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We devised a simple and effective method of electrochemical functionalization of horizontally aligned CNT films in diluted HCl and H2SO4 solutions upon their electrolysis under a constant current mode. We were able to cause notable generation of carbon–oxygen and carbon–chlorine functional groups on the CNT film anodes as proven by EDX, XPS, and Ra...
Citations
... By using an AC supply in which the voltage polarity was rapidly switched, the researchers were able to demonstrate stable Joule heating (up to 95°C in air) without membrane degradation or water splitting occurring, as is common in ionisable media like salt water under high DC potentials [120][121][122]. They achieved fluxes of just over 8 LMH with greater than 99% rejection and very high single-pass recovery values approaching 100%. ...
Membrane distillation (MD) is a thermal-based separation technique with the potential to treat a wide range of water types for various applications and industries. Certain challenges remain however, which prevent it from becoming commercially widespread including
moderate permeate flux, decline in separation performance over time due to pore wetting and high thermal energy requirements. Nevertheless, its attractive characteristics such as high rejection (~100%) of non-volatile species, its ability to treat highly saline solutions under low operating pressures (typically atmospheric) as well as its ability to operate at low temperatures, enabling waste-heat integration, continue to drive research interests globally. Of particular interest is the class of carbon-based nanomaterials which includes graphene and carbon nanotubes, whose wide range of properties have been exploited in an attempt to overcome the technical challenges that membrane distillation faces. These low dimensional
materials exhibit properties such as high specific surface area, high strength, tuneable hydrophobicity, enhanced vapour transport, high thermal and electrical conductivity and others. Their use in MD has resulted in improved membrane performance characteristics like
increased permeability and reduced fouling propensity. They have also enabled novel membrane capabilities such as in-situ fouling detection and localised heat generation. In this review we provide a brief introduction to membrane distillation and describe key membrane characteristics and fabrication methods. We then give an account of the various uses of carbon nanomaterials for MD applications, focussing on polymeric membrane systems. Future research directions based on the findings are also suggested.
... Several studies have been performed so far on the effect of their incorporation into polymers [polydimethylsiloxane (PDMS) [22,23], polyimides (PIs) [22][23][24], poly (imide siloxane) (PIS) [1,21,22], polyphenylsulfone (PSF) [22,25], polystyrene (PS) [21,25], CA [19], poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) [26][27][28][29], polyethersulfone (PES) [19][20][21], polyaniline (PANI) [2], etc.] on selectivity and gas separation properties, especially of H 2 , CO 2 , CH 4 , N 2 , O 2 from various gas mixtures. Nevertheless, CNTs could be privileged over other carbon allotropes due to their excellent overall mechanical properties, large specific surface area and smooth surface [1,2,4,30,31]. The hybrid inorganic-organic membranes are expected to combine the advantages of both polymer (enhanced gas transport properties) and CNTs (enhanced physical and mechanical properties) for gas separation. ...
... Moreover, the interfacial voids between MWCNTs and the polymer would only provide discontinuous and tortuous paths for gas transport if nanotubes were randomly dispersed in the polymer matrix [9]. To improve dispersion of CNTs in the polymer matrix one could use few methods like functionalization/modification of CNT outer walls and/or polymer matrix [19,21,23,24,30,31]. However, functionalization of pristine CNTs could be difficult due to generally low reactivity of regular hexagonal side-walls, although they have two distinct regions of enhanced reactivity, i.e. ends/tips rich in pentagons and/or free valences and graphene-wall defects [19]. ...
... Nevertheless, by exploiting MWCNT chemical modifications, many of their properties can be significantly enhanced, for instance: catalytic activity, dispersibility in solvents/matrices or capability of anchoring other functional molecules. Covalent functionalization is one of the most powerful CNT functionalization routes [3,24,27,30], and the main method covers oxidation, which equips CNTs with reactive functional groups, i.e. carboxyl, hydroxyl, ketone, lactone, anhydride, etc. This approach can be accomplished by many oxidants like ozone, concentrated oxidizing acids, oxygen at high temperature or radiation-supported [31]. ...
