Capacitive Deionization (CDI): How It Works and When to Use It

Capacitive Deionization (CDI): How It Works and When to Use It

Capacitive deionization (CDI) is an emerging electrochemical desalination technology that removes dissolved ions from water using the electrical double layer formed on porous electrode surfaces. CDI offers unique advantages for low-to-moderate salinity water treatment, including low energy consumption, no chemical requirements, and the ability to selectively remove specific ions.

How CDI Works

CDI operates in two phases:

• Adsorption (charging): A voltage (1.0-1.4 V) is applied across two porous carbon electrodes. Cations migrate to the negative electrode and anions to the positive electrode, where they are held in the electrical double layer. The water flowing between the electrodes is desalinated

• Desorption (discharge/regeneration): The voltage is reduced or reversed, releasing the adsorbed ions into a small volume of concentrate. The electrodes are regenerated for the next cycle

CDI Configurations

Flow-Through CDI

Water flows between the electrodes in a channel. Simple design but limited by low salt adsorption capacity per cycle.

Membrane CDI (MCDI)

• Ion exchange membranes placed in front of each electrode: cation exchange membrane on the cathode, anion exchange membrane on the anode

• Membranes prevent co-ion expulsion during charging, doubling salt removal efficiency

• MCDI achieves 2-3× higher salt removal per cycle compared to standard CDI

Flow-Electrode CDI (FCDI)

Uses flowing carbon slurry electrodes instead of fixed electrodes, enabling continuous operation without cycling. The charged slurry is regenerated in a separate chamber.

Advantages of CDI

• Low energy for low-salinity water: 0.1-1.0 kWh/m³ for water with <3000 ppm TDS

• No chemical consumption: Purely electrochemical process

• Selective ion removal: Can preferentially remove specific ions (hardness, nitrate, fluoride) with modified electrodes

• Low operating pressure: Operates near atmospheric pressure

• Energy recovery: Discharging electrodes generates electricity that can be recovered (20-50% of charging energy)

Limitations

• Limited to low-moderate salinity: CDI is not competitive with RO for water above 5000-10,000 ppm TDS

• Electrode degradation: Carbon electrode oxidation reduces capacity over time

• Lower water recovery than RO: Typically 50-80% recovery

• Scaling and fouling: Electrode surfaces are susceptible to scaling and organic fouling

Tech Inc. provides CDI test cells and electrode evaluation platforms for researchers developing advanced CDI electrode materials and system configurations.

Frequently Asked Questions

Is CDI better than RO?

CDI and RO serve different niches. CDI is more energy-efficient for low-salinity water (<2000 ppm) but cannot compete with RO for seawater or high-TDS water. CDI is ideal for brackish water polishing, selective ion removal, and applications where chemical-free operation is valued.

What electrode materials are used in CDI?

Activated carbon is the standard electrode material. Research focuses on higher-capacity materials including carbon aerogels, carbon nanotubes, graphene, MXenes, and metal oxide composites.