Efficient electrocatalytic oxygen reduction reaction of thermally optimized carbon black supported zeolitic imidazolate framework nanocrystals under low-temperature

Turning commercially available low-cost conducting carbon black materials into functional electrocatalytic electrode media using simple surface chemical modification is a highly attractive approach. This study reports on remarkably enhanced oxygen electrocatalytic activity of commercially available Ketjenblack (KB) by growing a non-precious cobalt metal-based zeolitic-imidazolate framework (ZIF-67) at room temperature in methanol solution followed by a mild thermolysis. The resulting Co@CoOx nanoparticle decorated nitrogen-doped KB derived from the optimized ZIF-67 : KB weight ratio of hybrid samples at 500-600 °C shows high performance for the oxygen reduction reaction (ORR) with impressive Eonset and E1/2 values of ∼0.90 and ∼0.83 V (vs. RHE), respectively in 0.1 M KOH electrolyte. Such ORR activity is comparable to, or better than many metal@metal-oxide-carbon based electrocatalysts synthesized under elevated carbothermal temperatures and using multicomponent/multistep chemical modification conditions. Therefore, a simple electrocatalyst design reported in this work is an efficient synthesis route that not only utilises earth-abundant carbon black but also comprises scalable room temperature synthesized ZIF-67 following mild thermolysis conditions under 600 °C

Structure‐guided Capacitance Relationships in Oxidized Graphene Porous Materials Based Supercapacitors

Supercapacitors formed from porous carbon and graphene-oxide (GO) materials are usually dominated by either electric double-layer capacitance, pseudo-capacitance, or both. Due to these combined features, reduced GO materials have been shown to offer superior capacitance over typical nanoporous carbon materials; however, there is a significant variation in reported values, ranging between 25 and 350 F g−1. This undermines the structure (e.g., oxygen functionality and/or surface area)-performance relationships for optimization of cost and scalable factors. This work demonstrates important structure-controlled charge storage relationships. For this, a series of exfoliated graphene (EG) derivatives are produced via thermal-shock exfoliation of GO precursors and following controlled graphitization of EG (GEG) generates materials with varied amounts of porosity, redox-active oxygen groups and graphitic components. Experimental results show significantly varied capacitance values between 30 and 250 F g−1 at 1.0 A g−1 in GEG structures; this suggests that for a given specific surface area the redox-active and hydrophilic oxygen content can boost the capacitance to 250–300% higher compared to typical mesoporous carbon materials. GEGs with identical oxygen functionality show a surface area governed capacitance. This allows to establish direct structure-performance relationships between 1) redox-active oxygen functional concentration and capacitance and 2) surface area and capacitance

Nonclassical crystal growth and growth rate hysteresis observed during the growth of curcumin in impure solutions

During the growth of crystals in impure solution, impurities can pin the flow of the elementary steps and decrease the growth rate or even arrest the crystal growth. In this work, for the first time, we showed that curcumin crystals can grow in impure solution that contains two structurally similar impurities, following a non-classical crystallisation pathway that deviates from the pinning mechanism. We showed that, in a highly impure solution that contains 20 wt% of impurities, a high supersaturation can initiate the crystal growth via sympathetic nucleation that involves the formation of new growth surfaces on the seed crystals. These new surfaces formed on the seed crystals at the expense of higher supersaturation act like active growth surfaces and dictate the entire crystal growth kinetics especially at lower supersaturations. We showed that, if we can artificially create new surfaces that look like giant macrosteps at the micron scale on the crystal surface, then these macrosteps can not only speed-up the crystallisation rate but also control the rate of transfer of impurities into the bulk crystals

Understanding and Optimizing Capacitance Performance in Reduced Graphene-Oxide Based Supercapacitors

Reduced graphene-oxide (RGO)-based electrodes in supercapacitors deliver high energy/power capacities compared to typical nanoporous carbon materials. However, extensive critical analysis of literature reveals enormous discrepancies (up to 250 F g-1 ) in the reported capacitance (variation of 100-350 F g-1 ) of RGO materials synthesized under seemingly similar methods, inhibiting an understanding of capacitance variation. Here, the key factors that control the capacitance performance of RGO electrodes are demonstrated by analyzing and optimizing various types of commonly applied electrode fabrication methods. Beyond usual data acquisition parameters and oxidation/reduction properties of RGO, a substantial difference of more than 100% in capacitance values (with change from 190 ± 20 to 340 ± 10 F g-1 ) is found depending on the electrode preparation method. For this demonstration, ≈40 RGO-based electrodes are fabricated from numerous distinctly different RGO materials via typically applied methods of solution (aqueous and organic) casting and compressed powders. The influence of data acquisition conditions and capacitance estimation practices are also discussed. Furthermore, by optimizing electrode processing method, a direct surface area governed capacitance relationship for RGO structures is revealed

