The Effects of Contact By the Catalyst Layer and the Gas Diffusion Layer Interface on the Performance of a PEFC with Marimo-like Carbon
Authors: Takamura Kohei , Sano Momoka , Gunji Hiroyuki , Eguchi Mika 
Published: December 2024

1. Introduction
The Pt catalyst supported on carbon black (Pt/CB) is widely used as a catalyst for the polymer electrolyte fuel cell (PEFC). As an alternative carbon support to the CB, we have used Marimo-like carbon (MC). MC has a spherical structure comprised of carbon nanofilaments (CNFs) radially grown from an oxidized diamond core 1). There are voids of several hundred nanometers between the CNFs of the MC. These voids are expected to improve the mass diffusivity. The cell components of the PEFC mainly include the catalyst layer (CL), gas diffusion layer (GDL), flow field plates, and gaskets. When the cell is assembled, the CL and GDL are compressed to the gasket thickness. This compression improves the contact between the CL and GDL interface and between the CL and the flow field plate interface. The CB has a particulate structure, whereas the MC has a fibrous structure. This difference in the structure of the carbon support transforms the interface structure of the CL. Therefore, in this study, we investigated the optimum conditions of the interface contact for the CL of the MC. The conditions of the interface contact were adjusted by controlling the gasket thickness. The results were evaluated based on the PEFC performance.
2. Experimental
The catalyst supports were the CB and MC. Pt/CB was prepared as a commercial Pt nanoparticle supported on the CB. MC was synthesized by thermal chemical vapor deposition (CVD). For the synthesis, methane (CH4) gas was used as the carbon source of the CNFs and Ni supported diamond oxide (Ni/O-dia.) was used as a growth catalyst. The Ni/O-dia. was contacted with the CH4 gas in a reactor rotating at 5 rpm. The CH4 gas was supplied at the flow rate of 100 CCM for 3 hours. During this time, the rotary reactor was heated to 550°C. Pt/MC was synthesized by the wet process2). To the suspension containing the MC, platinum chloride (H2PtCl6) and citric acid were added and stirred. Next, NaBH4 was added to the solution to reduce the Pt ions. The membrane electrode assembly (MEA) was fabricated by transferring the CL to the Nafion membrane by hot pressing. The area of the catalyst layer was 50 mm x 50 mm 3). The compression of the CL and GDL when the cells were assembled was evaluated by calculating the compression ratio. The compression ratio was calculated from the ratio of the total thickness of the CL and GDL before and after the cell assembly. The total thickness of the CL and GDL before the cell assembly was the sum of the measured interface contact of the CL and GDL thicknesses, respectively. The total thickness of the CL and GDL after cell assembly was regarded as the gasket thickness. The gaskets were fabricated from PTFE sheets with the gasket thicknesses ranging from 100 to 180 µm. The gaskets of various thicknesses were adopted for the sealing in order to control the compression ratios. The PEFC performance was evaluated at the cell temperature of 80°C. Humidified hydrogen gas was supplied to the anode and humidified air was supplied to the cathode.
3. Results and discussion
The compression ratio was calculated from the thickness of the CL, the GDL and the gasket. The thinner the gasket thickness, the higher the compression ratio. The compression ratio varied from 50 to 2% by using gasket thicknesses of 100 to 180 µm. The change in the compression ratio as a function of the gasket thickness was almost the same for the Pt/MC and Pt/CB.
The compression ratio affected the PEFC performance. Under-compression and over-compression conditions reduced the PEFC performance. The decrease in the PEFC performance under the low compression ratio condition was due to the reduced contact area of the interface between the CL and GDL and between the GDL and flow field plate. By increasing the compression ratio, the contact area was increased and the PEFC performance was improved. In the case of an excessive compression, the reduced gas diffusion paths in the CL and GDL decreased the PEFC performance. The change in the PEFC performance affected by these compression conditions was particularly high by the MC.
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2) M. Eguchi, et.al., Jpn. J. Appl. Phys., 52, 06GD06 (2013)
3) C. Simon, et.al., J. Electrochem, 164, F591-F599 (2017)