A Multifunction Lithium–Carbon Battery System Using a Dual

Dec 1, 2016 - Fuel cells, Li-ion batteries, and supercapacitors are attracting extensive attention, and it is highly desired to integrate the advantag...
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A Multifunction Lithium−Carbon Battery System Using a Dual Electrolyte Ziyang Guo, Ye Wang, Yanfang Song, Chao Li, Xiuli Su, Yonggang Wang,* Wen-bin Cai, and Yongyao Xia Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Fudan University, Shanghai 200433, China S Supporting Information *

ABSTRACT: Fuel cells, Li-ion batteries, and supercapacitors are attracting extensive attention, and it is highly desired to integrate the advantages of these devices into one system. Herein, a multifunction Li−carbon system was designed by using an aqueous−nonaqueous dual electrolyte to combine a nitrogen-doped ordered mesoporous carbon cathode with a metallic lithium anode. It is demonstrated that the nitrogen-doped ordered mesoporous carbon exhibits high performance in various applications of O2 reduction reaction, supercapacitors, and H2 evolution reaction, which makes the Li−carbon system exhibit multifunctionality. When operated in the ambient with O2, the system can work as a Li−air fuel cell or/and rechargeable battery with high energy density. When operated in an environment without O2, the battery can be used as a Li-ion supercapacitor which exhibits long-term cycling stability and improved energy performance. Finally, this cell can also be applied as a Li−water fuel cell for H2 evolution.

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Therefore, it is highly desired to integrate these advanced functions of energy conversion and storage devices into one system. Obviously, this target is a great challenge because of the totally different energy storage and conversion mechanisms. On the other hand, there are “electrochemical similarities” of the above-mentioned energy conversion and storage systems. Common features are that the energy-providing processes take place at the phase boundary of the electrode/electrolyte interface and that electron and ion transport are separated.9,10 For example, fuel cells, rechargeable batteries, and supercapacitors all consist of two electrodes in contact with an electrolyte solution.9,10,13 If we can find a couple of electrode materials that can be used in various energy storage and conversion systems, it still would be possible to build one system that can integrate the advanced functions of fuel cells, rechargeable batteries, and supercapacitors. Carbon electrode materials have been shown to be a promising choice. For example, recent studies have shown that carbon nanomaterials (carbon nanotubes, graphene) doped with nitrogen could be an efficient, low-cost, and metal-free alternative to Pt for oxygen reduction reaction (ORR).15−20 Furthermore, functional carbon materials could also effectively reduce the Gibbs free energy of hydrogen adsorption and result in an improved

lectrochemical energy conversion and storage systems, such as fuel cells, rechargeable batteries, and supercapacitors, have been attracting increasingly intense attention because they possess a number of desirable features, including pollution-free operation, high round-trip efficiency, and flexible power and energy characteristics to meet different grid functions.1−6 However, when used by themselves, fuel cells, rechargeable batteries, or supercapacitors can not serve all applications from the wide energy requirements of various devices. Fuel cells exhibit a very high energy density because the chemical energy stored in hydrogen (or several hydrocarbon fuels) is significantly higher than that found in common battery materials. Unfortunately, they cannot be directly used as the rechargeable systems. The hydrogen and hydrocarbon fuels must be prepared and stored with other technologies.7−10 Rechargeable batteries keep our devices working throughout the day, but they take hours to recharge when they run down. Charge storage of rechargeable batteries (e.g., Li-ions batteries) mainly depends on the cations’ (e.g., Li+) intercalation− deintercalation within electrode materials and thus is controlled by the cations’ diffusion within the crystalline framework of electrode materials.10−13 Supercapacitors can provide power density that is much greater than that of batteries because their charge storage is based on the surface reactions of electrode materials without ion diffusion within the bulk of materials. However, the surface charge storage makes the energy density of supercapacitors much lower than that of batteries.9,10,12−14 © XXXX American Chemical Society

Received: October 31, 2016 Accepted: December 1, 2016

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Figure 1. Schematic illustration of synthetic process for HS-N-OMC.

