A composite membrane, comprising: a) a microporous polymer structure present in an amount from 13 vol% to 65 vol% based on the total volume of the composite membrane, wherein the amount of microporous polymer structure based on the total volume of the composite membrane, % Vol MPM , is calculated according to the formula: % Vol MPM = M / A porous layer Matrix skeletal density MPM Composite Membrane thickness at 0 % RH = % in which M/A porous layer is the mass per area of the microporous polymer structure, Matrix skeletal density MPM is the matrix skeletal density of the microporous polymer structure and Composite Membrane thickness at 0%RH is the thickness of the composite membrane at 0% relative humidity; and b) an ion exchange material at least partially embedded within the microporous polymer structure and rendering the microporous polymer structure occlusive; wherein a thickness of the composite membrane is below 17 micrometers while an acid content of the composite membrane is between 1.2 meq/cm 3 to 3.5 meq/cm 3 , wherein the acid content is calculated according to the formula: Acid Content = Thickness at 0 RH − M / A microporous layer Matrix Density MPM × Density ionomer EW ionomer × 1 Composite Membrane thickness at 0 % RH = mole eq cc in which Thickness at 0 RH is the thickness of the composite membrane at 0% relative humidity, M/A microporous layer is the mass per area of the microporous polymer structure, Matrix Density MPM is the matrix skeletal density of the microporous polymer structure, Density ionomer is the density of the ion exchange material, EWionomer is the equivalent weight of the ion exchange material and Composite Membrane thickness at 0% RH is the thickness of the composite membrane at 0% relative humidity and cc is cm 3 and the parameters are measured as described in the description. a) a microporous polymer structure present in an amount from 13 vol% to 65 vol% based on the total volume of the composite membrane, wherein the amount of microporous polymer structure based on the total volume of the composite membrane, % Vol MPM , is calculated according to the formula: % Vol MPM = M / A porous layer Matrix skeletal density MPM Composite Membrane thickness at 0 % RH = % in which M/A porous layer is the mass per area of the microporous polymer structure, Matrix skeletal density MPM is the matrix skeletal density of the microporous polymer structure and Composite Membrane thickness at 0%RH is the thickness of the composite membrane at 0% relative humidity; and b) an ion exchange material at least partially embedded within the microporous polymer structure and rendering the microporous polymer structure occlusive; wherein a thickness of the composite membrane is below 17 micrometers while an acid content of the composite membrane is between 1.2 meq/cm 3 to 3.5 meq/cm 3 , wherein the acid content is calculated according to the formula: Acid Content = Thickness at 0 RH − M / A microporous layer Matrix Density MPM × Density ionomer EW ionomer × 1 Composite Membrane thickness at 0 % RH = mole eq cc in which Thickness at 0 RH is the thickness of the composite membrane at 0% relative humidity, M/A microporous layer is the mass per area of the microporous polymer structure, Matrix Density MPM is the matrix skeletal density of the microporous polymer structure, Density ionomer is the density of the ion exchange material, EWionomer is the equivalent weight of the ion exchange material and Composite Membrane thickness at 0% RH is the thickness of the composite membrane at 0% relative humidity and cc is cm 3 and the parameters are measured as described in the description. <EOS>