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Development and Characterization of Dye-Sensitized Ion-Exchange Membranes and Progress Toward Integrated Photodialysis Devices

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This dissertation focuses on the initial results, development, and design concepts of an integrated photodialysis devices based on photoacid-sensitized ion-exchange membranes. The central themes of this work are understanding ion transport and the J–E characteristic under applied bias of ion-exchange membranes in the dark and in the light.

Chapter 1 provides a brief description of the need for an integrated photodialysis device by describing recent natural disasters where a rapidly deployable desalination device could have saved lives. Minimum device parameters to outperform a solar distillation device are discussed. This concludes with a brief comment on prior work to develop artificial ion pumps.

Chapter 2 details two studies of the J–E characteristics of a commercial BPM and how to design a better water dissociation catalyst. The first study contains an analysis of how the addition of supporting electrolyte can affect the built-in potential of BPMs and postulates on the contribution of water dissociation at low applied bias. The second study describes the synthesis of several poly(2,6-dimethyl-1,4-phenylene oxide) derivatives that contained aminopyrene, phosphonic acid, and tertiary amine functional groups and their efficacy as water dissociation catalysts at low and high applied bias.

Chapter 3 is our seminal report on photoacid-sensitized Nafion® where the synthesis and characterization of the functional polymer is described. The photoresponse of the sensitized membrane suggests that cations are transported against a concentration gradient of protons and that on average each had a turnover number >18. The modified membranes also exhibited photovoltaic properties where the direction of photogenerated current was independent of the polarization direction of the external bias. A control Nafion® membrane containing ionically associated non-photoacidic dyes was found to exhibit less than 5% of the photoresponse of the photoacid-modified membrane, supporting that the majority of the photoresponse was not due to non-radiative decay of the excited-state dyes.

Chapter 4 details the incorporation of an anion-selective layer to photoacid-sensitized Nafion® to form a photoacid-sensitized BPM. Performances of these bipolar membranes was highly dependent on the conformity of the interfacial contact between the layers. The highest performing membrane exhibited a photovoltage of nearly 140 mV.

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