Battery and energy materials
Which statement about the electrolyte of a polymer electrolyte fuel cell (PEFC, also called a PEM fuel cell) is correct?
- AIt is a ceramic membrane that conducts oxide ions (O²⁻), so it is operated above 700 °C to raise its conductivity
- BIt is a molten carbonate in which carbonate ions move to the fuel electrode, so it is operated at about 650 °C
- CIt is a fluorinated polymer membrane with sulfonic acid groups that conducts protons when wet, so it runs at about 100 °C or below
Show answer and explanation
Answer: C. It is a fluorinated polymer membrane with sulfonic acid groups that conducts protons when wet, so it runs at about 100 °C or below
The electrolyte of a PEFC is a polymer membrane with a fluoropolymer backbone and side chains bearing sulfonic acid groups (−SO₃H). When the membrane absorbs water, hydrophilic channels lined with sulfonic acid groups form, and protons move through them together with water. Conductivity drops sharply when the membrane dries, so humidification is needed, and to avoid losing water from the membrane the operating temperature is kept at about 100 °C or below, typically about 80 °C. Using a ceramic membrane that conducts O²⁻ above 700 °C describes the solid oxide fuel cell (SOFC), which uses stabilized zirconia and similar materials. A molten carbonate electrolyte operated at about 650 °C is used in the molten carbonate fuel cell (MCFC). The differences in electrolyte determine each type's operating temperature, usable fuels and catalysts.