Dynamic Modeling of the Water Balance in the Cathode Gas Diffusion Layer of Polymer Electrolyte Fuel Cells

Transcription

1 Dynami Modeling of the Water Balane in the Cathode Gas Diffusion Layer of Polymer Eletrolyte Fuel Cells D. Fofana, K. Agbossou, Y. Dubé, J. Hamelin This doument appeared in Detlef Stolten, Thomas Grube (Eds.): 18th World Hydrogen Energy Conferene 21 - WHEC 21 Parallel Sessions Book 1: Fuel Cell Basis / Fuel Infrastrutures Proeedings of the WHEC, May , Essen Shriften des Forshungszentrums Jülih / Energy & Environment, Vol Institute of Energy Researh - Fuel Cells (IEF-3) Forshungszentrum Jülih GmbH, Zentralbibliothek, Verlag, 21 ISBN:

2 Proeedings WHEC Dynami Modeling of the Water Balane in the Cathode Gas Diffusion Layer of Polymer Eletrolyte Fuel Cells Daouda Fofana, Kodjo Agbossou, Yve Dubé, Jean Hamelin, Institut de reherhe sur l hydrogène, Université du Québe à Trois-Rivières, 3351 boul. Des Forges, Trois-Rivières (QC), Canada Development of PEMFC for use as a portable devies like mobile phones, omputer notebooks, digital ameras, et and transport devies are reeiving more overage in reent researh due to their lo ost-per kw, fast start-up, loer operating temperature and zero pollution. The operation in these appliations is generally dynami. So the development of dynami models is highly important. During fuel ell operation, management of ater produed is ruial to enhane PEMFC performane. Generally, ater is produed at the athode/membrane interfae due to the eletrohemial reations. Also, due to the humidifiation of gas at the anode and athode inlets to supply ater to the membrane, ondensation takes plae in the gas diffusion layer (GDL) preventing the diffusion of gas to the reative sites. The lak of gas in the reative sites inreases diffusion loss, and leads to a derease of the ell voltage. So its ater-transport harateristis must be studied in order to redue potential loss, flooding and enhane PEMFC performane. The ontinuity equation (or mass onservation) hih desribes the hange of mass onentration of i ith respet to time at a fixed point in x diretion of porous medium due to the motion of i and hemial reations produing i is give by the onept mathematial of equation [1,2]: ε (1 ) s Pi Ni + = Ri RT t x (1) The quantity ε (1 s) represents the volume oupied by gas speies in the porous medium. The soure or sink terms R is the rate of generation or onsumption of speies i. i When ater is present in the GDL, transport in the porous eletrode is indued by apillary fore and gas flo-driven shear fore. The governing equation for liquid ater an be expressed as [1,2]: s N ερ + = M R t x (2) Where ρ, M, N and R are density, moleular eight of liquid ater, molar flux of liquid ater and the interfaial mass-transfer rate of ater beteen liquid and ater vapour respetively.

