Promoting Surface-mediated Oxygen Reduction Reaction of Solid

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Promoting Surface-mediated Oxygen Reduction Reaction of Solid Catalysts in Metal-O2 batteries by Capturing Superoxide Species Peng Zhang, Liangliang Liu, Xiaofeng He, Xiao Liu, Hua Wang, Jinling He, and Yong Zhao J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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Journal of the American Chemical Society

Promoting Surface-mediated Oxygen Reduction Reaction of Solid Catalysts in Metal-O2 batteries by Capturing Superoxide Species Peng Zhang, Liangliang Liu, Xiaofeng He, Xiao Liu, Hua Wang, Jinling He, Yong Zhao* Key Lab for Special Functional Materials of Ministry of Education; National & Local Joint Engineering Research Center for High-efficiency Display and Lighting Technology; School of Materials Science and Engineering; Collaborative Innovation Center of Nano Functional Materials and Applications; Henan University, Kaifeng, 475004, P. R. China

Abstract The oxygen reduction reaction (ORR) in aprotic electrolyte is the essential reaction in metal-oxygen batteries. Capturing and shifting the absorbed metal superoxide intermediates/products from cathode surface is a longstanding challenge to clarify the ORR mechanism, accelerate the ORR, and improve the stability and energy density of metal-oxygen batteries. Herein, a bio-inspired pathway that cathode solid catalysts and soluble anthraquinone (AQ) molecules initiate an “enzyme-coenzyme” cooperative catalysis mechanism is developed to greatly boost the ORR activity of solid catalysts over 10-folds, which AQ acts as a scavenger to capture and shift the absorbed superoxide species from cathode surface to aprotic electrolyte. Taking lithium-oxygen (Li-O2) battery as a model system, the cell discharge ORR mechanism is well illustrated and capacities are significantly improved over 3 times in the presence of AQ molecules. This concept represents the first demonstration of stabilizing and solvating superoxide species to substantially accelerate ORR catalysis of solid catalysts and enhance the performance of metal-O2 batteries through biomimicking coenzyme-assisted reactions.

Keywords: coenzyme, lithium oxygen battery, quinone, capacity

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Introduction Oxygen reduction reactions (ORR) catalysis in aprotic medium that supports the absorbed metal superoxide (MO2, such as LiO2 and NaO2) as intermediate or product is the essential process to determine the performance of metal-air (exactly metal-O2) batteries, which have attracted great attention owing to their ultrahigh theoretical energy densities for long-term mobile power supplies.1-6 Oxygen electrodes assembled with various kinds of solid catalysts have been developed to increase the rate of surface ORR through optimizing the bond strength between active sites and O2/intermediates.7 Unfortunately, during cell discharging, a dense and insulating metal peroxide (M2O2) or MO2 products film on solid catalyst is easily formed, in which the generation of M2O2 is derived from the fast disproportination or reduction reaction of absorbed MO2 on cathode surface.8 Consequently, the ORR catalysis initiated by solid catalysts is totally terminated, leading to the low capacities and early cells death.9-15 Attempts over past years have been done to induce the solution-mediated ORR pathway via modulating the solubility and/or reactivity of MO2 species, which is suggestive for more competitive than surface ORR pathway to partially postpone the growth of dense M2O2/MO2 film.7, 16-17 For example, the selection of high donor number (DN) solvents18-21, high acceptor number additives22-24 or lithium salts25 with high DN values are able to stabilize LiO2 or NaO2 in the aforementioned electrolytes, which promote the growth of Li2O2/Na2O2/NaO2 through solution-mediated ORR pathway.26 However, such solvents and additives are puzzled with their high reactivity with metal anode, resulting in low columbic efficiency and instability of batteries. Decreasing the surface current densities via preparing the cathodes with sufficient active sites, desirable pore sizes and volumes is another feasible method to promote the solutionmediated ORR for the growth of products.27-30 Recently, the soluble mediator catalysts with redox potentials lower than thermodynamic ORR potentials, such as ethyl viologen (EV),31-32 tris(2,4,6-trichlorophenyl)methyl (TTM),33 2-phenyl-4,4,5,5tetramethylimidazoline-l-oxyl-3-oxide (PTIO),16 2,5-di-tert-butyl-1,4-benzoquinone (DBBQ),34 and N,N′-bis(salicylidene)ethylene-diamino-cobalt(II) (CoII-salen),35 have 2


