Emerging Nanostructured Materials for Energy and Environmental Science

Materials play a huge role in the development of human lifestyle at all ages. By the emergence of nanomaterials, a new field of science and technology was born, known as “nanoscience and nanotechnology.” This is mainly because of the peculiar characteristics of the nanomaterials that are applied in modern science and technology. Nowadays there are mainly two major global threats, namely, (i) energy and (ii) environment cleanliness. Most of the energy used these days are from fossil fuels, and this energy is running out, and therefore there is a huge hunt for new sources of energy; mostly they are renewable energy and that should be environment-friendly. Energy generation and storage are big challenges and can be done via solar cells, fuel cells, and batteries. The next issue of environmental pollution is mainly from the enormously developing industries. Most of these industries discharge effluents (mainly from textile and tanning industries) into the mainstream water bodies, leading to a polluted water for supply. This particular issue, depending on the type of chemicals/dyes/organics that it contains, is very harmful to the living things, including human beings. The level of danger can go to the extent of inducing cancer. These effluents have to be treated properly and converted into harmless products (such as water, CO₂, etc.) before letting them into water. Process like advanced oxidation process assisted by photocatalyst can be a good solution for cleaning the industrial wastes. Ability of the photocatalyst with appropriate characteristics is very crucial for this particular application. Another category of threat to environment is the corrosion caused by the electrochemical reactions when the metal/structure interacts with their surroundings. What is more importantly required to meet out the energy and environment issues is to develop better performing materials that can improve the working efficiency of solar cells, batteries, and photocatalysts for photocatalysis. Boon for this is the blooming of “nanomaterials” that come with improved properties compared to their bulk counterpart. Development of nanomaterials is enormous in all these fields of applications. In this aspect, this chapter describes the applications of nanomaterials in the fields of energy (solar cells, batteries, and fuel cells) and environment issues (gas sensing, photocatalysts in photocatalysis, and corrosion). The emergence of these nanomaterials for these applications is discussed in this chapter.

With the increase in worldwide consumption of nonrenewable energy resources (i.e., fossil fuels) and emission of toxic gases, it is our foremost concern to concentrate our research on sustainable and renewable energy. This motive paved the way to develop several renewable energy production and storage systems, like solar cells, supercapacitors, fuel cells, and lithium-ion batteries. These devices, with high specific power, long cycle life, portability, and ease of fabrication, have been able to secure worthy positions in the field of energy science and technology. Over the last decades, attempts have been taken to use nanostructured carbon-based materials, like graphene and carbon nanotubes (CNTs), with the aim of improving the efficiency of the abovementioned energy storage systems.

In this book chapter, focus has been directed toward the recent progress and advancement on the efficiency of the electrode materials of these renewable energy storage systems via application of CNTs, graphene, or nanohybrid fillers. The ability of these materials to exhibit superior capacity toward photon absorption, capability toward generation of photocarriers, photovoltaic properties, and separation of charge carriers to form heterojunctions makes them ideal applicants in solar cells. The capacitance of a supercapacitor varied with the specific surface area, synthetic approach, pore size, pore size distribution, and posttreatment of these materials. Moreover, high electron conductivity and high surface area of these nanomaterials led to improvements in (a) electrode reaction rates in fuel cells, lithium-ion batteries, and supercapacitors and (b) charge storage capability of supercapacitors and lithium-ion batteries.

As a consequence to the remarkable development of the science and technology, an exponential demand for energy leads to the exploitation of nonrenewable energy sources including fossil fuel paves the way to stern environmental crises. Global warming is one of the principal threats, due to the accumulation of greenhouse gases, resulting from the use of fossil fuels. Because of limited availability, the fossil fuels have been rapidly exhausting, compelling researchers to accelerate the search for environment-friendly, renewable, and sustainable energy sources like the solar cell, wind, and electrochemical energy storage systems. Electrochemical energy storage systems (EESs), more specifically rechargeable batteries and supercapacitors being efficient alternatives, have attracted tremendous attention. Rechargeable batteries not only serve as energy storage devices but also capable of providing the dispatchable energy for transportation, i.e., electrical vehicles (EVs and hybrid EVs).

