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The influence of the amine structure (secondary, tertiary, pyridinic) in amine-functionalized polymeric membranes on the mechanism of CO2 transport across the membrane is investigated in this work using operando surface enhanced Raman spectroscopy (SERS) and in-situ transmission FTIR spectroscopy. Specifically, the mechanism of CO2 transport across poly-N-methyl-N-vinylamine (PMVAm), poly-N, N-dimethyl-N-vinylamine (PDVAm), and poly(4-vinylpyridine) (P4VP) membranes was investigated by measuring CO2 permeances/selectivities of the membranes and simultaneously detecting CO2 transport intermediates (e.g., carbamate, bicarbonate) formed in the membrane under operating conditions using SERS and FTIR spectroscopy. While permeation measurements suggest that CO2 moves across all membranes via a facilitated transport mechanism, operando SERS and in-situ FTIR results suggest that the molecular-level details of the facilitated transport process are highly sensitive to the structure of the amine functional group. For membranes with secondary (PMVAm) and tertiary (PDVAm) amines, CO2 moves across the membrane as a mixture of both carbamate and bicarbonate species. For P4VP, which has pyridinic amine groups, no CO2-derived intermediates were detected suggesting a new facilitated transport mechanism involving weak interactions between CO2 and the pyridinic nitrogen group without transformation of CO2 into carbamate, bicarbonate, or other intermediate species. Such a facilitated transport mechanism has not been reported in the literature to our knowledge. 

Magnesium-ion-conducting solid polymer electrolytes have been studied for rechargeable Mg metal batteries, one of the beyond-Li-ion systems. In this paper, magnesium polymer electrolytes with magnesium bis(trifluoromethane)sulfonimide (Mg(TFSI)₂) salt in poly(ε-caprolactone-co-trimethylene carbonate) (PCL-PTMC) were investigated and compared with the poly(ethylene oxide) (PEO) analogs. Both thermal properties and vibrational spectroscopy indicated that the total ion conduction in the PEO electrolytes was dominated by the anion conduction due to strong polymer coordination with fully dissociated Mg²⁺. On the other hand, in PCL-PTMC electrolytes, there is relatively weaker polymer–cation coordination and increased anion–cation coordination. Sporadic Mg- and F-rich particles were observed on the Cu electrodes after polarization tests in Cu|Mg cells with PCL-PTMC electrolyte, suggesting that Mg was conducted in the ion complex form (Mg_xTFSI_y) to the copper working electrode to be reduced which resulted in anion decomposition. However, the Mg metal deposition/stripping was not favorable with either Mg(TFSI)₂in PCL-PTMC or Mg(TFSI)₂in PEO, which inhibited quantitative analysis of magnesium conduction. A remaining challenge is thus to accurately assess transport numbers in these systems.