Covalent organic frameworks (COFs) are ideal platforms to spatially control the integration of multiple molecular motifs throughout a single nanoporous framework. Despite this design flexibility, COFs are typically synthesized using only two monomers. One bears the functional motif for the envisioned application, while the other is used as an inert connecting building block. Integrating more than one functional motif extends the functionality of COFs immensely, which is particularly useful for multistep reactions such as electrochemical reduction of CO2. In this systematic study, we synthesized five Ni(II)- and Zn(II)-porphyrin-based COFs, including two pure component COFs (Ni100 and Zn100) and three mixed Ni/Zn-COFs (Ni75/Zn25, Ni50/Zn50, and Ni25/Zn75). Among these, the Ni50/Zn50-COF exhibited the highest catalytic performance for the electroreduction of CO2 to CO and formate at −0.6 V vs RHE, as was observed in an H-cell. The catalytic performance of the COF catalysts was further extended to a zero-gap membrane electrode assembly (MEA) operation where, utilizing Ni50/Zn50, CH4 was detected along with CO and formate at a high current density of 150 mA cm–2. In contrast, under these conditions predominantly H2 and CO were detected at Ni100 and Zn100 respectively, indicating a clear synergistic effect between the Ni- and Zn-porphyrin units.@en

Covalent organic frameworks (COFs) are a subclass of hyper-crosslinked polymers that contain ordered nanoporosity within their polymer network. Control over pore size and shape, as well as structure regularity, is obtained through careful selection of the monomeric building blocks and the synthesis reaction conditions. This has led to a vast library of available COF structures that are each designed for a specific application, typically in the direction of controlled capture and release. The work presented in this thesis addresses challenges in the development of COFs as active materials in energy transition applications. The goal of this thesis was to draw structure-property relationships of novel COFs in order to establish COF design rules for applications such as: CO2 capture, separation and conversion, as well as electrochemical energy storage...@en

In order to capture and separate CO2 from the air or flue gas streams through nanoporous adsorbents, the influence of the humidity in these streams has to be taken into account as it hampers the capture process in two main ways: (1) water preferentially binds to CO2 adsorption sites and lowers the overall capacity, and (2) water causes hydrolytic degradation and pore collapse of the porous framework. Here, we have used a water-stable polyimide covalent organic framework (COF) in N2/CO2/H2O breakthrough studies and assessed its performance under varying levels of relative humidity (RH). We discovered that at limited relative humidity, the competitive binding of H2O over CO2 is replaced by cooperative adsorption. For some conditions, the CO2 capacity was significantly higher under humid versus dry conditions (e.g., a 25% capacity increase at 343 K and 10% RH). These results in combination with FT-IR studies on equilibrated COFs at controlled RH values allowed us to assign the effect of cooperative adsorption to CO2 being adsorbed on single-site adsorbed water. Additionally, once water cluster formation sets in, loss of CO2 capacity is inevitable. Finally, the polyimide COF used in this research retained performance after a total exposure time of >75 h and temperatures up to 403 K. This research provides insight in how cooperative CO2-H2O can be achieved and as such provides directions for the development of CO2 physisorbents that can function in humid streams.

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Nanoporous polymers are becoming increasingly interesting materials for electrochemical applications, as their large surface areas with redox-active sites allow efficient adsorption and diffusion of ions. However, their limited electrical conductivity remains a major obstacle in practical applications. The conventional approach that alleviates this problem is the hybridisation of the polymer with carbon-based additives, but this directly prevents the utilisation of the maximum capacity of the polymers. Here, we report a layer-by-layer fabrication technique where we separated the active (porous polymer, top) layer and the conductive (carbon, bottom) layer and used these “layered” electrodes in a supercapacitor (SC). Through this approach, direct contact with the electrolyte and polymer material is greatly enhanced. With extensive electrochemical characterisation techniques, we show that the layered electrodes allowed a significant contribution of fast faradic surface reactions to the overall capacitance. The electrochemical performance of the layered-electrode SC outperformed other reported porous polymer-based devices with a specific gravimetric capacitance of 388 F·g−1 and an outstanding energy density of 65 Wh·kg−1 at a current density of 0.4 A·g−1. The device also showed outstanding cyclability with 90% of capacitance retention after 5000 cycles at 1.6 A·g−1, comparable to the reported porous polymer-based SCs. Thus, the introduction of a layered electrode structure would pave the way for more effective utilisation of porous organic polymers in future energy storage/harvesting and sensing devices by exploiting their nanoporous architecture and limiting the negative effects of the carbon/binder matrix. @en

Flow-induced disturbances were applied during the synthesis of a naphthalene diimide covalent organic framework (NDI-COF), which resulted in different COF polymer networks. We discovered that a high intensity of stirring resulted in more aggregated structures on both the micro- and nano-length scale. Subsequently, these structures absorbed light over longer wavelengths due to a relatively higher contribution of intermolecular interactions between the NDI-segments.

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Covalent organic frameworks (COFs) are an emerging material family having several potential applications. Their porous framework and redox-active centers enable gas/ion adsorption, allowing them to function as safe, cheap, and tunable electrode materials in next-generation batteries, as well as CO2 adsorption materials for carbon-capture applications. Herein, we develop four polyimide COFs by combining aromatic triamines with aromatic dianhydrides and provide detailed structural and electrochemical characterization. Through density functional theory (DFT) calculations and powder X-ray diffraction, we achieve a detailed structural characterization, where DFT calculations reveal that the imide bonds prefer to form at an angle with one another, breaking the 2D symmetry, which shrinks the pore width and elongates the pore walls. The eclipsed perpendicular stacking is preferable, while sliding of the COF sheets is energetically accessible in a relatively flat energy landscape with a few metastable regions. We investigate the potential use of these COFs in CO2 adsorption and electrochemical applications. The adsorption and electrochemical properties are related to the structural and chemical characteristics of each COF, giving new insights for advanced material designs. For CO2 adsorption specifically, the two best performing COFs originated from the same triamine building block, which-in combination with force-field calculations-revealed unexpected structure-property relationships. Specific geometries provide a useful framework for Na-ion intercalation with retainable capacities and stable cycle life at a relatively high working potential (>1.5 V vs Na/Na+). Although this capacity is low compared to conventional inorganic Li-ion materials, we show as a proof of principle that these COFs are especially promising for sustainable, safe, and stable Na-aqueous batteries due to the combination of their working potentials and their insoluble nature in water.

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A new concept of the formation of charge transfer (CT) complexes between an intrinsically electron-donating conjugated microporous polymer and a small molecule acceptor is reported. Spirobifluorene-based mesoporous organic polymers with high porosity and Brunauer–Emmett–Teller surface area are synthesized by the Suzuki-coupling reaction of spirobifluorene and pyrene monomers. The simple doping of the synthesized mesoporous, electron-rich, conjugated polymer with 7,7,8,8-tetracyanoquinodimethane as an acceptor leads to efficient CT complexation in the electron-donating mesoporous spaces. This results in a high-speed synthesis (within 5 s), thermally stable compound (up to about 200 °C), and good control of the concentration of donor–acceptor pairs in the CT complex.

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