(Invited) Electrolytic Application of Boron-Doped Diamond Powders
Authors: Kondo Takeshi 
Published: December 2024

Boron-doped diamond (BDD) electrodes or diamond electrodes are known to be useful for a durable electrode for electrolysis with a high efficiency base on their wide potential window and physical and chemical stabilities. As a result of electrolysis of concentrated sulfuric acid at diamond electrode, reactive oxidizing species, such as peroxodisulfate, peroxomonosulfate, and hydrogen peroxide, can be generated at a high current efficiency. Such electrolyzed sulfuric acid exhibits strong oxidizing power, and is used for mineralization of organic compounds. Carbon fiber reinforced plastics (CFRP) are widely used because it is light in weight and corrosion resistant with high strength and stiffness. Technology for recycling carbon fibers from waste CFRP is desired because it can reduce the amount of carbon fibers disposed of in landfills as waste. When CFRP is treated with electrolyzed sulfuric acid, only the resin is completely decomposed, and nearly intact carbon fibers can be recovered. Therefore, the electrolyzed sulfuric acid method should be a promising candidate for carbon fiber recycling technology. Usually, diamond electrodes are used for the production of electrolyzed sulfuric acid. The diamond electrode consists of a polycrystalline BDD thin film deposited on a conductive substrate by chemical vapor deposition (CVD) method. Therefore, the size of diamond electrode is limited depending on the size and power of the CVD equipment. In this study, we developed a new method to fabricate a large-sized diamond electrode that can be used for electrolyzed sulfuric acid production using BDD powder (BDDP).
BDDP was prepared by deposition of a BDD layer on the surface of diamond powder with a particle size of 3-6 μm via microwave plasma-assisted CVD. The BDDP was added to tetraethyl orthosilicate/ethanol solution, followed by addition of ultrapure water and nitric acid and stirring to prepare a BDDP/silica sol solution. The BDDP/silica sol solution was spray-coated on a hydrophilic titanium substrate, and after baking at 150 °C for 1 h, a spray-coated diamond electrode consisted of a BDDP/silica layer formed on the substrate was obtained. Constant-current electrolysis of 50% sulfuric acid was performed in a two-compartment electrochemical cell with the spray-coated diamond electrode as an anode. Concentrations of peroxodisulfate, peroxomonosulfate, and hydrogen peroxide in the sulfuric acid after electrolysis were estimated by titration method, and the current efficiency of the reactive oxidizing species formation was calculated.
Scanning electron microscopy observation revealed that the BDDPs were packed densely on the surface of the spray-coated diamond electrode. 10 mL of 50% sulfuric acid was electrolyzed at a constant current density of 20 mA/cm2 for 90 min at a spray-coated diamond electrode (electrode area: 0.5 cm2). During the electrolysis, the electrode potential was stable around +3 V vs. Ag/AgCl, indicating that no deterioration of the electrode occurred even at highly positive potentials in concentrated sulfuric acid. Current efficiency of reactive oxidizing species formation was calculated from the concentration estimated by titration to be 41%. These results confirm that the spray-coated diamond electrode can be used for electrolyzed sulfuric acid production. A large-sized spray-coated diamond electrode was also prepared in the same way using a 20×20 cm2-sized titanium substrate. Electrolysis of 50% sulfuric acid was performed at selected points on the electrode surface (electrode area for electrolysis: 1 cm2), and a constant current density of up to 32 mA/cm2 could be applied for 90 min, with a current efficiency of 67% for reactive oxidizing species formation. Therefore, the spray coating method is expected to be useful for scaling up of a diamond electrode for sulfuric acid electrolysis.