Nitrogen-Doped Carbon Nanofibres as Alternative Catalysts for the Oxygen Reduction Reaction in PEM Fuel Cells

Abstract:

Several large existing and emerging catalytic applications such as automotive catalysts and fuel cells are currently based on the use of platinum group metals (PGM). PGMs are identified by the European Commission as critical raw materials. Automotive catalysts are the major application of PGMs, where they are essential for reducing emissions from petrol and diesel engines. The European Union targets to replace the internal combustion engine in all passenger cars after 2050. Electric or fuel cell powered vehicles are the most promising solutions to replace the combustion engines. However, also conventional DMFC (direct methanol fuel cells) and PEMFC (polymer electrolyte membrane fuel cells) require large amounts of PGM. Reducing or even replacing the PGMs in fuel cells is therefore necessary in order to ease the dependency of such critical raw materials. Nitrogen-doped carbon nanofibers (N-CNFs) prepared in the presence of iron have shown promising activity for the oxygen reduction reaction (ORR), taking place at the fuel cell cathode. However, in acidic electrolyte the ORR-activities reported so far are lower than for conventional Pt/C catalysts. In order to improve the ORR-activity in acidic electrolyte it is therefore important to develop an understanding of the factors governing the catalytic activity in the N-CNFs. In this work, a chemical vapor deposition method has been employed to tailor the properties of the N-CNFs by in situ N doping. The ORR-activity was examined by performing linear sweep voltammetry in 0.5M H2SO4. Detection of H2O2 formed during the ORR was enabled using a rotating ring disc electrode. Determination of the elemental composition and nitrogen groups present at the surface of the N-CNFs was done by XPS .The synthesis at 650⁰C favored the presence of pyridinic nitrogen groups in the N-CNFs, and gave the most active oxygen reduction catalysts with an ORR-potential of 0.9V at -0.001mA. Extensive characterization has been performed in order to understand the role of the nitrogen groups on the ORR-activity of the N-CNFs, and will be discussed in connection with the differences in catalytic activity. Results from a 10-cell PEM fuel cell stack will also be presented.