Viologens: a versatile organic molecule for energy storage applications

Organic redox compounds illustrate to be a fascinating class of active materials in energy-storage applications. The structural diversity as well as molecular tailoring helps in fine-tuning of the electrochemical properties at molecular level which is highly desired for improvisation of performance. In reality it is very challenging to achieve the desired physico chemical properties. Viologens represent a unique class of redox active molecules that undergoes two one-electron reductions in steps. Its redox properties are highly explored in various fields such as electrocatalysis, electrochromism, photochromism, herbicides, as well as to some extent in energy-storage systems. The redox behavior as well as its cationic charges are highly useful in energy-storage systems in improvising the device performance. Different energy systems require different state of viologens, i.e., dicationic/monocation radical/redox couple. For ex. viologen dication shows electrostatic attractions towards polysulfides in Li S batteries, similarly dication radical prevents Li dendrite growth in Li-ion batteries. In the case of redox flow batteries, supercapacitors as well as in Li-air batteries, the redox behavior of viologens were useful in improving the performance. Despite these advantages, there are some challenges existing in viologen based systems which has to be conquered at the molecular level. In this regard, a systematic study on structure-activity relationship becomes a crucial issue to address the bottlenecks. This review analyses the role of viologens in various energy storage systems and the mode of chemical interactions responsible for enhancing the overall performance of electrochemical storage devices comprehensively.