A collaborative research effort between Tohoku University and the Japan Atomic Energy Agency has yielded a significant advancement in thermal energy storage, developing layered manganese dioxide (MnO2) nanosheets capable of high-density heat storage at temperatures below 100°C. This innovation directly addresses the critical challenge of efficiently capturing and storing low-grade excess heat, a major contributor to energy waste in industrial and residential sectors globally.

The newly engineered MnO2 nanosheets operate on a novel dual-mode heat storage mechanism, as explained by Tohoku University graduate student Hiroki Yoshisako, who led the research. "Our nanosheets operate using a dual-mode heat storage mechanism, where water molecules are simultaneously absorbed (intercalated) and adsorbed from the atmosphere," Yoshisako stated. This dual functionality distinguishes the material from conventional approaches.

While the intercalation of water molecules into MnO2 layers at around 130°C was previously understood, the team discovered a second, unexpected mechanism: surface adsorption emerging at temperatures below 60°C. This was achieved by breaking down bulk layered manganese dioxide into ultrathin nanosheets, enhancing surface interactions with water molecules. This dual mechanism dramatically improves performance, increasing the total amount of storable water molecules by 1.5 times and boosting the energy storage density by approximately 30% compared to bulk MnO2, enabling effective operation at significantly lower temperatures.

The research group, which included Norihiko L. Okamoto and Tetsu Ichitsubo from Tohoku University and Kazuya Tanaka from the Japan Atomic Energy Agency, also constructed a geometric model to predict water adsorption sites based on nanosheet thickness. Their analysis revealed distinct behaviors for interlayer water, exhibiting solid-like characteristics, and surface-adsorbed water, which behaves more like a liquid.

Norihiko L. Okamoto emphasized the broader implications of their findings, stating, "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." This robust design principle, tailoring heat storage performance based on nanoscale structures, promises to have a profound impact on future thermal management solutions, critical for advancing a carbon-neutral society and enhancing the reliability of renewable energy systems. The detailed findings of this study were published in the journal Communications Chemistry on June 3, 2025.