Performance <70 words. This paper tries to investigate the

Performance Optimization ofPEM Fuel Cell by Using Modified Bruggeman Correlation Model PrakashThapa1, Gye Choon Park*2, Sung Gi Kwon 3, Jin Lee41,2,3,4 Department of Electrical Engineering, MokpoNational University, [email protected],    [email protected], [email protected]

krr3, [email protected] Abstract Background/Objectives: In Proton ExchangeMembrane Fuel Cell (PEMFC), the modified Burggeman correlation is usedto estimate the effective conductivity and diffusivity of both catalyst and gasdiffusion of PEM fuel cell. Methods/Statistical analysis: It should be <70words. This paper tries toinvestigate the sensitivity of the gaseous diffusion to the various exponentialvalue of tortuosity factor m and n. This model provides empirical correlationfor the effective properties of composite system. Findings: It should be<170 words.

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The effectiveconductivity and diffusivity in the catalyst layers (CLs) and gas diffusionlayers (GDLs), is crucial for accurately predicting the fuel cell performanceand optimizing design parameters in the numerical modeling/simulation.When the value of mand n are increased the normalized function’s as well as saturation function’s values are decreased. Which indicatesthat the distribution of pore size of porous medium and the water saturationhas greater impact on the gas diffusion. Thus these parameter has greaterimpact on the performance optimization on the PEM fuel cell. The simulationresults shows that the performance of Polymer Exchange Membrane Fuel Cell(PEMFC) is varies with the m and n. At higher current density values, the valueof m=n=1.5 give the better performance.

 Improvements/Applications: In <30 words.For the betterimprovements of PEM fuel cell performance, conductivity and diffusivity of hydrogengas must be increased. Conductivity can be increased by increasing thetemperature of the gas as well as stack temperature whereas the diffusivity canbe increased highly concentrated hydrogen and oxygen gas and more porousmembrane. Keywords: PEM Fuel Cell, Conductivity,Diffusivity, Bruggeman Correlation, Tortuosity Factors  1.   Introduction A fuel cell is an electrochemical device that can convertchemical energy into electrical energy and produce heat and water as abyproduct through the electrochemical process. For the hydrogen gas generationeither PEM water electrolyzer or alkaline electrolyzer is used but for theelectricity generation we use PEM fuel cell stack. PEM fuel cell technology isone of the alternative resources which provides high quality power indistributed generation system 1. Due to smaller size, light weight, highpower density, low operating temperature, safe construction, more efficient andfast start-up, PEM fuel cell is more preferable than other type of fuel cell 2.

PEM fuel cell gives more than 40% efficiency in most of the stationary andtransport applications. The schematic diagram of PEM fuel cell is shown infigure 1 3    2. ProtonExchange Membrane Fuel Cell (PEMFC) Figure 1 showsthe geometry of a single fuel cell, which consists of a membrane, catalystlayers, gas diffusion layers, gas channels, and two collector plates. The mainfunctions of these components are: collector plates with flow channels are usedfor reactants and products transport, electron conduction and heat removal; gasdiffusion layers are for reactant distributions, electron conduction, andliquid water removal; catalyst layers are used to promote electrochemicalreactions where reactants are consumed, and products and heat are generated;and the membrane is used to conduct protons from the anode catalysts layer tothe cathode catalyst layer.

The electrochemical reaction occurring in the anode in whichhydrogen gas is consumed to produce protons and electrons 4, i.e.,                                                    Anode:                                                                (1)The produced electronsare passes through an external circuit to the cathode by providing electricalpower, while the protons transport through the membrane to the cathode.

At thecathode catalyst layer, oxygen combines with the protons and electrons toproduce water, i.e.,                                                   Cathode:                                                (2)And overall cellreaction occur during the electrochemical reaction process is given by,                                        Water                                        (3) Above these reactions involve on borderbetween ionically conductive electrolyte and electrically conductive electrode.For the better electrochemical reaction and the gases to arrive as well aswater to leave, the electrodes must be porous medium. Under steady stateconditions, the thickness of the cell is negligible compared to its otherisothermal approximation and the membrane is assumed to be fully hydrated.Moreover, the anode reaction over potential is neglected in the present study,because over potential due to the anode reaction to be negligible 5.Therefore, overall cell potential is obtained by subtracting the losses fromreversible cell voltage which is given by the following expression;                                                                                           (4)Where,  is the reversible cell voltage and  is the activation loss,  is Ohmic loss,  is concentration loss and  is the diffusion loss.

