No abstract

The single cell performances of polymer electrolyte fuel cells (PEFCs) using Pt catalyst supported on Nb-SnO 2 (Pt/Nb-SnO 2 ) with/without graphitized carbon black (GCB) were compared with that of a cell using Pt/GCB. The Pt-mass specific power of the Pt/Nb-SnO 2 cathode under low humidity conditions was superior to that of the Pt/GCB cathode, because the hydrophilic SnO 2 support helped to increase the proton conductivity of the ionomer, which led to high Pt effectiveness. The addition of GCB to the Pt/Nb-SnO 2 cathode improved the cell performance under high humidity, and the Pt-mass specific power value reached more than 10 kW g Pt −1. The improvement conditions was attributed to the formation of gas diffusion paths by adding the hydrophobic GCB. The Pt/Nb-SnO 2 cathodes, with/without GCB, had greater durability than that of the Pt/GCB cathode during the startup / shutdown potential sweep evaluation. The migration of Pt particles on the Nb-SnO 2 support was suppressed by the high stability of Nb-SnO 2 and strong orientation between Pt and Nb-SnO 2 support. The degradation of GCB in the Pt/Nb-SnO 2 cathode was smaller than that in the Pt/GCB cathode because of the absence of Pt on the added GCB. Therefore, the added GCB was still able to provide gas diffusion paths even after extensive cycling.

We measured highly accurate ohmic resistances of single cells at high frequency (HF) using Pt/Nb-SnO2 cathodes under several different conditions, including various cathode potentials and various atmospheres, in order to study the relationships between the ohmic resistances of single cells and the electrical conductivity of the Pt/Nb-SnO2 catalyst for polymer electrolyte fuel cells (PEFCs). The ohmic resistance of a cell using Pt/Nb-SnO2 increased with increasing cathode potential, in the range of typical cathode operation (0.4 V to 1.0 V vs. the reversible hydrogen electrode). The increase of Pt loading on the Nb-SnO2 support from 9 wt% to 17 wt% was effective in decreasing the ohmic resistance and in improving the cell performance. We propose that Pt oxidation might impede the effect of shrinking the depletion layer of the Nb-SnO2 surface. The addition of graphitized carbon black (GCB) to the Pt/Nb-SnO2 cathode was able to improve the cell performance by constructing both electronic pathways and gas diffusion pathways in the catalyst layer, under low and high current density operation, respectively. The performance of the Pt/Nb-SnO2 + GCB cathode was superior to that of the Pt/GCB cathode due to these effects.

No abstract

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 A. Masao et al., Electrochem. Solid-State Lett., 12, B119 (2009). F. Takasaki et al., J. Electrochem. Soc., 158, B1270 (2011) Y. Senoo et al., RSC Adv., 4, 32180 (2014). K. Kakinuma, et al., Electrochim. Acta, 110, 316 (2013). Y. Senoo et al., Electrochem. Commun., 51, 37 (2015). Y.Chino et al., J. Electrochem. Soc., 162, F736 (2015). Figure 1

