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Title: Tape casting of ceramic GDC/YSZ bi-layer electrolyte supports for high temperature co-electrolysis
Author: Soleimany Mehranjani, Alireza
ISNI:       0000 0004 6347 9469
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2017
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High temperature co-electrolysis of carbon dioxide and steam may provide an efficient, cost effective, and environmentally friendly production of syngas from curtailed wind energy. To achieve cost competitive high performance (e.g. with minimum internal resistance) electrolysis cells, it is critical to develop materials and cell configuration optimal for coelectrolysis. In addition, a cost-effective fabrication procedure is important in allowing broader commercialisation of Solid Oxide Electrolysis Cells (SOECs). The initial part of this work emphasises on the feasibility of SOECs plant for converting curtailed wind energy to syngas to enhance the grid flexibility. We first obtained operating parameters for the conversion plant based on the most recent literature data on the performance of high temperature co-electrolysis for syngas production. In addition, an evaluation of the interaction between variable generation and typical electricity demand patterns was presented; and, limitations in the flexibility of traditional electric generators were considered. Furthermore, in a projection of wind generation made for 2020, we estimated the maximum power value of the curtailment wind profile to be 23.9 GW. It was remarked that the cost increase for constraining wind in future could make SOEC conversion technology more commercially attractive. An estimation of the total investment costs for grid connected electrolysis system was made by considering the share of operating cost. The share of electricity price in the total cost of syngas production was estimated to be 61%. It was shown that using cost effective electricity could significantly reduce the syngas production price. The total investment costs for grid connected electrolysers were projected to be 0.38 M£/MW in 2020. It was highlighted that the scope of electrochemical conversion of CO2 to fuel offers flexible demand that is not yet sufficiently understood. There are still technical barriers that need to be addressed in the field of manufacturing processes, grid integration and system operation. A key factor in operating solid oxide electrolysis cells (SOECs) is the ability to provide a sufficiently high level of oxide ion conduction through the electrolyte in the cell. Commonly, high performance cells use Y-stabilised ZrO2 (YSZ) or Gd-doped CeO2 (GDC10). Whilst GDC10 has higher oxide ion conductivity than YSZ, it suffers from electronic conduction due to the partial reduction of Ce4+ to Ce3+ during operation at high temperature and low oxygen partial pressure environment. Here we describe the fabrication of a bilayer GDC10/8YSZ electrolyte support using tape casting and single step co-sintering. A cost effective fabrication procedure is important in allowing broader commercialisation of Solid Oxide Cells for fuel cell/electrolysis applications. A bi-layer 8YSZ/GDC electrolyte is suggested as an effective solution to avoid ceria reduction in a fuel (reducing) environment, thereby preventing current leakage across the electrolyte, while maintaining high oxide ion conduction. Bilayer zirconia/ceria processing has proven problematic due to thermochemical instability at high sintering temperatures. We first prepared and optimised the slip formulations for tape casting process, this was necessary to achieve high green density and uniform tapes. Furthermore, the shrinkage profile of the two bulk materials in bilayer electrolyte were matched using a Fe2O3 sintering additive. Additions of 5 mol% of Fe2O3 in the GDC layer and 2 mol% of Fe2O3 in the YSZ layer prevents delamination during co-sintering. The addition of Fe2O3 promotes densification behaviour, enabling achievement of a dense bilayer (~90%) at a reduced sintering temperature of 1300 °C; ~ 150 °C below conventional sintering temperatures. Bilayer 8YSZ/GDC10 electrolytes with relative thickness of 73/154 μm was successfully fabricated by tape casting and low-temperature co-sintering at 1300 °C. No significant microstructural defects or delamination were observed after co-firing The effect of the Fe2O3 sintering aid on the crystal structure of two bulk materials used in bilayer electrolyte were investigated by X-ray diffraction. Results showed that, both materials with Fe2O3 additions maintain their fluorite structure. The analysis revealed a reduction in unit cell volume for both Fe2O3-doped samples. While using Fe2O3 sintering aid was found to improve the sinterability of the two bulk materials, increasing the dopant concentration above the solubility limit leads to the formation of an iron rich phase, which was subsequently analysed by energy-dispersive X-ray spectroscopy. Elemental analysis at YSZ/GDC interface revealed asymmetric elemental diffusion behaviour when using Fe2O3 to co-sinter YSZ/GDC bilayers, with lower diffusivity of Zr and Y ions in the GDC layer compared to that of Ce and Gd ions detected in the YSZ layer, showing the positive effect of Fe2O3 on limiting the interdiffusion behaviour. Electrochemical impedance measurements in air revealed the total conductivity of the Fe2O3 containing bilayer electrolytes increased by an order of magnitude compared to Fe2O3-free bilayers. This was attributed to two factors; first, by limiting the overall elemental interdiffusion length from ~15 to ~5μm and, second, by achieving better contact between the YSZ and GDC layers and higher sintered density when using a Fe2O3 additive as a sintering aid. The cost-effective low-temperature processing technique presented in this study is expected to help widen the material selection and resolve the thermochemical issues associated with high-temperature co-sintering allowing a broader commercialisation of SOECs.
Supervisor: Rachael, Rothman Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available