Implications of Structural Disorder for the Electrocatalytic Properties of MoS₂ Materials

Molybdenum disulfide materials have been extensively studied as catalysts for the hydrogen evolution reaction (HER) and other electroreduction processes. Similar to other materials, improvements in the catalytic activity of MoS₂ have been reported upon amorphization and introduction of defects, i.e., disordering. However, the interrelationships between catalytic properties, on the one hand, and the modes and degree of structural disorder within MoS₂, on the other hand, are yet to be comprehensively rationalized. To explore these effects in a systematic manner, we used a test set of MoS₂ materials with different levels of interlayer disorder, i.e., turbostratic misalignment of layer stacking and/or interlayer expansion, and in-layer disorder, i.e., disruption of the molybdenum-sulfur units and bond lengths within layers. These structural features were correlated with the electrocatalytic activity for the hydrogen evolution and nitrate reduction reactions under acidic (pH 1), buffered neutral (pH 7), and alkaline (pH 13) conditions. We demonstrate that the interlayer disorder promotes the intrinsic activity of MoS₂ for the HER in neutral and alkaline solutions, but not in acidic solutions. In contrast, in-layer disorder facilitates H₂ evolution under acidic conditions only. The NO₃^- reduction intrinsic activity is enhanced upon introduction of very significant levels of in-layer disorder only, with no systematic effects of the interlayer disorder identified. At the same time, the interlayer disorder has a pronounced positive effect on the selectivity of NO₃^- reduction toward NH₃/NH₄⁺. These trends might facilitate future design of more effective noble-metal free electrocatalysts for these and other important processes.