3.1 Step-by-Step Procedure
- Preparing, polishing, and assembling the RDE Tip. Fixed gold working electrodes are preferred.
1. For standard cleaning, affix a microfiber polishing cloth to a polisher or a flat, stationary surface. Place a small amount of 0.05 μm polishing slurry onto the microfiber cloth. In a figure-8 pattern, polish the gold disk for about 4 min. Rinse the disk thoroughly with deionized water when polishing is completed. Sonicate in deionized water for 3 × 3 minutes, replacing the deionized water between each step to ensure the removal of leftover alumina polishing paste.
2. Repeatedly wipe electrode surface with a water-soaked Kimwipe, followed by a 2-propanol, then water-soaked Kimwipe.
- Preparation and application of the catalyst ink
1. Using a small, clean spatula, measure 3.5 mg of iridium oxide into a clean the vial. Cap the vial to prevent contamination.
2. Using a pipette, dispense a total of 7.6 ml of deionized water into the vial. After replacing the tip, use the pipette to dispense a total of 2.4 ml of 2-propanol into the vial.
3. Ice ink for 5 min before adding ionomer. Use a clean 10–100 μl micropipette to dispense 20 μl of Nafion® dispersion (1100 EW at 5 wt%) into the catalyst ink. Tightly cap the vial. NOTE: Depending on the type of material, the Nafion content may have to be altered for optimal activity.
4. Sonicate the ink for 20 min. Validate dispersion of the ink by observing if there is catalyst settling. One example protocol is the following:
⁃ Sonicate the catalyst ink for 30 s in a horn sonicator, followed by 20 min in a bath sonicator.
⁃ NOTE: Use an ice bath to prevent heating of the ink. Not icing the ink can have a significant impact on the dispersion quality, how the ink dries, and the resulting activity (Alia and Anderson, 2019).
5. Place the RDE tip and rod into the rotator upside down so that the ink can be drop cast on the RDE tip.
6. Once sonicated, use the micropipette to dispense a 10 μl drop of catalyst ink onto the gold disk of the working electrode rotating at 100 rpm. Turn up the rotator slowly from 100 to 700 rpm. Wait until dry (20 min) before turning off the rotation.
⁃ NOTE: To avoid spillage anchor elbows on a stationary surface and ensure that the rotator/shaft is not damaged (rotation on-axis). The drop of catalyst ink should not contain bubbles, should dry on the surface of the gold disk, and should not contact the PTFE shroud. After drying, the coating should be uniform (without a coffee ring). If any of these issues are observed, repolish the electrode and repeat.
- Glassware cleaning. Should be used for all glassware used at any point in RDE preparation or testing.
1. Soak all glassware in concentrated sulfuric acid, then ALNOCHROMIX™ overnight.
2. Immerse the electrochemical cell and components (stoppers, cleaned counter electrode, cleaned reference electrode if hydrogen) in deionized water and boil the water.
3. Remove the water, refill, and repeat the procedure 8 times.
4. Following the conclusion of each test, store glassware in a cleaned beaker filled with deionized water. Periodically reclean beaker/cell.
- 0.1 M Perchloric Acid Electrolyte. Recommend completing in a washdown fume hood.
1. Fill a clean 1-L volumetric flask 2/3 full with deionized water.
2. Weigh 14.39 g of concentrated perchloric acid into a clean beaker and add the perchloric acid into the volumetric flask.
3. Rinse the beaker that contained the perchloric acid multiple times with deionized water and pour the diluted perchloric acid into the volumetric flask until the solution is near to the 1 L mark. Slowly fill the volumetric flask to the 1 L mark.
4. Mix the flask to ensure that the electrolyte solution is homogeneously mixed.
- RDE Cell Setup. For temperature control during testing, jacketed cells could be used if cleanliness can be maintained.
1. Rinse the electrochemical cell at least twice with the prepared perchloric acid electrolyte.
2. Fill the electrochemical cell with a measured volume of electrolyte solution so that the reference electrode, gas bubbler, and counter electrode are all submerged in the electrolyte solution and not in contact with each other. The working electrode should be submerged, but below the electrode/rotator junction to prevent contact corrosion, contamination, and resistance accumulation. If there are any unused ports on the electrochemical cell, cover the ports with cleaned glass. Record volume of solution for effluent analysis purposes.
3. Assemble cell components.
⁃ High surface area counter electrode (mesh spot welded to wire) is preferred to minimize counter contributions to electrochemical measurements, including counter/working plating.
⁃ CAUTION: The high purity gold counter electrode is fragile.
⁃ Gold counter electrodes are preferred since upon dissolution, they will not improve OER activity or add contaminants (Alia and Pivovar, 2018).
4. If using a non-hydrogen reference electrode, calibrate the reference. Hydrogen reference electrodes are preferred to prevent contamination and data inaccuracies due to miscalibration and potential drift during testing (Garsany et al., 2010). For non-hydrogen references, sulfates (mercurous sulfate) are preferred to chloride (silver chloride, calomel) due to individual contaminant effects.
