Tough Electrodes: Carbon Nanotube Fibers as the Ultimate Current

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Perspective

Tough electrodes: carbon nanotube fibers as the ultimate current collectors/active material for energy management devices Juan J. Vilatela, and Rebeca Marcilla Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 22 Sep 2015 Downloaded from http://pubs.acs.org on September 28, 2015

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Chemistry of Materials

Juan J. Vilatelaa,*, Rebeca Marcillab a) IMDEA Materials Institute, Eric Kandel 2, Getafe, Madrid, 28005, Spain. b) IMDEA Energy Institute, Avda. Ramón de la Sagra, 3, Móstoles, Madrid, 28935, Spain. ABSTRACT: The assembly of nanocarbons (carbon nanotubes, graphene) into macroscopic architectures has led to a new type of mesoporous graphitic materials with an exceptional combination of high surface area, electrical conductivity and toughness. With a porosity close to that of an activated carbon and tensile properties in the high-performance range, macroscopic carbon nanotube (CNT) fibers are ideal current collectors for lightweight robust devices, such as supercapacitors, batteries, biofuel cells, solar cells and energy scavengers. In this Perspective, we discuss the basic properties of CNT ensembles compared to other carbon electrodes and present examples of their operation as part of electrochemical and electronic devices. We show the possibility to assemble the nanocarbon building blocks hierarchically and combine them with additional phases (e.g. metal oxides, semiconductors and polymer electrolytes) to produce large-surface electronic junctions and interfaces for electrochemical energy storage. The challenges ahead include understanding better the interactions at the interface between nanocarbon electrode and active phase, and how they can be chemically tailored to exploit more efficiently interfacial charge transfer and accumulation processes.

1.

Electrodes for structural electronics

There is a growing interest in electronic devices that can withstand large mechanical deformations/stresses during operation. This trend if often referred to as structural electronics, though perhaps a more accurate description is tough electronics, since in most applications the active phase is not a structural element. The vision is that the development of flexible, stretchable or strong electronics can result in a new generation of energy storage devices, sensors, actuators, displays, etc., with innovative possibilities of integration and performance. This calls for the use of new materials that can perform the electronic function while also having more mechanical robustness than traditional ones. There is, for example, rapid progress in the synthesis of ductile polymer-based semiconductors, electrolytes, and in general molecularly-stretchable electronics.1 Similarly, new assembly routes to circumvent the brittleness of inorganic materials by forming percolating networks of high aspect ratio elements rather than monolithic structures are now common, for example by dispersing Ag nanowires in an elastomeric polymer matrix above electrical percolation.2 In the quest for tough electronics the architecture of the material responsible for energy conversion/storage in the device is specific to the application, but the need for a conductive electrode is shared across all of these devices. Its basic function is to collect and/or transfer the current produced by the device with minimal losses (hence the use of highly conductive metals in non-structural electronics); however, in the context of multifunctional devices, the

electrodes must also be light and mechanically durable. Furthermore, if the electrode has a high porosity/surface area, it can also take part in energy storage at its interface with an electrolyte, by direct intercalation of ions, or by hosting another phase that can undergo a redox reaction. Its electronic structure and its role in separation of photogenerated electron-hole pairs are also relevant when used as electrode in photovoltaic devices. But in general, the most important requirements on electrodes for tough electronics can be summarized as high electrical conductivity (with thermal a surplus), surface area, chemical stability and mechanical robustness. It is thus not surprising that nanocarbons (carbon nanotubes and graphene) have been extensively used in this field, as they seem to match these requirements perfectly. They combine, in principle, exceptionally high carrier mobility (>100000 cm2/V),3 tensile strength (100GPa),4 Young’s modulus (1TPa)4 and toughness (>1000J/g), with a low density (2.1g/cc) and a high surface area approaching the activated carbon range.5 Indeed by assembling these building blocks, particularly long CNTs as continuous fibers, their axial properties have been efficiently exploited on a macroscopic length-scale. Normalized by their low density (1-2g/cc), CNT fiber arrays have tensile properties in the high performance range,6,7,8 electrical conductivity similar to steel and a large specific surface. As such, they have been a natural choice of electrode material in a wide range of energy-related devices.

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Figure 1. Different methods for spinning CNT fibers and scanning electron micrographs of representative samples. From top to bottom: direct spinning from the gas-phase during CNT synthesis by CVD,9 drawing from a forest of aligned CNTs10,11 and wet spinning of nanocarbons dispersed in liquid. 12,13 Reprinted with permission from refs 9-13. Copyright 2010 John Wiley & Sons, Inc. Copyright 2012 Elsevier. Copyright 2000, 2004 and 2013 AAAS.

