A reduction in emissions of CO₂ into the atmosphere is required to slow global warming. CO₂ capture and electrolysis technologies are a promising approach to reduce CO₂ emission and generate renewable chemicals. The CO₂ capture process, which relies on alkylamine solvents, requires significant energy input, typically involving thermally cycling the system to 120-150^°C to generate a pure stream of CO₂.^1-3 The subsequent valorization of the gas-phase CO₂ to value-added products introduces further energy losses and system complications.

We pursued therefore on a new scheme involving the electrochemical upgrade of the chemisorbed CO₂ directly from an amine capture electrolyte.⁴

Amine solutions capture CO₂ via reaction:

where the nucleophilic N and electrophilic carbon form a bond.

The direct electrochemical upgrade of amine-CO₂ (RNHCOO^-) to value-added products is of intense interest; yet, in early studies, we found that this goal was curtailed when the adduct of the amine molecules with CO₂ fails to place the CO₂ sufficiently close to the site of the heterogeneous reaction.

We turned our attention to the composition of electrochemical double layer to investigate poor heterogeneous electron transfer dynamics. Since heterogeneous electron transfer is an inner sphere reaction⁵, the distance between RNHCOO^- and the electrode in the electrochemical double layer is expected to play a crucial role in the electrolysis reaction. Systematic tuning of the electrochemical double layer can be achieved through the introduction of properly sized electrolyte ions. Here we focused on alkali cations with the goal of disrupting the undesired charge-blocking layer and achieving electron transfer to carbamate, and thus direct electrolysis.

We examined a suite of alkali cations (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) to gain insight into how the size of the cationic species affects the electrochemical double layer and the reduction of the amine-CO₂ adduct to CO. The CO Faradaic efficiency for the Li⁺ and Na⁺ electrolyte was similar to those of the amine electrolyte, which is below 5%. Cs⁺ containing electrolyte shows the best performance, with 30% CO FE at –0.66 V vs. RHE. To explore how tailoring the electrochemical layer by cationic species affects the electron transfer, we investigated the interfacial electric field strength via the Stark tuning. The Stark tuning slope of the frequency shift depends on the local electric field strength, which allows a direct reflection of the interfacial electric field with respect to the cationic species. ^6-8 We can correlate this electric field strength with the thickness of the electrochemical double layer.⁹ The Stark tuning slopes for K⁺, Rb⁺ and Cs⁺ are significantly larger than amine-CO₂ adduct (RNH₃⁺), Li⁺ and Na⁺, which agrees with the picture that the thickness of the electrochemical layer for K⁺, Rb⁺ and Cs⁺ is lower than that of RNH₃⁺, Li⁺ and Na⁺. We found that different constituents of the electrochemical double layer can hinder or promote the heterogenous electron transfer, a finding we attribute to the distance between the reactants and the electrode.

We then sought to improve the current density and Faradaic efficiency to reach meaningful levels of utilization. With the aid of an alkali cation and accelerated mass transport achieved through improved system design – temperature (~80^°C) and concentration (~2.5 M of amine-CO₂) – we achieved amine-CO₂ conversion to CO with 72% Faradaic efficiency at 50 mA/cm². The performance is competitive with that of existing CO₂RR systems such as alkaline flow cells⁵²,  gas-fed membrane electrode assemblies⁵⁶, and direct carbonate reduction systems¹⁰ (Table 1). 

Table 1. Energy cost for an alkaline flow cell, gas-fed MEA cell, direct carbonate electrolysis and direct amine-CO₂ electrolysis

This work indicates interest in amine-CO₂ systems: the approach levers the ability to perform electrolysis directly from a capture liquid and can reduce the energy cost (and carbon footprint) of the CO­₂ capture process and simplify the process flow.  

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