Figure 2. TEM images of (a, b) HS-N-OMC, (c, d) HS-OMC, (e, f) LS-N-OMC, and (g, h) LS-N-C.

hydrogen reduction reaction (HER) activity.21−25 In addition, it is well-known that carbon materials not only are the most widely used anode materials ever for rechargeable lithium-ion or sodium-ion batteries26−32 but also have been the common material of choice for the porous cathode in rechargeable Li−air batteries.33−36 Moreover, carbon materials have also attracted remarkable attention when applied as active electrode materials in supercapacitors.14,37−40 Apart from a carbon electrode, metallic lithium can be considered as another electrode candidate because it has been widely applied in Li−air fuel cells, rechargeable Li-metal batteries, and Li-ion supercapacitors.41−44 Therefore, it can be assumed that the

combination of metallic lithium electrode and carbon electrode by using a proper electrolyte could provide an energy storage and conversion system that displays multifunction. However, this has not been reported to date. Herein, we design an aqueous−nonaqueous double-electrolyte lithium−carbon battery with multifunction, in which a high surface area nitrogen (N)-doped ordered mesoporous carbon (HS-N-OMC) cathode in aqueous electrolyte and a Li-metal anode in nonaqueous electrode are connected by a ceramic lithium superionic conductor thin film. When there is O2 in the environment, the cell can be operated as a Li−air fuel cell, in which the discharge depends on the catalytic reduction of O2 in 37

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Figure 3. (a) CV curves of HS-N-OMC in O2-saturated (red) and N2-saturated (blue) 0.1 M KOH solution; (b) LSV curves of HS-N-OMC as compared to HS-OMC, LS-N-OMC, LS-N-C, and Pt/C for ORR catalytic activity at an RDE (1600 rpm) in O2-saturated 0.1 M KOH solution; (c) CV curves of four porous carbons at a scan rate of 5 mV s−1 and (d) their galvanostatic charge−discharge curves at 0.25 A gc−1; (e) HER polarization plots of these porous carbon catalysts in 1 M KOH; (f) initial and postpotential linear voltammograms of HS-N-OMC (500 cycles) and HS-OMC (500 cycles) in 1 M KOH. Potential sweeps were cycled between 0.2 and −0.25 V versus RHE (not iR-corrected).

the HS-N-OMC cathode and the oxidization of Li-metal anode. With an additional catalytic electrode for oxygen evolution reaction (OER) in aqueous electrolyte, the cell is converted into a rechargeable Li−air cell, in which charge involves the oxidization of H2O on OER electrode and the Li-deposition on the anode. When operated in an environment without O2, the cell can be cycled as a Li-ion supercapacitor, in which the

charge−discharge depends on the ions’ adsorption−desorption in HS-N-OMC cathode and dissolution−disposition of the metallic anode. Finally, this cell can be used as a Li−water fuel battery for H2 evolution, in which the operation mechanism is based on the HER in the HS-N-OMC cathode and the oxidization of the Li-metal anode. 38

