Article pubs.acs.org/est
Use of Soft Electrodes in Capacitive Deionization of Solutions Silvia Ahualli,* Guillermo R. Iglesias, María M. Fernández, María L. Jiménez, and Á ngel V. Delgado Department of Applied Physics, School of Science, Campus Fuentenueva University of Granada, 18071 Granada, Spain ABSTRACT: All efforts to obtain, reuse or purify water are extremely significant for society. Recently, researchers have begun to delve in an idea born decades ago: the desalination of water using highly porous electrodes. It is based on a fundamental aspect of electrical double layers, namely, their huge capacitance. The ions of a solution can be partially removed under the application of an electric field when the solution fills the space between porous electrodes, either bare (CDI, or capacitive deionization), coated with ionic exchange membranes (MCDI) or chemically treated (invertedCDI or i-CDI). One of the challenges of the last years was to explore new materials and arrangements to improve the efficiency of the system. In this work, we propose a new approach inspired in the electrokinetics of soft particles: a layer of polyelectrolyte (cationic on one electrode, anionic on the opposite one) coats the carbon electrodes, converting them in a sort of “soft” electrode pair. We present a theoretical model and a set of experiments showing how soft electrodes can be successfully employed in capacitive deionization.
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INTRODUCTION
by the membrane, so that they cannot leave the EDL under the applied voltage.13 In this work, we present an alternative to MCDI based on the coating of the electrodes with a polyelectrolyte layer. This film presumably covers most of the solid/solution interface, and particularly the macropores. The designation given to this kind of electrodes, soft electrodes or SEs,14 is due to their similarity to the preparation of nanoparticles coated by polymer layers, that is, soft particles.15−17 The cationic (anionic) polyelectrolyte coating is applied to the electrode to be initially connected to the positive (negative) terminal of the power source. Only a thin layer of polyelectrolyte (a membrane is about a hundred times thicker) is necessary, with the subsequent reduction in the internal resistance. Other methods also resort to alterations of the electrodes, notably the so-called inverted capacitive deionization (i-CDI).18 In this, the carbon electrodes are chemically modified in order to ensure spontaneous formation of EDLs on each electrode, without the need of external power source, which is used only in the desorption step. The method has demonstrated a significant improvement in the performance stability of CDI. In this work, we describe the cycle of desalination of NaCl solutions using SEs, in comparison to the results obtained with bare electrodes. Different situations are described including constant voltage and constant current during adsorption, and short circuit vs inverted potential for the releasing step.
In 1960 a pioneering work on the concept of water desalination was first proposed by Blair, Murphy and co-workers1−3 under the name of “electrochemical demineralization of water”. These studies were the starting point of a new approach called Capacitive Deionization4,5 (or CDI), a cost-effective, energy efficient alternative, useful for desalinating brackish water (
