MnO2 Nanotubes Synthesized with One-step Hydrothermal Method as Efficient Bi-functional Cathode Catalyst for a Rechargeable Zinc air Battery

Transcription

1 Advances in Engineering Research, vlume 129 6th Internatinal Cnference n Energy, Envirnment and Sustainable Develpment (ICEESD 217) MnO2 Nantubes Synthesized with One-step Hydrthermal Methd as Efficient Bi-functinal Cathde Catalyst fr a Rechargeable Zinc air Battery Haran Li1, Xuemei Li1, Qi Nie1, Keryn Lian2 and Jinli Qia1,* 1 Cllege f Envirnmental Science and Engineering, Dnghua Universtiy, 2999 Ren min Nrth Rad, Shanghai 2162, China 2 Department f Materials Science and Engineering., University f Trnt, Trnt, Ontari, Canada *Crrespnding authr. qiajl@dhu.edu.cn, keryn.lian@utrnt.ca Tel: Fax: Keywrds: Bifunctinal catalyst, MnO2 nantube catalysts, rechargeable zinc-air battery. Abstract. This paper reprts the preparatin and electr-catalytic activity f MnO2 nantube catalysts fr bth xygen reductin reactin (ORR) and xygen evlutin reductin (OER) in alkaline media. The catalysts were synthesized by a simple hydrthermal methd under cntrlling the different temperatures. Results f electrchemical characterizatin with emphasis n the activity f the catalyst are presented, which was demnstrated by cyclic vltammetry (CV) and linear sweep vltammetry (LSV) emplying a rtating disk electrde (RDE) technique. The experiments shw that the as-prepared MnO2 catalyst with nantube structure exhibited high electr-catalytic activity and stability during battery discharge, charge and cycling prcesses in 6 M KOH electrlyte. The primary Zn-air battery shwed a discharge peak pwer density ~293 mw cm, which crrespnds t the energy density ~725 ma hg-1. The rechargeable Zn-air battery exhibited a small charge discharge vltage plarizatin f ~.7 V at 1 ma cm, high reversibility and stability ver shrt charge and discharge cycles, and ~1.1 V fr a lng charge and discharge cycles test. Intrductin With the rapid develpment f the wrld ecnmy, the energy crisis and envirnmental prblems have becme increasingly prminent [1]. T vercme this challenge, strategies have t be prpsed t make a transitin frm a fssil fuel based ecnmy t a clean energy ecnmy. Electrchemical devices such as fuel cells, batteries and super capacitrs are mst ptential alternatives fr energy cnversin and strage instead f the burning f fssil fuels. Therefre, many effrts have been fcused n batteries and fuel cells, which can be used fr EVs prpulsin and large-scale energy strage. Amng the different energy strage systems, the rechargeable zinc-air battery evked a great interest due t its many attractive features including high energy density, cst-effective and envirnmentally friendly design, as well as lw perating risks [2]. The excellent prperties f rechargeable zinc-air batteries lead t ptential applicatins in areas, such as statinary and prtable pwer applicatins including electric vehicles [3,4]. Zinc-air batteries have a high theretical energy density f 186 Wh kg-1 (including xygen), abut five times higher than the current lithium-in technlgy [5]. They can be ptentially manufactured at very lw cst (<1 $ kw-1 h-1 estimated, abut tw rders f magnitude lwer than lithium-in). Fr many applicatins, zinc-air batteries ffer the highest available energy density f Cpyright 217, the Authrs. Published by Atlantis Press. This is an pen access article under the CC BY-NC license ( 92

