The present work is devoted to the design, production, and extensive characterization of novel non-fluorinated self-assembling composite membranes conceived as potential competitors of Nafion®, the conventional electrolyte currently adopted in Proton Exchange Membrane Fuel Cells (PEMFCs). The materials selected as constituents were polybenzimidazole (PBI), graphene oxide (GO), and its functionalized derivatives, i.e., sulfonated GO (SGO) and phosphonated GO (PGO). In the last decade, PBI and GO have been largely studied in this field, both alone and combined, due to their spectrum of properties and intriguing versatility. Moreover, they would conform to recent European policies, which are tightening the use of fluorine-containing materials due to deep environmental concerns. Nonetheless, the coupling of these species complied always and only with PBI serving as a matrix and the GO-related moieties as low-content fillers supporting the proton transfer task, typically assigned to phosphoric acid, used to dope PBI. This research attempts to overturn such a practice by proposing the employment of contents of GO, SGO, or PGO ranging from 25 wt% to 75 wt%. Therefore, five different PBI-to-GO/SGO/PGO mass ratios, equal to 3:1, 2:1, 1:1, 1:2, and 1:3, were explored, fostering a step beyond other composite membranes of this kind by entrusting proton conduction features to the numerous oxygenated functionalities of GO and its analogues. In this fashion, doping with phosphoric acid, the deriving issues of potential leaching and corrosion, as well as intrinsic operational constraints, i.e., temperatures of activation larger than 150 °C, can be avoided. Consequently, the developed non-fluorinated composite membranes could also be an answer to the so-called “conductivity gap”, the specific temperature range, between 80 and 120 °C, where a shortage of adequate electrolytic materials exists. The influence of such an innovative combinatorial strategy on the morphological, microstructural, and compositional traits of the composite membranes was inferred from optical and scanning electron microscopies, XRD, ATR-FTIR, EDS, and XPS, whereas their thermo-mechanical behaviors were assessed through thermogravimetric experiments and tensile tests. Then, all samples were subjected to the evaluation of the typical functional features required for proton exchange membranes. In this regard, ion exchange capacity evaluation, water immersion tests, and electrochemical impedance spectroscopy were performed. The latter, conducted at controlled temperatures of 60, 80, 100, and 120 °C, enabled the determination of both in-plane and through-plane proton conductivities, as well as the deduction of the degree of anisotropy. The collected results, compared with the ones of pure PBI, GO, SGO, and PGO, highlighted the reciprocal compatibility and the effective combination of PBI and GO, which was not undermined by the presence of further functional groups in the cases of SGO and PGO. Specifically, the three batches of samples indicated an almost even distribution of morphological and compositional properties that conformed to the innovative mass ratios. In all cases, PBI enhanced the thermal resistance of the self-assembling composite membranes and allowed them to easily sustain an aqueous environment. On the other side, GO/SGO/PGO raised the water sorption ability of the samples and, especially, empowered proton conductivity, even at temperatures that forbid the use of Nafion®. Such an effect was particularly stressed by the introduction of sulfonic and phosphonic acid groups without excessively altering the anisotropy of the samples. Nonetheless, the peculiar combination of materials appeared to be unsatisfactory from the mechanical standpoint. The elaboration of the experimental outcomes allowed to confirm the reliability of the designed preparation procedures, but also to comprehend that a further optimization of the formulations is necessary to achieve non-fluorinated composite membranes able to overcome Nafion® as the electrolyte of PEMFCs.
