Effect of Particle Size and Structure on the Activity of Non-Precious Metal Catalysts for Oxygen Reduction Reaction

Abstract

Introduction Over the decades, many scientists have tried to replace Pt with other alloys, non-Pt catalysts and non-precious metal catalysts (NPMC), because when fuel cells are produced for 500,000 units per year, the catalyst cost accounts for more than 50% of a fuel cell stack cost [1]. Thus, inexpensive catalysts such as NPMC are the final goal of catalyst studies. Recently, Woo et al. reported that FeNC catalyst with smaller particle size, lager mesopore size, and threedimensionally interconnected pore structures improved ORR(oxygen reduction reaction) activity [2]. Therefore, this study uses disordered mesoporous spherical silica (DMSS) as a hard template to produce NMPC catalysts having the three-dimensional disorder pore structure with various particle sizes by the nano-replication method. The nano-replication method has advantages that have a structure and physical properties similar to a template, so it is suitable for studying factors and structural properties that affect the activity of the catalysts [3]. Materials and Methods The FeCl3·6H2O(Sigma Aldrich) and 1,10-phenanthroline(Sigma Aldrich) mixed in ethanol solution using the sonicator for several minutes. These precursors and H2SO4(35wt.%, DEAJUNG) perfectly filled the pore volume of OMS template with Fe:N=1:2(mole ratio). Sequentially, the mixture was poured into a zipper bag contained 2g of OMS for the first incipient wetness impregnation process. The impregnated product was dried for 2 h at room temperature and then moved to an oven at 80°C for 4 h and aged at 160°C for 12 h. After the first impregnation process, the process was repeated with a 50wt.% of the precursor solution. The powder that completed second impregnation process was carbonized through a process of two hours of maintaining the temperature at 200°C and three hours at 900°C under argon condition. Each ramping rate is 1.25°C /min for the first step and 2°C /min for the second step. The carbonized product was stirred in HF (10wt.%) to remove silica, selectively. Then, final products were filtered with DI-water until neutral pH and dried at 80°C for more than 12 h. Results and Discussion The SP_FeNCs were derived from DMSS with three different particle sizes. The smallest diameter was 189nm, the midst was 540nm and the largest diameter was 913nm, which diameters were estimated by iSolution Lite x64 and calculated more than 100 particles for average diameter. These catalysts had similar surface areas of 1100-1300 cm2/g, which suggests all catalytic external active sites were similar. However, each catalyst showed different half-cell activity in the 0.1M of KOH solution. Among three different size catalysts, medium size catalyst displayed the best half-wave potential, the largest particle showed second activity and the smallest particle showed the worst activity in the kinetic regions. All three catalysts showed a superior ORR activity than commercial Pt/C catalyst. In addition, SP_FeNC series had similar high current densities of around 6.5mA/cm2 at 0.3V in the limiting regions. These results indicated that the smaller particle size can improve the activity in the kinetic reaction and the disordered pore structure provide better mass transfer paths. However, the smallest SP_FeNC_1 showed inferior activity than that of SP_FeNC_2 due to the severe agglomeration of primary particles up to micron size. Therefore, it is better for FeNC to have the optimum particle size and well-distributed particles to improve the catalytic activity toward ORR under alkaline condition.