Critical Aspects of Energetic Transition Technologies and the Roles of Materials Chemistry and Engineering

Abstract

The perspectives of technological advances needed for short term energetic transition are briefly reviewed and discussed critically. In particular, the technologies for the greenhouse gas emission-free production of electrical energy, its storage and transport, the production, transport, storage and use of hydrogen, and the use of biomass derived technologies are shortly and critically reviewed. Critical aspects are emphasized. The role of chemistry, and in particular materials chemistry and engineering, in short-term developments are underlined.

1. Introduction

Although not all investigators are fully convinced of this [1,2], it is common opinion today that the evident global warming effect is mostly due to anthropogenic emissions of greenhouse gases (GHGs), in particular CO₂ coming from the combustion of fossil fuels. This forces to an urgent technological revolution towards zero GHG emissions, in particular in the energy production and use sector, although this implies significant risks [3].

On the other hand, although some new technologies are ready to be used to substitute previous ones generating large amounts of GHG, a number of other technological solutions are at the moment only hypothesized, being still at early levels of development and readiness. It is also sure that the use of social media to spread partial information and advertising, mixed with normal technology and science information, results in confusing pictures, where the differentiation between real perspectives, good ideas, and dreams is not so easy. This is particularly harmful for politicians, which are hurried to take decisions or to define and fund technological development programs. It is also necessary to underline that self-sustainable technologies must be finally developed, especially when state funding and incentives will no longer be possible.

In

Table 1. Comparison of CO₂ emissions by sector from Dou et al. [4] and IEA [5] in 2021 (Gt of CO₂).

Dou et al. [4]		IEA [5]
Sector	Gt	Gt	Sector
Power	14.1	14.6	Electricity and Heat Production
Industry	10.3	9.4	Industry
Residential Consumption	3.5	1.7	Residential Consumption of Building Sect
Ground Transport	6.3	6.1	Road + Rail + Pipeline Transport
Domestic + Intern. Aviation	0.6	0.7	International Aviation
International Shipping	0.7	0.8	Interational Shipping

, data for global CO₂ emissions by activity sector are compared, as reported by Dou et al. [4], evaluated according to the GRACED technique, and coming from the IEA (International Energy Agency [5]).

The data agree, indicating that the production of power for electricity generation, and the direct consumption of fuels for industrial and residential energy and heat production, are the activities most involved in CO₂ emissions (>70%). The transport sector, instead, is definitely less involved, with its main impacts being from ground transportation and a more limited impact from shipping and aviation. Among industrial activities directly contributing to CO₂ emissions, steelmaking, cement manufacture, and petrochemical industries are mostly involved.

2. Decarbonization versus Defossilization—On the Use of Biomasses in the Energy Transition

The term most commonly used to define the next energy transition is “decarbonization” [6], which would imply the abandonment of all technologies that emit carbon oxides and hydrocarbons. Others, however, prefer to point out that the elimination of technologies that emit fossil-derived carbon compounds only, is needed. Technologies emitting carbon oxides of atmospheric origin do not contribute to the accumulation of greenhouse gases and could still be useful. This is why we may talk about “defossilization” [7]. The latter approach would allow the use of biomass (lignocellulosic matter, as well as food, agricultural and garden wastes, livestock waste, and biogas) as energy sources, which would be excluded by those who push for full decarbonization.

The main limit of the biomass-based approach to producing fuels and energy lies in the supposed small availability of non-edible plant raw materials that are not competitive with the food market. Additionally, deforestation must be avoided. In fact, authors agree that the careful management of natural resources and forests could make it possible, especially in some countries [8,9], to make large quantities of biomass available without reducing the mass of living and breathing plant material.

On the other hand, policies for fertilization, promoting the cultivation of energy-related plants in arid and desert areas, are also considered and can even allow a significant growth of living and CO₂-consuming vegetal mass, together with the availability of biomass (e.g., wood and inedible oils [10,11]) for chemical purposes and help the economies of underdeveloped countries [12].

Additionally, the hypothesized production of algae in large quantities for energy purposes is a technology yet to be developed commercially. It presents difficulties and problems, including environmental ones, that have not been resolved but are still considered promising [13].

The conversion of lignocellulosic and algal biomasses may allow the production of energy and fuels, as well as chemical intermediates, to feed industrial chemistry. This is the case, e.g., of bioethanol, a biofuel that can also be efficiently converted into bio-ethylene, i.e., the same molecule, which is today the main petrochemical intermediate in the origin of the production of many chemicals [14]. Similarly, bio-propylene [15] and bio-aromatics [16] can also be produced from biomasses, thus allowing hydrocarbon-based industrial organic chemistry to survive energy transition and defossilization.

Capturing and exploiting biogases represents another way of using biomass, in the form of wastes, for energy purposes [17]. After a relatively simple purification process, biogas can be burned to produce energy or heat, or it can be upgraded to biomethane, which can be introduced into the gas grid and used like natural gas, for example, to produce hydrogen [18].

Excluding these technologies a priori seems more the result of a dogmatic position, perhaps influenced by the enormous commercial interests involved, rather than the result of a rational and scientific approach. As we shall see, at least in the foreseeable future, it will not be possible to exclude biomass-based technologies to fully cut GHG emissions.

