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A Mechanistic Interpretation for Charge Storage in Conducting Polymers

Northcutt, Robert G
http://rave.ohiolink.edu/etdc/view?acc_num=osu1449157903

Abstract Details

2015, Doctor of Philosophy, Ohio State University, Mechanical Engineering.
This research focuses on the characterization of active redox sites in p-doped conducting polymers (CP) and applies this knowledge to develop a mechanistic interpretation for charge storage in CPs. The scientific goal of this thesis is to develop a novel understanding of ion transport and associated mechanics in CPs and leverage this knowledge to design electrodes with templating techniques that can produce polypyrrole-based membranes with charge storage capacity near its theoretical limit and yet undergo minimal mechanical strain during charging and discharging cycles. The specific translation of ion transport into mechanical deformation is analyzed using static and dynamic characterization techniques. The polypyrrole (PPy) membranes from this research can be fabricated into a flexible supercapacitor to demonstrate its advantages in energy storage applications. The hypothesis of this research is that polypyrrole membranes fabricated using phospholipid vesicles as soft-templates have higher specific surface area/volume (SA:V) ratio and decreased density, resulting in reduced mechanical strain. Additionally it is proposed that morphology effects ion ingress/egress and distribution throughout the polymer matrix. The higher SA:V ratio and improved ion transport kinetics is expected to lead to higher storage capacity, smaller strain during charging/discharging, and increased operational lifetime. The higher charge storage capacity of phospholipid-templated polypyrrole membranes would manifest itself as higher specific capacitance and o er systemic advantages as sensors and battery/supercapacitor electrodes. The development of this research hypothesis is based on active redox sites in a conducting polymer. A redox site is defined as a binding location for a single cation in an anion-doped CP membrane, and the number of redox sites directly represents the theoretical maximum number of cations the CP can store. This maximum cannot be achieved in thick polymer membranes as the polymer bulk obstructs ion transport into and out of the redox sites. Thus, the total accessibility of redox sites to ion ingress/egress determines the experimentally feasible number of charges that can be stored in a CP. Since charge storage is typically expressed in terms of its speci c capacitance, the number of redox sites can be used to compute the theoretical maximum for specific capacitance. Additionally, the number of accessible redox sites denotes practically achievable specific capacitance of a CP membrane. Based on this mechanistic interpretation, the ratio of accessible active redox sites to the total number of redox sites is defined in this thesis as the `filling efficiency' (denoted as Greek Phi) of the polymer membrane. The ability to directly calculate the number of ions moving into and out of a CP membrane while simultaneously measuring mechanical response allows for the calculation of a `chemomechanical coupling coefficient'. This coefficient describes the ion transport-dependent mechanical strain and can be used as a metric for optimizing total system strain based on its intended application (actuation (or) energy storage). Therefore filling efficiency, specific capacitance and chemomechanical coupling coefficient provide a novel set of metrics to determine the suitability of a CP as an energy storage electrode. These metrics quantify the effect of nanostructuring on electrode performance in energy storage. Contemporary knowledge does not account for redox sites and their influence on charge storage capacity of CPs. The inability to determine filling efficiency was limited by a lack of understanding of the polymerization mechanism, which has been expanded on through this work. Further, the ability to experimentally determine ion transport dependent strain has been limited by a lack of contemporary techniques to map surface topography and charge storage simultaneously. The limitation has been overcome in this research through the use of correlated electrochemical characterization (CA/CV) and shear force (SF) imaging. These new developments, combined with nanostructuring techniques developed through this thesis allows for the validation of the hypothesis and leads to the successful accomplishment of the scientific goals. The research objectives of this thesis were as follows: -Fabrication of nanostructured PPy-based membranes via phospholipid templating -Characterization of specific capacitance in PPy-based membranes and the influence of electrolyte concentration, membrane thickness, and nanostructuring conditions -Characterize static and dynamic mechanical response of PPy-based membranes during reversible charge storage -Develop a SECM-SF electrode for high- fidelity imaging of CP membranes (nm, pA accuracy) -Develop a methodology for measuring dynamic response (charge, actuation) of PPy-based membranes during redox processes with high- fidelity These research goals will be accomplished through the following tasks: Fabrication -Electropolymerize PPy in the vicinity of phospholipid vesicles -Verify increased SA:V ratio with SEM -Microfabricate SECM-SF high fidelity electrode with direct, nm scale measurement of strain Characterization -Perform CV and calculate specific capacitance for a range of electrolyte concentrations (50mM-500mM NaCl), membrane thickness (0.1-2 C/cm¿2) -Use SECM-SF electrode to perform CA on PPy-based membranes at oxidation and reduction potentials while measuring ion transport in membrane and volumetric expansion simultaneously -Produce high-resolution SF-imaging of PPy membrane surface to improve chemomechanical coupling coefficient accuracy and allow for high-resolution 3D surface mapping -Perform dynamic characterization techniques (introduced as Z-tracking) which simultaneously measures system input (redox potentials) and system output (charge and actuation response) and analyze the system poles and residues as a function of aerial density. This research has led to the following original contributions: -A mechanistic model for charge storage in CPs -A nanotemplated PPy membrane with phospholipid vesicle soft-templates -A characterization procedure to simultaneously measure localized electrochemical and mechanical response in CP membranes with high fidelity -A direct correlation between nanostructuring, charge storage, and volumetric expansion through a chemomechanical coupling coefficient with high accuracy -A unique measurement technique (Z-tracking) for dynamic measurement of mechanoelectrochemistry in materials which undergo faradaic processes -A dynamic analysis of PPy-based membranes with a mathematical model describing actuation and charge storage phenomena as a function of the system poles and residues
Vishnu-Baba Sundaresan, Ph.D. (Advisor)
Marcello Canova, Ph.D. (Committee Member)
Carlos Castro, Ph.D. (Committee Member)
Anne Co, Ph.D. (Committee Member)
185 p.
Mechanical Engineering
Conducting Polymer; Polypyrrole; Mechanoelectrochemistry; SECM; SF; Vesicle-Templating

Recommended Citations

Citations

  • Northcutt, R. G. (2015). A Mechanistic Interpretation for Charge Storage in Conducting Polymers [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1449157903

    APA Style (7th edition)

  • Northcutt, Robert. A Mechanistic Interpretation for Charge Storage in Conducting Polymers. 2015. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1449157903.

    MLA Style (8th edition)

  • Northcutt, Robert. "A Mechanistic Interpretation for Charge Storage in Conducting Polymers." Doctoral dissertation, Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1449157903

    Chicago Manual of Style (17th edition)

osu1449157903
307
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This open access ETD is published by The Ohio State University and OhioLINK.