Simultaneous Operability of Thermocatalytic and Electrocatalytic CO2 Reduction Reactions
Authors: Zhong Siyu , Holtappels Peter , Dittmeyer Roland 
Published: September 2023

The direct electrochemical reduction reaction of carbon dioxide (eCO2RR) captured from the ambient air into base chemicals and fuels holds great potential for an elegant implementation of a net-zero emission carbon cycle. This assumes that the energy used to drive the capturing and conversion process is largely free of carbon dioxide emissions, i.e., from renewables such as wind and photovoltaics. Moreover, high Faradaic efficiency for the desired products at sufficiently high current density in eCO2RR has to be demonstrated to compete with more conventional multi-step Power-to-X conversion schemes both in terms of overall energy efficiency and cost.[1]
eCO2RR is known for its wide variety of products.[2] However, hydrogen evolution reaction (HER), which is more thermodynamically reactive, inevitably seizes protons and electrons in the cathode. In addition, the reaction pathway of C2+ products is more complex than that of C1 products because it is strongly dependent on the catalyst surface and electrocatalytic operating conditions. For example, the formation of C-C bonds is interfered with the formation of C-H and C-O bonds on the catalyst surface, resulting in the direct generation of multi-carbon products technically challenging.[3] Due to these kinetic barriers, the C2+ products show much lower the energy efficiency, Faraday efficiency, and partial reduction current density of than those of the C1 products. The mechanistic aspects of the reaction pathways for the various C2+ products have not been fully deciphered, which ultimately limits the commercial application of eCO2RR conversion to C2+ products.[4]
Noting that industrial thermocatalytic processes produce and supply long-chain hydrocarbons in a long-term stable manner, we propose the production of C2+ products by combining simultaneous thermocatalysis and electrocatalysis. In the ongoing work, hydrogen from the water electrolysis is further converted to syngas via a reverse water-gas shift reaction (rWGS), followed by an established conversion route to fuels and chemicals such as Fischer-Tropsch or methanol synthesis and methanol to olefins, gasoline or jet fuel.[5] Here, a reactor and test bench capable of operating at 20 bar and 200 degrees Celsius have been fabricated to enable simultaneous thermocatalytic and electrocatalytic production of C2+ products. Furthermore, different catalysts can be employed to explore the catalytic processes on the catalyst surface.

References

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[2] J. Chen, T. Wang, Z. Li, B. Yang, Q. Zhang, L. Lei, P. Feng, Y. Hou, Recent progress and perspective of electrochemical CO2 reduction towards C2-C5 products over non-precious metal heterogeneous electrocatalysts, Nano Research 14 (2021) 3188-3207.
[3] K.D. Yang, C.W. Lee, K. Jin, S.W. Im, K.T. Nam, Current status and bioinspired perspective of electrochemical conversion of co2 to a long-chain hydrocarbon, J Phys Chem Lett 8 (2017) 538-545.
[4] Y. Zheng, A. Vasileff, X. Zhou, Y. Jiao, M. Jaroniec, S.Z. Qiao, Understanding the roadmap for electrochemical reduction of co2 to multi-carbon oxygenates and hydrocarbons on copper-based catalysts, J Am Chem Soc 141 (2019) 7646-7659.
[5] H. Kirsch, U. Sommer, P. Pfeifer, R. Dittmeyer, Power-to-fuel conversion based on reverse water-gas-shift, Fischer-Tropsch Synthesis and Hydrocracking: Mathematical modeling and simulation in Matlab/Simulink, Chemical Engineering Science 227 (2020) 115930.