RESEARCH ACTIVITY

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.

Control of crystallite size in layered MnO₂ materials enables a dual-mode heat storage mechanism: conventional interlayer water intercalation and newly activated surface adsorption. As the nanosheets become thinner, surface interactions with water molecules are enhanced, manifesting as liquid-like or ice-like states depending on structural conditions. This structural refinement significantly improves heat storage performance, particularly below 100°C, offering a pathway for efficient utilization of low-grade thermal energy. ©Hiroki Yoshisako et al.

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