Monday, 24 July 2017
Grand Ballroom East (The Diplomat Beach Resort)
Two primary mechanisms responsible for Ni-based anode degradation in SOFCs are carbon accumulation (or coking) and reduction/oxidation cycling. Numerous strategies have been developed to suppress carbon formation on SOFC anodes including steam reforming and doping the anodes themselves with materials intended to prevent carbon growth. Once coking begins, however, removing carbon can cause even more anode degradation due to partial oxidation of the Ni anode itself. In experiments described below, operando Raman spectroscopy coupled with EIS and voltammetry measurements examine the propensity of carbon deposits to form from both methane and steam reformed methane in electrolyte supported SOFCs operating at temperatures between 700˚C and 800˚C. Vibrational spectra acquired from functioning cells shows that steam reformed methane limits but does not eliminate carbon accumulation. Changes in the mid- and low-frequency EIS data correlate closely with the spectroscopic observations and suggest that carbon accumulation affects both fuel oxidation activation energies as well as mass transport through the anode. Removing carbon electrochemically also induces Ni oxidation as reported directly in vibrational Raman experiments. Subsequent ex-situ experiments show a heterogeneous distribution of species throughout the anode and evidence of irreversible material degradation at the electrolyte/anode boundary. Steam by itself, however, proves to be remarkably effective at removing carbon while leaving the Ni anode largely intact. The ability of steam to remove carbon, presumably through solid carbon gasification, while allowing the Ni to remain in its elemental state shows microscopic reversibility of Ni oxidation and Ni reduction by H2O/H2 strongly favors reduction. Taken together these studies present a clear roadmap for restoring Ni-based SOFC anodes to their original operational state without forcing repetitive redox cycling that ultimately results in cell failure.