Direct quantification of ion composition and mobility in organic mixed ionic-electronic conductors

Our paper “Direct quantification of ion composition and mobility in organic mixed ionic-electronic conductors” was published in Science Advances.

Understanding ion behavior is important for OMIECs. While past methods were mostly indirect, our team introduced operando X-ray fluorescence (XRF) spectroscopy to directly monitor ion composition and mobility during OMIEC operation.

EG mixed PEDOT:PSS was used as the model system in this study. Thick films showed a similar composition to our previous ex situ results (ACS Appl. Mater. Interfaces 2023, 15, 25, 30553–30566), with only cations detected and their concentration modulating by 5-10% between dedoped and doped states. Notably, operando composition results may differ from ex situ data, and thin films revealed the presence of anions, highlighting the interfacial effect on film composition.

We also made new discoveries about ion transport: 1) The cation transport in the first cycle can be separated into an initial rapid electrowetting and a slower proton exchange stage; 2) operando XRF results show consistency with previous optical moving front results in 20% EG-PEDOT:PSS, but in 5% EG-PEDOT:PSS, the XRF moving front lags behind the optical moving front; the difference might be due to residue proton transport; 3) exploring different thicknesses in 5% and 20% EG-PEDOT:PSS, we observed faster moving fronts with thinner films; plotting ion mobility against inverse thickness offering insights into bulk mobility (intercept) and interfacial effects (slope).

This was a great team effort led by grad student Ruiheng Wu, with mentorship from Jonathan Rivnay and postdoc Xudong Ji. We also thank for the help from our previous members Bryan Paulsen and Josh Tropp, and to our Argonne collaborators Qing Ma.