Access Type

Open Access Embargo

Date of Award

January 2019

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemical Engineering and Materials Science

First Advisor

Eranda . Nikolla

Abstract

The work reported in this thesis focused on providing a fundamental understanding, at the molecular level, required to guide the effective design of electrocatalysts towards a superior reaction kinetics on the electrode of solid oxide electrolysis cells SOECs devices (SOECs) during the CO2 electrolysis. High temperature SOECs are electrochemical energy conversion technologies, that have emerged as promising alternatives to mitigate environmental issues associated with combustion-based technologies, such as the rising atmospheric CO2 levels. The conversion of excess CO2 into high-energy molecules, such as CO can be efficiently achieved through the use of SOEC – which facilitates the electrochemical reduction of CO2 to CO using energy from renewable sources. In addition to CO2 reduction, co-electrolysis with H2O in SOECs is also a possibility, facilitating the generation of syngas (CO + H2), a valuable precursor for the synthesis of synthetic fuels.However, the main issue associated with this process is the large overpotential (i.e., surplus applied potential) required to allow the electrochemical transformations in the SOEC during the CO2 reduction.

To tackle this problem, a combination of theoretical and experimental approaches is used, aiming to obtain insights on the underlying factors governing the electrode reactions associated with the electrochemical CO2 reduction process (i.e., CO2 reduction in the cathode and oxygen evolution (OER) in the anode). Theory-guided structural/compositional-performance relationships were developed and validated, fostering the development of electrocatalysts that can lower the energy barrier associated with the relevant electrode reactions and reducing the overall overpotential losses during electrochemical reduction of CO2 in SOECs.

The OER is governed by surface oxygen exchange process, and as such, an understanding about the processes governing this reaction was first obtained. Using La2NiO4 as model electrocatalysts and through a combination of well-controlled synthesis method, detailed surface characterization and effecting thermochemical and electrochemical testing, it was possible to bridge the gap between electrocatalysts formulation and the calculated energetics associated with oxygen catalytic processes on their surface. The studies showed that electro/chemical oxygen exchange is surface structure sensitive on this type oxides, with the B-site terminated surfaces being the most active for oxygen electrocatalysis. Furthermore, it was demonstrated that nanostructured Co-doped La2NiO4 is a highly active electrocatalysts for oxygen exchange, owing to its best compromise between the energetics associated with O2 dissociation and surface oxygen vacancy formation, which can be predicted by the binding energy of O2 on a surface oxygen vacancy.

Finally, we experimentally demonstrated that O2 binding energy is also a good activity descriptor for the electrochemical CO2 reduction on SOECs. The electrochemical activity of Ni, Pd and Fe were assessed, revealing that Fe possess a higher electrochemical activity and selectivity towards CO2 reduction. This is owing to the stronger oxyphilic nature of Fe as compared to Ni and Pd. However, electrochemical stability studies demonstrated that long term stability issues, as consequence of the surface oxidation of Fe, emerges as side effect of the oxyphilic nature of Fe. Potential approaches to alleviate this issue are proposed

Available for download on Wednesday, January 12, 2022

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