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been explored to reduce O2 by its reduction state to form MO2 intermediates/products in electrolytes.34, 36 This directly chemical ORR pathway can reduce the cathode surface passivation and improve the discharge capacity even in the electrolytes with low DN number solvents. Typically, DBBQ has been proved to increase the capacities of Li-O2 batteries about 80-folds with weakly solvating ether as electrolyte solvent and carbon paper (CP) as cathode.34 However, the competitive ORR process is always existed between solid and soluble catalysts, the ORR onset potentials for general solid catalysts (2.6‒2.9 V vs Li/Li+) are always a little higher than or equivalent to the reduction potentials of redox mediators. The surface ORR pathway can’t be inhibited, and the consequent generation of absorbed MO2 species by solid-catalyst causes the formation of dense ORR product film, especially at high current densities.37-39 Thus, capturing and solvating the absorbed MO2 species from the electrode surface into electrolyte should be highly impressive for high-performance metal-O2 batteries. However, a longstanding challenge that captures and shifts the short lifetime MO2 intermediates or stable MO2 products remains, resulting in the impossibility to accelerate surface ORR from solid catalysts. In natural enzymatic system, coenzymes play an important role to transport electrons and units efficiently, resulting in the switch and acceleration of enzymatic reactions.4041

For example, quinone cofactors, such as plastoquinone and ubiquinone, serve as both

proton/electron transporters in chloroplasts and mitochondira for photosynthesis and cellular respiration.42 Those quinone coenzymes function as the key centers facilitating proton coupled electron transfer reactions for energy transduction in biosynthesis, as well as trapping sites for superoxide radical (O2·-) in the cell metabolism.43 The critical factor that coenzymes can absorb and transport proton/electron/superoxide is due to their specific quinone structure, such as Coenzyme Q-10, plastoquinone and Vitamin K. The redox-active quinone and conjugated structure are responsible for them to donate and accept electrons or bind superoxides for the modulation of enzymatic reactions. Inspired by the unique properties of coenzyme in nature, we propose a catalytic 3



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pathway that a functional quinone molecule acts as “coenzyme” to capture and stabilize the absorbed MO2 species from electrode surface to electrolyte, inducing the substantial acceleration of ORR catalysis on solid catalyst and the growth of ORR products in large-size (Fig. 1). Taking ORR in aprotic Li-O2 battery as a model system, the soluble anthraquinone (AQ) and electrode solid catalyst demonstrate a “coenzyme-enzyme” cooperative ORR pathway in low or high DN solvents. AQ molecules can capture and stabilize LiO2 to form the AQ-LiO2 species in the electrolyte, which expose the surface of solid catalysts and greatly boost their ORR activity (over 10 folds) comparing to that of conventional catalysis. As a result, the cell ORR mechanism is well illustrated, and the cell capacities in the presence of AQ are maximized over three folds with both conventional granular carbon and metal oxides cathodes, which is the state-of-the-art soluble redox chemicals up to now. The discovered mechanism shows a first and attractive demonstration that the surface ORR catalysis and capacities of Li-O2 batteries can be substantially increased through biomimicking coenzyme effect.

Fig. 1| Schematic illustration of oxygen reduction reaction (ORR) pathway in metal-O2 batteries. (a) Surface mediated ORR catalysis. In conventional cell discharging, a dense and insulating metal peroxide (M2O2) or superoxide (MO2) products film is formed on the surface of solid catalysts. Consequently, ORR catalysis initiated by solid catalysts is terminated, leading to the low capacities and early death of cells; (b) Biomimicking “enzyme-coenzyme” cooperative ORR catalysis. Soluble anthraquinone (AQ) molecules and electrode solid catalysts initiate a “coenzyme-enzyme” cooperative ORR pathway, in which AQ acts as scavenger/transporter to capture and shift the absorbed MO2 species from cathode surface to aprotic electrolyte. It greatly boosts the ORR activity of solid catalyst and the growth of large-size M2O2/MO2. 4


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Results and Discussion The ORR in the presence of Quinone

Fig. 2| Cyclic voltammogram curves of the electrolyte (1M LiTFSI in TEGDME) with 10 mM (a) AQ, (b) 10 mM DBBQ, and (c) 10 mM NQ, demonstrating the totally different ORR mechanism through comparison of CVs based on without (black, O2) and with AQ, DBBQ and NQ (Ar (red) and O2 (blue)). CVs were carried out at planar Au electrodes. The inset is the correspondingly enlarged range between 2.3-3.0 V vs Li+/Li.

