O2 Reduction Mechanism on Non-Noble Metal Catalysts for PEM Fuel Cells. Part II: A Porous-Electrode Model To Predict the Quantity of H2O2 Detected by Rotating Ring-Disk Electrode

During O-2 electroreduction, some O-2 molecules are only reduced by two electrons to form H2O2 while the rest is reduced by four electrons to form water. The relative H2O2 production is quantified with the % H2O2. The latter number means that if the reduction of 100 O-2 molecules yields, e.g., 20 H2O2 and 160 H2O molecules, then the % H2O2 is 20%. Here, O-2 electroreduction in a porous electrode is modeled to calculate the polarization (I-V curve) and % H2O2-voltage curves (%-V curve) for non-noble metal catalysts that are assumed to reduce O-2 following either (i) 2e reduction + H2O2 disproportionation or (ii) 2e + 2e electroreduction. Using a set of base-case parameters, a series of I-V and %-V curves has been calculated for case i upon variation of (a) porous electrode thickness, (b) electrode rotation rate, (c) rate constant of H2O2 disproportionation, and (d) rate constant of O-2-to-H2O2 electroreduction. For case ii typical I-V and %-V curves have also been modeled for one electrode thickness, one rotation rate, and the experimental rate constants of O-2-to-H2O2 and H2O2-to-H2O electroreductions obtained in part I of this paper (DOI 10.1021/jp900837e) for the Fe/N/C catalyst. Analytical solutions for the % H2O2 are also given when the porous electrode is under either kinetic or diffusion control. It is concluded that the experimental I-V and %-V curves of the Fe/N/C catalyst of part I (DOI 10.1021/jp900837e) cannot be explained by mechanisms i and ii and that the 4e direct electroreduction of O-2 to H2O is the main path occurring on the Fe/N/C catalyst investigated in part I (DOI 10.1021/jp900837e).