Transforming Pristine Carbon Fiber Tows into High Performance Solid‐State Fiber Supercapacitors

Abstract

A facile activation strategy can transform pristine carbon fiber tows into high-performance fiber electrodes with a specific capacitance of 14.2 F cm−3. The knottable fiber supercapacitor shows an energy density of 0.35 mW h cm−3, an ultrahigh power density of 3000 mW cm−3, and a remarkable capacitance retention of 68%, when the scan rate increases from 10 to 1000 mV s−1. The rapid advances in portable and wearable electronics in recent years have stimulated the development of small, lightweight, flexible, and wearable energy storage devices to meet the growing demands of human society.1-4 Supercapacitors (SCs) are widely used energy storage devices with the benefits of high power density, long cycle life, and fast charge/discharge rates.5-7 However, conventional SCs using liquid electrolytes sealed in bulky packaging materials are often too large, heavy and inflexible for flexible electronics applications.1 In this context, a new family of SCs, the fiber SCs, with 1D porous fibers as electrodes, have emerged as a promising solution for portable and wearable electronics. Their unique wire-shaped architectures render them distinct advantages of small size, lightweight, high flexibility, and excellent wearability as compared to conventional SCs.1-4 The energy storage of SCs is based on two principles: electrical double-layer capacitance (EDLC) and pseudocapacitance. Highly conductive electrode materials with large ion-accessible surface area can render small electrical resistance and short diffusion pathways for fast electron and ion transportation in SCs, which leads to high specific capacitance and excellent rate capability for SCs.8 In this sense, carbon materials have been widely utilized as electrode materials for macro-scale double-layer capacitors due to their high surface area, good electronic conductivity, and chemical stability.9 Similarly, a wide range of 1D porous carbonaceous fibers, including carbon nanotube (CNT) fibers, activated carbon fibers, graphene fibers, as well as their hybrids, have been used as electrodes to construct fiber SCs.1-4 However, most of carbonaceous fiber-based SCs have shown poor rate capability, i.e., high specific capacitance can only be achieved at a very low charge/discharge current density and their specific capacitance drops significantly with increasing charge/discharge current density or voltage scan rate. This has significantly limited their practical applications.10-19 For instance, Chou and co-workers reported a wire-shaped SC consisting of two CNT fiber electrodes in H2SO4-poly (vinyl alcohol) (PVA) gel electrolyte, which retained only 35% of its initial capacitance when the scan rate was increased from 5 to 1000 mV s−1.14 Lee and co-workers fabricated solid-state coaxial fiber SCs using all-carbon electrodes in polymer gel electrolyte, which have a high specific capacitance of 86.8 mF cm−2 at 2 mV s−1. However, they retained only 46% of the capacitance at 200 mV s−1.15 More recently, we designed a fiber SC using nitrogen-doped reduced graphene oxide (rGO)/CNT hybrid fibers, which had a high energy density comparable to a thin-film lithium micro-battery.17 However, its capacitance retention was about 50% when the current density was increased from 26.7 to 1000 mA cm−3. Alternatively, Kim et al. built fiber SCs using Pt wires as current collectors and polymer/CNT composite yarns as active materials, which showed a higher capacitance retention rate of 70% when the scan rate was increased from 10 to 1000 mV s−1.20 Nonetheless, compared to all-carbon fiber electrodes, metal wires are heavier in weight and less flexible upon bending.1, 15 Moreover, they account for a considerable weight and volume of assembled fiber SCs without contributing to energy storage, resulting in a lower energy density.1 Thus, it is more desirable to develop all-carbon fiber electrodes for building high-performance fiber SCs. Carbon fiber (CF) is a material consisting of graphite filaments about 5–10 μm in diameter. It has high stiffness, high tensile strength, low weight, and is widely used to reinforce composite materials in aircraft, wind turbine blade automotive parts and sports equipment, generating billions of dollars of revenue annually. CF is usually supplied in the form of a continuous tow, which is a bundle of many graphite filaments. Due to its high electronic conductivity and lightweight, CF has been used to construct flexible SCs.1, 21 However, pristine CF has a low surface area (<10 m2 g−1) and poor porosity.22, 23 Thus, its applications in SCs are often limited to current collectors or scaffolds for loading other capacitive materials.1 Herein, we report a facile strategy to introduce a large surface area and high mesoporosity on pristine CF tows (CFTs) by exfoliation, annealing, and reduction. Without