Questo lavoro è dedicato alla progettazione, produzione e caratterizzazione dettagliata di membrane composite auto-assemblanti non-fluorurate innovative concepite come potenziali concorrenti del Nafion®, l’elettrolita convenzionale attualmente adottato nelle Celle a Combustibile con Membrana a Scambio Protonico (PEMFCs). I materiali scelti come costituenti sono stati il polibenzimidazolo (PBI), l’ossido di grafene (GO) e i suoi derivati funzionalizzati, quali l’ossido di grafene solfonato (SGO) e fosfonato (PGO). Nello scorso decennio, PBI e GO sono stati studiati ampiamente in questo campo, sia individualmente che insieme, grazie alle loro caratteristiche e versatilità. Inoltre, si conformerebbero alle recenti politiche Europee, che stanno restringendo l’utilizzo di materiali a base fluoro in virtù di profonde preoccupazioni ambientali. Tuttavia, l’accoppiamento di queste specie ha previsto sempre e solo PBI come matrice e la porzione legata al GO come riempitivo a basso contenuto a supporto del trasferimento protonico, tipicamente assicurato dall’acido fosforico dopante. Questo studio tenta di ribaltare tale pratica proponendo l’utilizzo di contenuti di GO, SGO o PGO tra il 25 wt% e il 75 wt%. Pertanto, cinque diversi rapporti in massa tra PBI e GO/SGO/PGO, pari a 3:1, 2:1, 1:1, 1:2 e 1:3, sono stati investigati, favorendo un passo oltre comparabili membrane composite attraverso l’affidamento della conducibilità protonica alle numerose funzionalità ossigenate di GO e dei suoi analoghi. In questa maniera sarebbe possibile evitare la prassi del doping con acido fosforico, i derivanti problemi di possibile lisciviazione e corrosione, e i vincoli operazionali intrinseci (temperature di attivazione maggiore di 150 °C). Parallelamente, le nuove membrane composite non-fluorurate potrebbero rappresentare una risposta al cosiddetto “gap di conducibilità”, lo specifico intervallo di temperatura, tra 80 e 120 °C, in cui si osserva una carenza di materiali elettrolitici adeguati. L’influenza dell’innovativa strategia combinatoria sulle proprietà morfologiche, microstrutturali e composizionali delle membrane composite è stata dedotta attraverso la microscopia ottica ed elettronica a scansione, XRD, ATR-FTIR, EDS e XPS, mentre il comportamento termo-meccanico è stato indagato mediante esperimenti TGA e test di trazione. Successivamente, tutti i campioni sono stati sottoposti alla valutazione delle tipiche proprietà funzionali richieste a delle membrane a scambio protonico. In tal senso, sono state eseguite prove di capacità di scambio ionico, di immersione in acqua, e di spettroscopia di impedenza elettrochimica. Queste ultime, condotte a temperature controllate di 60, 80, 100 e 120 °C, hanno permesso di determinare la conducibilità protonica sia lungo il piano che trasversalmente ad esso, così da identificare il corrispettivo fattore di anisotropia. I dati collezionati e il confronto con campioni di riferimento di PBI, GO, SGO e PGO puri hanno evidenziato la reciproca compatibilità e l’effettiva combinazione di PBI e GO, un aspetto che non è stato indebolito dalla presenza di ulteriori gruppi funzionali in SGO e PGO. Nel dettaglio, i campioni dei tre set hanno indicato una distribuzione di caratteristiche morfologiche e composizionali sufficientemente conforme alla variazione degli innovativi rapporti in massa. In tutti i casi, PBI ha elevato la resistenza termica e ad ambienti acquosi delle membrane composite auto-assemblanti. D’altro canto, GO/SGO/PGO ne hanno potenziato la tendenza di assorbimento e, specialmente, la conducibilità protonica, persino a temperature che vieterebbero l’uso del Nafion®. Quest’ultimo effetto è stato particolarmente enfatizzato dall’introduzione di gruppi solfonici e fosfonici, senza alterare eccessivamente l’anisotropia dei campioni. Ciononostante, la peculiare combinazione di materiali è sembrata insoddisfacente dal punto di vista meccanico. L’elaborazione dei risultati sperimentali ha permesso di certificare l’affidabilità delle procedure progettate, ma anche di capire la necessità di ottimizzazione delle formulazioni per ottenere membrane composite non-fluorurate capaci di spodestare il Nafion® come elettrolita di una PEMFC.
