Synthesis of electrolytic manganese dioxide (EMD) and biomass waste-derived carbon for hybrid capacitors

Renewable energy (RE) is expected to be the primary energy supplier in the future energy mix. This has created the necessity for low-cost, safe, and reliable energy storage to guarantee a continuous energy supply by the intermittent RE sources. Due to the inbuilt rich chemistry of manganese dioxide (MnO2) and the advantageous characteristics; of low cost, environmentally friendliness, and nontoxic, it can be adapted for a wide range of applications such as biosensors, humidity sensors, catalysts, and so on. Among the different forms of MnO2, electrolytic manganese dioxide (EMD) is well-demanded energy storage material. However, the limitations such as lower capacitance, irreversibility, and cyclability of EMD in comparison with other metal oxides such as cobalt and nickel oxides, have hindered its application in capacitor energy storage, which was one of the focuses of this thesis. Therefore, this Ph.D. research project aimed at synthesizing modified EMD materials as the positive electrode for hybrid capacitor applications. The modified EMD was coupled with the biomass-derived activated carbon (AC) which is synthesized as the negative electrode to fabricate hybrid capacitors. This Ph.D. research work has contributed to the existing knowledge through the following: 1) synthesizing pristine EMD using galvanostatic electrodeposition and studying its suitability for capacitor applications via experimental and theoretical analysis, 2) biopolymer alginate assisted EMD synthesis and optimization via experimental and computational modeling, 3) studying the effect of varying surfactants to improve the electrochemical characteristics of EMD, 4) synthesis of biomass waste-derived activated carbon and modeling their parameters for capacitance prediction. The results indicated the challenge and importance of the delicate tailoring of the EMD characteristics for capacitor application. Pristine EMD was synthesized under different electrodeposition experiment conditions by varying applied current density (100, 200, 300 A m-2) and deposition duration (4, 5, 6 h). The electrodeposition was carried out in a low acidic medium electrolytic bath where a lead (Pb) anode and stainless steel (SS) cathode were used. The EMD was deposited on the Pb anode via Mn2+ oxidation to form Mn4+ and its oxide MnO2. The physicochemical and electrochemical characterization of the obtained EMD powder concluded that the material deposited at 200 A m-2 for 5 hours, showing the spindle-like morphology was suitable over others for supercapacitor (SC) application. The pristine EMD at these experimental conditions delivered 98 F g-1 capacitance at 1 mA cm-2 applied current density tested in 2 M NaOH aqueous electrolyte and proved its potential development by modifying its characteristics. Therefore, the pristine EMD was modified by introducing the biopolymer alginic acid crosslinking to improve its electrochemical performance. The alginic acid was added to the electrolytic bath at varying concentrations; 0, 0.1, 0.25, 0.5, and 1 g l-1, to optimize the added bio-polymer amount to maximize the capacitance. At 0.5 g l-1, the pristine EMD morphology was rearranged to a cactus-shaped with flutes. The calculated specific capacitance of the modified EMD was ~5 times higher (487 F g-1) than the pristine EMD. The molecular dynamics simulation results determined the polymer-ion interactions in the electrolytic bath and provided evidence, showing that the alginic acid could act as a template for binding the Mn2+ ions in a relatively ordered manner for the growth of the EMD deposit. 