Streaming current is a very inefficient way of generating energy. In order to generate an appreciable current, you have to use very diluted electrolytes (e.g. 0.1 M). It is only the charge density at the slip layer that counts, and even with very narrow pores, this accounts only for a tiny fraction of the whole electrolyte volume. You should compare this to the charge density in battery materials that is measured in tens of M.

It will be much more efficient to convert the volume flow to electricity in a turbine