⁃ Use the RDE setup (cleaned glassware, 0.1 M perchloric acid electrolyte) just prior to electrochemical testing.
⁃ Saturate the electrolyte with hydrogen and use a polycrystalline platinum electrode as the working.
⁃ Complete 10 cyclic voltammograms in the potential range −0.1–1 V versus a reversible hydrogen electrode (approximate from past calibrations) at 100 mV s−1, then one cathodic linear sweep voltammogram at 10 mV s−1 in the same potential range.
⁃ Use the hydrogen oxidation/evolution intercept as the reference to hydrogen calibration.
5. Connect an inert gas (nitrogen/argon) cylinder with the gas bubbler. Slowly increase the regulator pressure from the cylinder until gas begins bubbling in the electrochemical cell. The gas should purge through the electrochemical cell for at least 15 min prior to beginning electrochemical measurements. A bubbler/scrubber is also preferred to remove line contaminants prior to the gas entering the electrochemical cell.
6. Using the potentiostat, attach the appropriate cables to the working electrode port, reference electrode, and counter electrode.
- Electrochemical Testing. Electrochemical measurements may begin once the electrochemical cell has been purged with inert gas, the RDE components are immersed in the electrolyte, and the potentiostat is on and connected to the RDE setup.
1. OER activity measurements
⁃ If potentiostat allows, immerse the RDE working electrode at 1.2 V (vs. RHE). Otherwise, immerse under closed or open circuit voltage conditions.
⁃ Condition for 50 cycles, 1.2–1.8 V vs. RHE at 100 mV s−1 and 2,500 rpm.
⁃ Remove the working electrode, rinse with water, and allow to dry to remove bubbles that formed on the electrode surface or within the catalyst layer. Re-immerse the working electrode into the electrolyte for the activity evaluation.
⁃ Reduce the scan rate to 20 mV s−1 to collect activity data. Scan from 1.2 to a variable upper potential limit.
⁃ NOTE: It is preferable to correct for internal resistance (current interrupt) in the program file and not during data analysis to minimize inaccuracies. The upper potential can vary, and the intent is to balance the current range (fixed when correcting for internal resistance) and to capture the entire kinetic region (moderate current range) while minimizing noise (low current range). The potential of comparison (1.5–2 V) may also need to change based on catalyst loading and intrinsic activity. Tafel plots should be consulted to ensure that activities were compared at a potential where the observed current was within the kinetic region and not biased by capacitance or transport.
2. Cyclic voltammogram measurements.
⁃ Potential cycle 0–1.5 V vs. RHE at 100 mV s−1 for five cycles or until the current response becomes stable. Limit excessive cycling at the low potential to minimize the impact of near-surface reduction on OER activity/stability.
⁃ Record voltammograms 0–1.5 V vs. RHE at 20 mV s−1 and 10 mV s−1 for three cycles.
⁃ Complete surface area determinations if equipment and safety considerations allow (Zhao et al., 2015; Alia et al., 2016b).
3. Stability testing. The purpose of this test is to assess the short-term stability of the electrocatalyst (test results should not be used to predict long-term durability). Rotate the working electrode at 2,500 rpm, or the highest rotation speed allowed to limit the impact of transport.
⁃ Stability testing is marginally relevant for device-level testing. Longer-term testing at high potential is preferred if inferring electrolysis-relevant durability. Test the stability of the catalyst at 2 V vs. RHE for 13.5 h (Alia and Anderson, 2019).
⁃ NOTE: Longer durations and high potentials are preferred in RDE durability testing. If these conditions (2 V for 13.5 h) cannot be completed due to poor catalyst stability, short-term stability testing (example 1.55 V for 1 h) can be used to assess whether a catalyst is suitable for device-level performance testing.
⁃ Following the conclusion of the stability test, withdraw the RDE working electrode, rinse it with water, and allow it to dry to remove bubbles that formed on the electrode surface or within the catalyst layer. Pipette 10 ml of the solution into a centrifuge tube for analysis with inductively coupled plasma mass spectrometry (ICP-MS). Replicates and repeated tests are encouraged.
⁃ Discard electrolyte, rinse electrochemical cell (water, then fresh perchloric acid electrolyte), and fill with perchloric acid electrolyte. Re-immerse the working electrode into the electrolyte for the activity evaluation and cyclic voltammograms after the stability test (repeat procedures in steps 1 and 2).
⁃ NOTE: For catalyst stability, report dissolution data (ICP-MS) and OER activity following the potential hold. It is not preferable to report the raw current decrease during the potential hold due to the significant impact of transport on electrode activity over time, which is not a relevant loss mechanism. Potential holds are preferred for stability tests, since potential drives catalyst dissolution; RDE current holds can result in large differences in potential exposure that narrow in device-level testing (catalyst integration and site access differences, use of a membrane); RDE transport losses can increase potential and catalyst dissolution in a way that is not device relevant; and at particularly low current density, RDE may not be reflective of device-level operation or catalyst layer stresses (Alia et al., 2019; Alia, 2021; Alia et al., 2021).