Mechanisms of Oxygen Reduction Reaction on Nitrogen-Doped Graphene for Fuel Cells

Fuel cells can directly convert chemical energy into electric energy with high conversion efficiency, high power density, quiet operation, and no pollution. Among many factors affecting the chemical-electrical energy conversion, oxygen reduction reaction (ORR) on cathode is the pivot in fuel cell. This reaction is a kinetically slow process, which dominates the overall performance of a fuel cell. The ORR can proceed through two ways. One is a direct four-electron pathway, in which O2 is reduced directly to water without involvement of hydrogen peroxide, O2 þ 4Hþ þ 4e f 2H2O. The other is a less efficient two-step two-electron pathway in which hydrogen peroxide is formed as an intermediate, O2 þ 2Hþ þ 2e f H2O2. To achieve a high efficiency fuel cell, the four-electron pathway is expected to occur. Because the ORR process is very slow in nature, catalysts must be used to facilitate the fourelectron pathway to boost the efficiency of fuel cells. Traditionally, such electrocatalysts are platinum and its alloys, 4 but they are expensive and susceptible to time-dependent drift and CO poisoning, which limits large-scale application of the fuel cell. There have been intensive research efforts, 11 to reduce or replace Pt and Pt based alloys electrodes in fuel cell. Recently, it has been demonstrated that vertically aligned nitrogen-containing carbon nanotubes (VA-NCNTs) and nitrogen-containing graphene sheets (N-graphene), reduced graphene oxide/platinum supported electrocatalysts (Pt/RGO), and stabilizing metal catalysts at metal metal oxide graphene triple junctions (Pt-ITO-graphene) show a much better electrocatalytic activity, with long-term operation stability, and tolerance to crossover and poison effects, than platinum electrodes forORR. But the detailed electrocatalytical mechanisms of these nitrogen doped carbon nanomaterials remains unclear. A fundamental understanding of the catalytic mechanism will provide guideline for further increasing the efficiency of these catalysts and discovering new catalysts. There have been some reports on mechanisms of ORR on electrode of fuel cells. A suitable mechanism table was formulated for the prominent pathway of ORR in proton exchange membrane (PEM) fuel cell and the kinetics of the proposed nonelectrochemical reactions on platinum were studied using B3LYP density functional theory (DFT). The DFTmethods were also employed to elucidate the mechanisms of ORR on carbon supported Fe-phthalocyanine (FePc/C) and Co-ptthalocyanine (CoPc/C) catalysts in 0.1 M NaOH solution. Anderson et al. studied the oxygen reduction on graphene, nitrogen-doped graphene and cobalt graphene nitride systems 20 using B3LYP hybrid DFT method. Electroreduction of oxygen to hydrogen peroxide was presented in their study, which shows two-electron pathway. Another simulation method was also applied to study the ORR mechanism on electrodes for fuel cell. Car Parrinello molecular dynamics simulations had been performed to investigate the ORR on a Pt (111) surface. However, to our knowledge, there are no reports about the ORR mechanisms of the four-electron pathway on the catalytic electrode of