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Letter 2

How to Control the Discharge Products in Na-O Cells: Direct Evidence toward the Role of Functional Groups at the Air Electrode Surface Hossein Yadegari, Christopher J. Franko, Mohammad Norouzi Banis, Qian Sun, Ruying Li, Gillian R. Goward, and Xueliang Sun J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02227 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

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The Journal of Physical Chemistry Letters

How to Control the Discharge Products in Na-O2 Cells: Direct Evidence toward the Role of Functional Groups at the Air Electrode Surface Hossein Yadegari,1 Christopher J. Franko,2 Mohammad Norouzi Banis,1 Qian Sun,1 Ruying Li,1 Gillian R. Goward2 and Xueliang Sun1,* 1 Department of Mechanical and Materials Engineering, University of Western Ontario, London, Ontario N6A 5B9, Canada 2 Department of Chemistry and Chemical Biology, McMaster University, Hamilton, Ontario L8S 4M1, Canada

Corresponding Author Corresponding author: Xueliang Sun; E-mail: [email protected]

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Abstract Sodium-oxygen batteries have received a significant amount of research attention as a lowoverpotential alternative to lithium-oxygen. However, the critical factors governing the composition and morphology of the discharge products in Na-O2 cells are not thoroughly understood. Here we show that oxygen containing functional groups at the air electrode surface have a substantial role on the electrochemical reaction mechanisms in Na-O2 cells. Our results show that presence of functional groups at the air electrode surface conducts the growth mechanism of discharge products toward a surface-mediated mechanism, forming a conformal film of products at the electrode surface. In addition, oxygen reduction reaction at hydrophilic surfaces more likely passes through a peroxide pathway which results in formation of peroxidebased discharge products. Moreover, in-line XRD combined with solid state

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Na NMR results

indicate the instability of discharge products against carbonaceous electrodes. The findings of this study help to explain the inconsistency among various reports on composition and morphology of the discharge products in Na-O2 cells and allow the precise control over the discharge products.

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Sodium-oxygen (Na-O2) batteries have been attracting an extensive amount of attention during the past half-decade as an alternative battery system to lithium-oxygen (Li-O2).1-6 Lower charging overpotential of superoxide Na-O2 cells is considered as one of the main advantages of these cells over the peroxide Li-O2 cells.2-3 However, a variety of discharge products (i.e. NaO2, Na2O2, Na2O2.xH2O and Na2CO3) with different charging overpotentials have been reported in literature for Na-O2 cells despite of employing relatively similar experimental conditions.3 In addition, both of sodium superoxide (NaO2) and peroxide (Na2O2) have been predicted by the theoretical computational techniques as the major discharge products of the Na-O2 cells.7-8 The electrochemical reaction of superoxide Na-O2 cells (O2 + Na+ + e- → NaO2) is a classic reversible redox reaction and hence requires a small overpotential (~200 mV) during the charge cycle.5 While peroxide Na-O2 cells exhibit a large charging overpotential (~1000 mV) which is comparable to that of Li-O2 counterparts.9-10 Nevertheless, the determining experimental factors which lead to the formation of either products in Na-O2 cells are not thoroughly understood. Multiple reaction mechanisms have been proposed by number of researchers to explain the origin of products diversity in Na-O2 cells. Kim et al. proposed a mechanism based on proton abstraction from the electrolyte solvent by dissolved superoxide (O2-) ions.11 According to the authors, the solvent undergoes oxidative decomposition to form hydroxide (OH-) ions which then reacts with the hydrogen peroxide (H2O2) produced from disproportionation reaction of HO2 in the presence of Na+ ion, resulting in formation of Na2O2.2H2O as the product of the cell. A similar reaction mechanism based on the degradation of NaO2 by an ether-based electrolyte and formation of Na2O2.2H2O was also proposed by Sayed et al.12 In addition, formation of Na2O2.8H2O as a result of keeping a discharged cell in open circuit potential for an extended period of time (100 h) was observed by Black et al.13 3



