Cathode Gas Channel

Figure 18. Pseudo-2-D simulation results at 0.4 A/cm where the feed gases are dry and countercurrent, (a) Water partial pressure profiles at four positions in the fuel-cell sandwich as a function of distance along the channel the positions are at the anode and cathode gas channels (I and IV) and catalyst layers (II and III), respectively. Also plotted is the value of fS, the net flux of water per proton flux, as a function of position. The data are from Janssen. (Reproduced with permission from ref 55. Copyright 2001 The Electrochemical Society, Inc.) (b) Membrane water content as a function of position both along the gas channel and through the thickness of the membrane for the same simulation conditions as above. The data are from Weber and Newman. (Reproduced with permission from ref 55 and 134. Copyright 2004 The Electrochemical Society, Inc.)...

Distributions of water and reactants are of high interest for PEFCs as the membrane conductivity is strongly dependent on water content. The information of water distribution is instrumental for designing innovative water management schemes in a PEFC. A few authors have studied overall water balance by collection of the fuel cell effluent and condensation of the gas-phase water vapor. However, determination of the in situ distribution of water vapor is desirable at various locations within the anode and cathode gas channel flow paths. Mench et al. pioneered the use of a gas chromatograph for water distribution measurements. The technique can be used to directly map water distribution in the anode and cathode of an operating fuel cell with a time resolution of approximately 2 min and a spatial resolution limited only by the proximity of sample extraction ports located in gas channels. 

Figure 31. Water mole fraction profiles along the cathode gas channel measured by a microGC in a cathode underhumidified PEFC using 30 /am membrane (EW < 1000).

Eor the purpose of modeling, consider a planar SOEC divided into anode gas channel, anode gas diffusion electrode, anode interlayer (active electrode), electrolyte, cathode interlayer (active electrode), cathode gas diffusion electrode, and cathode gas channel. The electrochemical reactions occur in the active regions of the porous electrodes (i.e., interlayers). In an SOFC, oxidant reduction occurs in the active cathode. The oxygen ions are then transported through the electrolyte, after which oxidation of the fuel occurs in the active anode by the following reactions. 

In the standard condition with 80% RH (Fig. 6b), a water content, X [H20/S03H], of around 8 in the membrane and partial dehydration at the anode were observed at a current density of 0.2 A/cm2. This suggested that electro-osmosis was influential, even at this current density, because the membrane (Nation 1110, du Pont, 254 Hm) was thicker than usual. In the dry condition (40% RH), the water content in the membrane was around 3 (Fig. 6c) and the water content profile was flat. This can be attributed to molecular diffusion in the membrane, as fast chemical diffusion at this water content has been reported by Zawodzinski et al.41 It was also noted in both cases of 80% RH and 40% RH that the water generated in the cathode catalyst layer was not sufficiently transported to the membrane. This means that little water generated at the cathode condensed and was exhausted to the cathode gas channel as liquid. In the case of 92% RH shown... 

SOFC can be manufactured in different geometrical configurations, i.e. planar, tubular or monolithic. Regardless of the geometrical configuration, a solid oxide fuel cell is always composed of two porous electrodes (anode and cathode), a dense electrolyte, an anodic and a cathodic gas channel and two current collectors. For the sake of simplicity the planar configuration is taken as reference, as shown in Figure 3.1. 

The pressure is assumed to be the same for both the anode and cathode gas channels. The reversible potential at standard state conditions is obtained from the change in the standard Gibbs free energy. 

For the calculation of the electrochemical reaction rates the cathode gas composition is required. Considering the MCFC as a black box, the anode feed gas is completely oxidized with air which is fed into the catalytic combustion chamber, either electro-chemically or in an ordinary combustion reaction. With this, the amount and composition of the exhaust gas is independent of the electric cell performance and can be calculated directly from the conditions of the anode feed and the air feed. According to the assumption of spatially concentrated conditions inside the cathode channel, the exhaust conditions correspond to the conditions in the cathode gas channel. Thus, the cathode gas composition is determined from a combustion calculus. 

