Development of Alternative SOFCs for Higher Fuel Utilization

This PhD thesis provided detailed insight into application of alternative SOFC anode materials. It confirmed that conventional Ni-based anodes can be replaced by alternative materials without compromising the overall cell performance, and thus achieving significantly higher stability under degradation-inducing conditions, such as for instance high fuel utilization. High operating temperature of SOFCs bring an advantageous benefit of high electrical efficiency, thus enabling to convert various fuels efficiently into electricity in a highly-efficient manner. Nevertheless, the fuel utilization, which is one of the most relevant parameters to determine the cell efficiency, must be kept under critical value in practical systems in order to avoid the performance degradation by Ni reoxidation on the anode side. For realizing the advantage of SOFCs associated with potential for high electrical efficiency, the increase of the fuel utilization must be taken into account to improve the system efficiency.

For this purpose, the research performed within this work was primarily motivated to develop alternative SOFC anodes for higher fuel utilization, on which, to date, only few studies have focused. Alternative materials were developed and investigated for their applicability in anodes with an increased stability. Based on thermochemical calculations, three noble metals thus including Rh, Pt and Pd were chosen for alternative impregnated catalysts among several candidates (see Appendix for details). The SOFC performance achieved employing these materials were analyzed by means of galvanostatic voltage measurement, polarization curves and overall losses, as well as by microstructural analysis. In addition, thermochemical calculations and system simulations were carried out.

Firstly, a design of an alternative oxide-based SOFC anode was developed using electron-conducting LST and mixed ionic-electronic conducting GDC. Ni catalyst nanoparticles were dispersed onto the backbones via impregnation. The Ni-impregnated anodes with the LST-GDC backbones showed better redox stability compared to the conventional Ni-ScSZ anodes, due to the redox-stable oxide conducting backbones. In addition, co-impregnation of GDC with Ni nanoparticles further suppresses anode IR losses and non-ohmic overvoltage to achieve better durability and performance, due to more finely dispersed co-loaded GDC nanoparticles. The Ni-GDC co-impregnated anodes could exhibit comparable I-V performance to conventional Ni-ScSZ anodes (approx. 1.0 V at 0.2 A/cm² with 3%-humidified H2). However, undesired performance degradation was observed during the high fuel utilization durability tests with highly-humidified H2 fuel, because of the Ni oxidation occurrence. In summary, whilst the

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electrochemical properties at very high fuel utilization must be further improved, the alternative anodes with the Ni impregnation showed promising characteristics including comparable I-V performance and better redox stability as follows:

1. Utilization of oxide conducting backbones (LST and GDC in this research) achieved better redox stability (degradation rate for the Ni-GDC co-impregnated anodes: 0.36%/10 cycles) than the conventional Ni-based anodes (degradation rate for the Ni-ScSZ anode: 5.3%/10 cycles).

2. Catalyst impregnation on the oxide electrode backbones enabled the catalyst particles dispersed leading to improvement of the I-V performance.

For further improvement of the electrochemical and microstructural stability at high fuel utilization, the noble-metal impregnated anodes were developed with oxide-based conducting SOFC anode backbones. Noble metal (Rh, Pt, or Pd) catalyst nanoparticles were dispersed onto these backbones via co-impregnation with GDC. These noble-metal co-impregnated anodes not only exhibited comparable I-V performance to the conventional Ni-cermet anodes, but also had better tolerance against oxidation (i.e.

under high fuel utilization conditions) for 1000 hours, compared to the alternative anodes with the Ni impregnation. In summary, whilst there are still issues to be solved such as catalyst particle agglomeration, additional costs by noble metals, and further improvement of electrochemical performance (in particular under highly-humidified H2

fuel), these noble-metal co-impregnated anodes with stable backbone structure showed promising and innovative characteristics for high fuel utilization operation of SOFC systems as follows:

1. Alternative noble metal catalysts (Rh, Pt, or Pd in this research) were considered to be stable at high fuel utilization operation by applying the thermochemical calculations.

2. Noble metal catalyst impregnation on the oxide backbones enabled SOFCs operable at very high fuel utilization.

Besides novel materials development and analysis for SOFCs, their applicability for steam-electrolysis, considering very high steam/hydrogen ratios, was additionally examined. In this sense, they were employed as materials for fuel electrode of high-temperature SOEC electrolysers. The SOEC characteristics of the fuel electrodes using the LST-GDC backbones and the impregnated metal catalysts (Ni or Rh), were investigated and compared with the conventional based electrodes. Whilst Ni-reoxidation was observed both in the conventional Ni-based electrodes and the Ni-GDC

§8 CONCLUSIONS & OUTLOOK

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co-impregnated electrodes after the 80-hour SOEC durability tests, the noble-metal (Rh) co-impregnated electrodes remained its Rh metallic phase catalyst unchanged, which resulted in the sufficiently stable operation. Although further research and development are needed in order to approve their long-term durability and enable their commercial applications, the alternative electrodes with stable backbone structure showed promising characteristics for reversible SOFC/SOEC systems as follows:

1. Utilization of oxide conducting backbones (LST and GDC in this research) improved the problem of the irreversible microstructure degradation caused by Ni oxidation on the conventional Ni-based anodes.

2. Impregnated noble metal (Rh) particles remained its metal-phase catalyst after the SOEC degradation tests, whilst impregnated Ni particles were oxidized.

For consistent study from performance evaluation using alternative materials to cell efficiency quantification, a simulation study was performed using the alternative SOFC anodes, developed in this research. The resulting performance with the conventional Ni-ScSZ anodes showed higher I-V performance than that with the alternative anodes at the similar value of fuel utilization, whilst the former cell is inoperable at very high fuel utilization. In comparison of the cell efficiency between the two models, the efficiency with the alternative anodes at Uf ≅ 95.0% could exceed the one with the conventional anodes at Uf ≅ 75.0% by 2.1%. Considering the cost increase by utilization of the noble metal catalysts, the value is $2.2 to $27 per one 700 W SOFC system, which is not such a large proportion of the total price of the systems. Although further development is desirable, the alternative anodes have potential to increase the fuel utilization, thus enabling higher electrical efficiency in the future.

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