Abstract: A pair of boiler electrodes defines a volume therebetween to be filled with electrolyte to be heated by electrical current passage between the electrodes through the electrolyte. A microporous insulating barrier permeable to the electrolyte is disposed between the electrodes to limit current flow in the electrical current path between the electrodes to a level within the maximum allowable range of the electrode material, the barrier being spaced from the surface of the electrodes at least sufficiently to prevent concentration of current on the electrode surfaces. The barrier includes channels in the vertical direction to accommodate steam removal from the electrolyte.