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for LS-N-OMC, and 729 m2 g−1 for LS-N-C. The low microporous surface area of LS-N-OMC is due to the absence of silica source (i.e., TEOS) in its precursor. Because of its typical framework, including ordered mesoporous structure and large surface area with N-doping, HS-N-OMC may serve as a unique and promising catalytic material for ORR and HER and an active electrode material for supercapacitors. Therefore, we investigated the ORR, HER, and capacitance behavior of HS-N-OMC compared with that of HS-OMC, LS-N-OMC, and LS-N-C. The ORR performance of the HS-N-OMC catalyst was first evaluated in 0.1 M KOH solution. Figure 3a shows the cyclic voltammetry (CV) curves of HS-N-OMC in the electrolyte saturated with O2 or N2, and all data were recorded by cycling the potential at a scan rate of 5 mV s−1 until reproducible CVs were obtained. It can be found that the CV curve exhibits a clear cathodic reduction peak at about 0.8 V versus reversible hydrogen electrode (RHE) (calculation of RHE is shown in Supporting Information) in O2-saturated KOH solution, but not in the N2-saturated KOH solution, suggesting the catalytic activity of HS-N-OMC toward ORR. The ORR catalytic activity of HS-N-OMC was further investigated and compared with that of HS-OMC, LS-N-OMC, LS-N-C, and commercial Pt/C (20 wt % Pt on carbon black) using linear sweep voltammetry (LSV) with a rotating disk electrode (RDE). As shown in Figure 3b, the half-wave potential of HS-N-OMC is much higher than that of other carbon samples and is just 100 mV more negative than that of the Pt/C catalyst, indicating its higher catalytic performance toward ORR. In particular, the HS-N-OMC electrode exhibits better long-term stability and higher resistance to the methanol poisoning effect than commercialized Pt/C catalyst (Figure S7). To clarify the capacitive behavior, CV measurements at the rate of 5 mV s−1 were employed to investigate these sample in 1 M KOH solution without dissolved O2. As shown Figure 3c, all the CV curves present a nearly rectangular shape, indicating typical characteristics of electrical double-layer capacitors. Furthermore, the responding rectangular area in CV curves of HS-N-OMC is much higher than that of other samples, suggesting its highest specific capacitance. Consistent with the CV results, all galvanostatic charge−discharge tests (Figure 3d) show symmetric features with a fairly linear slope. A specific capacitance as high as 264 F gc−1 at a current density of 0.25 A gc−1 (all of the results for the specific capacitance and current densities were calculated with the total mass of porous carbon in the electrodes) was obtained for HS-N-OMC, versus 229 F gc−1 for HS-OMC, 225 F gc−1 for LS-N-MC, and 145 F gc−1 for LS-N-C (Figure 3d). Although HS-OMC displays the highest surface area (1831 m2 g−1) among the four samples, its specific capacitance is lower than that of HS-N-OMC with a surface area of 1231 m2 g−1 and is close to that of LS-N-OMC with a surface area of 556 m2 g−1. This result demonstrates the N dopant can efficiently improve the capacitance, which is consistent with previous reports.38,45 It is worth noting that the LS-N-C with a surface area of 874 m2 g−1 just exhibits a low specific capacitance of 145 F gc−1, which is ascribed to its surface area mainly arising from micropores (Table S2) and disordered porous structure. At a scan rate of 5 mV s−1, these disordered micropores (less than 1 nm) cannot be easily accessed by ions.12 Additionally, the rate and cycle performances of these carbons are shown in Figures S8 and S9, which demonstrate the excellent electrochemical stability of HS-N-

The direct organic−inorganic coassembly process to synthesize the high surface area nitrogen-doped ordered mesoporous carbon is illustrated in Figure 1. In this synthesis, resol molecules was used to bridge the Pluronic F127 template, silica oligomers, and dicyandiamide via hydrogen bonding and electrostatic interactions through an evaporation-induced selfassembly (EISA) process. During thermosetting at 100 °C for formation of rigid phenolic resin and subsequent pyrolysis at 700 °C in N2 for carbonization, dicyandiamide provides N species while resol and silica oligomers can form a stable framework, thus ensuring the successful synthesis of the freestanding N-doped ordered mesoporous carbon (OMC)−silica composite. After etching of the silica templates by HF solution, large amounts of micropores appeared in the ordered mesopore walls to form a high surface area N-doped ordered mesoporous carbon (donated as HS-N-OMC). We also synthesized three other kinds of carbon for comparison: a high surface area OMC with micropores on the mesopore walls, but without N-doping (denoted as HS-OMC); a low surface area N-doped OMC without micropores on the mesopore walls (denoted as LS-NOMC); and a low surface area N-doped porous carbon without ordered porous structure (denoted as LS-N-C). Detailed synthesis processes for these porous carbons are given in the Experimental Section in the Supporting Information. Figure 2 presents transmission electron microscopy (TEM) images of the four carbon samples. As shown in Figure 2a−d, both HS-N-OMC and HS-OMC show large domains of highly ordered, stripelike, two-dimensional (2D) hexagonal mesostructures. Furthermore, the morphology of HS-N-OMC (Figure 2a,b) is quite similar to the structure of HS-OMC (Figure 2cd), suggesting that N-doping does not destroy the ordered mesoporous structure. In addition, the TEM images of LS-N-OMC (Figure 2e,f) also exhibit 2D hexagonal arrays of mesopores. However, as shown in Figure 2g,h, there is not any ordered mesostructure for the LS-N-C sample, which was prepared without the F127 template. The field emission scanning electron microscopy (SEM) images of these samples are given in Figure S1. Energy dispersive X-ray (EDX) investigation demonstrates the presence of N element in HSN-OMC, LS-N-OMC, and LS-N-C (Figure S2). X-ray photoelectron spectroscopy (XPS) technology was employed to further confirm the successful N-doping in these carbons, and the resulting N 1 s and O 1s XPS spectra are shown in Figures S3 and S4. According to the XPS investigation, the relative surface concentrations of N element are 5.00% for HSN-OMC, 0% for HS-OMC, 3.45% for LS-N-OMC, and 4.66% for LS-N-C (Table S1). The 0% N in HS-OMC is attributed to the absence of N-precursor (i.e., dicyandiamide) in its preparation (see Experimental Section for details). Furthermore, the Fourier transform infrared spectrophotometry (FTIR) spectra of HS-N-OMC, LS-N-OMC, and LS-N-C further demonstrate that large amounts of N functional groups are formed in these samples (Figure S5). The N2 adsorption−desorption isotherm and the pore-size distribution of HS-N-OMC, HS-OMC, LS-N-OMC, and LS-NC are shown Figure S6, and the corresponding pore characteristics including the Brunauer−Emmett−Teller (BET) surface area, pore volume, and pore diameter are summarized in Table S2. It can be seen from Table S2 that HS-N-OMC, HSOMC, LS-N-OMC, and LS-N-C exhibit the overall surface area of 1231,1831, 556, and 874 m2 g−1, respectively. As shown in Table S2, the specific surface area from micropores are 624 m2 g−1 for HS-N-OMC, 931 m2 g−1 for HS-OMC, 280 m2 g−1 39

