Solution-Processable Redox-Active Polymers of Intrinsic Microporosity for Electrochemical Energy Storage

A New Combined Computational and Experimental Approach to Characterize Photoactive Conjugated 3D Polymers

A tractable new computational protocol is proposed to elucidate oligomeric-scale detail from experimental spectra, providing insight into the local and longer-range electronic and molecular structures of amorphous materials. The protocol uses an in-house code Ambuild to grow kinetically-controlled representative oligomeric clusters of an amorphous polymeric material. Generating many clusters, the statistical prevalence of different structural motifs is identified, and used to develop a 'subset' of structures that capture a broad range of important morphologies. Subsequent electronic structure calculations allow the prediction of IR, NMR, and UV-vis spectra of the bulk materials, providing significant insight into oligomeric scale topologies and helping develop structure-property relationships by identifying the underlying structural origins of different spectral features observed experimentally. Two known, and two novel, pyrene-based conjugated microporous polymers (CMPs) are synthesized and characterized as a test bed for this newly-proposed protocol. Meaningful IR, NMR, and UV-vis absorption spectral data, and experimentally comparable computationally derived spectra are obtained. Whilst IR and NMR reliably probe the local environment, UV-vis absorption spectroscopy is found to be particularly sensitive to the longer-range structural motifs observed on an oligomeric scale, providing significant structural insight into the synthesized materials with reasonable computational cost.

Electroactive Ionic Polymer of Intrinsic Microporosity for High-Performance Capacitive Energy Storage

Here, an ionic polymer of intrinsic microporosity (PIM) as a high-functioning supercapacitor electrode without the need for conductive additives or binders is reported. The performance of this material is directly related to its large accessible surface area. By comparing electrochemical performance between a porous viologen PIM and a nonporous viologen polymer, it is revealed that the high energy and power density are both due to the ability of ions to rapidly access the ionic PIM. In 0.1 m H2SO4 electrolyte, a pseudocapacitve energy of 315 F g-1 is observed, whereas in 0.1 m Na2SO4, a capacitive energy density of 250 F g-1 is obtained. In both cases, this capacity is retained over 10 000 charge-discharge cycles, without the need for stabilizing binders or conductive additives even at moderate loadings (5 mg cm-2). This desirable performance is maintained in a prototype symmetric two-electrode capacitor device, which has >99% Coloumbic efficiency and a <10 mF capacity drop over 2000 cycles. These results demonstrate that ionic PIMs function well as standalone supercapacitor electrodes and suggest ionic PIMs may perform well in other electrochemical devices such as sensors, ion-separation membranes, or displays.

Observation of super-Nernstian proton-coupled electron transfer and elucidation of nature of charge carriers in a multiredox conjugated polymer

Nernstian proton-coupled electron transfer (PCET) is a fundamental process central to many physical and biological systems, such as electrocatalysis, enzyme operation, DNA biosynthesis, pH-/bio-sensors, and electrochemical energy storage devices. We report herein the discovery of super-Nernstian PCET behavior with two protons per electron transferred in the electrochemical doping of a redox conjugated polymer, phenazine-substituted ladder poly(benzimidazobenzophenanthroline) (BBL-P), in aqueous electrolyte. We show that the super-Nernstian response originates from existence of multiredox centers that have a gradient of pKa on the conjugated polymer. Our use of various pH-dependent in operando techniques to probe the nature of charge carriers in n-doped BBL-P found that polarons are the charge carriers at low to intermediate levels of doping (0.1-1.0 electron per repeat unit (eru)) whereas at higher doing levels (1.3 eru), polarons, polaron pairs, and bipolarons co-exist, which evolve into strongly coupled polaron pairs at the highest doping levels (>1.5 eru). We show that PCET-assisted n-doping of BBL-P results in very high redox capacity (>1200 F cm-3) in acidic electrolyte. Our results provide important new insights into PCET in organic materials and the nature of charge carriers in n-doped conjugated polymers while having implications for various electrochemical devices.

Single-atom platinum with asymmetric coordination environment on fully conjugated covalent organic framework for efficient electrocatalysis

Two-dimensional (2D) covalent organic frameworks (COFs) and their derivatives have been widely applied as electrocatalysts owing to their unique nanoscale pore configurations, stable periodic structures, abundant coordination sites and high surface area. This work aims to construct a non-thermodynamically stable Pt-N2 coordination active site by electrochemically modifying platinum (Pt) single atoms into a fully conjugated 2D COF as conductive agent-free and pyrolysis-free electrocatalyst for the hydrogen evolution reaction (HER). In addition to maximizing atomic utilization, single-atom catalysts with definite structures can be used to investigate catalytic mechanisms and structure-activity relationships. In this work, in-situ characterizations and theoretical calculations reveal that a nitrogen-rich graphene analogue COF not only exhibits a favorable metal-support effect for Pt, adjusting the binding energy between Pt sites to H* intermediates by forming unique Pt-N2 instead of the typical Pt-N4 coordination environment, but also enhances electron transport ability and structural stability, showing both conductivity and stability in acidic environments.

