Two-dimensional electrically conductive metal-organic frameworks (MOFs) have emerged as promising model electrodes for use in electric double-layer capacitors (EDLCs). However, a number of fundamental questions about the behaviour of this...
Metal-organic frameworks (MOFs) are among the most promising materials for next-generation energy storage systems. However, the impact of particle morphology on the energy storage performances of these frameworks is poorly...
Electroconductive metal–organic frameworks (MOFs) have emerged as high-performance electrode materials for supercapacitors, but the fundamental understanding of the underlying chemical processes is limited. Here, the electrochemical interface of Cu3(HHTP)2 (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) with an organic electrolyte is investigated using a multiscale quantum-mechanics/molecular-mechanics (QM/MM) procedure and experimental electrochemical measurements. Our simulations reproduce the observed capacitance values and reveals the polarization phenomena of the nanoporous framework. We find that excess charges mainly form on the organic ligand, and cation-dominated charging mechanisms give rise to greater capacitance. The spatially confined electric double-layer structure is further manipulated by changing the ligand from HHTP to HITP (HITP = 2,3,6,7,10,11-hexaiminotriphenylene). This minimal change to the electrode framework not only increases the capacitance but also increases the self-diffusion coefficients of in-pore electrolytes. The performance of MOF-based supercapacitors can be systematically controlled by modifying the ligating group.
Two-dimensional electrically conductive metal-organic frameworks (MOFs) are candidate electrode materials for use in electric double-layer capacitor (EDLC) structure-property investigations due to their well-defined crystalline structures. Their promising capacitive performance was first illustrated by EDLCs constructed with the layered framework Ni<sub>3</sub>(HITP)<sub>2 </sub>(HITP = 2,3,6,7,10,11-hexaiminotriphenylene) and an organic electrolyte. Despite this promise, there have been few follow-up studies on the use of these frameworks in EDLCs, raising questions about the generality of the results. Here, we demonstrate the high capacitive performance of the layered framework Cu<sub>3</sub>(HHTP)<sub>2</sub> (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) in EDLCs with an organic electrolyte and compare its performance with Ni<sub>3</sub>(HITP)<sub>2</sub>. Cu<sub>3</sub>(HHTP)<sub>2</sub> exhibits a specific capacitance of 110 – 114 F g<sup>–1 </sup>at low current densities of 0.04 – 0.05 A g<sup>–1</sup> and shows modest capacitance retentions (66 %) at current densities up to 2 A g<sup>–1</sup>, mirroring the performance of Ni<sub>3</sub>(HITP)<sub>2</sub>. However, we also explore the limitations of Cu<sub>3</sub>(HHTP)<sub>2</sub> in EDLCs, finding a limited cell voltage window of 1.3 V and only moderate capacitance retention over 30,000 cycles. This illustrates that these materials require further development to improve their EDLC performance, particularly to reach similar cycling performance levels as porous carbons. Despite this, our work underscores the utility of framework materials in EDLCs and suggests that capacitive performance is largely independent of the identity of the metal node and organic linker molecule, instead being dictated by the three-dimensional structure of the framework. These important insights will aid the design of future conductive MOFs for use in EDLCs.
Supercapacitors are fast-charging energy storage devices of great importance for developing robust and climate-friendly energy infrastructures for the future. Research in this field has seen rapid growth in recent years. Therefore, consistent reporting practices must be implemented to enable reliable comparison of device performance. Although several studies have highlighted the best practices for analysing and reporting data from such energy storage devices, there is yet to be an empirical study investigating whether researchers in the field are correctly implementing these recommendations, and which assesses the variation in reporting between different laboratories. Here, we address this deficit by carrying out the first interlaboratory study of the analysis of supercapacitor electrochemistry data. We find that the use of incorrect formulae and researchers having different interpretations of key terminologies are the primary causes of variability in data reporting. Furthermore, we highlight the more significant variation in reported results for electrochemical profiles showing non-ideal capacitive behaviour. From the insights gained through this study, we make additional recommendations to the community to help ensure consistent reporting of performance metrics moving forward.
