(Digital Presentation) Effects of Thermal Convection on Species Transport in Photocatalytic Suspension Reactors

Abstract

Z-scheme photocatalytic suspension reactors with aqueous redox shuttles present a promising pathway for safe and low cost solar water splitting to produce hydrogen. Soluble redox shuttles act as electron relays between light absorbing particles present in the two compartments. In recent work we have demonstrated a theoretical pathway to attain up to 4% solar-to-hydrogen efficiency with passive diffusion driven species transport [1]. However, to attain larger solar-to-hydrogen efficiencies it is crucial to enhance the rate of species transport between the two reaction compartments. We report on computational and experimental investigations to explore the effects of natural convection on species transport in Z-scheme photocatalytic systems. A transient, two-dimensional fluid flow, heat- and mass-transfer model was developed for a two-compartment system with a porous separator; horizontal and vertical reactor arrangements were considered. Volumetric absorption of visible and infrared components of incident sunlight by the suspension was modeled. Transient species concentration profiles were obtained as a function of the operating solar-to-hydrogen efficiencies, porous separator morphology and the size of the compartments. Model results predict enhanced rates of transport of redox shuttle species between two compartments in the presence of thermal convection in addition to diffusion. At 10% solar-to-hydrogen efficiency, minimum redox shuttle concentration with convection decreases by 80% as compared to passive diffusion alone for a vertically stacked reaction compartment with 2 cm height. Experimental measurements have been also performed on two-compartment prototypes (horizontal and vertical) to probe the effects of natural convection on species mixing. The source of illumination was a xenon-lamp solar simulator (Newport 94023A) with an AM1.5G filter and calibrated such that the incident illumination power at the top of the reactor was 1 sun (1000 W/m2). The reactor was assembled by connecting two polycarbonate tubes (5 cm x 5 cm) with a height of 5 cm and placing a thin separator (Nafion 117 and regenerated cellulose) between the tubes to separate the compartments. A base was 3D-printed and painted black to secure the bottom compartment and maximize absorption of the incident illumination and promote natural convection. Thermocouples (K-type) were placed at various heights within the two compartments to track local temperatures within the fluid, including along the bottom surface of the lower compartment. Over a 3-hour period of 1 sun irradiance, the temperatures were recorded, and a favorable temperature gradient for natural convection was observed (ΔT ~ 2 °C) for vertically-stacked reaction compartments. The flow behavior was visualized through the injection of a colored dye at the bottom surface of the reactor and above the membrane after the 3-hour period. Image processing of the convection video is used to help quantify the rate of mixing within the compartments. Model predictions for flow behavior and temperature profiles were compared to and validated against experimental measurements. Collectively, this study implements a unique approach to predict and evaluate the effect of natural convection on species transport in Z-scheme photocatalytic reactors.[1] Bala Chandran, R., Breen, S., Shao, Y., Ardo, S., & Weber, A. Z. (2018). Evaluating particle-suspension reactor designs for Z-scheme solar water splitting via transport and kinetic modeling. Energy and Environmental Science, 11(1), 115–135. https://doi.org/10.1039/c7ee01360d Figure 1