As electrodes and electrolyte are in very close contact, they are assumed to have the same temperature 0 s. The anode and cathode gas channels have a finite heat transfer to the solid part with temperatures A and 0 c, respectively. 

Figure 5.9. Water partial pressures and / as a function of position in the gas channel. The feed is countercurrent with dry hydrogen and air with the conditions T = 80°C, i = 0.6 A/cm, psi—pc= 1.5 bar, hydrogen and air stoichiometries of 4 and 2, respectively. The partial pressures are given for the anode and cathode gas channels (aGC and cGC) and GDL / membrane interfaces (aM and cM). The water vapor pressure at this temperature is about 0.2 bar. (The figure is reproduced from Ref. [71] with permission of The Electrochemical Society, Inc.)...

Given the local values of the channel fluxes q, the chaimel temperatures 0o,0h, and the charmel pressures Pa,Pc, it is possible to determine the channel gas concentrations Co,Cc, Cf, and Cg in moles/m. It is assumed that the gases are ideal, obey Dalton s law, and move in the chaimel with a common velocity. Consider first the cathode gas channel. There are two cases to consider, depending on whether the cathode channel gases are saturated or unsaturated. 

Results water vapor concentration in cathode gas channel affects ice formation in cathode catalyst layer the membrane plays important role in start-up by absorbing product water and becoming hydrated... 

Ice formation and inner-cell temperature increase dependence on water vapor concentration in cathode gas channel, initial water content in membrane, current density, and start-up temperature... 

By assuming that n,as/ o,as is fixed and equal to that of the normal atmospheric air at sea level (i.e. 3.76), the partial pressures of nitrogen and oxygen in the cathode channel can be obtained from the known total pressure of the cathode channel. Thus, the steady-state values of the partial pressures of the gas species at the anode and the cathode gas channels are derived as functions of the SOFC operating conditions such as the temperature, the anodic and cathodic pressures, the load current, the FU and the OU. Therefore, it is possible to simulate the steady-state operation of the SOFC with the desired operating conditions. 

The fuel-cell model considers three compartments the anode, the cathode gas channels, and the sohd phase. The soHd phase includes all immobile parts of the real fuel cell the solid part of the electrodes, the electrolyte, the channel walls,... 

The assumptions for the cathode gas channels are similar to those applied to model the anode gas channels, with two exceptions. The first difference is that no reforming reactions occur in the cathode gas channels, and the second is that the main gas flow direction is along Z2 instead of Zi- Consequently, the equations are similar, so they are given here without further comments ... 

ElFective concentration of species i in agglomerate Average molar heat capacity of anode/cathode gas channel... 

Height of anode/cathode gas channels, electrolyte layer... 

Heat exchange parameter of anode/cathode gas channel to solid... 

Most DMFCs work with a liquid fuel supply at the anode. A general survey on this DMFC type is given in Mergel et al. Two key differences between DMFC and PEMFC flow field requirements remain on the one hand, the anodic fuel is liquid and, on the other hand, the amount of water that might block the cathodic gas channels is severely increased compared to PEMFCs. 

The heat equation in the anode and cathode gas channels involves convection and conduction heat transfer modes and no heat generation. The equation is expressed as... 

Cojch reactant concentration in the cathode gas channel Dq = oxygen diffusion coefficient in cathode electrode a = thickness of cafhode electrode... 

Humidified hydrogen and oxygen either pure or in the form of air flow through the anode and cathode gas channel, respectively. At the anode side. 

Gas flow in anode and cathode gas channels is assumed to be incompressible fluid flow with constant fluid viscosity (p) and is governed by the Navier-Stokes equation given as follows ... 

Humidified hydrogen enters the anode gas channel while oxygen in the form of air enters the cathode gas channel. 

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