Layer-by-Layer Assembly of Polyaniline Nanofibers and MXene Thin-Film Electrodes for Electrochemical Energy Storage.

The growing demand for compact energy storage devices may be met through the use of thin-film microbatteries, which generally rely on charge storage in thin or conformal layers. A promising technique for creating thin-film electrodes is layer-by-layer (LbL) assembly, based on the alternating adsorption of oppositely charged species to a surface to form a nanostructured electrode. Thin-film energy storage devices must have a high energy density within a limited space, so new electrode structures, materials, and assembly methods are important. To this end, both two-dimensional MXenes and polyaniline nanofibers (PNFs) have shown promising energy storage properties. Here, we report on the LbL assembly of positively charged PNFs and negatively charged TiCT MXenes into hybrid electrodes for thin-film energy storage devices. The successful assembly is demonstrated in which MXenes and PNFs are deposited in films of 49 nm/layer pair thickness. The resulting composition was 77 wt % PNFs and 23 wt % MXenes. The charge storage process was deconvoluted into faradaic/non-faradaic contributions and separated into contributions from PNFs and MXenes. A sandwich cell showed a maximum areal capacity, energy, and power of 17.6 μA h cm, 22.1 μW h cm, and 1.5 mW cm, respectively, for PNF/MXene multilayers of about 2 μm thickness. This work suggests the possibility of using LbL PNF/MXene thin films as electrode materials for thin-film energy storage devices used in next-generation small electronics.