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Figure 4. (a) Schematic illustration of a rechargeable Li−air battery with the “double catalytic cathodes” configuration, consisting of a Li anode, organic electrolyte, a LTAP film, alkaline electrolyte, a HS-N-OMC cathode for discharge, and a commercial RuO2/IrO2 coated Timesh electrode for charge; (b) the long-time continuous discharge curve of HS-N-OMC cathode at 500 mA gc−1; (c) discharge curves of HSN-OMC cathode at different current densities, from 250 to 3000 mA gc−1; (d) cycle performance of the rechargeable Li−air battery with double catalytic cathodes at a current density of 500 mA gc−1 and a duration of 2 h per cycle.

air catalytic electrode are separated by a Li1.35T1.75Al0.25P2.7Si0.3O12 (LTAP) film to form the dualelectrolyte Li−air cell, which was recently proposed even if the dual-electrolyte sysytem is complex.47−49 A HS-N-OMCbased ORR catalytic electrode is between the aqueous electrolyte solution and the air atmosphere, and a commercial RuO2/IrO2 OER catalytic electrode is in the aqueous electrolyte solution for discharge and charge, respectively, forming the “double catalytic cathode” system. We first investigated this system as a Li−air fuel cell using only HSN-OMC as an air cathode. Figure 4b shows the long-time discharge curve of the Li−air fuel cell at a current density of 500 mA gc−1 (all of the results for the specific capacities and current densities for the Li−air fuel cell and rechargeable Li−air battery were calculated with the total mass of porous carbon in the electrodes). It can be detected that the operating voltage of this battery can remain above 2.90 V (vs Li/Li+) with almost no obvious decrease even after 100 h of continuous discharge, indicating a high specific capacity of 50 000 mAh gc−1. Obviously, the achieved capacity is much higher than that of conventional Li−air batteries using a nonaqueous electrolyte.33−36,42 Figure 4c presents the discharge profiles for this Li−air fuel cell at different current densities. With the growth of applied current densities, the operation of discharge voltage slightly decreased, which is mainly due to the resistance of LTAP. When the applied current density is 250 mA gc−1, the battery exhibits a stable discharge curve of approximately 3.25 V. Unexpectedly, even at the very high current density of 3000 mA gc−1, the operating voltage of HS-N-OMC cathode can still remain above 2.12 V, suggesting the excellent rate performance. This good performance derives from H−N−OMC’s porous structure which facilitates an efficient diffusion of O2 gas and