The development of cathode catalysts that have both high activity for the oxygen reduction reaction (ORR) and high durability is an important subject for the application of PEFCs in fuel cell vehicles. At present, Pt nanoparticle catalysts supported on carbon black (Pt/CB) are typically being used in PEFC cathodes. While CB supports are essential for the high performance of PEFCs because of their large surface area, high electrical conductivity and well-developed pore structure, they corrode seriously during startup and shutdown of the fuel cell. Graphitized carbon black (GCB), which has a high degree of graphitization, has been used to mitigate such corrosion. According to our previous studies, the high stability of a Pt catalyst supported on GCB (Pt/GCB) was confirmed by use of simulated startup/shutdown evaluation.1 Electrochemical catalysts for PEFCs are required to have both electrically conductive paths and gas diffusion paths in order to form effective catalyst layers (CL). Therefore well-developed pore structures are needed in addition to high conductivity. We have developed Pt catalysts supported on doped SnO2 (Pt/Sb-SnO2, Nb-SnO2, and Ta-SnO2) with fused aggregate structures, which are similar to that of CB, constructed by the fusion of nearest-neighbor particles to form a branched structure.2,3 The Pt/doped SnO2 catalyst provides high electronic conductivity due to the low grain boundary resistance and also has a large micropore volume. Due to the effect of the doped SnO2 with fused aggregate structure, the high performance of these cathodes, which was superior to that of Pt/CB, was confirmed by use of single cell measurements.4,5 In this research,6 we report a detailed investigation of a single cell using Pt/Nb-SnO2 with the fused aggregate structure under actual operating conditions. For instance, we confirmed that the steady-state current-potential (I–E) curves of membrane-electrode assemblies (MEAs) were strongly dependent on the humidity condition of the supplied gases. We also investigated the effects of the addition of GCB into the Pt/Nb-SnO2cathode for the improvement of the cell performance. In Fig.1, the single cell performances using Pt/Nb-SnO2 with/without GCB as function of relative humidity were compared with that of a cell using Pt/GCB at 80◦C and hydrogen/air under 1 atm. The Pt-mass specific power of the Pt/Nb-SnO2 cathode under low humidity conditions was superior to that of the Pt/GCB cathode, because the hydrophilic SnO2 support helped to increase the proton conductivity of the ionomer, which led to high Pt effectiveness. The addition of GCB to the Pt/Nb-SnO2 cathode improved the cell performance under high humidity, and the Pt-mass specific power value reached more than 10 kW gPt −1. The improved performance was attributed to the formation of gas diffusion paths due to the addition of hydrophobic GCB. Fig. 2 shows that the Pt/Nb-SnO2 cathodes, with/without GCB, had greater durability than that of the Pt/GCB cathode after 60,000 cycles of potential sweep cycle evaluation (1.0-1.5 V, 0.5 V s-1). Fig. 3 shows transmission electron microscopic (TEM) images of the Pt/Nb-SnO2 CL and Pt/GCB CL both before and after durability evaluation. The Pt nanoparticle size on the Pt/Nb-SnO2 surface increased to 4.9 ± 0.8 nm (after 60000 cycles) from 3.0 ± 0.6 nm (initial state) in diameter during durability evaluation, becoming spherical in shape (Fig. 3(a) and 3(b)). In contrast, it can be observed that the Pt particles on GCB formed elongated clusters, most likely due to the aggregation of spheres, after 60000 cycles in Fig. 3(d). Moreover, we confirmed that the Pt (111) lattice planes were parallel to those of SnO2 (110), and that the Pt nanoparticles were well oriented on the Nb-SnO2 surface in the initial state, as shown in Fig. 4. We consider that such interaction between the Pt nanoparticles and the Nb-SnO2 support could also have suppressed the migration of the Pt nanoparticles during the durability evaluation. We conclude that the Pt/Nb-SnO2 cathodes, both with and without GCB, exhibited outstanding durability during the startup / shutdown potential sweep evaluation in PEFCs.6 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, and the JSPS KAKENHI grant Number B24350093. References M. Uchida et al., Physical Chemistry Chemical Physics, 15, 11236 (2012). K. Kakinuma et al., Electrochimica Acta, 56, 2881 (2011). Y. Senoo et al., RSC Advances, 4, 32180 (2014). K. Kakinuma et al., Electrochimica Acta, 110, 316 (2013). Y. Senoo et al., Electrochemistry Communications, 51, 37 (2015). Y. Chino et al., Journal of the Electrochemical Society, 162, F736 (2015). Figure 1

Metal oxides are gaining growing interest as support materials for Pt catalysts in polymer electrolyte fuel cells (PEFCs). They are expected to have higher stability in the oxidative electrochemical environment of a PEFC cathode compared to carbon supports of standard Pt/C catalysts [1]. In this context, SnO2 is one of the most promising candidates. Several studies could already demonstrate the improved durability of SnO2 supported Pt cathode catalysts compared to Pt/C but their oxygen reduction reaction (ORR) activity is usually moderate. Doping with Antimony or Niobium improves the poor conductivity of the SnO2 and simultaneously increases the catalytic activity of the supported Pt catalyst [2]. Strong metal-support interactions (SMSI) as well as conductivity effects are suggested to explain this observation. It remains unclear, however, which material properties are stimulating SMSI and how much does the support conductivity influence the ORR activity of Pt based catalysts.   We have prepared un-doped as well as Ta, Nb, W and Pt doped SnO2 thin film electrodes by reactive magnetron sputtering [3]. Using these SnO2 films as Pt support enabled us to separate between influences coming from the support conductivity and other support related effects. The surface composition of the doped SnO2 thin films was evaluated using X-ray photoemission spectroscopy. The binding energies of Sn 3d5/2, W 4f7/2, Pt 4f7/2, Nb 3d5/2 and Ta 4d5/2 are corresponding to SnO2, WO3, PtO, Nb2O5 and Ta2O5 showing that the dopants are present in the highest oxidation state with exception for Pt. Electrical properties of the doped thin films were analyzed by four-point probe measurements. For all dopants an increase of the conductivity by 1-3 orders of magnitude compared to the un-doped SnO2 prepared under the same conditions could be observed and is a reliable indication for the homogeneous distribution of the dopant in the SnO2 lattice. The highest conductivities of approximately 7*103 Scm-1 were obtained for Ta0.01Sn0.99O2 and Nb0.021Sn0.979O2. Electrochemical characterizations in 0.1 M HClO4 were carried to evaluate the ORR activity of the prepared Pt/SnO2 catalysts. An advance Hupd method [4] has been applied for the determination of the electrochemical active surface area (ECSA) of the oxide supported Pt catalysts. Using this method reproducible ECSA values of 30±1 m2g-1 were found for all catalysts having the same Pt loading of 2 µgcm-2. Among the supports studied, Pt deposited on Nb0.017Sn0.983O2 displayed the highest catalytic ORR activity which is superior compared to our Pt/GC catalysts even though this support does not show the highest conductivity. Pt supported on doped SnO2 films having the highest conductivities showing moderate ORR activities comparable to Pt/GC and Pt/undoped SnO2. Using these results we can demonstrate the influence of the dopant towards the catalytic activity of supported Pt catalysts and developed structure-reactivity relationships which indicate that the dopant is not only modifying the conductivity of the support but also triggers the activity of the supported catalyst. Acknowledgment The authors thank Umicore GmbH and Co KG and the Competence Center for Energy and Mobility Switzerland (CCEM) for financial support within the project DuraCat. References [1]           A. Rabis, P. Rodriguez, T.J. Schmidt, ACS Catal. 2, 864 (2012). [2]           K. Katinuma, Y. Chino, Y. Senoo, M. Uchida, T. Kamino, H. Uchida, S. Deki, M. Watanabe, Electrochim. Acta 110, 316 (2013). [3]           A. Rabis, D. Kramer, E. Fabbri, M. Worsdale, R. Kötz, T.J. Schmidt, J. Phys. Chem. 118, 11292 (2014). [4]           T. Binninger, E. Fabbri, R. Kötz, T.J. Schmidt, J. Electrochem. Soc. 161, H121 (2014).

