Minimization of Ionic Transport Resistance in Porous Monoliths for Application in Integrated Solar Water Splitting Devices

Monolithic solar water splitting devices consist of photovoltaic materials integrated with electrocatalysts and produce solar hydrogen by water splitting upon solar illumination in one device. Upscaling of monolithic solar water splitting devices is obstructed by high ohmic losses in the electrolyte due to long ionic transport distances. A new design overcomes the problem by introducing micron sized pores in a silicon wafer substrate coated with electrocatalysts. A porous solar hydrogen device was simulated by applying a current corresponding to ca. 10% solar-to-hydrogen efficiency. Porous monoliths of 550 µm thickness with varying pore size and spacing were fabricated by laser ablation and electrochemically characterized. Ohmic losses well below 100 mV were reached at 14.4% porosity with 77 µm pores spaced 250 µm apart in 0.25 M KOH electrolyte. In 1 M KOH, 100 mV was reached at 6% porosity with 1 mm pore spacing. Our results suggest ohmic losses below 50 mV can be achieved when using 10 µm thick substrates at 0.2% porosity. These findings make it possible for monolithic solar water splitting devices to be scaled without loss of efficiency.