Fabrication of graphene-based smart nanomaterials for green and sustainable electrochemical energy storage and conversion

Green and sustainable electrochemical energy storage and conversion have been increasing in demand worldwide. Lithium ion batteries (LIBs) and supercapacitors are the prime devices for electrochemical energy storage and conversion. After discovery of the extraordinary physicochemical properties of graphene by Andrey and his co-workers in 2004, graphene-based nanomaterials have been shown to be one of the most promising and novel nanomaterials for green and sustainable electrochemical energy storage and conversion systems. Therefore, there is a growing interest in using graphene-based nanomaterials as electroactive materials for supercapacitors and LIBs electrodes. Graphene is a promising nanomaterial as it possesses many exciting physicochemical properties including high electrical conductivity and large surface area. However, pure graphene does not exhibit good performance in LIB and supercapacitor systems. On the other hand, metal oxide nanoparticles could give rise to superior electrochemical performance for LIBs and supercapacitors. Besides, metal oxide nanoparticles are environmentally important as they have tremendous efficacy in the removal of heavy metal ions from aqueous solution. Further, several metal oxide nanoparticles, especially silver-based metal oxide nanoparticles exhibit remarkable efficiency against various microbial pathogens. However, it is highlighted that metal oxide nanoparticles suffer from poor electric conductivity and therefore, direct use of pure metal oxide nanoparticles as electroactive materials in LIBs and supercapacitors is not functional. Interestingly, the incorporation of metal oxide nanoparticles into graphene yielding graphene-based nanomaterials could improve performance of LIBs and supercapacitors. In this thesis, four promising nanomaterials, namely, (i) hematite nanorods, (ii) 1D-Sn (IV) hydroxide nanofluid, (iii) highly oxidized/silver/tin(IV) oxide nanocomposites and (iv) poorly oxidized/silver/tin(IV) oxide nanocomposites have been fabricated and their different applications including as supercapacitor electrodes have been investigated. First of all, a general background has been presented on graphene-based nanomaterials and electrochemical energy storage systems emphasising the state-of-the-art in LIBs and supercapacitors, and this content has been placed in Chapter 1 as “Introduction”. Chapter 2 has been constructed with a published paper describing hematite nanorods synthesis through a green approach with application to Pb(II) ions removal from aqueous systems. Chapter 3 contains the second published paper concerning the supercapacitor studies of the hematite nanorods from the section 2. Chapter 4 contains a third published paper discussing the synthesis of a promising nanomaterial, 1D-Sn(IV) hydroxide nanofluid, and the study of several physicochemical properties of this material, including its nonlinear optical property (NLO). Chapter 5 shows a fourth published paper that demonstrates the synthesis of two nanomaterials; (i) highly oxidized silver oxide/silver/tin(IV) oxide (HOSBTO or Ag3+-enriched AgO/Ag/SnO2) and (ii) poorly oxidized silver oxide/silver/tin(IV) oxide (POSBTO or AgO/Ag/SnO2) nanocomposite, using the aforementioned tin(IV) hydroxide nanofluid as main reaction precursor. In addition, both nanocomposites have been tested against four hazardous food pathogens. Section 6 is the final chapter which conveys the conclusions and plan for future activities. In this section, it is discussed that graphene-based nanomaterials have already been synthesized by incorporation of as-reported metal oxides/hydroxide with graphene. As a consequence, three different types of graphene-based nanocomposites, namely, (i) hematite nanorods decorated graphene nanocomposites, (ii) tin(IV) oxide embedded reduced graphene oxide nanocomposites, and (iii) silver oxide/silver/tin(IV) oxide/graphene nanocomposites have been developed. Further, structural, morphological and chemical characterizations of the as-described graphene-based nanocomposites have already been carried out using several microscopic techniques such as SEM, EDX, TEM, HR-TEM, SAED, XRD, XPS, TGA and FT-IR. Finally, each of the graphene-based nanomaterials has been used to fabricate electrodes of the LIBs and the supercapacitors. A deep and detailed electrochemical study of these LIBs and supercapacitors has been conducted by performing cyclic voltammetry (CV), glavanostatic charge-discharge (GCD) cycling and electrochemical impedance spectroscopy (EIS) studies. Consequently, the electrochemical performance of these LIBs and supercapacitors has been evaluated. Based on the full studies of LIBs and the supercapacitors on each of the graphene-based nanomaterials, six manuscripts are being prepared for submission in suitable journals. The metal oxides-based graphene nanocomposites have exhibited commendable performance towards both LIBs and supercapacitors. Therefore, mixed incorporation of these transition metal oxides towards graphene may give rise to other graphene-based nanocomposites worth exploring for developing high performance LIBs and supercapacitors in future.