Jennifer Hack

This perspective discusses the current status and future needs for the use of X-ray and neutron imaging as complementary methods for helping solve key challenges facing electrolyser materials.

Water management plays a key role in ensuring optimum polymer electrolyte fuel cell (PEFC) performance, and flow field design can influence the ability of a cell to balance maintaining hydration, whilst avoiding flooding and cell failure. This work deepens the understanding of water evolution in different PEFC flow channel designs, namely single serpentine (SS), double serpentine (DS) and parallel, using our novel high-speed neutron computed tomography method. We developed our previously-reported method by introducing continuous cell rotation, enabling 18 s per tomogram during 1 h holds at 300, 400 and 500 mA cm⁻². The volume of water evolved in the cathode, membrane electrode assembly and anode was quantified, and key mechanisms for water droplet formation in the different flow channel designs were elucidated. The parallel flow field design had the poorest water management, with 47% of the cathode flow channel becoming filled after 1 h at 400 mA cm⁻². This significant flooding blocked reactant sites and contributed to unstable cell performance and, ultimately, cell failure at higher current densities. The SS cell displayed the best water management, with only 11% of the cathode channel filled with water after 1 h at 500 mA cm⁻², compared with 28% of the DS cathode channel. 3D visualisation and analysis of droplet behaviour elucidated how water ‘slugs’ in the SS were removed in the gas stream, whereas three of the four parallel cathode flow channels became entirely filled with water plugs, blocking gas flow and exacerbating cell flooding. The new insights gained here are expected to extend to novel flow field designs and image-based models, with the use of operando neutron CT demonstrated as a powerful technique for both visualising and quantifying water management in operating PEFCs, as well as deepening the knowledge of droplet behaviour in different flow field types.

With the urgent need to move away from fossil fuels towards more sustainable technologies, alternative battery technologies are needed to alleviate the existing supply and safety concerns associated with Li-ion batteries¹. Metal-air batteries are one such alternative that hold promise thanks to their high theoretical specific energies². Whilst zinc-air batteries have one of the lowest theoretical specific energies (~1350 Wh kg^-1 for zinc-air versus 5200 Wh kg^-1 for Li-air²), zinc is an abundant element and there are few safety concerns associated with this type of battery. Furthermore, the theoretical specific energy of a zinc-air cell is still around three times higher than Li-ion (upwards of 270 Wh kg^-1)³. Currently, primary zinc-air cells are commercialised for application in hearing aids, but the commercialisation of secondary cells is still limited by poor cycle lifetimes and degradation of the anode⁴. Thus, alongside the need to develop better bifunctional catalysts, improvements need to be made to the anode to avoid dendrite formation and improve the reversibility between zinc and zinc oxide (ZnO) during cycling; there is significant volume expansion and morphology change during reaction from zinc to ZnO. In order to visualise the changes occurring in zinc-air battery electrodes, X-ray computed tomography (CT) has been used in a number of studies^5-7. It is non-destructive and internal features of the sample can be visualised and measured without the need to open the sample. This allows for time-resolved quantification of changes occurring to the various phases within zinc-air cells, like zinc or ZnO. In this work, we explore the morphologies of different anode electrode types, including foils, free-standing electrodes and slurries, to highlight the various existing anode design concepts for secondary zinc-air cells. Imaging methods, including X-ray CT and scanning electron microscopy (SEM), are used to visualise the structure of these electrodes, and quantitative analysis allows for the morphology to be correlated to their electrochemical performance. We discuss the advantages and disadvantages of the different electrode types and, finally, share our perspectives on the future of anode design for zinc-air cells. References ¹Y. Hau, X. Liu, S. Zhou, Y. Huang, H. Ling and S. Yang, Resour. Conserv. Recycl., 2021, 168, 105249. ²M. A. Rahman, X. Wang and C. Wen, J. Electrochem. Soc., 2013, 160, A1759-A1771. ³J. -S. Lee, S. Tai Kim, R. Cao, N. -S. Choi, M. Liu, K. T. Lee and J. Cho, Adv. Energy Mater., 2011, 1, 34-50. ⁴J. Zhang, Q. Zhou, Y. Tang, L. Zhang and Y. Li, Chem. Sci., 2019, 10, 8924-8929. ⁵T. Arlt, D. Schröder, U. Krewer and I. Manke, Phys. Chem. Chem. Phys, 2014, 16, 22273-22280. ⁶J. Hack, D. Patel, J. J. Bailey, F. Iacoviello, P. R. Shearing and D. J. L. Brett, J. Phys: Mater., https://doi.org/10.1088/2515-7639/ac3f9a. ⁷B. Bozzini, C. Mele, A. Veneziano, N. Sodini, G. Lanzafame, A. Taurino and L. Mancini, ACS Appl. Energy Mater., 2020, 3, 4931-4940. Figure 1

