Electrochemistry Communications 60 (2015) 38–41

Contents lists available at ScienceDirect

Electrochemistry Communications

j ourna l homepage: www.e lsev ie r .com/ locate /e lecom

Short communication

Highly active Co-doped LaMnO3 perovskite oxide and N-doped carbonnanotube hybrid bi-functional catalyst for rechargeable zinc–air batteries

Dong Un Lee, Moon Gyu Park, HeyWoong Park, Min Ho Seo, Vugar Ismayilov, Raihan Ahmed, Zhongwei Chen ⁎Department of Chemical Engineering, Waterloo Institute for Nanotechnology, Waterloo Institute for Sustainable Energy, University of Waterloo, 200 University Avenue West, Waterloo,ON, Canada, N2L 3G1

⁎ Corresponding author. Tel.: +1 519 888 4567x38664E-mail address: zhwchen@uwaterloo.ca (Z. Chen).

http://dx.doi.org/10.1016/j.elecom.2015.08.0011388-2481/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 July 2015Received in revised form 29 July 2015Accepted 3 August 2015Available online 10 August 2015

Keywords:HybridBi-functionalOxygen reduction reactionOxygen evolution reactionRechargeableZinc–air battery

Herein, non-precious cobalt doped lanthanum manganese perovskite oxide nanoparticles are used as a growthsubstrate for nitrogen-doped carbon nanotubes to form efficient and durable hybrid bi-functional catalyst(LMCO/NCNT). LMCO/NCNT demonstrates significantly enhanced onset and half-wave oxygen reduction reaction(ORR) potentials (−0.11 and−0.24 V vs. SCE, respectively), and oxygen evolution reaction (OER) current density(27 mA cm−2 at 0.9 V vs. SCE). Likewise, practical rechargeable zinc–air battery testing using atmospheric airreveals superior discharge voltages obtained with LMCO/NCNT, particularly at current densities higher than30 mA cm−2, and significantly lower charge voltages at all current densities tested, compared to state-of-art com-mercial platinum on carbon catalyst. In addition, very stable charge and discharge voltages of 2.2 and 1.0 V, respec-tively, are obtained over 60 cycles. The excellent performance and durability of the hybrid catalyst are attributed tovery uniformly distributed LMCO nanoparticles on the surface of NCNT resulting in enhanced surface area andmaterial utilization.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Due to continuing dependence of human activity on non-renewablesources of energy and their adverse environmental consequences, ad-vanced next generation energy conversion and storage systems such asrechargeable metal-air batteries having high performance, low capitaland maintenance costs, and environmental benignity have recentlygained tremendous attention [1,2]. Rechargeable zinc–air batteries areparticularly interesting due to extremely high energy density, abundanceof zinc, virtually unlimited oxygen supply, and safe battery operation [3].However, wide commercialization of rechargeable zinc–air batteries hasbeen hampered by intrinsically sluggish kinetics of oxygen reduction re-action (ORR) and oxygen evolution reaction (OER), which governsbattery's discharge and charge processes, respectively. To increase therates of these reactions, precious metal based catalysts such as platinumsupported on carbon (Pt/C) [4] and iridium oxide (IrO2) [5] and theiralloys [6,7] have been used. However, extremely high costs, insufficientelectrochemical stability, and uni-functionality severely limit their usein a large number of applications [8,9]. Thus, affordable yet highlyefficient catalyst bi-functionally active towards both ORR and OER withenhanced durability must be developed for rechargeable zinc–air batte-ries [10–12].

Recently, different classes of materials such as non-precious transi-tion metal based oxides[13], metal-free nanostructured carbons [14],

; fax: +1 519 746 4979.

and hybrids that combine the two [15] have been reported as bi-functionally active catalysts which could potentially replace preciousmetal based catalysts. Among them, hybrid catalysts, particularly non-precious transition metal oxide combined with hetero-atom dopednanocarbon, have become one of the focal points of catalyst researchdue to their high bi-functional activity, and superior electrochemicalstability upon battery cycling [16,17]. The inorganic constituent of thehybrid catalysts are often perovskite oxides, which are known to beORR and OER active due to the first cation ordering, disorder-free chan-nels that provide oxygen vacancies to enhance the mobility of oxygenions. However, the lost costs of hybrid catalysts still lead to slight perfor-mance sacrifice compared to precious metal based catalyst. In order toobtain a significantly cost competitive hybrid catalyst, the presentwork incorporates a small amount of cobalt dopant into low costmanganese-based perovskite oxide nanoparticles (LMCO), which alsomakes them sufficiently active towards both ORR and OER due to itsunique electronic structure [18–20]. Subsequently, LMCO are thendirectly used as a substrate for growth of nitrogen-doped carbon nano-tubes (NCNT) to create a hybrid catalyst (LMCO/NCNT) as active anddurable bi-functional catalyst for rechargeable zinc–air batteries.

