In-Operando Detection of Carbon Depositions and Carbon Formation Predictions for Industrial-Sized SOFCs Fueled with Synthetic Diesel Reformate
Authors: Subotic Vanja , Schluckner Christoph , Stoeckl Bernhard , Reichholf David , Lawlor Vincent , Pofahl Stefan , Schroettner Hartmuth , Hochenauer Christoph 
Published: February 2020

Solid oxide fuel cells (SOFCs) present an attractive and innovative technology for environmentally-friendly electricity and heat generation. Their high operating temperatures and the excellent catalytic performance make SOFCs the most efficient fuel cell type, with a high level of fuel flexibility. Feeding the SOFCs with conventional or alternative carbon-containing fuels carries the risk of carbon formation and its deposition on the anode, which can reduce cell’s activity, cause irreversible cell degradation, and decrease the cell’s lifetime. In order to prolong the cell’s lifetime and to ensure the undisturbed operation of SOFCs, an in-depth understanding of this degradation phenomenon is required.

The problematic of carbon deposition phenomenon is detailed explained and supported with numerical and experimental results for planar, industrial-sized, anode-supported solid oxide fuel cells within this study. Possible reactions to carbon build-up are presented and the methods and calculation principles used to predict the carbon formation are demonstrated. The key parameters that enable carbon predictions are: the components involved in a used gas mixture, temperature, the steam/carbon-ratio, equilibrium state, and C-H-O ternary diagram. All of the mentioned carbon prediction methods and analysis of the basic calculation principles are explicitely explained. For the detailed analysis and prediction of coking propensity, various diesel reformate mixtures, containing H2, CO, CO2, CH4, H2O and N2, are used. In order to examine the impact of water vapor on carbon deposition, the amount of H2O in the fuel mixture is varied between 0 vol%, 11.3 vol% and 20 vol%. The operating conditions, under which carbon can be formed and deposited on the anode side can thus be generated.

In the course of the experimental analysis of the carbon deposition phenomenon, the polarized single cells were fed with synthetic diesel reformate mixtures, thus representing realistic operating conditions. In order to obtain information about the degradation of cell performance, such as the critical operating time or degradation voltage, the methane volume fraction in the fuel mixture was varied between 2.3 vol% (as expected in APUs), 9 vol%, 14 vol% and 20.3 vol%. The carbon build-up process on the Ni-catalyst, and, in fact, anywhere in the cell’s fuel supply manifold, was detected at an early stage by means of electrochemical impedance spectroscopy, current/voltage measurement, temperature measuremens and gas analysis. In addition to the in-situ characterization techniques used, an ex-situ post-mortem analysis was introduced to support the knowledge gained in the experimental analysis and to confirm the occurrence of the carbon identified via employed in-situ characterization techniques. Methane, including the decreasing steam/carbon ratio, was identified as having a major impact on the degradation rate at the typical operating temperature of 800°C for anode-supported solid oxide fuel cells. The critical operating time for carbon to form and be deposited on the anode was found to be strongly dependent on the amount of methane in the fuel mixture. For lower amounts of methane in the fuel mixture, such as 2.3 vol%, degradation of the cell performance was observed after 44 h. After accelerated carbon deposition process, due to an increase in the methane concentration of the gas mixture in use, the total critical operating time was significantly reduced. The approaches that allow early identification of carbon build-up in order to avoid irreversible deterioration of cell performance and its microstructure were originated and they are provided. A detailed microscopic investigation showed that carbon was uniformly formed over the Ni- and YSZ-sites. Massive carbon deposits not only caused the degradation of the cell performance, but also changed the Ni-YSZ structure. The crushing of YSZ-particles and their deposition on the Ni-catalyst were determined to be degradation phenomenona that occur if the carbon formation process in not interrupted at an early stage.