In this Perspective, we first summarize the synthetic routes to spin CNT fibers and compare their basic properties with those of different carbon-based electrodes in the context of tough electrodes. Then we present selected examples reported of the use of CNT fibers as electrodes in robust supercapacitors, batteries, biofuel cells, solar cells and energy harvesters. Next we discuss strategies to engineer the interfaces between nanocarbon and the other phase(s), highlighting those using in-situ growth methods leading to hybrid structures with improved interfacial charge transfer and accumulation processes. Finally, we discuss open challenges in terms of characterization of the

complex hierarchical structure of these materials, as well as their opto-electronic-electrochemical properties. 2

Carbon-based electrodes for tough electronics

Carbon-based materials have been traditionally attractive for electrodes because of their low density, high chemical stability, high electrical conductivity, and depending on the application, their high surface area. Most commercial electrodes in electric double layer capacitors (EDLCs), for example, consist of a mixture of activated carbon obtained by carbonization of coconut fibers, carbon black and a binder to form a porous structure which is then infiltrated with an electrolyte. The carbon host is either a


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Figure 2. Comparison of CNT fiber, carbon fiber and activated carbon as candidates for tough electrodes. a) SEM micrographs showing the networks structure of CNT fibers compared to monolithic CF and AC powder (modified from 14). b) The long length of CNTs enables both a large porosity and high-performance properties (courtesy of B. Mas). c) Comparison of BET surface area and toughness of the different carbon-based electrodes. SBET and toughness data for CNT fibers from references 15, 16, 17 and 18 (direct spun); 19 and 20 (coagulation-spun); 21, 22, 23 and 13 (liquid-crystal spun); for CNT membranes from references 24 and 25; for CF from reference 26 and for AC from references 27 and 28.

powder, a paste or a brittle porous monolith. Similarly, in electrochemical devices (pseudocapacitors, batteries, fuel cells), both the metallic electrodes and the carbonaceous material often used as conductive filler, have limited mechanical resilience. Transparent electrodes used in photovoltaic devices are often also brittle materials, such as highly doped metal oxides. Although suitable for conventional monofunctional devices, these electrodes/current collectors cannot be subjected to significant mechanical deformations. High aspect ratio nanocarbons in the form of long CNTs or large graphene sheets are ideal candidates for tough electronics because they combine the molecular properties discussed above with the possibility to assemble them into a network structure such as a fiber or a membrane that can thus withstand large stresses and deformations. Such network is an interesting scaffold to host other materials, integrated either by in-situ deposition or exploiting the wide range of functionalization protocols developed for CNTs.

Membranes of randomly oriented CNTs are typically produced by vacuum filtration and often referred to as buckypaper. CNT fibers, on the other hand, are continuous macroscopic filaments of CNTs predominantly aligned parallel to the fiber axis. The fibers can be produced by four methods: direct spinning from the gas phase during chemical vapour deposition,29 drawing from an array of vertical CNTs,11 wet-spinning from lyotropic liquid crystalline dispersions30 and wet-spinning from polymer-CNT dispersions.12 A schematic representation of these process and representative electron micrographs are presented in Figure 1. The direct and wet-spinning processes have been under scale-up for various years, with current samples sizes in the m2 range announced for the former.31 The porosity and mechanical properties of CNT fibers are strongly determined by inter- and intra-particle properties, and by the morphology of the network in terms of orientation and size of the nanobuilding blocks. Because of



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the long length of the constituent CNTs in the range of millimeters,32 both contact points and porous regions coexist in CNT fibers. The contact points correspond to domains of adjacent well-stacked CNTs in bundles or other fiber subunits, and are responsible for most stress and charge transfer. The pores result from the imperfect packing of elements, and represent those areas exposed to foreign molecules. Figure 2a presents an electron micrograph showing this CNT fiber network structure, and a comparison with two traditional carbon electrodes: a carbon fiber with monolithic structure and only its external surface exposed, and activated carbon showing large porosity and a structure of aggregated particulates. Typical electrical conductivities of these materials are also included. Figure 2b shows a schematic of the CNT fiber network structure giving rise to both porosity and continuity of large crystalline graphitic elements. The performance of these CNT fibers as electrodes depends in general on their textural properties (e.g. pore size distribution and specific surface area (Sbet)), capacitance, electronic structure and redox potential relative to the other phases in the device. Used in tough electronics, their mechanical properties and light weight are also relevant. The precise mechanical requirements are dictated to some extent by the function of the device and whether it is intended to operate in flexion, at large strains (stretchable) or have a high tensile strength. But simplifying, the material’s property that best captures these mechanical requirement is probably specific toughness, T, taken as the energy absorbed by the material up to fracture, normalized by its specific gravity. It contains indirectly a measure of both strength and strain-to-fracture. Figure 2c presents plots of Sbet vs T for a variety of carbon-based materials from literature data. At the high end of surface area values are activated carbons, with Sbet up to 2000 m2/g,27 but negligible toughness, even when combined with a polymer binder ( 90 MPa at

