Co3O4 Quantum Dots Intercalation Liquid‐Crystal Ordered‐Layered‐Structure Optimizing the Performance of 3D‐Printing Micro‐Supercapacitors

Abstract The effects of near surface or surface mechanisms on electrochemical performance (lower specific capacitance density) hinders the development of 3D printed micro supercapacitors (MSCs). The reasonable internal structural characteristics of printed electrodes and the appropriate intercalation material can effectively compensate for the effects of surface or near‐surface mechanisms. In this study, a layered structure is constructed inside an electrode using an ink with liquid‐crystal characteristics, and the pore structure and oxidation active sites of the layered electrode are optimized by controlling the amount of Co3O4‐quantum dots (Co3O4 QDs). The Co3O4 QDs are distributed in the pores of the electrode surface, and the insertion of Co3O4 QDs can effectively compensate for the limitations of surface or near‐surface mechanisms, thus effectively improving the pseudocapacitive characteristics of the 3D‐printed MSCs. The 3D printed MSC exhibits a high area capacitance (306.13 mF cm−2) and energy density (34.44 µWh cm−2 at a power density of 0.108 mW cm−2). Therefore, selecting the appropriate materials to construct printable electrode structures and effectively adjusting material ratios for efficient 3D printing are expected to provide feasible solutions for the construction of various high‐energy storage systems such as MSCs.

The microstructures and morphology of samples were observed by field emission scanning electron microscopy (FE-SEM, ZeissSupra55) under the acceleration voltage of 5.0 kV and transmission electron microscopy (TEM, JEM-2100 instrument).High-resolution TEM (HRTEM) images, selected area electron diffraction (SAED) images, and elemental mapping were captured on a Tecnai G2 F30 at an acceleration voltage of 300 kV.The crystal phase was performed by X-ray diffraction (XRD) on a Bruker D8 Advanced X-ray Diffractometer (Cu-Kα radiation: λ = 0.15406 nm).Fourier transform infrared (FT-IR) spectra were captured on TENSOR27.Raman spectra were carried out on INVIA REFLEX.The X-ray photoelectron spectra (XPS) was obtained on a Thermo Scientific ESCALAB 250 apparatus.The value of specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method (ASAP 2469 model).The pore size was obtained from the adsorption-desorption branch of the nitrogen isotherms by the Barrett-Joyner-Halenda method.

Synthesis of V2O5 NWs
0.36 g of V2O5 powder was dispersed in 30 ml of deionized water.Then, 10 ml 30% hydrogen peroxide was added, and stirred at room temperature for 2 h, transferred to Teflon lined sealed autoclave at 200 °C for 96 h.V2O5 NWs were prepared by natural cooling, deionized water washing for 6 times and freeze-drying.

Synthesis of Co3O4 QD
200 mg of cobalt acetate was dispersed in 10 ml of benzyl alcohol solvent and stirred continuously at room temperature for 1 hour.Subsequently, 8 ml of ammonium hydroxide solution was added and stirred vigorously for 10 minutes.The solution turned reddish brown.
Then stir at 170 ℃ for 4 hours.As the volume gradually decreases, an inky black suspension is finally obtained.An appropriate amount of ether solvent is added to the reaction solution and centrifuged to collect the black precipitate.Before characterization, the sample is washed with ethanol and dried.

