Nanostructured Materials for Sustainable Energy: Design, Evaluation, and Applications

The rational design of novel two-dimensional (2D) materials has long been sought, with a particular objective in catalysis, energy storage, and conversion. The discovery of 2D metal phosphorus trichalcogenides (MPX₃) opens lots of success and breakthroughs in diverse fields such as photocatalysis, electrocatalysis, battery storage, and membrane technologies. In this chapter, a comprehensive overview of the synthesis strategies and applications involving 2D MPX₃-based nanomaterials is presented. The recent development in bottom-up and top-down approaches for obtaining low-dimensional nanostructures and their limitations are briefly discussed. Their common [P₂X₆]^4- unit, which exhibits the advantage of monitoring the adsorption-desorption chemistry in several reactions along with the rich toolbox to entertain various metal elements in their structure, makes them act as a suitable candidate in catalysis and energy storage device. Thus, the present chapter highlights the advances of 2D MPX₃ nanomaterials and promising potential applications in sustainable energy conversion.

The interactive materials can rapidly respond to external signal changes and self-regulate their structures. Liu’s group and her collaborators developed sixty-two nanomaterial formulations using top-down and bottom-up methods. Some examples of energy generation or production of hydrogen are given in this chapter. Electrocatalysts and general procedures in common use through water electrolysis are summarized. Our electrocatalyst utilization for hydrogen production showed an increase of 30-65 % relative to the base metal/metal oxide. The power density and energy outputs of hydrogen fuel cells and microbial fuel cells were increased by one and three times, respectively. In energy storage, the utilization of heterogeneous metal oxides as the cathode and anode catalysts to improve the supercapacitor’s energy density was described. The average energy density of our approach in the symmetrical supercapacitor device was determined to be 138 W h kg^-1 at 450 W kg^-1. Other examples include the use of ceramics in extreme environments. One example cited was new materials composed of metal boride used in microwave absorption in a wide range of wavelengths. The complex permittivity and permeability of the ternary materials indicated that the microwave absorption characteristics are excellent at absorption efficiency and impedance matching characteristics, with maximum absorption of -47.8 dB at 6.0 GHz and a thickness of 2.4mm, shopping that modification of refractory materials can generate materials with superior performance and with applications as heat shields or communication antenna materials in extreme environments. This review contains some examples of engineered nanomaterials from our group and literature examples that showcase current opportunities and challenges in the sustainable energy environment in terms of design and deployment of electrocatalysts and devices for hydrogen production, generation of energy, or energy storage with a minimal carbon footprint.

The usage and consumption of fossil fuels have caused large amounts of greenhouse gases to be released leading to accelerated global warming and climate change. To mitigate these issues, we proposed a solution to develop novel, sustainable materials to enable long-lasting energy storage and conversion. One family of these materials are composed of organic-carbon redox-active composites, which display high performance and high-power density when used as electrodes in electrochemical capacitors (EC). These redox active organic-carbon composites can be produced using bottom-up or top-down approaches with tunable morphologies and architectures using sustainable active materials not requiring extensive mining. In addition to the current common conducting polymers, there are many other promising redox active materials including small molecules, macrocycles, and covalent organic frameworks, etc. In this chapter, the following key perspectives are discussed; established and emerging redox active species, carbonaceous materials, fabrication methodologies, structural characterization, and electrochemistry of promising composite electrodes. The future directions are discussed with a focus on the fundamental understanding of composite components and interactions, and improvement of materials design and evaluation with an emphasis in the EC fields.

