Characterization of Ion Transport and -Sorption in a Carbon Based Porous Electrode for Desalination Purposes

Abstract

New and more efficient water desalination technologies have been a topic of incipient research over the past few decades. Although the focus has been placed on the improvement of membrane-based desalination methods such as reverse osmosis, the development of new high surface area carbon-based-electrode materials have brought substantial interest towards capacitive deionization (CDI), a novel technique that uses an electric field to separate the ionic species from the water. Part of the new interest on CDI is its ability to store and return a fraction of the energy used in the desalination process. This characteristic is not common to other electric-field-based desalination methods such as electro-deionization and electrodialysis reversal where none of the input energy is recoverable. This paper presents work conducted to analyze the energy recovery, thermodynamic efficiency, and ionic adsorption/desorption rates in a CDI cell using different salt concentration solutions and various flow rates. Voltage and electrical current measurements are conducted during the desalination and electrode regeneration processes and used to evaluate the energy recovery ratio. Salinity measurements of the inflow and outflow stream concentrations using conductivity probes, alongside the current measurements, are used to calculate ion adsorption efficiency. Two analytical species transport models are developed to estimate the net ionic adsorption rates in a steady-state and nonsaturated porous electrode scenario. Finally, the convective and electrokinetic transport times are compared and their effect on desalination performance is presented. Steady test results for outlet to inlet concentration ratio show a strong dependence on flow rate and concentration independence for dilute solutions. In addition, transient test results indicate that the net electrical energy requirement is dependent on the number of carbon electrode regeneration cycles, which is thought to be due to imperfect regeneration. The energy requirements and adsorption/desorption rate analyses conducted for this water-desalination process could be extended to other ion-adsorption applications such as the reprocessing of lubricants or spent nuclear fuels in a near future.

Two-Dimensional Porous Electrode Model for Capacitive Deionization

“…Characterizing ionic transport in porous electrodes has been the subject of many research efforts in diverse applications including capacitive water desalination, − microbial fuel cells, , energy storage in supercapacitors and batteries, − and energy recovery via salinity gradients. − Capacitive desalination, also known as capacitive deionization (CDI), is a relatively new technology that leverages porous electrodes for electrostatic ion adsorption and has potential as an energy-efficient and cost-effective method of the desalination of water with a low to moderate salt content. − The active component of CDI is a pair of engineered porous carbon electrodes. Upon application of ∼1 V across these electrodes (the charging process), salt ions are removed from feedwater and held electrostatically within electric double layers (EDLs) inside pores.…”

THE INFLUENCE OF NaCl SOLUTION FLOW RATE ON ION ABSORPTION OF CAPACITIVE ELECTRIC CARBON PLATE

“…Electrolyte ions are then absorbed through the pores on the entire area of the plate. When an electric potential is applied with the opposite pole, this movement will be detected as an electric current on the voltammogram imaging (Demirer et al, 2013). In the testing process of this research, the moving current at a value of 0.00575A indicates the absorption of ions in the carbon plate and becomes a zero value Amperes indicating the ions are released again towards electrolyte solutions (Rommerskirchen et al, 2015).…”

“…When the brine passes through the two electrodes, the Na+ and Cl-ions will separate themselves into the electrode poles that attract them (Fritz et al, 2019). This capacitive desalination system works at a relatively small voltage so that this system is an energy-efficient and environmentallyfriendly system (Demirer et al, 2013).…”

Electrodialysis for water desalination: A critical assessment of recent developments on process fundamentals, models and applications

“…Recently, the use of capacitive electrodes instead of conventional ones has been studied [52,53]. These electrodes are composed by an active carbon layer and a current collector.…”