GAS DIFFUSION LAYER AND REACTANT GAS CHANNEL INFLUENCE ON THE PERFORMANCE OF A HT-PEM FUEL CELL

Transcription

1 GAS DIFFUSION LAYER AND REACTANT GAS CHANNEL INFLUENCE ON THE PERFORMANCE OF A HT-PEM FUEL CELL V. IONESCU Department of Physics and Electronics, Ovidius University, Constanta, , Romania, ionescu.vio@gmail.com Received August 31, 2015 Proton exchange membrane fuel cells (PEMFC) are highly efficient power generators, being used recently in a clean hybrid power supply system. Comsol Multiphysics, a commercial solver based on the Finite Element Method (FEM) was used for developing a three dimensional model of a high temperature PEMFC in order to study operation mode and performance of the fuel cell. Cathode gas flow velocity influence on the cell performance was investigated. Gas channel geometry for cell model was optimized by varying the channel width/gdl width ratio λ at values between 0.3 and 0.8, choosing a low operating gas flow velocity of 0.06 m/s and computing the following parameters: water and oxygen molar concentrations at cathode catalyst layer, pressure drop variation across anode GDL and velocity field distribution across the cathode gas channel. Key words: proton exchange membrane fuel cell, polarization curve, gas flow velocity, molar concentration. 1. INTRODUCTION The fuel cell powered unit for combined heat and power generation (FC- CHP) can be a new, smarter technology developed in order to substitute in the future the classical oil-fired boilers. A Fuel Cell CHP (FC-CHP) system consists of three primary sub-systems: the fuel cell stack, the fuel processor and the power conditioning system. The fuel processor converts the fuel, for instance natural gas or methanol, into a hydrogenrich feed stream that is supplied to the fuel cell stack which in turns generates electrical and thermal energy [1]. The power conditioning system is used to convert the power generated by the stack as non-linear DC voltage in a form of electrical power which is useful for the end-user. The fuel is typically either an alcohol, a hydrocarbon or a substance derived from it (e.g. Hydrogen), which can be supplied continuously. Rom. Journ. Phys., Vol. 61, No. 7 8, P , 2016

2 1236 V. Ionescu 2 Proton exchange membrane fuel cells (PEMFC) are the most developed FC- CHP technology, powering around 90% of systems shipped to date [2]. They are most widely used in residential heating systems (1 3 kw thermal), such as those in the Japanese EneFarm program [3]. PEM technology offers high efficiency, durability and reliability, and costs have fallen rapidly due to mass production. Current research is aimed at system simplification: removing the platinum could avoid complex engineering solutions [4, 5], while high-temperature (HT-PEM) cells can operate on dry hydrogen at temperatures up to 200 o C, removing the need for humidifiers [6]. Simplified and cheaper fuel processor construction as HT-PEMFC stack can tolerate up to 5 vol.% of carbon monoxide (CO) with minor cell performance loss [7]. Various studies have been devoted to developing mathematical bidimensional models for the transport of reactants and product water in a PEMFC [8]. A 2D isothermal, steady state PEM fuel cell model was implemented with Comsol Multiphysics software in order to study the cell performance at different PBI membrane widths [9]. Berning and Djilali [10] presented a 3-dimensional, multi-phase and multi-component model for anode and cathode of PEMFC and it was described here the two phase flow inside the GDL by the unsaturated flow theory (UFT) according to a uniform gas phase pressure is assumed within the GDL. The 3D HT-PEMFC numerical model used in this paper was derived from 3D single-phase isothermal model developed by E.U. Ubong et al. [11] to predict the performance of a HT-PEMFC with a PBI membrane. The objective of this work is to investigate the influence of GDL porosity and channel width/gdl width ratio on the cell performance with the help of a 3D model using commercial FEM package Comsol Multiphysics (version 4.2). 2. MODEL SET-UP The 3D computational domain includes a section of the PBI membrane and both cathode and anode gas flow channels, GDLs, and catalyst layers (Fig. 1). At steady state, single-phase, isothermal model of PEM fuel cell consists of five principles of conservation: mass, momentum, species, energy and charge. COMSOL Multiphysics are used to solve this complex HTPEM fuel cell model. Laminar flow in the channels is described by the Navier-Stokes equations (dimensionless formulation), for the steady state in case of no external forces: 1 ( v ) v p v ( v) T 0; v 0 (1) Re where unknown depended variables are p pressure and v velocity; R e is a dimensionless Reynolds number.

