Hydrocarbon Fuels in Solid Oxide Fuel Cells:  In Situ Raman Studies of Graphite Formation and Oxidation

Abstract

In situ Raman spectroscopy and linear sweep voltammetry were used to characterize graphite formation on Ni/YSZ cermet anodes in solid oxide fuel cells (SOFCs) operating at 715 °C. The membrane electrode assemblies were run continuously with Ar-diluted H 2 and exposed to intermittent bursts of hydrocarbons. The appearance and disappearance of carbon deposits was monitored as a function of cell potential and hydrocarbon fuel identity. The hydrocarbon fuels employed in these studies included methane, ethylene, and propylene. Kinetic modeling predicts that of these three fuels, propylene is the most reactive under the conditions of the SOFC experiments. Methane was predicted to be virtually unreactive in the gas phase. Limited exposure of the SOFC anode to methane led to no observable carbon deposits and no appreciable change in SOFC electrochemical performance. Extended exposure to a continuous methane feed resulted in the formation of highly ordered graphite as evidenced by a single feature (assigned as the "G" band) at 1585 cm -1 in the Raman spectrum. The addition of ethylene to the incident fuel leads initially to the formation of highly ordered graphite as evidenced by the rapid growth of the G band and a small "D" band (at 1365 cm -1 ) in the Raman spectrum. Subsequent additions of ethylene created more disorder and led to deteriorating SOFC performance. Small amounts of propylene added to the fuel feed formed disordered carbon having significant amounts of tetrahedrally coordinated carbon, and SOFC performance suffered reversible degradation. Applying an overpotential to the anode led to the disappearance of carbon deposits with the intensity of the D band diminishing more rapidly than the G band. The disappearance rates depended directly on the anode overpotential.

Carbon deposition behaviour in metal-infiltrated gadolinia doped ceria electrodes for simulated biogas upgrading in solid oxide electrolysis cells

“…Numerous studies report Solid Oxide Fuel Cell (SOFC) degradation and failure due to carbon Journal Name ARTICLE 11 formation on the electrode. [21,33,54,56,58,[68][69][70][71] Recent studies, including this, have also shown that the same problem persists under electrolysis conditions. [17,55,63,72] It has been hypothesised that doped ceria can provide sufficient electrocatalytic activity alone such that the metal component is only required for efficient current collection.…”

Influence of Anode Thickness on Cell Performance in Internal Reforming Operation of SOFCs

“…The band at 1585 cm -1 arises from highly ordered graphite, which is generally labeled as "G". The band at 1350 cm -1 , labeled as "D", is attributed to the disordered graphite carbon (7)(8)(9). The spectra of 10-ȝm thick anode exhibited the specific peaks of D-and G-band at both surface and interface, while these peaks were observed only at the surface for the anode with 30-ȝm in thickness.…”

Designing a miniaturised heated stage for in situ optical measurements of solid oxide fuel cell electrode surfaces, and probing the oxidation of solid oxide fuel cell anodes using in situ Raman spectroscopy

“…In contrast Liu et al 4 demonstrated the first experiments enabling in situ Raman spectroscopy under practical operating conditions, followed by Pomfret et al 2,[5][6][7] who demonstrated an apparatus for in situ Raman spectroscopy on operating fuel cells, which could be referred to as in operando. Pomfret et al [5][6][7] demonstrated the successful measurement of carbon deposition from hydrocarbon fuels at various current densities, and observed transient graphitic carbon during hydrocarbon oxidation on the anode compared with build-up of carbon at open circuit conditions. The practical considerations involved in constructing and using an in operando SOFC Raman apparatus are highly challenging.…”

“…The apparatus used by Pomfret et al 2,[5][6][7] and latterly Eigenbrodt et al [8][9][10] involved a long-working distance objective with an SOFC mounted in a closed-end quartz tube situated in a furnace. The heated stage used by Maher et al 1 has the advantage of fitting on a microscope-mounted mapping stage enabling 2D spatial mapping of the surface, while the apparatus described by Pomfret et al 2,[5][6][7] enables the study of operational fuel cells and the effects of cell bias on surface properties.…”

“…The heated stage used by Maher et al 1 has the advantage of fitting on a microscope-mounted mapping stage enabling 2D spatial mapping of the surface, while the apparatus described by Pomfret et al 2,[5][6][7] enables the study of operational fuel cells and the effects of cell bias on surface properties.…”