Redox-Mediated ORR and OER Reactions: Redox Flow Lithium

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Redox-mediated ORR and OER Reactions: Redox Flow Lithium Oxygen Batteries Enabled with A Pair of Soluble Redox Catalysts Yun Guang Zhu, Xingzhu Wang, Chuankun Jia, Jing Yang, and Qing Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01478 • Publication Date (Web): 08 Aug 2016 Downloaded from http://pubs.acs.org on August 15, 2016

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Redox-mediated ORR and OER Reactions: Redox Flow Lithium Oxygen Batteries Enabled with A Pair of Soluble Redox Catalysts

Yun Guang Zhu, Xingzhu Wang, Chuankun Jia, Jing Yang, Qing Wang*

Department of Materials Science and Engineering, Faculty of Engineering, National University of Singapore, 117576, Singapore. E-mail: [email protected]

Abstract We demonstrate a redox flow lithium oxygen battery (RFLOB) using a pair of soluble redox

catalysts,

2,5-di-tert-butyl-p-benzoquinone

(DTBBQ)

and

tris-{4-[2-(2-

methoxyethoxy)ethoxy]-phenyl}-amine (TMPPA). The catalytic effects of DTBBQ on oxygen reduction reaction (ORR) and TMPPA on oxygen evolution reaction (OER) were investigated

and

unambiguously

substantiated

with

various

electrochemical,

morphological and spectroscopic characterization methods. It is observed that, upon discharging, oxygen is rapidly reduced by DTBBQ·- and Li2O2 is formed in the presence of Li+; upon charging, Li2O2 is oxidized by TMPPA·+ releasing oxygen. Such redoxmediated ORR and OER reactions enable the formation and oxidation of Li2O2 in a separate gas diffusion tank (GDT) other than on the cathode of the cell, and thus obviate 1 ACS Paragon Plus Environment

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surface passivation and pore clogging of the electrode. The cell presents a high energy density with DTBBQ as the ORR redox catalyst, and good rechargeability when paired with TMPPA as the OER redox catalyst. While the robustness and redox potential of two molecules are to be further optimized, RFLOB demonstrated in this study provides an intriguing means for high-density and large-scale energy storage.

Keywords: Redox catalysis, Lithium-oxygen battery, Redox flow battery, Oxygen reduction reaction, Oxygen oxidation reaction, Redox targeting

1. Introduction Lithium oxygen batteries (LOBs), due to their exceedingly high theoretical energy density, have attracted considerable attention in recent years.[1-2] However, the development and practical applications of LOBs have been severely hindered by a few critical issues, such as large overpotentials, instability of electrolyte at high voltage, and consequently poor cycling stability.[3-7] In order to solve these problems, researchers have been focusing on the design and modification of cathode and electrocatalysts to facilitate oxygen reduction (ORR) and evolution reactions (OER).[8-14] While enhanced performance has been achieved, some intrinsic limitations have to be circumvented before the further development of aprotic LOBs. For instance, one of the limiting factors is the surface passivation and the associated clogging problem on the cathode during discharging process.[15] The deposition of Li2O2 or other byproducts (e.g. Li2CO3 or LiOH) on the catalysts and cathode impedes the transfer of electrons and congests the mass transport of oxygen and electrolyte.[16-18] As a result, the discharging depth and

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reachable capacity are badly impaired. To address these issues, soluble redox catalysts have recently been proposed, which work between the current collector and the deposited Li2O2 particles to promote the OER or ORR reactions, and to some extent mitigate the passivation problem[19-27]. Nevertheless, the pore clogging problem remains especially at high discharging depth  the insoluble and insulating Li2O2 gradually builds up in the pores of the cathode and blocks the transport of oxygen and redox electrolyte. Recently, based on the “redox targeting” concept, we have developed a novel rechargeable redox flow lithium battery (RFLB) for large-scale energy storage.[28-31] With the help of redox mediators, the energy storage materials are shifted from current collector of the electrodes to energy storage tanks separated from the cell. This concept has been successfully implemented for non-aqueous Li-oxygen battery to solve the above pressing issues, leading to a conceptually new device — redox flow lithium oxygen battery (RFLOB).[32] As shown in Figure 1a, in the presence of a pair of ORR and OER redox catalysts in the catholyte, the formation and decomposition of Li2O2 in RFLOB take place in a gas diffusion tank (GDT), spatially apart from the cathode of the cell, which elegantly solve the surface passivation and pore clogging problems. In addition, unlike the conventional enclosed battery architecture, such a decoupled configuration for independent power generation (electrochemical reaction) and energy storage (chemical reactions) renders RFLOB most of the merits of redox flow batteries such as scalability and operation flexibility, besides the above advantages.[28-30] In the previous study, iodine was used as the OER and ethyl viologen as ORR redox catalyst, respectively. While they function reasonably well in RFLOB, these redox catalysts are however either corrosive[33], presenting large overpotential for the oxidation of Li2O2,[32] or instable upon prolonged


