Self-Supported NiS Nanoparticle-Coupled Ni2 P Nanoflake Array Architecture: An Advanced Catalyst for Electrochemical Hydrogen Evolution

Multiple Bubble Removal Strategies to Promote Oxygen Evolution Reaction: Mechanistic Understandings from Orientation, Rotation, and Sonication Perspectives

Bubble coverage of catalytically active sites is one of the well-known bottlenecks to the kinetics of the oxygen evolution reaction (OER). Herein, various bubble removal approaches (electrode orientation, rotating, and sonication) were considered for the OER performance evaluation of a state-of-the-art Ir-based electrocatalyst. Key parameters, such as catalyst mass loss, activity, overpotential, and charge- and mass-transfer mechanisms, were analyzed. First, it was suggested that a suitable orientation of the working electrode facilitates coalescence and sliding bubble effects on the catalyst surface, leading to better electrochemical performance than those of the traditional rotating disk electrode (RDE) configuration. Then, the convection and secondary Bjerknes force were explained as the responsible phenomena in improving the OER activity in the RDE and sonication methods. Finally, simultaneous implementation of the methods enhanced the catalyst mass activity up to 164% and provided fast charge-transfer kinetics and low double-layer capacitance, which eventually led to a 22% reduction in overpotential, while the catalyst loss slightly increased from 1.93 to 3.88%.

Heterostructured Ni3S2-Ni3P/NF as a Bifunctional Catalyst for Overall Urea-Water Electrolysis for Hydrogen Generation

Urea oxidation reaction (UOR) has been proposed to replace the formidable oxygen evolution reaction (OER) to reduce the energy consumption for producing hydrogen from electrolysis of water owing to its much lower thermodynamic oxidation potential compared to that of the OER. Therefore, exploring a highly efficient and stable hydrogen evolution and urea electrooxidation bifunctional catalyst is the key to achieve economical and efficient hydrogen production. In this paper, we report a heterostructured sulfide/phosphide catalyst (Ni₃S₂-Ni₃P/NF) synthesized via one-step thermal treatment of Ni(OH)₂/NF, which allows the simultaneous occurrence of phosphorization and sulfuration. The obtained Ni₃S₂-Ni₃P/NF catalyst shows a sheet structure with an average sheet thickness of ∼100 nm, and this sheet is composed of interconnected Ni₃S₂ and Ni₃P nanoparticles (∼20 nm), between which there are a large number of accessible interfaces of Ni₃S₂-Ni₃P. Thus, the Ni₃S₂-Ni₃P/NF exhibits superior performance for both UOR and hydrogen evolution reaction (HER). For the overall urea-water electrolysis, to achieve current densities of 10 and 100 mA cm^-2, cell voltage of only 1.43 and 1.65 V is required using this catalyst as both the anode and the cathode. Moreover, this catalyst also maintains fairly excellent stability after a long-term testing, indicating its potential for efficient and energy-saving hydrogen production. The theoretical calculation results show that the Ni atoms at the interface are the most efficient catalytically active site for the HER, and the free energy of hydrogen adsorption is closest to thermal neutrality, which is only 0.16 eV. A self-driven electron transfer at the interface, making the Ni₃S₂ sides become electron donating while Ni₃P sides become electron withdrawing, may be the reason for the enhancement of the UOR activity. Therefore, this work shows an easy treatment for enhancing the catalytic activity of Ni-based materials to achieve high-efficiency urea-water electrolysis.

Uniform Loading of Nickel Phosphide Nanoparticles in Hierarchical Carbonized Wood Channel for Efficient Electrocatalytic Hydrogen Evolution

The development of self-supporting high-efficiency catalysts is a major challenge for the efficient production of H2 via water splitting. In this manuscript, a freestanding Ni2P-Ni12P5/carbonized wood (CW) composite electrode was prepared by a simple hydrothermal method and high-temperature calcination using pine wood with uniform channel as support and a large number of hydroxyl groups as nucleation center. The morphology and structural characteristics indicated that the Ni2P and Ni12P5 nanoparticles were uniformly distributed within the hierarchical porous structure of the CW. In acid media, the as-prepared Ni2P-Ni12P5/CW exhibits an excellent catalytic activity with a low overpotential of 151 mV at 10 mA cm−2 and a reasonably good long-term stability.

