TEM images of the pyrolyzed aerogels in Figure 2 confirmed that the RF matrix, which consisted of nanoscale carbon particles characteristic of a pyrolyzed aerogel, acted as a support for the TMD sheets. Electron diffraction (Figures 2a and b, inset) and energy-dispersive X-ray spectroscopy (Supplementary Figures S9 and S10) of the doped aerogels demonstrate that the sol-gel process does not chemically modify the TMDs. After incorporation, these TMD crystals range in size from 5–100 nm in the (002) stacking plane and up to micron scale in length. While it does not have a high exfoliation efficiency, acetonitrile is effective at preserving large area sheet sizes by physisorbing to the chalcogenide atoms in TMDs to reduce van der Waals forces before subsequent intercalation36. This mild reduction is believed to prevent scissoring of TMDs and lead to large area sheet dispersion (Figures 2b and d)37. The WS 2 composite exhibited the highest degree of exfoliation, as evidenced by additional TEM images in Supplementary Figure S1.

Figure 2 TEM images of MA-17 (a and b), WA-17 (c and d), and RFA (e and f). e and f are characteristics of the gels as synthesized, while (a–d) demonstrate the presence of exfoliated sheets. Insets in the TEM images a and c show electron diffraction of the TMD sheets dispersed in the aerogel. Full size image

BET analysis of the aerogels (Figure 3, Supplementary Figure S2 and Table 1) demonstrated that the neither the addition of TMD sheets in this accelerated synthesis, nor the amount of TMD added significantly impacted the surface area or the morphology of the gel. All the TMD-loaded aerogels maintained high surface areas greater than 400 m2 g−1, with a maximum for WA-17 at 620 m2 g−1. Furthermore, processing the aerogels for supercapacitor electrodes did not significantly affect their surface area: WA-17 retained 94% of its original surface area after milling and sieving, and electrode sheets made from combining the same milled and sieved sample with PTFE tape and carbon black retained 99% of the original surface area of the aerogel (Supplementary Figure S7). These results suggest that the aerogel represents a mechanically stable support throughout processing.

Figure 3 Nitrogen sorption isotherms with BET surface area (a) and BJH pore size distribution (b) of MA-17 and WA-17. In addition, FTIR transmittance data (c) demonstrate functional groups within the aerogel composites. Full size image

Table 1 Summary of pyrolyzed TMD aerogel composite properties Full size table

Comparing the theoretical maximum surface areas for the TMD’s (750 and 483 m2 g−1, see Supplementary Information Section 1) to our control aerogel of pure pyrolyzed RF (776 m2 g−1), it is clear that the carbon aerogel constituted the majority of the surface area6. In addition, the nitrogen sorption isotherms of all the aerogels, including the unloaded control sample, exhibited type H1 hysteresis38, which is characteristic of largely uniform diameter spherical particles. This further confirms that the carbonaceous matrix constituted the bulk of the surface area in the TMD-loaded samples. Indeed, the TEM images in Figure 2 show that the TMD’s incorporated as large sheets of material supported by the homogeneous network of carbonized RF polymer. The BJH pore size distributions of the TMD-loaded aerogels were roughly unimodal and peaked below 100 Å pore radius, revealing their mesoporous nature. In contrast, the RFA featured a bimodal pore distribution with larger pores on average than the other samples, as evidenced by the peaks at 90 and 135 Å, and a tail extending past 250 Å.

In FTIR spectra of MA-17 and WA-17 (Figure 3c), the absence of epoxy functional groups at 1220 cm−1 and alkoxy groups at 1095 cm−1, which form during polycondensation of resorcinol and formaldehyde, shows that pyrolysis successfully removed these oxygen-containing moieties39. The broad band centered at 1510 cm−1 along with the weaker band at 1630 cm−1 correspond to C=C–C stretching in an aromatic ring and is evidence of carbonized sp2-bonded structures in the aerogels. The band at 1340 cm−1 represents O–H bending in phenol groups, which have been observed to survive heat treatment even at 1000 °C, well above the pyrolysis temperature of 800 °C for our aerogels40. Notably, both aerogels exhibit a small peak at 680 cm−1, corresponding to a C–S mode, which suggests that the rapid synthesis and subsequent pyrolysis produces chemical bonding between the RF matrix and the TMD sheets.

