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Lithium-ion technologies show great promise to meet the demands that the transition towards renewable energy sources and the electrification of the transport sector put forward. ...However, concerns regarding lithium-ion batteries, including limited material resources, high energy consumption during production, and flammable electrolytes, necessitate research on alternative technologies for electrochemical energy storage. Organic materials derived from abundant building blocks and with tunable properties, together with water based electrolytes, could provide safe, inexpensive and sustainable alternatives. In this study, two conducting redox polymers based on poly(3,4-ethylenedioxythiophene) (PEDOT) and a hydroquinone pendant group have been synthesized and characterized in an acidic aqueous electrolyte. The polymers were characterized with regards to kinetics, pH dependence, and mass changes during oxidation and reduction, as well as their conductance. Both polymers show redox matching, i.e. the quinone redox reaction occurs within the potential region where the polymer is conducting, and fast redox conversion that involves proton cycling during pendant group redox conversion. These properties make the presented materials promising candidates as electrode materials for water based all-organic batteries.
Li-ion batteries (LIBs) raise safety and environmental concerns, which mostly arise from their toxic and flammable electrolytes and the extraction of limited material resources by mining. Recently, ...water-in-salt electrolytes (WiSEs), in which a large amount of lithium salt is dissolved in water, have been proposed to allow for assembling safe and high-voltage (>3.0 V) aqueous LIBs. In addition, organic materials derived from abundant building blocks and their tunable properties could provide safe and sustainable replacements for inorganic cathode materials. In the current work, the electrochemical properties of a conducting redox polymer based on poly(3,4-ethylenedioxythiophene) (PEDOT) with hydroquinone (HQ) pendant groups have been characterized in WiSEs. The quinone redox reaction occurs within the potential region where the polymer is conducting, and fast redox conversion that involves lithium cycling during pendant group redox conversion was observed. These properties make conducting redox polymers promising candidates as cathode-active materials for safe and high-energy aqueous LIBs. An organic-based aqueous LIB, with a HQ-PEDOT as a cathode, Li4Ti5O12 (LTO) as an anode, and ca. 15 m lithium bis(trifluoromethanesulfonyl)imide water/dimethyl carbonate (DMC) as electrolyte, yielded an output voltage of 1.35 V and high rate capabilities up to 500C.
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•A hybrid organic lithium ion battery using a water-in-salt electrolyte (WISE)•Electrochemical characterization of a conducting redox polymer (CRP) in WiSEs.•The battery yielded a voltage of 1.35 V and high rate capabilities, up to 500C.•Quinones can be forced to cycle lithium in water electrolyte.
Quinones have a capacity for high energy storage and exhibit facile and reversible electrochemistry in several widely different electrolytes. They are, therefore, one of the most popular compounds ...currently used in organic materials based electrical energy storage. Quinone electrochemistry is, however, strongly affected by the composition of the electrolyte. This report summarizes our systematic investigation of the redox chemistry of a series of quinones with electron-withdrawing and electron-donating substituents in aqueous solution and in acetonitrile (MeCN) with tetrabutylammonium (TBA+)-, Li+-, and H+-based electrolytes. As a general trend, proton cycling, TBA+ cycling, and Li+ cycling resulted in the highest, the lowest, and intermediate redox potentials, respectively. We attribute this trend to stabilization of the reduced state, namely benzene-1,4-bis(olate) (Q2–), by the different counterions. Density functional theory (DFT) calculations showed that, in the fully reduced state, two Li+ counterions accommodated 35% of the injected electron charges while proton counterions accommodated 69% of the injected charge, thus significantly stabilizing the reduced state. However, with the bulky TBA+ as the cycling ion, this stabilization was not possible and the reduction potential was decreased. In addition, we showed that stabilization of the counterion also affected the Coulombic interaction between the successively injected charges, resulting in the well-known disproportionation of the semiquinone radical intermediate state with proton cycling, while Li+ and TBA+ cycling generally resulted in two consecutive redox reactions. Finally, we showed that the electrolyte strongly influences the effects of substitution with electron-donating and electron-withdrawing substituents. A strong relationship between the redox potential and the electron-withdrawing power of the substituent was observed in the MeCN solution. However, this relationship was completely lost in aqueous solution. The reason for the loss of the relationship was addressed using a DFT explicit-solvent model and is discussed.
