Electroactive nanofibrillated cellulose aerogel composites with tunable structural and electrochemical properties

Abstract

This work presents conductive aerogel composites of nanofibrillated cellulose (NFC) and polypyrrole (PPy) with tunable structural and electrochemical properties. The conductive composites are prepared by chemically polymerizing pyrrole onto TEMPO-oxidized cellulose nanofibers dispersed in water and the various nanostructures are obtained employing different drying methods. Supercritical CO₂ drying is shown to generate high porosity aerogel composites with the largest surface area (246 m² g⁻¹) reported so far for a conducting polymer–paper based material, whereas composites produced by ambient drying attain high density structures with mechanical properties significantly surpassing earlier reported values for cellulose–conducting polymer composites when normalized with respect to the content of reinforcing cellulose (Young's modulus = 0.51 GPa, tensile strength = 10.93 MPa and strain to failure = 2.5%). Electrochemical measurements clearly show that differences in the porosity give rise to dramatic changes in the voltammetric and chronoamperometric behavior of the composites. This indicates that mass transport rate limitations also should be considered, in addition to the presence of a distribution of PPy redox potentials, as an explanation for the shapes of the voltammetric peaks. A specific charge capacity of ∼220 C g⁻¹ is obtained for all composites in voltammetric experiments performed at a scan rate of 1 mV s⁻¹ and this capacity is retained also at scan rates up to 50 mV s⁻¹ for the high porosity composites. The composites should be applicable as electrodes in structural batteries and as membranes in ion exchange applications requiring exchange membranes of high mechanical integrity or high porosity.