Porous Graphene Materials for Energy Storage and Conversion Applications

Porous graphene materials possess a unique structure with interconnected networks, high surface area, and high pore volume. Because of the combination of its remarkable architecture and intrinsic properties, such as high mechanical strength, excellent electrical conductivity, and good thermal stability, porous graphene has attracted tremendous attention in many fields, such as nanocomposites, lithium batteries, supercapacitors, and dye-sensitized solar cells. This chapter reviews synthesis methods, properties, and several key applications of porous graphene materials

Synthesis and characterization of modified graphene for energy storage applications

This thesis presents the synthesis and characterization of modified graphene materials and investigates their role in sustainable energy storage applications by using both experimental methods and density functional theory simulations. The outcomes obtained provide a better understanding of the structure-property relationship in modified graphene and its role in electrochemical process in rechargeable batteries, benefiting the development of high-performance electrode materials

Structure-Property Relationship in 3D Graphene-Based Macrostructures

Three-dimensional (3D) graphene-based macrostructures (GBMs) have shown great potential in a wide range of clean energy-related technologies (including batteries, supercapacitors, fuel cells, solar cells, etc.) and environmental remediation applications (such as absorption, adsorption, catalysis, sensors, etc.) during recent years. However, from a practical viewpoint, a fundamental understanding of the structure-property relationship of 3D GBMs is essential to precisely tune their physicochemical properties, thereby outspreading their application potential. The current chapter targets this aspect amongst others, with a special emphasis on the effects of pore geometry on the physicochemical properties of 3D GBMs

First-principles study of sulfur species adsorption on microporous reduced graphene oxide (rGO)

Lithium-sulfur (Li-S) battery has revolutionized the energy storage arena due to its high theoretical energy density, which is 5 times greater than Li-ion battery, non-toxic nature and the high abundance of sulfur in nature. However, due to the fact that the entire mechanism is governed by a complex chemistry where the anode, electrolyte and the cathode interact with each other, several issues have prevented the practical applications of Li-S batteries so far. In this study, a systematic first-principles density functional theory calculation is employed to gain insight into how microporous reduced graphene oxide (rGO) influences the adsorption of unlithiated S8 and long chain lithium polysulfides (Li2Sx, x≥4) in terms of van der Waals interaction and chemical bonding. At the early stage of lithiation, the interaction between non-polar S8 and the substrate is dominated by physical interaction, thus smallest pore size induces the strongest interaction. Graphene domains and polar groups or rGO have the same adsorption strength towards S8, however functional groups develop a slight charge barrier at the interface. As discharge proceeds, functional groups enhances the interaction between long chain polysulfides by forming strong Li-O bonds and improve the conductivity of the cathode by facilitating the electron transfer at the interface

Effects of heteroatom doping on the performance of graphene in sodium-ion batteries: A density functional theory investigation

Heteroatom doped-graphene is a potential candidate as an anode material in sodium-ion batteries (SIBs). However, one of the major issues holding back its development is that a complete understanding of the doping effects accounting for the Na-ion storage of heteroatom-doped graphene has remained elusive. In this work, first principles calculations have been conducted to systematically investigate the electronic and geometric effects in various heteroatom-doped graphene. Graphene doping with pyridinic-N, pyrrolic-N, F and B improves the electrochemical Na storage due to the electronic effect which originates from electron deficient sites (i.e. defects or electron deficient atoms). On the other hand, P doping improves the Na storage ability of graphene due to the geometric effect caused by bond length mismatch. In contrast, the introduction of graphitic-N and S into graphene is inefficient for Na storage because of their inability to accept electrons from Na. Interestingly, the diffusion energy barriers obtained for Na on doped graphene are lower than that for the pristine graphene. Furthermore, co-doping strategy is predicted to achieve even better Na storage capacity due to the synergistic effect

Investigation of mechanical and electrical properties of 3D porous graphene hydrogels

As a 3D porous carbon monolith, graphene hydrogel (GH) has attracted increasing attention in various applications like energy storage devices, pollutant adsorption, catalysis, and tissue engineering. Among different synthesis methods of GHs, hydrothermal reduction demonstrates an advantage over others as it is a water-only simply routine producing reduced graphene oxide (rGO) with less impurity. Self-assembly and reduction play an important role in determining the final structure and properties of GHs. However there is a lack of understanding of the nature of the linkage (covalent or π-π stacking) between graphene sheets that causes the hydrogel formation and how self-assembly affects the properties. In this study, we have investigated the influence of the reduction degree and the bonding mechanism of graphene sheets on morphology, and mechanical and electrical properties. The effects of pH value on self-assembly of GHs, in terms of electrostatic repulsion force between GO sheets were examined

