International audienc

Metal organic frameworks (MOFs) have unique properties that make them excellent candidates for many high-tech applications. Nevertheless, their nonconducting character is an obstacle to their practical utilization in electronic and energy systems. Using the familiar HKUST-1 MOF as a model, we present a new method of imparting electrical conductivity to otherwise nonconducting MOFs by preparing MOF nanoparticles within the conducting matrix of mesoporous activated carbon (AC). This composite material was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), gas adsorption measurements, and electron paramagnetic resonance (EPR) spectroscopy. We show that MOF nanoparticles grown within the carbon matrix maintain their crystalline characteristics and their surface area. Surprisingly, as a result of the composition process, EPR measurements revealed a copper signal that had not yet been achieved. For the first time, we could analyze the complex EPR response of HKUST-1. We demonstrate the high conductivity of the MOF composite and discuss various factors that are responsible for these results. Finally, we present an optional application for using the conductive MOF composite as a high-performance electrode for pseudocapacitors.

In this study, we present the positive effect of 1,10phenanthroline as an electrolyte additive that is strongly adsorbed on activated carbon electrodes, thereby adding effective redox activity to their initially capacitive interactions with electrolyte solutions. We obtain a stable capacitance of 320 F/g for the negative electrode and 190 F/ g electrode for full symmetric supercapacitor cells, operating up to 3.4 V in nonaqueous media, during many thousands of cycles. This corresponds to a specific capacity of 180 (mA h)/g electrode . The high voltage and capacity of these systems can pave the way for developing high-energy-density pseudocapacitors that may be able to compete with battery systems. We explored the mechanisms of the electrode interactions using electrochemical tools, including impedance spectroscopy.

The main goal of this work was to modify activated carbon (AC) with carbon nanodots (C-dots) and to explore the modified composites as electrode materials for supercapacitors. C-dots were synthesized by sonication of polyethylene glycol followed by sonochemical modification of AC matrices with the preprepared C-dots. Sonication introduces the C-dots into the pores of the AC. The effect of the introduction of the C-dots into the AC and their incorporation into the pores was studied. The porosity of the AC/C-dots and the AC reference materials was explored, as well as the impact of the C-dot loading on the performance of the electrodes comprising these AC/C-dots. It was found that the AC/C-dot electrodes demonstrate a specific capacitance of 0.185 F/cm2 (per specific electrode area), three times higher than the capacitance of unmodified AC electrodes per specific electrode’s area. It was established that the new electrode’s material, namely, AC/C-dots, exhibits very stable electrochemical behavior. Many thousands of cycles could be demonstrated with stable capacity and a Coulombic efficiency of around 100%.

We report fabrication of flexible all-solid-state transparent electrochromic patterned microsupercapacitors based on twodimensional layered nanostructured molybdenum oxide (MoO 3−x )/ poly(3,4-ethylenedioxythiophene)−polystyrenesulfonate (PE-DOT:PSS) nanocomposite electrodes. Exceptional electrochemical performance of the transparent microsupercapacitors includes fast kinetics and response times, high specific capacitances (up to 79.2 C/ g, 99 F/g, and 2.99 mF/cm 2 ), and Coulombic efficiencies of 99.7% over 2500 cycles. Such exceptional performance is attributed to the synergistic effects of PEDOT:PSS providing high electrical conductivity and high charge storage capacity along with its segregated interfacial nanostructure facilitating the intercalation of the ionic species, H + (Na + , K + ) and SO 4 2− , into the high surface area tunnel structure of the 2D MoO 3−x nanosheets. Supercapacitors using MoO 3−x PEDOT:PSS electrodes exhibit optical transmittance above 70% (λ = 380−730 nm). The electrochromic performance of the transparent microsupercapacitor is due to both PEDOT:PSS and cation (H + ) intercalation in the tunnel structure of MoO 3−x .

This study aims to investigate the effect of the potential window on heat generation in carbon‐based electrical double layer capacitors (EDLCs) with ionic‐liquid (IL)‐based electrolytes using in operando calorimetry. The EDLCs consisted of two identical activated‐carbon electrodes with either neat 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethane‐sulfonyl)imide ([Pyr14][TFSI]) electrolyte or 1.0 m [Pyr14][TFSI] in propylene carbonate (PC) as electrolyte. The instantaneous heat generation rate at each electrode was measured under galvanostatic cycling for different potential windows ranging from 1 to 4 V. First, the heat generation rates at the positive and negative electrodes differed significantly in neat IL owing to the differences in the ion sizes and diffusion coefficients. However, these differences were minimized when the IL was diluted in PC. Second, for EDLC in neat [Pyr14][TFSI] at high potential window (4 V), a pronounced endothermic peak was observed at the beginning of the charging step at the positive electrode owing to TFSI− intercalation in the activated carbon. On the other hand, for EDLC in 1.0 m [Pyr14][TFSI] in PC at potential window above 3 V, an endothermic peak was observed only at the negative electrode owing to the decomposition of PC. Third, for both neat and diluted [Pyr14][TFSI] electrolytes, the irreversible heat generation rate increased with increasing potential window and exceeded Joule heating. This was attributed to the effect of potential‐dependent charge redistribution resistance. A further increase in the irreversible heat generation rate was observed for the largest potential windows owing to the degradation of the PC solvent. Finally, for both types of electrolyte, the reversible heat generation rate increased with increasing potential window because of the increase in the amount of ion adsorbed/desorbed at the electrode/electrolyte interface.

Supercapacitors are energy storage and conversion devices that display high power.

Ionic liquids (ILs) are attractive candidates for high‐voltage electrochemical energy storage systems, owing to their high electrochemical stability. Recently, a unique eutectic mixture of ILs was reported to demonstrate outstanding performance in supercapacitor systems at low temperatures. Yet, many publications using this or similar IL mixtures reported only a limited voltage or cyclability when utilizing them with practical activated carbon electrodes. With supercapacitors consisting of symmetric electrodes, in which voltages higher than 3 V are applied, fast capacity fading and activity termination are observed. In order to exceed the limit of 3 V for supercapacitors that use electrolyte solutions possessing wide electrochemical windows, we thoroughly investigated the (unexpected) failure mechanism, using several analytical methods. This is the most important aspect of the paper. By this, we discovered a pronounced difference in the electrochemical behavior of the negative and the positive electrodes, which has significant implications on the operation of full symmetric cells at high voltages. Finally, we propose a solution that enables stable operation of cells up to 3.4 V. By balancing the mass of the electrodes, we prevent high‐voltage failure and control the voltage split to use the full electrochemical window of each electrode and obtain a higher cell voltage of 3.4 V and an energy density higher than 40 Wh/kg (of the electrode materials). The most important aspect of this work was a rigorous study of the failure mechanism.