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Efforts for durability enhancement for polymer electrolyte membranes and electrochemical devices
Kim, ChulOong "Christoph"
Kim, ChulOong "Christoph"
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2023
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Abstract
Global warming and climate change are urgent global challenges driven by the notable increase in atmospheric carbon dioxide (CO2) resulting from fossil fuel combustion. International agreements, such as the Paris Agreement and the Kyoto Protocol, signify collective efforts to curtail greenhouse gas emissions and combat global warming. To achieve near-zero net CO2 emissions, transitioning to renewable energy sources, particularly hydrogen, is pivotal. Hydrogen offers promise as an energy carrier and industrial feedstock, with applications ranging from methanol and ammonia production, steel manufacturing, and using hydrogen as an energy carrier for fuel cell device applications. However, establishing a hydrogen-based energy grid necessitates prioritizing 'green hydrogen' production, generated using renewable energy sources, and development of efficient and economically feasible hydrogen-based energy devices, such as fuel cells and electrolyzers. For economic feasibility of the hydrogen economy, significant enhancement in durability coupled with performance of the electrochemical devices is crucial.
In this study, efforts were made to enhance the chemical durability of proton exchange membrane fuel cells (PEMFCs) through the utilization of a heteropoly acid functionalized perfluorosulfonic acid (PFSA-VDF-HPA) membrane. A composite membrane was fabricated by applying the doctor-blade method onto a Kapton® liner, blending PFSA-VDF-HPA and 3M-PFSA. Notably, the density of HPA particles varied between the two sides of the membrane, with the liner-side exhibiting a higher concentration. When the HPA-dense liner-side was integrated into a fuel cell, specifically facing the anode, a significant improvement in chemical durability was observed under accelerated stress conditions (AST), utilizing a 25 cm2 active area fuel cell.
To delve deeper into the properties of the PFSA-VDF-HPA membrane, four variations were synthesized by controlling the HPA loading through adjustments in the VDF anchoring group ratio of the starting polymer. Additionally, expanded-polytetrafluoroethylene (ePTFE) support was introduced to create reinforced membranes, to fabricate thin membranes (ca. 12μm), to mitigate mechanical degradation and the risk of electrical short circuits during chemical durability tests. The inclusion of ePTFE led to a more crystalline polymer structure. However, despite increased crystallinity, the synergistic effects of sulfonic acid and HPA functionalities contributed to higher water uptake and consequently, improved proton conductivity. Chemical durability testing was conducted under AST conditions with a larger active area (50 cm2). The results indicated that HPA loading was not the sole determinant of chemical durability enhancement, but an overall improvement was evident.
In the final phase of this study, the performance and durability of anion exchange membrane water electrolyzers (AEMWE) were explored with a focus on transitioning away from precious platinum group metal catalysts and fluorinated polymers that are often produced with carcinogenic solvents. Three transition metal oxides—Co3O4, Mn2O3, and MnO2—with low kinetic overpotential and high catalytic activity were selected for the oxygen evolution reaction side of the AEMWE. The addition of these electrocatalysts improved kinetics and performance, although a limitation arose due to the maximum loading capacity of the catalysts. This limitation stemmed from the transition metal oxides' lack of electrical conductivity, effectively acting as insulators. Subsequently, 100 h durability tests were conducted at 750 mA/cm2 and 50˚C for each catalyst, revealing conditioning of the membrane electrode assembly (MEA) for all samples, with Co3O4 and MnO2 exhibiting more pronounced conditioning effects. No signs of membrane thinning nor statistically meaningful ion-exchange capacity were registered. Comparing before and after durability test electrochemical impedance spectroscopy (EIS) measurements and Fourier Transform Infrared Spectroscopy (FTIR) mapping indicated micron-sized rearrangements in the bulk membrane, with Co3O4 and MnO2 showing more significant changes compared to minimal alterations in Mn2O3. Furthermore, a shift in the OH peak in FTIR post-test suggested that water became less tightly hydrogen-bound to the polymer after 100 h operation.
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