UCLA

With the continued demand for sustainable alternative energy fuels to replace limited fossil fuels, it is essential to explore new material properties to attain a zero-carbon emission economy. Porous metallic materials possess attractive properties for electrochemical storage and energy conversion such as superior surface area, high conductivity and large open volume, facilitating the mass transport of guest molecules. Among the different electrochemical energy storage applications, lithium metal batteries have been a competitive candidate for the next-generation of batteries. Despite its popularity, stability issues of lithium metal anode in the liquid electrolyte and the formation of lithium whiskers have kept it from practical use. Furthermore, finding scalable catalyst materials to electrochemically reduce carbon dioxide (CO2R) to multi-carbon products such as ethylene and ethanol is key for a promising pathway to a carbon-neutral economy. The objective of this thesis is to utilize porous metallic copper for electrochemical applications including (1) macroporous Cu as current collectors for Li metal batteries and (2) mesoporous Cu as a catalyst for higher-reduced products for CO2 reduction. The first part of this thesis proposes three-dimensional (3D) macroporous Cu current collectors as an effective method to mitigate whisker growth during the Li deposition process in a lithium metal battery (LMB). Three key parameters of the 3D current collectors, namely, the surface area, the tortuosity factor, and the surface chemistry are quantitatively studied to optimize the performance of these current collectors for LMB applications. Four types of porous copper networks with different sizes of well-structured microchannels are synthesized and electrochemically tested. It is found that a moderate surface area that can lower the current density of Li deposition while minimizing the formation of SEI is ideal to suppress lithium whisker growth and optimize performance. Furthermore, tortuosity should be kept at minimum in order to facilitate Li diffusion and lead to a more uniform Li morphology. In considering these parameters, it was determined that the 20Arc Cu sample (~14um average pore size) with a balance of moderate surface area and reasonable pore size had the best performance of all Cu samples tested. Lastly, a lithiophilic Zn layer is coated on the best and worst performing current collectors (~14�m and ~30�m average pore sizes, respectively). It is shown that Zn coating can improve the Li morphology and the coulombic efficiency from 99.16% to 99.56% for a low surface area 3D current collector. Chapter 3 is a follow-up study of Chapter 2 with a focus on strategies to increase the lifetime cycling of the current collectors via asymmetric lithiophilic Ag coating. For this purpose, 3D porous Cu frameworks of 10 �m (30-Cu) and 14 �m (20-Cu) average pore sizes were used. Unfortunately, Li deposition on these structures is generally top-favored, increasing the chances for dendrite formation. To overcome this issue, a lithiophilic Ag coating with gradient composition, gradually decreasing from the bottom to the top of the framework, was utilized on these 3D Cu structures to favor Li deposition at the bottom of the porous network. By comparing the coulombic efficiency (CE), stability, and lifetime of these asymmetrically against both uncoated and symmetrically coated samples, it was found that the lifetime and CE of the current collectors can be increased. Particularly, the 20-Cu sample improved its lifetime by 30% and 10% at 2mA/cm2 (10mAh/cm2) and 4mA/cm2 (10mAh/cm2) current rates, respectively. This was corroborated with post-cycling SEM images that showed a more ‘bottom-favored’ deposition and denser Li morphology on the asymmetrically coated samples. The last part of the thesis focuses on the development of scalable mesoporous Cu catalyst materials to electrochemically reduce carbon dioxide (CO2R) to multi-carbon products such as ethylene and ethanol. This work utilizes a scalable method of making powdered mesoporous Cu samples with dilute amounts of Al via dealloying of Cu-Al alloys with different compositions (10-22 at. % Cu). Dealloying produced mesoporous-macroporous Cu powdered structures with composition-dependent microstructure. These samples were electrochemically tested as catalysts for CO2 reduction in a membrane electrode assembly. Tuning the catalyst pore size distribution increased overall CO2R current density and C2+ selectivities. Tandem catalytic enhancement was also observed through dilute amounts of secondary metal coating, with the presence of a silver coating increasing C2+ reaction rates by up to 23.3%, and combined ethylene/ethanol production by up to 14.8% at a 500mV lower potential. Through the results in this work, we hope to contribute to the strategies for design and optimization of porous materials for current collectors in lithium metal batteries as well as the importance of synergistic optimization of catalyst pore architecture and chemical composition in CO2R catalyst design to enhance the production valuable products.