Efficiently capturing and storing excess heat, particularly below 200°C, is paramount to achieving a carbon-neutral society. Every year, factories and homes produce excess heat, much of which gets wasted. Likewise, as the world gets more reliant on renewable energy sources, the need to capture and store heat grows.

A collaboration between Tohoku University and the Japan Atomic Energy Agency has made significant strides in this regard, developing nanosheets of layered manganese dioxide (MnO₂) that can store heat even below 100°C.

"Our nanosheets operate using a dual-mode heat storage mechanism, where water molecules are simultaneously absorbed (intercalated) and adsorbed from the atmosphere," explains Tohoku University graduate student Hiroki Yoshisako.

Yoshisako led the research group along with Norihiko L. Okamoto and Tetsu Ichitsubo from Tohoku University, along with Kazuya Tanaka from the Japan Atomic Energy Agency.

The layered manganese dioxide operates thanks to intercalation, a reversible process where guest molecules or ions insert themselves into the layered structure of a host material without causing major structural changes. While it was previously known that water molecules enter MnO₂ layers at around 130°C, the team was surprised to learn of a second mechanism - surface adsorption emerging at temperatures below 60°C by breaking down layered manganese dioxide into ultrathin nanosheets.

This dual mechanism increases the total amount of storable water molecules by 1.5 times and enhances the energy storage density by approximately 30% when compared to bulk MnO₂. As a result, the nanosheets can effectively operate at much lower temperatures.

The team also constructed a geometric model that predicts the number of water adsorption sites based on the nanosheet thickness. Analysis revealed that interlayer water exhibits solid-like characteristics, while surface-adsorbed water behaves more like a liquid.

Okamoto stresses that their findings offer a robust design principle for tailoring heat storage performance based on nanoscale structures and will have a positive impact on the future development of thermal management solutions.

"Our breakthrough opens new avenues for next-generation thermal management solutions - ranging from solar heat storage systems for nighttime use to portable low-temperature waste heat recovery devices, and decentralized thermoelectric power generation that can operate regardless of time or location."

Details of the study were published in the journal Communications Chemistry on June 3, 2025.