In Situ Raman Studies of Carbon Removal from High Temperature Ni–YSZ Cermet Anodes by Gas Phase Reforming Agents

In situ vibrational Raman scattering has been employed to examine rates of carbon formation and removal from Ni/YSZ cermet anodes in functioning, electrolyte-supported solid oxide fuel cells (SOFCs). Specifically, Raman scattering characterized the ability of different gas phase species commonly used as reforming agents to remove carbon that had accumulated on Ni/YSZ cermet anodes at 730 °C. Anodes held at open circuit voltage (OCV) were exposed first to a dry methane feed and then to an inert carrier gas containing either H 2 O (g), CO 2, or O 2 . Carbon deposits began to form within 5 s of methane exposure. Vibrational Raman spectra showed that the carbon deposits consisted of highly ordered graphite as evidenced by a single pronounced feature in the spectra at 1556 cm –1 . Changing the incident gas phase environment over the anode to Ar containing either H 2 O (2%), CO 2 (6%), or O 2 (6%) led to quantitative removal of the carbon and partial or complete oxidation of the Ni as evidenced by the growth of a NiO vibrational band (at 1080 cm –1 ) in the Raman spectra. Carbon removal rates from the Ni/YSZ anode were fastest with vapor phase H 2 O, then O 2, and finally slowest with CO 2 . The extent of Ni oxidation was much more pronounced with O 2 than with either H 2 O or CO 2 . These chemical processes observed directly in the Raman spectra were reflected in the device’s open circuit voltage (OCV). Correlating findings from these two methods— in situ Raman spectroscopy and voltage measurements—provided a direct connection between the chemical composition of SOFC anodes and the electrochemical condition of the device. These results inspire confidence that any of the reforming agents used—H 2 O, O 2, and CO 2 —will remove carbon from Ni anodes quantitatively on a time scale of ∼10 to ∼125 s. However, H 2 O and CO 2 appear less likely to damage the cell following carbon removal, as H 2 O and CO 2 do not quantitatively oxidize the Ni in the cermet anode. In contrast, exposure to O 2 leads to much more extensive Ni oxidation and an OCV that approaches 0.0 V, implying that the Ni/NiO equilibrium sustained by H 2 O and CO 2 is driven completely to NiO by O 2 .