The Hallinan lab studies polymers for advanced energy sustainability. We are interested in two major classes of nanostructured materials, due to the emergent properties that can arise from having such nanostructure. The classes are block copolymers and polymer-grafted nanoparticles, both of which we synthesize in-house. These materials have potential applications in several areas of advanced energy. For example, significant improvements in safety, cost, and energy density of batteries are possible by replacing the currently used liquid electrolyte with nanostructured solid electrolytes.[1, 2] The major hurdles associated with solid electrolytes are slow ion transport and detrimental surface interactions at electrode surfaces (including oxidative degradation and lithium dendrites). We have developed new experimental techniques, such as time-resolved 7Li magnetic resonance imaging (MRI, see Figure 1), to accurately measure transport in solid electrolytes. Traditional approaches use a complicated set of electrochemical measurements that inherently have large error and rely on simplifying assumptions that do not apply to solid electrolytes. The spectroscopic techniques that we employ are more direct and do not require inappropriate simplifying assumptions.[4, 5] Our studies have identified possible solutions to slow transport and lithium dendrites, that involve using solid electrolytes with much higher ion contents than have been investigated up to now. We are also applying our expertise/techniques to study promising new solid electrolytes synthesized by several collaborators at major research universities.
Another area is our work with gold nanoparticle (Au NP) monolayers as surface-enhancing substrates for Raman spectroscopy. Surface-enhanced Raman spectroscopy (SERS) is a mobile spectroscopic method, potentially applicable for remote sensing. However, existing technology suffers from poor reproducibility due to the lack of control of the structure of the enhancing substrate. Au NP monolayers assembled with our patented methods overcome this issue and enable quantitative use of SERS.[7-10] As shown in Figure 2, we have demonstrated the ability to quantitatively detect chemicals at parts-per-million (ppm) levels, with less than 3% signal variation across an enhancing substrate. Since a spectrum is generated, SERS can be used for detection in multicomponent environments and identification of unknown compounds, especially at low concentrations. Raman is also an ideal solution for detection in aqueous environments, as it is not sensitive to the presence of water (in contrast to infrared spectroscopy). We are applying SERS with our Au NP monolayers to a wide range of problems, including spectroscopic study of lithium battery interfaces to hazards detection and food safety.
Reaction kinetics are particularly important at early times, for example during rapid or pulsed battery charging. The study of electrode reaction kinetics with rotating disk electrodes that are used in liquid electrolytes is not possible in solid electrolytes. Therefore, we have developed and validated a pulsed voltammetry method. In the limit of low applied voltage, this method converges to that measured with electrochemical impedance spectroscopy, which probes all interfacial resistance, both that due to exchange transfer as well as resistance from an interphase between electrode and electrolyte. At higher applied potentials, the interphase can be broken and pure electrode kinetics probed. We have examined oxidative degradation of polymer electrolytes and plating/stripping kinetics on lithium metal.
Water transport in polymer membranes is important for water purification, lithium-air batteries, and CO2 separations. Polystyrene-b-poly(ethylene oxide) (PS-b-PEO) is a tough, amphiphilic block copolymer with good mechanical properties for membrane-based separations and is expected to have high flux and good selectivity for water transport. We have investigated diffusion in this block copolymer as a function of water activity. Membrane water content has a significant effect on membrane crystallinity and block copolymer morphology. This impact on structure in turn affects water diffusion in the polymer.
 O. Oparaji, S. Narayanan, A. Sandy, S. Ramakrishnan, D. Hallinan, Macromolecules, 51 (2018), DOI: 10.1021/acs.macromol.7b01803.
 D. T. Hallinan, and N. P. Balsara, Annual Review of Materials Research 43 (2013), DOI: 10.1146/annurev-matsci-071312-121705.
 D. T. Hallinan, I. Villaluenga, and N. P. Balsara, MRS Bull. 43 (2018), DOI: 10.1557/mrs.2018.212.
 S. Chandrashekar, O. Oparaji, G. Yang, and D. Hallinan, J. Electrochem. Soc. 163 (2016), DOI: 10.1149/2.0681614jes.
 O. Oparaji, M. Minelli, C. Zhu, E. Schaible, A. Hexemer, and D. T. Hallinan Jr, Polymer 120 (2017), DOI: https://doi.org/10.1016/j.polymer.2017.05.055.
 O. Oparaji, X. Zuo, and D. T. Hallinan Jr, Polymer 100 (2016), DOI: 10.1016/j.polymer.2016.08.026.
 D. Hallinan Jr, and G. Yang, edited by USPTOUS, 2015).
 G. Yang, and D. T. Hallinan, Scientific Reports 6 (2016), DOI: 10.1038/srep35339.
 G. Yang, I. N. Ivanov, R. E. Ruther, R. L. Sacci, V. Subjakova, D. T. Hallinan, and J. Nanda, ACS Nano 12 (2018), DOI: 10.1021/acsnano.8b05038.
 G. Yang, J. Nanda, B. Wang, G. Chen, and D. T. Hallinan, ACS Applied Materials & Interfaces 9 (2017), DOI: 10.1002/macp.201700417.
 G. Yang, R. L. Sacci, I. N. Ivanov, R. E. Ruther, K. A. Hays, Y. Zhang, P.-F. Cao, G. M. Veith, N. J. Dudney, T. Saito, D. T. Hallinan, and J. Nanda, J. Electrochem. Soc. 166 (2019), DOI: 10.1149/2.0391902jes.
 D. T. Hallinan Jr, A. Rausch, and B. McGill, Chem. Eng. Sci. 154 (2016), DOI: 10.1016/j.ces.2016.06.054.
 M. D. Berliner, B. C. McGill, M. Majeed, and D. T. Hallinan Jr., J. Electrochem. Soc. 166 (2019), DOI: 10.1149/2.0851902jes.
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