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Introduction

1.1 Terminology

Energetics: This scientific field focuses on the properties and transformation of energy in physical, chemical, or biological processes. It encompasses the study of energy flows and conversions in various systems, ranging from molecular to ecological scales. Energetics is fundamental in understanding how energy is harnessed and utilized in different contexts, from industrial processes to natural phenomena.

Energy Carrier: Hydrogen serves as an energy carrier, facilitating the transfer of energy from its generation source to the point of use in various forms, such as a chemical product, electricity, or mechanical energy. Similar to electricity, which is also an energy carrier, hydrogen does not create energy but stores and transports it. Energy carriers like hydrogen and electricity enable the convenient use of energy in forms more practical than their original sources. Hydrogen, distinct from electricity, can be stored in large quantities for future use or transported to where it is needed.

Hydrogen Economy: The hydrogen economy extends beyond traditional industrial uses of hydrogen as a feedstock. It encompasses the use of hydrogen for decarbonizing various sectors currently reliant on fossil fuels, such as transportation and steel production. This concept is integral to the low‐carbon economy, aiming to minimize fossil fuel combustion and associated emissions that contribute to climate change.

Nonrenewable Energy: Major nonrenewable energy resources include coal, oil, natural gas, and nuclear energy. These fossil fuels, formed over millions of years from decomposed plants and animals, are finite and nonreplenishable. Our reliance on these resources, which we have significantly depleted over the past two centuries, poses a sustainability challenge, as they cannot be replaced once exhausted.

Renewable Energy: Renewable energy sources encompass wind, solar, biomass (including sewage and ethanol), hydropower, and geothermal power. Biomass, derived from organic plant and animal materials, is a key renewable resource that replenishes through natural processes within a human timescale. Renewable resources, integral to Earth’s ecosphere, are sustainable when their recovery rate matches or exceeds consumption. Life‐cycle assessments are crucial in evaluating the sustainability of these resources. While oceans and agricultural outputs are often considered renewable, wood is a borderline case due to its longer regeneration time.

Sustainability: Defined by the UN World Commission on Environment and Development, sustainability is the ability to meet present needs without compromising future generations’ ability to meet theirs. The UCLA Sustainability Committee further elaborates on this concept, emphasizing the integration of environmental health, social equity, and economic vitality. Sustainable practices acknowledge the finite nature of resources and advocate for their conservative and wise use, considering long‐term priorities and the interconnectedness of environmental, social, and economic factors. Ultimately, sustainability is about leaving a viable world for future generations.

Decarbonization: Decarbonization refers to the process of reducing carbon dioxide emissions through the use of low‐carbon power sources, which emit fewer greenhouse gases (GHGs). This term should not be confused with decarburization, which pertains to the loss of carbon in the surface layer of a material.

Global Warming: Global warming is typically used to describe the human‐induced warming of the Earth’s system. In contrast, climate change can encompass both natural and anthropogenic changes in the Earth’s climate.

Greenhouse Gas (GHG): Greenhouse gases, or GHGs are atmospheric gases that trap heat. During daylight, the sun’s rays warm the Earth’s surface. At night, the Earth cools, releasing heat back into the atmosphere. However, GHGs, including carbon dioxide and chlorofluorocarbons, retain some of this heat, contributing to the greenhouse effect by absorbing infrared radiation.

Powerfuels: Powerfuels are synthetic gaseous and liquid fuels produced from green electricity. They are envisioned for use in sectors where alternatives are currently unviable, such as aviation and certain shipping methods. The development and commercialization of powerfuels are in nascent stages, with cost being a significant challenge. Currently, they are priced at €3–5/L, making them 5–10 times more expensive than fossil fuels.

Precession: Precession refers to the slow change in the direction of the axis of a rotating body, such as the Earth. This phenomenon, which can be observed in planets and stars, results in cyclic climate changes on Earth, characterized by extended periods of warming and cooling.

Energy Storage: Energy storage encompasses devices that minimize imbalances between energy demand and production. Commonly known as accumulators or batteries, these devices can store energy in various forms, including radiation, chemical, gravitational potential, electrical potential, electricity, elevated temperature, latent heat, and kinetic energy. The process involves converting energy from forms that are challenging to store into more storable and economically viable forms.

Color Code for Hydrogen: The classification of hydrogen production methods is often distinguished by their associated GHG emissions or environmental impact. In the energy sector, professionals refer to hydrogen using various color codes, each denoting the relationship between hydrogen production methods and the resultant GHG emissions.

Green Hydrogen: Green hydrogen refers to hydrogen produced through electrolysis using renewable energy sources such as wind, water, or solar power. This process emits no GHGs, contributing to a reduction in our carbon footprint. The purity of hydrogen, crucial for fuel cell applications, is often described as “five nines” (99.999%). The cost of producing green hydrogen ranges approximately from $2.5 to 6.0/kg.

Gray Hydrogen: Gray hydrogen is produced from natural gas via steam‐methane reforming, similar to blue hydrogen, but without any measures to capture the carbon dioxide by‐products. The cost of gray hydrogen production is around $1.5/kg, varying with natural gas prices and carbon emission factors.

Blue Hydrogen: Blue hydrogen is generated from natural gas through steam methane reforming, where natural gas reacts with steam and a catalyst to produce hydrogen and carbon monoxide. Further reaction with water yields more hydrogen and carbon dioxide. When the CO2 emissions are captured and stored, the process is considered carbon‐neutral, and the resulting hydrogen is termed “blue hydrogen.” Blue hydrogen production costs about $2.5/kg, positioning it between gray and green hydrogen in terms of expense. The feasibility of converting existing hydrogen production facilities to blue hydrogen production depends on the availability of CO2 storage capacity.

However, blue hydrogen production is subject to controversy due to methane emissions associated with natural gas extraction and transportation. Methane, though shorter‐lived in the atmosphere than CO2, is a potent GHG, with one ton of methane equating to 28–36 tons of CO2 in terms of global warming potential.

Pink Hydrogen: Pink hydrogen is produced via electrolysis powered by nuclear energy, which does not emit carbon dioxide. However, the process generates radioactive waste, necessitating safe storage for thousands of years.

Yellow Hydrogen: Yellow hydrogen is produced through electrolysis using energy from the electrical grid. The carbon emissions associated with yellow hydrogen vary significantly based on the energy sources powering the grid.

Turquoise Hydrogen: Turquoise hydrogen refers to hydrogen generated from methane pyrolysis, a process that splits methane into hydrogen and solid carbon using heat in reactors. The solid carbon by‐product can be sold separately, adding economic value. While still in the early stages of commercialization, turquoise hydrogen has the potential to be cost‐efficient and environmentally conscious, especially when powered by clean energy sources.

Solid Hydrogen: Solid hydrogen typically refers to hydrogen in its solid state, achieved at temperatures below its melting point of 14.01 K (−259.14 °C). Another application of the term “solid hydrogen” pertains to metal hydride storage devices, which use a solid‐state medium for hydrogen storage.

1.2 Sustainability and Climate Change

Sustainability, a concept deeply rooted in the understanding that human activities are a primary driver of recent climatic changes, emphasizes the need for responsible stewardship of the environment. The escalation of GHG emissions, particularly carbon dioxide, has led to a significant increase in global temperatures, a phenomenon faster than any previously recorded. Scientific consensus indicates that anthropogenic factors, especially since the Industrial Revolution in the 1800s, are the primary contributors to these emissions. The burning of fossil fuels, releasing heat‐trapping gases, is largely responsible for the observed climate change, encompassing everything from global warming to habitat destruction. This change is not just a scientific observation but is becoming...