Electric field induced release of guest molecules from clathrate hydrates and its consequences for electrochemical CO₂ conversion

When CO₂ is encapsulated in clathrate hydrate, where the concentration is nearly-two orders of magnitude greater than in saturated aqueous solution, the rate of the CO₂ reduction reaction (CO₂RR) increases, and the corresponding Faradaic efficiency becomes up to three times higher than that of the competing hydrogen evolution reaction (HER) at applied potential of −0.5 to −0.7 vs RHE. The energy efficiency of the CO₂RR is correspondingly increased. The enhanced CO₂RR in clathrate is ascribed to non-equilibrium release of the CO₂ due to the electric field near the electrode, analogous to what has been observed recently for tetrahydrofuran [Li et al. J. Phys. Chem. Letters 125, 13802 (2021)]. This raises the chemical potential of CO₂ beyond that of saturated aqueous solution, thereby reversing the relative rates of the CO₂RR and the HER. The experimental demonstration of this release mechanism for the application of CO₂ up-conversion illustrates how the electric field can not only influence electrochemical reactions at the surface but also the reactant supply from clathrate hydrates to the surface. Experimental focus rests on the low applied-potential regime, where the consequences of the reactant release mechanism are well-observable unobscured by mass-transport effects.