produced a rapid mechanical and electrochemical degradation due to the limited porosity available for silicon expansion. Using a similar Si CVD method, Fu et al. produced CNT–Si sheets which reached 3322 and 2487 mAh/g discharge and charge capacities, with a first cycle CE of 75% and 97% for the next 30 cycles.66 Lithium-sulfur. Despite the advantages of Li-S storage technology, the practical application of sulfur as the cathode requires circumventing drawbacks such as the inherently poor electrical conductivity of sulfur (5×10−3 S/cm), the formation of soluble polysulfide Li2Sx at the cathode and its contemporary migration in the electrolyte, and the relatively large volume variation of sulfur during charge/discharge processes. In the last years, these issues have been mitigated by developing carbon-sulfur composites, and only recently, this strategy has been also applied to flexible sulfur electrodes. Liu et. al have reported highly



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Figure 8. CNT fibers used as electrodes in biofuel cells. (Top) Schematic of the bioelectrochemical system and a comparison of CF and CNT fiber electrodes.67 Reprinted with permission from ref 67. Copyright 2010 Nature Publishing Group (Bottom) Alternative electrode architecture obtained by infiltration of the enzymes and polymer mediators during twisting of fibers into a biscrolled structure. 68 Reprinted with permission from ref 68. Copyright 2014 Nature Publishing Group

flexible and robust CNT–graphene/sulfur (CNTs–RGO/S) free-standing composite as cathode for Li/S batteries, with tensile strength up to 72 MPa, modulus of 9.7GPa and a toughness around 0.25J/g (Figure 7).65 The free-standing CNTs–RGO/S cathode was able to deliver a peak capacity of 911.5 mAh g–1sulfur (∼483 mAh g–1electrode) and maintain 771.8 mAh g–1sulfur after 100 charge–discharge cycles at 0.2C, indicating a capacity retention of 84.7%, both higher than the cathodes assembled without CNTs. Even after 100 cycles, the cathode showed a tensile strength of 62.3 MPa. Metal-air battery. Lithium air batteries have attracted much attention in recent years due to their outstanding

theoretical energy density of about 3500 Wh/Kg. Air-electrodes usually contain a carbonaceous material, a catalyst and a binder to keep the integrity of the electrode. Recently, commercial CNT fibers mats have been used as airelectrode in a lithium-air battery, in which the typical liquid electrolyte was substituted by PEO-based polymer electrolyte.69 Although this solid version of Li–O2 cell exhibited lower electrochemical and discharge performance compared to their liquid electrolyte Li–O2 cell, the advantages of such a system rise well above its flaws. For instance, although not fully demonstrated in the article, the use of both solid polymer electrolyte and mechanical stable CNTs open the possibility to obtain structural or flexible


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lithium-batteries. Moreover, the attachment of organic or inorganic catalysts70 to the CNT fibers is an effective ideal technique to increase energy storage in such batteries and which remains largely unexplored for these electrodes. 3.3 Biofuel cells CNT fibers have also been used as electrodes in biofuel cells and shown to provide high output power compared with conventional electrodes. But more importantly, they have led to devices with high mechanical robustness, relevant for many envisaged applications of micro biofuel cells, such as implants in the body that can power cardiac pacemakers and other biomedical devices. CNT fibers modified with enzymes (biocatalyst) and redox polymer mediator are used as both anode and cathode in a biofuel cell where glucose is electro-oxidized at the anode and oxygen reduced to water at the cathode under physiological conditions, as schematically shown in Figure 8. To render the electrodes hydrophilic and thus enable the immobilization of the enzymes, the cross-linker and the redox polymer, CNT fibers are subjected previously to either plasma oxidation67 or coated with poly(3,4-ethylenedioxythiophene) (PEDOT) via a vapor-phase polymerization.71 Though specific to testing conditions, output powers of these biofuel cells are in the range of 750 W/cm2, and up to 2.18mW/cm2 (793 W/cm3) when the diffusion distance between fuel and oxidant is minimized in biscrolled electrode fibers (Bottom Figure 8) produced by applying twist during infiltration of the active species into CNT films (Figure 8 - bottom).68 The reported tensile strength and toughness of such bi-scrolled electrodes (without electrolyte) are around 70MPa and 1.4J/g, respectively. They are comparable to those of the PEDOT phase in the fibers, as indeed it represents 88wt. %. These device mechanical properties are a good achievement and probably sufficient for biological applications, but clearly there is ample room for improvement. Key for the operation of CNT fiber-based biofuel cells and their high output power, compared for example with CF electrodes (Figure 8 - top), is their combination of a high surface area with a sufficiently open porosity to enable first, successful infiltration of polymer redox mediators and enzymes during assembly and then, efficient diffusion of glucose and oxygen and their evolved species during operation. This porous structure can be efficiently exploited by PEDOT-CNT hybridization. PEDOT produces a more robust connection of biocatalyst with the CNTs and also provides increased electrode stability in human serum by avoiding the incorporation of chemical species as urate, chloride and ascorbic acid that can damage the electrode.67 Further enhancements in electrochemical performance can arise from controlled assembly of the hybrid structure. In the process, strategies that go beyond plasma oxidation or the deposition of a polymer to increase hydrophilicity, and instead, promote the interaction between redox polymer and CNT fiber through surface tailoring might prove useful to increase energy conversion.