Electrocatalysis is highly important in contemporary energy technologies. Many energy-related electrochemical reactions, such as hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR), require efficient low-cost electrocatalysts to enhance their reaction kinetics. Nanostructured iron, cobalt, and nickel chalcogenides are a class of promising electrocatalysts for these reactions. To synthesize these electrocatalysts, atomic layer deposition (ALD) has recently emerged as a novel powerful technique, particularly for its unique ability of conformal film coating. During the past few years, numerous new ALD processes have been realized to synthesize the chalcogenides, including FeS_x, CoS_x, NiS_x, Fe_xCo_1−xS_y, FeSe₂, CoSe₂, and NiSe₂, and their promising applications for electrocatalysis have also been well demonstrated. To provide an overview of this dynamic research area, this Chapter summarizes the latest progress in the synthesis and electrocatalysis applications of the ALD iron, cobalt, and nickel chalcogenides with an outlook on future trends.

The calixarene-based ligand—4-tert-butylsulfonylcalix[4]arene has the capacity of coordinating with various metal cations either individually by itself or together with various carboxylate/thiol-containing coligands. Interestingly, different types of metal cations, various counterions and variable solvents not only generated diverse coordinated cages with special morphology and topology, but also provided a tool in controlling their various porosities, hierarchical architectures, and physiochemical properties. Particularly, those calixarene-based coordinated cages with different porous structures, properly sized hierarchical cage/cavity-like spaces, as well as selected active elemental components greatly promoted applications to electrocatalytic reactions such as hydrogen evolution reaction, oxygen evolution reaction and oxygen reduction reaction. In this chapter, we not only summarized and discussed the preparation of calixarene-based coordinated cages and their applications to electrocatalysis, but also provided academic and industrial concerns and suggestions in future researches.

Ammonia is a very important chemical to human society, and its production mainly relies on the traditional Haber-Bosch (HB) method. However, the HB process is carried out under high temperature and pressure, resulting in a large amount of energy consumption and gas emissions. Electrochemical synthesis of ammonia (ESA) is a promising route due to its cleanliness and sustainability with low energy consumption. Solid oxide electrolysis cells (SOECs) working at high temperature have the advantages of increasing catalytic activity, easy to assemble and friendly to environment. Therefore, in this chapter recent experimental and theoretical studies on EAS using proton and oxygen ion conducting SOECs are reviewed. At the current stage, the ammonia production rates reported are in a range of 10^-13-10^-9 mol s^-1 cm^-2, which is far from the feasible commercial value. A lot of efforts have been made to further improve the ammonia production rate, including the development of new materials and microstructure design for electrode and electrolyte, plasma assisted ESA, and so on. Then we reviewed the recent progress of mechanism from experiments observation and density theoretical calculation (DFT) for the electrochemical nitrogen reduction reaction. In the end, the challenges and outlook for ESA using high-temperature SOEC are discussed. There is still a long way to go before the commercialization of ESA.

The effective utilization of solar energy for environmental pollution control is an important topic of sustainable development. However, so far, most of photocatalytic materials only work under ultraviolet light irradiation, which inhibits their practical application. In contrast, nanostructured tungsten disulfide (WS₂) demonstrates a full-solar-light-spectrum response from ultraviolet to visible and even near-infrared regions, showing a great promise for photocatalytic degradation of organic pollutants. In this chapter, the principles of photocatalysis are revealed first, followed by an ample discussion on the structure, optical properties, and synthesis routes of WS₂-based nanomaterials. Then, the design strategies of WS₂-based nanomaterials toward highly efficient photocatalytic degradation are elucidated, after which the application of WS₂-based photocatalysts for the degradation of dyes, antibiotics, and phenols is briefly discussed. Furthermore, the challenges and potential future research directions are outlined.