Similar as the quinone families as coenzymes in biosynthesis, they are hypothesized to have the equivalent functions to capture LiO2 intermediates in the ORR process. In order to confirm the capture capability of quinones to LiO2 intermediates, the absorption energy (AE) between them was evaluated by using density functional theory (DFT) calculation. As shown in Fig. S1, three typical quinones, DBBQ, naphthaquinone (NQ) and AQ, demonstrate the strong absorption capabilities to LiO2 (AE: -1.15 eV for DBBQ, -1.11 eV for NQ, -1.01 eV for AQ). It reveals that all of them are potentially capable of capturing LiO2 intermediates through the bond formation between LiO2 and quinones during electrochemical ORR process. Following with the theoretical calculation, cyclic voltammograms (CVs) were conducted to test the electrochemical behaviors of quinones in the presence and absence of O2 (Fig. 2). All of AQ (Fig.2a), DBBQ (Fig. 2b) and NQ (Fig. 2c) exhibit the reversible redox behaviors at Au electrode under Ar atmosphere in 1 M lithium bis(trifluoromethane)sulfonimide (LiTFSI)/tetraethylene glycol dimethyl ether-based electrolyte (TEGDME) electrolyte. ORR currents are increased remarkably in O2 environment comparing to those of cases without quinone. The clear difference of CVs is the ORR onset potentials in the 5



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presence of different quinones, as shown in the insets of Fig.2. The starting ORR potentials (black line in Fig. 2) initiated by Au electrode surface is ~2.7 V vs Li/Li+. The starting ORR potentials in the presence DBBQ and NQ (blue line in Fig. 2b and 2c) are consistence to their one-electron-transfer reduction potentials (~2.7‒2.8 V, red line in Fig. 2b and 2c). The overlapped potentials revealed that the reduction of DBBQ/NQ (DBBQ→DBBQ-, NQ→NQ-) and O2 at electrode surface are proceeded simultaneously. In contrast, the starting ORR potentials (~2.7 V, blue line in Fig. 2a) in the presence of AQ are much higher than one-electron-transfer reduction potentials of AQ (2.52 V; AQ→AQ-Li+, a midpoint potential of ~2.36 V, red line in Fig. 2a), and one new cathodic peak (~2.4 V) appears simultaneously. In the ORR process, the redox state of AQ molecules are unchanged, while ORR currents are remarkably increased over 10 times at 2.4 V compared to those of bare Au electrode. CVs was also conducted with polished carbon as working electrode in the presence and absence of AQ, DBBQ and NQ molecules. The similar potential tendency as Au electrode were obtained (Fig. S2). It proved the remarkable increase of surface ORR activity from any electrodes. It should be noted that the redox potential of AQ is below 2.3 V vs Li+/Li (on Au electrode) in TBA+TFSI- electrolyte, while it is positively shifted to ~2.4 V vs Li+/Li in Li+TFSI- electrolyte (Fig. S3). Similar phenomenon is also observed for those of DBBQ in the TBA-TFSI and LiTFSI electrolytes (Fig. S3). It suggested the coupling strength of Li+ and AQ-/DBBQ- was stronger than those of TBA+ and AQ-/DBBQ-, resulting in the positive shift of potentials from the original AQ/AQ- redox. It is consistent with the classic EC’ mechanism.44-45 Herein, the positive shift of ORR potentials in AQ-system is based on the combination from AQ- to AQ-Li+ and from O2 to LiO2 to AQ-LiO2. Moreover, the starting ORR potentials of AQ-based electrolyte at Au and carbon electrodes is ~2.68 V and ~2.77 V vs Li+/Li respectively, which are consistence to those of starting ORR potentials initiated by bare Au and carbon electrodes. In contrast, both of the starting reduction potentials of AQ-based electrolyte is ~2.52 V vs Li+/Li at Au and carbon electrodes, and their corresponding positive shift of potential values are ~0.16 V and ~0.25 V (Fig. S2), respectively. It indicated that the substantial increase 6


