(585ax) Pulse/Pulse-Reverse Electrodeposition of Copper Electrocatalysts for CO2 Reduction to Ethylene

There have been many attempts to find efficient approaches to reduce CO₂ to various organic compounds due to the industrial need for a carbon source and the general need to mitigate the large amounts of CO₂ generated by human activities. However, a recent DOE report indicates that major scientific challenges still exist to realize development of an efficient, inexpensive, and durable catalytic system that efficiently facilitates complex, multi-electron and atom/ion transfer events [1]. Modern carbon emission mitigation efforts have focused primarily on carbon capture and sequestration (CCS) [2]. Despite the challenges extant in developing technologies for CO₂ conversion to value-added products, such capabilities are anticipated to reduce risk and offset the cost of CCS development and thus encourage broader industrial adoption of CO₂mitigation processes.

This talk presents recent work toward development of efficient, selective, and active copper electrocatalysts for reduction of carbon dioxide to ethylene. Copper is well known as a unique CO₂ reduction electrocatalyst, capable of forming alcohol (e.g., ethanol) and hydrocarbon (e.g., methane and ethylene) products in addition to aldehydes, carboxylic acids, carbon monoxide and hydrogen [3-6]. Ethylene is of specific interest, as its role as a platform feedstock for the chemicals and plastics industries affords it an appreciably higher market value than many other potential products [7]. However, the catalytic activity and selectivity achieved to date with Cu catalysts and CO₂-saturated aqueous carbonate solutions have to our knowledge not been sufficient to enable development of an industrially viable process. Faraday and MIT are currently investigating pulse/pulse-reverse electrodeposition methods as a means to fabricate copper catalysts with greater activity and ethylene selectivity in order to enable large-scale electrocatalytic conversion of CO₂.

The microstructure of metallic catalysts is known to influence various properties including selectivity and activity; for copper in particular, a strong effect on the selectivity for ethylene versus methane as a function of crystallographic orientation has been reported [8,9]. Given that pulsed electrodeposition is known to have a significant effect on the microstructure of the resulting metal films [10-12], the technology is a natural candidate for fabrication of novel, high-performance copper CO₂ reduction catalysts. In this work, catalyst activity and selectivity were further enhanced through the use of a modified literature activation protocol involving aerobic thermal oxidation followed by electrochemical reduction [13]. This talk will present data confirming that catalytic properties can be enhanced by tuning both the pulsed waveform used for deposition as well as the conditions used in the oxidation/reduction activation protocol. In particular, the electrodeposition and activation parameters have been demonstrated to have significant effects on the total faradaic efficiency of carbon conversion and the selectivity of the CO₂reduction reaction for ethylene over methane.

As an adjunct means of optimizing system performance, we are focusing development toward the use of aqueous solutions containing CO₂-solubilizing additives such as room-temperature ionic liquids (RTILs) and saturated amines (monoethanolamine, diethanolamine, N-methyldiethanolamine, etc.). These materials have been extensively studied in academia and industry as a liquid-phase capture media for removing CO₂ from post-combustion exhaust streams (e.g., bulk generation power plants) [14-17], and have the potential to increase the effective CO₂ concentration at catalyst active sites and thus enhance the faradaic efficiency of the CO₂ reduction reaction relative to H₂ formation from water electrolysis. There are also advantages to developing an advanced technology for electrocatalytic conversion of CO₂using capture media already in large-scale industrial use, since it would dramatically reduce the logistical and economic costs of integration into existing facilities.

Two primary conclusions can be drawn from the preliminary data gathered to date: (1) EMIM and BMIM ionic liquids and saturated primary amines (e.g., monoethanolamine) in aqueous carbonate solution participate significantly in anodic reactions at the potentials required for CO₂ reduction to hydrocarbons, and are thus not suitable additives for enhancing electrocatalytic CO₂ conversion; and (2) saturated secondary and tertiary amines, though stable under relevant electrocatalysis conditions, completely inhibit electrocatalytic conversion of CO₂at ambient temperature and pressure, and are thus unsuitable additives at these conditions. We hypothesize that it may be possible to overcome this inhibitory behavior via operation at elevated temperatures (e.g., 40 to 70 °C), enabling further enhancement in the performance of the pulse-deposited copper electrocatalysts under development.

The authors acknowledge the financial support of NASA Contract No. NNX14CC53P and US DOE Contract No. DE-SC0015812.

[1] Basic Research Needs: Catalysis for Energy, 8/6-8/2007. www.sc.doe.gov/bes/reports/list.html