Figure 9. CNT-based Schottky solar cell. a) Deposition of a CNT film onto patterned Si and (b-c) its use as a front electrode. d) Superior photovoltaic efficiency is obtained with CNT fibers compared with randomly oriented CNTs.73 Reprinted with permission from ref 73. Copyright 2013 John Wiley & Sons, Inc.

3.4 Solar conversion devices Jia et al produced one of the first examples of Schottky solar cells with CNT fibers by fixing 17-30nm diameter twisted filaments of SWNT arrays on n-doped Silicon with transparent tape.72 The short fibers had tensile strength of 0.8-1GPa and moduli of 8- 10 GPa, toughness around 67J/g (Calculated from reference 72 assuming linear stress-strain up to 15% and density of 1g/cc.) and conductivity of 1.8x10 5 S/m. These devices had limited power conversion efficiencies around 3%, but demonstrated a simple integration route for Schottky junction solar cells with CNT fiber electrodes as hole collectors. The approached was later improved by Di et al, who used thin film (50-100nm) of fewlayer CNTs drawn from a vertically aligned array and deposited it on a pre-patterned n-doped Si wafer.73 In this configuration, schematically shown in Figure 9, the thin CNT film has a high transmittance and can thus operate as a front electrode in the Schottky cell. They obtained a



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short-circuit current density of 33.4mA cm-2, open circuit voltage of 540mV and a fill factor of 58.3%. The measured power conversion efficiency (PCE) was in the range of 10% and typically a factor of 3-4 higher than that obtained using randomly dispersed CNTs of equivalent composition. Improvements using PMMA and HNO3-doping of the CNT electrode increased PCE to 13.1%.74 This architecture of depositing CNT fibers on monolithic semiconductors exploits the mechanical properties of the CNT film material only in terms of the robustness of the thin 50nm films enabling a simple deposition process over cm2 areas, but which results in conventional planar and stiff devices. However, recent work combining CNT fiber films with thin Ti foil, TiO2 nanotubes and halide perovskites has demonstrated flexible solar cells with PCE of up to 8.3%. The construction, shown in Figure 10, maintains the role of CNT fibers as high-transmittance front electrode, though combined with spiro-OMeTAD (2,2’,7,7’tetrakis-(N,N-di-p-methoxyphenylamined) 9,9’-spirobifluorene) to enhance hole collection.75 Using thin Ti as work electrode supporting TiO2 nanowires and hosting a perovskite absorber, Wang et al demonstrated flexible planar solar cells retaining about 83% of their efficiency after 100 bending cycles (Figure 10c). While the properties of these devices are still largely dominated by the much larger volume fraction of the Ti foil, TiO2 nanotubes and the perovskite, the flexibility attained is only possible because of the resilience of the CNT electrode.

Figure 10. CNT fibers used in a Ti-TiO2-perovskite flexible solar cell. (a) Demonstration of flexibility, (b) schematic of device architecture and (c) photovoltaic performance. Modified from.75 Reprinted with permission from ref 75. Copyright 2015 Elsevier

A popular approach to exploit the flexibility of CNT fibers consists in intertwining two CNT fibers, with one of them containing the photoactive material(s). Using either TiO2 loaded with a dye to form a dye-sensitized solar cell,76 or a perovskite (Figure 11),77 these devices have PCE values in the range 3%. Compared to metals, the use of CNT fiber-

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based electrodes provides resistance to oxidation by the dye (for DSSC), presumably a lower net device weight, but more importantly, large flexibility. These small two-fiber devices showed tolerance to cyclic bending without substantial degradation of power output. Larger samples of a few intertwined arrays based on CNT fibers combined with a Ti cathode and a polymer electrolyte and woven into cm2 devices showed only 80% retention of a PCE < 2% after 1000 bending cycles (Figure 11).78 Such results demonstrate the versatility of the intertwined architecture, the ease of weaving of CNT fiber-based devices, but also the challenge in preserving materials’ properties when increasing number of fibers and covering areas of even a few cm2.

