AIMR develops new battery with low environmental impact made from magnesium and paper — Expected use in wearable devices

Professor Hiroshi Yabu of the Advanced Institute for Materials Research (AIMR) at Tohoku University, Senior Research Scientist Shimpei Ono of the Central Research Institute of Electric Power Industry, AZUL Energy Inc. (a venture company originating from Tohoku University), and British startup AMPHICO, developed a magnesium-air battery (a type of fuel cell and metal-air battery) using a unique and safe electrode catalyst and paper as a base. In other words, the research group realized a high-performance "metal-air paper battery" that uses almost no heavy metals or plastics that have a high environmental impact. The battery exhibits a voltage of 1.8 V, an output of >100 mW/cm², and a capacity of 968.2 Wh/kilogram (Mg), which is sufficient to be activated by saltwater for driving a wearable device. In addition, the research group demonstrated that it can be used as a power source of a wearable SpO₂ meter that monitors a decrease in the blood oxygen level and a smart life jacket equipped with a GPS sensor that allows to locate a person in need of rescue in the event of drowning. The information was published in the online news edition of RSC Applied Interfaces and was selected for the cover image.

Metal-air batteries, which use easily soluble metals such as zinc and magnesium as the anode instead of hydrogen, are a type of fuel cell and are expected to be a next-generation battery with a mass energy density three times higher than that of lithium-ion batteries (LIBs). Low-environmental-impact metal-air paper batteries that are activated by electrolytes and generate electricity by forming a cathode on the surface of paper and placing a zinc anode on the opposite side have been reported in the literature. However, they require a toxic alkaline electrolyte, and even with salt water, the output was at the microwatt per square centimeter level, still far from being able to drive practical devices.

Three factors that affect the performance of metal-air paper batteries are: the efficiency of the oxygen reduction reaction (ORR) of the cathode; the use of a metal anode that allows higher voltage between the cathode and anode; and the reduction of the battery cell resistance. So far, the research group has developed an AZUL (AZaphthalocyanine Unimolecular Layer) catalyst, a high-performance ORR catalyst with metallic azaphthalocyanine supported on carbon and has developed a high-performance metal-air battery without using precious and heavy metals such as platinum and manganese oxide which are commonly used as ORR catalysts.

The research group created a metal-air-paper battery by coating paper, such as filter paper, with a cathode catalyst and using magnesium, which has a high voltage and low environmental impact, as the anode and sandwiching it between current collectors. The current-voltage characteristics and current-output performance of the cells were evaluated using salt water as the electrolyte. Results showed that the cells with higher paper density had a higher ability to absorb salt water through capillary force, but lower output due to lower electrolyte retention and higher resistance. Furthermore, while paper with a lower density took longer to absorb salt water, the cell output was higher. Therefore, optimizing paper density is important for improving the performance of metal-air paper batteries. The optimized cell achieved an open circuit voltage of 1.8 V, an output of 103 mW/cm², and a capacity of 968.2 Wh/kilogram (Mg). By mixing carbon nanofibers (CNF) as a conductive assistant to the cathode, it was possible to create a metal-air paper battery that could be connected directly to a device without the use of a current collector.

Aiming for the social implementation of metal-air paper batteries, the research group conducted demonstration tests on applications of wearable SpO₂ meters and smart life jackets with GPS. AZUL Energy designed and fabricated a novel wearable SpO₂ meter and conducted a demonstration test to confirm whether it could be powered by the metal-air paper battery. Remote monitoring of the oxygen level in living organisms was successful. There is also growing demand for smart life jackets equipped with GPS sensors to identify the location of rescuers in the event of drowning during commercial fishing or water-based recreation activities. This metal-air paper battery uses salt water as an activator to generate electricity, so it functions as a sensor and power source when submerged in water. A GPS sensor designed and fabricated by AZUL Energy was incorporated into a highly water-repellent life jacket developed by AMPHICO. When the power supply using the metal-air paper battery as the power source was submerged in salt water, it was demonstrated that the signal from the GPS sensor could be used to locate the position on Google Earth. In addition to high performance, the battery is made of ecofriendly materials, such as magnesium which is abundant in soil and seawater, safe catalysts, paper, and carbon, so it has a low environmental impact and is safe for disposal. The battery is expected to be used in various wearable devices and emergency power sources.

Journal Information
Publication: RSC Applied Interfaces
Title: Rare-metal-free high-performance water-activated paper battery: a disposable energy source for wearable sensing devices
DOI: 10.1039/d4lf00039k