Nitrogen-Doped Holey Graphene for High-Performance Rechargeable

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Letter

Nitrogen-Doped Holey Graphene for HighPerformance Rechargeable Li-O Batteries 2

Jianglan Shui, Yi Lin, John W. Connell, Jiantie Xu, Xueliu Fan, and Liming Dai ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.6b00128 • Publication Date (Web): 06 Jun 2016 Downloaded from http://pubs.acs.org on June 10, 2016

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Nitrogen-Doped Holey Graphene for High-Performance Rechargeable Li-O2 Batteries Jianglan Shui,†, ‡ Yi Lin, *, ɸ, § John W. Connell, *, £ Jiantie Xu, † Xueliu Fan, † and Liming Dai*, †

† Department of Macromolecular Science and Engineering, Case School of Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio, 44106, USA ɸ National Institute of Aerospace, 100 Exploration Way, Hampton, VA 23666, USA § Department of Applied Science, The College of William and Mary, Williamsburg, VA 23185, USA # Mail Stop 226, Advanced Materials and Processing Branch, NASA Langley Research Center, Hampton, VA 23681, USA ‡Present Address: School of Materials Science and Engineering, Beihang University, Beijing 100191, China Corresponding Author Prof. L. Dai, [email protected] Dr. Y. Lin, [email protected] Dr. J. W. Connell, [email protected]

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ABSTRACT

Li-air batteries represent cutting edge electrochemical energy storage devices, but their practical applications have been precluded by the high cathode cost, the low discharge/charge efficiency, and/or the short battery lifetime. Here, we developed a low cost, but very efficient, air electrode from porous nitrogen-doped holey graphene for rechargeable non-aqueous Li-O2 cells. The resultant Li-O2 cell can deliver a high round-trip efficiency (85%) and a long cycling life (> 100 cycles) under controlled discharge-charge depths, or a high capacity of 17,000 mAh/g under the full discharge-charge condition, superior to most other carbonaceous air cathodes. The observed superb performance for the air electrode based on the nitrogen-doped holey graphene can be attributed to its efficient metal-free catalytic activity and three-dimensional mass transport pathway. Therefore, this work represents a new approach to low-cost, efficient, metal-free, binder-free, and hierarchically porous air electrodes useful for energy conversion and storage from N-doped holey graphene.

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Rechargeable non-aqueous Li-O2 batteries are advanced electrochemical devices for energy storage, promising for long range electric vehicle applications due to their high energy density (3600 Wh kg−1 based on the reaction: 2‫ ݅ܮ‬ା + Oଶ + 2e ↔

‫݅ܮ‬ଶ ܱଶ E0 = 2.96 V).1-2 During discharge, oxygen gas is reduced to Li2O2 through oxygen reduction reactions (ORR); then a reverse reaction for oxygen evolution (OER) takes place in the charge process. The ORR/OER processes are sluggish with high overpotentials and low energy efficiencies, which further cause significant electrolyte decomposition and hence a short battery lifespan.3-4 Considerable efforts have been placed on developing catalytically active oxygen electrodes or using additives, like LiI or DBBQ, in the electrolyte solvent to lower the reaction overpotentials.5-8 A few studies have focused on the use of non-carbon electrodes to avoid possible oxidation of carbon into inert carbonate.9 All them have one common purpose to extend the cycling life of Li-air batteries. Catalysts developed so far can be classified into noble-metals,9-10 transition metal-oxides,11 carbonaceous materials,12-15 and a few others.16 Noble metal Ru based catalysts usually possess the lowest overpotential while transition metal-oxides have a much poorer catalytic performance. Delicately designed carbonaceous catalysts could exhibit considerable catalytic activities close to that of noble metals with additional advantages, including low cost, high durability, and lightweight.17 Owing to its atomically thin sheet structure, high graphitization degree, excellent electrical conductivity, large surface area, and easy functionalization by either doping with heteroatoms or decorating with catalyst nanoparticles, graphene (Gr) has 3 ACS Paragon Plus Environment

