Nanoscale Surface Engineering of Solid Oxide Electrochemical Cell Electrodes for Stability Enhancement, Characterization, and Reactivation

Schmauss, Travis Anthony

doi:https://doi.org/10.21985/n2-fwdw-5026

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Nanoscale Surface Engineering of Solid Oxide Electrochemical Cell Electrodes for Stability Enhancement, Characterization, and Reactivation

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Solid oxide fuel and electrolysis cells (SOFCs and SOECs) must be engineered with the entire lifetime of their performance in mind. Electrochemical activity will decrease as degradative processes take effect, leading to higher overpotentials and decreased power outputs. Materials science and engineering can stave off these inefficiencies through an understanding of degradative mechanisms while allowing the development of strategies to overcome, reverse, or mitigate them. Degradative processes occur most acutely in the electrodes of the cells, both in the oxygen and the fuel electrode; this dissertation explores both. For the former, atomic layer deposition (ALD) is investigated as a strategy to abate the degradation associated with the coarsening of nanoparticle-infiltrated systems, electrode architectures that have very high initial performance for the oxygen reduction reaction, but which, due to the mobility of the surface particles at the operating temperatures of SOFCs, rapidly reduce their catalytic area. The system under consideration is ALD-ZrO2 atop the perovskite Sr0.5Sm0.5CoO3-δ infiltrated into Gd-doped ceria. ALD coverage is found to be highly prone to deposition gradients based on the complexity of electrode microstructure: higher aspect ratios lead to increased rates of impingement on electrode surfaces, each collision a chance for thermal decomposition of the organometallic precursor. The understanding borne of the limitations of ALD then leads to the development of a method that leverages a truly ideal ALD precursor, trimethylaluminum, which forms conformal Al2O3 even through microstructurally complex infiltrated electrodes. In this case, however, Al2O3 is electrochemically inert, and its function is instead in serving as a mold, cast around electrode surface features, one that can then be digested and quantified using acids and plasma spectroscopy to back-calculate the exact surface area covered. This technique fits a niche for measuring the surface areas of relatively low (~cm2) absolute surface area sintered electrodes, useful for solid oxide electrode development and possibly for other heterogenous catalysts. Attention switches to the fuel electrode, in which a promising class of electrode based on the perovskite structure, but with reducible cations, forms socketed surface metal particles upon exposure to fuel environments. The systems explored are the (Ni,Fe) and (Ru,Fe) alloy forming Fe, Ni, and Ru-doped strontium titanate perovskites (STFN and STFR). One principle benefit of an all-oxide-supported fuel electrode is redox stability; this is investigated by redox-cycling STFN after long periods of time (250 h) in reducing atmosphere, in which performance degrades due to phase decomposition of the perovskite to a Ruddlesden-Popper phase as Fe concomitantly reduces, further enriching the (Ni,Fe) exsolved particles. After redox cycling, performance is regained, but Ni does not fully re-incorporate into the perovskite lattice. Upon subsequent re-reduction, the existing NiO phase left behind acts as a nucleation site for metal reduction, and the areal density of exsolved particles decreases while average particle size increases. While redox cycling cannot completely reverse decomposition, it can completely burn out solid carbon deposited via operation in C-based fuel environment; this effect is demonstrated with STFR benchmarked against a conventional cermet anode. Finally, an analytic study is presented that focuses on SOFCs in combination with carbon capture in the context of vehicles. Given the unique ability of the SOFC to output a concentrated stream of CO2 when operating on energy-dense, C-based fuels, the combination of SOFCs with onboard carbon capture is appealing for the decarbonization of difficult-to-abate vehicles. The primary advantage of the combination is the ability to serve large amounts of energy dense fuels with a relatively small electrochemical conversion device, therefore the best fit is for extremely large classes of vehicles like maritime merchant ships which require extreme amounts of onboard energy.

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