Yp 50

N-Methyl pyrrole (>99%), 1-Bromopropane (>98%), acetonitrile (>99.5%) were obtained from Aldrich and used without further purification. The PC solvent (battery grade, extra dry <20 ppm of water) and Lithium difluoro(oxalato)borate (>99%) were purchased from Jiangsu Guotai Super Power New Materials Co. Ltd. (China). The prepared electrolyte salt (Py₁₃DFOB) was placed in a glove box filled with high pure argon (<1 ppm O₂ and <1 ppm H₂O), then dissolved in the PC with 1 mol L⁻¹ concentration, added 3Å molecular sieves to remove trace moisture for 1 week. The final water content is <20 ppm testing by Karl Fischer titration method (Mettler-Toledo C20, Switzerland). The impurities of halide and alkali metal ions were <2 ppm confirmed by Inductive Coupled Plasma Emission Spectrometer (ICP) test.
The activated carbon electrodes were prepared by mixing 82% activated carbon (Kuraray YP-50), 10% carbon black (VXC72) as the conductor, 4% carboxymethylcellulose sodium (CMC), and 4% styrene butadiene rubber (SBR) as the binder. The mixture was stirred to a sticky state followed by coating on aluminum foils. The electrodes were punched into disks with a diameter of 18 mm, then dried under vacuum at 120°C for more than 12 h prior to be used. The mass of each electrode is about 6 mg and the thickness is ~60 um (including 20 um aluminum foil).

The blocking electrode was coated with a novel carbon ink [24 (link)]. The electrode was a J-cuff style electrode [34 (link)], with platinum as the base metal between two silicone sheets with an exposed window. The carbon ink consists of 3 g carbon black (YP-50 Kuraray, Canoga Park, CA) combined with 6 g of N-methyl pyrrolidone (NMP) and finally 3 g of 10% polyvinylidene fluoride diluted in NMP. A small amount (5–10 μl) of the carbon ink was deposited on the platinum surface and baked at 200 °F for 20 min. The resulting electrode has very high capacitance [24 (link)]. The total charge capacity of each electrode was measured before each experiment via cyclic voltammetry using a Solartron Inc., Model 1280B Potentiostat with a sweep rate 10 mV s⁻¹, voltage range of −0.255 to +1.20 V, and sampled at 10 Hz. The electrodes were placed in a bath of Ringers solution with a Ag/AgCl reference (BASi RE5B) and a large platinum return in a three-electrode measurement configuration. The Q value, or the total amount of charge that can be delivered, was calculated by integrating the current in the capacitive region of the electrode.

All electrochemical tests were performed on a Biologic VMP300 potentiostat. For the three-electrode test, plastic Swagelok cells (PFA-420-3) were used (Supplementary Fig. 15). MXene hydrogels on glassy carbon, overcapacitive activated carbon (YP-50, Kuraray, Japan), Ag/AgCl in 3.5 M KCl, and 3 M H₂SO₄ (0.5 mL) were used as working electrode, counter electrode, reference electrode, and electrolyte respectively. For the MSC device test, vacuum-dried MXene hydrogels and PVA-EG-H₂SO₄ were the electrode materials and gel electrolyte, respectively. The room-temperature electrochemical tests were done in ambient conditions with an average temperature of 25 °C. The low-temperature electrochemical tests were conducted in a lab fridge. EIS was measured ranging from 10 mHz to 1000 kHz under a potential amplitude of 10 mV.

The morphology and structure of the samples were studied using field-emission SEM (FE-SEM, FEI Nova 450 Nano), optical microscopy (Nikon), high-resolution transmission electron microscopy (HRTEM, TECNAI), AFM (Shimadzu), X-ray diffraction (X'Pert Pro, PANanalytical), XPS (ESCALab250), Raman spectroscopy and photoluminescence (LabRAM HR800). For the optical microscopy and AFM measurements, the samples were dropped on SiO₂/Si and dried at room temperature. The electron conductivity was measured using a Keithley 4,200. All of the electrochemical tests were carried out using ECLab and CHI660E. For the typical three-electrode set-up, Ag/AgCl (CHI, USA) was used as the reference electrode, YP-50 (Kuraray, Japan) was used as the counter electrode and a Celgard film served as the separator (Celgard, USA). All of the tests were conducted in Swagelok cells (Swagelok, USA). The electrochemical impedance was measured from 1 to 1 MHz with a potential amplitude of 10 mV. The organic electrolyte was fabricated by dissolving LiClO₄ in ethylene carbonate/dimethyl carbonate at a concentration of 1 M.