Submitted 22/08/21
Revised 24/09/21
Accepted 29/09/21

Zhexu Xi “Underlying Structure-Activity Correlations of 2D Layered Transition Metal Dichalcogenides-Based Electrocatalysts for Boosted Hydrogen Generation” Theoretical Physics Letters, vol. 9, no. 15.
DOI - 10.1490/100236.980ptl

 

Introduction
Nowadays, demand for usable energy worldwide has dramatically risen due to rapid growth in population, which inevitably triggers the overuse of traditional fossil fuels as well as a series of environmental issues [1, 2]. Accordingly, it is of great importance to find another, the less polluting energy source to tackle the current problems. Hydrogen (H2), owing to its zero-polluting combustion byproduct (water) and high energy density, holds high potential as an alternative to fossil energy [3]. For H2 production pathways, water electrolysis (electrocatalytic water splitting) is also known as a renewable and clean industrial approach [4]. Currently, the best electrocatalyst for the Hydrogen Evolution Reaction (HER) is Pt, which markedly minimizes the overpotential and exhibits optimal catalytic activity. However, the high cost and limited reserves of Pt seriously restrict the further development of Pt-based catalysts [3, 5]. Thus, a novel HER electrocatalyst with rich abundance and similar reactivity to Pt has captured wide attention.
Two-dimensional transition metal dichalcogenides (2D TMDs), also generally expressed in the form of MXn (M = Mo, W, Ti, V, and Zr; X= S, Se, and Te), have recently been verified to be the most prospective promising alternatives to Pt due to extraordinary catalytic performance [4-6]. First, the atomically thin 2D layered structure offers plentiful exposed active sites and a high specific area for HER [3][7]. Second, the unique characteristics of TMDs are primarily related to the tailor-made electronic structures, which can provide a more accurate and comprehensive understanding in terms of their HER catalytic mechanisms [7, 8]. Third, although the unsatisfactory in-plane activity of TMDs has been reported to restrict the applications in HER electrocatalysts, more strategies based on the structural modification of TMDs have been implemented to improve the catalytic performance, including phase transition and defect engineering [8, 9].
Herein, we focus on the role of 2D TMDs as ideal replacements for Pt in HER enhancement. First, we summarise the theoretical understanding of the overall electrocatalytic HER system. Second, based on the recent discoveries in this rapidly advancing research area, we make a comprehensive analysis regarding the nanoscale modulation strategies of 2D-TMDs-based electrocatalysts in three aspects: 1) composition (different kinds of 2D TMDs that have superior HER catalytic performance, as well as how structural modulation strategies are implemented), 2) characterisation (various instrumental techniques used for measurement, analysis and quantification of 2D TMDs), and 3) construction (novel nanoengineered 2D TMDs based on versatile modulation strategies that boost the HER activity) [3-5, 7-9, 10]. Finally, we propose the perspectives and challenges of TMDs-based electrochemical water splitting technologies, which can provide more insights into the rational design and fabrication of HER-related catalysts.

 

Summary and perspectives

 

We comprehensively summarised the modification strategies and the state-of-the-art advances of HER electrocatalysts based on 2D TMDs. Following the composition-characterisation-construction guideline, we offered three methodologies for HER enhancement: 1) to increase the active sites; 2) to improve the intrinsic conductivity and activity; 3) to optimise the electronic structure. These strategies can boost HER performance individually or in a synergistic way to highlight their roles in structural design and electronic modulation. Both theoretical and experimental findings play vital roles in more insight into the TMDs-related HER system, as comprehensively summarized in Fig. 8.
Fig. 8 schematic principles of the optimal design and modulation of TMDs-based HER electrocatalysts based on the composition-characterisation-construction guideline
As can be inferred, the oriented, versatile modification strategies with simple, various techniques render more comprehensive structure-activity regimes by precisely
Theoretical Physics Letters, 30(09): 09.-15.
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modulating the structural morphologies and tuning the electronic band structures of TMDs. The advantages of TMDs, including huge surface area, ultrathin thickness and multiple phase types, can strikingly boost the discoveries and comprehension of this correlation.
However, there is still a long way to go before the broad application of TMDs-based catalysts in water electrocatalysis:
1) Regarding the nano-level synthesis of TMDs, there is a lack of systematic theoretical guidance and well-tuned fabrication methods;
2) The correlations in HER catalytic activity and nanostructures of TMDs are unclear;
3) More intelligent algorithms are urgently needed to narrow the gap between experimental and simulated results.
4) Lastly, the long-term stability of catalysts should be highlighted. The large-scale application needs electrocatalysts with extraordinary long-term stability and durability.
Overall, the 2D TMDs exhibit the great potential to replace the noble-metal HER electrocatalysts (Pt) for efficient water electrochemical splitting. By the rational optimal design of TMDs. It is possible to achieve a wide-ranging commercial application.