Plasma-Induced Defect Engineering and Cation Refilling of NiMoO₄ Parallel Arrays for Overall Water Splitting

Abstract

Developing highly active water splitting electrocatalysts with ordered micro/nanostructures and uniformly distributed active sites can meet the increasing requirement for sustainable energy storage/utilization technologies. However, the stability of complicated structures and active sites during a long-term process is also a challenge. Herein, we fabricate a novel approach to create sufficient atomic defects via N2 plasma treatment onto parallel aligned NiMoO4 nanosheets, followed by refilling of these defects via heterocation dopants and stabilizing them by annealing. The parallel aligned nanosheet arrays with an open structure and quasi-two-dimensional long-range diffusion channels can accelerate the mass transfer at the electrolyte/gas interface, while the incorporation of Fe/Pt atoms into defect sites can modulate the local electronic environment and facilitate the adsorption/reaction kinetics. The optimized Pt-NP-NMC/CC-5 and Fe-NP-NMC/CC-10 electrodes exhibit low overpotentials of 71 and 241 mV at 10 mA cm–2 for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), respectively, and the assembled device reveals a low voltage of 1.55 V for overall water splitting. This plasma-induced high-efficiency defect engineering and coupled active site stabilization strategy can be extended to large-scale fabrication of high-end electrocatalysts.

Plasma modified and tailored defective electrocatalysts for water electrolysis and hydrogen fuel cells

“…According to the location of heteroatoms in the lattice, heteroatomic doping can be classified into two categories 118 : (1) “dopants in lattice” meaning heteroatoms substituting the original anions or cations in the lattice and (2) “dopants sitting on the surface” meaning heteroatoms trapped by atomic vacancies and sitting on the surface of the base materials. Liu et al have adopted N 2 plasma etching to create abundant and uniformly distributed defect/vacancy sites on the parallel‐aligned NiMoO 4 array surfaces, followed by filling of these atomic defects with Fe/Pt heteroatoms to obtain high electrocatalytic activity 119 . The O vacancies in Co 3 O 4 enhance the OER activity but sometimes the stability of O vacancies is not sufficient.…”

“…In this case, enough defects are needed to produce an identifiable reaction promotion phenomenon. For example, the plasma‐assisted Fe/Pt heteroatom‐doped NiMoO 4 nanosheets have a larger active site density and improved adsorption/reaction kinetics due to the incorporated Fe/Pt heteroatoms as dominant actual active sites 119 . By means of the cold hydrogen plasma reduction method, single Mo atoms on cogenetic monolayer MoS 2 are created for high‐performance HER electrocatalysts 128 .…”

“…Besides, owing to the atomic scale defects, the electrical conductivity is enhanced and the plasma‐derived open channels enable the internal active surfaces to be exposed to the electrolyte. The plasma‐activated heterostructure composed of metal, metal alloy, metal nitride, and O vacancy‐riched metal hydroxide delivers superior HER performance comparable to the benchmark Pt/C catalyst in alkaline solutions due to the synergistic effects of multiple components 119 . Ni(OH) 2 , MoNi 4 alloy, and Ni 3 N weaken the OH bond of absorbed water to facilitate water dissociation (Volmer step), while metallic Ni, MoNi 4 , and Ni 3 N exhibit optimized Gibbs free energy for hydrogen adsorption.…”

Tuning the structure, optical, and magnetic properties of nanostructured NiMoO4 by nitrogen plasma treatment

“…Because the nitrogen plasma treatment is predicted to create a large number of crystallographic defects and vacancies on the NMO NPs surface [30,35], EPR is used to evaluate the effect of the treatment duration on the oxygen vacancy density [50]. The intensity of oxygen vacancies signal identified at g = 2.175 (Fig.…”

“…The extremely engaged atoms then react with the investigated material to introduce a unique bond, resulting in effective hetero atom doping and the existence of multiple oxygen vacancies [31][32][33][34]. Recently, Liu et al [35] developed a unique method for synthesizing significant atomic defects on NMO-like nanosheets using N 2 plasma treatment, then filling these defects with heterocation dopants, and stabilizing them with sintering.…”