Preparation of the boron-doped nanodiamond

To determine the deposition time of the MPCVD process for the BDND preparation, the electrical conductivity of the BDND before heat treatment was measured as a function of the deposition time (Fig. 1). The electrical conductivity of the as-deposited BDND was increased rapidly from 1.8 × 10−9 S cm−1 to 3.5 × 10−4 S cm−1 with a deposition time of 1 h. The BDND prepared with 8 h deposition time exhibited a conductivity sufficient for an electrochemical electrode material (2.0 × 10−2 S cm−1). Therefore, we set the MPCVD deposition time for the preparation of the BDND as 8 h.

Figure 1 Electrical conductivity of the as-deposited BDND as a function of the CVD deposition time for the BDND preparation. Full size image

The UV Raman spectrum of the ND showed a sharp peak at 1325 cm−1 and a broad peak at around 1650 cm−1, as similarly reported by Mochalin29 (Fig. 2a). The peak at 1325 cm−1 can be deconvoluted into a main peak at 1320 cm−1 and a shoulder peak at 1220 cm−1 with a small unknown peak at 1300 cm−1. The peak at 1625 cm−1 was found to be a combination of three bands at 1580, 1640, and 1720 cm−1, which were attributed to graphitic carbon, OH bending, and C=O stretching originated from surface functionalities29. In the case of the as-deposited and heat-treated BDND, broad peaks at approximately 1410 and 1580 cm−1 for D and G bands, respectively30, were observed, which indicated that a significant amount of sp2 carbon components existed even after the heat treatment (Fig. 2b,c). Since the presence of the sp2 carbon structure may be detrimental for the electrochemical properties of the BDD (e.g., wide potential window), heat treatment in air at 425 °C was performed to remove the sp2 carbon impurities.

Figure 2 UV Raman spectra of the (a) ND, (b) as-deposited BDND, and (c) heat-treated BDND. Solid thin and thick lines indicate experimental and simulated curves, respectively. The spectra were decomposed into Voigt profiles (dashed lines). Full size image

Figure 3 shows the transmission electron microscopy (TEM) images of the ND, as-deposited BDND, and heat-treated BDND. The diameter of the ND was 5–10 nm, and the lattice spacing was determined from the image to be 0.2 nm, which is in agreement with the diamond (111) planes31 (Figs. 3a,b, S1a and S2a). In the image of the as-deposited BDND, although primary particles could not be clearly seen, the lattice spacing of 0.2 nm for diamond and a layer spacing of 0.367 nm attributable to graphite were observed (Figs. 3c,d, S1b and S2b). This layer spacing was slightly larger than that reported for graphite (0.335 nm)32, which can be attributed to the lower crystallinity of the graphitic components. From this result, the CVD process can be considered to give rise to graphitic components covering the particle and deposition of the BDD on the ND agglomerate. After heat treatment in air at 425 °C for 8 h, the graphitic component was partly removed, resulting in the exposure of the diamond surface (Figs. 3e,f, and S1c). The average diameters obtained by dynamic light scattering measurement of the as-deposited and heat-treated BDND were 111 and 120 nm, respectively. Elemental analysis using inductively coupled plasma atomic emission spectrometry (ICP-AES) revealed that the boron concentration increased from 440 to 620 ppm after the heat treatment. Since the BDND has an (undoped) ND core, the boron concentration in the BDD layer is considered to be larger than these values.

Figure 3 TEM images of the (a,b) ND, (c,d) as-deposited BDND, and (e,f) heat-treated BDND. Full size image

Figure 4a shows the Brunauer–Emmett–Teller (BET) specific surface area estimated from the nitrogen gas sorption measurement of the BDND sample after heat treatment in air for various time periods following the CVD process. The as-deposited BDND exhibited a BET specific surface area of 182 m2 g−1, which increased remarkably to 658 m2 g−1 after heat treatment in air at 425 °C for 8 h. The specific surface area was almost the same after 10 h of heat treatment; therefore, heat treatment for 8 h was found to be the optimum condition. This increase in the specific surface area after heat treatment is most likely due to removal of the sp2 carbon components contained in the BDND. The as-deposited BDND consists of an ND agglomerate covered with BDD and sp2 carbon materials, which is expected to cause a decrease in the specific surface area from that of the original ND. However, the heat treatment would remove the vulnerable sp2 carbon part, leaving a stable sp2 carbon structure, e.g., a graphene-based structure. This is supported by the fact that the Raman spectra of the BDND before and after heat treatment were similar (Fig. 2b,c). The remaining sp2 carbon structure should contribute to increase significantly the specific surface area (Fig. 4b). Although the sp2 carbon components could not be removed completely from the as-deposited BDND with heat treatment for 8 h, the remaining sp2 carbon components can be considered to contribute to the large specific surface area, which was almost half of that obtained for an AC sample used in this work, i.e., 1318 m2 g−1.