Dendritic sp Carbon-Conjugated Benzothiadiazole-Based Polymers with Synergistic Multi-Active Groups for High-Performance Lithium Organic Batteries

Green organic materials composed of C, H, O, and N elements are receiving more and more attention worldwide. However, the high solubility, poor electrical conductivity, and long activation time limit the development of organic materials in practice. Herein, two stable covalent organic materials with alkynyl linkage between benzene rings and benzothiadiazole groups with different amounts of fluorine atoms modification (defined as BOP-0F and BOP-2F), are designed for lithium-ion batteries. Both BOP-0F and BOP-2F can achieve superior reversible capacities of ≈719.8 and 713.5 mAh g-1 over 100 cycles on account of the redox activity of alkynyl (two-electron involved) and benzothiadiazole units (five-electron involved) in these organic materials. While BOP-2F electrodes exhibit much more stable cycling performance than BOP-0F electrodes, especially without pronounced capacity ascending during initial cycling. It can be assigned to the synergy effect of alkynyl linkage and fluorine atom modification in BOP-2F. The lithium storage and activation mechanism of alkynyl, benzothiadiazole, and fluorine groups have also been deeply probed by a series of material characterizations and theoretical simulations. This work could be noteworthy in providing novel tactics for the molecular design and investigation of high-efficiency organic electrodes for energy storage.

Recyclable Conjugated Polyelectrolyte Hydrogels for Pseudocapacitor Fabrication

In alignment with widespread interest in carbon neutralization and sustainable practices, we disclose that conjugated polyelectrolyte (CPE) hydrogels are a type of recyclable, electrochemically stable, and environmentally friendly pseudocapacitive material for energy storage applications. By leveraging ionic-electronic coupling in a relatively fluid medium, one finds that hydrogels prepared using a fresh batch of an anionic CPE, namely, Pris-CPE-K, exhibit a specific capacitance of 32.6 ± 6.6 F g-1 in 2 M NaCl and are capable of 80% (26.1 ± 6.5 F g-1) capacitance retention after 100,000 galvanostatic charge-discharge (GCD) cycles at a current density (J) of 10 A g-1. We note that equilibration under a constant potential prior to GCD analysis leads to the K+ counterions in the CPE exchanging with Na+ and, thus, the relevant active material Pris-CPE-Na. It is possible to remove the CPE material from the electrochemical cell via extraction with water and to carry out a simple purification through dialysis to produce a recycled material, namely Re-CPE-Na. The recycling workup has no significant detrimental impact on the electrochemical performance. Specifically, Re-CPE-Na hydrogels display an initial specific capacitance of 26.3 ± 1.2 F g-1 (at 10 A g-1) and retain 77% of the capacitance after a subsequent 100,000 GCD cycles. Characterization by NMR, FTIR, and Raman spectroscopies, together with XPS and GPC measurements, revealed no change in the structure of the backbone or side chains. However, rheological measurements gave evidence of a slight loss in G' and G''. Overall, that CPE hydrogels display recyclability argues in favor of considering them as a novel materials platform for energy storage applications within an economically viable circular recycling strategy.

Solution-processable polymers of intrinsic microporosity for gas-phase carbon dioxide photoreduction

Four solution-processable, linear conjugated polymers of intrinsic porosity are synthesised and tested for gas phase carbon dioxide photoreduction. The polymers' photoreduction efficiency is investigated as a function of their porosity, optical properties, energy levels and photoluminescence. All polymers successfully form carbon monoxide as the main product, without the addition of metal co-catalysts. The best performing single component polymer yields a rate of 66 μmol h^-1 m^-2, which we attribute to the polymer exhibiting macroporosity and the longest exciton lifetimes. The addition of copper iodide, as a source of a copper co-catalyst in the polymers shows an increase in rate, with the best performing polymer achieving a rate of 175 μmol h^-1 m^-2. The polymers are active for over 100 h under operating conditions. This work shows the potential of processable polymers of intrinsic porosity for use in the gas phase photoreduction of carbon dioxide towards solar fuels.

Porous Organic Polymer with Hierarchical Structure and Limited Volume Expansion for Ultrafast and Highly Durable Sodium Storage

Sustainable organic electrode materials, as promising alternatives to conventional inorganic electrode materials for sodium-ion batteries (SIBs), are still challenging to realize long-lifetime and high-rate batteries because of their poor conductivity, limited electroactivity, and severe dissolution. It is also urgent to deeply reveal their electrochemical mechanism and evolution processes. A porous organic polymer (POP) with a conjugated and hierarchical structure is designed and synthesized here. The unique molecule and structure endow the POP with electron delocalization, high ionic diffusivity, plentiful active sites, exceptional structure stability, and limited solubility in electrolytes. When evaluated as an anode for SIBs, the POP exhibits appealing electrochemical properties regarding reversible capacity, rate behaviors, and long-duration life. Importantly, using judiciously combined experiments and theoretical computation, including in situ transmission electron microscopy (TEM), and ex situ spectroscopy, we reveal the Na-storage mechanism and dynamic evolution processes of the POP, including 12-electron reaction process with Na, low volume expansion (125-106% vs the initial 100%), and stable composition and structure evolution during repeating sodiation/de-sodiation processes. This quantitative design for ultrafast and highly durable sodium storage in the POP could be of immediate benefit for the rational design of organic electrode materials with ideal electrochemical properties.

Nanostructural Organization in a Biredox Ionic Liquid

Ionic liquids generally display peculiar structural features that impact their physical properties, such as the formation of polar and apolar domains. Recently, ionic liquids functionalized with anthraquinone and TEMPO redox groups were shown to increase the energy storage performance of supercapacitors, but their structure has not yet been characterized. In this work, we use polarizable molecular dynamics to study the nanostructuration of such biredox ionic liquids. We show that TEMPO nitroxyl functions tend to aggregate, while the anthraquinone groups favor stacked arrangements. The latter eventually percolate through the whole liquid, which sheds some light on the mechanisms at play within biredox ionic liquid-based supercapacitors.