A Ta-TaS₂ monolith catalyst with robust and metallic interface for superior hydrogen evolution

The excessive reliance on fossil fuels has led to severe environmental issues. Hydrogen (H₂), a clean energy carrier with zero carbon emissions, can be produced via water electrolysis powered by renewable energy, contributing to global carbon neutrality. Polymer electrolyte membrane (PEM) electrolyzers offer high efficiency and production rates, but face challenges related to stability, cost, and efficiency. Current commercial water electrolysis relies on scarce and expensive noble metals like platinum (Pt) and iridium (Ir), which exhibit poor stability at high current densities. Developing cost-effective and durable noble-metal-free catalysts or reducing noble metal usage has been a long-standing goal, yet remains challenging, especially at the high current densities required by industry. Beyond high current operation, long-term electrode stability is crucial. Traditional methods involving anchoring catalysts on conductive substrates using binders often lead to catalyst detachment under hydrogen bombardment at high current densities, resulting in short electrode lifespans and high interface resistance, which further reduces efficiency due to Joule heating. Growing the catalyst directly on the substrate improves adhesion but may not eliminate interface resistance. The synthesis of a monolith catalyst offers a solution that tackles these challenges by directly growing a metallic transition metal dichalcogenide (m-TMDC) on a substrate of the same metal via an oriented-solid-phase synthesis (OSPS) method. This approach eliminates the interface resistance and enhances the binding force.

Numerous studies have focused on improving hydrogen evolution reaction (HER) catalysts. Researchers have explored various strategies, including using two-dimensional materials like tantalum disulfide (TaS₂) and niobium disulfide (NbS₂), and employing single-atom catalysts and alloying to enhance activity and stability. However, achieving high activity and durability at industrially relevant current densities (≥1000 mA cm⁻²) remains a significant hurdle. The use of binders to attach catalysts to substrates often compromises long-term stability at high current densities, while direct growth methods don't always eliminate interface resistance. The literature extensively discusses the importance of interfacial engineering for electrocatalysis, but the covalent bonding approach presented in this paper for creating robust and conductive interfaces offers a novel strategy.

This study introduces a novel monolith catalyst (MC) architecture to overcome the limitations of existing HER catalysts. The Ta-TaS₂ MC was synthesized using an oriented-solid-phase synthesis (OSPS) method. This method involves pre-oxidizing a tantalum (Ta) substrate, followed by oriented sulfurization along the oxidation path, and finally an electrochemical treatment to create a porous structure. The process is illustrated in detail in Figure 2a and the supplementary information. The structure and properties of the Ta-TaS₂ MC were thoroughly characterized using various techniques: X-ray diffraction (XRD) confirmed the formation of the 3R-phase TaS₂; X-ray photoelectron spectroscopy (XPS) provided elemental analysis; high-resolution transmission electron microscopy (HRTEM) visualized the vertical growth of TaS₂ on the Ta substrate; energy dispersive X-ray spectroscopy (EDS) elemental mapping confirmed the clear interface; and scanning transmission electron microscopy-high-angle annular dark field (STEM-HAADF) microscopy further investigated the interface structure. The mechanical properties were examined using a universal tester, measuring the adhesive force of the Ta-TaS₂ interface. The electrical conductivity was measured to assess charge transfer capabilities. Contact angle measurements were conducted to assess wettability. Electrochemical measurements were performed in a three-electrode system using 0.5 M H₂SO₄ electrolyte. Linear sweep voltammetry (LSV) determined the catalytic activity, chronoamperometry evaluated stability, and electrochemical impedance spectroscopy (EIS) examined charge transfer resistance. Density functional theory (DFT) calculations were used to understand the electronic structure and hydrogen adsorption free energy of the Ta-TaS₂ MC. The computational details are described in the methods section, specifying software, functionals, and convergence criteria.

The Ta-TaS₂ MC exhibited superior HER performance compared to conventional Ta/TaS₂ composites and a porous Pt foil. It achieved a current density of 2000 mA cm⁻² at an overpotential of only 398 mV, significantly lower than the values for Ta/TaS₂ (920 mV) and porous Pt (740 mV). The Ta-TaS₂ MC demonstrated remarkable stability, showing negligible performance decay after 200 hours of operation at high current densities (Fig 4d). The analysis of Δη/Δlog|j| ratios indicated excellent performance at large current densities (Fig 4b). Mechanical testing revealed a significantly stronger adhesive force (39.9 N/m²) for the Ta-TaS₂ MC compared to Ta/TaS₂ (12.3 N/m²) and Pt/C/GC (13.4 N/m²), confirming the robust interface. The MC exhibited high electrical conductivity (~3 × 10⁶ S/m), comparable to metals, showcasing excellent charge transfer kinetics (Fig 3b). The near-zero contact angle indicated excellent wettability, promoting efficient mass transfer. The superior performance was attributed to the covalently bonded interface between Ta and TaS₂, which provided both mechanical robustness and exceptional electrical conductivity, facilitating efficient charge transfer and minimizing energy loss. The scaled-up synthesis of Ta-TaS₂ MC (Fig 5a) demonstrated the potential for industrial applications. In a water electrolyzer with IrO₂ anode, the Ta-TaS₂ MC cathode achieved a current density of 1000 mA cm⁻² at 1.98 V, outperforming a porous Pt foil || IrO₂ couple (2.20 V). The electrolyzer maintained excellent performance for over 24 hours at high current densities (Fig 5d), with a Faraday efficiency near 100% (Fig 5c). The low cost and abundance of Ta and Nb compared to Pt offer a significant advantage for industrial applications.

The results demonstrate the success of the monolith catalyst design in overcoming the limitations of traditional HER catalysts. The strong covalent bond between Ta and TaS₂ creates an interface with both superior mechanical strength and electrical conductivity, leading to high activity and durability at industrially relevant current densities. The significantly lower overpotential and enhanced stability of the Ta-TaS₂ MC compared to other catalysts highlight the effectiveness of this approach. The scalability of the synthesis process further supports its potential for large-scale industrial hydrogen production. The excellent performance of the Ta-TaS₂ MC in a water electrolyzer, outperforming Pt-based systems, underscores its practical relevance. The findings of this work offer a promising strategy for developing highly efficient and sustainable electrocatalysts for water splitting and other energy applications.

This study successfully designed and synthesized a novel Ta-TaS₂ monolith catalyst exhibiting superior HER performance. The covalently bonded interface resulted in high activity (2000 mA cm⁻² at 398 mV overpotential), remarkable durability (negligible decay after 200 h at high current densities), and scalability. The Ta-TaS₂ MC outperformed commercial Pt-based catalysts in a water electrolyzer, demonstrating its potential for industrial applications. Future research could explore other TMDCs and optimize the synthesis process to further enhance the catalyst's performance.

While the Ta-TaS₂ MC demonstrated excellent performance, further investigation into long-term stability under even more demanding industrial conditions, such as higher temperatures and pressures, would strengthen the conclusions. The study focused on a specific electrolyte (0.5 M H₂SO₄); exploring performance in other electrolytes might provide valuable insights into the catalyst's versatility. The synthesis method currently employs laser patterning; investigation into alternative scalable and cost-effective patterning techniques would be beneficial for large-scale manufacturing.