Chemical energy storage

Energy is the lifeblood of human society. It keeps our machines running and our vehicles moving, drives our production processes and keeps our homes at a pleasant temperature. To provide this lifeblood, we have created an energy system which keeps us supplied with our everyday energy needs. In turn, this energy system is strongly dependent on the use of fossil fuels as a primary energy source. Unfortunately, this is not a sustainable situation; the reserves of available fossil fuels are limited and their consumption has a severe impact on the environment. To develop an energy system which is sustainable, we should take advantage of the most abundant source of free energy available: the sun. Naturally, this would require a complete overhaul of our current energy system. Chemistry, as a means for capturing, converting and storing this solar energy, will inevitably play a crucial part in meeting that challenge.

The start of the book, chapter 1.1, is named “The Solar Refinery” and introduces us to our energy system and the biological origin of the fossil fuels we depend upon as a primary source of energy. It shows us how chemistry is present throughout our energy system and provides us with an introduction to the fundamentals of chemical reactions and catalysis. A design for a sustainable future energy system is then proposed, utilizing chemistry as the platform. In chapter 1.2, different energy storage systems are discussed, with a clear distinction between grid-scale storage of electrical energy and mobile energy storage. The use of liquid (fossil) hydrocarbons for mobile energy applications holds a particular advantage, as the high energy density that these provide is hard to match by other means. No chemistry is discussed in chapter 1.3, which instead underlines the important task of scientists, to make society aware of the challenges we are facing in the long run and the necessary changes that this requires today.

In chapter 2.1, methods for conversion of lignocellulose into biofuels are discussed, starting with the optimum choice for the crops that serve as a source of the biomass and the properties of lignocellulose. The production of biofuels from lignocellulose is considered, ideally by initial transformation of lignocellulose into a select number of “platform chemicals” that can then be used as feedstocks in the production of biofuels. Also different types of biofuels are discussed. Biomass as a feedstock is also covered in chapter 2.2, which extends the scope to include lipids and proteins as biomass, as well as non-fuel products. The topic of chapter 2.3 is thermal conversion of biomass, through torrefaction, pyrolysis, gasification or combustion, all of which are aimed at utilizing the energy stored in the biomass. Chapter 2.4 concludes the section on biomass with a discussion of the hydrothermal carbonization (HTC) process to obtain “biocoal”.

The third section of the book concerns electrochemistry and begins with an introduction to the thermodynamic fundamentals of the electrochemical cell in chapter 3.1. Chapter 3.2 covers the concept of water-splitting into hydrogen and oxygen through electrocatalysis, particularly the oxygen evolution reaction (OER) at the anode of the electrolysis cell. Density functional theory (DFT) is presented as a means for evaluating these reactions. The use of fuel cells for direct conversion of chemical energy into electricity is covered in chapter 3.3. This includes the theoretical background, the components and the characteristics of a fuel cell, as well as a comparison between several types of fuel cells. Chapter 3.4 returns to water-splitting, but this time through photochemical means. It explains the operation of photosynthesis and hydrogen conversion in nature and how this knowledge can aid us in the design of synthetic catalysts, to allow direct production of solar fuels from sunlight. Chapter 3.5 discusses the thermodynamics and kinetics of rechargeable batteries, as well as materials optimization to further improve their operation.

Section four is devoted to the workhorse of many chemical bulk processes: heterogeneous catalysts. Chapter 4.1 provides the basics, with an introduction to reaction kinetics and the role of catalysts. Next follows a discussion of the properties of solid catalysts, their structure characteristics and their synthesis in chapter 4.2, and an overview of catalyst characterization methods in chapter 4.3. This background is then used in chapter 4.4, which explains the importance of heterogeneous catalyst modeling for catalyst development using four case studies. Finally, in chapter 4.5, methods for studying reaction mechanisms are discussed, as well as several small molecule reactions that are fundamental to energy conversion processes.

Chapter 5.1 discusses devices for the production of liquid fuels from CO₂ by artificial photosynthesis, and the required components that would go into their design. Alternatively, CO₂ may be activated by the sun’s heat through thermochemical means, which is discussed in chapter 5.2. The last two chapters are concerned with synthesis gas and its use in the production of synthetic fuels. Chapter 5.3 discusses the advantages of methanol as an energy carrier, the production of synthesis gas via steam reforming and the production of methanol from synthesis gas. Lastly, chapter 5.4 discusses the conversion of synthesis gas into hydrocarbon fuels via the Fischer-Tropsch process.

An interesting book, it provides valuable insight into the requirements and possibilities for developing a sustainable energy system utilizing solar energy. For some time, humanity has been aware that our current energy system is on a dead end track. And while a number of promising technologies have thus far been suggested as a solution, in reality our energy system is much too complex for a simple fix. In this book, the needs of a future energy system are considered one by one and sustainable technologies are discussed by how well they fit these needs. In addition to a comparison of their industrial feasibility, the book also provides a thorough introduction to the fundamentals of each technology, followed by an overview of recent advances and an outlook for further development. The editor, Robert Schlögl, is an expert in the related fields of heterogeneous catalysis and chemical energy conversion and as a result, particular detail is given to the description of catalysts. As such, the book will be interesting for readers that are looking for both a solid fundamental basis on sustainable energy technologies as well as an overview of the current state of the art. In addition, anyone simply curious about our energy system and the current progress towards making it sustainable will find this book an interesting read as well.