Application of Cesium Dihydrogen Phosphate in Intermediate Temperature Electrochemical Devices

Plymill, Austin Blake

doi:https://doi.org/10.21985/n2-m4xv-q271

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Application of Cesium Dihydrogen Phosphate in Intermediate Temperature Electrochemical Devices

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Electrochemical cell devices are increasingly being sought for energy conversion and storage applications due to their high efficiencies and their potential for operating free of greenhouse gas (GHG) emissions. Solid Acid Electrochemical Cells (SAECs), which most commonly employ CsH2PO4 (CDP) as the electrolyte component, are uniquely suited to meet the demands of these energy applications due to their operability at intermediate temperatures. Cesium dihydrogen phosphate displays high ionic conductivity at the moderate temperature of 250 °C and stability over a wide range of environments. This intermediate operating temperature gives SAECs advantages in reaction kinetics and fuel flexibility over cooler operating systems and advantages in cost, portability, and system complexity over warmer operating systems. This thesis primarily explores (i) electrocatalysis of the hydrogen oxidation reaction (HOR) in existing and novel catalyst systems, showing that SAECs are highly effective for hydrogen extraction from ammonia, and (ii) the technoeconomic feasibility of using SAECs for ammonia synthesis. First, a systematic analysis of HOR on composite electrodes in CDP-based SAECs is performed. Evaluations of HOR on the conventional catalysts, Pt and Pd, are performed in addition to analysis of the more novel Ru, Pt-Pd-Ru alloys, and metal phosphide systems. The three monometallic systems display comparable HOR activity at low oxidation potentials, while Ru is poisoned at high potentials. Introduction of small amounts of Pt and/or Pd to Ru eliminates the poisoning effect. Nickel-based phosphide catalysts show promise as active earth abundant HOR catalysts. Next, ammonia electro-decomposition is explored. Use of either Pt or Ru alone as NH3 electrooxidation catalysts results in poor H2 production rates. However, integration of a Ru-based reforming catalyst layer with a Pt-based H2 electrocatalyst layer in a hybrid cathode structure results in the production of high-purity hydrogen at an unprecedented rate, opening the potential for using ammonia as a hydrogen carrier. Finally, a technoeconomic analysis of using SAECs for ammonia synthesis is performed. This analysis shows that by leveraging cheap electricity, modular design, and distributed locations, SAECs can be market competitive with existing and possible future technologies for ammonia synthesis.

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