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    Nanofluid as Cooling Medium In Polymer Electrolyte Membrane (PEM) Fuel Cell: A Study On Potentials And Possibilities Irnie Azlin Zakaria 1,a , Zeno Michael 1,b , Wan Ahmad Najmi Wan Mohamed 1,c   1 Faculty of Mechanical Engineering, Universiti Teknologi Mara ( UiTM ),Shah Alam, Malaysia a, b zenomichael@, c Keywords:  Thermal management; PEM fuel cell; Nanofluid Abstract . Tremendous need for an optimum conversion efficiency of a Polymer Exchange Membrane Fuel Cell (PEMFC) operation has triggered varieties of advancements namely on the thermal management engineering scope. Excellent heat dissipation is correlated to higher  performance of a fuel cell thus increasing its conversion efficiency. This study reveals the potential advancement in thermal engineering of a fuel cell stack related to nanofluid technology. Nanofluids are seen as a potential evolution of nano technology hybridisation with fuel cell serving as a cooling medium. The thermophysical characteristics have been reviewed and challenges with regards to fuel cell application is discussed. Nanofluid has been successfully tested on many thermal management systems isolated from thermoelectrical environments such as fuel cell. The main challenge is formulating a nanofluid coolant with high thermal conductivity but with strict limit on electrical conductivity of less than 5 µ S/cm. Lack of electrical conductivity data for various nanofluids in open literature is another challenge in nanofluid application in fuel cell. 1. Introduction There is a critical need to enhance heat transfer in fuel cell for better performance acquisition. Conventional methods on liquid cooled fuel cell improvement such as cooling channel design optimization, flow field optimization and cooling system enhancement has been explored by researchers [1, 2]. However, optimization on the geometrical design either on fuel cell stack or heat exchanger has come to a saturated zone where the improvement is seen to be not vigorous enough in order to fulfill criteria of compact, simplified, and lighter weight fuel cell system with superior  power density compared to conventional liquid cooling strategy . Application of nanofluids in fuel cells is a new area that has just recently been explored by the industry mainly due to the need for improved thermal management without jeopardizing system sizing of fuel cells [3]. However, available literatures are very limited in explaining the operating  behaviour of nanofluids in a fuel cell. To date this issue has only been addressed by an industry from the perspective of nanofluid durability in an electrically active environment. A Polymer Electrolyte Membrane fuel cell (PEMFC) converts hydrogen energy directly into electrical energy via electrochemical reactions with oxygen.   The PEMFC is best suited for many applications due to their high power density and excellent dynamics characteristics as compared to other types of fuel cells [4]. Electrochemical reactions occur at catalyst layer surface which is interfaced between gas diffusion layer and membrane as illustrated in Figure 1 . Hydrogen is diffused through the gas diffusion layer and catalytically split to its constituent of protons and electrons in catalyst layer. The polymer membrane, sandwiched between two bipolar plates known as anode and cathode, is impermeable to gasses and allows only protons to travel through by the mechanism of charge transport using water molecules. The free electrons will then travel through electrically conductive electrodes, current collector or external circuitry and will eventually be the net result (electric current) of this electrochemical reaction. At the cathode side, the hydrogen protons and electrons will meet and react with oxygen molecules thus producing water as a by-product of this electrochemical reaction.  Advanced Materials Research Vol. 1109 (2015) pp 319-323Submitted:25.07.2014© (2015) Trans Tech Publications, SwitzerlandRevised:04.11.2014doi:10.4028/  All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, (ID:,09:23:55)    The electrochemical reactions happen simultaneously in both anode and cathode and represented as follows :- At anode side :  H  2 2  H  +  +  2  e -   (1) At cathode side : ½ O 2 + 2  H  +  + 2 e -  H  2 O (2)   Thus giving overall reaction of 2  H  2 O + O 2   2  H  2 O (3) Formation of water and internal resistance of electrical circuit generates heat as another by-product and the concentration is normally higher at cathode side contributed to formation of water. This  phenomena requires a good thermal management in order to avoid membrane overheat as this can lead to stack performance deterioration. Figure 1 : Basic construction of a typical PEM fuel cell [5] Figure 2 : Electrical and thermal power relationship based on conversion efficiency   2. Thermal management of a PEMFC Adequate thermal management is needed to ensure that the fuel cell operates at its highest efficiency. Effective thermal management of PEMFC stack in automotive application is very exigent due to elevated requirement of both power output and