2. Materials and methods

2.1. Preparation of LMCO/NCNT hybrid

The synthesis procedure of LMCO/NCNT hybrid is illustrated inFig. 1a. In the first stage, LMCO nanoparticles were synthesized by

Fig. 1. (a) Schematic illustration of synthesis of LMCO/NCNT hybrid catalyst. (b) SEM image, (c) XRD spectrum, and (d) full XPS survey of LMCO nanoparticles. (e) SEM, and (f, and g) TEMimages of LMCO/NCNT hybrid.

39D.U. Lee et al. / Electrochemistry Communications 60 (2015) 38–41

dissolving 1.0 mM La(NO3)3·6H2O, 0.9 mM of Mn(NO3)2·4H2O, and0.1 mM Co(NO3)2·6H2O in 35 mL distilled de-ionized (DDI) water, thepH of which was adjusted to 13 by adding 6.0 M potassium hydroxide(KOH). The solution transferred into a Teflon-lined container inside anautoclave was placed in an oven at 220 °C for 48 h. The resulting productwaswashedwithDDIwater and ethanol,filtered, dried at 60 °C overnightthen calcined in air at 850 °C for 12 h. The same procedure was used ex-ceptwithout the addition of Co(NO3)2·6H2Oand increased concentrationofMn(NO3)2·4H2O to 1.0mMto obtain LMOnanoparticles. In the secondstage, LMCO nanoparticles were used as a substrate to directly growNCNT by injection chemical vapor deposition (CVD). The detailed synthe-sis procedure is outlined elsewhere in our previous report [21].

2.2. Material characterization

Scanning electron microscopy (SEM) (LEO FESEM 1530) and trans-mission electronmicroscopy (TEM) (JEOL 2010F)were employed to ob-serve the material's morphology. X-ray diffraction (XRD) (Brunker AXSD8 Advance), and X-ray photoelectron spectroscopy (XPS) (ThermalScientific K-Alpha XPS spectrometer) were conducted to confirm theformation of perovskite LMCO.

2.3. Half-cell electrochemical testing

The electrocatalytic activity was evaluated by rotating disk elec-trode (RDE) voltammetry using a potentiostat (CH Instrument760D) and a rotation speed controller (Pine Instrument Co., AFMSRCE).LMCO/NCNT modified glassy carbon disk electrode with catalyst loadingof 0.41 mg cm−2 was used as the working electrode. As comparisons,LMO and LMCOwere mixed with Vulcan XC-72 in 3:7 catalyst to carbonblackweight ratio, then coated onto glassy carbonwith the same loadingas LMCO/NCNT. The details of half-cell conditions and testing procedureare outlined elsewhere [21].

2.4. Single-cell rechargeable zinc–air battery fabrication and testing

A rechargeable zinc–air battery prototype was designed and fabri-cated. A polished zinc plate, and catalyst-coated gas diffusion layer(GDL) (Ion Power Inc.) with loading of 0.78 mg cm−2 were used asthe anode and cathode, respectively. Microporous membrane (Celgard5550) and 6.0 M KOH were used as the separator and electrolyte,respectively. The actual area of GDL exposed to the electrolyte was2.84 cm2. The detailed procedures of preparation of catalyst-coatedGDL and the battery performance testing are outlined elsewhere [21].