3 332 Proeedings WHEC21 The eletrohemial kinetis is governed by the Voltmer-Butler equation, and sine the athode kinetis is suffiiently slo, it an be expressed by Tafel kinetis, i.e in the form [1,2]: I P αf = (1 s) ai exp η RT C RT O2 ons O2 ref (3) Where the fator (1 s) represents the redued surfae area due to liquid ater that overs the ative reation sites., i the exhange urrent density, a the reative surfae area, O 2 O2 P ons the gas pressure of loal oxygen, C ref the referene oxygen onentration, η the athode overpotential and α the transfer oeffiient for oxygen redution. The shemati illustration of apillary pressure P and gradient dp / ds are represented in fig.1. The apillary pressure is defined as the differene beteen gas and liquid-phase pressures. It interesting to note that the gradient dp / ds is not onstant in the range of and 1 of saturation. In order to bring out the variation of dp / ds e represented from saturation of liquid ater in GDL the graph of apillary pressure P and the gradient dp / ds fig.2. The absolute valued of apillary pressure is higher at the GDL/membrane interfae here saturation is higher than the GDL/ hannel interfae. It is interesting to note that the gradient of apillary pressure hih is responsible for driving fore for liquid ater flo is onfirmed. Furthermore, the gradient dp / ds hih has most time taken as a onstant [3-5] to redue mathematial diffiulty or to take into aount the assumption of lo saturation is not onstant. The main onlusion of these studies is the assumption of gradient dp / ds onstant should be used for uniform saturation or very lo saturation but hen saturation an be varied in the range of and.8 this assumption an not be used. In order to study the effet of ondensation on the polarisation urve, e alloed ondensation in fig.3 at pressure of 325 kpa. This figure shos the polarizations urves of fuel ell for both ondensation and ithout ondensation on GDL at 1% of relative humidity. It an be seen in this figure that the dynami polarization urve ithout ondensation is better than the polarisation urve ith ondensation before purge. When ater vapour ondenses in the GDL, liquid ater is produed and redues the volume available for diffusion. The onentration or pressure of reatant dereases at eletrohemial reation sites resulting in lo voltage as it an be seeing in fig.3. The inrease of liquid ater is related to saturation inrease hih in return inreases the ativation overpotential due to relation beteen them. Fig.4 presents the study state profile of ell voltage ith ontrol of ondensation phase. The main reason for this derease in performane is due to the fat that gas pressure at inlet is still onstant so that the pressure or the onentration of oxygen dereases ith the distane before reahing the DGL/membrane interfae and also the liquid ater ondensed in GDL. The poor onentration of oxygen at the interfae and the liquid ater ondensed redue the eletrohemial reation rate and also the ell performane. The model results reinfore some important points suh as GDL, ondensation effet, evaporation effet on fuel ell behaviour. The ondensation of liquid ater affets fuel ell

4 Proeedings WHEC performane by redued the voltage magnitude. The liquid ater saturation level aumulates at the interfae GDL/membrane is responsible for suh situation than that in GDL due to the small thikness of GDL The gradient dp / ds depends strongly on the range of saturation. It should not be taken as a onstant for large range saturation. The non-linear equation of liquid ater should be resolved for dynamial model ithout assumption of onstant gradient dp / ds. P, dp/ds, (Atm) P dp/ds Saturation Figure 1: Graph of Capillary pressure P and gradient dp / ds P (Atm), dp/ds (Atm) P 1 P 2 P 3 dp 1 /ds dp 2 /ds dp 3 /ds x (µm) Figure 2: Shemati representation of P and dp / ds at (1):6 s,(2):1 s and (3):136 s.

5 334 Proeedings WHEC21 Cell P otential (V ) 1.5 V=f(I,t) ithout ondensation V=f(I,t) ith ondensation Current Density (A/m²) Figure 3: Effet of ondensation on polarization urve of fuel ell at 325 kpa ith ondensation and ithout ondensation at relative humidity (RH) of 1%. C ell v oltage (V ) With ondensation ithout ondensation Time (min) Figure 4: Steady state profile of ell voltage ith ondensation phase ontrol (GDL thikness of 3 µm,.5a/m²). Referenes [1] D. Natarajan & T.V. Nguyen A to-dimensional, To-Phase, Multiomponent, Transient Model for the Cathode of a Pemf USING CONVENTIONAL Gas distributors, journal of the Eletrohemial Soiety, 148(12) A1324-A1335 (21) [2] Dilip Natarajan, Trung Van Nguyen Three-dimensional effets of liquid ater flooding in the athode of a PEM fuel ell Journal of Poer Soures 115 (23) 66 8 [3] Jin Hyun Nam 1, Massoud Kaviany Effetive diffusivity and ater-saturation distribution in single- and to-layer PEMFC diffusion medium International Journal of Heat and Mass Transfer 46 (23) [4] Shih-Ming Chang, Hsin-Sen Chu, Transient behavior of a PEMFC Journal of Poer Soures 161 (26)

6 Proeedings WHEC [5] R. Madhusudana Rao & al. A to-dimensional steady state model inluding the effet of liquid ater for a PEM fuel ell athode Journal of Poer Soures 173 (27)