Although LIBs possess energy densities higher than those of the conventional batteries, their lower power densities and poor cycling lives are critical challenges for their applications in electric vehicles (EVs) and grid-scale storage. The present book chapter is an attempt to provide a detailed description of several aspects of the development of Li-ion battery, i.e., preferred electrode (cathode as well as the anode) materials, separators, electrolyte media, and their additives with associated challenges. This chapter spotlights the mechanism for Li-ion storage (lithiation/delithiation processes) with various vital parameters that determine the overall performance of a battery including the shape and size of electrode materials. The recent advancement in designing several nanostructures for high-energy electrodes are highlighted in detail.

The use of fossil fuels such as coal, oil, and natural gas has allowed a fast and unprecedented development of human society. However, this has led to a continuous increase in anthropogenic CO₂ emissions, which affect human life and the ecological environment through global warming and climate changes. There are various strategies to mitigate the atmospheric concentration of CO₂, such as capture, separation, and utilization. Among them, CO₂ hydrogenation to obtain different products through catalytic processes is a strategy of great interest. Thus, the catalytic combination of CO₂ and hydrogen not only mitigates anthropogenic emissions into Earth’s atmosphere, but it also produces carbon compounds that can be used as fuel or precursors for the production of different chemicals.

This chapter reviews the use of different nanomaterials for CO₂ hydrogenation. Three different processes are distinguished, depending on the final product: (i) CO₂ hydrogenation to carbon monoxide, (ii) methanol production by CO₂ hydrogenation, and (iii) CO₂ hydrogenation to methane. It has been included both nanomaterials that act as support and those that can replace the active metal phase. Concerning CO₂ hydrogenation to CO, one-dimensional transition metal carbides have received increasing attention because their unique electronic structure allows similar catalytic properties to the expensive noble metals. Attending the high thermal requirements of CO synthesis, emerging metal oxides nanocatalysts are focused to prevent the metal sintering by increasing the metal-support interactions. Controlling the support’s morphology at nanoscale can enhance both catalytic activity and stability at high temperatures up to twice with respect to those conventional micro-sized catalysts. Regarding to methanol production, the nanomaterials most commonly used as supports are those based on carbon, e.g., carbon nanotubes, carbon nanofibers, and graphene oxide. The main advantage of using these materials is their high surface area, which improves metallic phase dispersion, higher thermal and electrical conductivities, and greater mechanical resistance. In addition, the use of intermetallic nanoparticles as an active phase is very promising. The combination of two metals in the same nanoparticle greatly increases the interface between components, which clearly leads to a synergistic effect between them. The use of these nanomaterials improves the activity and selectivity to methanol between 2 and ~50%, compared with classical catalysts. Moreover, similar strategies are equally valid in methane production. Catalysts based on nanoparticles, such as Ni or NiO, supported on traditional metal oxides have been recently reported to improve catalytic activity in CO₂ methanation with high resistance to coke deposition. Other supports, such as carbon nanofibers and carbon nanotubes previously mentioned, have shown excellent results, with CO₂ conversions higher than 90% and complete selectivity to methane. Finally, TiO₂-based catalysts are a promising solution for methane production by the still undeveloped photocatalytic reduction. This reaction can be performed under mild temperatures and pressure conditions, which is a clear advantage for methane synthesis.

Hydrogen storage is a critical bottleneck to hydrogen economy. Presently none of the solid-state hydrogen storage materials (metal hydrides) reaches the capacity vs performance target (6.5 wt.% at 85 °C/5-12 bar, 1500 cycles) for the commercialization of light duty H₂ fuel cell vehicles. A few reversible hydrogen storage materials (e.g. MgH₂, LiBH₄/MgH₂ composite) possess adequate capacity, but their performance needs to be improved significantly. Metal oxide additives improve the hydrogen storage performance of metal hydrides, but the additive-hydride reaction mechanism remains not well understood. In this context, the present chapter discusses how various metal oxide additives interact with metal hydrides and facilitate the low temperature de/ab-sorption of hydrogen. The metal oxide additives may either directly catalyze the reaction without making any chemical changes or they catalyze indirectly by making active in situ products. In this chapter, various oxides and hydride combinations of the latter category are analyzed, and factors governing the improved hydrogen ab-/desorption performance are highlighted.