Figure 11. Photovoltaic devices based on intertwined CNT fibers. (Top) Example of an architecture where the CNT fiber electrode hosts a preovskite, and demonstration of its applicatoin as a multifilament flexible device.77 Reprinted with permission from ref 77. Copyright 2015 John Wiley & Sons, Inc. (Bottom) DSSC consisting of a CNT fiber twisted around a dye-sensitised Ti-TiO2 wire electrode and demonstration of operation under bending. 76 Reprinted with permission from ref 76. Copyright 2012 American Chemical Society.

The above examples illustrate the potential of CNT fibers as electrodes in different photovoltaic devices, where their oxidation resistance, large surface area and flexibility is a clear advantage over metals. While intertwined fiber devices show promising results, the contact between active material and electrode (i.e. between fibers) relies on both filaments being tense enough to ensure proximity but without short circuiting and could thus be unreliable or difficult to reproduce. Electronic characterization of the multiple junctions in intertwined systems, and particularly the effect of their sensitivity to mechanical deformations, would be of interest to better understand their properties and to reduce their intrinsic source of variability. Planar PV devices have a comparatively simpler construction; however their use as front electrodes so far has implied that they are integrated as thin (30 layers) made by vacuum filtration of a suspension and toughness of around 0.8J/g (N.B. Toughness calculated from stress-strain curves in the original reference and assuming a density of 1g/cc for both materials). Interestingly, the authors noted that the ZnO micro rods were much thinner when grown on the few-layered CNTs. Piezoelectric measurements were performed on ZnO/CNT array samples with either Al or Ti as second electrode. Pressing the sample produced piezoelectric voltages of around 3mV, whereas manually bending them increased output voltage up to 50mV on account of the larger strain applied. The use of Al or Ti as electrodes seeks to form a

rectifying Schottky barrier that thus enables harvesting of only the current flowing in one direction. The theoretical position of the work functions of these metals and ZnO are around 4.2-4.3, compared with CNTs at 5eV,83 and would thus support the formation of the junction proposed and qualitatively agree with the rectifying behavior observed under I-V measurements. We note, however, other effects that might contribute to the I-V behavior observed, such as the formation of an oxide at the metallic electrode, the concentration of dopants in the ZnO and the presence of defects on the CNT surface changing their work function. As pointers to increase output power we highlight the need to establish a clear equivalent circuit of the system, carry out interfacial characterization by CV and impedance spectroscopy, and the use of additional techniques to experimentally confirm the electronic band positioning (ideally under ambient conditions). 4.

Relevance of the synthetic approaches, interfacial control and mechanical properties

Synthetic approaches One of the challenges in the synthesis of the materials discussed in this Perspective is simply determining the structure and uniformity of the final samples. Because of the confluence of length scales, adequate determination of the infiltration of foreign molecules into the mesopores of macroscopic fibers arrays is problematic and requires a combination of various characterization techniques. FIBmilled composite sections can be inspected by SEM,84 though the contrast between phases is often low and the resolution of the interface poor. WAXS-SAXS is useful to probe the CNT network structure and the orientation of phases, and if combined with synchrotron microdiffraction can distinguish the external and internal structure of composite filaments.85 It can help establish, for example, epitaxial relations for in-situ grown hybrids similar to those



