(Invited) Optimizing CO2 Reduction in Aprotic Media: Insights Via Kinetic Modeling of Bimetallic CuxZn1-X Electrocatalysts
Authors: Oppel Niklas , Witzel Ruth , Röse Philipp , Krewer Ulrike 
Published: December 2024

Bimetallic CuxZn1-x electrocatalysts, as an earth-abundant material, have emerged as promising candidates for improving the efficiency and selectivity of CO2 reduction in aqueous electrolytes.[1] However, the use of aqueous electrolytes has several drawbacks, including limited CO2 transport due to its low solubility in the electrolyte solution and, most importantly, the competing hydrogen evolution reaction (HER).[2] These challenges can be effectively circumvented by using aprotic organic electrolytes instead, which offer higher CO2 solubility and minimize HER.[3] Aprotic electrolytes (e.g., acetonitrile) show distinct differences in the reaction pathway, yielding either CO or oxalate. Studies under these conditions are scarce, and fundamental knowledge of the performance-limiting processes at the electrode-electrolyte interface is lacking. This knowledge gap hinders further development and process design for efficient CO2R in aprotic media.
In this contribution, we present a comprehensive analysis of CO2R on Cu and CuxZn1-x electrocatalysts in acetonitrile. By combining standard electrochemical methods such as cyclic voltammetry, polarization curve, and electrochemical impedance spectroscopy, with mathematical modeling that couples microkinetics with continuum transport, we elucidate reaction and transport phenomena and reveal the underlying performance limitations. Based on this approach, we provide insights into the local reaction environment and discuss in detail the rates, surface coverages of adsorbed species, and local concentration profiles at the electrode. We investigate Cu and α-brass at different CuxZn1-x ratios as planar as well as nanoparticle coated electrodes. For Cu, we find that adsorbed CO2 is being consumed faster than it replenishes, suggesting that CO2 adsorption is the bottleneck in aprotic electrolytes,[4] as opposed to CO2 transport as in alkaline solution[5]. With increasing Zn content of the α-brass, higher CO2R performance is achieved. According to our simulations this can be attributed to i) higher utilization of the electrocatalyst and ii) improved CO2 adsorption capability compared to pure Cu.
The gained insights allow for a better understanding of the effects of the catalyst composition on the performance and highlight the importance of CO2 adsorption on the performance of CO2 electroreduction in organic electrolytes. This enables knowledge-based optimization of the electrocatalysts and tailoring of reaction conditions. Overall, our results provide valuable insights into the poorly understood organic electrolytes for CO2R as a viable alternative to aqueous electrolytes and pave the way for the analysis of more complex reaction systems such as electro-organic syntheses or CO2R with protic additives.

References:

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2021, 1, 663–680.
[2] G. Marcandalli, M. C. O. Monteiro, A. Goyal, M. T. M. Koper, Acc. Chem. Res.
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[3] M. König, J. Vaes, E. Klemm, D. Pant, iScience
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[4] N. Oppel, P. Röse, S. Heuser, M. Prokein, U.-P. Apfel, U. Krewer, Electrochim. Acta
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Figure. a)Experimental cyclic voltammograms at varying Zn-content in CO2 saturated 0.1M TBABF4 in Acetonitrile at a scan rate of 200 mV s-1, iR-corrected 3rd cycle is shown;b) Schematic of the 1D model system with the reaction plane located at the electrode-electrolyte interphase at x = 0 and the end of the diffusion layer, at x = δ.

Figure 1