Flexible asymmetric supercapacitors based on ultrathin two-dimensional nanosheets with outstanding electrochemical performance and aesthetic property

Flexible asymmetric supercapacitors with excellent electrochemical performance and aesthetic property are realized by using ultrathin two-dimensional (2D) MnO2 and graphene nanosheets as cathode and anode materials, respectively. 2D MnO2 nanosheets (MSs) with a thickness of ca. 2 nm are synthesized with a soft template method for the first time, which achieve a high specific capacitance of 774 F g−1 even after 10000 cycles. Asymmetric supercapacitors based on ultrathin MSs and graphene exhibit a very high energy density up to 97.2 Wh kg−1 with no more than 3% capacitance loss after 10000 cycles in aqueous electrolyte. Most interestingly, we show that the energy storage device can have an aesthetic property. For instance, a “Chinese panda” supercapacitor is capable of lighting up a red light emitting diode. This work has another, quite different aspect that a supercapacitor is no longer a cold industry product, but could have the meaning of art.


Preparation of MnO 2 nanosheet (MS) and graphene (GA) printing ink
The MS printing ink was prepared by mixing 70wt% MS powder (synthesized with the soft template method) as active material, with 20wt% acetylene black as conductive additive, and 10wt% LA133 (purchased from Chengdu Indigo Power Sources Co., Ltd.) as binder. LA133 aqueous solution was firstly prepared by dissolving LA133 in deionized water and stirring 30min. Next acetylene black and MS were successively added in the LA133 solution under stirring. After going on stirring 8h for well dispersing, a homogeneous aqueous MS printing ink with a solid content of about 15±2wt% was produced.
Similarly, the GA printing ink was prepared by using 80wt% GA powder as active material with 10wt% acetylene black as conductive additive and 10wt% LA133 as binder through a process the same as preparing MS printing ink.

Fabrication of MS and GA electrodes
Take MS screen-printed electrodes as an example, in this work a simple manual screen printing method was utilized because of its easy operation, low cost and high quality. Firstly, a certain size of ITO-PET substrate was cut out as the printing stock and fixed on a flat and smooth table with ITO face upwards. Then a designed screen plate (the screen mesh count is 200mesh) was put on the ITO-PET with through-hole part above the ITO face. And the prepared MS printing ink was transferred onto the screen plate. Next hand a sdueegee and pressure to the screen, with a certain speed do scraping movement. With the pressure applied by the sdueegee, the ink went through the through-holes printing onto ITO film. Finally, after drying the ITO-PET printed with MS printing ink at 85 ℃ for 8h to remove solvent, the screen-printed MS electrode was obtained. By adjusting the concentration of ink, sdueegee speed, and sdueegee pressure, etc. The thickness of MS pattern can be controlled from 6 to 17 μm measured by using a micrometer. Those used in characterizing electrochemical performance usually have a thickness of about 10 μm. Typically, the loading mass of MS and GA is about 0.4 and 0.9 mg cm -2 , respectively.
The fabricating procedure of GA electrodes using GA printing ink with screen printing method was the same to the fabrication of MS electrodes, this electrode-made method is also suitable to most electrode materials.

Fabrication of flexible screen-printed supercapacitors
Before printing, it is necessary to prepare the three key elements known as printing ink, screen plate and printing stock. The ink is prepared by dispersing powders of electrode material in water or organic solvents. Occasionally, binders and conductive additives are added to enhance the adhesivity and conductivity when necessary. In this work, the aqueous printing inks of manganese dioxide and graphene (GA) powders have been prepared. The screen plate consists of blind-holes and through-holes. The ink can be printed on the printing stock through the through-holes with certain artwork form. The through-holes can be designed as various patterns, pictures or letters, such as a cute "panda", "Tsinghua University" letters and star-shaped or dot patterns as shown in Figure 2A. This character leads to its critical role, like "the magic paint brush", in creating various vivid supercapacitors such as a "panda" supercapacitor. The printing stock can be soft paper, cloth, plastic, hard glass, ceramic, etc.  The screen-printed supercapacitor was fabricated by two electrodes sandwiched with electrolyte, and some double-side tapes were necessary during packaging. Except for a blank ITO-PET part was setting aside to be used as the lead wire, all around the edge of the screen-printed electrode was firstly taped up with some double-side tapes.
Next the window in the middle was brushed with transparent Ca(NO 3 ) 2 -SiO 2 composite gel electrolyte. Then non-stick part of double-side tapes was stripped away and the other piece of electrode was folded on it. Finally, a screen-printed supercapacitor was assembled and its structure was showed in Figure S1.
It has to be pointed out that this screen printing method is suitable for inks made of almost all electrode materials in kinds of solvents, and the printing stock can be soft, hard, flat, curved as well as various shape and size. In addition, electrode materials printed on the ITO layer with especially beautiful contact are thickness-controllable and stable.
Most interestingly, the versatile tool of "screen plate" provides the electrodes or supercapacitors preferred transparency and pattern. Hence, the supercapacitor presented here is not only an excellent energy storage device with flexibility but also an artwork.

Characterization of screen-printed flexible electrodes Unprinted ITO-PET substrate
The transmissivity of unprinted ITO-PET substrate is characterized by ultraviolet and visible spectrophotometer (UV-vis) ( Figure S5). Based on the wavelength ranging from 450 to 800 nm, the average transparency value of unprinted ITO-PET substrate is about 85%.  Figure S6 shows the photograph of screen-printed MS electrode with different square lattice patterns. It needs to be pointed out that the four square lattice patterns (A, B, C, D) of electrode are formed by 1×1, 0.75×0.75, 0.5×0.5 and 0.3×0.3(mm 2 ) printed squares with an arrangement rule. And they have the same arrangement rule as the magnified one in Figure 2A. And we can readily calculate their rates of coverage: k A =4/9, k B =1/4, k C =1/9, k D =1/25. Here α is about 1.0 (the opacity of MS electrode material film printed on ITO-PET) and T 0 is 85% (transmissivity of unprinted ITO-PET presented in Figure S5). The transparency (T C =76%) value of electrode with certain pattern in Figure S6C or Figure 2A

Screen-printed GA flexible electrode
Figures S8 shows the electrochemical performance of the screen-printed GA electrode. It was measured with a three-electrode system and 2 M Ca(NO 3 ) 2 aqueous solution was used as electrolyte. The electrodes used have a square lattice pattern with the same arrangement rule to Figure S6C (where the size of printed square point is 0.5×0.5(mm 2 ) and its transparency calculated by equation (1) is about 76%). They appear similar to the MS electrodes utilized in tests.

Characterization of screen-printed supercapacitors
Unprinted supercapacitor Figure S9 presents the UV-vis spectrum of unprinted supercapacitor assembled with two ITO-PET substrate and Ca(NO 3 ) 2 -SiO 2 composite gel electrolyte. The average transmissivity of this unprinted supercapacitor is up to 72%.

Movie S1
In this movie an asymmetric MS/GA supercapacitor with "panda" design is manufactured with aqueous Ca(NO 3 ) 2 -SiO 2 composite gel electrolyte. After charging the supercapacitor up to the voltage of 2V at a scan rate of 10 mV s -1 , it lights up a red light emitting diode (the operating voltage is 1.8V ~2.0V).