 is calculated from a modified version of theNernst equation, which is modified with an extra term to account for changes intemperature from the standard reference temperature 6. Which is given by Eq.(5).             (5)Where P and Trepresent the effective pressure and temperature respectively. Activationlosses can calculated by using empirical equation 7;                                                                                 (6) Where  are empirical coefficient and I is the cellcurrent and T is the absolute temperature.

 is the undissolved oxygen concentration whichcan be expressed as 8; Ohmic Overpotential Due to membraneresistance (Ionic Resistance)The voltage drop dueto the membrane resistance to the flow of ions produce the ohmic overpotentialloss in the fuel cell.where  is the ionic resistance as a function of membrane conductivity,  is the membrane height and  is the ionic conductivity of membrane withwater content and temperature 9.Where  be the degree of membrane humidification and is the cell temperature. Electronic ResistanceThe potential loss dueto the electronic bipolar plates and electrodes current collectors is calledelectronic resistance losses and is given by,But the ohmicresistance of the electronic materials is given by,Where,  is the material resistivity, l is the lengthand A is the cross-sectional area of the conductor.The ohmicoverpotential due to electronic and ionic resistance is 10 given by, The resistance protonis calculated by the following expression,The resistivity of themembrane depends on the water activity and cell temperature. Empirical formula for  is express as follows 11. J is the currentdensity with in the cell. The value of l can be fitted for a particular cell.

To obtain the value of l we can use the Sharifi model, which is given by 7;Where, 3. GasDiffusion and Catalyst Layer Both gas diffusion andcatalyst layers are porous media, the diffusion of oxygen gas at the cathodeterminal is given by 12;The diffusion ofhydrogen gas at the anode terminal is given by 13; The production ofwater at the cathode terminal side is given by 14; 4. Bruggeman CorrelationAccording to theFlick’s law, the gaseous diffusion in gas diffusion layer (GDL) and catalystlayers (Cl) of PEM fuel cell is given by,                                                                      (4)Where,  is the porosity of GDL,  be the concentration and  be the effective diffusivity of the reactantsgaseous.Similarly, accordingto the Bruggemann correlation, the effective diffusivity in a porous structurecan be expressed as 15,                                                                           (5)Where,  is the tortuosity factor of porous medium. Thetortuosity ( is defined as the ratio of the actual flowpath length and the thickness of the porous medium along the flow direction.When the impact ofliquid water saturation is taking into account then above Bruggeman correlationequation become,                                               (6)Where,  are normalized functions.Similarly, tortuosityfactor for Burggeman exponents m=0.

5 and saturation exponents, n=1.5. Thusabove equation 7 becomes;Then normalizedfunction for modified Bruggeman correlation 16 with tortuosity factor (m=n=1)is given by the following equation;                                                       (8) Where, be the effective conductivity and is the concentration of the liquid.   5.

Simulation ResultsThe figure 2 and 3 shows the simulatedpolarization curve by using different value of m and n. At higher currentdensity the PEMFC has significant effect of the tortuosity factor m and n. 

>  The normalized function g(s) Vs averageporosity value  and average water saturation function g(s) Vsaverage saturation   forvarious values of m and n are shown in figure 5 and 6.When the value of m and nare increased the normalized function’s as well as saturation function’s valuesare decreased. Which indicates that the distribution of pore size of porousmedium and the water saturation has greater impact on the gas diffusion.

Thusthese parameter has greater impact on the performance optimization on the PEMfuel cell.    6.ConclusionThe tortuosity factor for Burggemanexponent’s m and n has significant effect on the performance of PEM fuel cellat current density. So for the PEMFC design process the value of m and n mustbe equal to 1.5. Above this value the fuel cell performance will be decreased.

The sensitivity of the fuel cell performance to the value of n in GDL and CL isneglected as compared to that in the cathode GDL and CL but the sensitivity incathode CL is stronger than that in the cathode GDL. So for the performanceoptimization of fuel cell, concentration of the gas should be increased andwater removal from the cathode GDL and CL during gaseous diffusion, must beimprove. 7. AcknowledgementThis work was supported by KEPCO ResearchInstitute grant funded by Korea Electric Power Corporation (R16DA11) andBusiness for Cooperative R&D between Industry, Academy and ResearchInstitute funded by Korea Small and Medium Business Administration (C0442952).  8. References 1Peng andS.J.

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