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Although the proposed reaction mechanisms indicate the potential sources of side-products in Na-O2 cells, yet the dominant factor governing the morphology and composition of bulk discharge products in the cells remains a mystery. In the present study, we reveal the effect of oxygen containing functional groups at the air electrode surface on the morphology and composition of discharge products in Na-O2 cells. We find that the discharge reaction in Na-O2 cells follow distinct mechanisms on hydrophobic and hydrophilic carbonaceous surfaces. A solution-mediated growth mechanism during the discharge cycle of Na-O2 cells using a diglyme electrolyte leads to formation of crystalline NaO2 products on a hydrophobic air electrode. On a hydrophilic air electrode, however, a conformal layer of products is being produced via a surface-mediated growth mechanism. Our results highlight the significant role of oxygen containing functional groups at the air electrode surface on conducting the discharge reaction path in Na-O2 cells. Hydrophobic and hydrophilic air electrodes in this study were prepared using a commercial carbon cloth diffusion layer as starting material (see Supporting Information for experimental details). Figure 1 compares the normalized wide-scan survey X-ray photoemission spectra (XPS) for hydrophobic and hydrophilic air electrodes. Hydrophilic air electrode exhibits O 1s peak more than 15 times larger than that of hydrophobic electrode due to the high concentration of oxygen containing functional groups at the electrode surface. Based on the deconvoluted XPS spectra of the C 1s and O 1s regions, the oxygen-containing functional groups are mainly composed of carboxyl and carbonyl/carbocyclic groups.14 In addition, the intensity ratio of disordered and graphitized bands (ID/IG) in Raman spectra (Figure S1) increases from 0.92 to 1.06, indicating an increased amount of defects at the electrode surface due to the presence of oxygen-containing functional groups. The air electrodes were also characterized by solid state 1H 4


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NMR (figure S2). The hydrophobic electrode shows a small signal corresponding to the edge protons around the graphite sheets. A second higher frequency signal appears for the hydrophilic counterpart which is consistent with the added hydroxyl groups seen in the XPS spectrum. Meanwhile, both electrodes exhibit similar macroscopic and microscopic morphological characteristics (Figure S3).

Figure 1. Normalized wide-scan survey XPS spectra for hydrophobic and hydrophilic air electrodes. Deconvoluted C 1s and O 1s are presented as insets. The electrochemical performance of the hydrophobic and hydrophilic air electrodes for oxygen reduction reaction (ORR) was examined by chronopotentiometry. Figure 2a shows the discharge curves of the electrodes in Na-O2 cells. Hydrophobic air electrode shows a constant-potential discharge plateau, while hydrophilic electrode displays a sloping discharge curve with a 5



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significantly lower capacity value. It is also noticeable that the discharge reaction on the hydrophilic electrode starts at higher potentials compared to that of on the hydrophobic electrode, indicating different electrochemical processes at the electrode surfaces. Similar discharge behavior was observed using hydrophobic and hydrophilic electrodes in Li-O2 cells (Figure S4a). Moreover, direct observation of the discharged hydrophobic and hydrophilic air electrodes illustrates a substantial deference between the morphology of the discharge products on the different surfaces in both cells. Crystalline cubic discharge products are formed on the hydrophobic air electrode in the case of Na-O2 cell (Figure 2b and c), whereas a conformal film covers the hydrophilic air electrode (Figure 2d and e). Similarly, crystalline toroid-shaped products can be seen on the hydrophobic air electrode discharged in a Li-O2 cell (Figure S4b and c), while a uniform conformal film of products are formed on the discharged hydrophilic surface (Figure S4d and e). SEM micrographs of a hydrophobic air electrode discharged to the same capacity as a fully-discharged hydrophilic electrode in a Na-O2 cell are shown in Figure S5. Unlike hydrophilic surfaces, square-shaped films of products can be seen at the hydrophobic electrode surface discharged to a limited capacity. These square-shaped products will probably grow into cubic NaO2 by continuing the discharge process. Significant changes in morphology of the discharge products indicate different growth mechanism on hydrophobic and hydrophilic surfaces. A solution-mediated mechanism results in formation of micrometer-size crystalline discharge products on hydrophobic air electrodes in both Li- and Na-O2 cells. On the other side, a surface-mediated mechanism produces a conformal film of discharge products. A similar transition in growth mechanism as a function of trace amounts of protic additives in the cell electrolyte has been reported for both Li- and Na-O2 cells.4,

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Enhanced solvation of superoxide intermediates by protic additives was shown to 6


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transit the growth mechanism from surface-mediated to solution-mediated. However, our results illustrate that the surface properties of the air electrode may also affect the growth mechanism of the products during the discharge cycle of the cells.

