476010 Nanoscale Engineering and Model-Guided Design of Advanced Energy Storage and Conversion Technologies Utilizing Ultrathin Polymer Films
Due to CO2-induced climate change, there is currently great interest in using sustainable energy resources such as sunlight. The major challenge is that the collection and storage of solar energy should be economical. A promising approach to address this grand challenge is the application of inexpensive novel polymers in solar power conversion and storage systems. Therefore, my research focuses on scientific exploration of innovative material development in polymers for energy storage and harvesting systems. By developing new chemical vapor deposition (CVD) processes, I have successfully integrated nanoscale polymer thin films (<30nm) into the nanostructured electrodes of dye sensitized solar cells (DSSCs) and supercapacitors, and have improved device performance and stability. Concurrently, much of the current material research in the areas of energy storage and conversion has been based on scientific intuition and trial and error experimentation—a slow and inefficient process with many of the directions taken to address device challenges being without theoretical guidance. In fact, many of the recent advances in the DSSC and supercapacitor fields have relied on the computational design and screening of materials. Therefore, a major focus of my research has been to use first principles modeling to gain insight into the key physical and electrochemical processes occurring in these energy systems, and then apply that knowledge towards enhancing device performance by optimizing material properties and architecture. This holistic synergetic approach between experiment and modeling has allowed me to gain a more complete understanding into the fundamental processes within these devices along with rapidly optimizing material selection, device architecture, and performance.
In the area of energy conversion, DSSCs represent one of the newer generations of solar technologies. Current DSSC technology utilizes an organic liquid electrolyte; however, liquid electrolytes suffer from practical issues of leakage, evaporation, and stability as well as fundamental issues of rapid charge recombination loss at the photoanode-electrolyte interface that limits photocurrent and photovoltage. One promising solution is to replace the liquid with a solid polymer electrolyte material. Polymer electrolytes are an attractive alternative because they do not suffer from the practical disadvantages of liquid electrolytes and could potentially enhance the cell’s I-V behavior. However, one of the major challenges with using polymers is the difficulty in integrating them within the tortuous mesoporous TiO2 photoanode of the DSSC. The photoanode consists of a sintered network of TiO2 nanoparticles, which results in an electrode layer of at least 10–15 µm thick with 10–25 nm interconnected pores. Conventional liquid processing techniques like spin coating or solvent casting lead to ineffective filling of the pores due to wettability and surface tension issues, resulting in premature pore blockage in this high aspect ratio (>1000) material. My work has addressed these challenges by utilizing a novel initiated chemical vapor deposition (iCVD) technology to directly synthesize and grow polymer electrolytes within the mesoporous TiO2 photoanode1. In this way, there is tight integration of the polymer electrolyte with the photoanode with good contact at the electrode-electrolyte interface. In tandem with these experiments, I’ve formulated first-principles mathematical models composed of coupled partial differential equations to accurately describe the dynamic internal and interfacial processes during DSSC operation to understand how properties were affected by polymer chemistry and to determine how chemistry affects overall performance. The models accurately predict the J-V behavior of polymer electrolyte DSSCs and has been used for parametric sensitivity studies and parameter estimation to derive cell parameters that can be directly related to polymer properties. The work discovered that the polymer's Lewis acid/base character has a significant impact on the interaction of the polymer with the TiO2 at the interface, which significantly influences charge conduction and electrode potential. These insights offered for the first time a way to combine modeling and experiments to design polymer electrolytes that would lead to more enhanced DSSCs with much higher power conversion efficiencies, greater than 50% improvement in conversion efficiency when compared to a liquid electrolyte DSSC and this work was published in the Journal of Power Sources2. In fact, very recently, I’ve successfully utilized these insights to carefully select a new polymer electrolyte that enabled an increase in both photocurrent and photovoltage, a breakthrough that was previously not possible with other polymer electrolytes studied that showed only an increase in either the current or voltage but not both. This work has been formally published as an invited paper in a Special Issue on Recent Advances in Energy Conversion and Storage Devices in Chemical Engineering Science3. Extending from this work, I’ve formed successful collaborations that take advantage of my strengths and expertise in DSSC materials, fabrication, and analysis. This has led to another contribution in a Special Issue on Solar Cells in AIMS Materials Science4 and a submitted paper to the Journal of Physical Chemistry C5.
