In some embodiments, the molybdenum doped carbon material is G-Mo and/or CNT-Mo. Although the molybdenum doped carbon material is described in terms as CNT-Mo and G-Mo, it may be appreciated that the teachings of the present disclosure may be utilized for other carbon-based materials, such as but not limited to, graphite, activated carbon, reduced graphene oxide, carbon nanofibers, and carbon black. In an embodiment, the substrate may be made from one or more of materials such as, stainless steel, aluminum, nickel, copper, platinum, zinc, tungsten, and titanium. In one embodiment, the substrate may be an aluminum current collector.

A method of making a nanocomposite electrode using the molybdenum doped carbon material is disclosed. The order in which the method is described is not intended to be construed as a limitation, and any number of the described method steps can be combined in any order to implement the method. Additionally, individual steps may be removed or skipped from the method without departing from the spirit and scope of the present disclosure.

The method includes dissolving at least one binding compound, and at least one conductive additive to form a slurry. In an embodiment, the slurry was obtained by mixing polyvinylidene fluoride (HSV 900 PVDF) binder in a mixture containing at least one conductive additive at a particular temperature with constant stirring until homogenous mixture obtained. In one embodiment, the slurry is stirred at a temperature of 70° C. until the slurry is homogeneous.

The method further includes mixing 3-25 wt. % of the molybdenum doped carbon material into the slurry for creating a synthesis mixture. In an embodiment, various concentrations of molybdenum doped carbon material are added in the resultant slurry. In an embodiment, the concentration of the molybdenum doped carbon material in the resultant slurry may be 5, 10, and 20% (w/w).

The method includes coating the synthesis mixture onto the substrate to form the nanocomposite electrode. In one embodiment, the coating is performed using an automatic coating machine. In an embodiment, the coating is less than 100 μm, preferably 500 nm-100 μm, 20-80 μm, or 30-50 μm. In an embodiment, the synthesis mixture coats at least 80% of the substrate, preferable 90%, or 100%.

The method includes drying the nanocomposite electrode at a temperature less than 100° C. In an embodiment, after coating the substrate, the developed electrode is dried in a standard oven at 70° C. Appropriately sized electrodes are then cut from dried electrode. In accordance with one embodiment, a precision pneumatic disk cutter may be used to cut and obtain, from the substrate, electrodes corresponding to a die size of the precision pneumatic disk cutter. In an embodiment, the substrates have a diameter of 1-6 cm, preferably 2-5 cm, 3-4 cm. Although diameter implies a circular shape of the substrate, the electrodes may be but are not limited to circular, rectangular, and square shaped.

Embodiments of the present disclosure also relate to a supercapacitor device including the nanocomposite electrode and an electrolyte. In an embodiment, the supercapacitor device comprises two nanocomposite electrodes disposed facing one another. The substrate of each nanocomposite electrodes is coated with the mixture on an inside facing surface and the outer surfaces of the nanocomposite electrodes are not coated with the mixture. The inside facing surfaces are separated by the electrolyte.

In an embodiment, the electrolyte is a polyol compound mixed with at least one selected from the group consisting of alkali metal hydroxides, alkaline earth hydroxides, alkali metal salts, and alkaline earth salts including but not limited to sodium hydroxide, potassium hydroxide, lithium hydroxide, barium hydroxide, calcium hydroxide, sodium chloride, potassium bromide, magnesium chloride to form a gel electrolyte. The polyol compound is a compound containing multiple hydroxyl groups, such as but not limited to glycerol, ethylene glycol, and propylene glycol. In an embodiment, the electrolyte may be a glycerol-KOH gel electrolyte. The KOH is 1-6 M in the glycerol, preferably 2-5 M or 3-4 M. Gel electrolytes compared to solution electrolytes increase the flexibility and elasticity of the electrode and thereby improve the cyclability and allow for use in wearable electronics.

