Current Amplification Driven by Reversible Redox Cycling in a Thin-Layer Reactor Using Boron-Doped Diamond Electrodes

Current amplification plays an essential role in electrochemistry by improving the productivity of the electrochemical production of industrial materials and enhancing the sensitivity of environmental and biomedical sensing. Various approaches have been explored to enhance steady-state current, such as thin-layer reactors, microelectrodes, and rotating-disk electrodes. Thin-layer reactors have several advantages, including the ability to generate larger currents using bulk-sized electrodes and simple fabrication processes. In this study, we developed a thin-layer reactor using boron-doped diamond (BDD) electrodes with an interelectrode distance of several tens of micrometers, which is comparable to the thickness of a diffusion layer. The use of BDD electrodes enabled reversible redox cycling in the thin-layer reactor, resulting in more than 2-fold current amplification compared to conventional thin-layer reactors. This effect was observed only when BDD electrodes were used for both the working and counter electrodes, and the interelectrode distance was 200 microm and below. Based on the experimental results of the present study, we propose a novel concentration profile model and reaction mechanism for BDD thin-layer reactors that cannot be explained by conventional thin-layer reactor models. The model involves a three-step reversible redox cycle: (1) consumption of reduced species and generation of oxidized species at the working electrode, (2) regeneration of reduced species at the counter electrode, and (3) resupply of reduced species to the working electrode. The BDD thin-layer reactor also demonstrated twice the sensitivity, and a detection limit one-tenth those of the conventional bulk-reactor in electrochemical detection based on the proposed model.