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Defects in metal-organic frameworks (MOFs) have great impact on their nano-scale structure and physiochemical properties. However, isolated defects are easily concealed when the frameworks are interrogated by typical characterization methods. In this work, we unveil the presence of solvent-derived formate defects in MOF-74, an important class of MOFs with open metal sites. With multi-dimensional solid-state nuclear magnetic resonance (NMR) investigations, we uncover the ligand substitution role of formate and its chemical origin from decomposed N,N-dimethylformamide (DMF) solvent. The placement and coordination structure of formate defects are determined by 13 C NMR and density functional theory (DFT) calculations. The extra metal-oxygen bonds with formates partially eliminate open metal sites and lead to a quantitative decrease of N 2 and CO 2 adsorption with respect to the defect concentration. In-situ NMR analysis and molecular simulations of CO 2 dynamics elaborate the adsorption mechanisms in defective MOF-74. Our study establishes comprehensive strategies to search, elucidate and manipulate defects in MOFs. Defects in metal-organic frameworks impact their structure and properties. Here authors uncover formate defects in MOF-74 that originate from decomposed DMF solvent. NMR shows that the defects partially eliminate open metal sites and lead to a decrease of gas adsorption; the adsorption mechanism of CO 2 in defective MOF is also elucidated.

Supercapacitors store charge through the electrosorption of ions on microporous electrodes. Despite major efforts to understand this phenomenon, a molecular-level picture of the electrical double layer in working devices is still lacking as few techniques can selectively observe the ionic species at the electrode/electrolyte interface. Here, we use in situ NMR to directly quantify the populations of anionic and cationic species within a working microporous carbon supercapacitor electrode. Our results show that charge storage mechanisms are different for positively and negatively polarized electrodes for the electrolyte tetraethylphosphonium tetrafluoroborate in acetonitrile; for positive polarization charging proceeds by exchange of the cations for anions, whereas for negative polarization, cation adsorption dominates. In situ electrochemical quartz crystal microbalance measurements support the NMR results and indicate that adsorbed ions are only partially solvated. These results provide new molecular-level insight, with the methodology offering exciting possibilities for the study of pore/ion size, desolvation and other effects on charge storage in supercapacitors. Observing ionic species at the electrode/electrolyte interface in supercapacitor devices is difficult. In situ NMR is now used to directly quantify anionic and cationic species within a working microporous carbon supercapacitor electrode.

Supercapacitors are emerging as energy-efficient and robust devices for electrochemical CO 2 capture. However, the impacts of electrode structure and charging protocols on CO 2 capture performance remain unclear. Therefore, this study develops structure-property-performance correlations for supercapacitor electrodes at different charging conditions. We find that electrodes with large surface areas and low oxygen functionalization generally perform best, while a combination of micro- and mesopores is important to achieve fast CO 2 capture rates. With these structural features and tunable charging protocols, YP80F activated carbon electrodes show the best CO 2 capture performance with a capture rate of 350 mmol CO2 kg –1 h –1 and a low electrical energy consumption of 18 kJ mol CO2 –1 at 300 mA g –1 under CO 2 , together with a long lifetime over 12000 cycles at 150 mA g –1 under CO 2 and excellent CO 2 selectivity over N 2 and O 2 . Operated in a “positive charging mode”, the system achieves excellent electrochemical reversibility with Coulombic efficiencies over 99.8% in the presence of approximately 15% O 2, alongside stable cycling performance over 1000 cycles. This study paves the way for improved supercapacitor electrodes and charging protocols for electrochemical CO 2 capture. Supercapacitors are emerging as energy-efficient devices for CO 2 capture. This work investigates the effects of charging protocols and electrode structures on electrochemical CO 2 capture and explores the potential of devices for practical CO 2 capture, especially in the presence of O 2 .