In an embodiment, the supercapacitor device made with 15-25 wt % G-Mo as the molybdenum doped carbon material has an energy density of 40-60 Wh/kg, preferably 45-55 Wh/kg, or 47-52 Wh/kg at a specific power of 250-300 W/kg, preferably 260-290 W/kg, or 270-280 W/kg. In an embodiment, the supercapacitor device made with 15-25 wt % G-Mo as the molybdenum doped carbon material has a specific capacitance of 450-500 F/g, preferably 460-490 F/g, or 470-480 F/g at 0.5-5 A/g. In an embodiment, the supercapacitor device made with 15-25 wt % G-Mo as the molybdenum doped carbon material maintains at least 90%, preferably 95%, or 100% of said specific capacitance after 10,000 charge-discharge cycles.

In an embodiment, the supercapacitor device made with 15-25 wt % CNT-Mo as the molybdenum doped carbon material has an energy density of 30-50 Wh/kg, preferably 35-45 Wh/kg, or 37-42 Wh/kg at a specific power of 250-300 W/kg, preferably 260-290 W/kg, or 270-280 W/kg. In an embodiment, the supercapacitor device made with 15-25 wt % CNT-Mo as the molybdenum doped carbon material has a specific capacitance of 450-500 F/g at 0.5 A/g, preferably 460-490 F/g, or 470-480 F/g at 0.5-5 A/g. In an embodiment, the supercapacitor device made with 15-25 wt % CNT-Mo as the molybdenum doped carbon material maintains at least 88%, preferably 95%, or 100% of said specific capacitance after 10,000 charge-discharge cycles. In some embodiments, a higher concentration of the molybdenum doped carbon material, such as 15-25 wt %, improves the performance of the electrodes and supercapacitors, which may be due to the higher ionic conductivity that creates a convenient pathway for faster ion diffusion through the material.

In an embodiment, the supercapacitor is flexible and can be bent at 90°, or 180° (FIG. 5A). In an embodiment, 2-10, preferably 3-9, 4-8, or 5-7 of the supercapacitor devices are connected in parallel and/or series (FIG. 5C). In another embodiment, the supercapacitor device is electrically connected to a sensor in a wearable electronic device and functions as a battery.

Examples

The following examples describe and demonstrate nanocomposite electrodes, a supercapacitor comprising said nanocomposite electrodes, and exemplary embodiments of the synthesis of molybdenum-doped carbon nanotubes and graphene are described herein. The examples are provided solely for the purpose of illustration and are not to be construed as limitations of the present disclosure, as many variations thereof are possible without departing from the spirit and scope of the present disclosure.

Example 1: Materials

Kuraray active carbon (AC) and conductive carbon (CC) for supercapacitor electrode, HSV 900 PVDF (polyvinylidene fluoride) binder for Li-ion battery electrodes, Timical super C65 application (conductive additive) were obtained from MTI (USA). 1-Methyl-2-pyrrolidone (NMP), ammonium molybdate tetrahydrate (AM), ((NH₄)₆Mo₇O₂₄·4H₂O), hydrochloric acid (HCl) carbon nanotubes (CNT), and graphene (G) were provided from Merck. Glycerol and potassium hydroxide were received from Sigma-Aldrich.

Example 2: Synthesis of CNT-Mo and G-Mo Electrodes

CNT-Mo and G-Mo nanocomposites were synthesized by reaction of AM with CNT, and G, respectively. 100 mg of CNT or G were heated at 500° C. to 30 minutes to obtain crack formation on the structures, then dispersed in deionized water via ultrasonication. The pH of the solution was set 2.0 by adding hydrochloric acid and 1.0 g of AM was added to the suspension under stirring, which was then magnetically stirred at room temperature for 3 days. After filtration and washing samples were dried in an oven at 80° C. where both CNT-Mo and G-Mo were obtained (FIG. 1).