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attracted a great deal of interest for electrochemical catalysis.18-19 However, the pristine graphene without functionalization are not sufficiently active for ORR or OER, except using additive in the electrolyte.14, 20-21 After being decorated with noble metals or their oxide nanoparticles (e.g., Pt, Ru, RuO) or with transition metal oxides, the graphene-based composite materials can show considerable catalytic activities in Li-air batteries.11, 22-24 By simply doping graphene with heteroatoms, such as N, S, O and P, the heteroatom-doped graphene materials could also act as highly active metal-free catalysts for ORR and OER.13, 15, 25 Different catalytic mechanisms are involved for graphene materials decorated with different types of catalytic centers. For graphene-supported Ru, RuO2 and transition metal oxides, it is believed that metal or metal oxides are catalytic sites and graphene mainly acts as the support. For heteroatom-doped carbon, however, carbon works as the active catalytic material. Compared to metal/metal oxide nanoparticles, the heteroatom-doped catalytic sites are much smaller at the atomic scale, leading to an extremely high catalytic site density on graphene even at a low doping level (e.g., few weight percent dopants). This could significantly reduce the cost and amount of catalysts need to be used for a high catalytic activity. To date, it is still a big challenge to develop highly efficient heteroatom-doped graphene materials to exhibit comparable performance to that of noble metals or their oxide catalysts in rechargeable non-aqueous Li-O2 batteries, though we have recently reported a vertically-aligned nitrogen-doped coral-like carbon nanofiber (VA-NCCF) array acted as an efficient metal-free oxygen electrode for non-aqueous Li-O2 4 ACS Paragon Plus Environment

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batteries.17 Theatrical calculations indicate N-doping on graphene can facilitate the nucleation of Li2O2 clusters, though the importance of specific N dopant is still under controversial.26-27 N-doped graphene materials firstly showed only a considerable discharge capacity in primary batteries.28-29 Recently, N/S co-doped graphene nanosheets demonstrated cycling capability, but the reduced overpotentials of discharge/charge were not much compared with the pristine graphene.13 To further improve the cell performance of graphene based air electrodes in Li-air batteries, one needs to consider the complicated reaction environment around the cathode, where three-phase, namely the gas phase (oxygen), liquid phase (electrolyte), and solid phase (electron transfer to/from and through the electrode), reactions take place simultaneously. Since solid Li2O2 is the discharge product, a high-performance air electrode should have sufficient free space for a high uptake of Li2O2, efficient pathways for oxygen/electrolyte/electron transports, and large number of uniformly distributed catalytic sites for catalyzing Li2O2 formation and decomposition. In this study, we doped holey graphene produced by a scalable method

30

with

nitrogen (N-HGr) via thermal annealing under NH3 gas (see, Experimental). The resultant N-HGr was then used to develop a highly efficient, metal-free, binder-free, hierarchically porous air electrode by a filtration method (see Experimental). The resultant electrode showed a high energy efficiency (85% in the middle of charge-discharge) and a long cycling life (> 100 cycles) under controlled discharge-charge depth (800 mAh/g) with a full capacity as high as 17,000 mAh/g in rechargeable non-aqueous Li-O2 batteries – comparable to noble metal Pd catalyst31 5 ACS Paragon Plus Environment

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and better than most other reported metal-free carbonaceous catalysts,13,

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or

graphene materials decorated with transition metal oxides.11, 23 The observed superior performance for the air electrode based on the nitrogen-doped holey graphene can be attributed to the rough holey structure, coupled with the highly porous electrode architecture and N-doping, leading to an enhanced density of catalytic active sites and electrolyte/oxygen/electron transports.

Figure 1. Morphology and porosity characterization of the holey graphene. a, b) SEM and c, d) TEM images of the O-doped holey graphene (O-HGr) sheets. e) Incremental pore volume and f) incremental pore area distributions of the pristine Gr and O-HGr materials.


