Defect-induced B4C electrodes for high energy density supercapacitor devices

Boron carbide powders were synthesized by mechanically activated annealing process using anhydrous boron oxide (B2O3) and varying carbon (C) sources such as graphite and activated carbon: The precursors were mechanically activated for different times in a high energy ball mill and reacted in an induction furnace. According to the Raman analyses of the carbon sources, the I(D)/I(G) ratio increased from ~ 0.25 to ~ 0.99, as the carbon material changed from graphite to active carbon, indicating the highly defected and disordered structure of active carbon. Complementary advanced EPR analysis of defect centers in B4C revealed that the intrinsic defects play a major role in the electrochemical performance of the supercapacitor device once they have an electrode component made of bare B4C. Depending on the starting material and synthesis conditions the conductivity, energy, and power density, as well as capacity, can be controlled hence high-performance supercapacitor devices can be produced.


Spin counting procedure:
In order to accurately count the number of spins, there are many important issues that should be considered before and after the EPR experiment. Crucial issues, which have to be carefully taken into account, are: (i) Samples should be always weighed before the experiment to avoid complications due to different sample amounts in EPR tubes. If it is not possible to have always the same amount of sample in an EPR tube, then for normalization each spectrum should be multiplied by a filling factor (deduced from the mass of the sample in the EPR tube). (ii) The sample position should be always adjusted to the center of the microwave cavity. (iii) If there is a background EPR signal (e.g., from impurities in the resonator), it has to be subtracted from the EPR signal of the sample. (iv) One should always be careful not to saturate the EPR signal by applying too high microwave power. The where (Area) D and N D are the area of the EPR signal of the related defect centre and the number of spins of the sample, respectively. In this work we used MnO powder as standard sample, which has 1.75  10 15 spins/g.

Equations for the Electrochemical Performance Parameters:
The performance properties of the supercapacitors such as specific capacitance, energy density, power density are calculated from the discharge part of the GCPL curves. The capacitance, C (F) can be obtained by the well-known Equation (3)  And finally the the coulombic efficiency (%) is calculated by Eq. (7) as follows: where t d and t c are the discharging and charging times, respectively.
Additional Electrochemical experiments: Figure S5: (a) Voltage holding test for two kinds of supercapacitors, namely for S1@G3 and S1@G6. (b) Leakage current profile of S1@G3 and S1@G6 supercapacitor device at 1 V volt holding voltage obtained from the floating test.
From the voltage holding and floating experiments it is obviously clear that the more defective sample (S1@G3) -according to EPR-has the less impact on the capacitance than the S1@G6. Since B 4 C is highly new type of material for supercapacitor designs as electrode, the voltage hold and leakage performance is highly promising. Of course, we are aware of the results are not as impressive as graphene or metal oxides electrodes, however in next studies, we believe the results will be improved substantially by further modification of materials such as synthesizing nanosized B 4 C or producing composite of B 4 C with other kinds of energy materials as well as metal ion doping. Figure S6: (a) CV measurements at various scan rates, (b) rate capability of S1@G3 obtained from GCPL measurements (c) Nyquist plots obtained from PEIS measurements before and after 100 CV cycles with different scan rates at 20 mV/s and 100 mV/s.
Supercapacitor from the electrode of S1@G3 has been thoroughly tested by CV, GCPL and PEIS as esteemed Reviewer suggested and the results are presented in Fig. S6 (a-b-c). From such results basically the supercapacitor performance is quite promising but not outstanding. CV curves measured at 5 mV/s up to 200 mV/s in Fig.   S6(a) is almost identical even at higher scan rates confirming the good electrochemical stability of the electrodes. The shape with presence of redox humps in the CV curves confirms the battery-like pseudocapacitive mechanism. On the other hand, the specific capacitance at the highest specific current rate was obtained as 96.9 F/g at 0.1 A/g, respectively which was obtained by considering the Eq. 4. Also Nyquist plot were plotted both before and after 100 CV scans and the differences are not much while anyway the impedance values are in the range of kOhm.
Nevertheless, after 100 cycle the impedance become much smaller which shows the more the cycle time the more the electrons will transfer and find easier path for conduction. Similar results have been reported for supercapacitors composed of NiO nanofiber electrodes and the effect has been explained by the possible loss of adhesion of some active materials with the current collector during the cycling 1 .
In order to fully understand the properties of the electrodes in designed supercapacitors all Nyquist plots obtained from PEIS were fitted to suitable equivalent circuits by the aid of ZFit.
Supp. Table 1: Equivalent circuit elements and their numerical values obtained from fitting of the Nyquist plots given in Fig. 6 in the main text. a and s are the crucial internal parameters/units to obtain good fit. Figure S7: The equivalent circuits that are obtained by the aid of Zfit software. a) Equivalent circuit for S1@G3 and, b) equivalent circuit for S2@G6, S1@A3 and, S1@A6. Refer Fig. 6 in main text for the fitted curves.

R1
Specific Capacity vs potential results obtained from GCPL test Figure S8: Specific capacity revealing more than 2000 mAh/g at first cycles.

Performance comparison with other carbonaceous materials
A compact table listing the specific capacitance, power and energy density values of other carbon-based materials compared to this work.
Supp. Table 2: Electrochemical performance of this work and various carbon-based electrodes used in supercapacitor devices.