Carbon nanotube-based materials as capacitive deionization electrodes

Abstract

Capacitive deionization (CDI) is an emerging desalination technology that uses ion electrosorption at electrically charged electrode interfaces and has gained increasing recognition as a sustainable and cost-effective solution for water purification. Among the various electrode materials, carbon nanotube (CNT)-based structures have attracted considerable research interest because of their outstanding physicochemical properties, including high specific surface area, superior electrical conductivity, and excellent electrochemical stability. Significant efforts have been devoted to improving the CDI performance of CNT-based electrodes using material engineering and structural design. A comprehensive analysis of recent advances in performance optimization strategies for CNT-based CDI electrodes is provided, and their pivotal role in driving technological progress in CDI is evaluated. Persistent challenges and promising research to overcome current limitations are also considered.

1. Introduction

As global demand for clean water continues to rise, natural freshwater resources are increasingly under pressure, and water scarcity has emerged as one of the key challenges of the 21st century. Addressing this issue through the development of efficient water purification technologies, such as seawater and brackish water desalination, has become a research priority. Various water treatment technologies have been explored to date, including reverse osmosis, electrodialysis, and thermal distillation. However, these conventional technologies are often associated with inherent limitations, such as high capital and operating costs, substantial energy consumption, and adverse environmental impacts, which hinder their large-scale deployment.

CDI is a comparatively new desalination process that removes dissolved salts from brackish water by electrosorption onto electrode surfaces when ions form electric double layers. The process has demonstrated considerable potential for producing drinking water from brackish water resources. Similar to electric double-layer capacitors (EDLCs) used for energy storage, CDI devices are often referred to as flow-through capacitors, as they typically operate with brackish water continuously flowing through the cell. Regeneration can be readily achieved when the electrodes become saturated with salt ions by reversing the applied potential or by short-circuiting the electrodes. CDI offers several important advantages over conventional desalination methods, including the absence of chemical additives during operation and the application of low voltages, which helps avoid water electrolysis. Thus, CDI represents a cost-effective and environmentally friendly alternative to conventional desalination methods, such as thermal distillation, reverse osmosis, and electrodialysis. This technology holds significant promise for providing affordable and pollution-free drinking and agricultural water. Previous reviews have categorized CDI as a replacement desalination technology, with membrane-based, flow-through, hybrid, and entropy battery systems classified as its major subcategories.

Ideal CDI electrode materials should possess the following characteristics: (1) a high specific surface area to provide abundant ion electrosorption sites; (2) a well-optimized pore size distribution, featuring high microporosity for enhanced salt electrosorption capacity along with an interconnected macroporous network for facile ion transport; (3) high electrical conductivity to ensure uniform voltage distribution across the electrodes and minimize resistive heat generation; (4) high wettability to promote effective contact between the electrode materials and ions, thereby maximizing pore utilization during the CDI process; and (5) excellent electrochemical stability, which is critical for the design of CDI electrodes capable of withstanding redox reactions and achieving long operational lifetimes. Among various candidates, carbon nanotubes (CNTs) have attracted considerable attention as promising CDI electrode materials.