Ionic transport inside porous carbon electrodes underpins the storage of energy in supercapacitors and the rate at which they can charge and discharge, yet few studies have elucidated the materials properties that influence ion dynamics. Here we use in situ pulsed field gradient NMR spectroscopy to measure ionic diffusion in supercapacitors directly. We find that confinement in the nanoporous electrode structures decreases the effective self-diffusion coefficients of ions by over two orders of magnitude compared with neat electrolyte, and in-pore diffusion is modulated by changes in ion populations at the electrode/electrolyte interface during charging. Electrolyte concentration and carbon pore size distributions also affect in-pore diffusion and the movement of ions in and out of the nanopores. In light of our findings we propose that controlling the charging mechanism may allow the tuning of the energy and power performances of supercapacitors for a range of different applications. It is challenging to probe ion dynamics in supercapacitor electrodes, which has significant implications in optimizing their performance. Here, the authors develop in situ diffusion NMR spectroscopy to measure and illustrate the diffusion of the charge-storing ions in working supercapacitors.

Understanding how ions interact with electrodes in electric double-layer capacitors (EDLCs) is key to advancing energy storage, yet many fundamental aspects remain unclear. Here, we employ operando small-angle X-ray scattering (SAXS) to investigate charge storage in metal-organic framework (MOF)-based supercapacitor electrodes as a model system. Using Ni 3 (2,3,6,7,10,11-hexaiminotriphenylene) 2 (Ni 3 (HITP) 2 ) MOF electrodes and 1 M aqueous sodium bis(trifluoromethanesulfonyl)imide (NaTFSI) as the electrolyte, we show that TFSI - anions are immobilised near MOF pore walls via fluorine-hydrogen interactions with N-H functional groups of the MOF. We quantify the concentration of pinned anions and demonstrate that their immobilization persists across different applied cell voltages, resulting in a cation-dominated charge storage mechanism governed solely by Na + adsorption and desorption. Charge balancing is unaffected by whether voltage is applied stepwise or gradually, with no dynamic differences between in-pore and out-of-pore environments and no ion intercalation taking place. Understanding charge storage in supercapacitors remains a challenge. Here, authors use operando X-ray scattering to show that selective anion immobilization in MOF-based electrodes leads to a cation-driven charge storage mechanism.

Carbon dioxide capture is essential to achieve net-zero emissions. A hurdle to the design of improved capture materials is the lack of adequate tools to characterise how CO 2 adsorbs. Solid-state nuclear magnetic resonance (NMR) spectroscopy is a promising probe of CO 2 capture, but it remains challenging to distinguish different adsorption products. Here we perform a comprehensive computational investigation of 22 amine-functionalised metal-organic frameworks and discover that 17 O NMR is a powerful probe of CO 2 capture chemistry that provides excellent differentiation of ammonium carbamate and carbamic acid species. The computational findings are supported by 17 O NMR experiments on a series of CO 2 -loaded frameworks that clearly identify ammonium carbamate chain formation and provide evidence for a mixed carbamic acid – ammonium carbamate adsorption mode. We further find that carbamic acid formation is more prevalent in this materials class than previously believed. Finally, we show that our methods are readily applicable to other adsorbents, and find support for ammonium carbamate formation in amine-grafted silicas. Our work paves the way for investigations of carbon capture chemistry that can enable materials design. A hurdle for designing improved capture materials is the lack of adequate tools to characterise how carbon dioxide adsorbs. Here the authors developed a method to understand how carbon dioxide is captured by materials. Their 17 O solid-state NMR spectroscopy reveals clear signatures for different capture products.

