Diamond electrodes with ablated surfaces

Abstract

An electrode and method of forming the electrode, wherein the electrode is formed from boron doped diamond, the electrode has a total solution accessible electrode area that has undergone an ablation surface modification process and comprises at least 60% diamond stabilized non-diamond carbon. Electrochemical cell electrodes are also disclosed.

Description

[Technical Field]

[0001] The present invention relates to the field of diamond electrodes, in particular diamond electrodes formed from boron-doped diamond, and electrochemical cells comprising such electrodes. The invention also relates to the field of methods for forming such electrodes and methods for using such electrochemical cells. [Background technology]

[0002] Ozone is a highly oxidizing molecule that can exist in the gas or dissolved phase, and its uses include sterilization and disinfection, deodorization, and decolorization. One of the major advantages of ozone over other oxidizing agents is that it leaves no harmful residues. Most commonly, ozone is used in dissolved form in the water treatment industry, where it is used to remove contaminants in waste and drinking water treatment. However, due to its limited half-life of approximately 20 minutes, ozone must be generated in situ. Due to its simplicity, electrochemical ozone generation (EOP) has gained general acceptance because ozone can be generated directly from water by electrochemical oxidation. 7 The mechanism involves the generation of surface-bound hydroxyl radicals, which decay to generate adsorbed oxygen radicals. The oxygen radicals can react together to form adsorbed oxygen molecules or can react with water molecules to form adsorbed OOH radicals; the latter ultimately leads to oxygen evolution. Once adsorbed oxygen is generated, it can further react with the adsorbed oxygen radical to form ozone or simply desorb from the surface as oxygen. The final pathway taken by water oxidation depends on the binding energy of the radical species and the oxygen molecule on the target electrode surface. Because ozone generation appears to be the preferred pathway for water oxidation, electrodes with high overpotentials for the oxygen evolution reaction (OER) may result in higher current efficiencies.

[0003] Historically, Pt and PbO2 were the early electrode materials used for ozone generation at subambient temperatures to enhance ozone solubility, although others, such as SnO2, have also been used (see Foller et al., "The electrochemical generation of high concentration ozone for small-scale applications," Ozone Sci. Eng., 1984, 6, 29-36). Commercially available EOP systems employing PbO2 anodes exist, but they suffer from electrode corrosion and Pb contamination problems, as is common with other metal-based EOP systems. For this reason, boron-doped diamond (BDD) electrodes, which are well known for exhibiting enhanced durability, do not raise contamination concerns, and have high overpotential for OER, have become one of the most popular choices for commercial EOP devices (see Arihara et al., Electrochemical Production of High-Concentration Ozone-Water Using Freestanding Perforated Diamond Electrodes, J. Electrochem. Soc., 2007, 154, E71, and Arihara et al., Electrochem. Solid-State Lett., 9, D17 2006).

[0004] In commercial applications, it is desirable to generate ozone from electrolyte-free solutions (pure water) to avoid the reduction in current efficiency of the ozone generation process due to competing side reactions involving electrolysis of electrolyte counterions and to provide a route to reagentless ozone generation. 13,14However, the low conductivity of the solution necessitates the use of a solid electrolyte, such as a proton-transporting Nafion™ membrane sandwiched between two BDD electrodes (zero-gap cells). The electrodes are most often used in porous form, involving the placement of through-holes in thick, free-standing BDDs or the growth of thin-film BDDs on perforated substrates (see Kraft et al., Electrochemical ozone production using diamond anodes and a solid polymer electrolyte, Electrochem. Commun., 2006, 8, 883-886). Pore formation is important in allowing solution access to the Nafion™ membrane. This creates a Nafion™-solution-BDD "triple point," which some believe is also the site where ozone is produced. While the cost of producing thin-film BDDs is lower than that of free-standing materials, growing pinhole-free BDDs on geometrically challenging substrates is challenging; pinholes can lead to delamination of the BDD membrane and premature electrode failure, even at moderate current densities. With zero-gap cells, BDD electrodes are favorably competitive with PbO2 negative electrodes in terms of current efficiency, but they require higher operating voltages.

[0005] The surface of the BDD electrode has been discussed in the EOP literature. Some researchers have commented on the preferred use of a non-diamond, carbon-free surface, i.e., a clean BDD, but the sp 2 There is limited experimental research suggesting that bound carbon impurities can increase ozone output (Watanabe et al., Tailored design of boron-doped diamond electrodes for various electrochemical applications with boron-doping level and sp 2(See, for example, "Bonded Carbon Impurities," Phys. Status Solidi Appl. Mater. Sci., 2014, 211, 2709-2717; and Honda et al., "An electrolyte-free system for ozone generation using heavily boron-doped diamond electrodes," Diam. Relat. Mater., 2013, 40, 7-11). This work aims to preserve the durability of the electrode and the structural stability of the BDD film by using sp 2 They emphasize the importance of keeping the proportion of bonded carbon impurities low. 2 The amount of bound carbon impurities should be controlled low enough to maintain the durability of the diamond. Raman spectra show that BDD-B and D contain some non-diamond sp 2 It indicates that it contains bonded carbon impurities, but sp 2 The proportion of bonded carbon is extremely low' and 'sp 2 It is important to note that the presence of structures reduces the stability of BDDs. 2 Bound carbon was introduced during chemical vapor deposition (CVD) growth by varying the ratio of boron to carbon feedstock from 0.1% to 5%. 2 Since the bonded carbon is grown-in, it is present throughout the film, not just on the surface. In these studies, the boron concentration and sp 2 It should be noted that the bound carbon content was simultaneously varied and the effect of changes in material conductivity (due to variations in boron concentration) was not taken into account on the results obtained. Summary of the Invention

