Unexpected bell-shaped double layer capacitance promoted by nitrate anions at a-CNx / aqueous electrolyte interface and simulated with the lattice-gas model

Abstract

Amorphous carbon nitride thin fims, a-CN_x, are potentially low cost candidates for electrochemical nitrate treatment by comparison to boron doped diamond electrodes. In aqueous media, a-CN_x electrodes are characterized by a large potential window and, in acidic pH solutions, no surface charge capacitive contribution. In perchloric acid at pH 1, nitrate reduction occurs at the negative limit of a potential domain without any significant redox reaction over almost 1 Volt. In this domain, in the presence of a nitrate salt, a bell-shaped behaviour was observed for the interfacial capacitance. Differences were evidenced according to the solvation state of the cations (Na⁺, K⁺, Li⁺, Cs⁺, (CH₃)₄N⁺, (C₂H₅)₄N⁺), depending on the cation size. Experimental capacitance data were simulated by using the phenomenological theory developed by Kornyshev in the case of ionic liquids. A good agreement was obtained assuming a compact layer in series with the highly structured diffuse layer and taking into account the short-range ion-ion interactions. Thus, at the a-CNx/aqueous electrolyte interface, nitrate anions are engaged into strong anion-cation interactions (ion pairing) especially with small sized cations, leading to nitrate anion trapping in the multilayered interfacial region with a possible negative effect on the nitrate ions electroreduction.