The utilization of boron doped diamond (BDD) as an electrode material for the electrochemical reduction of CO₂ has been studied in recent decades. Its stability and ability to suppress hydrogen evolution makes it an attractive choice for the electrochemical reduction of CO₂. It has been confirmed that, when using a bare BDD electrode, very high selectivity and productivity can be achieved in the production of formic acid. Moreover, by modifying the surface of a BDD electrode with copper (Cu) particles, we have been able to produce compounds with a high number of carbon atoms. In this article, we summarize the results of our work on the electrochemical reduction of CO₂ using BDD electrodes, with the specific aim of the producing compounds with a high number of carbon atoms.

The reductive desorption of 2-aminoethanethiol (AET) monolayers formed on a polycrystalline gold electrode has been studied using cyclic voltammetry. Three cathodic peaks were observed and they were assigned to reductive desorption from each small domain of Au(111), (100), and (110) over the polycrystalline surface. The kinetics of adsorption were likely under the condition where the kinetic control and the diffusion control were balanced over the concentration range 5–50 μM (M = mol dm⁻³), whereas it was described by the kinetic control model at 100 μM. The thermodynamics of adsorption was well-described by the Langmuir isotherm. The saturation surface coverage was found to be 5.3 × 10⁻¹⁰ mol cm⁻², which suggested that AET adsorbed lying flat on a polycrystalline gold electrode.

Two sizes of polyacrylonitrile beads were prepared as precursors of carbon beads by varying the reaction solvent ratio of N,N-dimethylformamide to methanol for dispersion polymerization reaction. The spherical structure of the carbon beads were maintained after carbonization and activation. Charge-discharge tests and AC-impedance analyses revealed that the electric resistance between the particles is low without conducting supplements. In addition, an appropriate pore size brought by the KOH activation of smaller carbon beads lead to a better capacitor performance at high current conditions.

The influence of immersion in a hexaammineruthenium chloride ([Ru(NH₃)₆]Cl₃) aqueous solution on the redox reaction on a polyethyleneimine (PEI) thin film modified with gold nanoparticles (AuNPs) is investigated for the electrochemical analysis of the conjugated reaction area of electronic and electrochemical conduction. The PEI thin film is electrodeposited on a glassy carbon (GC) electrode in an ethylenediamine acetonitrile solution. AuNPs were prepared by Frens’s method and loaded onto the PEI thin film on the GC (GC/PEI) electrode. The redox reaction of [Ru(NH₃)₆]³⁺ on AuNPs loads onto a PEI film on a GC (GC/PEI/AuNPs) electrode is observed by cyclic voltammetry and electrochemical impedance spectroscopy. The apparent electron transfer rate constant at a single AuNP, calculated from the charge transfer resistance and AuNP number density, increases during immersion of GC/PEI/AuNPs in the [Ru(NH₃)₆]³⁺ solution. It is suggested that the redox reaction occurs not only at the AuNPs by the tunneling effect, but also at the GC electrode due to the ionic transport of [Ru(NH₃)₆]³⁺ during the immersion in the [Ru(NH₃)₆]³⁺ solution. Care should be taken that the electrochemical reaction is measured as soon as possible when using PEI films before the electrolyte can penetrate the film completely, because the ionic conduction of the [Ru(NH₃)₆]³⁺ solution in the PEI thin film cannot be ignored during immersion in the electrolyte solution.

The bimetallic alloy nanoparticle catalysts composed of Pd and Cu were synthesized for electrochemical reduction of CO₂. The effect of the catalyst composition on the CO₂ reduction activity and the selectivity was investigated, and at the high Pd/Cu ratio CO₂ reduction to formate rather than hydrogen evolution proceeded predominantly. The combination of Pd with Cu also changed the stability of the catalyst. By using the bimetallic catalyst, stable reduction current was obtained in the long-term electrolysis while a monometallic Pd catalyst suffered from deactivation due to poisoning by CO. These improvements are attributable to the electrical interaction between Pd and Cu. The resultant lowering shift of a d-band center reduced binding strength of CO, leading to the achievement of stable reduction of CO₂ with a small overpotential.