[0006] The present invention provides high levels of sp 2 We describe the optimization of BDD electrodes for the production of bactericidal species through the generation of bound surface carbon. In contrast to the current literature, the present invention provides at least 60% sp 2We propose a BDD electrode with a surface covered with bonded carbon, a level well beyond what is currently taught to be the optimal coverage for BDD EOP operation. 2 BDD electrodes with high bonded carbon surface content provide increased power output due to EOP, which we attribute to the increased density of possible radical / oxygen binding sites on the BDD surface. Surprisingly, such electrodes can achieve such high sp 2 Despite the high bonded carbon content, the structural stability, and therefore the stability of the ozone output, can be maintained for a long time, e.g., at least 20 hours, under the extremely high current density (voltage) operating conditions of the EOP. We have identified the sp SiO 2 produced using our method as diamond-stabilized non-diamond carbon (DSC) because of its ability to withstand the extreme operating conditions of the EOP. 2 We refer to bonded carbon regions. A preferred method for introducing such high DSC surface coverage is by a combination of ablation processing and post-ablation chemical treatment to produce the DSC. In this method, thermal damage of the BDD, for example by the high-energy emission of a laser beam, results in the conversion of diamond to graphite. Subsequent chemical treatment yields the DSC. The DSC exists as a layer of graphitic sheets of substantially vertically aligned carbon (typically up to 10 nm) at the points of intimate contact with the underlying BDD, at least partially sealed by an amorphous carbon shell, which can be less than 10 nm thick (Cobb et al., Assessment of acid and thermal oxidation treatments for removing sp 2 bonded carbon from the surface of boron doped diamond, Carbon NY, 2020, 167, 1-10).

[0007] In a first aspect, there is provided an electrode formed from boron-doped diamond, the electrode having a total solution-accessible electrode area comprising at least 60% diamond-stabilised non-diamond carbon. This type of solution-accessible electrode region is applicable to various types of boron-doped diamond materials, including free-standing CVD BDD, thin film coatings of CVD BDD on conductive substrates (with preferred thicknesses in the range of 0.5-50.0 μm), and BDD materials produced via high temperature and pressure (HPHT) synthesis of BDD particles that are then sintered or compressed into a polycrystalline matrix material. In one option, the total solution accessible electrode area comprises at least any of 70%, 80%, 90%, and 95% diamond stabilized non-diamond carbon. In one option, the diamond stabilized non-diamond carbon comprises oriented graphite bonded to a diamond surface having an amorphous carbon layer, the graphite being oriented at an angle of greater than 20° relative to the plane of the total solution accessible electrode area where it is bonded to the diamond surface. In one option, the boron doped diamond is selected from any of CVD diamond, HPHT diamond and compressed HPHT diamond.

[0008] The boron doped diamond is optionally in the form of a coating layer on a conductive or non-conductive substrate. In one alternative, the diamond-stabilized non-diamond carbon is formed on the solution-accessible electrode regions by an ablation technique followed by application of an oxidation treatment to the solution-accessible electrode regions. In a further alternative, the oxidation treatment comprises treatment for at least 10 minutes in a liquid comprising any of sulfuric acid and potassium nitrate; sulfuric acid and hydrogen peroxide; nitric acid and hydrochloric acid; hydrofluoric acid; hypochlorous acid; nitric acid, perchloric acid and sulfuric acid; and a permanganate selected from any of potassium permanganate, ammonium permanganate, calcium permanganate, sodium permanganate, and silver permanganate. The oxidation treatment optionally includes electrochemical oxidation.

[0009] The solution-accessible electrode area optionally further comprises any of slots, depressions and non-flat surface features. One option is to consider the possibility that the bulk boron-doped diamond away from the solution-accessible electrode region has significantly fewer sp 2 Contains bonded carbon. sp in bulk material 2 The amount of bound carbon may be at least five times less than the amount in the solution-accessible surface area.

[0010] In a second aspect, there is provided an electrochemical cell comprising a first electrode as described above in the first aspect, a second opposing electrode, a flow path configured to conduct a fluid, drive circuitry configured to apply a potential across the electrodes such that a current flows between the electrodes when the fluid flows through the flow path, and a sealed enclosure within which the electrodes are disposed, the enclosure configured to contain the fluid within the flow path. In one option, the electrochemical cell is configured to generate ozone when in use. One option is to use a voltage in the range of 5 to 10 V and a current of 0.01 to a maximum of 0.05 A / cm. 2 Over a range of current densities, at a nominal temperature of 25°C and 240 ml -1 When using deionized water (greater than 15 MΩ cm) at a flow rate of 0.1 MPa, the peak ozone current efficiency is greater than 25%. As a further option, the peak ozone current efficiency is selected from any of at least 30% and at least 35%. In one option, the ozone output gradient decreases by no more than 10% during at least 5 hours of continuous operation. In a further option, the 5 hour period is measured after an initial period of use of at least 20 hours. As an option, during use, the ozone productivity gradient is 0.01-0.05 A / cm under an applied voltage of 5-15 V operating in water with a resistivity greater than 15 MΩ cm at a flow rate of 240 ml / min. 2 Over the current density range of R 2 by least squares linear regression fitting with a coefficient of determination >0.95.

[0011] In a third aspect, there is provided a method of forming an electrode, the method comprising providing a boron doped diamond and applying a surface modification process to form a total solution accessible electrode area comprising at least 60% diamond stabilized non-diamond carbon. If necessary, the surface modification step includes an ablation processing step. In one option, the ablation process includes laser ablation, in this example, using a wavelength of 355-1064 nm, a pulse length of 10-500 ns, a pulse frequency in the range of 50 Hz-25 MHz, and a traverse speed of 0.1-100,000 mm / s at a dose of 10 J / cm. 2 This is done, if necessary, with a laser having a fluence exceeding 100 .mu.m. Alternatively, the ablation process includes electrical discharge machining, in this example performed with an electrode gap of 0.05-3.00 mm, an applied voltage of 5-60 V, and a current of 0.1-5.0 A, with a pulse time of 10-300 μs, as desired. The ablation process may also be used to form slots, depressions and / or non-planar surface features in the solution-accessible electrode regions as needed.