The Raman spectrum of WA-17 shows both the D band at 1345 cm−1 and the G band at 1603 cm−1 (Figure 4c). The D band stems from carbon-carbon sp3 stretching with A 1g symmetry, associated with disordered atoms, while the G band originates from the doubly degenerate (iTO and LO phonon modes) carbon stretching with E 2g symmetry. Interestingly, the intensity ratio of these modes and the location of the G band provide information about both the amount of sp3 bonding and the graphitic grain size domain. As the G band decreases in wavenumber and the intensity ratio of the D band to G band decreases, carbon bonding shifts from graphite to nanocrystalline graphite to amorphous carbon41. This analysis suggests that these aerogels contain approximately 5% sp3 bonding with graphitic grain sizes of 11 nm. Prior reports have shown that the addition of transition metal ions into a carbon aerogel can catalyze graphitization during pyrolysis at temperatures greater than 1000 °C (Ref. 42). However, we do not observe any catalytic graphitization of the aerogel from Raman spectroscopy.

Figure 4 Raman characterization of MoS 2 (a) and WS 2 (b) dispersed within the RF matrix of the aerogel (c) for samples synthesized from 17 mg mL−1 TMD dispersions. The Raman scattering of the aerogel (c) was collected from the WS 2 composite. For each RF-supported TMD, the bulk Raman spectra is displayed offset for comparison. In addition, the supported TMD and bulk Raman spectra were collected without adjusting the spectrometer grating to prevent alignment-induced shifts in wavenumber. All wavenumbers were further calibrated with a silicon wafer. Vertical dotted lines represent peak centers of the in-plane (E1 2g ) and out of plane (A 1g ) modes of the exfoliated TMD’s to emphasize the shift from their bulk counterparts due to exfoliation in the case of MoS 2 and WS 2 . Full size image

All the TMD’s exhibit E1 2g and A 1g symmetry Raman active modes, which correspond to in-plane and out-of-plane stretching modes, respectively43. Similar to carbon, the distance and intensity ratio between these scattering modes gives information about the degree of electrical coupling between layers. The addition of more monolayers tends to increase the energy of the out-of-plane A 1g mode. While the TEM data do not suggest high exfoliation of the TMD’s in the gels, the shift between peaks implies that there is a decrease in interlayer coupling, which could lead to increased adsorption or intercalation of ions during supercapacitor operation. For WS 2 , the spacing between the E1 2g and A 1g peaks decreases from 69 cm−1 to 64 cm−1 after sonication, which implies that monolayers are electrically coupled to two nearby sheets (Figure 4b)43,44. We note that this does not necessarily mean that the sheets were highly exfoliated, only that interlayer coupling in the (001) direction decreased during processing. For MoS 2 , there is a less distinct shift, which suggests that the exfoliated material is only slightly shifted from its bulk counterpart (Figure 4a). The exfoliated, dispersed (bulk) E1 2g peak sits at 378 cm−1 (381 cm−1) and the A 1g sits at 404 cm−1 (407 cm−1), leading to a difference of 26 cm−1 (26 cm−1). A comparison to literature for the out-of-plane A 1g shows that each MoS 2 remained coupled to only one other layer. However, the E1 2g and peak spacing suggest the material retained its bulk-like characteristics43,45.

