Non-PGM Electrocatalysts for PEM Fuel Cells: Thermodynamic Stability of Potential ORR CoNx-C Electrocatalytic Sites

Abstract

We present here a thermodynamic assessment of the stability behavior in acidic environments at 298 and 353 K (80 °C) of several Co-based sites that may potentially be active for the oxygen reduction reaction (ORR) at the cathode of proton exchange membrane (PEM) fuel cells. These sites are: i) the porphyrin-like CoN4/C site located in a micropore, ii) two different 1,10-phenanthroline-like CoN(2+2)/C sites located either in a micropore or embedded into a graphene sheet, and iii) the CoN(4+2)/C site having (at its center) the structural motif of the hexa-aza-macrocyclic complex of Co(II) embedded into a graphene sheet. The calculations of the equilibrium constants ( K c ) for the reactions of demetallation indicate that all evaluated active sites are chemically stable in an acidic medium at both 298 and 353 K. At 298 K, K c values for CoN4/C and for the two examples of CoN(2+2)/C sites are very similar. The same conclusion is also true at 353 K. However, the simple introduction of two supplementary nitrogen atoms in the structure of the embedded hexa-aza CoN(4+2)/C site vs that of the embedded phenanthroline-like CoN(2+2)/C site significantly affects its stability toward Co2+ demetallation, rendering CoN(4+2)/C much less likely to demetallate than CoN(2+2)/C either at 298 or 353 K.