density. Insignificant temperature difference between ambient and operating temperature of PEMFC, especially in hot climate country as compared to internal combustion engine has make the thermal management issue worse. Apart from that, cooling liquid is a solitary heat removal agent since reactant and product stream waste heat is almost negligible [2]. There are several challenges associated with its thermal efficiency, especially the low operating temperature. PEMFC operating temperature is in the range of 60 ° C to 80 ° C. Higher temperature than this may leads to membrane overheat or excessive drying which will then result in  performance reduction of a fuel cell. Lower temperature is also not favourable as it can lead to flooding issue due to lower water saturation pressure at lower temperature and also reduction in kinetic energy which is not favourable in term of reactant reactions [2, 6]. Optimal thermal management acquisition is desperately needed in PEMFC in order to increase the efficiency as electrical power is directly accompanied by almost equivalent thermal power, depending on the conversion efficiency as illustrated in figure 2. Heat generated in fuel cell is governed by : Q gen = (1.254 - V cell ).I.n cell  (4) 320NANO-SCITECH 2014    Where it is assumes that all product water leaves the stack as vapor at 25 ° C. Heat generated from the fuel cell is dissipated through several ways such as conducted through solid parts in fuel cell such as gas diffusion layer and bipolar plates and also to the reactant gas and surrounding through convection and radiation. However, the prime bulk of heat goes to the cooling medium through convection. Heat transfer or termed as heat 'absorbed' by cooling fluid is expressed by Newton's law of cooling [7] Q= h A ∆ T (5) where Q denotes heat flow, h is the heat transfer coefficient, A is the effective heat transfer area and ∆ T is the temperature difference between coolant out and coolant into fuel cell stack. Apart from increasing ∆ T and maximizing effective heat transfer area, A heat transfer coefficient h can be further enhanced   through improving the transport properties of the heat transfer material via addition of nano particles to coolant termed as nano coolant or sometimes nanofluid to the cooling liquid. 3. Possibility of nanofluid as a fuel cell coolant Unlike conventional heat transfer fluid, fuel cell coolant not only requires good thermal conductivity behaviour for excellent heat dissipation and low viscosity for minimal pumping loss  but also minimal electrical conductivity property. Corrosion inhibitor as an additive in conventional heat transfer fluids is normally organic acid salt or metal which is ionic in nature. The presence of these ions in solution may provide a path for a stray electric current which is hazardous to fuel cell operator as it can cause electric shock and also reducing performance of a stack due to the fluid capability of providing a short cut to electricity generated by fuel cell thus reducing the amount of  power generation [8] Apart from the low electrical conductivity requirement which is as low as 1.5 to 2 µ S/cm  based on Mc Mullen[9], fuel cell coolant must also be compatible to all materials that comes in contact in fuel cell. He has also table up a general guideline for a fuel cell coolant as in table 2. Mohapatra on the other hand has addressed that base fluid is responsible to determine the freezing point, flash point, material compatibility and thermophysical properties of fluid but the additive will help to reduce the the electrical conductivity level and he has examined a matrix of  base fluid and organic corrosion inhibitor and Polymer Ion suppressant. The study proposed the use of Polymer Ion suppressant as this can reduce the electrical conductivity to less than 0.3 µ S/cm measured after 2 weeks time of coolant composition [10]. Deionized water has been widely used as coolant in fuel cell due to its excellent properties in specific heat, thermal conductivity and low viscosity. However, there is another requirement for other properties such as freezing point and electrical conductivity which make deionized water as a less preferred coolant selection. Other development of coolant for fuel cell such as Ethylene Glycol, Alcohol, Propylene Glycol, Glycerine are now being reviewed by researchers [8, 10, 11].  Nanofluid application and challenges has been thoroughly reviewed by [3, 12] which covers applications mainly as a heat transfer enhancement and friction reduction devices. However, available literatures are very limited in explaining the operating behaviour of nanofluids in an electrically active environment such as fuel cell. A significant gap in reporting the electrical conductivity is also identified. Electrical conductivity requirement of less or equivalent to 2 µ S/cm for fuel cell application needs to be satisfied in order to craft the feasibility of nanofluid application in fuel cell. Advanced Materials Research Vol. 1109321    Table 2 :   Criteria for PEM fuel cell [9]   Criteria Description Specification Electrical Conductivity Conductive coolant will reduce