Hydrous amorphous RuO₂ nanoparticles supported on reduced graphene oxide for non-aqueous Li–O₂ batteries

Abstract

Hydrous amorphous RuO₂ nanoparticles supported on reduced graphene oxide were developed as a cathode material for non-aqueous Li–O₂ batteries, resulting in substantially reduced overpotential as well as excellent cycleability. A new type of nanoneedle-like Li₂O₂ morphology was produced during discharge. The enhanced catalytic activity can be attributed to the amorphous and hydrous nature of the catalyst.

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With the increasing demand for high energy density storage devices, non-aqueous Li–O₂ batteries have been extensively investigated as a next generation energy storage/conversion system.¹ Generally, the reaction between lithium ions and oxygen produces lithium peroxide (Li₂O₂) during discharge. This reaction is called the oxygen reduction reaction (ORR), while the reverse reaction, which involves the electrochemical decomposition of Li₂O₂ to lithium ions and oxygen during charging, is known as the oxygen evolution reaction (OER).² Since the charge–discharge processes of Li–O₂ batteries are surface reactions, the kinetics of these fundamental reactions can be strongly promoted by incorporating appropriate catalysts and catalyst supports.

Carbon based materials, such as super P, ketjen black, carbon nanotubes, carbon nanofibers and mesoporous carbon, have been most widely applied as catalyst supports in light of their excellent characteristics. Among them, reduced graphene oxide (rGO) is considered to be an ideal support for metal nanoparticles since it provides a large surface area, high mechanical strength, high conductivity, and thermal and chemical inertness.³

It has been reported that the catalysts have an immense influence on the electrochemical properties of Li–O₂ batteries by affecting the formation and properties of the discharge product Li₂O₂, including its growth, particle size distribution, and morphology, based on the reaction mechanism.⁴ Above all, ruthenium oxide (RuO₂) is one of the most extensively investigated catalysts due to its outstanding catalytic activity toward ORR/OER.⁵

On the basis of our previous work, non-crystalline RuO₂ catalyst has been considered a critical factor for obtaining electrochemically improved Li–O₂ batteries; the amorphous RuO₂ delivers better catalytic activity toward OER than crystalline RuO₂.⁶ At the same time, while hydrous RuO₂ is known to be an excellent catalyst for OER in aqueous media, only a few reports have been published on the use of hydrous RuO₂ in non-aqueous media. Recently, Guo et al.⁷ have shown that hydrous binary oxides containing γ-MnO₂ and RuO₂ exhibited superior catalytic activity in both aqueous and non-aqueous Li–O₂ batteries. Jung et al.⁸ developed hydrated ruthenium oxide supported on rGO (RuO₂·0.64H₂O–rGO hybrid), demonstrating its extraordinary catalytic performances using a TEGDME electrolyte, and confirmed the stability of H₂O in hydrated RuO₂.

In this regard, here we propose hydrous amorphous RuO₂ as a catalyst and rGO as a catalyst support for development as a cathode material for non-aqueous Li–O₂ batteries. A new type of Li₂O₂ morphology was found and the resulting electrochemical performances were evaluated.

The experimental details for synthesizing the rGO/hydrous amorphous RuO₂ composite⁹ is shown in the ESI.†

Based on a straightforward reaction as presented below,¹⁰

RuCl₃ + 3NaOH = Ru(OH)₃ + 3NaCl

the synthesized cathode material is regarded to be the composite of rGO and hydrous RuO₂. The X-ray diffraction pattern in Fig. S1 (ESI†) reveals that the synthesized rGO/hydrous RuO₂ has no detectable diffraction patterns, except a broad peak centered at around 24° corresponding to the typical pattern of rGO, demonstrating that the crystal structure of the hydrous RuO₂ is amorphous form in phase.¹¹ Therefore, the obtained material for Li–O₂ battery cathode consists of rGO as a catalyst support and hydrous amorphous RuO₂ as a catalyst.