[0012] The method optionally further comprises applying an oxidation step to the total solution-accessible electrode area, the oxidation step comprising applying an oxidizing environment to the solution-accessible electrode area. The oxidation step optionally comprises treatment for at least 10 minutes in a liquid comprising any of sulfuric acid and potassium nitrate; sulfuric acid and hydrogen peroxide; nitric acid and hydrochloric acid; hydrofluoric acid; hypochlorous acid; nitric acid, perchloric acid and sulfuric acid; and a permanganate selected from any of potassium permanganate, ammonium permanganate, calcium permanganate, sodium permanganate, and silver permanganate. Alternatively, the oxidation step optionally comprises electrochemical oxidation. The method involves applying any of the following steps to the solution-accessible electrode area: electrochemical anode process, oxygen plasma treatment, and chemical / electrochemical reduction step, as appropriate. Optionally, the method comprises providing a first electrode as described above in the first aspect, providing a second opposing electrode, providing a flow path configured to conduct a fluid, providing drive circuitry configured to apply a potential between the electrodes such that when fluid flows through the flow path, a current flows between the electrodes, and providing a sealed enclosure in which the electrodes are disposed, the enclosure configured to contain the fluid within the flow path. The method optionally includes disposing a solid electrolyte between the first and second electrodes. As a further option, the method includes treating the solid electrolyte by hydrating and protonating the solid electrolyte prior to use.

[0013] In a fourth aspect, there is provided a method of using an electrochemical cell, the method comprising providing an electrochemical cell as described above in the second aspect, flowing a fluid through a flow path, and applying a potential between the electrodes such that a current flows between the electrodes. The method optionally further comprises applying an electrical potential between the electrodes via dry electrical contacts such that the only electrochemically active electrode material is the boron-doped diamond electrode. By way of example, non-limiting embodiments are hereinafter described with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic diagram of an exemplary electrochemical EOP cell. [Figure 2] FIG. 10 shows an example of how to calculate the total solution-accessible electrode area. [Figure 3] a) Ozone output gradient of the electrochemical EOP cell versus accessible DSC fraction, expressed as % solution-accessible area of ​​the BDD electrode; b) Peak current efficiency of the electrochemical cell versus accessible DSC fraction (%) of the BDD electrode surface. [Figure 4] 1 shows an optical image of a corroded glassy carbon electrode after use in an EOP electrochemical cell. [Figure 5]FIG. 10 shows the ozone output gradient versus electrode thickness for the perforated electrode design. [Figure 6] a) is a graph showing UV absorbance at 258 nm versus time for an exemplary BDD electrode cell; b) is a graph showing dissolved ozone concentration versus current for an exemplary BDD electrode cell; and c) is a graph showing current efficiency versus applied current for an exemplary BDD electrode cell. [Figure 7] a) Photograph of an electrode formed from compressed HPHT particulates cut to the desired shape and glued into a half cell; b) Diagram of dissolved ozone concentration versus current; c) Diagram of current efficiency versus applied current. [Figure 8] Figures a) and b) show UV absorbance at 258 nm versus time for CVD BDD and HPHT BDD electrode cells, respectively, and c) shows dissolved ozone concentration versus current after long-term stability testing for CVD and HPHT electrode cells. [Figure 9] 1 is a flow diagram showing exemplary steps for forming a BDD diamond electrode. [Figure 10] 1 is a flow diagram showing exemplary steps for forming a BDD diamond electrochemical cell. [Figure 11] 1 is a flow diagram showing exemplary steps for using a BDD diamond electrochemical cell. DETAILED DESCRIPTION OF THE INVENTION

[0015] As mentioned above, up to now, in BDD for EOP, sp 2 It has been emphasized that the proportion of bound carbon impurities must be kept low. The inventors have found that introducing a large amount of DSC onto the surface not only improves the properties of the BDD electrode for EOP, but also, unexpectedly, enables the electrode to maintain long-term (at least 20 hours) power stability during EOP. It is sufficient that the DCS is present only in the solution-accessible electrode region of the electrode; it is sufficient that it is present in the sp 2It has been shown that DSC can be introduced using a combination of ablation and post-ablation chemical treatment. Ablation introduces thermal damage to the BDD surface, allowing the formation of sp-like materials such as graphite. 2 Form bonded carbon, and in subsequent processing, sp 2 It has been proposed that the bound carbon can be stabilized to form DSCs. These subsequent treatments can be performed before the BDD electrode is placed in the electrochemical cell, or in situ within the electrochemical cell as part of a conditioning process. As noted above, DSCs exist as layers of vertically aligned graphitic sheets of carbon (typically up to tens of nanometers thick) sealed at the points of intimate contact with the underlying BDD by an amorphous carbon shell (which can be less than 10 nanometers thick). An example of the fabrication of a BDD diamond electrode with a stable layer for DSC against EOP is provided below.

[0016] Cell Design Figure 1 shows a schematic of the electrochemical cell design. Each cell 1 contains two separate half-cells 2 and 3. The half-cells 2 and 3 were 3D printed using Form3 (FormLabs) and UV-cured clear polymethyl methacrylate (PMMA) resin (FormLabs Standard Clear, FormLabs) with a 50 μm layer height. They were then washed in isopropyl alcohol (reagent grade, Fisher Scientific) for 10 minutes, followed by UV curing at 60 °C for 20 minutes. This step ensured the removal of excess resin and optimized the material properties of the resin parts. Next, the support material was removed, and the surfaces were polished to a smooth surface using successively increasing grades of CarbiMet paper (Buhler, USA).

[0017] The perforated electrode cell is composed of two identical halves containing perforated BDD electrodes 4, 5, respectively, a recess to accommodate the BDD electrodes, a 3D-printed gasket 6, slots for copper tab contacts 7, and channels for solution flow spanning the backside of the electrodes, as shown in Figure 1. Electrodes 4 and 5 were glued into the recesses of each half-cell 2 and 3 using UV resin (FormLabs Clear, FormLabs). Electrical contacts to the electrode contacts were made using silver-filled epoxy (Conductive Epoxy, CircuitWorks, Chemtronics). The resulting half-cells were identical and labeled A and B. Each half-cell 2 and 3 could function as either a negative or positive electrode. Prior to cell testing, the BDD electrodes 4 and 5 were polished with alumina fines (0.05 μm, Buehler, Germany) on a wet micro-cloth pad (Buehler) and then on a wet alumina-free micro-cloth pad. For sealing, an elastomeric 3D-printed gasket (FormLabs Elastic 50A, FormLabs, USA) was placed in the recesses on each half-cell 4 and 5, sealing around the Nafion™ membrane 8. A 2 cm diameter circular reinforced Nafion™ perfluorinated membrane (Nafion™ 424 reinforced with 0.33 mm poly(tetrafluoroethylene) fibers, Sigma-Aldrich) was placed between half cells 3 and 4, which were then bolted together.

