Simultaneous Suppression of Metal Corrosion and Electrolyte

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Simultaneous Suppression of Metal Corrosion and Electrolyte Decomposition by Graphene Oxide Protective Coating in Magnesium-Ion Batteries: Toward a 4V-wide Potential Window S. J. Richard Prabakar, Chunguk Park, Amol Bhairuba Ikhe, Kee-Sun Sohn, and Myoungho Pyo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16103 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017

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ACS Applied Materials & Interfaces ACS Appl. Mater. Interfaces

Simultaneous Suppression of Metal Corrosion and Electrolyte Decomposition by Graphene Oxide Protective Coating in Magnesium-Ion Batteries: Toward a 4V-wide Potential Window S. J. Richard Prabakar,† Chunguk Park,† Amol Bhairuba Ikhe,† Kee-Sun Sohn,‡,* and Myoungho Pyo,†,*

†

Department of Printed Electronics Engineering, Sunchon National University, Chonnam 57922, Republic of Korea

‡

Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 05006, Republic of Korea

ABSTRACT: Despite remarkable developments in electrolyte systems over the past two decades, magnesium-ion batteries still suffer from corrosion susceptibility and low anodic limits. Herein we describe how graphene oxide (GO), coated onto non-noble metals (Al, Cu, and stainless steel) via electrophoretic deposition, can solve this problem. In all phenyl complex (APC) electrolytes, GO-coating results in a significant suppression of corrosion and extends the anodic limits (up to 4.0 V vs. Mg/Mg2+) with no impact on reversible Mg plating/stripping reactions. The same effect of GO-coating is also established in magnesium aluminum chloride complex (MACC) electrolytes. This remarkable improvement is associated with the electrostatic interaction between the ionic charges of electrolytes and the surfacefunctional groups of GO. In addition, GO-coating does not aggravate the cathode performance of Mo6S8, which allows the use of non-noble metals as current collectors. We also discuss the role of GO in increasing anodic limits when it is hybridized with α-MnO2. 1

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KEYWORDS: Non-noble metal, graphene oxide, electrophoresis, corrosion, magnesium-ion batteries

■ INTRODUCTION Magnesium is one of the key elements in the quest for an efficient alternative to lithium-ion batteries. The use of the reversible Mg/Mg2+ redox reaction for anodes and a Mg2+ insertion/extraction process for cathodes in magnesium-ion batteries (MIBs) has benefits that include natural abundance, high theoretical capacity (3832 mAh·cm-3), and a reduction in safety concerns (no dendrite formation).1,2 However, the formation of a passivation layer, which blocks Mg2+ transport, on the Mg surface in common electrolytes requires the development of new Mg-compatible electrolyte systems.3,4 The first successful version of this type of electrolyte was realized by combining MgR2 (RMgCl) Lewis base and AlCl3-nRn Lewis acid (R = alkyl).5-7 This electrolyte allowed reversible Mg plating/stripping on a Mg metal anode and Mg2+-intercalation into a Mo6S8 Chevrel cathode with a potential window of ca. 2.2 V.6 The electrochemical properties of this type of electrolyte, anodic limits in particular, were further improved by tailoring Lewis bases and/or Lewis acids.8-12 For example, all phenyl complex (APC, 2PhMgCl + AlCl3) solutions show an increase of almost 1.0 V in the anodic limit, which allows 3V-class MIBs, at least in principle.8 The potential viability of the Lewis acid-Lewis base couples has stimulated active research to formulate other types of electrolytes. These include an inorganic 2