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Fig. 1a-d shows SEM and TEM images for the oxygen-doped holey graphene (i.e., O-HGr) with a hole size from several nanometers to tens of nanometers and rough edges. In consistent with the SEM and TEM observations, Brunauer–Emmett– Teller (BET) measurements show more pore volume under the pore size of 30 nm for O-HGr than the pristine Gr (Fig. 1e). Surface areas of the pristine Gr and O-HGr were 337 m2/g and 724 m2/g respectively (Fig. S1). The pristine Gr material has a thickness of 3~4 nm observed by TEM (Fig. S2). The relatively large surface area for O-HGr with respect to Gr is attributable to its relatively large volume of pores over the entire pore size range (Fig. 1e, f). Since micropores (≤ 2 nm) have been proved critical to hosting N-containing catalytic sites,33 the strong peak near the micropore region seen in the incremental pore area distribution curve in Fig. 1f suggests that the newly created O-HGR could support pronounced N-doping along the hole edge of numerous micropores in the holey graphene to significantly boost its catalytic activity. N-doped carbonaceous materials have been well demonstrated to be efficient and low cost metal-free catalysts for ORR and OER, even superior to Pt/C in the alkaline electrolyte.[34, 35] For the use of metal-free carbon-based catalysts for electrocatalysis, the graphitization degree as well as the nature and number of defects play important roles in regulating their catalytic activities. Raman spectra given in Fig. S3 show that the pristine Gr has a lower graphitization degree than that of O-HGr and r-HGr, presumably because amorphous carbon was oxidized in air in order to create holes. Defects could be created again during ammonia treatment (N-doping), leading to the lower graphitization degree for N-HGr and is also indicative of the effectiveness of 7 ACS Paragon Plus Environment

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the N-doping (Fig. S3). X-ray photo-electron spectroscopic (XPS) measurements revealed a N-doping level of 1.3% N for N-HGr (Fig. S4). In comparison, N-r-HGr has only 0.5% and N-r-Gr has only 0.1% N, 2 and 10 times less than that of N-HGr, respectively. This confirms the importance of pre-oxygen doping and creation of holey structure for facilitating the N-doping. Indeed, the holey structure and N-doping play predominant roles in making these metal-free graphene materials into efficient oxygen electrodes, as discussed below.

Figure 2. Discharge-charge voltage profiles. a) N-HGr vs. r-Gr. b) N-HGr vs. r-HGr. c) N-HGr vs. N-Gr. d) O-HGr vs. r-HGr. The current density was 40 mA/g.


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Fig. 2a shows the discharge-charge voltage profile of the first cycle for r-Gr and N-HGr at a current density of 40 mA/g. The discharge voltage of N-HGr electrode (2.93 V) was very close to the thermodynamic value of 2.96 V. A total overpotential of 0.54 V and an energy efficiency of 85% were recorded with N-HGr, which were 0.68 V smaller and 16% higher than the overpotential (1.22 V) and the efficiency (69%) of r-Gr, respectively. The observed performance of the N-HGr air electrode is even better than those reported for graphene composites decorated with various transition metal oxides or Au/NiCo2O4 at the similar current density.11,

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Therefor, N-doping coupled with the holey structure is an efficient strategy to make graphitic carbon materials more catalytically active for ORR and OER.18 This is because the nano-sized holes with rough hole edges could provide numerous defects over the graphene basal plane as active sites for not only N-doping but also oxygen reduction and evolution via metal-free electrocatalysis.25, 35 To decuple the N-doping effect from the effect of holey structure on the Li-air cell performance, we compared N-HGr with r-HGr or N-Gr. Fig. 2b shows an overpotential reduction by 0.53 V for N-HGr compared to 1.07 V for r-HGr, indicating a significant improvement in catalytic activity by N-doping the holey graphene. In addition, we found a decrease in the cell resistance with N-doping of the holey graphene (Fig. S5). Fig. 2c shows that N-HGr has also a much lower overpotential (0.45 V) than that of N-Gr (0.99 V), indicating the importance of the holes to facilitate N-doping as indicated by XPS data. Comparing the N-HGr with the published N-doped graphene electrode, the electrochemical properties, such as the 9 ACS Paragon Plus Environment

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energy efficiencies, full discharge/charge capacities, and cycling life, were all improved due to introducing the nano-holey structure to the N-doped graphene.36-38 Nano-holes by themselves could also facilitate the mass transfer to slightly reduce the overpotential from 1.22 V for r-Gr (Fig. 2a) to 1.07 V for r-HGr (Fig. 2b). The importance of porosity as the mass transfer pathway of the electrode to the electrochemical performance has been previously discussed for hierarchically structured 3D porous N-doped graphene.39 In addition to N-doping, O-doping on the holey graphene (O-HGr) could also cause a small, but noticeable, increase in catalytic activities with the more obvious effect on the ORR (Fig. 2d).

