Facile construction of ultrafine nickel-zinc oxyphosphide nanosheets as high-performance electrocatalysts for oxygen evolution reaction

Cu-induced NiCu-P and NiCu-Pi with multilayered nanostructures as highly efficient electrodes for hydrogen production via urea electrolysis

Since urea is commonly present in domestic sewage and industrial wastewater, its use in hydrogen production by electrolysis can simultaneously help in water decontamination. To achieve this goal, the development of highly active and inexpensive urea electrolysis catalysts is necessary. This study deals with the preparation of multilayered nickel and copper phosphides/phosphates (NiCu-P/NF and NiCu-Pi/NF) supported on Ni foam (NF) and their application as new electrocatalyst types for the electrolysis of urea-containing wastewaters. In these materials, Cu atoms induce the formation of multilayer nanostructures and modulate electron distribution, allowing for the exposure of additional active sites and acceleration of the process kinetics. NiCu-P/NF is used as a cathode and NiCu-Pi/NF as an anode in an electrolysis cell and exhibits significant catalytic activity and stability in the urea oxidation reaction (UOR) and the hydrogen evolution reaction (HER). The NiCu-Pi/NF||NiCu-P/NF electrolysis cell, operating with an alkaline urea-containing aqueous electrolyte, achieves a current density of 10 mA cm^- at a potential of 1.41 V, which is less than required by the RuO₂||Pt/C cell utilizing commercial noble metal-based electrodes. The study provides a novel strategy for designing efficient catalysts to produce hydrogen by urea electrolysis.

Shining Light on Anion-Mixed Nanocatalysts for Efficient Water Electrolysis: Fundamentals, Progress, and Perspectives

This review introduces recent advances of various anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, (oxy)hydroxides, and borides) for efficient water electrolysis applications in detail. The challenges and future perspectives are proposed and analyzed for the anion-mixed water dissociation catalysts, including polyanion-mixed and metal-free catalyst, progressive synthesis strategies, advanced in situ characterizations, and atomic level structure-activity relationship. Hydrogen with high energy density and zero carbon emission is widely acknowledged as the most promising candidate toward world's carbon neutrality and future sustainable eco-society. Water-splitting is a constructive technology for unpolluted and high-purity H₂ production, and a series of non-precious electrocatalysts have been developed over the past decade. To further improve the catalytic activities, metal doping is always adopted to modulate the 3d-electronic configuration and electron-donating/accepting (e-DA) properties, while for anion doping, the electronegativity variations among different non-metal elements would also bring some potential in the modulations of e-DA and metal valence for tuning the performances. In this review, we summarize the recent developments of the many different anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, oxyhydroxides, and borides/borates) for efficient water electrolysis applications. First, we have introduced the general information of water-splitting and the description of anion-mixed electrocatalysts and highlighted their complementary functions of mixed anions. Furthermore, some latest advances of anion-mixed compounds are also categorized for hydrogen and oxygen evolution electrocatalysis. The rationales behind their enhanced electrochemical performances are discussed. Last but not least, the challenges and future perspectives are briefly proposed for the anion-mixed water dissociation catalysts.

Nonmetal-doping of noble metal-based catalysts for electrocatalysis

In response to the shortage of fossil fuels, efficient electrochemical energy conversion devices are attracting increasing attention, while the limited electrochemical performance and high cost of noble metal-based electrode materials remain a daunting challenge. The electrocatalytic performance of electrode materials is closely bound with their intrinsic electronic/ionic states and crystal structures. Apart from the nanoscale design and conductive composite strategies, heteroatom doping, particularly for nonmetal doping (e.g., hydrogen, boron, sulfur, selenium, phosphorus, and tellurium), is also another effective strategy to greatly promote the intrinsic activity of the electrode materials by tuning their atomic structures. From the perspective of electrocatalytic reactions, the effective atomic structure regulation could induce additional active sites, create rich defects, and optimize the adsorption capability, thereby contributing to the promotion of the electrocatalytic performance of noble metal-based electrocatalysts. Encouraged by the great progress achieved in this field, we have reviewed recent advancements in nonmetal doping for electrocatalytic energy conversion. Specifically, the doping effect on the atomic structure and intrinsic electronic/ionic state is also systematically illustrated and the relationship with the electrocatalytic performance is also investigated. It is believed that this review will provide guidance for the development of more efficient electrocatalysts.

NiCo2O4 nanoparticles inlaid on sulphur and nitrogen doped and co-doped rGO sheets as efficient electrocatalysts for the oxygen evolution and methanol oxidation reactions

The present work depicts the fabrication of NiCo₂O₄ decorated on rGO, and doped and co-doped rGO and its electrocatalytic activity towards the oxygen evolution reaction and methanol oxidation reaction. The NiCo₂O₄ catalyst with S-doped rGO outperformed the other catalysts, indicating that the sulphur atoms attached on rGO possess low oxophilicity and optimum free energy. This results in facile adsorption of the intermediate products formed during the OER and a rapid release of O₂ molecules. The same catalyst requires an overpotential of 1.51 V vs. RHE to attain the benchmark current density value of 10 mA cm^-2 and shows a Tafel slope of 57 mV dec^-1. It also reveals outstanding stability during its operation for 10 h with a minimum loss in potential. On the other hand, NiCo₂O₄/S,N-rGO reveals superior activity with high efficiency and stability in catalyzing methanol oxidation. The catalyst delivered a low onset potential of 0.12 V vs. Hg/HgO and high current density of 203.4 mA cm^-2 after addition of 0.5 M methanol, revealing the outstanding performance of the electrocatalyst.

