Morphological tuning of membrane processing by temporal proton-metal cation substitution in perfluorosulfonic acid membranes

K+ Transport in Perfluorosulfonic Acid Membranes and Its Influence on Membrane Resistance in CO2 Electrolysis

“…Although K + is still soaked into the membrane (against the direction of electrodiffusion), no metal ions are transported from the anode side. These observations are well in line with our earlier results, proving quantitative potential‐driven substitution (PDS) in situ for Li + ions in PFSA [25] . The different protonation levels at cathode (low protonation due to K + drag into the membrane) and anode (high protonation due to OER) are also reflected in the Nyquist plot shape: In a MEA setup, differentiation between anode and cathode side resistance (expressed by two separated half‐cycles) is often impossible due to a very close neighborhood and intimate interactions of the whole system [37] .…”

“…The spontaneous interaction of K + with the sulfonic acid groups is highly exothermic, [25] which is expressed by the observed temperature increase.…”

“…For electrolysis applications, where the use of a liquid, aqueous salt based electrolyte is unavoidable, there is a clear preference to use monovalent cations: It is known that the thermal stability of the PFSA polymer is increased when alkaline metals are incorporated in the membrane whereas threefold positively charged cations (Fe 3+ and Al 3+ ) induce membrane degradation [24] . Moreover, we showed in an earlier study that Li + ions incorporated in PFSA can be driven out of the membrane quantitatively by applying an electrochemical potential [25] …”

“…These observations are well in line with our earlier results, proving quantitative potential-driven substitution (PDS) in situ for Li + ions in PFSA. [25] The different protonation levels at cathode (low protonation due to K + drag into the membrane) and anode (high protonation due to OER) are also reflected in the Nyquist plot shape: In a MEA setup, differentiation between anode and cathode side resistance (expressed by two separated half-cycles) is often impossible due to a very close neighborhood and intimate interactions of the whole system. [37] Due to the differences in protonation, the electrodes become slightly differentiable and therefore, the Nyquist plot shows a shoulder, which can be traced back to the anode (Figure 6b, arrow).…”

“…[24] Moreover, we showed in an earlier study that Li + ions incorporated in PFSA can be driven out of the membrane quantitatively by applying an electrochemical potential. [25] In this study, we work with a water electrolysis setup and employ it as a model system for CO2RR applications. Our model will help to differentiate between resistance contributions, especially stemming from the membrane in different operation modes, that are not accessible in a CO 2 electrolysis setup.…”