Evaluation of the Durable Performance of the Pt/Marimo Carbon Catalyst in PEFC

Abstract

1.IntroductionImproving the durability of polymer electrolyte fuel cell (PEFC) has been a challenge. The deterioration of carbon can be cited as a cause of reduced durability. Carbon black is considered to be easily oxidized and deteriorated, because of the low crystallinity of amorphous carbon. We applies Marimo carbon, a new carbon material comprising entangled carbon fibers.(1) There is a cavity of several hundred nanometers in Marimo carbon. The diffusion of water and fuel gas is expected to be improved compared with conventional carbon catalysts. The fibers of Marimo carbon are arranged in a cup stack structure. It is possible to easily loading platinum. We study an improvement in the power generation performance of PEFCs using Marimo carbon.(2) In this study, an accelerated degradation test of a catalyst employing Marimo carbon was performed using electrochemical measurements to evaluate changes in catalyst performance. 2.ExperimentalMarimo carbon was synthesized by chemical vapor deposition. The loading of Marimo carbon with Pt was performed using the nano colloid solution method.(3) Marimo carbon and NaOH were added to distilled water, and the mixture was stirred during ultrasonication. Citric acid and chloroplatinic acid were then added, and the mixture was stirred during ultrasonication. Next, sodium borohydride was added as a reducing agent, and the mixture was stirred during ultrasonication. Finally, centrifugation and drying were performed to prepare a Pt/Marimo carbon. Accelerated degradation tests were performed with a scan speed of 500 mV s-1, and a potential range of 1.0 - 1.5 V vs. RHE. After each test cycle, the catalyst performance was evaluated by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) with a rotating ring disc electrode (RRDE). Catalyst performance was also evaluated by I-V testing using a single cell. For comparison, similar measurements were performed using a Pt/carbon black catalyst. 3.Results and discussionLSV results before and after the accelerated degradation test are presented. The number of cycles ranged from 0 to 7200, and RRDE rotation speed was 3600 rpm. In the case of carbon black, the value of the diffusion limited current density after 7200 cycles was confirmed to become small. However, in the case of Marimo carbon, the value of the diffusion limited current density was not changed even after 7200 cycles. The lowering of the catalytic activity due to the oxidation of carbon or elimination of the catalyst from the electrode. The resistance overvoltage was compared using Tafel plots. Prior to the accelerated degradation test, no difference was found in the slopes of both catalysts. After the accelerated degradation test, the slope for Marimo carbon did not change. However, the slope for carbon black increased, and thus, the resistance increased. The electrochemically active surface area (ECSA) was calculated CV of each accelerated degradation test. The ECSA of carbon black after 7200 cycles was reduced to 34 % of its initial value. However, Marimo carbon exhibited a high ECSA value, i.e., 80 % of its initial value after 7200 cycles and 63 % of its initial value after 23400 cycles. For Marimo carbon, there was no change in the oxidation peak of carbon, before and after the accelerated degradation test. In the case of carbon black, the oxidation peak of the carbon after the accelerated degradation test increased. From these results, carbon black is easily oxidized because of its amorphous nature and low crystallinity. Marimo carbon is not easily oxidized because Marimo carbon fibers comprise laminated graphite of higher crystallinity. The durability of Marimo carbon is higher than its of carbon black.Reference (1) K. Nakagawa et al., J. Mater. Sci. 44,221-226 (2009). (2) M. Eguchi et al., Trans. Mat. Res. Soc. Japan 38, 349-352 (2013). (3) K. Baba et al., Jpn. J. Appl. Phys. 52, 06GD06 (2013).