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cycling[20, 34-35]. In this work, we report a new pair of redox molecules, tris-{4-[2-(2methoxyethoxy)ethoxy]-phenyl}-amine (TMPPA) and 2,5-di-tert-butyl-p-benzoquinone (DTBBQ) as the OER and ORR catalyst respectively, representing a new attempt to tackle these issues (Figure 1b). 2. Results and discussion Triphenylamine (TPA) is a widely studied redox molecule, which forms radical cation and dication upon oxidation (Figure S2). As the molecule tends to dimerize losing its electrochemical

stability,[36]

tris-{4-[2-(2-methoxyethoxy)ethoxy]-phenyl}-amine

(TMPPA) was synthesized to replace the labile TPA molecule for OER reaction. In TMPPA, the para-positions of the three phenyl rings are blocked with alkoxyl chains, which stabilizes the molecule and lowers the redox potential. The synthetic details of TMPPA can be found in the Supporting Information. In addition, DTBBQ was chosen to catalyze the ORR reaction, which has been reported to be stable and have a relatively low overpotential for the reduction of oxygen.[35] In order to confirm the feasibility of these two redox molecules, cyclic voltammetric (CV) measurements were conducted. As shown in Figure 1c, DTBBQ and TMPPA present reversible redox reaction at around 2.63 and 3.63 V (vs. Li/Li+), respectively, just straddling the equilibrium potential of Li2O2 at 2.96 V (vs. Li/Li+). This suggests thermodynamically, DTBBQ·- could reduce O2 and work as an ORR mediator promoting the formation of Li2O2 during discharging process, while TMPPA·+ could oxidize Li2O2 and serve as an OER mediator catalyzing the decomposition of Li2O2 during charging process. To elucidate the working mechanism of RFLOB, a diagram showing the redox-targeting 4 ACS Paragon Plus Environment

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reactions of these two molecules towards Li2O2 is depicted in Figure 1d. During the discharging process, both molecules in the catholyte flow into the cathodic compartment in the cell and are reduced, which determine the discharging potential of RFLOB. When the catholyte circulates back to the GDT, the formed DTBBQ·- further reduces O2 where the gas is provided, and forms Li2O2. In this process, the chemical reaction between DTBBQ·- and O2 only involves charge transfer at the liquid-gas interface in the tank. In contrast, the catalyst in conventional Li-O2 battery must be in good contact with both O2 and current collector so that charges could be extracted, which is however difficult when the catalyst is covered by insulating Li2O2. The detailed discharging process of RFLOB is described by the following two reactions: DTBBQ + e − → DTBBQ 2DTBBQ

-

-

(Electrochemical reaction in the cell)

(1)

+ 2Li + + O 2 → 2DTBBQ + Li 2 O 2 (Chemical reaction in the GDT)

(2)

During the charging process, both molecules are oxidized on the electrode, which determines the charging potential of the cell. The formed TMPPA·+ then oxidizes Li2O2 releasing O2 when the catholyte circulates back to the GDT. The detailed charging process is described by the following two reactions: TMPPA - e − → TMPPA 2TMPPA

+

+

(Electrochemical reaction in the cell)

+ Li 2 O 2 → 2TMPPA + 2Li + + O 2 (Chemical reaction in the GDT)


(3) (4)

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Figure 1. (a) Configuration of the redox flow lithium oxygen battery (RFLOB) single cell. (b) Molecular structures of DTBBQ and TMPPA. (c) The cyclic voltammograms (CV) of 10 mM DTBBQ (blue curve) and TMPPA (green curve) in TEGDME (1 M LiTFSI) electrolyte. The working electrode was glassy carbon disc and both the counter and reference electrodes were lithium foil. The scan rate is 0.08 V/s. (d) Diagram describing DTBBQ and TMPPA-catalyzed ORR and OER reactions during the charging and discharging processes of RFLOB.

