In Operando Mechanistic Studies of Heterogeneous Electrocatalysis on Solid Oxide Electrochemical Cell Materials
Eichhorn, Bryan W
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This dissertation details the development and utilization of in operando protocols for observing electrochemical reactions on solid oxide electrochemical cells (SOCs) in order to better understand the fundamental chemistry governing their operation. Two key reactions in SOC processes are studied using ambient pressure X-ray photoelectron spectroscopy (AP-XPS), the oxygen reduction and evolution reactions (ORR and OER). Measurements made on lanthanum strontium manganite (La1-xSrxMnO3±∂, LSM), a standard electrode material show that the surface composition does not match the bulk stoichiometry. Sr extrudes onto the LSM surface in the form of SrO and greater Mn reduction is observed. These phenomena are further augmented by application of a cathodic bias (promoting ORR), while an anodic bias (promoting OER) results in the oxidation of Mn and no significant changes in Sr segregation. Surface potentials on the LSM are measured to locate regions of electrochemical activity when promoting ORR and OER. These measurements yield in operando spectroscopic evidence that all electrochemical activity occurs at the electrode/electrolyte interface and that LSM is more electrocatalytically active toward ORR than OER. We further compare surfaces between a pure LSM material and a composite of LSM and yttria-stabilized zirconia ((ZrO2)1-2x(Y2O3)x, YSZ) in different gaseous environments which approximate standard operating conditions. The LSM/YSZ composite exhibits a larger concentration of surface oxygen vacancies in each environment allowing for greater oxygen reactivity. A method for measuring surface Co oxidation states with XPS is explored. In situ thermal redox studies on cathode material, lanthanum cobaltite (LaCoO3-∂), show a potential correlation between Co reduction and the Auger parameter. An in operando technique for monitoring SOCs with near infrared (NIR) imaging is presented. Ce oxidation states are tracked in an operating SOC using ceria (CeO2-x) electrodes in studies analogous with previous AP-XPS research. However, the NIR experiments take place in fully ambient conditions as opposed to the model, near ambient conditions used in the AP-XPS experiments. Ce reduction is observed within an electrochemically active region commensurate with that found with AP-XPS, simultaneously supporting the use of NIR imaging for in operando studies on these SOCs, and the model AP-XPS experiments previously conducted.