Transition metal dichalcogenides (TMDCs), such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2), have emerged as promising catalysts for critical electrochemical reactions, including the hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and CO2 reduction. Their layered structure, tunable electronic properties, and cost-effectiveness make them attractive alternatives to noble metal catalysts. The catalytic performance of TMDCs is highly dependent on their active sites, phase engineering, and structural modifications, which can be systematically optimized for enhanced efficiency.
### Active Sites: Edge vs. Basal Plane
The catalytic activity of TMDCs is strongly influenced by the nature of their active sites. For HER, the edge sites of MoS2 and WS2 exhibit significantly higher activity than the basal planes. Density functional theory (DFT) calculations reveal that the Gibbs free energy of hydrogen adsorption (ΔGH*) on edge sulfur atoms is close to thermoneutral, a key descriptor for efficient HER catalysis. In contrast, the basal plane is largely inert due to weak hydrogen adsorption. Experimental studies confirm that edge-terminated nanostructures, such as vertically aligned MoS2 nanosheets, achieve lower overpotentials (e.g., ~200 mV at 10 mA/cm²) compared to their planar counterparts.
For ORR and CO2 reduction, both edge and basal plane sites contribute, but their roles differ. Edge sites facilitate charge transfer and intermediate stabilization, while basal planes can be activated through defects or doping. For example, nitrogen-doped MoS2 basal planes exhibit improved ORR activity by introducing catalytically active sites that enhance oxygen adsorption and reduction kinetics.
### Phase Engineering: 1T vs. 2H
The phase of TMDCs plays a critical role in determining their catalytic properties. The semiconducting 2H phase is the most stable but suffers from limited conductivity. In contrast, the metallic 1T phase, achieved through chemical intercalation or strain engineering, shows superior charge transport and catalytic activity.
For HER, 1T-MoS2 demonstrates an overpotential reduction of up to 50% compared to 2H-MoS2, attributed to its higher electrical conductivity and more favorable hydrogen adsorption energetics. The 1T phase also exposes additional active sites due to structural distortions. However, metastable 1T phases require stabilization strategies, such as covalent functionalization or substrate interactions, to prevent reversion to the 2H phase.
In ORR, the 1T phase enhances the four-electron transfer pathway, critical for efficient oxygen reduction to water. Studies show that 1T-WS2 achieves a half-wave potential within 60 mV of commercial Pt/C catalysts, with improved long-term stability in acidic media. For CO2 reduction, the 1T phase promotes multi-electron transfer, enabling higher selectivity for valuable products like methanol or methane.
### Performance Metrics and Optimization
The catalytic performance of TMDCs is evaluated using metrics such as overpotential, Tafel slope, turnover frequency (TOF), and faradaic efficiency. For HER, the best-performing MoS2 catalysts achieve overpotentials below 150 mV at 10 mA/cm² and Tafel slopes of ~40 mV/dec, approaching Pt-like activity. Alloying with transition metals (e.g., Co, Ni) or introducing sulfur vacancies further optimizes ΔGH* and active site density.
For ORR, the key metrics include onset potential, kinetic current density, and electron transfer number. Defect-engineered MoS2 with sulfur vacancies exhibits an onset potential of 0.85 V vs. RHE and a four-electron selectivity exceeding 90%. Hybrid structures, such as MoS2/graphene composites, enhance charge transfer and mass transport, improving current density by up to 5x compared to pristine TMDCs.
CO2 reduction performance is assessed by product selectivity, partial current density, and stability. Phase-controlled MoS2 selectively produces CO with faradaic efficiencies exceeding 80% at moderate overpotentials (~300 mV). Doping with transition metals (e.g., Fe, Cu) shifts selectivity toward hydrocarbons by stabilizing key intermediates like *COOH.
### Challenges and Future Directions
Despite progress, challenges remain in scaling TMDC catalysts for industrial applications. The long-term stability of 1T phases under operational conditions requires further study, as does the scalability of edge-rich nanostructures. Advanced synthesis techniques, such as plasma-assisted growth or electrochemical exfoliation, may address these issues by enabling precise control over phase and morphology.
Future research should focus on elucidating reaction mechanisms at atomic-scale active sites using in situ spectroscopy and computational modeling. Coupling TMDCs with conductive scaffolds or single-atom catalysts could further enhance activity and durability. By systematically optimizing active sites, phase, and interfaces, TMDCs may soon rival noble metals in catalytic applications, enabling sustainable energy conversion technologies.
The versatility of TMDCs as catalysts for HER, ORR, and CO2 reduction underscores their potential in addressing global energy challenges. Through continued innovation in material design and synthesis, these layered materials could play a pivotal role in the transition to a carbon-neutral economy.