Membrane/anode interface

Consideration of the gas inlet streams saturated by water vapor, which pass in part through the gas-diffusion electrode to reach the electrode/membrane interface and in part exhaust the electrode gas chaimel, yielded the following expression for the mole fraction of water in the gas mixture at the anode/membrane interface (designated as interface 2 [87]) ... 

IB. Effects of Aluminum and Silica. There is a synergy between aluminum and silica that can lead to the formation of complex ciystalline aluminosilicates within the membrane. Again, the consequence is a loss in current efficiency. Aluminum, if present in sufficient concentration, will also precipitate at the anode/membrane interface. This causes a very slight increase in voltage that can be reversed when the electrolyzer shuts down. 

Depending on the pH of the anolyte, iron precipitates on or close to the anode surface of the membrane. Under acidic conditions, it penetrates into the membrane and contributes to a rising voltage. Iron from anticaking agents has a tendency to accumulate in the anode compartment and form a brown deposit at the anode/membrane interface. Suitable means of analysis to achieve the sensitivity required are not usually deployed in a chlorine plant, and it is then necessary to make use of a specially equipped analytical laboratory. 

Water content at the interface is computed using Equation 9.13a for the anode-membrane interface as... 

Hydrogen gas concentration at the anode-membrane interface is given by Equation 11.31 as... 

The three-dimensional simulation model for the fuel cell based on steady-state, single-phase and incompressible flow analysis is presented here. The other basic assumptions that have been made in formulating the model are as follows (1) gas flows in the channels are assumed to be incompressible, (2) water generation takes place only at the anode membrane interface, (3) there is no water generation and water transport in the electrolyte, (4) water exists only in the gas phase in the fuel cell, and (5) humidified hydrogen and air are assumed to be ideal gases. 

Continuity in heat flux is used at all interfaces except at tire cathode and membrane interface. For example, at the channel and gas diffusion layer interface, at the channel and bipolar plate interface, and at the anode-membrane interface, the continuity condihon is given as... 

Higher-density currents result in water mass-transport limitations and the appearance of additional low-frequency impedance relaxation. This diffusion impedance due to water transport within the polymer electrolyte morphology is often represented by the finite diffusion processes affecting both the anodic (Zq ) and cathodic (Z ) processes (Figure 12-16B). An increase in anodic impedance is often related to partial drying out of the anode/membrane interface. 

Figure 16. SEM images of the fracture surface of an RH-cycled (80-120% RH) sample, showing crack initiation and growth from a craze site near the anode and membrane interface. A crazing site found on Hi-RH cycled samples. The half-penny shaped craze closely resembles crazes formed on free surfaces.

Here / is the current density with the subscript representing a specific electrode reaction, capacitive current density at an electrode, or current density for the power source or the load. The surface overpotential (defined as the difference between the solid and electrolyte phase potentials) drives the electrochemical reactions and determines the capacitive current. Therefore, the three Eqs. (34), (35), and (3) can be solved for the three unknowns the electrolyte phase potential in the H2/air cell (e,Power), electrolyte phase potential in the air/air cell (e,Load), and cathode solid phase potential (s,cath), with anode solid phase potential (Sjan) being set to be zero as a reference. The carbon corrosion current is then determined using the calculated phase potential difference across the cathode/membrane interface in the air/air cell. The model couples carbon corrosion with the oxygen evolution reaction, other normal electrode reactions (HOR and ORR), and the capacitive current in the fuel cell during start-stop. 

Here subscripts a and c denote anode and cathode respectively, iref is the reference exchange current density, y is the concentration dependence exponent, [ ] and [ ]ref represent the local species concentration and its reference concentration, respectively. Anode transfer current, Ra, is the source in the electric potential equations at the anode/electrolyte interface with positive sign on membrane (electrolyte) side and negative sign on solid (anode) side. Similarly, near the cathode interface, the source on membrane (electrolyte) side is negative of the cathode transfer current, Rc and that on solid (cathode) side is positive of Rc. The activation over-potentials, in Equations (5.35) and (5.36) are given by... 

Since the membrane is assumed to be impermeable to gases, Eqs. (118), (123), and (125) can be written for the anode and the cathode compartments separately with the boundary condition of zero flux at the catalyst layer/membrane interfaces. 

