POM ‐based Nanomaterials for Supercapacitors

Electrochemical energy-storage systems are essential to support the ongoing green transition. In particular, supercapacitors are of special relevance as high-power density devices capable of delivering energy in a few minutes or seconds. These are composed of electrodes, the device 'soul', that dictates its overall performance. To achieve the highest performance possible, high surface area and conductivity, along with chemical stability of the materials, are of utmost importance. Although mostly used, carbon-based electrodes often operate in toxic organic electrolytes. Therefore, there is a need for new high-power/energy materials for supercapacitor electrodes. Transition-metal-based materials emerged as alternatives because of their operation in aqueous media and redox nature responsible for fast and reversible reactions with superior storage capability. However, due to their high economic and negative environmental impacts new, viable and chemically stable candidates are required. Polyoxometalates (POMs), unique in their multi-metal structure, are capable of fast and continuous multi-electron transfer processes. Owing to their redox chemistry and adjustable structure/composition, POMs are promising metal oxide polyanion clusters to improve the performance in energy-storage devices. Anchoring POMs into conductive substrates was reported a key strategy to ensure chemical stability, improve conductivity and storage capability of electrodes. POMs also offer unique advantages as electrolyte additives for all solid-state supercapacitors, since redox mediator electrolytes can mitigate drawbacks of traditional water/organic-based electrolytes while enhancing devices performance. This chapter aims at a comprehensive description of the electrochemical behaviour of Keggin-type POM structure materials. Brief conclusions and comments on the current challenges and future perspectives will also be addressed.