The XRD and SAED of both MoS 2 and WS 2 -loaded aerogels demonstrated that the TMD’s remained crystalline throughout the rapid sol-gel processing and the subsequent high-temperature pyrolysis (Figures 2 and 5). For these sulfur-based TMD’s, we identified the sharp peaks in XRD as highly crystalline 2H phases. The underlying broad peak centered at 2θ=17° originates from amorphous carbon within the aerogel6. By examining the peak broadening, we further quantified the size of the TMD crystals loaded into the aerogels, using the Scherrer equation (Supplementary Information, Section 2). This analysis suggested that on average, the thickness of the WS 2 crystallites in the (002) axis is about the same for all the WS 2 -loaded aerogels, around 100 nm or 160 layers; whereas the thickness of the MoS 2 crystallites in the MoS 2 -loaded aerogel is somewhat lower at 64 nm or 104 layers (Table 1). This agrees well with the cross-sectional TEM images of the pyrolyzed aerogels (Figure 2).

Figure 5 XRD patterns of MA-17 and WA-17. Vertical solid lines show the corresponding X-ray diffraction (XRD) peaks and relative intensities of 2H-MoS 2 and 2H-WS 2 from the International Centre for Diffraction Data cards. Full size image

From electrochemical tests of our pyrolyzed TMD aerogel supercapacitor electrodes, we evaluated specific volumetric capacitance based on galvanostatic discharge profiles at each applied current, using the full voltage window of 0.9–0.1 V (Supplementary Information, Section 3 and Figure 6d). As we vary the mass loading of TMD’s into the aerogel, we observed significant differences in the densities of the aerogels: 0.33, 0.61, and 0.90 g cm−3 for RFA, MA-17, and WA-8.6, respectively. The wide range reflects the significant differences in the TMD densities—7.5 g cm−3 for WS 2 and 5.06 g cm−3 for MoS 2 —as well as their molecular weights. For example, while the WS 2 comprises only 3.3 mol% of WA-17, it represents 41.5 mass-%. Thus, the resulting capacitances represent the interplay between the molar percentage of the TMD and the density, capacitance, surface area, and conductivity of the added TMD, as discussed below.

Figure 6 Electrochemical characterization of the pyrolyzed aerogels with different WS 2 mass loadings fabricated into coin cell supercapacitors, including an equivalent circuit diagram (a), Nyquist plots from EIS (b, inset shows more detail of high-mid frequency range), cyclic voltammogram at sweep rate 20 mV s−1 (c), and specific volumetric capacitance (d) as a function of applied current density from galvanostatic tests. Full size image

While undoped and doped aerogels exhibited similar gravimetric capacitances (87.5 and 84.5 F g−1 maxima, respectively), the volumetric capacitance increased significantly upon the addition of TMD’s (Table 1). The WA-34 exhibited the greatest volumetric capacitance of the samples at 59.8 F cm−3 (64.7 F g−1), 127% greater than RFA at 26.3 F cm−3 (87.5 F g−1). Similarly, the MA-17 featured a large volumetric capacitance compared to the RFA, at 52.5 F cm−3 (84.5 F g−1), as did the other WS 2 -loaded aerogels. This marked improvement may be attributed to the reduction of interlayer coupling in the TMD’s accompanying sol-gel processing, as previously shown in Raman analysis. In addition, the enhanced volumetric capacitance of the TMD-loaded aerogels suggests that they represent promising, scalable materials for high-density supercapacitor applications, such as hybrid vehicles or portable electronics where space is constrained1,2,5,46. It is worth noting that all of our 2H TMD-loaded aerogel devices perform markedly better than devices based on pure, bulk 2H TMD (2–3 F g−1 to 40 F g−1) and similarly to devices based on the 1T metallic phase of MoS 2 (~80 F g−1) without pyrophoric materials19,47.