Figure 2. (a) Discharge curves of hydrophobic and hydrophilic air electrodes in Na-O2 cells. (be) SEM micrographs for hydrophobic (b and c) and hydrophilic (d and e) air electrodes discharged in Na-O2 cells. Combining thermodynamics with kinetics, galvanostatic intermittent titration technique (GITT) retrieves unique information about the discharge mechanism in metal-O2 cells.16 The discharge curves of Na-O2 cells employing hydrophobic and hydrophilic air electrodes are compared with corresponding GITT curves with different relaxation times in Figure 3a and b. Discharge capacity of the cells with hydrophobic air electrode increases with increase of the relaxation 7



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time, while the discharge capacity in the case of hydrophilic air electrodes remains the same. The correlation between discharge capacity and relaxation time supports the solution-mediated growth mechanism on hydrophobic air electrodes. In the case of hydrophilic air electrodes, however, the capacity is rather limited by charge transfer at the electrode surface which is in accordance with the sloping potential during the discharge. In fact, the surface-mediated growth mechanism at the hydrophilic air electrode gradually increases the thickness of conformal film of products at the electrode surface which results in continues increase of the charge transfer resistance. The increase of the charge transfer resistance in turn leads to constant potential drop during the discharge process. In addition, direct observation of the products on the hydrophobic air electrodes discharged using GITT mode with different rest times (Figure S6) indicates that the increased capacity is a result of increased density of the discharge products. The increased capacity in the cells discharged under GITT mode contradicts the proposed limiting factor of pore clogging during the discharge cycle of Na-O2 cells (at least on the low surface area electrodes).17 Instead, GITT results suggest that the discharge capacity in Na-O2 cells with hydrophobic air electrode is mainly limited by mass transfer, since applying longer rest time periods between consecutive discharge steps increases the discharge capacity. In fact, the discharge capacity in Na-O2 cells is dictated by an equilibrium between the amount of O2- dissolved into the cell electrolyte with the amount of precipitated NaO2. Consecutive discharge/rest periods during a discharge process under GITT mode result in multiple saturation/depletion of O2- in the cell electrolyte. Meanwhile, new NaO2 nucleation sites grow on the air electrode surface from O2--saturated electrolyte during the rest periods which increases the density of the discharge products at the electrode surface (Figure S6). Moreover, longer rest periods result in further depletion of O2- in the cell electrolyte which 8


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in turn postpone the potential drop due to the oversaturation of O2- in the cell electrolyte and result in increased discharge capacity of the cells. Similar phenomenon has been reported for LiO2 cells.16 In a constant current discharge process, however, the formation rate of O2- exceeds the precipitation rate of NaO2, leading to the potential drop and premature discharge termination.

Figure 3. Discharge curves for hydrophobic (a) and hydrophilic (b) air electrodes using regular constant current mode (0 min rest time) as well as GITT mode with different rest times (5 and 15 min rest times). The surface properties of the air electrode have a more significant effect on the charge characteristics in Na-O2 cells compared with those in Li-O2 (Figure S7a and b). Both hydrophobic and hydrophilic air electrodes show comparable charging overpotentials in Li-O2 cells, while the charging features in Na-O2 cells remarkably depends on the air electrode. Na-O2 9



