Enhanced Electrocatalytic Oxygen Reduction to H₂O with High Selectivity via 2D 1T-MoS₂ Integrated with Functionalized Carbon Nanohorns

Technical Summary: Enhanced Electrocatalytic Oxygen Reduction to H₂O with High Selectivity via 2D 1T-MoS₂ Integrated with Functionalized Carbon Nanohorns

Problem Statement
The development of anion-exchange membrane fuel cells (AEMFCs) is hindered by the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode and the tendency for incomplete oxygen reduction, which limits efficiency. While platinum-based catalysts are the current benchmark, their high cost restricts commercialization. Consequently, research focuses on low-cost, platinum-group-metal-free alternatives. Two-dimensional transition metal dichalcogenides (TMDCs), specifically MoS₂, show promise due to their tunable electronic properties and active edge sites. However, MoS₂ often suffers from limited electrical conductivity, weak interfacial interactions when combined with supports, and structural restacking. Furthermore, achieving a highly selective four-electron reduction pathway (directly to H₂O) over the less desirable two-electron pathway (producing peroxide) remains a significant challenge for non-precious metal catalysts.

Methodology
The authors engineered a robust heterostructure by integrating metallic 1T-MoS₂ nanosheets with covalently functionalized carbon nanohorns (CNHs). The synthesis involved a multi-step approach:

1. Functionalization of CNHs: Carbon nanohorns were covalently modified with 4-amino-N,N,N-trimethylbenzenaminium (TMA) groups via diazonium chemistry. This process introduced positively charged ammonium groups onto the CNH surface, rendering them water-dispersible.
2. Preparation of 1T-MoS₂: Metallic 1T-MoS₂ nanosheets were produced via lithium intercalation followed by exfoliation in water, resulting in negatively charged nanosheets.
3. Heterostructure Assembly: The positively charged CNH-N and negatively charged 1T-MoS₂ were mixed in an aqueous solution. The assembly was driven by strong electrostatic interactions and van der Waals forces, forming the CNH-N–MoS₂ hybrid.
4. Characterization: The materials were characterized using thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), dynamic light scattering (DLS), zeta potential measurements, Raman spectroscopy (including mapping), and transmission electron microscopy (TEM).
5. Electrochemical Evaluation: ORR performance was assessed in 0.1 M KOH using cyclic voltammetry (CV), rotating ring-disk electrode (RRDE) measurements, and electrochemical impedance spectroscopy (EIS).
6. Computational Modeling: Density functional theory (DFT) calculations were performed to analyze electronic structures, charge transfer mechanisms, and Gibbs free energy diagrams for ORR pathways.

Key Results

• Structural Confirmation: TGA and XPS confirmed the successful covalent attachment of TMA groups to CNHs, evidenced by a significant weight loss in TGA and the appearance of quaternary ammonium nitrogen signals in XPS. Raman spectroscopy indicated an increase in sp³ defects due to functionalization, while the subsequent assembly with MoS₂ led to a reduction in the D/G band ratio, suggesting strong interfacial coupling and defect passivation. TEM images revealed intimate contact between CNH-N aggregates and MoS₂ nanosheets, with EDS confirming the coexistence of C, Mo, and S.
• Electrocatalytic Performance: The CNH-N–MoS₂ heterostructure exhibited superior ORR activity compared to pristine CNHs, functionalized CNH-N, pure MoS₂, and a physical mixture of CNH and MoS₂.
  • Onset Potential: The heterostructure achieved an optimized onset potential of 0.80 V vs. RHE, significantly higher than the 0.72 V observed for CNH-N and 0.77 V for pure MoS₂.
  • Selectivity: The catalyst demonstrated a dominant four-electron reduction pathway with approximately 94% selectivity toward H₂O, comparable to commercial Pt/C. In contrast, individual components and the physical mixture showed significantly lower selectivity (higher peroxide yields).
  • Kinetics: Electrochemical impedance spectroscopy revealed a reduced charge-transfer resistance (Rct) of ~66 Ω for the heterostructure, lower than the physical mixture (107 Ω), indicating enhanced interfacial charge transfer.
• Mechanistic Insights (DFT): Computational results showed that TMA functionalization induces a substantial redistribution of electron density, facilitating a threefold increase in electron transfer from the carbon support to the MoS₂ interface compared to the non-functionalized system. The work function analysis indicated a decreased electron injection barrier. Crucially, the Gibbs free energy diagrams revealed that the functionalized interface lowers the overpotential for the four-electron pathway (1.51 eV) relative to the two-electron pathway (1.80 eV), rationalizing the experimental observation of high H₂O selectivity.

Significance and Claims
The paper claims that the integration of metallic 1T-MoS₂ with covalently functionalized CNHs creates a synergistic effect that stabilizes the active 1T phase and optimizes the adsorption energies of ORR intermediates. The study highlights that controlled chemical functionalization and heterointerface engineering are critical for enhancing catalytic activity and selectivity. The resulting CNH-N–MoS₂ heterostructure is presented as a promising class of metal-free (or low-metal-content) electrocatalysts that offer high efficiency and selectivity for fuel cell applications, addressing the need for cost-effective alternatives to platinum-based catalysts. The work underscores the importance of establishing strong interfacial interactions to overcome the limitations of individual components in electrocatalysis.