Thus, water permeable, highly porous layers (i.e., comprising over about 60% and preferably over about 75% void volume) of such hydrophobic materials as cellulose acetate or poly(n-butylmethacrylate-co-2-acrylamido-2-methylpropane sulfonic acid-co-2-acetoacetoxyethyl methacrylate) can be used as the binder or matrix for the redox couple.