OMC and its potential as a useful electrode with long cycling durability. Finally, the HER performance of the HS-N-OMC catalyst and reference samples, including HS-OMC, LS-N-OMC, LS-NC, and Pt/C, was also evaluated by using the RDE technique in an Ar-saturated 1 M KOH solution. As shown in Figure 3e, the HS-N-OMC catalyst exhibited a HER activity that is higher than that of the HS-OMC, LS-N-OMC, and LS-N-C and had an overpotential of below 370 mV at a current density of 12 mA cm−2, which is still inferior to that of the commercial Pt/C catalyst. The incorporation of N element into the hierarchical micropores and mesoproes of carbon led to a profound enhancement of the HER activity, as reflected by the large shift of the polarization curve of the HS-N-OMC catalyst to a lower overpotential compared with the HS-OMC catalyst without N dopant. An accelerated degradation study was also carried out to compare the stability of the HS-N-OMC catalyst with that of HS-OMC to clarify the stability of N dopant for HER. Specifically, CV scanning was employed as an accelerated degradation method, and LSV curves of HS-N-OMC and HSOMC catalysts before and after 500 CV cycles were tested for accessing the stability. As shown in Figure 3f, HS-N-OMC exhibits a negative shift that is smaller than that of HS-OMC over 500 CV cycles, demonstrating that N doping even can improve the HER catalytic stability. A superior ORR catalyst is highly desirable for use in metal− air battery applications;18,46 therefore, we employed the asprepared HS-N-OMC as air catalytic electrode and metallic Li as anode to build a Li−air fuel cell and a rechargeable Li−air battery by using an aqueous−nonaqueous dual-electrolyte system. As shown in Figure 4a, the organic electrolyte used for the Li anode and the alkaline aqueous electrolyte used for 40

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Figure 5. (a) Schematic illustration of a hybrid Li-ion supercapacitor which consists of a Li anode in the organic electrolyte and a HS-N-OMC electrode in the alkaline electrolyte, separated by a LTAP film; (b) discharge−charge curves of HS-N-OMC cathode at different current densities, from 100 to 500 mA gc−1; (c) gravimetric energy and power characteristics of HS-N-OMC electrode at different current densities, from 100 to 500 mA gc−1; (d) cycle performance of the hybrid Li-ion supercapacitor based on HS-N-OMC electrode at a current density of 500 mA gc−1.

stability (Figure 4d). Furthermore, the voltage gap between discharge and charge is stabilized at ∼0.85 V during 100 cycles, indicating relatively low discharge−charge overpotential. In addition, Figure S11 further demonstrates the rechargeable performance of this Li−air battery. As mentioned above, when there is O2 supply from the environment, the combination of a HS-N-OMC-based cathode and a Li-anode with a dual-electrolyte system can be operated as a Li−air fuel cell or a rechargeable Li−air battery. This operation depends on the high activity of HS-N-OMC for ORR. According to previous reports,38,45 N-doped carbon materials also have been widely applied as active electrode materials for supercapacitors. In addition, the results shown in Figure 3c,d also demonstrate that the capacitance performance of HS-N-OMC is superior to that of other carbon materials. Therefore, if there is no O2 supply, the combination of a HS-NOMC-based cathode and a Li-anode with a dual-electrolyte system can be operated as a hybrid Li-ion supercapacitor. On the basis of the above consideration, we investigated the capacitance performance of the above system without O2 in the environment, where the operation depends on the adsorption− desorption of anions in the porous structure of H-N-OMC cathode (i.e., double-layer capacitance behavior) and the dissolution−deposition of Li-metal, accompanied by the Li+ diffusion between nonaqueous electrolyte and aqueous electrolyte through the LTAP film (Figure 5a). Owing to the different charge storage mechanisms in the cathode and anode, this system should be a hybrid Li-ion supercapacitor. Figure 5b presents the typical charge−discharge curves of the hybrid Liion supercapacitor with a HS-N-OMC cathode between 2.3 and

electrolyte to the active sites, large surface area which provides enough active sites toward ORR, and high content N-doping which further increases ORR activity. According to recent reports, oxidation under the positive potentials needed for OER (or recharge process) usually caused the loss of electrocatalytic activity for ORR catalysts.49,50 To circumvent the deactivation problem of ORR catalyst upon exposure to OER potential regimes, we have explored our Li−air battery in a “double catalytic cathode” configuration.49,50 OER and ORR electrocatalysts are located in two separate electrodes for charge and discharge, respectively (Figure 4a). The ORR electrode connects to the Li anode only for discharge, thus avoiding exposure to positive potentials. In this configuration, the double catalytic cathodes are decoupled and could be optimized individually. Also, we have found that the commercial RuO2/ IrO2 coated Ti-mesh electrode shows highly catalytic activity, superior long-term stability, and relatively low polarization for OER or charge process (Figure S10). The reaction equations corresponding to the complete cell process can be summarized as the following equation: 4Li + O2 + 2H 2O ⇌ 4Li+ + 4OH−

