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Sulphur, Carbon, and re-Oxidation Tolerant Anodes and Anode Supports for Solid Oxide Fuel Cells

Final Report Summary - SCOTAS-SOFC (Sulphur, Carbon, and re-Oxidation Tolerant Anodes and Anode Supports for Solid Oxide Fuel Cells)

Executive Summary:
The solid oxide fuel cell (SOFC) is a clean and efficient technology for generation of power, heat and/or cooling. When substituting conventional generating technology, SOFC offers significant reduction in the emissions of CO2, NOX and particulate matter. To make the technology more mature for the market, system efficiency lifetime are important parameters. Today system introduced failures are one of the reasons for insufficient lifetime. For small, a few kW residential combined heat and power supply (m-CHP) three failure mechanisms have been identified, namely loss of fuel, breakdown of the desulfurizer and reformer, which degrade the fuel electrode (anode), when using a state-of-the art nickel cermet. Those failures have to date been addressed at the system level.
The project aims to demonstrate a new type of solid oxide fuel cell (SOFC), based on a ceramic strontium titanate oxide (ST), which addresses the three major failure mechanisms: sulphur tolerance, re-oxidation (RedOx) stability and coking. In this project, the integration of the most promising ST materials into existing cell designs, electrolyte supported and anode supported cells of 100 cm2 and 144 cm2 size, respectively, have been pursued.
At the early stages of the project, three materials compositions were defined as candidate anode materials, based on suitable electrical conductivity, chemical stability and processing characteristics: A-site modified strontium titanates (lanthanum and calcium substituted strontium titanate (LSCT) and an yttria substituted strontium titanate (SYT)) and one B-site, Niobium modified strontium titanate (STN)). Several kg of powders for the processing development have been produced for each of these materials. Investigations of the electrical and mechanical properties of the porous anode supports revealed that insufficient processing quality control had a clear impact, limiting the mechanical robustness of full anode supported cells at sizes exceeding 25 cm2. Thus, priority was given to electrolyte supported cells up to 100 cm2, using the various ST materals and with different infiltrated electro catalysts, such as nickel and/or ruthenium together with ceria or gadolinium modified ceria (CGO).
The initial testing showed promising initial performances, but severe degradation over several hundreds of hours. The latter could clearly be attributed to a decrease of the electrochemical activity of the anode. Stable cell performance over several hundreds of hours of operation could be achieved by using Ni/CGO or Ru/CGO as electro catalysts. Ni/CGO and Ru/CGO electro catalyst combinations were then infiltrated into the most promising titanate material, LSCT, and electrolyte supported cells of 100 cm2 cells tested for tolerance to sulphur exposure and loss of fuel in 5-cell short stacks operated on pre-reformed pipeline natural gas. Upon sulphur exposure (8 ppm), both types of electrodes showed an initial decrease in performance, which then stabilized for Ru/CGO infiltrated cells. Therefore, at this stage, the cell concept cannot be evaluated as superior to state-of the-art solid oxide fuel cells in terms of sulphur tolerance. Fuel shut down and subsequent re-oxidation of the fuel electrode, both with and without cooling down the stack temperature, did not affect the cell or stack performance. Thus, the cell concept is proven to be tolerant towards RedOx cycles. The RedOx tolerance was finally confirmed in a system test, using a 60 cell stack operated by pipeline natural gas. The resulting power output (AC) was 800 W with an electrical system efficiency of 21 %. This, to the knowledge of the project partners, is the first demonstration of a ceramic anode based cell in a real solid oxide fuel cell system and proves the feasibility of the concept.
Project Context and Objectives:
The solid oxide fuel cell (SOFC) is a clean and efficient technology for generation of power, heat and/or cooling. When substituting conventional generating technology, SOFC offers significant reduction in the emissions of CO2, NOX and particulate matter. While SOFC technology is approaching state-of-the-art, there are still important challenges which must be overcome to successfully commercialise the technology. For small, a few kW residential m-CHP applications three failure mechanisms have been identified, namely loss of fuel, breakdown of the desulphurizer and reformer, which impact the fuel electrode (anode). In order to make the SOFC cells more tolerant and durable, sulphur and coking tolerant and redox stable cells are required, which are not available based on the state of the art Nickel cermet anodes. An alternative to Ni, resulting from multiple years of European research is a ceramic oxide namely modified strontium titanate. However, its application in real cells and stacks was not performed. Successful demonstration of such a new full ceramic SOFC, with superior fuel electrode (anode) robustness towards the above mentioned failures would enable system simplification, which is particularly relevant for small systems, e.g. combined heat and power (CHP). Furthermore, replacing carcinogenic nickel and nickeloxide in the fabrication is an important aspect when production in Europe is considered.
The project aims to demonstrate a new type of full ceramic solid oxide fuel cell (SOFC), which addresses three major failure mechanisms that, to date, have to be addressed at the system level: sulphur tolerance, redox stability and coking. The project addresses both the cell demonstration, as well as critical issues related to the operation of micro CHP FCs, namely Start Up/Shut down(redox stability, C tolerance required) and Grid outage/system failures(redox, sulphur, C-tolerance required). Simplifying the system reduces statistically based failures and thus increases lifetime and decreases costs. In this way, the project aims to contribute to the following targets in the application area of stationary fuel cells: 45% electrical efficiency, 80% CHP efficiency and increased fuel flexibility. Lifetime targets (40000 h lifetime) and cost competitiveness are not major objectives, but will be considered in the final assessment.

Further information can be found in the attachement.


Project Results:
The scientific-technical results of the project are covering manly three areas:
• Materials properties relevant for the evaluation of the materials, here electrical conductivity and mechanical stability are of most importance
• The development of prototype cells, in particular the upscaling of the cell dimensions
• Application relevant testing, which was performed in short stack configuration using natural gas.
• The performance of protoype cells in a real 1 kW system environment
The main results and achievements are in the following presented along these lines in the attachment.

Potential Impact:
The overall evaluation of the project results proves the feasibility of a novel electrode concept based on a ceramic backbone and using infiltrated nano-particles as electrocatalysts. The cells are re-oxidation tolerant and thus can withstand typical fuel cut offs in micro-CHP systems. Based on the promising stability towards re-oxidation, the ceramic anode is considered particularly suited for small scale applications, e.g. below 1 kW. In this context small systems operating on other hydrocarbon based fuels e.g. Methanol and Ethanol or DME become interesting applications, e.g. for remote power. However, there is still some R&D necessary so that the anodes can compete with the state-of-the-art Ni-cermet anodes.

More information on the further use and exploitation of the project results can be found in the attachment.
List of Websites:
A brief project description including the Deliverables classified “public” can be found on the project website:
www.scotas-sofc.eu

Relevant contact details can be found in the attachement.