Electrochimica Acta 211 (2016) 545–551

Cost-effective carbon supported Fe2O3 nanoparticles as an efficientcatalyst for non-aqueous lithium-oxygen batteries

M.C. Wu, T.S. Zhao*, P. Tan, H.R. Jiang, X.B. ZhuDepartment of Mechanical and Aerospace Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China

A R T I C L E I N F O

Article history:Received 29 January 2016Received in revised form 28 April 2016Accepted 21 May 2016Available online 24 May 2016

Keywords:Non-aqueous lithium-oxygen batteriesoxygen evolution reactioniron oxidecatalystcharge voltage

A B S T R A C T

In this work, we synthesize inexpensive Vulcan XC-72 carbon supported Fe2O3 nanoparticles (Fe2O3/XC)as an oxygen evolution reaction (OER) catalyst for non-aqueous lithium-oxygen batteries. It isdemonstrated that the battery with the Fe2O3/XC cathode exhibits a charge voltage plateau of 4.01 V at acurrent density of 200 mA g�1, which is 0.43 V lower than that with the pure XC carbon cathode. Thebattery also presents an outstanding rate capability, giving a charge voltage plateau of 3.99, 4.01 and4.15 V at the current density of 100, 200 and 400 mA g�1, respectively. Furthermore, the battery can beoperated for 50 cycles at a fixed capacity of 500 mA h g�1 without obvious degradation, showing itssuperior cycling stability. The results suggest that the iron oxide is a cost-effective catalyst for non-aqueous lithium-oxygen batteries.

ã 2016 Elsevier Ltd. All rights reserved.

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1. Introduction

The non-aqueous lithium-oxygen battery has a remarkably highenergy density up to �11.1 W h g�1 (based on lithium anode),which is comparable to that of gasoline (�13 W h g�1). Because ofits exceptional energy potentiality, non-aqueous lithium-oxygenbatteries have been regarded as one of the most promising powersources for portable devices and electric vehicles [1]. However, tomake this technology commercially viable, many critical issuesneed to be addressed, including a low energy efficiency, shortcycling life and poor rate capability, which are thought to be mainlyinduced by the high overpotentials during discharge-chargecycling [1–3]. In conventional non-aqueous lithium-oxygenbatteries, carbon materials were widely applied as cathodematerials due to their large specific surface area, good oxygenreduction activities, appropriated pore size and volume as well aseconomic merits [3–11]. However, carbon materials suffer fromOER large polarization during charge process. Thus, tremendousefforts have been devoted to reducing the large overpotentials(especially for the charging process, >1 V) by developing highlyeffective cathode [12]. One effective approach is to design catalystswith high electrocatalytic activities for oxygen reduction reaction(ORR) and oxygen evolution reaction (OER).

* Corresponding author. Tel.: +852 2358 8647.E-mail address: metzhao@ust.hk (T.S. Zhao).

http://dx.doi.org/10.1016/j.electacta.2016.05.1470013-4686/ã 2016 Elsevier Ltd. All rights reserved.

Thus far, various catalysts, including precious metals and alloys,transition metal oxides, heteroatom-doped carbonaceous materi-als, have been investigated and demonstrated to improveelectrochemical performance for non-aqueous lithium-oxygenbatteries [13–19]. Among them, transition metal oxides (e.g., MnOx,CoOx) have attracted great interest, due to their low cost, highabundance and considerable activities [18–20]. In particular, Mnand Co based oxides have been extensively studied due to theirintriguing electrochemical properties [17–19,21–25]. Anotherearth-abundant metal oxide, i.e., iron oxide, however, receivesless attention in non-aqueous lithium-oxygen batteries [26–28].Zhang et al. [27] prepared Fe2O3 nanocluster decorated graphenehybrid as a cathode and showed enhanced electrochemicalperformance, but the large polarization still needs to be improved.Chen et al. [28] synthesized a hierarchical mesoporous g-Fe2O3/carbon nanocomposite as the cathode. They demonstrated that theg-Fe2O3/carbon nanocomposites presented a lower charge anddischarge overpotential, higher discharge capacity as well as bettercycling stability than Super P. These results suggest that iron oxidecan also be a promising electrocatalyst for non-aqueous lithium-oxygen batteries and worth further investigation. In addition, thehigh electronic conductivity is also important for the catalyst topossess high electrocatalytic activities. Therefore, carbon materialswere widely employed as conductive matrix or supports [29–32].Moreover, a homogeneous distribution of a catalyst on the carboncan, to some extent, decrease the direct contact between thecarbon and Li2O2 product, thus partially reducing the formation of

