Fluorination of Boron-Doped Diamond Film Electrodes for Minimization of Perchlorate Formation

Abstract

This research investigated the effects of surface fluorination on both rates of organic compound oxidation (phenol and terephthalic acid (TA)) and ClO formation at boron-doped diamond (BDD) film anodes at 22 °C. Different fluorination methods (i.e., electrochemical oxidation with perfluorooctanoic acid (PFOA), radio frequency plasma, and silanization) were used to incorporate fluorinated moieties on the BDD surface, which was confirmed by X-ray photoelectron spectroscopy (XPS). The silanization method was found to be the most effective fluorination method using a 1H,1H,2H,2H-perfluorodecyltrichlorosilane precursor to form a self-assembled monolayer (SAM) on the oxygenated BDD surface. The ClO formation decreased from rates of 0.45 ± 0.03 mmol m min during 1 mM NaClO oxidation and 0.28 ± 0.01 mmol m min during 10 mM NaCl oxidation on the BDD electrode to below detectable levels (<0.12 μmoles m min) for the BDD electrode functionalized by a 1H,1H,2H,2H-perfluorodecyltrichlorosilane SAM. These decreases in rates corresponded to 99.94 and 99.85% decreases in selectivity for ClO formation during the electrolysis of 10 mM NaCl and 1 mM NaClO electrolytes, respectively. By contrast, the oxidation rates of phenol were reduced by only 16.3% in the NaCl electrolyte and 61% in a nonreactive 0.1 M KHPO electrolyte. Cyclic voltammetry with Fe(CN) and Fe redox couples indicated that the long fluorinated chains created a blocking layer on the BDD surface that inhibited charge transfer via steric hindrance and hydrophobic effects. The surface coverages and thicknesses of the fluorinated films controlled the charge transfer rates, which was confirmed by estimates of film thicknesses using XPS and density functional theory simulations. The aliphatic silanized electrode also showed very high stability during OH production. Perchlorate formation rates were below the detection limit (<0.12 μmoles m min) for up to 10 consecutive NaClO oxidation experiments.

Selective Seawater Splitting Using Pyrochlore Electrocatalyst

“…The extent of deviation increases with further increase in overpotential, indicating increased ACSFR activity. The reduced OER (i.e., increased ACSFR) selectivity with increase in potential is further confirmed by DPD colorimetry (Supporting Information section S3; Figure b). The reduced OER and increased ACSFR selectivity with increase in potential are further confirmed by DPD colorimetry, wherein 3, 11, 20, and 32% ACSFR current efficiencies are achieved at 1.70, 1.75, 1.80, and 1.85 V vs RHE, respectively (Figure b).…”

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

“…Our arguments that hydrophobic molecular layers inhibit inner-sphere reactions are supported by two prior studies. Gayen et al showed that hydrophobic, perfluorinated monolayer on BDD blocked ClO 3 – from reaching the electrode surface, while still allowing for • OH formation . Zhao et al also reported that modification of PbO 2 anodes with hydrophobic fluorine resin increased oxygen evolution overpotential and thereby increased • OH production efficiency .…”

“…Gayen et al 43 showed that hydrophobic, perfluorinated monolayer on BDD blocked ClO 3 − from reaching the electrode surface, while still allowing for • OH formation. 43 Zhao et al also reported that modification of PbO 2 anodes with hydrophobic fluorine resin increased oxygen evolution overpotential and thereby increased • OH production efficiency. 17 Taken together, they suggest that the use of hydrophobic layers to inhibit inner-sphere reactions in aqueous media may be a general phenomenon.…”

“…Similarly, the O–H dissociation constants K a of aliphatic alcohols are small: K a ≈ 10 –18 for tert -butanol, corresponding to free-energy changes of ∼100 kJ mol –1 . These values show that although oxidized BDD (O-BDD) surfaces may have a number of species, − these do not necessarily provide labile sites for inner-sphere electrochemical reactions. , One potentially important property controlling electrochemical pathways on diamond is surface hydrophobicity, which impacts the organization of the adjacent water molecules. , Surface hydrophobicity can be manipulated via direct fluorination of diamond − (F-BDD) and by functionalization with polyfluorinated ligands (PF-BDD) . These surface modifications have been shown to yield very hydrophobic surfaces with only very weak interaction with water. , The high C–F bond strength (calculated at 506 kJ mol –1 ) suggests that using fluorinated surfaces or ligands provides a robust way to potentially alter the relative rates of O 2 and • OH production.…”

“…35,36 Surface hydrophobicity can be manipulated via direct fluorination of diamond 37−42 (F-BDD) and by functionalization with polyfluorinated ligands (PF-BDD). 43 These surface modifications have been shown to yield very hydrophobic surfaces with only very weak interaction with water. 35,36 The high C−F bond strength (calculated at 506 kJ mol −1 ) 44 suggests that using fluorinated surfaces or ligands provides a robust way to potentially alter the relative rates of O 2 and • OH production.…”