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magnesium aluminum chloride complex (MACC, mMgCl2 + nAlCl3) with no Grignard reagents,13-15 a magnesium-only complex (mRMgCl + nMgCl2) with no Lewis acid,16,17 a ternary mixture (MgCl2 + APC),18 etc.19,20 Most of these electrolytes showed a reasonable anodic stability in between 2.8 and 3.5 V vs. Mg/Mg2+ on Pt and glassy carbon (GC). The high anodic limits reported for inert electrodes are rather unrealistic, however, because severe corrosion of the non-noble metals (current collectors and/or coin cell casings) can occur in chloride-containing electrolytes before the potential for electrolyte decomposition is reached. For example, the potential window of 2.3 V for Mg(AlCl2R1R2)2 (R1=ethyl, R2=butyl) on Pt was significantly decreased to 1.7 V on Cu.21 The same trend was also demonstrated in APC,10,20 MACC, and (hexamethyldisilazide)2Mg/AlCl3,22 which indicated that chloride is the culprit for corrosion.23 Herein, we offer the first report that graphene oxide (GO) films on non-noble metals (Al, Cu, and stainless steel (SS)) can efficiently suppress both the corrosion reaction and the electrolyte decomposition in APC and MACC. This work contrasts with a previous report on carbon-coated metals via plasma carburization,24 wherein the benefit of a µm-thick carbon coating over the direct application of a carbon electrode is ambiguous (To the best of our knowledge, this is the only report that has described corrosion protection via surface modification in MIBs.). The extension of the anodic limit is also not covered in that previous report. In contrast, this work describes that the oxygen-functional groups of GO can prohibit the access of anionic species to metallic substrates, which suppresses autocatalytic pitting corrosion and retards electrolyte oxidation. We also describe that the electrochemical performance of cathodes (Mo6S8 and α-MnO2) on GO-coated Al (GO/Al) in APC. 3




■ RESULTS AND DISCUSSSION GO films were evenly coated onto non-noble metals (Al, Cu, and SS) via electrophoretic deposition (EPD). Commercial GO (For physicochemical properties of GO, see Figure S1 and S2) was dispersed in H2O (1.0 mg·mL-1) and DC potentials were applied between two electrodes (2.3 cm separation) for various durations. GO contained no graphitic domains (Figure S1), implying that GO was composed of few-layer graphenes. Survey X-ray photoelectron microscopy (XPS) indicated the oxygen content of ca. 30.7 at% in GO (Figure S2A). Deconvolution of C peaks of GO (Figure S2B) revealed the different chemical states of carbon that were composed of C=C, (33.4 at%), C-C (23.2 at%), C-OH and C-O-C (24.9 at%), and C=O and O-C=O (18.5 at%). EPD is a simple and cost-effective method which is used for a variety of applications (e.g., fabrication of thin ceramic nanocomposites and nano-sized functional materials on solid substrates).25-27 GO was uniformly dispersed in H2O (zeta potential of ca. -20 mV at pH = 4), which was stable for long periods of standing. A DC potential of 4.0 V was chosen because higher potentials resulted in the irregular GO film formation with gas evolution. Deposition for 4 - 5 min produced a smooth film. Shorter deposition time resulted in insufficient GO-coating and deposition for longer than 5 min led to the formation of aggregates (Figure 1). Photographs in Figure 1 show the change in the film quality with an increase of GO-loading. Inset shows a cross-sectional view of GO/Al (5 min deposition), indicating a GO film thickness of ca. 200 nm. When compared with bare Al, an increase in the film thicknesses was also evidenced by a clear color change from pale yellow to purple and bright green with deposition time. Note that the mass-loading was quite small with an 4


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average surface coverage of ca. 60 µg·cm-2 for 5 min deposition.

Figure 1. Amounts of GO deposited on Al with EPD time. For EPD, two metal foils were immersed into a GO solution (1.0 mg·mL-1) at a separation of 2.3 cm. Photographs show the change in the film quality with an increase of GO-loading. Inset shows a cross-sectional view of GO/Al (5 min deposition), indicating a GO film thickness of ca. 200 nm. In order to ascertain the possible variation in oxygen contents of GO during EPD, XPS spectrum of GO/Al was examined and compared with that of GO (Figure S2A). The XPS survey spectra showed that the oxygen content of GO/Al was slightly decreased to 26.2 at% from that of GO (30.7 at%) with a concomitant increase in carbon contents. Considering the accuracy in quantitation of XPS, however, such a small decrease implied that the most oxygen functional groups were inert during EPD (Note also that GO was deposited on a positive electrode.). Prior to comparing the effect of GO-coating on corrosion prevention, we first 5




