Investigation Of The High Performance Of Non-precious Metal Oxygen Reduction Reaction Catalysts Derived From Polyimide

The large scale application of PEMFCs requires highly active and durable catalysts for oxygen reduction reaction （ORR）. Currently, the state-of-the-art catalysts for fuel cell are based on precious metals, for example, platinum. However, disadvantages, such as the scarcity, prohibitive cost and limited durability of these noble metals hamper the large-scale commercialization of PEMFCs. Accordingly, extensive efforts have been directed to the development of low-cost non-precious metal catalysts （NPMCs） with high performance to reduce the cost of PEMFCs. Among various NPMCs, the Fe/N/C catalysts exhibit excellent performance on ORR, considered as a promising substitute for Pt or Pt alloys.Triazine-based framework polymers were famous as 2D or 3D porous solids for their intrinsic microporosity and high surface area. After pyrolysis, such a nitrogen-containing polymer leads to a uniform distribution of nitrogen sites on the catalysts surface and an increase in the surface density of catalytic sites that conduce to the improvement in ORR activity. In this paper, the polyimide obtained from the polymerization between melamine and pyromellitic dianhydride was employed as the nitrogen precursor to fabricate catalysts. These catalysts has a high nitrogen content and numerous porosity, and thus exhibit well electrocatalytic performance for ORR.Ferric chloride hexahydrate（FeCl3）, MA（Melamine）, PMDA （pyromellitic Dianhydride） and carbon support were employed to fabricate the Fe/N/C oxygen reduction reaction catalysts. The catalysts prepared with the （Black pearls （BP） 2000, Ketjenblack （KJ） EC-300J, Vulcan XC-72） were denoted as PI-Fe-BP, PI-Fe-EC and PI-Fe-XC. The Brunauer-Emmett-Teller （BET） specific surface areas of these three catalysts were determined by the low-temperature nitrogen absorption method. The catalyst PI-Fe-BP shown a highest BET surface area of 624 m2·g-1, and a highest micropore area of 292 m2·g-1. And the PI-Fe-BP catalyst show the most positive onset potential of 0.92 V, largest limiting current density of 4.0mA·cm-2 and the maximum power output of 310 mW·cm-2. With the increase of the surface area, the activity of these catalysts improved. That was because that the high surface and numerous pores ought to provide high surface density of catalytic sites exposed to oxygen molecules and thus lead to the enhanced ORR activity.In order to enhance the density of active sites, molten salts （MS） were induced as the dispersing matrices to prepare the self-support oxygen reduction reaction catalysts. The samples GNP15₁₁、GNP₂5₁ and GNP₅0₁ were obtained with variable MS/reactant weight ratios of 15:1、25:1 and 50:1 respectively. The oxygen reduction polarization of GNP₅0₁ catalyst is observed a largest reduction peak in the potential range of 0.65-0.75 V at more negative potential zone where mass transfer dominates the current density （i.e., diffusion limiting current density）, while the GNP₁5₁ catalyst show the most positive onset potential of 0.97 V. The catalyst GNP₅0₁ shown a highest BET surface area of 1230.6 m2·g-1, and a highest pore volume density of 0.67cm3·g-1. Raman spectra shown that the GNP₁5₁ has a highest intensity ratio of the G to the D band （IG/ID） of 1.08. Based on the investigations mentioned above, we could ascribe the enhanced oxygen reduction peaks at the diffusion limiting zone to the improved oxygen adsorption of different GNP based Fe/N/C catalysts, mainly caused by the higher microporosity and surface area. And large amounts of microporous structure which impedes the integrity of carbon framework reduces the electric conductivity and leads to the reduction of kinetic current.