The development of carbon dioxide electrochemical reduction (CO2ER) has mainly focused on aqueous electrolytes. However, the low solubility of apolar CO2 in polar water negatively impacts the electrochemical process, especially mass transport. Organic-based electrolytes, such as methanol, acetonitrile, and dimethylformamide, have been explored as an alternative due to increased CO2 solubility. Yet, insights into other organic electrolytes are scarce. Dinitrile solvents have decreased polarity in comparison to the mononitrile solvent acetonitrile and thus can potentially further increase CO2 solubility and advance the reaction’s performance. Herein, the novelty of dinitrile-based electrolytes for the application of CO2ER is explored. Acetonitrile, adiponitrile, and sebaconitrile were used to study the effects of a secondary nitrile group addition and the length of the aliphatic, nonpolar chain on salt solubility, electrolyte conductivity, and CO2 solubility. Electrochemical effects were evaluated through cyclic voltammetry (CV) and chronoamperometry (CA) measurements with copper (Cu) and copper oxide (CuO) commercial nanoparticle catalysts. DFT calculations were employed to further explore the CO2 interactions with the selected solvents. Finally, it was found that tuning the polarity of the nitrile solvent results in a profound effect on the physiochemical properties of the electrolyte solutions, markedly CO2 solubility and the CO2ER.

Accumulating carbon dioxide (CO2) concentrations in the atmosphere due to anthropogenic sources has become an increasing threat to the environment because of the resulting effects of global warming. It has therefore become imperative to develop and utilize alternative sustainable sources for fuels and feedstock chemicals. CO2 electrochemical reduction (CO2ER) has been identified as a promising process which can convert CO2 to added value chemicals, such as hydrocarbons. In combination with the use of renewable energy resources, the technology has potential to be a net zero carbon process. Commonly, aqueous electrolytes are used for the reaction such as bicarbonate or phosphate-buffered saline solutions. However, these exhibit low CO2 solubilities, which hinders the performance of the reaction by limiting mass transfer. Organic solvent electrolytes possess relatively higher CO2 solubilities. These are also able to suppress the competing hydrogen evolution reaction (HER) due to lowered proton concentrations. Organic nitrile-solvent-based electrolytes, more commonly studied for Li-ion batteries, are known to possess high thermal and electrochemical stability. Furthermore, the polarity and CO2 miscibility can be tuned in dinitrile solvents by variation of the chain length. As such, the effect of chain length was explored on organic dinitrile-based solvent electrolytes for the application of CO2 electroreduction. The selected dinitrile solvents were Adiponitrile (n=4), Suberonitrile (n=6), and Sebaconitrile (n=8), where n indicates the number of methylene groups in the chain. These dinitriles were also compared to Acetonitrile, a commonly used organic mononitrile solvent. Conductivity was first studied as a function of salt concentration and temperature for all solvents. Then, CO2 solubility was evaluated for the different solvents. Finally, electrochemical tests utilizing copper and copper-based bimetallic catalysts were conducted. The catalysts explored consisted of synthesized copper (I) oxide (Cu2O) and copper-silver (CuAg) nanoparticles, obtained through a chemical reduction method in water using sodium borohydride (NaBH4) at ambient conditions. These particles were evaluated against commercial Cu2O and CuAg nanoparticle counterparts. The Cu-based nanoparticles were characterized using XRD, TEM, EDX, and EELS techniques. The obtained results are summarized and discussed.

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