2 Advances in Engineering Research, vlume 129 any primary battery systems. Hwever, in a rechargeable zinc-air battery, xygen evlutin reactin (OER) and xygen reductin reactin (ORR) ccur n the air electrde during battery charge and discharge prcess, respectively. The sluggish reactin kinetics and large ver ptential assciated with OER and ORR greatly limit the perfrmance f rechargeable zinc-air batteries [2,4].T date, fr mst fuel cells, the mst practical cathde catalysts are still Pt-based materials fr imprving the sluggish kinetics f xygen reductin reactin. As yet, the high cst and the pr stability f Pt-based nble metal catalysts hinder this technlgy's cmmercializatin. As a result, an affrdable, active and stable bifunctinal catalyst is an urgent issue fr imprving the perfrmance f rechargeable zinc-air batteries. In this article, we reprted the synthesis f MnO2 nantube catalysts, and their electr-catalytic activity fr bth ORR and OER including charge-discharge prperties when used as air cathde electrde were studied. Experimential Catalyst preparatin. MnO2 nantube catalysts were prepared thrugh a facile hydrthermal methd. In a typical experiment,.7963 g KMnO4 and 2 ml cncentrated HCl (37%) were added t 5 ml deinized water t frm the precursr slutin, then the slutin was transferred int a 1 ml tefln-lined stainless steel autclave. The autclave was sealed and hydrthermally treated at 1, 14, 18 and 22C fr 12 h. After the autclave was cled dwn t rm temperature naturally, the MnO2 nantube sample was cllected and washed 3-5 times with deinized water and ethanl. Then the prducts were dried in air at 75C fr 24 hurs. After that, the resulting prducts were calcined in air at 35C fr 1 h. Electrde preparatin and electrchemical measurements. In this experiment, we use a rtating disc electrde (RDE) half-cell setup t investigate the ORR and OER catalytic activity f the MnO2 nantube catalyst samples. The wrking electrde was fabricated by casting Nafin-impregnated catalyst ink nt a glassy carbn disk electrde (5.6 mm in diameter). 5 mg f the catalyst was ultrasnically dispersed int 1 ml ethanl and 8 µl 5 wt% Nafin slutin t frm a catalyst ink. After that, 8 µl f the catalyst ink was drpped nt disk electrde and dried naturally at rm temperature. The wrking electrde was allwed t achieve a catalyst lading f.1 mg cm, which was then immersed in a glass cell cntaining.1 M KOH aqueus electrlyte during the whle measurements. Electrchemical activity f the MnO2 nantube catalyst samples were tested thrugh linear sweep vltammetry. A Pt/C electrde was used as the reference electrde. Catalyst activity tward ORR and OER was evaluated in xygen-saturated electrlyte slutin frm1.67 t.1 V vs RHE. The ptential f the reference electrde was nrmalized with respect t the ptential f the reversible hydrgen electrde (RHE). The rtatin rate is 15 rpm and the scan rate is 5 mvs-1. A cmmercial Pt/C catalyst (3 wt%, JM) was tested using the same prcedures. Materials Characterizatin. The mrphlgies f varius catalyst samples were evaluated using an FEI Sirin 2 field-emissin scanning electrn micrscpe (SEM) perating at 5 kv. And transmissin electrn micrscpy (TEM) images were taken by a JEOL JEM 21F micrscpe at an accelerating vltage f 2 kv. Zinc-air batteries measurements. A hme-made zinc air battery was used t validate the practical catalyst activity and stability. The air cathde was prepared by spraying the catalyst (with a lading f 2 mg cm) nt a piece f carbn paper with an expsed active area f 4 cm2. Briefly, 3 mg f catalyst was dispersed in 1mL f ethanl by snicatin fr 3 minutes. 8 μl f 5 wt% Nafin slutin was added fllwed by additinal snicatin fr 1h. Then the catalyst mixture was sprayed nt the carbn paper and dried in an ven at 6 C fr 1 hur. The catalyst lading was cntrlled by calculating the weight f the air electrde with the carbn paper befre and after 921

