Temperature dependence of CO desorption and oxidation from cathodic electrochemically treated carbon paper supported Pt electrodes

The Proton Exchange Membrane Fuel Cells (PEMFCs) are considered good alternatives as power generation systems for different kinds of applications because of their high efficiency in energy conversion and suitability. One of the problems appears when using hydrogen obtained from reforming, because it contains CO, which can strongly compete with H2 for adsorption on Pt surface .This contamination lead to the metal poisoning and decreases the anode performance[1]. Three mechanisms have been suggested to explain improved CO tolerance of Pt based catalyst compared to pure Pt: the bifunctional mechanism, the ligand effect mechanism, and the “detoxification” mechanism [2].Whereas the bifunctional and ligand effect mechanisms explain the improved CO tolerance by a favoured electrochemical oxidation of CO, the “detoxification” mechanism proposes a lower equilibrium CO coverage at the surface, simply via the CO adsorption/desorption process. Therefore, both the electrochemical CO oxidation and the equilibrium attained through the adsorption/desorption process should be considered as physicochemical processes affecting the CO tolerance [3].Studies have shown that particle size, exposed crystal faces and oxidation state of carbon supports are important factors which have influence on specific activity of the Pt electrocatalyst for CO and methanol oxidation [4] Studies show the effect of electrochemical treatment of GC support [5] and oxidation of the HOPG substrate [4] on CO and methanol oxidation, because a large influence on the particle size and better distribution of platinum on the substrate. But, as we know, these supports are not real support for fuel cells. The study of the electrooxidation mechanisms of COad on Pt based catalysts is very important for designing more effective CO-tolerant electrocatalysts for fuel cell.To the best of the authors’ knowledge, there is no study about the effect of temperature on cathodic oxidation of carbon paper as a support for Pt electrodeposition on CO electrooxidation . The goal of this study is to investigate the effect of temperature on CO tolerancefor cathodic electrochemically treated carbon paper supported Pt with considering two approaches: electrochemical CO oxidation and the 95 equilibrium coverage attained through the adsorption/desorption process.0.25 cm2 carbon paper (TGPH-090;Toray) was used as the support for electrochemical catalyst preparation. Oxidation on carbon papers were performed by applying -2 V for cathodic oxidation for 5 min in 0.5M H2SO4 solution. Pt was electrodeposited onto the carbon paper by electrodeposition in an electrolyte solution of 0.2M H2SO4 +2mM H2PtCl6.6H2O . Single-pulse chronoamperometry electrodepsition by adjusting potential profile of 0V (1 s) and 1.15 V (600 s) (vs.Ag/AgCl(sat)) was done. All chemicals used in this investigation were of analytical grade (Merck). The oxidized CPs were characterized by micro-ATR-FTIR. Primary alcohol, ether, phenol, carboxylic, quinne and conjugated ketone, Phenol oraal tertiary alcohol and lactone functional group peaks can be observed. The effect of the temperature on CO electrochemical oxidation at different temperatures was analyzed by CO stripping voltammetry in the range of 25–85 0C. Figure 1 and figure 2 show the results obtained from CO stripping voltogramms for non-treated and cathodic electrodes, respectively. At elevated temperatures, a small current from upd-H oxidation was observed before CO removal. This current increases with the temperature, indicating the decrease of COad coverage at higher temperatures, as is clearly demonstrated in the θCO-temperature plot in Figure 3. It seems that the presence of oxygen functionalized groups on the surface of carbon paper support may make weaken the strength band between the CO and the Pt catalyst. As it has shown (Fig.3) the CO coverage is lower for treated electrode at room temperature. Indeed, the changes of CO coverage with temperature on cathodic electrode is higher than non-treated one (Fig.3). 96 Increasing the temperature may have synergetic effect on lowering the equilibrium coverage attained through the adsorption/desorption process for cathodic electrode in compare with nontreated one. As it has shown in figure 1and figure 2 effect of the temperature was similar for both electrodes, the anodic current (onset and peak potential) related to the CO oxidation reaction shifts towards more negative potentials as the temperature increases. This is related to an increment of the water dissociation reaction as well as to a faster surface CO diffusion toward the most active site with the rise of the temperature. The activation energy for the CO oxidation reaction can be calculated using the temperature-depending Nernst equation (Eq.1), which was suggested by Herrero et al. [6]: (Eq.1) The plot of the peak potential of CO electrooxidation vs. the absolute temperature for CO-stripping is given in Fig 4. From Fig. 4 and employing the Eq.1, activation energies for CO oxidation reaction were determined from the value of the potential at 0 K [7]. Consequently, different activation energies for Pt catalysts on cathodic and non-treated carbon paper are achieved. The obtained activation energy for cathodic and non-treated electrode was 125 kJ mol-1 and 180 kJ mol- 1, respectively. Results show that the presence of oxygen containing groups on carbon paper were likely contributing to CO electrooxidations similar to bifunctional mechanism. Indeed, more desorption of adsorbed carbon monoxide and consequently lower coverage were shown for 97 cathodic electrode because of the synergetic effect between treated electrode and increasing the temperature. We can conclude that in addition to bifunctional mechanism, detoxification can contribute for CO tolerance on cathodic electrochemically treated carbon paper supported Pt electrodes, especially on elevated temperature.