(1)

Figure 4d gives the galvanostatic discharge−charge curves of Li−air battery at a current density of 500 mA gc−1 and duration of 2 h per cycle (1 h discharge and 1 h charge per cycle) in the double catalytic cathode configuration using HS-N-OMC-based electrode for ORR and the commercial RuO2/IrO2-based electrode for OER. There is almost no obvious variation in both the discharge and charge voltages of this battery for 100 cycles at a fixed time of 2 h per cycle, suggesting the good cycle 41

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Figure 6. (a) Schematic illustration of a Li−water fuel cell which consists of a Li anode in the organic electrolyte and a HS-N-OMC electrode in the alkaline electrolyte, separated by a LTAP film; (b) discharge curves of HS-N-OMC cathode at different current densities, from 250 to 2000 mA gc−1; (c) long-time continuous discharge curve of HS-N-OMC cathode at the current density of 500 mA gc−1; (d) discharge curves of HS-N-OMC cathode at a current density of 750 mA gc−1 and a duration of 3600 s initially and after 500 CV sweeps. Potential sweeps were cycled between 2.3 and 2.75 V.

producer of hydrogen (Li−water fuel cell) when operated in environment without O2. The operation mechanism of this cell is based on the HER in HS-N-OMC cathode and the oxidization of Li-metal anode (Figure 6a). Figure 6b gives the discharge curves for the HS-N-OMC-based Li−water fuel cell at different current densities, with a limited cycle period of 3600 s. As shown in Figure 6b, the discharge voltage decreased as the applied current densities increased, which can be also mainly attributed to the resistance of LTAP. However, even at the highest current density, i.e., 2000 mA gc−1 (all of the results for the specific capacities and current densities for Li−water fuel cell were calculated with the total mass of porous carbon in the electrodes), the Li−water fuel cell with HS-N-OMC cathode still exhibited a stable discharge curve of above 1.5 V. Herein, the good performance of our HS-N-OMC catalyst derives from its porous structure, which facilitates an efficient diffusion of generated H2 gas and electrolyte to the active sites. Electrochemical durability is one of the important parameters for the long-term application of a hydrogen production device. The long-time continuous discharge test was therefore carried out to assess the stability of the Li−water fuel cell with HS-N-OMC cathode. As shown in Figure 6c and Figure S12, no significant potential drop of this cell was observed under galvanostatic discharge for 20 h at 500 mA gc−1, indicating a good catalytic stability of this cell. To further demonstrate the stability of this system, another accelerated degradation study was tested using CV sweeps with a scan rate of 5 mV s−1. After CV scanning for 100 cycles, the voltage curve under galvanostatic discharge for 3600 s at 750 mA gc−1 was compared with the discharge curve before CV scanning (Figure 6d). As shown in Figure 6d, there is an insignificant change for the discharge curve after 100 CV

