Enhanced performance and electrocatalytic kinetics of Ni-Mo/graphene nanocatalysts towards alkaline urea oxidation reaction

Nickel-molybdenum/graphene (Ni-Mo/G) nanocatalysts have been fabricated for alkaline urea electrooxidation. The optimal Ni2Mo1/G catalysts exhibit fine mesoporous geometric structure (pore size: 3.75 nm, particle size: 20 ∼ 40 nm) and superior electronic structure with richer MoOx species and Ni3+ electroactive sites. Owing to the structural/electronic effects, the Ni2Mo1/G catalysts achieve higher activity and stability (128 mA cm−2 at 0.53 V; 76% retention for 1000 s at 0.50 V) but comparable kinetics (onset potential: 0.39 V; Tafel slope: 120 mV dec−1) comparing with the Ni/G nanocatalysts (107 mA cm−2 at 0.51 V; 55% retention for 1000 s at 0.50 V; 0.36 V; 105 mV dec−1). Cyclic voltammetry (CV) results reveal the typical catalyst regeneration mechanism (EC') and diffusion-kinetics mixed processes for the Ni2Mo1/G catalysts towards alkaline urea electrooxiation, showing the charge transfer coefficient (α) and diffusion coefficient (D) respective values of 0.73 and 1.11 × 10−5 cm2 s−1. Linear sweep voltammetry (LSV) and Tafel results display the average Tafel slopes (113 mV dec−1; 110 mV dec−1) and the chemical reaction orders (1.03; 0.60/-0.38/0.32) towards KOH and urea concentrations respectively. Chronoamperometry (CA) results indicate a favorable electrocatalytic endurance by proper Mo doping. The structural/electronic effects tend to produce more Ni3+ electroactive species and promote the oxidation of urea intermediates via the bi-functional mechanism of the MoOx. The direct urea-hydrogen peroxide fuel cell (DUHPFC) with the Ni2Mo1/G anode catalysts has also shown superior performance.