It is noteworthy that during the discharging process, TMPPA·+ is firstly reduced to TMPPA followed by the reduction of DTBBQ to DTBBQ·-; while during the charging process, both molecules are oxidized consecutively — DTBBQ·- is oxidized to DTBBQ at first, followed by the oxidation of TMPPA to TMPPA·+. So it is unlikely that both TMPPA·+ and DTBBQ·- coexist in the electrolyte. As a result, the reaction between the two redox mediators is well prohibited, and they would be stable with each other during the charging and discharging processes.


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The catalytic effect of TMPPA on the oxidation of Li2O2 was corroborated by UV-Vis spectroelectrochemical measurement. As shown in Figure 2a, two absorption bands appeared at around 383 and 740 nm upon oxidizing TMPPA at 3.80 V vs. Li/Li+, attributed to the formation of the TMPPA·+ radical cation.[37] The inset of Figure 2a shows the photographs of 20 mM TMPPA and TMPPA·+ solutions, with the former in light brown while the latter in dark blue, consistent with the UV-Vis measurement. The two characteristic peaks of TMPPA·+ became more pronounced when the oxidation time was extended from 1 to 10 min, and retained almost unchanged when the oxidation ceased. In contrast, the TMPPA solution presents distinct spectra in the presence of 1 mg Li2O2 powder — no absorption peak was observed upon different treatment, suggesting the absence of TMPPA·+. This could be rationalized by the reaction between TMPPA·+ and Li2O2: the formed TMPPA·+ is reduced by Li2O2 and as a result no blue color presents. The above spectroelectrochemical study implies that TMPPA·+ radical cation could effectively oxidize Li2O2, an essential necessity for an OER catalyst. Ex-situ UVVis measurement for the reaction between TMPPA·+ and Li2O2 was also conducted. As shown in Figure S3. the characteristic peaks of TMPPA·+ vanished after dispersing Li2O2 powder in the solution, substantiating the reactions between the two. Similar UV-Vis measurement has also been attempted to investigate the reaction between DTBBQ and O2. However, due to the small difference of absorption between DTBBQ and DTBBQ·-, the results are not entirely clear. In addition, introducing O2 into the optical cell without interfering the absorption measurement is another challenge. As a result, we resort to other techniques such as rotating disk electrode (RDE), to monitor the ORR and OER reactions of the redox mediators.


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Figure 2. (a) The UV-Vis spectra of 1 mM TMPPA and 1 mM TMPPA plus Li2O2 suspension after consecutive electrochemical treatment in 1 M LiTFSI in TEGDME. The inset is the photograph of 20 mM TMPPA and TMPPA·+ solution in TEGDME. (b) RDE measurement of the OER reaction between 2 mM TMPPA and Li2O2 dispersion (0.5 mg/mL). (c) RDE measurement of the ORR reaction between 2 mM DTBBQ and O2. The electrolyte was 0.10 M LiTFSI in TEGDME saturated with N2 or O2 in the above two RDE measurements. The rotation rate of the above RDE measurements is 1600 rpm. The RDE potential scan rate is 10 mV/s. The Ag/Ag+ reference electrode in RDE measurement was calibrated to be ~3.60 V vs. Li/Li+. (d) Electron density associated with the redox shuttle and the materials. Blue and yellow zones correspond to electron density deduction and enhancement regions, respectively.

The OER activity of TMPPA·+ was further scrutinized on a RDE. In the absence of TMPPA, no current was detected in the OER potential range despite the presence of Li2O2 in the electrolyte (Figure 2b). When 2 mM TMPPA was added into the above electrolyte, a steady-state current of ~0.26 mA/cm2 appeared at potential above ~3.70 V 8 ACS Paragon Plus Environment

