Electrochemical Supercapacitors from Diamond

Boron-doped diamond has been utilized as an electrode material to construct an electric double layer capacitor (EDLC) as well as an electrode support to form a pseudocapacitor. In 1.0 M NaSO4 solution, the capacitance of diamond EDLC is in the range of 3.6–7.0 μF cm–2, comparable with those of EDLCs based on other carbon materials. During a charge/discharge process for 1000 cycles at a scan rate of 100 mV s–1, the capacitance only decreases 5%, indicating high stability and a long lifetime of such an EDLC. To improve the capacitance of diamond EDLCs, diamond was coated with a MnO2 film to construct a pseudosupercapacitor. The MnO2 films were electrodeposited at a constant potential of 0.9 V vs Ag/AgCl in 0.2 M MnSO4 solution. The mass of MnO2 deposited per unit area, called the area density, calculated from the deposition charge, was controlled via the deposition time. The MnO2 films were characterized using various techniques like SEM, XPS, Raman spectroscopy, etc. In 1.0 M NaSO4 solution, the capacitance of the MnO2/diamond-based pseudosupercapacitor rises with an increase of the mass of MnO2 on diamond. Its maximum capacitance was found to be reached at a MnO2 area density of 24 μg cm–2. The capacitance obtained from voltammetry is 384 μF, or 326 F g–1 at a scan rate of 10 mV s–1, which is comparable with the value of 406 μF, or 349 F g–1, obtained from charge/discharge process at a current density of 3 A g–1 in the potential range 0 to 0.8 V. The capacitance was reduced by 34% after 1000 subsequent charge/discharge cycles carried out at a scan range of 100 mV s–1. The comparison of the performance of the MnO2/diamond pseudosupercapacitor with that of those pseudosupercapacitors based on MnO2 and other carbon materials indicates that diamond could be suitable for electrochemical supercapacitor applications.