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Liu et al. Energy Mater 2023;3:300011 https://dx.doi.org/10.20517/energymater.2022.68 Page 3 of 10

TiO /CNTs was achieved by the hydrolysis of the titanium butoxide on the surface of CNTs, and the
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[27]
method was modified by a previous report . Ru nanoparticles were then loaded on the TiO /CNTs via a
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[28]
modified aqueous reduction route . In detail, in a flask (250 mL), a certain amount of RuCl ·3H O was
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dissolved in 100 mL aqueous solution and then 100 mg of TiO /CNTs was added to form a mixture. This
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mixture was stirred for 24 h at room temperature. Afterwards, 10 mL aqueous solution of NaBH was added
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dropwise to the previous solution to keep the molar ratio at 3 by controlling the volume of NaBH and the
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weight of RuCl ·3H O. The Ru/TiO /CNTs black powder was generated after 1 h vigorous stirring. Then, the
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resulting solid product was washed with water and ethanol via centrifugation several times and finally dried
in a vacuum oven at 80 °C overnight.
Physical characterizations
X-ray diffraction (XRD) (GBC mini materials analyser MMA) patterns were performed at a scan rate of
4° min and analysed with Traces™ software coupled with the Joint Committee on Powder Diffraction
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Standards (JCPDS) powder diffraction files. The morphological information of the samples was
characterized by field emission scanning electron microscopy (FE-SEM, JEOL 7500) and transmission
electron microscopy (TEM, JEOL ARM-200F). The XPS result was analysed using CasaXPS software, and
all the data were calibrated by C 1s at 284.6 eV for graphite.
Li-O battery measurements
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For the fabrication of the cathode, catalyst (80 wt.%), Ketjen Black (KB10 wt.%), and poly(1,1,2,2-
tetrafluoroethylene) (PTFE) (60% dispersion, 10 wt.%) were mixed in an isopropanol solution to get a
homogeneous slurry, which was then coated on carbon paper. Afterwards, the electrodes were dried in a
vacuum oven at 120 °C overnight. All the lithium-oxygen batteries were assembled in an Ar-filled glove box
(Mbraun, Unilab, Germany) with both water and oxygen contents below 0.1 ppm. 2032-type coin cells with
9 air holes on the cathode cape were employed as Li-O batteries. Apart from cathode, in a typical Li-O
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battery, lithium metal acted as the counter electrode while 1 M LiCF SO dissolved in tetraethylene glycol
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dimethyl ether (TEGDME) worked as electrolyte. All the assembled coin cells were stored in an O -purged
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homemade chamber which was connected to the battery tester (LAND CT 2001) for 2 h before each test.
The galvanostatic discharge-charge tests were then conducted in the potential range of 2.0-4.5 V (vs. Li /Li).
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The loading amount in each cathode was about 0.6~0.8 mg cm .
Examination of the discharged and recharged electrodes was realized by disassembling the batteries in the
glove box, followed by rinsing the cycled electrode with tetraethylene glycol dimethyl ether (TEGDME) and
removing the solvent under vacuum. For ex-situ SEM and XPS measurements, the collected electrodes were
put in a plastic package and laminated in the glove box before taking them out to the outside instruments.

RESULTS AND DISCUSSION
Structure and morphology analysis
The fabrication process of the Ru/TiO /CNTs cathode consisted of two steps [Figure 1]. TiO /CNTs was
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firstly prepared by the hydrolysis of the titanium butoxide on the surface of CNTs. Ultrafine Ru
nanoparticles were then loaded on TiO /CNTs via an aqueous reduction reaction. The rational designed
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Ru/TiO /CNTs cathode was anticipated to effectively catalyze the reversible Li O formation and
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decomposition.
The CNTs had a diameter of 15 nm and showed a very distinct layered structure of nano-walls [Figure 2A].
Both light and dart fields were utilized to well characterize the microstructure of Ru/CNTs and Ru/TiO /
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CNTs. Ultrafine Ru nanoparticles with a size of 2-3 nm were decorated on the surface of CNTs in the
sample of Ru/CNTs [Figure 2B and C]. The existence of TiO in the sample of Ru/TiO /CNTs
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