adding other capacitive materials, the specific volumetric capacitance of the activated CF tows (A-CFTs) shows a 22-fold increase up to 14.2 F cm−3 in aqueous H2SO4 electrolyte together with a high discharge rate up to 10000 mV s−1. A-CFTs can be used to assemble high performance solid-state fiber SCs in polymer electrolyte with excellent flexibility and weavability. Particularly, the assembled fiber SCs show a superior rate capability with 68% capacitance retention when the scan rate increases from 10 to 1000 mV s−1, the best among all-carbon fiber SCs reported so far. Activation of pristine CFTs involves three steps: oxidative exfoliation by KMnO4/H2SO4, annealing in the air, and reduction by a mixture of hydrogen iodide (HI) and acetic acid (AcOH), as showed in Scheme 1. Previous studies have shown that harsh oxidation treatments (such as using KMnO4/H2SO4) can unzip multi-walled CNTs (MWCNTs) or exfoliate graphite.24 By optimizing the oxidation condition for MWCNTs, it is possible to partially transform the outer few walls of MWCNTs into numerous small graphene oxide (GO) pieces attached to their inner walls, whereas the inner walls remain intact with high electrical conductivity.24 Inspired by this approach, we first oxidized pristine CFTs by a modified Hummers’ method,24 which was followed by further annealing in the air. To reduce electrical resistance of oxidized CFTs, they were subsequently chemically reduced using the mixture of HI/AcOH. It should be noted that a similar oxidation–reduction method was previously used to prepare MnOx/CNT hybrid films.25 After undergoing the above activation steps, mechanical strength and electrical conductivity of CFTs remain nearly unchanged (Figures S1 and S2, Supporting Information). Their electrical conductivity decreases from 629 to 405 S cm−1 after oxidation, and then is restored to 590 S cm−1 after reduction. Scanning electron microscopy (SEM) images show that pristine CFTs consist of bundles of graphite filaments with a smooth surface (Figure 1a,b). In contrast, A-CFTs have a rough surface with some broken segments, indicating that their surface is indeed modified after activation (Figure 1c,d). To further examine their modified surface, A-CFTs were characterized by transmission electron microscopy (TEM). TEM images (Figure 1e,f) show abundant exfoliated ultrathin carbon sheets attached to the intact fiber core, forming a core–shell structure. SEM and TEM analysis also indicates that porous structures are created on the A-CFT surface (Figure S3, Supporting Information). A minor change in the crystalline structure of pristine CFTs and A-CFTs was also observed by X-ray diffraction (Figure S4, Supporting Information). The surface of pristine CFTs and A-CFTs was further characterized by Raman spectroscopy (Figure S5, Supporting Information) and X-ray photoelectron spectroscopy (XPS) (see Figure 2a,b). After activation, the oxygen content in A-CFTs is only 8%, comparable to that in pristine CFTs, indicating that most of functional groups introduced by oxidative exfoliation have been successfully removed by the reduction of HI/AcOH. The C1s XPS spectrum of A-CFTs can be deconvoluted into three peaks centered at 284.3, 286.1, and 287.6 eV, which are assigned to CC, CO, and CO bonds, respectively. These peaks suggest that there are still residual oxygen-containing functional groups on the A-CFT surface. The surface areas of pristine CFTs and A-CFTs were measured using N2 physisorption. Figure 2c,d shows their adsorption isotherms and the corresponding pore-size distributions. Brunauer–Emmett–Teller (BET) surface area of A-CFTs increases by one order of magnitude from 6 to 92 m2 g−1 compared to that of pristine CFTs. The BET pore size distributions show that there are many more mesopores in A-CFTs, which is consistent with SEM and TEM observations (Figure S3, Supporting Information). The electrochemical performances of pristine CFTs and A-CFTs were first investigated using a three-electrode cell in 1.0 m aqueous H2SO4 electrolyte. Tightened pristine and activated CFTs of about 220 μm in diameter were directly used as working electrodes. Figure 3a presents their cyclic voltammetry (CV) curves at 100 mV s−1. The CV curve of A-CFTs exhibits a much larger area compared to that of pristine CFTs, indicating a significant increase in capacitance. A-CFTs also show pseudocapacitive characteristics at the potential window between 0 to 0.6 V with nonrectangular CV curves, which may come from residual oxygen-containing functional groups on their surface.15, 16 A-CFTs have a specific volumetric capacitance of 14.2 F cm−3 at 10 mV s−1, which is 22-fold higher than that of pristine CFTs (0.6 F cm−3). This capacitance value is higher than that of many previously reported carbonaceous fibers, such as CNT yarn (5 F cm−3)26 and wet-spun graphene fiber (1.5 F cm−3).27 The large specific volumetric capacitance of A-CFTs can