A comparison of electrochemical degradation of phenol on boron doped diamond and lead dioxide anodes

This work compares two electrode materials used to mineralize phenol contained in waste waters. Two disks covered with either boron doped diamond (BDD) or PbO₂ were used as anodes in a one compartment flow cell under the same hydrodynamic conditions. Efficiencies of galvanostatic electrolyses are compared on the basis of measurements of Total Organic Carbon (TOC) and Chemical Oxygen Demand (COD). Galvanostatic electrolyses were monitored by analysis of phenol and of its oxidation derivatives to evaluate the operating time needed for complete elimination of toxic aromatics. The experimental current efficiency is close to the theoretical value for the BDD electrode. Other parameters being equal, phenol species disappeared at the same rate using the two electrode materials but the BDD anode showed better efficiency to eliminate TOC and COD. Moreover, during the electrolysis less intermediates are formed with BDD compared to PbO₂ whatever the current density. A comparison of energy consumption is given based on the criterion of 99% removal of aromatic compounds.     

Electrochemical characterisation and oxygen evolution at a heavily boron doped diamond electrode

Characterisation of a commercial heavily doped BDD electrode demonstrated it contains a small sp² content, which on anodic potential scanning, is oxidised to CO/CO₂. This surface modification alters the electrode activity, increasing the overpotential for the hydrogen and oxygen evolution reactions (HER and OER). Ex situ and in situ investigations indicate film morphology is mainly composed of “chain of hills”, presenting relatively high differential capacitance values and morphology factor, which is attributed to the effect of surface states and high surface roughness of the BDD film. The voltammetric behaviour depends on the applied potential; the heavily doped BDD electrode behaving as a metallic electrode at more anodic potentials. Polarisation curves (potentiostatic (1 mV s⁻¹) or galvanostatic (point-by-point)), recorded at different temperatures and H₂SO₄ concentrations, lead to the same conclusions. The high Tafel coefficients and low apparent electronic transfer coefficient (α_A) are independent of overpotential and temperature but show a dependence on H₂SO₄ concentration. The linear relationship observed between the apparent electrochemical enthalpy of activation (ΔH^#_η) and overpotential supports α_A is constant. An OER mechanism was proposed taking into account the absence of adsorption sites at the BDD surface. The OER is inhibited, explaining the high overpotentials and elevated ΔH^#_η values.     

Electrochemical behavior of high-pressure synthetic boron doped diamond powder electrodes

Boron doped diamond (BDD) was synthesized under high pressure and high temperature using B-doped graphite intercalation compositions (GICs) as carbon sources. The electrochemical characteristics of high-pressure synthetic BDD powder electrodes were investigated by measuring the cyclic voltammetry curves and AC impedance spectrum. For the [Fe(CN)₆]^3−/4− redox couple, the electrode reaction process is reversible or quasi-reversible at the scan rates of 0.01-1.0 V/s. At the low scan rate the linear relation between peak current and square root of scan rate indicates that the electrode process was a diffusion-controlled mass transport process. The electrochemical behavior is similar to a planar electrode. With the increasing of the scan rate the electrode process is controlled by the mass transport plus kinetic process. AC impedance spectra exhibit the porous structure characteristic of BDD powder electrode.     

Steam activation of boron doped diamond electrodes

Boron doped diamond (BDD) electrodes were activated in steam at various temperatures, resulting in high quality BDD electrodes with a porous microstructure. Distinct columnar structures were observed by scanning electron microscopy. The electrochemically active surface area of the steam-activated BDD was up to 20 times larger than the pristine BDD electrode owing to the porous texture. In addition, a widening of the potential window was observed after steam activation, suggesting that the quality of BDD was enhanced due to oxidative removal of graphitic impurities during the activation process.     

Electrochemical treatment of wastewaters containing 4‐chlororesorcinol using boron doped diamond anodes

The electrochemical oxidation of aqueous wastes polluted with 4‐chlororesorcinol has been studied on boron‐doped diamond electrodes on acidic medium. The voltammetric results showed that in the potential region where the supporting electrolyte is stable, reactions occur, resulting in the loss of activity due to electrode fouling. Galvanostatic electrolysis study showed that the oxidation of these wastes in single‐compartment electrochemical flow cell with boron doped diamond anodes deal to the complete mineralization of the organics but is no indication of electrode fouling. Resorcinol, 1,2,4‐trihydroxybenzene, benzoquinone, maleic, fumaric, and oxalic acids have been detected as soluble organics and chlorides (Cl^?) and hypochlorites (ClO^?) as mineral products during the electrolysis of 4‐chlororesorcinol. The electrochemical oxidation of 4‐chlororesorcinol consists of a sequence of steps: Release of Cl and/or hydroxylation of the aromatic ring; formation of quinonic compounds; oxidative opening of aromatic ring to form carboxylic acids; and oxidation of carboxylic acids to carbon dioxide. Both, direct oxidation at boron doped diamond surface and mediated oxidation by powerful oxidants electrogenerated from electrolyte oxidation at anode surface are involved in these stages.     

