Carbon Nanotube Based Robust and Flexible Solid-State Supercapacitor.

All solid-state flexible electrochemical double-layer capacitors (EDLCs) are crucial for providing energy options in a variety of applications, ranging from wearable electronics to bendable micro/nanotechnology. Here, we report on the development of robust EDLCs using aligned multiwalled carbon nanotubes (MWCNTs) grown directly on thin metal foils embedded in a poly(vinyl alcohol)/phosphoric acid (PVA/H3PO4) polymer gel. The thin metal substrate holding the aligned MWCNT assembly provides mechanical robustness and the PVA/H3PO4 polymer gel, functioning both as the electrolyte as well as the separator, provides sufficient structural flexibility, without any loss of charge storage capacity under flexed conditions. The performance stability of these devices was verified by testing them under straight and bent formations. A high value of the areal specific capacitance (CSP) of ∼14.5 mF cm-2 with an energy density of ∼1 µW h cm-2 can be obtained in these devices. These values are significantly higher (in some cases, orders of magnitude) than several graphene as well as single-walled nanotube-based EDLC's utilizing similar electrolytes. We further show that these devices can withstand multiple (∼2500) mechanical bending cycles, without losing their energy storage capacities and are functional within the temperature range of 20 to 70 °C. Several strategies for enhancing the capacitive charge storage, such as physically stacking (in parallel) individual devices, or postproduction thermal annealing of electrodes, are also demonstrated. These findings demonstrated in this article provide tremendous impetus toward the realization of robust, stackable, and flexible all solid-state supercapacitors.

High Adsorption of Benzoic Acid on Single Walled Carbon Nanotube Bundles.

Removal of harmful chemicals from water is paramount to environmental cleanliness and safety. As such, need for materials that will serve this purpose is in the forefront of environmental research that pertains to water purification. Here we show that bundles of single walled carbon nanotubes (SWNTs), synthesized by direct thermal decomposition of ferrocene (Fe(C5H5)2), can remove emerging contaminants like benzoic acid from water with high efficiencies. Experimental adsorption isotherm studies indicate that the sorption capacity of benzoic acid on these carbon nanotubes (CNTs) can be as high as 375 mg/g, which is significantly higher (in some cases an order of magnitude) than those reported previously for other adsorbents of benzoic acid such as activated carbon cloth, modified bentonite and commercially available graphitized multiwall carbon nanotubes (MWNTs). Our Molecular Dynamics (MD) simulation studies of experimental scenarios provided major insights related to this process of adsorption. The MD simulations indicate that, high binding energy sites present in SWNT bundles are majorly responsible for their enhanced adsorptive behavior compared to isolated MWNTs. These findings indicate that SWNT materials can be developed as scalable materials for efficient removal of environmental contaminants as well as for other sorption-based applications.

Thermodynamic evidence of a transition in ZIF-8 upon CH4 sorption.

We present the results of an experimental study of methane sorption in ZIF-8. We measured isotherms at five different temperatures between 87 K and 107 K. We have observed three sub-steps in each of the isotherms. The intermediate sub-step had not been observed experimentally in previous studies of this system. This newly determined experimental feature suggests that a transition is taking place in the sorbed system (this newly observed sub-step occurs over a loading interval where published computer simulation results for CH4 in ZIF-8 had identified a structural transition occurring in the sorbent). We have studied the kinetics of adsorption for this system (we measure the time required for the system to reach equilibrium after gas is added to the experimental cell as a function of sorbent loading). We observed a sharp peak in the equilibration time at high loadings, below saturation. We have explored the isosteric heat of adsorption, and its dependence on sorbent loading, for this system. We found a broad peak in the isosteric heat at loadings corresponding to the intermediate isotherm sub-step. Previously reported computer simulations for the isosteric heat dependence on loading for CH4 in ZIF-8 are in good agreement with our experimental results for this quantity.