Chapter 1: Introduction

Energy and how it is sourced are increasingly important topics in the day-to-day operations of nearly all modern societies. In this context, the world is currently on a pathway to what is commonly referred to as an “energy transition”.¹  The development of society over the last century has predominantly been driven by the use and exploitation of economically inexpensive fossil fuels such as natural gas, coal, and crude oil. However, this has not come without a price, and unfortunately these have been at the expense of our climate. We are often confronted with ecological disasters, especially for communities in the global south that have looked to exploit their natural resources to drive the development of their own societies (e.g. reflecting the first world’s path to higher development). This is not to say that the utilisation of fossil fuels (e.g., in making energy supplies less expensive) has not led to significant advancement in a short time. Still, it is essential to recognise that this has also resulted in ecological damage and pollution on every continent.²  The adverse effects of fossil fuel exploitation are increasingly visible in many forms, with perhaps the most notable and commonly discussed in the public sphere being climate change.³  In the eyes of the general public, fossil fuel use, associated emissions, and climate change represent the main argument for this so-called energy transition, which is a fundamental necessity for the existence of humanity.

When we refer to the energy transition, what do we actually mean? Energy and its accessibility are a prerequisite for the socio-economic development of a society. In this respect, the energy transition refers to an ongoing global shift from the use of traditional fossil fuel-based energy sources (e.g., coal, oil, and natural gas) to cleaner, more sustainable, and lower environmental impact alternatives, including renewable energy (RE) sources such as solar, wind, hydro, geothermal, and bio-energy.⁴  The energy transition is driven by recognition over the last two to three decades of the urgent need to address several interconnected challenges and/or drivers, specifically climate change, air pollution, energy security, and sustainable economic development for all. These challenges/drivers are summarised briefly.

The consumption, primarily via combustion, of fossil fuels releases significant amounts of greenhouse gases (GHGs) into the atmosphere, perhaps the most well-known and detrimental of these being carbon dioxide (CO₂). GHGs enhance the trapping of heat in the Earth’s atmosphere, which in turn increases the mean global temperature, commonly referred to as global warming. This temperature increase results in a disruption of natural climatic cycles that have been established over millennia. The energy transition aims to reduce GHG emissions (e.g., via substitution of fossil for RE) and mitigate the adverse impacts of climate change by replacing carbon-intensive energy sources with low- or zero-emission alternatives.

Fossil fuel combustion also emits various other pollutants detrimental to the environment and human health. These include sulphur dioxide (SO₂), nitrogen oxides (NO_x), particulate matter (e.g., inhalable coarse particles, designated as PM₁₀, with diameters of 10 micrometres (µm)), and anthropogenic volatile organic compounds (V_OC; such as aliphatic hydrocarbons, ethyl acetate, glycol ethers, and acetone). These can all contribute to air pollution and have detrimental effects on human health and the environment, leading to problems such as respiratory disease and atmosphere chemical cycles.⁵  Transitioning to cleaner energy sources can help improve air quality, reduce associated diseases, and enhance overall public and environmental health.

Fossil fuels are recognised as being finite resources. Geographically, they are often concentrated in specific regions, such as the Middle East, which have historically created geopolitical tensions and potential conflicts over access and control. The transition to energy supplies from RE and diversification of energy generation (solar, wind, wave, etc.) can lead to a greater emphasis on domestic energy production and thus reduce dependence on imported fossil fuels. Under the correct direction and policy, the use of RE can boost energy security and economic stability.⁶ 

Over the last few decades, the RE sector has experienced remarkable growth. The field of solar energy, for example, has been promoted by policies, technology learning rates, and market demand, which has led to job creation and contributed to driving economic development. The energy transition and associated investment in clean energy technologies and infrastructure will continue stimulating innovation, attracting investments, and enhancing growth in sustainable industries, leading to further job creation and long-term economic benefits.⁷  Within European Union (EU), there are efforts to foment local job creation and economic development in the RE and clean technology sector, avoiding its migration overseas as happened with the nascence of the photovoltaic market.⁸  Furthermore, given expectations in technology learning rates,⁹  the energy transition is also expected to deliver additional advantages, including reduced energy costs over time, increased energy efficiency through technological advancements, and decentralised energy systems that empower local communities and individuals to generate their own clean energy.^10–12 

