Isolating the impact of CO concentration in syngas mixtures on SOFC performance via internal reforming and direct oxidation

Abstract

One of the distinct advantages of solid oxide fuel cells (SOFCs) is their ability to directly oxidize CO in addition to H2, which allows them to be run on syngas mixtures. However, membrane-electrode-assembly (MEA) models typically neglect CO electrochemistry in the presence of H2 and H2O, assuming that the water–gas-shift reaction proceeds faster than sluggish CO electro-oxidation. In this paper, however, we demonstrate with a comprehensive 1D-MEA model that CO electro-oxidation cannot be neglected in syngas mixtures, particularly at high current densities for high CO-content syngas. We first demonstrate that incoming CO is not all shifted to form H2 before reaching the triple-phase boundary, as previously assumed, due to the equilibrium limitation of the water–gas-shift reaction at 800 °C. Furthermore, we confirm that direct oxidation of CO contributes non-negligible current relative to H2 at high anode overpotentials in syngas mixtures. Together these results show that CO electro-oxidation plays an important role in SOFC performance not only via water–gas-shift reforming, but also via direct oxidation even when H2 is present. This work suggests that accurate models for both surface reforming and direct electro-oxidation of CO in SOFC anodes must be included in order to capture performance when using syngas mixtures.