Electrochemical Applications of Conductive Boron-Doped Diamond Powders and Nanoparticles

Conductive boron-doped diamond (BDD) electrode is known to exhibit several excellent electrochemical properties including wide potential window, low background current, as well as physical and chemical stabilities. Based on these properties, the BDD electrodes are expected to be used for a functional electrode material, which enables highly sensitive electrochemical analysis and highly efficient electrolytic processes. The BDD electrodes usually consist of a polycrystalline BDD thin film deposited on a conductive substrate, such as a conductive silicon wafer. Thus, the BDD electrodes are used as hard and planar electrodes, and this situation limits the range of application. In order to expand the application field of BDD electrode, we have developed BDD powder (BDDP) and boron-doped nanodiamond (BDND). BDDP was prepared by deposition of a BDD layer on the surface of commercially available diamond powder via microwave-plasma assisted chemical vapor deposition. BDND was also fabricated by the similar way using detonation nanodiamond as the substrate material.

A BDDP-packed electrolysis flow cell was developed for application to an efficient electrolytic water treatment system. A cylinder equipped with a filter was filled with BDDP (particle size 40-60 μm), and the packed bed was used as the anode. 0.1 M Na₂SO₄ containing 50 μM methylene blue (MB) as a model of organic pollutants was treated with the BDDP-packed electrolysis flow cell. As a result, the decomposition rate of MB was found to increase as the BDDP amount increased. This suggests that the entire BDDP-packed bed functions as an anode. In addition, no decrease in the decomposition rate was observed even when repeated electrolysis experiments were performed. Therefore, it is expected that the BDDP-packed electrolysis flow cell will be useful for an efficient and durable electrolytic water treatment system.

BDND can be used as a conductive material with excellent durability and large specific surface area. Since the BDND exhibits wide potential window in aqueous electrolytes, it is expected to be used for an electrode material for aqueous supercapacitor with a large cell voltage. The use of BDND as the electrode material and saturated NaClO₄ as the electrolyte demonstrated a large cell voltage up to 2.8 V, which should enable fabrication of an aqueous supercapacitor with high energy and power densities.

BDND can be also useful for a catalyst support of polymer electrolyte fuel cell (PEFC) cathode catalyst, which is durable to corrosion by application of high potential. Pt-supported BDND (Pt/BDND) was found to show good electrochemical activity, including oxygen reduction reaction activity. Durability tests using high potential cycling (+1.0–+1.5 V vs. NHE) showed that the electrochemically active surface area (ECA) retention rate during the test was greater for Pt/BDND than for conventional Pt/C.