0.42 of pyrolusite and 0.58 of ramsdellite fractions present in the modified material were quantitatively determined using the neutron powder diffraction (NPD) data. The slight increments of the lattice spacing observed in high-resolution transmission electron microscopy (HRTEM) images were well aligned with the NPD results of unit cell volume expansions of the EMD-polymer composite showing the polymer intercalation within the EMD structure influencing its characteristics. At 2 mA cm-2, the fabricated hybrid capacitor delivered 52 F g-1 specific capacitance, 14 Wh g-1 specific energy, 500 W g-1 specific power, and 94 % capacitance retention over 5000 cycles. The results highlighted the importance of the functional molecular structure of the biopolymer alginic acid to produce a binary composite of EMD-polymer as a capacitor material. Further, the pristine EMD was modified by electrodepositing the MnO2 using surfactant mediated electrolyte solutions. The electrochemical performance of the synthesized EMD in the presence of three novel cationic surfactants was compared with the pristine EMD and the EMD co-deposited with commonly used cetyltrimethylammonium ammonium bromide (C-AB) surfactant. The three surfactants with different molecular structures are Tetradecyltrimethylammonium bromide (T-AB), Didodecyldimethylammonium bromide (D-AB), Benzyldodecyldimethylammonium bromide (B-AB) used at varying concentrations (15, 30, 60 g l-1) in the electrolytic bath. Among the B-AB surfactant at 30 mg l-1, the EMD (EMD/B-AB30) showed the highest capacitance of 602 F g-1 tested at 1 mA cm-2 current density. The molecular dynamics simulation indicated that when the B-AB surfactant was attached to the Pb electrode via electrostatic, Van der Walls interactions, then the nucleation of MnO2 particles occurred surrounding the surfactant molecule. The unique molecular structure influenced the nucleation formation well-ordered, whereas, for pristine EMD, the nucleation was random. The hybrid capacitor comprises the best performed modified EMD (EMD/B-AB30), and biomass waste-derived AC exhibited 91 F g-1 specific capacitance, an outstanding energy density of 32.4 Wh kg-1 for a corresponding power density of 971 W kg-1. Valorization of the biomass waste, Mango seed husk (MS), and the Grape marc (GM) was carried out by converting the waste into AC for capacitor electrodes. The MS was carbonized, followed by chemical activation using KOH as the activating agent. Activation temperature was varied at 800, 900, 1000, and 1100 °C temperatures, among at 1100 °C highest surface area of 1943 m2 g-1, and the specific capacitance of 135 F g-1 was obtained for the MS-AC. The MS-AC experimental data were incorporated in four machine learning (ML) algorithms; linear regression (LR), decision tree (DT), support vector regression (SVR), and multi-layer perceptron (MLP) for capacitance prediction. Among, the MLP model showed the best correlation (R2 = 0.9868) between the experimental and predicted capacitance values and proved its potential application for computing the complex non-linear relationships between the input and output datasets. Further, the porous carbon materials were derived from GM using four synthesis routes by varying the parameters of activating agent (KOH and ZnCl2), dopant (Nitrogen), and carbonization (450, 600 °C) and activation (450, 800 °C) temperatures. Among the different GM-AC products, the GM carbon, doped with urea and activated by KOH (KACurea), exhibited better morphology, hierarchical pore structure, larger surface area (1356 m2 g-1), and the highest specific capacitance of 139 F g-1 in 2 M NaOH aqueous electrolyte. The miscellaneous collection of datasets based on AC experiments was used for specific capacitance and power prediction using the MLP ML model. Overall, this thesis showed that the EMD could be produced in bulk to be used for hybrid capacitor applications. Particularly, it provided insights about the specie interactions in the electrolyte solution that improved the material performance. This built the platform for further studies on altering the additive concentrations and combinations for developing high-performing EMD materials. This Ph.D. work also highlighted the opportunities to valorize the biomass waste to produce AC with desired characteristics of hierarchical pore structure, larger surface area, etc., to replace the conventional AC electrodes. Finally, the electrochemical performance of the hybrid capacitor fabricated using best performed EMD material (EMD/B-AB30) and biomass-waste derived AC (MS-AC 1100) surpassed the energy density values of the existing supercapacitors, proving its potential development in commercial applications