revisited the corrosion behaviors of non-noble metals in APC (Figure 2A). As expected, linear sweep voltammetry (LSV) revealed that non-noble metals are susceptible to corrosion at potentials far below the anodic limit of APC. Al and Cu showed the passage of a significant amount of corrosion currents at low potentials (e.g., 2.02 and 0.48 mA·cm-2 at 2.5 V, respectively). SS was also unstable in APC when compared with a voltammogram on Pt. Figure 2B demonstrates a continuous decrease of anodic currents with an increase of film thicknesses on Al. Thin GO film (38 nm in thickness, EPD time = 1 min) resulted in an abrupt decrease in current (0.04 mA·cm-2 at 2.5 V), which was followed by a gradual decrease of corrosion currents and finally reached a negligible level (0.001 mA·cm-2 at 2.5 V) in a coating of 192 nm thick GO. Interestingly, besides the suppression of corrosion, GO films also conspicuously alleviated electrolyte oxidation. GO/Al (Hereafter, GO/Al denotes 192 nm thick GO-coated Al.) showed an anodic current of 0.004 mA·cm-2 at 4.0 V, which was in sharp contrast with 1.56 mA·cm-2 on bare Pt (Figure 2A). The low level of currents on GO/Al was clearly contrasted with the LSV on commercial carbon-coated Al (C/Al). The curve showed an on-set of anodic currents at ca. 3.0 V and 2.12 mA·cm-2 at 4.0 V (Figure 2A). Though carboncoating on Al could retard the anodic process to some extent, therefore, the effectiveness seemed to be incomparably low when compared with GO/Al. The simultaneous inhibition of corrosion and electrolyte decomposition by GOcoating was nonspecific to the substrates. GO-coated Cu (GO/Cu) and SS (GO/SS) showed the same behavior, proving that the dual effects were not associated with the underlying substrates (Figure 2C). GO-coated Pt (GO/Pt) also revealed a significant suppression of electrolyte decomposition, showing a decrease of the anodic current from 1.56 mA·cm-2 (bare Pt) to 0.009 mA·cm-2. Slightly higher currents on GO/Pt appeared to reflect the effect 6


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Figure 2. Linear sweep voltammograms on (A) various metals and C/Al, (B) GO/Al, and (C) GO-coated metals (GO thickness = 200 ± 10 nm) in APC. Numbers in (B) indicate the thickness of GO films in nm. (D) Cyclic voltammograms of Al and GO/Al in APC. Scan rate = 10 mV·s-1.

of surface roughness. EPD for a fixed time (5 min) might result in the insufficient GO-coating, especially on a rough substrate. We repeatedly used a Pt foil after cleaning, which could make the surface rougher. Furthermore, GO did not aggravate the reversible Mg plating/stripping reaction (Figure 2D). Despite a slight negative shift in the on-set potential 7




for Mg plating (-0.20 V for GO/Al and -0.06 V for Al), the reversibility on GO/Al was comparable to that on Al (Coulombic efficiencies: 85 % for GO/Al vs. 87 % for Al). Note that anodic stability was also retained after Mg plating/stripping in GO/Al (For reversible Mg plating/stripping and high anodic stability during potential cycling on GO/SS, GO/Cu, and GO/Pt, see Figure S3). Field emission scanning microscopy (FESEM) studies, coupled with energy dispersive X-ray (EDX) examination, clearly disclosed the inertness of Al by GO-coating (Figure 3. For low magnification FESEM images, see Figure S4.). While the formation of deep trenches due to severe corrosion was evident for bare Al (Figure 3B), the surface state of GO/Al (Figure 3C) was indiscernible from that of pristine Al (Figure 3A). EDX maps revealed that the trenches were composed of high concentrations of O (22.2 %), Cl (2.3 %), and Mg (2.2 %). Al was detectable outside the trenches (73.3 %). In contrast, EDX maps of GO/Al showed a featureless distribution with Al (97.7 %) and O (1.9 %). Cl was not detected, and only a small amount of Mg (0.4 %) was present. XPS studies also revealed the effectiveness of GO in corrosion protection (Figure 3D). The absence of Cl2p and Mg1s peaks in GO/Al was contrasted with the high intensity of the two elements in bare Al. However, the relative concentrations of Al and O in the XPS spectra did not agree with the EDX results. The lower content of Al and the higher content of O in GO/Al were rather opposite to the EDX results. This contradiction can be explained by the difference in detection depths. Since a typical analysis depth when using XPS (< 10 nm) is much smaller than that when using EDX (few µm), XPS measurement would be more sensitive to the Al2O3 surface layer (2~3 nm thick). The native Al2O3, which remained intact in GO/Al could have led to a lower content of Al and a higher content of O contents in the XPS results. 8