Two-dimensional electrically conductive metal-organic frameworks (MOFs) have emerged as promising model electrodes for use in electric double-layer capacitors (EDLC). Here, we demonstrate the high capacitive performance of the framework Cu<sub>3</sub>(HHTP)<sub>2</sub> (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) with an organic electrolyte and compare its behaviour with the previously reported analogue, Ni<sub>3</sub>(HITP)<sub>2</sub> (HITP = 2,3,6,7,10,11-hexaiminotriphenylene). At low current densities of 0.04 – 0.05 A g<sup>−1</sup>, Cu<sub>3</sub>(HHTP)<sub>2</sub> electrodes exhibit a specific capacitance of 110 – 114 F g<sup>−1</sup> and show modest capacitance retentions (66 %) at current densities up to 2 A g<sup>−1</sup> , mirroring the performance of Ni<sub>3</sub>(HITP)<sub>2</sub> and suggesting that capacitive performance is largely independent of the identity of the metal node and organic linker molecule. However, we find a limited cell voltage window of 1.3 V and only moderate capacitance retention (86 %) over 30,000 cycles at a moderate current density of 1 A g<sup>−1</sup>, both significantly lower than state-of-the-art porous carbons. These important insights will aid the design of future conductive MOFs with improved performance in EDLCs.
Two-dimensional electrically conductive metal-organic frameworks (MOFs) have emerged as promising model electrodes for use in electric double-layer capacitors (EDLCs). However, a number of fundamental questions about the behaviour of this class of materials in EDLCs remain unanswered, including the effect of the identity of the metal node and organic linker molecule on capacitive performance and the limitations of current conductive MOFs in these devices relative to traditional activated carbon electrode materials. Herein, we address both these questions via a detailed study of the capacitive performance of the framework Cu<sub>3</sub>(HHTP)<sub>2</sub> (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) with an acetonitrile-based electrolyte, finding a specific capacitance of 110 – 114 F g<sup>−1</sup> at current densities of 0.04 – 0.05 A g<sup>−1</sup> and a modest rate capability. By, directly comparing its performance with the previously reported analogue, Ni<sub>3</sub>(HITP)<sub>2</sub> (HITP = 2,3,6,7,10,11-hexaiminotriphenylene), we illustrate that capacitive performance is largely independent of the identity of the metal node and organic linker molecule in these nearly isostructural MOFs. Importantly, this result suggests that EDLC performance in general is uniquely defined by the 3D structure of the electrodes and the electrolyte, a significant finding not demonstrated using traditional electrode materials. Finally, we probe the limitations of Cu<sub>3</sub>(HHTP)<sub>2</sub> in EDLCs, finding a limited cell voltage window of 1.3 V and only a modest capacitance retention of 81 % over 30,000 cycles, both significantly lower than state-of-the-art porous carbons. These important insights will aid the design of future conductive MOFs with greater EDLC performances.
Supercapacitors are fast-charging energy storage devices of great importance for the development of robust and climate-friendly energy infrastructures for the future. Research in this field has seen rapid growth in recent years, hence consistent reporting practices must be implemented to enable reliable comparison of device performance. Although several studies have highlighted the best practices for analysing and reporting data from such energy storage devices, there is yet to be an empirical study that investigates whether researchers in the field are correctly implementing these recommendations, and which assesses the variation in reporting between different laboratories. Here, we address this deficit by carrying out the first interlaboratory study of the analysis of supercapacitor electrochemistry data. We find that the use of incorrect formulae and researchers having different interpretations of key terminologies are the primary causes of variability in data reporting. Furthermore, we highlight the more significant variation in reported results for electrochemical profiles showing non-ideal capacitive behaviour. From the insights gained through this study, we make additional recommendations to the community to help ensure consistent reporting of performance metrics moving forward.
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