Figure 3. SEM images of the binder-free electrode cross-sections. a, b) relatively dense N-HGr electrode from NMP, and c, d) highly porous N-HGr electrode from DMF. e) Discharge-charge voltage profiles of the dense and the porous N-HGr electrodes. The current density was 40 mA/g. f) AC-impedance spectra of the dense and the porous N-HGr electrodes. 10 ACS Paragon Plus Environment

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Different from the application in a supercapacitor where the dense packing of the holey graphene is favorable for a high volumetric capacitance, a more porous structure is critical for the holey graphene electrode in a Li-O2 battery. The porosity of oxygen electrode affects cell performance through tuning the mass transfers (oxygen and

electrolyte).

The

above

discussed

Li-O2

cells

were

all

based

on

N,N-Dimethylformamide (DMF)-derived porous oxygen electrodes. For comparison, N-Methyl-2-pyrrolidone (NMP) was also used as solvent and could disperse N-HGr much better than DMF. The resultant electrode (filter cake) was thinner and denser (Fig. 3a-d and Fig. S6). However, the dense electrode exhibited higher overpotentials (0.17 V in ORR and 0.73 V in OER) and larger resistance than the porous electrode (Fig. 3e, f), suggesting the necessity to maintain a level of porosity in the architecture of graphene based oxygen electrodes. Fig. 4 shows the rechargeable performance of N-HGr in non-aqueous Li-O2 cells. Two Li-O2 cells were discharged to a depth of 2000 mAh/g, and then one of them was charged back (Fig. S7). The discharged and recharged oxygen electrodes were taken out of the cell in an Ar-filled glove box and washed thoroughly with dimethyl carbonate (DMC) to remove the residual electrolyte, followed by drying in vacuum at 60 °C. As can be seen in Fig. 4a, the discharge product Li2O2 could be detected by X-ray diffraction with its spherical shape observed by both TEM and SEM (Fig. 4b, c).


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Figure 4. Identification of the discharge product Li2O2, proving a rechargeable capability of N-HGr in Li-O2 cells at a discharge depth of 2000 mAh/g (cf. Fig. S6). a) XRD patterns show the formation and decomposition of Li2O2 at the discharged and recharged electrodes, respectively. b) TEM and c) SEM images of a N-HGr electrode with deposited Li2O2 particles as indicated by arrows. d) SEM image of the recharged N-HGr electrode showing the disappearance of Li2O2 particles.

After recharging to complete a reversible ORR/OER cycle, Li2O2 was fully decomposed and no longer observable by XRD or SEM (Fig. 4a, d). Two small peaks labelled with “C” around 46° and 55° were also observed on discharged and 12 ACS Paragon Plus Environment

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recharged electrodes, which were consistent with (101) and (004) face index of graphite and are, most probably, arising from the locally ordered graphene stacks assembled under the strong mechanical compression applied onto the N-HGr electrode in the coin cell. This was confirmed by the same peaks observed on the blank electrode before being electrochemically tested, but subjected with a compression in the sealed coin cell.

Figure 5. Cycling performances of N-HGr, r-Gr and r-HGr at current densities of a, b) 40 mA/g and c, d) 200 mA/g under a controlled capacity of 800 mAh/g. e) Three full discharge/charge cycles of N-HGr within a voltage window of 2.2~4.4 V under a 13 ACS Paragon Plus Environment

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current density of 100 mA/g.

