Activated carbon is an electron-conducting amphoteric ion adsorbent

TL;DR

This paper introduces an amphoteric Donnan model for activated carbon electrodes that accurately predicts ion electrosorption behavior by incorporating surface chemical groups, improving understanding of capacitive deionization processes.

Contribution

It presents a novel EDL model based on surface chemistry, replacing the attraction term with chemical information, to better describe ion adsorption in activated carbons.

Findings

The model accurately fits experimental ion electrosorption data.

Surface chemical groups influence EDL structure and desalination performance.

The model links activation conditions to electrode performance.

Abstract

Electrodes composed of activated carbon (AC) particles can desalinate water by ion electrosorption. To describe ion electrosorption mathematically, accurate models are required for the structure of the electrical double layers (EDLs) that form within electrically charged AC micropores. To account for salt adsorption also in uncharged ACs, an "attraction term" was introduced in modified Donnan models for the EDL structure in ACs. Here it will be shown how instead of using an attraction term, chemical information of the surface structure of the carbon-water interface in ACs can be used to construct an alternative EDL model for ACs. This EDL model assumes that ACs contain both acidic groups, for instance due to carboxylic functionalities, and basic groups, due to the adsorption of protons to the carbon basal planes. As will be shown, this "amphoteric Donnan" model accurately describes various data sets for ion electrosorption in ACs, for solutions of NaCl, of CaCl2, and mixtures thereof, as function of the external salt concentration and of the cell voltage between two AC electrodes in a process called capacitive deionization (CDI). The amphoteric Donnan model can be used to relate the conditions of activation treatment and electrode preparation (which both influence the carboxylic acid content) to the EDL-structure and desalination performance of ACs.