(4bs) Understanding Complexity in Membrane Systems for Efficient Separations and Advanced Energy Technologies

Research Interests: My principal research interest is understanding the chemical and structural properties of polymeric membranes that underpin their transport of small molecules and ions in complex systems. Polymeric membranes are employed in a variety of gas separation, water purification, energy storage, and energy generation applications. Due in part to the largely Edisonian development of membranes for particular applications, substantial opportunity exists to understand how chemical and structural membrane properties influence the fundamental transport and reaction processes that ultimately govern membrane performance. I have endeavored to develop laboratory methods by which the ramifications of complexity may be systematically explored and understood. Specifically, I have focused on: 1) precise measurement of multicomponent transport in membranes, 2) systematic synthesis to control membrane structure and chemistry, and 3) deconvoluting the contributions of coupled physicochemical properties to transport phenomena.

Precise Measurement of Multicomponent Transport: The permeation of mixtures of aqueous solutes through membranes is of critical importance in many applications, including fuel cells and solar-driven electrochemical carbon dioxide reduction (artificial photosynthesis). However, laboratory diffusion measurements are typically made on single solutes, and techniques capable of quantifying multicomponent transport often require burdensome aliquot sampling. A novel in situ FTIR spectroscopy technique was developed for quantitative measurement of multicomponent organic permeation through membranes in a simple diffusion cell without aliquot sampling. The technique was demonstrated by quantifying the permeability of ion exchange membranes, such as Nafion and Selemion, to single and multicomponent mixtures of alcohols and organic ions using a conventional diffusion cell. Membrane permeabilities were extracted from time-resolved half-cell concentration data using fee volume models, and membrane permeabilities and selectivities calculated from the single component permeation experiments were compared to those calculated for solutes in multicomponent permeation experiments to understand emergent transport phenomena in these aqueous systems.

Systematic Synthesis to Control Membrane Structure and Chemistry: In artificial photosynthesis devices, which photoelectrochemically produce liquid fuels from carbon dioxide and sunlight, a hydrated membrane is used to minimize the crossover of carbon dioxide reduction products (e.g., alcohols), and to promote the passage of electrolyte ions so that current can flow in the device. A systematic series of new anion exchange membranes based on poly(vinyl imidazolium) polymers were prepared by UV-photopolymerization to understand the impact of crosslink density on membrane performance in artificial photosynthesis devices. The water content, ion conductivity, and permeability to alcohols (via the in situ FTIR technique described above) were evaluated. The permeability of membranes to ions and alcohol products was correlated with membrane water content, which serves as a useful metric to estimate the free volume between polymer chains through which solutes diffuse. Experimental membranes were tested in artificial photosynthesis devices and compared to commercial anion exchange materials. These initial studies on poly(vinyl imidazolium) membranes offer insight into polymer design considerations for anion exchange membranes used in artificial photosynthesis.

Deconvoluting the Contributions of Coupled Physicochemical Properties to Transport Phenomena: Fouling is a major obstacle to more widespread implementation of membranes for water treatment. Conventional laboratory studies use a constant transmembrane pressure (TMP) technique, where the pressure difference between the feed and permeate sides of the membrane is fixed, and the flux declines as the membrane fouls. As the flux varies, however, so too does the rate at which foulants are brought to the membrane surface, so the effect of membrane physicochemical properties on the fouling behavior is obfuscated by changing hydrodynamic conditions. In contrast, fouling experiments in which the flux through the membrane is held rigorously constant and the TMP evolves as the membrane fouls are more difficult to perform, but offer a methodology by which the effects of membrane surface properties and hydrodynamics may be disentangled. The efficacy of hydrophilic surface modifications in mitigating emulsified oil fouling of ultrafiltration membranes was investigated under constant flux and constant TMP conditions. In constant TMP experiments, where the modified and unmodified membranes were operated with the same TMP, the modified membrane exhibited a higher flux (lower resistance) than the unmodified membrane. In constant flux operation, however, where both membranes were operated at the same flux, the modified membrane exhibited a higher TMP (higher resistance) than the unmodified membrane. In light of the constant TMP result, the constant flux result was counterintuitive as it suggested that fouling was worsened by the hydrophilic coating. The result was understood, however, by realizing that the modified membrane had a higher initial TMP, and consequently a higher initial resistance, than the unmodified membrane due to the mass transfer resistance of the applied surface modification. It is proposed, therefore, that membrane design and subsequent surface modification for fouling mitigation should be coupled. For example, a membrane with larger pores than required for an application may be surface-modified to achieve fouling resistance and to reduce the pores to the desired size.

Teaching Interests: At LBNL, I have had the opportunity to mentor undergraduate, graduate, and postdoctoral students from diverse backgrounds and with a broad range of skill sets. While my position as a Staff Scientist as LBNL has afforded me outstanding facilities at a premier research facility, a desire to teach and to organically grow an academic family has motivated me to seek faculty appointment opportunities. My principal goal as a faculty appointee is to cultivate world-class chemical engineering graduates and researchers who will become successful, productive members of the industrial and academic scientific community. In this regard, I view both classroom instruction and laboratory research as opportunities to develop within students the skills that they will need to succeed professionally upon graduation. Aside from the technical content of a chemical engineering education, such skills include proficiency in written and oral communication, systematic critical thinking and problem solving, and hypothesis-driven research strategies grounded in the scientific method. I am prepared and qualified to teach any undergraduate chemical engineering courses. At the graduate level, I would teach courses related to my areas of expertise in transport and polymer science. In cases where a comparable course is not available, I will aim to develop courses in membrane science, separations, and advanced energy technologies. At LBNL, I have endeavored to promote diversity in the workforce by training students from disadvantaged backgrounds and minority groups, and by participation in laboratory committees focused on the development of diversity initiatives and in hiring. Finally, I will always promote safety and responsibility as the most critical lessons that students can take away from the research laboratory. At LBNL, I have served in leadership capacities for laboratory safety as well as participated in several committees to improve safety training and protocols for safe laboratory work.