Various inorganic-organic hybrid membranes based on regular and modified poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) matrix and iron-encapsulated multi-wall carbon nanotubes (Fe@MWCNTs) were designed and prepared for air separation. Addition of Fe@MWCNTs into the PPO matrix has changed gas transport parameters (D, P, S and α) through the hybrid membranes. Indeed, we have demonstrated that application of magnetic field during the stage of nanotube-polymer membrane preparation has enhanced alignment of the nanotube filler in the polymer matrix and improved separation performance of the membranes. Additionally, hydroxylation of Fe@MWCNTs and sulfonation of PPO matrix-as applied to enhance the nanotube-polymer interphase via hydrogen bonding-also accompanied by external magnetic field have allowed to obtain membranes with higher permeability, diffusivity and permselectivity. Mechanical properties, such as tensile strength (R m) and Young's modulus under tensile (E), as well as magnetic parameters of the membranes were further found as improved by the increase of Fe@MWCNT content and the presence of magnetic field.
... However, to the best of our knowledge, all reported CNT Joule heaters have been demonstrated in low-ionizable conditions, where the application of the high voltages (10-20 V pp ) required to power the element did not ionize the surrounding media, which ensured the CNTs maintained their stability and heat-transfer properties 7,8,12 . However, in ionizable mediums, such as high-salinity brines, the application of even low anodic potentials (<2 V d.c. ) to the CNT film can lead to surface charging, water splitting and rapid degradation [23][24][25] . Although polymer coatings could be used to shield the CNTs from the environment, these coatings block the porous structure of the film and form an additional thermal barrier between the heated CNT network and the fluid. ...
Water shortages and brine waste management are increasing challenges for coastal and inland regions, with high-salinity brines presenting a particularly challenging problem. These high-salinity waters require the use of thermally driven treatment processes, such as membrane distillation, which suffer from high complexity and cost. Here, we demonstrate how controlling the frequency of an applied alternating current at high potentials (20 Vpp) to a porous thin-film carbon nanotube (CNT)/polymer composite Joule heating element can prevent CNT degradation in ionizable environments such as high-salinity brines. By operating at sufficiently high frequencies, these porous thin-films can be directly immersed in highly ionizable environments and used as flow-through heating elements. We demonstrate that porous CNT/polymer composites can be used as self-heating membranes to directly heat high-salinity brines at the water/vapour interface of the membrane distillation element, achieving high single-pass recoveries that approach 100%, far exceeding standard membrane distillation recovery limits.
... Electrolysis of diluted HCl and H 2 SO 4 aqueous solutions by electrodes made of CNT macroassemblies caused a notable introduction of functional groups at the anode. The electrodes themselves are very attractive in terms of structural properties and also require lower overpotential for faradaic splitting of water 119 . Alternatively, macroscopic assemblies from CNTs can be turned into resistive heaters at sufficiently high bias voltage. ...
The direct-spinning method of creation of CNT macroassemblies has received a lot of attention because of its simplicity to produce high-performance material without apparent limits to its size. CNT fibers or films have shown unparalleled properties and opened new areas of research and commercial development. The process designed more than a decade ago has already given interesting information about the basic science of nanomaterials, which in parallel led to the creation of the first prototypes with high potential of implementation in everyday life. Because of this, there has been growing interest in this technique with research articles coming into view from all around the world on a frequent basis. This review aims to summarize all the progress made in the direct-spinning process on a spectrum of fronts ranging from the study of complex synthesis parameters, material properties to its viable applications. The strong and weak points of the "Cambridge process" are carefully evaluated to put forward what challenges are most pressing. The future overlook puts the state of the art into perspective and suggests the prospective research directions.
... Powdered CNTs dispersed in solvents have been used to coat cotton fabric and create CNT/textile heaters [21]. CNT-based heaters have also found applications as efficient and durable electrodes for Faradaic water splitting [22]. CNT sheet heaters have been fabricated as transparent film heaters, due to their high transmittance and good heating capabilities [12,[15][16][17]. ...
... Together with the increase in their external dimensions, the active surface area is expanded and sensing parameters of ACNTs are improved. The process of swelling of ACNT fibers has already been observed by our group [15], however, it is the first preliminary report in which its origin has been analyzed. ...
... The process of swelling can be partially related to electrolytic hydrogen evolution. Since the ACNT fiber is a porous material, hydrogen is evolved not only on the outer surface of fibers but also in the free space existing between individual nanotubes [15]. when recording cyclic voltammograms (CVs) of potassium hexacyanoferrate(II), which was employed as a reference compound (Fig.1b). ...