Emissions reduction and greenhouse gas removal from the atmosphere are both necessary to achieve net-zero emissions and limit climate change 1 . There is thus a need for improved sorbents for the capture of carbon dioxide from the atmosphere, a process known as direct air capture. In particular, low-cost materials that can be regenerated at low temperatures would overcome the limitations of current technologies. In this work, we introduce a new class of designer sorbent materials known as ‘charged-sorbents’. These materials are prepared through a battery-like charging process that accumulates ions in the pores of low-cost activated carbons, with the inserted ions then serving as sites for carbon dioxide adsorption. We use our charging process to accumulate reactive hydroxide ions in the pores of a carbon electrode, and find that the resulting sorbent material can rapidly capture carbon dioxide from ambient air by means of (bi)carbonate formation. Unlike traditional bulk carbonates, charged-sorbent regeneration can be achieved at low temperatures (90–100 °C) and the sorbent’s conductive nature permits direct Joule heating regeneration 2 , 3 using renewable electricity. Given their highly tailorable pore environments and low cost, we anticipate that charged-sorbents will find numerous potential applications in chemical separations, catalysis and beyond. Charged-sorbents are a new class of designer sorbent materials for the capture of carbon dioxide from the atmosphere, and can be regenerated at low temperatures with direct heating generation using renewable electricity.

Industrial processes prominently feature π-acidic gases, and an adsorbent capable of selectively interacting with these molecules could enable important chemical separations 1 – 4 . Biological systems use accessible, reducing metal centres to bind and activate weakly π-acidic species, such as N 2 , through backbonding interactions 5 – 7 , and incorporating analogous moieties into a porous material should give rise to a similar adsorption mechanism for these gaseous substrates 8 . Here, we report a metal–organic framework featuring exposed vanadium( ii ) centres capable of back-donating electron density to weak π acids to successfully target π acidity for separation applications. This adsorption mechanism, together with a high concentration of available adsorption sites, results in record N 2 capacities and selectivities for the removal of N 2 from mixtures with CH 4 , while further enabling olefin/paraffin separations at elevated temperatures. Ultimately, incorporating such π-basic metal centres into porous materials offers a handle for capturing and activating key molecular species within next-generation adsorbents. Nitrogenases use transition metals to selectively capture weak π acids such as N 2 by employing backbonding interactions. Here, a metal–organic framework with exposed vanadium sites is presented that uses this approach for selective capture of N 2 from CH 4 , with impressive selectivity and capacity.

Emissions reduction and greenhouse gas removal from the atmosphere are both necessary to achieve net-zero emissions and limit climate change. There is thus a need for improved sorbents for the capture of carbon dioxide from the atmosphere, a process known as direct air capture. In particular, low-cost materials that can be regenerated at low temperatures would overcome the limitations of current technologies. In this work, we introduce a new class of designer sorbent materials known as ‘charged-sorbents’. These materials are prepared through a battery-like charging process that accumulates ions in the pores of low-cost activated carbons, with the inserted ions then serving as sites for carbon dioxide adsorption. We use our charging process to accumulate reactive hydroxide ions in the pores of a carbon electrode, and find that the resulting sorbent material can rapidly capture carbon dioxide from ambient air by means of (bi)carbonate formation. Unlike traditional bulk carbonates, charged-sorbent regeneration can be achieved at low temperatures (90–100 °C) and the sorbent’s conductive nature permits direct Joule heating regeneration using renewable electricity. Given their highly tailorable pore environments and low cost, we anticipate that charged-sorbents will find numerous potential applications in chemical separations, catalysis and beyond.

Industrial processes prominently feature π-acidic gases, and an adsorbent capable of selectively interacting with these molecules could enable important chemical separations. Biological systems use accessible, reducing metal centres to bind and activate weakly π-acidic species, such as N2, through backbonding interactions, and incorporating analogous moieties into a porous material should give rise to a similar adsorption mechanism for these gaseous substrates. In this work, we report a metal-organic framework featuring exposed vanadium(II) centres capable of back-donating electron density to weak π acids to successfully target π acidity for separation applications. This adsorption mechanism, together with a high concentration of available adsorption sites, results in record N2 capacities and selectivities for the removal of N2 from mixtures with CH4, while further enabling olefin/paraffin separations at elevated temperatures. Ultimately, incorporating such π-basic metal centres into porous materials offers a handle for capturing and activating key molecular species within next-generation adsorbents.