SEARCH Articles Figures Tables Metal-Complex Dyes Sulfonic Acid Groups Nafion sulfonic acid groups Nitro groups, reduction sulfonic acids Pendant sulfonic acid group Proton conduction mechanism hydrophilic sulfonic acid group Replacement of sulfonic acid groups Replacement sulfonic acid groups Sulfonate group Sulfone groups Sulfonic acid group titration Sulfonic acid groups fuel cell membrane stability Sulfonic group With additional coordinating groups Sulfonic acid