The microstructural characteristics of the rGO and rGO/hydrous amorphous RuO₂ composite were observed by conventional scanning electron microscopy (SEM), and the results are shown in Fig. 1. The rGO, the catalyst support, appears to have a thin sheet-like morphology with aggregation, showing that individual sheets are closely associated each other (Fig. 1a). The catalyst, hydrous amorphous RuO₂, was applied to the rGO to be developed into a rGO/hydrous amorphous RuO₂ composite. The resulting cathode material showed nanoparticles with sizes of about 20–100 nm homogeneously anchored onto the surface of the rGO while preserving its sheet-like morphology (Fig. 1b).

Fig. 1 FE-SEM images of (a) rGO (b) rGO/hydrous amorphous RuO₂ composite.

Further investigations were carried out by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). Fig. S2 (ESI†) indicates that the nanoparticles are successively mounted on the surface of rGO sheet without any morphological alteration of the rGO. TEM-EDS analysis was conducted on the nanoparticle region and clarified the presence of Ru. In the mapping images, the signals of the C, O and Ru elements are represented as bright colored spots, respectively, indicating that all the elements are uniformly well-distributed within the particles.

Since the performance of Li–O₂ batteries is not only affected by the catalyst support but is also greatly affected by the OER/ORR activities of the catalyst, the voltage profiles of the as-prepared samples were observed, and are compared in Fig. 2. The rGO itself exhibited an overpotential of 1.5 V (energy efficiency 65%), which is a lower value than that of the most commonly used carbon-based catalyst support, super P, with 1.6 V (energy efficiency 63%). The rGO/hydrous amorphous RuO₂ composite exhibited outstanding performance in comparison with that of rGO, showing an overpotential of 0.8 V (energy efficiency 80%).

Fig. 2 Voltage profiles of (a) super P (b) rGO (c) rGwO/hydrous amorphous RuO₂ composite.

As shown, the respective ORR potentials of super P, rGO and rGO/hydrous amorphous RuO₂ composite are 2.68, 2.71 and 2.76 V, indicating that the rGO and hydrous amorphous RuO₂ have no noticeable effect; whereas the OER potentials are 4.32, 4.21 and 3.53 V, respectively, exhibiting significant differences in reaction potential. Accordingly, it can be concluded that the rGO is a more effective catalyst support than super P, and the catalytic activity of hydrous amorphous RuO₂ is highly functional toward OER, rather than ORR.

The proposed reaction mechanism of Li–O₂ batteries is the reversible formation and decomposition of Li₂O₂ on the surface of cathode materials.¹² Therefore, it is essential to verify the reaction product on the as-prepared cathode material. For this purpose field-emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS) analysis were conducted and the results are shown in Fig. 3 and S3.† Fig. 3 presents the FE-SEM images of the as-prepared rGO/hydrous amorphous RuO₂ composite, after the first discharging and charging. Each electrode was washed with dimethyl ether (DME) in order to remove residual Li salt from the electrolyte. As can be seen in Fig. 3b, ORR products were deposited on the surface of the cathode material with no sign of pristine sheet-like morphology.

Fig. 3 FE-SEM images of rGO/hydrous amorphous RuO₂ composite (a) pristine (b) after first discharging (c) after first charging.

It has been reported that there are several types of Li₂O₂ morphologies, such as toroidal-shaped, spherical particles, elongated particles, close-packed nanosheets, thick layer and porous ball-like shapes.¹³ Surprisingly, the discharge products of the rGO/hydrous amorphous RuO₂ composite cathode exhibited an unusual morphology, a mixture of agglomerates and nanoneedle-like particles, which has not been previously reported as far as we know. The marked region suggests agglomerates might be produced by the accumulation of nanoneedle-like particles. Since the morphological variations of Li₂O₂ are considered to have a great effect on the electrochemical performances of Li–O₂ batteries, we presume the superior electrochemical performances of the rGO/hydrous amorphous RuO₂ composite can be attributed to this new type of morphology. It is possible that the formation of nano-sized Li₂O₂ prevents air blocking in the cathode, as well as making it easy for Li₂O₂ to decompose upon charging, leading to reduced OER overpotential.

Moreover, Jung et al.¹⁴ reported that the –OOH moieties of β-FeOOH can possibly reduce OER overpotential by decreasing oxygen pre-absorption and/or the chemisorption energy. In this respect, the hydrous nature of the catalyst may play a role in the dissociation process of the discharge product, reducing the overpotential of OER.

