Application of Acoustic Emission to Study Electrochemical Energy Conversion and Storage Devices
Authors: Maier Maximilian , Tan Chun , Castanheira Luis , Hinds Gareth , Shearing Paul R. , Brett Daniel J.L. 
Published: February 2020

Electrochemical energy conversion and storage devices find wide application in industry, transportation, and consumer goods. Due to the varying requirements of these different applications, there is no one size fits all solution and technologies of interest span from electrolyser and fuel cell technology to batteries such as Li-ion and Li-S. The future of the ‘Hydrogen Economy’ is contingent on establishing renewable sources of hydrogen, and electrolysers are a key technology to fulfil this goal. Polymer electrolyte membrane (PEM) technology, widely adopted in fuel cells, is of special significance to electrolysers due to numerous advantages over alternatives such as alkaline electrolysers; developments in PEM fuel cell technology can be rapidly applied to water electrolysers due to similarities in materials and electrode architecture. Lithium-based batteries are a complementary energy storage technology, used in numerous fields such as electronics, medical devices, and the automotive industry.

During the operation of these energy conversion and storage devices, complex chemical and mechanical processes occur, each with specific significance to lifetime, performance, and safety. In electrolysers, two-phase flow occurs in the flow channels and gas diffusion layers and electrochemical activity occurs at the catalyst layer. In Li-ion batteries, the intercalation of Li ions into negative and positive electrode, cracking of electrode materials, the formation of gas bubbles due to electrolyte decomposition and solid electrolyte interphase (SEI) formation are processes of great importance. Various approaches have been described in order to monitor and investigate these processes, e.g. X-ray computed tomography, scanning electron microscopy and electrochemical methods. However, diagnostic tools often come with specific drawbacks: high costs, no possibility for in-situ measurements or destructive measurements.

These issues call for novel diagnostic techniques, which allows for fast, low-cost, in-situ and non-destructive measurements. This work describes the use of acoustic emission (AE) as a diagnostic technique for electrochemical energy storage devices. In AE, a piezoelectric sensor is used to detect mechanical perturbations from a sample. By comparing the amount of acoustic activity, its duration, energy, and frequency spectrum, conclusions can be drawn regarding the nature of the observed process.

In this work, we have successfully demonstrated the potential of acoustic emission as a diagnostic tool for PEM water electrolysers; in particular to study two-phase flow within the flow channels during gas formation. A close link between bubble size and number within the flow channel of a PEM water electrolyser and the AE measured on the surface of the end plates has been established. Two locations along the flow channel were investigated through AE measurements and high-speed visual imaging. For the upper location, AE measurements showed that the number of acoustic hits reaches a maximum around 0.5 A cm2 and decreases before reaching a plateau at higher current densities. The average frequency of these hits decreases with current density, which can be attributed to an increase in average bubble size. High-speed imaging at the upper location is consistent with the acoustic emission results. The number of bubbles detected follows the same trend as the number of acoustic hits with current density. This implies that AE can monitor relative changes in the number of bubbles flowing through an electrolyser and can, therefore, detect changes in flow regime. Image analysis also showed a trend towards increasing bubble size at higher current densities, which supports the frequency data from acoustic emission; hence, AE can be used to detect changes in bubble size in a PEM electrolyser.

Furthermore, AE was used to monitor processes in Li-ion and Li-S batteries during charge and discharge. In Li-ion cells, strong acoustic activity was found between 3.7 and 4.0 V during the first cycles, a voltage range which is related to electrolyte decomposition and gas evolution. An increased number of AE hits has also been found during SEI formation. In Li-S batteries, AE events were demonstrated to coincide with the chemical transformation between different polysulphide species.