Johdepolymeerien ja niiden komposiittirakenteiden höyryfaasipolymerisaatio ja sovellutukset

Johdepolymeerien joustavuus yhdistettynä taipuviin substraatteihin antavat mahdollisuuden käyttää näitä materiaaleja esimerkiksi taivutettavissa superkondensaattoreissa, joissa niiden sähköiset ominaisuudet pääsevät myös hyötykäyttöön. Johdepolymeerejä ei ole vielä markkinoilla tällaisissa laitteissa, koska näiden polymeerien liukoisuus teollisiin liuottimiin ei ole hyvä. Höyryfaasipolymerisaatiossa liukoisuus ei kuitenkaan ole ongelma, koska monomeeri höyrystetään, jonka jälkeen se voidaan polymerisoida hapettimen läsnä ollessa kiinteään muotoon halutulle substraatille, joka voi olla myös pietsosähköinen. Höyryfaasipolymerisaation etuna on siis tämän lisäksi helppokäyttöisyys, muokattavuus ja sen tuloksena saadut ohuet, tasaiset ja ominaisuuksiltaan metodin parametrien määrittämät kalvot. Höyryfaasissa polymerisoitaessa substraattien ei periaatteessa tarvitse olla kaksiulotteisia, koska kaasu pystyy kulkeutumaan kolmiulotteisessakin rakenteessa. PEDOT omaa erityisesti hyvän sähkönjohtavuuden ja kun se yhdistetään grafeenioksidin hyvään kapasitanssiin, saadaan ainakin tietyiltä ominaisuuksiltaan parempi kalvo kuin mitä materiaalit ovat erikseen. Perustutkimus eri parametrien ja monomeerien vaikutuksista muodostuvaan kalvoon antavat myös paljon hyödyllistä tietoa esimerkiksi morfologian suhteen. PEDOT:n ja kenties myöhemmin myös komposiittien yhdistäminen pietsosähköisiin substraatteihin on sekin mielenkiintoinen tutkimuskohde, koska tällä tavalla voitaisiin saada lisää hukkaenergiaa hyötykäyttöön

Electrochemical characterization of redox activity and stability of various tris(2,2‘-bipyridine) derived complexes of iron(II) in aqueous solutions

Tris(2,2'-bipyridine) Fe(II) complexes with different 4,4'-placed substituents were studied electrochemically in aqueous solutions. Digital simulation of the experimental cyclic voltammograms enabled the evaluation of the redox potentials, electrochemical kinetics as well as complex stability. The substituent effect on the formal potential of the complexes was investigated, showing that electron-withdrawing substituents shift the formal potential to a positive direction from the potential of the unsubstituted [Fe(II)(bpy)3]2+ complex (0.875 V vs. Ag/AgCl). Respectively electron-donating substituents shift the formal potential to a negative direction. The most positive formal potential (0.97 V vs. Ag/AgCl) was obtained with 4,4'-dicarboxyl substituted and the lowest 0.56 V vs. Ag/AgCl with 4,4'-di-OMe substituted [Fe(II)(bpy)3]2+. We show here that the stability of the compounds in the oxidized form can be evaluated by voltammetry. None of the studied complexes was stable enough for flow battery applications, but knowledge of their decomposition rates was obtained via simulations, considering that all oxidized species undergo a chemical reaction, resulting in a loss of redox-active species. The counterion of the complex affected the solubility and stability of the complex, as the presence of tetrafluoroborate resulted in faster decomposition than the presence of sulfate. Battery testing of the most stable Fe(II) complex revealed a voltage drop upon discharge, lowering the energy efficiency. Battery cycling showed a capacity decay most likely related to the chemical reaction occurring to the oxidized species. Even though the studied complexes are not suitable for aqueous flow battery applications as such, knowledge of a substituent, counterion, and electrolyte effect on their performance is needed to develop these complexes further and to improve their stability via structural design. We show here that voltammetry is a suitable tool for fast initial evaluation of the stability of the materials.peerReviewe

Exploration of Vitamin B6-Based Redox-Active Pyridinium Salts towards the Application in Aqueous Organic Flow Batteries

We have examined the utility of pyridoxal hydrochloride, a vitamin B6 vitamer and a biobased feedstock, as a starting point towards organic redox flow battery materials. Pyridoxal hydrochloride was synthetically converted to a series of diverse vitamin B6-based redox-active benzoyl pyridinium salts. These compounds were electrochemically characterized through cyclic voltammetry (CV) measurements in neutral and basic aqueous electrolytes (1 M KCl and 0.1 M NaOH). Based on the CV results, which demonstrated reversibility under basic conditions, two of the most promising salts were subjected to laboratory-scale redox flow battery tests involving galvanostatic cycling at 10 mM with 0.1 M NaOH as the supporting electrolyte. These results showed that the battery was charged completely, corresponding to the transfer of two electrons to the electrolyte, but no discharge was observed. Both CV analysis and electrochemical simulations confirmed that the redox wave observed in the experimental voltammograms corresponds to a two-electron process. To explain the irreversibility in the battery tests, we conducted bulk electrolysis with the benzoyl pyridinium salts, affording the corresponding benzylic secondary alcohols. This process involves the transfer of two electrons and two protons to reduce the ketone group to alcohol. Computational studies suggest that the reduction proceeds in three consecutive steps: first electron transfer (ET), then proton-coupled electron transfer (PCET) and finally proton transfer (PT). 1H NMR deuterium exchange studies indicated that the last PT step is not reversible in 0.1 M NaOH, rendering the entire redox process irreversible. The apparent reversibility observed in CV at the basic media likely arises from the slow rate of the PT step at the timescale of the measurement