High utilization efficiencies of alkylbenzokynones hybridized inside the pores of activated carbon for electrochemical capacitor electrodes

Benzoquinone derivatives (BQDs) are hybridized inside activated carbon (AC) pores via gas-phase adsorption to prepare electrochemical capacitor materials. In this study, 2 mmol of BQDs are hybridized with 1 g of AC. The hybridization of alkylbenzoquinones (ABQs) with AC enhances the volumetric capacitances of the hybrids from 117 to 201 F cm−3 at 0.05 A g−1 and the capacitances are retained up to 73% at 10 A g−1. Meanwhile, the volumetric capacitances are increased to 163 F cm−3 at 0.05 A g−1 by the hybridization of halobenzoquinones (HBQs) and the capacitance retentions at 0.05 A g−1 are ∼62%, which are higher than that of AC (46%). The results of electrochemical measurements suggest that HBQs exist as agglomerates while ABQs are finely dispersed inside the pores. The ABQs have good contact with the conductive carbon pore surface compared to the HBQs. Consequently, most of the ABQ molecules undergo reversible redox reactions (i.e., high utilization efficiencies), and a large contact area facilitates charge transfer at the large contact interface, thereby endowing the hybrids of ABQs with fast charging and discharging characteristics. HBQ molecules can be finely dispersed by liquid-phase adsorption, but the finely dispersed HBQ molecules are mobile inside the pores at room temperature and gradually form agglomerates. The difference in the existing form of BQDs is explained by the dominant interaction affecting the BQD molecules. ABQs have a strong interaction with the carbon pore surface while the intermolecular interaction is dominant for HBQs.


S1. Electrode preparation and assembly of electrochemical cells
Fig. S1 shows the method to prepare working and counter electrodes and the cell assembly of the threeelectrode cell used in this study. An electrode sheet was prepared using carbon black (CB: DENKA BLACK Li, Denka Company Ltd.) and polytetrafluoroethylene (PTFE: PTFE 6-J, Du Pont-Mitsui Fluorochemicals Company, Ltd). In order to prepare working electrodes, CB and PTFE were mixed with AC or the AC/BQD hybrids, and the resulting mixture containing 8.5 mg of AC was weighed. The mixture was formed into a 12×12 mm square sheet and the sheet was sandwiched by SUS304 mesh (100 mesh, Nilaco) at 30 MPa for 150 s. Since BQDs were adsorbed inside the pores of AC without the volume change of AC particles (for details, see section S3), the weight ratio of AC (i.e., excluding the weight of BQD), CB, and PTFE was adjusted to 18:1:1 and the use of the mixture containing 8.5 mg of AC led to the same electrode thickness for the electrodes of AC and the AC/BQD hybrids. The weights of the working electrodes are summarized in Table S1. A counter electrode was prepared using AC (MSC30, Kansai Coke and Chemicals Co., Ltd.), CB, and PTFE, in the same manner as the method to prepare the working electrode, and 20 mg of the mixture was sandwiched by SUS304 mesh. As shown in Fig. S1, a three-electrode beaker cell was prepared using the working and counter electrodes, a Ag/AgCl reference electrode containing saturated aqueous KCl solution, and 15 mL of aqueous 1 M H2SO4 electrolyte.
For the two-electrode cell measurements, the same beaker cell was used and the cell voltage (the difference in the potential between the anode and cathode) was controlled in the range from 0 to 0.8 V.
To confirm the potentials of the anode and cathode, the Ag/AgCl reference electrode was used during two-electrode cell measurements. An asymmetrical two-electrode cell was prepared using AC/TMBQ as an anode and AC as a cathode. The anode contained 8.5 mg of AC and the weight of the cathode was balanced considering the capacitances of AC/TMBQ and AC measured at 2 A g −1 using a three-electrode cell (Fig. 2c). For comparison, a symmetrical two-electrode cell was prepared using AC and both electrodes contained 8.5 mg of AC. The weights of the anodes and cathodes for both asymmetrical and symmetrical cells are summarized in Table S2.

