Tailored sustainable synthesis of MoxC by electrochemical etching of a Mo2TiAlC2 MAX phase toward electrochemical applications

Abstract

Molybdenum carbide stands out as a promising material for energy-related applications, particularly as a hydrogen evolution reaction (HER) catalyst and a supercapacitor (SC) electrode material. However, traditional synthesis methods for molybdenum carbide are often complex, requiring multi-steps or severe conditions, which result in limited material accessibility and scalability. Herein, the work proposes a novel method for preparing molybdenum carbide materials with excellent HER catalytic performance and efficient SC capabilities by electrochemically etching a quaternary carbide (Mo₂TiAlC₂ MAX) precursor. The etching mechanism from Mo₂TiAlC₂ MAX to Mo_xC is systematically investigated by the experiment and the density functional theory (DFT) method. The results show that Ti and Al atoms can be completely removed by electrochemical etching in an alkaline K₂S solution. The resulting molybdenum carbide (Mo_xC) nanosheets exhibit remarkable HER catalytic activity due to their abundant metal vacancy defect structure. Specifically, Mo_xC delivers improved overpotentials (157 mV at 10 mA cm⁻²) in 0.5 M H₂SO₄ compared to Mo₂TiAlC₂. Theoretical calculations unraveled that Mo_xC exhibits a reduced Gibbs free energy for the HER. Mo_xC is demonstrated as an optimal candidate for supercapacitor (SC) applications, exhibiting a high specific capacitance of 394 F g⁻¹ at 2 mV s⁻¹ and excellent cycling stability, retaining nearly 94.7% of its initial capacity after 5000 cycles. The as-fabricated asymmetric supercapacitor exhibits a maximum energy density of 12.4 W h kg⁻¹ at a power density of 89.1 W kg⁻¹ and the capacity retention rate can still reach up to 80.2% even after 6000 cycles at a current density of 1 A g⁻¹.