Sodium-oxygen batteries exhibit several advantages compared with their lithium-oxygen analogues [1]. However, conflicting findings regarding the precise mechanism of oxygen reduction in the presence of sodium ions have been reported [2-3]. A key parameter determining the reaction pathway is the solvent used to dissolve the sodium electrolyte [4]. In addition, studies have usually been carried out using polycrystalline electrode surfaces [5]. In order to examine the part played by surface structure in sodium-oxygen electrochemistry, for the first time, a method of preparing clean, well-defined Pt{hkl} electrodes for adsorption studies in aprotic solvents is described. The oxygen reduction reaction (ORR) in NaClO₄ containing dimethyl sulfoxide (DMSO) as the electrolyte highlights the structural sensitivity of the ORR under these conditions. Using cyclic voltammetric and in situ spectroscopic characterisation methods (surface-enhanced Raman spectroscopy (SERS) and shell-isolated nanoparticles for enhanced Raman spectroscopy (SHINERS) the various stages of oxygen reduction as a function of i) potential, ii) sodium ion concentration and iii) sweep rate are investigated. It is found that on Pt{111} and Pt{110}-(1x1) terraces, reversible and sharp electrosorption/desorption peaks are observed which we ascribe to a long lived surface sodium peroxide intermediate formed at potentials negative of the initial sodium superoxide surface phase. In contrast, on Pt{100} and polycrystalline platinum electrodes, this sodium peroxide species appears to be unstable and either disproportionates or dissolves into the bulk electrolyte. On all surfaces studied, the initial reduction pathway corresponds to the formation of a surface sodium superoxide phase. In the absence of sodium ions, superoxide formation takes place exclusively followed by superoxide formation into the bulk of the electrolyte with no evidence of peroxide formation.

Furthermore the use of electrochemical attenuated total reflection surface enhanced infrared spectroscopy (ATR-SEIRAS), for a direct detection of the intermediates and reaction products formed during the oxygen reduction process is presented [6, 7]. Compared to conventional infrared spectroscopy techniques, ATR-SEIRAS is particularly attractive for its strong sensitivity and selectivity to the interfacial region. Here we report on the detection of metastable, solvated, and surface adsorbed alkali metal–oxygen (M–O₂) discharge species [7]. Oxygen–oxygen stretching bands (ν_O–O) of superoxide species formed during Na–O₂ battery discharge have been challenging to observe by conventional infrared (IR) techniques, and because of this, there has been limited use of IR techniques for in situ monitoring of the discharge products at the cathode in Na–O₂ (and Li-O₂) batteries. In situ IR spectroscopy studies, together with a coupled-cluster method including perturbative triple excitations [CCSD(T)] calculations, establishes that certain M–O and O–O stretching bands (ν_M–O and ν_O–O) of metal superoxide and peroxide molecular species are IR active, although these vibrational modes are silent or suppressed in their crystalline forms. An in situ IR spectroscopy based approach to distinguish between “solution mediated” and “surface confined” discharge pathways in non-aqueous M–O₂ batteries is demonstrated.

References

[1] P. Hartmann, C. Bender, M. Vračar, A. Dűrr, A. Garsuch, J. Janek and P. Adelhelm, Nat. Mater., 12 (2012) 228

[2] C. Bender, D. Schröder, R. Pinedo, P. Adelhelm and J. Janek, Angew. Chem.-Int. Ed.., 55, (2016) 4640

[3] P. Hartmann, M. Heinemann, C. Bender, K. Graf, R. Baumann, P. Adelhelm, C. Heiliger and J. Janek, J. Phys. Chem. C, 119, (2015) 22778

[4] I.M. Aldous, L.J. Hardwick, Angew. Chem. Int. Ed., 55, (2016) 8254

[5] S. Ma, W. McKee, J. Wang, L. Guo, M. Jansen, Y. Xu, Z. Peng, Phys. Chem. Chem. Phys., 19, (2017) 12375

[6] J.P. Vivek, N. Berry, G .Papageorgiou, R. Nichols, L.J. Hardwick, J. Amer. Chem. Soc. 138 (2016) 3745

[7] J.P. Vivek, N.G. Berry, J. Zou, R. J. Nichols, L. J. Hardwick, J. Phys. Chem. C, 121 (2017) 19657

Figure: Cyclic Voltammetry of 0.1 M NaClO₄ in DMSO (saturated with O₂) on different platinum single crystal facets and polycrystalline platinum, Pt(poly), Pt(111), Pt(110), Pt(100).