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of surface ORR activity was dependent on the electrode surface, confirming the surfacepromoted ORR pathway by AQ molecule. The electrochemical behaviors of AQ, DBBQ and NQ were also investigated in dimethyl sulfoxide (DMSO)-based electrolyte (Fig. S4). A newly cathodic peak (~2.6 V) in the presence of AQ and O2 can also be detected, while the similar positive shift of potentials were not observed in the presence of NQ and DBBQ. It indicated that the similar species are existed in TEGDME and DMSO electrolytes. For the assignment of new ORR peak in the presence of AQ, there are two possible explanations. One is the formation of AQ-O2 species that is electrochemically reduced to AQ-O2- species, and the new cathodic peak is assumed to be AQ-O2-. Previous reports showed that metal phthalocyanine (Fe-PC, Co-PC)46-47 and CoII-salen35 might bind O2 to induce the positive shift of their initial reduction potentials. Another possibility is the formation of AQ-LiO2 species.34,48 It means that O2 is firstly reduced and then combines Li+ to form LiO2 species on electrode surface, which are captured by AQ molecules to form AQ-LiO2 species in the electrolyte. In the forward case, we conducted DFT calculation to check the absorption energy (AE) between O2 and AQ (Fig. S5). The AE between O2 and AQ is too low (˂‒0.2 eV) to form the stable species. Thus the second case shall be the reality that the newly cathodic ORR peak is assigned to be AQ-LiO2. The aforementioned DFT calculation for evaluating the AE of AQ-LiO2 (-1.01 eV) suggests this possibility. AQ molecules capture and shift the absorbed LiO2 from electrode surface to electrolyte with the formation of AQ-LiO2 species, which can maintain the cleanliness of the electrode surface and accelerate ORR catalysis initiated by solid catalysts. The proposed reaction process is quite similar to those of enzymatic biosynthesis with coenzyme. Besides, the stability of different quinones was investigated. As shown in Fig. S6, when the potential window increase to 4.5 V vs Li+/Li during anodic scan, no oxidation currents can be observed for AQ based electrolyte on Au electrode. In contrast, the clearly increased oxidation currents can be obtained in the DBBQ (>3.8 V vs Li+/Li, Fig. S6b) and NQ (>4.3 V vs Li+/Li, Fig. S6c) based electrolytes. Moreover, the 7



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oxidation currents of DBBQ and NQ based electrolytes are also larger than that of AQ at higher potential (4.3-4.5 V vs Li+/Li) with graphite carbon as the working electrode (Fig. S6d-S6f). Equally important, the overlapping CV curves (100 cycles) of AQ between 2.0-4.0 V vs Li+/Li further prove the electrochemical stability of AQ (Fig. S7). These results are in accordance with the computed oxidation potential of AQ derivatives (above 4.5 V vs Li+/Li).49

The ORR mechanism in the presence of Quinone As the results referred above, the surface-mediated ORR pathway with AQ is totally different from the previous solution-mediated ORR pathway with redox mediators in references. The reported redox mediators, such as EV,31 TTM,33 and DBBQ,34 accept an electron from the electrode firstly, and then chemically reduce O2 in the electrolyte. In contrast, the redox state of AQ molecule is unchanged in the surface-mediated ORR pathway. AQ performs as a scavenger and transporter to capture and shift LiO2 from the electrode surface to electrolyte. It is regenerated after the formation of Li2O2 through the disproportionation reaction of AQ-LiO2 species.

Fig. 3 | Simulated situation of O2 and LiO2 adsorbed on (a) AQ, (b) DBBQ. Adsorption energy was provided for all situations. Red spheres represent “O”, green ones represent “Li”, pink ones represent “H”, and brown ones represent “C”.