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cells with hydrophobic air electrode exhibit a low overpotential plateau which is characteristic of NaO2 decomposition.2, 4 Hydrophilic air electrode, however, display a sloping charge curve with poor coulombic efficiency. Similar charge curves were obtained using restricted discharge capacities in both Li- and Na-O2 cells (Figure S8). Such a substantial difference between charging behaviors can be correlated to the chemical composition of the discharge products.9 The discharge products of Li-O2 cells have been shown to be Li2O2 on both hydrophobic and hydrophilic air electrodes which explain similar charging overpotentials.18 In the case of Na-O2 cell, NaO2 was detected by X-ray diffraction (XRD) on the discharged hydrophobic air electrode, whereas no obvious peaks were detected on the discharged hydrophilic air electrode (Figure S9). However, based on the morphology of the discharge products as well as charge curve characteristics, the product of the cell using hydrophilic air electrode is not NaO2. Different discharge products formed in Na-O2 cells using hydrophobic and hydrophilic air electrodes indicate distinct electrochemical reaction paths. In addition, different electrochemical responses were observed using cyclic voltammetry (CV), shown in Figure S10. Hydrophobic air electrode exhibits reversible O2/O2- redox peak, whereas a single oxidation peak appears around 3.4 V vs. Na/Na+ in the case of hydrophilic electrode. Comparison of linear sweep voltammetry (LSV) curves for hydrophobic and hydrophilic electrodes in Na-O2 cell (Figure S11) also demonstrate a more positive onset potential for hydrophilic air electrode which is consistent with discharge potentials (see Figure 2a). The more positive potential observed for ORR at hydrophilic air electrode can be related to the catalyzed oxygen reduction through the peroxide pathway. It is widely accepted that the presence of oxygen-containing functional groups facilitates the oxygen adsorption at the electrode surface and hence catalyzes the ORR reaction

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by decreasing the activation energy.19-22 The ORR at hydrophilic electrode surface then can be proposed as follow: O2 (ads) + e- + Na+ → Na+-O2- (ads)

(1)

Na+-O2- (ads) + e- + Na+ → Na2+-O22- (ads)

(2)

It should also be noted that Na+ may be replaced by acidic protons from functional groups at the electrode surface in equation (1) or (2) to form a variety of products with general formula of NaxH2-xO2 (0≤x≤2). Accordingly, abstraction of acidic protons from functional groups may result in formation of H2O2:19 O2 + H+ + e- → HO2* (3) 2HO2* → H2O2 + O2 (4) The produced H2O2 may also chemically react with the formed NaO2, resulting in formation of Na2O2.2H2O through an indirect reaction pathway: 2NaO2 + 2H2O2 → Na2O2.2H2O + 2O2

(5)

Examining the hydrophobic and hydrophilic air electrodes by XPS after discharge and charge cycles in Na-O2 cells (Figure S12) indicates that the surface of hydrophilic air electrode undergoes severer changes compared to that of hydrophobic electrode. Hydrophilic air electrode exhibits a significant increase in the carboxyl/carbonate peaks after discharge cycles which could be related to the oxidation of hydroxyl groups via a nucleophilic attack by O2- to form carbonatebased side products:

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-COH + O2- + Na+ → NaHCO3

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(6)

The charged electrode, however, shows a relatively lower amount of carbonate groups which can be related to the CO2 release during the charge cycle.21 In addition, an increase in the intensity of peak around 286 eV for the charged hydrophilic electrode can be related to the formation of epoxide O-C-O groups.23-24 In comparison, hydrophobic air electrode only demonstrates a slight increase in carboxyl/carbonate groups after discharge and charge cycles. The XPS results prove that the functional groups at the electrode surface involve in the electrochemical processes of the cell. Similarly, higher concentration of carboxyl/carbonate groups were found on a defect-rich glassy carbon electrode compared with a highly oriented pyrolytic graphite, after the electrodes were potentiostatically discharged in Li/O2 electrochemical cells.24 Furthermore, hydrophilic air electrode may also facilitate the reduction of residual moisture in the cell electrolyte. Figure S13 compares the discharge/charge responses of the hydrophobic and hydrophilic air electrodes with and without 20 ppm of added water into the cell electrolyte. The discharge capacity of the hydrophobic air electrode slightly increases with added water into the cell electrolyte. In contrast, adding water decreases the discharge capacity in the case of hydrophilic air electrode. It should also be mentioned that water content of the cell electrolyte did not show any notable effect on the morphology of the products (Figure S14). Different responses of the hydrophobic and hydrophilic air electrodes toward the added water into the cell electrolyte can be related to the reduction of water at hydrophilic surface, forming NaOH as a side-product: 2H2O + 2e- + 2Na+ → H2 (g) + 2NaOH

