Transition metal dichalcogenide (TMD) materials have recently demonstrated exceptional supercapacitor properties after conversion to a metallic phase, which increases the conductivity of the network. However, freestanding, exfoliated transition metal dichalcogenide films exhibit surface areas far below their theoretical maximum (1.2%), can fail during electrochemical operation due to poor mechanical properties, and often require pyrophoric chemicals to process.
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On the other hand, pyrolyzed carbon aerogels exhibit extraordinary specific surface areas for double layer capacitance, high conductivity, and a strong mechanical network of covalent chemical bonds. In this paper, we demonstrate the scalable, rapid nanomanufacturing of TMD (MoS 2 and WS 2) and carbon aerogel composites, favoring liquid-phase exfoliation to avoid pyrophoric chemicals. The aerogel matrix support enhances conductivity of the composite and the synthesis can complete in 30 min. We find that the addition of transition metal dichalcogenides does not impact the structure of the aerogel, which maintains a high specific surface area up to 620 m 2 g −1 with peak pore radii of 10 nm. While supercapacitor tests of the aerogels yield capacitances around 80 F g −1 at the lowest applied currents, the aerogels loaded with TMD’s exhibit volumetric capacitances up to 127% greater than the unloaded aerogels. In addition, the WS 2 aerogels show excellent cycling stability with no capacitance loss over 2000 cycles, as well as markedly better rate capability and lower charge transfer resistance compared to their MoS 2-loaded counterparts. We hypothesize that these differences in performance stem from differences in contact resistance and in the favorability of ion adsorption on the chalcogenides.
In response to rapid improvements in renewable energy generation, electrochemical supercapacitors with high power densities and rapid cycling have emerged as a promising technology to bridge the energy density storage and variable energy density demands of grid management and hybrid vehicles. State of the art batteries employ lithium intercalation that enables 3–30 times more charge storage than supercapacitors. However, supercapacitors can provide 2–3 orders of magnitude greater power. While typical lithium-ion batteries store electrochemical potential via phase transformative redox reactions, supercapacitors do so by means of an electrical double layer in a working electrolyte and/or rapid charge transfer processes with no concomitant phase transition, that occurs in response to an applied voltage.
Thus, supercapacitors do not face the performance-limiting factors of reaction kinetics, ion transport through bulk electrode material, and accompanying volume changes that are characteristic of batteries. Improvements in efficient, scalable, and economical syntheses are needed to drive supercapacitor applications.Because double-layer formation dictates power densities, nanostructured materials, such as pyrolyzed carbon aerogels, with high specific surface areas (SSA) have emerged as the premier supercapacitors. Carbon aerogels are amorphous, sp 2- and sp 3-bonded carbon structures formed from the reaction of resorcinol and formaldehyde with high SSAs (500 m 2 g −1), narrow pore sizes, and low densities.
Upon pyrolysis, these aerogels exhibit improved electrical conductivity (up to 50 S cm −1) and SSA with benign chemistry, making them an intriguing material for supercapacitors. To improve their performance, research groups have attempted to load them with high-conductivity or high-capacitance materials. However, their lengthy synthesis time (typically 24 h or longer) prevents economical scalability, limiting their widespread use.Transition metal dichalcogenides (TMDs) are layered, graphite-like van der Waals structures composed of a transition metal layer sandwiched between two chalcogenide layers that have shown promise as supercapacitor active materials due to the range of oxidation states available to transition metals. A recent report demonstrated that MoS 2 capacitance significantly improves after conversion from the semiconducting 2H phase to the metallic 1T phase, due to enhanced electrical conductivity—the in-plane conductivity of 2H MoS 2 is 0.2 S cm −1 and that of the 1T phase is 10–100 S cm −1—and increased ion intercalation mobility. Similarly, 2H MoS 2 and WS 2-based supercapacitor performance improved after the creation of a mesoporous with enhanced conductivity. However, it is important to note that the most successful 1T devices suffered from a low SSA of 9 m 2 g −1, and required the use of pyrophoric n-butyllithium to induce a phase change, both of which mitigate the scalability of their impressive performance. In addition, freestanding TMD films do not exhibit mechanical stability in electrochemical environments, which sets a limit to the maximum size of unsupported films and inhibits high surface area applications.To address the aforementioned limitations, we present in this work a synthesis that emphasizes scalable processes to encapsulate exfoliated TMD sheets in an aerogel matrix via benign and ultrafast resorcinol-formaldehyde (RF) sol-gel chemistry.
