Layer-separated MoS2 bearing reduced graphene oxide formed by an in situ intercalation-cum-anchoring route mediated by Co(OH)2 as a Pt-free electrocatalyst for oxygen reduction

Abstract

A significant improvement in the electrochemical oxygen reduction reaction (ORR) activity of molybdenum sulphide (MoS₂) could be accomplished by its layer separated dispersion on graphene mediated by cobalt hydroxide (Co(OH)₂) through a hydrothermal process (Co(OH)₂–MoS₂/rGO). The activity makeover in this case is found to be originated from a controlled interplay of the favourable modulations achieved in terms of electrical conductivity, more exposure of the edge planes of MoS₂ and a promotional role played by the coexistence of Co(OH)₂ in the proximity of MoS₂. Co(OH)₂–MoS₂/rGO displays an oxygen reduction onset potential of 0.855 V and a half wave potential (E_1/2) of 0.731 V vs. RHE in 0.1 M KOH solution, which are much higher than those of the corresponding values (0.708 and 0.349 V, respectively) displayed by the as synthesized pristine MoS₂ (P-MoS₂) under identical experimental conditions. The Tafel slope corresponding to oxygen reduction for Co(OH)₂–MoS₂/rGO is estimated to be 63 mV dec⁻¹ compared to 68 mV dec⁻¹ displayed by the state-of-the-art Pt/C catalyst. The estimated number of electrons transferred during oxygen reduction for Co(OH)₂–MoS₂/rGO is in the range of 3.2–3.6 in the potential range of 0.77 V to 0.07 V, which again stands out as valid evidence on the much favourable mode of oxygen reduction accomplished by the system compared to its pristine counterpart. Overall, the present study, thus, demonstrates a viable strategy of tackling the inherent limitations, such as low electrical conductivity and limited access to the active sites, faced by the layered structures like MoS₂ to position them among the group of potential Pt-free electrocatalysts for oxygen reduction.