The K-KO₂ alkali metal-oxygen cell chemistry exhibits a very high theoretical specific energy of 935 Wh/kg¹_. The free energy of formation of KO₂ is -239.4 KJ mol^-1 (compared to -570.8 KJ mol^-1 for Li₂O₂ in a Li-O₂ cell) and further, it is thermodynamically stable. However, these cells have been reported to exhibit a steep drop off in capacity following the first cycle¹ with the reported state-of-the-art specific energy being 100 Wh/kg_KO2². Reasons for the steep decline in capacity include: 1) lack of selectivity at the cathode (peroxide/carbonate/hydroxide formation in addition to the desired superoxide) resulting in lower reversibility¹, and 2) reactions with the electrolyte solvent (ether/carbonate) to form undesirable byproducts (which then form insulating layers on the K anode³). The mitigation of the first issue requires a fundamental understanding of the oxygen reduction reaction (ORR) at the cathode of these cells. Herein, a rotating ring-disk electrode (RRDE) investigation of the K-O₂ ORR is presented. The voltage window for the desired one electron reduction from K to KO₂ was identified and a gap of 0.3V was measured between this reaction and the subsequent second electron transfer to produce K₂O₂, in line with the 0.28V difference in their E⁰ values. Subsequent cyclic voltammograms (CV) and linear sweep voltammograms (LSV) were performed within the KO₂ voltage window (-0.9V to -1.3V vs. Ag/Ag⁺). Calculations of the number of electrons in this window using the Nicholson-Shain and Koutecky-Levich relationships on the CVs and LSVs respectively confirmed the formation of KO₂ as the product in a one electron process. Nevertheless, changes in the i_D/i_R plots at more cathodic potentials, with the plots displaying typical one step and multi-step behaviors⁴ respectively at different potential regimes, indicated that the mechanism involved more than the single electron transfer step. It was hypothesized that given favorable binding orientation on the surface, chemical disproportionation of KO₂ occurs to form K₂O₂. The RRDE data was fitted to kinetic models incorporating one -step and multi-step reactions in the appropriate voltage regions and the various elementary reaction rate constants were calculated. The rate constant for the first electron transfer reaction to form KO₂ was found to be similar in both models, with a higher desorption rate for the KO₂ at lower potentials (closer to the limiting region). The formation rate of K₂O₂ is predicted to increase closer to the limiting region as the surface coverage of KO₂ increases. Thus, the chemical disproportionation reaction to form K₂O₂ is identified as yet another confounding factor in the practical realization of K-O₂ cells and partially explains the observations regarding the inverse correlation between charging/discharging rate and the capacity of the cells².

References:

(1) Ren, X.; Wu, Y. J. Am. Chem. Soc. 2013, 135 (8), 2923.

(2) Xiao, N.; Ren, X.; He, M.; McCulloch, W. D.; Wu, Y. ACS Appl. Mater. Interfaces 2017, 9 (5), 4301.

(3) Ren, X.; Lau, K. C.; Yu, M.; Bi, X.; Kreidler, E.; Curtiss, L. A.; Wu, Y. ACS Appl. Mater. Interfaces 2014, 6 (21), 19299.

(4) Damjanovic, A.; Genshaw, M. A.; Bockris, J. O. J. Chem. Phys. 1966, 45 (11), 4057.