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Figure 3. FESEM images and corresponding EDX maps of Al surfaces (A) before and (B) after electrochemical cycles between -1.0 and +4.0 V vs. Mg/Mg2+ in APC (10 mV·s-1, 10 cycles). (C) Al surface of GO/Al after electrochemical cycles. GO was removed by gentle scrubbing with a cotton swab. (D) XPS spectra of (solid line) Al and (dotted line) Al of GO/Al after electrochemical cycles.

In order to understand the origin of the corrosion and the electrolyte decomposition suppression, GO was thermally reduced by treating GO/Al at 300 oC under Ar (rGO/Al). After thermal reduction, rGO showed a sharp drop in the oxygen content (17.2 at%. Figure S2A). The cyclic voltammogram on rGO/Al was compared with that on GO/Al (Figure 4A). The Coulombic efficiency for the Mg plating/stripping on rGO/Al was slightly lower (72 %) than that on GO/Al. However, a more striking difference was observed in the current levels 9




between a potential range of 1.1 and 4.0 V. The current response was no longer negligible. Significantly higher anodic currents were observed at a level of a few hundred µA·cm-2. The FESEM image of an Al surface after cycling also showed several scattered pits that were 20 µm in diameter. EDX studies on rGO/Al revealed an elemental distribution around these pits that was similar to that seen in bare Al, although not as severe (Figure S5). Therefore, the relatively high current flow and corrosion susceptibility in rGO/Al strongly supported that the oxygen-functional groups on GO played a key role in the suppression of corrosion and electrolyte decomposition.

Figure 4. (A) Cyclic voltammograms of rGO/Al in APC. Inset shows the FESEM image of an Al surface after electrochemical cycles. Scan rate = 10 mV·s-1. Scale bar = 20 µm. Cyclic voltammograms of (B) I-/I3- and (C) BV2+/BV·+ on Pt and GO/Pt. Scan rate = 5 mV·s-1.

In APC, the major anionic species is [AlCl2Ph2]-, which is known to be responsible for electrolyte decomposition.8 Although the source of Cl- remains controversial, it is a general consensus that in-situ-generated Cl- causes corrosion.23 The major cationic species [Mg2(µ10


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Cl)3]+ is associated with the Mg plating/stripping process. Since our experimental results indicated that the cation-involved process is not noticeably hindered (Mg plating/stripping), but the anion-involved reaction is significantly suppressed (electrolyte decomposition) on GO/Al in APC, we suspected the electrostatic interaction caused by the oxygen-functionality of GO. In order to verify this speculation, we first examined the electrochemical activity of an I-/I3- redox couple on GO. Figure 4B compares the cyclic voltammograms of I-/I3- on Pt and on GO/Pt. As expected, the I-/I3- couple showed a well-defined two-step redox process on bare Pt (I3- + 2e- ⇄ 3I- at E1/2 = -0.23 V; 3I2 + 2e- ⇄ 2I3- at E1/2 = +0.30 V).28 On the other hand, on GO/Pt, the low potential redox process was completely suppressed and the high potential redox-pair was shifted to an E1/2 of +0.49 V with a small peak current. The inactivity of the low-potential redox process was believed due to a high charge-density of the anionic species (I-), which could be repelled from the electrode surface by GO. The highpotential redox process was not completely blocked, because GO allowed the access of I3- of a low charge-density to some extent. Note the substantially small peak currents and the over-potential of ca. 0.2 V for the high-potential redox process on GO/Pt. Despite a significant distortion in the voltammogram of an anionic probe on GO, it was not the case for the cationic species. Figure 4C demonstrates the voltammograms of benzyl viologen (BV2+) on Pt and on GO/Pt. In contrast to I-/I3-, BV2+ produced almost identical voltammograms on both substrates (BV2+ + e- ⇄ BV·+ at E1/2 = -0.7 V). Neither a shift in the peak position nor a decrease in the peak height was observed. The former was particularly interesting, because this implies that the GO film was so thin that it would not increase the internal resistance (IR drop). As a whole, the voltammetric results shown in Figure 4 prove that the negative polarity of a GO surface efficiently blocked anion access, but 11




