Operando Surface Enhanced Infrared Spectroscopic Investigations of Interfacial Restructuring and Oxygen Electrochemistry in Ionic Liquid Electrolytes for Metal-Air Batteries

Alkali metal-oxygen (M-O 2 ) chemistry has received significant research interest as a promising next-generation battery, but many challenges need addressing before the technology can become viable in practice. Key among these obstacles are the understanding and control of the mechanisms of the oxygen reduction and evolution reactions (ORR and OER), and the tendency for the electrolyte materials to degrade due to attack by reactive intermediates. As such, a greater understanding of the ORR/OER in M-O 2 cells, and how these factors contribute to the poor reversibility on cycling, is required in order to develop stable electrolytes. Attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) is highly sensitive to the interfacial region and can probe sub-monolayer adsorbates and species within ca. 10 nm of the electrode surface, which makes it valuable for studying interfacial processes. Our group demonstrated the application of ATR-SEIRAS to identify ORR products in nonaqueous M-O 2 batteries, wherein a ZnSe prism internal reflection element (IRE) was utilised to extend the measurement range down to ca. 700 cm -1 . 1 While ZnSe offers enhanced optical properties over some conventional IREs, the lower wavenumber cut-off means that the sub-700 cm -1 vibrational frequencies of some superoxide and peroxide species and intermediates generated during ORR/OER cannot be observed. Recently, a micromachined Si wafer IRE has been developed to extend the measurement range of ATR-SEIRAS to well below 700 cm -1 . 2 In this work, we report ATR-SEIRAS investigations into the O 2 electrochemistry and the potential dependent interfacial restructuring of 1-methyl-1-propylpyrrolidinium bis{(trifluoromethyl)sulfonyl}imide ([Pyrr 13 ][TFSI]) utilising a thin-layer Au electrode on the micromachined Si wafer IRE to extend the measurement range towards the far-IR region. While the potential-dependent restructuring of the IL/Au electrode interface has been investigated by ATR-SEIRAS previously, 3 the optical transmission of the Si wafer IRE used here reveals previously unreported changes in the 1000–500 cm -1 spectral region (Figure 1). We also discuss here how the redox couple of dissolved O 2 strongly alters the double-layer restructuring, and the associated activation energy barriers, in the IL electrolyte. The ORR in the IL electrolyte in the presence of Li + or Na + cations is also explored, wherein the identification of sub-700cm -1 (su)peroxy intermediates/products, critical to understanding the overall mechanisms, is made possible using the Si wafer IRE. References J. P. Vivek, N. G. Berry, J. Zou, R. J. Nichols, and L. J. Hardwick, J. Phys. Chem. C , 121 , 19657–19667 (2017). T. A. Morhart, B. Unni, M. J. Lardner, and I. J. Burgess, Anal. Chem. , 89 , 11818–11824 (2017). K. Motobayashi, Y. Shibamura, and K. Ikeda, Electrochem. Commun. , 100 , 117–120 (2019). Figure 1. Operando ATR-SEIRAS measurements of [Pyrr 13 ][TFSI] (at a thin-layer Au electrode) at 0.9 V vs reference spectra at -2.7 V (dark blue and dark red spectra) at a) 1500-900 cm -1 and b) 860-500 cm -1 . The Si wafer IRE (red spectrum) allows detection of [TFSI] - peaks below 1000 cm -1 , which are not visible using the Si prism (light blue spectrum) due to the poor IR transmission in this region. Wafer spectra intensities are multiplied by a factor of 2 and the grey trace is an ex situ ATR-FTIR spectrum of the bulk IL. Figure 1