3 Technical Analysis

Although a stack ASR of -1 flcm2 is a reasonable value in the present international context, the value needs improvement to reduce stack volume/kW. The ASR of the individual cells of the standard production was 0.3-0.4 flcm2. Allowing for the resistance of the interconnect, a stack ASR of 0.4-0.5 £2cm2 may be realised. This result was actually met by one of the 10-cell substacks constituting the stack. Analysis of stack performance 121 showed, that the main performance problem was related to dimensional instability of the LSCV interconnect in reducing atmosphere, and the inability of the sealing glass to accommodate the resulting relative in-plane movement between cells and interconnects. Gas mixing obviously was a main problem, leading to a reduction of the active fuel cell area. Current interrupt measurements showed that for stack sections with less satisfactory performance an increased area specific resistance could be attributed to in-plane current conduction in the thin electrodes.

The performance analysis clearly shows, that the p02-related dimensional instability exhibited by the LSCV interconnect cannot be tolerated in high performance stacks. It is necessary to intensify the research for ceramic interconnect materials with much better dimensional stability in reducing atmosphere. Metallic interconnect is an alternative to ceramics, but reduced temperatures are required. The increase in electrode overpotential with decreasing temperatures, however, necessitates further electrode development in order to reach the ASR achieved at higher temperatures.

Few materials have been identified as potential candidates for metallic interconnect, and all have high chromium contents. Oxidation and volatilization of Q2O3 influences ASR and cathode performance adversely, necessitating further development. The presently most advanced metallic interconnect material uses dispersed yttria to reduce the oxidation rate M. Fabrication of Cr203 based alloys usually requires relatively expensive powder metallurgical processes and the presence of yttria boosts the cost of the complex shaped component further due to machining problems.

Cofiring - in the sense of simultaneous sintering of adjacent layers - is often mentioned as a means of reducing the number of sintering steps. Cofiring which includes sintering of the electrolyte membrane is difficult, because sintering temperatures much below 1300°C of YSZ will result in incomplete sintering; cofiring of the YSZ with NiO-based anodes at temperatures above 1300°C may affect the bend strength of the electrolyte adversely assumably due to NiO dissolution in the electrolyte layer, and cofiring with LSM based cathodes at temperatures even below 1200°C may result in the formation of zirconates. The latter prevents full densification of the electrolyte and will affect cathode performance adversely. Cofiring of Ni-based anodes and Cr based interconnect materials at relevant temperatures may also lead to undesired reactions. However, simultaneous firing of non-adjacent layers, e.g. anode and cathode sintering on the two sides of an already sintered electrolyte is a means for reduction of the overall number of sinterings.

Probably, a commercialized SOFC will be fuelled with natural gas. If todays standard anode, the Ni-YSZ-cermet, is exposed to natural gas, carbon precipitation will occur and the anode will be destroyed. Therefore, steam reforming is necessary. The economic calculations show that it is very important to avoid external reforming units which will add to the cost of the SOFC-system (balance of plant). As Ni is a good steam reforming catalyst, internal reforming is in principle possible. However, at 1000°C the endothermic reforming process will be too fast resulting in unacceptable steep temperature gradients at the fuel inlet. Consequently, it is necessary to develop anodes which are either more suitable for internal reforming than todays standard, or preferably able to facilitate direct oxidation of the methane in prereformed natural gas /8/. Prereforming of natural gas will always be necessary to remove hydrocarbons of two or more carbon atoms to avoid carbon deposition in the gas preheating tubes. Natural gas usually contains a few per cent (~5%-l0%) of C2's or higher hydrocarbons.

Materials

Materials

Labor & Maintenance 26,5%

Fig. 3. Assessed distribution of stack fabrication cost after planned optimizations

Labor & Maintenance 26,5%

Fig. 3. Assessed distribution of stack fabrication cost after planned optimizations

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