Reimagining Solid Acid Fuel Cells: From Electrolyte Discovery to Cathode Design

Wang, Louis Shen

doi:https://doi.org/10.21985/n2-wcf4-ww59

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Reimagining Solid Acid Fuel Cells: From Electrolyte Discovery to Cathode Design

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Solid acid fuel cells confer unique advantages over nearby technologies, such as polymer electrolyte membrane fuel cells (PEMFCs) or solid oxide fuel cells (SOFCs), due to the solid acid electrolyte – a solid-state, anhydrous, intermediate-temperature proton conductor.Despite these encouraging unique properties, solid acid fuel cells have performed unfavorably in comparison to the aforementioned technologies both due to constraints specific to the electrolyte as well as the electrochemical kinetics at the cathode. In the first chapter, a review is conducted of the properties of solid acid electrolytes including the superprotonic transition and the related structures, proton transport properties, and degradation behavior. Focus is placed on cesium dihydrogen phosphate (CDP), as this solid acid compound is currently the only technologically relevant electrolyte. Additionally, the foundational methods for characterizing solid acid electrolytes – x-ray diffraction, differential scanning calorimetry and thermogravimetric analysis, and electrochemical impedance spectroscopy – are discussed. In Chapters 2 and 3, a new approach to the modification of solid acid phases is demonstrated with substantial impact on the phase behavior and structure of the materials. By introducing off-stoichiometry in CDP, we discovered a new remarkable superprotonic phase and revealed that the superprotonic phase of CDP is highly amenable to non-stoichiometry. The new superprotonic compound Cs7(H4PO4)(H2PO4)8, or CPP features extraordinary H4PO4+ cations on select Cs sites of a structure that otherwise resembles cubic superprotonic CDP. CPP was found to be stable in dry Ar atmospheres from 90- 151 °C, but the material’s conductivity is only moderate in comparison to that of CDP. In the composition space between CDP and CPP, the cubic superprotonic phase of CDP was found to accommodate cesium deficiency in the form of Cs vacancies which were charge balanced by excess protons. The non-stoichiometric cubic phase, α-CDP(ss), displayed eutectoid phase behavior, forming at 155 °C at its eutectoid composition, x = 0.18. The proton conductivity of the α-CDP(ss) phase was found to be relatively insensitive to composition; when coupled with the eutectoid phase behavior, this result presents the opportunity to extend the lower operating temperature limit of CDP-based devices. More generally, the non-stoichiometry demonstrated here in CDP presents a powerful new approach to the modification of solid acids. The later half of the thesis is devoted to solid acid electrochemical devices, with a focus on fuel cells. Chapter 4 provides a review of solid acid devices (SADs) and highlights the progress made thus far in SAFCs. Additionally, the fundamental principles underlying fuel cell operation and the important electrochemical techniques for characterizing SAFCs are summarized. In Chapter 5, the limitations of the SAFC cathode are examined through the lens of a 1-D model. Key to the modeling approach taken here was the experimental measurement of parameters characterizing the cathode, including the measurement of electrochemical kinetic parameters for the oxygen reduction reaction on Pt nanoparticle catalysts. The measurements crucially revealed that the primary factor hampering SAFC performance compared to that of PEMFCs is a low cathodic charge transfer coefficient, and that this factor more than negates the effect of thermal enhancement of the exchange current density. The 1-D model was used to examine the internal mechanisms of the cathode and used to evaluate the impacts of various advances in materials properties and changes in cathode microstructure. In addition to the critical importance of the charge transfer coefficient, it was found that significant improvements to cell performance could be achieved by discovering a solid acid electrolyte with reduced humidification requirements.

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