To investigate the cycling performances, a N-HGr air electrode was firstly cycled in a Li-O2 cell with a current density of 40 mA/g under controlled discharge/charge depths of 800 mAh/g. In the initial charge stage (< 400 mAh/g), the charge overpotentials kept dropping until the 20th cycle (Fig. 5a). This interesting phenomena was also observed by Zhang et al.,40 but the exact reason for that is still unknown. In the subsequent cycles, the voltage profiles were stabilized with the initial half charging voltage below 4.0 V and the left half charging voltage between 4.0~4.2 V (Fig. 5a). The capacities of both charge/discharge showed no decay over 100 cycles (Fig. S8a). These steady performances indicate a good cycling capability of the N-HGr air electrode. In contrast, r-Gr and r-HGr electrodes showed relatively poor cycling stabilities due to lack of the holey structure and/or N-doping (Fig. 5b, Fig. S8b). The efficiency and cycling capability of cells were also tested at a high discharge/charge current density (e. g., 200 mA/g). It was found that the N-HGr air electrode showed 30 stable cycles between 2.2~4.4 V, and then decayed quickly due to the worse polarization condition at the higher cycling current. We also found a small loss of N dopant from 1.3% to 0.7% on the N-HGr electrode after 5 cycles (Fig. S9). The overpotential of the N-HGr electrode increased to 0.85 V in the middle or 1.27 V at the end of the first charge-discharge cycle, and the associated energy


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efficiency dropped to 77% or 69% (Fig. 5c), though considerably lower than RuO2 based catalysts at the similar or even higher current intensities.22, 41-42 However, the N-HGr electrode still performed much better than non-doped r-Gr electrodes, indicating, once again, the enhanced catalytic capability by N-doping of graphene holes. Under the full discharge/charge condition, N-HGr showed a discharge capacity of ~17400 mAh/g at the current density of 100 mA/g (Fig. 5e), demonstrating a large energy storage capability for the N-HGr electrode. In addition, the initial half charging processes of all the three cycles were under 4.0 V, considerably lower than 4.3~4.4 V of other carbon based air electrodes and even carbon-supported AuPt under similar full discharge/charge conditions,3, 8, 43-44 indicating the high catalytic activity of the N-HGr electrode. Although N-HGr electrode had a large capacity under full discharge-charge condition, the cycling stability was less than satisfactory with only three cycles due to complete exhaustion of the electrolyte.3,

45

The dead cell has

exhausted all the electrolyte (60 µl) in 45 days under full discharge-charge condition, while the cell under controlled discharge/charge depths (Fig 5a, b) still had liquid electrolyte in the separator after 83 days cycling (Fig. S10), indicating the promising long working life for the N-HGr electrode under a relatively low specific capacity. In summary, metal-free N-doped holey graphene was for the first time made into hierarchically porous, binder-free oxygen electrodes for rechargeable non-aqueous Li-O2 cells. The metal-free N-doped holey graphene electrodes exhibited a high efficiency (85%) and long cycling life (>100 cycles), superior to most other non-noble 15 ACS Paragon Plus Environment

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metal functionalized graphene catalysts and metal-free carbonaceous catalysts at the same charge-discharge current density. Nano-sized holes with rough edge on the holey graphene basal plane could not only facilitate N-doping but also provide numerous catalytic sites with improved electrolyte/oxygen/electron transports. The cell performance of N-HGr was much better than that of graphene. The porous N-HGr electrodes could be fabricated from holey graphene generated by oxidizing commercially available graphene in air at a large scale. This work demonstrated possibilities for large scale fabrication of low-cost and metal-free graphene electrodes for high-performance Li-O2 batteries with a good discharge/charge efficiency and a long battery life.

Supporting Information. Experimental methods, extra BET isothermal sorption curves, TEM image, SEM images, Raman spectra, XPS, and Li-O2 cell performance, used separate images. AUTHOR INFORMATION

Corresponding Author

* Prof. L. Dai, [email protected] * Dr. Y. Lin, [email protected] * Dr. J. W. Connell, [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT We thank the support from AFOSR (FA9550-12-1-0037), NSF-AIR (IIP-1343270), and NSF (CMMI-1400274). Y.L. and J.W.C are grateful for the support from Internal Research and Development (IRAD) funds at NASA Langley Research Center.

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(14) Park, J. E.; Lee, G. H.; Shim, H. W.; Kim, D. W.; Kang, Y.; Kim, D. W. Examination

of

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Nanoplatelets

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Nitrogen-Doped Holey Graphene for High-Performance Rechargeable

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