In Operando Optical Studies of SOFCs Operating with Butanol

In operando optical methods coupled with electrochemical measurements were employed to explore the effects of dry and humidified butanol on the performance and durability of anode supported Ni-YSZ membrane electrode assemblies in functioning solid oxide fuel cells (SOFCs). n‑Butanol serves as a useful model biofuel given its ready availability from agricultural feedstocks and its tendency to undergo complex reforming and pyrolysis in high temperature environments.   (Combustion and Flame, 158 (2011) 16)  Butanol’s complicated gas phase and surface chemistry, however, has hampered efforts to characterize and quantify electrochemical oxidation mechanisms as well as reactive pathways that lead to SOFC component degradation and failure.  In order to assess directly the effects of butanol in SOFCs operating at 700˚C and 800˚C, near-infrared thermal imaging was used to measure temperature gradients across the anode while vibrational Raman spectroscopy quantified the tendency for carbon to accumulate in different regions of the anode. FTIR emission spectroscopy monitored gas phase composition above the anode and identified adsorbed surface intermediates.  Butanol reforming led to modest cooling (DT = ‑4˚C) of the anode at OCV.  Humidifying the fuel mitigated this effect slightly (DT ~ -2˚C).  Polarizing the cell had very little effect on anode temperature changes that resulted from endothermic fuel surface reactions. (Phys. Chem. Chem. Phys. 16, (2014) 227)   Within the high temperature environment of the SOFC, butanol underwent significant gas phase reforming with methane, ethylene, formaldehyde and carbon monoxide all being observed in the SOFC fuel exhaust.  This heterogeneous mixture led to substantial carbon accumulation on the anode observed in operando, Subsequent studies, however, indicated that carbon distribution was very heterogeneous in the anode microstructure.  Post mortem, ex situ experiments showed that heterogeneity in carbon accumulation extended both across and through the anode microstructure with the highest concentration of surface carbon closest to the fuel inlet.  Such observations are consistent with recent reports describing carbon accumulation on Ni/ceria electrodes performing electrolysis with CO/CO₂ mixtures. (Phys. Chem. Chem. Phys. 16 (2014) 13063.) Carbon that formed from butanol was moderately well ordered, with Raman spectra showing a large graphitic “G” response and a small, but measurable “D” feature corresponding to grain boundaries and/or site defects.  (J. Phys. Chem. C 112 (2008) 5232)  Polarizing the cell slowed, but did not prevent, carbon from forming under a neat butanol feed, but humidifying the butanol suppressed carbon formation completely.  FTIR emission spectroscopy showed measurable increases in both CO_(g) and CO_2(ads) as a function of cell polarization.  The latter species, CO_2(ads), is a surface intermediate formed during electrochemical oxidation of adsorbed carbon. (J. Phys. Chem. Lett. 4 (2013) 1310).  Humidifying butanol (either 1:1 or 1:3 butanol:steam) led to significantly diminished electrochemical performance and decreased cell longevity.  These observations emphasize the need to consider carefully how incident fuel mixtures affect SOFC performance and durability.

The figure shows (a) the near infrared thermal image acquired from the anode at 800˚C while eposed to plain butanol and (b) Raman spectra acquired from the anode at 800˚C following exposure to both plain and humidified butanol at OCV.