23
Production of Nafion
®
membranes by DuPont, which consists of chemically
stable polytetrafluoroethylene (PTFE) backbone structure, made a major step in
development of PEMFC. Nafion
®
has a high acidity and high conductivity, which is
mainly affected by its water content, so that it increases significantly with the relative humidity (RH) of the reactants fed to the PEMFC. Therefore, PEMFCs are
usually operated near 100% RH, which limits their useful temperature range
to 90 °C.
The electrodes are carbon-supported Pt or Pt alloy catalysts and PFSA ionomers.
For operation with pure H 2 , carbon-supported platinum (Pt/C) anode catalysts are
used, whereas PtRu/C catalysts offer superior performance in the case of operation
with CO-contaminated H-rich reformates produced from reforming of hydrocarbons. Current catalyst loadings are of the order of 0.2–0.5 mg Pt cm
−2
for each
electrode. Due to their relatively low operating temperature, rapid startup capability
at ambient temperatures, and highly dynamic response, PEMFCs are the main focus
for automotive, portable, and stationary/residential applications.
3.1.2 Alkaline Fuel Cells
The AFC is characterized by the highest electrical efficiencies of all fuel cells, but it
works properly only with very pure gases, which is a drawback in most applications.
The potassium hydroxide (KOH) electrolyte used in AFCs (usually in concentrations of 30–45 wt.%) has an advantage over acid fuel cells in that the oxygen reduction kinetics are much faster in alkaline electrolyte than in acid. The AFC was used
in the Apollo missions and the Space Shuttle program and was planned to be used
in the European Hermes Project. In the Space Shuttles, the fuel cells are used for
producing energy, cooling of the Shuttle compartments and producing potable water.
AFCs are now normally run at operating temperatures below 100 °C. Another
useful feature of AFCs is a possibility to use non-noble metals as catalysts, given the
lower corrosive behavior than acids where the most often used catalyst is platinum
or platinum alloys.
The formation of carbonates that can ruin the electrolyte is one of the most problematic aspects of AFCs. Carbon dioxide in the air and the CO 2 formed by reaction
(oxidation of methanol, other organics, or the carbon support) react with the
electrolyte:
CO
OH
CO
H O
2
3
2
2
2
+
+
−
−
→
(3.7)
and the cell performance rapidly decreases. There are engineering solutions to solve
the problem of CO 2 in H 2 . New development for this type of fuel cell is a new alkaline electrolyte membrane that tolerates higher temperatures.
3 Electrochemical Energy Conversion in Fuel Cells
Production of Nafion
®
membranes by DuPont, which consists of chemically
stable polytetrafluoroethylene (PTFE) backbone structure, made a major step in
development of PEMFC. Nafion
®
has a high acidity and high conductivity, which is
mainly affected by its water content, so that it increases significantly with the relative humidity (RH) of the reactants fed to the PEMFC. Therefore, PEMFCs are
usually operated near 100% RH, which limits their useful temperature range
to 90 °C.
The electrodes are carbon-supported Pt or Pt alloy catalysts and PFSA ionomers.
For operation with pure H 2 , carbon-supported platinum (Pt/C) anode catalysts are
used, whereas PtRu/C catalysts offer superior performance in the case of operation
with CO-contaminated H-rich reformates produced from reforming of hydrocarbons. Current catalyst loadings are of the order of 0.2–0.5 mg Pt cm
−2
for each
electrode. Due to their relatively low operating temperature, rapid startup capability
at ambient temperatures, and highly dynamic response, PEMFCs are the main focus
for automotive, portable, and stationary/residential applications.
3.1.2 Alkaline Fuel Cells
The AFC is characterized by the highest electrical efficiencies of all fuel cells, but it
works properly only with very pure gases, which is a drawback in most applications.
The potassium hydroxide (KOH) electrolyte used in AFCs (usually in concentrations of 30–45 wt.%) has an advantage over acid fuel cells in that the oxygen reduction kinetics are much faster in alkaline electrolyte than in acid. The AFC was used
in the Apollo missions and the Space Shuttle program and was planned to be used
in the European Hermes Project. In the Space Shuttles, the fuel cells are used for
producing energy, cooling of the Shuttle compartments and producing potable water.
AFCs are now normally run at operating temperatures below 100 °C. Another
useful feature of AFCs is a possibility to use non-noble metals as catalysts, given the
lower corrosive behavior than acids where the most often used catalyst is platinum
or platinum alloys.
The formation of carbonates that can ruin the electrolyte is one of the most problematic aspects of AFCs. Carbon dioxide in the air and the CO 2 formed by reaction
(oxidation of methanol, other organics, or the carbon support) react with the
electrolyte:
CO
OH
CO
H O
2
3
2
2
2
+
+
−
−
→
(3.7)
and the cell performance rapidly decreases. There are engineering solutions to solve
the problem of CO 2 in H 2 . New development for this type of fuel cell is a new alkaline electrolyte membrane that tolerates higher temperatures.
3 Electrochemical Energy Conversion in Fuel Cells