidic electrolyte. All nickel phosphides show moderate HER activity in a potential range of -160 to -260 mV to achieve current densities of 10 mA/cm² ordered as c-NiP₂ ≥ Ni₅P₄ > NiP₃ > m-NiP₂ > Ni₂P, with NiP₃ activity showing some particle size influence. Phosphorus-rich c/m-NiP₂ appears most stable under acidic conditions during extended reactions. The HER activity of these different nickel phosphides appears influenced by a combination of factors such as particle size, phosphorus content, polyphosphide anions, and surface charge.

Hierarchical CoNiO2 Microflowers Assembled by Mesoporous Nanosheets as Efficient Electrocatalysts for Hydrogen Evolution Reaction

In order to alleviate the energy crisis and propel a low-carbon economy, hydrogen (H₂) plays an important role as a renewable cleaning resource. To break the hydrogen evolution reaction (HER) bottleneck, we need high-efficiency electrocatalysts. Based on the synergistic effect between bimetallic oxides, hierarchical mesoporous CoNiO₂ nanosheets can be fabricated. Combining physical representations with electrochemical measurements, the resultant CoNiO₂ catalysts present the hierarchical microflowers morphology assembled by mesoporous nanosheets. The ultrathin two-dimensional nanosheets and porous surface characteristics provide the vast channels for electrolyte injection, thus endowing CoNiO₂ the outstanding HER performance. The excellent performance with a fewer onset potential of 94 mV, a smaller overpotential at 10 mA cm^-2, a lower Tafel slope of 109 mV dec^-1 and better stability after 1000 cycles makes CoNiO₂ better than that of metallic Co and metallic Ni.

Transition Metal Phosphide Nanoarchitectonics for Versatile Organic Catalysis

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

Improving the HER Activity and Stability of Pt Nanoparticles by Titanium Oxynitride Support

Water electrolysis powered by renewables is regarded as the feasible route for the production of hydrogen, obtained at the cathode side through electrochemical hydrogen evolution reaction (HER). Herein, we present a rational strategy to improve the overall HER catalytic performance of Pt, which is known as the best monometallic catalyst for this reaction, by supporting it on a conductive titanium oxynitride (TiON_x) dispersed over reduced graphene oxide nanoribbons. Characterization of the Pt/TiON_x composite revealed the presence of small Pt particles with diameters between 2 and 3 nm, which are well dispersed over the TiON_x support. The Pt/TiON_x nanocomposite exhibited improved HER activity and stability with respect to the Pt/C benchmark in an acid electrolyte, which was ascribed to the strong metal–support interaction (SMSI) triggered between the TiON_x support and grafted Pt nanoparticles. SMSI between TiON_x and Pt was evidenced by X-ray photoelectron spectroscopy (XPS) through a shift of the binding energies of the characteristic Pt 4f photoelectron lines with respect to Pt/C. Density functional theory (DFT) calculations confirmed the strong interaction between Pt nanoparticles and the TiON_x support. This strong interaction improves the stability of Pt nanoparticles and weakens the binding of chemisorbed H atoms thereon. Both of these effects may result in enhanced HER activity.

Recent Advances in Layered-Double-Hydroxides Based Noble Metal Nanoparticles Efficient Electrocatalysts

With the energy crisis and environmental pollution becoming more and more serious, it is urgent to develop renewable and clean energy. Hydrogen production from electrolyzed water is of great significance to solve the energy crisis and environmental problems in the future. Recently, layered double hydroxides (LDHs) materials have been widely studied in the electrocatalysis field, due to their unique layered structure, tunable metal species and highly dispersed active sites. Moreover, the LDHs supporting noble metal catalysts obtained through the topotactic transformation of LDHs precursors significantly reduce the energy barrier of electrolyzing water, showing remarkable catalytic activity, good conductivity and excellent durability. In this review, we give an overview of recent advances on LDHs supporting noble metal catalysts, from a brief introduction, to their preparation and modification methods, to an overview of their application in the electrocatalysis field, as well as the challenges and outlooks in this promising field on the basis of current development.

Triphenylphosphine-Assisted Transformation of NiS to Ni2P through a Solvent-Less Pyrolysis Route: Synthesis and Electrocatalytic Performance

Straightforward synthetic routes to the preparation of transition metal phosphides or their chalcogenide analogues are highly desired due to their widespread applications, including catalysis. We report a facile and simple route for the preparation of a pure phase nickel phosphide (Ni₂P) and phase transformations in the nickel sulfide (NiS) system through a solvent-less synthetic protocol. Decomposition of different sulfur-based complexes (dithiocarbamate, xanthate, and dithiophosphonate) of nickel(II) was investigated in the presence and absence of triphenylphosphine (TPP). The optimization of reaction parameters (nature of precursor, ratio of TPP, temperature, and time) indicated that phosphorus- and sulfur-containing inorganic dithiophosphonate complexes and TPP (1:1 mole ratio) produced pure nickel phosphide, whereas different phases of nickel sulfide were obtained from dithiocarbamate and xanthate precursors in the presence or absence of TPP. A plausible explanation of the sulfide or phosphide phase formation is suggested, and the performance of Ni₂P was investigated as an electrocatalyst for supercapacitance and overall water-splitting reactions. The performance of Ni₂P with the surface free of any capping agents is not well explored, as common synthetic methods are solution-based routes; therefore, the electrocatalytic performance was also compared with metal phosphides, prepared by other routes. The highest specific capacitance of 367 F/g was observed at 1 A/g, and the maximum energy and power density of Ni₂P were calculated to be 17.9 Wh/kg and 6951 W/kg, respectively. The prepared nickel phosphide required overpotentials of 174 and 316 mV along with Tafel slopes of 115 and 95 mV/dec to achieve a current density of 10 mA/cm² for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively.

