Global warming, predominantly driven by combustion and utilization of fossil fuels, has catastrophic effects on our communities and health. The climate goal of limiting warming to 2 °C to likely prevent catastrophic and irreversible damage to our planet, is becoming more challenging and costly with each year that passes. If action is not taken soon, humankind will face far-reaching repercussions on the Earth's climate patterns and on all living beings.

Solar photoelectrochemical water splitting produces hydrogen, an environmentally benign gas that is a clean alternative to fossil fuels. Most conventional designs for solar photoelectrochemical water splitting reactors are wafer-based, i.e. similar to photovoltaic panels as electrode systems. One of the downsides of such a system is cost, partially due to the use of expensive electrode materials and fabrication techniques. In comparison, certain particle slurry reactors are projected to be the cost-competitive with H₂ produced from fossil fuels, on an energy equivalent basis. One inexpensive design consists of two transparent baggies that are stacked and filled with aqueous particle suspensions and a redox shuttle that mediates charge between the baggies. This design will be presented along with its beneficial characteristics.

Moreover, we will report on the synthesis and in-depth characterization of the following state-of-the-art materials for use in the proposed reactor design: Rh-doped SrTiO₃, C₃N₄, (Ga_1-xZn_x)(N_1-xO_x), and ZrO₂-TaON light-absorbing particles. An in-depth chemical composition study of the synthesized materials was performed using x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). The optical bandgap, i.e., the threshold for photons to be absorbed, and sub-bandgap absorption analysis of the aforementioned particles were determined using diffused reflectance spectroscopy (DRS). Charge dynamics were investigated using pulsed laser spectroscopy and electrochemical impedance spectroscopy. Photoelectrochemical characterizations were carried out in a three-electrode setup comprised of a drop-casted working electrode of the synthesized visible-light photocatalysts, a Pt counter electrode, and an Ag/AgCl reference electrode under simulated solar irradiation. The gases evolved in the reaction were quantified by inline mass spectrometry and compared to the electrochemical measurements. An assessment of the collection of data will be performed for its overall impact on the proposed new stacked reactor design consisting of particle slurries for solar water splitting. This work provides an experimental baseline for future work into optimization of particle and reactor conditions for the most efficient and cost-effective solar water splitting.

This work was supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Incubator Program under Award No. DE-EE0006963.