S2. Calculations of the volumetric current and the volumetric capacitance in the three-electrode cell measurements
Since 2 mmol of the BQDs were hybridized with 1 g of AC, the weight percentages of the BQDs in the AC/BQD hybrids (X) were calculated using the following equation: X % g g 1 g 100 2 10 mol g mol 2 10 mol g mol 1 g 100 1 where g and g mol are the weight of 2 mmol of BQD and the molecular weight of BQD, respectively. Therefore, the weight ratio of AC and BQDs in the AC/BQD hybrids is 100 X : X.
As shown in Fig. S2a, the electrode of AC consists of AC, binder (i.e., PTFE), and CB, and the weights g of AC, PTFE, and CB per 1 cm 3 of the electrode are defined as , , and , respectively.
The experimental electrode density of AC is defined as g cm and equal to the sum of , , and . Meanwhile, assuming that the adsorption of BQD is not accompanied by the volume change of the AC particles (for details, see Section S3), the weights of 1 cm 3  A g A g 100 100 X 1 The volumetric current of AC and the AC/BQD hybrids ( A cm : the current per 1 cm 3 of the electrode) was calculated according the following equation: A cm A g g cm A g 100 100 X 0.9 g cm

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As well as the volumetric current, the volumetric capacitance can be similarly calculated according to the following equations: The areal capacitance ( F cm ) was calculated using the area of the working electrode (1.44 cm 2 ,

S3. Calculation of the electrode densities of the AC/BQD hybrids
In our previous studies, we have reported that the adsorption of organic and organometallic compounds was not accompanied by the volume change of porous carbon particles because the experimental electrode densities are in good agreement with the theoretical ones. 1-6 Specifically, in the case of using p-DCBQ (2,5-dichloro-1,4-benzoquinone) and the same AC used in this study, the experimental electrode densities were the same as those of the theoretical ones despite the amount of p-DCBQ, where the amount of p-DCBQ in the AC/p-DCBQ hybrids was examined up to the saturation amount: i.e., 14.0 mmol per 1 g of AC (71.2 wt%). 4 Therefore, we used the theoretical electrode densities in this study. As shown in Fig. S2, the theoretical electrode densities ( g cm ) can be calculated as the sum of , , , and , according to the following equation: 3,7 g cm 0.9 X 100 X X 100 X 0.1 9 The experimental electrode density of AC is 0.33 g cm −3 . 4 The electrode densities of AC/p-DCBQ-L, AC/p-DCBQ-L-H, and AC/p-DCBQ-L# are regarded as the same values as that of AC/p-DCBQ.

