Effect of Surface Wettability and Pore Structure of Non-Precious Metal Catalysts on Alkaline Oxygen Reduction Reaction Activities at the Gas Diffusion Electrode
Authors: Kim Jong Gyeong , Jung Hyeon Seung , Cho Youngin , Pak Chanho 
Published: December 2024

Anion exchange membrane fuel cells (AEMFC) have been attracting attention for low-cost fuel cells with a wide material selection range and facilitated oxygen reduction reaction (ORR) from lower electrode potential due to high pH.1 At the cathode, 2 moles of water are consumed in ORR, and at the anode, 4 moles of water are produced in the hydrogen oxidation reaction. Water generated at the anode is back diffused to the cathode and participates in the ORR. This imbalance of water between the cathode and the anode would make the anode and cathode vulnerable to flooding and drying, respectively. Therefore, water management of the membrane electrode assembly is important for the operation of AEMFC.2 To solve this issue, approaches such as controlling the relative humidity of H2 and O2 or modifying the electrode structure were attempted.2, 3 These approaches are mainly considered in single cells, and additional research is needed on the effect of catalyst properties on electrodes in a half-cell.

Catalysts were prepared by the silica hard template method. Generally, 1 g of ordered mesoporous silica (SBA-15), which served as a hard template, was uniformly mixed with 0.87 g of 1, 10-phenanthroline, 0.65 g of FeCl3, and 3.49 g of citric acid and kept at 200 ℃ for 2 h. The red composite cooled to room temperature was carbonized in a furnace in an inert atmosphere at 900 ℃ for 3 h. Then, silica was removed using HF. To remove iron oxide and metal particles, acid leaching was performed overnight in 2 M HCl at room temperature. Afterward, heat treatment was performed again at 900 ℃ for 1 h. The obtained catalyst was named FeNC_CA, and FeNC without citric acid was prepared for comparison. GDEs of FeNC and FeNC_CA were prepared by ultrasonic spray method. Catalyst and ionomer (FAA-3; Fumasep) were dispersed in EtOH/water solution with a tip sonicator. Prepared ink was sprayed on carbon paper (Toray 060) to be 1.5 mg cm-2 of catalyst loaded.

FeNC and FeNC_CA have major differences in mesopore volume to micropore volume ratio (Vmeso/Vmicro) and oxygen content. For FeNC, the ratio of mesopore volume to micropore volume was 1:1, and for FeNC_CA, the mesopore volume was 3.4 times the micropore volume. X-ray photoelectron spectroscopy showed that FeNC_CA had a 1.7 at% higher oxygen content than FeNC. The high mesopore volume and oxygen content of FeNC_CA led to high ionomer capacity and wettability when implemented as GDE. Interestingly, FeNC had 20 wt.% ionomer content in solid (catalyst+ionomer) content as an optimum, while FeNC_CA showed optimal ionomer content at 35 wt.% ionomer content (Fig. 1). This deviation was assumed to be due to the wettability of the catalyst layer and the formation of ionomer–catalyst aggregate. FeNC_CA-4 showed a flooding issue at 20 wt.% ionomer content. At 35 wt.% of FeNC, the excessive ionomer on the active site caused ion transfer hindrance. These results indicated that to implement the activity of the catalyst into an electrode, it is necessary to consider the influence of catalyst surface properties and pore structure on electrode properties.