Water is produced at the cathode/membrane interface, and it must be transported to the anode and cathode flow channels to be removed. At present, we do not have a direct measurement of the water activity in the membrane (the partial pressure of water in the membrane, p> embrane do know the water content in the effluent streams. The experimental data may be integrated from the known initial water content (after the water injection) to the steady state current and partial pressures of water. Integration of Eq. (3.4) gives the steady state membrane water content for known water partial pressure at the anode and cathode. Effective mass transfer coefficients for water from the cathode/membrane interface to the cathode gas flow channel and from the cathode/membrane interface to the... 

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.)...

To examine the transport-mode-transition region in more detail, simulations were run at different current densities [71]. The resultant membrane water profiles are shown in Figure 5.11 where a vapor-equilibrated membrane at unit activity has a water content of L = 8.8 as calculated by the modified chemical model (see Section 5.5.1). The profiles in the figure demonstrate that the higher the current density the sharper the transition from the liquid-equilibrated to the vapor-equilibrated mode as well as the lower the value of the water content at the anode GDL/membrane interface. The reason why the transition occurs at the same point in the membrane is that the electro-osmotic flow and the water-gradient flow are both proportional to the current... 

Anode performance depends on the brine quality and the operating parameters such as pH, current density, NaCl concentration, and NaOH concentration (in diaphragm and membrane cells). The contribution of the anode to the cell inefficiency, as mentioned in Section 4.4, is directly related to the losses arising from the oxygen evolution reaction, and indirectly by chlorate formation. Thus, as the %02 increases, the pH at the anode-solution interface decreases, and hence, the amount of chlorate formed will decrease as the bulk pH is lowered. The amount of O2 generated at the anode is a function of the current density, pH, the composition and surface area of the anode coating, and the salt concentration. 

We can extend this logic to the case of iron. In the oxidizing atmosphere of the anolyte, the ions to consider are Fe ". With its very low solubility product (10 ), Fe(OH)3 is quite insoluble and precipitates at a relatively low pH. Accordingly, there are many reports of Fe(OH>3 precipitation within the anolyte. In membrane cells, deposits can form on the anolyte face of the membrane. Iron impurities in the catholyte also can cause deposits to form on the anode side. In the catholyte, iron exists as an anion, HFeO - Like the OH ion, this travels through the membrane by diffusion and migration. The HFeOj species oxidizes to Fe " " at the anolyte/membrane interface and precipitates as Fe(OH)3. 

These considerations are important to the action of iron because of its ability to form complex ions with hydroxyl. At low pH, hydrolysis is incomplete and Fe(OH) " and Fe(OH)J exist. At high pH, ferric ion forms Fe(OH)J (or FeOj-2H20), and the solubility of iron increases. The solubility is at a minimum between pH 5 and pH 10. This range includes the normal pH at the membrane interface and explains the lack of penetration by iron. The accumulation of Fe(OH)3 can raise the cell voltage but does not aifect the current efficiency. When the pH profile shifts to aflow the interfacial value to drop below five, ions can penetrate the membrane and deposit within the membrane but closer to the anode side. 

The basic components of the SOFC are the anode, the cathode and the electrolyte, as shown in Fig. 10.1. They are together referred to as the membrane electrode assembly (MEA). Fuel (hydrogen) is supplied to the anode side and air is supplied to the cathode side. At the cathode-electrolyte interface, oxygen molecules accept electrons coming from the external circuit to form oxide ions. The solid electrolyte allows only oxide ions to pass through. At the anode-electrolyte interface, hydrogen molecules present in the fuel react with oxide ions to form steam, and electrons get released. As a result of the potential difference set up between anode and cathode... 

Figure 1.3 Schematic for the calculation of voltage loss in a fuel cell (for discussion see text). ACL and CCL are the abbreviations for the anode and cathode catalyst layers, respectively. Yellow shaded areas indicate the local polarization voltage r]. For simplicity, the proton conductivity of catalyst layers is taken to be equal to the proton conductivity of the bulk membrane (otherwise the curve loses smoothness at the membrane interfaces). Note that the half-cell voltage loss is given by the value of the overpotential at the catalyst layer/membrane interface.

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