H₂ in the energy transition

Hydrogen was identified as a chemical element in its own right as early as 1766 by the English scholar Henry Cavendish. Cavendish realized that hydrogen was flammable, and that the product of its combustion was nothing but ordinary water. Already from the early ninteenth century the work of pioneers such as William Nicholson and Anthony Carlisle demonstrated that it was possible to use the energy carried by electricity to split water into its constituent elements: oxygen and hydrogen. The circle was closed already in 1842, when the Welsh scholar William Robert Grove managed to obtain an electric current from the direct electrochemical reaction between hydrogen and oxygen in a “gas voltaic battery”, the forerunner of modern fuel cells [1]. Despite these early developments, the use of hydrogen as a thermal or electrochemical fuel was not very successful in the context of the Industrial Revolution due to the abundant availability of fossil fuels (e.g., coal and oil), which were very inexpensive and easy to transport and use. However, it should be noted that already in 1875 the visionary genius of the French writer Jules Verne had glimpsed the possibility of using hydrogen to obtain energy. In his novel “The Mysterious Island”, Verne has Cyrus Harding, one of the protagonists, say: “… I believe that water will one day be employed as fuel, that hydrogen and oxygen which constitute it, used singly or together, will furnish an inexhaustible source of heat and light …” [2]. Today, in the twenty-first century, this vision is becoming a reality.

Global warming is one of the greatest threats to our future, causing serious damage to human health, economic activities and the natural environment. This warming significantly increases the frequency of extreme climatic events (including droughts and floods) and threatens biodiversity. One of the main causes of global warming is the increase in the concentration of greenhouse gases in the atmosphere, with particular reference to the CO₂ produced by burning fossil fuels in order to obtain the energy necessary to sustain the lifestyle and economic development of humanity. It is therefore clear that any strategy aimed at mitigating global warming must implement a massive restructuring of the entire system used today at the global level to convert and distribute energy, minimizing the use of fossil fuels and aiming instead at a massive exploitation of the so-called “renewable sources”, with particular reference to the energy provided by the sun and the wind. The entire international community agrees in recognizing the need to ensure that global warming does not exceed two degrees compared to pre-industrial levels in order to curtail damage to the environment and guarantee a sustainable future for humankind [3].

The European Union (EU) recently launched an ambitious plan, known as the “European Green Deal”, which aims to make our continent “climate-neutral” by 2050 [4]. The European Green Deal consists of numerous actions aimed at curtailing greenhouse gas emissions, investing in cutting-edge technologies in numerous fields including energy efficiency, mobility and sustainable development. The European Green Deal plays a crucial role also and above all because it aims at demonstrating for the first time in the history of humankind that it is possible: (i) to decouple the degree of development of a society from the amount of energy consumed per capita; and (ii) to support economic growth using only clean technologies. These results, already very important at the European level, also aim to trace a new path to guide the growth of developing countries (Fig. 1), with a particular reference to the young economic superpowers of the twenty-first century (e.g., China and India). In the future, the large population of these countries, much higher than that of the entire EU, will play a crucial role to determine the impact of humankind on the environment. It is therefore essential to guarantee to all countries the possibility of achieving a high standard of living and an advanced economy (Fig. 1) without repeating the mistakes, wastes and damage to the environment that have been inflicted in the last two centuries during the growth of today’s “developed countries” (which include, among others, the EU, the United States and Japan).

Fig. 1:

Human development and use of energy. The challenge: to implement a sustainable development and growth policies at the national and the global level. The size of the circles is proportional to the population of the corresponding country. Very populous countries (including China and India) currently have a low per capita energy consumption. It is necessary to devise new growth paths, to ensure that these countries achieve high levels of human development without reaching the enormous per capita consumption of energy recorded by various developed countries, including the USA. Figure provided as a courtesy by prof. Plamen Atanassov, University of California, Irvine, USA.

