Probing the Working Mechanism of Electrocatalyst-Assisted Nonaqueous Lithium-Oxygen Evolution Reaction

Introduction

   Nonaqueous lithium-oxygen (Li-O₂) catalysis plays critical roles in rechargeable lithium-air (or Li-O₂) batteries. Reducing the energy loss associated with Li-O₂ recharge (i.e. oxygen evolution reaction, OER) is one of the most important objectives of Li-O₂ research to date.^1-2 There are two schools of thought for catalyst effect on the OER. First, McCloskey et al.³ have reported that no enhanced OER kinetics were found in the presence of catalysts. On the other hand, a number of studies have reported that the application of catalysts can effectively decrease the Li-O₂ charge potential compared to the uncatalyzed carbon.^4-5 The large discrepancy reflects the lack of fundamental understanding on the OER mechanism and nonaqueous OER catalysis.

In this study, we exploit comprehensive analytical techniques to investigate if and how electrocatalysts promote reaction kinetics upon recharge.

Results and Discussion

We select a widely-investigated catalyst, ruthenium (Ru),^5-6 as an example to study if and how electrocatalyst promotes OER kinetics. We first investigate if the apparent efficacy of catalyst depends on the discharge capacity, charge rate and the solvent used. Figure 1 shows the galvanostatic voltage profiles of Li-O₂ cells using Vulcan carbon (VC) and Ru/C with controlled discharge capacity and charge rate in 1,2-dimethoxyethane (DME) and dimethyl sulfoxide (DMSO). First, the reduced overpotential associated with Ru obtained at low charge rate (ΔE = 200 mV at 20 mA/g_c) is smaller than that achieved under higher charge rate (ΔE = 500 mV at 100 mA/g_c). Second, the effectiveness of the catalyst is insensitive to the discharge capacity. Third, the reduced overpotential obtained in DMSO (ΔE = 800 mV) is significantly greater than that achieved using in DME (ΔE = 500 mV). These observations support our hypothesis that the apparent efficacy of the electrocatalyst is influenced by operating conditions and electrolyte. Figure 2 shows the potentiostatic intermittent titration technique (PITT) result of pre-discharged VC and Ru/C Li-O₂ cells in DME. The result suggests that the application of Ru catalyst reduces the time needed for nucleation⁷ (35 min) at a much lower potential (3.2 V_Li) than the uncatalyzed carbon (> 200 min) at a higher potential (3.58 V_Li). This observation sheds light into how Ru promotes the reaction of Li₂O₂-oxidation. Further investigation on how catalyst influence nonaqueous Li-OER using rotating ring-disk electrode (RRDE), on-line mass spectrometry (OEMS),⁸Fourier transform infrared spectroscopy (FT-IR) and electrochemical impedance spectroscopy (EIS) will be presented.

Acknowledgements

This research is supported by project RNEp1-13 of the Shun Hing Institute of Advanced Engineering, The Chinese University of Hong Kong and a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China, under Theme-based Research Scheme through Project No. T23-407/13-N.

References

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