In certain legal jurisdictions, frameworks have been introduced to drive the adoption of energy transition technologies. For example, the EU has established comprehensive frameworks to drive the energy transition and achieve its ambitious energy and climate goals (e.g., Fit for 55 legislative package,¹³  REPowerEU,¹⁴  etc.). Several vital legal drivers play a crucial role in shaping this energy transition; these include the Renewable Energy Directive II (RED II) for setting binding targets for member states on RE consumption,¹⁵  the Energy Efficiency Directive (EED) that establishes binding energy efficiency targets and requires member states to develop and implement energy efficiency policies and measures,¹⁶  and National Energy & Climate Plans (NECP)¹⁷  (introduced by the Regulation on the governance of the EU & climate action (EU)2018/1999), agreed as part of the Clean energy for all Europeans package adopted in 2019. NECP is particularly interesting as it commits EU member states to issue National Hydrogen Strategies complemented by other mechanisms such as REPowerEU. This is a constantly evolving, critical area of the transition with further developments ongoing (e.g., version III of RED currently under evaluation).

Therefore, the energy transition can be described as being an essential and critical ambition if humanity is going to be successful in addressing the challenges of climate change, combating air pollution, enhancing energy security, fostering economic growth, and building a more sustainable and resilient future, hopefully in an equitable fashion. However, the transition will be difficult, particularly to the needed levels (e.g. ≫90% RE).¹⁸  It will require a significant change in economic practices and political ideology, and thus collaborative efforts between governments, businesses, and collective bodies to accelerate the deployment of RE technologies, improve energy efficiency, and adopt sustainable and circular practices as part of a heavily integrated sector-coupled system (Scheme 1.1).

The energy transition requires the rapid and deep defossilisation of, firstly, the energy system and, secondly, a couple of RE with traditionally disparate economic sectors, including transportation, heavy industry, etc. Several technological concepts have been proposed that could contribute to this ambition. Perhaps the most famous and prominent in the public eye, at least, is the so-called “hydrogen economy”.^19,20  This concept envisions a future where H₂ plays a crucial role as a clean, versatile, and sustainable energy carrier. In this concept, H₂ is produced via RE sources (e.g., green electricity, photons, etc.) and can be used as a fuel and energy storage medium across multiple sectors (Scheme 1.2). H₂ produced in this manner is termed “green hydrogen”.

There are several aspects that make up the hydrogen economy concept. Some of them are summarised in the paragraphs below.

H₂ can be produced through various methods. The predominant methods are the highly polluting steam methane reforming and coal gasification routes. Alternative routes include biomass gasification, although it is expected that the majority of production in the future will arise from electrochemical water splitting. The emphasis in the hydrogen economy concept is on using RE-sources to generate green electricity, which can then be used for electrochemical splitting of water through a process termed electrolysis. This process involves splitting water molecules (H₂O) into H₂ and O₂, with H₂ being used and/or stored for later use. It is important to note that electrolysis can be performed in both a grid connected²¹  or an off-grid (e.g., direct connection to a solar PV field) manner,²²  with the level of “greenness” of the former being determined by the electricity mix of the grid (RE vs. fossil energy). As will be discussed later in this book, electrolysis is a well-known and, for some variants, a market-mature technology.²³  However, several challenges need to be addressed to make electrolysis suitable for the expected deployment levels (e.g., 10s of GWs of installed capacity in the EU in the coming decades)²⁴  and the demands of a heavily integrated, RE-driven energy system. Many other technologies are also being developed which demonstrate various degrees of technology and materials integration which in the future may provide alternative and more elegant and nature-inspired routes to the production of green H₂.²⁵  One of these technologies, as discussed in a dedicated chapter (Chapter 3), termed photoelectrochemical water splitting (PEC-WS),²⁶  combines the aspects of both photovoltaic and electrolyser technologies into highly integrated, complex devices, which provides exciting potentialities in terms of off-grid green H₂ production.