In the area of energy storage, there is currently much research interest in developing new, inexpensive, and lightweight energy storage devices that are environmentally friendly. Supercapacitors represent a viable storage solution that provides high power density. However, current supercapacitors suffer from lower energy density compared to other storage technologies like batteries because they only use physical, electrostatic interactions to store charge. A promising approach to enhancing the energy density in supercapacitors is by adding a redox polymer film on the porous carbon electrodes of these devices. These polymers, like polypyrrole, polythiophene and polyaniline, can participate in Faradaic redox reactions that provide further charge storage through more energy-dense chemical reactions, which can double charge storage of these devices. However, integrating polymer coating into these porous electrodes is non-trivial and challenging, but worthwhile because of the possible 2x performance improvement. Current methods for coating polymers onto carbon substrates such as chemical bath deposition, electrodeposition, and casting from suspension are unable to uniformly and conformally coat the microporous (< 2 nm) carbon electrodes throughout the entire electrode thickness, typically 100 µm, without blocking pore accessibility to ions. The high aspect ratios (>10,000) and tortuous pore structure along with liquid surface tension, solution viscosity, poor wettability, and solute steric hindrance make conformal coatings very challenging. Furthermore, poor solubility of conducting polymers in common solvents due to their rigid backbone structure limits processibility, and for electrochemical deposition, the need for a conductive substrate further limits broad utility. Overcoming these challenges is worthwhile because a highly porous structure with an ultrathin uniform conformal polymer coating would maintain the rapid movement of ions through the pores during charge/discharge reactions, preserving the high electrochemical high double layer capacitance (EDLC) while simultaneously dramatically improving the capacitance along with power and energy densities due to the polymer’s redox reactions. By developing novel oxidative chemical vapor deposition (oCVD) processes, I was able to synthesize redox (conjugated) polymers and form highly conformal, ultrathin (< 10 nm) coatings around the topologically complex surfaces of porous nanostructured carbons. This initial effort has led to a publication in ACS Nano in which I was a co-author6. Very recently, I have successfully showed, for the first-time, that oCVD can produce polyaniline, which has the highest theoretical capacitance of all redox polymers, and significantly improve the energy capacity and stability of porous carbons derived from carbide materials through a collaborative effort with Prof. Yury Gogotsi and his group, recognized as one of the world's experts in carbon nanomaterials and energy storage. This work is submitted to Advanced Materials7. The findings indicate that the performance of the polymer can be tuned based on the reaction conditions and optimal conditions enable oCVD to produce ultrathin (<30 nm) conducting polymer films that conformally coat pores as small as 1.7 nm with the resulting supercapacitors showing >100% increase in specific capacitance compared with bare supercapacitors and exhibiting excellent cyclability, lowering to only 90% of the initially stabilized value (~100 F/g) after 10,000 cycles—which is unprecedented for conducting polymers.
Looking forward, there is a wide range of research areas that could benefit from the integration of ultrathin polymer films using a holistic, synergetic modeling and experimental approach that I have developed and used throughout my research. These areas include functionalizing nanostructured surfaces, energy applications such as solar cells, batteries, fuel cells, and supercapacitors, along with research in separation and membranes. The combined mathematical modeling and iCVD & oCVD expertise is a unique attribute that will facilitate positive research contributions and developing new processing insight particularly with the design of polymeric electronic materials for nanoscale science and engineering.
I received my B.S. in Chemical Engineering at the University of Virginia where I did research for Prof. Teresa B. Culver in sustainable stormwater management. I am currently co-advised by Prof. Soroush and Prof. Lau at Drexel University. Academically, I enjoy learning and have maintaining a perfect 4.0 GPA (grade point average, equivalent to an overall A grade) in all my Ph.D. graduate courses at Drexel, including all the required core graduate chemical engineering courses (Mathematical Methods, Thermodynamics, Transport Phenomena, and Kinetics). I have greatly enjoyed helping teach undergraduate chemical engineering course while I was a TA, some of my favorite course to teach would be Kinetics and Thermodynamics.