3.0 V at different current densities. As depicted in Figure 5b, the HS-N-OMC cathode delivers a specific capacitance of ∼230 F gc−1 at a current density of 100 mA gc−1 (all of the results for the specific capacitances and current densities for Li-ion supercapacitor were calculated with the total mass of porous carbon in the electrodes). At the higher current densities, the specific capacitance only slightly decreases to 203 and 186 F gc−1 at the currents of 250 and 500 mA gc−1, respectively. The corresponding capacitance retention is 88.3 and 80.9%, respectively, indicating a promising rate performance. The gravimetric energy and power characteristics of the hybrid Liion supercapacitor with the HS-N-OMC electrode upon discharge are shown in Figure 5c. The HS-N-OMC electrode can deliver a gravimetric energy up to 116 Wh kgc−1 at 259 W kgc−1, 105 Wh kgc−1 at 665W kgc−1, and 96 Wh kgc−1 at 1327 W kgc−1 normalized to the weight of HS-N-OMC, which indicates this hybrid Li-ion supercapacitor can combine the features of supercapacitors and lithium batteries with high energy density and high power density. Additionally, the longterm cycling stability of the HS-N-OMC cathode was also evaluated in this study by repeating the galvanostatic charge− discharge experiment between 2.3 and 3.0 V at a current density of 500 mA gc−1 for 2000 cycles. As shown in Figure 5d, the HS-N-OMC cathode shows a slight decrease in the capacitance during 2000 cycles and the corresponding capacitance retention is as high as 83.3%, suggesting the perfect cycle performance of this cell. Recent studies indicate that N doping could obviously reduce the Gibbs free energy of hydrogen adsorption and result in an improved HER activity compared with that of pure carbon materials.21,22 Hence, our system can also act as an efficient 42

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ACS Energy Letters cycles compared with the profile before CV sweeps, indicating good stability of this cell in the HER process. In summary, a high surface area N-doped ordered mesoporous carbon (HS-N-OMC) based electrode was coupled with a Li-metal electrode through an aqueous− nonaqueous dual electrolyte to form a Li−carbon cell/battery system. Owing to the inherent various promising applications of HS-N-OMC in electrode materials for ORR, HER, and supercapacitors, the Li−carbon system exhibits multifunction at different operation conditions. When operated in the ambient with O2, the cell can first act as a Li−air fuel cell which delivers a stable voltage platform during the long-time discharge process (100 h continuous discharge performance with a stable operating voltage of ∼3.0 V at 500 mA gc−1) and good rate performance (operating voltage above 2.12 V even at 3000 mA gc−1) at limited discharge time conditions. With a commercial RuO2/IrO2 coated Ti-mesh electrode for OER in aqueous electrolyte, the cell is subsequently converted into a rechargeable Li−air battery, which shows long cycle life (a small charge−discharge voltage polarization of ∼0.85 V and excellent cycling stability for 100 cycles at 500 mA gc−1). When operated in the ambient without O2, the cell can be applied as a Li-ion supercapacitor which exhibits obviously improved energy density (e.g., 116 Wh kgc−1 at 100 mA gc−1) and long-term cycling stability (capacitance retention of 83.3% during 2000 cycles). Finally, this system can be also used as a Li−water fuel cell for H2 evolution, exhibiting a relatively low overpotential (e.g., 1.5 V discharge voltage platform even at 2000 mA gc−1) and a long-term stability for H2 evolution (20 h continuous H2 production with a stable operating voltage curve at 500 mA gc−1). In brief, we have demonstrated that the combination of the HS-N-OMC cathode and Li anode with a dual electrolyte can integrate the advanced functions of a fuel cell, rechargeable battery, and supercapacitor into one system. However, there are still many further improvements to be made to the performance of this Li−carbon cell system. For example, the low ionic conductivity of the solid Li-ion conducting glass− ceramic film (Li1.35T1.75Al0.25P2.7Si0.3O12) and the huge interface impedance consume much of the performance of this Li− carbon battery. Furthermore, its poor mechanical flexibility and fragile properties also hinder its application. Therefore, identifying a suitable alternative separator to LTAP film may be the next logical step for the development of this system. This initial proof of concept study potentially provides a new research idea for the design of advanced energy storage and conversion devices and the related materials.

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Yongyao Xia: 0000-0001-6379-9655 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge funding support from the Natural Science Foundation of China (21373060, 21622303), the State Key Basic Research Program of PRC (2014CB932301), and Shanghai Science & Technology Committee (13JC1407900).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00566. Experimental details, characterization of materials (SEM, BET, FT-IR, XPS, etc.), and electrochemical performance of materials (CV and LSV data, cycle and rate performance, etc.) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel./fax: 0086-21-51630318. E-mail: [email protected]. ORCID

Yonggang Wang: 0000-0002-2447-4679 43

DOI: 10.1021/acsenergylett.6b00566 ACS Energy Lett. 2017, 2, 36−44

Letter

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DOI: 10.1021/acsenergylett.6b00566 ACS Energy Lett. 2017, 2, 36−44