Polymer electrolyte fuel cells (PEFCs) are highly attractive power generation systems for fuel cell vehicles and residential co-generation systems. Platinum-based catalysts supported on carbon are important cathode catalysts for PEFCs, but the degradation of the carbon support in the high potential range is a key issue to be solved. Tin oxide (SnO2) is one of the good candidates for alternative non-carbon support with high electrical conductivity and stability in the high potential range.1 We have succeeded the synthesis of a Nb-doped tin oxide (Nb-SnO2) support with an aggregated network structure, like carbon black (CB), by a flame oxide-forming method.2-4 In this research, we evaluate the electrical conductivity and electrochemical activity of the Pt supported Nb-SnO2 with such unique morphology (Pt/ Nb-SnO2) by rotating disk electrode (RDE) and membrane electrode assembly (MEA). The obtained particles (crystallite size, 10-20 nm; specific surface area, 40-100 m2/g) were partially sintered with nearest neighbors and constructed an aggregated network structure with primary pore (diameter < 30 nm) and secondary pore (diameter > 30 nm). We evaluated the level of necking between nanoparticles with “Necking Index” (NI=SBET/SXRD) defined by the fraction of a specific surface area of particles measured by BET method (SBET) to that of the particles estimated from the mean crystallite size determined by the XRD method (SXRD). The apparent electrical conductivity (σapp.) of the support increased with decreasing NI, (Fig. 1, bold line), which rely on both the decrease of contact resistance by development of the necking among the particles and the change of microstructure from a closed-packed aggregate type to a chain-like aggregate type. Some particles exhibited a deviation from the simple correlation curve to higher conduction values even though the NI was nearly the same (Fig. 1: dotted line). The electrical conductivities of corresponding Nb-SnO2 increased with increasing their primary pore volumes due to a development of ramification of the chain-like aggregated structure with improvement of the electron conducting paths (Fig. 2). Moreover, we found that σapp. of Pt/ Nb-SnO2 catalysts were more than two orders of magnitude higher than those of the corresponding supports. The kinetically controlled current density (j k,Pt) for the oxygen reduction reaction (ORR) at 0.80 V of Pt/Nb-SnO2 (17 wt%, Pt particle size, 3.0 nm) increased with increasing σapp. and exceeded that of commercial Pt supported on carbon black catalyst (Pt/CB) (Fig. 3). The single serpentine pattern cell (Japan Automobile Research Institute (JARI) standard cell) with Pt/Nb-SnO2 also showed superior performance to that with Pt/CB. This work was supported by funds for the “HiPer-FC Project” of NEDO, Japan. References 1. A. Masao, S. Noda, F. Takasaki, K. Ito, K. Sasaki, Electrochem. Solid-State Lett., 12, B119 (2009). 2. K. Kakinuma, M. Uchida, T. Kamino, H. Uchida, M. Watanabe, Electrochim. Acta, 56, 2881(2011). 3. K. Kakinuma, Y. Chino, Y. Senoo, M. Uchida, M. Uchida, H. Uchida, S. Deki, M. Watanabe, 4. Y. Senoo, K. Kakinuma, M. Uchida, M. Uchida, H. Uchida, S. Deki, M. Watanabe, submitted.