The main product obtained by electrochemical reduction of CO2 depends on the electrode material, and in many cases the Faradaic efficiency for this is determined by the electrolyte. Only a few investigations in which attempts to produce different products from the same electrode material have been done so far. In this work, we focus on boron-doped diamond (BDD) electrodes with which plentiful amounts of formic acid and small amounts of carbon monoxide have been produced. By optimizing certain parameters and conditions used in the electrochemical process with BDD electrodes, such as the electrolyte, the boron concentration of the BDD electrode, and the applied potential, we were able to control the selectivity and efficiency with which carbon monoxide is produced. On one hand, with a BDD electrode with 1% boron used for the cathode and KClO4 for the catholyte, the selectivity for producing carbon monoxide was high. On the other hand, with a BDD electrode with 0.1% boron used for the cathode and KCl for the catholyte, the production of formic acid was the most evident. In situ attenuated total reflectance-infrared (ATR-IR) measurements during electrolysis showed that CO2 •– intermediates were adsorbed on the BDD surface in the KClO4 aqueous solution. Here, switchable product selectivity was achieved when reducing CO2 using BDD electrodes.

Electrochemical reduction of CO2 to useful compounds have been actively investigated. The faradaic efficiency and selectivity for products of CO2 reduction depend on some factors, such as electrode materials and electrolytes. Recently, boron‐doped diamond (BDD) was found to be a promising electrode material for production of formic acid by CO2 reduction, but the influence of electrolytes has not been sufficiently elucidated. Here, we studied the impact of various cations and anions on CO2 reduction using BDD electrodes. The faradaic efficiency for producing formic acid became higher by using larger alkali metal cations, because these hydrated cations would keep the pH near the cathode suitable for CO2 reduction by their buffer effect. Moreover, formic acid was efficiently obtained by using specifically‐adsorbed anions such as halides and sulfate. Finally, we achieved the maximum faradaic efficiency for producing formic acid as high as 95% in RbBr aqueous solution.

Electrochemical CO 2 reduction using an intermittent flow cell system with boron-doped diamond (BDD) as the working electrode is presented. A stop−start motion of the flow conditions in the intermittent cell is created using a piston pump, and this considerably increases the rate of electrochemical conversion of CO 2 to HCOOH compared to a continuous flow system. The system works by stopping the flow of the electrolytes at a controlled frequency, which allows for sufficient time for more CO 2 anion radicals (intermediate species) to be reduced into HCOOH instead of being washed away from the surface of the electrode. The findings presented here provide an important basis from which the design of CO 2 reduction systems for industrial-scale applications can be started.

Carbon capture, utilization, and storage (CCUS) have been attracting much attention as an initiative to achieve carbon neutrality. In the capture and storage strategy, amines are widely used for highly efficient separation and capturing of CO 2 since amines react with CO 2 to form carbamates. In the utilization strategy, CO 2 electroreduction is a promising method to convert CO 2 into value-added compounds because reaction conditions are relatively mild and easily controlled. In this work, toward combining CO 2 capture and storage technologies and CO 2 electroreduction, we focused on amine modification of the electrode surface, in which borondoped diamond (BDD) was chosen as a sustainable electrode material. CO 2 electroreduction using the amine-modified BDD was performed, and an applied potential dependence of the product selectivity was examined. In situ attenuated total reflectance-infrared measurements showed that the peak intensity of the stretching vibration of the carbonyl group at around 1640 cm −1 decreased as applied potentials became more negative. The effect of amine modification of the BDD surface lies in the formation of a C−N bond during CO 2 electroreduction, resulting in the enhancement of the selectivity of CO production up to 8 times compared to the unmodified BDD.

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