Hierarchical NiSe2 sheet-like nano-architectures as an efficient and stable bifunctional electrocatalyst for overall water splitting: Phase and morphology engineering

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Doping P atom with a lone pair: an effective strategy to realize high HER catalytic activity and avoid deactivation under wide H* coverage on 2D silicene and germanene by increasing the structural rigidity

In view of the weak aromatic characteristic resulting from the weak π-bonding ability (different from the analogous graphene), employing two-dimensional (2D) silicene and germanene monolayers could be one of the most promising ways to realize a new type of highly efficient and nonprecious catalyst for the hydrogen evolution reaction (HER). However, the HER activity of pristine silicene and germanene has to be improved, although both of them can exhibit a good change trend. Particularly, the hydrogen phenomenon can occur under moderate or high H* coverage on 2D silicene and germanene. To overcome these bottlenecks, in this study we identify the most effective strategy through doping P with a lone pair to significantly improve the HER catalytic activity under a high H* coverage, by screening a series of IIIA (i.e., B, Al, Ga, In and Tl) and VA (i.e., N, P, As, Sb and Bi) heteroatoms with different electronegativity under detailed DFT calculations. It is revealed that the doped P atoms and almost all the Si/Ge atoms can uniformly serve as highly active sites. Especially, in view of the existence of the lone pair, doping P effectively prevents hydrogenation (even under full H* coverage) by increasing the structural rigidity. Moreover, the P-doping concentration also plays a crucial role in obtaining high HER activity. The relevant mechanisms have been analyzed in detail. Clearly, all these fascinating findings are beneficial for realizing new HER electrocatalysts based on the excellent silicene or germanene nanomaterials, and even other Si/Ge-related materials in the near future.

Regulation of Morphology and Electrochemical Properties of Ni0.85Se via Fe Doping for Overall Water Splitting and Supercapacitor

The Fe-doped Ni0.85Se nanosheets array on Ni foam was synthesized successfully through one-step solvothermal method as effective binder-free multifunctional catalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER), overall...

Crystal facet and phase engineering for advanced water splitting

This review covers the principles and recent advances in facet and phase engineering of catalysts for photocatalytic, photoelectrochemical, and electrochemical water splitting. It suggests the basis of catalyst design for advanced water splitting.

Selenium-induced NiSe2@CuSe2 hierarchical heterostructure for efficient oxygen evolution reaction

Electrochemical water splitting is widely studied in the hope of solving environmental deterioration and energy shortage. The design of inexpensive metal catalysts exhibiting desired catalytic performance and durable stability for efficient oxygen evolution is the pursuit of sustainable and clean energy fields. Herein, a three-dimensional (3D) flower-like NiSe₂ primary structure, modified with highly dispersed CuSe₂ nanoclusters as the secondary structure, is obtained by regulating the growth trend of the nanosheets. Benefiting from the metallicity of selenides and the formation of a heterogeneous interface, NiSe₂@CuSe₂/NF shows comparable performance toward the oxygen evolution reaction (OER) in an alkaline environment. Upon regulating the synthesis conditions, the catalyst exhibits its optimal performance with ultralow overpotential for the OER when the Ni/Cu molar ratio is 1 : 0.2 and the hydrothermal temperature and hydrothermal time are 200 °C and 6 h, respectively. It provides a current density of 10 mA cm^-2 when a potential of 201 mV is applied without iR compensation. In this work, the hierarchical heterostructures of NiSe₂ and CuSe₂ are synthesized, which exhibit high electrocatalytic activity towards the oxygen evolution reaction and provides a new possibility for the extensive application of copper-based compounds in advanced energy fields.

Fe, Al-co-doped NiSe2 nanoparticles on reduced graphene oxide as an efficient bifunctional electrocatalyst for overall water splitting

Developing low-cost and highly efficient electrocatalysts for overall water splitting is of far-reaching significance for new energy conversion. Herein, dual-cation Fe, Al-co-doped NiSe2 nanoparticles on reduced graphene oxide (Fe, Al-NiSe2/rGO) were prepared as a bifunctional electrocatalyst for overall water splitting. The dual-cation doping can induce a stronger electronic interaction between the foreign atoms and host catalyst, for optimizing the adsorption energy of reaction intermediates. Meanwhile, the leaching out of Al from the crystal structure of the target product during the alkaline wash creates more defects and increases the active site exposure. As a result, the Fe, Al-NiSe2/rGO catalyst exhibits excellent catalytic activities for both the OER and HER with an overpotential of 272 mV @η10 for the OER in 1.0 M KOH and 197 mV @η10 for the HER in 0.5 M H2SO4, respectively. A two-electrode electrolyzer using Fe, Al-NiSe2/rGO as the anode and cathode shows a low voltage of 1.70 V at the current density of 10 mA cm-2. This study emphasizes the synergistic contribution of the dual-cation co-doping effect and more defects created by Al leaching to boost the performance of water splitting.

Decorated nickel phosphide nanoparticles with nitrogen and phosphorus co-doped porous carbon for enhanced electrochemical water splitting

A novel free-standing electrode consisting of nickel phosphide (Ni₂P) nanoparticles on nitrogen and phosphorus co-doped porous carbon (NPC) are synthesized on carbon cloth (CC). Polyaniline (PANI) and nickel (Ni) are sequentially electro-deposited on the surface of CC, which are then transformed into NPC and Ni₂P by an in-situ carbonization-phosphorization combined process. The electrode surface is distributed with large amounts of uniform macropores, which could expose more active sites and enhance the interfacial exchange with the electrolyte. The Ni₂P@NPC@CC electrode delivers early overpotentials of 92 and 280 mV vs. Reversible Hydrogen Electrode (RHE) at 10 mA cm^-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline condition, respectively. The electrolytic cell with Ni₂P@NPC@CC electrode both as anode and cathode can achieve 10 mA cm^-2 at a small bias of 1.54 V for the overall water splitting. Density functional theory (DFT) calculation indicates that combination with Ni₂P and NPC can decrease Gibbs free energy for H* adsorption (ΔG_H*) and increase charge density on the interface, thus could lead to the enhanced activity for water splitting. The free-standing and noble-metal free Ni₂P@NPC@CC electrode is stable, highly active and cost effective, thus have great potential for the hydrogen production.