(Invited) Boron-Doped Diamond Powder/Nanoparticle Materials for Electrochemical Applications

Abstract

Heavily boron-doped diamond (BDD) electrode exhibits several excellent electrochemical properties such as wide potential window, low background current, as well as extreme physical and chemical stabilities. Based on these properties, the BDD electrodes are expected to be used for highly sensitive electrochemical sensors and highly efficient electrolytic electrodes. The BDD electrodes are usually prepared by depositing a polycrystalline BDD thin film on a conductive substrate such as silicon wafer, and thus are used as hard planar electrodes. Furthermore, the size of the BDD electrode also has a limit depending on the CVD apparatus. These situations limit the range of applications of BDD electrodes, despite their excellent electrochemical properties. With the aim of expanding the range of applications of BDD in the electrochemical field, we have developed BDD powder (BDDP). BDDP is a conductive diamond material, which is prepared by the deposition of a BDD layer on the surface of commercially available diamond powder. BDDP has advantages over BDD thin films, such as a larger specific surface area and the ability to create electrodes by applying ink on a substrate, and can be used for applications that have not been possible with BDD thin-film electrodes. 1. BDDP-packed electrolysis flow cell 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 of 40-60 μm) and the packed bed served as an anode. Since the BDDPs are in contact with each other in the BDDP-packed bed, the entire BDDP-packed bed is considered to be electrically connected without a binder. On the other hand, the interparticle voids in the BDDP-packed bed form a network-like narrow channel, and the electrolyte passing through the channel can be treated efficiently. 0.1 M Na 2 SO 4 containing 50 μM methylene blue as a model for polluted water was treated with the BDDP-packed electrolysis flow cell at a voltage of 5 V applied to the two-electrode system of BDDP-packed bed anode and Pt cathode. As a result, it was found that as the amount of BDDP loaded increased, the decomposition rate of MB increased. This suggested that the entire BDDP-packed bed functioned as an anode. Moreover, no deterioration in the decomposition rate was observed even when repeated electrolysis experiments were performed using the BDDP-packed electrolysis flow cell. Therefore, the BDDP-packed electrolysis flow cell is expected to be useful for efficient and durable electrolytic water treatment systems. 2. Pt/BDDP and Pt/BDND for a durable PEFC cathode catalysts A major cause of deterioration of polymer electrolyte fuel cell (PEFC) cathode catalysts is corrosion of the catalyst support due to application of high potential during start-stop operations. Therefore, there is a need for the development of a catalyst support with excellent high potential resistance. In this study, we prepared Pt-supported BDDP (Pt/BDDP) as a durable cathode catalyst, and investigated its electrochemical properties and durability to high potentials. In the CV of Pt/BDDP with a BDDP size of 200 and 300 nm, redox peaks characteristics for Pt was observed, confirming that the BDDP functions as a conductive catalyst support. 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/BDDP than for conventional Pt/C. Therefore, the BDDP is expected to be used for a catalyst support durable to high potential application. In addition, Pt-supported boron-doped nanodiamond (Pt/BDND) showed also superior high potential durability compared to Pt/C.