Non-Precious Metal Catalysts with Different Spherical Particle Sizes towards for Oxygen Reduction Reaction in Alkaline Condition

Abstract

Over the decades, various catalysts such as Pt alloys, non-Pt catalysts and non-precious metal catalysts (NPMC) has been investigated to replace the state-of-the-art Pt catalyst for oxygen reduction reaction (ORR), because the cost of Pt catalysts accounts for more than 50% of a fuel cell stack cost when the fuel cell electric vehicles are produced for 500,000 units per year. Thus, economic catalysts such as FeNC as NPMC catalyst are the final goal of catalyst research and development. Recently, it was reported that FeNC having different morphologies derived by hard-template method displayed the dependence of ORR activity on the particle size, mesopore size. However, this study did not clearly conclude the particle size effect for ORR activity only due to the combination of several factors such as different pore structure and particle shape. Therefore, in this study, we investigate the effect of particle size only on the ORR activity using the FeNC catalysts prepared by using the hard template having the same pore structure and shape. The particle size of disordered mesoporous spherical silica (DMSS) is controlled by the base concentration in the Stöber process, which has the three-dimensional disorder pore and spherical shape. The three SP_FeNCs were derived from DMSS with three different particle sizes. The smallest diameter was 189 nm, the midst was 540 nm and the largest diameter was 913 nm, which diameters were estimated by counting 100 particles for average diameter. These catalysts had specific surface areas in the range of 1100-1300 cm2/g, respectively, which suggests all exposed catalytic active sites were similar. As anticipated, each catalyst showed different half-cell activity for ORR in the 0.1M of KOH solution. Among the three SP_FeNC catalysts, medium size (540 nm) catalyst displayed the best half-wave potential (0.881V), the largest particle (913 nm) and the smallest particle (189 nm) showed the similar activities as 0.844V and 0.845V, respectively. It is worthy to note that all three catalysts showed a superior ORR activity than commercial Pt/C catalyst in the alkaline condition. This tendency of activity for ORR could be attributed to the agglomeration of the smallest catalyst, even though the smallest catalyst is favorable to diffusion of reactant. That is, it was suggested that spherical particle FeNC catalyst has an optimum particle size to maximize the ORR activity.