Aligned carbon nanotube fibers are macroscopic materials with remarkable properties, such as high specific strength, stiffness, extreme flexibility as well as electrical and thermal conductivity. It is demonstrated that when subjected to negative potentials, these structures undergo the process of swelling in which the increase of their external dimension is observed. Swelling is believed to be caused by cation insertion in the process similar to intercalation. The efficiency of swelling was determined both in organic and aqueous solutions of different pH. Chronocoulometry was used as the technique to monitor the charging-discharging processes of swollen ACNT fibers in a presence of different electrolytes, i.e. LiCl, NaCl and KCl. The possibility of performing the charging-discharging cycles multiple times indicates that the swollen ACNT fibers can be considered as an advantageous material for electrodes in ion batteries and supercapacitors.
... As a kind of 2D macroscopic material of CNT, the array-based CNT film has high-load CNTs, less impurities and good arrangement, due to the unique solid-state transferring process, and exclusion of surface modification and dispersion [5]. Therefore, it is an efficient transforming structure of CNTs, and performs good mechanical and functional properties, hugely potential in the application of flexible displays [6,7], chemical and biological sensors [8,9], electrodes [10,11], loudspeakers [12], microwave adsorption materials [13], solar cells and supercapacitors [14,15], etc. ...
... Vertically aligned carbon nanotubes, VA-CNTs, commonly called carpets, have well-defined large surface area with a narrow range of tube length and diameters [10]. It was recently demonstrated that thin films of densely packed horizontally aligned CNTs, HA-CNTs, deposited on PET surface may serve as good electrode materials [11]. These fibers are packed horizontally to each other and in a dense manner to achieve full coverage of the support. ...
The process of conjugated polymer deposition on the surface of horizontally aligned multi-walled carbon nanotube, HA-CNT, electrode is described. Poly(3,4-ethylenedioxythiophene), PEDOT, was grown electrochemically under variable conditions in both aqueous and non-aqueous solutions of selected electrolytes. It is shown that the mechanism of nucleation highly depends on the reaction environment. The presence of a surfactant or non-aqueous medium favors the process of progressive nucleation in which the polymer growth is uniform and a homogeneous film of PEDOT is formed. It is demonstrated that the conditions make it is possible to cover the outer walls of individual HA-CNTs instead of forming thick polymer layer. The application of overpotential is proven to be a necessary condition to generate radical cations and cause relatively fast growth of polymer layer.
Membrane distillation (MD) is a sustainable approach for the treatment of challenging saline water by effective removal of non-volatile compounds at high water recovery, offering near-to-zero liquid discharge to environment. Progressive efforts have been made in recent literature to mitigate membrane fouling and enhance the wetting resistance of MD for long-term stable operation; however, extensive energy consumption is the key constraint that hinders MD to become an economically sustainable solution for industrialization. This review represents the evaluation of energy consumption in MD in comparison with other existing advanced water treatment technologies (e.g., reverse osmosis). An up-to-date review of low-energy MD utilization to minimize energy consumption is provided in this work. High energy consumption in MD can be compensated by the effective utilization of renewable energy sources such as solar energy, geothermal energy, or waste heat. However, due to the sporadically unequal distribution and unstable availability of these low-grade sources, the dependence on the abundance of these energy sources may limit the flexibility in commercial MD applications. A recent approach to reduce specific thermal energy through direct heating of the membrane or spacer is also discussed in this review. The development of the membrane materials/configurations was highlighted for mitigating the effects of temperature polarization and improving energy efficiency by localized heating at/near the membrane surface by using photothermal, electrothermal, or induction materials.
Self-heating membrane distillation has been considered as an alternative technology to alleviate water crisis, while the unsatisfactory energy efficiency and distillate flux limit the practical application. Herein, an all-in-one conductive hollow fiber membrane without supporting layer was fabricated, in which carbon nanotube (CNT) guaranteed the rapid electro-thermal conversion and polydimethylsiloxane (PDMS) endowed the composite membrane with superhydrophobicity to enhance membrane stability. The narrow channel of the hollow fiber membrane encircled the feed solution in a localized space to achieve efficient vaporization, which was benefit for both high higher distillate flux and low energy requirement. As a result, the CNT/PDMS hollow fiber membrane obtained a high distillate flux of 24.27 ± 0.89 kg m⁻² h⁻¹ and salt rejection rate > 99.9 % under 2.5 W input power with a flow rate of 12.81 mm s⁻¹. Meanwhile, this membrane displayed a superior salt-resistance with increasing of the feed solution concentration from 10 g L⁻¹ to 100 g L⁻¹. For actual seawater desalination, the concentration of representative ions in collected water met the requirement standard of soft drinking water, indicating a great feasibility in practical application. Thus, this energy-efficient hollow fiber membrane provides a promising avenue to fabricate high-performance self-heating membrane for membrane distillation.