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vs. Li/Li+, corresponding to the first oxidation of TMPPA. This oxidation current is significantly higher than that of TMPPA alone in the absence of Li2O2 suspension (~0.16 mA/cm2), implying the catalytic reaction between TMPPA·+ and Li2O2 which regenerates TMPPA and results in an enhanced oxidation current. In addition, the reaction is insensitive to atmosphere — identical current was observed in both N2 and O2-saturated electrolytes, indicating TMPPA could be invulnerable to oxygen for the period of measurement. Similar RDE measurement was conducted to investigate the ORR reaction of DTBBQ. As shown in Figure 2c, the reduction of 2 mM DTBBQ in O2-saturated electrolyte prompted a significantly higher current than that in N2-saturated electrolyte, when the potential is lower than 2.70 V (vs. Li/Li+). This phenomenon, analogous to the catalytic effect observed for TMPPA in Figure 2b, suggests DTBBQ·- could effectively reduce O2. It should be noted that the direct reduction of O2 on Pt electrode is suppressed in the presence of DTBBQ presumably due to the competitive adsorption of DTBBQ on Pt. The reduction current of DTBBQ did not reach the saturated value due to the second reduction reaction of DTBBQ when the potential is further lowered. In addition, it was observed that in the absence of DTBBQ the current due to the direct reduction of O2 on RDE firstly increased and then decreased with rotation rate in O2-saturated electrolyte (Figure S4), despite a thinner diffusion layer on the disk electrode surface. Such a phenomenon could be rationalized by the “Li2O2 blocking effect”.[23] That is, the produced Li2O2 deposited and blocked the further ORR reaction on the RDE electrode surface. In contrast, the current monotonously increased with rotation rate in the presence of DTBBQ, indicating the formed Li2O2 by redox catalytic reaction is likely not


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deposited on the disk electrode but presented in the surrounding electrolyte. Ab initio calculations were carried out to understand the charge transfer between the redox shuttle and the materials. During the charging process, TMPPA·+ prefers a parallel geometry when interacting with Li2O2 (0 0 0 1) surface. The charge transfer associated with their interaction is characterized by the electron density difference map as shown in Figure 2d, where the blue and yellow zones correspond to electron density deduction and enhancement regions, respectively. Evidently, electron flows from Li2O2 to TMPPA·+ upon interaction, consistent with the experiment results. The charge transfer amount is quantified by the Bader charge calculation to be around 0.23e-. During the discharging process, the oxygen is reduced by DTBBQ·- and such a reaction is taken place on the substrate of Li2O2, on which a layer of oxygen molecules is chemically adsorbed. The charge redistribution is clearly evidenced with the electron density difference map as shown in Figure 2d. Upon their interaction, the electron will move from DTBBQ·- to the substrate and such an electron movement agrees well with the UV-Vis and RDE experimental results. In order to assess the stability of TMPPA and DTBBQ upon prolonged cycling, galvanostatic test was performed using these two redox molecules as liquid cathode in lithium half cells. As shown in Figure 3a-b, both molecules presented excellent electrochemical stability in inert atmosphere. Especially, the alkoxyl substituents on the phenyl rings of TMPPA effectively block the polymerization of the molecule and greatly stabilize the formation of TMPPA radical cation and dication. After 100 cycles, the capacity retention of TMPPA and DTBBQ are 87.4% and 89.4%, respectively. A RFLOB with a separate GDT tank and cell stack was built to investigate the 10 ACS Paragon Plus Environment

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functionality of the two molecules as redox catalysts. As the setup shown in Figure S1, 10 mM TMPPA and 10 mM DTBBQ were dissolved in the catholyte circulating between the GDT and cathodic compartment of the cell by a peristaltic pump. The two electrode compartments in the cell stack were separated with a Nafion/PVDF membrane, which is conductive to Li+ while impermeable to other redox species so that the crossover is precluded.[31] Here the Li+-conducting membrane is critical to prevent the shuttle effect of redox molecules and protect Li metal as revealed by Bergner et al.[38], while as a matter of fact it is often ignored in many studies. Nickel foam was used as an inert support for the deposition of Li2O2 in the GDT tank, in which the O2 pressure was kept at 1 atm. To unambiguously demonstrate the redox-mediated ORR and OER reactions, the cell was deliberately cycled at a controlled capacity of 6 mAh, significantly higher than the capacity of the two molecules (~2 mAh). Figure 3c shows the voltage profiles of the cell upon discharging and charging at a current density of 0.50 mA/cm2. When the cell was firstly discharged, a voltage plateau at ~2.70 V was observed corresponding to the reduction of DTBBQ in the cell (Eq. 1). As evinced previously, the ORR reaction of DTBBQ·- with O2 in the GDT tank contributes to the extended capacity, which forms Li2O2 in the tank meanwhile regenerates the molecule (Eq. 2). Upon charging, the voltage rises gradually and levels off at ~3.70-3.80 V, attributed to the oxidation of TMPPA in the cell (Eq. 3) and the associated OER reaction between TMPPA·+ and Li2O2 in the GDT tank evolving O2 (Eq. 4). In a separate test, in which there was not TMPPA and DTBBQ in the catholyte, the cell only delivered negligible capacity (Figure S5a), at testing the critical roles of the two redox mediators. In the subsequent cycles (Figure 3c), a noticeable upward shift of the discharging voltage 11 ACS Paragon Plus Environment

