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

Nature Materials volume 22, pages 746–753 (2023)Cite this article

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Abstract

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 23Na and 1H 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 23Na 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 23Na 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 SO3− 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 23Na NMR intensity changes, is close to 100% when using a 1 M NaCl electrolyte. Operando 1H 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.

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Fig. 1: Na+ adsorption sites in PEDOT:PSS and ex situ 23Na NMR spectra.
Fig. 2: Operando NMR setup and electrochemistry.
Fig. 3: Operando 23Na NMR spectra as a function of voltage.
Fig. 4: Correlation between doping and 23Na NMR quadrupolar splitting (Δv).
Fig. 5: Coupling between injected charge and Na+-ion concentration.
Fig. 6: Operando 1H NMR of a 0.30 mm PEDOT:PSS film.

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Data availability

The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Code availability

The MATLAB code used for processing the operando dataset is available from the corresponding author on request.

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Acknowledgements

We acknowledge the Cambridge Trust and the China Scholarship Council for funding (D.L. and Y.J.) and thank Z. Liu, Y. Yang, P. B. Groszewicz, D. M. Halat and Y. Zhou for helpful discussions and C. M. Proctor and A. Polyravas for advice concerning the PEDOT:PSS film preparation. D.L., Y.J. and C.P.G. acknowledge funding support from EPSRC under grant no. EP/M009521/1. We thank O. Pecher for assistance with the NMR hardware. S.S. acknowledges support from the CCP for NMR crystallography and EPSRC grant EP/M022501/1.

Author information

Author notes

  1. Pieter C. M. M. Magusin

    Present address: HU University of Applied Sciences Utrecht, Institute for Life Sciences & Chemistry, Utrecht, Netherlands

  2. Evan Wenbo Zhao

    Present address: Magnetic Resonance Research Center, Institute for Molecules and Materials, Faculty of Science, Radboud University Nijmegen, Nijmegen, Netherlands

  3. Shunsuke Yamamoto

    Present address: Graduate School of Engineering, Tohoku University, Sendai, Japan

  4. These authors contributed equally: Dongxun Lyu, Yanting Jin.

Authors and Affiliations

  1. Yusuf Hamied Department of Chemistry, University of Cambridge, Cambridge, UK

    Dongxun Lyu, Yanting Jin, Pieter C. M. M. Magusin, Evan Wenbo Zhao & Clare P. Grey

  2. Science and Technology Facilities Council, Harwell Science and Innovation Campus, Didcot, UK

    Simone Sturniolo

  3. Electrical Engineering Division, Department of Engineering, University of Cambridge, Cambridge, UK

    Shunsuke Yamamoto, Scott T. Keene & George G. Malliaras

  4. Cavendish Laboratory, University of Cambridge, Cambridge, UK

    Scott T. Keene

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Contributions

C.P.G. and G.G.M. conceived the idea and supervised the project. D.L., Y.J. and E.W.Z. planned the experiments. Y.J. and D.L. constructed the operando NMR setup and carried out the electrochemistry measurements. Y.J. and D.L. carried out the pulsed-field-gradient NMR experiments. D.L. and P.C.M.M.M. performed the multiple-quantum magic-angle spinning and variable-temperature NMR measurements. C.P.G. and D.L. built and fit the diffusive averaging model. S.Y. and S.T.K. prepared the samples and S.T.K. performed the vertical electronic transport measurements. S.S. carried out the simulations of 23Na quadrupolar parameters. All the authors discussed the results and contributed to writing the manuscript.

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Correspondence to Clare P. Grey.

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Extended data

Extended Data Fig. 1 Studies of hydration.

Static 23Na NMR spectra of a (0.20 mm thick) PEDOT:PSS polymer film with area 0.3 cm2 as a function of the volume of added aqueous 1 M NaCl solution. Two central transitions (CT) peaks are resolved at low water contents as shown in the inset on the right hand side, a sharp one at 0 ppm and a broader one at -0.4 ppm associated with the two satellite (ST) peaks. We note that in studies of another p-doped polymer, this NaCl concentration was found to minimize water uptake (and thus swelling), while also maximizing ionic conductivity and thus response in the gating experiments41.

Extended Data Fig. 2 Operando NMR experiment of the thick PEDOT:PSS film (1.15 mm).

Operando 23Na NMR spectra of a 1.15 mm thick PEDOT:PSS film as a function of potential. 23Na NMR spectra acquired during voltage holds of 0, -0.6 and 0.6 V (voltage profile shown on the right-hand side). Each spectrum takes about 7 s to acquire, while the voltage is held constant for 40 minutes at each step. A thick film (1.15 mm) was used to maximize the S/N ratio. Films of this thickness have also been used in inkjet-printed PEDOT:PSS films for organic neuromorphic transitors42.

Extended Data Fig. 3 Fits to the operando 23Na NMR spectra of the thick film.

a, Charge and 23Na NMR quadrupolar splitting (Δv) versus time during sequential voltage steps for the (1.15 mm) thick films. In a), the voltage was stepped sequentially to -0.6 V (red shading; resulting in a total charge stored of -0.044 C), + 0.6 V (blue shading;  + 0.055 C) and then 0 V (red shading), with voltage profiles shown in Extended Data Fig. 2. b, Correlation between the charge stored and quadrupolar splitting (Δv) for the thick films on doping and dedoping. The experimental data in a) were fit as described in the SI to extract the time constants associated with ion migration and parasitic currents.

Supplementary information

Supplementary Information

Supplementary Figs. 1–13, Sections 1–7, Equations (1)–(17) and Table 1.

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Lyu, D., Jin, Y., Magusin, P.C.M.M. et al. Operando NMR electrochemical gating studies of ion dynamics in PEDOT:PSS. Nat. Mater. 22, 746–753 (2023). https://doi.org/10.1038/s41563-023-01524-1

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