Electrochemical Capture and Conversion of Carbon Dioxide into All-Carbon Nanostructures

Abstract

To address the rising levels of atmospheric carbon, recent efforts have considered the capture of CO2 from release points, such as power plants, and conversion into chemicals including formic acid, methanol, CO, and ethylene. In this technique, CO2 acts as the chemical feedstock for the manufacturing of useful chemicals and provides the potential for a viable secondary market for otherwise pollutant gases, which are normally expensive to sequester. However, these routes for CO2 conversion are bottlenecked by the cost of operation versus the perceived economic benefit to society, and these low-value materials produced at low efficiencies, often from expensive catalyst materials, undermine the rationale of this approach. These issues can be resolved with the development of techniques that capture and convert atmospheric CO2 into more valuable materials that can be developed into high-value products.In contrast to the hydrogenation of CO2 into hydrocarbons and alcohols, liquid-phase electrochemical splitting of CO2 into its individual elemental constituents has been investigated beginning with aqueous electrolytes. However, low solubility of CO2 in aqueous solution and similar reaction potentials for water splitting were problematic. CO2 splitting in room temperature ionic liquids (RTILs) has been studied for their attractive electrochemical window and high solubility of CO2, but the high cost and toxicity of RTILs makes their commercial adoption impractical. In contrast to these methods, molten carbonates boast low cost and high ionic conductivity with a low vapor pressure, and have been proven as viable electrolytes for the capture and electrochemical splitting of CO2 dating back now seven decades. This method relies on the decomposition of dissolved CO2 between two biased electrodes, where elemental carbon is captured at the cathode, and the resulting structures of the deposited carbons are largely dependent on process parameters including electrolyte, current density, and electrode materials.The last 25 years have been lauded as perhaps the most exciting for carbon researchers due to the emergence of nanostructured carbon materials with extraordinary mechanical, thermal, and electronic properties that cannot be replicated in other known materials. The most mature family of carbon nanomaterials in the literature and industrial applications is carbon nanotubes, which are typically grown in a gas-phase chemical vapor deposition method. In contrast to this mature field, the growth of carbon nanostructures from the liquid-phase electrochemical reduction of CO2 remains only a new field of research, with the most recent work demonstrating growth of large-diameter (>100 nm) CNTs and few-layer graphene flakes from CO2 conversion. These initial works demonstrate the capability to leverage CO2 as a precursor in carbon nanostructure growth, even though forward-looking efforts to achieve high quality, precisely tuned materials such as small diameter CNTs or single-layered graphene at high yields will require control of the process beyond the systems-level approaches reported so far. This presents an exciting frontier that exists at the intersection of these two communities – those who have studied the mechanistic details of catalytic processes relating to nucleation and growth of nanostructures, and those who are focused on systems-level directions to develop platforms which can address important global issues.SkyNano has developed technologies to capture and convert CO2 into all-carbon nanostructures such as small diameter CNTs mediated by molten salt electrolysis. This talk will focus on our work thus far, including the engineering of inert anodes with thin film deposition of materials to activate catalytic activity at the cathode as the basis for CNT growth, the utilization of pre-deposited catalyst onto cathode architectures and mechanistic understand of dynamic catalyst processes that take place over time, such as Ostwald ripening. We will also discuss our efforts to utilize flue-gas sourced CO2 in collaboration with the Tennessee Valley Authority, in an effort to transition from the laboratory to industrial scale. Finally, this talk will address challenges we see for the field of CO2-to-carbon conversion, and the pathways we believe exist for those interested in scaling from the laboratory to the marketplace. Figure 1