allowed cation access, which resulted in a suppression of both corrosion and electrolyte decomposition, but maintained the reversibility of Mg plating/stripping. Since the origin of simultaneous corrosion protection and electrolyte-decomposition suppression is the electrostatic repulsion between the anions and the electrode surface, GOcoating strategy were also valid in MACC (Figure 5A). While severe corrosion of Al and the electrolyte decomposition on Pt were distinct, GO/Al showed negligible anodic currents within a potential window (10 µA·cm-2 at 4.0 V) in an unconditioned MACC electrolyte.

Figure 5. (A) Linear sweep voltammograms on Al, Pt, and GO/Al in an unconditioned MACC electrolyte. Scan rate = 10 mV·s-1. (B) EIS spectra of Al and GO/Al in APC.

In order to confirm that the excellent protection of corrosion and suppression of electrolyte-decomposition does not result from an increase of DC resistances by GO-coating, electrochemical impedance spectroscopy (EIS) studies were carried out on Al and GO/Al in APC (Figure 5B). A DC potential of 0.8 V was applied with a sinusoidal wave (±10.0 mV). The 12


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EIS spectra revealed that the serial resistance (Rs) in bare Al (157.4 Ω) increased only by a small amount in GO/Al (219 Ω), which implied that the GO film was thin enough not to impede a charge transfer reaction. The effect of GO coating on cathode performance was also investigated. Chevrel phase Mo6S8 was synthesized according to a previous report (For characterization of Mo6S8, see Figure S6).29 The galvanostatic charge/discharge (C/D) profiles of Mo6S8 on Al in APC revealed that the 1st discharge (Mg2+ insertion) was followed by continuous oxidation at +1.2 V during charge, due to the corrosion of the underlying Al substrate (Figure 6A). The Mo6S8 on GO/Al, on the other hand, exhibited a conventional C/D profile. The 1st discharge capacity of 127 mAh·g-1 was decreased to 82 mAh·g-1 during the 2nd discharge due to a partial entrapment of Mg2+. The potential difference between the charge and discharge plateaus was comparable to the results in previous reports,6,29 which reflected a GO film that was thin enough not to impede the charge transfer, as shown above (Figure 5B). The subsequent C/D cycles showed stable capacity responses (Figure 6A, inset), which indicated the robustness of GO on Al. FESEM images of the corresponding Al surfaces also revealed the maintenance of the role of GO during C/D of Mo6S8. While the aggressive pitting corrosion was evident without GO, GO/Al showed no sign of Al surface damage. Since the potential window of APC can be extended to 4.0 V on GO-coated nonnoble metal substrates, we finally examined the possible implementation of a high-voltage cathode (α-MnO2) in an APC-based MIB (For characterization of α-MnO2, see Figure S7).30 Unfortunately, though the underlying Al was inert by GO-coating, electrolyte decomposition occurred on the surface of α-MnO2 and/or on the conducting carbon at +2.73 V (Figure 6B). 13




