Plasma-Structured Molybdenum Oxycarbides Enabling Ultrastable Acidic Hydrogen Evolution up to 10 a cm-2
Authors: Wu Shiwen 
Published: December 2024

Fabricating electrocatalysts capable of stable operation at high current densities is crucial for the industrial proton exchange membrane water electrolysis. However, current catalysts suffer from high overpotentials and rapid degradation when working in acid electrolytes at high current densities. Despite these advantages, critical challenges still exist in designing efficient catalysts for HER in acidic electrolytes at high current densities: 1) Poor long-term stability. The corrosive environment of strong acids, combined with the heat and mechanical stresses from hydrogen (H2) bubble generation at high current densities, severely challenges the stability of existing catalysts, including those noble metal-based ones. Catalysts with both excellent thermodynamic stability and robust physical adhesion to substrates are required to ensure long-term stability under these harsh conditions. 2) High overpotential at large current densities. At high current densities, high activation energy is required to drive HER because of the rapidly consumed H3O+ near the catalyst surface and the blocked active sites by the high-level H2 bubble coverage at catalyst surfaces, leading to increased overpotential and reduced energy efficiency.

Existing transition metal-based catalysts generally exhibit optimal activities only at low current densities (1-100 mA cm−2) with limited stability, making their operation in acidic electrolytes at industrial-level current densities (> 3 A cm−2) quite challenging. Non-equilibrium plasmas, characterized by highly energetic reactive species in a far-from-equilibrium state and the capability to operate at low temperatures, offer distinct advantages in the development and fabrication of innovative materials. The existence of non-equilibrium plasmas can significantly influence nucleation and crystal growth processes in material synthesis, facilitating thermodynamically unfavorable reactions that are difficult or impossible to achieve using conventional equilibrium thermal methods.

In this paper, we report the in-situ growth of vertically standing nanoedge-enriched molybdenum oxycarbide nanosheets (MoOxCy) through plasma-enhanced chemical vapor deposition (PECVD) based on earth-abundant salts (i.e., MoO3 and NaCl) as precursors to achieve ultrahigh-throughput acidic electrocatalytic hydrogen evolution at high current densities up to 10 A cm-2. The plasma-structured nanoedge-enriched MoOxCy catalysts offer the following benefits toward HER: 1) Significantly improved long-term stability of catalysts because of the non-equilibrium plasma-engineered structures, which are kinetically unfavorable to obtain through conventional equilibrium thermal methods; 2) Strong localized electric field near the ultrasharp edges of the catalysts, enabling fast diffusion of H3O+ from the electrolyte to catalyst surface; 3) Efficient hydrogen bubble detachment from the nanoedge-enriched structure with high roughness, maintaining continuous exposure of catalytic sites to the surrounding electrolyte. Benefiting from the unique plasma-induced morphology and chemical composition, the MoOxCy exhibits outstanding HER performance with a low overpotential (η) of 415 mV at an ultrahigh current density (j) of up to 10 A cm-2 for 1,000 h in 0.5 M H2SO4. This performance leads to an ultrahigh H2 throughput of 4,477.4 L cm-2, substantially outperforming the state-of-the-art transition metal- and even noble metal-based catalysts. This work paves new avenues for the development of high-efficiency catalysts for practical industrial applications in electrocatalytic hydrogen evolution.