Influence of Operating Conditions on the Mass Activity Characteristics and Performance of Carbon-Free Pt Black Electrodes for PEFC
Authors: Umedu Keita , Shimada Iori , Osada Mitsumasa , Takahashi Nobuhide , Fukunaga Hiroshi 
Published: December 2024

In recent years, the development of polymer electrolyte fuel cells (PEFCs) has been progressing. To expand their application to heavy-duty vehicles (HDVs), enhancing the power density and heat dissipation efficiency of PEFCs through high-temperature operation has become essential. Therefore, there is a need for high durability to enable the high-temperature operation of electrodes. In this context, we focused on carbon-free ionomer-free electrodes utilizing unsupported catalysts such as Pt black and low-Pt yet highly durable platinum nanosheets (Pt-ns). These electrodes offer high durability by avoiding the use of carbon and ionomer, which are factors contributing to performance degradation. Moreover, these electrodes have a different proton conduction pathway compared to conventional electrodes, relying on water on the Pt surface. Therefore, water management becomes crucial, considering the characteristics of carbon-ionomer-free electrodes. This study aims to elucidate the characteristics of the performance in the low-current region of Pt black electrodes and the effect of operating conditions on power generation characteristics.

The performance of ionomer-free Pt black electrodes and ionomer-added Pt black electrodes was compared in terms of their IV characteristics. In the high-current region (>1000 mA) where concentration overpotential dominates, ionomer-free Pt black electrodes exhibited a gentler slope, indicating superior gas diffusion compared to ionomer-added Pt black electrodes. Conversely, in the low-current region (<100 mA), ionomer-free Pt black electrodes showed higher power generation performance, attributed to ionomer-induced catalyst poisoning reported in literature.

Additionally, the IV characteristics of ionomer-free electrodes using different types of unsupported Pt catalysts, Pt black, and Pt particles were examined. Ionomer-free Pt particle electrodes displayed lower performance across the entire current range compared to ionomer-free Pt black electrodes, with a convex curvature of the IV curve indicating concentration overpotential. Analysis of electrochemical surface area (ECSA) and mass activity (MA) showed that ionomer-free Pt particle electrodes exhibited lower ECSA and MA compared to ionomer-free Pt black electrodes, indicating a lack of effective reaction sites, likely due to limited proton conduction.

The IV characteristics of ionomer-free and ionomer-added Pt black electrodes at different relative humidities were investigated. Ionomer-free Pt black electrodes demonstrated consistent performance across relative humidities in the range of 1 to 10 mA, but as humidity increased, the curvature of the IV curve steepened, indicating higher activation overpotential. In the region above 100 mA, the curvature of the IV curve became concave with increasing current under low humidity, suggesting localized current density distribution due to proton conduction resistance as electrolyte water increased.

Acknowledgments

Part of this paper is based on results obtained from a project, JPNP20003, subsidized by the New Energy and Industrial Technology Development Organization (NEDO).