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was observed rendering a reduced voltage hysteresis of ~0.90 V, implying the DTBBQ molecule may have undergone certain change in the 1st charging process so that the redox potential of DTBBQ became more positive. Such a change of cell voltage became more pronounced when the cell was cycled for longer time (Figure S5a). Thus the chemical lability of DTBBQ at high potential will have to be mitigated in the harsh oxygen environment. In addition, the high voltage plateau in the beginning of the 2nd and 3rd discharging processes indicates the formed [TMPPA·+] in the prior charging process has not been fully reduced by Li2O2 in GDT. As a result, these oxidized species are reduced first in the subsequent discharging process, which is also indicative of slow reaction between the two. Moreover, the above evolution of cell performance may also be caused by the degradation of Li metal and membrane upon prolonged testing. It should be noted that, although the RFLOB was only tested for three cycles (cycle number itself is of little value if the depth of charge/discharge is not defined), the total duration of the test is 180 h, much longer than some of the best performing Li-O2 battery recently reported.[39] The electrochemical property of the pair of redox catalysts was further examined with galvanostatic intermittent titration technique (GITT). As shown in Figure 3d, the RFLOB cell presented a discharge voltage plateau at ~2.60 V and a stabilized rest cell voltage at ~2.88 V. Interestingly, the stabilized voltage is noticeably higher than the formal potential of DTBBQ determined by CV measurement (2.63 V, Figure 1c), indicating rapid reaction between DTBBQ·- and O2 which brings down [DTBBQ·-] as compared to [DTBBQ] in the catholyte. As a consequence, the equilibrium potential of the molecule bumped up as determined by the Nernst equation. This is also consistent with the short stabilizing time of DTBBQ even at the beginning of discharging process (Figure S6a-b). In comparison,


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the charging process exhibited different kinetics. The slow relaxation at the initial charging process (Figure S6c) suggests relatively sluggish reaction between TMPPA·+ and Li2O2, consistent with the galvanostatic test as discussed previously. With the buildup of [TMPPA·+] in the catholyte, the reaction becomes faster and the stabilizing time of cell voltage is considerably shortened (Figure S6d).

Figure 3. The galvanostatic voltage profiles of (a) 20 mM TMPPA and (b) 20 mM DTBBQ measured in static lithium half cells in an argon-filled glove box; (c) 10 mM TMPPA and 10 mM DTBBQ in a RFLOB cell; (d) GITT curves of a RFLOB cell in the first cycle. The electrolyte is 1 M LiTFSI in TEGDME. The current density is 1.25 mA/cm2 for static cells and 0.50 mA/cm2 for RFLOB cells.

The formation of Li2O2 in the GDT tank as a discharging product was confirmed by field emission scanning electron microscopy (FESEM), transmission electron microscopy


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(TEM) and Raman spectroscopy measurement. Figure 4a-c reveal the surface morphology changes of the Ni foam taken from the GDT tank at different stages of discharge and charge. After discharging the cell, a layer of precipitate appeared on the surface of Ni foam (Figure 4b) and vanished after recharging (Figure 4c). As depicted by TEM and selected area electron diffraction (SAED) measurement, the discharging product was agglomerated and amorphous (Figure 4e), different from the nanocrystalline form when ethyl viologen is used as ORR catalyst.[20, 32] The distinct phase structure indicates the formation of Li2O2 is strongly affected by the redox catalyst, while further studies are required to elucidate the underlying mechanism. Raman spectra of the product at different stages of discharge and charge are shown in Figure 4f-h. The two characteristic bands of Li2O2 at around 256 (Li-O) and 790 (O-O) 1/cm are clearly observed,[3] unambiguously substantiating the reversible formation of Li2O2 as a major discharging product in RFLOB.