Carbon dioxide (CO₂) reforming or dry reforming of methane (CH₄) to produce syngas, hydrogen (H₂) and carbon monoxide (CO), is a promising low-carbon energy technology that converts two greenhouse gases (CO₂ and CH₄) into valuable chemicals and fuels. Recently, solar-driven dry reforming of methane (DRM) has attracted increasing attention as it further lowers the carbon footprint by using renewable energy as the input. Most of studies in this area focus on solar thermochemical DRM, i.e. using sunlight merely as the thermal energy source. In this chapter, an innovative photothermocatalytic DRM (PTC-DRM) approach is introduced, which integrates photocatalysis and thermocatalysis to maximize the solar energy efficiency through the design of a unique catalyst that can promote the synergies between photo- and thermo-catalysis. Platinum (Pt) loaded on cerium oxide (CeO₂) support is one example of such catalysts. This chapter presents recent advancements in Pt/CeO₂-based catalysts and their effectiveness and stability in the PTC-DRM process powered by concentrated sunlight. First, the contribution of photocatalysis in the PTC-DRM process on a Pt/CeO₂ catalyst is confirmed and the PTC-DRM mechanisms unraveled. Secondly, the effects of dopants such as silicon (Si) in the CeO₂ support on the PTC-DRM performance are discussed, which is mainly due to the generation of surface oxygen vacancies and increased light absorption. Thirdly, a new materials innovation is introduced, i.e. the incorporation of Zn promoter and atomic layer deposition (ALD) enabled ultrathin MgO coating on Pt/CeO₂, which significantly enhances the efficiency and stability of PTC-DRM. Lastly, this chapter presents an approach of engineering surface acidity of the support by mixing Al₂O₃ with CeO₂, and the resultant Pt/Al₂O₃-CeO₂ catalyst achieves a near unity H₂/CO ratio in the produced syngas. The studies discussed in this chapter provide valuable insights on the importance of materials innovations to the advancement of sustainable energy technologies, in particular, solar energy harvesting and low-carbon fuels production.

The recent spike in interest in replacing fossil energy with cleaner, more sustainable energy has directed research attention towards enhancing solar energy efficiency. Photovoltaic (PV) devices that transform solar radiation energy into electricity are known as solar cells. With global urbanization rising, more modernized housing and commercial spaces are likely to be created in cities, demanding even more energy for heating and cooling. Solar collectors are intended to create effective power using solar heating and radiative cooling systems. Because solar heating and radiative cooling have similar operating principles, integrating these two technologies into a single system is highly desired for everyday use. Due to superior thermal conductivity, nanofluids can significantly improve the efficiency of solar collectors. Nanomaterials applications for solar energy harvesting are also expanding due to their mechanical characteristics and photonic applications, which are more relevant for developing and producing dual-mode heating systems. With further advancements of efficient nanofluids, solar collectors can deliver more heat for solar heating and efficient solar heat-cycle cooling.

Alcohols are originally introduced to scCO₂ foam flooding as a cosolvent to increase the surfactant solubility in scCO₂. Other than increasing surfactant solubility, alcohols can also distribute at the interface region and further influence the water/scCO₂ (foam interface) interfacial properties. In this report, we use the molecular dynamics (MD) simulation to study the alcohol effect on the foam interface properties and their partitioning in various phases. Alcohols with varying tail lengths (C₂OH-C₁₆OH) under a wide range of concentrations are introduced to water/AOT/scCO₂ interface systems to study their effects. Temperature and pressure are set as a typical reservoir condition (333 K and 200 bar). We find that alcohols can distribute in water, interface region, and scCO₂ phases, and their partitioning in various phases is dependent on the alcohol tail length. Alcohol tail length has a negligible effect on its distributions at the interface under the same concentration in scCO₂ (). On the other hand, the alcohol concentration in the water phase () increases as tail length decrease. The ability to reduce interfacial tension (IFT) is similar for various alcohols when is relatively low (before reaching the inflection point). Longer chain alcohols reach the inflection point under lower . In other words, the lowest available IFT increases as alcohol chain length increases. The mean square displacement of AOT decreases as increases, and such a decrement trend is more significant in the systems with long-chain alcohols (C₁₆OH). In addition, long-chain alcohols (C₈OH and C₁₆OH) also help orientate the AOT tail group, while a negligible change in the AOT tail orientation is observed for the systems with short- and mid-tail alcohols.