Fabrication and Cell Analysis of a Pt/SiO2 Platinum Thin Film Electrode

Platinum thin film electrodes, such as nanostructured thin film (NSTF) by 3M [1], have attracted much attention as alternative catalysts for polymer electrolyte fuel cells (PEFCs). Although NSTF electrodes showed higher efficiency and power density than conventional Pt/C electrodes under suitable operating conditions, it has been reported that they tend to show larger performance decline under dry or wet conditions [2]. To examine all those properties of NSTF on other thin film electrodes and reveal their general features, in this research we fabricated a platinum thin film electrode on the surface of SiO₂ nanofibers. Performances of the cell with the electrode were studied and its various properties were analyzed.

     Platinum was deposited on a SiO₂ nanofiber web (Japan Vilene Company, Ltd., 200 nm in fiber diameter and 6 mm in web thickness) by atomic layer deposition. An SEM image of the cross section of the Pt/ SiO₂ nanofiber electrode is shown in Fig. 1. The SiO₂ nanofibers are covered with a platinum thin film of 3 nm in thickness. Nafion membrane was sandwiched between the Pt/SiO₂ nanofiber electrode as a cathode and a conventional Pt/C electrode as an anode to form an MEA. Note that no ionomer was impregnated on the Pt/SiO₂ nanofiber electrode. Fuel cell polarization curves were obtained using 1 cm² single-cell hardware with cross-flow flowfields under various conditions. The electrochemical surface area (ECSA), proton conductivity and oxygen transport resistance were analyzed by cyclic voltammetry, electrochemical impedance measurement, and diffusion-limited current measurement using diluted oxygen, respectively.

     Polarization curves and analyzed results were shown in Fig. 2 and Table 1, respectively, in comparison with those of conventional Pt/C electrode. The Pt/SiO₂ cell showed comparable performance to the Pt/C cell at 100% RH in spite of the much smaller ECSA. At 40% RH, however, the Pt/SiO₂ cell showed lower performance than the Pt/C cell.

     A transmission line model of the electrode was applied to the Pt/SiO₂ nanofiber electrode to analyze the performance loss. By following the model, the reaction resistance was calculated using values of the cathode resistance and the ohmic resistance, which were obtained from slopes of the polarization curves and the proton conductivity, respectively. The analyzed results under various RH conditions are shown in Fig. 3. The reaction resistance significantly increases with decreasing RH, while the ohmic resistance increase is limited. This indicates that the performance loss of the Pt/SiO₂ nanofiber electrode under dry condition is mainly due to the increase in the reaction resistance.

     Oxygen transport resistance, performances in wet conditions, and the effects of ionomer impregnation on the Pt/SiO₂ electrode will also be discussed.

[1] M. K. Debe, J. Electrochem. Soc., 2013, 160, F552

[2] A. Kongkanand, et al., J. Electrochem. Soc., 2012, 159, F405