A practical guide to using boron doped diamond in electrochemical research

Abstract

Conducting, boron doped diamond (BDD), in addition to its superior material properties, offers several notable attributes to the electrochemist making it an intriguing material for electrochemical research. These include the widest solvent window of all electrode materials; low background and capacitive currents; reduced fouling compared to other electrodes and; the ability to withstand extreme potentials, corrosive and high temperature/pressure environments. However, BDD is not your typical electrode material, it is a semi-conductor doped degenerately with boron to present semi-metallic characteristics. Input from materials scientists, chemists and physicists has been required to aid understanding of how to work with this material from an electrochemical viewpoint and improve electrode quality. Importantly, depending on how the BDD has been grown and then subsequently treated, prior to electrochemical measurement, the resulting material properties can vary quite significantly from one electrode to the next. This likely explains the variability seen by different researchers working on the same experimental systems. The aim of this "protocols" article is not to provide a state-of-the-art review of diamond electrochemistry, suitable references are provided to the interested reader, but instead serves as a reference point for any researcher wishing to commence work with diamond electrodes and interpret electrochemical data. It provides information on how best to characterise the material properties of the electrode before use and outlines the interplay between boron dopant density, non-diamond-carbon content, grain morphology, surface chemistry and redox couple identity. All should ideally be considered when interpretating electrochemical data arising from the diamond electrode. This will aid the reader in making meaningful comparisons between data obtained by different researchers using different diamond electrodes. The guide also aims to help educate the researcher in choosing which form of BDD is best suited to their research application.

Assessment of Boron Doped Diamond Electrode Quality and Application to <em>In Situ</em> Modification of Local pH by Water Electrolysis

“…Hence a stringent protocol for assessing the quality of the electrode prior to use is adopted to assess NDC content as discussed in ), it will be charge depleted, at potentials negative of the flatband potential, resulting in a decrease in electrochemical performance 7,42 . The easiest way to qualitatively assess metallic doping levels is to look for the presence of a Fano signature which causes asymmetry in the sp 3 peak, in the Raman spectra, as shown in Figure 1(A-C). This is due to interference between the discrete phonon state and the electronic continuum and is seen at boron doping levels > 10 20 B atoms cm -343…”

“…6. Observe the peak ~ 1,332 cm -1 in the spectrum which indicates sp 3 diamond (Figure 1); the broader the peak the more defects present 3,25 . 7.…”

“…4. Measure the peak current of the forward scan, i p , and correlate with that expected from the Randles Sevcik equation 2 (quoted at 298 K) 3,30 , assuming the electrode is disk-shaped in geometry and large enough (diameter 1 mm) that linear diffusion dominates. ) and v is scan rate (V sec -1 ).…”

“…It can be seen that for an NDC-containing electrode the onset of H 2 O oxidation and reduction has been brought in significantly, narrowing the solvent window. Also of note is the appearance of anodic peaks due to the oxidation of NDC and a cathodic peak due to ORR which is catalyzed on NDC but not on sp 3 carbon. For a high quality BDD electrode with negligible NDC the solvent window is expected to be >>3 V in aqueous KNO 3 solution.…”

“…In recent years, sp 3 carbon (diamond) doped with sufficient boron to render the material "metal-like" has become a popular choice for a wide range of electroanalytical applications due to its excellent electrochemical (and thermal and mechanical) properties 1,2,3 . These include corrosion resistance under extreme solution, temperature and pressure conditions 4 ultra-wide solvent windows, low background currents, and reduced fouling, in comparison to other commonly used electrode materials [5][6][7]3 . However, increasing non-diamond-carbon (NDC: sp 2 ) content results in a decreasing solvent window, increasing background currents 7,8 , changes in both structural integrity and sensitivity towards different inner sphere redox species, e.g.…”