The Study of Porous Geometry Design for Capacitive Deionization Devices through Vertically-Aligned Carbon Nanotube Electrodes
Authors: Mutha Heena K , Hashempour Mazdak , Wardle Brian L , Thompson Carl V. , Wang Evelyn N 
Published: February 2020

In the face of an increasing water crisis, we are developing tools to optimize capacitive deionization (CDI) for brackish water desalination. CDI can be a competitive technology for brackish water treatment due to its higher energy efficiencies compared to reverse osmosis and its more inherent resistance to fouling [1]. However, CDI is still a developing technology where adsorption capacity and salt removal rates into porous, tortuous carbon electrodes is still low. We have designed vertically-aligned carbon nanotube (VA-CNT) electrodes, with minimal tortuosity, to investigate the role of porous geometry on the performance of flow-by CDI devices, specifically examining changes in diffusion resistance, salt adsorption rate and capacity.

Previously, a breadth of carbon materials have been investigated for CDI including activated carbon, carbon aerogels, ordered mesoporous carbons, carbide-derived carbons, and carbon nanotubes, among many others [2]. High salt adsorption capacities have been attributed to the presence of micropores (< 2 nm), [3] and optimal materials seem to have a presence of macroporous pathways with microporous structures. [4, 5] However, many carbon materials have tortuous pores making it challenging to decouple the role of pore diameter on salt adsorption rate. VA-CNTs are an exciting material for investigating this issue due to the ability to manipulate the inter-CNT spacing, to study changes in the ion transport rate as a function of geometry, while maintaining mininimal tortuosity and intrinsic capacitance.

In this work, we synthesize VA-CNT electrodes that are sparse (inter-CNT spacings upto 100 nm), and more dense CNTs (inter-CNT spacings upto 25 nm). These CNTs are grown using a standard chemical vapor deposition of ethylene on silicon wafers with iron catalyst. By varying the partial pressure of ethylene and the oxygen content in the furnace, we synthesize CNTs of varying densities. The VA-CNT forests is delaminated from the substrate with a high-temperature oxygen etch, seperating the carbon from the corrosive catalyst. The free-standing films are mounted against Ti metal plates serving as the current collectors, and constrained by a PEEK porous mesh (200 μm window, 40 μm wire diameter). We use a flow-by CDI experimental set up to study the role of varying voltages, electrode thicknesses, CNT densities, and chemical functionalization on desalination performance. We find that in a 1mM NaCl solution, CNT electrodes can adsorb from 5-15 mg salt/g carbon, at rates of 0.2-1 mg/g-min. Through densification, we maintain gravimetric performance, but increase our volumetric salt adsorption capacity from 0.2 to 0.3 g/cm3. We achieve charge efficiencies upto 80% for these systems. We combine this experimental investigation with an electric double layer model for macroporous electrodes to inform the design of carbon electrode materials for optimal ion adsorption and throughput in a flow-by CDI device.

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