40 D.U. Lee et al. / Electrochemistry Communications 60 (2015) 38–41

3. Results and discussion

The SEM image of LMCO itself prior to hybridizingwithNCNT revealsnanoparticles having an average diameter of 100 nm (Fig. 1b), which in-creases the exposure of active surface area for not only ORR and OER, butalso provides more nucleation sites for the growth of NCNT. The XRDspectrum of LMCO nanoparticles reveals peak splitting of the reflectionsat 32.5, 41, 52.5, and 58°, and relatively lower peak intensity of the(214) reflection at 58° compared to those of (110) and (024) (Fig. 1c), in-dicative of a rhombohedral symmetric structure, which has been shownto be very effective for catalyzing OER in our previous investigation[22]. In fact, the spectrum closely matches that of LaMn0.9Co0.1O3 (ICSD#164468), confirming successful formation of LMCOnanoparticles. In ad-dition, theXPS spectrumof LMCOclearly showspeaks at binding energiesthat correspond to the elements La, Mn, and Co (Fig. 1d), further corrob-orating the formation of LMCO. After injection CVD growth of NCNT onthe surface of LMCO, the morphology of LMCO/NCNT hybrid catalyst isrevealed by SEM where uniformly distributed LMCO nanoparticleswithin the NCNTmatrix are observed (Fig. 1e). Interestingly, the highlyintertwined network of NCNT helps to knit together relatively smallerLMCO nanoparticles into spherical clusters, which enhances materialutilization and facilitate charge transfer during electrochemical reactionthrough highly conductive carbon nanotubes. Similarly, the TEM analysisof LMCO/NCNT shows prolific growth of NCNT and uniformly distributedLMCOnanoparticles throughout the carbon network (Fig. 1f). In addition,relatively brighter spots are observed inside LMCO/NCNT clusters, whichindicates that they are porous in nature. Highly porous LMCO/NCNT actsas a reservoir for electrolyte and improves diffusion of reactants duringoxygen electrocatalysis. Furthermore, NCNT having very high aspectratio is ideal for bridging individual LMCO/NCNT clusters, providinginter-particle connection for improved charge transport and materialutilization.

The half-cell electrochemical evaluation of LMCO/NCNT in terms ofORR results in polarization curves obtained at various rotation speedsof the disk electrode that clearly show kinetic-, mixed- and diffusion-

Fig. 2. (a) Half-cell ORR polarization curves obtained at various rotation speeds (400, 900, 1600rotation speed of 1600 rpm, and (d) OER curves of LMO, LMCO, NCNT and LMCO/NCNT obtain

controlled regions with uniform spacing between limiting currentdensities (Fig. 2a). The Koutechy–Levich (K–L) analysis conductedat −0.6, −0.7, and −0.8 V using the obtained ORR curves resultsin the electron transfer number (n) of 3.9, which is very close to muchmore favorable four-electronORR reaction pathwaywhere the formationof corrosive peroxide species is minimized (Fig. 2b). The effect of intro-ducing a small quantity of cobalt dopant to LMO on the activity of ORRis clearly observed by a positive shift of the half-wave potential of thecurve obtainedwith LMCO(Fig. 2c). Furthermore, the half-wavepotentialobtained with LMCO/NCNT hybrid is significantly improved to −0.24 Vas well as muchmore positive shifted onset potential and greater magni-tude of limiting current density of 5.25 mA cm−2

. The onset and half-wave potential improvements are likely due to highly active ORR sitescreated by pyridinic and pyrrolic nitrogen species incorporated in NCNTduring the injection CVD growth [23], and the improved current densityis likely due to more readily accessible ORR active sites of high surfacearea NCNT network as observed in the microscopic analyses above.These improvements are observed to be derived from the synergisticcombination of LMCO and NCNT. A similar trend is observed with OERactivities of the catalysts, where cobalt doped LMO results in an im-proved OER current density most likely due to better eg orbital filling,resulting in an electronic structure that weakens the absorption of O2

[18,22]. Further, the growth of NCNT onto LMCO synergistically en-hances the OER activity, resulting in the highest current density of27 mA cm−2 obtained at 0.9 V (Fig. 2d).

To demonstrate practical usage of LMCO/NCNT hybrid catalyst, it isutilized as the active air electrodematerial of a rechargeable zinc–air bat-tery, which uses oxygen in atmospheric air as the main source of fuel in-stead of pure oxygen. Galvanodynamically charging and discharging thebattery up to 70 mA cm−2 result in voltage profiles with sufficientlyhigh performance, demonstrating effectiveness of the hybrid as a bi-functional catalyst (Fig. 3a). Compared to charge and discharge profilesobtained with state-of-art commercial Pt/C catalyst, the hybrid showslightly lower open circuit potential of 1.25 V, and this in turn resultsin slightly lower discharge potentials at lower current densities.

, and 2500 rpm), and (b) Koutechy–Levich plot of LMCO/NCNT hybrid. (c) ORR obtained ated at rotation speed of 1600 rpm.

Fig. 3. (a) Rechargeable zinc–air battery galvanodynamic charge and discharge voltageprofiles, and (b) galvanostatic cycling performance obtained at current density of18 mA cm−2 of LMCO/NCNT hybrid and commercial Pt/C catalyst.