Example 3: Electrode/Electrolyte Preparation

Carbon hosting electrode slurry was prepared by mixing of 80% (w/w) of active carbon (AC), 10% of (w/w) PVDF, and 10% (w/w) of conductive carbon (CC) and further homogenized at constant stirring for 2 h at 70° C. Different ratios 5, 10, and 20% of CNT-Mo or 5, 10, and 20% G-Mo were mixed to the carbon slurry and further stirred for 1 h at 70° C. to prepare G-MoX and CNT-MoX, where X=5, 10 and 20 respectively. Then, the obtained carbon composites were uniformly coated on an aluminum current collector by using an automatic coating machine (MRX Shenzhen Automation Equipment). A standard oven was used to dry the sheet at 70° C. and then they were cut into circular electrodes utilizing a disk cutter. Gly/KOH (1M) electrolyte was prepared and homogenized by under stirring at 50° C. and then stored under vacuum environment before use.

Example 4: Materials Characterization and Electrochemical Measurement

The morphologies of the materials were obtained directly by Transmission Electron Microscopy (TEM) (Morgagni 268) and Scanning Electron Microscopy (SEM) (TESCAN VEGA3). Energy-dispersive X-ray spectroscopy (EDX) measurements were carried out with an EDAX. X-ray diffraction (XRD, Shimadzu XRD-7000) was used to investigate the crystalline structure of the products with monochromatic high-intensity Cu Kα radiation (λ=1.5406 Å) and 20=5-70° by a scan rate of 0.5° min⁻¹. The FTIR spectra of the electrode were studied in the range of 450-4000 cm⁻¹by Perkin Elmer Fourier-transform infrared spectrophotometer, Spectrum Two™.

Supercapacitor devices were fabricated using the configuration: Composite Electrode/Gel electrolyte/Composite Electrode. The Gly/KOH gel was used as electrolyte material to supply ions to the system to store the energy. The cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) experiments were carried out using Gly/KOH in CNT-Mo and G-Mo-based supercapacitors. The supercapacitor devices were assembled into the split cell assembly. GCD analysis was done at different currents ranging from 0.5 to 5 mA and cut-off voltage was set between-1 to 1 V. The supercapacitor cell was then assessed at different scan rates ranging from 10 to 150 mV s⁻¹. Cyclic voltammetry (CV) of the devices was investigated by using a Palmsens emstat-4 electrochemical analyzer.

The structural morphology of the samples was investigated by TEM as shown in FIG. 1. A pure tube-like CNT structure with a narrow diameter is represented in FIG. 1a. It is clear from the TEM image MoO₃nanoparticles interconnected with multiwall CNT after 3 days of vigorous stirring (FIG. 1C). FIG. 1B shows pure graphene sheets with a wrinkled-like structure. The TEM image shows that the Mo-bonded nanolayers are transformed into a thick structure with a Mo-expanded graphene surface. The X-ray diffraction (XRD) patterns of the products pure graphene and carbon nanotubes shown in (FIG. 6A). The diffraction peaks of graphene and carbon nanotubes reflections occurring at ~25.9° and 43.4° belong to (002) and (100) planes, respectively. According to the (JCPDS-ICDD card no. 00-008-0415) the typical main peak is observed which belongs to carbon. The change in the intensity of peaks is an effect of the inserting of Mo in the structure. It is noted that the samples G-Mo20 and CNT-Mo20 composite exhibits a similar pattern to the pure G and CNT except for a few weak peaks from α-MoO₃at around 12°, 20°, 52° and 68° assigned to the (020), (110), (002) and (202) crystallographic planes of the α-MoO₃phase (JCPDS-CDD card 05-0508), respectively, possibly due to the low diffracting intensity of the latter.