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Figure 4. (a-c) FESEM images of the surface of Ni foam taken from the GDT tank (a) before and (b) after discharge, and (c) after charge. (d, e) low and high-resolution TEM images of the precipitate collected from the surface of Ni foam after discharge. The inset of (e) shows the SAED pattern. (f ̶ h) Raman spectra of Ni foam obtained at the same condition as those in (a-c).

The above results have clearly manifested the importance of the concurrent use of dual redox mediators for both ORR and OER. We notice most of the previous studies just focused on redox mediators either for OER or ORR. An OER redox mediator, such as TTF, can lower the charging overpotential by redox targeting reaction between the oxidized TTF and Li2O2 (or LiO2).[19, 40, 41] However, such a redox-assisted OER process 15 ACS Paragon Plus Environment

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does not help alleviate the passivation and pore clogging problems of cathode, for which the insoluble and insulating discharging product is still inevitably formed on the cathode. On the other hand, the sole ORR redox mediator used in the conventional Li-O2 cell facilitates the formation of discharging product on the cathode, while the issues on pore clogging and those for the OER process remain.[42] The above has concertedly verified the functions of the pair of redox mediators and affirmed the operating principle of RFLOB. With redox-mediated ORR and OER reactions, the formation of Li2O2 is shifted from the cathode to a separate GDT tank, with which the formation of Li2O2 on the cathode is generally precluded. As the SEM images shown in Figure S7, nearly no discharge product was observed on the cathode after 20 mAh discharging when compared with the fresh carbon felt. The new operation mode has a number of advantages over the conventional lithium oxygen battery. Besides the intrinsic passivation and clogging issues which are resolved in RFLOB, the new device offers great operation flexibility and scalability. For instance, the RFLOB system could deliver substantial amount of energy should there be sufficient lithium anode and large GDT tank. With DTBBQ as the ORR catalyst a volumetric energy density as large as 7090 Wh/L could in theory be attained if only consider the volume of GDT tank (the GDT tank could eventually be filled with Li2O2. See the calculation in Supporting Information), which is ~10 times the next highest hybrid redox flow lithium batteries (RFLB) based on Li-iodide and LiFePO4 system

[43]

, and more than two orders of

magnitude higher than the best vanadium flow batteries.

3. Conclusion 16 ACS Paragon Plus Environment

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Redox flow lithium oxygen battery, as a radical approach to solve the cathode surface passivation and pore clogging problems, has been demonstrated with a new pair of redox catalysts. For the first time, benzoquinone derivative DTBBQ was employed as an ORR catalyst to promote the formation of Li2O2 in lithium oxygen batteries. The judiciously designed triarylamine derivative, TMPPA demonstrated superior electrochemical property for the oxidation of Li2O2. The concurrent use of both redox mediators in flow battery configuration led to a disruptively innovative operation mode for lithium oxygen batteries. Such a new device offers great advantages in terms of scalability and operation flexibility for large-scale high-density energy storage. Nonetheless, TMPPA and DTBBQ are obviously not robust enough for RFLOB. Degradation of the molecules, especially that of DTBBQ upon long-term cycling causes the shift of voltage profiles, and presents the main obstacle for their practical use. In addition, the redox potentials of the two molecules are not perfectly matching that of Li2O2, which induces relatively large overpotential loss during discharging and charging. Hence systematical mechanistic studies will be required to understand the side reactions of the two molecules in O2 environment meanwhile further tune their redox potentials to mitigate voltage hysteresis. Supporting

Information.

Detailed

experiments,

computation

method,

energy

calculations, and some supporting data are provided in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org Acknowledgements This research was supported by the National Research Foundation, Prime Minister’s Office, Singapore, under its Competitive Research Program (CRP Award No. NRFCRP8-2011-04 and NRF-CRP10-2012-06). 17 ACS Paragon Plus Environment

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TOC A new pair of redox catalysts is investigated for OER and ORR in Li-O2 batteries. The redox flow Li-O2 battery (RFLOB) enabled with the pair of redox mediators elegantly resolves the pressing clogging and passivation issues confronted by Li-O2 batteries and provides an unprecedently high energy density, ~10 times the next highest hybrid redox flow lithium battery.


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Redox-Mediated ORR and OER Reactions: Redox Flow Lithium

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