Nitrogen doping of ash-free coal and effect of ash components on properties and oxygen reduction reaction in fuel cell

Abstract

Nitrogen doping of ash-free coal (HPC) and raw coal in a stream of ammonia was studied in application to a cathode catalyst for a fuel cell. The relation to structure morphology, surface properties and catalytic activity for the oxygen reduction reaction (ORR), the effects of addition of the ash components of the raw coal to HPC and the comparison with carbonization in a stream of helium were determined. Tests were carried out with linear sweep voltammetry measurement in streams of oxygen and argon gases and a scan region of 0.04–1.04V using a three-electrode setup. Inorganic elements dominant in the ash of the raw HPC coal were iron, aluminum, potassium, magnesium, silicon and calcium. The nitrogen doping of HPC at 773–1073K decreased the oxygen content and increased the nitrogen content and the pyridinic nitrogen with the treating temperature compared to the carbonization in a stream of helium based on the XPS analysis. The addition of metals to HPC and subsequent nitrogen doping decreased the nitrogen content of HPC but increased the pyridinic nitrogen in the nitrogen distribution except for magnesium and calcium due to interaction with the pyridinic nitrogen. From the Raman analysis, the nitrogen doping at 773–1073K decreased the ID/IG ratio (deficient carbon degree) compared to the carbonization, but the addition of the ash components, especially 0.5% Fe+0.5% Al, increased it, resulting from the formation of many edges of deficient carbons in the presence of the ash. The TEM observation showed that the nitrogen doping of HPC at 1073K created onion-like fullerene structures but the addition of iron and aluminum destroyed this structure and formed a nanocarbon structure. The presence of iron and aluminum increased the pyridinic nitrogen and the disordered defect carbons formed along the edges of the graphite layers. Furthermore, the nitrogen doping at 1073K increased the onset potential of the ORR from 0.37 (carbonizing) to 0.67V. The addition of both iron and aluminum further increased the onset to 0.79V for the ORR which was the same potential as that of the raw HPC containing the ash components. The presence of ash in the raw coal, especially iron and alumina, promoted the electronic performance for the oxygen reduction. Consequently, the nitrogen-doped combined iron and aluminum with nanocarbons were the active species of the HPC catalysts for the ORR.