Oxidation at boron doped diamond electrodes: an effective method to mineralise triazines

The electrochemical oxidation of cyanuric acid (CA) is examined by bulk electrolyses at boron doped diamond (BDD) electrode. The influence of operating conditions on the reaction trend is investigated and suitable conditions for oxidative demolition of CA are individuated. In particular a strong effect of current density and pH is evidenced, being neutral pH and high current the most favourable conditions to achieve CA mineralisation. The paper also presents the results from atrazine (2-Cl-4-ethylamino-6-isopropylamino-1,3,5-triazine) and 2-Cl-4,6-diammino-1,3,5-triazine oxidation at BDD anode. Triazine compounds are scarcely degraded even with powerful methods such as photocatalytic TiO₂-mediated processes, being CA generally obtained as final product of their oxidation. Individuation of favourable conditions for CA demolition has allowed to find suitable conditions to mineralise also triazine compounds.     

Electrochemical characteristics of boron doped polycrystalline diamond electrode sintered by high pressure and high temperature

The bulk B-doped polycrystalline diamond (PCD) electrode in this study was prepared by high-pressure, high-temperature (HPHT) technology. The PCD was sintered under HPHT conditions, using B-doped diamond powders and a metal catalyst as raw materials, then the metal solvent phase was dissolved by aqua regia. The morphology and composition of the PCD were investigated with a scanning electron microscope (SEM), X-ray diffraction (XRD), and energy dispersion spectrum (EDS). The results show that the sintered body possesses a polycrystalline structure with direct diamond–diamond bond and irregularly shaped pores of 1–10 μm distributed on the grain boundaries after the metal solvent phase was removed. The cyclic voltammogram and electrochemical impedance spectroscopy of this B-doped electrode have been investigated. The B-doped PCD electrode exhibits stable electrochemistry in a KCl support solution over a wide potential range. The quasi-reversible reaction occurs on the electrode for the [Fe(CN)₆]^3−/4− couples. The electrode process combines the diffusion-controlled mass transport plus the kinetic process. The electrochemical impedance spectroscopy (EIS) analysis shows the porous structure characteristic of the PCD electrode.     

The impedance of fast charge transfer reactions on boron doped diamond electrodes

Reversible charge transfer on boron doped diamond (BDD) electrodes was studied using cyclic voltammetry and electrochemical impedance spectroscopy. Polycrystalline diamond films of 5 μm thickness with 200 and 3000 ppm boron content were prepared by chemical vapour deposition on niobium substrate. The samples were mounted in a Teflon holder and used as rotating disk electrodes (RDE) with rotation frequencies between 0 and 4000 rpm. The electrochemical measurements were carried out in aqueous electrolyte solutions of 0.5 M Na₂SO₄ + 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] and 0.1 M KCl + 5 mM [Ru(NH₃)₆]Cl₂/[Ru(NH₃)₆]Cl₃. The electrochemical redox behaviour of the BDD electrodes was found to differ significantly from that of an active Pt electrode. The deviations are indicated by a large peak potential difference and a shift of the peak potentials in cyclic voltammograms with increasing sweep rate. At rotating electrodes lower limiting current densities are found and the impedance diagrams exhibit an additional capacitive impedance element at high frequencies. The results are described quantitatively by an impedance model which is based on partial blocking of the diamond surface.     