(7)

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Formation of insulator side-products in turn block the reaction sites at the electrode surface and restricts the discharge capacity of the cell by imposing an increasing overpotential during the discharge process. In contrast, reduction of water at hydrophobic electrode surface requires higher overpotential to overcome the repulsion barrier which makes it unfavorable in competition with ORR. Therefore, additional water in the cell electrolyte using a hydrophobic air electrode play the role of “phase-transfer catalyst” and increases the discharge capacity.4 Decomposition of the initially formed discharge products at the air electrode surface is another potential source of side-products and may be partially accountable for the wide range of products reported in Na-O2 cells. To monitor the decomposition process of the discharge products, hydrophobic and hydrophilic air electrodes discharged in Na-O2 cells were placed in an air-tight XRD sample holder and in-line consecutive diffractograms were recorded within 13 hours (Figure 4). While NaO2 was detected as the major product on the fresh discharged hydrophobic air electrode, Na2O2.2H2O peaks started to appear within 1 hour and continuously raised (Figure 4a). NaO2 on hydrophobic air electrode thoroughly transformed into Na2O2.2H2O after 13 h even in the controlled atmosphere of air-tight XRD sample holder which is consistent with previous reports.11, 25 It should also be noted that the resultant Na2O2.2H2O is also unstable and gradually degrades within time at a lower decomposition rate compared with NaO2 (Figure S16 and 17). On the other side, the products remained on the hydrophilic air electrode are different from hydrophobic air electrode, composing of H2O2, Na2O2.2H2O and Na2CO3 (Figure 4b). The inline XRD results indicate the instability of NaO2 against the carbonaceous air electrodes which in turn results in detection of variety of products in different studies.

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Figure 4. Consecutive in-line XRD patterns for hydrophobic (a) and hydrophilic (b) air electrodes discharged in Na-O2 cells. Solid State 23Na NMR was also implemented to study the discharge products of the hydrophobic and hydrophilic air electrodes. Discharged electrodes were placed in air tight NMR rotors and spun under N2 gas. The NMR spectra show NaO2 as the major discharge product on hydrophobic electrodes when examined recently after discharge (figure 5a). In-line with the time dependent XRD data, NaO2 then decomposes over several hours as the electrode sits in the NMR magnet. Distinct from the XRD data, the NMR experiment shows the main decomposition product of NaO2 to be Na2CO3. This is analogous to previous studies showing NaO2 degrading in the 14


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presence of carbonaceous electrodes.6 To confirm this pathway, synthesized NaO2 was ground with hydrophobic electrodes and examined by 23Na NMR (figure S18). NaO2 is shown to slowly degrade into Na2CO3 in the presence of the hydrophobic air electrode, and at similar rates to the observed degradation in the cycled air electrode, confirming the suspected degradation pathway. Na2O2.2H2O is not seen in the grinding experiments as there was no source of water or electrolyte to provide the protons for such a breakdown pathway. Na2CO3 is observed to be a more dominant degradation product in the 23Na NMR spectra than in the XRD pattern. This is expected as the Na2CO3 is formed in a non-crystalline, amorphous like, state which the XRD naturally underestimates (Figure 2). NMR has no such demand for crystallinity and therefore images the Na2CO3 more effectively. Conversely, the

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Na spectrum

of Na2O2.2H2O is not known and may be overlapping with the other electrochemical product lineshapes.

Heteronuclear correlation experiments for separating the

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Na lineshapes are

underway. Both NMR and XRD have useful application to the study of Na-O2 cells, and in tandem paint a clearer picture of the competing degradation pathways for NaO2 on the hydrophobic air electrodes. The hydrophilic cathode was also examined by

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Na NMR (Figure

5b). In-line with the XRD data, no NaO2 is observed and Na2CO3 is seen as a major discharge product. As before, the NMR spectra suggest larger amounts of Na2CO3 than the XRD diffractograms. Also in agreement with XRD, where only a single, un-changing phase is formed, there is negligible evolution of discharge products seen by NMR over the time scale of the experiments (about every 45 min for both NMR and XRD data).