[0018] Fabrication of perforated BDD electrodes with DSC The perforated BDD electrodes 3 and 4 were fabricated using a 355 nm Nd:YAG 34 ns laser micromachining system (E-355H-ATHI-0 system, Oxford Lasers) on freestanding polycrystalline BDD wafers (420 μm, unless otherwise specified, with a boron dopant density >10 B atoms / cm). 3 The as-grown wafers were machined from a silicon wafer (Sapphire, Element Six). The growth surface of the as-grown wafers had a roughness of about 15 μm RMS, while the nucleation surface roughness was about 100 nm RMS. Both were measured using white light interferometry (WLI). A trepanning system was used to widen the laser spot diameter to 50 μm, widening the cut trench. This ensured that the cut edge was perpendicular to the BDD, preventing the beam from interfering with the trench sides and eliminating the need for a kerf. Electrodes 3 and 4 were laser cut using two passes at a fluence of 760 J / cm per pass. To facilitate electrical contact, the BDD was cut into circles of different diameters (12 or 24 mm) with 3x2 mm tabs. Through-holes of different shapes were cut from the center. To improve Ti / Au contact during application, the nucleation surfaces of the contact tabs were laser roughened at a fluence of approximately 30 J / cm using a 532 Nd:YAG 15 ns laser micromachining system (A-Series, Oxford Lasers).

[0019] Perforated HPHT compact electrode production Freestanding HPHT BDD electrodes were fabricated using a binderless HPHT compression process (at 6.6 GPa and 1700°C in a cubic anvil press) of HPHT-synthesized BDD microparticles. The microparticles were grown from an Fe / Ni / C melt with 4.8 wt% AlB added as a boron source (as described in detail in Wood et al., High pressure high temperature synthesis of highly boron-doped diamond microparticles and porous electrodes for electrochemical applications, Carbon NY, 2021, 171, 845-856). This method produces freestanding cylindrical compressed electrodes, referred to herein as "compact," with a diameter of approximately 16 mm, a wall thickness of 2 mm, and a mass of approximately 2-3 x 10 20 B atoms / cm 3The compact had a boron content of about 1000 .mu.m and a resistivity of about 650 mΩ. 2 mm thick compacts were sliced ​​using electrical discharge machining (EDM) to obtain electrodes about 500 μm thick. To facilitate electrical contact through the back of the electrode for substantially dry electrical contact, the electrodes were cut into 8 mm diameter circles with rectangular tabs (3 x 2 mm). Further through-holes were cut using the same laser conditions as for the CVD electrodes.

[0020] Electrode treatment for forming DSC After laser processing, all electrodes were immersed in concentrated H2SO4 saturated with KNO3, a strong oxidizing solution, at approximately 200 °C for 30 minutes, followed by rinsing with ultrapure water, and then placed in the concentrated H2SO4 solution at 200 °C for another 30 minutes. This process removes all weakly bonded sp 2 Remove the bonded carbon and create a very strong form of sp 2 leaving behind bonded carbon, DSC (see Cobb et al., referenced above). Strong oxidizing conditions can be produced by several methods, including concentrated H2SO4 and H2O2, aqua regia [HNO3 and HCl], HF and HNO3, other oxidizers such as potassium permanganate, and thermal processes such as plasma etching or other conditions that etch graphite under controlled conditions. Electrical contacts were applied by sputtering (Moorfield MiniLab 060 Platform Sputter system) a thin layer of Ti (10 nm) onto the laser-roughened contact tabs on the lapped surface of the perforated electrode and the backplane electrode, followed by a second layer of Au (400 nm). The contacts were annealed at 400 °C for 5 h to create an ohmic connection.

[0021] Ozone measurement Ultrapure water (Milli-Q, resistivity >15 MΩ / cm) was pumped from a reservoir through the ozone cell at a flow rate of approximately 330 mL / min using a diaphragm pump. A galvanostatic current ranging from 0.1 to 0.6 A was applied in 0.1 A increments using a Voltcraft VLP-2602 OVP power supply (Voltcraft). Aliquots of ozonated water were collected after 30 s. Ozone concentrations were measured by recording absorbance at 258 nm on a Lambda850 UV / Vis spectrometer (Perkin Elmer) using a quartz cuvette (Hellma Analytics) with a 1 cm optical path and a molar absorption coefficient of 2900 / (M·cm). Prior to use, one side of the cell was run at 0.6 (for the 12 mm electrode) or 0.3 A (for the 8 mm electrode) for 5 minutes to hydrate and protonate the Nafion™ membrane. The pretreatment was applied to the A-side BDD electrode connected as the negative electrode immediately before recording three replicate calibrations with the A-side as the negative electrode (and the B-side as the positive electrode). The pretreatment was then applied again to the B-side BDD electrode connected as the negative electrode immediately before recording three replicate calibrations with the B-side as the negative electrode. The selected conditions provided a stable response between replicate calibrations that produced consistent current / voltage and current / ozone output data.

[0022] Long-term testing For long-term stability testing, UV absorbance at 258 nm was recorded as a function of time over 20 hours of continuous operation at a constant current of approximately 0.3 A (12 mm cell) and approximately 0.15 A (8 mm cell). Calibration was performed in triplicate before and after the long-term test, with electrode A and then electrode B acting as the negative electrodes for ozone generation. A diaphragm pump was used to recirculate a 10 L DI water reservoir through the ozone cell. Water from the reservoir passed through the ozone cell and exited into a flow-through cuvette (45 FL; FireflySci) in the UV-vis system, removing ozone through the system and then returning to the 10 L reservoir. The ozone removal system was a high-surface-area quartz tube system between two 254 nm UV lamps (dissolved ozone is converted to oxygen at 254 nm). 4 ).