Recent advances in graphene based materials as anode materials in sodium-ion batteries

Sodium-ion batteries (SIBs) have emerged as a promising alternative to Lithium-ion batteries (LIBs) for energy storage applications, due to abundant sodium resources, low cost, and similar electrochemical performance. However, the large radius of Na+ and high molar mass compared to Li+, result in large volume strain during charge/discharge and low reversible capacity and poor cycling stability. Due to exceptional physical and chemical properties, graphene has attracted increasing attention as a potential anode material for SIBs. When integrated with other nanomaterials in electrodes, graphene can improve the electrical conductivity, accommodate the large volume change and enhance reaction kinetics. This paper provides a systematic review of recent progress in the application of graphene based anodes for SIBs, with a focus on preparation, structural configuration, Na+ storage mechanism and electrochemical performance. Additionally, some challenges and future perspectives are provided to improve the sodium storage performance of graphene based electrodes

Understanding the structure-property relationships in hydrothermally reduced graphene oxide hydrogels

Graphene hydrogel (GH) has attracted increasing attention in energy storage and conversion, pollutant adsorption, catalysis, sensors and tissue engineering applications. However, a good understanding of the structure-property relationship is essential to precisely tune their properties. In this work, a pH assisted hydrothermal process was used to synthesize reduced graphene oxide (rGO) hydrogels with different three-dimensional (3D) porous structures. We systematically investigated the structure-property relationships in the GH, with a focus on the effects of geometrical dimensions of the pore structure. We found that the best mechanical properties were achieved in a compact microstructure consisting of small pores but thick walls. Despite having a lower C/O ratio, the compact structure gave rise to the highest electrical conductivity, attributed to the highly interconnected 3D porous structure providing conductive pathways. On the other hand, the hydrogels prepared under basic conditions exhibited higher C/O ratio but lower mechanical and electrical properties due to the disordered pore structure with large pores and thin walls

Interaction between functionalized graphene and sulfur compounds in a lithium-sulfur battery - a density functional theory investigation

Lithium–sulfur (Li–S) batteries are emerging as one of the promising candidates for next generation rechargeable batteries. However, dissolution of lithium polysulfides in the liquid electrolyte, low electrical conductivity of sulfur and large volume change during electrochemical cycling are the main technical challenges for practical applications. In this study, a systematic first-principles density functional theory calculation is adopted to understand the interactions between graphene and graphene with oxygen containing functional groups (hydroxyl, epoxy and carboxyl groups) and sulphur (S8) and long chain lithium polysulfides (Li2S8 and Li2S4). We find the adsorption is dominated by different mechanisms in sulphur and lithium polysulfides, i.e. van der Waals attraction and formation of coordinate covalent Li–O bonds. The adsorption strength is dependent on the inter-layer distance and electron rich functional groups. Through these mechanisms, sulphur and lithium polysulfides can be successfully retained in porous graphene, leading to improved conductivity and charge transfer in the cathode of Li–S batteries

Coating Fe2O3 with graphene oxide for high-performance sodium-ion battery anode

Sodium-ion batteries (SIBs) have recently shown the potential to meet the demands for large scale energy storage needs as an attractive alternative to lithium-ion batteries due to the high abundance of sodium resources around the world. The major hurdle of SIBs resides in developing viable anode materials with a high energy density and an appropriately long cycle life. Here a simple and low-cost method for synthesizing Fe2O3/graphene oxide (Fe2O3/GO) composites made out of Fe2O3 nanoparticles sandwiched between graphene oxide (GO) layers is reported. The unique structure of the Fe2O3/GO composites served a synergistic effect to alleviate the stress of Fe2O3 nanoparticles, prevent nanoparticles aggregation, maintain the mechanical integrity of the electrode, and facilitate mass transfer of Na ions during batteries operating. Consequently, the Fe2O3/GO composites as anode for SIBs attained a reversible specific capacity of ca. 420 mAh g-1 after 100 cycles at 0.1C (1C=1007 mA g-1) and a good rate capability at various current densities. Moreover, the Coulombic efficiency of the SIBs could rapidly increase in the early cycles. Due to the facile synthesis method and high electrochemical performance, the Fe2O3/GO composites would have a significant potential as anode materials for rechargeable SIBs