(Invited) Clay: A Ubiquitous Sustainable Material for Energy Applications
Authors: Mukhopadhyay Kausik 
Published: February 2024

Silicates and their precursors make up the majority of the land mass on Earth in the form of inorganic components of soil, silt, and sediment. Clay minerals, essentially hydrous silicates or aluminosilicates, are widely used in a variety of applications and industries, including ceramic goods, cement, drilling fluids, molding sands, anti-corrosion paints, paper manufacturing process, water treatment, polymer-clay composites for automotive industries, and even in medicine and pharmaceutical applications. Lately, it has found use in developing cost-effective ink, drug delivery, food packaging using oxygen barrier properties, and battery separators.
Structure-property features of clays reveal that one can functionalize clays leveraging their porous structures, tunable surface areas, remarkable thermal and mechanical stabilities, and abundant reserves, which make clay one of the cheapest natural materials from a sustainable source and one of the most sought-after materials for a variety of applications. Clays generally have layered structures with the possibility of interchangeable intercalated ions and tunable chemical properties. They are composed of alternating tetrahedral silica (T) and octahedral alumina (O) sheets arranged in a 1:1 ratio (T-O). The tetrahedral sheets are formed from Si4+ ions coordinated with oxygen; however, the Al3+ metal ion is the central atom in the octahedral sheet. Bentonite or Montmorillonite is one of the most used clay minerals owing to its abundance and low cost. Notably, naturally occurring Bentonite has some octahedral sites occupied by the Mg2+ and Fe2+ ions. The presence of these ions allows tuning chemical properties and mechanical strength by functionalizing with organic molecules within the octahedral layers. Over the decades, polymer-based clay composites have been developed using this technique. Herein, we shed light on a new strategy to create hybrid clay films made from small organic molecules with charges at two ends, known as zwitter ions. Using this novel approach, we have developed cost-effective, environmentally benign, and biocompatible clay films with unique physical and chemical properties, where the clay galleries are infused with intercalating zwitter ions. The insertion of these ions increases the interlayer spacing while simultaneously allowing the formation of mechanically robust and thermally and chemically durable clay films. These hybrid clay films can be used as membranes and have been characterized using powder X-ray diffraction, X-ray photoelectron spectra, rheology, ATR-IR spectra, DSC-TGA, dynamic mechanical analyses, and imaging techniques. The effect of varying the carbon chain length and changing functional groups in these zwitter ion species has been verified to significantly impact the clay slurry's rheology and control the clay films' interlayer spacing. Electrochemical impedance spectroscopy studies of the hybrid films confirm that intercalation of zwitter ions in the clay structures leads to improved ionic conductivity of the clays. A series of impedance spectroscopy experiments conducted using various electrolytes reveal remarkable improvements in the ionic conductivity of the hybrid clay films. Contrary to metal ion batteries due to dendritic growth causing thermal degradation of the batteries, the hybrid clay films possess high thermal and mechanical stability that makes them versatile for energy materials. A cost-effective solution to engineering such hybrid films provides an efficient and recycle-free avenue to develop a new generation of materials from sustainable sources for energy applications, especially for battery separators and capacitors for the future.