For further investigation, the XPS spectra of Li 1s and O 1s were obtained and are depicted in Fig. S3 (ESI†). After discharging, the binding energy peak of 55 eV in the Li 1s spectrum and 531.5 eV in the O 1s spectrum were observed, indicating the presence of Li₂O₂.¹⁵ In the reversible reaction after charging it was observed that the intensity in both the Li 1s and O 1s spectra were reduced, signifying the decomposition of Li₂O₂. This is strongly supported by the result from FE-SEM in Fig. 3c indicating that the ORR products deposited on the surface of the cathode material were utterly decomposed after charging, revealing the sheet-like morphology of rGO and the nanoparticle morphology of the catalyst. Accordingly, it is noteworthy to mention that the main reaction of the rGO/hydrous amorphous RuO₂ composite is the formation and decomposition of the Li₂O₂ on the cathode surface, and this reversible reaction implies that the prepared cathode material is electrochemically active in Li–O₂ batteries.

To account for the origin of this new type of morphology, the cathode material was annealed at 300 °C under oxidative atmosphere for dehydration of hydrous amorphous RuO₂ catalyst and its characteristics were compared, as shown in Fig. S4 (ESI†). The FE-SEM image of rGO with dehydrated amorphous RuO₂ after the first discharging indicated that a thick agglomerated film was produced which covered the entire electrode surface, without formation of any needle-like product. The voltage profile showed poor performance, with a capacity of 900 mA h g⁻¹ with an overpotential of 1.22 V, which can be attributed to massive clogging by the discharge product, limiting Li₂O₂ formation, thus leading to the capacity decay of Li–O₂ batteries.

Consequently, the factor affecting Li₂O₂ morphology is speculated to be the presence of a hydrated species in the cathode material. It has been accepted that water molecules react with Li₂O₂ to form LiOH, which has an adverse effect on the battery performance. However, there is no evidence of LiOH formation in the XPS analysis (55.5 eV in the Li 1s and 531.2 eV in the O 1s spectra) as depicted in Fig. S4 (ESI†), and the stability was further proved by the cycleability result below. The formation mechanism of the nanoneedle-like morphology has not been proved yet, and it is only speculated that this stable water molecules somehow prevent the over-growth of Li₂O₂ and thus facilitate the formation of nanoneedle-like Li₂O₂. Further investigations will be needed to determine the role of hydrated species in the Li–O₂ batteries.

To prove not only the reversibility of the reaction but also the stability of the hydrous amorphous RuO₂ catalyst, the electrochemical performance per number of cycles was determined and is presented in Fig. 4. The capacity was retained over 55 cycles with a limited capacity of 1000 mA h g⁻¹ at a current density of 100 mA g⁻¹, and indicate that the discharge and charge terminal voltages of the electrode were preserved without a significant reduction in discharge or increase in charging (Fig. 4a). The energy efficiency result in Fig. 4b shows that the rGO/hydrous amorphous RuO₂ composite was substantially maintained, up to 63% after 55 cycles, which is higher than that of rGO.

Fig. 4 (a) Cycleability and terminal voltage profiles of rGO/hydrous amorphous RuO₂ composite with a limited capacity of 1000 mA h g⁻¹ (b) energy efficiency of rGO and rGO/hydrous amorphous RuO₂ composite as a function of cycle numbers.

In summary, rGO/hydrous amorphous RuO₂ composite was prepared and developed as a cathode material for Li–O₂ batteries for the first time. The rGO was demonstrated to be a better catalyst support than that of super P, and the superior catalytic activity of hydrous amorphous RuO₂ greatly enhanced the electrode performance. Along with the amorphous characteristics of the catalyst, it can be assumed that the enhanced catalytic activity produced by the hydrous nature of the catalyst contributed to producing a new type of Li₂O₂ morphology, involving a mixture of agglomerates and nanoneedle-like particles. Further studies are necessary to resolve the formation mechanism of the nanoneedle-like Li₂O₂ morphology.

Acknowledgements

This research was supported by the Government-Funded General Research & Development Program by the Ministry of Trade, Industry and Energy, Republic of Korea.

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Footnote

† Electronic supplementary information (ESI) available: Details of the synthesis and characterization results. See DOI: 10.1039/c6ra03347d

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