[0023] Results and Discussion Metrics: Total Area and DSC Content: Figure 2 shows examples of calculated total areas for both immersed (electrode suspended in electrolyte and accessible to solution on all sides except the Nafion™ side) and sealed (electrode glued or sealed in a cell, leaving only a portion of one side accessible to solution) electrodes. The checkered area represents the wetted backside area, the white area represents the internal slot area, and the striped area represents the side blocked from the solution by the Nafion™ membrane. The black areas represent other areas not accessible to solution. For perforated electrodes, the face of the BDD that is in contact with the Nafion™ membrane and under pressure is considered to be solution-excluded. Therefore, the total solution-accessible electrode area is the wetted backside area plus the internal ablation area of ​​the through slots machined into the BDD (referred to as the internal slot area). The total solution-accessible electrode area is calculated as shown in Equation 1: Total solution accessible electrode area = wetted back surface area + internal slot area (Equation 1) Expressed as the ratio of the internal slot area to the solution-accessible electrode area, the DSC quantity is calculated as shown in Equation 2:

number

[0024] Electrolyte ozone generation gradient: EOP gradient (mg A cm 2 / L) is the applied current density (A / cm 2 ), y-axis, vs. measured ozone concentration (mg / L), x-axis, calculated by applying a least-squares linear fit. Ozone concentrations were measured in all cells for constant currents ranging from 0.1 to 0.6 A in 0.1 A increments. Due to the different electrode geometries investigated, the applied current had to be normalized by the solution-accessible electrode area (see above) to facilitate comparisons. Electrodes with steeper slopes are more efficient at ozone generation. Current efficiency: The current efficiency (ε) was calculated using equation 3:

number

[0025] Effect of introducing diamond-stabilized non-diamond carbon on perforated electrodes: Increasing the DSC content from 0% to 20% has little effect on the ozone output, as shown in Figure 3a. However, increasing the DSC ratio from 60% to 80% results in a sharp increase in both ozone output slopes (from 2 to 4.7 mg cm). 2 / (A·L)). The increasing ozone output gradient with increasing DSC ratio indicates the importance of DSC for ozone production. As shown in Figure 3b, a similar trend exists for peak current efficiency; on average, cells with higher DSC content have higher peak current efficiencies. From this data, the optimal electrode is a sp such as glassy carbon or graphite. 2Bonded carbon electrode material (i.e., 100% sp 2 bonded carbon), which is why these sp 2 This is not possible due to the fact that bonded carbon electrodes are not DSC or stable and are likely to corrode oxidatively under high negative potentials. 2 The only previous EOP publication on bonded carbon electrodes used glassy carbon as the negative electrode. A relatively high current efficiency of 35% was achieved, but the electrode was limited to 400 mA / cm to prevent oxidation corrosion. 2 It should be noted that the DSC was operated in an acidic medium at a maximum (low) current density of 1000 kJ / s. This low current density required the use of very large electrodes (20 x 2.5 cm rods) to generate meaningful concentrations of dissolved ozone. In this study, when electrodes identical in shape to the 70% DSC BDD electrodes were cut from glassy carbon wafers and used for EOP under our conditions (see above), a single calibration could not be completed without substantial corrosion damage evident to the naked eye, as shown in Figure 4. Laser ablation of the BDD followed by post acid oxidative chemical treatment introduces a thin layer of extremely robust DSC into the solution-accessible electrode area, which can survive these high-potential applications. The BDD is essentially a conductive vehicle from which and on which the DSC is formed. The presence of DSC is a key factor in the ozone output for a given current density: a low ratio of sp 2 Compared to the combined carbon content BDD, this improves the efficiency of the BDD for EOP and allows for a more compact electrode for the same ozone output. sp 2 and sp 3Because both the bonded carbon electrode and the bonded carbon electrode have a high applied potential for the oxygen evolution reaction (OER), the origin and efficiency of the enhanced ozone output are unlikely to be due to reduced competition from the OER. Instead, the observed improvement is thought to be related to the strength of radical adsorption to the electrode surface, which is a crucial factor in surface-driven processes such as ozone generation. The BDD electrode is unique in that hydroxyl radicals are known to adsorb very weakly on the electrode surface, allowing them to desorb relatively easily from the electrode and react with species in solution. As a result of this property, the BDD electrode has been extensively studied for advanced oxidation. Adsorbed hydroxyl radicals are a key step in the formation of adsorbed oxygen radicals, which ultimately leads to ozone formation. Hydroxyl radicals are more strongly adsorbed in regions of the electrode with a high proportion of DSC, resulting in a higher concentration of adsorbed radicals and, ultimately, more ozone production.

[0026] Material thickness and triple point length: Material thickness was also investigated as a method for producing BDD electrodes with high DSC ratios without changing the electrode geometry. Perforated electrodes with the same design as the 70% DSC electrode in Figure 3 were cut from BDD samples with thicknesses of 200, 300, 420, and 700 μm, resulting in DSC ratios ranging from 53 to 80%. Figure 5 shows the ozone gradient versus electrode thickness for a single perforated electrode design. Increasing thickness from 200 to 700 μm results in an increasing ozone output gradient. This also provides another example of how changing the DSC percentage in the design can lead to improved cell EOP performance.

[0027] Another interesting aspect of this study is that it does not appear to be consistent with the "triple point" theory proposed by (Kraft et al. Electrochemical ozone production using diamond anodes and a solid polymer electrolyte, Electrochem. Commun., 2006, 8, 883-886). In this study, the authors state that only the region of the BDD electrode in close proximity to both the Nafion™ membrane and the solution is active in producing ozone. Since all four electrodes in Figure 5 have the same slot length (and therefore the same triple point length, which is 170 mm in this design), if the triple point theory were true, they should all have the same ozone output gradient, which is not the case.

[0028] Long-term testing: The 70% DSC cell was then subjected to a long-term stability test by operating at 0.3 A for 20 hours with the B side as the negative electrode, and the UV absorbance measured at 258 nm was continuously monitored. As shown in Figure 6a, the UV absorbance, and therefore the dissolved ozone concentration, appeared relatively constant over the 20 hours, demonstrating the long-term operational stability of the BDD electrode. The numerous unavoidable spikes in the absorbance data are due to air bubbles in the flow system generated during EOP. After the long-term stability test, the calibration plot was recorded again (Figure 6b, red line) using a flow rate of approximately 300 ± 50 mL / min and compared with the calibration plot recorded before the long-term test (Figure 6b, black line); no significant differences were observed. Both recorded calibration plots had good linearity, with R 2The (adjusted) values ​​were 0.9823 and 0.9936, respectively. The maximum dissolved ozone produced was 1.7 mg / L (before the long-term test) compared to 1.5 mg / L (after the long-term test), and the applied current of 0.6 A similarly remained relatively constant, within the typical error range observed for these cells. In both calibration plots, the amount of ozone produced can be seen to increase linearly with increasing applied current, with slopes of 3.4 ± 0.2 mg / (L·A) and 2.7 ± 0.1 mg / (L·A) before and after the long-term test, respectively (Figure 6b). Similarly, the current efficiency vs. current plot (Figure 6c) did not change significantly as a result of the long-term test, with maximum efficiencies of 39% and 34%, respectively, before and after the long-term test (Figure 5c). This data demonstrates the potential for long-term product life when using BDD electrodes for ozone generation.