FIG. 6B demonstrates the FTIR spectrum of ammonium molybdate (AM), G, CNT, G-Mo20, and CNT-Mo20. The spectrum of carbon (CNT and G) illustrates a soft plot, while ammonium molybdate shows broad feature around 3600-2600 cm⁻¹corresponds to the stretching mode of the O—H bond indicating the presence of OH groups. Two significant peaks of AM exhibit at ~1416 and 1634 cm⁻¹were ascribed to the N—H vibration band and water molecular, respectively. At 573, 812, and 878 cm⁻¹, AM shows strong vibrations due to stretching of Mo—O bond and stretching and bending bonds of Mo—O—Mo. For G-Mo20 and CNT-Mo20 samples, the peaks of OH stretching vibration almost disappeared, while peaks of molybdate appeared with little shifting and low intensity. The intensity of vibrations of Mo—O and Mo—O—Mo reduced noticeably and shifted to 523 and 837 cm⁻¹for CNT-Mo, while G-Mo showed a weak band at 843.2 cm⁻¹and disappeared other peaks.

The SEM images (FIGS. 2A and B) show that Mo is homogeneously distributed in carbon nanotubes and graphene nanocomposite electrode. The homogeneous distribution of elements in the active electrode materials was well obtained from elemental mapping of CNT-Mo20 and G-Mo20 via EDX analysis. It was observed that carbon, oxygen, and molybdenum are uniformly distributed in the electrode structure. The EDX spectrum (FIG. 7) shows successful doping of Mo onto the carbon nanotubes and graphene which is consistent with the result of XRD and FTIR analysis.

The CV measurements of G-Mo-based and CNT-Mo-based supercapacitors at a dispersion ratio of 5, 10, and 20% were studied at a broad potential window of −1 to +1 V range at a scan rate of 10 mV s⁻¹. CV patterns of all devices (FIGS. 3A and B) demonstrated similar behavior with quasi rectangular shape indicating a suitable electric double-layer capacitor (EDLC) behavior. For G-Mo electrodes, the area between forward-reverse scan of CV curve increased with increasing Mo concentration, where the highest capacitance obtained from the G-Mo20-based electrode. The oxidation-reduction peaks of Mo are not observed in the CV curves referring to the low content of redox species and thus the pseudo-capacitance effect could be suppressed by the EDLC. However, to show the effectiveness of the electrolyte in a supercapacitor, the ion diffusions capability of the anhydrous Gly-KOH was examined at variable scan rates from 10 to 150 mV s⁻¹. These curves exhibited that the supercapacitor has reversible charge-discharge capacitive properties. The CV curves of the CNT-Mo20-based device (FIG. 3B) had similar profiles in the same potential window, where CNT-Mo20 was larger than those of the CNT-Mo5 and CNT-Mo10 based devices. At various scan rates from 10 to 150 mV s⁻¹, CV curves reserved their quasi-rectangular form up to 40 mV. The curves tend to distort at the higher scan rates starting from 60 mV s⁻¹show the CNT-Mo-based electrodes are not favorable at rates compared to G-Mo-based electrodes. Electrochemical impedance spectroscopy (EIS) reveals the frequency-based Nyquist graphs of the fabricated supercapacitors by fitting to the equivalent circuit. In the high-frequency area, Nyquist plots exhibited a semicircle, followed by a straight line in the low-frequency area (FIGS. 3C and D). The equivalent series resistance (ESR), which includes many factors such as the resistance of the electrolyte, the electrical resistance of the electrode material, and contact resistance between the electrolyte and the electrode material, the interfacial resistance between the current collector, and the diffusion resistance of the ions in the electrolyte was measured (Table 1). See G. Lee, S.-K. Kang, S. M. Won, P. Gutruf, Y. R. Jeong, J. Koo, S.-S. Lee, J. A. Rogers, J. S. Ha, Fully Biodegradable Microsupercapacitor for Power Storage in Transient Electronics, Adv. Energy Mater. 7 (2017) 1700157; and V. Rajendran, A. M. V. Mohan, M. Jayaraman, T. Nakagawa, All-printed, interdigitated, freestanding serpentine interconnects based flexible solid state supercapacitor for self powered wearable electronics, Nano Energy. 65 (2019) 104055, each incorporated herein by reference in their entirety. The internal resistance of the CNT-Mo was higher than G-Mo, which becomes lower conductivity for the electrodes. A lower ESR value (G-Mo) indicates higher ionic conductivity that electrode/electrolyte creates a convenient pathway for faster diffusion. The charge transfer resistance (Rct) has been calculated using the equivalent circuit diagram and the data fitted by special software (CorrTest Electrochemical Analyzer). The Rct for G-MoX and CNT-MoX shows that increase concentrations of G-Mo and CNT-Mo enhance the conductivity of the device electrodes (Table 1).

