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Title: Eletrodeposited nickel anodes for solid oxide fuel cells
Author: Jamil, Zadariana
ISNI:       0000 0004 7657 8407
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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Solid oxide fuel cells (SOFCs) are among the most efficient and environmentally-friendly technologies available for generating electricity from hydrogen, natural gas and other renewable fuels. A highly efficient SOFC electrode needs to be ionically and electronically conductive with a sufficient open porosity, allowing the transportation of reactants and product to and from the reaction sites. This can be achieved by carefully selecting the electrode materials and fabrication methods. Conventionally, the state of the art electrode, Ni/YSZ, is fabricated using powder mixing or impregnation methods. However, the powder mixing requires a high processing temperature that is not suitable for low melting point metals and limits the control of electrode microstructure and introduces undesirable defects due to thermal expansion mismatch of the elements. Although, impregnation can possibly overcome these problems, it requires multiple cycles of impregnation and calcination to achieve a sufficient amount of metal in the electrodes. Hence, it is time- and energy-consuming and unsuitable for mass production. In this study, a novel fabrication method using electroless and electrodeposition of Ni/Ag/GDC for SOFC anodes is proposed. First, a pre-sintered porous Ce(0.9)Gd(0.1)O(2-x) (GDC) scaffold on a YSZ electrolyte was metallized with silver using Tollens' reaction, followed by electrodeposition of nickel from a Watt's bath. The aims of this study are to (i) provide an alternative metallizing technique to impregnation; (ii) understand and optimise the electroless and electrodeposition onto Ag substrates; and (iii) characterise the electrochemical performance of the electrodeposited electrodes and their microstructures. The electroless and electrodeposition method has shown potential for mass production as it offers simpler and much faster metallising process, approximately 7 times, than impregnation at lower temperature (< 100 °C). It was observed that the electrodeposited Ni is well distributed across the GDC scaffolds. At the same time, this method allows the control of porosity and microstructure of the electrodes. Hence, the electrodes can be designed with desired characteristics to achieve high performance. A systematic cyclic voltammetry study of nickel electrodeposition from a Watts bath on silver foils was carried out to understand the influence of operating conditions on the electrodeposition process. It can be concluded that suitable operating conditions for nickel electrodeposition into porous Ag/GDC scaffolds and catalytic membranes are: 1.1 M Ni2+ concentration in Watts bath; deposition potential between -0.65 to - 1.0 V vs. Ag/AgCl; a temperature at 55 °C; sodium dodecyl sulphate (SDS) as the surfactant; pH 4.0 ± 0.2 and an agitation rate of 500 rpm. Pulse and continuous electrodeposition modes allow nickel to be deposited throughout porous Ag/GDC scaffolds, however the pulse electrodeposition mode is favoured as it results in an even Ni distribution within the porous scaffolds at minimum H2 pitting. The electronic conductivity of electrodeposited Ni/Ag/GDC electrodes was good even at relatively low Ni content (3.5 vol%). The electrochemical performance of the SOFC anodes was measured in both symmetrical and fuel cell configurations. The lowest area specific resistance (ASR) of the electrodeposited Ni/Ag/GDC anodes was 0.69 [ohm].cm-2 at 750 °C in humidified 97 vol% H2. This value is comparable to the ASR values of Ni/GDC anodes fabricated using conventional powder mixing and co-precipitation (operated at 600-850 °C), however with 5 to 6 times lower Ni content. It was also shown that the performance of the electrodeposited anodes was independent to the range of Ni content (5-18 vol%) studied, suggesting that the electrochemical reactions primarily occur at double phase boundaries due to mixed ionic and electronic conductivity properties of the GDC. This is also supported by the short term study, the performance did not change although Ni agglomerated and coarsened. Moreover, the electrodeposited electrodes were successfully integrated into a fuel cell and operated in both H2 and syngas. The microstructures of these anodes were analysed using scanning electron microscopy (SEM), focused ion beam - SEM (FIB - SEM) and energy dispersive x-ray spectroscopy (EDX). Nano-particles of Ni formed in the porous GDC scaffolds provided triple phase boundaries (TPB). The 3D imaging and quantification analysis for an electrodeposited electrode sample were studied and then compared with the results obtained from the experimental chemisorption method. This provides a brief overview of the possibility of using experimental chemisorption method as an alternative for microstructural analysis.
Supervisor: Brandon, Nigel P. Sponsor: Kementerian Pengajian Tinggi, Malaysia
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral