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Carbon fibers accept aerial apparent areas and affluent functionalities for interacting with ions, molecules, and particles. However, the ascendancy over their porosity charcoal challenging. Conventional syntheses await on aggregate polyacrylonitrile with sacrificial additives, which macrophase-separate and aftereffect in ailing controlled pores afterwards pyrolysis. Here, we use block copolymer microphase separation, a fundamentally disparate admission to synthesizing absorptive carbon fibers (PCFs) with well-controlled mesopores (~10 nm) and micropores (~0.5 nm). After entering any carbon precursors or dopants, poly(acrylonitrile-block-methyl methacrylate) is anon adapted to nitrogen and oxygen dual-doped PCFs. Attributable to the commutual arrangement and the awful optimal bimodal pores, PCFs affectation essentially bargain ion carriage attrition and an ultrahigh capacitance of 66 μF cm−2 (6.6 times that of activated carbon). The admission of appliance block copolymer precursors revolutionizes the amalgam of PCFs. The avant-garde electrochemical backdrop announce that PCFs represent a new belvedere actual for electrochemical activity storage.
Carbon fibers are above abstracts for adjustable and wearable electronics (1, 2), as able-bodied as for aerospace and airspace applications (3), because of their outstanding automated strength, aerial flexibility, low density, accomplished electrical conductivity, actinic stability, aerial temperature tolerance, and baby thermal amplification accessory (3–5). Carbon fibers mostly accept been adopted as acknowledging scaffolds to allay the poor electrical appliance of high-capacitance pseudocapacitive abstracts in supercapacitors (6–9). Besides confined as supports that accord little to the supercapacitor capacitance, in the aftermost decade, carbon fibers accept been the focal point of growing efforts to about-face them into alive apparatus (1, 10, 11). The absolute use of carbon fibers as independent electrodes removes the allegation for time-consuming processes to aggregate added materials, such as pseudocapacitive materials, conductive additives, and anchor (12, 13), that acquaint added interfacial attrition and are adverse to ultrafast charge/discharge. However, there are problems associated with this approach. Best carbon fibers are solid carbon filaments produced pyrolytically from angle (14), polyacrylonitrile (PAN) (15) (Fig. 1, A, D, and E), and biorenewable polymers [e.g., artificial (16), lignin (17), and others (18, 19)]. Their bland surfaces with bound able apparent areas (<10 m2 g−1) cede them around butterfingers of autumn a ample aggregate of electrochemical activity (20, 21). Therefore, strategies to admission the porosity of carbon fibers are awful adorable to apprehend their abeyant as alive abstracts in electrochemical activity storage.
Conventional (A and B) and new (C) methods for synthesizing carbon fibers from assorted polymer precursors. (A) Authentic PAN is electrospun into a cilia mat, breakable at 280°C in air to crosslink PAN (blue), and afresh pyrolyzed at 800°C in N2 to accomplish carbon fibers (gray). An alone polymer cilia (purple) is abstract for illustration. (B) PAN is alloyed with sacrificial PMMA (red) to anatomy a polymer blend. Afterwards oxidation, the polymer alloy macrophase-separates and forms aberrant domains. Afterwards pyrolysis, PMMA is removed, consistent in aberrant pores. (C) PAN-b-PMMA block copolymer microphase-separates into compatible PMMA nanodomains (red) in a cast of PAN (blue) afterwards blaze and self-assembly. Afterwards pyrolysis, the PCFs accommodate well-controlled and analogously broadcast pores. SEM images of the carbon fibers from (D and E) PAN, (F and G) PAN/PMMA, and (H and I) PAN-b-PMMA.
To admission their capacitance, carbon fibers allegation accept (i) awful compatible pores (22) of assertive sizes (preferably micropores of <1 nm and mesopores of ~10 nm) (21, 23) and (ii) hierarchical absorptive structures to admittance attainable admission by ions to the micropores (24). There are mainly two means to actualize absorptive carbon fibers (PCFs). The aboriginal class is to posttreat carbon fibers via activation and actinic exfoliation (25, 26). Activation with acerb actinic agents (e.g., HNO3 and KOH) roughens the carbon cilia surfaces, but it usually needs awful reductive chemicals (e.g., hydrogen and hydrazine) and complicated postprocesses to restore the electrical conductivity. Actinic exfoliation is accession accomplished way to actuate carbon fibers, but with this method, it is difficult to ascendancy the porosity and pore size, as able-bodied as to bottle the cilia integrity.
The additional action focuses on designing carbon cilia precursors. A array of PCFs accept been able by electrospinning PAN attenuated with sacrificial homopolymers (27, 28), SiO2 nanoparticles (29, 30), and carbon additives (11). Although these methods aftermath PCFs, their ascendancy over the pore admeasurement and accord is unsatisfactory because of the macrophase break of the polymer blends (Fig. 1, B, F, and G, and fig. S1A) and the adversity in distributing the additives analogously in the PAN matrix. Furthermore, the abatement of asleep particles involves the use of awful baneful and acerb chemicals, and often, it is arduous to absolutely abolish the congenital particles (29, 30). Therefore, the development of an able and able adjustment for acclamation the above obstacles, as able-bodied as the conception of compatible and hierarchical absorptive structures, is acute to beforehand the electrochemical achievement of carbon fibers (22).
The use of PAN-containing block copolymers as precursors is a able way to actualize hierarchical absorptive structures in carbon fibers (31–35). In adverse to the polymer blends, block copolymers microphase-separate (36) to anatomy awful compatible mesoporous structures (34, 37, 38) with pore sizes in the ambit of 2 to 50 nm (39–42). Furthermore, block copolymers can actualize commutual mesopores and micropores. The hierarchical pores are important for aerial capacitive achievement because mesopores serve as ion-buffering reservoirs and ion carriage pathways which abate the ion circulation distances from the aggregate electrolyte to the micropores, consistent in an added amount adequacy (6, 21, 22).
Here, we present a adjustment for synthesizing hierarchical PCFs with awful controlled structures by demography advantage of block copolymer self-assembly (Fig. 1, C, H, and I). Our adjustment adopts a metal-free capricious addition-fragmentation alternation alteration (RAFT) polymerization of poly(acrylonitrile-block-methyl methacrylate) (PAN-b-PMMA) followed by electrospinning, oxidation, and pyrolysis. The adjustment is chargeless from the use of etchants or activation chemicals. The as-prepared PCFs accept hierarchically commutual meso- and micropores with a aerial apparent breadth of 503 m2 g−1 and a affluent nitrogen agreeable of 12.8%. The PCFs accomplish a aerial apparent area–normalized capacitance of 66 μF cm−2, essentially college than any of the avant-garde PCFs acquired from PAN, PAN blends, and added carbon cilia precursors (table S1).
PAN-b-PMMA [111-b-62 kDa; polydispersity base (PDI), 1.14] with 64 aggregate % of PAN was actinic and acclimated as the carbon precursor. The as-electrospun polymer fibers were white and adjustable (Fig. 2A). Because of the accelerated bread-and-butter dehydration during electrospinning, the consistent polymer fibers had asperous surfaces (Fig. 2B and fig. S1B). The boilerplate bore of the polymer fibers was 911 ± 122 nm (fig. S1H). By crosslinking and cyclization of PAN at animated temperatures in air (43), the blaze action counterbalanced the coarse structures (Fig. 2C and fig. S1C), and the cilia diameters did not change decidedly afterward. The crosslinking of PAN prevented PAN-b-PMMA from basic continued range–ordered nanostructures (44), and the consistent absorptive structures were commutual and appropriately benign for ion carriage in supercapacitors.
SEM images of PAN-b-PMMA fibers (A and B) afterwards electrospinning, (C) afterwards blaze and self-assembly at 280°C in air, and (D to F) afterwards pyrolysis at 800°C in N2. Inset: A agenda photograph of a allotment of as-spun polymer cilia mat. (C) The ablaze and aphotic domains are PAN and PMMA, respectively. The acceptable adverse amid PAN and PMMA in the SEM angel is due to the fractional abasement of PMMA in air. (E and F) High-magnification SEM images of analogously broadcast mesopores in PAN-b-PMMA-CFs. (G and H) TEM images of mesopores in a PAN-b-PMMA-CF. (I) High-resolution TEM angel of absorptive carbons at the bend of a PAN-b-PMMA-CF.
After oxidation, PAN-b-PMMA fibers apparent microphase-separated poly(methyl methacrylate) (PMMA) domains in a PAN cast (Fig. 2C and fig. S1C). PAN and PMMA showed accomplished adverse and were calmly apparent in the scanning electron microscopy (SEM) images, attributable to the fractional abasement of PMMA during the blaze action as apparent by the thermogravimetric assay (TGA) (fig. S2). The thermally annealed polymer fibers were subjected to pyrolysis in an N2 atmosphere, consistent in PCFs termed PAN-b-PMMA-CFs (Fig. 2, D and E, and fig. S1D). After the use of actinic exfoliation or activation, the pyrolysis of PAN-b-PMMA yielded a connected absorptive carbon structure. The carbon crop was ~30.5% according to TGA (fig. S2). Afterwards pyrolysis, the boilerplate cilia bore decreased to 519 ± 96 nm (fig. S1I) and the coarse arrangement remained intact. The carbon cilia bore was essentially above than the declared bore of 364 nm, bold abounding pyrolysis and alliance of polymers into non-PCFs (eq. S8), advertence that the consistent carbon fibers were awful absorptive and had a low density. The estimated porosity of carbon fibers was ~50.8%, in acceding with Brunauer-Emmett-Teller (BET) assay (~50.6%; eqs. S9 and S10). The absorptive anatomy was accepted by high-magnification SEM images (Figs. 1I and 2F and fig. S1D), in which mesopores were empiric throughout the fibers. PAN-b-PMMA-CFs were adjustable and remained complete back the cilia mat was angled to assorted angles (fig. S1, E to G).
Transmission electron microscopy (TEM) added accepted the commutual pores in the carbon fibers (Fig. 2, G to I). The mesopore admeasurement was bent to be ~10 nm, as accent in the TEM angel (Fig. 2H). At the bend of the PCF, layered carbon structures were empiric (Fig. 2I). The array of a audible carbon bandage (dsingle) was estimated by averaging over a cardinal of carbon layers. For instance, the array of nine layers of carbon was ~3.46 nm (Fig. 2I), agnate to an interplanar acceding of ~0.38 nm. The abstinent amount agreed able-bodied with the amount of ~0.37 nm affected from x-ray diffraction (XRD) analysis. Both PAN-b-PMMA-CFs and PAN/PMMA-CFs (i.e., carbon fibers acquired from PAN/PMMA) apparent a abounding amplitude at bisected best of the (100) aiguille of 0.11 in radian (fig. S3A and table S2, XRD section), which corresponded to a crabbed admeasurement (La) of ~2.8 nm according to the Debye-Scherrer blueprint (eq. S2). The crabbed sizes of both PAN-b-PMMA-CFs and PAN/PMMA-CFs were alert that of PAN-CFs (carbon fibers acquired from PAN). PAN-b-PMMA-CFs had a crystallite admeasurement (Lc) of 0.98 nm, advertence about three to four π-stacked graphitic layers. In addition, Raman spectra of PAN-CFs, PAN/PMMA-CFs, and PAN-b-PMMA-CFs appear the appropriate “G band” at ~1560 to 1600 cm−1 and “D band” at ~1310 to 1350 cm−1 (fig. S3B), agnate to ordered graphitic structures and confused domains, respectively. The affected D bandage to G bandage acuteness arrangement (ID/IG) of all carbon fibers was 1.16 (table S2, Raman section), advertence that their graphitization degrees were identical.
The microstructures of block copolymer and carbon fibers were added accepted by small-angle x-ray drop (SAXS; Fig. 3A). The SAXS spectrum of the as-electrospun PAN-b-PMMA fibers was bald of any audible features. Afterwards blaze at 280°C, PAN-b-PMMA fibers apparent a ample Bragg aiguille in the ambit of 0.172 to 0.218 nm−1, agnate to center-to-center breadth spacings of ~37 to 29 nm. The ample SAXS aiguille appropriate that the microphase-separated structures of PAN-b-PMMA were acceptable confused and lacked all-embracing adjustment (38). Afterwards pyrolysis at 800°C, the position of the Bragg aiguille confused to 0.224 to 0.308 nm−1 and the center-to-center spacings decreased to ~28 to 20 nm (table S2, SAXS section). The about-face of the Bragg aiguille in SAXS spectra agreed with that in the fast Fourier transform (FFT) spectra (fig. S3C) from the SEM images (Fig. 2, C and E). FFT spectra showed center-to-center spacings of ~38 to 29 nm and ~29 to 20 nm for the breakable and pyrolyzed PAN-b-PMMA fibers, respectively. In contrast, the pyrolyzed PAN and PAN/PMMA fibers did not affectation any apparent features, advertence the absence of categorical nanostructures. The drop acuteness (I) and beachcomber abettor (q) followed Porod’s law (45), I ~ qx. For PAN-b-PMMA fibers, the power-law base x ≈ −3.73, in adverse with a antecedent address (35) of x ≈ −4.00 for block copolymer powders. Afterwards pyrolysis, x added to ~−3.43, suggesting that the surfaces of the carbon fibers were awful fractal and asperous (table S2, SAXS section).
(A) SAXS spectra of electrospun (e-spun), oxidized, and pyrolyzed PAN-b-PMMA fibers, in allegory with the carbon fibers pyrolyzed from PAN and PAN/PMMA blend. a.u., approximate units. (B) N2 at 77.4 K and (C) CO2 at 273.2 K adsorption-desorption isotherms of all carbon fibers. STP, accepted temperature and pressure. (D) Affected pore admeasurement distributions of all carbon fibers appliance NLDFT.
N2 and CO2 adsorption-desorption abstracts of the PCFs (Fig. 3, B and C) appear blazon IV and I isotherm characteristics, respectively. At about N2 pressures (P/P0) of 0.6 to 0.9, the arresting blazon H1 hysteresis of PAN-b-PMMA-CFs adumbrated the attendance of mesopores according to the International Union of Authentic and Applied Chemistry (IUPAC) allocation (46). The pore admeasurement administration was evaluated appliance a nonlocal body anatomic approach (NLDFT; Fig. 3D). PAN-b-PMMA-CFs had a BET apparent breadth of 503 m2 g−1 (as affected in the beeline ambit of P/P0 = 0.01 to 0.1; Fig. 3B) and a absolute pore aggregate of 0.45 cm3 g−1 (table S2, BET section). Because of the block copolymer microphase separation, PAN-b-PMMA-CFs apparent a ample cardinal of well-controlled mesopores with a attenuated pore admeasurement administration centered at ~9.3 nm (Fig. 3D). The SEM angel assay appear an boilerplate mesopore admeasurement of ~10.1 nm (fig. S4, B to E), in acceptable acceding with the boilerplate pore admeasurement acquired from the NLDFT model. Notably, agnate to the pore admeasurement administration acquired from the NLDFT model, the angel assay additionally showed a accept aiguille at ~3 to 4 nm (fig. S4E), acceptance the actuality that the NLDFT archetypal was acceptable for pore admeasurement assay of our PCFs. On the contrary, PAN/PMMA-CFs showed meso- and macropores with a abundant broader administration (~2 to 200 nm), and the PAN-CFs had no apparent mesopores. Thus, PAN-b-PMMA provided essentially bigger ascendancy over the mesopore admeasurement than PAN and PAN/PMMA did. Furthermore, the compatible mesopores in PAN-b-PMMA-CFs offered abundant channels to interconnect with the micropores (peaked at 0.5 nm), arch to a 100% admission in the micropore aggregate (table S2, BET section). As a result, PAN-b-PMMA-CFs had a specific apparent breadth of 503 m2 g−1, added than alert the apparent areas of PAN-CFs (213 m2 g−1) and PAN/PMMA-CFs (245 m2 g−1). Demography calm the SEM, SAXS, and BET results, we are assured that PAN-b-PMMA-CFs are able with a hierarchical absorptive structure, a affection that is basal to facilitate ion circulation and to accomplish high-rate adequacy in supercapacitors.
We advance that the commutual meso- and micropores in PAN-b-PMMA-CFs accommodate able pathways for accelerated ion circulation and accredit outstanding capacitive achievement (Fig. 4A). To assay the capacitive performance, we accumulated two-electrode symmetric supercapacitor cells. The electrodes were bogus by sandwiching carbon fibers amid two pieces of Ni cream after any conductive additives or polymer binders. PAN-b-PMMA-CFs apparent about ellipsoidal circadian voltammograms with no apparent redox peaks, alike at a aerial browse amount of 100 mV s−1, suggesting a near-ideal capacitive behavior (Fig. 4B and fig. S5A). In allegory with PAN/PMMA-CFs and PAN-CFs, PAN-b-PMMA-CFs had the bigger breadth amid by circadian voltammetry (CV) and appropriately the accomplished capacitance at a browse amount of 50 mV s−1 (Fig. 4B). Gravimetric capacitances were affected based on chronopotentiometry (CP) (Fig. 4C and fig. S5C) and CV (fig. S5B). At 1 A g−1, the accomplished gravimetric capacitance of PAN-b-PMMA-CFs was 360 F g−1 and the boilerplate amount over eight accessories was 334 ± 17 F g−1. At a aerial accepted body of 10 A g−1, the gravimetric capacitance of PAN-b-PMMA-CFs accomplished 226 ± 6 F g−1 (Fig. 4D), alert those of PAN/PMMA-CFs (111 ± 19 F g−1) and PAN-CFs (90 ± 9 F g−1). At an acutely aerial accepted body of 100 A g−1, the capacitance was 202 ± 10 F g−1 and retained >60% of the amount at 1 A g−1.
(A) Schematic analogy of a binder-free and conductive additive–free electrode composed of PCFs. (B) CV curves at a browse amount of 50 mV s−1. (C) CP curves at a accepted body of 10 A g−1. (D) Gravimetric capacitance against accepted body alignment from 1 to 100 A g−1. (E) Gravimetric capacitance and (F) BET apparent area–normalized capacitance (CA) of PAN-b-PMMA-CFs at 1 A g−1, in allegory with those of added carbon fibers appear in the abstract (black circles).
To added authenticate the accomplished electrochemical performances of the PCFs, we accumulated and activated three-electrode beef in 6 M KOH aqueous solution. The electrochemical achievement (fig. S5, E and F) corroborated that in a two-electrode testing configuration. Back the accepted body was added from 10 to 100 A g−1, the gravimetric capacitance of PAN-b-PMMA-CFs alone alone by 11%. The outstanding amount adequacy accepted that the commutual meso- and micropores in PAN-b-PMMA-CFs provided able pathways for accelerated electrolyte aggression and ion diffusion.
PAN-b-PMMA-CFs apparent an outstanding gravimetric capacitance of 360 F g−1 at a aerial accepted body of 1 A g−1 (Fig. 4E). This amount is college than those of the ahead appear carbon fibers (table S1), as able-bodied as those of the ahead appear nonfibrous absorptive carbons from PAN-containing block copolymers. For example, Zhong et al. (35) appear nanoporous nitrogen-enriched carbon acquired from poly(acrylonitrile-b-butyl acrylate) with a gravimetric capacitance of ~166 F g−1 at 0.1 A g−1. Yan et al. (34) appear block copolymer–derived mesoporous carbon particles (in a nonfibrous format) as supercapacitor electrode abstracts with 254 F g−1 at 0.5 A g−1. Our PCFs displayed 33% bigger capacitive achievement than KOH-activated carbon fibers, e.g., the N and O dual-doped, KOH-activated PCFs appear by Li et al. (47) with a capacitance of ~270 F g−1 at 1 A g−1. Our block copolymer–based PCFs were not subjected to any actinic activation or postsynthesis treatments. The added capacitance of PAN-b-PMMA-CFs is mainly attributed to the cilia arrangement and the categorical hierarchical micro- and mesoporous structures. After any binders, the pores accommodate connected circulation pathways for the ions, and the carbon cast offers advice pathways for the electrons (Fig. 4A).
BET apparent area–normalized capacitance (CA) of PAN-b-PMMA-CFs is as aerial as 66 ± 3 μF cm−2, college than the ahead appear PCFs and best absorptive carbons (Fig. 4F and table S1). We agenda that the CA of PAN-b-PMMA-CFs is abundant college than the archetypal amount for electrical double-layer capacitance (5 to 20 μF cm−2) and 6.6 times that of activated carbon (10 μF cm−2) (48). The aerial capacitance stems from the pseudocapacitive reactions brought by the awful attainable heteroatoms (i.e., O and N). Two capacitance adverse methods (Trasatti and Dunn’s methods; fig. S6) accede that ~37% of the absolute capacitance is from pseudocapacitance (Fig. 5A). The PCFs affectation accomplished continued aeon life, as evaluated by the voltage captivation tests (fig. S7) and the constant-current charge-discharge cycling assay for 10,000 cycles (Fig. 5B). The PCF-based supercapacitors affectation a ability body of 9.6 kW kg−1 at an activity body of 4.5 Wh kg−1 (Fig. 5B, inset), conspicuously college than that of the commercially attainable supercapacitors (typically 1 to 2 kW kg−1 at a agnate activity body of 5 Wh kg−1) (49).
(A) Dunn adjustment assay of capacitance accession of PAN-b-PMMA-CFs. The black regions appearance the accepted contributions from the electrical double-layer capacitive (magenta) and pseudocapacitive (cyan) processes. Inset: A histogram shows the percentages of electrical double-layer capacitance (62.2%) and pseudocapacitance (37.8%). (B) Cycling adherence of PAN-b-PMMA-CFs at a accepted body of 100 A g−1. Inset: Ragone artifice of PCFs. The gravimetric activity body and the ability body are affected according to Eqs. 4 and 5, respectively. (C) Nyquist plots of carbon fibers in the abundance ambit of 100 kHz to 0.1 Hz with an across-current perturbation of 10 mV. Inset: Nyquist plots in the middle- and high-frequency range. The broadcast credibility are beginning data, and the solid curve are the applicable curves. The impedances of the Nyquist plots are normalized to the alive areas of the activated electrodes. (D) Beeline applicable to the absolute allotment of impedance (Z′) against the −1/2 ability of the angular abundance (ω−0.5) plots in a abundance ambit of 1 to 10 Hz to abstract the ion circulation attrition (σ).
Electrochemical impedance spectroscopy (EIS) reveals the ion and allegation carriage dynamics. In anniversary Nyquist artifice (Fig. 5C, adapted to an agnate ambit archetypal in fig. S5D), an abridged semicircle in the high-frequency arena is followed by a 45°-inclined Warburg circulation band and a abrupt beeline band in the low-frequency region. The agnate ambit archetypal (50) auspiciously describes the arresting appearance and the capacitive behavior of our PCF electrodes (fig. S5D). As apparent in Fig. 5C, all carbon fibers affectation about vertical low-frequency lines, advertence about ideal capacitive behavior (51). Among the three electrodes advised herein, PAN-b-PMMA-CFs affectation the steepest beeline band in the low-frequency range. This affection is activated to the ion circulation resistance, highlighting the arete of the hierarchical absorptive structures in PAN-b-PMMA-CFs and advertence that the ion circulation in PAN-b-PMMA-CFs is the best efficient. To quantify the ion circulation resistance, the Warburg accessory σ (ohm s−0.5) can be extracted by applicable the absolute allotment of impedance (Z′) against the −1/2 ability of the angular abundance (ω−0.5) in the abundance ambit of 1 to 10 Hz (Fig. 5D). The abruptness of the adapted band equals the Warburg accessory (σ), a constant barometer the circulation attrition back the ions broadcast through the electrodes (52). PAN-b-PMMA-CFs affectation a baby σ amount of 0.87 ohm s−0.5. In contrast, both PAN-CFs and PAN/PMMA-CFs affectation aerial σ ethics of 2.01 and 1.78 ohm s−0.5, appropriately (table S2, electrical backdrop section). The high-frequency Z′ ambush and the abridged semicircle are associated with agnate alternation attrition (Rs) and allegation alteration attrition (Rct), appropriately (Fig. 5C, inset) (50). PAN-b-PMMA-CFs affectation an ambush and a semicircle abate than those of PAN-CFs and PAN/PMMA-CFs, advertence lower Rs and Rct. These after-effects prove that the analogously broadcast mesopores are advantageous for ion diffusion, and hence, PAN-b-PMMA-CFs affectation aerial capacitances and accomplished amount capabilities.
To the best of our knowledge, this is the aboriginal address of awful compatible PCFs acquired from block copolymers. Our after-effects appearance that PAN-b-PMMA is a awful able forerunner to aftermath hierarchical meso- and micro-PCFs. In adverse to added carbon cilia precursors, PAN-b-PMMA requires no acerb chemicals for postsynthesis activation, nor any additives to admission the apparent breadth and to ascendancy the pore size. The attenuated pore admeasurement administration and aerial apparent breadth of PAN-b-PMMA-CFs are ascribed to the microphase break of the block copolymer. The mesopore admeasurement of PCFs can be fine-tuned by the polymer atomic weight. The mesopore admeasurement is accepted to admission as the atomic weight of PMMA is increased. The change in mesopore admeasurement will added adapt the porosity, apparent area, and electrochemical achievement of the PCFs.
As supercapacitor electrodes, PAN-b-PMMA-CFs affectation achievement above to both PAN-CFs and PAN/PMMA-CFs. The aerial capacitive achievement is due to the commutual micro- and mesoporous carbon structures with aerial porosity, aerial apparent area, and low resistance. The micropores of ~0.5 nm and the compatible mesopores of ~10 nm are favorable for high-capacitance ion accumulator (21–23) with two capital advantages. First, the micropores on the mesopore walls accommodate awful ion-accessible apparent areas to advance the electrochemical double-layer capacitance (Fig. 4A) (24). Second, the commutual mesopores acquiesce assertive ion carriage and abate the distances for ion circulation from the aggregate electrolyte to the micropores. In accession to the structural advantages, the hierarchical absorptive anatomy has a apparent actinic agreement with accomplished electrolyte wettability, as appear by x-ray photoelectron spectroscopy (XPS) (fig. S8 and table S2, XPS section). The gravimetric and area-normalized capacitances of PAN-b-PMMA-CFs are college than any ahead appear PCF electrodes. The ultrahigh ethics aftereffect from the accessory furnishings of the commutual hierarchical absorptive structures, the well-controlled pore sizes, and the electrochemically alive nitrogen and oxygen anatomic groups that are calmly attainable to ions (24). Because the PCFs are flexible, they are acceptable for adjustable electronics.
In summary, we authenticate a confusing adjustment of appliance block copolymers for synthesizing PCFs with categorical bimodal pores and outstanding electrochemical properties. PAN-b-PMMA produces PCFs via self-assembly and pyrolysis, eliminating the annoying postsynthesis accomplish that added arrangement or actinic activation methods require. Moreover, PAN-b-PMMA offers arresting ascendancy over the pore admeasurement uniformity, bigger than PAN/PMMA and added polymer blends. Importantly, the area-normalized capacitance of the PCFs alcove 66 μF cm−2, outperforming all ahead appear carbon fibers, attributable to their hierarchically commutual meso- and micropores, affluent nitrogen and oxygen contents, and independent characteristics. Notably, our PCFs absorb aerial capacitances at ultrahigh accepted densities, because the analogously broadcast mesopores facilitate ion circulation above the fibers (22). The versatility of the adjustment extends the borderland of PCF nanotechnology and enables the development of avant-garde applications above electrochemical activity storage, such as catalysis, separation, purification, and wearable sensors (3, 11).
Acrylonitrile (AN; ≥99%), methyl methacrylate (MMA; ≥99%), 2,2′-azobis(2-methylpropionitrile) (AIBN; ≥98%), cumyl dithiobenzoate (CDB; ≥99%), benzene (≥99.9%), aluminium oxide (activated, neutral, Brockmann Activity I), N,N-dimethylformamide (DMF; ≥99.7%), and dimethyl sulfoxide (DMSO; ≥99.9%) were purchased from Sigma-Aldrich. The monomers were anesthetized through alumina columns to abolish inhibitors afore use. All added chemicals were acclimated as received.
According to our antecedent report, the PAN-b-PMMA block copolymer was actinic by RAFT polymerization (31). First, a admixture of MMA (35.0 ml, 310 mmol), CDB (84.28 mg, 0.3094 mmol), and AIBN (25.42 mg, 0.1548 mmol) was attenuated in benzene (51.6 ml) in a 100-ml Schlenk flask. The admixture was subjected to three cycles of freeze-pump-thaw (FPT), followed by back-filling with N2. Then, the alembic was placed in an oil ablution at 60°C and afflicted for 24 hours. The consistent PMMA macro–chain alteration abettor (macro-CTA) was precipitated in booze and broiled beneath exhaustion for 12 hours to absolutely clear the actual solvent. The antiseptic PMMA macro-CTA (Mn, SEC = ~62 kDa; PDI, 1.04) was acclimated to amalgamate PAN-b-PMMA block copolymers. PMMA macro-CTA (0.65 g, 11 μmol), AN (2.6 ml, 43 mmol), AIBN (0.44 mg, 2.7 μmol), and DMSO (7.22 ml) were alloyed in a 40-ml Schlenk alembic able with a alluring alive bar. The admixture was degassed by three FPT cycles and afresh acrimonious in an oil ablution at 65°C beneath N2 atmosphere for 24 hours. A PAN-b-PMMA block copolymer (Mn, SEC = ~173 kDa) with a PDI of 1.14 and a PAN aggregate atom of 64% was obtained. The PAN-b-PMMA block copolymer was antiseptic analogously to PMMA macro-CTA.
PAN-b-PMMA block copolymer was attenuated in DMF at a absorption of 14 weight %. The band-aid was afflicted at 65°C for 2 hours and afresh electrospun to polymer cilia mats at a agriculture amount of 1.5 ml/hour beneath a voltage of 18 kV (Acopian Technical Company, Easton, PA). The polymer fibers were calm on an in-house–built rotary Al disc. Afterwards electrospinning, the cilia mat was bald off from the beneficiary and broiled in a exhaustion oven at 60°C for 6 hours. Blaze and pyrolysis of fibers were conducted in a tube boiler (Lindberg/Blue M, Asheville, NC). The cilia mat was aboriginal breakable by heating from allowance temperature to 280°C at a amount of 1°C min−1 and kept at 280°C for 8 hours in air. Afterwards oxidation, the cilia mat was acrimonious afresh from allowance temperature to 800°C at a amount of 10°C min−1, followed by carbonization at the aforementioned temperature for 1 hour in a N2 breeze (200 accepted cubic centimeters per minute). For comparison, authentic PAN and a polymer alloy of PAN and PMMA (denoted as PAN/PMMA; 64 aggregate % of PAN) were electrospun into polymer cilia mats and underwent the identical pyrolysis accomplish at 800°C to admission carbon cilia mats.
The electrochemical capacitive achievement of the carbon cilia mats was evaluated in a symmetric two-electrode configuration. To accumulate the testing electrodes, carbon cilia mats with a accumulation loading of at atomic 1.0 mg were sandwiched amid two pieces of bankrupt nickel cream after any conductive additives or binders. An aqueous KOH band-aid (6 M) was acclimated as the electrolyte. The two electrodes were placed in the aqueous electrolyte. The ambit amid the two electrodes was ~2 cm to abstain any concrete acquaintance or electrical abbreviate circuit.
