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Current Research Projects

My group research activities focused on three areas: Li-ion batteries, Na-ion batteries, alkaline fuel cells, and electroanalytical techniques. These research activities cover topics from fundamental electrochemistry and materials synthesis to electrochemical devices..

Fundamental Electrochemistry

Novel Electroanalytical Techniques for Phase Transformation Electrodes
Phase transformation materials (such as LiFePO4, Li4Ti5O12) have been recognized as the next generation electrode materials for electric vehicle and renewable energy storage applications. However, the mechanism for such excellent performance is still not fully understood due to the lack of accurate electroanalytical techniques to study the kinetics of Li insertion/extraction in these materials. All the existing electroanalytical techniques rely on classic Fickian diffusion in a solid solution phase, and thus are not valid for phase transformation electrodes. Using traditional electroanalytical techniques, only an apparent diffusion coefficient, rather than the true diffusion coefficient, can be obtained in the two-phase region.

Figure 1  
Figure 1. Li-ion diffusion coefficient of LiFePO4 measured using current solid solution GITT and our phase transformation GITT. The phase region is marked based on the lithiation equilibrium potential-composition curve.  

The “apparent” diffusion coefficients of LiFePO4 in two-phase regions obtained using existing electroanalytical techniques are four orders of magnitude lower than those in the single phase region (black square in Figure 1). We developed phase transformation electroanalytical techniques by integrating mixed-control phase transformation theory with existing electroanalytical techniques. For the first time, the true Li-ion diffusion coefficients and interface mobilities of LiFePO4 electrode materials were obtained by my group using phase transformation electroanalytical techniques. The similar Li+ diffusion coefficients in single and phase transformation regions (Figure 1) validated our phase transformation electroanalytical techniques. The developed phase transformation electroanalytical techniques can be widely applied to other ion-insertion electrodes including hydride, magnesium, and sodium batteries. Three papers have been published in the Journal of Physical Chemistry C and the Journal of Power Sources.


Li-ion Battery

Virus enabled Anodes for Li-ion Batteries

Si has a nearly ten-fold increase in capacity over current graphite anodes, but suffers from poor cycling stability due to large volume changes during lithiation/delithiation cycles. In this study, I (the corresponding author) collaborated with Prof. James Culver at the Institute for Bioscience & Biotechnology Research and Prof. Reza Ghodssi in the ECE Department to develop a novel, three-dimensional tobacco mosaic virus (TMV) assembled silicon anode.

  Figure 1
  Figure 2. TEM image of a single Si/Ni/TMV1cys nanowire and SEM images of the silicon anodes.

We demonstrated that metal coatings on patterned TMV1cys templates can be used as 3-D current collectors, and Si can be deposited on patterned 3-D current collectors using sputtering and electrodeposition to form nanowire anodes (Figure 2). Unlike previously reported methodologies that utilized biological templates for the synthesis of nanomaterials and relied on powder mixing and ink casting for the electrode fabrication, the method presented in this study involves the direct fabrication of a nanostructured silicon electrode. Every silicon nanowire is connected to the patterned current collector, resulting in a high capacity, long cycling stability, and a high rate capability. The findings on TMV enabled electrodes were published in ACS Nano, Chemical communications, Advanced Functional Materials, and Electrochimica Acta. This research has been highlighted on the US News, the DOE website, USNBC, Discovery, and many other news outlets in the U.S. and Europe.

Scaffold Si based anodes for Li-ion batteries

To mass produce Si anodes at a low cost, a unique air spray method of fabricating Si anodes with a porous carbon (or polymer) scaffold structure has been developed by our group (Figure 3). . Such carbon scaffold Si anodes are fabricated via carbonization of porous Si-PVdF precursors which are directly deposited on the Cu or carbon-nanofiber current collector.

  Figure 1
  Figure 3. Schematic of the structurally sustainable carbon scaffold Si anode (a) before lithiation, (b) after lithiation.

