An electrochemical device comprises a first type of membrane disposed between first and second reservoirs containing an input solution, and a second type of membrane, different from the first type, is disposed between a first redox-active electrolyte chamber and the first reservoir and disposed between a second redox-active electrolyte chamber and the second reservoir. The first type of membrane and one of the second type of membranes form a membrane pair and the pair has an area specific resistance below y=5065.3x3−1331.1x2+90.035x+39 Ohm cm2 when the pair is equilibrated in an electrolyte and for at least part of a range where 0