Synthesis of Ti3C2TX nanosheets
Ti3C2TX nanosheets was prepared in a typical method. 1 g Ti3AlC2 was added to the etchant solution, which was consist of 6 mL HCl, 1 mL HF and 3 mL deionized water (DI water).The reaction was stirred at 400 rpm for 15 h at 41 °C.Then, the resultant was washed with DI water repeatedly until pH is neutral (4500 rpm, 5 min) and the accordion-liked Ti3C2TX was obtained.
Subsequently, 25 mL DI water containing 1.5 g of LiCl was added.After reaction for 2 h, the resultant was washed with DI water repeatedly (3500 rpm, 5 min) and delaminated manually by hand shaking agitation and centrifugation to obtain Ti3C2TX suspension.
Preparation of VCGQD-1, VCGQD-2, VCGQD-3 hydrogels 10 mg 8.5 mg ml −1 graphene oxide dispersion (Tanfeng Tech.Inc.) and 45 mg CNTs (XFNANO, 14 wt%) dispersion were mixed.Then, 45 mg V2O5 NW and 2 mg Co3O4 QD were dispersed in above mixture.After ultrasonic for 60 min, the sequence of stirring for 60 min was repeated for many times until the mixture was uniform, the mixture was vacuum filtered through a polytetrafluoroethylene (PTFE) membrane filter with a pore diameter of 0.45 μm.Subsequently, the ink on the membrane filter was collected and stirred to obtain VCGQD-1 gel.VCGQD-2(4 mg Co3O4 QD) and VCGQD-3 (6 mg Co3O4 QD) were prepared by the same process.
Preparation of the Gel Electrolyte: Add 4.56 g polyvinyl alcohol (Xilong, PVA-124, AR) powder (PVA)was added to 30 ml deionized water and soaked, and then was heat to 80 °C until the solution becomes transparent.Meanwhile, 6.39 g KOH (Shanghai Aladdin Biochemical Technology Co., Ltd.) was dissolved in 15 ml deionized water.Afterwards the KOH solution was then slowly added dropwise to the PVA solution until the transparent gel state was achieved.
After boiling, the 20 min bubbles were discharged.The final solution was cooled down to room temperature and dropped on the interphalangeal region of the printed MSCs to fully wet the electrode.
Preparation of 3D Printing MSCs: Firstly, according to the gelation strategy of modified nanocomposites, VCGQD electrodes and MXene electrodes for extrusion 3D printing is prepared.1D CNT in ink is used as fluid collector and conductive agent to form interconnected conductive network.2D GO nanosheet is used as adhesive.VCGQD hydrogels were prepared by Co3O4 QD, V2O5 NWs, CNTs and GO via micro ultrasonic and vacuum filtration.After the gels formulation were completed, they were deposited layer by layer on the polyethylene terephthalate (PET), and thick interdigital electrodes were constructed for MSCs using extrusion 3D printing.Then dry, remove the solvent, solidify the architecture of 3D printing, and use hydrazine hydrate vapor reduction process to reduce graphene oxide (rGO) for 12 h.
Finally, the PVA-KOH gel electrolyte was dripped into the projection area of the microelectrode, and the directional network was completely filled in to finish the preparation of MSCs.
According to the formula, the ink was loaded into a 5 ml syringe barrel, then transferred into a 50 ml centrifuge tube, and centrifuged at 2500 rpm for 3 min to remove the internal bubbles before printing.3D printing was performed using a desktop robot (XM-331) according to a pre-programmed program.The inner diameter of the passing needle was 210-230 μm.The ink is extruded by air pressure.The optimum extrusion pressure is 30-60 psi.The movement speed of the nozzle is 5-7 mm s −1 .First of all, 3D printing technology was used to print the designed pattern and electrode gel onto the PET substrate.The preset line spacing is 200-400 μm.The nozzle height is maintained at 200-300 μm or so.Then PVA-KOH gel electrolyte was used to cover the whole MSCs.Finally, the additional PET layer is used as a passivation layer to cover the gel electrolyte to enhance stability.

Calculations
The mass-specific capacitance (C/F g −1 ) of the device can also be calculated using： where m is the mass of the activated materials, I is the discharge current, tdischarge is discharge time, and △V is the potential drop during discharge.
The area-specific capacitance (C/mF cm −2 ) of the device can also be calculated using： where A is the surface area of the device, I is the discharge current, tdischarge is discharge time, and △V is the potential drop during discharge.
The kinetic of capacitive contribution can be obtained by calculating the CV curves at different scan rates.The relationship between current (i) and scan rate (v) can be written as: where a and b are constant that can be obtained from log(v) versus log(i) plots.The situation where b=0.5 represents an ideal diffusion-controlled process and when b=1.0 indicates a surface capacitive-controlled process.The capacitive contributions at different scan rates can be calculated by the equations described as below: where i is the current density at a voltage (V), v is the scan rate (mV s −1 ), k1 and k2 can be obtained from the slope and intercept, respectively.Where k1v can be attributed to the current from surface capacitance contribution, while k2v 1/2 is indexed to the diffusion process.

3 . 2 Figure
Figure S1.a) Apparent viscosity of the MXene hydrogels as a function of shear rate; b) PHS experiment; c) The G′ and G″ of MXene hydrogels.