Molybdenum disulfide (MoS2) and carbon nanofiber (CNF) hybrids have emerged as promising electrocatalysts for hydrogen evolution reaction (HER) and nitrogen reduction reaction (NRR). These hybrids leverage the synergistic effects between MoS2 and CNFs, combining the catalytic activity of MoS2 with the high conductivity and structural stability of CNFs. The performance of these hybrids is influenced by edge site exposure strategies, heterojunction effects, and the electrolyte environment, making them versatile for both acidic and alkaline media.
MoS2 is a layered transition metal dichalcogenide with intrinsic catalytic activity primarily localized at its edge sites, where sulfur-terminated edges provide favorable hydrogen adsorption free energy. However, bulk MoS2 suffers from limited edge site availability and poor electrical conductivity. To address these limitations, integrating MoS2 with CNFs enhances charge transfer and exposes additional active sites. Strategies to maximize edge site exposure include vertical alignment of MoS2 nanosheets, defect engineering, and phase modulation from the thermodynamically stable 2H phase to the metastable 1T phase, which exhibits metallic conductivity.
Vertical alignment of MoS2 on CNFs ensures a high density of exposed edge sites perpendicular to the substrate, facilitating reactant access and electron transport. Defect engineering, such as sulfur vacancies, further increases catalytic activity by creating unsaturated Mo sites that serve as additional active centers. The 1T phase, achieved through chemical exfoliation or doping, improves conductivity and lowers the energy barrier for HER. However, the metastable nature of 1T-MoS2 necessitates stabilization strategies, such as covalent bonding with CNFs or encapsulation within conductive carbon matrices.
Heterojunction effects between MoS2 and CNFs play a critical role in enhancing electrocatalytic performance. The intimate contact between MoS2 and CNFs facilitates rapid electron transfer from CNFs to MoS2, reducing charge recombination and improving reaction kinetics. Additionally, the interfacial electric field formed at the MoS2-CNF junction promotes charge separation, further boosting catalytic efficiency. The hybridization also mitigates MoS2 aggregation, ensuring a high surface area and long-term stability.
In acidic media, MoS2-CNF hybrids exhibit superior HER performance compared to pure MoS2. The overpotential required to achieve a current density of 10 mA cm-2 is typically reduced by 100-200 mV in hybrids, with Tafel slopes decreasing from approximately 120 mV dec-1 for pure MoS2 to 40-60 mV dec-1 for MoS2-CNF hybrids. The improved kinetics are attributed to enhanced charge transfer and increased edge site availability. In alkaline media, the performance gap between hybrids and pure MoS2 widens further due to the sluggish water dissociation step in HER. The conductive CNF network facilitates proton-coupled electron transfer, lowering the energy barrier for water dissociation and improving overall activity.
For NRR, MoS2-CNF hybrids demonstrate higher Faradaic efficiency and ammonia yield compared to pure MoS2. The nitrogen reduction pathway involves multiple proton-electron transfer steps, requiring efficient charge transport and active site availability. The hybrid structure promotes nitrogen adsorption on Mo sites while facilitating electron transfer through CNFs, leading to improved ammonia production rates. In acidic conditions, the ammonia yield for MoS2-CNF hybrids can reach 30-50 μg h-1 mgcat-1, compared to 10-20 μg h-1 mgcat-1 for pure MoS2. In alkaline media, the yield is slightly lower due to competitive hydrogen evolution but remains superior to pure MoS2.
The stability of MoS2-CNF hybrids under operational conditions is another advantage over pure MoS2. The carbon matrix protects MoS2 from corrosion and mechanical degradation, ensuring sustained performance over extended periods. In HER, MoS2-CNF hybrids maintain their activity for over 100 hours with minimal degradation, while pure MoS2 often suffers from structural collapse and active site poisoning. For NRR, the hybrids exhibit consistent ammonia production rates over multiple cycles, whereas pure MoS2 shows significant activity loss due to surface oxidation and sulfur leaching.
Comparative studies highlight the superiority of MoS2-CNF hybrids in both HER and NRR applications. The table below summarizes key performance metrics for MoS2-CNF hybrids and pure MoS2 in acidic and alkaline media.
Performance Metric | MoS2-CNF Hybrid | Pure MoS2
HER Overpotential @ 10 mA cm-2 (Acidic) | 150-200 mV | 250-350 mV
HER Tafel Slope (Acidic) | 40-60 mV dec-1 | 100-120 mV dec-1
HER Overpotential @ 10 mA cm-2 (Alkaline) | 180-250 mV | 300-400 mV
HER Tafel Slope (Alkaline) | 50-70 mV dec-1 | 110-130 mV dec-1
NRR Ammonia Yield (Acidic) | 30-50 μg h-1 mgcat-1 | 10-20 μg h-1 mgcat-1
NRR Faradaic Efficiency (Acidic) | 15-25% | 5-10%
NRR Ammonia Yield (Alkaline) | 20-40 μg h-1 mgcat-1 | 5-15 μg h-1 mgcat-1
NRR Faradaic Efficiency (Alkaline) | 10-20% | 3-8%
The development of MoS2-CNF hybrids represents a significant advancement in electrocatalysis, addressing the limitations of pure MoS2 while offering tunable properties for diverse applications. Future research may focus on optimizing hybrid compositions, exploring novel synthesis routes, and scaling up production for industrial deployment. The insights gained from these hybrids can also guide the design of other transition metal dichalcogenide-carbon composites for energy conversion and storage applications.