Platinum–manganese oxide as catalysts for DMFC and the importance of order of electrodeposition

Over the past few decades, direct methanol fuel cells (DMFCs) have generated tremendous interest owing to their advantages. However, the successful commercial application of DMFCs is still hindered by several technological challenges, including poor kinetics due to catalyst poisoning [1]. Pt-based electrocatalysts have been suggested with an aim to mitigate such poisoning effect [2]. Among these various promoters, metal oxides are considered as the best dueto their low cost [3]. El-Deab et al suggested that novel nanoparticles-based binary catalyst composed of Pt and manganese oxide directly electrodeposited onto GC for the efficient electrooxidation of formic acid[4] and CO[5]. According to our knowledge there is not anyresearch for investigating methanol oxidation on Pt-MnOx which is electrodeposited on carabon paper. In the present study, MnOx has been chosen as a second component in these condition and the influence of the order of electrodeposition of the two species on the electrocatalytic oxidationof methanol is investigated. Carbon paper (Carbon Toray 090) was used as the working electrodes and MnOx and Pt were electrodeposited on it. Electrochemical measurements were performed in a compartment three-electrode using a computer controlled electrochemicalanalyzer. A Pt electrode and an Ag/AgCl/KCl(sat.) were employed as the counter and reference electrodes, respectively. All potentials in this investigation will be displayed in reference to the Ag/AgCl. All chemicals used in this investigation were of analytical grade (Merck).Fourelectrodes were prepared by electrodeposition on carbon substrate: MnOx, Pt, Pt on MnOx and MnOx on Pt. For MnOx and Pt on MnOx electrodes, MnOx was electrodeposited onto the carbonpaper substrate by anodic electrodeposition in an electrolyte solution of 0.5M H2SO4 +0.5MMnSO4.4H2O under potentiostatic condition of 1.15 V for 25 min at 450C. For MnOx on Pt, MnOx was electrodeposited under the same condition only for 1 min. For Pt bare and Pt on MnOx, Pt was electrodeposited by cathodic LSV electrodeposition in an electrolyte solution of 0.2M H2SO4 +2mM H2PtCl6.6H2O at 5 mV/s between -0.5 and 0. 5V at room temperature.Fig1 (A-D) shows a SEM micrograph of the electrodes and the presence of these particles were characterized by EDS spectrum in Fig 1 (E-G). Fig(2-a) shows CVs measured in 0.1 M H2SO4. The CVs clearly reveal the characteristic features due to hydrogen adsorption/desorption, double layer charging, oxide formation, and oxide reduction on Pt surface. Although the region of H2 ads/des is the same for all electrodes but the current densities are different. The presence ofMnOx decreases the coverage of Pt, the peaks might be damp (Fig2-MnOx on Pt) but the deposition of Pt on different supports make some changes (Fig1-C-D,F-G) so, the catalytic activities are affected. According to Fig.2 double layer capacitance of MnOx on Pt is larger, reflecting that surface areaof MnOx/C is larger than carbon paper. So the order of deposition would have an important role in the catalytic activity.The promoting activity of Pt in the presence of MnOx for the methanol electrooxidation reaction was investigated by CVs measurements (Fig2.B). The high peaks withhigh reactivation current clearly show the promoting activity of MnOx for methanol electrooxidation compare with Pt. Moreover, the onset potential on the Pt on MnOx sample islower than that for the MnOx on Pt which shows the importance of order of deposition. The modification of Pt anodes with manganese oxide results in improvement of the activity for methanol electrooxidation. Results show higher promotional activity of the MnOx on Pt in compare with Pt bare. The higher catalytic activity of Pt on MnOx compared to its mirror image reveals that the order of the electrodeposition is an essential parameter.

Keywords:

DMFC , Manganese oxide , methanol oxidation , order of electrodeposition