S4. TG-DTA analyses of BQDs
To determine the adsorption temperatures of BQDs, thermogravimetry-differential thermal analysis (TG-DTA) was performed on Simultaneous Thermogravimetry/Differential Thermal Analyzer (DTG-60, Shimadzu). BQDs were measured at a ramp rate of 10 ºC min −1 up to 500 ºC under nitrogen gas (100 mL min −1 ). The results are shown in Fig. S3. The melting point of m-DCBQ is 122 ºC and m-DCBQ shows a distinct peak at ca. 120 ºC (Fig. S3a), corresponding to fusion. However, the weight of m-DCBQ decreases below its melting temperature due to sublimation. Therefore, in order to perform gas-phase adsorption, the adsorption temperature of m-DCBQ was set to 105 ºC. Similarly, the adsorption temperatures of the other BQDs were determined, as summarized in Table S4.  where L is the mean crystal size (nm), K is the Scherrer constant depending on the crystallite shape normally taken as 0.9, λ is the X-ray wavelength (0.154 nm for Cu Kα), β is the full width of the diffracted peak profile at half-maximum height (in radians 2θ), and θ is the Bragg reflection angle in radians.
Therefore, very small particles do not show distinct peaks in the XRD pattern. All hybrids do not show any distinct peaks derived from the crystalline BQDs but show a broad peak in their XRD patterns; i.e., the BQD molecules exist inside the pores of AC and there are few BQD molecules on the particle surface of AC.
Since the positions of the broad peaks in the XRD patterns of the AC/BQD hybrids have nothing to do with those of the bulk BQDs, the positions of the broad peaks would be related to the distance between the BQD molecules and the carbon pore surfaces, which are interacted through π-π stacking. If this is the case, the distance must have positive correlation with the size of functional groups on the BQD molecules; large functional groups distance BQD molecules from the carbon surface. The distance between the carbon surface and benzene interacted through π-π stacking is reported to be 0.324 nm. 9 Meanwhile, tetrafluoro-1.4-benzoquinone interacts with phenanthrene through π-π stacking and the distance is ca. 0.33 nm. 10 The interaction between phenanthrene and tetrafluoro-1.4-benzoquinone is similar to that of the BQDs and carbon surfaces for the AC/BQD hybrids and the distance of 0.33 nm corresponds to 27º (2θ). The bulky functional groups in BQD molecules further distance the molecules from the carbon surface, resulting in the shift of the broad peak to the low angle in the XRD pattern, which is observed for the hybrids of bulky ABQs: e.g., AC/ t BuBQ and AC/p-D t BuBQ.

S6. Results of nitrogen adsorption/desorption measurements for AC and the AC/BQD hybrids
The nitrogen adsorption/desorption isotherms and pore size distributions of AC, AC/BQ, and the AC/ABQ hybrids are shown in Fig. S5a and b, respectively. Fig. S5c and d show the nitrogen adsorption/desorption isotherms and pore size distributions, respectively, for AC and the AC/HBQ hybrids. The amount of adsorbed nitrogen is normalized per 1 g of AC in Fig. S5. The nitrogen adsorption/desorption isotherms of AC and all the hybrids show the steep uptake below P/P0 of ca. 0.05 due to the micropore filling. In addition, a gradual increase of adsorbed nitrogen is observed up to P/P0 of ca. 0.4 without hysteresis. The steep uptake is based on the micropores while the gradual increase without hysteresis is derived from small mesopores. Therefore, the type of the isotherms is the combination of types I and IV.    S16

S7. SEM observation of AC and the AC/BQD hybrids
The scanning electron microscopy (SEM) images were collected on JCM-7000 NeoScope (JEOL) at an accelerating voltage of 15 kV. The SEM images of AC and the AC/BQD hybrids are shown in Fig.   S6−S8. By comparing with the SEM images of AC (Fig. S6a−c), a difference in morphology between AC and the AC/BQD hybrids was not observed in the SEM images. Considering the results of the XRD and nitrogen adsorption/desorption analyses, there is few BQD molecules on the particle surface of AC for the AC/BQD hybrids.

S9. A method to calculate the utilization efficiencies of the BQDs
The utilization efficiencies of the BQDs were calculated by integrating the quantity of electricity in the anodic peak area, as shown in Fig. S10. The voltammograms collected at 1 mV s −1 were used for the calculation. First, a baseline was drawn by extrapolating the constant current, which is attributed to the formation of the electric double-layer. Then, the peak current was integrated, as indicated in the figure.
The calculated quantity of current was then compared with the theoretical value, which was calculated considering that all the hybridized BQD molecules underwent reversible redox reactions based on the two-electron redox reaction.

S11. Areal capacitances and gravimetric, volumetric, and areal capacities of AC and the AC/BQD hybrids
The areal capacitances of AC and the AC/BQD hybrids are shown in Fig. S16a. Their gravimetric, volumetric, and areal capacities are shown in Fig. S16b, c, and d, respectively. Their BET surface areas and pore volumes are summarized in Table S6 together with the normalized values. Although there is a slight difference in the pore size distribution (Fig. S19b), a significant difference is not observed in their isotherms (Fig. S19a), BET surface areas, and pore volumes.