Fundamental Aspects of Oxygen Reduction in Non-Aqueous Electrolytes: Formulation of a General Mechanism. Sanjeev Mukerjee, K. M. Abraham, C. Allen, and M. Trahan Department of Chemistry and Chemical Biology Northeastern University Center for Renewable Energy Technology Northeastern University, 360 Huntington Avenue, Boston, MA 02115, USA   The theoretical specific energy of the Li-air battery is 5280 Wh/kg which approaches the energy density limit of electrochemical energy conversion and storage systems. However, such a high energy density can be harnessed in a practical battery only if a four-electron reduction of O2 to form Li2O can be reversibly achieved. The primary discharge product of the Li-air battery has been identified to be Li2O2 (1-4) involving an overall two-electron O2 reduction which makes the theoretical specific energy 3500 Wh/kg. It is also widely recognized that a cathode catalyst is needed to recharge the battery resulting in the search for oxygen evolution catalysts (1, 5-8) as an active area of research. Contrary to this traditional belief we have recently demonstrated that the Li-air battery can be recharged without a catalyst in tetraethylene glycol (TEGDME)-based electrolytes, albeit with larger charge voltage polarizations than with a cathode catalyst (4). We have also shown (2, 3) unambiguously that the electrolyte influences the mechanism of the ORR in non-aqueous electrolytes with both the conducting salt cation and the solvent influencing the relative stabilities of the reduction products and their rechargeability. That study has led to the realization that Li+-conducting electrolytes in DMSO, a solvent with very high Lewis basicity as measured by Gutmann donor number of 29.8, is a good media to fully elucidate ORR and OER mechanisms via stabilization of the electrochemical reaction products and by facilitation of the reverse processes. We have found that the first ORR product almost always is superoxide, O2-(1), which forms very stable complexes with soft Lewis acid cations like tetrabutyl ammonium (TBA+). The stability of this TBA+― O2-(1) complex is so high that it resists further reduction of the dioxygen entity resulting in a reversible one-electron O2/O2-(1) redox couple. When the DMSO electrolyte contains Li+-conducting ions, the Li+― O2-(1) initially formed can be further reduced to Li2O2 and finally to Li2O, realizing the four electron reduction of O2. Furthermore, all of these ORR products can be oxidized albeit with different levels of kinetic irreversibility. Oxygen reduction and evolution reactions (ORR and OER) have also been studied in ionic liquids containing singly charged cations having a range of ionic radii, or charge densities. A strong correlation was found between the ORR products and the ionic charge density including those of the ionic liquids. The observed trend is explained in terms of the Lewis acidity of the cation present in the electrolyte using an acidity scale created from 13C NMR chemical shifts and spin lattice relaxation (T1) times of 13 C=O in solutions of these charged ions in propylene carbonate (PC). The ionic liquids lie in a continuum of cascading Lewis acidity scale with respect to the charge density of alkali metal, IL and TBA cations with the result that the ORR products in ionic liquids and in organic electrolytes containing any conducting cations can be predicted on the basis of a general theory based on the Hard Soft Acid Base (HSAB) concept. References: 1. K. M. Abraham and Z. Jiang, J. Electrochem. Soc., 1996, 143, 1-5. 2. C. O. Laoire, S. Mukerjee, K. M. Abraham, E. J. Plichta and M. A. Hendrickson, J. Phys. Chem. C, 2009, 113, 20127-20134. 3. C. O. Laoire, S. Mukerjee, K. M. Abraham, E. J. Plichta and M. A. Hendrickson, J. Phys. Chem. C, 114, 9178-9186. 4. C. O. Laoire, S. Mukerjee, E. J. Plichta, M. A. Hendrickson and K. M. Abraham, J. Electrochem. Soc., 158, A302-A308. 5. A. Debart, J. Paterson Allan, J. Bao and G. Bruce Peter, Angewandte Chemie, 2008, 47, 4521-4524. 6. Y.-C. Lu, Z. Xu, H. A. Gasteiger, S. Chen, K. Hamad-Schifferli and Y. Shao-Horn, J. Am. Chem. Soc., ACS ASAP. 7. J. Xiao, W. Xu, D. Wang and J.-G. Zhang, J. Electrochem. Soc., 157, A294-A297. 8. J. Read, J. Electrochem. Soc., 2002, 149, A1190-A1195. < volver