This process employs the ultrasonication of TMD precursors—MoS 2 and WS 2—in acetonitrile to produce exfoliated 2H MoS 2 and WS 2, respectively, followed by the rapid, acid-catalyzed gelation of RF within 30 min at mild temperatures. After supercritical drying and high-temperature pyrolysis, the resulting carbon aerogel provides a high surface area, mechanically stable, and electrically conductive support for TMDs that is well suited for electrochemical devices. To demonstrate these applications, we perform supercapacitor tests on our TMD-loaded carbon aerogels that yield favorable specific capacitances around 80 F g −1, volumetric capacitances approaching 60 F cm −3, and low operational electrochemical impedance. When exfoliated TMD sheets dry, they restack to form low surface area (9 m 2 g −1) films. As such, we designed the rapid synthesis to avoid allowing the TMD solution to dry before the gelation can trap any exfoliated sheets. In addition, while we employ sonication, to disperse the TMDs, there are a range of other potentially scalable exfoliation methods including shear mixing, direct synthesis, and intercalation, to name a few.outlines the TMD aerogel synthetic scheme. In the first step, a temperature-controlled bath sonicator (22 °C; Branson 1510R-DTH, Danbury CT, USA) exfoliates and disperses TMD sheets in acetonitrile (ACN, EMD Millipore, Billerica MA, USA) at concentrations from 8.6 to 34 mg mL −1.
However, we were able to synthesize aerogels at TMD loadings of up to 100 mg mL −1 in acetonitrile. After sonication for 60 min, we transferred this solution to a polypropylene tube and added resorcinol (R, Sigma-Aldrich, St. Louis, MO, USA), formaldehyde (F, 37 wt-% methanol-stabilized aqueous solution, Sigma-Aldrich), and hydrochloric acid (C, 37 wt-%, Macron, Center Valley PA, USA) to achieve molar ratios of R:F=1:2, R:C=8.4:1, and R:ACN=1:76, which result in a 2:1 ratio by weight of resorcinol to TMD for a 17.1 mg mL −1 starting dispersion of TMD.
This corresponds to molar ratios for resorcinol to MoS 2 (powder, Sigma-Aldrich) and WS 2 (powder, Alfa Aesar, Haverhill MA, USA) of 2.9:1 and 4.5:1, respectively. For WS 2-loaded gels, we prepared additional samples by the same technique using initial WS 2 dispersions of 8.6 and 34 mg mL −1, corresponding respectively to 4:1 and 1:1 weight ratios, or 9.0:1 and 2.25:1 molar ratios, of resorcinol to WS 2. We also prepared a control sample with no TMD.
The mixture of reagents was quickly placed in the bath sonicator set to 40 °C for 30 min. During this time, the resorcinol undergoes electrophilic aromatic substitution at the 2, 4, and 6 positions with formaldehyde to form methylene and methylene-ether bridges. We then washed the aerogel with ethanol three times over 36 h to remove the acetonitrile and dried it with supercritical CO 2 in an autoclave (E3100, Quorum Technologies, Laughton, East Sussex, UK). Because it has low density and surface tension, supercritical CO 2 displaces the ethanol and preserves pore structure to produce a high surface area product. Finally, we pyrolyzed the aerogels in a tube furnace at 800 °C in an argon atmosphere for 4 h, which drives off oxygen moieties, yielding a high conductivity sp 2- and sp 3-bonded support of carbon spheres. We found that annealing at 1000 °C destroyed the TMDs.
In addition, we note that this pyrolysis step mimics the current industrial synthesis of supercapacitors. Before pyrolysis, the MoS 2 and WS 2 loaded aerogels exhibit a dull, deep blue, and green color, respectively, whereas the pure RF aerogel has a brick-red color.
Following pyrolysis, the aerogels all exhibit a dark black color, indicative of carbonization. Henceforth, the pyrolyzed pure RF aerogel will be abbreviated RFA, the pyrolyzed MoS 2-loaded aerogel will be abbreviated MA-17, and the pyrolyzed WS 2-loaded aerogel will be abbreviated WA-8.6, WA-17, or WA-34 according to the concentration of the initial TMD dispersion. Compared with the chemical exfoliation of TMD’s via pyrophoric n-butyllithium and the long gelation times in other syntheses, our sol-gel synthesis represents a rapid, mild, and benign process.