Novel non-fluorinated self-assembling composite membranes as potential electrolytes for alternative energy generators
Di VIRGILIO, MATTEO
2024/2025
Abstract
The present work is devoted to the design, production, and extensive characterization of novel non-fluorinated self-assembling composite membranes conceived as potential competitors of Nafion®, the conventional electrolyte currently adopted in Proton Exchange Membrane Fuel Cells (PEMFCs). The materials selected as constituents were polybenzimidazole (PBI), graphene oxide (GO), and its functionalized derivatives, i.e., sulfonated GO (SGO) and phosphonated GO (PGO). In the last decade, PBI and GO have been largely studied in this field, both alone and combined, due to their spectrum of properties and intriguing versatility. Moreover, they would conform to recent European policies, which are tightening the use of fluorine-containing materials due to deep environmental concerns. Nonetheless, the coupling of these species complied always and only with PBI serving as a matrix and the GO-related moieties as low-content fillers supporting the proton transfer task, typically assigned to phosphoric acid, used to dope PBI. This research attempts to overturn such a practice by proposing the employment of contents of GO, SGO, or PGO ranging from 25 wt% to 75 wt%. Therefore, five different PBI-to-GO/SGO/PGO mass ratios, equal to 3:1, 2:1, 1:1, 1:2, and 1:3, were explored, fostering a step beyond other composite membranes of this kind by entrusting proton conduction features to the numerous oxygenated functionalities of GO and its analogues. In this fashion, doping with phosphoric acid, the deriving issues of potential leaching and corrosion, as well as intrinsic operational constraints, i.e., temperatures of activation larger than 150 °C, can be avoided. Consequently, the developed non-fluorinated composite membranes could also be an answer to the so-called “conductivity gap”, the specific temperature range, between 80 and 120 °C, where a shortage of adequate electrolytic materials exists. The influence of such an innovative combinatorial strategy on the morphological, microstructural, and compositional traits of the composite membranes was inferred from optical and scanning electron microscopies, XRD, ATR-FTIR, EDS, and XPS, whereas their thermo-mechanical behaviors were assessed through thermogravimetric experiments and tensile tests. Then, all samples were subjected to the evaluation of the typical functional features required for proton exchange membranes. In this regard, ion exchange capacity evaluation, water immersion tests, and electrochemical impedance spectroscopy were performed. The latter, conducted at controlled temperatures of 60, 80, 100, and 120 °C, enabled the determination of both in-plane and through-plane proton conductivities, as well as the deduction of the degree of anisotropy. The collected results, compared with the ones of pure PBI, GO, SGO, and PGO, highlighted the reciprocal compatibility and the effective combination of PBI and GO, which was not undermined by the presence of further functional groups in the cases of SGO and PGO. Specifically, the three batches of samples indicated an almost even distribution of morphological and compositional properties that conformed to the innovative mass ratios. In all cases, PBI enhanced the thermal resistance of the self-assembling composite membranes and allowed them to easily sustain an aqueous environment. On the other side, GO/SGO/PGO raised the water sorption ability of the samples and, especially, empowered proton conductivity, even at temperatures that forbid the use of Nafion®. Such an effect was particularly stressed by the introduction of sulfonic and phosphonic acid groups without excessively altering the anisotropy of the samples. Nonetheless, the peculiar combination of materials appeared to be unsatisfactory from the mechanical standpoint. The elaboration of the experimental outcomes allowed to confirm the reliability of the designed preparation procedures, but also to comprehend that a further optimization of the formulations is necessary to achieve non-fluorinated composite membranes able to overcome Nafion® as the electrolyte of PEMFCs.| File | Dimensione | Formato | |
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2025, PhD Thesis_Di Virgilio.pdf
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https://hdl.handle.net/10589/239870