Cathode gas

To improve the power demand of the electrolysis, it was suggested in the open literature in May, 1963 that the H2-rich cathode effluent gas be recycled to the anode (Fig. 1) [7]. In fact, a complex array was proposed (Fig. 2) to increase the enrichment, the final product recovery as DzO. It was later reported, however, that no selectivity is found at Pd anodes that is, no selectivity for H2/HD oxidation [8]. Because of this, the incentive to recycle the evolved cathode gas was weakened and apparently was not applied. 

Besides the reaction involving H2 and O2 to produce H2O, the equation shows a transfer of CO2 from the cathode gas stream to the anode gas stream, with 1 mole CO2 transferred along with two Faradays of charge or 2 gram moles of electrons. The reversible potential for an MCFC, taking into account the transfer of CO2, is given by the equation... 

C. Xu, T. S. Zhao, and Y. L. He. Effect of cathode gas diffusion layer on water transport and cell performance in direct methanol fuel cells. Journal of Power Sources 171 (2007) 268-274. 

U. Pasaogullari, C. Y. Wang, and K. S. Chen. Two-phase transport in polymer electrolyte fuel cells with bilayer cathode gas diffusion media. Journal of the Electrochemical Society 152 (2005) A1574-A1582. 

E. Antolini, R. R. Passos, and E. A. Ticianelli. Effects of the cathode gas diffusion layer characteristics on the performance of polymer electrolyte fuel cells. Journal of Applied Electrochemistry 32 (2002) 383-388. 

The key to the development of C02-resistant protonconducting oxides was the maximization of the en-tropic stabilization of protonic defects. If this approach also led to stable hydroxides with sufficiently high conductivity, AFCs using such electrolytes may operate even with air as the cathode gas. This would be tremendously advantageous, because fuel cells with nonacidic electrolytes may operate with non-noble-metal catalysts such as nickel for the anode and silver for the cathode. 

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.)...

Specifically, Figure 16 shows that the current density in a cell with dry cathode gas feed drops nearly instantaneously once the cell voltage is relaxed from 0.6 to 0.7 V due to the fact that the electrochemical double-layer effect has a negligibly small time constant. Further, there exists undershoot in the current density as the oxygen concentration inside the cathode catalyst layer still remains low due to the larger consumption rate under 0.6 V. As the... 

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. 

P. Burckhard found that the electrolysisof tetrasodium pyrophosphate is attended by the evolution of much oxygen at the anode, and at the platinum cathode, gas bubbles are formed which inflame spontaneously in air, and platinum phosphide is formed. P. Walden found the eq. electrical conductivity of aq. soln. of tetrasodium pyrophosphate at 25° for soln. with one gram of the salt in v litres of water ... 

Another interesting phenomenon observed so far is that after purge the cell HFR gradually decreases in a time scale of hours, which is called HFR relaxation after purge. Typical results of FIFR relaxation are shown in Fig. 20. When the 60-s purge is completed, the valves at the inlet and outlet of the cell for both anode and cathode gas lines are closed and the cell temperatures are maintained constant at the purge cell temperature during the whole relaxation process. 

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... 

The reversible voltage Vfcksv at constant anodic and cathodic gas compositions can be calculated by Equations (2.13) and (2.16)... 

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. 

To use this formula, the assumption has been made that the fuel consists of a binary mixture of hydrogen and water, while the cathodic gas is a binary mixture of oxygen and nitrogen. The diffusion coefficient for binary mixtures D y eff is estimated by the equation proposed by Hirschfelder, Bird and Spotz [12], and the Knudsen diffusion coefficient for species i is given by free molecule flow theory [11], Finally, combining Equations (6.15-6.18) the anodic and the cathodic concentration overvoltages are given by (see also Equations (A3.20) and (A3.21)) ... 

In this case, the operating temperature of the fuel cell (used in the Nemst potential, and overpotential calculations) is assumed to be the outlet cathode gas temperature. Initial fuel cell parameters used in the various simulations are summarized in Table 8.5. 

Here, for the time being, we explicitly assume that no internal energy is being generated through chemical reactions (Gg = 0). Similar model equations are used for the lumped cathode gas model. The solid body thermal equation is ... 

Fig. 9.12 Node definitions for the cell, anode gas, cathode gas and interconnect calculation domains.

Fig. 9.15 Nodal cathode gas temperature histories for the one-dimensional model.

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