SnO2-Supported Electrocatalysts on Conductive Fillers for PEFCs
Authors: Nakazato Yoshiki , Iwami Masahiro , Okumura Makito , Noda Zhiyun , Hayashi Akari , Sasaki Kazunari 
Published: February 2020

Introduction 

Electrocatalyst layer of polymer electrolytefuel cells (PEFCs) has complex 3-dimensional structure, and therefore control and optimization of microstructure is necessary for higher I-V performance. Carbon black supported cathode catalyst is generally used, however carbon supports can be degraded through oxidation on the cathode side. Thus, SnO2 support which is conductive and stable under the strongly-acidic cathode condition could be alternative support materials [1-3]. The objective of this study is thus to develop MEAs with higher performance and durability using SnO2 supports on conductive fillers for optimizing microstructural design of electrocatalysts.

Experimental

 Sn0.98Nb0.02O2 was synthesized via the ammonia co-precipitation method or the homogeneous precipitation method. Pt nanoparticles were decorated on such a metal oxide support by using platinum acetylacetonate complex as a precursor. Microstructure of MEAs was modified by changing Nafion-to-electrocatalyst ratio (hereafter, Nafion ratio) systematically. The performance of MEAs was evaluated by measuring I-V characteristics and separating each overvoltage. Two series of MEAs were prepared and evaluated: MEAs with different conductive fillers (VGCF and CNT) and MEAs with different SnO2loadings on the conductive fillers.

Results and Discussion

I-V performance with different Nafion ratios is shown in Fig. 1. MEA with Pt/SnO2(Nb)/VGCF + 23wt. % Nafion exhibited the highest I-V performance in this study. In case Nafion ratio decreased from 23wt. %, I-V performance was degraded. However, the differences of each I-V performance were small. When Nafion ratio increased from 23wt. %, I-V performance decreased notably mainly due to higher concentration overvoltage. This is because open pores as gas transport pathway were filled with Nafion ionomer. I-V performance with different conductive fillers and different SnO2 loadings are shown in Fig. 2. I-V performance of MEA with CNT as the conductive filler was comparable to that of MEA with VGCF as the conductive filler. In case SnO2 loading increased, I-V performance was improved. This is attributed to better dispersion of Pt nano-particles. In case CNT was used as the conductive filler, the distribution of SnO2 particles on CNTs was often inhomogeneous so that Pt nano-particles were aggregated. Accordingly, surface area of Pt was reduced and activation overvoltage became higher. When SnO2 loading was increased, surface area of SnO2 was increased too and Pt nano-particles were highly dispersed, leading to lower activation overvoltage. In this study, cell voltage of the MEA with Pt/SnO2(Nb)/VGCF (SnO2 loading: 50wt. %) and MEA with Pt/SnO2(Nb)/CNT (SnO2 loading: 70wt. %) was 95% of that of MEA with 46.4wt. % Pt/KB at 0.2 A/cm2. High I-V performance of MEAs with SnO2support on conductive fillers is thus confirmed.

Reference

[1]K. Kanda, Z. Noda, Y. Nagamatsu et al., ECS Electrochem. Lett., 3(4) F15-F18 (2014)

[2]T. Tsukatsune, Y. Takabatake, Z. Noda et al., J.Electrochem.Soc., 161(12) F1208-F1213 (2014)

[3]F. Takasaki, S. Matsuie, Y. Takabatake et al., J.Electrochem.Soc., 158(10) B1270-B1275 (2011)

Figure 1