Figure 4 (a) BET specific surface area of AC, ND, and BDND with various heat treatment times (0, 5, 8, and 10 h); (b) A schematic illustration of the BDND preparation. Full size image

Electrochemical investigation for aqueous supercapacitors

The electrochemical properties of the BDND electrode were examined in 1 M H 2 SO 4 . According to the result of the CV measurement using a three-electrode system, the BDND electrode exhibited a wide potential window of about 3 V (Fig. 5b). Wide potential windows in aqueous electrolytes are a typical characteristic of BDD electrodes6,33. Therefore, the wide potential window obtained in this work for the BDND probably stems from the BDD part in the BDND. A redox peak pair around +0.3 V vs. Ag/AgCl can be attributed to the quinone/hydroquinone group existing in the edge site of the graphitic components34,35. Figure 5a illustrates the CV of an AC electrode, which exhibits a narrower potential window of 1.5 V than that of the BDND electrode. Similarly, a redox peak pair for the quinone/hydroquinone group was observed. Figure 5c,d display the CV obtained using a symmetric two-electrode system. The BDND electrode was found to exhibit a wide potential window of 1.8 V (Fig. 5d), whereas that of the AC electrode was 0.8 V in 1 M H 2 SO 4 (Fig. 5c). The characteristics of the BDD are most likely responsible for the large cell voltage of the BDND in aqueous electrolyte. Although the BDND contained a large amount of sp2 carbon components, the potential window was found to be as wide as that of BDD thin film electrodes6. The double-layer capacitance of the BDND electrode (10 mV s−1) was calculated to be 15.1 F g−1, whereas that of the AC electrode was 20.4 F g−1. Figure S3 shows the CV of the as-deposited BDND and BDND after heat treatment for 8 h with a symmetric two-electrode system in 1 M H 2 SO 4 . The double-layer current was found to be 3.27 times larger for the heat-treated BDND than for the as-deposited BDND, which can be explained by the difference in the specific surface area of both BDNDs (Fig. 4a). Electrochemical impedance spectroscopy measurement was performed to estimate the electrochemical properties of the BDND electrode (Fig. S4). The Nyquist plot indicated a blocking behavior with a slope in the low frequency region, which is typical of conventional EDLCs.

Figure 5 CVs in 1 M H 2 SO 4 with (a,b) a three-electrode and (c,d) a symmetric two-electrode system: (a,c) AC and (b,d) BDND electrodes. The scan rate was 10 mV s−1. Full size image

The charge–discharge rate performance was investigated using CV with various scan rates (Fig. S5). Even at fast scan rates up to 1000 mV/s, the CVs for the BDND showed a constant current region. Figure 6a,b show the gravimetric and volumetric capacitance of the BDND and AC electrodes calculated from the CV data as a function of the scan rate using the formula C = I/v, where C is the capacitance, I is the charging current in the constant region, and v is the scan rate. The gravimetric capacitance of the BDND electrode was larger than that of the AC electrode when the scan rate was 100 mV/s or faster (Fig. 6a). From this result, the BDND electrodes can be considered to be suitable for high-rate charging–discharging. Since AC had well-developed micropores in the particles, its capacitance deteriorated for fast scan rates36. In contrast, the absence of such pores in the BDND allowed for the entire surface to contribute efficiently to the fast charging–discharging. The higher suitability of the BDND is further evinced when considering the volumetric capacitance (Fig. 6b) because the bulk density of the BDND electrode layer (0.52 g/cm3) was larger than that of the AC electrode layer (0.23 g/cm3)27. The energy density was calculated using the capacitance extracted from Figs. 6a,b according to the following equation: E = CV2/2/3600, where E is the energy density (in Wh/kg), C is the capacitance (in F/kg), and V is the cell voltage (in V). The cell voltage used for the calculation was 1.8 and 0.8 V for the BDND and AC, respectively. This difference caused the energy density of the BDND to be much larger than that of the AC (Fig. 6c,d). The large energy density of the BDND remained stable even at fast scan rates due to its relatively large capacitance of the BDND in the fast scan rate region (Fig. 6a,b).

