Direct, In Situ Optical Studies of Ni−YSZ Anodes in Solid Oxide Fuel Cells Operating with Methanol and Methane

By Eigenbrodt, Bryan C.; Pomfret, Michael B.; Steinhurst, Daniel A.; Owrutsky, Jeffrey C. & Walker, Robert A.
Published in The Journal of Physical Chemistry C NULL 2011

Abstract

Near-infrared imaging and vibrational Raman scattering have been used to measure the susceptibility of Ni-based cermet anodes to carbon formation in solid oxide fuel cells (SOFCs) operating with methane and methanol fuels at 715 °C. These two complementary optical methods afford previously unavailable opportunities to monitor chemical and physical processes occurring in situ and in real time with molecular specificity and spatial resolution. Imaging and spectroscopic data show that when the cell is held at open circuit voltage carbon forms within one minute of methanol or methane being introduced to the anode chamber. Raman spectra identify these deposits as highly ordered graphite based on a single sharp feature in the vibrational spectrum near 1580 cm-1. While graphite formed from methane remains highly ordered regardless of exposure duration, graphite formed from sustained exposure to methanol begins to show evidence of structural disorder inferred from the appearance of a weak feature at 1340 cm-1. This lower frequency vibrational band has been assigned previously to the presence of grain boundaries and/or site defects in a graphite lattice. Correlating the growth of intensity in the Raman spectra with exposure time quantifies the kinetics of carbon deposition and suggests that carbon formed from methanol grows via two distinct mechanisms. Thermal imaging data show that carbon deposition is endothermic and reduces anode temperatures. This effect is more pronounced for methanol (ΔT = -5.5 °C) than methane (ΔT = -0.5 °C). These results agree with data from vibrational Raman experiments showing that exposure to methanol leads to significantly more carbon deposition. Polarizing the cell reduces the amount of carbon deposited. This effect is reversible and more significant for methanol. The effects of the graphite formed from methanol are evident in electrochemical impedance data but less apparent in voltammetry experiments. In contrast, graphite formed from methane has only modest impact on device performance. Collectively, these studies address long-standing questions about the tendency of methanol to form carbon on eletrocatalytic SOFC anodes and the consequences of this chemistry on device performance.

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