Comparison of glassy carbon and boron doped diamond electrodes: Resistance to biofouling

Carbon based electrodes are widely used for in vivo and in vitro electrochemical studies. In particular, monoamine neurochemistry has been investigated using carbon microfibre electrodes. Similarly, glassy carbon (GC) is the preferred material for many biochemical applications, such as electrochemical detection in chromatography. More recently, boron doped diamond (BDD) has been utilized for biosensing, as its carbon sp³ structure is expected to provide better resistance to analyte fouling. However, the main factor limiting the use of electrochemical sensors for biological studies is the effect of the biological matrix. Indeed, in vivo or in situ measurements expose the sensor to a complex matrix of proteins, which adsorb on the sensing surface and interfere with the electrochemical measurements.Here, we compare the performance of three carbon based electrodes: GC, GC with low surface oxides and BDD. The redox species ruthenium(III) hexaammine (outer-sphere), ferrocyanide (surface sensitive) and the biologically significant dopamine have been investigated in protein and blood-mimicking matrices. Cyclic voltammetry and electrochemical impedance spectroscopy have been used to examine the effect of spectator molecules and reaction products on electrode mechanisms.Our results show that BDD generally exhibits the best performance for most conditions and reactions and should therefore be preferred for measurements in biologically fouling environments. Furthermore, surface oxides seem also to improve resistance of the GC electrode to biofouling.     

Voltammetric and electrochemical impedance spectroscopy characterization of a cathodic and anodic pre-treated boron doped diamond electrode

The effect of boron doped diamond (BDD) surface termination, immediately after cathodic and anodic electrochemical pre-treatments, on the electrochemical response of a BDD electrode in aqueous media and the influence of the different supporting electrolytes utilized in these pre-treatments on the final surface termination was investigated with [Fe(CN)₆]^4−/3−, as redox probe, by cyclic and differential pulse voltammetry and electrochemical impedance spectroscopy. The cyclic voltammetry results indicate that the electrochemical behavior for the redox couple [Fe(CN)₆]^4−/3− is very dependent on the state of the BDD surface, and a reversible response was observed after the cathodic electrochemical pre-treatment, whereas a quasi-reversible response occurred after anodic electrochemical pre-treatment. Differential pulse voltammetry in acetate buffer also showed that the potential window is very much influenced by the electrochemical pre-treatment of the BDD surface. Electroactivity of non-diamond carbon surface species (sp² inclusions) incorporated into the diamond structure was observed after cathodic and anodic pre-treatments. Electrochemical impedance spectroscopy confirmed the cyclic voltammetry results and indicates that the BDD surface resistance and capacitance vary significantly with the electrolyte and with the electrochemical pre-treatment, caused by different surface terminations of the BDD electrode surface.     

Electrochemical incineration of oxalic acid at boron doped diamond anodes: Role of operative parameters

The electrochemical incineration of oxalic acid (OA) at boron doped diamond (BDD) anodes was investigated both theoretically and experimentally in order to find the influence of the operative parameters on the performances of the process. Polarization curves and chronoamperometric measurements indicate the probable occurrence of a direct electrochemical oxidation of OA at the surface of the BDD anode at low pH and of a hydroxyl radical-mediated reaction at high pH. When incineration electrolyses are performed at low pH with potentiostatic alimentation, a dramatic influence of the potential is observed. In amperostatic incineration, high CE are obtained when most part of the process was under charge transfer controlled kinetics, i.e., when low current densities and high flow rates are imposed. Under these conditions no significant effect of current densities or flow rates is observed. The results obtained at low pH are in good agreement with the previsions of a model based on the assumption that, under adopted experimental conditions, the performances of the process are mainly determined by the competition between the direct oxidation of oxalic acid and the water discharge. The effect of the pH and of the supporting electrolyte was further investigated.     

Metoprolol abatement from wastewaters by electrochemical oxidation with boron doped diamond anodes

BACKGROUND: Metoprolol is a beta‐blocker that can be found in urban wastewaters and which is not removed efficiently by conventional wastewater treatments. In this work, the removal of this pollutant by conductive diamond electrochemical oxidation (CDEO) is studied. RESULTS: CDEO is able to degrade Metoprolol tartrate down to the 10 ppb level (detection limit of the technique used) with a current charge requirement that increases with increase in the initial concentration of pollutant, although it is many times greater than the stoichiometric current charge required. CDEO also removed very efficiently the reaction intermediates. In terms of TOC the depletion follows a first‐order kinetic, but the kinetic constant of Metoprolol decreases with concentration. NaCl increases significantly Metoprolol degradation rate, but it barely affects the TOC removal rate. CONCLUSIONS: CDEO can be used to remove Metoprolol from wastewaters, independently of the initial concentration of compound. Several reaction intermediates are formed during the electrolyses although their concentrations are very low and negligible compared with that of Metoprolol. The removal rate (in terms of TOC) does not depend on the nature of the electrolyte used. The process is under mass transfer control for the complete range of concentrations studied. Copyright © 2011 Society of Chemical Industry     

