Electrocatalytic oxidation of formic acid on Pd/MWCNTs nanocatalysts prepared by the polyol method

MWCNTs supported Pd nanocatalysts have been prepared by different variants of the polyol method aiming at a systematic study of the effect of preparation conditions (presence of citric acid as stabilizer and aging of the catalysts under stirring after the reduction) and support functionalization on the catalytic activity and stability of the prepared catalysts compared to commercial Pd/C. The catalysts were characterized with XRD and TEM to identify their crystal phases and nanostructure properties. All catalysts exhibit the fcc structure characteristic for Pd bulk metal. The Pd nanoparticles were unevenly distributed on the carbon materials with some agglomeration in case of CNTs whereas they were well distributed with high dispersion in Pd/C. CO oxidation on the prepared catalysts is particle size independent whereas formic acid electro-oxidation shows particle size dependency. Heat treatment of the catalysts prepared in the presence of citric acid at 200 °C enhances their activity toward formic acid oxidation while the heat treatment at 400 °C has an adverse effect on catalytic activity and stability which is discussed in terms of increased particle size. The results obtained in this study suggest the superiority of Pd nanocatalysts supported on CNTs compared to a commercial catalyst (Pd/C, E-TEK) in terms of both mass specific activity as well as surface specific activity despite their structurally less favorable properties. Chronoamperometry shows that Pd nanocatalysts prepared on functionalized carbon nanotubes without aging and in the absence of citric acid have the highest stability among all catalysts under study, the results agree with cyclic voltammetry measurements. All these features make the prepared Pd nanocatalysts suitable for practical applications as anode catalysts for direct formic acid fuel cells (DFAFCs).