A Study on Mesoporous Non-precious Metal Catalysts for Oxygen Reduction Reaction: Modification of Porosity and Electronic State of Catalysts

Abstract

The increased interest in anion exchange membrane fuel cells (AEMFCs) and the accelerated progress of related research fields emphasize the necessity for AEMFCs as a system to meet the target of low-cost fuel cells. Alkaline conditions with high pH allow the affordable materials range to replace the expensive fuel cell components. Nevertheless, high-performing AEMFC is still impossible without precious metal catalysts, therefore, developing new cathode and anode catalysts and membrane electrode assemblies (MEAs) is essential. Among them, on the cathode side, research on non-precious metal catalysts (NPMCs) that can improve the sluggish kinetics of oxygen reduction reaction (ORR) is attracting attention. Accordingly, catalytic properties such as pore structure and active site modification of non-precious metal catalysts have been adjusted, and the impact of these properties on ORR activity has been revealed, however, the understanding remains at the half-cell level rather than MEA. Verification of the catalytic activity of the newly developed catalyst is mainly accomplished through a rotating disk electrode (RDE) system with a thin film catalyst layer coated on glassy carbon as a working electrode in an O2-saturated electrolyte. Subsequently, catalysts with high ORR activity selected from RDE could be applied to the MEA. However, despite the high ORR activity in RDE, the performance is often not sufficiently implemented in the MEA. This discrepancy might originate from factors that are not considered in the RDE, such as the difference in operating conditions of half-cell and single cell, changes in properties when the catalyst is fabricated as a three-dimensional electrode, and the distribution and influence of the ionomer. Therefore, when designing a catalyst, it is necessary to consider both the characteristics of the material and the catalyst layer when implemented as an electrode. However, predicting these properties is a complex and challenging task. Therefore, the catalyst layer needs to be assessed with a system that can determine the electrochemical properties of the three-dimensional catalyst layer, the gas diffusion electrode (GDE) half-cell. In this paper, the pore structure and active site state of FeNC, one of the NPMCs, were controlled. The effect of catalyst characteristics on electrochemical properties was investigated through RDE and GDE half-cell. When synthesizing FeNC using ordered mesoporous silica as a hard template, citric acid with carboxyl functional groups contributed to reducing micropore and increasing mesopore fractions. When Cu, S, and P were added to FeNC, the electronic state of the active site (Fe) was affected even without direct interaction with heteroatoms. The change in free energy between the ORR intermediate and modified active sites resulted in high ORR activity in RDE. Interestingly, the electrochemical properties of the catalyst in the GDE half-cell were consistent with or contrasted with those of RDE. Observation of electrochemical properties according to differences in operating conditions of RDE and GDE half-cell, electrode structure, and phase of oxygen (purged in electrolyte or gaseous) as a reactant provided insight to deliberate the connectivity between catalyst and electrode.