Letter Cite This: Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/journal/estlcu
High-Performance Capacitive Deionization via Manganese OxideCoated, Vertically Aligned Carbon Nanotubes Wenbo Shi,*,†,‡,∞ Xuechen Zhou,†,∇,∞ Jinyang Li,§ Eric R. Meshot,∥ Andre ́ D. Taylor,†,⊥ Shu Hu,†,# Jae-Hong Kim,†,∇ Menachem Elimelech,†,∇ and Desiree L. Plata*,†,‡ †
Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States § Key Laboratory of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, P. R. China ∥ Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, United States ⊥ Department of Chemical and Biomolecular Engineering, New York University, Brooklyn, New York 11201, United States # Energy Sciences Institute, Yale University, 810 West Campus Drive, West Haven, Connecticut 06516, United States ∇ Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment (NEWT), Yale University, New Haven, Connecticut 06520, United States
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S Supporting Information *
ABSTRACT: Discovering electrode materials with exceptional capacitance, an indicator of the ability of a material to hold charge, is critical for developing capacitive deionization devices for water desalination. Maganese oxides (MnOx) have shown promise as capacitive electrode materials, but they exhibit a trade-off in which a higher loading of the active MnOx comes at the cost of lower conductivity. To address this challenge and achieve high salt adsorption, we fabricated electrodes comprising vertically aligned core−shell nanostructures using atomic layer deposition (ALD) to coat thin films of MnOx onto vertically aligned carbon nanotubes (VACNTs). The inherently hierarchical, anisotropic, three-dimensional macroporous structure of VACNTs and the tunable coating, a hallmark of ALD, enabled co-optimization of the hybrid material’s specific capacitance with respect to mass and geometric area. The specific capacitance was optimized in this study to 215 ± 7 F/g and 1.1 ± 0.1 F/cm2 in a 1 M NaCl electrolyte at a scan rate of 5 mV/s. This material exhibited a remarkable sodium ion adsorption capacity of 490 ± 30 μmol of Na/g of material (2-fold higher than that of pristine VACNTs) at a functioning voltage of 1.2 V, which may ultimately enable expanded desalination applications of capacitive deionization.
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INTRODUCTION The growing population and corresponding rise in clean water scarcity create appeal for transformative desalination technology development and innovations.1−3 Capacitive deionization (CDI), during which salt is removed via an electrochemical method, has emerged in an effort to tackle the limitations of the complex infrastructure and high cost of other welldeveloped desalination technologies (e.g., reverse osmosis and nanofiltration).4−6 To realize CDI, electrode materials that maximize charge storage capacity well beyond current achievements must be developed. In particular, nanostructured electrode materials, which have provided substantial advances in electrochemical energy storage systems due to inherently large specific surface areas and tunable morphologies and compositions,7−9 might be enabling components in future CDI devices for water treatment. Conventional CDI devices utilize the electrical double-layer capacitance of porous carbon materials,10 and recent advances © XXXX American Chemical Society
have demonstrated that adding pseudocapacitive metal oxides11−16 to form hybrid materials could boost the salt adsorption capacity. Pseudocapacitance (i.e., “charge storage capacity”) refers to storage of charges based on fast reduction− oxidation (redox) reactions occurring at the electrode− electrolyte interface, rather than the accumulation of ions in the electrical double layer, as in classical capacitance. In the field of supercapacitors, carbon nanotube-manganese oxide (CNT-MnOx) materials are a well-studied, prominent hybrid system17−19 that leverages the high electric conductivity and large surface area of the CNTs and the low cost, earth abundance, and high theoretical specific capacitance of MnOx. More importantly, when combined, these individual strengths Received: July 31, 2018 Revised: September 21, 2018 Accepted: September 25, 2018
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DOI: 10.1021/acs.estlett.8b00397 Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX
Letter
Environmental Science & Technology Letters
Figure 1. Vertically aligned coaxial CNT-MnOx hybrid synthesis and characterization. (a) Schematic illustration of the MnOx coating strategy on VACNTs via ALD and proposed model of electrosorption of ions on the coated VACNTs. (b) Cross-sectional scanning electron microscopy image demonstrating the alignment of pristine VACNTs. (c) Increase in VACNT diameter after 150 ALD cycles. (d and e) Transmission electron microscopy images demonstrating the tunable MnOx coating by changing the ALD cycle number at 75 and 150 cycles, respectively. (f) Scanning transmission electron microscopy image (high-angle annular dark-field imaging) of VACNT-MnOx with 75 ALD cycles and (g) corresponding energy-dispersive X-ray spectroscopy elemental mapping confirming the presence of the MnOx coating (Mn colored blue and O colored red).
VACNT-Mn3O4 did not exhibit compelling capacitance values as expected, possibly because of poor CNT quality (e.g., number density, alignment, and height) originating from VACNT direct growth on a conductive substrate. Previously, we demonstrated practical strategies for overcoming these limitations by producing densely packed, well-aligned, tall VACNT forests.45,46 Here, we hypothesize that a cyclic ALD technique for coating MnOx onto those VACNT structures (Figure S1), and subsequently transferring that product to a conductive substrate in a nondestructive way, would produce a vertically aligned, coaxial electrode configuration (Figure 1a) with improved sodium ion accessibility and electrical storage performance. As posited, coaxially coating VACNT-MnOx endowed them with outstanding capacitance with respect to both mass and geometric area, providing promise for superior candidate electrode materials for CDI water desalination.
can offset or overcome the critical limitations of each component in isolation (e.g., the insulator behavior of MnOx20 or the low-specific capacitance feature of CNTs). Although CNT-MnOx materials have been demonstrated with high capacitance in supercapacitor applications, the available studies utilizing CNT-MnOx as CDI electrode materials are limited, and perhaps because of poorly designed material architectures, the reported adsorption capacities are low (
