• Energy Research

Supercapacitors are high-power energy storage devices that will play an important role in the transition to a low-carbon society. In recent years, layered electrically conductive metal-organic frameworks (MOFs) have emerged as one of the most promising electrode materials for next-generation supercapacitors. Their crystalline and tuneable structures facilitate structure-performance studies, which are challenging to conduct with traditional porous carbon electrodes. In this work, the electrochemical performances of layered conductive MOFs in supercapacitors are investigated to both improve our understanding of these materials and to develop structure-performance relationships. Having demonstrated that the layered conductive MOF Cu3(HHTP)2 (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) exhibits good performance in supercapacitors, measurements on samples with different particle morphologies reveal that ‘flake’ particles, with small length-to-width aspect ratios, are optimal for these devices. This is due to improved ion accessibility and dynamics through the short pores of the ‘flake’ particles, resulting in a higher power performance compared to particle morphologies with longer pores. Electrochemical quartz crystal microbalance (EQCM) and three-electrode experiments are then performed with Cu3(HHTP)2 and a series of electrolytes with different cation sizes to investigate both the charging mechanism of this MOF and how electrolyte ion size impacts electrochemical performance. It is shown that cations are the dominant charge carriers in Cu3(HHTP)2, with co-ion desorption occurring upon positive charging and counterion adsorption during negative charging. Large ions lead to porosity saturation in MOF electrodes, reducing charge storage and forcing solvent molecules to participate in the charge storage mechanism. The impact of modifying MOF-electrolyte interactions on the electrochemical capacity of layered MOF supercapacitors is then investigated by altering both the electrolyte cation and the MOF electrode functionality. These experiments allow for the systematic probing of the influence of different functional groups on supercapacitor performance, and reveal that MOFs with hydroxy ligating groups, together with Li⁺ electrolytes, constitute the best electrode-electrolyte combination for maximising capacitive performance. Finally, an interlaboratory study is conducted to assess the variability in the reporting of performance metrics across different laboratories. Overall, this work provides unique insights into the performances of layered conductive MOFs for supercapacitor applications, and will guide the design of improved electrode materials for next-generation supercapacitors.

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.

Experimental data supporting "Understanding Electrolyte Ion Size Effects on the Performance of Conducting MOF Supercapacitors". This data set contains five folders: - Elemental Analysis Data (contains elemental analysis data from samples; .pdf, .xlsx, .txt). - XRD Data (contains laboratory XRD data from samples; .txt, .xlsx). - Gas Sorption Data (contains results from gas sorption measurements; .txt). - Electrochemistry Data (contains CV, GCD and EIS data from three-electrode and two-electrode electrochemical cells; .txt, .xlsx). - EQCM Data (contains CV, frequency change, and mass change data from EQCM cells; .txt, .xlsx). 'README' text files are included in each folder containing detailed metadata on experiments.

