Air cathode

The plant incorporating the air cathode electrolyzer must include a high performance air scmbbing system to eliminate carbon dioxide from the air. Failure to remove CO2 adequately results in the precipitation of sodium carbonate in the pores of the cathode this, in turn, affects the transport of oxygen and hydroxide within the electrode. Left unchecked, the accumulation of sodium carbonate will cause premature failure of the cathodes. 

Most of the voltage savings in the air cathode electrolyzer results from the change in the cathode reaction and a reduction in the solution ohmic drop as a result of the absence of the hydrogen bubble gas void fraction in the catholyte. The air cathode electrolyzer operates at 2.1 V at 3 kA/m or approximately 1450 d-c kW-h per ton of NaOH. The air cathode technology has been demonstrated in commercial sized equipment at Occidental Chemical s Muscle Shoals, Alabama plant. However, it is not presentiy being practiced because the technology is too expensive to commercialize at power costs of 20 to 30 mils (1 mil = 0.1 /kW). 

The ordn uses for polypropylene are varied. It is used in the fabrication of personnel body armor (Refs 6 7) in slurry-type expls for the demolition of concrete structures (Ref 11) as a microporous hydrazine-air (cathode) separator in fuel cells (Ref 9) as a propint binder matl, particularly in caseless ammo, (Refs 5 8) and as a candidate to act as a proplnt aging inhibitor for the 155mm RAP round (Ref 10) Refs 1) Beil 1, 196, (82), [167], 677 and (725) 2) A.V. Topchiev V.A. Krentsel,... 

Recent testing in phosphoric acid fuel cells has shown improved performance using promoted Ft on carbon catalysts in the air cathode. The promoters are oxides of the base transition metals, e.g., Ti (O,... 

V (2 ), Cr ( ), Zr (1 ), or Ta (1 ). The role of these promoters in the air cathode is unclear, and some have suggested that the active catalysts are alloys of the Ft with the transition metal (1,4) which form during heat-treatment of the oxide impregnated precursor. In the first section of this paper, we review the work from the Lawrence Berkeley Laboratory on the study of the mechanism of promotion of air cathode performance by these transition metal additives. 

Yang CC. 2004. Preparation and characterization of electrochemical properties of air cathode electrode. Int J Hydrogen Energy 29 135-143. 

Fig. 5. Exploded view of an ion-exchange membrane electrochemical oxygen separator. Oxygen removal characteristics of the flow-through type oxygen removal system are shown. Air cathode area = 100 cm2, water temperature = 40 °C.

Fig. 6. Polarization of the oxygen separator. Air cathode area = 10 cm2 water temperature = 40 °C air feed = 4 dm3/min.

C02 is produced as the primary product and this precludes the use of alkaline electrolytes due to the precipitation of CO - in the pores of the anode and consequent electrode fouling. Acid electrolytes lead to problems of corrosion and slow kinetics for the reduction of 02 at the air cathode. 

Figure 2.3 Schematic diagram of the experimental setup for measuring amounts of liquid water and water vapor in the air cathode exhaust at a close point of cathode exit of an operating DMFC stack.

Figure 2.5 Amount of liquid water in the air cathode exhaust at cathode exit point of an operating DMFC stack as a function of stack operating temperature and air feed actual stoichiometry.

Hence the maximum air feed actual stoichiometry is a function of water vapor partial pressure corresponding to the air exhaust release temperature (pw) and the total pressure on the air cathode exhaust (ptotai)- Figure 2.8 shows the maximum air feed actual stoichiometry calculated from Equation 2.5, using water vapor partial pressure from the CRC Handbook of Chemistry and Physics [D.R. Lide (ed.), 72nd edn, 1991-92], as a function of air cathode exhaust release temperature. 

Figure 2.8 Maximum allowed air feed actual stoichiometry for a DMFC power system as a function of air cathode exhaust release temperature.

From the foregoing discussion, it is clear that, in a DMFC, the air cathode has to be operated under rather challenging conditions, that is, with a low air feed rate at nearly full water saturation. This type of operating conditions can easUy lead to cathode flooding and thus poor and unstable air cathode performance. To secure better air cathode performance, we have made great efforts to improve the ell cathode structure and cathode flow field design to facilitate uniform air distribution and easy water removal. The performance of our 30-cell DMFC stacks operated with dry air feed at low stoichiometry is reported in the following section. 

Figure 2.14 Air cathode pressure drop across the 30-cell stack operated at 60°C with a 0.5 M methanol solution feed at 125 mLmin at the anode and with 0.76 atm dry air feed at 7.35 SLPM at the cathode.

The high overpotential for O2 evolution could be avoided if the reaction were replaced with a different anodic reaction. This replacement could in turn reduce AE, the minimum cell potential difference, which depends on the nature of the electrode reactions. Such a strategy has already been applied with success in the chlor-alkali industry, where the CI2-H2 couple (A = 1.35 V) has been replaced with CI2-O2 (A ri0.90 V) (O2 is reduced at the so-called air cathode). 

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