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Figure 5. 23Na NMR spectra of discharged hydrophobic (a) and hydrophilic (b) air electrodes at different intervals after discharge. Below, Reference spectra of Na2CO3 and NaO2. The presented results in this study illustrate the significant role of oxygen containing functional groups on the composition and morphology of the discharge products in Na-O2 cells. Presence of these functional groups not only affects the morphology of the discharge products, but also alters the composition of the cell products. Compared with Li-O2 cells with relatively more stable Li2O2 product, Na-O2 cells are more sensitive toward oxygen containing functional groups at the air electrode surface. While crystalline NaO2 products are formed on hydrophobic air electrodes, a conformal film of products is deposited on the hydrophilic air electrode surface. In addition, electrochemical studies suggest distinct ORR mechanisms at hydrophobic and hydrophilic surfaces. ORR at hydrophilic electrodes more likely passes through a peroxide pathway catalyzed by oxygen-containing functional groups at the electrode surface. On the hydrophobic

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surfaces, however, superoxide intermediates dissolve into the cell electrolyte in lack of adsorbing functional groups and precipitate as NaO2 after combining with Na+ ions. Moreover, the trace amount of water in the cell electrolyte may be electrochemically reduced at defect sites on the hydrophilic electrodes to produce NaOH in the presence of Na+ ions. These results explain the inconsistency among the different reports on composition of the discharge products in Na-O2 cells. Presence of uncertain amount of functional groups at the air electrode surface combined with slight differences in water content of the cell electrolyte in various studies lead to the formation of discharge products with variety of compositions and morphologies. The inherent instability of NaO2 formed at hydrophobic air electrode surfaces is another potential reason for inconsistent results. In-line XRD results demonstrated that NaO2 completely transforms into Na2O2.2H2O within a few hours. In addition, progressive formation of Na2CO3 was revealed by solid state 23Na NMR. Nevertheless, the mechanism behind the decomposition of NaO2 and formation of Na2O2.2H2O or Na2CO3 is not thoroughly understood at this time and requires more fundamental studies. ASSOCIATED CONTENT Supporting Information. Electrochemical data, additional SEM micrographs, and extended characterization results including Raman, XRD, XPS and NMR. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT

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This research was supported by Natural Sciences and Engineering Research Council of Canada, Canada Research Chair Program, Canada Foundation for Innovation, Canadian Light Source (CLS) and the University of Western Ontario. REFERENCES 1. Xia, C.; Fernandes, R.; Cho, F. H.; Sudhakar, N.; Buonacorsi, B.; Walker, S.; Xu, M.; Baugh, J.; Nazar, L. F., Direct Evidence of Solution-Mediated Superoxide Transport and Organic Radical Formation in Sodium-Oxygen Batteries. J. Am. Chem. Soc. 2016, 138, 11219-11226. 2. Hartmann, P.; Bender, C. L.; Vracar, M.; Durr, A. K.; Garsuch, A.; Janek, J.; Adelhelm, P., A Rechargeable Room-Temperature Sodium Superoxide (NaO2) Battery. Nat Mater 2013, 12, 228-232. 3. Yadegari, H.; Sun, Q.; Sun, X., Sodium-Oxygen Batteries: A Comparative Review from Chemical and Electrochemical Fundamentals to Future Perspective. Adv. Mater. 2016, 28, 7065-7093. 4. Xia, C.; Black, R.; Fernandes, R.; Adams, B.; Nazar, L. F., The Critical Role of PhaseTransfer Catalysis in Aprotic Sodium Oxygen Batteries. Nature chemistry 2015, 7, 496501. 5. McCloskey, B. D.; Garcia, J. M.; Luntz, A. C., Chemical and Electrochemical Differences in Nonaqueous Li-O2 and Na-O2 Batteries. J. Phys. Chem. Lett. 2014, 5, 1230-1235. 6. Reeve, Z. E.; Franko, C. J.; Harris, K. J.; Yadegari, H.; Sun, X.; Goward, G. R., Detection of Electrochemical Reaction Products from the Sodium-Oxygen Cell with Solid-State 23Na NMR Spectroscopy. J. Am. Chem. Soc. 2017, 139, 595-598.

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Cells - ACS Publications - American Chemical Society

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