Carbon monoxide and methanol oxidations on electrochemically treated carbon paper supported Pt electrodes

The Proton Exchange Membrane Fuel Cells (PEMFCs) and Direct Methanol Fuel Cells (DMFCs) 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 PEMFC’s 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. On the other hand the oxidation of methanol produces CO-type intermediates in the anode of DMFC, which also lead to the Pt metal poisoning [1]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[2]. Unfortunately, the non-treated carbon is often hydrophobic, so there is particular interest to carry out treatments on the carbon surface to activate it by additional functional groups. Studies show the effect of electrochemical treatment of GC support [3] and oxidation of the HOPG substrate on CO and methanol oxidation [4]. But, as we know, these supports are not real support for fuel cells. In this study, the carbon paper (CP), as electrode support, is activated by applying anodic and cathodic potential to introduce oxygenated surface compounds. The Pt was electrodeposited on treated and non-treated electrode and the effect of treatment for CO tolerance was investigated. To the best of the authors’ knowledge, studies about the effect of anodic oxidation for CP as a support on Pt electrodeposition is scarcely found in the literature [5] and there is no research for these oxidation of CP as catalyst support for enhancing the Pt activity for CO tolerance in fuel cells electrode. 0.25 cm2 CP (TGPH-090;Toray) was used as the support for electrochemical catalyst preparation. Oxidation on carbon papers were performed by applying +2V for anodic oxidation and -2 V for cathodic oxidation for 5 min in 0.5M H2SO4 solution. Pt was electrodeposited onto the CP by electrodeposition in an electrolyte solution of 100 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. Oxidation of carbon leads to a formation and growth of oxide layer, causing the surface of individual fibers becomes roughened and more defects appear on the surface which is in accordance with the SEM pictures. The surface of the cathodic electrode has more defect than anodic electrode that shows the changes in surface functional groups is different for cathodic and anodic electrooxidation. The oxidized CPs were characterized by micro-ATR-FTIR for both oxidized electrodes. Primary alcohol, ether, phenol, carboxylic, quinne and conjugated ketone functional group peaks can be observed. The strength of these peaks is weaker for anodic electrode which may result from the higher oxidation of carbon paper during the cathodic oxidation. Indeed, Phenol oraal tertiary alcohol and lactone peaks only appear after cathodic treatment which shows additional oxygen functional groups on CP which is prepared by cathodic oxidation. Cyclic voltammetry diagrams of the oxidized CPs (Fig.1) show the magnitude of the background current is highly sensitive to the electrochemical treatment and it is small for non-treated CP in compare with oxidized ones, but in the case of the anodic a high capacitive-like background current is observed. The CV of anodic shows evidence of surface oxidation due to the presence of one anodic and cathodic peak current is assigned to the hydroquinone–quinone redox couple (in the ranges0.3–0.4 V ) [6] In cathodic electrode the weak reduction peak at 0.3–0.4 V together with the results of FTIR and SEM analyses, indicate that higher oxidation states of carbon were produced. [1] 101 On the other hand, more small particles are noticeable when the deposition is performed on anodic CP(Fig.2). Further, although the metal particles seems bigger on cathodic electrode, uniformly dispersion of Pt with low agglomeration is visible (Fig.2.c). It seems that it is because of the different type of oxygen groups and amount of them, which have influence on nucleation and agglomeration of the particles on treated supports. The higher coulombic charge of the hydrogen adsorption/desorption zone for oxidized electrodes in Fig.3 is related to an increase in ECSA of the catalyst due to lower particle size. [1]. The CO oxidation abilities of the electrodes were evaluated 102 by CO-stripping experiment (Fig.4). The cathodic electrode has lower onset potential of CO electro-oxidation (0.44V) and peak potential of CO electro-oxidation (0.59V) than that of anodic (0.65V) and non-treated electrode (0.70V). These negative shifts show that cathodic electrode has more effective for CO oxidation in compare with anodic and non-treated electrode. The CV curves of methanol electrooxidation for electrodes (Fig.5) were consisted of two well-defined peaks at the forward and backward scans which were related to oxidation of methanol molecules and oxidation of intermediates, respectively. The ratio of the forward anodic peak current (If) to the backward anodic peak current (Ib) which can be used to gauge the tolerance exhibit larger ratios of If/Ib(1.50,1.41) for cathodic and anodic electrodes in compare with non-oxidized electrode. The onset potential of methanol oxidation occurs at 0.135 V, which is relatively 100 mV negative than non-treated electrode. Results show that cathodic electrode has more tolerance for intermediate species in compare with anodic and non-treated electrode. The stability of the electrodes in methanol was also evaluated by amperometric measurements.(Fig.6).Treated electrodes are more stable than non-treated one and cathodic electrode was able to maintain the highest current density and higher stability among all electrodes. We have revealed how the treatment of carbon with cathodic and anodic oxidizing improves the activity of the electrocatalyst in methanol and CO electrooxidations on Pt surface. Our results show that in addition to the size of the catalyst the type of the oxidation and the amount of functional groups and type of them has great effect on the activity of the electrodes on CO tolerance when the carbon support is oxidizing and oxygenated functional groups has two ways to effect on catalytic activity: first, by changing the size and metal dispersion in electrodeposition of Pt on carbon paper. Second, these groups were likely contributing to methanol and CO electro-oxidations similar to bifunctional mechanism .These additional oxygen functional groups on cathodic oxidized CP make this electrode more tolerant in compare with other electrodes in this study.