The Performance of PEFC with Marimo-like Carbon Considering Relative Humidity

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1.IntroductionThe 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, which is a type of particulate carbon, we have used Marimo-like carbon (MC) which is a type of fibrous carbon. MC has a spherical structure comprised of carbon nanofilaments (CNFs) radially grown from an oxidized diamond core 1). The CNFs in the MC have a cup-stack primary structure of graphene sheets; thus, the sheet edge can effectively function as a supporting site for the Pt catalyst. There are voids of several hundred nanometers between the CNFs of the MC. These voids are expected to improve the diffusion of hydrogen gas and air and the removal of produced water. The diffusivity of hydrogen gas and air and the removal of produced water are different between the MC and CB due to their structural differences. These differences in mass diffusivity changed the state of water management and the PEFC performance. In this study, the performance of a PEFC with Pt/MC while considering the relative humidity (RH) of the hydrogen gas and air were investigated.2.ExperimentalThe catalyst supports were CB and MC. 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 1000 CCM for 3 hours. During this time, the rotary reactor was heated to 550°C. Pt/MC was synthesized by reducing Pt ions in a suspension with MC. Pt/CB was prepared as a commercial Pt nanoparticle supported on the CB. Nafion was used as the ionomer in this study. The catalyst ink was prepared by adding a Nafion dispersion to the Pt/MC and Pt/CB. The ionomer/carbon (I/C) ratio was 0.1 and 0.7 for the MC and CB, respectively 3). The catalyst layer (CL) was prepared by spraying the catalyst ink onto a 50 mm × 50 mm PTFE sheet. The membrane electrode assembly (MEA) was fabricated by transferring CL to the Nafion membrane by hot pressing. The PEFC performance was evaluated at the cell temperature of 80.0°C. Hydrogen gas and air were saturated at this temperature using a humidifier. Humidified hydrogen gas was supplied to the anode and humidified air was supplied to the cathode. The relative humidity of the hydrogen gas and air was controlled by the humidifier temperature.3.Results and discussionThe structure of the MC was observed by a scanning electron microscope (SEM). SEM images of the MC showed numerous CNFs. Voids of several hundred nanometers were observed between the CNFs. The Pt supporting state of the Pt/MC was observed by a transmission electron microscope (TEM). TEM images of the Pt/MC showed Pt supported on the surface of the CNFs.The PEFC performances of the Pt/MC and the Pt/CB were compared. At a high current density, the PEFC performance of the Pt/MC was higher than that of the Pt/CB. The PEFC performance in the high current density region consumed a large amount of hydrogen gas and air. This result was due to the high hydrogen gas and air diffusivity and the high removal of produced water from the MC.The relative humidity of the air was controlled from 20 to 100% RH by supplying it to a humidifier at a temperature of 42.7 to 80.0°C. The humidification conditions of the air affected the PEFC performance of both the Pt/MC and the Pt/CB. Under the low humidification condition, the PEFC performance was reduced due to the decrease in proton conductivity of the Nafion membrane. Under the high humidification conditions, the PEFC performance decreased due to flooding. The change in the PEFC performance under these humidifying conditions was particularly high by the MC.1) N. Hirata, et.al., RSC Adv., 11, 39216-39222 (2021)2) M. Eguchi, et.al., Jpn. J. Appl. Phys., 52, 06GD06 (2013)3) M. Eguchi, et.al., polymers, 4, 1645-1656 (2012)