3 Advances in Engineering Research, vlume 129 the spray cating. 1 ml f 6 M KOH was used as the electrlyte in the zinc air battery, and a plished zinc plate (purity > 99.99%, thickness: 1. mm, Shengshida Metal Mater. C. Ltd, China) was used as the ande. A Galvan dynamic methd with a current density ranging frm t 1 ma were used t btain the discharge plarizatin and pwer density plts. Battery testing and cycling experiments were perfrmed under 25 C using the recurrent galvanic pulse methd, where ne cycle cnsisted f a discharging step (5 ma cm fr 1 minutes) was cnducted fllwed by a charging step f the same current and duratin time. Results and Discussin Fig. 1 (a) SEM image f MnO2; (b) TEM image f MnO2. Surface Structure f MnO2 nantube catalyst. In fig. 1(a-b), SEM and TEM images are prvided, where the mrphlgies f the catalyst sample was examined. Fig. 1(a) reveals the typical SEM image f the MnO2 nanmaterials. It can be seen that MnO2 nantubes can be successfully synthesized by applying the simple ne-step hydrthermal methd. Mrever, this SEM image demnstrated that MnO2 nantubes have clear texture structure n its surface under hydrthermal temperature f 14 C. Such texture structure made up f cuntless prus with the diameter between.43~.45 nm. As shwn in Fig. 1(b), MnO2 nantubes cnsisting f average diameter and length f 12 nm and 45~6 nm, respectively, can be clearly bserved. Mre imprtantly, the mechanical strength will be increased and the surface area will increase due t the small the particle size, which will directly imprve the stability and activity f the catalyst. Linear sweep vltammetry. Fr further investigating the electr-catalytic activity fr bth ORR and OER, LSV study was cnducted in O2 saturated.1 M KOH slutin using a rtating disk electrde. Fig. 2(a) shws the LSV curves at different hydrthermal temperatures fr MnO2 nantube catalyst. It can be seen that hydrthermal temperature has great effect n MnO2 catalysts perfrmances. As shwn in Fig. 2 (a), the nset ptential fr ORR was detected at.94 V fr MnO2 nantube catalyst synthesized at 14C, whereas the nset ptentials f.91 V,.79 V and.78 V were bserved fr MnO2 nantubes synthesized at 1, 18 and 22 C, respectively. At.4 V, the MnO2 nantube catalyst synthesized at 1, 14, 18 and 22C affrded an ORR current density f 1.2 ma cm, 6.1 ma cm, 5.2 ma cm and 5.8 ma cm, respectively. Apart frm the ORR activity, gd OER activity is particularly critical fr MnO2 bi-functinal catalysts. Frm Fig. 2(a), it can be seen that the nset ptential was detected at 1.45 V fr MnO2 nantube catalyst synthesized at 14C, whereas it was 1.51 V, 1.56 V and 1.53 V fr MnO2 nantube catalyst synthesized 1, 18 and 22C, respectively. The OER current densities fr MnO2 nantube catalyst synthesized 1, 14, 18 and 22C at 1.7 V are 16, 9, 12 and 14 ma cm, respectively. 922

4 Advances in Engineering Research, vlume E = O 2/OH 12 1 C 14 C 18 C 22 C - ORR OER -4 (a) Ptential / V (vs RHE) -1 MnO2-14 C Pt/C -3-4 (b) Ptential / V (vs RHE) Fig. 2 (a) ORR and OER plarizatin curves f MnO2 by 1, 14, 18 and 22 ; In additin, the MnO2 nantubes catalyst synthesized at 14C can give a much high catalytic ORR current density f 6.1 ma cm accmpanied by a defined diffusin-limiting current plateau at.4 V, which is almst 1.5 times f Pt/C catalyst (Fig. 2(b)). Primary Zn-air batteries. T practically cnfirm that the MnO2 materials culd be used as electr-catalysts fr Zn air battery, here the MnO2-14 catalyst as-synthesised was used as the ORR catalyst laded n the carbn fibre paper as cathde. With the cathde prepared, we install it with a Zn fil (.5 mm thickness) as ande and 6 M KOH as the electrlyte t make up a primary Zn-air battery. As shwn in Fig. 3(a), the primary battery shwed a high pen circuit vltage f ~1.325 V. At a vltage f 1. V, it gave a high current