Figure 6. (A) C/D profiles of (dotted line) Mo6S8/Al and (solid line) Mo6S8/GO/Al in APC. Inset shows capacity retention of Mo6S8/GO/Al during 10 C/D cycles. Inset FESEM images also show the differences in Al surface states for Mo6S8/Al and Mo6S8/GO/Al after 10 C/D cycles. Scale bar = 20 µm. Current density = 50 mA·g-1. (B) C/D profiles of α-MnO2/GO/Al (solid line) with and (dotted line) without GO-hybridization. Inset shows capacity retention of GOMnO2/GO/Al during 10 C/D cycles. The weight fraction of GO in the active material (GOMnO2) was ca. 25 %. Current density = 20 mA·g-1.

However, the anodic limit could be further increased to 3.2 V by hybridizing GO with MnO2 via co-precipitation (GO-MnO2/GO/Al), which indicated that the GO-coating method is also applicable to porous cathode films. In contrasts to MnO2/GO/Al, GO-MnO2/GO/Al showed no obvious sign of electrolyte decomposition at 2.73 V and the voltage-window could be extended to 3.2 V. The C/D profiles of GO-MnO2/GO/Al between 1.0 and 3.2 V showed reversibility, but a continuous capacity loss during 10 cycles. The initial discharge capacity of 192 mAh⋅g-1 was decreased to 81 mAh⋅g-1 (42 % retention) after 10 cycles. However, this relatively fast capacity fading was not attributed to GO-hybridization, rather it was due to 14


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the inherent instability of α-MnO2 (Jahn-Teller distortion).31,32 To prove this, α-MnO2/GO/Al (i.e., with no GO hybridization) was cycled between 1.0 and 2.5 V, in which the electrolyte decomposition was avoided (Figure S8). Due to a lower charge potential limit, smaller discharge capacities were obtained. The capacity retention of α-MnO2/GO/Al was even worse than that of GO-MnO2/GO/Al (190 mAh⋅g-1 was reduced to 32 mAh⋅g-1 after 10 cycles), indicating that the relatively fast capacity fading in GO-MnO2/GO/Al was not due to GOhybridization with α-MnO2 at least. We are currently investigating the best method for GOprotected cathodes to enable the implementation of high-voltage cathodes in APC and MACC.

■ CONCLUSIONS In conclusion, we show that GO can protect non-noble metals from corrosion and can extend the anodic limits to 4.0 V in APC and MACC electrolytes. This is because oxygenfunctional groups on GO prohibit anion access for the charge-transfer process. GO/Al does not aggravate the electrochemical performance of a cathode, which allows the use of nonnoble metals as current collector and coin cell casings in a corrosive electrolyte. The electrochemical properties of Mo6S8 on GO/Al are not altered from those on inert current collectors. The positive roles of GO are also gained when it is hybridized with a high-voltage cathode. The concept presented here could be applicable to other MIB electrolytes where an anionic species limits the use of non-noble metals and/or the application of high-voltages.

■ EXPERIMENTAL SECTION Electrophoretic deposition of graphene oxide. All reagents were purchased from Aldrich, unless otherwise mentioned. Single layer graphene oxide (thickness 0.7–1.2 nm, 99% 15




