(2cv) Rational Design of Ion Exchange Membranes for Sustainable Water and Energy

Rapid climate change has become a global challenge impacting many people’s lives. Increased atmospheric CO₂ levels have been recognized as a major contributor to this issue. Unfortunately, various human basic needs (e.g., water and energy) have been developed around fossil fuels, e.g., they provide the energy needed to do most things, including water purification. Dense polymeric membranes are nonporous selective layers that have been widely applied in many applications, including reverse osmosis and gas separation. Ion exchange membranes (IEM) are unique dense membranes that provide selective ion transport via their fixed charge groups. Over several decades, membrane scientists have acquired an in-depth understanding of ion transport in these membranes. However, the application of these membranes in technologies, such as electrochemical desalination (ED) and direct alcohol fuel cells (DAFC), has been challenging due to complex transport processes. For instance, one of the major challenges with an IEM is the crossover of undesired ions and/or fuels (e.g., alcohols in DAFC), which often reduces the overall performance.

My primary research interest is developing novel crosslinked IEMs for current and future applications. During my Ph.D., I led the development of a novel crosslinked AEM (XL-AEM) that showed decent performance in direct urea fuel cells (DUFC), which oxidize urea to generate energy. We hypothesized this result was likely due to urea uptake, within our membrane, saturating at a low concentration, which led to a low urea crossover. As a principal investigator (PI), I would like to utilize this synthetic approach in developing IEMs for emergent applications. In the short-term, I would like to develop a membrane targeting DAFC. DAFC is an eco-friendly device that oxidizes alcohol to generate energy. Like DUFC, it lacks an IEM to suppress the crossover of alcohols. However, designing an IEM can be more challenging as the diameters of alcohols are smaller than that of urea and, therefore, they are more prone to crossover. Therefore, the primary objective of this investigation is to reduce the fractional free volume while increasing the hydrophobicity of the membrane. To achieve this goal, I will perform compositional optimization by cloud point analysis among a crosslinker, a hydrophilic monomer, and a hydrophobic monomer. I will then evaluate these materials in a DAFC by preparing membrane-electrode assemblies.

As a second research goal, I would like to investigate the impact of backbone chemistry in polysulfone-based IEMs on membrane performance in ED. ED removes undesired ions from the feed solution by alternating AEM and CEM between an anode and a cathode. While ED can be a powerful desalination approach, the applicability has been limited to relatively dilute solutions due to the membrane selectivity. As ions (e.g., Na⁺, Cl^-) are relatively smaller and less hydrophobic than urea and alcohols, grafting on a low free volume polymer, like polysulfone, is a reasonable choice. Polysulfones are polymers consisting of diphenyl sulfone (sulfonyl with para-linked phenyl groups) and diphenol (e.g., bisphenol A, BPA) repeat units. During my post-doc investigation, I have noticed a series of polysulfones can be synthesized by replacing BPA with other diphenols, including tetramethyl BPA and tetramethyl BPF. These methyl groups can become active sites for grafting a negatively-charged monomer (carbonic or sulfuric acid) or a positively-charged monomer (choline or imidazole) to form a CEM or AEM, respectively. As a PI, I would like to synthesize a series of polysulfones with different diphenols and graft a series of charged monomers to evaluate the impact of the difference in backbone chemistry on the ion transport and selectivity. Ultimately, these data will be transferred to design IEMs for ED.

As a long-term goal, I want to advance pervaporation membrane by infusing polysulfone into a crosslinked membrane. Pervaporation is a promising separation technique for biofuels (extracting ethanol from water). Several investigations have been performed using rubbery polymers (e.g., polydimethylsiloxane) as an active layer to absorb ethanol and reject water. Unfortunately, the selectivity of these membranes often suffers due to membrane swelling. To overcome this issue, I propose to develop a series of mixed matrix membranes (MMM) by infusing a halogenated polysulfone into the carboxylate-containing crosslinked membrane (XL-COOH). The selection of the halogenated polysulfone will be dependent on the results from the second research goal. The preparation of the XL-COOH will be similar to the synthetic approach from the primary research interest. Acrylic acid will be considered to provide alcohol groups that can form a covalent bond with the halogen groups on the polysulfone to anchor the polysulfone within the crosslinked network to provide additional mechanical properties and reduce the free volume of the overall MMM.

These dense polymeric membrane research activities will impact many membrane applications, including direct alcohol fuel cells, electrochemical desalination, and biofuel pervaporation. Ultimately, these applications can contribute to the suppression of rapid climate change.

Teaching Interests

The classroom experience is considered one of the most valuable learning experiences for students and a great opportunity for the professor to guide students based on their interests, such as research, teaching, and industry. Typically, a traditional chemical engineering classroom consists of the instructor lecturing on a new concept for most of the class time and going over some example problems. While it is an effective model for transferring knowledge within a limited time, it limits teacher-student interactions to students who are brave enough to ask a question during the class. Therefore, this model is challenging for introverted students who want to get involved.

As an undergraduate student, I was an international student from a country where English is not a common language and a student who had transferred to engineering from pharmacy school. Initially, I felt quite challenged in the classroom as I found the learning environment quite different from a health professional school. So, I reached out to the instructors for help, and one recommendation was to find a study group. Unfortunately, following this advice was not particularly straightforward as study groups are often formed among students from similar backgrounds under the premise of contributing to the group.

The flipped classroom (active learning) approach is an alternative model to enhance the classroom experience. Briefly, students are required to watch pre-recorded content before class, and they utilize class time to ask questions while working on in-class examples, homework, and group projects. While well-organized flipped classes show a clear advantage over traditional classroom experience, it is often challenging to meet the expectations of the students. One of the major hurdles is that students may not listen to the recorded content and may ask naïve questions that were addressed in the recorded content. Moreover, it misses the opportunity to go over the problem together in class, which students often find more comfortable as they can interact with the instructor.

As a prospective instructor, I propose to use a semi-flipped class model. I believe the instructor has a responsibility for engaging students, and I can achieve this goal by blending active learning with the traditional model. As a working model, I would like to divide the lecture into three parts: (1) listening to a short, pre-recorded lecture focused on the motivation and background, (2) an in-class lecture focused on the application, and (3) a group activity. I find motivation and background to be crucial in learning a new concept. To achieve this objective, I think a short video (< 20 min) can be a powerful tool as students can watch and stop the video at their convenience, use it as supplementary material while reading the textbook, and interact with me by leaving comments and questions. The first part of the in-class lecture will focus on applying knowledge by going over some problems together (< 30 min). The last part of the class will focus on the group activities (< 20 min). During this time, students will be working on a problem to be turned in with the remaining homework. This way, every student will be exposed to a study group and have the opportunity to develop long-term study group relationships with their peers.

Research mentorship is a valuable learning experience for students who have a particular passion for acquiring new knowledge. During the last semester of my undergraduate studies, I received an opportunity to participate in summer research, and I consider this a turning point in my life. Although the chemistry was challenging, I found that my PI trusted my ability to conduct polymer synthesis. I believe this trust and faith is a core part of every mentoring relationship. As long as mentees feel the mentor trusts their capability with enough care, the mentees will continue to respect the leadership and try to support the mentor. During my Ph.D. and Post-doc, I have mentored more than 20 graduate and undergraduate students. During this experience, I have learned everyone has different determination and capability; therefore, I modify and adjust my approach to mentoring each individual student. As an assistant professor, I plan to apply this mentoring approach to support students in achieving what they would like to accomplish in research.