I believe that both teaching and research are greatly enhanced when they are integrated together as an educational program. Throughout my Ph.D., I have mentored one high school and nine undergraduate students on research projects related to my research and their educational curriculum. I have trained and guided these students to perform polymer thin film synthesis and processing using CVD approaches as well as materials/device characterization under the broad theme of energy. I have also given back to the Drexel and Philadelphia community by helping to initiate research projects with the Drexel Smart House, a physical building and living environment run by Drexel undergraduate students with the goal of exploring and understanding different sustainable technologies for the future and showcasing those technologies to the larger Philadelphia community. I have also been the President of our department's Graduate Student Association as well as Vice President of the College of Engineering Graduate Association and helped give back to the graduate college.
I am enthusiastic and passionate about research, which is shown tangibly by the number of publications and by the wide range of major professional conferences I have presented at, such as the American Institute of Chemical Engineers (AIChE), American Vacuum Society (AVS), American Chemical Society (ACS), and the Electrochemical Society (ECS)--including one invited talk at the ECS conference in Cancun, Mexico that acknowledged the importance of my DSSC work. I have presented 11 oral presentations and 5 poster sessions. Furthermore, I have formed collaborations with W.L. Gore, Prof. Gogotsi in Drexel’s Material Science department, Prof. Ji in Drexel’s Chemistry Department, Prof. Li in Penn’s Chemical Engineering Department, and Prof. Kalra in the Chemical and Biological Engineering Department at Drexel.
As a testament to my work ethic, I have been the recipient of numerous honors, including the Department of Chemical and Biological Engineering's 2016 Outstanding Ph.D. Student Award, the College of Engineering's 2016 Outstanding Graduate Student Award, and most recently Drexel University's highest honor with the 2016 Research Excellence Award for Outstanding Research given to only four students out of the entire university's graduate population. I have also been the recipient of numerous merit fellowships, including the 2016 Leroy L. Resser Endowed Fellowship, the 2015 George Hill, Jr. Endowed Fellowship, and the 2015 Koerner Family Award. In addition, I have been a GAANN fellow for the Department of Education's National Academy of Engineering Grand Challenges from 2014–2016. I was also one of the six national finalists for the highly competitive W.L. Gore Fellow Award in 2015.
(1) Janakiraman, S., Farrell, S. L., Hsieh, C.-Y., Smolin, Y. Y., Soroush, M. & Lau, K. K. S. Kinetic analysis of the initiated chemical vapor deposition of poly(vinylpyrrolidone) and poly(4-vinylpyridine). Thin Solid Films 595, Part B, 244-250, (2015). http://www.sciencedirect.com/science/article/pii/S0040609015005088
(2) Smolin, Y. Y., Nejati, S., Bavarian, M., Lee, D., Lau, K. K. S. & Soroush, M. Effects of polymer chemistry on polymer-electrolyte dye sensitized solar cell performance: A theoretical and experimental investigation. Journal of Power Sources 274, 156-164, (2015). http://www.sciencedirect.com/science/article/pii/S0378775314016371
(3) Kuba, A. G., Smolin, Y. Y., Soroush, M. & Lau, K. K. S. Synthesis and integration of poly(1-vinylimidazole) polymer electrolyte in dye sensitized solar cells by initiated chemical vapor deposition. Chemical Engineering Science. http://www.sciencedirect.com/science/article/pii/S000925091630238X
(4) Johnson, N. M., Smolin, Y. Y., Shindler, C., Hagaman, D., Soroush, M., Lau, K. K. S. & Ji, H.-F. Photochromic Dye-Sensitized Solar Cells. AIMS Materials Science 2, 503-509, (2015)
(5) Johnson, N. M., Smolin, Y. Y., Hagaman, D., Soroush, M., Lau, K. K. S. & Ji, H.-F. Suitability of N-Propanoic Acid Spiropyrans and Spirooxazines for Use as Sensitizing Dyes in Dye-Sensitized Solar Cells. Journal of Physical Chemistry C, Submitted, (2016)
(6) Nejati, S., Minford, T. E., Smolin, Y. Y. & Lau, K. K. S. Enhanced Charge Storage of Ultrathin Polythiophene Films within Porous Nanostructures. ACS Nano 8, 5413-5422, (2014). http://dx.doi.org/10.1021/nn500007c
(7) Smolin, Y. Y., Van Aken, K, Boota, M., Soroush, M., Gogotsi, Y.,Lau, K. K. S. Engineering Ultrathin Polyaniline in Carbide Derived Carbon Supercapacitors using Oxidiative Chemical Vapor Deposition. Advanced Materials, Submitted, (2016)
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