Ultraviolet-visible (UV-vis) spectrum spectroscopy was carried out to prove the formation of AQ-LiO2 species. As shown in Fig. S8a, the peak at ~260 nm assigned to the π→π* absorption shifts to the longer wavelength (~280 cm-1) after adding KO2 in testing solution (KO2 can release O2- in electrolyte, the concentration of AQ, KO2 and LiTFSI in TEGDME is about 0.05 mM, 0.1 mM, and 1 mM, respectively). In addition, 8


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the peak at ~330 nm assigned to the n→π* absorption does not shift, indicating the unchanged structure of functional group (C=O). The above peak characteristics for the π→π* and n→π* absorption suggest the increased electron conjugation of AQ molecules, probably ascribing to AQ-LiO2 species. The peak shift (~265→~270 nm) of the π→π* absorption is also observed in the DMSO medium (Fig. S8b). The shift values in DMSO is lower than those in TEGDME. It can be attributed to the stronger solvation capability of DMSO than that of TEGDME, as reported previously.20 In contrast, the peak of DBBQ at ~265 nm assigned to the π→π* absorption does not shift, while n→π* absorption is changed and a new peak appears at ~330 nm (Fig. S8c and S8d). It suggests that no electron conjugation is occurred, and DBBQ is reduced to DBBQ- by KO2.50 In the presence of Li+ and KO2, DBBQ-LiO2 is proposed to be formed in the solution, which is suggested by DFT calculations. Owing to the higher redox potentials (DBBQ/DBBQ-) than that of O2-/O2 in Li+-based electrolyte (Fig. S3), DBBQ- owns the stronger combination energy to Li+ than O2. As a result, DBBQ-LiO2 easily reacts with DBBQ--Li+. As shown in Fig. 3, the AE of AQ to LiO2 (-1.01 eV) is lower than that of AQ--Li+ to O2 (-1.52 eV), indicating the stronger interaction between Li+ and O2-. In contrast, AE of DBBQ to LiO2 is -1.15 eV, higher than that of DBBQ--Li+ to O2(-1.13 eV), suggesting the weaker combination between Li+ and O2-. Thus, the order of AEs to Li+ is DBBQ-＞O2-＞AQ-, in accordance with their redox potentials (Fig. S3).

Fig. 4 | (a) HNMR spectroscopy of AQ, AQ-Li+, AQ-KO2 and AQ-KO2-Li+ in DMSO solvents; (b) number of electrons on different carbon atom sites. There are four types of C atom sites in AQ molecule, which are marked in Fig. 4b (inset), and the Arabic numerals in Fig. 4b represent the H bonded C atom sites. 9



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In order to further clarify the existence of AQ-LiO2 species, hydrogen nuclear magnetic resonance (HNMR) spectroscopy was conducted to evaluate the chemical shifts of quinones in the presence of KO2 and/or Li+. As shown in Fig. 4a, chemical shifts of AQ molecules (black line) at 8.25 ppm and 7.95 ppm are assigned to H atoms at 1/4 sites and 2/3 sites (marked at molecular structure in Fig. 4b), respectively. As theoretical calculation in Fig. 4b, the symmetrical C atoms at 1 and 4 sites have the identical electron numbers, indicating the same chemical shifts of H atoms at 1 and 4 sites. Similarly, chemical shifts for H atoms of AQ at 2 and 3 sites are the same. When AQ is reduced to AQ- by KO2 (formula 1 and 2, Fig. 4a), four chemical shifts (8.25, 7.95, 7.60, 7.37 ppm) appear (AQ-KO2, blue line). Accordingly, from high-field to lowfield, the chemical shifts of H correspond to 4-3-2-1 sites of C atoms. When both Li+ and KO2 are added in the AQ solution (AQ-KO2-Li+, pink line), three chemical shifts (8.05, 7.71, 7.50) appear. The change of chemical shifts from AQ-KO2 to AQ-KO2-Li+ indicates that the state of AQ- is changed. The reaction process is summarized as follows: AQ +KO2→AQ- +O2

(1)

AQ- +Li+→AQ--Li+

(2)

AQ--Li++O2→AQ-LiO2

(3)