Figure 6 Capacitance (a) per unit weight of active material and (b) per unit volume of electrode layer; and energy density (c) per unit weight of active material and (d) per unit volume of electrode layer as a function of the scan rate. The capacitance was estimated from the CV data recorded in 1 M H 2 SO 4 with a symmetric two-electrode system. The energy density was calculated according to the capacitance (panels a and b) and cell voltage (0.8 V and 1.8 V for AC and BDND, respectively). Full size image

To estimate the stability of the BDND electrode in repetitive charging–discharging cycles, a voltage cycle test was performed from 0 to 1.8 V at 10 mV/s. Figure 7a shows capacitance retention as a function of voltage cycle number, indicating sufficient stability of the electrode with a 3.5% loss of the capacitance after 10,000 cycles. The stability of the BDND electrode was also investigated with a floating test (Fig. 7b). When a cell voltage of 1.8 V was applied to the cell, the capacitance was found to decrease to 38% of the initial value after a floating time of 12 h. In the case of a cell voltage of 1.6 V, however, the capacitance was maintained at 91% of its original value. This result indicates that aqueous EDLCs having a BDND electrode and 1 M H 2 SO 4 can be operated practically at a large cell voltage such as 1.6 V.

Figure 7 (a) Capacitance retention of the BDND electrode cell using 1 M H 2 SO 4 as a function of the CV cycle number of the voltage cycling test. The cell voltage was scanned from 0 to 1.8 V at 10 mV/s. (b) Capacitance retention of the BDND electrode cell using 1 M H 2 SO 4 as a function of time of floating. The cell voltage was held at 1.6 and 1.8 V during the test. The capacitance was estimated by CV at 10 mV/s. Full size image

Electrochemical investigation in saturated NaClO 4

Since the use of saturated NaClO 4 as an aqueous electrolyte has been reported to expand the potential window37, we examined the electrochemical properties of the BDND electrode in saturated NaClO 4 using CV with a three-electrode system (Fig. 8a). The potential window was confirmed to be wider for saturated NaClO 4 than for 1 M H 2 SO 4 at the BDND electrode. Consequently, the cell voltage in a two-electrode system was found to expand to 2.8 V in saturated NaClO 4 at both BDND and AC electrodes (Fig. 8b). Figure 9 shows the capacitance of the BDND electrode in 1 M H 2 SO 4 and saturated NaClO 4 estimated from the CV recorded in a two-electrode system as a function of the scan rate. According to the result, the capacitance of the BDND electrode in saturated NaClO 4 was similar to that in 1 M H 2 SO 4 . In contrast, the energy density of the BDND electrode cell with saturated NaClO 4 was much higher than that exhibited in 1 M H 2 SO 4 due to the larger cell voltage (Fig. 9c,d).

Figure 8 (a) CV in 1 M H 2 SO 4 and saturated NaClO 4 at the BDND electrode. The potential sweep rate was 10 mV s−1. (b) CV in 1 M H 2 SO 4 and saturated NaClO 4 with a symmetric AC and BDND two-electrode system. The scan rate was 10 mV s−1. Full size image

Figure 9 Capacitance (a) per unit weight of active material and (b) per unit volume of electrode layer; and energy density (c) per unit weight of active material and (d) per unit volume of electrode layer of the BDND electrode cell as a function of the scan rate. Full size image

Figure 10 illustrates the Ragone plots showing the gravimetric and volumetric power density versus the energy density of the BDND and AC electrode cells using 1 M H 2 SO 4 or saturated NaClO 4 as electrolyte. For the cells using 1 M H 2 SO 4 , the BDND demonstrated significantly larger energy and power densities based on the large cell voltage. The energy density of the BDND electrode cell was further enhanced by using saturated NaClO 4 as an aqueous electrolyte (approximately 20 Wh/kg with a power density of 102–104 W/kg or 0.01 Wh/cm3 with a power density of 0.1–10 W/cm3). Although the use of saturated NaClO 4 improved the energy density of the AC electrode cell, the power density in the fast scan rate region was much lower than that of the BDND electrode cell. Therefore, the proposed aqueous supercapacitor using BDND as an electrode material is expected to be an energy storage device suitable for high-speed charging–discharging.