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observed in semi-crystalline polymer-CNT fiber composites.86 Volume fractions of the phases can be determined from TGA, provided individual decomposition peaks can be resolved and the phases do not catalyse oxidation, as is often the case with metal oxides.87 Porosity studies from gas adsorption are also of great interest. Reported BET/BJH measurements are scarce, but point to a wide range of mesoporosity from a few nm to around 100nm. 15, 17, 18, 19 The mesoporous structure observed in CNT fibers is probably sufficiently large for infiltration of most molecules of interest during composite or hybrid synthesis. However, CNT fibers are typically highly anisotropic and so is their porosity.88 In this sense, stronger/stiffer fibers are expected to have a reduced surface area and pore size on account of the better packing of their building blocks. We also point to elastocapillary effects in these systems, whereby because of the high aspect ratio of CNTs, their interaction with solvents can be sufficiently strong to change the pore structure by “wedging” bundle junctions open.15 Furthermore, infiltrated mesopores behave as capacitive junctions which can close under electric fields.89 These two effects imply that the CNT mesoporous network is not static and can be altered, for example, by precursor diffusion into the pores and electric fields such as those applied during electrodeposition. A final comment about porosity is the fact that small pore sizes imply slow infiltrations. Dynamic measurements of wetting/wicking demonstrate behaviour close to LucasWashburn, with (one dimensional) infiltration rate scaling roughly as square root of average pore size.19,90 This dependence could make fabrication of uniform samples of higher thickness considerably more difficult by liquid/polymer infiltration compared to gas-phase processes. In general, more experimental work on liquid/polymer interaction with CNT porous media is desired, especially in order to distinguish between effects arising from device fabrication and those corresponding to inherent properties of the materials and their interfaces. Interfacial control The synthesis protocol varies for the different devices, but for all of them it is possible to distinguish between those produced by “simple” mixing of components to form a composite, or those produced by in-situ growth or selective attachment of one of the phases to form a hybrid. The distinction, championed by Eder,91 embodies the differences between top-down and bottom-up synthetic approaches, and is helpful to understand interfacial charge/energy transfer processes in such materials.92,93 In most composites discussed in this Perspective and in general in the literature, the integration of nanocarbon and additional phases is typically done by solvent-based processes such as vacuum filtration, casting or infiltration by immersion.68, 76, These methods rely on capillary forces introducing a volume of the polymer or inorganic phase into the fibers, followed by solvent removal/evaporation under conditions favouring uniform coverage of the internal surface of the CNT fibers.

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From the perspective of interfacial processes, the resemblance of such devices to composites is in the sense that the phases are in sufficient contact to take part in load and charge transfer but in which no control of these processes is expected.91 CNT fibers pressed on n-Si wafers, for example, result in a relatively small series resistance in the range of 62but probably on account of the ease with which CNT fibers reshape under transverse compression9 and flatten against a planar surface. This is adequate for the operation of the photovoltaic device reported, and would correspond to a relatively small contact resistance per unit area 1 cm-2 if the contact area is taken as the projection of the CNT fiber on the Si wafer, but the integrity of the contact under slight mechanical deformations is questionable. Interestingly, a simple contact produced by filtration can produce heterojunctions between nanocarbons and semiconductors94,95 of high thermal stability and surprisingly low resistance, however, this interaction is most likely not sufficiently mechanically robust for many of the multifunctional devices envisaged for the future and discussed here. The greater potential of CNT fiber electrodes lies in the ability to engineer the interface between CNTs and the energy-active phase, forming a hybrid with controlled electronic junctions. This requires selective attachment of this phase onto the CNT surface using surface chemistry functionalization, including non-covalent linking agents,96,97 or by direct growth of the phase in the presence of the CNTs and its nucleation on their surface. These strategies are routinely used to produce hybrids of nanocarbons in dispersions or in other non-fibrous arrangements, growing the active phase by methods such as sol-gel,91 CVD,98 ALD99 and in-situ polymerization. Adapting them to CNT fibers is feasible in most cases, except when the integration requires inherently to form a CNT dispersion. Growth of a uniform TiO2 on the internal and external surfaces of CNT fibers was for example, achieved by using benzyl alcohol as a linking agent100 taking advantage of its strong  interaction with CNTs.101 As an example of a hybridisation technique we note the electro-polymerization method to deposit PANI on the surface of CNT fibers, discussed above. This method results in controlled integration of the polymer on CNT fibers,42 and when applied to fibers pre-functionalised with carboxyl groups through nitric acid treatment,50 it leads to the formation of a smoother PANI layer and a large increase in the capacitance of the device. Interestingly though, the higher capacitance of the functionalised CNT fiber/PANI hybrid was equally attributed to: a) a higher fiber conductivity, b) faster monomer deposition and c) different polymer morphology; which illustrates the difficulty in pinning down the effects of a simple chemical treatment on the final complex hybrid structure. Another example of successful hybridization is the synthesis of uniform conformal coatings of hydrogenated amorphous Si grown in-situ by CVD on CNT fibers.64 This battery anode hybrid material was demonstrated to withstand several lithiation cycles without degradation of elec


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Figure 13. Schematic of the main features of CNT fibre-based energy management composites. A key aspect is the close proximity of the CNTs and the energy-active phase, which improves energy accumulation and transfer across the interface. The exceptionally large surface area of the fibres enables them to act as current collector/active materials and mechanical reinforcement.