H₂ has a theoretically high energy density and can be stored and transported efficiently.²⁷  It can be compressed, liquefied, or stored in materials like metal hydrides²⁸  or chemical compounds²⁹  (which, it is important to note, require energy, thus reducing the net energy stored). Additionally, existing natural gas pipelines can be repurposed for H₂ transportation, enabling the integration of H₂ into existing energy infrastructure.³⁰  Both pipelines and storage tanks for H₂ require specific materials innovation. Recent reviews have been published that discuss the respective challenges associated with both pipes and tanks.^31–33  H₂ can be subsequently utilised in a wide range of applications. It can be burned in hydrogen-powered internal combustion engines³⁴  or used more efficiently in fuel cells (FC) to generate electricity (e.g., to drive mobility drivetrains).³⁵  When H₂ reacts with O₂ in a FC, the only by-product is water (H₂O), making it a clean and emission-free energy carrier. H₂ can thus power vehicles, provide heat and electricity for infrastructure (such as buildings, off-grid telecommunications, etc.), and even be used in industrial processes.

When produced from RE, H₂ can act as an energy connection molecule between conventionally disparate sectors like energy production and heavy industry. It offers a solution to defossilise sectors that are difficult to electrify directly with RE, including heavy-duty transportation,³⁶  long-distance shipping,³⁷  and certain industrial processes such as existing “legacy” steel manufacturing plants.³⁸  As intermittent RE sources like solar and wind experience fluctuations (day/night or seasonal cycles), excess (and potentially low-cost) electricity can be used to power water electrolysis (an approach often referred to as “peak shaving” – see the concept of “power-to-X” below). This approach can act to reduce both the levelized cost of electricity (LCOE) and the levelized cost of hydrogen (LCOH). In principle, H₂ can then be stored and supplied to different sectors for use and, if required, converted back to electricity when RE generation is low. Thus, this can act to provide a means for both energy storage and system flexibility (with increased resilience).

In the field, the “power-to-X” (PtX) concept is also receiving significant attention. It essentially refers to converting surplus renewable electricity, for example, during peak shaving, into other valuable forms of energy or chemical compounds (the “X”).³⁹  In power-to-gas (PtG), the surplus of RE powers water electrolysis units to produce H₂ (and O₂) (i.e., power-to-hydrogen (PtH)).⁴⁰  H₂ can further convert CO_x (e.g., captured from industrial process exhausts or directly from the atmosphere) to produce synthetic chemicals and/or fuels. One option is the production of methane (CH₄) (i.e., power-to-methane; PtM), which itself can be stored, transported, and used as a renewable substitute for natural gas in heating and power generation whilst utilising existing significant natural gas infrastructure.⁴¹  The chemistry and catalysis involved in this methane synthesis reaction, the Sabatier reaction, will be discussed in Chapter 8.

The power-to-liquids (PtL) concept encompasses the production of various synthetic liquids, chemicals, and fuels.⁴²  Regarding fuels, this can include synthetic gasoline, diesel, kerosene, and other multi-use oxygenates (e.g., methanol, dimethyl ether) using surplus RE-electricity.⁴³  As for PtM, waste carbon dioxide is converted catalytically with green H₂. A potential advantage of this approach is that these synthetic fuels can be viewed as drop-in replacements for fossil fuels without significant changes to existing infrastructure. This approach is particularly focused on the synthesis of synthetic alternatives to diesel⁴⁴  and aviation fuels.⁴⁵  Regarding chemicals (i.e., power-to-chemicals (PtC)), this approach involves the production of platform and/or valuable chemicals and/or raw materials using RE. The range of target products is extensive (from carbonates to polyurethanes, alcohols, ammonia, etc.), although most of these technologies focus on converting carbon oxides and biomass-derived molecules. A classic example (Figure 1.1) of a product is methanol (CH₃OH; >100 million tonnes per year industrial platform chemical; often denoted as power-to-methanol).⁴⁶  The production of methanol, in particular, has received particular interest, with Olah et al. proposing an extension of the hydrogen economy, termed the “methanol economy”.⁴⁷ 