41D.U. Lee et al. / Electrochemistry Communications 60 (2015) 38–41

However, the hybrid outperforms Pt/C at current densities higher than30 mA cm−2 exhibiting higher discharge voltages, due to its superiorrate capability. In terms of charging, lower OER overpotential of the hy-brid results in far superior charge voltages at all current densities tested.In addition to the excellent charge and discharge capabilities, the hybridcatalyst demonstrates very stable electrochemical durability in terms ofboth charge and discharge voltages (1.0 and 2.2 V, respectively) upongalvanostatic cycling up to 60 cycles at 18 mA cm−2 without virtuallyany voltage losses (Fig. 3b). On the other hand, even though Pt/Cdemonstrates slightly higher discharge voltage of 1.1 V for the firstthree cycles, both charge and discharge voltages degrade significantlyover cycling, resulting in very poor energy efficiency. The stable volt-ages obtained with LMCO/NCNT is attributed to high ORR and OERactivities, which greatly reduces the overpotentials associated withthese reactions thereby preventing catalyst degradation and subse-quent deactivation of the air electrode.

4. Conclusions

Highly efficient and durable bi-functional hybrid catalyst consistingof cobalt doped lanthanum manganese oxide nanoparticles combinedwith nitrogen doped carbon nanotubes (LMCO/NCNT) has been suc-cessfully synthesized. The half-cell evaluation of LMCO/NCNT showssignificantly improved ORR onset and half-wave potentials (−0.11and−0.24 V vs. SCE) and OER current density (27 mA cm−2). Practicalrechargeable zinc–air battery performance testing using atmosphericair reveals higher discharge voltages at current densities higher than30 mA cm−2, and lower charge voltages at all current densities testedcompared to state-of-art commercial platinum on carbon catalyst, andvery stable charge and discharge voltages (2.2 and 1.0 V, respectively)over 60 cycles. The excellent catalytic activity of the hybrid is attributed

to uniformly distributed LMCOnanoparticles throughout thenetwork ofNCNT, resulting in enhanced surface area and material utilization.

Conflict of interest

The authors state that there is no conflict of interest.

Acknowledgment

This work was financially supported by the Natural Sciences andEngineering Research Council of Canada (NSERC) though grants toDr. Z. Chen and the University of Waterloo.

References

[1] Z.-L. Wang, D. Xu, J.-J. Xu, X.-B. Zhang, Oxygen electrocatalysts in metal–air batteries:from aqueous to nonaqueous electrolytes, Chem. Soc. Rev. 43 (2014) 7746–7786.

[2] M.A. Rahman, X.Wang, C.Wen, High energy density metal–air batteries: a review, J.Electrochem. Soc. 160 (2013) A1759–A1771.

[3] Y. Li, H. Dai, Recent advances in zinc–air batteries, Chem. Soc. Rev. 43 (2014)5257–5275.

[4] U.A. Paulus, T.J. Schmidt, H.A. Gasteiger, R.J. Behm, Oxygen reduction on a high-surface area Pt/Vulcan carbon catalyst: a thin-film rotating ring-disk electrodestudy, J. Electroanal. Chem. 495 (2001) 134–145.

[5] Y. Lee, J. Suntivich, K.J. May, E.E. Perry, Y. Shao-Horn, Synthesis and activities of rutileIrO2 and RuO2 nanoparticles for oxygen evolution in acid and alkaline solutions, J.Phys. Chem. Lett. 3 (2012) 399–404.

[6] B. Lim, M. Jiang, P.H. Camargo, E.C. Cho, J. Tao, X. Lu, Y. Zhu, Y. Xia, Pd–Pt bimetallicnanodendriteswith high activity for oxygen reduction, Science 324 (2009) 1302–1305.

[7] Y.-C. Lu, Z. Xu, H.A. Gasteiger, S. Chen, K. Hamad-Schifferli, Y. Shao-Horn, Platinum–goldnanoparticles: A highly active bifunctional electrocatalyst for rechargeable lithium–airbatteries, J. Am. Chem. Soc. 132 (2010) 12170–12171.

[8] Y. Shao, G. Yin, Y. Gao, Understanding and approaches for the durability issues ofPt-based catalysts for PEM fuel cell, J. Power Sources 171 (2007) 558–566.