trochemical properties of the electrode, implying a high resilience of the aSi-CNT interface to the large volumetric expansion occurring during charge/discharge and a constant equivalent series resistance on account of the strong interaction at the interface. This example demonstrates good mechanical properties retention after several cycles when the lithiation level in the anode was low. Further work should be done to preserve mechanical properties also at higher lithiation levels that would result in higher energy density batteries, for example by introduction of a small number of cross-links to provide a stronger interaction than the weak van der Waals association between its elements. With a large arsenal of tools to engineer interfaces in CNT fiber-based hybrids, the question rapidly leads to the techniques available to study the charge accumulation, injection and separation in these materials. The hybrids tend to be free-standing structures where the CNTs form a lowresistance percolating network and a defined interface with the energy-active phase. They resemble traditional nanoporous electrodes and can be similarly studied using a combination of EIS, CV, and I-V measurements, in the dark and under irradiation. Some of these measurements are reported in the examples discussed above, but often with an emphasis on device performance that leaves behind the relevant details of the interfacial charge transfer/accumulation processes behind their operation. Oxidized CNT fiber-PANI produced by anodic electropolymerization, for example, showed the appearance of two new PANI anodic peaks under cyclic voltammetry.50 These anodic contributions could be central to the enhancement in charge storage capacity observed in the hybrid, but no assignation to specific redox processes has been made yet. Electronic measurements on CNT fiber solar and piezoelectric devices confirm that when combined with metal oxides or semiconductors CNTs tend to form Schottky junctions on account of the higher work function of graphitic materials compared with nSi, ZnO, and TiO2. However, the engineering of interfaces, for example by growing

the inorganic phase at specific crystallographic orientations relative to the CNTs, calls for a more in-depth analysis of the role of interface states in the junction transport properties.94 It would also be desirable to combine transport measurements with XPS/UPS or Kelvin probe measurements that thus allow to determine a complete picture of the electronic band structure of the hybrid materials including the contribution of defects (in both the CNT and the inorganic phase) and surface functional groups. Once the electronic and electrochemical techniques provide a clear picture of the charge transfer and accumulation processes at the interface, they could be combined with in-situ or operando methods similar to those used in heterogeneous catalysis.102 Raman spectroscopy, for example, could provide insight into both charge transfer to the CNTs as well as interfacial strains arising from volumetric expansion of one of the phases. Mechanical properties While the addition of CNT fibers to the active phase is often beneficial for the properties of the final composite or hybrid material, the active phase can often also contribute by improving stress transfer between CNT bundles relative to the bare CNT fiber. This is particularly relevant for polymer phases that can readily infiltrate into the CNT fiber pores and increase tensile properties16,103 and provide additional energy absorption and dissipation mechanism.104,105 In this line, Horn et al have shown, for example, spinning of PVA/lysosome/CNT fibers combining antibacterial properties and a remarkable toughness of 660.9J/g.106 Increases in tensile strength and modulus were also observed after pulsed electrodeposition of PANI in CNT fibers for EDLC,42 and vapor-phase polymerization of PEDOT on CNT sheets as a step during preparation of biofuel cell elec68 trodes. With respect to mechanical properties, we further note that whereas flexibility is largely a consequence of size and shape (flexural strength scales as 1/diameter cubed), other properties of interest require a more active role of the noncarbonaceous phase, such as compressive strength and



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modulus and fracture toughness, which are considerably dominated by matrix properties. Interestingly, despite their immense importance in engineering components, these properties have been somewhat largely overlooked in CNT fiber-based arrays. This might be to some extent due a stronger interest in the field in using CNT fibers in woven textiles for application in innovative wearable electronics, and to the relatively simple integration involved in matrix-free woven fabrics. 5.

