Low-cost Nanomaterials

Increasing worldwide demand on energy and the limited amounts of nonrenewable fossil fuels have stimulated intense research and development efforts on renewable energy, in the areas of solar cells, energy storage, fuel cells, and water splitting, to name a few. However, widespread applications of renewable energy have not been very successful mainly due to the high cost of materials and underdeveloped processing and fabrication techniques.

One of the greatest challenges for human society and civilization is the development of powerful technologies to harness renewable solar energy to satisfy the ever-growing energy demands. Semiconductor nanomaterials have important applications in the field of solar energy conversion. Among these, TiO₂ represents one of the most promising functional semiconductors and is extensively utilized in photoelectrochemical applications, including photocatalysis (e.g., H₂ generation from water splitting) and photovoltaics (e.g., dye-sensitized solar cells, DSSCs). As such, many efforts have focused on developing and exploiting cost-effective nanostructured TiO₂ materials for efficient solar energy applications.

This paper presents a review of nanostructured nitrogen doping (N-doped) TiO₂ nanomaterials and their application into dye-sensitized solar cells (DSCs). Such N-doped TiO₂ nanomaterials aim at enhancing the performance of TiO₂ photoanodes for DSCs. Herein, we summarize the different synthesis methods, nanostructures, and physiochemical properties of N-doped TiO₂. Also, the differences in electron transport behavior in DSCs based on N-doped and pure TiO₂ photoanodes were involved. Further understanding of the nanostructured N-doped TiO₂ photoanodes will promote the development of energy conversion and other related areas.

Dye-sensitized solar cells (DSCs) are potential candidates to silicon solar cells due to their merits of simple fabrication procedure, low-cost, and good plasticity. Till date, great advances have been achieved in the design of dyes, redox couples, and counter electrode (CE) catalysts for DSCs and the highest energy conversion efficiency is up to 12.3 %. In this part, our attention focuses on CE catalysts. Besides Pt, carbon materials, conductive polymers, transition metal compounds (carbides, nitrides, oxides, sulfides, phosphides, selenides), and composite catalysts, denoted as Pt-free catalysts, have been introduced into DSCs as CE catalysts. In the following sections, we give a summary of Pt-free CE catalysts and highlight the advantages and disadvantages of each variety of Pt-free catalysts.

Quantum dot sensitized solar cells, but in general semiconductor sensitized photovoltaic devices, have erupted in recent years as a new class of systems, differentiated for several reasons of the most common dye-sensitized solar cells. In this chapter, we review the enormous potentialities that have impelled the research in this field. We highlight the differences between quantum dot and dye-sensitized solar cells that we divide in five aspects: (i) Preparation of the sensitizer; (ii) Nanostructured electrode; (iii) Hole Transporting Material; (iv) Counter electrode, and (v) Recombination and surface states. Some of the optimization works performed in each one of these lines is revised, observing that further improvement can be expected. In fact, the recent breakthrough in photovoltaics with organometallic halide perovskites, originated by the intensive study on quantum dot-sensitized solar cells, is also revised, stressing the potentiality of these systems for the development of low cost photovoltaic devices.

Pyrite has long been proposed as a green solar cell material. Even with its promising properties, studies on pyrite have lagged behind many other semiconducting materials. Unanswered questions about the affects of defects and how to grow pure crystalline material still exist. With the rise of nanochemistry and more powerful computational methods, pyrite is seeing an explosion of new studies. This chapter first presents pyrite and its green promise as a material, followed by the materials characteristics. It then moves into synthesis of pyrite, starting with old methods and then transitioning into different methods of nanocrystal creation. Finally, photo-devices created out of pyrite materials are discussed. The chapter then wraps up with a summary and what still needs to be done for pyrite to achieve its golden status.