[9] X. Yu, S. Ye, Recent advances in activity and durability enhancement of Pt/C catalyticcathode in PEMFC: part II: degradationmechanism and durability enhancement of car-bon supported platinum catalyst, J. Power Sources 172 (2007) 145–154.

[10] Z. Chen, A. Yu, D. Higgins, H. Li, H. Wang, Z. Chen, Highly active and durable core–corona structured bifunctional catalyst for rechargeable metal–air battery application,Nano Lett. 12 (2012) 1946–1952.

[11] M. Prabu, P. Ramakrishnan, H. Nara, T. Momma, T. Osaka, S. Shanmugam, Zinc–airbattery: understanding the structure and morphology changes of graphene-supported CoMn(2)O(4) bifunctional catalysts under practical rechargeable condi-tions, ACS Appl. Mater. Interfaces 6 (2014) 16545–16555.

[12] J. Masa, W. Xia, I. Sinev, A. Zhao, Z. Sun, S. Grutzke, P. Weide, M. Muhler, W.Schuhmann, Mn(x)O(y)/NC and Co(x)O(y)/NC nanoparticles embedded in anitrogen-doped carbon matrix for high-performance bifunctional oxygen electrodes,Angew. Chem. Int. Ed. Engl. 53 (2014) 8508–8512.

[13] D.U. Lee, B.J. Kim, Z. Chen, One-pot synthesis of a mesoporous NiCo 2 O 4 nanoplateletand graphene hybrid and its oxygen reduction and evolution activities as an efficientbi-functional electrocatalyst, J. Mater. Chem. A 1 (2013) 4754–4762.

[14] H.W. Park, D.U. Lee, Y. Liu, J. Wu, L.F. Nazar, Z. Chen, Bi-functional N-doped CNT/graphene composite as highly active anddurable electrocatalyst formetal air batteryapplications, J. Electrochem. Soc. 160 (2013) A2244–A2250.

[15] Y. Liang, Y. Li, H. Wang, H. Dai, Strongly coupled inorganic/nanocarbon hybrid mate-rials for advanced electrocatalysis, J. Am. Chem. Soc. 135 (2013) 2013–2036.

[16] H.W. Park, D.U. Lee, P. Zamani, M.H. Seo, L.F. Nazar, Z. Chen, Electrospun porousnanorod perovskite oxide/nitrogen-doped graphene composite as a bi-functionalcatalyst for metal air batteries, Nano Energy 10 (2014) 192–200.

[17] Y. Li, M. Gong, Y. Liang, J. Feng, J.-E. Kim, H. Wang, G. Hong, B. Zhang, H. Dai, Advancedzinc–air batteries based on high-performance hybrid electrocatalysts, Nat. Commun. 4(2013) 1805.

[18] Y. Zhu,W. Zhou, Z.G. Chen, Y. Chen, C. Su, M.O. Tadé, Z. Shao, SrNb0.1Co0.7Fe0.2O3− δ pe-rovskite as a next‐generation electrocatalyst for oxygen evolution in alkaline solution,Angew. Chem. Int. Ed. 54 (2015) 3897–3901.

[19] W. Zhou, M. Zhao, F. Liang, S.C. Smith, Z. Zhu, High activity and durability of novel pe-rovskite electrocatalysts for water oxidation, Mater. Horiz. (2015)http://dx.doi.org/10.1039/c5mh00096c.

[20] K.A. Stoerzinger, W.S. Choi, H. Jeen, H.N. Lee, Y. Shao-Horn, Role of strain and con-ductivity in oxygen electrocatalysis on LaCoO3 thin films, J. Phys. Chem. Lett. 6(2015) 487–492.

[21] D.U. Lee, H.W. Park, M.G. Park, V. Ismayilov, Z. Chen, Synergistic bifunctional catalystdesign based on perovskite oxide nanoparticles and intertwined carbon nanotubesfor rechargeable zinc–air battery applications, ACS Appl. Mater. Interfaces 7 (2014)902–910.

[22] M.H. Seo, H.W. Park, D.U. Lee, M.G. Park, Z. Chen, Design of highly active perovskiteoxides for oxygen evolution reaction by combining experimental and ab initio studies,ACS Catal. 5 (2015) 4337–4344.

[23] Z. Chen, D. Higgins, H. Tao, R.S. Hsu, Z. Chen, Highly active nitrogen-doped carbonnanotubes for oxygen reduction reaction in fuel cell applications, J. Phys. Chem. C113 (2009) 21008–21013.