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Inspired by “water ripples” in nature and the flocculation phenomenon in colloid chemistry, a novel liquid drop/colloid flocculation approach is developed to fabricate an extremely flexible and compressible 3D macroscopic graphene-based architecture (hydrogels or aerogels), via a new coagulation-induced self-assembly mechanism. This facile and universal technique can be achieved in a neutral, acidic, or basic coagulation bath, producing microsized hydrogels with various structures, such as mushroom, circle, disc shapes, etc. The method also allows us to introduce various guest materials in the graphene matrix using transition metal salts as the coagulating bath. A mushroom-shaped NiCo oxide/GS hybrid aerogel (diameter: 3 mm) is prepared as an example, with ultrathin NiCo oxide nanosheets in situ grown onto the surface of graphene. By employing as binder-free electrodes, these hybrid aerogels exhibit a specific capacitance of 858.3 F g–1 at 2 A g–1, as well as a good rate capability and cyclic stability. The asymmetric supercapacitor, assembling with the hybrid aerogels as cathode and graphene hydrogels as anode materials, could deliver an energy density of 21 Wh kg–1 at power density of 4500 W kg–1. The ease of synthesis and the feasibility of obtaining highly flexible aerogels with varied morphologies and compositions make this method a promising one for use in the field of biotechnology, electrochemistry, flexible electronics, and environment applications.

Over the past few years, graphene-based composites have been proved to be one of the most prominent materials of modern times. Graphene based aerogels are the most widely explored 3D-graphene structures for wastewater treatment, owing to its excellent physical and chemical properties. There has been a considerable development in the research of graphene-based materials with the exponential progress of scientific publications in the last five years. This review starts with a background of graphene-based composite, its structural properties and the importance of aerogel material in wastewater treatment. In continuation, a number of recent and emerging experimental procedures for the preparation with particular focus on fabrication techniques and advanced applications in wastewater remediation are elaborated. Techniques, including hydrothermal reduction, chemical reduction, electrochemical reduction, cross-linking, chemical vapor deposition etc. are discussed in detail. Finally, the challenges and research lacunas of graphene based emerging materials and their future perspective in economy are discussed.

Supercapacitors are emerging as highly promising electrochemical energy storage devices, which can be sources of clean and sustainable energy. Off late, a lot of studies are ongoing in the field of carbon-based hydrogels for use in electrodes for supercapacitors. A lot of hybrid materials employing carbon frameworks and pseudo-capacitive materials are finding a lot of importance in the current scientific environment, as they synergistically improve the electrochemical performance of the supercapacitor electrode. It is also highly imperative to carefully choose the components of the hybrid, since the final properties and performance of the material will highly depend on the individual properties of the components. In the light of these important parameters, my study focuses on the synthesis of hybrid materials with carbon-based materials for a base. In the initial phase of my work, soft materials like three-dimensional graphene hydrogel hybrid systems were used. Polyaniline and manganese oxides nanoparticles were embedded in a graphene hydrogel matrix and their performance was studied for supercapacitor applications. Zeolite imidazole framework-derived (ZIF-67) nickel cobalt mixed oxides were then embedded in graphene hydrogels and were found to perform better than the Manganese oxide/polyaniline/graphene hydrogel. A biomass-derived carbon/Ni Co metal nanoparticle system was designed to mimic this graphene/Ni Co mixed oxide system, with a new and non-conventional use of a coconut tree fibre, coconut leaf sheath; a cheap substitute for graphene. My study shows the possibility of using a well-studied, optimised and functionalised biomass-derived carbon as a cheap substitute for graphene and the detailed comparison studies have shown that the energy-power density matrix for a biomass derived carbon framework embedded with ZIF-derived Ni-Co nanoparticles is comparable with a graphene/ZIF-derived Ni Co mixed oxide system. This highlights the untapped potential in our abundantly available renewable resources like biomass. Doctor of Philosophy

A novel structure of graphene-based hybrid hydrogels was constructed, in which α-Ni(OH)2 nanoflowers with nanopetals thicknesses of approximately 20 nm were uniformly anchored on a three-dimensional graphene framework. Benefiting from the unique morphological nickel hydroxide nanoflowers and hydrogels, the nickel hydroxide nanoflowers/graphene hydrogels exhibited great specific capacitances (1 A·g–1; 1632 F·g–1), great rate capabilities, and longer cycle life (after 1000 cycles, 95.2% capacitance retention) when used as electrodes in supercapacitors.

