Architecture of electrodes for electrochemical CO2 reduction to syngas and C2+ products

Abstract

The atmospheric CO$_2$ has been increased as a result of anthropogenic emission from industries and fossil fuel power plants, which causes climate changes. The electrochemical CO$_2$ reduction reaction (CO$_2$RR) converts CO$_2$ to value-added chemicals and fuels in the room temperature and atmospheric pressure. Moreover, CO$_2$RR using renewable electricity represents a perspective long-term solution due to the required operation conditions. The syngas and C$_2$$^+$ products are attractive chemicals because of their large global market size and high impact on CO$_2$ emissions reduction. However, electrochemical CO$_2$RR in the liquid-phase CO$_2$ system had showed the low current densities because of low solubility of CO$_2$ in the aqueous electrolytes and high stability of CO$_2$. In addition, there are remaining challenges such as low selectivity, high overpotential, and low catalyst durability for the commercial applications. Many researchers have studied cathode materials, reaction systems, and operation conditions because of the high impacts of the parameters on the CO$_2$RR. In this thesis, I designed gas diffusion electrodes (GDEs) with selective for syngas and C$_2$$^+$ products, which can be a scalable electrode for gas-phase CO$_2$ systems.

The co-electrolysis of H$_2$O and CO$_2$ can be derived from hydrogen evolution reaction (HER) and CO$_2$RR in the aqueous electrolytes using an electrolyzer. I designed the cathodes for syngas production, which can control the syngas composition with stable performance in a flow cell. Ag/TiO$_2$ composite catalysts were synthesized to prepare the cathodes because Ag and TiO$_2$ are selective for CO and H$_2$, respectively, and durable in the alkaline electrolytes. For modulating H$_2$/CO ratios, Ag/TiO$_2$ catalysts were synthesized by changing Ag content and the properties of TiO$_2$, which were carried out by using anatase and rutile phases and introducing oxygen vacancies in anatase phase. The H$_2$/CO ratio generally varies according to Ag content and applied potentials. Likewise, the diverse syngas compositions were achieved on the Ag/TiO$_2$ catalysts with different Ag content. However, 40 wt% Ag/TiO$_2$ anatase and Ag/TiO$_2$ rutile catalysts exhibited stable H$_2$/CO ratios in the wide potential range with high selectivity and reaction rate. Furthermore, H$_2$/CO ratios were controlled by using different Ag/TiO$_2$ catalysts which possessed oxygen vacancies and different phases. The H$_2$/CO ratio increased on Ag/TiO$_2$ anatase with more oxygen vacancies on the surface and in the bulk because the oxygen vacancies in TiO$_2$ increased selectivity and activity for H$_2$ production.

The relatively low selectivity for C$_2$$^+$ products compared to C$_1$ products has been reported because of complex reaction pathways involving multi-steps of the electron and proton transfers. The GDE consists of catalyst layers with catalyst and binder and porous gas diffusion layer (GDL). The materials and electrode structures are important to achieve stable and high performance of CO$_2$RR. Cu is typically used for C$_2$$^+$ production due to its unique ability to C-C coupling which is an essential reaction step. The facet-controlled Cu$_2$O with surface exposed (100) or (111) have showed high selectivity for C$_2$$^+$ products compared to spherical Cu$_2$O. In this thesis, I designed electrodes using Cu$_2$O catalysts to improve selectivity and activity for C$_2$$^+$ products in the near-neutral electrolyte using a flow cell. Firstly, Cu$_2$O catalysts with surface exposed (100) and (111) were synthesized by wet chemical reduction without surfactants and additional capping agent. The Cu$_2$O cube with (100) exhibited high performance and stability for C$_2$$^+$ production due to the stronger binding CO$_2$RR intermediates and less strong H*. The Cu(100) showed higher binding energy of CO but similar binding energy of H compared to Cu(110), which resulted in higher selectivity and activity of C$_2$$^+$ products. Therefore, electrode structure with Cu$_2$O cube was more tailored by ionomer binders which can play important roles of securing the catalyst particles on the GDL and forming porous catalyst layers. I explored effects of binders and catalyst content on the electrode structure to modulate the C$_2$$^+$ production. The N-containing anion exchange ionomers (AEIs) with minimum content improved activity and stability due to the increased hydrophobicity and charge transfer resistance. The hydrophilicity in the catalyst layers and hydrophobicity in the GDL should be maintained for high activity and stability of the systems. The selectivity and activity for C$_2$$^+$ production can also be changed by pre-reduction conditions such as gas atmospheres, applied potentials, and electrolysis durations. Therefore, I investigated their relationships to reveal the effect of pre-reduction conditions on the electrode structures leading to different CO$_2$RR performances. The Cu$_2$O cube catalysts maintained the initial shape after pre-reduction and CO$_2$RR. However, the fragmented Cu$_2$O particles appeared and its degree was increased at the harsh conditions including high current density and increased duration. The Cu$_2$O electrode treated under mild current density and longer durations showed enhanced C$_2$$^+$ production due to the increased step sites and fragmentation of Cu$_2$O.