Direct prediction of the desalination performance of porous carbon electrodes for capacitive deionization

S Porada,P M Biesheuvel,V Presser,M Oschatz,L Borchardt,J S Atchison,M Bryjak,S Kaskel,K J Keesman

doi:10.1039/c3ee42209g

Abstract

Desalination by capacitive deionization (CDI) is an emerging technology for the energy- and cost-efficient removal of ions from water by electrosorption in charged porous carbon electrodes. A variety of carbon materials, including activated carbons, templated carbons, carbon aerogels, and carbon nanotubes, have been studied as electrode materials for CDI. Using carbide-derived carbons (CDCs) with precisely tailored pore size distributions (PSD) of micro- and mesopores, we studied experimentally and theoretically the effect of pore architecture on salt electrosorption capacity and salt removal rate. Of the reported CDC-materials, ordered mesoporous silicon carbide-derived carbon (OM SiC-CDC), with a bimodal distribution of pore sizes at 1 and 4 nm, shows the highest salt electrosorption capacity per unit mass, namely 15.0 mg of NaCl per 1 g of porous carbon in both electrodes at a cell voltage of 1.2 V (12.8 mg per 1 g of total electrode mass). We present a method to quantify the influence of each pore size increment on desalination performance in CDI by correlating the PSD with desalination performance. We obtain a high correlation when assuming the ion adsorption capacity to increase sharply for pore sizes below one nanometer, in line with previous observations for CDI and for electrical double layer capacitors, but in contrast to the commonly held view about CDI that mesopores are required to avoid electrical double layer overlap. To quantify the dynamics of CDI, we develop a two-dimensional porous electrode modified Donnan model. For two of the tested materials, both containing a fair degree of mesopores (while the total electrode porosity is ∼95 vol%), the model describes data for the accumulation rate of charge (current) and salt accumulation very well, and also accurately reproduces the effect of an increase in electrode thickness. However, for TiC-CDC with hardly any mesopores, and with a lower total porosity, the current is underestimated. Calculation results show that a material with higher electrode porosity is not necessarily responding faster, as more porosity also implies longer transport pathways across the electrode. Our work highlights that a direct prediction of CDI performance both for equilibrium and dynamics can be achieved based on the PSD and knowledge of the geometrical structure of the electrodes.

Highlights

Providing access to affordable and clean water is one of the key technological, social, and economical challenges of the 21st century.[1,2,3] For the desalination of water, commercially available methods include distillation,[4] reverse osmosis,[5] and electrodialysis.[6]
Predictive tools have to include the salt adsorption capacity and desalination rate as functions of the pore size distribution of the carbon electrode material, and of the geometrical measures of the electrodes, such as interparticle porosity. We present such a method based on a two-dimensional porous electrode theory, in combination with a predictive salt adsorption capacity analysis based on the pore size distribution
The carbide-derived carbons (CDCs) materials used for this study are produced from selective etching of silicon or titanium atoms out of a carbide precursor (SiC or TiC), a procedure which results in a material with a high BET SSA which, in the case of OM SiC-CDC, is as high as 2720 m2 gÀ1 (Table 1)

Summary

Introduction

Providing access to affordable and clean water is one of the key technological, social, and economical challenges of the 21st century.[1,2,3] For the desalination of water, commercially available methods include distillation,[4] reverse osmosis,[5] and electrodialysis.[6] Novel approaches include ion concentration polarization in microporous media,[7] systems based on batteries,[8,9] forward osmosis,[10] and capacitive deionization (CDI).[11,12,13,14,15]. CDI is based on an electrochemical cell consisting of an open-meshed channel for water ow, in contact with sheets of porous electrodes on both sides. A er some time, when the electrodes reach their adsorption capacity (which depends on cell voltage), a discharge cycle is initiated by reducing or reversing the cell voltage, thereby releasing the salt as a concentrated stream. In the discharging step of the cell, energy recovery is possible.[16,17]

Objectives

Results

Conclusion