In the global carbon neutrality mission, electrochemical water splitting to produce ‘green hydrogen’ powered by renewable energy plays a crucial role. As the most important part of the hydrogen evolution reaction (HER), the electrocatalyst determines the feasibility of industrial electrolyzer applications. In recent years, considerable progress has been made in this field, witnessing diverse catalyst design and synthesis. However, scarce and expensive platinum-based catalysts still monopolize the market even though the stability is hardly satisfactory. The emergence of low-dimensional nanomaterials as potential candidates has a tendency to replace platinum-based catalysts. Relying on the unique electronic structure generated by quantum confinement, low-dimensional nanomaterials exhibit extraordinary physical and chemical characteristics. It has been comprehensively optimized in terms of dimensionality, surface chemistry, electron transport pathway, morphology, and catalyst-electrolyte interaction. Herein, this thesis first systematically introduces the progress of low-dimensional nanomaterials in electrocatalytic hydrogen evolution. Then, I conducted different strategies to optimize the catalytic characteristics of low-dimensional nanomaterials and achieve a breakthrough in the hydrogen evolution mechanism.

The initial research focused on zero-dimensional (0D) and one-dimensional (1D) nanomaterials. 0D metal nanoparticles usually have excellent catalytic properties but are easily corrupted in strong electrolytes; 1D carbon nanomaterials, although their intrinsic activity is limited, are incredibly stable in the electrolytic medium. Novel 0D-1D heterogeneous nanostructures that combine the unique traits of both 0D metal nanoparticles and 1D carbon nanomaterials are anticipated to optimize catalytic efficiency by leveraging the strengths of each component. Here, a valence engineering strategy is devised to construct 0D-1D hetero-nanomaterials with polyvalent cobalt encapsulated in nitrogen-doped carbon nanofibers (Co/N–CNFs). The diverse cobalt valence states of the Co/N–CNF catalysts contribute to their excellent catalytic effect and high durability in electrochemical processes. The optimal Co/N–CNF catalyst fabricated exhibits low overpotentials of hydrogen evolution (241 mV) and oxygen evolution (380 mV) at 10 mA cm^-2, performing efficient overall water splitting.

Although 0D-1D polyvalent nano-heterostructures have achieved good results, the single catalytic site limits their reaction pathways. The reaction kinetics of HER is largely determined by balancing the Volmer step in alkaline media. Bifunctionality as a proposed strategy can divide the work of water dissociation and intermediates (OH^* and H^*) adsorption/desorption. Therefore, by introducing new low-dimensional nanocomponents based on previous research, the dual active sites formed will have the opportunity to stimulate the characteristic of the bifunctional effect. However, sluggish OH^* desorption plagues water re-adsorption, which leads to poisoning effects of active sites. Some active sites may even directly act as spectators and do not participate in the reaction. Furthermore, the activity comparison under approximate nanostructure between bifunctional effect and single-exposed active sites is not fully understood. Here, a facile three-step strategy is adopted to successfully grow two-dimensional (2D) molybdenum disulfide (MoS₂) on cobalt-containing nitrogen-doped carbon nanotubes (Co-NCNTs), forming obvious 1D-2D dual active domains. The active sites on domains of Co-NCNTs and MoS₂ and the tuned electronic structure at the heterointerface trigger the bifunctional effect to balance the Volmer step and improve the catalytic activity. The HER driven by the bifunctional effect can significantly optimize the Gibbs free energy of water dissociation and hydrogen adsorption, resulting in fast reaction kinetics and superior catalytic performance. As a result, the Co-NCNTs/MoS₂ catalyst outperforms other HER electrocatalysts with low overpotential (58 and 84 mV at 10 mA cm^-2 in alkaline and neutral conditions, respectively), exceptional stability, and negligible degradation.

The above research works yield outstanding HER performance through the characteristic superposition effect. Still, the intrinsic activity of the characteristic catalytic sites of low-dimensional nanomaterials has not been significantly improved. In an electrocatalytic hydrogen production system, well-designed edge site coordination environment could greatly improve the original reaction kinetics, break the intrinsic catalytic limit, and effectively reduce the energy barriers for a significant increase in the hydrogen production. Here, I report a novel catalyst designed and fabricated by electrochemical deposition of Ru single atoms onto the edge sites of 2D MoS₂ and further coordination with S to form Ru single-edge atoms (Ru SEA). The Ru SEA replacing Mo acts as a new trigger point for water dissociation to improve the catalytic activity and proton supply rate. Based on the Tafel slope obtained, the Ru SEA creates a local acid-like chemical environment that is more conducive to the kinetics of the hydrogen evolution reaction due to the change from the original MoS₂ Volmer-Heyrovsky catalytic pathway to the Ru SEA Volmer-Tafel catalytic pathway. Rather than limiting the activity at high-current-density (including ampere-level), the Ru SEA configuration exhibits superior performance and long-term durability compared with other MoS₂- and single-atom-based catalysts. The SEA material modification markedly improves the activity of the catalyst and has a high potential to become a dominant strategy in renewable fuel generation.

In short, this thesis has achieved the optimization of the characteristic mechanism of low-dimensional nanomaterials in the field of electrocatalytic hydrogen evolution and comprehensively proposed advanced catalyst design strategies from the aspects of synergistic complementation, distinctive effect activation, and intrinsic activity enhancement. It outlines new opportunities for future research directions in electrocatalytic hydrogen evolution.