Influence of temperature on the electrochemical window of boron doped diamond - a comparison of commercially available electrodes
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-09-24 |
| Journal | Scientific Reports |
| Authors | Maeve McLaughlin, Emma Dunlop, Alexander C. PakpourâTabrizi, DĂ©bora C. Faria, Richard B. Jackman |
| Institutions | Schlumberger (United Kingdom), University College London |
| Citations | 18 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis study investigates the stability and determination methods of the Electrochemical Window (ECW) of heavily Boron Doped Diamond (BDD) electrodes under high-temperature (HT) and high-pressure (HP) conditions, crucial for applications in extreme environments.
- Temperature Dependence: The ECW of all tested BDD electrodes (polished/unpolished, H/O terminated) narrows significantly as the temperature increases from 21 °C to 125 °C, confirming that the electrolyte redox reactions are thermally activated.
- Surface Roughness Dominance: Unpolished BDD electrodes consistently exhibited the widest ECW compared to polished electrodes. This is attributed to the lower proportion of sp2 carbon content on the rougher surface, which minimizes unwanted electrocatalytic activity.
- Termination Impact: The type of surface termination (Hydrogen, BDDH, or Oxygen, BDDO) resulted in smaller changes to the ECW and activation energy compared to the effect of surface roughness (polished vs. unpolished).
- Methodology Standard: The Linear Fit method (intersection of linear fits of the CV curve) is proposed as a more reliable standard for comparing ECW values across different literature reports, as it is less sensitive to mass transport and electrolyte concentration effects than the arbitrary Jcut-off method.
- Operational Stability: Experiments were successfully conducted at 5 bar pressure to suppress electrolyte boiling and bubble formation, ensuring stable CV measurements up to 125 °C.
- Engineering Relevance: The results provide critical data for selecting appropriate BDD electrode types (unpolished offers widest ECW/most versatility) for HT/HP electroanalytical applications where the detection range of ions is limited by the narrowed ECW.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Temperature Range (CV) | 21 to 125 | °C | Electrochemical measurements |
| Operating Pressure | 5 | bar | Used to prevent bubble formation |
| Boron Doping Concentration ([B]) | > 1020 | atoms/cm3 | Electrochemical grade BDD |
| Unpolished Roughness (RA) | ~50 | ”m | Polycrystalline BDD (pBDD) |
| Polished Roughness (RA) | ~50 | nm | Polycrystalline BDD (pBDD) |
| Electrolyte | 1 | M | Phosphate buffer (pH 7) |
| Scan Rate (CV) | 0.1, 0.5, 1.0 | V/s | Used for Cyclic Voltammetry |
| ECW (Unpolished BDDH, 21 °C) | 3.2 | V | Linear Fit Method |
| ECW (Unpolished BDDH, 125 °C) | 2.2 | V | Linear Fit Method |
| ECW Narrowing (Max observed) | 0.88 | V | Difference between Jcut-off 0.5 mA/cm2 and 5.0 mA/cm2 at 21 °C |
| Activation Energy (Unpolished BDDO) | 7.41 x 107 ± 0.23 x 107 | eV | Highest reported activation energy for oxidation potential |
| Contact Angle (Polished BDDH, after polishing) | 55 ± 1.5 | ° | Indicates damaged H-termination (more hydrophilic) |
| Contact Angle (Polished BDDO, after polishing) | 29 ± 1.0 | ° | Indicates predominantly O-termination (hydrophilic) |
| Diamond Raman Peak | 1332 | cm-1 | Characteristic sp3 diamond carbon |
| Non-Diamond Raman Peak | 1575 | cm-1 | Characteristic sp2 carbon (G peak) |
Key Methodologies
Section titled âKey MethodologiesâThe experimental procedure involved precise substrate preparation, termination, electrode construction, and high-temperature electrochemical testing:
-
Substrate Preparation:
- BDD substrates (10x10x0.5 mm, > 1020 B atoms/cm3) were laser cut into 3 mm diameter disks.
