Oxygen reduction reaction (ORR) is one of the important electro-catalytic reactions, because of its practical application for the cathode of polymer electrolyte fuel cells (PEFCs). The rotating ring-disk electrode (RRDE) method provides us information on applied-potential-dependent redox phenomena that proceed at the electrode surface and, has been widely used for studying ORR kinetics of the various electrodes. In contrast, few studies have been reported for dynamic behaviors of dissolved O2 molecules in electrolytes during ORR. In this study, we combine the online electrochemical mass spectrometry (OLEMS) [1] with the rotating disk electrode (RDE) method (RDE-OLEMS) to investigate potential-dependent phenomena for the dissolved O2 present in immediate vicinity of the Pt electrode during ORR.
Experimental
Fig.1(a) presents a schematic representation of the newly developed RDE-OLEMS system to analyze dissolved O2 present in the vicinity of a Pt working disk electrode (ø5mm, t = 4mm). The alumina polished Pt polycrystalline electrode was set into a RDE set-up. Then, the RDE-OLEMS measurements were performed in O2-saturated 0.1 M HClO4 with given disk rotation rates. Dissolved O2 molecules presented in the vicinity of the electrode surface was collected through a porous Teflon to prevent introduction of aqueous components into the vacuum system. During the RDE-OLEMS measurements, the distance between a PEEK Tip and Pt electrode surface was set to be ca. 50 µm by using a micrometer. The collected O2 was introduced into a quadrupole mass spectrometer (Q-mass) and, then, applied-potential-dependent O2 ion current (m/z=32) was recorded and compared with corresponding linear sweep voltammogram (LSV).
Results and Discussion
Fig.1(b) shows LSV curves of the Pt electrode recorded at anodic-potential-sweep (10 mV/s from 0.0 to 1.1 V vs. RHE) with the PEEK Tip configuration (Fig.1(a)). The LSV curves clearly showed diffusion-limiting currents in the potential region of 0.2 to 0.6 V: the Levich plots (inset) indicates that current densities change linearly with a square-route of the disk rotation rates. The results show that the dissolved O2 molecules can be supplied to the electrode surface in the developed RDE-OLEMS layout (with the PEEK Tip located at ca. 50 µm from the electrode surface). Fig.1(c) shows the RDE-OLEMS results (red) for the collected O2 (m/z=32) recorded at cathodic-potential-sweep from 1.05 to 0.05 V with a scan rate of 0.5 mV/s and a disk rotation rate of 1200 rpm. A simultaneously recorded LSV curve (cathodic-potential-sweep; black) is shown as a reference. It can be clearly seen that the applied-potential-dependent change in O2 ion current (MSCV) well corresponds to the LSV curve. The results clearly indicate that concentration of the dissolved O2 present in the vicinity of the electrode decrease with increasing ORR currents, especially in the diffusion limiting current region. In our poster presentation, we will discuss the results of RDE-OLEMS measurements for Pt and Pt-based alloy single crystal surfaces.
Acknowledgement
This work was partly supported by a Grant-in-Aid for scientific research (A) from the Japan society for the promotion of science (T. W.).
References
[1] A. H. Wonders et al., Journal of Applied Electrochemistry 36 (2006) 1215-1221.
Fig.1 (a)Schematics of RDE-OLEMS system and PEEK Tip. Inset; magnified image around the PEEK Tip. (b)Anodic sweep LSV curves for ORR. inset; Levich plot. (c)RDE-OLEMS results for O2 (red; m/z=32) and corresponding LSV (Black).