Engineering catalysts, interfaces, and semiconductors for sustainable hydrogen production via solar driven water splitting [electronic resource]
Keywords: Chemical Engineering.

Hydrogen is an extremely important industrial chemical used primarily for ammonia synthesis and hydrocarbon upgrading in the petrochemical industry. Currently, the main process used to produce hydrogen is steam reforming of natural gas, which results in the release of CO2. Sustainable hydrogen production methods have the potential to substantially reduce greenhouse gas emissions. Photoelectrochemical (PEC) water splitting, a process which produces H2 and O2 from sunlight and H2O, is a particularly promising route for synthesizing hydrogen using solar energy without direct release of CO2. However, there are many challenges to overcome before PEC water splitting is economically viable for implantation at scale. Designing active catalysts, corrosion protection schemes, and wide band gap photoelectrodes are critical to the success of PEC water splitting. The first part of this thesis focuses on the development of new materials and strategies for improving the activity of hydrogen evolution catalysis. We begin by improving the activity of already excellent [Mo3S13]2- through catalyst-support interactions. Through collaboration with researchers performing density functional theory, we were able to attribute the observed activity increases on different supports to changes in the catalyst's hydrogen binding energy. This study showed the power of a combined theory-experiment approach to developing new catalyst materials. Then, we develop a new synthesis technique to create thin films of cobalt phosphide (CoP) which are highly active for the hydrogen evolution reaction (HER). This synthesis route provides a platform for both fundamental studies of a variety of transition metal phosphide electrocatalysts and for the fabrication of high-performance photoelectrodes. In fact at the time of publication, the CoP-silicon photocathode produced via this method was among the most active non-precious metal silicon photocathodes ever created. Continuing with strategies to improve the activity of non-precious metal photoelectrodes, we pair a silicon semiconductor with a catalyst-coated high surface area transparent conducting support to create a highly efficient photocathode. Broadly, we show the importance of synthesizing photoelectrodes which decouple a high surface area catalyst and a low surface area semiconductor to maxiimize the efficieny of each component and therefore maximize the effieincy of the overall device. Building on our success with silicon electrodes, we turn our attention to improving the activity and enhancing the durability of a wide band gap copper gallium diselenide (CGSe) photocathode. CGSe is a promising semiconductor for use as the top absorber of a PEC water splitting cell, but requires the integration of buffer layers and protecting schemes to enhance its suitability for PEC water splitting. First, we report the development of a thin film, conformal coating of molybdenum disulfide (MoS2) and titanium dioxide (TiO¬2) that is used to enhance the operating lifetime of polycrystalline CGSe photoelectrodes 8-fold, from 43 h to 337 h in 0.5M sulfuric acid electrolyte. Most importantly, the translatable, low temperature synthesis methods developed here to fabricate the conformal protecting layer of MoS2 can be extended to a variety of other polycrystalline photocathodes. We then apply the MoS2/TiO¬2 protecting layer and a platinum nanoparticle catalyst to a cadmium sulfide coated copper gallium diselenide (CdS/CGSe) to create the most efficient CGSe photoelectrode reported to date. Finally, we use a platinum nanoparticle catalyst and CdS/CGSe with a gallium arsenide solar cell to create a water splitting device which operates at 1.6% theoretical solar-to-hydrogen (STH) efficiency, a step along the way to approaching the maximum achievable STH efficiency of 26%. In summary, this thesis covers a variety of topics ranging from the investigation of fundamental phenomena in HER catalysis to the development of applied water splitting devices that may someday have commercial potential. Most importantly, the materials and methods developed in this thesis provide indispensable knowledge for the water splitting community and make progress down the path to low cost, sustainable hydrogen production.