286 | Energy Environ. Sci., 2017, 10, 286--295 This journal is © The Royal Society of Chemistry 2017 Cite this: Energy Environ. Sci., 2017, 10, 286 A bifunctional solid state catalyst with enhanced cycling stability for Na and Li–O 2 cells: revealing the role of solid state catalysts† Hossein Yadegari,‡ a Mohammad Norouzi Banis,‡ ab Andrew Lushington, a Qian Sun, a Ruying Li, a Tsun-Kong Sham c and Xueliang Sun* a Solid state catalysts play a critical role in peroxide alkali metal–O 2 cells. However, the underlying mechanism behind the catalytic activity remains controversial due to the different nature of oxygen reduction and evolutions reactions (ORR, OER) in non-aqueous cells compared to those in classic aqueous based reactions. In the present study, we reveal a detailed spectroscopic and electrochemical picture of the mechanism of catalytic activity in Na– and Li–O 2 cells. We demonstrate that ORR and OER catalytic activity in alkali metal– O 2 cells primarily originates from the stabilization of O 2 À intermediates on the catalyst surface during the electrochemical reaction. Monitoring the electronic state of the solid state catalyst during the ORR and OER revealed a dynamic interaction occurring between the catalyst and the discharge product. The morphology and composition of discharge products is also illustrated to be influenced by solid state catalysts. The findings of the present study suggest that catalysts with a higher oxygen-bonding capability may exhibit a higher catalytic activity in alkali metal–O 2 cells. Broader context Alkali metal–oxygen (Li– and Na–O 2 ) batteries have attracted a great deal of attention over the past decade. The high theoretical energy density of these battery systems which is comparable with that of gasoline makes them desirable candidates for potential applications in electrical transportation. However, multiple basic challenges associated with the working mechanisms of alkali metal–oxygen cells limit their cycle life and hinder them from further development. The large overpotential required for charging the cells with a peroxide discharge product is among the major challenges facing the alkali metal–oxygen batteries. An extensive amount of effort has been devoted to develop and employ solid-state catalysts in order to reduce the charging overpotential and improve the cycling stability of the cells. Nevertheless, a little is known about the mechanism of the catalytic activity in these cells which makes it a controversial topic in the field. The present study reveals detailed spectroscopic evidence toward the working mechanisms of solid-state catalysts in alkali metal–oxygen cells. The obtained results suggest a correlation between the ability of the catalyst surface for stabilizing superoxide (O 2 À ) intermediates with the catalytic activity. Introduction Alkali metal (Li and Na)–O 2 cells are considered as the next generation of electrochemical energy storage technology with potential applications for electrical transportation. 1–5 The high energy density produced by alkali metal–O 2 cells is based on coupling a high energy alkali metal (negative electrode) with a breathing oxygen electrode (positive electrode). The resulting reaction, known as an oxygen reduction reaction (ORR), produces superoxide (O 2 À ) ions which combine with alkali metal ions, from the negative electrode, to form a solid metal oxide complex as the discharge product. 4,6 The superoxide then further reduces (chemically or electrochemically) to peroxide (O 2 2À ) in Li–O 2 cells and produces lithium peroxide (Li 2 O 2 ) as the major discharge product of the cell. 7 In the case of a Na–O 2 cell, however, the larger radii of the sodium ion stabilizes the superoxide intermediate and result in the formation of either sodium superoxide (NaO 2 ) or peroxide (Na 2 O 2 ) as the product of the cell. 8–13 The formed solid discharge products of the cells are then forced to decompose back to molecular oxygen and alkali metal during the charge cycle. However, the oxygen evolution reaction (OER) in the air electrode of alkali metal–O 2 peroxide cells requires a large overpotential, thereby significantly reducing the energy efficiency of the cell. 1–3,5 a Department of Mechanical and Materials Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada. E-mail: xsun@eng.uwo.ca b Canadian Light Source, Saskatoon S7N 2V3, Canada c Department of Chemistry, University of Western Ontario, London, Ontario N6A 5B7, Canada † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c6ee03132c ‡ Hossein Yadegari and Mohammad Norouzi Banis have equivalent contribu- tions to this work. Received 25th October 2016, Accepted 2nd December 2016 DOI: 10.1039/c6ee03132c www.rsc.org/ees Energy & Environmental Science PAPER Published on 02 December 2016. Downloaded on 19/01/2017 01:56:37. View Article Online View Journal | View Issue