Dotted crystallisation: nucleation accelerated, regulated, and guided by carbon dots

Crystallisation from solution is an important process in pharmaceutical industries and is commonly used to purify active pharmaceutical ingredients. Crystallisation involves phase change and the mechanisms involved are random which makes the process stochastic. This creates a variation in the time required to reach a fixed percentage of yield from batch to batch. It is essential to regulate the batch crystallisation process and make it more predictable for industrial applications for the ease of process chain scheduling of upstream and downstream unit operations. In this work, we propose a new technique called dotted crystallisation, where carbon dots are used to dictate and regulate events associated with nucleation and crystallisation processes. Following the rules of two-step nucleation theory, the carbon dots intentionally added to a supersaturated solution of curcumin anchors the crystallising compound to form prenucleation clusters that evolve into stable nuclei. Using curcumin as a model compound, we showed that the nucleation of this compound in isopropanol can be regulated, and the nucleation rate can be improved via addition of small quantities of carbon dots to the supersaturated solution. Our results confirmed that the nucleation rate of curcumin by dotted crystallisation was roughly four times higher than the nucleation rate by conventional cooling crystallisation and produced smaller sized crystals with a narrow size distribution

Graphene-Based Nanocomposites for Energy Storage

Since the first report of using micromechanical cleavage method to produce graphene sheets in 2004, graphene/graphene-based nanocomposites have attracted wide attention both for fundamental aspects as well as applications in advanced energy storage and conversion systems. In comparison to other materials, graphene-based nanostructured materials have unique 2D structure, high electronic mobility, exceptional electronic and thermal conductivities, excellent optical transmittance, good mechanical strength, and ultrahigh surface area. Therefore, they are considered as attractive materials for hydrogen (H2) storage and high-performance electrochemical energy storage devices, such as supercapacitors, rechargeable lithium (Li)-ion batteries, Li–sulfur batteries, Li–air batteries, sodium (Na)-ion batteries, Na–air batteries, zinc (Zn)–air batteries, and vanadium redox flow batteries (VRFB), etc., as they can improve the efficiency, capacity, gravimetric energy/power densities, and cycle life of these energy storage devices. In this article, recent progress reported on the synthesis and fabrication of graphene nanocomposite materials for applications in these aforementioned various energy storage systems is reviewed. Importantly, the prospects and future challenges in both scalable manufacturing and more energy storage-related applications are discussed

Design of 3D Graphene-Oxide Spheres and Their Derived Hierarchical Porous Structures for High Performance Supercapacitors

Graphene-oxide (GO) based porous structures are highly desirable for supercapacitors, as the charge storage and transfer can be enhanced by advancement in the synthesis. An effective route is presented of, first, synthesis of three-dimensional (3D) assembly of GO sheets in a spherical architecture (GOS) by flash-freezing of GO dispersion, and then development of hierarchical porous graphene (HPG) networks by facile thermal-shock reduction of GOS. This leads to a superior gravimetric specific capacitance of ≈306 F g(-1) at 1.0 A g(-1) , with a capacitance retention of 93% after 10 000 cycles. The values represent a significant capacitance enhancement by 30-50% compared with the GO powder equivalent, and are among the highest reported for GO-based structures from different chemical reduction routes. Furthermore, a solid-state flexible supercapacitor is fabricated by constructing the HPG with polymer gel electrolyte, exhibiting an excellent areal specific capacitance of ≈220 mF cm(-2) at 1.0 mA cm(-2) with exceptional cyclic stability. The work reveals a facile but efficient synthesis approach of GO-based materials to enhance the capacitive energy storage

Adsorption Sites and Binding Nature of CO₂ in Prototypical Metal−Organic Frameworks: A Combined Neutron Diffraction and First-Principles Study

We report a detailed study of CO₂ adsorption in two important metal−organic framework (MOF) compounds (Mg-MOF-74 and HKUST-1). In both MOFs, the open metal ions were identified as the primary binding sites through neutron diffraction measurements. The relatively strong metal−CO₂ binding was attributed to an enhanced electrostatic interaction, and vibrational mode analysis shows that the adsorbed CO₂ molecule is strongly attached through one of its oxygen atoms while the rest of the molecule is relatively free. This high orientational disorder is the reason for the large apparent O−C−O bond bending angle derived from diffraction measurements. Our calculations give only a small degree of bond bending, suggesting that the CO₂ adsorption on the open metal site is still largely physisorption. Interestingly, the overall metal−CO₂ binding strength is right in the range which can facilitate both adsorption (CO₂ capture) and desorption (MOF regeneration) under typical flue gas conditions