Unlike the conventional slurry casting method, binder and conductive additives are not used in the preparation of the carbon scaffold Si anodes. The carbon scaffold Si anode has a close-knit porous carbon structure that can not only accommodate the Si volume change, but can also facilitate the charge transfer reaction. The carbon scaffold micron-Si anode can retain 1280 mAhg-1 capacity after 118 full cycles between 0 and 1.5 V under a 0.05C cycling rate. These results were published in Chemical Communications, Journal of Materials Chemistry, Electrochemistry Communications, and Electrochimica Acta.

 


Beyond Li-ion Battery

High energy density Li-S, Na-S, and Li-air batteries

Figure 1  
Figure 4. History and future of Li batteries (top) and cycling stability of Li-S batteries (bottom)potential-composition curve.  

In addition to cutting-edge research in Si anodes for advanced Li-ion batteries, our group has also explored future batteries. The challenges facing the commercialization of Li-S batteries are poor cycling stability and low coulombic efficiency, which are induced by the dissolution of polysulfides into the electrolyte and the shuttle reaction. The current technique is to physically restrain the polysulfide dissolution using barrier materials. However, this only mitigates these challenges. Our group solved this problem by breaking S8 into S4 and S2 at temperatures above 500oC and bonding them onto carbon so that soluble high order polysulfides could not be formed. The disordered carbon nanotubes, impregnated with sulfur at temperature of 500oC, demonstrated great cycling stability and 96% coulombic efficiency (Figure 4). This manuscript has been published in Nano Letters. In addition, high performance of Na-S batteries has also been fabricated in our group recently.


Alkaline fuel cell

Synthesis of alkaline anion exchange membranes (AAEMs) for fuel cell and metal-air battery applications

  Figure 1
  Figure 5. Synthesis of alkaline anion exchange membranes using a bottom-up approach through miniemulsion copolymerization of three functional monomers.

The investigation of alkaline anion exchange membranes for alkaline fuel cells (AFCs) has been recently revived because of the significant advantages of AFCs over acid PEMFCs in terms of high kinetics for oxygen reduction and fuel oxidation in alkaline environment and lower cost through the use of non-precious metal catalysts. AAEM, a key component in an AFC system, requires both high OH- conductivity and superior durability. However, current AAEMs lack both conductivity and mechanical strength because the conventional AAEMs are synthesized by modifying existing nonconductive polymers. To overcome this problem, our group synthesized AAEMs using a bottom-up approach through miniemulsion copolymerization of selected functional monomers to fulfill the requirements of ion conductivity and mechanical strength (Figure 5). In the poly-(methyl methacrylate-co-butyl acrylate-co-vinylbenzyl chloride) (PMBV) copolymer, hydrophobic methyl methacrylate (MMA) provides mechanical support. A hydrophobic Butyl acrylate with a low glass transition temperature (Tg) allows for the flexibility of the polymer chain. The vinylbenzyl chloride (VBC) after quaternization and ion-exchange provides OH- conductivity. The obtained AAEM demonstrated superior alkaline fuel cell performance: 180 mW cm-2 peak power density at 70oC capable of working for up to 2 days. This work was published in ChemSusChem, J. Power Sources, and Macromolecular Chemistry & Physics.


Awards for Research: Received by Students

  1. Ann Sun (undergraduate):
    - Philip Merrill Presidential Scholar 2010
    - U.S. Department of Energy Mickey Leland Energy Fellowship 2010
  2. Emily Li (undergraduate):
    - Department of Energy Mickey Leland Energy Fellowship 2010
  3. Bob Latimer, Kevin Bates, Lucas Hedinger, Adam Gradzki, Sandhya Patel, John Weston Breda, Whitney Hollinshead, Leslie Mok, and Brett Koller
    Team Thirsty Turtles, advised by ChBE assistant professor Chunsheng Wang
    - took first place at the American Institute of Chemical Engineers’ (AIChE) mid-Atlantic Regional Conference's Chem-E Car Competition, earning them a spot in the finals at the organization's national meeting this fall.

 

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