Richa Singhal

A free-standing carbon nanofiber interlayer for high-performance lithium–sulfur batteries

Free-standing porous carbon nanofibers with tunable surface area and pore structure have been investigated as an interlayer between the sulfur cathode and the separator to inhibit the shuttling of the intermediate polysulfides in lithium–sulfur (Li–S) batteries. Specifically, the effects of thickness, surface area, and pore size distribution of carbon nanofiber (CNF) interlayers on the performance of Li–S batteries have been studied. The carbon nanofiber interlayer not only reduces the electrochemical resistance but also localizes the migrating polysulfides and traps them, thereby improving the discharge capacity as well as cyclability. It was found that the optimum thickness of the interlayer is a critical factor to achieve good cell performance, in addition to the surface area and pore structure. A high initial discharge capacity of 1549 mA h g−1 at C/5 rate, which is 92% of the theoretical capacity of sulfur, with 98% average coulombic efficiency and 83% capacity retention after 100 cycles was obtained with a meso–microporous carbon nanofiber interlayer.

Porous Carbon Mat as an Electrochemical Testing Platform for Investigating the Polysulfide Retention of Various Cathode Configurations in Li-S Cells.

Two optimized cathode configurations (a porous current collector and an interlayer) are utilized to determine the better architecture for improving the cycle stability and reversibility of lithium-sulfur (Li-S) cells. The electrochemical analysis on the upper-plateau discharge capacity (QH) and the lower-plateau discharge capacity (QL) is introduced for assessing, respectively, the polysulfide retention and the electrochemical reactivity of the cell. The analysis results in line with the expected materials chemistry principles suggest that the interlayer configuration offers stable cell performance for sulfur cathodes. The significance of the interlayer is to block the free migration of the dissolved polysulfides, which is a key factor for immobilizing and continuously utilizing the active material in sulfur cathodes. Accordingly, the carbon mat interlayers provide sulfur cathodes with a high discharge capacity of 864 mA h g(-1) at 1 C rate with a high capacity retention rate of 61% after 400 cycles.

Using common salt to impart pseudocapacitive functionalities to carbon nanofibers

A novel and simple method of incorporating pseudocapacitive surface functionalities on free-standing carbon nanofibers using common salt (sodium chloride) is presented. The blend of sodium chloride (NaCl) and polyacrylonitrile is electrospun together, followed by pyrolysis and mild acid treatment to obtain functionalized free-standing (binder-free) carbon nanofibers. The synthesized materials have a low surface area of only 24 m2 g−1, however the electrochemical studies show a five-fold increase in specific capacitance on incorporation of NaCl compared to that without NaCl. The XPS characterization demonstrates that the presence of NaCl leads to enhanced oxygen on the surface of carbon nanofibers, particularly in the form of carboxyl groups. These carboxyl groups then facilitate the adsorption of sulfur functional groups on acid treatment. A high specific capacitance of 204 F g−1, areal capacitance of 1.15 F cm−2, and volumetric capacitance of 63 F cm−3 in 1 M H2SO4 are obtained, which are attributed to the surface functional groups participating in the pseudocapacitive redox reactions. The fabricated nanofibers demonstrate good capacitance retention at high current densities and high cyclability.

Binder-free hierarchically-porous carbon nanofibers decorated with cobalt nanoparticles as efficient cathodes for lithium–oxygen batteries

The development of efficient cathodes is a great challenge inhibiting the advancement of lithium–oxygen (Li–O2) batteries. In the present study, binder-free, high surface area hierarchically-porous carbon nanofibers decorated with cobalt nanoparticles (Co–PCNF) are investigated as cathodes for Li–O2 batteries. We fabricate the nanofibers using a facile electrospinning technique followed by thermal treatment with in situ incorporation of cobalt nanoparticles. This method provides a free-standing, electron-conducting network with a hierarchical pore structure and effective dispersion of cobalt nanoparticles, which is directly used as a cathode in Li–O2 cells without any binders. Li–O2 cells with Co–PCNF as the cathode exhibit a high initial discharge capacity of 8800 mA h g−1 at the current density of 100 mA g−1, and can be recharged for more than 50 cycles with a limited discharge capacity of 500 mA h g−1. In comparison, porous carbon nanofibers without cobalt provide a discharge capacity of 6670 mA h g−1, and a cycle life of only 35 cycles. The post mortem analysis of discharged cathodes revealed Li2O2 as the major discharge product, and suggested a LiO2-mediated reaction mechanism responsible for the excellent performance of Co–PCNF.

Cobalt nanoparticle-embedded porous carbon nanofibers with inherent N- and F-doping as binder-free bifunctional catalysts for oxygen reduction and evolution reactions

Efficient, low-cost, non-precious metal-based, and stable bifunctional electrocatalysts are key to various energy storage and conversion devices such as regenerative fuel cells and metal-air batteries. In this work, we report cobalt nanoparticle-embedded porous carbon nanofibers with inherent N- and F-doping as binder-free bifunctional electrocatalysts with excellent activity for both the oxygen reduction and oxygen evolution reaction (ORR/OER) in an alkaline medium. Single-step electrospinning of a solution of the polymer mixture (carbon precursor) and the cobalt precursor followed by controlled pyrolysis with an intermediate reduction step in H2 (to reduce cobalt oxides to cobalt) was utilized to synthesize an integrated freestanding catalyst. The fabricated catalyst with effective structural and electronic interaction between the cobalt metal nanoparticles and the N- and F-doped carbon defect sites showed enhanced catalytic properties compared to the benchmark catalysts for ORR and OER (Pt, Ir, and Ru). The ORR potential at the current density of -3 mA cm-2 was 0.81 VRHE and the OER potential at a current density of 10 mA cm-2 was 1.595 VRHE , resulting in a ΔE of only 0.785 V.

Optimization of Biodiesel Production by Response Surface Methodology and Genetic Algorithm

The biodiesel production from alkali-catalyzed transesterification of karanja oil was investigated. In this study, the effect of three parameters, i.e., reaction temperature, catalyst concentration, and molar ratio of methanol to oil on biodiesel yield was studied. Central composite design (CCD) along with response surface methodology (RSM) was used for designing experiments and estimating the quadratic response surface. Catalyst concentration was found to have a negative effect on biodiesel yield, whereas molar ratio showed positive effect. Temperature and molar ratio showed significant interaction effect. The reaction conditions were optimized for maximum response, i.e., biodiesel yield from RSM. The program for the RSM model, coupled with genetic algorithm (GA), was developed for predicting the optimized process parameters for maximum biodiesel yield to obtain a global optimal solution. The results were found to be similar from both of the methods.

Modelling of Non-Isothermal Batch Reactor Using Fuzzy Logic

Abstract In the present work, fuzzy logic (FL) has been used to model the performance of a batch reactor system and, hence, the conversion profile at varying operating conditions. The transient temperature vs. time data of exothermic reaction was taken from open literature [1] for modelling with FL. The FL model with properly chosen fuzzy sets, type of membership function and rule base relate time with temperature very well in the reactor under experimental condition. The simulated result through FL model was compared with experimental findings, mathematical model and artificial neural network (ANN) model. The FL model predictions were found to be in very good agreement with experimental findings with average absolute error less than 1%.