In-Situ Magnetic Alignment of Tandem Semiconductor Microwire Slurries for Solar Hydrogen Generation

Abstract

Technoeconomic models have consistently shown that a photoactive slurry reactor for water-splitting could offer the cheapest route to storing solar energy as hydrogen fuel. However, abundant challenges with this approach have thus far limited such particle-based reactors to prohibitively low efficiencies. Many of the weaknesses of a particulate slurry reactor system can be addressed with monolithically integrated semiconductor tandem particles, which can achieve higher photovoltages while more efficiently utilizing the solar spectrum for water-splitting. However, subcell layers in a tandem must be current-matched and thus the orientation of such particles to the illumination may affect their performance. While ideally matched complementary bandgaps are ultimately sought, the unassisted solar water-splitting in the tandem particulate form factor was first demonstrated with a TiO2/Si microwire system. Nickel hydrogen evolution catalyst was selectively deposited on one end of the wire via a photodeposition process. A proof-of-concept was then demonstrated to use an electromagnet along with the paramagnetic Ni HER catalyst to align the microwire particles in a slurry under active agitation. A simple model is used to predict the conditions under which alignment could lead to more efficient tandem particle slurry performance.