With the formation of AQ-LiO2 (formula 3), the electrons number on C3 decreases to the level close to C4, occupying the high field sites (8.05), while the H chemical shifts at C1 and C2 locate at low-field (7.71 and 7.50). Theoretical calculation well matches the location of chemical shifts of H atoms in the presence AQ-LiO2 in AQKO2-Li+ sample (Fig. 4b). Though the disproportionation reaction can occur in AQKO2-Li+ medium, the excess of KO2 can regenerate AQ- through reaction (1), ensuring the stable yield of AQ-LiO2 during the HNMR characterization. In order to clearly illustrate the difference of adsorption ability of LiO2 by AQ and DBBQ, H chemical shifts of DBBQ in the presence of KO2 and/or Li+ were also investigated as the reference (Fig. S9). The H chemical shifts of DBBQ (DBBQ, black line) at quinone ring and -CH3 groups locate at round 6.48 and 1.22 ppm, respectively. 10


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With addition of KO2 into DBBQ solution (DBBQ-KO2, blue line), the old peak at 6.48 remains and a new peak appears at 3.91 ppm, indicating the formation of DBBQ--Li+. Different from above AQ-KO2-Li+, chemical shifts of DBBQ are not changed in the presence of KO2 and Li+ (DBBQ-KO2-Li+, pink line). It means that no DBBQ-LiO2 is existed, ascribing to that DBBQ-LiO2 is totally consumed by DBBQ--Li+. It consists with the results shown in Fig. 3 and Fig. S9.

Fig. 5 | Schematic of ORR mechanism in the presence of (a) AQ and (b) DBBQ for Li-O2 batteries.

According to aforementioned theoretical and experimental results, the ORR mechanism in the presence of AQ is clarified. As shown in Fig. 5a, O2 accepts an electron from electrode surface firstly, and then combines Li+ to form LiO2 (formula 4). AQ plays the role of coenzyme that captures and stabilizes LiO2 intermediate (formula 5). After Li2O2 releasing (formula 6), AQ is regenerated as new scavenger and transporter. The reaction process is summarized as follows: O2+e-+Li+→LiO2

(4)

AQ+LiO2→AQ-LiO2

(5)

2AQ-LiO2→2AQ+Li2O2 +O2

(6)

DBBQ+e-+Li+→DBBQ-Li+

(7)

DBBQ-Li++O2→DBBQ-LiO2

(8)

DBBQ--Li++DBBQ-LiO2→DBBQ+Li2O2

(9)

In comparison, the ORR mechanism in presence of DBBQ is also clarified (Fig. 5b), a little difference from that in the previous reports.34 DBBQ is firstly reduced and combined with Li+ to form DBBQ--Li+ (formula 7), which reduces O2 to form DBBQLiO2 (formula 8). Although DBBQ-LiO2 is theoretically stable in electrolyte (as discussed in Fig. S1), it quickly transforms to Li2O2 that experiences the fast 11



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oxidation‒reduction reaction (formula 9) instead of the disproportionation reaction (2DBBQ-LiO2→2DBBQ+Li2O2+O2). The conclusion is strongly supported by the data of HNMR and UV-vis spectroscopy (Fig. S8c, S8d and S9), in which the signal of DBBQ-LiO2 species is not detected. The difference of two ORR mechanisms is ascribed to the gaps between the surface-mediated ORR potentials initiated by solid catalysts and their reduced potentials of AQ and DBBQ. Higher one (DBBQ↔DBBQ-, a midpoint potential of ~2.63 V vs Li/Li+ in TEGDME) reduces O2 by its reduced form, because the ORR starting potentials at electrode is overlapped with the reduction potentials of DBBQ. In contrast, the lower one (AQ↔AQ-, a midpoint potential of ~2.36 V vs Li/Li+ in TEGDME) is not reduced and works as a carrier of LiO2 from the electrode surface. Importantly, if the solid catalyst at cathode of Li-O2 batteries has high ability to absorb and reduce O2, both DBBQ and O2 are reduced electrochemically to DBBQ- and O2- on solid catalysts. DBBQ--liked soluble catalysts have less ORR competition ability than those of solid catalysts, ascribing to that the indirect electron transfer ORR pathway is much slower than that of direct pathway. It is therefore concluded that the adsorbed LiO2 is formed inevitably. Although the theoretical adsorption capabilities of LiO2 by DBBQ and AQ are similar in the DFT calculation, DBBQ converts to DBBQ--Li+ electrochemically at the cathode in battery operation. DBBQ--Li+ chemically reacts with LiO2 or DBBQ-LiO2 instead of the surface-mediated ORR pathway. Fortunately, AQ remains its oxidized state in the surface-mediated ORR pathway, and LiO2 intermediates are stabilized by AQ to form AQ-LiO2 in electrolytes. Consequently, the formation of dense Li2O2 film is prevented on the cathode in contrast to those of redox mediator catalysts, even if the solid catalysts have the high O2 adsorption ability and ORR activity. Li-O2 battery capacity in the presence of Quinone