Most of the high-efficiency organic photovoltaic devices (OPVs) reported to date have been fabricated based on the concept of a bulk heterojunction, where a conjugated polymer (donor) and a soluble fullerene (acceptor) form an interpenetrating network featuring a large donor–acceptor interfacial area. In this chapter, we first introduce the fundamentals of OPVs and then review the recent progress related to OPVs based on conjugated polymers. We then discuss the annealing approaches that have been used to optimize the morphologies of the photoactive layers, including thermal annealing and solvent annealing, and describe the engineering of the interfaces at the contacts between the polymer blends and the metal electrodes. Next, we outline the two most common optical methods for improving the light absorption efficiency of OPVs: the use of optical spacers and the triggering of surface plasmons. Finally, we summarize the development of low-band-gap polymers for the absorption of long-wavelength photons from solar irradiation and provide a brief outlook of the future use of OPVs.

Polymer solar cell (PSC) is the latest of all photovoltaic technologies which currently lies at the brink of commercialization. The impetus for its rapid progress in the last decade has come from low-cost high throughput production possibility which in turn relies on the use of low-cost materials and vacuum-free manufacture. Indium tin oxide (ITO), the commonly used transparent conductor, imposes the majority of the cost of production of PSCs, limits flexibility, and is feared to create bottleneck in the dawning industry due to indium scarcity and the resulting large price fluctuations. As such, finding a low-cost replacement of ITO is widely identified to be very crucial for the commercial feasibility of PSCs. In this regard, a variety of nanomaterials have shown remarkable potential matching up to and sometimes even surpassing the properties of ITO. This chapter elaborates the recent developments in ITO replacement which include, but are not limited to, the use of nanomaterials such as metal nanogrids, metal nanowires, carbon nanotubes, and graphene. The use of polymers and metals as replacement to ITO are described as well. Finally, recent progress in large-scale experiments on ITO-free PSC modules is also presented.

Polymer light-emitting diodes (PLEDs) and organic photovoltaics (OPVs) are considered as next generation electronics due to the low-cost, flexibility, and lightweight features. However, there are challenges such as large-area processing technologies, film coating quality, and long-term stability toward scalable and low-cost polymer electronics. This chapter deals with the various scalable processing methods and evaluates the coating performance as well as electrical performances in polymer electronics fabricated by the solution processes. Special attention on coating instability is elaborated in the context of important material components in PLEDs and OPVs. Proper coating techniques can be chosen by considering the thickness requirement of each functional layer with good reproducibility. Additionally, we will evaluate mechanical/optical characteristics of the polymer anode for ITO-free electrodes; and introduce the metal mesh in combination with conductive polymers as the ITO-free transparent electrode for large area applications.

Hydrogen represents a clean and high gravimetric energy density chemical fuel that could potentially replace fossil fuels and natural gas in electricity generation and powering vehicles. Central to the success of hydrogen technology and economy, the sustainability, efficiency and cost of hydrogen generation are the major factors. Industrial hydrogen is currently obtained from steam methane reforming and water-gas shift reaction, however, this method still relays on fossil fuels. Therefore, it is important to develop efficient, low-cost, and scalable method to produce hydrogen in a sustainable manner. Photoelectrochemical (PEC) water splitting to produce hydrogen is one of most promising and sustainable approaches. The development of low-cost and efficient nanostructured photoelectrodes is the key to achieve this goal. In this chapter, we will give a brief background on PEC water splitting and review the recent advancement of developing low-cost nanostructured photoelectrodes.

Hydrogen is an attractive fuel for many applications because of its high energy density as molecular hydrogen, as well as the clean exhaust produced when burned with oxygen. One significant challenge to the widespread adoption of hydrogen, for mobile applications in particular, is the inability to efficiently store large amounts of readily accessible hydrogen in small volumes at ambient temperature and pressure. This chapter describes the current research on one particularly interesting candidate for hydrogen storage, nanostructured magnesium. The synthetic methods currently used to control the size and shape of nanostructured magnesium are described, as are the measured kinetics of hydrogen storage, the modeling used to explain the observed kinetics, and theoretical models that can be used to guide experimental efforts.