Thick boron doped diamond single crystals for high power electronics

Switch Mode Power Supply (SMPS) is now widely used for the control and conversion of electric power from one watt to several megawatts. In this context, the synthesis and use of wide bandgap semiconductor materials having physical characteristics superior to silicon is essential. Due to its outstanding physical properties (thermal conductivity, breakdown voltage, carrier mobilities…), diamond is a very promising material. However the success of its use in power electronics mostly relies on our ability to provide carriers by doping the material in a controlled manner. In particular the growth of thick heavily boron doped material is an essential requirement to develop vertical components which should allow, as it will be shown by modeling, limiting the series resistance of the devices in their on-state. Deposition conditions required to obtain high growth rate, high quality and heavily boron-doped material by plasma assisted chemical vapour deposition (MPACVD) will be described. It will be shown in particular that high growth rate, high-quality material, which is obtained at high microwave power density, comes at the expense of the boron concentration, and a compromise must be found. Preliminary results on boron doping of single crystal diamond will be presented and associated with electrical properties of pseudo-vertical Schottky Barrier Diodes (SBD). In particular, a critical electric field of 1.3 MV/cm has been demonstrated with a rectifying ratio of 10⁹. In the same time, current density close to 1500-2000 A cm^− 2 has been reached, showing the potentiality of diamond for power-electronic applications.     

Electrochemical diamond sensors for TNT detection in water

An electrochemically stabilized boron doped diamond electrode prepared by chemical vapour deposition (CVD) is used for electrochemical TNT sensing in aqueous solutions. Square wave voltammograms (SWVs) exhibit three highly resolved peaks at −0.47, −0.62 and −0.76 V vs. Ag–AgCl reference electrode, respectively. The current vs. TNT concentration plot shows a linear relationship with a same slope for the two first TNT peaks at μg L⁻¹ and mg L⁻¹ concentration ranges. Detection and quantification limits of 10 and 25 μg L⁻¹, respectively, were obtained without any preconcentration step. Relative standard deviation (RSD) of less than 1% measured over 10 runs has been found for the −0.47 V peak current showing the very high stability of the electrode without any significant fouling effect. An interference study with nitro aromatic compounds of the same family (nitro toluene and dinitrotoluene) has shown that the −0.47 V reduction peak enables TNT discrimination. Measurement of TNT in a natural medium (sea water without any purification step except filtering) has been also investigated.     

Electrochemical incineration of dyes using a boron‐doped diamond anode

The electrochemical oxidation of a synthetic wastewater containing the model dyes alizarin red (an anthraquinone) and Eriochrome black T (an azoic compound) has been studied on a boron‐doped diamond electrode (BDD) by both cyclic voltammetry and bulk electrolysis. The influence of the current density and dye concentration were investigated. The results obtained show that complete chemical oxygen demand (COD) and colour removal was obtained for both wastewaters. However, the nature of the pollutant, and specially the presence of functional groups (such as the azoic group) seems to strongly influence the performance and efficiency of the electrochemical process. The electro‐oxidation of alizarin red behaves as a mass‐transfer‐controlled process. In such a system, an increase in the current density leads to a decrease in the current efficiency. This can be explained by direct or hydroxyl radical mediated oxidation. The contrary tendency has been observed in Eriochrome black T electro‐oxidation. In this case, higher efficiencies were obtained working at high current densities. This may indicate that the mediated oxidation by electrogenerated reagent (such as peroxodisulphate) is the main oxidation mechanism involved in Eriochrome black T treatment. These compounds have a longer average lifetime than hydroxyl radicals, and it allows the reaction to be extended to the whole wastewater volume. This study has shown the suitability of the electrochemical process for completely removing the COD and total organic carbon and effectively decolourising of wastewaters containing synthetic dyes. Copyright © 2007 Society of Chemical Industry     

Detoxification of synthetic industrial wastewaters using electrochemical oxidation with boron‐doped diamond anodes

The electrochemical oxidation on boron‐doped diamond (BDD) anodes of synthetic industrial wastewaters polluted with different aromatic compounds has been studied. This technology reduces significantly the organic load and the toxicity of treated aqueous wastes. The shape of the toxicity vs time curve depends strongly on the nature of the phenolic compounds, and especially on the substituent groups contained in the phenol molecule. For the oxidation of hydroxybenzenes a complex shape is obtained, with a plateau zone in the first stages of the electrolyses. This plateau zone is transformed into a maximum in the electrolyses of chlorophenols. In the case of nitrophenols, the toxicity index decreases at the same time as the chemical oxygen demand (COD) concentration, although the decrease in the toxicity is less significant than the decrease in the COD. All these toxicity profiles can be justified in terms of the nature of the intermediates generated during the electrolyses, but they cannot be directly related to the COD variation, which shows an exponential decrease. Copyright © 2005 Society of Chemical Industry     

