Efficient Light-Driven Charge Storage in Titania Aerogels: From Photochemical Batch Studies to Capillary Flow Reactors

A great challenge in material design for photocatalysis is the separation of the light-harvesting and catalytic steps. The use of photochargable materials is a promising approach to separate light-driven energy storge from a subsequent dark reaction. In this work, the exceptional capability of as-synthesized titania aerogel to store electrons upon irradiation were systematically investigated, including sacrificial agent, irradiation time, and incident photon flux. Methanol, owing the largest number of α-H atoms, was found to be the most effective sacrificial agent compared to ethanol and isopropanol. The use of methanol along with a high photon flux and a prolonged irradiation time resulted in significant electron storage of 64.8 µmol e^- in 100 mg titania aerogel. Insights gained from studies in a semi-batch reactor with a Xe-arc lamp were translated to a flow capillary reactor irradiated with UVA LEDs. A thorough characterization of the radiation field of both reactors was the basis for an objective comparison of data obtained in the different experimental setups. Using the same charging conditions in the capillary reactor as in the semi-batch reactor yielded a low charging performance, due to a significant transmission loss of photons as a result of a low loading. To account for the short optical pathlength of the capillary reactor, photocharging conditions were tailored, specifically through an increase of the titania aerogel loading and a reduced dispersion volume. This enhanced the photocharging rate in the capillary reactor, eventually exceeding that of the batch system by a factor of two. The photonic efficiencies was increased fourfold through doubling the catalyst loading. These findings demonstrate both the potential and challenges of transferring and scaling light-driven processes from batch to flow, and to establish critical design insights for light-driven charge storage in continuous systems.