The fundamental properties of supercapacitors (SCs) with descriptions restricted to the metal oxides systems and the effect on the electrochemical performance and synthesis are described in this chapter. Metal oxides such as manganese oxide (MnO), vanadium oxide (V₂O₅) and ruthenium oxide (RuO) have demonstrated great potential in the field of energy storage due to their structural as well as electrochemical properties, thus attracting huge attention in the past decade and in recent years. The major contributing factor to the electrochemical properties is their capability to achieve relatively high pseudocapacitive performance derived from their theoretical values resulting from their multiple valence state changes. The developments of the metal oxide (MO)-based electrode materials and their composites are being explored from the synthetic point of view as well as their emerging applications as energy storage materials. Therefore, the need to further exploration of MO-based electrodes is motivated by their considerably low-cost and environmentally friendly nature as compared to other supercapacitive electrode materials. This chapter accounts to the overview of various nanostructured metal oxide materials for application as energy storage materials in supercapacitors.

The large scarcity of natural fuels in earth crust has triggered to search alternative energy reservoirs for the future generation of human life. Because of large abundancy, solar energy is considered as big hope for the future generation energy utilization for commercial as well as home applications. The scientific revolution achieved in synthesis and processing of semiconductor nanomaterials, organic conducting polymers have led into new dimension in fabrication of future-generation solar cells. Reduction in the dimension of semiconductor nanomaterials significantly influences on their structural and optical properties which is helpful for the excellent photon harvesting. Also, their large surface area is further favourable to assist with the attachment of several organic or inorganic compounds in order to functionalize them effectively. Developments that have been made in semiconducting organic polymers still encourage the fabrication of highly efficient, flexible solar cell devices on conducting substrates. Formation of nanocomposites, hybrids, alloy system, doping, etc. are successfully carried out on different kinds of inorganic semiconductor nanomaterials for the photovoltaic applications. The day-by-day improvement in terms of efficiency and new materials development predicts that the breakthrough to achieve highly stable, high-efficiency solar cell is about the near future. In this aspect, this chapter summarizes the development in the solar cells research of each category with general aspects. The important parameters and process that affects the performance of each category is outlined.

Microbial fuel cell (MFC) technologies have been globally noticed as one of the most promising sources for alternative renewable energy, due to its capability of transforming the organics in the wastewater directly into electricity through catalytic reactions of microorganisms under anaerobic conditions. In this chapter, the state of the art of review on the various emerging technological aspects of nanotechnology for the development of nanomaterials to make the existing microbial fuel cell technology as more sustainable and reliable in order to serve the growing energy demand. Initially, a brief history of the development and the current trends of the microbial fuel cells along with its basic working mechanism, basic designs, components, fascinating derivative forms, performance evaluation, challenges and synergetic applications have been presented. Then the focus is shifted to the importance of incorporation of the nanomaterials for the sustainable development of MFC technology by means of advancements through anode, cathode, and proton exchange membranes modifications along with the various ultimate doping methods. The possibilities of applied nanomaterials and its derivatives in various places in MFCs are discussed. The nanomaterials in MFCs have a significant contribution to the increased power density, treatment efficiency, durability, and product recovery due to its higher electrochemical surface area phenomenon, depending on the fuel cell components to get modified. The promising research results open the way for the usage of nanomaterials as a prospective material for application and development of sustainable microbial fuel cells. Though the advances in nanomaterials have opened up new promises to overcome several limitations, but challenges still remain for the real-time and large-scale applications. Finally, an outlook for the future development and scaling up of sustainable MFCs with the nanotechnology is presented with some suggestions and limitations.

Water contamination has been a global challenge for many decades. A variety of inorganic and organic pollutants have indeed degraded the quality of water over the years. Among these various pollutants, fluoride ions stand out as the inorganic pollutants of prime concern in ground- and drinking water since concentrations which exceed 1,5 mg/L can lead to skeleton fluorosis and other bone disorders. The defluoridation of ground- and drinking water has therefore become a topic of great importance. This chapter therefore serves to review the current global distribution of fluoride in ground- and drinking water. Some attention is also paid to the natural and anthropogenic sources of fluoride as well as the chemistry of fluorides which renders them toxic. The various toxic effects of fluoride are highlighted. The defluoridation techniques employed thus far are also critically reviewed and discussed. Much emphasis is devoted to adsorption technology which is currently the most popular technology due to its simplicity and effectiveness and the various adsorbents that have been tested for defluoridation. Each adsorbent is critically analysed with respect to its merits and demerits. Lastly, the benefits of nanotechnology and nanostructured adsorbents are discussed, and future research prospects on the use of such a technology are presented.

