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Best practices for OER experiments
EC‐MS Technical Note #16
Introduction
The EC-MS Application Note #4 gives a thorough introduction on the benefits of using the EC-MS for OER studies. In this document, we list some best practices recommended to obtain the best possible results with your Spectro Inlets EC-MS.
Experiment preparation
• For any materials investigated it is crucial for the user to ensure that the tested ma-terial can sustain the electrochemical environment, e.g. the catalyst material and/or support may dissolve under OER conditions. Note, catalyst oxidation (which may also change conductivity) is included in the total measured current by the poten-tiostat, while the MS is insensitive to such effects. We recommend EC-MS users to confer with Pourbaix diagrams [1] for all the elemental constituents of the investigated materials.
• Prior any experiment it is recommend for users to clean the EC-MS cell according to Cleaning Procedures EC-MS Technical Note #12. Always store electrolytes in clean glassware/PFA bottles and refrain from using alkaline electrolytes more than a day old, as CO2 uptake from the ambient air may change electrochemical responses.
• Characterization of the catalyst is recommended to be done both prior and after any prolonged electrochemical experiment. This can be done most easily electrochemi-cally (e.g. by CO-stripping [2], see CO-stripping Technique EC-MS Application Note #1), or from the double-layer capacitance. For the latter, we encourage users to record initial and final pseudo stable CVs (at varying scan rates) as such EC-MS data provide essential information of change in activity and relative catalyst area.
EC-MS specific considerations
• Use standard chips in acid and neutral conditions, and non-aqueous chips in alkaline electrolyte (do not use highly concentrated alkaline electrolyte or HF).
• Note, for alkaline OER using carbon-based catalyst/supports, any CO2 formed will re-act with the electrolyte producing (bi)carbonates, which vapour pressure is too high for detection by the EC-MS. Hence, full quantification by O2 calibration is recom-mended for such studies.
• Match scan rate and bandwidth filter ensuring minimum feedback waves in the CV is observed, for more info on this important issue see Potentiostat instability EC-MS Technical Note #3.
• Use CVs to identify redox processes onset/offset potentials and see if insights match Poubaix diagrams [1] of catalyst/support constituents. This provides the user with insights on material transitions (and stability). CVs taken at various scan rates can be used for area evaluation. Note, repeated oxidation/reduction of the catalyst may roughen the catalyst or completely detach/dissolve electrode materials.
• For quantitative EC-MS evaluation of OER performance (see EC-MS quantification EC-MS Application Note #2) conducting potentiostatic measurements (with evalua-tion of series resistance) is recommended, as doing so eases and improve accuracy of the charge integration. Repeated redox events from cycling/stepping may increase dissolution. We recommend stepping potentials using (using the EC-lab ”SPEIS” tech-nique). This approach renders, unless too large currents are accessed (< 0.3 mA) or too poorly conducting electrolyte is used, reproducible results. We rely on post-correction for the IR-compensation of the potential, consequently reproducing exact potential set-points is difficult.
References
[1] K. A. Persson, B. Waldwick, P. Lazic, and G. Ceder, “Prediction of solid-aqueous equilib-ria: Scheme to combine first-principles calculations of solids with experimental aqueous states,” Phys. Rev. B, vol. 85, p. 235 438, 23 Jun. 2012. DOI: 10.1103/PhysRevB.85. 235438.
[2] A. López-Cudero, A. Cuesta, and C. Gutiérrez, “Potential dependence of the saturation CO coverage of Pt electrodes: The origin of the pre-peak in CO-stripping voltammo-grams. part 1: Pt (111),” Journal of Electroanalytical Chemistry, vol. 579, no. 1, pp. 1–12, 2005. DOI: 10.1016/j.jelechem.2005.01.018.