TABLE 1

ESR, Rct, and ionic conductivity with carbon electrodes

including G-MoX and CNT-MoX.

Device
E_SR(Ω)
R_ct(Ω)
σ (S cm⁻¹)

G-Mo5
10.63
59.85
0.00133

G-Mo10
9.5
28.38
0.00150

G-Mo20
8.53
18.64
0.00166

CNT-Mo5
34.78
109.52
0.00041

CNT-Mo10
24.25
38.7
0.00058

CNT-Mo20
9.25
9.79
0.00153

The ionic conductivity of the device with the electrodes bearing G-MoX and CNT-MoX were calculated using Eq. (1) at room temperature.

σ
⁡
(

S
/
cm

)

=

L
(
cm

)

/

R
⁡
(
Ω
)

×

A
⁡
(

cm

2

)

(
1
)

The temperature resistance behavior of the fabricated device ((G-Mo20)-based electrode) was studied by CV and EIS measurements under a temperature range between 21° C. and 70° C. (FIGS. 3E and F). Depending on the temperature increase (from 21° C. to 50° C.), the CV curves almost preserved the area under the forward-reverse scans at a scan rate of 10 mV s⁻¹. As the temperature rises to 70° C., the supercapacitor capacitive performance increases, as shown by the CV profile and reduced Rct value in EIS spectra. These results shows the fabricated supercapacitors devices have a great potential to work at a broad temperature domains and give more efficiency at higher temperatures.

Galvanostatic charge-discharge (GCD) profiles of the fabricated devices including various types of electrodes (pure carbon, G-Mo, and CNT-Mo) with Gly/KOH anhydrous gel electrolyte were measured at different currents from 0.5 mA to 5 mA within the potential range of −1 V to +1 V (FIG. 4). FIG. 4A shows the GCD profile of pure carbon electrode containing supercapacitor using Gly/KOH anhydrous gel electrolyte. The G-Mo20-based and CNT-Mo20-based devices exhibited a longer discharge time and a lower voltage drop as compared the other devices. The discharge time of the CNT-Mo20-based electrode was obtained as 982 s at a current of 0.5 mA (FIG. 4B). This was 1.5 times higher than that of the CNT-Mo5-based electrode (544 s), and 1.8 times higher than that of the CNT-Mo10 device (640 s). While the G-Mo20-based electrode exhibited a performance of 993 s (FIG. 4C) which was 1.3 and 1.6 times higher than that of the G-Mo5-based device (598 s) and CNT-Mo10-based device (769 s), respectively. The G-Mo20 and CNT-Mo20 electrodes had a longer discharge time (5.2 times) and superior electrochemical stability (FIG. 4D) when compared to pure carbon-based electrodes. The GCD charts varied in behavior depending on the current, demonstrating that it could work with great performance even at high current.

In electrochemical energy storage systems, the operating performance of devices in different voltage ranges is an important factor. GCD measurements performed at different voltage ranges for both systems (G-Mo20 and CNT-Mo20) are shown in FIGS. 4E and 4F, respectively. The results obtained show that both systems reveal very efficiently in measurements made at different voltages. In addition, the CV studies G-Mo20 and CNT-Mo20 (not shown) confirming the GCD measurements at different voltages at the scan potential windows of −1.0-0.0, −1.0-0.5, and −1.0-1.0 V. It was observed that CV curves maintained their quasi-rectangular profile in all voltage windows.