From a technological viewpoint (Fig. 2), hydrogen represents one of the most promising solutions to achieve the energy transition to be implemented in the context of the European Green Deal. Hydrogen is an excellent vector for energy in the broader framework of the so-called “hydrogen economy” [5]. The energy produced from renewable sources, with a particular reference to those that are intermittent and non-programmable (including the sun and the wind) can in fact be used to split water, producing hydrogen. The latter can then be stored for long periods of time before being used to produce the energy as the necessity arises, for example operating a fuel cell. Hydrogen can also be used to reduce greenhouse gas emissions produced by numerous economic activities, including for example the iron and steel industry and several chemical industries [6]. Hydrogen has numerous properties of high interest for applications. First, when hydrogen is used to obtain energy, it does not release CO₂ at the point of use. 1 kg of hydrogen contains the same energy as 3.7 L of gasoline, and with it: (i) a car can travel about 100 km; and (ii) it is possible to cover the energy needs of a normal house for 1–2 days. A tank capable of containing 1 kg of hydrogen weighs about 20 kg, and occupies a volume of about 25 L. To date, the production of 1 kg of hydrogen costs between 2 and 10 Euros, depending on the process used. The latter is of crucial importance to assess the environmental impact of the produced hydrogen [7].

Fig. 2:

Overview of the various methods by which hydrogen is produced. Each production method corresponds to a certain “color” of hydrogen. Reproduced with permission from [7] – Copyright IOP Publishing, Ltd.

Traditionally, hydrogen was produced through thermochemical processes (Fig. 2) based on the use of fossil fuels such as coal and natural gas (“black/grey hydrogen”) and which released large amounts of CO₂ into the atmosphere. These approaches, although well-known and consolidated, are completely useless in the context of the energy transition even if the produced CO₂ is sequestered underground (“blue hydrogen”). It is also possible to produce hydrogen using nuclear energy (“pink hydrogen”). Although the production of this latter hydrogen does not give rise to greenhouse gas emissions, to date it is still not entirely clear whether the associated environmental impact is acceptable, with particular reference to issues relating to the lifecycle of the nuclear plant and the management of the nuclear wastes. On the other hand, the production of hydrogen through processes that do not trigger greenhouse gas emissions is of considerable interest (Fig. 2), including: (i) the splitting of water by biological agents; (ii) the photo-electrochemical splitting of water; (iii) the thermochemical splitting of water; and above all (iv) the electrolysis of water using electricity obtained from renewable sources (in particular the sun and the wind). The “green hydrogen” produced by these latter processes plays the most important role in the energy transition (Fig. 2).

As of today, considerable efforts are underway at the international level to bring down the costs associated with the production of “green hydrogen”. We would like to highlight the “Energy Earthshot – Hydrogen Shot™” initiative promoted by the Department of Energy of the United States Government [8], which aims at producing 1 kg of “green hydrogen” at a cost of 1 dollar by 2031. If this initiative were successful, the cost of hydrogen would become competitive with that of traditional fuels (with a particular reference to gasoline). It is also underlined that the energy density of hydrogen stored in the latest generation of tanks at a pressure of 700 bar (about 6–7 MJ/kg) is much higher than that of the most advanced lithium batteries (around 0.7–1 MJ/kg), further promoting the diffusion of vehicles powered by hydrogen fuel cells. To date, the technologies necessary to use hydrogen in numerous applications (including automotive, domestic energy production and the chemical industry) are maturing rapidly, making their practical implementation feasible between 2025 and 2040.

The two technologies that represent the cornerstone of the “hydrogen economy” and more generally of a massive use of hydrogen in economic activities are the electrolyzers and the fuel cells (Fig. 3). Electrolyzers consume electricity to split water into its basic components i.e., hydrogen and oxygen. Fuel cells, on the other hand, implement the opposite process: they recombine hydrogen and oxygen obtaining electricity and, as the waste, only water. The operation of fuel cells and electrolyzers does not produce greenhouse gases. It is emphasized that fuel cells work through electrochemical processes; therefore, their energy conversion efficiency (up to about 60 %) is much higher than that of traditional technologies which instead exploit the heat obtained by burning fossil fuels such as coal and gasoline (here the practical efficiency rarely exceeds 20–25 % even in ideal conditions). Fuel cells and electrolyzers share the same basic structure, which includes an electrolyte layer sandwiched between two planar electrodes (Fig. 3). At the interface between each electrode and the electrolyte is a layer of electrocatalyst (Fig. 4). The latter promotes the specific electrochemical process that takes place at the electrode, increasing the conversion efficiency of the entire device.