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546 M.C. Wu et al. / Electrochimica Acta 211 (2016) 545–551

the lithium carbonate, which induces large OER polarization andthe failure of the battery [33,34]

Here, we prepared a Vulcan carbon supported Fe2O3 nano-particles nanocomposite OER electrocatalyst for non-aqueouslithium-oxygen batteries via a facile method by simultaneouslycalcinating ferric nitrate with commercial XC-72 carbon in the airatmosphere. Unlike conventional method in which the catalystsare mixed with carbon matrix, we directly deposit the catalystnanoparticles on the carbon power, avoiding the catalystagglomeration as well as the poor contact between carbon andthe catalyst [35–39]. We used linear sweeping voltammetry (LSV)and galvanostatic discharge-charge profiles to examine theelectrocatalytic activities of the Fe2O3/XC cathode. The formationand decomposition of the discharge product were detected, andthe morphologies of the cathode after discharge and recharge wereobserved to obtain an insight into the effects of the Fe2O3 catalyst.Furthermore, the cycling performance was tested to demonstrateits remarkable cyclic stability.

2. Experimental

2.1. Cathode preparation

The cathode was fabricated as follows: i) Fe(NO3)3�9H2O wasadded to commercial Vulcan XC-72 to give Fe2O3/XC ratio of 1:9and mixed with polytetrafluorethylen (PTFE) in ethanol at a dryweight ratio of 9:1 (for pure XC cathode, ferric salts were not addedwith a XC/PTFE ratio of 9:1); ii) the slurry is then ultrasonicallystirred for 1 hour; iii) the slurry is uniformly casted onto the carbonpaper. After air dried, the cathode was transferred to a mufflefurnace and calcinated in the air at 350 �C for 2 hours with a

Fig. 1. Characterization of the Fe2O3/XC cathode. (a) Raman Spectra; (b) High resolution X(d) lattice fringes of Fe2O3 nanoparticles.

temperature increase rate of 2 �C per minute and the obtainedcathodes are hereafter denoted as Fe2O3/XC and XC cathode,respectively. The loading of active materials is �0.9 mg cm�2.

2.2. Material characterization

The morphologies of the cathode were observed by using ascanning electron microscope (SEM, JWOL-6700F) at an accelera-tion voltage of 5.0 kV. Transmission electron microscopy (TEM)images were obtained by operating a high-resolution JEOL 2010FTEM system with a LaB6 filament at 200 kV. Nitrogen adsorption-desorption was used to examine the BET surface area of the activematerials. Raman measurement was performed with a RM 3000(Renishaw) micro-Raman spectrometer at 514.5 nm. X-ray photo-electron spectroscopy (XPS) characterization was performed byusing Physical Electronics PHI 5600 multi-technique system usingan Al monochromatic X-ray at a power of 350 W. Thermogravi-metric analysis (TGA) was measured using a Perkin-Elmer system(TGA Q500) under air atmosphere from 25 to 800 �C, with a heatingrate of 10 �C per minute. A Fourier-transform infrared (FTIR) testwas carried out on a spectrometer (Vertex 70, Bruker) in thefrequency range of 400–2000 cm�1.

2.3. Electrochemical measurements

A lithium-oxygen battery was assembled by a home-madesetup with a lithium metal foil, a glass-fiber separator (WhatmanGF/C), 100 mL electrolyte containing 1.0 M bis(trifluoromethane)sulfonimide lithium (LiTFSI, Sigma-Aldrich, 99.5%) in tetraethyleneglycol dimethyl ether (TEGDME, Sigma-Aldrich, 99%) with thewater concentration at less than 5 ppm and an as-prepared

PS spectra of Fe; (c) TEM image (inset: the histograms of particle size distribution);

