Electrochemical data (raw) for Carbon-POM supercapacitors

Electrochemical data for Carbon-POM supercapacitors including charging-discharging, stability, capacitance (scan rate) and CV.Specific CapacitanceThe Na₃PW₁₂O₄₀ CPH device has the lowest capacitance (seen as smallest current), whilst H⁺ and TBA⁺ are similar. Cyclic voltammograms at the operating voltages (0-4.5 V) were recorded at 25, 50 100, 150, and 200 mVs^-1scan rates, and used to calculate specific capacitance according to the following equation:(1)where<i> C</i> is capacitance (F), ν is the scan rate (V/s), <i>V</i>_<em>i</em>and <i>V</i>_<em>f</em>are the limits of the sweep potential window, <i>m</i> is the mass of the active material, and <i>I(V)</i> is the voltage-dependent current response.²³<br>StabilityThe rate of discharging was used to calculate the capacitance, thus the % capacitance retained compared to the first scan for each cycle could be determined. When fewer than three data points for the discharging segment were available (due to rapid discharging rate), the data were fit using a spline interpolation function.<br><br>where <i>τ</i>: time (s), <i>R</i>: resistance (ohm), <i>C</i>: capacitance (F), <i>V</i>: voltage, and <i>I</i>: current.<br>Fabrication of POM-carbon hybrid electrodesA pestle and mortar were used to grind POM crystals, which were then mixed with Solaronix low-temperature carbon black ink in various wt% of POM. Then, the ink was doctor bladed onto a cleaned fluorine-doped tin oxide (FTO) using a circular 1 cm diameter punch and scotch tape. Substrates were then annealed for 30 minutes at 110 °C, with a 20-minute ramp time. The film thickness of the hybrid electrodes was measured with a profilometer (Dektak), and their conductivity was measured with a four-point probe (Ossila).For symmetric supercapacitor devices, two electrodes were fabricated as described above with a square punch (0.5 x 0.5 cm) and clamped facing each other. Before clamping the electrodes were soaked with a few drops of supporting electrolyte, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Emim][NTf₂]) and separated with a Nafion fumapen® F-950 membrane. Solder was deposited onto each electrode to provide a contact for the measurements.Cyclic voltammetry of POM-carbon hybrid electrodesCyclic voltammetry (CV) studies were carried out using a 3-electrode set-up using a low-volume cell. The working electrode was a synthesised hybrid electrode, the counter electrode was a Pt wire, and the reference electrode was an Ag wire or Ag wire in saturated LiCl in ethanol. Scan parameters were: 0.01 V s^-1 scan rate and 0.01V potential step, with a 30 s equilibration period unless stated otherwise. Supporting electrolytes were 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide unless stated otherwise.CVs of symmetric carbon-POM supercapacitors were measured in a 2-electrode set-up, with the reference electrode (R.E.) connected to the working electrode (W.E.). CVs were measured at 25, 50 100, 150, and 200 mV s^-1 with a 0.005 V step.Galvanostatic C-D MeasurementsSymmetric cells were connected in a 2-electrode configuration. Initially, cells were discharged to 0 V by applying a negative bias, then, currents of 0.19, 0.16, 0.12, and 0.09 A g^-1 were applied to charge the cell until a cut-off of 4 V, where a negative bias of the same magnitude was applied to discharge the cell.Stability cyclingCells were connected to an Autolab potentiostat, and a potential of 4 V was applied for 1000 cycles whilst the current was measured.