Zinc–air batteries have gained significant attention as safe battery alternatives, with high theoretical energy densities and a high abundance of their constituent materials. However, barriers to their widespread adoption include the need to improve their cycling lifetime, as well as stability and avoiding degradation mechanisms such as zinc dendrite growth and hydrogen-producing side reactions. X-ray computed tomography (CT) is a widely used technique for the study of batteries. In situ / operando x-ray CT has been increasingly used to study the zinc anode of zinc–air batteries to evaluate the interesting morphological changes occurring during the reaction from zinc (Zn) to zinc oxide (ZnO) during discharge (vice versa during charge). However, several studies have been carried out using synchrotron x-ray sources, which have limited availability for users. In this work, we present a comprehensive study of the discharge of commercial, primary zinc–air batteries using a laboratory-based x-ray source for in situ x-ray CT measurements. Four different discharge rates are investigated (C/30, C/60, C/90 and C/150), with tomograms collected at various stages throughout each discharge. Results confirm that with decreasing C-rate (i.e. decreasing discharge current) a greater volume of zinc is reacted, with average mass utilisations of 17%, 76%, 81% and 87% for C/30, C/60, C/90 and C/150, respectively. Furthermore, quantification using x-ray CT datasets showed that there is a direct correlation between the volume of zinc remaining in the cell and the state-of-charge of the cell, which deviated from linearity for the longer C-rates. Finally, a potential new mechanism for shape change is discussed, where a Zn particle is replaced with a pore of a similar volume. As well as improvements in statistical relevance gained from multiple repeats for each C-rate, the results presented here could be used in both modelling of battery performance, as well as consideration for future anode design concepts.

The relative vast abundance and more equitable global distribution of terrestrial sodium makes sodium-ion batteries (NIBs) potentially cheaper and more sustainable alternatives to commercial lithium-ion batteries (LIBs). However, the practical capacities and cycle lives of NIBs at present do not match those of LIBs and have therefore hindered their progress to commercialisation. The present drawback of NIB technology stems largely from the electrode materials and their associated Na⁺ ion storage mechanisms. Increased understanding of the electrochemical storage mechanisms and kinetics is therefore vital for the development of current and novel materials to realise the commercial NIB. In contrast to x-ray techniques, the non-dependency of neutron scattering on the atomic number of elements (Z) can substantially increase the scattering contrast of small elements such as sodium and carbon, making neutron techniques powerful for the investigation of NIB electrode materials. Moreover, neutrons are far more penetrating which enables more complex sample environments including in situ and operando studies. Here, we introduce the theory of, and review the use of, neutron diffraction and quasi-elastic neutron scattering, to investigate the structural and dynamic properties of electrode and electrolyte materials for NIBs. To improve our understanding of the actual sodium storage mechanisms and identify intermediate stages during charge/discharge, ex situ, in situ, and operando neutron experiments are required. However, to date there are few studies where operando experiments are conducted during electrochemical cycling. This highlights an opportunity for research to elucidate the operating mechanisms within NIB materials that are under much debate at present.

Polymer electrolyte fuel cells (PEFCs) are a promising replacement for the fossil fuel–dependent automotive and energy sectors. They have become increasingly commercialized in the last decade; however, significant limitations on durability and performance limit their commercial uptake. Catalyst layer (CL) design is commonly reported to impact device power density and durability; although, a consensus is rarely reached due to differences in testing conditions, experimental design, and types of data reported. This is further exacerbated by aspects of CL design such as catalyst support, proton conduction, catalyst, fabrication, and morphology, being significantly interdependent; hence, a wider appreciation is required in order to optimize performance, improve durability, and reduce costs. Here, the cutting‐edge research within the field of PEFCs is reviewed, investigating the effect of different manufacturing techniques, electrolyte distribution, support materials, surface chemistries, and total porosity on power density and durability. These are critically appraised from an applied perspective to inform the most relevant and promising pathways to make and test commercially viable cells. This holistic view of the competing aspects of CL design and preparation will facilitate the development of optimized CLs, especially the incorporation of novel catalyst support materials.