Metal-Free Mesoporous Cathode for Lithium-Oxygen Batteries: Noble Carbons As Bifunctional Catalyst

The use of renewable energy is an urgent and serious request to ensure the energy supply in the 21^st century.¹ The rechargeable lithium-oxygen batteries (LOBs) are one of the most promising secondary batteries to be applied to smart grid and electronic vehicles which are electrified from mixed energy generated by renewable and traditional fossil fuels. However, LOBs are suffering from unoptimized combinations of catalysts and electrolytes: 1) carboneous materials, which are good oxygen reduction reaction (ORR) catalysts and support for other catalysts, are one of main causes of side-reactions associated with electrolyte decomposition and 2) from point of commercial viability, the price of LOBs should be affordable: We cannot use expensive noble metals.^2-4

Here, we show that mesoporous nitrogen doped carbons (meso-NdCs) as efficient bifunctional catalysts which can process well both ORR and oxygen evolution reaction (OER).⁵ The meso-NdCs were synthesized by carbonization of all-organic ionic liquid and using SiO₂ nanoparticles (14 nm of diameter) as a hard template. The electrochemical properties of meso-NdCs are characterized by galvanostatic charge-discharge curves. The formation/de-formation of lithium peroxide and other side products were checked by ex-situ X-ray diffractometer and ex-situ Fourier-transform infrared spectroscopy. As the results, the meso-NdCs shows good catalytic activities in both ORR and OER. Especially at OER, meso-NdCs show the recharge plateau at 3.7 V versus Li/Li⁺: the lowest overpotential as non-metal catalyst (0.7 V). Indeed, we found that the cycle property can be improved by synthetic condition due to higher conductivity enhanced by enrichment of atomic graphitic parts. Indeed, stability against active reduced oxygen species and fewer defects could give positive effects for this better electrochemical property. This study suggests that by changing the amount of dopant and the degree of graphitization, we can control both an electrochemical catalytic activity and the cycling property of the carboneous materials as possible catalysts in LOBs.

We propose that meso-NdCs are the new candidate as affordable, efficient, metal-free bifunctional catalysts to be used in rechargeable energy storage devices. This is very important because several elements, such as sulfur and phosphor, are promising candidates to be doped in and improve the electrochemical catalytic properties of carbon, which is considered as an unfavorable catalyst for LOBs so far.

 

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5. K. Sakaushi, T.-P. Fellinger, M. Antonietti, manuscript submitted.