Electrochemical reduction of nitrate on boron-doped diamond electrodes: Effects of surface termination and boron-doping level.

This study is among the first to systematically study the electrochemical reduction of nitrate on boron-doped diamond (BDD) films with different surface terminations and boron-doping levels. The highest nitrate reduction efficiency was 48% and the highest selectivity in the production of nitrogen gas was 44.5%, which were achieved using a BDD electrode with a hydrogen-terminated surface and a B/C ratio of 1.0%. C-H bonds served as the anchor points for attracting NO3- anions close to the electrode surface, and thus accelerating the formation of NO3-(ads). Compared to oxygen termination, hydrogen-terminated BDD exhibited higher electrochemical reactivity for reducing nitrate, resulting from the formation of shallow acceptor states and small interfacial band bending. The hydrophobicity of the hydrogen-terminated BDD inhibited water electrolysis and the subsequent adsorption of atomic hydrogen, leading to increased selectivity in the production of nitrogen gas. A BDD electrode with a boron-doping level of 1.0% increased the density of acceptor states, thereby enhancing the conductivity and promoting the formation of C-H bonds after the cathodic reduction pretreatment leading to the direct reduction of nitrate.

Electrochemical properties of fluorinated boron-doped diamond electrodes via fluorine-containing plasma treatment.

Since a boron-doped diamond (BDD) exhibits excellent electrode properties such as wide potential window, low back-ground current, and high physical and chemical durability, it has been studied as an electrode material for various electrochemical applications. The electrochemical behavior of BDD depends on the surface termination, which can be easily converted by chemical reactions. Fluorine termination has attracted interest because it exhibits unique surface properties such as high hydrophobicity and a low coefficient of friction, and the electrochemical properties also drastically change. However, so far, it has not been elucidated why fluorinated BDD exhibits specific electrochemical properties. In this article, fluorine-terminated BDD was fabricated by a fluorine-containing plasma treatment, and the electrochemical properties were systematically investigated. Together with experiments, we have calculated the interfacial structures and electronic states of hydrogenated, oxygenated, and fluorinated BDD electrodes. As a result, fluorinated BDD showed lower electrochemical reactivity than hydrogenated and oxygenated BDD. Especially, electron transfer between anionic compound and fluorinated BDD was significantly suppressed. Considered together with theoretical calculations, this reactivity could be attributed to the larger interfacial band bending in fluorinated BDD and electrostatic interactions between BDD and redox species.

Switchable Product Selectivity in the Electrochemical Reduction of Carbon Dioxide Using Boron-Doped Diamond Electrodes.

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.

In Situ Spectroscopic Study on the Surface Hydroxylation of Diamond Electrodes.

Carbon-based materials are regarded as an environmentally benign alternative to the conventional metal electrode used in electrochemistry from the viewpoint of sustainable chemistry. Among various carbon electrode materials, boron-doped diamond (BDD) exhibits superior electrochemical properties. However, it is still uncertain how surface chemical species of BDD influence the electrochemical performance, because of the difficulty in characterizing the surface species. Here, we have developed in situ spectroscopic measurement systems on BDD electrodes, i.e., in situ attenuated total reflection infrared spectroscopy (ATR-IR) and electrochemical X-ray photoelectron spectroscopy (EC-XPS). ATR-IR studies at a controlled electrode potential confirmed selective surface hydroxylation. EC-XPS studies confirmed deprotonation of C-OH groups at the BDD/electrolyte interface. These findings should be important not only for better understanding of BDD's fundamentals but also for a variety of applications.

Comparison of performance between boron-doped diamond and copper electrodes for selective nitrogen gas formation by the electrochemical reduction of nitrate.

