Enhanced H2O2 Production at Reductive Potentials from Oxidized Boron-Doped Ultrananocrystalline Diamond Electrodes

Abstract

This work investigates the surface chemistry of H₂O₂ generation on a boron-doped ultrananocrystalline diamond (BD-UNCD) electrode. It is motivated by the need to efficiently disinfect liquid waste in resource constrained environments with limited electrical power. X-ray photoelectron spectroscopy was used to identify functional groups on the BD-UNCD electrode surfaces while the electrochemical potentials of generation for these functional groups were determined via cyclic voltammetry, chronocoulometry, and chronoamperometry. A colorimetric technique was employed to determine the concentration and current efficiency of H₂O₂ produced at different potentials. Results showed that preanodization of an as-grown BD-UNCD electrode can enhance the production of H₂O₂ in a strong acidic environment (pH 0.5) at reductive potentials. It is proposed that the electrogeneration of functional groups at oxidative potentials during preanodization allows for an increased current density during the successive electrolysis at reductive potentials that correlates to an enhanced production of H₂O₂. Through potential cycling methods, and by optimizing the applied potentials and duty cycle, the functional groups can be stabilized allowing continuous production of H₂O₂ more efficiently compared to static potential methods.

Keywords: boron-doped diamond; catalysis; hydrogen peroxide; liquid disinfection; oxidized boron-doped diamond; reactive oxidative species; reactive oxygen species; surface modification.

Figure 1

Oxygen content of each electrode given by the O 1s integrated peak area normalized to the C 1s integrated peak area. O 1s/C 1s values displayed were taken as the average of three measurements. The dashed line corresponds to the unpolarized control electrode oxygen content. The error bars correspond to the standard deviation of the measurements for each electrode.

Figure 2

Change in carbon bonding at surface of (A) reduced (R), (B) oxidized (O), and (C) oxidized then reduced (OR) BD-UNCD electrodes compared to the control electrode indicated by change in C 1s regional spectra taken from XPS.

Figure 3

(A) Comparison of relative amount of functional groups between O and OR electrodes. The O electrodes were oxidized at the given oxidation voltage for 20 min. O electrodes that were subsequently reduced at −1.5 V vs Ag/AgCl for 20 min are renamed OR electrodes. The relative amount of functional groups before, and after reduction of the oxidized electrodes indicates presence, and stability dependent on the oxidation voltage. Oxidation voltage of 0.5 V corresponds to the relative amount of functional groups on the control electrode. The green arrows indicate instability of the oxidized surface in reducing environment at the given voltages. (B) Charge transfer for the R1.5 and OR electrodes at discrete times during reduction at −1.5 V vs Ag/AgCl sat. KCl for 20 min. The charge transferred for the OR1.5 and OR2.5 electrodes compared to R1.5 indicates there is greater charge transferred during reduction if BD-UNCD is “pre-anodized” at either 1.5 or 2.5 V. However, OR1.5 and OR2.5 compared to OR3.5 shows that too high of an oxidation voltage during “pre-anodization” removes this effect. Charge transfer is proportional to oxygen reduction, and therefore hydrogen peroxide production in acidic aqueous environments.

Figure 4

CV scans at 500 mV/s in oxygen-saturated H₂SO₄ (0.5 M): (A) between −0.7 and 2.5 V, (B) between −0.5 V and an increasing anodic vertex potential.

Figure 5

CV scans at 500 mV/s in oxygen-saturated H₂SO₄ (0.5 M). The anodic vertex potential is 2.5 V and the cathodic vertex potential is (A, B) higher than −0.7 V and (C, D) lower than −0.7 V.

Figure 6

(A) H₂O₂ concentration produced and corresponding Coulombic efficiency after 30 min of electrolysis at different static voltages in a two-electrode cell. (B) H₂O₂ concentration produced using a potential cycling method for 30 min where the anodic cycle was constant at 100 s and the cathodic cycle, given by the horizontal-axis, was varied. When the cathodic cycle was equal to 1800 s, there was no anodic cycle. (C) H₂O₂ concentration produced using a potential cycling method for 30 min where the cathodic cycle was constant at 100 s and the anodic cycle, given by the horizontal-axis, was varied. When the anodic cycle was equal to 1800 s, there was no cathodic cycle.

Figure 7

Chronoamperometric curves recorded at different time of the electrolysis during the cathodic step. The cathodic step lasted 100 s, whereas the anodic step lasted either 100 s (dash line) or 500 s (solid line).

Figure 8

(A) Proposed schematic of current vs time curve profiles during oxygen reduction. (B) Suggested mechanism for oxygenated functional group enhancement of H₂O₂ generation.