Editorial—Special Issue “Catalysis for Energy Production”

The rapid increase in anthropogenic greenhouse gas concentrations in the last several decades means that the effects of climate change are fast becoming the familiar horsemen of a planetary apocalypse. Catalysis, one of the pillars of the chemical and petrochemical industries, will play a critical role in the effort to reduce the flow of greenhouse gases into the atmosphere. This Special Issue is timely, as it provides a collection of high-quality manuscripts in a diverse range of topics, which include the production of green hydrogen via water electrolysis, the steam reforming of ethanol, propane or glycerol, the dry reforming of methane, and the autothermal reforming of diesel surrogate fuel. The topic of the transformation of biomass waste to chemicals is also well represented as is the tackling of CO2 emissions via novel utilization technologies. The Editors are grateful to all authors for their valuable contributions and confident that this Special Issue will prove valuable to scholars, university professors and students alike.

Hydrogen adsorption trends on two metal-doped Ni2P surfaces for optimal catalyst design

In this study, we looked at the hydrogen evolution reaction on the doubly doped Ni3P2 terminated Ni2P surface. Two Ni atoms in the first three layers of the Ni2P surface model were exchanged with two transition metal atoms. We limited our investigation to combinations of Al, Co, and Fe based on their individual effectiveness as Ni2P dopants in our previous computational studies. The DFT calculated hydrogen adsorption free energy was employed as a predictor of the materials' catalytic HER activity. Our results indicate that the combination of Co and Fe dopants most improves the catalytic activity of the surface through the creation of multiple novel and active catalytic sites.

TiO2 Photocatalysis for the Transformation of Aromatic Water Pollutants into Fuels

The growing world energy consumption, with reliance on conventional energy sources and the associated environmental pollution, are considered the most serious threats faced by mankind. Heterogeneous photocatalysis has become one of the most frequently investigated technologies, due to its dual functionality, i.e., environmental remediation and converting solar energy into chemical energy, especially molecular hydrogen. H2 burns cleanly and has the highest gravimetric gross calorific value among all fuels. However, the use of a suitable electron donor, in what so-called “photocatalytic reforming”, is required to achieve acceptable efficiency. This oxidation half-reaction can be exploited to oxidize the dissolved organic pollutants, thus, simultaneously improving the water quality. Such pollutants would replace other potentially costly electron donors, achieving the dual-functionality purpose. Since the aromatic compounds are widely spread in the environment, they are considered attractive targets to apply this technology. In this review, different aspects are highlighted, including the employing of different polymorphs of pristine titanium dioxide as photocatalysts in the photocatalytic processes, also improving the photocatalytic activity of TiO2 by loading different types of metal co-catalysts, especially platinum nanoparticles, and comparing the effect of various loading methods of such metal co-catalysts. Finally, the photocatalytic reforming of aromatic compounds employing TiO2-based semiconductors is presented.

Structural Diversity in Molecular Nickel Phosphide Carbonyl Nanoclusters

The reaction of [Ni₆(CO)₁₂]^2– as a [NBu₄]⁺ salt in CH₂Cl₂ with 0.8 equiv of PCl₃ afforded [Ni₁₄P₂(CO)₂₂]^2–. In contrast, the reactions of [Ni₆(CO)₁₂]^2– as a [NEt₄]⁺ salt with 0.4–0.5 equiv of POCl₃ afforded [Ni_22–xP₂(CO)_29–x]^4– (x = 0.84) or [Ni₃₉P₃(CO)₄₄]^6– by using CH₃CN and thf as a solvent, respectively. Moreover, by using 0.7–0.9 mol of POCl₃ per mole of [NEt₄]₂[Ni₆(CO)₁₂] both in CH₃CN and thf, [Ni_23–xP₂(CO)_30–x]^4– (x = 0.82) was obtained together with [Ni₂₂P₆(CO)₃₀]^2– as a side product. [Ni_23–xP₂(CO)_30–x]^4– (x = 0.82) and [Ni₂₂P₆(CO)₃₀]^2– were separated owing to their different solubility in organic solvents. All the new molecular nickel phosphide carbonyl nanoclusters were structurally characterized through single crystal X-ray diffraction (SC-XRD) as [NBu₄]₂[Ni₁₄P₂(CO)₂₂] (two different polymorphs, P2₁/n and C2/c), [NEt₄]₄[Ni_23–xP₂(CO)_30–x]·CH₃COCH₃·solv (x = 0.82), [NEt₄]₂[Ni₂₂P₆(CO)₃₀]·2thf, [NEt₄]₄[Ni_22–xP₂(CO)_29–x]·2CH₃COCH₃( x = 0.84) and [NEt₄]₆[Ni₃₉P₃(CO)₄₄]·C₆H₁₄·solv. The metal cores’ sizes of these clusters range from 0.59 to 1.10 nm, and their overall dimensions including the CO ligands are 1.16–1.63 nm. In this respect, they are comparable to ultrasmall metal nanoparticles, molecular nanoclusters, or atomically precise metal nanoparticles. The environment of the P atoms within these molecular Ni–P–CO nanoclusters displays a rich diversity, that is, Ni₅P pentagonal pyramid, Ni₇P monocapped trigonal prism, Ni₈P bicapped trigonal prism, Ni₉P monocapped square antiprism, Ni₁₀P sphenocorona, Ni₁₀P bicapped square antiprism, and Ni₁₂P icosahedron.

Five new molecular nickel phosphide carbonyl clusters have been obtained displaying overall sizes in the range 1.16−1.63 nm and very diverse environments for the phosphide atoms.