density f 2.6 ma cm. The peak pwer density culd reach as high as mw cm at.7 V, much better than that f mst recently reprted Zn-air primary battery [6]. In additin, ur catalyst has much higher peak pwer density cmpared with Pt/C (12.8 mw cm 2) under the same measuring cnditins. Recently, Zn-air fuel cells r Zn-air flw batteries have been prpsed and demnstrated t pwer electric vehicles with high pwer, lng driving distance and cmmercial viability [7]. The abve results indicate that the nantube catalyst develped in this wrk shuld be ideally suited fr such traditinal primary Zn-air batteries wing t the decent high ORR activity and durability Pt/C Vltage / V MnO2 27 cycle (b) j = 1 ma cm Pwer density / mw cm Cell ptential / V (a) V.7 V 1. cycling: 5 min discharge, 5 min chagre Time / h Fig. 3 (a) Plarizatin curve and crrespnding pwer density plts f the primary Zn-air battery using the MnO2 catalyst; (b) Charge and discharge plarizatin (V-i) curves f the rechargeable Zn-air battery. 923

5 Advances in Engineering Research, vlume 129 Rechargeable Zn-air batteries. Fr further demnstrating MnO2 nantube catalyst develped abve in practical applicatin, an electrchemically rechargeable Zn-air batteries was cnstructed and tested by utilizing the as-prepared MnO2 nantube materials as the cathde catalyst. Fig. 3(b) shws the charge and discharge plarizatin curves f the rechargeable Zn-air battery. As can be seen clearly frm the Fig. 3(b), ur rechargeable Zn-air battery exhibits a remarkable stable cycling stability with a narrw charge-discharge vltage gap f.7 V, and the discharge vltage can be able t maintain abve 1.3 V. This suggests that the hmemade Zn-air battery exhibits a stable cycling stability when it was charged and discharged at cntrlled current densities (1 ma cm) and cycling pattern (5 minute per charge r discharge perid). Even after 44 hurs, ur rechargeable Zn-air battery can still shw a narrw charge discharge vltage gap (~1.1 V), indicating the lng term durability f the catalyst. Cnsidering that the battery xidant feeding was by an un-enfrced atmsphere air instead f pure xygen r enfrced air flw, the results described abve are significantly imprved. Cnclusins In summary, the MnO2 nantubes as a new kind f air electrde catalyst have been synthesized via a simple ne-step hydrthermal methd. Particularly, the MnO2 nanmaterial was used as a highly active bi-functinal cathde catalyst fr rechargeable Zn-air battery. The maximum pwer density can achieve as high as mw cm, and a high current density ma cm at 1. V. Mrever, the rechargeable battery shws the high perfrmance with an excellent cycling stability. All f these battery tests have cnfirmed that the MnO2 bi-functinal catalyst develped in this wrk is feasible and prmising fr practical applicatin f Zn-air batteries. Acknwledgements The authrs acknwledge the financial supprt frm the Prject f Intrducing Overseas Intelligence High Educatin f China (21617), the Graduate degree thesis Innvatin Fundatin f Dnghua University (EG21731) and the Cllege f Envirnmental Science and Engineering, State Envirnmental Prtectin Engineering Center fr Pllutin Treatment and Cntrl in Textile Industry, Dnghua University. References [1] S.I. Smedley and X.G. Zhang: J. Pwer Surces Vl. 165 (27), p [2] V. Neburchilv, H.J. Wang, J.J. Martin and W. Qu: J. Pwer Surces Vl. 195 (21), p [3] J. Gldstein, I. Brwn and B. Kretz: J. Pwer Surces Vl. 8 (1999), p [4] P. Sapkta and H. Kim: J. Ind. Eng. Chem. Vl. 15 (29), p [5] Y.G. Li, H.J. Dai: Chem. Sc. Rev. Vl. 43 (214), p [6] G.J. Du, X.D. Liu, Y. Zng, T.S. Hr, A.Y. Yu and Z.L. Liu: Nanscale Vl. 5 (213), p [7] J. Hu, L. Wang, L.N. Shi and H. Huang: J. Pwer Surces Vl. 269 (214), p