Electrical applications of carbon fiber –fuel cells (3)

Phosphoric acid fuel cell (PAFC): This cell uses stabilized phosphoric acid on a SiC matrix as the electrolyte, operating at 160-210C. Platinum or alloys of Pt are used as the catalyst at both electrodes supported on carbon black. The fuel must be H2 rich and contain <2% CO. The efficiency is 36-89%.

Anode  H2 → 2H+ + 2e

Cathode O2/2 + 2H+ + 2e → H2O

Cell    H2 +O2/2 → H2O

A typical phosphoric acid fuel cell is shown in Figure 23.10.

Water is produced at the cathode, whereas in the alkali fuel cell, it is produced at the anode. At present, a 200 KW PAFC costs about 3000$ per kw.

 

Molten carbonate fuel cell (MCFC): This type of cell uses lithium, sodium and/or potassium carbonate electrolyte soaked in a ceramic matrix of LiAlO2 and operates at 650C, a temperature that is necessary to achieve sufficient conductivity of the electrolyte, but with the dual benefits of achieving a higher efficiency and not requiring a noble metal catalyst. However, corrosion due to the molten carbonate is a problem.

The high working temperature eliminates the need for a separate reformer and CH4 can be steam reformed, where the reaction occurs simultaneously with the electrochemical oxidation of the H2 within the anode compartment. The heat required for steam reforming is supplied by the heat generated in the cell reaction and improved conversion is favoured by operating the process at 5-105 N/m2. A schematic layout of a MCFC is shown in figure 23.11 showing deployment of the Ni catalyst.

The electrochemical reactions which occur are:

Anode   H2 + CO3→ H2O +CO2 + 2e

Cathode  O2/2 + CO2 + 2e → CO3

Cell     H2 +O2/2 + CO2 → H2O +CO2

It is normal practice to recycle the CO2 generated at the anode to the cathode, where it is consumed.

The commercialization of this electrode is being followed by Energy Research Corporation, International Fuel cells corporation and M-C power corporation in the USA; and other companies.

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