[0029] HPHT electrode: The effectiveness of the HPHT BDD compact for ozone generation was also investigated by comparing an 8 mm HPHT electrode with an identically shaped 8 mm CVD electrode. The HPHT electrode (shown in Figure 7a) was 500 μm thick, and the corresponding CVD electrode was 420 μm thick, with DSC ratios of 82 and 79%, respectively. For both cells, the flow rates were 335 ± 4 and 302 ± 6 mL / min for the CVD and HPHT cells, respectively. It can be seen that the amount of ozone produced increased linearly with increasing applied current, with the slopes being 3.13 ± 0.06 mg / (L·A) and 2.23 ± 0.07 mg / (L·A) for the CVD and HPHT cells, respectively, as shown in Figure 7b. For the CVD cell, a maximum dissolved ozone of 1.64 ± 0.03 mg / L was produced at a maximum applied current of 0.6 A, compared with the HPHT cell, where a slightly lower maximum dissolved ozone of 1.17 ± 0.07 mg / L was produced at a maximum applied current of 0.6 A. The small difference is most likely due to the lower resistivity of the CVD electrode (60 mΩ·cm) compared to the higher resistivity of the HPHT electrode (650 mΩ·cm). In the CVD material, particles grow across each other during synthesis, creating good electrical connectivity between particles. In contrast, during compression of the particles, they are forced together. Additional uncompensated resistance is also introduced as a result of the penetration of the solution into the submicron-sized pores of the compressed HPHT material. Figure 7c compares the current efficiency between the HPHT and CVD cells. For both cells, the current efficiency increases linearly with applied current from 0.1 to 0.3 A, at which point the slope decreases and the current efficiency begins to plateau. Between 0.4 and 0.6 A, the rate of increase in current efficiency decreases significantly with further applied current, reaching maximum efficiencies of 37% and 23% for the CVD and HPHT cells, respectively. Both cells were then subjected to a long-term stability test by operating at 0.15 A for 20 hours, using flow rates of 194 ± 1 and 193 ± 1 mL / min for the CVD and HPHT cells, respectively, while continuously monitoring UV absorbance at 258 nm (Figures 8a and 8b). After the long-term stability test, the calibration plot was recorded again (Figure 8c) and compared with Figure 6b. For the CVD cell, a slight increase in maximum dissolved ozone produced was observed at an applied current of 1.84 ± 0.07 mg / L compared to 1.64 ± 0.03 mg / L for the HPHT cell at 0.6 A. For the HPHT cell, the maximum ozone output of 1.19 ± 0.09 mg / L at 0.6 A remained the same, within the error range. For both cells, the slope of the fitted calibration data was not significantly affected by the 20-hour run time: for HPHT, 2.23 ± 0.07 mg / (L A) (before 20 hours) vs. 2.30 ± 0.08 mg / (L A) (after 20 hours), and for CVD, 3.13 ± 0.06 mg / (L A) (before 20 hours) vs. 3.09 ± 0.08 mg / (L A) (after 20 hours). Both CVD and HPHT calibration plots recorded after the long-term stability study had excellent linearity, with R 2 (adjusted) were 0.9882 and 0.9817, respectively, indicating the potential for long product life without performance degradation when using CVD or HPHT BDD electrodes for ozone generation.

[0030] 9, a flow diagram illustrating exemplary steps for forming a BDD diamond electrode for an EOP is shown. The following numbering corresponds to that of FIG. 9: S1. A boron doped diamond is provided. As mentioned above, this may be in the form of a CVD diamond, a HPHT diamond, or a compact of HPHT diamond. S2. A surface modification process is applied to the BDD to form a total solution-accessible electrode area, such that the total solution-accessible electrode area comprises at least 60% diamond-stabilized non-diamond carbon. As mentioned above, the surface modification process typically involves sp 2This involves an ablation process to form bonded carbon and subsequent formation of DSC. When laser ablation is used as part of the surface modification process, typical laser parameters for BDD are in the following range: 10 J / cm with a pass speed of 0.1 to 100,000 mm / s. 2 Using a laser with a fluence exceeding 1000 ns, the wavelength is 355 to 1064 nm, the pulse frequency is in the range of 50 Hz to 25 MHz, and the pulse length is 10 to 500 ns. When EDM ablation is used, the BDD workpiece is placed in a dielectric solution, usually deionized water. An electric potential is applied between the BDD and the positive electrode, which is typically a wire electrode, usually brass, with a diameter of 0.1 to 1 mm, at a feed rate of 1 to 15 m / min. A typical electrode gap of 0.05 to 0.3 mm, an applied voltage of 5 to 60 V, a current of 0.1 to 5 A, and a pulse time of 10 to 300 μs are used. Typical material cutting speeds vary depending on the resistivity and thickness of the BDD material, but are typically in the range of 0.1 to 10 mm / min. Ablation processing has the advantage that the freestanding BDD can be post-patterned and chemically treated, and DSC surface coverages of over 60% can be easily achieved in a cost-effective manner. As taught by Einaga et al., attempts to introduce such high coverages via CVD significantly compromise the structural integrity and robustness of the resulting material for EOP. The sp 2 Bound carbon is highly unlikely to be DSC; long-term EOP tests were not reported in the study by Einaga et al. This is due to the higher sp 2 This is also the reason why ultrananocrystalline diamond, which contains bound carbon content, has never proven suitable for EOP applications. The ablation region can be a shallow depression, pattern, or other non-planar surface feature cut into the face of the electrode, 0.05 to 5 μm deep. For example, the ablation region can be a vertical wall in the form of a slot through the electrode or a trench in the electrode. The surface modification process may include the application of a chemical treatment process to the ablation area, such as the application of an oxidizing environment, such as immersing the electrode in a solution of boiling sulfuric acid and potassium nitrate for at least 10 minutes. Additional suitable surface modification steps include electrochemical anode processes, oxygen plasma treatments, and chemical / electrochemical reduction steps applied to solution-accessible electrode regions, which, where appropriate, may be performed in situ in the electrochemical cell in which the electrode is to be used. 10, a flow diagram illustrating exemplary steps for forming a BDD diamond electrochemical cell is shown. The following numbering corresponds to that of FIG. 10: S3. A first electrode is provided, consisting of a BDD having a total solution-accessible electrode area of ​​at least 60% comprising the diamond-stabilized non-diamond carbon described above. A solid electrolyte, such as Nafion™, may be disposed between the first and second electrodes. The solid electrolyte may be hydrated and protonated prior to disposing it between the electrodes or in situ in the electrochemical cell. S4. A second electrode is provided, which typically also comprises a BDD with a total solution-accessible electrode area comprising at least 60% diamond-stabilized non-diamond carbon, although this is not required. S5. A flow path configured to carry a fluid is provided. S6. A drive circuit is provided that is configured to apply a potential between the electrodes such that a current flows between the electrodes when a fluid passes through the flow path. Note that the first and second electrodes are connected to the drive circuit via dry electrical contacts such that, in use, the only electrochemically active electrode material is the boron-doped diamond electrode. S7. A sealed enclosure is provided, with the electrodes disposed therein, the sealed enclosure being configured to contain, in use, a fluid in the flow path. Referring now to FIG. 11, a flow diagram illustrating exemplary steps for using a BDD diamond electrochemical cell is shown. S8. An electrochemical cell is provided as described above. S9. Pass the fluid through the flow path. S10. A potential is applied between the electrodes so that a current flows between them, typically to produce ozone from the fluid.

