Combining the two topics for the first time, this book begins with an introduction to the recent challenges in energy conversion devices from a materials preparation perspective and how they can be overcome by using atomic layer deposition (ALD). By bridging these subjects it helps ALD specialists to understand the requirements within the energy conversion field, and researchers in energy conversion to become acquainted with the opportunities offered by ALD. With its main focus on applications of ALD for photovoltaics, electrochemical energy storage, and photo- and electrochemical devices, this is important reading for materials scientists, surface chemists, electrochemists, electrotechnicians, physicists, and those working in the semiconductor industry.
Julien Bachmann is Professor of Inorganic Chemistry at the Friedrich-Alexander University of Erlangen-Nürnberg in Erlangen, Germany. He obtained his chemistry diploma from the University of Lausanne, Switzerland, and a PhD in inorganic chemistry from the Massachusetts Institute of Technology in Boston, USA. After an Alexander von Humboldt postdoctoral fellowship at the Max Planck Institute of Microstructure Physics in Halle, Germany, he was hired as a Junior Professor of Applied Physics at the University of Hamburg, Germany, before joining the faculty in Erlangen.
PART I. INTRODUCTION
Challenges in Energy Conversion Devices
Basics of Atomic Layer Deposition (ALD)
PART II. PHOTOVOLTAICS
ALD for Passivation in Silicon Solar Cells
ALD for Light Absorption
ALD for Interface Engineering
ALD for Charge Transport
PART III. ELECTROCHEMICAL ENERGY STORAGE
ALD for Supercapacitors
ALD of Electrocatalysts in Fuel Cells and Electrolyzers
ALD of Lithium Ion Battery Materials
ALD of Oxide Conductors in Fuel Cells
PART IV. PHOTOELECTROCHEMICAL AND THERMOELECTRIC ENERGY CONVERSION
ALD in Photoelectrochemical Devices
ALD of Thermoelectric Materials
Friedrich-Alexander University of Erlangen-Nürnberg, Department of Chemistry and Pharmacy, Egerlandstrasse 1, 91058 Erlangen, Germany
The Past of Energy Conversion
The ability to harvest energy from the environment and utilize it is a defining characteristic of life. It also represents an activity of central importance to mankind. In fact, several major (pre)historic events are intimately related to the control of novel energy forms. Mastering fire and subsequently the labor of domestic animals are two most crucial achievements of the Paleolithic and the Neolithic, respectively. Much later, craftsmanship became powered by water or wind. Subsequently, the industrial revolution originated from the invention of the heat engine, with coal and steam featured as the most prominent energy carriers (Figure 1). In the twentieth century, they were replaced with petroleum derivatives in the internal combustion engine, used for decentralized energy conversion. Simultaneously, electrical power was established as the most versatile energy form. However, even to date, its generation still mostly relies on coal and steam in a highly centralized infrastructure based on very large energy-converting units (power plants).
Figure 1 Diagram illustrating the principle of energy harvesting from fossil fuels and highlighting the massive losses inherent to the heat engine.
The Future of Energy Conversion
This hybrid situation, in which electrical power is generated from fossil fuels and complemented by them, especially for mobile applications, might represent an intermediary stage toward a society in which electricity has become the universal energy carrier and fossil fuels are eventually outdated. One can envision that in a not-too-distant future, mankind will harvest solar power, be it directly or via wind, water, and biomass (all of which originate from it) into electrical current without the intermediacy of any heat engine. Given that mankind consumes approximately 18 TW (18 × 1012 J s-1, or roughly the equivalent of 18 000 large "traditional" power plants) whereas sunlight provides 120 000 TW (120 × 1015 J s-1), capturing only a small fraction of the incoming solar energy would suffice .
Of course, this transformation of our energy economy can only be gradual, and it is unlikely to be driven significantly by a shortage of fossil fuels and the corresponding increase of their costs. Transitioning toward renewable energy sources, however, would present a number of opportunities (Table 1). First and foremost, it would halt anthropogenic emissions of greenhouse gases (or, at least, cut down on them very significantly) and thereby avoid dramatic climate changes with all their social and geopolitical consequences. Second, it would eliminate the heat engine, another relic of the nineteenth century, the efficiency of which is limited by thermodynamics independently of all engineering feats. Third, it would put energy harvesting at the (financial and technical) reach of individual private persons and small corporations. This possible decentralization of the whole energy infrastructure would reverse the trend followed in the past centuries and would represent a formidable empowerment of the individual citizens .
