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Investigations of interfaces for electrochemical devices using tunable block copolymer electrolytes
Buggy, Nora C.
Buggy, Nora C.
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2021
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Abstract
Polymer electrolyte-based electrochemical energy conversion devices, such as electrolyzers or
fuel cells, are of interest for large scale hydrogen production and conversion, respectively. Much
eort has gone toward improving the anion exchange membrane (AEM), the solid polymer
electrolyte which separates the anode and cathode and transports anions and water across the
electrochemical device. In addition to the AEM, device performance is dictated in part by
sluggish reaction kinetics that can result from poorly formed structures in the electrode catalyst
layer (CL). The CL is comprised of a heterogeneous mixture of conductive supporting material,
catalyst particles, and anion exchange ionomer (AEI). Electrode design can be improved by
synergistically developing ionomer chemistry with fundamental knowledge of their interactions
with catalysts and supporting materials. The limited work carried out in this area has focused on
using platinum group metal catalysts, which can be replaced with cheaper more abundant
catalysts in AEM-based devices due to the enhanced reaction kinetics realized in base. Hence, in
this work, tunable block copolymer-based AEIs are developed and their interactions, structure
and performance with a silver catalyst are investigated.
First, a novel polyethylene-based block copolymer AEM was developed. The polycyclooctene
midblock of the ABA triblock copolymer
polychloromethylstyrene-b-polycyclooctene-b-polychloromethylstyrene (PCMS-b-PCOE-b-PCMS)
was hydrogenated to yield a polyethylene (PE) midblock. The new PE-based AEM had high ionic
conductivity, moderate water uptake, and decent alkaline stability. Notably, the mechanical
strength of the AEM improved in liquid water. X-ray scattering studies revealed that in liquid
water the PE backbone rearranges to form larger crystalline domains, leading to enhanced
mechanical properties.
Next, interactions between silver nanoparticles (AgNPs) and block copolymer-based ionomers
were investigated. This study utilized the PCMS-b-PCOE-b-PCMS triblock copolymer precursor
and ionomers derived from quaternization with trimethylamine or N-methylpiperidine. Using
FTIR, interactions with AgNPs were determined to occur more strongly with the phenyl groups,
vinyl groups, and the pendant quaternary ammonium cations (QACs). Changes in thermal characteristics and crystallinity were highly dependent on the QAC, uncovering differences in the
nature of the interactions between silver and trimethylammonium or methylpiperidinium.
Ionomer thin film morphology was characterized on a silver surface to model an idealized
catalyst interface using grazing incidence small angle X-ray scattering. This work utilized two
diblock and two triblock copolymer precursors of PCMS and polyisoprene (PIp). The morphology
of the block copolymer precursor was found to align vertically to the interface, but after
quaternization, the morphology became more disordered due to dipole-dipole interactions between
pendant QACs. Environmental studies were used to elucidate water uptake via changes in the
radius of gyration.
The final set of studies implemented a half-cell to study the kinetics of the oxygen evolution
reaction (OER) on electrodes coated in a silver-ionomer ink. Optimization of the ionomer
chemistry is realized through a series of backbone and QAC modifications. The best performing
electrode was integrated in a water electrolyzer with the PE-based AEM developed in the first
study. In the final chapter, the hypotheses and structure-property-performance relationship are
revisited, and future work is proposed.
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