Recent Trends in Synthesis and Investigation of Nickel Phosphide Compound/Hybrid-Based Electrocatalysts Towards Hydrogen Generation from Water Electrocatalysis

Sustainable and high performance energy devices such as solar cells, fuel cells, metal-air batteries, as well as alternative energy conversion and storage systems have been considered as promising technologies to meet the ever-growing demands for clean energy. Hydrogen evolution reaction (HER) is a crucial process for cost-effective hydrogen production; however, functional electrocatalysts are potentially desirable to expedite reaction kinetics and supply high energy density. Thus, the development of inexpensive and catalytically active electrocatalysts is one of the most significant and challenging issues in the field of electrochemical energy storage and conversion. Realizing that advanced nanomaterials could engender many advantageous chemical and physical properties over a wide scale, tremendous efforts have been devoted to the preparation of earth-abundant transition metals as electrocatalysts for HER in both acidic and alkaline environments because of their low processing costs, reasonable catalytic activities, and chemical stability. Among all transition metal-based catalysts, nickel compounds are the most widely investigated, and have exhibited pioneering performances in various electrochemical reactions. Heterostructured nickel phosphide (Ni_xP_y) based compounds were introduced as promising candidates of a new category, which often display chemical and electronic characteristics that are distinct from those of non-precious metals counterparts, hence providing an opportunity to construct new catalysts with an improved activity and stability. As a result, the library of Ni_xP_y catalysts has been enriched very rapidly, with the possibility of fine-tuning their surface adsorption properties through synergistic coupling with nearby elements or dopants as the basis of future practical implementation. The current review distils recent advancements in Ni_xP_y compounds/hybrids and their application for HER, with a robust emphasis on breakthroughs in composition refinement. Future perspectives for modulating the HER activity of Ni_xP_y compounds/hybrids, and the challenges that need to be overcome before their practical use in sustainable hydrogen production are also discussed.

Layered Ternary and Quaternary Transition Metal Chalcogenide Based Catalysts for Water Splitting

Water splitting plays an important role in the electrochemical and photoelectrochemical conversion of energy devices. Electrochemical water splitting by the hydrogen evolution reaction (HER) is a straightforward route to producing hydrogen (H2), which requires an efficient electrocatalyst to minimize energy consumption. Recent advances have created a rapid rise in new electrocatalysts, particularly those based on non-precious metals. In this review, we present a comprehensive overview of the recent developments of ternary and quaternary 6d-group transition metal chalcogenides (TMCs) based electrocatalysts for water splitting, especially for HER. Detailed discussion is organized from binary to quaternary TMCs including, surface engineering, heterostructures, chalcogen substitutions and hierarchically structural design in TMCs. Moreover, emphasis is placed on future research scope and important challenges facing these electrocatalysts for further development in their performance towards water splitting.

NiSe-Ni0.85 Se Heterostructure Nanoflake Arrays on Carbon Paper as Efficient Electrocatalysts for Overall Water Splitting

Fabricating cost-effective, bifunctional electrocatalysts for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in basic media is critical for renewable energy generation. Here, NiSe/CP, Ni_0.85 Se/CP, and NiSe-Ni_0.85 Se/CP heterostructure catalysts with different phase constitutions are successfully prepared through in situ selenylation of a NiO nanoflake array oriented on carbon paper (CP) by tuning the original Ni/Se molar ratio of the raw materials. The relationship between the crystal phase component and electrocatalytic activity is systematically studied. Benefiting from the synergetic effect of the intrinsic metallic state, facile charge transport, abundant catalytic active sites, and multiple electrolyte transmission paths, the optimized NiSe-Ni_0.85 Se/CP exhibits a remarkably higher catalytic activity for both the HER and OER than single-phase NiSe/CP and Ni_0.85 Se/CP. A current density of 10 mA cm^-2 at 1.62 V and a high stability can be obtained by using NiSe-Ni_0.85 Se/CP as both the cathode and anode for overall water splitting under alkaline conditions. Density functional theory calculations confirm that H and OH^- can be more easily adsorbed on NiSe-Ni_0.85 Se than on NiSe and Ni_0.85 Se. This study paves the way for enhancing the overall water splitting performance of nickel selenides by fabricating heterophase junctions using nickel selenides with different phases.

Engineering NiS/Ni2P Heterostructures for Efficient Electrocatalytic Water Splitting

Developing high-active and low-cost bifunctional materials for catalyzing the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) holds a pivotal role in water splitting. Therefore, we present a new strategy to form NiS/Ni₂P heterostructures. The as-obtained NiS/Ni₂P/carbon cloth (CC) requires overpotentials of 111 mV for the HER and 265 mV for the OER to reach a current density of 20 mA cm^-2, outperforming their counterparts such as NiS and Ni₂P under the same conditions. Additionally, the NiS/Ni₂P/CC electrode requires a 1.67 V cell voltage to deliver 10 mA cm^-2 in a two-electrode electrolysis system, which is comparable to the cell using the benchmark Pt/C||RuO₂ electrode. Detailed characterizations reveal that strong electronic interactions between NiS and Ni₂P, abundant active sites, and smaller charge-transfer resistance contribute to the improved HER and OER activity.