purity) was obtained from Cheap Tubes Inc., USA, and was used without further purification. GO dispersion for EPD was made by ultrasonication in distilled water (1.0 mg·mL-1). The Al (thickness 20 µm, Wellcos, Korea), Cu (thickness 25 µm, Wellcos, Korea), SS (type 304, thickness 25µm, Alfa Aesar, USA), Pt (thickness 25 µm, Alfa Aesar, USA) foils were cleaned with ethanol and dried at 60 °C in vacuum before use. C/Al (Wellcos, Korea) was used as received. The GO-coated metal substrates (Al, Cu, SS, and Pt) were fabricated by applying the DC voltages with respect to Al (or Cu) electrodes that were separated by 2.3 cm. The two electrodes were immersed in the GO solution and connected to a DC power supply (ED Rectifier, Toyotech, DP 30-05CF, Korea). EPD was performed at various conditions (voltage and deposition time). The GO-deposited foils were immediately transferred to a vacuum oven and dried at 60°C overnight to avoid the substrate corrosion due to water condensation inside the GO layers. Synthesis of Mo6S8 and α-MnO2. Chevrel phase Mo6S8 and α-MnO2 were synthesized as reported previously.29,30 The structure and phase purity was confirmed by XRD measurements. All peaks in the XRD patterns for Mo6S8 and α-MnO2 (Figure S6A and S7A) could be indexed by rhombohedral (space group: R-3, JCPDS: 270319) and tetragonal (space group: I4/m, JCPDS 440141) phases, respectively. Characterizations. XRD patterns were recorded using a Rigaku ULTIMA 4 equipped with Cu Kα radiation at a scan rate of 2°·min-1. XPS studies were carried out using a Thermo Fisher (K-Alpha) electron spectrometer with an Al Kα X-ray source (excitation energy = 1486.6 eV). FESEM investigations were carried out using a JEOL JSM-7100F equipped with an EDX spectroscope.

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Electrochemical investigations. All the experiments were conducted in an Ar-filled glove box (O2, H2O < 1 ppm), unless otherwise indicated. Cyclic voltammetry measurements were performed using a custom-built open cell (3-electrode type), where freshly cleaned/polished Mg foils served as counter and reference electrodes, and either bare or GO-coated metal substrates were used as working electrodes. The APC electrolyte was 0.8 M PhMgCl and 0.4 M AlCl3 (2:1) dissolved in dry tetrahydrofuran. The MACC electrolyte was prepared by mixing the equal volumes of 0.06 M AlCl3 and 0.12 M MgCl2 in tetrahydrofuran. For cathode performance studies, the electrodes were made by mixing 80 % active material (Mo6S8, α-MnO2, or GO-MnO2), 10 % poly(vinylidene fluoride) (MTI, USA), and 10 % super-P carbon (MTI, USA). The slurry was coated onto GO/Al (or Al) foils (doctor blade method) and punched into a desired size after drying. The GO-incorporated MnO2 electrode was prepared by co-precipitating GO and MnO2 that were dispersed in H2O (20 wt% GO). The GO content in GO-MnO2 was confirmed by thermogravimetry and found to be 25 %. The clear supernatant was decanted and the precipitates were washed, dried, and used as an active material. The galvanostatic charge/discharge tests were conducted in a 2-electrode cell with a freshly cleaned/polished Mg foil electrode. For electrochemical probe tests, a conventional 3-electrode system was utilized. Cyclic voltammograms of I-/I3- were recorded in an acetonitrile solution containing 5 mM LiI , 5 mM I2, and 0.1 M LiClO4. For cationic probe tests, benzyl viologen (BV2+) dichloride (1 mM) was dissolved in an acetonitrile solution containing 0.1 M tetrabutylammonium hexafluorophosphate. GO/Pt electrode served as a working electrode, while Ag/Ag+ (0.1 M AgNO3, acetonitrile) and Pt foil were used as reference and counter electrodes, respectively. All the electrochemical studies were performed using an automatic WBCS 3000 battery 17




cycler (WonATech). EIS spectra were recorded by applying a sinusoidal wave with amplitude of ±10.0 mV for frequencies ranging from 1 MHz to 1 Hz.

■ ASSOCIATED CONTENT ⓢ Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxxxxxxxxxx FESEM image and XRD pattern (GO), XPS spectra (GO, GO/Al, and rGO/Al), cyclic voltammograms (GO/SS, GO/Cu, and GO/Pt), FESEM and EDX images of Al, characterizations of Mo6S8 and α-MnO2, and C/D profiles of α-MnO2/GO/Al.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (M. Pyo), [email protected] (K.-S. Sohn). Tel: 82-61-7503638. Fax: 82-61-750-3608. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

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■ ACKNOWLEDGMENTS This research was supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future (2015M3D1A1069710). This research was also partially supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A6A1030419).

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Graphical Abstract


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Simultaneous Suppression of Metal Corrosion and Electrolyte

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