Novel thiophene-based donor-acceptor scaffolds as cathodes for rechargeable aqueous zinc-ion hybrid supercapacitor

Well-defined π-conjugated thiophene donor-acceptor molecules play an important role in different fields ranging from synthetic chemistry to materials science. Their chemical structure provides specific electronic and physicochemical properties, which can be further tuned by the introduction of functional groups. Herein, we design and synthesize two novel thiophene-based π-conjugated donor-acceptor molecules TDA-1 and TDA-2 through Aldol and Knoevenagel condensations. In our proof-of-concept study we report for the first time on the use of small organic molecules employed in aqueous zinc-ion hybrid supercapacitors (Zn-HSCs),which exhibit capacitance as high as 206.7 and 235.2 F g-1 for TDA-1, and TDA-2, respectively.


2) Characterization
The structure, composition and texture properties of materials were investigated by X-ray powder diffraction (XRD) with a Bruker D8 X-ray diffractometer, and Fourier transform infrared (FT-IR) with a JASCO, FT-IR 4700 Fourier Transform Infrared Spectrometer equipped with ATR Diamond. Thermogravimetric analyses (TGA) decomposition curves are recorded in the range 25-800 °C operating under nitrogen atmosphere, with a thermal step of 10 °C/min on a Mettler Toledo TGA/SDTA851e system. The specific surface area was measured using a Micromeritics ASAP 2050 surface area and porosity analyzer. Prior to the BET measurements, the samples were outgassed for 10 hours at 100 °C. Adsorption isotherms were calculated for nitrogen adsorption at 77 K and pressures up to 1 bar. Scanning Electron Microscopy (SEM) images were recorded with a FEI Quanta FEG 250 instrument S3 (FEI corporate, Hillsboro, Oregon, USA). Chemical composition was assessed by X-ray Photoelectron Spectroscopy with a Thermo Scientific K-Alpha X-ray photoelectron spectrometer equipped with an aluminium X-ray source (energy 1.4866 keV) at a vacuum level of 10 −8 -10 −9 mbar in the main chamber. The spot size of the X-ray beam was fixed at 400 µm.
Both, 1D and 2D-NMR spectra were recorded on a Bruker Ascend-500 Magnet, equipped with a Bruker Prodigy CryoProbe, using a Bruker AVANCE Neo as electronic bay. 1 H NMR are reported in ppm using solvent as internal standard for proton referencing (500 MHz, residual CHCl 3 at 7.26 ppm). 13 C{1H} NMR spectra were also reported in ppm using solvent as internal standard (125 MHz, CDCl 3 at 77.3 ppm).

Fabrication of Zn-HSC
Zinc foil was directly used as anode electrode after polished with gauze and pouched into electrodes with 12 mm diameter. The cathode electrode composes of 60 wt% of TDA-1 or -2, 30 wt % of Super P conductive additives and 10 wt% of polyvinylidene fluoride as binder, 1-methyl-2-pyrrolidone was used as a dispersing agent. The mixed slurry was smeared on a porous stainless steel current collector and dried in a vacuum oven at 80 ℃ overnight (mass loading of active materials on stainless steel between 0.7 and 1.2 mg cm -2 ). Zinc-ion hybrid supercapacitors were assembled with electrolyte of 2 M Zn(CF 3 SO 3 ) 2 aqueous solution, Nonwoven separator MPF30AC as separator and coin battery shell.

Electrochemical characterizations and calculations of Zn-HSC
Electrochemical performance of Zn-HSC was studied using cyclic voltammetry Synthesis of TDA-1: In a typical experiment, 2-thiophenecarboxaldehyde (5 mmol) was added to 2,4,6-trimethyl-1,3,5-trazine (5.7 mmol) in 70 ml of ethanol followed by the addition of 0.35 mmol of potassium tert-butoxide (t-BuOK). A light-yellow precipitate separates immediately, but the solution was stirred for another hour. Finally, the precipitate was filtered and washed with ethanol and was dried giving a yellow solid (yield 80%).

Synthesis of TDA-2:
In a typical experiment, 2-thiophenecarboxaldehyde (5 mmol) was added to BTAN (5.7 mmol) in 70 ml of ethanol followed by the addition of 0.35 mmol of potassium tert-butoxide (t-BuOK). A yellow precipitate separates immediately, but the solution was stirred for another hour. Finally, the precipitate was filtered, washed with ethanol and was dried giving a yellow solid (yield 80%). Figure S1. 2D 1 H-13 C HSQC NMR spectra of TDA-1 Figure S2. 2D 1 H-13 C NMR spectra of HMBC for TDA-1.