Renewable energy has the ultimate capacity to
resolve the environmental and scarcity challenges of the world’s energy
supplies. However, both the utility of these sources and the economics of their
implementation are strongly limited by their intermittent nature; inexpensive
means of energy storage therefore needs to be part of the design. Distributed
thermal energy storage is surprisingly underdeveloped in this context, in part
due to the lack of advanced storage materials. Here, we describe a novel family
of thermal energy storage materials based on pyrazolium cation, that operate in
the 100-220°C temperature range, offering safe, inexpensive capacity, opening
new pathways for high efficiency collection and storage of both solar-thermal
energy, as well as excess wind power. We probe the molecular origins of the
high thermal energy storage capacity of these ionic materials and demonstrate
extended cycling that provides a basis for further scale up and development.