Since the first experiment conducted by William Grove in 1839, fuel cell, a device that converts the chemical energy stored in fuels into electricity through electrochemical reactions with oxygen or other oxidizing agents, has attracted worldwide attention in the past few decades. However, despite extensive research progress, the widespread commercialization of fuel cells is still a big challenge partly because of the low catalytic performance and high-cost of the Pt-based electrocatalysts. In addition, the hydrogen storage is another critical issue for the commercialization of hydrogen-powered fuel cells. Among the metal catalysts, Pd has been found to be a promising alternative because of its excellent catalytic properties and lower cost than Pt. Moreover, Pd-based materials exhibit high hydrogen storage capabilities. In this chapter, we summarize recent progress in the synthesis of one-dimensional (1D) Pd-based nanomaterials and their applications as electrocatalysts on both anodic and cathodic sides of fuel cells, and their applications in hydrogen storage. We demonstrated here that various 1D Pd-based nanomaterials, such as nanorods, nanowires, and nanotubes have been successfully prepared through different synthetic routes. The nanostructured 1D Pd-based materials exhibit high catalytic performance for electrooxidation of small organic molecules and oxygen reduction reaction (ORR). Moreover, high capacities for hydrogen storage have also been reported with 1D Pd-based nanomaterials.

Production, storage and deployment of affordable and clean energy is one of the biggest challenges facing humanity. Although a majority of current energy requirements is obtained from fossil fuels, the supply is finite and could last only for a specified period depending on the nature of resources.

Energy storage is more important today than at any time in the human history. The battery systems that are pursued for clean renewable energy-based grid or the electrification of transportation need to meet the requirements of low cost and high efficiency. Li-ion battery is the most advanced battery system, but it is expensive and insufficient as a resource for widespread application. Na-ion battery is seen as a promising alternative due to the abundance of Na resource. However, the realization of the Na-ion intercalation/deintercalation mechanism is also challenging because Na ions are 40 % larger in radius than Li ions. This makes the finding of suitable host materials with high storage capacity, rapid ion uptaking rate, and long cycling life not easy. In the recent 3 years, several electrode materials were found to have energy density close to those used in Li-ion batteries. These scientific advances have greatly rekindled worldwide passion for Na-ion battery system. In this chapter, the development of the electrode materials for Na-ion batteries is briefly reviewed, with the aim of providing a wide view of the problems and future research orientations of this system.

Due to their many fascinating properties and low-cost preparation by chemical reduction method, particular attention has been paid to the graphene-based materials in the application of energy storage devices. In the present chapter, we focus on the latest work regarding the development of flexible electrodes for batteries and supercapacitors based on graphene as well as graphene-based composites. To begin with, graphene as the sole or dominant part of flexible electrode will be discussed, involving its structure, relationship between structure and performance, and strategies to improve their performances; The next major section deals with graphene as conductive matrix for flexible electrode, the role of graphene to offer efficient electrically conductive channels and flexible mechanical supports will be discussed. Another role of graphene in flexible electrode is as active additives to improve the performance of cellulose and carbon nanofiber papers, examples will be given and such strategy is promising for further reducing the cost of flexible electrodes. Finally, prospects and further developments in this exciting field of graphene-based flexible energy storage devices will be also suggested.

Phase change materials (PCMs) have received considerable attention for the application of thermal energy storage and transfer. This chapter discusses synthesis and characterization of several types of PCM particles, as well as the use of PCMs to enhance the performance of heat transfer fluids. Two different PCM microcapsules are introduced first: one comprises solid–liquid PCM paraffin encapsulated in polymer shell; the other involves solid–solid PCM neopentyl glycol (NPG) core and silica shell. Then the synthesis of low-melting metallic nanoparticles and NPG nanoparticles without shells are discussed. The last part of this chapter is dedicated to a new type of phase-changeable fluids, nanoemulsion fluids, in which the dispersed nanodroplets can be liquid–vapor PCM or liquid–solid PCM, depending on the PCM properties and the operating temperature. Material synthesis and property characterizations of these phase-changeable fluids are two main aspects of this chapter.