Global climate change due to combustion of fossil fuels and a lack of grid-scale energy storage will continue to negatively affect our environment and impede our ability to diversify energy sources and increase energy independence. However, if we can harness the power of the Sun, much like plants, then we can achieve a sustainable, net carbon neutral future with inherent energy storage capacity in chemical bonds.

Solar photoelectrochemical water splitting produces H₂, which can be used as a fuel. Proposed technologies to achieve this photochemistry typically use either a separate photovoltaic and electrolyzer, or an integrated wafer-based system. Unfortunately, these systems are costly, relying on relatively large quantities of precious metal electrocatalysts and intensive fabrication/processing methods. In contrast to these designs is the particle slurry reactor where the hydrogen evolution reaction and the oxygen evolution reaction are separated into different compartments, connected ionically and electronically via a solution redox shuttle, and stacked optically in series. This design is predicted to be cost-competitive with hydrogen produced from fossil fuels on an energy equivalent basis.    

Herein, we report on the reductive and oxidative depositions of electrocatalysts (e.g. Pt, Au, CoOPi, NiMo, and NiFe(O)OH) on state-of-the-art semiconducting nanoparticles (e.g. TiO₂, WO₃, and BiVO₄). The unique aspect of this work is that this deposition occurs in the bulk without physically contacting the particles with electrodes. Instead, bipolar electrochemistry is used to electrodeposit these catalysts, which is a wireless, non-contact technique. After deposition, the composite materials were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD) to determine the efficiency of the deposition process. Using a variety of electrochemistry techniques, electrodes of these materials will be fabricated and evaluated for the efficiency for either the hydrogen or oxygen evolution reaction. Deposition of electrocatalysts was controlled by changing the particle and electrolyte concentrations, the solution viscosity, and the applied bias. We show that these controls allow tuning of the morphology, geometry, and material deposition of the electrocatalysts. This work expands on the robustness of bipolar electrochemical techniques and suggests that a flowable bipolar electrochemical system may allow more control and flexibility of electrocatalyst deposition than other bulk deposition methods. 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.