The addition of TMDs also improved the rate performance of the aerogels. At the highest tested current, the specific capacitance of WA-8.6 was 24% of its maximum value, compared to 15% for RFA. However, WA-17, WA-34, and MA-17 exhibited more severe drop-offs than RFA. We hypothesize that the poorer capacitance retention of these samples, as well as RFA, is correlated with their higher charge transfer resistance, a value that is measured from impedance spectroscopy, as discussed below. In addition, WA-17 exhibited excellent cycling stability, and increased in performance during repeated charging and discharging (Supplementary Figure S6). The WA-17-specific capacitance more than doubled between cycles 200 and 400, remaining 33% higher than its initial value at the final tested discharge. This enhanced capacitance during cycling may be due to additional exfoliation of the TMD or increased pseudocapacitance during charging and discharging, as observed by Bissett et al. for MoS 2 -graphene composite electrodes48.

We model the experimental EIS data, shown as Nyquist plots in Figure 6b, Supplementary Figures S4a and b, with the equivalent circuit in Figure 6a, which consists of an equivalent series resistance R ESR followed by a constant phase element Q in parallel to a charge transfer resistance R CT and a finite linear diffusion element M a . R ESR comprises the resistances associated with the bulk electrolyte, bulk electrode, and ‘external’ parts of the system such as the current collector, terminals, and leads. It is represented in the Nyquist plot by the intercept of the curve with the real impedance axis. R CT comprises the resistances due to electron transfer at interfaces in the device and specific adsorption of ions onto the active material, and is measured as the diameter of the best-fit semicircle at mid to high frequencies. Although R CT is typically associated with the kinetics of Faradaic reactions at the electrode-electrolyte interface, the lack of peaks or troughs in the CV sweeps (Figure 6c), as well as the lack of voltage plateaus in the galvanostatic discharge profiles (Supplementary Figure S3), suggest that no such reactions occur under our testing conditions. The constant phase element (CPE) accounts for frequency dispersion of capacitance that arises from the inhomogeneities of porous and rough electrodes49. This causes a slight depression and angling of the semicircular arc that is characteristic of an R|C component. Finally, M a is a particular mass-transport impedance where the diffusion layer has a finite thickness and a reflecting (non-permeable) boundary condition. This accounts for the resistance of electrolyte in pores and interfacial double-layer capacitance along pore walls50. In the Nyquist plot, M a manifests as the kinked line following the semicircle, which deviates from the vertical line of an ideal capacitor.

The Nyquist plots show that R CT increases with WS 2 loading, and that R CT is much greater for MA-17 than WA-17 (~21 Ω vs. 3.9 Ω), despite the former having larger pore sizes, which would reduce ion transport resistance. We hypothesize that these trends in R CT are related to the formation of Schottky barriers between semiconducting TMDs and metals51. Zhang et al. 52 have observed the analogous formation of a Schottky junction at the interface of MoS 2 with sp2 hybridized carbon in graphite, which they attributed to the existence of metallic edge states in MoS 2 nanosheets. Fermi level pinning may be exacerbated in these devices because the aerogels are not composed of pristine graphite, but carbonized RF polymer, whose highly defective structure hosts many charge trapping sites. In addition, while WS 2 and MoS 2 have similar bulk contact resistances, the incorporation of WS 2 likely does not impact the overall charge transfer resistance of the aerogel as severely as MoS 2 due to differences in molar loading (Table 1). Alternatively, R CT is associated with ion adsorption within the pores of the active material53. On this subject, an EIS study by Bissett et al. 47 on supercapacitors with exfoliated TMD membrane electrodes in aqueous Na 2 SO 4 electrolyte, showed that the ion adsorption in MoS 2 occurs on a much slower timescale compared to WS 2 . It is worth noting that R CT values comparable to ours have been reported previously for coin cells containing bulk (2H phase) MoS 2 as the active material54.

