(PEFC 15 Symposium Student Poster Award Winner) Electrochemical Evaluation of Pt Catalysts Supported on Nb-Doped SnO2 in Membrane-Electrode Assemblies for PEFCs

Platinum catalysts supported on carbon black (Pt/CB) have been widely applied as the cathode catalysts for PEFCs. It is well known that the CB degrades under high potential operating conditions. In order to improve the durability of the support, SnO₂ have been proposed as an alternative to carbon.¹ The enhancement of the electrical conductivity of SnO₂ by doping of aliovalent cations (e.g., Sb⁵⁺) was shown to be accompanied by an improvement of the ORR activity.² We also reported that the electrical conductivity of Nb-doped SnO₂ nanoparticles improved by the fused aggregation of particles with nearest-neighbors and by construction of a CB-like randomly branched structure (fused aggregate structure).³ Moreover, the single cell performance using a Pt cathode catalyst supported on the Nb-SnO₂ (Pt/Nb-SnO₂) was reported to be higher than that using commercial Pt/CB.^4-6 In this research, we evaluated the cell resistance using a Pt/Nb-SnO₂  cathode catalyst layer as a function of cell potential, current density and AC frequency, and investigated the reason for the unique electrically conducting behavior of the catalyst layers.

The Nb-SnO₂ supports with fused aggregate structure were synthesized by the flame combustion method and were loaded with nanometer-sized Pt catalysts by the colloidal method (Pt loading; 16.5 wt% and 9.6 wt%). The catalyst inks for cathodes were prepared with the Pt/Nb-SnO₂, Nafion^® binder, ethanol and pure water, and were sprayed onto the Nafion membrane by the pulse-swirl-spray technique. The Pt loading amounts on the cathode catalyst layers were 0.16 mg cm^-2 (16.5 wt%) and 0.08 mg cm^-2 (9.6 wt%), which meant that the Nb-SnO₂ loading amounts were almost the same (1.0 ± 0.1 mg-support cm^‑2). The Japan Automobile Research Institute (JARI) standard cell (active geometric area 29.2 cm²) was used for the electrochemical measurements. The impedance spectra were potentiostatically measured under steady-state operation, with hydrogen and oxygen/air at 80◦C under ambient pressure. The utilizations of the reactant gases were 70% for H₂, 40% for O₂ and 40% for air. Each cell resistance was estimated from the intercept of the x-axis of the Nyquist plots.

Figure 1 showed the cell potentials and ohmic resistances obtained by impedance spectra at 80 ^oC, 100% RH. In general, the cell ohmic resistance derives mainly from the proton conductivity of the membrane and is maintained stable value at 100% RH, as indicated in the cell resistance using heat-treated Pt catalyst supported on graphitized CB (Pt/GCB-HT). In contrast, the cell ohmic resistances using both Pt/Nb-SnO₂ cathodes decreased with increasing current density. The resistance of the cell using 16.5 wt% Pt/Nb-SnO₂ at 150 mA/cm² was approximately the same as that using the Pt/GCB-HT cathode. The resistance using the 16.5 wt% Pt/Nb-SnO₂ cathode was less than half of that using the 9.6 wt% Pt/Nb-SnO₂ cathode. Figure 2 showed the Nyquist plots for these cells at 0.65 V. The intercept of x-axis of each Nyquist plot appeared at different frequency. In addition, we found that the ohmic resistances of the cell using Pt/Nb-SnO₂ cathodes were strongly dependent on the cell potential, as shown in Fig. 3. In our previous research, we reported that the electrical conductivities of Nb-SnO₂ supports were dependent on the atmospheric condition, and increased with increasing amounts of loaded metallic Pt. The conductivity of the catalyst reached the values more than three orders of magnitude larger than that of the bare support.³ Such phenomena should strongly be related to the thickness of the depletion layer of the Nb-SnO₂ supports.³ which should not pertain to the GCB support, in principle. The cell potential dependence of the resistance using the Pt/Nb-SnO₂ cathode indicated that the electrically conductive paths in the Pt/Nb-SnO₂ catalyst layers should be improved with decreasing cell potential due to reduction of Pt oxide to metallic Pt.

Acknowledgement

This research was supported by funds for the “Research on Nanotechnology for High Performance Fuel Cells” (HiPer-FC) project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

References

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2. F. Takasaki et al., J. Electrochem. Soc., 158, B1270 (2011)

3. Y. Senoo et al., RSC Adv., 4, 32180 (2014).

4. K. Kakinuma, et al., Electrochim. Acta, 110, 316 (2013).

5. Y. Senoo et al., Electrochem. Commun., 51, 37 (2015).

6. Y.Chino et al., J. Electrochem. Soc., 162, F736 (2015).