Nanocomposite electrodes based on pre-synthesized organically grafted platinum nanoparticles and carbon nanotubes. III: Determination of oxygen reduction reaction selectivity and specific area of porous electrode related to the oxygen reduction reaction ranging from 2 m2 gPt−1 to 310 m2 gPt−1

Abstract

Our recent paper has reported the determination of diffusion area, ADiffO2, related to oxygen reduction reaction (ORR) in porous electrodes structures based on Platinum Organically Grafted Electrocatalyst (Pt-OGE) nanoparticles and carbon nanotubes. It was clearly shown that diminishing the electrocatalyst coverage density at the carbon nanotube surface resulted in a strong increase of ADiffO2, though some of the trends observed as a function of platinum loading were not clearly understood. Previous work considered the ORR selectivity, i.e. n, the average electron number exchanged per O2 molecule as being 4 whatever the Pt loading and the electrode feature. The present paper addresses this point using a recently reported new method for the determination of selectivity that can be implemented directly on porous electrodes, without using rotating disc or ring disc electrodes. Then, the paper considers the determination of specific ADiffO2 (S-ADiffO2) as a function of electrocatalyst loading on both, porous electrode based on Pt-OGE's grafted with Low Molecular Weight Organic (Pt-LMWO) and with High Molecular Weight Organics (Pt-HMWO). For five kinds of porous electrode structures combining carbon nanotubes and involving both kinds of Pt-OGE's, the n corrected trends for S-ADiffO2 expressed in m2gPt−1, are discussed. It is highlighted that for a given porous Pt-OGE/carbon nanotubes electrode structure two feature parameters can be found: a S-ADiffO2 value independent of Pt loading for which the value of n is found to be ≈4, and a critical electrocatalyst loading below which n<4 and S-ADiffO2 deviates from its feature value. These trends are explained in the frame of small sized electrodes arrays at which spherical diffusion regime occurs. As a conclusion, it is finally proposed that S-ADiffO2 could be considered as a feature parameter of interest to characterize the ORR electrocatalysts in porous electrode structures.