M.C. Wu et al. / Electrochimica Acta 211 (2016) 545–551 547

cathode in an argon-filled glovebox (Etelux, Lab 2000) at water andoxygen contents below 1 ppm. Before use, the separator was driedin vacuum and the electrolyte was dried with molecular sieves toeliminate the moisture effect. After assembly, the inlet of thebattery was tightly connected to high purity oxygen (H2O � 1 ppm,CO2� 1 ppm) with a constant flow to exhaust the remaining argon.Then, the outlet of the battery was sealed and the battery wasexposed to oxygen at a constant pressure of about 1 atm. Thegalvanostatic discharge-charge tests were carried out on a batterycycling system (Neware, CT-3008 W) at within a voltage window of2.0–4.5 V (vs. Li/Li+). For the linear sweeping voltammetry (LSV)test, the battery was firstly discharged at a constant current of0.042 mA for 4 h, and then scanned from open circuit voltage to4.5 V at a rate of 1 mV s�1. All tests were performed underatmospheric temperature (23 � 2 �C).

3. Results and discussion

The Raman spectra of the XC and Fe2O3/XC cathode are shownin Fig. 1a. For the both cathodes, two strong peaks at �1350 and�1580 cm�1 can be assigned to the D and G bands of carbon,respectively. While in the Fe2O3/XC cathode, another three broadpeaks around 350, 500, and 700 cm�1 are observed, which areunique to the maghemite (g-Fe2O3) species [40], indicating thatthe iron oxide formed in the cathode is g-Fe2O3. The Fe2O3formation was further confirmed through XPS analysis. The high-resolution spectrum of Fe is shown in Fig. 1b. Two peaks located at724.6 and 712.3 eV, can be assigned to the Fe 2p1/2 and Fe 2p3/2,respectively, indicating the existence of g-Fe2O3 [41]. Moreover,the Fe 2p3/2 peak is associated with a satellite situated at about219 eV, suggesting the absent of Fe2+ [22–24]. A TEM image (Fig.1c)shows that Fe2O3 nanoparticles with an average sizes of3.6 � 0.14 nm are uniformly dispersed on the carbon surface. Themeasured lattice spacing of 2.09 Å in Fig. 1d corresponds to the(400) crystal planes of g-Fe2O3, indicating the crystalline nature ofthe particles. The TEM-EDX mapping (Fig. S1) further demonstratesthat the Fe2O3 nanoparticles are well distributed. Therefore, thisdirect thermal decomposition of iron nitrate is a facile and effectiveapproach to prepare carbon supported Fe2O3 nanocatalysts. Basedon the N2-sorption measurement (see Fig. 2a), the specific surfaceof pure XC and Fe2O3/XC sample is calculated to be 232.89 and172.58 m2g�1, respectively. The decreased specific surface area ofFe2O3/XC can be attributed to the decoration of dense Fe2O3nanoparticles on the XC carbon. From the pore size distribution of

Fig. 2. (a) Nitrogen-adsorption–desorption isotherms of XC and Fe2O3/XC composite (in

XC and Fe2O3/XC in the inset of Fig. 2a, it is seen that the pores ofthe XC and Fe2O3/XC range from several nanometers to more thanone hundred meters and the pore distribution does not change a lotafter the introduction of Fe2O3 nanoparticles. The exact weightcontent of Fe2O3 in the Fe2O3/XC composite is determined by TGAin air atmosphere to be 8.13% (see Fig. 2b), almost consistent withthe designed value (10%).

As carbon materials can offer substantial ORR catalytic activitiesfor non-aqueous lithium-oxygen batteries and the main over-potential is resulted from the charging process [12,44], Fe2O3 isintroduced mainly functioning as an OER catalyst. The electro-calalytic activities of the Fe2O3 toward OER was evaluated by linearsweep voltammetry (LSV) as shown in Fig. 3a. The cathodecontaining Fe2O3 exhibits a much larger OER current density andlower onset potential compared with that of the pure XC cathode,demonstrating that Fe2O3 is an effective OER catalyst for non-aqueous lithium-oxygen batteries. It is worth noting that the largeranodic current density is not caused by the oxidation of theelectrolyte, as the current density of the cathode without dischargeproduct between the sweeping voltage windows is negligible. Theelectrocalatytic effect of the Fe2O3 was further studied byglavanostatic discharge-charge profiles. The first discharge-chargecycle curves at a current density of 200 mA g�1 of both XC andFe2O3/XC cathodes are compared and presented in Fig. 3b. It is seenthat the XC and Fe2O3/XC cathodes deliver a similar capacity of�2000 mA h g�1 with an almost identical discharge plateau, as thecurrent density and capacity are normalized to the weight of XCcarbon. In fact, if the Fe2O3 catalyst is taken into consideration, thecapacity slightly decreases to 1900 mA h g�1, which is a little lowerthan that of the XC cathode. Similar results have been reported byYilmaz et al. and Tan et al., in which the specific capacity decreasesafter the introduction of RuO2 catalyst [37,45]. To study the effect ofFe2O3 loading on the electrochemical performance, we preparedanother Fe2O3/XC cathode with a Fe2O3 loading of 17.27% (Fig. S2b).The battery with an 17.27% Fe2O3 present a similar dischargebehavior but with a slight decrease in the specific capacity(Fig. S2a). The decreased capacity may be caused by the decrease inthe specific surface area (Fig. S2c) [36] as well as the change in thedischarge product morphology [46]. On charge, the battery with XCcathode rises quickly to a voltage plateau of about 4.44 V andreaches 4.5 V at the end of charging, with a low coulombicefficiency of only 94.2%, remaining some undecomposed dischargeproduct [45]. For the Fe2O3/XC cathode, the charge voltage remainsat 4.01 V, remarkably 0.43 V lower than that of XC and comparable

