Current generation via quantum proton transfer

Researchers at the Nagoya Institute of Technology (NITech) in Japan have demonstrated that a specific material can act as an efficient battery component for sodium-ion batteries that will compete with lithium-ion batteries for several battery characteristics, especially speed of charge.

The findings were published in Scientific Reports in November of 2018 and was headed by Naoto Tanibata, Ph.D., an Assistant Professor at the Department of Advanced Ceramics at NITech.

The popular lithium-ion batteries have several benefits - they are rechargeable and have a wide application spectrum. They are used in devices such as laptops and cell phones as well as in hybrid and fully electric cars. The electric vehicle - being a vital technology for fighting pollution in rural areas as well as ushering in clean and sustainable transport - is an important player in the efforts to solve the energy and environmental crises. One downside to lithium is the fact that it is a limited resource. Not only is it expensive, but its annual output is (technically) limited (due to drying process). Given increased demand for battery-powered devices and particularly electric cars, the need to find an alternative to lithium - one that is both cheap as well as abundant - is becoming urgent.

Sodium-ion batteries are an attractive alternative to lithium-based ion batteries due to several reasons. Sodium is not a limited resource - it is abundant in the earth's crust as well as in seawater. Also, sodium-based components have a possibility to yield much faster charging time given the appropriate crystal structure design. However, sodium cannot be simply swapped with lithium used in the current battery materials, as it is a larger ion size and slightly different chemistry. Therefore, researchers are requested to find the best material for sodium ion battery among vast number of candidates by trial-and-error approach.

NIMS and Hokkaido University jointly discovered that proton transfer in electrochemical reactions is governed by the quantum tunneling effect (QTE) under the specific conditions. In addition, they made a first ever observation of the transition between the quantum and classical regimes in electrochemical proton transfer by controlling potential. These results indicated the involvement of QTE in electrochemical proton transfer, a subject of a long-lasting debate, and may accelerate basic research leading to the development of highly efficient electrochemical energy conversion systems based on quantum mechanics.

Many of the state-of-the-art electronic devices and technologies that have realized present modern lives were established based on the fundamental principles of quantum mechanics. Quantum effects in electrochemical reactions in fuel cells and energy devices are, however, not well understood due to the complex movement of electrons and protons driven by electrochemical reaction processes on the surfaces of electrodes. As the result, application of quantum effects in electrochemical energy conversion is not as successful as the fields of electronics and spintronics, which surface and interfacial phenomena are equally critical in all of these fields. Assuming that electrochemical reactions are closely associated with quantum effects, it may be feasible to design highly efficient energy conversion mechanisms based on these effects: including QTE, and devices that take advantage of such mechanisms.

In this study, the NIMS-led research team focused on oxygen reduction reaction (ORR) mechanisms--the key reaction in fuel cells--using deuterium, an isotope of hydrogen having a different mass. As a result, the team confirmed proton tunneling through activation barriers within a small overpotential range. Furthermore, the team found that an increase in overpotential leads to electrochemical reaction pathways to change to proton transfer based on the semiclassical theory. Thus, this research team discovered the novel physical processes: the transition between the quantum and classical regimes in electrochemical reactions.

This research shows the involvement of QTE in proton transfer during the basic energy conversion processes. This discovery may facilitate investigations of microscopic mechanisms of electrochemical reactions which are not understood in detail. It may also stimulate the development of highly efficient electrochemical energy conversion technology with a working principle based on quantum mechanics, capable of operating beyond the classical regime.