Improving the activity and CO tolerance by the nitrides-modified Pd/C electrocatalysts for methanol and formic acid oxidation reactions

Abstract

Direct methanol and formic acid fuel cells attracted extensive attention with high specific energy and low operating temperature. The efficient electrocatalyst was one of the important factors to limit their commercial applications. Here, the nitrides-modified Pd/C catalysts (Pd–NbN/C, Pd–Mo2N/C, Pd-VN/C and Pd-BN/C) were prepared using sodium citrate as a reducing agent in ethylene glycol solution. The mass activities followed the trend of Pd–NbN/C > Pd–Mo2N/C > Pd-VN/C > Pd-BN/C > Pd/C, where Pd–NbN/C exhibited the highest mass activity (4052.19 A gPd−1) for MOR, 19.2 times that of the commercial Pd/C (210.53 A gPd−1). Electrochemical measurements (LSV, Tafel, EIS, CA) indicated that the addition of nitrides increased the charge-transfer kinetics, reaction rate and CO tolerance of catalysts for MOR in alkaline media. The rate-determining step was COads oxidation process. Combined the structural analysis (HRTEM, XRD, XPS, ICP) with electrochemical results, the enhanced catalytic performance was ascribed to that higher ECSAs and the charge transfer between Pd and nitrides, leading to the negative shift of the d-band center towards the Fermi level. The linear correlations were found for the d-band center VS the mass activities and the onset potential of CO oxidation VS the d-band center, indicating that the activity and anti-CO poisoning ability could be enhanced by controlling the d-band structure of catalysts for MOR. Furthermore, the higher mass activity for FAOR in acidic media suggested that nitrides-modified Pd/C may be the promising bi-functional materials.