Experimental data supporting "Capturing Carbon Dioxide from Air with Charged-Sorbents". This dataset contains the raw data used to produce the following figures in the manuscript and supplementary information: - Main text figures 1 - 3. - SI figures S1 - 15, S18 - 20 'README' text files are included in each subfolder containing detailed metadata on each experiment. Further details: 1. 1H, 13C NMR data: These data were derived from 4 samples (Blank cloth, Negatively charged cloth, Positively charged cloth, and Uncharged cloth) prepared by electrochemical charging process or soaked process. The data were collected with the aim of characterising the incorporation of hydroxide ions and the mechanistic pathway responsible for strong CO2 binding in the charged sorbent. The original carbon fibre cloth was purchased from Kynol company with a drying step in the vacuum oven. Solid-state NMR experiments were performed with a Bruker Advance spectrometer operating at a magnetic field strength of 9.4 T, corresponding to a 1H Larmor frequency of 400.1 MHz. A Bruker 4 mm HX double resonance probe was used in all cases. 1H NMR spectra were referenced relative to neat adamantane (C10H16) at 1.9 ppm and 13C NMR spectra were referenced relative to neat adamantane (C10H16) at 38.5 ppm (left-hand resonance). All of the NMR tests were conducted with a sample magic angle spinning rate of 12.5 kHz. A 90° pulse-acquire sequence was used in each experiment. For 13C NMR experiments, recycle delays were set to be more than five times the spin-lattice relaxation time for each sample to ensure that the experiments were quantitative. Charged-sorbents with different water contents were prepared for the NMR characterization. The sorbents were kept in a closed container for 24 h under different relative humidities (RH). Saturated Mg(NO3)2 solutions were used to maintain 53% RH at 25 °C, respectively. 2. N2 and CO2 adsorption isotherms data: These data were derived from 4 samples (Blank cloth, Negatively charged cloth, Positively charged cloth, and Uncharged cloth) prepared by electrochemical charging process or soaked process. The data were collected with the aim of characterising BET surface area, pore size distribution and CO2 uptake. N2 isotherms were collected using an Autosorb iQ gas adsorption analyzer at 77 K. The BET surface area was determined by the BET equation and Rouquerol’s consistency criteria implemented in AsiQwin. All pore size distribution fittings were conducted in AsiQwin using N2 at 77 K on carbon (slit-shaped pores) quenched solid density functional theory (QSDFT) model. CO2 sorption isotherms were also collected on an Autosorb iQ gas adsorption analyzer. Isotherms conducted at 25, 35, and 45 °C were measured using a circulating water bath. Samples were activated at 100 °C in vacuum for 15 h prior to gas sorption measurements. The data were analyzed by plotting the adsorption amount of gas versus the partial pressure. Pore size distribution was simulated with model from the Autosorb iQ software. 3. Thermogravimetric gas sorption data: The data were collected with the aim of characterizing the stability of charged sorbent. Thermogravimetric CO2 adsorption experiments were conducted with a flow rate of 60 mL/min using a TA Instruments TGA Q5000 equipped with a Blending Gas Delivery Module. Samples were activated under flowing N2 for 30 min at various temperatures prior to cooling to 30 ºC and switching the gas stream to CO2 mixtures. Cycling experiments were carried out on a Mettler Toledo TGA / DSC 2 Star system equipped with a Huber mini chiller. For tests with high-concentration CO2, the adsorption and desorption of CO2 were performed at 30 °C and 100 °C for 20 min under 30% CO2 and 70% N2 with a flow rate of 140 mL/min, respectively. For DAC tests, adsorption was carried out at 30 °C for 60 min, with 400 ppm CO2 in dry air; and Desorption was carried out at 130 °C for 60 min with 100% N2. 4. Adsorption microcalorimetry data: These data were derived from positively charged cloth and blank cloth. The data were collected with the aim of characterising the heat released during CO2 uptake in positively charged cloth. The simultaneous measurement of the heat of adsorption and the adsorbed amount of carbon dioxide was performed by means of a heat flow microcalorimeter (Calvet C80 by Setaram), connected to a high-vacuum (residual pressure <10−4 mbar) glass line equipped with a Varian Ceramicell 0–100 mbar gauge and a Leybold Ceramicell 0–1000 mbar gauge. Before the measurement, both PCS-OH and blank carbon cloth (ca. 150 mg before activation) were activated for 24 h under high vacuum (residual pressure < 10-3 mbar) at 100 °C (temperature ramp 3 °C/min). The adsorption microcalorimetry measurements were performed at 30 °C by following a well-established step-by-step procedure described in detail elsewhere. This procedure allows, during the same experiment, the determination of both integral heats evolved (-Qint) and adsorbed amounts (na) for small increments of the adsorptive pressure. The partial molar heats obtained for each small dose of gas admitted over the sample are computed by applying the following ratio: ΔQint/Δna, kJ mol-1. The (differential) heats of adsorption are then reported as a function of CO2 adsorbed amount, to obtain the (differential) enthalpy changes associated with the proceeding adsorption process. The equilibration time in the microcalorimetric measurement was set to 24 hours for small equilibrium pressures (< 30 mbar), whereas it was reduced to 2 hours for larger doses for PCS-OH. The equilibration time was reduced to 2 hours (regardless of the equilibrium pressure) for the bare carbon cloth, as equilibration is expected to occur faster in absence of specific adsorption sites. 5. X-ray diffraction (XRD) data: These data were derived from positively charged cloth and blank cloth. The data were collected with the aim of characterising the crystalline KOH or related products on the sample. Powder X-ray diffraction (PXRD) patterns were collected on a Malvern Panalytical Empyrean instrument equipped with an X'celerator Scientific detector using a non-monochromated Cu Kα source (λ = 1.5406 Å). The data were collected at room temperature over a 2θ range of 3–80 °, with an effective step size of 0.017 °. 6. Titration data: The data were collected with the aim of characterising the amount of hydroxide ions in the positively charged sorbent. First, 88 mg of sample was immersed in 2 mL deionized water and sonicated for 20 min at 25 °C. The pH value was then recorded with a pH meter (Insmark IS128C, calibrated with Buffer solutions before use) at 25 °C as the initial point. Second, 100 µL HCl (0.1 M) was slowly added. The mixture was sonicated for 20 min at a constant 25 °C and the pH of the solution was recorded. The second step was repeated until the end of the titration. There was no weight loss due to evaporation during the titration. 7. The DAC data: The data were collected with the aim of characterising the low-pressure CO2 uptake of positively charged sorbent through chemisorption. The tests were carried out in a sealed box (volume ~600 mL) with a CO2 sensor (Aranet4) to record the concentration of CO2, temperature and RH at every one-minute interval. Before each cycle, the box was exposed to fresh air until the CO2 concentration, RH and temperature stabilized. The sorbent was then placed in the box, which was sealed during measurements.