Protonexchange

Proton exchange is the process of transfer of a proton (H+) between chemical species. In solution, it underpins acid–base equilibria, buffer behavior, and catalysis. Proton exchange reactions are often described by pH, pKa, and reaction rate constants, and they can involve solvents such as water, alcohols, or ionic media. In biological systems, proton exchange across membranes contributes to the proton motive force that drives ATP synthesis.

Proton exchange membranes (PEMs) are solid polymer electrolytes that conduct protons while electronically insulating the electrodes. They are central to proton-exchange membrane fuel cells and to electrolyzers that produce hydrogen from water. The best-known PEM material is Nafion, a perfluorosulfonic acid polymer developed by DuPont in the 1960s. PEMs conduct protons primarily by hydration: protons hop between water molecules and sulfonic acid groups (the Grotthuss mechanism) along hydrated channels. Conductivity increases with humidity and temperature up to a practical limit; typical operation is around 60–80°C with high humidity. Doping and water uptake promote mobility; mechanical and chemical stability are important design considerations.

Materials outside Nafion include sulfonated poly(ether ether ketone)s, polybenzimidazole doped with phosphoric acid for higher-temperature PEMs, and various composite or inorganic fillers to enhance stability and conductivity.

Applications include fuel cells for transportation and stationary power, and electrolyzers for hydrogen production. Challenges include achieving high conductivity at lower humidity, reducing methanol crossover in direct methanol fuel cells, chemical stability, cost, and long-term durability.