Fig. 3:

Fuel cells and electrolyzers, the cornerstones of the hydrogen economy.

Fig. 4:

Main features of membrane-electrode assemblies (MEAs) and of their functional components.

There are several families of fuel cells and electrolyzers, which differ on the basis of the functional materials used and the operating temperature [9]. Systems operating at high temperatures (typically between 600 and 1000 °C) suffer from significant limitations since: (i) they must be large in order to operate efficiently; and (ii) after a very limited number of turn on/switch off cycles they get damaged irrecoverably. On the other hand, fuel cells operating at low temperatures are much more compact and their operation is much more flexible. They are capable of adapting easily to the energy input profile (for example, in electrolyzers connected to intermittent sources of renewable energy) or to the contingent needs of a user (for example, in fuel cells for vehicles). The “heart” of most modern fuel cells and electrolyzers is the so-called “membrane-electrode assembly” (MEA). In a MEA, a thin polymer electrolyte membrane separates two porous electrodes covered by suitable electrocatalyst materials. There are two main families of low temperature fuel cells/electrolysers: (i) systems based on proton-exchange membranes (Fig. 4); such systems are efficient and durable, but require the adoption of electrocatalysts comprising noble metals such as platinum and iridium; and (ii) systems based on anion-exchange membranes (Fig. 4), which do not require the use of precious metals, but which include membranes that are still primitive and not very durable. As of today, research is very active to develop low-temperature fuel cells/electrolyzers capable to overcome the limitations described above and achieve the ambitious objectives necessary for the practical implementation of the energy transition. Some of these objectives are, for example [10]: (i) to reduce the cost of an electrolyzer down to 100 €/kW; and (ii) to develop a fuel cell capable of producing at least 8 kW per gram of noble metal used in the electrocatalysts.

The EU is spending major efforts to promote the large-scale use of hydrogen in the economy and in the mobility and energy sectors. The Commission coordinates several EU institutions and programs on hydrogen. Among them, the “Clean Hydrogen Partnership” [5] stands out, which involves both industry (in the “Hydrogen Europe – HE” partnership) and research institutions (in the “Hydrogen Europe Research – HER” network). As of today, a high priority is given to the development of the hydrogen infrastructure, with a massive ramp-up of the production capacity of “green hydrogen”. It is expected that the latter will reach 10 million tonnes per year by 2030 [11]. The EU is also using other instruments to facilitate the transnational practical application of hydrogen-based technologies, including the “Important Projects of Common European Interest – IPCEIs”. European research on hydrogen is coordinated by networks such as HER and the “European Energy Research Alliance – EERA” [12], which aims at covering all the various aspects of the energy transition including not only technological but also social issues. In this framework, the EU member states provide an important contribution, offering substantial resources (in the order of 5–15 billion Euros per state up to 2030) for the development of the hydrogen economy.

Italy is also playing its part in this sector, deploying resources derived from the National Recovery and Resilience Plan (PNRR) [13] and which amount to no less than 4 billion Euros until 2026 only to implement hydrogen production and distribution. Italy is also removing the barriers that hinder the practical implementation of the hydrogen economy, for example by: (i) proposing a specific certification for “green hydrogen”; (ii) developing the hydrogen refueling infrastructure; and (iii) promoting the strategic collaboration between the different “hydrogen valleys”. Hydrogen research in Italy is in great turmoil, with numerous projects financed with funds obtained both from the EU (Horizon Europe projects, or supported by other European bodies such as EIT Raw Materials) and from national sources (PNRR, FISR and PRIN projects), without forgetting the important role of the Regions. These are interesting times: the lives of all of us are undergoing continuous and radical changes. However, thanks to hydrogen we will be able to hope that some of these changes can lead us to a better future, leaving to our children as our legacy a world wherein well-being and development can coexist with the protection of our environment.