Palladium-copper bimetallic nanocatalyst for electrochemical ethanol oxidation and oxygen reduction in alkaline media

Abstract

The necessity of replacement of traditional energy sources with renewable and green alternatives has initiated vast research of direct alcohol fuel cells (DAFC). Ethanol is non-toxic and its crossover through a membrane is lower than methanol due to its larger molecule size. However, the splitting of C–C bond in ethanol is energetically difficult and much effort in electrocatalysts’ improvement is still needed to take the advantage of the high mass energy density of ethanol [1,2]. Electrochemical oxidation of alcohols in requires a noble metal, Pt or Pd, to adsorb the molecule, but also some other oxophilic metal to facilitate further reaction of the adsorbed intermediates Their promoted oxidative desorption can be achieved by introducing oxygen-containing species at the surface, which may also modify the electronic structure of the noble metal centers and thus weaken the strong adsorbate–noble metal bond. Copper is inactive for alcohol oxidation but its addition to palladium enhances the ethanol oxidation reaction (EOR) rate [3,4]. Various forms of Pd-Cu electrocatalysts exhibited improved mass activity for the EOR and oxygen reduction reaction (ORR), but the effect of Cu addition on the specific activity is not so clear. Therefore, in the present work, specific activity for EOR and ORR of synthesized Pd-Cu nanoparticles [5] supported on high area carbon was examined. As reference catalysts, synthesized Pd/C and commercial Pt/C were used. For the electrochemical characterization the nanocatalysts were applied on a glassy carbon (GC) substrate in the form of a thin–film. Pd/C and Pt/C were characterized by cyclic voltammetry, Cuupd, and COads stripping in an acid and an alkaline solution. Electrochemically active surface area (ECSA) of the Pd-Cu nanocatalyst was calculated from the charge of desorption of CO in the alkaline solution. Cyclic voltammetry showed that in the presence of Cu atoms on the Pd surface, the onsets of CO desorption were negatively shifted. This indicates that Cu atoms provide oxygen-containing species at adjacent Pd sites at a lower potential than that achieved on pure metals. Nanocatalyst’s activity for EOR was investigated under potentiodynamic and potentiostatic conditions. Adding of Cu to Pd enhances the intrinsic activity of Pd for the EOR, with the greatest effect achieved for one Cu atom to 2-4 Pd atoms. Bimetallic catalysts surpassed Pd/C by mass activity, as well. The activity of Pt/C for EOR was higher compared with Pd-based catalysts, both as specific and mass activity, but with a significant decline over 30 min potentiostatic stability test. Therefore, the bifunctional and electronic effect contributed to the good performance of the nanoalloy for EOR. For ORR, Pd-Cu/C showed a negative half-wave potential shift compared to Pd/C and Pt/C of 11 and 30 mV, respectively. However, it was found that the specific ORR activities of Pd-Cu/C and Pd/C are the same at low current densities, i.e. up to a potential of 0.90 V, but higher than the specific activity of Pt/C by a factor of 5.