Fundamental Studies on Oxygen and Hydrogen Peroxide Reduction Reaction with Different Pt/C Catalyst Loadings Using Rrde Technique

Abstract

Introduction Polymer electrolyte fuel cells (PEFCs) are expected as one of highly efficient power generation systems. Moreover, the product is only water, using hydrogen gaseous as fuel. However, many problems remain to be solved for its commercialization. H2O2 formation on anode and cathode catalyst surface in PEFCs is one of the most important issues for the durability of the MEA in PEFCs. We reported that lager amount of H2O2 formed as an intermediate in oxygen reduction reaction (ORR) on Pt/C catalysts with lower catalyst loadings, and Fe2+ cation accelerated a deterioration of the electrolyte membrane1, 2). In addition, similar phenomenon was reported in the case of Pt/C catalysts3). In present study, we evaluated the ORR activity and H2O2 formation rate on prepared model electrodes with different Pt/C loadings by RRDE technique. Experimental The RRDE consisted of a glassy carbon (GC) disk and Pt ring sealed in a polytetrafluoroethylene (PTFE) holder used in ORR and hydrogen peroxide reduction reaction (HPRR) measurements. Commercially available 45.6 wt% Pt/C (TKK Inc, TEC-10E50E) catalyst was used for RRDE measurement. The ultrasonicated Pt/C catalyst suspension was carefully dropped on the mirror polished GC disk electrode surface with a microsyringe at each Pt/C loading densities. A thermostated three-compartment electrochemical cell was used for all electrochemical measurements. Counter electrodes were Pt and Au wire for ORR and HPRR measurements, respectively. A reversible hydrogen electrode (RHE) was used as a reference electrode. Linear sweep voltammetry (LSV) was performed from 0.05 to 1.0 V at 10 mV s-1 in oxygen saturated 0.1 M HClO4 for the ORR measurement on prepared electrodes. And then, the potential of the ring electrode held at 1.2V where it achieved to diffusion limit, and the rotating rate of the RRDE were between 400 and 3000 rpm. HPRR measurement was carried out at the same disk potential range and scan rate, in Ar saturated 0.1 M HClO4 with H2O2 additive, while ring electrode potential held at 0.1V. Results and Discusssion Cyclic voltammograms (CV) per mass of Pt on prepared electrodes with different catalyst loadings shows similar behavior. A characteristic CV curves such as Hupd adsorption/desorption and Pt oxide formation, and its reduction were clearly observed. The each CV current per mass of Pt was almost equal to each catalyst loading. Therefore, the prepared electrodes were successfully highly dispersed and modified on GC disk electrodes. Hydrodynamic voltammograms for ORR at the different Pt/C catalyst loadings was obtained and achieved to diffusion limit above the catalyst loading of 2.82 µgPt cm-2. The kinetic current (IK ) for ORR can estimated by the Koutechy-Levich equation, and a Mass activity (MA) and a specific activity (SA) was obtained. The MA and the SA decreased under the catalyst loading of 7.05 µgPt cm-2. This decrease indicated that the amount of the Pt/C catalyst is not enough to remove the O2 diffusion effect under the catalyst loading of 7.05 µgPt cm-2. H2O2 formation rate were calculated from LSV in the ORR. H2O2 formed below 0.8V, and the amount was larger with lowering disk potential and decreasing catalyst loadings. Fig. 1 shows LSVs in HPRR. The disk currents (ID ), which is a reduction current of H2O2, decreased with decreasing of catalyst loadings, similar to the ORR. On the other hand, IR , which is reduction currents of H2O2 and O2 formed on the disk electrode, increased with decreasing of catalyst loadings, while it is generally known that O2 produced from the H2O2 oxidation reaction above 0.8V. The increase of the IR caused by decreasing catalyst loadings indicated that chemically O2 formation such as disproportionation reaction occurred on the Pt/C catalyst surface. 4. References1) M. Inaba, H. Yamada, J. Tokunaga and A. Tasaka, Electrochemical and Solid-State Letters, 7 (12), A474-A476 (2004).2) M. Inaba, H. Yamada, R. Umebayashi, M. Sugishita and A. Tasaka, Electrochemistry, 75 (2), 207-212 (2007).3) H. Itaya, S. Shironita, A. Nakazawa, M. Inoue and M. Umeda, International Journal of Hydrogen Energy， 41(1), 534-542, 2015. Figure 1