Development of Cell-Protecting Methods for Carbon-Removal from Porous Ni-YSZ Anodes and Regeneration of the Cell Performance
Authors: Subotic Vanja , Stoeckl Bernhard , Schluckner Christoph , Pofahl Stefan , Lawlor Vincent , Schroettner Hartmuth , Hochenauer Christoph 
Published: February 2020

Solid oxide fuel cells are highly efficient units, which directly convert the chemical energy of gaseous fuels into electrical energy without additional conversion steps. The overall efficiency can be increased by using heat losses occurring during the operation at high temperatures between 600–1000°C. SOFCs further offer a great fuel flexibility and compared to other fuel cell types they can internally reform hydrocarbons. Internal conversion of carbon-containing fuels over nickel as the most widely used catalyst tends to promote carbonaceous deposits on the anode surface as well as inside the anode, causing deactivation of the fuel cells. Nevertheless, undisturbed operation with conventional fuels and early detection of the cell degradation are highly important for the commercial usage of solid oxide fuel cells in industry, such as in auxiliary power units or especially for mobile application. Formation and deposition of carbonaceous species on the anode surface of solid oxide fuel cells may be prevented under specific operating conditions such as high steam/carbon ratio or high current density. The increasing amount of steam in fuel can chemically remove carbon adsorbed on the anode surface thus reducing the total amount of carbon deposits. However, this would significantly dilute the fuel thus reducing the overall power and the cell efficiency. Further possibility to avoid carbon formation is to operate the cells under load higher than the critical current density. However, increase of the steam amount or variation of the cell load is neither realistic nor advantageous for the commercial purpose. Without the need for the variation of the operating conditions, such as higher amount of steam or higher load, early detection of degradation phenomenon and applying of appropriate cell-protecting regeneration strategies can improve the operating reliability of the SOFC systems. In order to prolong the cell’s lifetime and to accelerate the market access of SOFCs, diverse approaches for carbon removal in a cell-protecting manner and regeneration of the cell performance were developed and detailed examined. They are outlined within this study. The practicability of the developed methods for industrial application was also considered.
The regeneration strategies are based on application of: (1) gaseous components, for the gasification of carbon deposits, and (2) electrochemical methods, which remove carbon during the cell loading. Operation under gaseous components as hydrogen, water vapour or oxygen-enriched gas mixtures allowed complete carbon removal. Nevertheless, under operation in a fuel cell mode, only the approach that combined hydrogen and water vapour removed carbon from the anode while also protecting it from further undesired degradation. It is important to note that very high amount of water can deteriorate the cell performance and even provoke unwanted nickel oxidation, which thus must be considered. However, the allowed water vapor amount must thus me determined. Furthermore, feeding the anode with a very low amount of oxygen in a wide temperature range showed high carbon removal rates but resulted in further deterioration of the cell performance, and eventually led to mechanical degradation. Both methods for carbon removal in a cell-protecting manner and approaches that induced further microstructure degradation are presented within this study.