Pulse/Pulse-Reverse Electrodeposition of Copper Electrocatalysts for CO2 Reduction to Ethylene
Authors: Skinn Brian , Sen Sujat , Brown Steven Michael , Snyder Stephen , Brushett Fikile R. , Garich Holly 
Published: February 2020

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 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 CO2 mitigation processes.
Recent work toward development of efficient, selective, and active copper electrocatalysts for reduction of CO2 to ethylene is presented. Copper is well known as a unique CO2 reduction electrocatalyst, capable of forming alcohol (e.g., ethanol) and hydrocarbon (e.g., methane, ethylene) products in addition to aldehydes, carboxylic acids, carbon monoxide and hydrogen [[3],[4]]. 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 [[5]]. However, the catalytic activity and selectivity achieved to date with Cu catalysts and CO2-saturated aqueous carbonate solutions have 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 [[6],[7]]. Given that pulsed electrodeposition is known to have a significant effect on the microstructure of the resulting metal films [[8],[9]], 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 [[10]]. Photographs of a representative catalyst coupon at each stage of fabrication are presented in Figure 1. 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.
We are focusing development toward the use of aqueous solutions containing CO2-solubilizing additives such as room-temperature ionic liquids and cyclic amines. These materials have been extensively studied as a liquid-phase capture media for removing CO2 from post-combustion exhaust streams (e.g., power plants) [[11],[12]], 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 advantages to developing an advanced technology for electrocatalytic conversion of CO2 using capture media already in large-scale industrial use, since it would dramatically reduce the logistical and economic costs of integration into existing facilities. The data gathered to date provide encouraging evidence that enhanced catalytic performance can be realized through the use of such CO2-solubilizing additives in tandem with pulse-deposited copper electrocatalysts.
The authors acknowledge the financial support of NASA Contract NNX14CC53P and US DOE Contract DE-SC0015812.

References

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Figure 1