Interestingly, R CT of the RFA control is similar to WA-34 (5.7 Ω). This observation is consistent with previous studies of TMD-carbon composite supercapacitors where the addition of the TMD lowered R CT from that of the plain carbon as well as the bulk TMD55,56. Like WA-34 and MA-17, the RFA has larger pores which would lower ionic resistance, but no TMD’s to contribute to contact resistance within the electrode. In this case, we hypothesize that the larger R CT is related to the significantly higher specific surface area (776 m2 g−1) and lower bulk density (0.33 g cm−3) of the RFA due to the absence of TMD’s, resulting in a more sparse 3D network of active material with poorer electronic conductivity whose effect is great enough to counteract the easier ion movement. In support of this claim, Yang et al. recently conducted a comprehensive study of pyrolyzed RF aerogel supercapacitors where the pore size was tuned by catalyst concentration57. They found that charge transfer resistance tended to increase in tandem with pore size and confirmed the high electronic resistance of samples with large pores by four-point probe measurements.

In contrast to R CT , R ESR is similar for all tested aerogels, ranging from 0.56–0.76 Ω. This is not surprising given the identical composition of the pyrolyzed RF matrix and identical construction of the coin cell devices for all samples. The slight increase in R ESR for MA-17 compared to WA-17 reflects the difference in the electrical conductivity of the constituent bulk TMDs—0.9 S cm−1 for WS 2 versus 0.2 S cm−1 for MoS 2 at 300 K (Refs. 22,58). Mechanical integrity may also be responsible for the differences in series resistance, as the MA electrode amalgam had a stronger tendency to crack and flake apart during the flattening process, presumably due to the weak interlayer bonding of the TMD and its higher molar loading compared to WA-17.

Another performance metric is the knee frequency f k , which is the frequency at which the semicircle transitions into the sloped linear region in the Nyquist plot, corresponding to a local minimum of the phase angle. Physically, the knee frequency signifies the point below which ions can penetrate more easily into pores of the active material, covering its entire surface to produce capacitive behavior. Within the WS 2 aerogels, f k increases with decreasing TMD loading—20, 28, and 54 Hz for WA-34, WA-17, and WA-8.6 respectively—although it drops to 14 Hz for the unloaded RFA. The larger charge transfer resistance of WA-34 and RFA likely accounts for the lower f k , even though they have larger pores on average compared to the other samples, which would suggest less hindrance to ion diffusion within the electrode59. In fact, the very short 45°-sloped Warburg region preceding the steeper part of the line in these samples also suggests lower resistance to ion diffusion in the pores. The much lower f k of MA-17 (4 Hz) compared to WA-17 is indicative of the former’s much higher charge transfer resistance as well.

The wider pore size distribution of WA-34 and RFA also explains why these samples exhibit smaller slopes—corresponding to a lower phase angle—in the low-frequency region. With variation in pore sizes, the AC signal does not penetrate equally at a given frequency, since it is easier for ions to access larger pores than smaller pores, resulting in a shift from the theoretical vertical line of a capacitor. Song et al. developed a model to describe this particular frequency dispersion using a dimensionless frequency-dependent ‘penetrability’ and a pore size distribution function, showing that the slope of the line in the Nyquist plot decreases for pore distributions with greater standard deviation60,61.

The current–voltage plots from the 20 mV s−1 CV sweeps are shown in Figure 6c and Supplementary Figure S4c. While an ideal capacitor exhibits a rectangular shape, the voltammograms of the aerogel samples exhibit rounded corners, indicating resistance to ion diffusion that slows the response of the current to changes in the direction of the voltage sweep. In agreement with EIS, the MA-17 shows the largest ion diffusion resistance, represented by a lens-shaped voltammogram. Similarly, WA-34 and RFA also have significantly distorted CV curves, while WA-8.6, and WA-17 have the most rectangular curves. The more rectangular CV shape of WA-8.6 and WA-17 is also corroborated by their higher knee frequencies compared to the other two samples. The lack of peaks and troughs in the voltammograms of the three aerogels indicate that no redox reactions occur over the tested voltage range and that the mechanism of capacitance is purely double layer. While Na+ ions are known to intercalate between the layers of TMD particles, they are unlikely to do so except at extremely low scan rates62,63.