Enhancing Electrochemical Efficiency of Hydroxyl Radical Formation on Diamond Electrodes by Functionalization with Hydrophobic Monolayers

Abstract

Electrochemical formation of high-energy species such as hydroxyl radicals in aqueous media is inefficient because oxidation of H2O to form O2 is a more thermodynamically favorable reaction. Boron-doped diamond (BDD) is widely used as an electrode material for generating •OH radicals because it has a very large kinetic overpotential for O2 production, thus increasing electrochemical efficiency for •OH production. Yet, the underlying mechanisms of O2 and •OH production at diamond electrodes are not well understood. We demonstrate that boron-doped diamond surfaces functionalized with hydrophobic, polyfluorinated molecular ligands (PF-BDD) have significantly higher electrochemical efficiency for •OH production compared with hydrogen-terminated (H-BDD), oxidized (O-BDD), or poly­(ethylene ether)-functionalized (E-BDD) boron-doped diamond samples. Our measurements show that •OH production is nearly independent of surface functionalization and pH (pH = 7.4 vs 9.2), indicating that •OH is produced by oxidation of H2O in an outer-sphere electron-transfer process. In contrast, the total electrochemical current, which primarily produces O2, differs strongly between samples with different surface functionalizations, indicating an inner-sphere electron-transfer process. X-ray photoelectron spectroscopy measurements show that although both H-BDD and PF-BDD electrodes are oxidized over time, PF-BDD showed longer stability (≈24 h of use) than H-BDD. This work demonstrates that increasing surface hydrophobicity using perfluorinated ligands selectively inhibits inner-sphere oxidation to O2 and therefore provides a pathway to increased efficiency for formation of •OH via an outer-sphere process. The use of hydrophobic electrodes may be a general approach to increasing selectivity toward outer-sphere electron-transfer processes in aqueous media.

Electrochemical oxidation of butyl paraben on boron doped diamond in environmental matrices and comparison with sulfate radical-AOP

“…BDD anodes are known to enhance the generation of ROS compared to other anodic materials (Pecková et al, 2009); this facilitates the effective conversion of organic micropollutants of emerging concern and, subsequently, their mineralization. However, the exact mechanisms of O 2 and • OH production have not been completely elucidated (Henke et al, 2019;Kraft et al, 2003).…”

The Mechanism of Oxidation of Formic Acid in Acidic Solutions on Boron‐Doped Diamond Electrodes: A Quantum Chemical Study

“…The BDD is experimentally shown to be a very good electrode for the complete incineration of organics, such as oxalic, formic or maleic acids. Also the very recent experimental measurement of Henke et al [17] confirms that the production of HO radicals on BDD occurs via an outer-sphere electron-transfer process. [13] Our assumption of an outer sphere mechanism is based on the facts that under experimental conditions the BDD surface is passivated, which prohibits HO adsorption, and also that the oxidation of oxalic acid sets in at the potential where HO radical is generated.…”

“…20 nm from the electrode surface, the HO radicals are not adsorbed on BDD and are able to react with carboxylic acids diffusing from the solution towards the electrode. Also the very recent experimental measurement of Henke et al [17] confirms that the production of HO radicals on BDD occurs via an outer-sphere electron-transfer process. It is shown additionally that functionalization of BDD with hydrophobic ligands increases electrochemical efficiency of formation of HO radicals.…”

“…[14,[17][18][19][20][21][22][23][24] In the present study on the reaction of syn-FA with HO in water as basic initial geometries for the pre-reactive adducts, product complexes and the transition states were used the structures obtained in vacuo reported in the work of Anglada, [18] nevertheless some other possible configurations were also tested. [14,[17][18][19][20][21][22][23][24] In the present study on the reaction of syn-FA with HO in water as basic initial geometries for the pre-reactive adducts, product complexes and the transition states were used the structures obtained in vacuo reported in the work of Anglada, [18] nevertheless some other possible configurations were also tested.…”

“…The reactions (2a) and (2b) of formic acid with OH radical in vacuo were extensively studied in the past. [14,[17][18][19][20][21][22][23][24] In the present study on the reaction of syn-FA with HO in water as basic initial geometries for the pre-reactive adducts, product complexes and the transition states were used the structures obtained in vacuo reported in the work of Anglada, [18] nevertheless some other possible configurations were also tested. For the anti-FA conformer only preliminary calculations were performed for a set of structures similar to those obtained for the syn rotamer as energetically favored, to compare analogous reaction paths.…”