3 3 Gas diffusion layer and reactant gas channel influence on performance of HT-PEM fuel cell 1237 Fig. 1 3D HT-PEMFC computational model geometry. Porous gas diffusion layers (GDLs) and electrode flow can be given by a similar set of differential equations, when the Brinkman formulation is used: 1 T kv p v ( v) 0 R v 0 e 2 where k is the Brinkman parameter, defined as: H k, with H channel height Re and permeability. Conservation of species was solved for the flow channels, GDLs and porous electrode using the Maxwell Stefan equations in two different Transport of Concentrated Species interfaces. It solves for the fluxes of each species (H 2 and H 2 O in the anode compartment, O 2 and H 2 O in the cathode compartment) in terms of mass fraction. The Maxwell-Stefan equation, defined for each component of the mixture of the three gases, is presented below [14]: 3 M M p [ i Dij j j x j j i v] 0. (3) j 1 M j M p In Equation (3), x j is the molar fraction of each gas j, parameters ω i and ω j are the mass fractions of gases i and j respectively, parameter ρ is the overall mass density of the air mixture obtained from the ideal gas law, D ij is the binary diffusion coefficient, M is the total molar mass of the mixture and M j is the molecular weight of gas j. Conservation of the electric charge is based on two currents: an ionic current formed by the protons travelling through the membrane and an electronic current formed by the electrons passing through the solid matrix of electrodes. The current continuity equations are obtained by using Ohm s law [11]: (2)

4 1238 V. Ionescu 4 ( ) S s s s ( ) S m m m were Φ is the phase potential, σ is the effective electric conductivity ( S/m), S is the current source term (A m -3 ) and subscripts s and m denotes the properties of the solid phase and membrane, respectively. Table 1 Fuel cell design and operating parameters Parameter Value Gas channel dimensions Cell length (cm) 2 Channel width (mm) 1.7 Channel depth (mm) 1 Width of the shoulder (mm) 0.9 Catalyst layer information [12] Thickness (µm) 50 Permeability (m 2 ) Porosity 0.2 Open volume fraction for gas diffusion 0.4 Membrane PBI/H 3 PO 4 properties [13] Thickness (µm) 98 Conductivity (S/m) 1.44 GDL properties [14] Conductivity (S/m) 222 Bulk porosity 0.4 Permeability coefficient ( m 2 ) 1.8 Thickness (µm) 380 Operating conditions Reference pressure (Pa) Cell voltage (V) 0.9 Temperature ( o C) 150 Oxygen reference concentration (Mol/m 3 ) Hydrogen reference concentration (Mol/m 3 ) Inlet H 2 mass fraction (anode) Inlet H 2 O mass fraction (cathode) 0.18 Inlet O 2 mass fraction (cathode) Gas pair diffusivities and basic fluid parameters H 2 -H 2 O Binary diffusion coefficient (10-4 m 2 /s) N 2 -H 2 O Binary diffusion coefficient (10-5 m 2 /s) 4.48 O 2 -N 2 binary diffusion coefficient (10-5 m 2 /s) 4.18 O 2 -H 2 O binary diffusion coefficient (10-5 m 2 /s) 4.91 Anode inlet flow velocity (m/s) 0.17 Cathode inlet flow velocity (m/s) 0.42 Anode viscosity (10-5 Pa s) 1.12 Cathode viscosity (10-5 Pa s) 2.68 (4)

5 5 Gas diffusion layer and reactant gas channel influence on performance of HT-PEM fuel cell 1239 At anode catalyst layer, S m = j a and S s = j a ; at cathode catalyst layer, S m = j c and S s = j c. Here, j a and j c are the transfer current density corresponding to the electrochemical reaction at the anode and cathode catalyst layers, respectively. Transfer current densities were calculated by using a simplified Butler- Volmer equation [14]. The fuel cell design and operating parameters for base model are presented in Table 1. The boundary conditions for the model in this study are as follows: i) continuity at all internal boundaries, ii) no slip boundary condition for all the channel walls, iii) all initial values set to zero, iv) velocity and temperature defined at channel inlet, step function used for these two parameters in time dependent study, v) no back-pressure at channel outlet, convective flux boundary conditions applied, vi) constrain outer edges set to zero for both inlet and outlet, vii) bipolar plates on the most side of the cell set to electric ground and cell operation potential, viii) HTPEM fuel cell is insulated from environment. 3. RESULTS AND DISCUSSIONS The simulation results for the base case operating parameters presented in Table 1 were verified against experimental measurements of J. T. Wang et al. [15], as we could see in Fig. 2. Fig. 2 Comparison of the numerical model of base condition with experimental data.

