Probing the Enhancement Mechanism of Catalyst-Assisted Nonaqueous Oxygen Evolution Reaction in Lithium-Oxygen Batteries

Abstract

Introduction Rational catalyst design for Li-O2 recharge (i.e. oxygen evolution reaction, OER) has been hindered by a large large discrepancy on whether solid catalysts are effective in evolving O2 via decomposing solid lithium peroxide (Li2O2). One school of thought reported pronounced catalyst influence on improving the oxidation kinetics of Li2O2.1-3 Another school of thought reported that applying catalysts leads no improvement in the Li2O2-oxidation kinetics due to lacking of soluble species derived from Li2O2-oxidation.4These discrepancies and open questions preclude rational design of efficient catalysts for non-aqueous OER. Results and Discussion By using online electrochemical mass spectrometry (OEMS), we show that solid catalysts (e.g. ruthenium) effectively promote Li2O2-oxidation kinetics to evolve O2 in a solid-state environment.5 We select a widely-investigated solid catalyst, ruthenium (Ru),5, 6 as an example to study whether and how the solid-solid catalysis reaction kinetics was promoted. Figure 1 shows the efficacy of the solid Ru catalyst in solid-state and liquid-based Li-O2 cells using Vulcan carbon (VC) and Ru supported on VC (VC+Ru).5 First, the Ru catalyst reduces the overpotential of the Li-OER over the entire charge capacity (450 mV lower compared to pure VC) with the similar O2 evolution efficiency in liquid-based Li-O2 cell indicating that the solid catalysts could promote the Li2O2 decomposition kinetics. Second, in solid-state environment where no soluble species were available, the Ru catalyst also shows apparent effectiveness with higher OER efficiency (0.872 O2/2e-) compared to that of VC (0.650 O2/2e-).5 Based on these observations, we propose that the role of the solid catalyst for Li2O2-oxidation is to reduce the energy of the reaction intermediate states such as Li2-xO2 by forming a stable intermediate state i.e. Li2-xO2 (solid)-Ru, from which to decrease the overpotential needed for Li-extraction from Li2O2.5Further studies using operando UV-vis spectroscopy, X-ray absorption spectroscopy and X-ray photoelectron spectroscopy will be discussed to examine the proposed enhancement mechanism. Figure caption: Fig. 1. The efficacy of solid catalyst Ru on Li2O2 oxidation in solid-state and liquid-based cell. Galvanostatic voltage profiles of VC and VC+Ru in solid-state cell (a) and liquid-based cell (b), in-situ O2 evolution analysis by OEMS in solid-state cell (c) and liquid-based cell (d).5 Acknowledgements This work is partially supported by a grant from the Research Grants Council (RGC) of the Hong Kong Special Administrative Region (HK SAR), China, under Theme-based Research Scheme through Project No. T23-407/13-N, and partially supported by a RGC project No. CUHK14200615.