Tuning the Polarity of Dinitrile-Based Electrolyte Solutions for CO₂ Electroreduction on Copper and Copper-Based Catalysts

Abstract

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.

Mechanistic Basis of Conductivity in Carbon Dioxide-Expanded Electrolytes: A Joint Experimental–Theoretical Study

“…Prior findings from the literature show that, under conventional conditions, solution conductivity decreases linearly with decreasing ionic strength. 33 Thus, our findings in these CXEs contradict this expected behavior based only on ionic concentration. However, as previously stated, this is not the sole factor that contributes to conductivity, and thus other effects, identified by molecular dynamics simulations (vide infra), must also be taken into account.…”

“…This also appears to be the case with Et 2 O dilution; dilution of 4 blue points. 33 In addition to the slight deviation from linearity in the dilution-dependent conductivity data using Et 2 O, we also identified several other key insights. The more negative slope/ dependence of the conductivity-vs-concentration plot obtained for the case of dilution with Et 2 O can be superficially understood to arise due to the smaller dielectric constant of Et 2 O (4.27) in comparison to that of MeCN (36.64).…”