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Electrochemical water technologies for resource recovery using polymer electrolytes
Wu, Lipin (Ivy)
Wu, Lipin (Ivy)
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2023
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Abstract
Electrification of water technologies is needed to overcome the challenges of our current linear water economy amid climate change, aging infrastructure, and intensified water demand. Technologies such as electrochemical nutrient recovery and water electrolysis are promising methods to obtain valuable products from typically unutilized streams. For nutrient recovery, agricultural or municipal waste are wasted resources harboring valuable sustainable phosphorous and nitrogen, which can be separated by precipitation as struvite. However, work is needed to transition from chemical precipitation of nutrients to electrochemical precipitation, especially at a wide range of pH values. For water electrolysis, new anion exchange materials need to be developed with both high conductivity and durability for thousands of hours of operation. In this work, electrochemical nutrient recovery at acidic and neutral pH conditions were explored to elucidate critical factors governing struvite recovery outside of alkaline environments. Then, a family of highly tunable anion exchange triblock copolymers were designed and their tunability demonstrated in electrodialysis and single-cell anion exchange membrane water electrolysis (AEMWE).
First, coupling of electrochemical struvite precipitation with hydrogen production was investigated under acidic conditions. Acidic conditions are unfavorable for struvite precipitation, but by adjusting temperature, convection, and cell design, ammonium-deficient solids were produced at high temperatures while the competition between ion migration and bulk convection controlled struvite purity at intermediate temperatures by affecting local ion concentrations. Hydrogen was captured and was not found to correlate with struvite recovered. Furthermore, separation of the anode and cathode via a polymer electrolyte forms a N-heavy liquid stream which has potential to enhance overall recovery by concentrating unprecipitated N.
Amelogenin peptides were then explored to enhance electrochemical struvite precipitation at neutral pH. The peptides, anchored to a substrate, were found to facilitate Mg2+ ions to arrange in a conformation favorable for precipitation and increase overall struvite recovery. A change in crystal morphology from polyhedral to dendritic in the presence of peptides indicated a shift in the struvite crystallization mechanism. The peptides alter the local supersaturation near the crystals, resulting in elongated growth and overall higher struvite recovery.
Next, a triblock copolymer of polychloromethylstyrene-b-polyethylene-b-polychloromethylstyrene (PCMS-b-PE-b-PCMS) was developed with ease of processing and high tunability, and its electrodialysis performance studied as a function of the hydrophilic PCMS block length. It was demonstrated that the triblock copolymer anion exchange membrane could be tuned for selectivity by decreasing the hydrophilic PCMS block, resulting in lower water uptake and conductivity. However, heterogeneous distribution of water forming a narrow water region in the center of the pore enhances interaction between fixed cations of the membrane with free anions, enabling full ionic dissociation to give similar transport properties to that of more conductive AEMS containing greater quantities of PCMS. Therefore, controlling water properties by altering polymer chemistry increases permselective effects of AEMs containing low hydrophilic PCMS blocks.
The promising triblock copolymer material was then investigated for AEM water electrolysis. A series of triblock copolymers were designed with increasing hydrophobic PE content to balance durability with performance. The morphology of the triblock material shows phase separation between the hydrophilic cations from the hydrophobic backbone. Similar to the case in the previous study, a heterogeneously distributed water channel at the hydrophilic ends and nonwetted backbone enables full anionic dissociation with similar activation energies of conduction for all AEMs. The triblock ionomer and AEMs were evaluated in a single-cell electrolyzer at 50°C and 1 M potassium carbonate with high PE containing AEMs improving durability by three orders of magnitude. Post-mortem analysis showed the water molecules become more tightly bound to the polymer after 50 h of constant current electrolysis. This study highlights the utility of PE to provide improved durability with little sacrifices to performance. All together, these results show the breadth of water characteristics the PCMS-b-PE-b-PCMS block copolymer can exhibit based on simple synthesis changes.
Lastly, an idealized scenario was presented where nutrient recovery could occur onsite with the effluent feeding into a water electrolyzer to produce fertilizer and hydrogen, all powered by renewable energy. Recommendations for future work needed to realize this scenario are proposed.
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