We previously found (see here) the initially perhaps counterintuitive result that moving your electrodes further apart can sometimes decrease the ohmic resistance. A small 0.2 mm gap between a perforated electrode and the diaphragm was found to increase the energy efficiency compared to a zero gap. The reason is that a small gap allows electrolyte flow and gas bubbles to escape more easily.
A natural next question is, what is the optimal value of this gap? We found that a 0.06 mm gap gives still slightly better results than a 0.2 mm gap. Smaller values will likely not be significantly better as bubbles are of this size and likely they will get trapped.
For larger distances we found multiphase computational fluid dynamics simulations to be in good agreement with our experiments. We even managed to make an approximate analytical model of the gas fraction and the resistance as a function of the gap distance and height. This surprisingly simple model allows estimating how much the current density will decrease as a function of height.
