Ionomer-Free Electrocatalyst of Sulfonate Chemical Group Introduced on Carbon As a Proton-Conductive Support for PEMFC
Authors: Wu Dan , Yoshihara Ryo , Fujigaya Tsuyohiko 
Published: December 2024

The needs of find diversify energy resources and new energy media to replace the fossil fuel to overcome the environment pollution problem. Owing to their cleanliness and high energy densities, polymer electrolyte membrane fuel cell (PEMFC) are vital devices for realizing a sustainable society. Reduce the cost of Pt catalyst from catalyst layer (CL) in PEMFC is the key to realize commercialization. Especially, the Pt utilization efficiency is highly dependent on the structure of Pt interface in cathode, where proton and electrons are delivered though a proton-conductive carbon support, while the O2 diffused though the porous network. Therefore, optimizing Pt interface structure and porous network in CL are important for the efficiency and utilization. To control the structure of the CL, especially for the coverage of the ionomer, significantly related to the proton conduction and O2 diffusion, namely, increasing the ionomer content increases proton conductivity but decreases O2 diffusivity in the porous network. In addition, sulfate anions or sulfonate groups in ionomers strongly adsorb on the Pt surface and acts as barriers for O2 permeation, resulting in increased overpotentials at the cathode. Therefore, even if the ionomer content is optimized, the presence of ionomers on Pt catalysts inevitably causes Pt poisoning.

In this study, to avoid these issues, we prepared proton-conductive carbon supports, in which acid groups were grafted on the surface of carbon black (CB) to afford surface proton conduction. Then, these carbon supports with Pt loaded on the surface were tested as ionomer-free electrocatalysts for PEMFCs. As the acid grafted, sulfonated aryl diazonium was chosen since their radical addition process proceeds under mild condition.

The acid-grafted carbon named SCB was characterized by X-ray photoelectron spectroscopy (XPS), resulting a new S 2p and S 2s peaks at ~168mV and ~232mV, respectively, from sulfonated aryl diazonium. The loading of Pt nanoparticles was carried out using Pt(acac)2 as the Pt precursor to reducing Pt particles under H2 atmosphere, resulting a homogeneous dispersion of Pt nanoparticels with diameters of 2.6 ± 0.3 nm on SCB. While control Pt nanoparticles on CB (CB/Pt) using H2PtCl6·6H2O and ethylene glycol as the Pt source and reducing agent, respectively (polyol method) had nanoparticles with diameters of 2.1 ± 0.5 nm. Using SCB/Pt and CB/Pt, SCB/Pt ink without ionomer added and CB/Pt ink with ionomer 28wt% were casted on Nafion membrane to evaluate the single cell performance.

In-situ CV of SCB/Pt cell and CB/Pt cell were measured by using H2 and N2 at the anode and cathode, respectively. Interestingly, the ECSA of SCB/Pt (38.0 m2 g−1) was comparable to that of CB/Pt (37.1 m2 g−1). Polarization curves of SCB and CB cell are measured at 80 °C with 100% RH to confirm to activation of acid-grafted carbon support catalyst. In most of the current density regions, the potential of SCB/Pt cell was lower than that of CB/Pt cell and the maximum power density of SCB/Pt cell (0.49 W cm−2) was slightly lower than that of CB/Pt cell (0.58 W cm−2), which is likely caused by the higher proton resistance of SCB/Pt cell with 2.9 Ω cm−2, but 0.14 Ω cm−2 for CB/Pt cell. However, at lower voltages, SCB/Pt cell showed a higher current density (>1.8 A cm−2) than CB/Pt cell (∼1.7 A cm−2). As potential drops are often observed in the high current region when the O2 supply in the CL is insufficient, we concluded that superior O2 diffusion is a unique characteristic of this ionomer-free system.

Furthermore, to elucidate the superior performance of SCB/Pt cell at high current densities, the microporous structure of the cathode CL was analyzed using cross-section images collected using focused ion beam scanning electron microscopy (FIB-SEM). The three-dimensional (3D) reconstructed structures CLs revealed that the CL of SCB/Pt contained smaller agglomerations than that of CB/Pt. In addition, the calculated porosity for the CL of SCB/Pt (0.614) was larger than that for the CL of CB/Pt (0.561). It is indicated that the higher potentials observed at high currents are associated with the formation of a porous structure with larger pores in SCB/Pt CL as compared to that in CB/Pt CL [1].

[1] Yoshihara R, Wu D, Phua YK, Nagashima A, Choi E, Jayawickrama SM, et al. Ionomer-free electrocatalyst using acid-grafted carbon black as a proton-conductive support. Journal of Power Sources. 2022;529:231192.

Figure 1