[0031] While the present invention has been shown and described in detail with reference to preferred embodiments, those skilled in the art will understand that various changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims. For example, while the above examples used high percentage DSC-BDD electrodes on both sides of the Nafion™ membrane, it will be understood that only one electrode should be a high percentage DSC-BDD electrode. Another aspect of the present invention may be as follows. [1] An electrode formed from boron-doped diamond, the electrode having a total solution-accessible electrode area containing at least 60% diamond-stabilized non-diamond carbon. [2] The electrode described in [1], wherein the total solution-accessible electrode area comprises at least 70%, 80%, 90%, and 95% diamond-stabilized non-diamond carbon. [3] The electrode described in [1] or [2], wherein the diamond-stabilized non-diamond carbon comprises oriented graphite bonded to the diamond surface having an amorphous carbon layer, and the graphite is oriented at an angle of greater than 20° to the plane of the total solution-accessible electrode area at the location where it is bonded to the diamond surface. [4] The electrode according to [3], wherein the layer of amorphous carbon has a thickness of less than 10 nm. [5] The electrode according to any one of [1] to [4] above, wherein the boron-doped diamond is selected from the group consisting of CVD diamond, HPHT diamond, and compressed HPHT diamond. [6] The electrode according to any one of [1] to [5], wherein the boron-doped diamond is in the form of a coating layer on a conductive or non-conductive support. [7] An electrode according to any one of [1] to [6], wherein the diamond-stabilized non-diamond carbon is formed in the solution-accessible electrode region by applying an ablation processing technique followed by an oxidation treatment to the solution-accessible electrode region. [8] The oxidation treatment is Sulfuric acid and potassium nitrate; sulfuric acid and hydrogen peroxide; Nitric acid and hydrochloric acid; Hydrofluoric acid; Hypochlorous acid; Nitric, perchloric and sulfuric acids; and a permanganate selected from potassium permanganate, ammonium permanganate, calcium permanganate, sodium permanganate, and silver permanganate; The electrode according to [7], further comprising treating the electrode in a liquid containing any one of the following for at least 10 minutes. [9] The electrode according to [7], wherein the oxidation treatment includes electrochemical oxidation.

[10] The electrode described in any one of [1] to [9], wherein the solution-accessible electrode region further includes any one of a slot, a depression, and a non-flat surface shape.

[11] The bulk boron-doped diamond away from the solution-accessible electrode region has significantly less sp than the solution-accessible surface. 2 The electrode according to any one of [1] to

[10] above, which contains bonded carbon.

[12] A first electrode, which is the electrode according to any one of [1] to [9]; a second, opposing electrode; a flow path configured to carry a fluid; a drive circuit configured to apply a potential between the electrodes such that a current flows between the electrodes when the fluid passes through the flow path; and An electrochemical cell comprising a sealed enclosure in which the electrodes are disposed, the enclosure configured to contain the fluid within the flow path.

[13] The electrochemical cell according to

[12] , wherein the electrochemical cell is configured to generate ozone during use.

[14] In use, and voltage in the range of 5 to 10 V and 0.01 to maximum 0.05 A / cm 2 Over a range of current densities, at a nominal temperature of 25°C and 240 ml -1 The electrochemical cell according to

[12] or

[13] , wherein the peak ozone current efficiency is greater than 25% when using deionized water (greater than 15 MΩ cm) at a flow rate of 0.1 MPa.

[15] The electrochemical cell according to

[14] , wherein the peak ozone current efficiency is selected from the group consisting of at least 30% and at least 35%.

[16] The electrochemical cell according to any one of

[12] to

[15] , wherein during use, the ozone output gradient does not decrease by more than 10% over a period of at least 5 hours of continuous operation.

[17] The electrochemical cell of

[16] , wherein the 5-hour period is measured after an initial use period of at least 20 hours.

[18] In use, the ozone productivity gradient is 0.01 to 0.05 A / cm under an applied voltage of 5 to 15 V operating in water with a resistivity greater than 15 MΩ·cm at a flow rate of 240 ml / min. 2 Over the current density range of R 2 The electrochemical cell according to any one of

[12] to

[17] above, wherein the measurement is performed by least squares linear regression fitting having a coefficient of determination of >0.95.

[19] A method for forming an electrode, comprising: providing a boron-doped diamond; applying a surface modification process to form a total solution-accessible electrode area comprising at least 60% diamond-stabilized non-diamond carbon; A method comprising:

[20] The method according to

[19] , wherein the surface modification step includes an ablation processing step.

[21] The method according to

[20] , wherein the ablation processing step includes laser ablation.