6 1240 V. Ionescu 6 A good agreement was observed between experimental and computed polarization curve in the regions of ohmic and activation polarization. From here we could notice that ohmic portions of both curves had a distinctly steeper gradient due to the shoulder effects. The shoulders provide and additional contribution to the ohmic overpotential and a higher ionization potential drop compared to the case of the 2D cell model [12]. The liquid water concentration distribution at the operating voltage of 0.2 V for the cathode part of the cell is shown in Fig. 3, for different GDL porosities. Starting with the value ε = 0.2, we could see in the upper corner (outlet) of GDL and gas channel the formation of a local maximum in water concentration, water levels increasing with the direction of flow. This may be explained by the fact that for higher GDL porosity the electrochemical reaction rate is enhanced and more water is transferred at the outlet of the cell. It can be also seen from Fig. 3 that the liquid water concentration increases along the Z coordinate and the peak value of liquid locates at the cathode catalyst layer for the lowest values of GDL porosity. This phenomenon is more evident (appearing along to entire catalyst layer lenght) at ε = 0.1. Fig. 3 The distribution of the water molar concentration (Mol/m 3 ) along the cathode GDL and cathode gas channel for the cell model at base conditions, with a GDL porosity value varied between 0.1 and 0.4. Because air is readily available and does not need to be processed, it is used as an oxidant in most of the PEM fuel cells. Oxygen from the air, which has low concentration, is the main reactant gas and cell performance will be affected unless a sufficiently high pressure is applied at the cathode inlet to increase the concentration of oxygen reaching the catalyst layer [16]. Figure 4 shows the effect of changing the cathode gas flow velocity at values under 0.42 m/s on the fuel cell performance. When the gas flow velocity is increased from 0.02 to 0.12 m/s, the fuel cell performance is clearly enhanced, especially at lower operating fuel cell voltages.

7 7 Gas diffusion layer and reactant gas channel influence on performance of HT-PEM fuel cell 1241 Fig. 4 Effect of cathode gas flow velocity on cell performance at base conditions for different cathode inlet flow velocities (between 0.02 and 0.42 m/s). The rate of the electrochemical reaction is increased due to the increase in oxygen gas through the gas diffusion layer to reaction sites. Due to the low membrane humidification, this enhancing performance effect is minimal at high operating voltages, as Wang and Liu [17] demonstrated in their experimental work on PEM fuel cell performance. So, the air stream is able to supply oxygen with the required rate for gas flow velocity values over 0.1 m/s in the case of the cell model with gas channel dimensions from Table 1. In order to optimize the gas channel geometry, which is one of the key parameters in the PEM fuel cell, it was modified between 0.3 and 0.8 the channel width/gdl width ratio, defined as: Wch W W ch rib. (5) The width of the shoulder W rib has been kept constant at the value of 0.9 mm. For all the simulations witch will follow, cathode gas flow velocity was selected as 0.06 m/s and the operating voltage was chosed at 0.3 V. The rest of cell model parameters remained the same as in Table 1. The effect of seven values for channel width/gdl width ratio λ on polarization curve was plotted in Fig. 5, along with the corresponding power density curves. It was found an optimized ratio λ of 0.65 at which the cell model works with the best results, this model configuration having a maximum power density of 118 mw/cm 2.

8 1242 V. Ionescu 8 Fig. 5 Effect of channel width/gdl width ratio variation on cell performance, plotted as: (a) polarization curves and (b) power density curves. Figure 6 shows the variation of water and oxygen molar concentrations at cathode catalyst layer along the cell depth for the cell model configurations with ratio λ between 0.3 and 0.8. Water vapor is generated in the cathode catalyst layer due to electrochemical reactions and hence the water vapor molar concentration should increase along the cell depth. When the channel width was at the highest values (for λ = ), larger amount of air entered the cell and hence a lot less amount of dilution taken place due to water addition, as we could see from Fig. 6a. Oxygen molar concentration variation was also almost linear along the cell at the same values of λ (Fig. 6b). Fig. 6 Variation of water molar concentration (a) and oxygen molar concentration (b) at cathode catalyst layer for different values of λ.

9 9 Gas diffusion layer and reactant gas channel influence on performance of HT-PEM fuel cell 1243 Parasite loss, such as pressure drop in fuel micro flow channel should be reduced in order for a fuel cell to produce optimum performance [18]. In Fig. 7 is presented the pressure drop and velocity field distributions along the anode GDL and the cathode gas channel, respectively. Pressure drop of only 0.6 Pa at anode GDL was obtained from the simulation of cell model geometry with λ = 0.65, as we could see from Fig. 7a. Fig. 7 Pressure drop variation across anode GDL (a) and velocity field distribution across cathode gas channel (b) for different values of λ. In the laminar flow, the flow resistance in a channel is inversely proportional to the channel width when the channel depth is fixed. However, if the channel width is too big then the flow and mass transfer are limited by the thermal boundary layer effect [19]. From Fig. 7b we could observe that velocity was at its highest at the inlet and outlet of the flow field with the maximum velocity achieved at the outlet for the cell geometries with λ = 0.65 and 0.8. For the cell model with λ = 0.65, the fluid moved the fastest through the cathode flow channel. 4. CONCLUSIONS In this paper, a steady-state 3D computational model was established with success in order to study the performance of a single HT-PEM fuel cell under varying gas channel and GDL parameters. The model prediction was validated by comparison with experimental data and was in good agreement. The location of maximum water concentration at the cathode side of the cell appeard along the entire catalyst layer lenght for a GDL porosity of 0.1.

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