PtC also includes ammonia (NH₃) production (i.e., power-to-ammonia (PtA)), whereby the traditional Haber–Bosch process can be fed with green H₂; this process has historically relied on H₂ supplied by SMR.⁴⁸  These chemicals can be used in various industrial sectors, including fertilisers and plastics, via low molecular weight olefins. As discussed in Chapter 7, higher levels of process integration can potentially be achieved, and the conversion of nitrogen (N₂ or N-containing compounds such as nitrates and nitrites) to NH₃ can be achieved directly through a RE-powered electrochemical process, presenting a more elegant and potentially more efficient route to fertiliser production.⁴⁹ 

As part of the more global hydrogen economy concept, PtX offers several benefits, including the utilisation of surplus RE (e.g., peak shaving) and inexpensive electricity to perform chemical processes. It also gives scope for alternative energy storage and grid balancing options whilst contributing to the delivery of sustainable fuels and chemicals to several different sectors (e.g., the chemical industry). Furthermore, it provides an interesting concept for developing coupled circular economy approaches for the reuse and valorisation of CO₂ emissions. As for the hydrogen economy, PtX technologies face many challenges related to cost, scalability, infrastructure requirements, and the sustainability of feedstocks used in the conversion processes. Despite this, PtX is gaining increasing attention as an important contributing technology to the overall goal of cross-sectoral defossilisation.

Regarding circular economies and analogous concepts, “carbon capture & utilisation” (CCU) is a proposed concept that involves the capture of CO₂ emissions from industrial processes or directly from the atmosphere and converting them into high-value/low-volume and/or low-value/high-volume products or resources. This process can simultaneously act on reducing direct GHG emissions and transform an environmental liability into a potential asset providing economic and environmental benefits.^50–52  CCU encompasses various technologies and processes that aim at utilising captured CO₂ rather than simply storing it in certain geological locations.⁵¹  CCU typically begins with CO₂ capture from industrial sources such as cement factories and steel mills or directly from the atmosphere using direct air capture (DAC) technologies (Figure 1.2).⁵³ 

Depending on the subsequent conversion process, the captured CO₂ may have to be separated and purified from other gases for further utilisation. The captured CO₂ can then be used as a feedstock for synthesizing chemicals, materials, or fuels (e.g., power-to-X). This can proceed via thermochemical,⁵⁵  electrochemical,⁵⁶  photochemical⁵⁷  and/or biological⁵⁸  processes to yield industrially relevant molecules such as methanol, ethanol, and acetate or even be used in the synthesis of plastics (e.g., polycarbonates)⁵⁹  or concrete manufacturing for construction.⁶⁰  As discussed later in this book, captured CO₂ can be processed via electrochemical processes using specialised electrochemical reactor designs (see Chapter 5). This represents a higher level of device/process integration and avoids the necessity to perform water electrolysis as a first step in the thermochemical conversion of CO₂ and opens a range of other interesting projects. Not discussed in this book, CO₂ can also be mineralised by reacting it with certain minerals or waste materials to form stable carbonates (i.e., a carbonation process) and can be applied to make materials like concrete or construction waste, thus permanently storing CO₂ in a solid form.⁶¹  Utilising captured CO₂ for chemistry and fuel production has several potential benefits. It can significantly reduce CO₂ emissions by capturing and utilising them rather than releasing them directly into the atmosphere. This contributes to mitigating climate change and achieving emission reduction targets, as well as providing scope for legacy industry (e.g., steel) infrastructure to continue to operate if they implement end-of-pipe emissions remediation technology.

