Hydrogen iodide energy cycle to repeat solar hydrogen generation and battery power generation using single‐walled carbon nanotubes

We propose a new energy cycle called the “HI cycle” that involves the repeated generation of solar hydrogen and battery power. Solar hydrogen generation using an HI solution allows for the use of a narrower band gap photocatalyst compared to water. We demonstrated that the addition of single‐walled carbon nanotubes (SWCNTs) effectively enhances solar hydrogen generation from an HI solution with methylammonium lead iodide (MAPbI3). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations, along with energy‐dispersive X‐ray spectroscopy (EDS) analysis and Raman measurements, revealed that SWCNTs improve hydrogen generation by adsorbing by‐product iodine molecules. We also fabricated a zinc‐iodine battery using the paper form of I@SWCNTs recovered from the photocatalyst test cell and zinc metal. We demonstrated that the battery efficiently operated with an initial cell voltage of approximately 1.2 V. The battery’s capacity, corresponding to the amount of encapsulated iodine molecules, indicated that SWCNTs can effectively adsorb the by‐product iodine molecules within the photocatalyst test cell. We also discussed that the electrolyte solution after the discharge experiment should include not only iodide ions but also a significant amount of hydrogen ions, indicating that the solution after battery discharge returns to the starting point of the “HI cycle.” Raman measurements revealed that I@SWCNTs, formed during the solar hydrogen generation experiment, were transformed back into empty tubes during the discharge experiment. Therefore, SWCNTs can be repeatedly used in the new cyclic energy scheme referred to as the “HI cycle.”This article is protected by copyright. All rights reserved.