Development of a Novel Solar Photoelectrochemical Tandem Reactor With a Perforated Photocathode for Simultaneous Hydrogen Production and Waste Water Treatment

Hydrogen generation in solar photoelectrochemical reactors could provide an important contribution to future energy regimes by storing intermittent renewable energy in a versatile energy vector. Using waste water as electron donor potentially facilitates economic operation. Here, organic contaminants instead of water are oxidised at the anode and two products of value are obtained simultaneously: hydrogen and clean water. Three different reactor concepts were compared in terms of ohmic losses. Based on the results of the simplified analysis a novel planar and scalable solar reactor with an aperture area of 368 cm2 was developed. It features a perforated photocathode and a non-perforated photoanode, both cold gas sprayed, in tandem arrangement and accepts electrolyte temperatures of up to 80°C. Confirmed by ray-tracing simulations the slit design of the photocathode allows homogeneous illumination of the two involved photoelectrodes with DLR’s test platform SoCRatus (Solar Concentrator with a Rectangular Flat Focus). The photocathode compartment and the photoanode compartment are separated by a membrane. Thus, the membrane being located in the optical path has to show sufficiently high transparency for solar light, particularly in the UV-Vis range. A 1,418 h aging study was performed in order to assess the optical performance of a Nafion™ membrane N1110 exposed to an aqueous mixture at 80°C, which contained 10 vol.-% methanol as a model substance for organic contaminants and sulfuric acid to adjust pH 3. It could be verified that the membrane maintains high transparency in the considered wavelength region from 280 nm to 1,100 nm which suggests the feasibility of the reactor concept. The design electrolyte flow of 2.5 l/min through each of the two reactor chambers practically allows isothermal operation on the SoCRatus under 17.5-fold concentrated irradiation. The inlet and outlet geometry of the reactor aims at uniform flow patterns, a low pressure drop as well as effective product gas transport and was optimised for automatic manufacturing. Reference electrodes and temperature sensors are incorporated directly in the reactor body for extended analysis and operation options. The parts of the reactor ensure compatibility with a wide range of waste waters and involved chemicals as well as mechanical stability. Moreover, they are resistant to light exposure and weathering.