Utilizing organic redox-active materials as electrodes is a promising strategy to enable innovative battery designs with low environmental footprint during production, which can be hard to achieve ...with traditional inorganic materials. Most electrode compositions, organic or inorganic, require binders for adhesion and conducting additives to enable charge transfer through the electrode, in addition to the redox-active material. Depending on the redox-active material, many types and combinations of binders and conducting additives have been considered. We designed a conducting polymer (CP), with a soluble, trimeric unit based on 3,4-ethylenedioxythiophene (E) and 3,4-propylenedioxythiophene (P) as the repeat unit, acting as a combined binder and conducting additive. While CPs as additives have been explored earlier, in the current work, the use of a trimeric precursor enables solution processing together with the organic redox-active material. To evaluate this concept, the CP was blended with a redox polymer (RP), which contained a naphthoquinone (NQ) redox group at different ratios. The highest capacity for the total weight of the CP/RP electrode was 77 mAh/g at 1 C in the case of 30% EPE and 70% naphthoquinone-substituted poly(allylamine) (PNQ), which is 70% of the theoretical capacity given by the RP in the electrode. We further used this electrode in an aqueous battery, with a MnSO4 cathode. The battery displayed a voltage of 0.95 V, retaining 93% of the initial capacity even after 500 cycles at 1 C. The strategy of using a solution-processable CP precursor opens up for new organic battery designs and facile evaluation of RPs in such.
Organic materials receive increasing attention as environmentally benign and sustainable electrode‐active materials. We present a conducting redox polymer (CRP) based on ...poly(3,4‐ethylenedioxythiophene) with naphthoquinone pendant group, which is formed from a stable suspension of a trimeric precursor and an oxoammonium cation as oxidant. This suspension allows us to easily coat the polymer onto a current collector, opening up use of roll‐to‐roll processing or ink‐jet printing for electrode preparation. The CRP showed a full capacity of 76 mAh g−1 even at a high C rate of 100 C in acidic aqueous electrolyte. These properties make the CRP a promising candidate as anode‐active material; a polymer–air secondary battery was fabricated with the CRP as anode, a conventional Pt/C catalyst as cathode, and sulfuric acid aqueous solution as electrolyte. This battery yielded a discharge voltage of 0.50 V and showed good cycling stability with 97 % capacity retention after 100 cycles and high rate capabilities up to 20 C.
Breathing air into polymers: A polymer–air secondary battery was fabricated with the conducting redox polymer as anode, a conventional Pt/C catalyst as cathode, and sulfuric acid aqueous solution as electrolyte. This battery demonstrated good cycling stability with 97 % capacity retention after 100 cycles and high rate capabilities up to 20 C.
Redox-active covalent organic frameworks (RACOFs) can be employed in various functional materials and enesrgy applications. A crucial performance or efficiency indicator is the percentage of redox ...centres that can be utilised. Herein, the term redox-site accessibility (RSA) is defined and shown to be an effective metric for developing and optimising a 2D RACOF (
viz.
, TpOMe-DAQ made from 2,4,6-trimethoxy-1,3,5-benzenetricarbaldehyde TpOMe and 2,6-diaminoanthraquinone DAQ) as an anode material for potential organic-battery applications. Pristine TpOMe-DAQ utilises only 0.76% of its redox sites, necessitating the use of conductivity-enhancement strategies such as blending it with different conductive carbons, or performing
in situ
polymerisation with EDOT (3,4-ethylenedioxythiophene) to form a conductive polymer. While conductive carbon-RACOF composites showed a modest RSA improvement of 4.0%, conductive polymer-RACOF composites boosted the redox-site usage (RSA) to 90% at low mass loadings. The material and electrochemical characteristics of the conductive polymer-RACOF composite containing more-than-necessary conductive polymer showed a reduced surface area but almost identical electrochemical behaviour, compared to the optimal ratio. The high RSA of the optimally loaded composite was replicated in a RACOF-air battery with over 90% active redox sites. We believe that the reported approach and methods, which can be employed on a milligram scale, could serve as a general guide for the electrification and characterisation of RACOFs, as well as for other redox-active porous polymers.
A systematic method is presented which demonstrates how accessing more redox-active sites in a poorly conducting 2D COF can be done in a rational manner. An optimised and dramatically improved charge-storage composite was produced using this method.
A new qualitative model for estimating the properties of substituted cyclopentadienes and siloles in their lowest ππ* excited states is introduced and confirmed through quantum chemical calculations, ...and then applied to explain earlier reported experimental excitation energies. According to our model, which is based on excited‐state aromaticity and antiaromaticity, siloles and cyclopentadienes are cross‐hyperconjugated “aromatic chameleons” that adapt their electronic structures to conform to the various aromaticity rules in different electronic states (Hückel’s rule in the π2 electronic ground state (S0) and Baird’s rule in the lowest ππ* excited singlet and triplet states (S1 and T1)). By using pen‐and‐paper arguments, one can explain polarity changes upon excitation of substituted cyclopentadienes and siloles, and one can tune their lowest excitation energies by combined considerations of ground‐ and excited‐state aromaticity/antiaromaticity effects. Finally, the “aromatic chameleon” model can be extended to other monocyclic compound classes of potential use in organic electronics, thereby providing a unified view of the S0, T1, and S1 states of a range of different cyclic cross‐π‐conjugated and cross‐hyperconjugated compound classes.
