A redox flow cell is presented that utilizes a porous membrane separating a first half cell and a second half cell. The porous membrane is chosen to have a figure of merit (FOM) is at least a minimum FOM. A method of providing a porous membrane for a flow cell can include determining a figure of merit; determining a first parameter from a pore size or a thickness for the porous membrane; determining a second parameter from the pore size or the thickness that is not the first parameter for the porous membrane, based on the figure of merit; and constructing a porous membrane having the pore size and the thickness.