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Fig. 6 | Comparison of the cell capacities without quinone (black line), with DBBQ (red line), with NQ (green line) and with AQ (blue line) at (a) 100 mA g-1 and (b) 500 mA g-1 (the electrolyte was 1M LiTFSI in TEGDME and the concentration of quinones was controlled at 10 mM).

According to the above-mentioned ORR mechanism, the cell capacities are assumed to be significantly improved by incorporating AQ molecules. For the assembly of aprotic Li-O2 battery, commercial carbon (Ketjen Black, KB) was chosen to construct oxygen cathode for revealing the real situation. TEGDME was selected as the electrolyte solvent for cell assembly due to its superior stability towards Li metal and carbon over DMSO-based electrolyte.51 As shown in Fig. 6, the batteries with AQ exhibit the highest capacities at both low (~11000 mAh g-1 at 100 mA g-1, Fig. 6a) and high (~4700 mAh g-1 at 500 mA g-1, Fig. 6b) current densities, about 3.6 times as those of batteries without quinones. Addition of DBBQ and NQ also increases the cell capacities from ~3300 mAh g-1 to ~6400 mAh g-1 and ~4400 mAh g-1 at 100 mA g-1, respectively. It indicates AQ molecule has the outstanding capability to capture LiO2 intermediates from electrode surface, substantially boosting the ORR catalysis of cathode surface and prompting the solution growth of Li2O2 in battery operation. To confirm the role of AQ in battery operation, the morphology of discharge product was investigated by scanning electron microscopy (SEM). As shown in Fig. 7b, the discharge products without quinone exhibit the typical toroidal structure with the diameter less than 1 μm. With the aid of quinones, the sizes of discharge product increase to several micrometers (Fig. 7c-7f), indicating the role of quinones in prompting the solution-growth of Li2O2. It shall be noted that the discharge product in the presence of AQ is not the toroidal morphology but the spherical structure assembled by nanosheets (Fig. 7e). The packed nanosheets show the more loosely structure than 13



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those of toroidal products with DBBQ and NQ (Fig. 7c and S10). The distinct morphologies probably can be attributed to that the different discharge mechanism is evolved among the studied quinones in aprotic electrolyte. Meanwhile, the loosely packed nanosheets shall be easily decomposed in comparison with the close-packed one. As shown in Fig. S11, the cell charging overpotential with AQ is the lowest one although the highest capacity achieved.

Fig. 7 | SEM images of (a) the pristine KB electrode, (b) the discharged KB electrode without quinone, the discharged electrode of KB with DBBQ (c and d) and AQ (e and f) additives. The battery was discharged to 2 V in the electrolyte with 1 M LiTFSI in TEGDME.

The measurements of X-ray diffraction (XRD), infrared (IR) and Raman spectroscopy were carried out to confirm the physical structure of discharged species in the presence of quinones. XRD patterns (Fig. 8a, Fig. S12a) display two peaks at about 33° and 35°, well matching to that of Li2O2. The other two peaks (~43° and ~51°) are associated with stainless steel substrate. It proves that Li2O2 is the primary discharge products, which is further confirmed by IR (Fig. 8b, Fig. S12b) and Raman spectra (Fig. 8c). Tiny impure peaks are found in the IR spectra and they are associated with the minor byproduct of lithium acetate/formate and Li2CO3, similar as previous reports.5253

Raman peaks at around 250 cm-1 and 790 cm-1 are associated with O-O bonding of

Li2O2. There are no other peaks assigned to byproducts.

14


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Fig. 8 | (a) XRD patterns, (b) infrared and (c) Raman spectra of the discharge product of the batteries with quinones.