Outlook

The drive to produce materials for energy management devices that can withstand large and repeated strains/stresses calls for mechanically lightweight robust electrodes and current collectors. CNT fibers emerge as ideal candidates because of their network structure of high aspect ratio building blocks, which endows them with a combination of electrical conductivity in the range of metals, density similar to that of polymer, specific surface area close to activated carbon and toughness above that of carbon fiber (Figure 13). This Perspective presented selected examples of multifunctional energy management devices, that is, devices that combine mechanical robustness with an electrical, electrochemical or optoelectronic function and where CNT fibers are central in the charge transfer or accumulation processes. The examples include supercapacitors, batteries, biofuel cells, solar cells and energy harvesters. Some of these show comparable performance to traditional monofunctional energy devices, but with the added possibility to subject them to flexural or tensile strains unattainable with conventional electrodes. This success has often directed attention in the field to producing “new” devices, where the active phase is simply substituted by another material. CNT fibers used in solar devices, for example, have gone through the sequence of Si to dye-sensitizers to perovskites. This trend has the risk of overlooking the new possibilities that CNT fibers offer as electrodes and the exciting materials science challenges behind their combination with polyelectrolytes, metal oxides and semiconductors, as well as the interfacial processes in the final materials. However, CNT fibers are more than a porous conductive high-performance fibre, considering the possibility to precisely tailor their building blocks on a molecular scale by adjusting their diameter and number of layers, with the corresponding impact on bulk and interfacial properties. Fewer walls often imply superior mass-normalised properties on account of the limited contribution from “parasitic” internal layers to the mechanical and transport properties of CNTs. And although only a third of theoretically possible SWNTs are metallic, fibres of SWNTs produced by CVD in the direct spinning process, for example, show intrinsic predominance of metallic SWNTs.107 On the other hand, though specific surface would appear to benefit from a few layers as possible, the aggregation of SWNTs into bundles is likely to reduce the surface accessible to foreign molecules. An additional consideration is how the size and electronic properties of the CNT might affect the attachment of the second phase. More layers facilitate covalent functionalisation treatments, but at the expense of increasing diameter and thus reducing the match between domain sizes of the two phases. The large differences in electronic

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properties of CNTs depending on their number of layers and how they modify the charge accumulation/injection at the interface, add a further interesting aspect to these hybrid materials. Overall, our perspective is that macroscopic ensembles of CNTs (and other nanocarbons) offer a unique scaffold for bottom assembly of hybrids, where the attachment of the second phase can be controlled by: exploiting their high thermal conductivity to remove excess heat during crystallization,108 supramolecular assembly and surface chemistry modification by grafting,96,97 nanotube morphology (size/orientation/chirality) effects on nucleation86 and epitaxy, amongst others. The possibility to combine this toolkit with a range of controlled synthetic routes, such as electro-polymerization, vapour phase polymerization or atomic layer deposition,99 promises a new generation of multifunctional hybrids and a rich materials science ahead, of which current work in the field offers only a glimpse. From a practical point of view in relation to the realization of useful devices, we note that although most of the laboratory samples in this Perspective demonstrate successful use of CNT fibers as multifunctional electrodes in a wide variety of arrangements, we anticipate that the reproducible and uniform fabrication of larger samples using these methods outside the academic environment presents important challenges. Several methods rely on solvent infiltration, which could be difficult to control considering the complex pore structure of the materials. The hierarchical structure and combination of various phases in even a simple device such as a supercapacitor, call for a combination of characterization techniques that span across the nm to tens of microns scale, at least. In this respect, these materials would also seem and ideal test ground for existing multiscale modelling tools,109,110 that could operate from interfacial load and charge and processes a the nmscale, to the efficient integration of components into a final macroscopic device. The large difference between laboratory and semi-industrial CNT fibers will also deserve attention. Tailoring of CNT fibers at a molecular (purity,111 diameter/number of layers,112,113 and metallicity114) and assembly level is reasonably widespread and demonstrated on a kilometers/day scale,113 but semi-industrial processes are likely to be on a divergent path dictated by commercial factors that most likely will lead to very different CNT fibers available for devices. It is also relevant to note the strong interest in using CNT fibers as sensors. These fibres are intrinsically piezoresistive and experience electrical resistance when in contact with liquids.15,89 Recently they have been used for neural activity recording and stimulation in brain tissue115 as well as microelectrodes for selective detection of biologically relevant molecules.116 The principles of operation of these sensors often rely on charge storage and transfer mechanisms similar to those in energy storage and harvesting. It would appear thus that the development of tough sensors, requiring comparatively lower volumes of material than energy devices and benefiting directly from a fiber geometry, represent an interesting intermediate step in the development of energy management components.


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* [email protected].

The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

We are grateful for useful discussions with D. Eder, A. Cherevan, E. Senokos, S. Marchesan, D. Granados and A. Monreal. We thank B. Mas for designing the schematics in Figures 2b and 13 of this manuscript. Generous financial support is acknowledged from the European Union Seventh Framework Program under grant agreements 310184 (CARINHYPH project) and 322129 (Marie Curie Action MUFIN), the Spanish Ministry of Innovation and Competiveness under grant MAT2012-37552-C03-02 (MUDATCOM project) and “Juan de la Cierva” and “Ramón y Cajal” fellowships, and the Madrid regional government under grant S2013/MIT-3007 (MAD2D project).

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Tough Electrodes: Carbon Nanotube Fibers as the Ultimate Current

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