Approaching the activity limit of CoSe2 for oxygen evolution via Fe doping and Co vacancy

Electronic structure engineering lies at the heart of efficient catalyst design. Most previous studies, however, utilize only one technique to modulate the electronic structure, and therefore optimal electronic states are hard to be achieved. In this work, we incorporate both Fe dopants and Co vacancies into atomically thin CoSe₂ nanobelts for /coxygen evolution catalysis, and the resulted CoSe₂-D_Fe–V_Co exhibits much higher catalytic activity than other defect-activated CoSe₂ and previously reported FeCo compounds. Deep characterizations and theoretical calculations identify the most active center of Co₂ site that is adjacent to the V_Co-nearest surface Fe site. Fe doping and Co vacancy synergistically tune the electronic states of Co₂ to a near-optimal value, resulting in greatly decreased binding energy of OH* (ΔE_OH) without changing ΔE_O, and consequently lowering the catalytic overpotential. The proper combination of multiple defect structures is promising to unlock the catalytic power of different catalysts for various electrochemical reactions.

While electronic-structure engineering lies at the heart of catalyst design, most previous studies utilize only one technique to tune the electronic states. Here, authors demonstrate that Fe doping and Co vacancy work synergistically to approach the activity limit of CoSe₂ for oxygen evolution reaction.

Heterogeneous Co(OH)2 nanoplates/Co3O4 nanocubes enriched with oxygen vacancies enable efficient oxygen evolution reaction electrocatalysis

Heterogeneous Co(OH)₂ nanoplate/Co₃O₄ nanocube hybrids with rich oxygen vacancies have been constructed through a controllable approach. The high surface areas of such unique nanohybrids together with abundant oxygen vacancies provide more surface active sites, which can facilitate the charge transfer and boost the exchange of intermediates. Specifically, the resultant Co(OH)₂ nanoplate/Co₃O₄ nanocube hybrids display outstanding oxygen evolution reaction (OER) performances with a low overpotential of 281 mV at 10 mA cm^-2 and excellent durability after continuous CV of 3000 cycles, shedding light for large-scale applications in practical water splitting.

Self-supported nickel-cobalt nanowires as highly efficient and stable electrocatalysts for overall water splitting

The development and design of highly active and stable electrocatalysts based on cheap and Earth-abundant materials is critically important to enable water splitting as a desirable renewable energy source. Herein, we fulfill the significant electrochemical water splitting enhancement in both electrocatalytic activity and durability by constructing self-supported nickel-cobalt nanowire catalysts with abundant oxygen vacancies. Specifically, the rich oxygen vacancies can largely promote the oxygen evolution reaction (OER) activity of optimal Ni₁Co₁O₂ NWs with a relatively low overpotential of 248 mV to drive a current density of 10 mA cm^-2. More significantly, after the phosphorization of Ni₁Co₁O₂ NWs, the resultant Ni₁Co₁P NWs can also display excellent electrocatalytic hydrogen evolution reaction (HER) performances with an overpotential of only 101 mV to achieve a current density of 10 mA cm^-2. Furthermore, benefiting from the unique 1D nanowire structure, the synergistic effect, and the optimal Gibbs free energy for hydrogen evolution evolved from the phosphorization, the Ni₁Co₁O₂ NWs//Ni₁Co₁P NWs couple is thus highly active and stable for overall water electrolysis with a low voltage of 1.58 V at 10 mA cm^-2, showing extraordinary promise for practical overall water splitting electrolysis.

Hierarchical NiMo Phosphide Nanosheets Strongly Anchored on Carbon Nanotubes as Robust Electrocatalysts for Overall Water Splitting

Although a great achievement has been made in the field of electrochemistry, the exploration of high-efficiency catalysts for the generation of hydrogen and oxygen via overall water splitting is still a grand challenge. We herein report the successful construction of a new class of hierarchical catalysts with defect-enriched nickel-molybdenum phosphide nanosheets anchored on the surface of carbon nanotubes for efficient water splitting. Via the construction of a hierarchical nanostructure, more efficient electron mobility and mass transfer occurrence were achieved, resulting in a substantial enhancement of electrocatalytic performances. Interestingly, overpotentials of only 255 and 135 mV are required for the optimized Ni₁Mo₁P NSs@MCNTs to afford a current of 10 mA cm^-2 for oxygen evolution reaction and hydrogen evolution reaction, respectively. More significantly, the introduction of molybdenum and phosphorus is also significant for exposing surface active sites and modifying the bonding energy between hydrogen and metals; all of these advantages have endowed the Ni₁Mo₁P NSs@MCNTs//Ni₁Mo₁P NSs@MCNTs couple to display highly efficient water electrocatalysis property with a relatively low overall potential of 1.601 V at 10 mA cm^-2, shedding bright light for large-scale overall water electrocatalysis.

Phosphorus-doped cobalt-iron oxyhydroxide with untrafine nanosheet structure enable efficient oxygen evolution electrocatalysis

Although explosive progresses have been achieved in the field of water splitting, the design and development of stable and inexpensive electrocatalysts for oxygen evolution remain a formidable challenge. Herein, the cost-efficient two dimensional (2D) phosphorus-doped CoFe oxyhydroxide nanosheets (denoted as CoFeP NSs) are successfully engineered and showing exceptional oxygen evolution reaction (OER) activity and chemical stability in 1 M KOH solution. This unique 2D nanosheet structure facilitates the mass transfer and electron transport, resulting in the remarkable OER activity that delivers a current density of 10 mA cm^-2 at a low overpotential of 305 mV with an ultra-small Tafel slope 49.6 mV/dec. More significantly, the doped P also plays a vital role in modulating the surface active sites, leading to the substantial enhancement of electrocatalytic performances. Our study provides a facile one-pot method for the successful fabrication of 2D P-doped CoFe NSs which display superior electrocatalytic performance, shedding great promise for environment and energy-related fields.