- Acid Cleaning: Substrates were cleaned in a highly oxidizing solution (ammonium persulfate + concentrated sulfuric acid) heated to 200 °C for 10 minutes to remove adventitious carbon.
-
Surface Termination:
- Hydrogen (BDDH): Performed in an AX5010 Seki Technotron reactor using H-plasma for 10 minutes at 400 °C platen temperature, 700 W power, and 35 Torr pressure.
- Oxygen (BDDO): Performed via ozone treatment in an Ozone Cleaner NL-UV253 for 1 hour, under 10-6 mbar vacuum, with ozone generation at 10 g/h.
-
Electrode Construction:
- 3 mm BDD pieces were metallized (Ti-Pt-Au) and soldered to Be-Cu pins.
- The assembly was sealed into a PEEK bulkhead using Loctite Hysol 9483 epoxy (rated to 150 °C), ensuring the epoxy did not leak onto the active diamond surface.
-
Electrochemical Testing (CV):
- Setup: The PEEK bulkhead was placed in a PEEK flow cell sealed with Viton o-rings, housed in a steel box for heat transfer.
- Conditions: Electrolyte (1 M phosphate buffer, pH 7) was injected via a syringe pump system (Syrris Asia pump) through a heating coil inside an oven. The system was maintained at 5 bar pressure.
- Measurement: CV staircase scans were performed between -1.9 V and 1.5 V at scan rates of 0.1, 0.5, and 1 V/s across the temperature range (21 °C to 125 °C).
-
ECW Determination Methods:
- Jcut-off Method: ECW defined by the potential range between points where the current density reaches a predefined arbitrary value (tested at 0.5, 1.0, and 5.0 mA/cm2).
- Linear Fit Method: ECW defined by the intersections of linear fits applied to the three sections of the CV curve (pre-oxidation, pre-reduction, and background).
Commercial Applications
Section titled âCommercial ApplicationsâThe findings regarding BDD stability and ECW determination under extreme conditions are highly relevant for several high-value engineering sectors:
- Oil and Gas Exploration (HPHT Environments): BDD electrodes are suitable for downhole sensing applications, such as monitoring corrosion, detecting heavy metals (like mercury), or analyzing fluid composition in oil wells where temperatures can exceed 150 °C and pressures are high.
- High-Temperature Electroanalysis: Use in industrial processes or reactors requiring electrochemical monitoring or synthesis at elevated temperatures, where conventional electrodes fail due to thermal degradation or narrow ECW.
- Water Treatment and Remediation: BDD is widely used for advanced oxidation processes (AOPs). Understanding the ECW narrowing at high temperatures is crucial for optimizing the efficiency and selectivity of pollutant degradation in heated industrial wastewater streams.
- Sensor Development: Fabrication of robust, wide-potential-range sensors for harsh chemical environments, enabling the detection of multiple redox species simultaneously across a wide temperature range.
- Battery and Energy Storage Research: Characterization of electrolyte stability limits in non-aqueous or ionic liquid systems intended for high-temperature operation, where precise ECW determination is critical for safety and performance.
View Original Abstract
Abstract This work compares the electrochemical windows of polished and unpolished boron doped diamond (BDD) electrodes with hydrogen and oxygen terminations at a series of temperatures up to 125 °C. The experiment was run at 5 bar pressure to avoid complications due to bubble formation. An alternative method for determining the electrochemical window is compared to the most commonly used method, which defines the window at an arbitrary current density cut-off (J cut-off ) value. This arbitrary method is heavily influenced by the mass transport of the electrolyte and cannot be used to compare electrodes across literature where different J cut-off values have been used. A linear fit method is described which is less affected by the experimental conditions in a given measurement system. This enables a more accurate comparison of the relative electrochemical window from various diamond electrode types from reported results. Through comparison of polished and unpolished BDD electrodes, with hydrogen and oxygen surface terminations, it is determined that the electrochemical window of BDD electrodes narrows as temperature increases; activation energies are reported.