[22] The laser ablation is performed at a wavelength of 355 to 1064 nm, a pulse length of 10 to 500 ns, a pulse frequency in the range of 50 Hz to 25 MHz, and a passing speed of 0.1 to 100,000 mm / s at a power of 10 J / cm 2 The method according to

[21] above, wherein the method is carried out using a laser having a fluence of more than 1000 Hz.

[23] The method described in

[20] , wherein the ablation machining process includes electrical discharge machining.

[24] The method according to

[23] , wherein the electric discharge machining is carried out using an electrode gap of 0.05 to 3.00 mm, an applied voltage of 5 to 60 V, and a current of 0.1 to 5.0 A, with a pulse time of 10 to 300 μs.

[25] The method according to any one of

[20] to

[24] , wherein the ablation process is further used to form any of slots, depressions, and non-flat surface shapes in the solution-accessible electrode region.

[26] The method according to any one of

[19] to

[24] , further comprising applying an oxidation step to the total solution-accessible electrode area, wherein the oxidation step comprises applying an oxidizing environment to the solution-accessible electrode area.

[27] The oxidation step comprises: Sulfuric acid and potassium nitrate; sulfuric acid and hydrogen peroxide; Nitric acid and hydrochloric acid; Hydrofluoric acid; Hypochlorous acid; Nitric, perchloric and sulfuric acids; and a permanganate selected from potassium permanganate, ammonium permanganate, calcium permanganate, sodium permanganate, and silver permanganate; 27. The method according to claim 26, further comprising treating the cells for at least 10 minutes in a liquid containing any one of the following:

[28] The method according to

[26] , wherein the oxidation step comprises electrochemical oxidation.

[29] The method according to any one of

[19] to

[27] , further comprising applying any one of an electrochemical negative electrode process, an oxygen plasma treatment, and a chemical / electrochemical reduction process to the solution-accessible electrode region.

[30] A method for forming the electrochemical cell according to any one of

[12] to

[18] , Preparing the first electrode according to any one of [1] to [8] above; providing a second opposing electrode; providing a flow path configured to carry a fluid; providing a drive circuit configured to apply a potential between the electrodes such that a current flows between the electrodes when the fluid passes through the flow path; and providing a sealed enclosure configured to contain the fluid within the flow path, the sealed enclosure having the electrode disposed therein; A method comprising:

[31] The method according to

[30] , further comprising a solid electrolyte disposed between the first and second electrodes.

[32] The method according to

[30] , further comprising treating the solid electrolyte by hydrating and protonating the solid electrolyte before use.

[33] The method according to

[31] or

[32] , wherein the solid electrolyte comprises Nafion (trademark).

[34] A method for using an electrochemical cell, comprising: Preparing the electrochemical cell according to any one of

[12] to

[18] above; causing a fluid to flow through the flow path; and applying a potential between the electrodes such that a current flows between the electrodes; A method comprising:

[35] The method of

[34] , further comprising applying the potential between the electrodes via dry electrical contacts so that the only electrochemically active electrode material is the boron-doped diamond electrode.

Claims

1. An electrode for ozone generation formed from boron-doped diamond, comprising at least 60% diamond-stabilized non-diamond sp 2 An electrode for generating ozone having a total solution-accessible electrode area comprising carbon.

2. The total solution accessible electrode area is at least 70% diamond stabilized non-diamond sp 2 The electrode for generating ozone according to claim 1 , comprising carbon.

3. The diamond stabilized non-diamond sp 2 3. The electrode for ozone generation according to claim 1 or 2, wherein the carbon comprises oriented graphite bonded to the diamond surface having an amorphous carbon layer, the graphite being oriented at an angle of greater than 20° to the plane of the total solution-accessible electrode area at the location where it is bonded to the diamond surface.

4. 3. The electrode for ozone generation according to claim 1 or 2, wherein the boron-doped diamond is in the form of a coating layer on a conductive or non-conductive support.

5. 3. The electrode for generating ozone according to claim 1, wherein the solution-accessible electrode area further comprises any of a slot, a depression, and a non-flat surface shape.

6. a first electrode, the first electrode being the electrode of claim 1; a second opposing electrode; a flow path configured to carry a fluid; a drive circuit configured to apply a potential between the electrodes such that a current flows between the electrodes when the fluid passes through the flow path; and An electrochemical cell comprising a sealed enclosure in which the electrodes are disposed, the enclosure configured to contain the fluid within the flow path.

7. 7. The electrochemical cell of claim 6, wherein the electrochemical cell is configured to generate ozone in use.

8. In use, and voltage in the range of 5 to 10 V and 0.01 to maximum 0.05 A / cm 2 at a nominal temperature of 25°C and 240 ml -1 8. The electrochemical cell of claim 6 or 7, wherein the peak ozone current efficiency is greater than 25% using deionized water (greater than 15 MΩ cm) at a flow rate of 9. A method for forming an electrode for generating ozone, comprising: providing a boron-doped diamond; A surface modification process is applied to remove at least 60% of the diamond-stabilized non-diamond sp 2 forming a total solution-accessible electrode area comprising carbon; A method comprising:

10. The method of claim 9 , wherein the surface modification step includes an ablation step.

11. 11. The method of claim 10, wherein the ablation process is further used to form any of slots, depressions, and non-flat surface features in the solution-accessible electrode region.

12. 12. The method of claim 10 or 11, further comprising subjecting the total solution-accessible electrode area to an oxidation process step, the oxidation process comprising applying an oxidizing environment to the solution-accessible electrode area.

13. The oxidation step comprises: Sulfuric acid and potassium nitrate; sulfuric acid and hydrogen peroxide; Nitric acid and hydrochloric acid; Hydrofluoric acid; Hypochlorous acid; Nitric, perchloric and sulfuric acids; and a permanganate selected from potassium permanganate, ammonium permanganate, calcium permanganate, sodium permanganate, and silver permanganate; 13. The method of claim 12, comprising treating the cellulose fiber with a liquid comprising any one of the following for at least 10 minutes.

14. The method of claim 12 , wherein the oxidation step comprises electrochemical oxidation.

15. 1. A method of using an electrochemical cell, comprising: Providing an electrochemical cell according to claim 6; causing a fluid to flow through the flow path; and applying a potential between the electrodes such that a current flows between the electrodes; A method comprising:

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