Nanotechnology and Nanomaterials for Energy. Pierre-Camille Lacaze

had reached a level where it was possible to observe objects on a nanometric scale [RUS 86]. Scanning electron microscopy (SEM) developed in parallel, although commercial applications appeared at a later stage; this technique is based on the analysis of reflected secondary electrons. The first apparatus was launched onto the market by Cambridge Scientific Instruments in the 1960s. SEM makes it possible to observe the texture of material surfaces at nanometric level, constituting a breakthrough in terms of observing nano-objects in 3D.

      The decisive breakthrough into the “nanoworld”, however, was not made until the mid-1980s. Rohrer and Binnig’s invention of near-field microscopy [ROH 86], including scanning tunneling microscopy (STM) and atomic force microscopy (AFM), marked a further step in the observation of sub-nanometric objects: the long-pursued goal of atomic-level observation had finally been attained. These new microscopy techniques, which now play a central role in the study of nanomaterials, were increasingly used to produce high-resolution images. Compared to classic TEM and SEM techniques, they also present a practical advantage in that they do not need to be used in a vacuum, and can indeed be used in “everyday” atmospheric conditions.

These technological innovations in the field of observation went hand-in-hand with the discovery of new materials, on a nanometric scale, which displayed entirely new physical and chemical properties due to their very smallness. Thus, in 1985, Kroto et al. (Nobel Prize winners in 1996) discovered fullerenes: new carbon nanomaterials with exceptional properties [KRO 85]. The first fullerenes were obtained in the form of traces; they are now produced commercially on a large scale. Fullerenes consist of a spherical assembly of carbon atoms, assembled in hexagon and pentagon shapes. Often considered as “electron wells”, they have particularly interesting chemical properties and are widely used in various domains, notably in plastic photovoltaic cells; their application in this field in the 1990s resulted in significant gains in energy yield [YU 95].

      A few years later, in Japan, Lijima [LIJ 91] discovered the tubular structure of carbon nanotubes (CNTs), then of single wall carbon nanotubes (SWCNTs) [BET 93], which also consist of a hexagonal assembly of carbon atoms. These nanotubes vary from one to several nanometers in diameter, and have a length of several hundred nanometers. As we shall see later, the mechanical properties of these nanotubes largely surpass those of steel; furthermore, they also possess exceptional electronic properties.

      These discoveries rapidly attracted the attention of scientific agencies in the United States. An ambitious program of fundamental and applied research into nanotechnologies was launched in the year 2000, with both scientific and industrial support. A series of directives and recommendations issued by the NNI attracted attention on the world stage, and a number of other nations launched their own initiatives, highlighting the strategic importance of the domain.

      The scale of technological research increased considerably from the early 2000s on, and the number of publications in connection with nanomaterials or nanotechnologies continues to grow. While the creation and use of nanometric elements in the field of electronics are well established, other fields, including chemistry, the energy sector, biology and medicine, stand to make considerable gains from the progression of nanotechnologies. New fields of investigation are opened up on a regular basis, and their results look highly likely to revolutionize both theoretical knowledge and practical applications in these domains.

I.2. Outline of this book

      Our main aim in this book is to highlight new breakthroughs and areas of research in nanotechnologies which have appeared in recent years, notably since 2010; however, we shall begin by presenting a brief overview of earlier discoveries, essential to understanding later work. Many of the new findings presented here have yet to be used commercially, but their interest in terms of research and potential future applications is immense.

We have chosen to focus on problems and solutions relating to the energy sector. Our discussion is split into two parts: first, a description of nanomaterials and their properties, and second, a discussion of the ways these materials are, or may be, used in the energy sector for storage and conversion, electrocatalysis and photocatalysis.

      Chapter 1 is devoted to the subject of carbon nanomaterials, and includes a description of recent preparation methods, properties and major applications. Some of these materials (fullerenes, carbon nanotubes, nanodiamonds, etc.) largely predate the year 2000. However, they are yet to reveal all of their secrets, notably in terms of the remarkable properties they possess and in new contexts of application. Twodimensional families of nanomaterials, derived from graphene, are a far more recent discovery; graphene itself looks set to become the star material of the 21st century. More recently still, the family of 2D carbon composites has expanded to include materials such as graphdiyne, with a huge range of potential applications.

      Chapter 2 concerns the family of inorganic nanomaterials. This field, too, has undergone considerable development in recent years, and new properties have emerged with the discovery of materials at an increasingly small level. Atom clusters, which contain from ten to a few hundred atoms and are smaller than 1.5 nm in size, possess physico-chemical properties which set them apart from larger NPs; this is a result of their spatial confinement. Quantum dots (QD), which are generally made up of binary alloys of semiconductors (SC), constitute another important family of nanomaterials and are particularly notable for their luminescent properties. The latest set of 2D materials include dichalcogenides of transition metals, along with a significant number of inorganic lamellar materials; the interest of these materials is already apparent, particularly in terms of their applications in the energy sector.

      The second part of this book constitutes a critical study of new problems in the domain of energy, notably in terms of energy transition and preserving the environment. The fight to reduce greenhouse gas emissions raises two major problems: those of storing and converting energy. Nanotechnologies and nanomaterials stand to make a decisive contribution to solving these issues.

      Chapter 3 is devoted to the key question of energy storage, a major challenge linked to the development of renewable energies. This field encompasses large-capacity static storage, the production of batteries and supercapacitors for electric vehicles, and also microbatteries, designed to provide power for small portable electronic devices. All of these different systems are subject to different constraints, leading us to focus on different design choices.

Chapter 4 concerns energy conversion, notably in terms of photovoltaic sources and lighting. Solar power is nothing new, but huge progress has been made in recent years, notably with the development of organic photovoltaics and that of perovskites; these solutions provide a genuine alternative to inorganic silicon-based systems, and, importantly, are more cost-effective. Lighting and display devices constitute another important subject in the energy field, notably in connection with the general phenomenon of electroluminescence. The overarching goal is to reduce energy consumption while providing a better quality of lighting than that obtained using incandescent bulbs or neon tubes. Considerable advances have been made over the last few years; as before, parallel developments in inorganic and organic nanomaterials have found practical applications in this area. The development of new forms of memory electronics (not covered in this book) is another important area of research, where once again the reduction of energy consumption is a major goal.

      Chapter 5 concerns electrocatalysis and photocatalysis. Electrocatalysis relates primarily to the development of electrolysis cells (water splitting) and fuel cells, a main focus for future advances in converting chemical energy into electrical energy. Hydrogen is a particularly promising source of fuel: it is non-polluting, and constitutes a highly efficient means of transporting and storing energy. One especially promising area of research relates to the production of hydrogen by electrolyzing water, powered by a photovoltaic cell. This development is connected to the broader aim of developing systems which use non-precious metals to catalyze oxygen evolution reactions (OER)