be attributed to (1) the relatively high density up to 1.8 g cm−3, (2) the increased surface area, (3) the formation of more mesopores, and (4) the presence of residual functional groups, which may not only enhance the surface wettability and accessible electroactive surface area, but also provide additional pseudocapacitance.28 The capability of retaining high capacitance during fast charging/discharging is critical for realizing high-performance fiber SCs.8, 9 Figure 3b shows that when the CV scan rate increases from 10 to 1000 mV s−1, the capacitance retention rate of A-CFTs is 72%, approaching the best rate capability of carbon-based electrodes ever reported in aqueous electrolytes.6 Further, A-CFTs are robust enough to be charged and discharged over a wide range of the voltage scan rate (from 100 to 10 000 mV s−1, Figure 3c and Figure S6, Supporting Information). Figure 3d shows that their discharge currents raise linearly with the increase of scan rate up to 5000 mV s−1, indicating very fast charge transports within A-CFTs.21 Next, as schematically illustrated in Figure 4a, we assembled solid-state fiber SCs by twisting two identical A-CFT electrodes (2 cm in length) together after coating them with a thin layer of PVA/H3PO4 gel electrolyte. Figure 4b shows CV curves of the fiber SC at different scan rates from 10 to 1000 mV/s. All CV curves present a nearly rectangular-like shape, indicating the formation of efficient EDLC and good charge propagation between the two fiber electrodes. The presence of imperfectly rectangular CV curves suggests electrode polarization, which is similar to many previous reports.15, 16, 23 As suggested in previous literatures on functionalized carbonaceous fiber materials,15, 16, 23 this may be attributed to the presence of oxygen-containing functional groups. In particular, Figure 4c displays that the fiber SC allows a high scan rate up to 5000 mV s−1, and its discharge current has a linear dependence on the scan rate up to 2000 mV s−1 in the capacitive region, indicating its high power capability.5 These results suggest that not only A-CFT electrodes can provide fast charge transport, but also the interface between the two electrodes with the gel electrolyte enables effective ion diffusion, despite the fact that polymer gel electrolytes usually have lower ionic conductivity compared to liquid electrolytes. The volumetric capacitance of the fiber SC was calculated based on its total volume including two A-CFT electrodes and the coated gel electrolyte. Its volumetric capacitance determined from CV curves is 2.55 F cm−3 at 10 mV s−1, which is higher than that of many carbon-based SCs reported in the literatures.5, 15, 21, 29-31 More importantly, Figure 4d shows that the A-CFT fiber SC retains 68% of its initial capacitance when CV scan rate increases from 10 to 1000 mV s−1. The volumetric capacitance calculated from their galvanostatic charging/discharging curves at different current densities (Figures S7 and S8, Supporting Information) indicates a capacitance retention rate of 74% when the current density is increased from 55.5 to 1000 mA cm−3 (Figure 4e). These capacitance retention results exhibit the excellent rate capability of the fiber SC, which, to our knowledge, is the best among all-carbon (without using metal wires) fiber SCs reported so far. Figure 4f shows the Nyquist plot of the fiber SC with an open-circuit voltage. Its equivalent series resistance is about 15 Ω. The nearly vertical profile in the low-frequency region represents an ideal SC behavior. The absence of a semicircle in the high-frequency region suggests efficient ion diffusion and good electrical conductivity of A-CFT electrodes.21 We further examined the volumetric energy and power density of the fiber SC. The Ragone plot (Figure 5a) first compares the volumetric performance of the A-CFT fiber SC with a few commercial energy storage devices. It shows a maximum volumetric energy density (normalized to its total volume) of ≈0.35 mWh cm−3, which is comparable to that of commercial bulk SCs (2.75 V/44 mF, activated carbon).6 This volumetric energy density is moderate compared with other reported fiber SCs. It is higher than MnO2 coated carbon fibers (0.22 mWh cm−3),21 MWCNT coated carbon microfiber bundles (0.14 mWh cm−3),15 and carbon fibers coated by MnO2/ZnO (0.04 mWh cm−3),11 but still lower than nitrogen-doped rGO/SWCNT hyrbid fibers,17 coaxial CNT fibers,32 and V2O5/carbon fibers@WO3-PPy nanoarrays/carbon fibers.33 Its areal performance (up to 3 μWh cm−2) is better than many fiber SCs,13, 17, 32, 34-36 including those made from pen ink coated carbon fibers (1.76−2.7 μWh cm−2),34 ordered mesoporous carbon/CNT fibers (1.77 μWh cm−2),34 and rGO fibers (0.17 μWh cm−2).13 A unique advantage of the A-CFT fiber SC is its high volumetric power density of 3000 mW cm−3, which is better than that of most fiber SCs reported so far, including