Willing to change: Siloles and cyclopentadienes are cross‐hyperconjugated “aromatic chameleons” that adapt to the different aromaticity rules (Hückel's versus Baird's rules) for π2 versus ππ* states (see figure). It demonstrated that this concept is potentially useful in the design of new building blocks for organic electronics.
A conducting redox polymer (CPR) based on pyrrole with a hydroquinone pendant group was synthesized through electropolymerization of the corresponding monomer. The formal potential (E 0′) in aqueous ...solution at different pH as well as in MeCN containing equal amounts of pyridinium-triflates and the corresponding free pyridine with different pK a was investigated. E 0′ could be completely recovered in MeCN, and by utilizing pyridine bases with different donor–acceptor strengths, a decrease of 61 meV/pK a was found that corresponded exactly to the pH dependence of E 0′ in aqueous electrolyte. To separate the entropic and enthalpic contributions to E 0′, temperature-dependent electrochemistry was performed. Two different modes of operation with changing pH/pK a between the two media were revealed. In MeCN, E 0′ varies only because of the enthalpic contribution as the entropic contribution is unaffected by change in pK a. In water, there is primarily an entropic contribution to E 0′ with changing pH due to solvation of the proton. The presented results are expected to open up for new design possibilities of CRPs based on ion-coordinating redox groups for electrical energy storage.
An organic cathode material based on a copolymer of poly(3,4‐ethylenedioxythiophene) containing pyridine and hydroquinone functionalities is described as a proton trap technology. Utilizing the ...quinone to hydroquinone redox conversion, this technology leads to electrode materials compatible with lithium and sodium cycling chemistries. These materials have high inherent potentials that in combination with lithium give a reversible output voltage of above 3.5 V (vs Li0/+) without relying on lithiation of the material, something that is not showed for quinones previously. Key to success stems from coupling an intrapolymeric proton transfer, realized by an incorporated pyridine proton donor/acceptor functionality, with the hydroquinone redox reactions. Trapping of protons in the cathode material effectively decouples the quinone redox chemistry from the cycling chemistry of the anode, which makes the material insensitive to the nature of the electrolyte cation and hence compatible with several anode materials. Furthermore, the conducting polymer backbone allows assembly without any additives for electronic conductivity. The concept is demonstrated by electrochemical characterization in several electrolytes and finally by employing the proton trap material as the cathode in lithium and sodium batteries. These findings represent a new concept for enabling high potential organic materials for the next generation of energy storage systems.
A new technology for organic energy storage, referred to as the proton trap, is described and exemplified in a novel organic cathode material based on a poly(3,4‐ethylenedioxythiophene)‐based copolymer with hydroquinone side groups. The quinone/hydroquinone redox conversion is coupled to an intrapolymeric proton transfer, which makes the material compatible with a multitude of cycling chemistries, both organic and inorganic.
Thirty two differently substituted siloles
-
and 1,4-disilacyclohexa-2,5-dienes
-
were investigated by quantum chemical calculations using the PBE0 hybrid density functional theory (DFT) method. The ...substituents included σ-electron donating and withdrawing, as well as π-electron donating and withdrawing groups, and their effects when placed at the Si atom(s) or at the C atoms were examined. Focus was placed on geometries, frontier orbital energies and the energies of the first allowed electronic excitations. We analyzed the variation in energies between the orbitals which correspond to HOMO and LUMO for the two parent species, here represented as Δε
, motivated by the fact that the first allowed transitions involve excitation between these orbitals. Even though Δε
and the excitation energies are lower for siloles than for 1,4-disilacyclohexa-2,5-dienes the latter display significantly larger variations with substitution. The Δε
of the siloles vary within 4.57-5.35 eV (ΔΔε
= 0.78 eV) while for the 1,4-disilacyclohexa-2,5-dienes the range is 5.49-7.15 eV (ΔΔε
= 1.66 eV). The excitation energy of the first allowed transitions display a moderate variation for siloles (3.60-4.41 eV) whereas the variation for 1,4-disilacyclohexa-2,5-dienes is nearly doubled (4.69-6.21 eV). Cyclobutadisiloles combine the characteristics of siloles and 1,4-disilacyclohexa-2,5-diene by having even lower excitation energies than siloles yet also extensive variation in excitation energies to substitution of 1,4-disilacyclohexa-2,5-dienes (3.47-4.77 eV, variation of 1.30 eV).
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