Cu and Pd nanoparticles supported on a graphitic carbon material as bifunctional HER/ORR electrocatalysts

The development of efficient, available and robust substitutes for the Pt-based electrocatalysts is very important for a sustainable energetic future. Herein, we report a series of composites based on Cu, Pd and Cu-Pd nanoparticles (NPs) supported on high surface area graphite (HSAG), as electrocatalysts for the energy-related reduction reactions - oxygen reduction (ORR) and hydrogen evolution (HER) reactions. All the composites showed interesting ORR electrocatalytic activities in alkaline medium. The Pd/HSAG and Cu-Pd/HSAG composites exhibited the most promising performances, with onset potentials of 0.84 and 0.91 V and current densities of j(L, 0.3 V, 1600 rpm) =-3.5 and -4.2 mA cm(-2), respectively. All the composites showed selectivity for the 4-electron process and Tafel slopes in the range 48-77 mV dec(-1). The metal/HSAG composites revealed a great tolerance to methanol and moderate electrochemical stability. In highly acidic medium (0.5 mol dm(-3) H2SO4, pH = 0.3) only the Cu-Pd/HSAG and Pd/HSAG electrocatalysts presented electrocatalytic activity toward HER, with relative low overpotentials (eta(10) = 0.145 and 0.063 V, respectively), small Tafel slopes (75 and 42 mV dec(-1)) and similar exchange current densities (0.43 and 0.57 mA cm(-2)). These electrocatalysts also showed moderate electrochemical stability, in particular Cu-Pd/HSAG for which overpotential only changed between 0.033 and 0.038 V for j = 40 mA cm(-2). The results showed that only small loading of Pd NPs (1 wt.% Pd) was able to improve significantly the ORR/HER electrocatalytic activity, which is a very important outcome to rationalize the design of efficient and cost-effective electrocatalysts in future.