Efficient bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are required for metal air batteries, to replace costly metals, such as Pt and Ir/Ru based compounds, which are typically used as benchmarks for ORR and OER, respectively. Isolated single atomic sites coordinated with nitrogen on carbon supports (M-N-C) have promising performance for replacement of precious metal catalysts. However, most of monometallic M-N-C catalysts demonstrate unsatisfactory bifunctional performance. Herein, a facile way of preparing bimetallic Fe and Co sites entrapped in nitrogen-doped hollow carbon nanospheres (Fe,Co-SA/CS) is explored, drawing on the unique structure and pore characteristics of Zeolitic imidazole frameworks and molecular size of Ferrocene, an Fe containing species. Fe,Co-SA/CS showed an ORR onset potential and half wave potential of 0.96 and 0.86 V, respectively. For OER, (Fe,Co)-SA/CS attained its anodic current density of 10 mA cm at an overpotential of 360 mV. Interestingly, the oxygen electrode activity (ΔE) for (Fe,Co)-SA/CS and commercial Pt/C-RuO is calculated to be 0.73 V, exhibiting the bifunctional catalytic activity of (Fe,Co)-SA/CS. (Fe,Co)-SA/CS evidenced desirable specific capacity and cyclic stability than Pt/C-RuO2 mixture when utilized as an air cathode in a homemade Zinc-air battery. Ministry of Education (MOE) This work was supported by the AcRF Tier 1 grant (RG105/19), provided by Ministry of Education in Singapore. H.H. and J.C. like to thank National Natural Science Foundation of China (Grant No. 11874044).

In this study, Fe doped, Co and CoO encapsulated N doped carbon frameworks were prepared from simple hybrid zeolite imidazole frameworks (ZIF) with extra N enrichment. The facile strategy included preparation of ZIF-8 core and ZIF-67 shell and later replacing the metal centers of ZIF with Fe ions and increasing the N content with N rich melamine. The metal and metal oxide components got captured and encapsulated in the N doped mesoporous carbon frameworks through a pyrolysis process at different temperatures. Fe, Co and CoO were trapped in the N doped mesoporous carbon networks through annealing and denoted as FCNCx. The activity and electrochemical stability of such prepared materials towards ORR and OER were tested in basic media. After analyzing rotating disk electrode studies, FCNC900 was seen to perform superior bifunctional electrocatalytic performance for both ORR and OER which was higher than Pt/C catalyst. Promising ORR performance of FCNC900 can be simply be judged from E 1/2 =0.868 V (vs. RHE) and E onset =1.01 V (vs. RHE) while OER overpotential for same catalyst was 360 mV much smaller than others. Longer stability and high methanol tolerance of this catalyst was also investigated. Ministry of Education (MOE) This work was supported by the AcRF Tier 1 grant (RT17/16), provided by Ministry of Education in Singapore.

Coconut leaf sheath-derived nitrogen doped carbon framework is developed and incorporated with nickel and cobalt metal nanoparticles in the carbon matrix by a facile process of growing ZIF-67 metal organic framework particles on the graphitised carbon, followed by annealing it in inert atmosphere. Various parameters are modified to obtain three different samples. These samples are tested for high performance supercapacitors and oxygen evolution reaction (OER) catalysts. The optimised sample NiCo–C-1 gave a high specific capacity of 308 mAh g−1 at a current density of 1 A g−1 in a 2 M KOH electrolyte. An asymmetric supercapacitor assembly prepared from NiCo–C-1 as the positive electrode and the nitrogen-doped carbon as the negative electrode, exhibited an energy density of up to 31.8 Wh Kg−1 for a high power density of 6.2 kW kg−1 over a potential window of 0–1.55 V. Two of our best samples were also tested for OER, giving good water oxidation kinetics, revealed by their lower Tafel slopes of around 107 mV and a low over potential (η) of around 420 mV at a current density of 10 mA cm−2. Hence, this work opens great avenues for biomass-derived materials for high performance supercapacitors and catalysis. MOE (Min. of Education, S’pore)

