Experiments with Button-Type Solid Oxide Fuel Cells Using Electrochemical Impedance Spectroscopy to Detect Carbon Formation

Abstract

Carbon deposition is one of the operational problems in hydrocarbon fueled solid oxide fuel cells (SOFCs). Using electrochemical impedance spectroscopy (EIS), experiments are performed to detect carbon formation on button-type cermet Ni-YSZ/YSZ/LSM SOFCs. A mass spectrometer (MS) is used to monitor the outlet gas composition, and the distribution of relaxation times (DRT) is computed to analyse the measured impedance. This work presents preliminary results obtained for H2-CH4 mixtures at OCV and 800oC, showing an increase in the mid-frequency range (10-3 and 10-2 s) for methane concentrations exceeding 10 vol%.

Control of oxygen-to-carbon ratio and fuel utilization with regard to solid oxide fuel cell systems with anode-offgas recirculation: A review

“…Fuel depletion as well as carbon formation and deposition ('coking') are the two major faulty operation states, which have to be limited to minimize cell degradation and enhance durability [25][26][27][28]. Fuel depletion leads to a shortage of fuel at the anode resulting in a reduction in cell voltage and in re-oxidation of the anode, the latter leading to enhanced cell degradation and irreversible cell damage [8,16,19,27,[29][30][31][32].…”

“…In dependency of the local temperature, the lack of oxygen in a carbon containing fuel gas can lead to carbon formation at the anode [17,28]. Typical carbon formation processes are the Boudouard-reaction, reverse coal gasification and especially cracking of long-chain hydrocarbons [8,16,29,33,36,37].…”

“…This can lead to a disconnection of the paths for the conduction of the electrons, increased pressure losses and blockage of the reaction zone in the triple phase boundary [8,15,17,29,34]. To prevent carbon formation it is necessary to supply either enough water vapor or oxygen in the fuel gas in dependency of the upstream reforming process [28,29]. For example, water vapor enhances the water-gas shift-reaction leading to a reduced amount of carbon monoxide.…”

“…For example, water vapor enhances the water-gas shift-reaction leading to a reduced amount of carbon monoxide. This is described by the temperature-dependent parameters oxygen-to-carbon-ratio (O/C) or steam-to-carbon-ratio (S/C), which are stated below in equation ( 3) and ( 4) in dependence of a general alkane molecule CnHm 1 [28,38,39]. They represent one of the major characteristic parameters for operation of a SOFC-stack or system.…”

“…A system with CPOx-reforming is rather simple, but has a relative low electrical efficiency of about 35% due to a lower hydrogen-generation and a higher risk of carbon formation due to carbon monoxide generation [16,25,29,33]. SR can be realized with an external water supply (EWS) or anode exhaust-gas recirculation (AEGR) [28]. Anode exhaust gas is partly depleted considering oxidable gas components and contains a lot of water vapor due to the electrochemical conversion of H2 [12,16,43].…”