Cathode channel

X. Liu, H. Guo, and C. Ma. Water flooding and two-phase flow in cathode channels of proton exchange membrane fuel cells. Journal of Power Sources 156 (2006) 267-280. 

F. B. Weng, A. Su, C. Y. ITsu, and C. Y. Lee. Study of water-flooding behavior in cathode channel of a transparent proton-exchange membrane fuel cell. Journal of Power Sources 157 (2006) 674—680. 

Figure 8. Two-dimensional sketch of water management in a PEM fuel cell whereby the membrane—electrode assembly separates the anode feed channel from the cathode, and a diagram of water uptake profiles in anode and cathode channels.

A transparent PEM fuel cell with a single straight channel was designed by Ma et al.11 to study liquid water transport in the cathode channel (this study is also mentioned in Section 2.5). The pressure drop between the inlet and outlet of the channel on the cathode side was used as a diagnostic signal to monitor liquid water accumulation and removal. The proper gas velocities for different currents were determined according to the pressure drop curves. 

Figure 12 under the counter-flow mode at 0.5 A/cm2. The index with the MPL is larger than that without the MPL for an index of 0-1 as depicted in Fig. 12. This result also shows that the MPL enhances water back-transport from the cathode side to the anode side. However, when the index is negative, meaning that the internal water circulation from the anode channel to the cathode channel, the index with the MPL is slightly higher than the index without the MPL. Therefore, the MPL at the cathode suppressed water vapor absorption at the anode, which is explainable by membrane hydration attributable to the MPL at the cathode. Consequently, the MPL promotes membrane hydration, leading less internal water circulation from the anode to the cathode side.

The anode exhaust gas is mixed with air, and the nonoxidized components are totally oxidized in a catalytic combustion chamber. Because air is fed in excess, the exhaust gas from the burner still contains a significant amount of oxygen. This gas is then fed to the cathode channel where the electrochemical reduction of oxygen takes place. There, new carbonate ions are produced from carbon dioxide and oxygen according to the backward direction of the following cathode reaction ... 

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. 

For simplicity we will ignore momentum loss due to meander turns and assume that the channel is straight with the axis directed along z. Consider a cathode channel. Due to the electrochemical reaction each oxygen molecule is replaced with two water molecules in the flow. The continuity equation, therefore, reads (In [192]... 

Equations (138) and (139) allow to calculate va(z) and vc(z), if j(z) is known (the latter is determined from the solution of the internal problem). Then one can find the feed gas concentration along the channel. For example, in the cathode channel we have... 

The velocity in the cathode channel, therefore, increases. Physically, oxygen molecules are replaced with water molecules in the flow the total mass of... 

Consider the cathode channel (similar arguments are applicable to the anode channel of DMFC or hydrogen PEFC). For simplicity we assume that (1) the catalyst layer is thin enough, so that there are no voltage losses associated with proton transport across the layer and (2) the diffusion losses of oxygen in the backing and catalyst layers are negligible. 

Polarization Curves of Individual Segments ofa Cathode Channel S-shape Behavior as a Signature of Oxygen Starvation... 

Analysis of hydrodynamic equations for the flow in the fuel cell channel shows that this flow is incompressible [13]. In other words, the variation of pressure (total molar concentration) along the channel is small. Consider first the case of zero water flux through the membrane. Each oxygen molecule in the cathode channel is replaced with two water molecules. Pressure is proportional to the number of molecules per unit volume. To support constant pressure, the flow velocity in the channel must increase. The growth of velocity provides expansion of elementary fluid volume the expansion keeps pressure in this volume constant. 

Crossover of water from the anode to the cathode side supplies additional water molecules to the cathode channel and induces faster growth of velocity. Let a be the overall transfer coefficient of water from the anode to the cathode side (a number of water molecules transported per each proton through the membrane, taking into account back diffusion). Calculations [13] give... 

We take the same mass transport parameter, f, for each of the three gas species. The heat transport is primarily through the channel landing, and we take a value of the heat transport parameter, t, which is consistent with a solid-solid interface see [11]. The cathode channel total gas concentration, C, satisfies... 

Aiiode-Membnme RH Anode-Channel RH Cathode-Channel RH... 

Cathode channel water vapor concentration moles/m. ... 

TZ Ideal gas constant 8.3143 J/K mole. ta,rc Anode and cathode channel relative humidity. 

Wa,Wc . Liquid water flux in anode and cathode channels per unit orthogonal width (z) in moles/m s. ap. Cathode transfer factor, taken to be 1[4). 

At this level of the model, consider to be given the local current density i, the anode and cathode catalyst temperatures (da and 6c), the anode and cathode channel vapor concentrations (c and Cc), the anode channel hydrogen concentration (ch), and the cathode channel oxygen concentration (Co)-To be determined from the MEA model are the local cell voltage v, the diffusive water flux through the membrane / from cathode to anode, and the heat generated due to membrane resistance and cathode overpotential losses. As mentioned above, in this simplified model, the membrane resistivity is taken to be constant. Other resistances (except to in-plane currents in the bipolar plates in the stack model) are neglected. 

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. 

Assume first that the cathode channel is unsaturated and compute... 

If the value of computed above is greater than Psat 6o)/ TZ0 ), then the cathode channel gases are over-saturated. Assume that vapor will condense to prevent over-saturation and replace the concentrations c and Cc above with the following values ... 

Figure 9.6. Coolant temperature (left) and cathode channel temperature (right) for unit cell base case (solid line, cathode stoich 1.8) and anomalous case (dotted line, cathode stoich 1.3).

---