Operando NMR Visualization of Ion Dynamics in PEDOT:PSS

Abstract

While organic mixed ionic/electronic conductors are widely studied for various applications in bioelectronics, energy generation/storage, and neuromorphic computing, a fundamental understanding of the interactions between the ionic and electronic carriers remains unclear, particularly in the wet state and on electrochemical cycling. Here, we show that operando NMR spectroscopy can selectively probe and quantify ion and water movement during the doping/dedoping of poly(3,4-ethylene dioxythiophene) poly(styrene sulfonate) (PEDOT:PSS) films, the most widely used organic mixed conductor. Na+ ions near or within the PEDOT-rich domains experience an anisotropic environment resulting from the underlying partial PSS chain orientation in the polymer films, giving rise to a distinct quadrupolar splitting in the 23Na NMR spectrum. Operando 23Na NMR studies reveal a linear correlation between the quadrupolar splitting and the charge stored in the film, which is interpreted in terms of the roles that the Na+ ions at the PEDOT/PSS interfaces play in charge balance and electric double layer formation. The observed correlation is quantitatively explained by a competitive binding model, in which holes on the PEDOT backbone are bound to PSS, the hole concentration changes during doping/dedoping inducing variations in the Na+ binding percentage at the PEDOT/PSS interfaces. The Na+-to-electron coupling efficiency, measured via 23Na NMR intensity changes, varies noticeably depending on the cycling history of the film. Operando 1H NMR spectroscopy confirms that water molecules accompany the ions that are injected into/extracted from the films. 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.

Operando Characterization of Organic Mixed Ionic/Electronic Conducting Materials

“…(b) Operando NMR setup for OMIECs. Adapted from ref under CC BY 4.0 license. (c) In situ ssNMR determines cation coordinating with glycol chain and its transport rate in Li salt and PEO blends.…”

“…Copyright 2020 American Chemical Society. (d) Operando NMR differentiates the free water in electrolyte and the bound water in PEDOT:PSS and tracks the solvent uptake kinetics; Adapted from ref under CC BY 4.0 license. (e) PFG-NMR gives diffusion coefficient of ions in graphene.…”

“…Distinguishing elements from different environments allows one to track the transport of these elements in OMIECs. Jin et al has reported operando NMR in electrolyte gated PEDOT:PSS (setup shown in Figure b) and reveal the ion/solvent injection/extraction during electrochemical cycling . Using PSS-Na as well as aqueous NaCl solutions as standards, Na + in PEDOT:PSS in PEDOT-rich domains and PSS-rich domains were identified separately by Na-23 NMR.…”

“…By changing the magnetic field gradient, the diffusion coefficient D is readily fitted from the curve between normalized NMR peak intensity and b factor, shown in eq : where the b factor is a function related to the strength magnetic field gradient, the measured nuclei, and the pulse pattern, defined by eq : Here, g is the strength of the magnetic field gradient, γ is the nucleus gyromagnetic ratio, δ is the duration of an individual magnetic field gradient pulse, Δ is the observation time, and τ is the delay time between pulses . As for now, in situ PFG-NMR has been already reported to determine self-diffusion coefficient of Na + in PEDOT:PSS films (no external potential) and is consistent with previous results measured by moving front experiments …”

“…EPR has been applied to the in situ and operando detection of OMIEC materials as a method to directly detect electron spins, which is described in section 5.1. NMR has recently been reported to be applied to OMIEC materials 248 but has not yet been widely applied due to challenges in experimental implementation. In section 5.2, we will focus on the information that in situ/ operando NMR has the potential to bring to the field.…”