It has been reported that DBBQ can improve the battery capacities by 80-folds with a capacity of approximate 10 mAh cm-2 by using H2 treated CP as cathode and TEGDME as solvent.34 Here, the cell capacity with DBBQ is about 5.7 mAh cm-2 (loading mass of KB carbon is about 0.75 mg). The only difference of cell assembly between the reference34 and this work is CP and KB carbon for the construction of cathodes, respectively. KB carbon surface is riddled with defect sites, which can bond and reduce O2 to form adsorbed LiO2 after accepting one electron and Li+. It is therefore suggested that the growth of Li2O2 through surface-mediated ORR pathway can not be avoided by adding the redox mediators. It shall be noted that, in the battery operation process, DBBQ--Li+ is generated on the cathode surface. Once the adsorbed LiO2 forms, film-like

Li2O2

can

be

rapidly

formed

(DBBQ--Li++LiO2→Li2O2+DBBQ).

Accordingly, the effectiveness of soluble redox mediators, such as DBBQ and EV, is substantially reduced, when the cathode surface has the good ORR activity and capability of adsorbed LiO2 formation. To confirm this hypothesis, CP electrode etched by oxygen plasma (P-CP) was prepared. The capacity of DBBQ assisted battery is clearly reduced from 5.7 mAh cm-2 (CP) to 2 mAh cm-2 (P-CP) (Fig. S13), indicating the remarkably decreased amount of Li2O2 through surface-mediated ORR pathway. In contrast, the capacities of AQ-assisted batteries based on CP and P-CP cathodes only exhibit a little difference (2.6 mAh cm-2 for CP and 2.3 mAh cm-2 for P-CP). It indicates the key role of AQ in capturing LiO2 species from the electrode surface. To further clarify the coenzyme-like effect of AQ, a typical metal oxide (Co3O4) electrode with high O2 absorption ability and ORR activity was used as the cell cathode. Co3O4 based 15



Page 16 of 20

batteries in the presence of AQ offer the three and two times capacities compared to those of batteries without and with DBBQ, respectively (Fig. S14). The enhanced capacities for KB-carbon and Co3O4 cathodes in the presence of three types of quinones further confirm the ORR mechanism.

Conclusions and Outlook Inspired by natural enzymatic system, a new pathway is developed to accelerate ORR rate of solid catalysts through capturing the adsorbed MO2 species from the surface of solid catalysts. The capability of capturing LiO2 intermediate by AQ is clarified by theoretical calculation, electrochemical and physical characterizations. With the aid of AQ, the ORR activity of solid catalysts is increased one magnitude with the assistance of AQ-LiO2 formation. As a model metal-O2 battery system, the capacities of Li-O2 batteries can be improved over three times, surpassing state-of-the-art DBBQ redox mediator. The key requirements to initiate the “enzyme-coenzyme” cooperative catalysis ORR pathway in metal-O2 batteries are the suitable redox potential and high absorption ability of quinone to metal superoxide species. The other molecules that meet the two key factors, as well as other excellent characteristics including high stability, nontoxic, and solubility, may have bright future to remarkably enhance surface ORR rate and improve the cell capacities. Biomimicking enzymatic biosynthesis by using coenzyme-like transporter in the growth of M2O2/MO2 products paves a new way to greatly maximize the capacity of metal-O2 batteries or other energy devices. It also provides a guidance for accelerating chemical synthesis reactions with similar reaction mechanism.

ASSOCIATED CONTENT Supporting Information Experimental section, calculation details, electrochemical experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION 16


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Corresponding Authors [email protected] (Y. Z.) Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS The authors gratefully acknowledge National Natural Science Foundation of China (U1604122, 21773055, 51702086, 21203055, 21805070), Program for Science & Technology Innovation Talents in Universities of Henan Province (18HASTIT004), and the “1000 Youth Talents Plan” of China. The authors thank Dr. Lei Jiang from Techinical Institute of Physics & Chemistry, Chinese Academy of Sciences, Dr. Zhiyong Jiang from Henan University for his assistance with HNMR measurements, Dr. Yu Jia, Dr. Binbin Hu from Henan University and Dr. Binju Wang from Xiamen University for his valuable discussion.

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Promoting Surface-mediated Oxygen Reduction Reaction of Solid

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