set: pore size distribution); (b) thermogravimetric analysis of Fe2O3/XC composite.

Fig. 3. (a) Linear sweep voltammograms of OER in non-aqueous lithium-oxygen batteries with XC and Fe2O3/XC cathodes of 1 mV s�1 with and without initial dischargeproduct; (b) initial discharge-charge curves of XC and Fe2O3/XC cathodes at a current density of 200 mA g�1; (c) discharge-charge curves of Fe2O3/XC cathode at variouscurrent densities; (d) XRD pattern of the XC and Fe2O3/XC after discharge and charge at a current density of 200 mA g�1.

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to other noble metal base catalyst [47,48], and reaches to 4.39 Vafter completely charged, indicating that the OER is greatlyimproved by the Fe2O3 catalyst in the charge process. Fig. 3c showsthe rate performance of a battery with the Fe2O3/XC cathodewithin the voltage window of 2.0–4.5 V. When the current densityincreases from 100 to 400 mA g�1 the discharge voltage plateaudrops from �2.7 to �2.5 V, and the specific capacity decreases from2600 to 1100 mA h g�1, while the charge voltage plateau increasesfrom 3.99 to 4.15 V, demonstrating a good rate capability and thecharge voltage is comparable to that of the recent work on Fe2O3@Cnanocomposite [49]. To confirm the reversible formation anddecomposition of Li2O2 in the discharge and charge process, ex-situXRD was conducted to analyze the composition of the XC andFe2O3/XC cathode after fully discharge and charge. As shown inFig. 3d, two typical peaks at 33 and 35� are clearly observed afterdischarge for both cathodes, indicating that the main dischargeproduct is Li2O2. These peaks disappear after subsequent chargeprocess, suggesting that Li2O2 is removed from the cathodes. Also,FITR was carried out to test the product in the discharged/chargedelectrode and the results are shown Fig. S3. The absorbance peak ataround 430 cm�1 and 500 cm�1 (marked as the rhombus symbol)is derived from Li2O2 in the discharged cathodes, and itsdisappearance in the charged cathodes indicates the decomposi-tion of Li2O2 in the following charge process. Some by product likelithium carbonate was also detected which may be caused by thereaction between lithium peroxide with carbon electrode or thedecomposition of the electrolyte.

To further investigate how a Fe2O3/XC cathode could lead to alower charge overpotential, the morphologies of both XC andFe2O3/XC cathodes after discharge and charge were examined, aspresented in Fig. 4. After discharge to 2.0 V at a current density of200 mA g�1, the products in both cathodes exhibit a disc-likemorphology (Fig. 4c and d), which is consistent with the previousreported Li2O2 morphology [27–29]. After the charge process,some mud-like aggregates still remain in the XC cathode (Fig. 4e),which is consistent with the charge profile of a low coulombicefficiency (94.2%) and other reported results [52], indicating thepoor rechargeability of XC cathode. In contrast, for the Fe2O3/XCcathode, the generated Li2O2 product is completely decomposed,leaving a clear cathode (Fig. 4f), which is identical to the initial one.This implies that Fe2O3 can effectively catalyze the decompositionof Li2O2 during the charge process. The introduction of Fe2O3nanoparticles onto the carbon surface can induce many moredeposition sites for Li2O2, and this result in a thinner disc-likemorphology of Li2O2, providing more interfaces between thecathode/Li2O2 and Li2O2/electrolyte, and thus resulting in anenhanced rechargeability [53–55].