A novel three dimensional ternary composite graphene-based hydrogel was prepared using a simple hydrothermal reaction. The graphene/polyaniline/MnOx hydrogel named as PGM-HCl, obtained by the controlled morphology of MnOx by preparing it in an acidic environment using hydrochloric acid (HCl), offered an extremely high specific capacitance of 955 F g−1 at a current density of 1 A g−1 and a capacitance retention of 89% after 1000 cycles and 69.1% after 5000 cycles at 20 A g−1. The synergistic effects created by the pseudocapacitance of PANI, MnOx and the electrochemical double layer capacitance of graphene highly improved the overall electrochemical performance of these hydrogels compared to the performances of the individual constituents. The electrode prepared from this material gave an extremely high specific energy density of 61.2 W h kg−1 even at a high power density of 4.5 kW kg−1. The composite had the superior advantage of being used directly for binder free supercapacitor electrodes and made use of low cost raw materials making it a very promising candidate for cost effective, large scale and high energy applications. A hydrogel PGM was also prepared in the absence of HCl using a purely hydrothermal reaction under the same reaction conditions, for comparison. Thus, this study opens the exciting untapped potential in engineering and synthesizing such composites with existing low cost materials for high performance energy storage devices like supercapacitors.

PHB-silver nanocomposite (PHB-AgNc) was synthesized biologically by utilizing a dairy-industry by-product, cheese whey permeate as a substrate for Bacillus megaterium. The single-step synthesis of PHB-AgNc was further confirmed by UV–vis spectroscopy and GC-MS analysis. Further, the extracted PHB-Ag Nc was characterized by employing various techniques such as TEM, SEM, FTIR, NMR, Zeta Potential, and DLS analysis. Mechanical properties such as elongation at break, tensile strength, and Young's Modulus were found to be 1.305%, 35.42, and 1.058 N/mm2, respectively. The nanocomposite was found to be stable, polydispersive, and hydrophobic. It exhibited a degradation temperature of 340 °C and portrayed significant antimicrobial resistance against common food pathogens such as E.coli and Pseudomonas spp. Batch fermentation study was carried out for a period of 96 h in a 14 L bioreactor. The highest biomass and nanocomposite yield obtained was 5.8 and 2.4 g/L, respectively. The highest product productivity concerning biomass was found to be 0.012 h−1 at 12 h. The film's migration properties were tested for various food stimulants, and the values obtained were less than the overall migration limit established for food contact materials; hence, the film was found to be appropriate for food packaging applications.

Highly optimized nickel cobalt mixed oxide has been derived from zeolite imidazole frameworks. While the pure cobalt oxide gives only 178.7 F g-1 as the specific capacitance at a current density of 1 A g-1 , the optimized Ni:Co 1:1 has given an extremely high and unprecedented specific capacitance of 1931 F g-1 at a current density of 1 A g-1 , with a capacitance retention of 69.5% after 5000 cycles in a three electrode test. This optimized Ni:Co 1:1 mixed oxide is further used to make a composite of nickel cobalt mixed oxide/graphene 3D hydrogel for enhancing the electrochemical performance by virtue of a continuous and porous graphene conductive network. The electrode made from GNi:Co 1:1 successfully achieves an even higher specific capacitance of 2870.8 F g-1 at 1 A g-1 and also shows a significant improvement in the cyclic stability with 81% capacitance retention after 5000 cycles. An asymmetric supercapacitor is also assembled using a pure graphene 3D hydrogel as the negative electrode and the GNi:Co 1:1 as the positive electrode. With a potential window of 1.5 V and binder free electrodes, the capacitor gives a high specific energy density of 50.2 Wh kg-1 at a high power density of 750 W kg-1 .