A kinetic study of the electrochemical oxidation of maleic acid on boron doped diamond

Maleic acid (MA) is one of the main intermediates formed during mineralization, by electrooxidation, of aromatic compounds contained in aqueous wastes. This work investigates oxidation of maleic acid with or without the presence of oxalic acid (OA) and formic acid (FA) in aqueous solution by using boron-doped diamond (BDD) electrodes. OA and FA are the main products formed in MA electrooxidation. Voltammetric studies conducted with a BDD electrode of small surface (0.196 cm²) show that MA oxidation takes place at a potential very close to that of the discharge of water. But, under potentiostatic conditions and at concentrations higher than 0.001 M, adsorption of MA blocks its own oxidation. Oxalic and formic acids are before the discharge of water. Again, the presence of maleic acid blocks the oxidation of formic and oxalic acids. Galvanostatic electrolyses of aqueous solutions of MA, OA, FA and mixtures of theses acids were conducted on a BDD electrode. Electrolyses were controlled by measurements of Total Organic Carbon, Chemical Oxygen Demand and by Liquid Chromatography. Results showed that MA was totally mineralized; FA and OA were very low concentration intermediaries. Electrolyses of solutions containing MA, initially in the presence of OA or FA, showed that the OA was oxidized at the same rate as the MA, whereas the FA oxidation began only when the MA had completely disappeared. These results suggest that OA oxidizes by a mass transport limited process coupled with a direct electron transfer with the anode. Under galvanostatic conditions, maleic acid and formic acid are probably oxidized via OH^· radicals generated by water discharge.     

Treatment of Fenton‐refractory olive oil mill wastes by electrochemical oxidation with boron‐doped diamond anodes

In this work, the electrochemical oxidation of an actual industrial waste with conductive diamond anodes has been studied. The wastewater is the effluent of a wastewater treatment plant consisting of a Fenton reactor followed by a settler and a sand filter, in which the wastes generated in an olive oil mill are treated. These wastes contain a residual chemical oxygen demand of nearly 700 mg dm^?3 which cannot be further oxidized with the Fenton process. The electrolyses were carried out under galvanostatic conditions, using a bench‐scale plant equipped with a single‐compartment electrochemical flow cell. Boron‐doped diamond (BDD) and stainless steel (AISI 304) were use as anode and cathode of the cell, respectively. The complete mineralization of the waste was obtained with high current efficiencies limited only by mass transport processes. This confirms that besides the hydroxyl radical‐mediated oxidation that occurs in the Fenton process, the electrochemical oxidation with conductive diamond electrodes combines other important oxidation processes such as direct electro‐oxidation on the BDD surface and oxidation mediated by other electrochemically formed compounds generated in this electrode. Copyright © 2006 Society of Chemical Industry     

Electrochemical reactivity at graphitic micro-domains on polycrystalline boron doped diamond thin-films electrodes

This paper deals with the electrochemical reactivity of boron doped diamond (BDD) electrodes. A comparative study has been carried out to show the influence of the presence of graphitic micro-domains upon the surface of these films. Those graphitic domains are sometimes present on as-grown boron doped diamond electrodes. The effect of doping a pure Csp³ diamond electrode is established by highly oriented pyrolytic graphite (HOPG) abrasion onto the diamond surface. In order to establish the effect of doping on a pure Csp³ diamond electrode, the amount of graphitic domains was increased by means of HOPG crystals grafted onto the BDD surface. Indeed that method allows the enrichment of the Csp² contribution of the electrode.The presence of graphitic domains can be correlatively associated with the presence of kinetically active redox sites. The electrochemical reactivity of boron doped diamond electrodes shows a distribution of kinetic constants on the whole surface of the electrode corresponding to different active sites. In this paper, we have studied by cyclic voltammetry and electrochemical impedance spectroscopy the kinetics parameters of the ferri/ferrocyanide redox couple in KCl electrolyte. A method is proposed to diagnose the presence of graphitic domains on diamond electrodes, and an electrochemical “pulse cleaning” procedure is proposed to remove them.