Key Takeaways & Executive Findings
- •• • CNT-based electrodes achieve high salt adsorption capacity (SAC) up to 20 mg/g, outperforming conventional activated carbon (typically <10 mg/g) due to enhanced electrical conductivity and ion transport, enabling more energy-efficient desalination. • • Hybridization of CNTs with activated carbon (AC) at low content (e.g., 5 wt%) improves electrode conductivity by 30% and reduces charge transfer resistance, leading to faster electrosorption kinetics and higher charge efficiency (>80%). • • MOF-derived nitrogen-doped carbon/CNT heterostructures exhibit a SAC of 25 mg/g at 1.2 V in 500 mg/L NaCl solution, with capacitance retention of 95% after 100 cycles, demonstrating excellent stability for long-term operation. • • Flow-electrode CDI systems incorporating CNT-based materials achieve continuous desalination with salt removal rate of 0.5 mg/(cm2·min) at a current density of 20 mA/cm2, addressing scalability bottlenecks for industrial application.
Abstract
Capacitive deionization (CDI) is an emerging desalination technology that removes dissolved salts from brackish water via ion electrosorption at electrically charged electrode interfaces. It has gained recognition as a sustainable and cost-effective alternative to conventional methods such as reverse osmosis, electrodialysis, and thermal distillation, which often suffer from high energy consumption and environmental impact. Among electrode materials, carbon nanotubes (CNTs) are particularly attractive due to their high specific surface area, superior electrical conductivity, and excellent electrochemical stability. This review comprehensively analyzes recent advances in performance optimization strategies for CNT-based CDI electrodes, including material engineering and structural design. Key strategies include hybridization with activated carbon, graphene, metal oxides, and metal-organic frameworks (MOFs), as well as surface functionalization and three-dimensional architecture construction. These approaches enhance salt adsorption capacity, charge efficiency, and cycling stability. For instance, dispersing CNTs in activated carbon electrodes improves conductivity and ion transport, while MOF-derived nitrogen-doped carbon/CNT heterostructures exhibit high desalination performance. The review also evaluates the pivotal role of CNT-based electrodes in driving technological progress in CDI and discusses persistent challenges such as electrode fouling, scalability, and cost-effectiveness. Promising research directions, including flow-electrode systems and selective ion removal, are highlighted to overcome current limitations. Overall, CNT-based materials hold significant promise for advancing CDI as a viable water purification technology.
1. Introduction
Global water scarcity has intensified the need for efficient and sustainable desalination technologies. Conventional methods such as reverse osmosis and thermal distillation are energy-intensive and require significant capital investment, limiting their deployment in resource-constrained regions. Capacitive deionization (CDI) offers a promising alternative, operating at low voltages (<1.2 V) without chemical additives, thus reducing energy consumption and environmental footprint. However, the performance of CDI hinges on electrode materials, which must exhibit high specific capacitance, rapid ion transport, and long-term stability. Traditional carbon electrodes, such as activated carbon, suffer from limited conductivity and ion accessibility, hindering salt adsorption capacity and rate.
Carbon nanotubes (CNTs) have emerged as a superior electrode material due to their exceptional electrical conductivity, high surface area, and mechanical robustness. Yet, pristine CNT electrodes often suffer from aggregation and limited ion-accessible sites. This review addresses these bottlenecks by systematically analyzing material engineering strategies, including hybridization with activated carbon, graphene, and metal oxides, as well as structural design such as three-dimensional networks and MOF-derived heterostructures. These approaches enhance ion transport and provide additional active sites, thereby improving salt adsorption capacity and cycling stability. By evaluating recent advances and identifying persistent challenges, this review aims to guide the development of high-performance CNT-based electrodes for practical CDI applications.
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WANG Xiaomei (2026). Carbon nanotube-based materials as capacitive deionization electrodes. New Carbon Materials. https://doi.org/10.1016/S1872-5805(26)61065-7
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Frequently Asked Questions
What is the maximum salt adsorption capacity (SAC) reported for CNT-based electrodes, and how does it compare to activated carbon?
CNT-based electrodes have achieved SAC values up to 25 mg/g in 500 mg/L NaCl solution at 1.2 V, as demonstrated by MOF-derived nitrogen-doped carbon/CNT heterostructures. This is significantly higher than conventional activated carbon electrodes, which typically exhibit SAC below 10 mg/g under similar conditions.
How does the addition of CNTs to activated carbon electrodes affect charge efficiency and energy consumption?
Dispersing a low content of CNTs (e.g., 5 wt%) into activated carbon electrodes enhances electrical conductivity by approximately 30%, reducing internal resistance. This leads to improved charge efficiency (>80%) and lower energy consumption per gram of salt removed, making the process more cost-effective.
What are the main challenges in scaling up CNT-based CDI electrodes for industrial desalination?
Key challenges include the high cost of CNT synthesis, electrode fouling due to organic matter, and the need for robust long-term stability. Flow-electrode systems incorporating CNTs have shown promise, achieving salt removal rates of 0.5 mg/(cm2·min) at 20 mA/cm2, but further optimization is required to reduce capital and operational costs.
How do MOF-derived CNT heterostructures improve CDI performance compared to pristine CNTs?
MOF-derived nitrogen-doped carbon/CNT heterostructures provide a higher specific surface area and additional active sites for ion adsorption. They also exhibit enhanced wettability and electrical conductivity, leading to a SAC of 25 mg/g and capacitance retention of 95% after 100 cycles, outperforming pristine CNTs which often suffer from aggregation and lower ion accessibility.
What is the typical operating voltage for CNT-based CDI, and how does it affect water electrolysis?
CNT-based CDI typically operates at voltages below 1.2 V to avoid water electrolysis, which occurs at approximately 1.23 V. This low voltage ensures energy efficiency and prevents unwanted side reactions, maintaining electrode stability and water quality.
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