Interaction of Natural Organic Matter with Layered Minerals: Recent Developments in Computational Methods at the Nanoscale

The role of mineral surfaces in the adsorption, transport, formation, and degradation of natural organic matter (NOM) in the biosphere remains an active research area owing to the difficulties in identifying proper working models of both NOM and mineral phases present in the environment. The variety of aqueous chemistries encountered in the subsurface (e.g., oxic vs. anoxic, variable pH) further complicate this field of study. Recently, the advent of nanoscale probes such as X-ray adsorption spectroscopy and surface vibrational spectroscopy applied to study such complicated interfacial systems have enabled new insight into NOM-mineral interfaces. Additionally, due to increasing capabilities in computational chemistry, it is now possible to simulate molecular processes of NOM at multiple scales, from quantum methods for electron transfer to classical methods for folding and adsorption of macroparticles. In this review, we present recent developments in interfacial properties of NOM adsorbed on mineral surfaces from a computational point of view that is informed by recent experiments

Recent advances in cobalt based heterogeneous catalysts for oxygen evolution reaction

The future of the world energy lies in clean and renewable energy sources. Many technologies, such as solar cells, wind turbines, etc., have been developed to harness renewable energies in different forms of fuel. Amongst them, electrolysis of water to produce oxygen and hydrogen is one of the paramount developments towards achieving clean energy, which has attained significant attention due to its green and simple method for the production of fuels. In electrolysis of water, the half-reaction containing the oxygen evolution reaction (OER) is a reaction that is kinetically sluggish, which requires higher overpotential to produce O2, when compared to the other half-reaction, i.e. hydrogen evolution reaction (HER). Many electrocatalysts are studied extensively to be used in the OER process to get an economical yield out of it. Noble metal-based catalysts are the state-of-the-art catalyst used for OER currently. But due to their high cost and scarcity, they cannot be applied in a large-scale manner to be used in the future. The non-noble metals (transition metals and perovskites) are gaining interest by exhibiting on par or better OER performance compared to the noble metal used. Due to their low cost, ample resources, and several metals available, they have opened up a variety of areas with a different combination of metals to be used as a catalyst for OER. Amongst these metals, cobalt has received massive appreciation for performing as an excellent OER catalyst. Multi metals, multimetal mixed oxides, multimetal phosphides, perovskites, and carbon-supported catalysts containing cobalt have shown low overpotential with high long-term stability. Therefore, in this review, we go through different cobalt-based electrocatalysts for OER, the general mechanism governing the OER process, the challenges that we are facing today to enhance the catalytic performance, and future aspects to overcome such challenges.This study was supported by the NPRP grant ( NPRP8-145-2-066 ) from the Qatar National Research Fund (a member of the Qatar Foundation). The statements made herein are solely the responsibility of the authors. The author(s) would also like to acknowledge the support from Qatar University 's internal grant QUCG-CENG-19/20-7 .Scopu

Development of enzyme-free hydrogen peroxide biosensor using cerium oxide and mechanistic study using in-situ spectro-electrochemistry

During recent development, it has been demonstrated that cerium oxide nanoparticles (CNPs) have exhibited catalytic activity which mimics naturally existing enzymes such as superoxide dismutase (SOD) and catalase. The underlying mechanism is attributed to the modulation of oxygen vacancies on CNPs lattice by dynamic switching of the oxidation states between Ce3+ and Ce4+ due to the electron transfer resulting from the redox reaction between CNPs and reactive oxygen species such as hydrogen peroxide (H2O2). Thereby the redox potential of CNPs is dependent on the surface chemistry i.e. the surface concentration of Ce3+ and Ce4+ Currently, the ratio of Ce3+/ Ce4+ in CNPs is characterized ex-situ using XPS or TEM which involves sample drying and exposure to high energy X-rays and electron beam, respectively. Sample drying and high energy beam exposure could lead to sample deterioration. The goal of the study is to explore a technique to characterize CNPs in-situ and identify the surface chemistry of CNPs. The in-situ investigation of CNPs was carried using spectroelectrochemistry wherein the electrochemical and optical measurements are carried out simultaneously. Detailed optical characterization of two different CNPs having different catalytic activity were carried under oxidation and reduction environments. Analysis of spectra revealed widely different redox potential for CNPs which was a function of pH and composition of buffer solution. In second part of dissertation a suitable surface chemistry of CNPs is investigated to replace the enzyme in biosensor assembly to allow amperometric detection of H2O2 in physiological conditions. Upon electrochemical investigation of the physio-chemical properties of CNPs, it was found that CNPs having higher surface concentration of Ce4+ as compared to Ce3+ oxidation states, demonstrated increased catalytic activity towards H2O2. The addition of CNPs resulted in 5 orders of increment in amperometric current with a response time of 400 msec towards detection of H2O2 and exhibited excellent selectivity in presence of interfering species. Additionally, cerium oxide was successfully integrated into the biosensor assembly through the anodic electrodeposition, which allowed the transfer of electron generated from the CNPs in the redox reaction to the electrode and demonstrated successful sensing of H2O2. Furthermore, to achieve detection of H2O2 in physiological conditions, CNPs were integrated with nanoporous gold (NPG) which exhibited anti-biofouling properties. The anti-biofouling property of NPG was investigated using electrochemical techniques and showed excellent signal retention in physiological concentration of albumin proteins. The novel study targets at developing robust enzyme free biosensor by integrating the detection ability of CNPs with the anti-biofouling activity of NPG based electrode

Battery Systems and Energy Storage beyond 2020

Currently, the transition from using the combustion engine to electrified vehicles is a matter of time and drives the demand for compact, high-energy-density rechargeable lithium ion batteries as well as for large stationary batteries to buffer solar and wind energy. The future challenges, e.g., the decarbonization of the CO2-intensive transportation sector, will push the need for such batteries even more. The cost of lithium ion batteries has become competitive in the last few years, and lithium ion batteries are expected to dominate the battery market in the next decade. However, despite remarkable progress, there is still a strong need for improvements in the performance of lithium ion batteries. Further improvements are not only expected in the field of electrochemistry but can also be readily achieved by improved manufacturing methods, diagnostic algorithms, lifetime prediction methods, the implementation of artificial intelligence, and digital twins. Therefore, this Special Issue addresses the progress in battery and energy storage development by covering areas that have been less focused on, such as digitalization, advanced cell production, modeling, and prediction aspects in concordance with progress in new materials and pack design solutions