those made from rGO/MoS2 composite fibers (<200 mW cm−3),27 nitrogen-doped rGO/SWCNT composite fibers (≈1000 mW cm−3),17 MWCNT-rGO@cellulose composite yarns (<300 mW cm−3),18 MnO2 coated carbon fibers (400 mW cm−3),21 MnO2/ZnO coated carbon fibers (2.4 mW cm−3),11 and PPy/MnO2 coated carbon fibers (2000 mW cm−3).37 Moreover, its volumetric power density is more than two orders higher than that of the commercial thin-film lithium battery (4 V/500 μAh), and also higher than that of the commercial bulk SCs (2.75 V/44 mF, activated carbon).6 Cycling performance of the A-CFT fiber SC is shown in Figure 5b. It retains 91% of its initial capacitance after 10000 charge–discharge cycles, indicating impressive cycling stability with a long cycle life. Flexible electronics applications require fiber SCs to have a good mechanical flexibility without electrochemical performance compromises. Bending tests were performed to evaluate the flexibility and capacitance retention (capacitance retention is normalized to its initial value) of the A-CFT fiber SC. Its capacitance retention is maintained at nearly 100% when bending from 0° to 180° (Figure 5c). An 8 cm long A-CFT fiber SC was sewed into a glove to evaluate its performance at different deformation conditions of bending and knotting. Figure 5d shows that its performance has negligible changes after 1000 bending or knotting cycles, demonstrating excellent weavability, knottablity, and flexibility. Furthermore, because A-CFTs retain their mechanical strength and structural integrity after activation, fabricating fiber SCs in different lengths becomes a straightforward task. The video in the Supporting Information shows how two long A-CFT fibers can be assembled into a fiber SC using a handheld yarn spinner (“Lacis” Power, 4-ply Cord Maker and Fringe Twister, also see Figure 5e). As exhibited in Figure 5e, the assembled fiber SC can be woven into a dragonfly knot, which demonstrates its application potentials in smart textiles. When the length of the fiber SC increases from 1 to 20 cm, an almost linear increase in its capacitance is observed (Figure 5f), indicating the possibility of integrating long fiber SCs together to meet various energy output requirements. As a proof-of-concept, we have assembled five fiber SCs of 6 cm in length each in series made from multiple A-CFTs to power a green light-emitting diode (Figure S10, Supporting Information). In conclusion, we have presented a facile and scalable approach to directly transform pristine CFTs into high-performance fiber SCs. Through oxidative exfoliation by KMnO4/H2SO4, annealing in the air, and reduction by HI/AcOH, exfoliated ultrathin carbon sheets are created on the surface of CFTs without compromising their excellent electrical conductivity and mechanical strength. This leads to a significant increase in their surface area by one order of magnitude from 6 to 92 m2 g−1 and the formation of higher mesoporosity. Activated CFTs without loading any other capacitive materials exhibit a high specific volumetric capacitance up to 14.2 F cm−3 in aqueous H2SO4 electrolyte measured in a three-electrode cell, which is 22-fold higher than that of pristine CFTs, and much higher than many previously reported graphene fibers and CNT yarns. More importantly, it has an excellent rate capability with a discharge rate up to 10 000 mV s−1. Two A-CFTs can be assembled into a fiber SC in PVA/H3PO4 gel electrolyte, which exhibits a long cycle life, an energy density of 0.35 mWh cm−3, a high power density of 3000 mW cm−3, as well as remarkable capacitance retention of 68%, when the scan rate increases from 10 to 1000 mV s−1. This is the best rate capability among all reported all-carbon fiber SCs reported so far. Production of A-CFTs can be easily scaled up using industrial-scale CF manufacture processes, and their electrochemical performances are almost unchanged when subjected to bending or knotting. Thus, the weavable, bendable, and knottable A-CFT fiber SCs hold promises for realizing efficient energy storage in smart textiles and flexible electronics. It also opens the possibility of directly integrating A-CFT SCs into CF enforced composite materials for energy storage other than mechanical reinforcement alone. Furthermore, considering the combination of a large surface area with mesopores and excellent electrical conductivity and mechanical strength, A-CFTs may have much broader applications for functional textiles, smart cloths, flexible electronics, sensors, biofuel cells, to name a few. This work was supported by the Youth 1000 Talent Program of China, the National Natural Science Foundation of China (No. 51233008), Ministry of Education, Singapore (MOE2011-T2–2–062 and 2013-T1–002–132), and ifood program funded by Nanyang Technological University, Singapore. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. 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