the performance of fuel cell as well as increase the shock hazard for  personnel in contact <  2.0 µ S/cm Boiling Point Boiling point should be more than the highest bulk temperature of the coolant >  90 ° C Freezing Point Must br freeze tolerant under extreme cold condition <  -40 ° C Thermal conductivity Higher is better for heat transfer >  0.4 W/m.K Viscosity Lower is better for heat transfer and pumping power <  1.0 cP at 80 ° C Specific Heat Higher is better for heat transfer >  3 kJ/kg.K Durability More durable coolant will reduce the operating cost >  5000 hrs of operation ( 2 years of lifetime ) Material compatibility Coolant must be compatible with stainless steel, sillicone, EPDM, Viton and other fuel cell components' materials. - Toxicity Should be classified as non toxic for transportation Similar or less toxic than ethylene glycol ( EG ) Flammabality Should be classified as non flammable Flash point > 93.3 ° C Table 3 : Literature review on Electrical Conductivity of nanofluids   Ref Yr Nanoparticle Base fluid Vol fraction (%) Findings [13]  2013 Al 2 O 3 , CuO, Cu DI & EG 0.05 ~1 Limit of 2 µ S/cm only met by vol fraction of ≤0.05% in EG [14] 2012 Al 2 O 3  DI 1 ~ 4 EC increase with Temp increase ; lowest EC at 25 ° C is 500 µ S/cm [15]  2008 Al 2 O 3  DI 0.05 ~0.09 Lowest EC at 0.015 % is 99.6 µ S/cm [16]  2010 Graphene, f-TEG DI & EG 0.05~0.003 In water , min EC at 15 µ S/cm while in EG , for vol fraction of 0.005% and 0.07 % is ≤ 2 µ S/cm [17]  2013 Graphene, f-HEG 70:30 (DI:EG) 0.041~0.395 For 0.041 % vol fraction at 10 ° C, the EC already at 10 µ S/cm and increase with temp increment [18]  2009 Al 2 O 3  DI 0.5~3 Lowest EC of 10 µ S/cm recorded with 0.5% vol fraction at 24 ° C 4.0 Conclusion  Nanofluid application in PEM fuel cell offers a great potential as cooling medium due to its superior thermo-physical properties especially in thermal conductivity and convective heat transfer. However, electrical conductivity thermophysical characteristic needs to be further investigated in order to enable the adoption in fuel cell. 322NANO-SCITECH 2014    References [1] H. Morikawa, H. Kikuchi, and N. Saito, "Development and Advances of a V-Flow FC Stack for FCX Clarity," SAE International2009. [2] G. Zhang and S. G. Kandlikar, "A critical review of cooling techniques in proton exchange membrane fuel cell stacks," international journal of hydrogen energy, vol. 37, pp. 2412-2429, 2012. [3] R. Saidur, K. Y. Leong, and H. A. Mohammad, "A review on applications and challenges of nanofluids," Renewable and Sustainable Energy Reviews, vol. 15, pp. 1646-1668, 2011. [4] Y. Wang, K. S. Chen, J. Mishler, S. C. Cho, and X. C. Adroher, "A review of polymer electrolyte membrane fuel cells: Technology, applications, and needs on fundamental research," Applied Energy, vol. 88, pp. 981-1007, 2011. [5] A. Faghi and Z. Guo, "Challenges and opportunities of thermal management issues related to fuel cell technology and modeling," International Journal of Heat and Mass Transfer, vol. 48, pp. 3891-3920, 2005. [6] E. Hosseinzadeh, M. Rokni, A. Rabbani, and H. H. Mortensen, "Thermal and water management of low temperature Proton Exchange Membrane Fuel Cell in fork-lift truck  power system," Applied Energy, vol. 104, pp. 434-444, 2013. [7] G. Cengel, Heat and Mass Transfer : Fundamentals and Application, 4th ed.: Mc Graw hills companies, 2011. [8] A. V. G. carol S.Jeffcoat, Peter M.Woyciesjes,Filipe J.Marinho., "Heat Transfer Compositions With High Electrical Resistance For Fuel Cell Assemblies," United states Patent, 2009. [9] P. McMullen, S. Mohapatra, and E. Donovan. Advances in PEM Fuel Cell Nano-Coolant [Online]. [10] S. C.Mohapatra, "fuel cell and fuel cell coolant compositions," united state of america Patent 7,138,199, 2006. [11] T. Takashiba and S. Yagawa, "Development of fuel cell coolant," Honda R&D C0.Ltd2009. [12] W. Yu and H. Xie, "A Review on Nanofluids: Preparation, Stability Mechanisms,and Applications," Journal of Nanomaterials, vol. 2012, 2011. [13] K. G. K. Sarojini, S. V. Manoj, P. K. Singh, T. Pradeep, and S. K. Das, "Electrical conductivity of ceramic and metallic nanofluids," Colloids and Surfaces A: Physicochemical and Engineering Aspects, vol. 417, pp. 39-46, 2013. [14] R. S. L. Alina Adriana Minea, "Investigations on electrical conductivity of stabilized water  based Al2O3 nanofluids," Microfluid Nanofluid, vol. 13, 2012. [15] K.-F. VincentWong and T. Kurma, "Transport properties of alumina nanofluids,"  Nanotechnology vol. 19, 2008. [16] T. T. Baby and S. Ramaprabhu, "Investigation of thermal and electrical conductivity of graphene based nanofluids," Journal of Applied Physics, vol. 108, 2010. [17] M. Kole and T. K. Dey, "Investigation of thermal conductivity, viscosity, and electrical conductivity of graphene based nanofluids," Journal of Applied Physics, vol. 113, 2013. [18] S. Ganguly, S. Sikdar, and S. Basu, "Experimental investigation of the effective electrical conductivity of aluminum oxide nanofluids," Powder Technology, vol. 196, pp. 326-330, 2009. Advanced Materials Research Vol. 1109323
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