We use quantitative analytical methods to determine the rate, selectivity and yield toward products of electrochemical reactions during electrolysis at steady conditions. The timelength of such measurements is typically in the range of 1-2 hours. For longer measurements that may exceed 24 hours, we developed a newly built, fully automated electrolysis station as shown in the scheme and described below. Measurements are mostly performed at constant potential (potentiostatic operation), so the interpretation of data is more straightforward since all fundamental potential-dependent parameters are controlled. Application-relevant constant current (galvanostatic) operation is also possible.
The electrochemical reactor is a home-made divided cell (12 mL volume in each compartment, up to 4.5 cm2 working electrode area) which can be operated with continuous flow of electrolyte. High surface-to-volume ratio is preferred so the electrochemical reactions can have a more drastic impact on the concentration of products in the electrolyte. The reactor is designed to accommodate a wide range of electrodes (metal foils, sputtered thin-films, nanoparticles deposited on a conductive substrate, samples prepared by atomic layer deposition etc). The choices of the counter electrode and the membrane separator are always process-dependent, and our criteria are to avoid contaminations at the working electrode, to avoid the loss of products from the working to the auxiliary compartment etc. The electrolyte effluent from the working compartment is either disposed or recirculated depending on the process. Prior to entering the reactor, the electrolyte passes through a 2 mL vial which is used for sampling with a liquid handler at specific times. The liquid handler is customized to prepare samples in different vials that are compatible with our autosamplers for (i) HPLC (ii) ion chromatography (iii) headspace-GC/MS and (iv) ICP/MS. The liquid handler is equipped with two syringes capable of withdrawing accurately liquid volumes from 10-10 000 μL, thus capable of making additionally dilutions of the samples when appropriate. The liquid handler is additionally customized to allow sample injections directly into crimped headspace vials, to avoid evaporation of volatiles until the analysis with GC/MS. The experiments are always performed with continuous gas supply to ensure controlled conditions in the reactor but also that the formed gases are rapidly removed for analysis. The gaseous outlet is directed to an online GC for quantitative determination of gases.
Therefore, during long-term electrolysis, three functions are taking place at the same time:
- the current is recorded by the potentiostat
- the online GC is characterizing gaseous products periodically
- the liquid handler is collecting samples for offline analysis with HPLC, Headspace-GC/MS, Ion Chromatography or ICP/MS.
During batch operation, i.e. without continuous electrolyte flow, the liquid handler is not used and the electrolyte after the end of electrolysis is used for offline analysis. Our electrolysis station allows the automated operation of the electrochemical reactor while the associated analytical techniques cover typically the entire range of products that have to be analysed, including dissolved metal ions from catalyst degradation.
Details on the instruments used for our electrolysis station can be found here.