Table 1 Contrasting features of economies based on fossil fuels and renewable energy sources Fossil Renewable Nineteenth and twentieth centuries Twenty-first century?
Primary energy sources Coal, oil, natural gas (and uranium)a Sunlight (possibly via wind, water, and biomass) Available amounts of primary sources Limitedb Practically unlimitedb Secondary form of energy Heat (as steam or combustion gases) Electricity Storable form of energy Fuels as refined petroleum derivatives Fuels in or from (photo)electrochemical devices Emissions Greenhouse gas CO2a No net CO2 Major type of energy-converting devices Mechanical: heat engine Solid-state, no moving parts Most typical scaling behavior of power conversion efficiency Increases with unit size Decreases slightly when unit size increases Organization of energy-harvesting and distribution infrastructure Centralized: large units owned and operated by few large companies Distributed: small units owned and operated by companies and individuals
a Nuclear fission power relies on a fossil, nonrenewable source (fissile material such as 235U) but does not contribute to greenhouse gas emissions. Nuclear fusion, if or when technically realized, would represent a practically unlimited energy source.
b Fossil fuel reserves may last for some decades, centuries, or perhaps millennia. Solar power will be available for billions of years.
Technical Ingredients Needed
The vision of a fully renewable, decentralized energy economy can only be realized if technologies are available for the inexpensive and efficient conversion of energy between its various forms (Figure 2). Nowadays, electricity can be harvested from sunlight in photovoltaic devices (solar cells) with efficiencies of ~20% in mass-market products and up to 46% in the current record-holding laboratory device ; thus, photovoltaics have long beaten the efficiency numbers theoretically possible with heat engines. Electricity, however, is much more difficult to store and transport compared to fuels. Therefore, the intermittency of energy harvesting inherent to renewable sources (which are tributary of weather conditions) also renders efficient schemes for the reversible interconversion of electrical and chemical energy forms (electrical current and fuels) just as crucial. This is the role of electrochemical devices such as rechargeable batteries, electrolyzers, and fuel cells. Optionally, the direct conversion of sunlight into fuels in photoelectrochemical cells (it can also be called artificial photosynthesis) represents an integrated solution of simultaneous harvesting and storage. Finally, the direct conversion of heat into electrical power in thermoelectric devices may contribute to improving the overall energy efficiency of industrial processes by recovering a fraction of the waste heat that is otherwise lost and by enabling the operation of low-power devices off-grid.
Figure 2 Diagram illustrating energy harvesting (green), storage (blue), and exploitation (red) in a renewable energy economy. The forms of energy harvesting and storage displayed are treated in this book.
No single technology can possibly provide a general solution. Instead, each specific application defines specific requirements and calls for an individual solution best suited to it. For example, different types of electrochemical storage chemistries may be optimized with respect to maximizing energy density (for sustained, regular delivery over an extended period) or power density (for occasional but intense usage). Some are of particular interest in terms of volumetric density (compact), others in terms of gravimetric density (lightweight). The technical performance of energy-converting devices cannot be defined by a single parameter and must be considered in the context of other parameters of economic or practical nature. For example, the relative relevance of purchasing cost, maintenance costs, reliability, and longevity may be very different, depending on whether a certain storage device is used in consumer products, mobility, health care, or space applications. Correspondingly, the future of renewable energy will be diverse.
Scope of This Book
Accordingly, this book presents selected types of devices serving to interconvert the solar, electrical, chemical, and heat forms of energy in nonmechanical devices. Thus, wind, hydroelectric power, geothermal power, solar thermal conversion, heat pumps, and other approaches based on moving parts and "classical" engineering will not be treated. Instead, we focus on approaches based on solids and their interfaces, in which atomic layer deposition, a thin-film coating technique, is of relevance. We also exclude the generation of light from electricity (a scientifically challenging topic of industrial importance in lighting and displays but not directly involved in renewables), of heat from electricity (since resistive heating presents no fundamental scientific hurdle), and of heat from fuels (given that combustion is well suited to it and well established).
All other types of energy conversion rely on the transport of charge carriers (mostly electrons, but also ions) inside condensed phases and their transfer across interfaces (which may be associated with rearrangements in chemical bonding). The throughput of energy conversion (the power density) is often determined by the rate at which charge carriers are exchanged at the interface between two solids or between a solid and a liquid phase. In that case, increasing the geometric area of the interface while maintaining short transport distances toward each point of the interface is possible via a nanostructured interface (Figure 3). Nanostructuring presents the advantage...