Carbon supported Ag and Ag-Co catalysts tolerant to methanol and ethanol for the oxygen reduction reaction in alkaline media

Ag/C and Ag-Co/C catalysts with high activity towards the oxygen reduction reaction in alkaline media, different metal loadings and average sizes below 20 nm were synthesized by glycerol and borohydride reduction methods without stabilizers. Physicochemical characterization of the materials was performed by X-ray techniques (diffraction, energy dispersive and photoelectron spectroscopy) and transmission electron microscopy. Well dispersed small nanoparticles were obtained in all cases, mainly formed by Ag(0). For Ag-Co/C catalyst, it was observed that Co was not alloyed but presented as cobalt oxides. The electrocatalytic activity towards oxygen reduction reaction (ORR) in alkaline solution was evaluated by cyclic voltammetry and rotating disk techniques. A four electron transfer mechanism was established although increasing Ag loading produces a decrease of this number, indicating that hydrogen peroxide produced as intermediate in a first two electron step was not completely reduced. Alcohol tolerance of the catalysts was also established in methanol and ethanol solutions. Materials were not active for the electro-oxidation of alcohols, although it was observed that both methanol and ethanol were adsorbed on the catalyst. Highest activity and alcohol tolerance was observed for the 60 wt.% Ag loading material. Also the introduction of Co produces an increase in the activity (higher ORR limiting currents) and in the alcohol tolerance. In comparison to Pt and Pd, Ag and Ag-Co present more appropriate activities for ORR when alcohol tolerance is considered, being good candidates for the use as catalysts in alkaline direct alcohol fuel cells.