Single emulsion drops for photocatalytic water splitting as a membrane-free approach to product separation

The coevolution of hydrogen and oxygen during photocatalytic water splitting presents a challenge for efficient product separation. Here, we demonstrate membrane-free, drop-based photocatalytic water splitting with inherent product separation and competitive production rates. Drops composed of an aqueous solution containing a mediator and photocatalysts are surrounded by perfluorinated oil made (1) by mechanical agitation of a bulk mixture or (2) within a microfluidic device. The high oxygen solubility of the oil allows for inherent trapping of oxygen while immediately releasing hydrogen. We achieve average solar-to-hydrogen efficiencies of 0.024% ± 0.001% in the conventional suspension approach, 0.016% ± 0.002% in the bulk emulsion system, and 0.046% ± 0.008% if drops are irradiated in the microfluidic device. A three orders of magnitude decrease in the residence time of photocatalysts with a 1.34 times increase in production rate is possible in the microfluidic drop approach when compared to conventional suspensions. Oxygen is trapped and easily released by heating the oil effluent at 343 K.