For all the circular energy/chemical concepts introduced above, their development and implementation face challenges associated with device development and manufacture, production costs, infrastructure development, storage technologies, and ensuring the sustainability of H₂ and associated chemical/fuel production. However, advancements in technology, increasing RE capacity, and supportive policies are driving the growth, offering the potential to significantly reduce GHGs, enhance energy security, and promote sustainable economic growth. Additionally, the sustainability and environmental impact of any proposed concepts (e.g., CCU processes) need to be carefully evaluated to ensure that the overall lifecycle emissions and resource requirements are, in fact, beneficial.^62,63  To this end, and as discussed in Chapter 9, life cycle analysis (LCA) will be a key tool in providing this level of critical information and assessment.

With the energy transition in mind, a number of high-level R&D projects have been completed or are running which seek to advance the technology readiness levels (TRL) and address the technological challenges of the energy transition. In this respect, a number of these projects are summarised in Table 1.1.

The overarching objective of this book is to provide an introduction, discussion and concise review of a number of different chemical technologies that will contribute to the ongoing energy transition. The content is not complete as a book on the subject would extend to a number of volumes as is the nature of the RDI underway in the field. Instead, the book offers a number of chapters from leading researchers which provide a detailed review and discussion on the state of the art and the direction of innovation in their respective area of research.

The chapters of this book aim to provide insights into the various chemical processes and technologies that will play a crucial role in shifting from traditional fossil fuel-based energy sources to more sustainable and renewable alternatives. Chapters 2 and 3 are dedicated to hydrogen production, which, as mentioned before, is one of the central technologies for the energy transition. Chapter 2 compares the four main water electrolysis technologies with respect to some key techno-economic aspects such as the current–voltage curve, the economic current–voltage curve (which includes stack costs), gas crossover, heat management and gas separation, and durability. In Chapter 3, a deep overview of photo-driven hydrogen production is given. The main reactor designs and components are discussed in addition to procedures for photovoltaic characterisation, determination of stability and efficiency measurements and the reporting of all this information.

Chapter 4 goes beyond the conventional and looks at the use of the plasma phase to activate and split the key chemical bonds of the energy transition (e.g. O–H, C═O, etc.). Chapters 5 and 6 are devoted to electrochemical technologies as solutions for power-to-chemicals. In Chapter 5, the recent development in the field of the electrochemical conversion of CO₂ is discussed in depth with a thorough analysis through available literature. Chapter 6 looks into electrochemical conversion as an effective, environmentally friendly and economically viable process for biomass valorisation to more valuable compounds. Conversion of molecules such as furanics, glycerol and levulinic acid is described. Moreover, an overview of paired electrolysis (simultaneous use of the cathode and anode for synthesis) is provided. Chapter 7 compiles novel approaches to activate N₂ and produce NH₃ at lower temperatures and pressures than those used in the Haber–Bosch process. These approaches include direct electrochemical approaches, plasma-mediated systems, and indirect electrochemical approaches (lithium mediated). In this chapter, some of the energy, economic, and materials considerations for these novel technologies are compared.

In Chapter 8, the fundamental aspects of the thermo-catalysis used for CO₂ reduction to methane (a synthetic fuel) are described. Recent advances in understanding and controlling the CO₂ methanation performance are reviewed by examining key mechanistic proposals and structure–activity relationships on commonly used catalysts. The book concludes in Chapter 9 with a discussion of the increasingly critical tool of “life cycle assessment” (LCA) described and with examples given in the context of hydrogen production.

The chapters of this book aim to provide insights into the various chemical processes and technologies that will play a crucial role in shifting from traditional fossil fuel-based energy sources to more sustainable and renewable alternatives. It seeks to educate readers on the advancements and challenges in the field, empowering them to contribute to the ongoing global efforts towards a successful energy transition.