The cycling performance of the battery with the Fe2O3/XCcathode was tested at a current density of 200 mA g�1 with a fixedcapacity of 500 mA h g�1, and compared with that of the batterywith the pure XC cathode. As shown in Fig. 5a, the dischargevoltage plateau of XC electrode is �2.6 V, and decreases withcycling; while the charge voltage plateau is �4.4 V and increaseswith cycling. The charge voltage reaches the cutoff voltage of 4.5 V

Fig. 4. SEM image of (a)pristine XC (b) pristine Fe2O3/XC; (c) XC and (d) Fe2O3/XC cathode after discharged to 2.0 V; and (e) XC and (f) Fe2O3/XC cathode after charged at acurrent density of 200 mA g�1.

M.C. Wu et al. / Electrochimica Acta 211 (2016) 545–551 549

after several cycles before fully charged, remaining someundecomposed Li2O2. In addition, the electrolyte decompositionand carbon corrosion can occur under a high charge voltage to formirreversible products (e.g., Li2CO3) [56]. The accumulation of sideproducts and undecomposed Li2O2 would cover the active sites,increasing the transport resistance and eventually causing thedischarge capacity decay at 15th cycle. On the contrary, the batterywith Fe2O3/XC cathode presents a much lower charge voltageplateau and longer cycle life. As shown in Fig. 5b, the first dischargeplateau is also �2.6 V while the charge plateau is �4.0 V, which isconsiderably 0.4 V lower than that with XC cathode. Even at the20th cycle, the charge plateau remains at 4.1 V. However, thebattery could not be fully charged at the 39th cycle. Afterdisassembly, we found that the lithium metal was covered witha white layer at the interface with the separator, which would bethe reason of the capacity decay, because it was found that thelithium has limited reversibility due to oxygen crossover andelectrolyte decomposition [57,58]. To prove this, a battery was builtwith the used Fe2O3/XC cathode but with a new lithium metal, anew separator and fresh electrolyte. As shown by the dash line inFig. 5b, the discharge and charge curves of the rebuilt battery

recovered to the previous 20th level. Hence, the battery with theFe2O3/XC cathode can maintain its discharge capacity andcoulombic efficiency for 50 cycles without sign of degradation,as shown in Fig. 5c and d, respectively, demonstrating its goodcycling stability and the cycling number is much enhancedcompared with the previous work using Fe2O3 as catalyst(Table S1).

4. Conclusion

In summary, the Fe2O3/XC composite was prepared through afacile one step calcination in air. When employed as the catalyst innon-aqueous lithium-oxygen batteries, the Fe2O3/XC cathodeexhibits a superior electrocatalytic activity for OER, with aconsiderably 0.43 V lower charge overpotential than that of thepure XC cathode. In addition, a considerable rate capability (with acharge voltage plateau of 3.99, 4.01 and 4.15 V at the currentdensity of 100, 200 and 400 mA g�1, respectively) and muchenhanced cyclic stability (up to 50 cycles) were obtained. Theresults demonstrate that the Fe2O3/carbon composite is a cost-effective catalyst for non-aqueous lithium-oxygen batteries.

Fig. 5. Cycling stability of the XC and Fe2O3/XC cathodes: discharge/charge curves of a non-aqueous lithium–oxygen battery with (a) XC and (b) Fe2O3/XC cathode at200 mA g�1 with a fixed capacity of 500 mA h g�1; (c) discharge capacity and (d) coulombic efficiency as a function of cycle number.

550 M.C. Wu et al. / Electrochimica Acta 211 (2016) 545–551

Acknowledgement

The work described in this paper was fully supported by a grantfrom the Research Grants Council of the Hong Kong SpecialAdministrative Region, China (Project No. 16213414).

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.electacta.2016.05.147.

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Cost-effective carbon supported Fe2O3 nanoparticles as an efficient catalyst for non-aqueous lithium-oxygen batteries1 Introduction2 Experimental2.1 Cathode preparation2.2 Material characterization2.3 Electrochemical measurements

3 Results and discussion4 ConclusionAcknowledgementAppendix A Supplementary dataReferences