Molecular gels are ubiquitous soft solids formed by the self-assembly of small building blocks via the weak intermolecular interactions resulting in the formation of 3D nano- or micro fibrous network wherein solvent molecules are trapped that act as an excellent platform for environmental and energy applications. Appropriate molecular modification can alter the nanoscale assembly which could be utilized practically for various applications in the field of biology, medicine, and materials science. The nanofiber formation in gel can be transformed into conducting architectures or metallic nanowires via doping and annealing procedure. Since gels are formed by weak intermolecular interactions, the gel-to-solution transition can be triggered by various external stimuli such as temperature, mechanical action, light, ultrasound waves, acids, bases, ions, redox reagents, and biomolecules. However, gel-to-sol phase change associated with stimuli-responsive behavior can be used for tuning the molecular-level behavior. This chapter reviews the various practical applications of molecular gels such as removal of dyes, aromatic compounds, toxic metals, anions, hydrocarbons, crude oil, smart photonics, electrolytes, and artificial light-harvesting devices.

Self-cleaning in one of the most significant applications in the recent years to repel the contaminants like dirt, toxic pollutants, and microbes from any kind of surface. Hydrophobic and hydrophilic coatings are commonly used to fabricate a self-cleaning surface. Nanoparticles play a vital role in the design of self-cleaning glasses/goggles, windows, paints, building materials, medical devices, fabrics, and corrosion resistance materials. The basic principles, various applications, and key findings of hydrophobic/super-hydrophobic and hydrophilic/super-hydrophilic coated substrates are briefly discussed in this chapter. The utilization of different self-cleaning products available in the market is also highlighted. Moreover, the future challenges are described to fabricate an eco-friendly, cost-effective, highly stable, and durable self-cleaning surface.

The development of nanotechnology marked a new era in the field of catalysis. The fascinating electronic, structural, and magnetic properties of the nanomaterials arising from its shape, size, and unusually high surface-to-volume ratio make them unique from the bulk materials. These properties facilitate various chemical reactions including fuel cell and other electrochemical reactions for energy and environmental applications. In the recent years, fuel cells have emerged as alternative energy sources to meet the ever-increasing energy demands. Thus, designing suitable catalyst to boost the rate of the reactions associated with fuel cells or other energy storage devices has become very crucial. Different types of electrochemical reactions include oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and electrooxidation of small organic molecules like methanol, ethanol, formic acid, etc. On the other hand electrochemical CO₂ reduction to various value-added products is a promising strategy to address the atmospheric CO₂ levels. Herein, we have mainly emphasized the catalytic behavior of different types of morphology and size-dependent nanomaterials toward electrochemical reduction of oxygen and CO₂ and electrooxidation of formic acid and ethanol.

Various environmental hazards occurring in present days are the results of population explosion, industrial pollution, unsafe agricultural practices, and several miscellaneous reasons. Hence, remediation process becomes very crucial in limiting the pollution. The process of treatment of contaminated environmental media, i.e., soil and water, in order to remove the toxicants present in it is called as “remediation/environmental remediation.” “Bioremediation” is a process of swabbing contaminated media with biological agents/microbes or naturally extracted chemicals. Bioremediation depending on site of application is further categorized into ex situ bioremediation, in situ bioremediation, phytoremediation, and permeable reactive barrier (PRB). However, if the percentage of contaminant is higher in the media, microbes used for bioremediation will get digested by toxicants/contaminants resulting in the ineffectiveness to remove the bacteria. While the usage of nanoparticles in bioremediation process is one of the key factors for reducing the limitations of this technique, the combination/addition of nanoparticles along with biological agents and applying on the contaminated media can give better results than individual bioremediation techniques. Nanoparticles due to their specific physical and chemical properties possess high reactivity with contaminated area. Nanomaterials are used in different forms in bioremediation process like nanoiron, nanofibers, nanorods, nanotubes, nanoribbons, nanocomposites, nanoporous materials, nanofoam, and nanocrystalline materials. Due to the powerful potential executed by the combination of nanoparticles and biological agents in bioremediation, their usage in future gets widened.