Cathode gas flow

Hollow cathode gas flow sputtering operates in the pressure range of 0.1-1 mbar. The particles are thermalized at the substrate. Plasma activation of growth processes can be achieved either by applying a substrate bias or by pulsed mode operation of the discharge. 

Fig. 5.33. Schematic diagram of hollow cathode gas flow sputtering. Total pressure pt.ot, 0.1 — lmbar. Gas flow q( Ar) fsl — 5slm for 75 cm target length...

Diffusion of water along the membrane results in the distinctly different ignition fronts with counter-current flow. The water made at the cathode of the fuel cell is partitioned between the membrane and the cathode gas flow channel. The water in the membrane diffuses to the anode where the water activity is lower, and then it can enter the anode gas flow channel. 

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... 

The above calculated saturated pressure is in bars. The molar fraction of water vapor in the fully humidified inlet gas stream at the cathode gas flow channel is simply the ratio of the saturation pressure and the total pressure ... 

Figure 18.13 Water content volume fraction of MEA and GFCs as a function of current density at 50°C and 100% relative humidity inlet gas feeds (O) MEA (including GDLs) ( ) anode gas flow channel and ( ) cathode gas flow channel. Reproduced from [23] by permission of ECS-The Electrochemical Society.

The unconverted part of the energy is released as heat within the fuel cell stack, which generates a heat removal problem for practical systems. The heat may be removed by water cooling, which in turn makes the system more complex and expensive. Cooling by the anode and cathode gas flows are simpler alternatives. They become more attractive the smaller the fuel cell system is, because cost issues are more stringent in such instances. 

Furthermore, the cell has to heat up to above freezing temperature fast enough to avoid the ice formation from completely shutting down the electrochemical reaction at the cathode catalyst layer. To assist in this, the cathode gas flow rate can be increased (to blow ice away and to carry any excess water) and the inlet gases can be heated (to limit time below freezing temperatures). Because of the low vapor pressures of water at low temperatures, however, and the low heat capacities of the reactant gases, these approaches have limited utility in affecting the startup profiles of full-sized stacks. 

In summary, the experimental data fits well into a picture of the carbon fuel cell reaction that indicates an open circuit potential (calculated from Boudouard equilibrium in the anode chamber), a minimal polarization of the carbon anode at 0.1 AJeve , and a significant loss associated with the cathode. Improvement of the cathode and realistic assumptions about Nernst losses in the cathode gas flow indicate that 0.8 V (80% efficiency) should be readily achieved in practical industrial cells. The greater efficiency of the cathode offsets the greater Nemst potential loss at practical air cathode flow rates. 

---