Recent Advances in Defect-Engineered Transition Metal Dichalcogenides for Enhanced Electrocatalytic Hydrogen Evolution: Perfecting Imperfections

Abstract

Switching to renewable, carbon-neutral sources of energy is urgent and critical for climate change mitigation. Despite how hydrogen production by electrolyzing water can enable renewable energy storage, current technologies unfortunately require rare and expensive platinum group metal electrocatalysts, which limit their economic viability. Transition metal dichalcogenides (TMDs) are low-cost, earth-abundant materials that possess the potential to replace platinum as the hydrogen evolution catalyst for water electrolysis, but so far, pristine TMDs are plagued by poor catalytic performances. Defect engineering is an attractive approach to enhance the catalytic efficiency of TMDs and is not subjected to the limitations of other approaches like phase engineering and surface structure engineering. In this minireview, we discuss the recent progress made in defect-engineered TMDs as efficient, robust, and low-cost catalysts for water splitting. The roles of chalcogen atomic defects in engineering TMDs for improvements to the hydrogen evolution reaction (HER) are summarized. Finally, we highlight our perspectives on the challenges and opportunities of defect engineering in TMDs for electrocatalytic water splitting. We hope to provide inspirations for designing the state-of-the-art catalysts for future breakthroughs in the electrocatalytic HER.

Figure 1

(a) Illustration of the HER mechanism in acidic (left) and alkaline conditions (right). Reprinted with permission from []. Copyright 2019 American Chemical Society. (b) Volcano plot showing the relationship between ΔG_H* and the HER activity (represented by J₀). Adapted with permission from []. Copyright 2014 American Chemical Society. (c) SEM images of vertically aligned MoS₂ (left) and MoSe₂ (middle) nanosheets, and an illustration of their orientation on the substrate (right). Reprinted with permission from []. Copyright 2013 American Chemical Society. (d) Illustration of the atomic rearrangement in the unit cell of MoS₂ and the change in morphology during the phase transformation from the 2H to the 1T phase by lithium intercalation. Reprinted with permission from []. Copyright 2013 American Chemical Society.

Figure 2

(a) The estimated ΔG_H* under various sulfur vacancy concentrations and the distribution in the lattice based on the DFT calculations. Reprinted with permission from []. Copyright 2020 American Chemical Society. (b) STEM image showing uniformly distributed single S-vacancies (yellow dotted circles) in a MoS₂ film after etching with H₂O₂. Reprinted with permission from []. Copyright 2020 American Chemical Society. (c) A scheme illustrating the reaction between MoS₂ and Zn to introduce S-vacancies. Reprinted with permission from []. Copyright 2019 John Wiley and Sons. (d) Illustration for the formation of S-vacancies in MoS₂ at a moderate annealing temperature in hydrogen, and stripping of S atoms to reveal large areas of undercoordinated Mo atoms at higher temperatures. Reprinted with permission from []. Copyright 2019 American Chemical Society. (e) EPR spectra of MoS₂ after annealing at various conditions. The inset shows the EPR intensity at different annealing temperatures. Reprinted with permission from []. Copyright 2019 American Chemical Society.

Figure 3

Atomically resolved HAADF-STEM images of (a) MoSe₂, (b) NbSe₂, and (c) Mo_0.5Nb_0.5Se₂. The insets of (c) show the intensity profiles across the yellow dotted lines. V_Se, V_M, A_Se, and A_M represent the Se-vacancy, the metal-vacancy, the adatom at the Se site, and the adatom at the metal site, respectively. Reprinted with permission from []. Copyright 2021 American Chemical Society.

Figure 4

Illustration of the atomic arrangement in 2H- and 3R-MoS₂. Reprinted with permission from []. Copyright 2020 American Chemical Society.

Figure 5

(a) Illustration for the growth mechanism of spiral WS₂. The single-layer thickness of WS₂ nanosheet and interlayer spacing of WS₂ layers are represented by δ and α, respectively. Reprinted with permission from []. Copyright 2019 American Chemical Society. (b) The crystal structure (left) of pristine NiS₂, top view (middle) and side view (right) of the optimal binding site for H in P-doped NiS₂ with S-vacancies. (c) DFT calculations for the ΔG_H* of various possible binding sites of P-doped NiS₂ with S-vacancies. Reproduced with permission from []. Copyright 2021 Elsevier.