Materials and Structures for the Electrodes of Solid-State Electrochemical Energy Devices

Pandey, Shobhit A

doi:https://doi.org/10.21985/n2-36ba-3w65

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Materials and Structures for the Electrodes of Solid-State Electrochemical Energy Devices

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Electrochemical devices play a vital role in the efforts towards a sustainable green future. Solid acid based electrochemical cells, employing super protonic CsH2PO4 (CDP) as the electrolyte component, offer unique application advantages due to their operability at intermediate temperatures 250°C. At these temperatures, one can achieve improved reaction kinetics over cooler operating systems and advantages in cost, portability, and system complexity over warmer operating systems. This thesis focuses on both (i) improving the performance and stability of known solid acid fuel cells (SAFC) by applying thin films for cathode fabrication, and (ii) advanced synthesis, characterization, and stability of oxynitrides materials as potential catalysts in CDP based solid acid devices. These oxynitrides are then demonstrated as effective catalysts in proof-of-concept Solid Acid Electrolyzer cells (SAEC). Furthermore, SAECs complement SAFC by producing H2 from steam splitting at mid temperatures. First, in an application-focused study, Pt thin films by atomic layer deposition on CDP were applied for Solid Acid Fuel Cell (SAFC) cathodes. Low temperature ALD recipe using ozone at 150oC was optimized to coat Pt films on CDP, resulting in a growth rate of 0.09 ± 0.01 Pt wt%/cycle. Smooth, fully conformal films were obtained after 200 deposition cycles. ALD Pt@CDP based SAFC cathodes show good performance of ~0.5 V for a current density of 200 mA/cm2. The overpotentials were found to be relatively insensitive to the Pt loading, thereby minimum catalyst utilization. Excellent stability over 100hrs was observed. Next, several oxynitrides materials, both metallic and semiconducting, were synthesized via ammonolysis and evaluated for their stability with CDP for potential application in Solid acid devices. Out of 9 oxynitride systems pursued, two stable candidates: Tantalum and Molybdenum Oxynitrides, were evaluated in more detail. Controlled ammonolysis was performed to optimize recipes for phase pure products. Introduction of slight humidity during ammonolysis, termed as Wet ammonolysis, was demonstrated as an effective way for tuning the phase of final products. Specifically, wet ammonolysis was shown as a new route to prepare δ-phase molybdenum oxynitride from oxide precursor Subsequently, a comprehensive chemical and structural characterization study of all 4 phases of molybdenum oxynitrides was conducted. Nominally treated as nitrides in literature, we show that conventional ammonolysis of oxides, in fact, results in hydride incorporated Molybdenum oxynitride phases Mo1-x(N1-yOy)Hz. Combined synchrotron and neutron refinement informed by a gamut of chemical characterization techniques was used to determine the true chemistry, structure and overall chemical tunability of these molybdenum oxynitride phases Finally, a proof on concept application study of new SAEC devices was performed. Tantalum and Molybdenum oxynitrides from previous studies were demonstrated as promising oxygen evolution and hydrogen catalysts for steam splitting at 250oC. Several phases of these oxynitrides were evaluated. Ta-O-N based SAEC anodes resulted in phase insensitive yet promising OER performance of upto 2.5 mmol O2/cm2/h@1.5V, higher than reported Ir-RuO2 based PEM cells. Similarly, excellent HER performance and stability was observed using Mo-O-N based SAEC cathodes, with all phases giving, upto >10 mmoles of H2/hr/cm2 @0.75V. In addition, performance was found to be phase-sensitive, with hexagonal phase pushing the HER rate upto 15.8 mmoles of H2/hr/cm2.@0.75V

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