Designing Zn-doped nickel sulfide catalysts with an optimized electronic structure for enhanced hydrogen evolution reaction

A comprehensive review on the electrochemical parameters and recent material development of electrochemical water splitting electrocatalysts

Electrochemical splitting of water is an appealing solution for energy storage and conversion to overcome the reliance on depleting fossil fuel reserves and prevent severe deterioration of the global climate. Though there are several fuel cells, hydrogen (H2) and oxygen (O2) fuel cells have zero carbon emissions, and water is the only by-product. Countless researchers worldwide are working on the fundamentals, i.e. the parameters affecting the electrocatalysis of water splitting and electrocatalysts that could improve the performance of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) and overall simplify the water electrolysis process. Noble metals like platinum for HER and ruthenium and iridium for OER were used earlier; however, being expensive, there are more feasible options than employing these metals for all commercialization. The review discusses the recent developments in metal and metalloid HER and OER electrocatalysts from the s, p and d block elements. The evaluation perspectives for electrocatalysts of electrochemical water splitting are also highlighted.

Super-Hydrophilic Microporous Ni(OH)x/Ni3 S2 Heterostructure Electrocatalyst for Large-Current-Density Hydrogen Evolution

Exploiting active and stable non-precious metal electrocatalysts for alkaline hydrogen evolution reaction (HER) at large current density plays a key role in realizing large-scale industrial hydrogen generation. Herein, a self-supported microporous Ni(OH)x/Ni₃ S₂ heterostructure electrocatalyst on nickel foam (Ni(OH)x/Ni₃ S₂ /NF) that possesses super-hydrophilic property through an electrochemical process is rationally designed and fabricated. Benefiting from the super-hydrophilic property, microporous feature, and self-supported structure, the electrocatalyst exhibits an exceptional HER performance at large current density in 1.0 M KOH, only requiring low overpotential of 126, 193, and 238 mV to reach a current density of 100, 500, and 1000 mA cm^-2 , respectively, and displaying a long-term durability up to 1000 h, which is among the state-of-the-art non-precious metal electrocatalysts. Combining hard X-rays absorption spectroscopy and first-principles calculation, it also reveals that the strong electronic coupling at the interface of the heterostructure facilitates the dissociation of H₂ O molecular, accelerating the HER kinetics in alkaline electrolyte. This work sheds a light on developing advanced non-precious metal electrocatalysts for industrial hydrogen production by means of constructing a super-hydrophilic microporous heterostructure.

Engineering nickel vacancies in NiCo LDH nanoarrays accelerates hydrogen evolution and oxygen evolution reactions

Exploring efficient bifunctional electrocatalysts is crucial for constructing water splitting systems. In this work, a bifunctional catalyst, NiCo layered double hydroxide (LDH) nanosheets with nickel vacancies, was fabricated by a hydrothermal and chemical etching method, which requires 195 and 227 mV overpotentials for HER and OER to achieve 10 mA cm^-2 and exhibited sustained activity for 100 h with almost no degradation. This study provides a new idea for the rational design of efficient non-precious metal catalysts with defects for water splitting.

Doping-driven electronic structure and conductivity modification of nickel sulfide

The lack of electrical conductivity limits the electrochemical kinetic rate of the electrode material, resulting in the inability to reach its theoretical capacity. A facile method is adopted to improve the intrinsic conductivity of binary NiS₂/Ni₃S₄ hybrid nickel sulfide, with the doping of transition metal atoms Co, Mn and Ag. Through the introduction of heteroatoms, the electronic structure of the electrode material is modified and the electrical conductivity is significantly improved, thus enhancing its electrochemical performance. The improvement of conductivity is attributed to the formation of intermediate bands of transition metals and the redistribution of electrons, and the result is demonstrated by experimental and density functional theory (DFT) calculations. As a result, the NiS₂/Ni₃S₄ hybrid nickel sulfide after 0.5% amount of Co-doping reaches the highest specific capacitance of 2874 F g^-1 at 1 A g^-1, increasing specific capacitance of 653 F g^-1 as 29.4% of the specific capacitance of non-doped nickel sulfide. The Co doped nickel sulfide also exhibits remarkable cycling stability compared with non-doped nickel sulfide. The assembled 2% Co-doped nickel sulfide//rGO, 0.5% Mn-doped nickel sulfide//rGO and 0.5% Ag-doped nickel sulfide//rGO asymmetric supercapacitors show a specific energy density of 36.6, 36.1 and 36.0 W h kg^-1 at a power density of 800 W kg^-1. This study provides a useful insight into the fabrication of high performance pseudocapacitive materials.