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Self-Assembly and Mass Transport in Membranes for Artificial Photosynthesis

  • Author(s): Modestino, Miguel Antonio
  • Advisor(s): Segalman, Rachel A
  • et al.

Recent environmental factors have triggered a strong interest towards the development of scalable technologies that can increase the share of renewable sources into our energy mix. Artificial photosynthesis systems are a promising alternative as they can simultaneously capture and store solar energy in the form of a fuel. These systems are based on photoelectrochemical (PEC) cells that can take low energy density reactants such as water and/or carbon dioxide and transform them into energy dense hydrogen or carbon containing molecules via light-driven processes. Deployable solar-fuel generators need to be able to produce fuels in a robust, scalable, and efficient manner. Despite the large number of studies focusing on this technology since its inception in the early 1970's, a system that can satisfy those three requirements does not exist. Significant innovation is required to develop cost-effective components that can perform the light-absorption, catalytic redox reaction, ion transport and product separation requirements. Additionally, understanding of component performance in integrated devices is crucial for developing high efficiency solar-fuel generators.

This dissertation focusses on several aspects of component integration in solar-hydrogen generators. The initial focus involves the development of self-assembly techniques of nanometer scale units to obtain architectures necessary for solar-fuel devices. Starting with solutions of semiconducting nanorods and polymers, this work demonstrates that by controlling the evaporation rate during solvent casting, arrays of vertically aligned nanorods embedded in polymer films can be obtained over large areas (> 1 cm2). This architecture is desirable for the integration of hydrogen generating nanorods into integrated water splitting membranes, where H2 and O2 are evolved at physically separated sites. This work also describes how the structure of proton conducting membranes (Nafion®) is affected at inorganic interfaces such as the ones present in solar-fuels devices. The effects of thin-film confinement and wetting interactions are studied in Nafion thin-film model systems using a combination of X-ray scattering and mass transport characterization techniques. These studies show how confinement of Nafion films to thicknesses below 10 nm results in significant limitations to self-assembly, disruption of phase separation in the material and ultimately decrease in ionic conductivity. Wetting interaction also play a role in the orientation of conducting domains in the material. Hydrophobic surfaces results in a parallel orientation of ionic domains while films cast on hydrophilic substrates result in an isotropic orientation of domains. The differences in domain orientation also impact the mass transport behavior of the material.

Additionally, this dissertation covers several topics related with the integration of components for the fabrication of practical hydrogen generators. Here we describe the development of a microfluidic test-bed for the incorporation of catalytic and membrane components in scales amenable for research. This tool allows for the simple exchange and quantitatively assessment of the performance of integrated electrochemical fuel generating devices as well as each of the individual components that participate in the process. Lastly, this work also describes engineering solutions that allow both membrane-separated water electrolyzers and solar hydrogen generators to operate under buffered electrolytes. This is achieved by using supporting electrolytes to carry the ionic current through the membrane while controlled convective streams around the membranes are implemented to avoid the formation of large concentration gradients between reaction sides. This development opens up the space of operating electrolytes for the incorporation of wide range of components that are not stable under strong basic or acidic conditions.

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