[Paper Review] Capacitive Deionization -- defining a class of desalination technologies

P. M. Biesheuvel, Martin Z. Bazant+18 more|arXiv (Cornell University)|Jul 18, 2017

Membrane-based Ion Separation Techniques1 references28 citations

TL;DR

This paper proposes expanding the definition of Capacitive Deionization (CDI) to include all electrically driven, cyclic ion removal technologies—regardless of ion storage mechanism—based on shared operational principles and metrics. It establishes CDI as a unified technology class alongside reverse osmosis and electrodialysis, enabling standardized analysis and comparison across diverse materials and mechanisms.

ABSTRACT

Over the past decade, capacitive deionization (CDI) has realized a surge in attention in the field of water desalination and can now be considered as an important technology class, along with reverse osmosis and electrodialysis. While many of the recently developed technologies no longer use a mechanism that follows the strict definition of the term "capacitive", these methods nevertheless share many common elements that encourage treating them with similar metrics and analyses. Specifically, they all involve electrically driven removal of ions from a feed stream, storage in an electrode (i.e., ion electrosorption) and release, in charge/discharge cycles. Grouping all these methods in the technology class of CDI makes it possible to treat evolving new technologies in standardized terms and compare them to other technologies in the same class.

Motivation & Objective

• To address the growing diversity of ion storage mechanisms in desalination technologies that share the same cyclic operation and metrics as traditional CDI.
• To resolve ambiguity in terminology by defining CDI not by mechanism but by operational and analytical framework.
• To promote standardization in performance evaluation and comparison across emerging desalination technologies.
• To include redox-active materials, ion-selective polymers, and hybrid systems under the CDI umbrella based on shared operational principles.
• To support the development of the field by enabling consistent benchmarking and technology platform classification.

Proposed method

• Proposes a broadened definition of CDI based on cyclic electrode charging/discharging and ion electrosorption, regardless of the underlying ion storage mechanism.
• Defines CDI as a technology class unified by common metrics: equilibrium salt removal and stored charge as a function of cell voltage.
• Applies the same analytical and operational frameworks to all systems that follow the cyclic charging/discharging process, including carbon-based, intercalation, and redox-based electrodes.
• Includes both fixed and moving electrode configurations (e.g., rods, wires, flowing particles) in the CDI framework, as long as they operate cyclically.
• Recognizes selective ion removal via double layer effects, chemical affinity groups, and redox-active materials as valid CDI applications.
• Uses standardized performance metrics to compare technologies across different materials and mechanisms, enabling cross-platform evaluation.

Experimental results

Research questions

• RQ1 Can desalination technologies using non-EDL mechanisms (e.g., intercalation, Faradaic reactions) be meaningfully grouped under the CDI umbrella?
• RQ2 What common operational and analytical features justify classifying diverse ion removal mechanisms as part of the same technology class?
• RQ3 How can performance metrics like salt removal and charge storage be standardized across different CDI technologies?
• RQ4 To what extent do moving electrodes or alternative materials (e.g., transition metal hexacyanoferrates) conform to the proposed CDI framework?
• RQ5 What is the role of ion selectivity in expanding the scope of CDI beyond general desalination?

Key findings

• The paper successfully redefines CDI as a broad technology class based on operational and analytical consistency rather than mechanism, enabling inclusion of diverse materials and processes.
• Ion removal via redox-active intercalation materials (e.g., sodium manganese oxide, hexacyanoferrates) is valid CDI as long as it follows cyclic charging/discharging and uses shared metrics.
• Ion-selective molecules or polymers immobilized on electrodes, regenerated via voltage, are included in the CDI framework when they follow the same cyclic operation.
• Hybrid systems combining EDL and Faradaic processes are recognized as CDI if they adhere to the same performance evaluation standards.
• Moving electrodes—such as rods, wires, or flowing particles—can be classified as CDI if they undergo cyclic charging and ion electrosorption in their reference frame.
• Selective ion removal using high-affinity groups or membranes is equally valid under the expanded CDI definition, especially for targeted ion removal from complex water sources.