Effect of NH3 Etching on Fe-N-C Catalyst for Oxygen Reduction Reaction in Direct Methanol Fuel Cell

Abstract

Globally, because of the consumption of fossil fuel, the proton exchange membrane fuel cells (PEMFC) have received great attention. Direct methanol fuel cells(DMFC) have been applied into many fields, such as computers, global positioning systems(GPS) and portable devices. Platinum catalysts are considered as one of the most efficient catalysts for the oxygen reduction reaction(ORR),which is one of the most essential process for the DMFC. However, due to the high cost, the development of the DMFC calls for the non-precious metal catalyst, such as the Fe-N-C catalyst. We report a process to synthesize a Fe-N-C catalyst. Firstly, 80 ml 1.5M HCl, 2 ml aniline, 3 ml cyanamide, and 3 g ferric chloride were fixed and stirred for 20 min, with 5 g (NH4)2S2O8 dissolved into 80 ml 1.5M HCl dropped slowly. The mixture after being stirred for 4 h was mixed with the BP2000 carbon black dissolved into 80 ml 1.5M HCl for 40 min. Secondly,the mixture was heated at 80 degrees and stirred at 300rpm until dried. After being boiled in 2M H2SO4 for 5 h at 90 ℃ and milled at 400rpm for 3h, the sample was filtered and dried. Finally, half of the precursor was calcined for 2.5 h in N2, and 0.5 h in NH3 at 900℃, and the others were calcined in N2 for 3 h. After being heated for 5 h in 0.5 M H2SO4 at 90℃, and calcined for 3 h in N2 at 900℃ to gain the catalyst. The catalysts were named Fe-N-C00 and Fe-N-C30 separately. For RDE and RRDE tests on the Fe-N-C catalyst, the ink was prepared by mixing 15mg catalyst dissolved into 525µl ethanol and 145µl Nafion, which was stirred for 40min to achieve a good dispersion. 7µl ink was coated on the surface of RDE, making the loading of the catalyst 800µg cm-2. The staircase voltammetry(SCV) tests were conducted at the speed of 1600rpm in O2-saturated 0.5M H2SO4 solution. The half-wave of the potential of Fe-N-C00 catalysts were about 20mV lower than Fe-N-C30(0.685V vs 0.705V). According to the Koutecky-Levich equation, we calculated the electron transfer number was 3.97, and the productivity of H2O2 was 1.3%, which proved a four-electrons transfer process. The Fe-N-C catalyst used as the cathodic catalyst,which was sprayed onto a carbon paper gas diffusion(GDL,SGL 29BC), was applied into a single DMFC. A Nafion® 117 membrane from Dupont worked as the electrolyte. The membrane was pre-treated following a standard procedure before use. A commercial Pt-Ru (30% Pt-Ru/C)catalyst was used as anodic catalyst.The anode gas diffusion electrode (GDE), the cathode GDE and the membrane were put together in sequence and hot-pressed by 135℃and 5MPa for 5 min to form a MEA separately. The loading of the catalysts including Fe-N-C and Nafion was 4mg cm-2,and the ratio of the Fe-N-C catalyst was 55%. The polarization curve of the DMFC with the different MEA was tested to reflect the effect of Fe-N-C00 and Fe-N-C30 catalyst on DMFC. Figure 1