Alkali-cation-free electrochemical CO2 reduction to multicarbon products in aqueous electrolytes containing tetraalkylammonium cations

Abstract

The electrochemical reduction of CO2 to multicarbon (C2+) products is attracting attention for the sustainable production of fuel and chemicals. Conventionally, electrolytes containing alkali cations are typically used; however, salt precipitation associated with these cations often hinders stable CO2 electrolysis. Organic cations are promising alternatives to alkali cations. Herein, we conducted gaseous CO2 electrolysis in aqueous solutions containing tetraalkylammonium cations in the absence of alkali cations to evaluate the effect of organic cations on C2+ formation. When tetramethylammonium cations were present as the only cation species besides protons, the faradaic efficiency for CO2 reduction exceeded 89% across a broad current density range of 0.1-1 A cm−2. In particular, C2+ formation was efficient under high total current density conditions, reaching a faradaic efficiency of 69.6% and a partial current density of 0.7 A cm−2. By contrast, the use of larger cations such as tetraethylammonium and tetrapropylammonium cations resulted in lower ethylene selectivity. Numerical simulations based on the generalized modified Poisson-Nernst-Planck model suggested that the size of the tetraalkylammonium cations affects the electric field strength within the electric double layer, with smaller cations forming a stronger field that promotes ethylene formation.