CV and EIS were performed on a PARSTAT 4000 electrochemical workstation (Princeton Applied Research, AMETEK Inc.). CP abstracts were agitated out by a array assay arrangement (model 580, Scribner Associates Inc.). The abeyant window called for both CV and CP was 0 to 0.8 V. EIS was conducted in a abundance ambit from 100 kHz to 0.1 Hz with a 10-mV perturbation. The electrochemical abstracts of three-electrode beef were performed appliance a Gamry 600 (Gamry Instruments). The actinic carbon cilia mat, a allotment of bald nickel foam, and an Ag/AgCl (in saturated KCl) electrode were acclimated as the alive electrode, the adverse electrode, and the advertence electrode, respectively. The adherence was evaluated by constant-current (100 A g−1) charge-discharge tests and voltage-holding tests. For the voltage-holding tests, the supercapacitors were answerable at a accepted body of 4 A g−1, captivated at a best voltage of 0.8 V for 1 and 5 min, and afresh absolved at 4 A g−1. The charge-hold-discharge action was again for 20 cycles.
The gravimetric capacitance (C in F g−1) of supercapacitors was affected appliance the acquittal allocation of the calm CP curves (53, 54)(1)where I is the acquittal accepted (A), Δt is the acquittal time (s), ΔV is the abeyant window (V), and m is the sum of alive actual accumulation (g) of two electrodes.
Alternatively, C was evaluated from CV curves appliance the afterward equation(2)where Im(V) is the accepted body (A g−1), v is the browse amount (mV s−1), and V0 and Vt are the lower and high abeyant banned of the called abeyant window, respectively.
The boilerplate area-normalized capacitance (CA, μF cm−2) was affected on the base of the BET apparent breadth according to the afterward formula(3)where ABET is the BET-specific apparent breadth (m2 g−1).
The activity body (E, Wh kg−1) and ability body (P, kW kg−1) of the supercapacitors were evaluated by appliance the afterward formulas(4)(5)
Supplementary actual for this commodity is attainable at http://advances.sciencemag.org/cgi/content/full/5/2/eaau6852/DC1
Section S1. Bold and instrumentation
Section S2. Calculation of carbon cilia porosity appliance geometric analysis
Section S3. Calculation of carbon cilia porosity appliance BET analysis
Section S4. Calculation of the amount of mesopore interconnectivity
Fig. S1. Added SEM images, flexibility, and admeasurement administration of PAN-b-PMMA-CFs.
Fig. S2. Thermogravimetric analysis.
Fig. S3. Wide-angle XRD spectra, Raman spectra, and FFT spectra.
Fig. S4. Allegory of the pore admeasurement distributions from angel assay and NLDFT fitting.
Fig. S5. Added electrochemical achievement of PAN-b-PMMA-CFs.
Fig. S6. Capacitance accession analyses.
Fig. S7. Adherence achievement of PAN-b-PMMA-CFs.
Fig. S8. XPS spectra and acquaintance angles.
Table S1. Arbitrary of the electrochemical capacitive achievement of PCF electrodes.
Table S2. Arbitrary of the concrete and actinic characterization.
This is an open-access commodity broadcast beneath the agreement of the Creative Commons Attribution license, which permits complete use, distribution, and reproduction in any medium, provided the aboriginal assignment is appropriately cited.
Acknowledgments: We accede the use of accessories in Institute for Critical Technology and Applied Science (ICTAS), Virginia Tech. We acknowledge S. M. Martin for accouterment admission to the SAXS bold and the Apparent Assay Laboratory at Virginia Tech for XPS acquisition. Funding: This actual is based aloft assignment accurate by the Air Force Appointment of Scientific Analysis beneath accolade cardinal FA9550-17-1-0112 through the Young Investigator Program (YIP). The assignment was additionally accurate by the Virginia Tech ICTAS JFC Award. Author contributions: G.L. conceived the idea. G.L. and Z.Z. advised analysis and experiments. All authors performed research, analyzed data, wrote the paper, and contributed to and accustomed the final manuscript. Aggressive interests: G.L. and Z.Z. are inventors on a conditional apparent appliance accompanying to this assignment filed by the Virginia Tech Intellectual Property (VTIP) appointment (no. 62/660,881; filed on 20 April 2018). The authors acknowledge no added aggressive interests. Abstracts and abstracts availability: All abstracts bare to appraise the abstracts in the cardboard are present in the cardboard and/or the Supplementary Materials. Added abstracts accompanying to this cardboard may be requested from the authors.
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