A fuel cell whose electrolyte is a polymer membrane with sulfonic acid groups that conducts protons when hydrated. Abbreviated PEFC (also PEMFC); it is humidified to keep the membrane from drying and operates at about 80 °C.
Japanese name: 固体高分子形燃料電池
Polymer electrolyte fuel cell appears in 1 practice question across 1 topic, including Battery and energy materials.
Field: Battery and energy materials
In a PEFC, hydrogen is oxidized at the anode as H₂ → 2H⁺ + 2e⁻; protons pass through the electrolyte membrane and electrons through the external circuit to the cathode, where oxygen is reduced as O₂ + 4H⁺ + 4e⁻ → 2H₂O. The overall reaction forms water from hydrogen and oxygen; the standard Gibbs energy change for forming liquid water at 25 °C is −237.1 kJ per mol of water, so the theoretical emf is E° = −ΔG°/(2F) ≈ 1.23 V. The water produced is discharged on the air side, and no carbon dioxide is emitted during operation.
When current is drawn, the actual voltage of a single cell drops to about 0.6–0.8 V. The largest loss is the activation overpotential caused by the slowness of the oxygen reduction reaction at the cathode, which remains large even with platinum catalysts. As current increases, losses from the resistance of the membrane and contacts are added, and at high currents mass transport losses appear as oxygen and water cannot move fast enough. The upper limit of theoretical efficiency, ΔG/ΔH, is about 83% on a higher heating value basis, but operating at 0.7 V gives a voltage efficiency of about 47%. The required voltage is obtained by stacking hundreds of single cells.
A membrane electrode assembly (MEA) consists of the electrolyte membrane (about 10–50 µm thick) with, on each side, a catalyst layer of carbon carrying fine platinum particles mixed with polymer electrolyte, and a gas diffusion layer; it is sandwiched between separators with flow channels to make a cell. The membrane conducts protons poorly when dry, but too much water fills the pores of the catalyst and gas diffusion layers and stops gas from reaching them (flooding). Electro-osmotic drag, in which protons carry water with them to the cathode, also occurs, so water management, balancing humidification against drainage, is key to performance.
In a PEFC operating at low temperature, carbon monoxide adsorbs strongly on the platinum surface and blocks the catalyst (CO poisoning). The international standard for hydrogen for fuel cell vehicles (ISO 14687) requires CO to be kept to 0.2 ppm or less, and home systems that produce hydrogen by reforming city gas must include a step to remove CO. Because PEFCs start quickly at low temperature and have high power density, they are used in fuel cell vehicles and in home fuel cells such as the ENE-FARM systems sold in Japan since 2009. Reducing the amount of platinum is a major cost challenge.
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.
Yes. In Japan the name polymer electrolyte fuel cell (PEFC) is common, while in English-speaking countries proton exchange membrane fuel cell (PEMFC) is often used; both refer to fuel cells with a polymer membrane as the electrolyte.
A PEFC uses a polymer membrane that conducts protons and operates at about 80 °C, so it starts quickly and suits vehicles. An SOFC uses a ceramic that conducts oxide ions and operates above 700 °C, so it needs no platinum and generates power efficiently, but it takes time to start up.
Because at the low temperature of about 80 °C, the oxygen reduction and hydrogen oxidation reactions are slow, and sufficient current cannot be obtained without a highly active catalyst such as platinum. Research is under way on catalysts that use less platinum or none at all.
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