Operando NMR electrochemical gating studies of ion dynamics in PEDOT:PSS.

Although organic mixed ionic-electronic conductors are widely proposed for use in bioelectronics, energy generation/storage and neuromorphic computing, our fundamental understanding of the charge-compensating interactions between the ionic and electronic carriers and the dynamics of ions remains poor, particularly for hydrated devices and on electrochemical cycling. Here we show that operando 23 Na and 1 H nuclear magnetic resonance (NMR) spectroscopy can quantify cation and water movement during the doping/dedoping of films comprising the widely used mixed conductor poly(3,4-ethylene dioxythiophene) poly(styrene sulfonate) (PEDOT:PSS). A distinct 23 Na quadrupolar splitting is observed due to the partial ordering of the PSS chains within the PEDOT:PSS-rich domains, with respect to the substrate. Operando 23 Na NMR studies reveal a close-to-linear correlation between the quadrupolar splitting and the charge stored, which is quantitatively explained by a model in which the holes on the PEDOT backbone are bound to the PSS SO 3 - groups; an increase in hole concentration during doping inversely correlates with the number of Na + ions bound to the PSS chains within the PEDOT-rich ordered domains, leading to a decrease in ions within the ordered regions and a decrease in quadrupolar splitting. The Na + -to-electron coupling efficiency, measured via 23 Na NMR intensity changes, is close to 100% when using a 1 M NaCl electrolyte. Operando 1 H NMR spectroscopy confirms that the Na + ions injected into/extracted from the wet films are hydrated. These findings shed light on the working principles of organic mixed conductors and demonstrate the utility of operando NMR spectroscopy in revealing structure-property relationships in electroactive polymers.