Best practices using EC‐MS system for HER

The following points are important to consider when designing your HER EC-MS experi-ment(s):

• Utilize non-aqueous chips in alkaline and standard aqueous chips in acid (never use HF).

• Dependent on the catalyst/support, these materials may reduce during cycling, i.e. charge transfers observed by the potentiostat may go towards other reduction pro-cess which the EC-MS may or may not be able to measure. Hence, dependent on the materials and the experiment, full calibration using a calibration gas for exact quantification of faradaic efficiencies, TOFs and even stability insights is strongly rec-ommended. A procedure for calibration is described in EC-MS quantification EC-MS Application Note #2).

• For any materials investigated we urge the users to look up relevant material prop-erties, limitations and consider these in the context of the electrochemical environ-ment, especially the catalyst’s/support’s dissolution potential(s). We recommend EC-MS users to always confer with Pourbaix diagrams [1] for the elemental constituents of the investigated materials.

• Prior to any experiment we recommend users to clean EC-MS components in touch with the electrode/electrolyte following the guidelines described in Cleaning Proce-dures 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 up-take may change electrochemical responses. Note, even trace amounts of Pt (or Pd, Ir etc.) will dominate as the active site for HER. Pt is dissolved by fresh Piranha, but aqua regia is superior at removing these traces.

• Match scan rate and bandwidth filter to ensure minimum feedback waves in the CV is observed, for more info on this important issue see Potentiostat instability EC-MS Technical Note #3.

• Using potentiodynamic methods like LSV and CV is recommended to identify on-set/offset potentials of redox processes. This can be used, for example, to evalu-ate whether these match with what would be expected from Pourbaix diagrams [1]. Note that repeated oxidation/reduction of the catalyst may roughen the catalyst or completely detach/dissolve catalyst material. Also, be aware that significant parasitic current due to reduction processes can be imposed on HER current, when cycling to potentials where the electrode is oxidized.

• For quantitative EC-MS evaluation of HER performance we recommend conducting potentiostatic measurements to minimize the influence of (transient) parasitic pro-cesses in combination with a H₂ gas calibration procedure (see EC-MS quantification EC-MS Application Note #2). Note, that if it’s sought to measure the actual HER ac-tivity of a catalyst, the system should be free of mass transport limitations within the range investigated and the electrolyte should be saturated with H₂ throughout the experiment (i.e. via H₂ make-up gas) to avoid a Nernst shift of the potential scale. Prevent H₂ bubble formation by using currents > −0.3 mA and reduce the ohmic resistance of the system by using sufficiently conductive electrolyte. If necessary, we recommend post-correction for the IR-drop.

• Characterization of the catalyst is recommended both prior and post any prolonged electrochemical experiment, also when using the EC-MS. This can be done ex-situ, or electrochemically e.g. using pseudo stable CVs (at varying scan rates), CO-stripping [2] (see CO-stripping Technique EC-MS Application Note #1) or by integrating known redox features in the CVs.