Transport Properties of Flow-through Electrodes with Varying Composition and Particle Dimension for Electrochemical Desalination

Abstract

The growing need for energy-efficient desalination has motivated research into cation intercalation as a technology to remove salt ions from water. However, Prussian Blue analogues (PBAs), a promising class of intercalation compounds with high capacity and cycle life, demonstrate low electronic conductivity [1]. A common technique to increase electronic conductivity is the use of additives such as carbon black, [2,3] however adding conductive particles reduces active particle volume fraction and overall energy density. In addition to electronic conductivity, MacMullin number and hydraulic permeability affect energy consumed by ionic conduction and electrolyte pumping, respectively. These factors can be improved through the use of macroscopic pores to increase ion transport and fluid flow at the cost of removing active material to form the pores and lowering areal capacity [4,5]. Developing practical desalination technology requires the selection of conductive additives to facilitate the electron transport with simultaneous ion transport and fluid permeation. The present work studies how the volume fraction of either C45 or Ketjen Black conductive additive affects electrodes that incorporate either insulative nanoparticles or microparticles. Here, alumina particles are chosen here as a surrogate for insulative redox-active particles. Based on electronic conductivity measurements, smaller carbon black particles were found to form a percolating path through the electrode microstructure at lower volume fractions, increasing the electrode’s bulk electronic conductivity without sacrificing active material loading. The use of smaller alumina particles showed a similar effect, as electrodes made with nanoparticles possessed orders of magnitude higher conductivity at lower carbon black volume fractions (Fig. 1). A new application of the four-point probe technique was developed to measure the in-plane MacMullin number, which showed a weak decline with increasing carbon black content. Hydraulic permeability was also measured by applying a hydraulic head onto the cross-section of an electrode. Electrodes made with smaller particles (either conductive or insulative) were less permeable due to the particles’ ability to form smaller pores at similar porosities. Particle radii and inter-particle forces played a key role in electrode solidification, an idea supported by simulations of materials used in Li-ion batteries [6]. The van der Waals forces between the carbon black particles and alumina are generally weaker than those between carbon particles themselves, and this difference is enhanced for more numerous particles of smaller radii, allowing the carbon to percolate more easily. The three transport properties were also used to predict theoretical power requirements and find the attainable area-specific adsorption rate for a given specific energy consumption in a desalination cell, as faster adsorption is expected to require more energy [7]. The size of both conductive and insulative particles correlate with these properties and determine the rate and energy cost of salt removal. Electrodes composed of similar volume fractions of conductive and insulative material but different particle sizes display dramatically different electronic conductivity, ionic conductivity, and hydraulic permeability. Applying these lessons to the fabrication of porous electrodes could reduce energy consumption and lead to efficient desalination technology. Figure 1: A) Conductivity values at varying volume fractions of carbon black for the four combinations of C45 and Ketjen Black mixed with alumina microparticles and nanoparticles. B) MacMullin Numbers for the same electrodes. C) Results of permeability tests for the four particle mixtures.