The electrochemical nitrate reduction by using boron-doped diamond (BDD) and copper (Cu) electrodes was investigated at various potentials. Product selectivity of nitrate reduction was strongly dependent on the applied potential for both electrodes. The highest selectivity of nitrogen gas production was obtained at -2.0 V (vs. Ag/AgCl) by using a BDD electrode with a faradaic efficiency as high as 45.2%. Compared with Cu electrode, nitrate reduction on BDD electrode occurred at more positive potential, and the production of nitrogen gas was larger. The transformation of surface-adsorbed nitrate into molecular nitrogen would be accelerated on BDD electrode with hindering nitrite production. In addition, low concentration of surface-adsorbed hydrogen on the BDD would also retard the ammonia generation, leading to increase in the selectivity of nitrogen gas formation. Meanwhile, BDD electrode could hinder the hydrogen evolution reaction, which enhanced the efficiency for nitrate reduction and decreased energy consumption. BDD electrode has excellent stability to remain better performance for reducing nitrate during electrolysis without any variation of surface morphology or chemical components.

Stable and Highly Efficient Electrochemical Production of Formic Acid from Carbon Dioxide Using Diamond Electrodes.

High faradaic efficiencies can be achieved in the production of formic acid (HCOOH) by metal electrodes, such as Sn or Pb, in the electrochemical reduction of carbon dioxide (CO2 ). However, the stability and environmental load in using them are problematic. The electrochemical reduction of CO2 to HCOOH was investigated in a flow cell using boron-doped diamond (BDD) electrodes. BDD electrodes have superior electrochemical properties to metal electrodes, and, moreover, are highly durable. The faradaic efficiency for the production of HCOOH was as high as 94.7 %. Furthermore, the selectivity for the production of HCOOH was more than 99 %. The rate of the production was increased to 473 µmol m-2 s-1 at a current density of 15 mA cm-2 with a faradaic efficiency of 61 %. The faradaic efficiency and the production rate are almost the same as or larger than those achieved using Sn and Pb electrodes. Furthermore, the stability of the BDD electrodes was confirmed by 24 h operation.

Surface Hydrogenation of Boron-Doped Diamond Electrodes by Cathodic Reduction.

Boron-doped diamond (BDD) has attracted much attention as a promising electrode material especially for electrochemical sensing systems, because it has excellent properties such as a wide potential window and low background current. It is known that the electrochemical properties of BDD electrodes are very sensitive to the surface termination such as to whether it is hydrogen- or oxygen-terminated. Pretreating BDD electrodes by cathodic reduction (CR) to hydrogenate the surface has been widely used to achieve high sensitivity. However, little is known about the effects of the CR treatment conditions on surface hydrogenation. In this Article, we report on a systematic study of CR treatments that can achieve effective surface hydrogenation. As a result, we found that the surface hydrogenation could be improved by applying a more negative potential in a lower pH solution. This is because hydrogen atoms generated from protons in the CR treatment contribute to the surface hydrogenation. After CR treatments, BDD surface could be hydrogenated not completely but sufficiently to achieve high sensitivity for electrochemical sensing. In addition, we confirmed that hydrogenation with high repeatability could be achieved.

Photochromism-induced amplification of critical current density in superconducting boron-doped diamond with an azobenzene molecular layer.

A key issue in molecular electronics is the control of electronic states by optical stimuli, which enables fast and high-density data storage and temporal-spatial control over molecular processes. In this article, we report preparation of a photoswitchable superconductor using a heavily boron-doped diamond (BDD) with a photochromic azobenzene (AZ) molecular layer. BDDs electrode properties allow for electrochemical immobilization, followed by copper(I)-catalyzed alkyne-azide cycloaddition (a "click" reaction). Superconducting properties were examined with magnetic and electrical transport measurements, such as field-dependent isothermal magnetization, temperature-dependent resistance, and the low-temperature voltage-current response. These measurements revealed reversible amplification of the critical current density by 55% upon photoisomerization. This effect is explained as the reversible photoisomerization of AZ inducing an inhomogeneous electron distribution along the BDD surface that renormalizes the surface pinning contribution to the critical current.