3. Criteria for the Evaluation of Technologies, in Particular for the Production of Energy

The evaluation of the reliability and friendliness of technologies, in particular, but not only “green technologies”, to produce electrical energy can be conducted with respect to a number of different criteria.

3.1. Technological Readiness

The evaluation of the real availability or of future perspectives of technologies is usually quantified in terms of their TRL (Technology Readiness Level). According to many authors or entities, including the European Commission [19], the TRL ranges from one, when a basic idea is proposed, to nine, when industrial exploitation is raised. According to the IEA, the final step is at a TRL = 11, when a technology developed at the industrial level also demonstrates long-term stability, with the intermediate value of 10, when the technology still needs further integration efforts (

Table 2. Technology Readiness Levels, according to IEA.

TRL	European Union [19]	IEA [20]
1	Basic principles observed	Initial idea: basic principles have been defined
2	Technology concept formulated	Application formulated: the concept and application of the solution have been formulated
3	Experimental proof of concept	Concept needs validation: the solution needs to be prototyped and applied
4	Technology validated in lab	Early prototype: prototype proven in test conditions
5	Technology validated in relevant environment (industrially relevant environment in the case of key enabling technologies)	Large prototype: components proven in conditions to be deployed
6	Technology demonstrated in relevant environment (industrially relevant environment in the case of key enabling technologies)	Full prototype at scale: prototype proven at scale in conditions to be deployed
7	System prototype demonstration in the operational environment	Pre-commercial demonstration: solution working in expected conditions
8	System complete and qualified	First-of-a-kind commercial: commercial demonstration, full-scale deployment in final form
9	Actual system proven in the operational environment (competitive manufacturing in the case of key enabling technologies; or in space)	Commercial operation in the relevant environment: the solution is commercially available, but needs evolutionary improvement to stay competitive
10	--------------	Integration at scale: the solution is commercial but needs further integration efforts
11	--------------	Proof of stability: predictable growth

[20]).

It can be noticed that, in the past, many technologies, previously considered very promising, never reached the commercial level (thus stopping at TRL ≤ 8) while other technologies, having reached the commercial level, were shortly abandoned having proved to contain bugs, be uncompetitive, or even be harmful. On the other hand, it can be seen that some technologies that are presented today as reliable alternatives to the actual ones are still sometimes at very low levels of development and readiness. A main example is nuclear fusion technology, which is presented as the future way to produce infinite energy without environmental impact, and is the objective of impressive development work [21,22]. In spite of this, up to now, nuclear fusion experiments have not produced energy for more than a few tens of seconds. Thus, nuclear fusion as an energy production technology is still evaluated to be at a TRL of 2–3 [23] (still, its practical feasibility is not proven). The real environmental impacts [24] and safety risks [25] are, consequently, still not fully known, although they are certainly not negligible.

3.2. Additional Criteria for Technology Evaluation

A number of other criteria must be taken into consideration when the applicability of technology is evaluated. In

Table 3. Criteria for technology evaluation.

Criterium	Ref.
Readiness level
Continuity versus discontinuity
Predictability versus unpredictability
Application constraints	[26]
Materials intensity	[27,28]
Land occupation and land use change	[29,30]
Effects on human health	[26,31,32]
Impact on ecosystems	[26,31]
Water use or consumption	[33]
Safety risks	[32]
Diminishing land aesthetic values and loss of habitat quality	[26]
Social effects: job creation and people satisfaction	[27]
In life and end-of-life management of devices
Capital costs, operational costs, lifetime, levelized cost	[26,27,34]

, a list of such criteria is reported, together with references where some of such criteria are comparatively evaluated. In the next chapter, we will analyze different technologies, also taking into consideration these criteria.

4. Evaluation of Virtually “Zero GHG Emission” Electrical Energy Production Technologies

In Table 4, the most used power generation technologies are summarized together with the energy produced in 2020 using them, according to British Petroleum [35]. From these data, it is clear that more than 60% of the globally produced energy arises from fossil fuels. It is evident that the decarbonization of electric energy production implies a strong increase in the use of renewable technologies. On the other hand, another important fraction (around 10%) comes from nuclear fission, whose phasing-out is also considered welcome by many experts.

Additionally, in the near future, electric vehicles and electrolytic hydrogen-fueled vehicles will likely become most used, implying a further significant increase in the need for stationary electrical energy production. It is evident that the decarbonization of electrical energy production implies a significant increase and further development of already available “green technologies” and maybe the development of several others. According to Tamor and Stechel, carbon neutrality in 2050 for the USA implies quadruplication of electrical energy production as well as the production of an enormous amount of biofuels [36]. From Table 4, it becomes also clear that technologies that in principle do not emit CO₂ are actually responsible for its emission, e.g., in system-building steps. This is the case of hydropower, which may consume enormous amounts of cement and building materials that are produced with CO₂ emissions, as well as silicon-based photovoltaic, due to string CO₂-emitting silicon manufacturing technologies.

Table 4. Power generation technologies and their application in 2020.

Fuel	Technology [27]	Emission fuel gCO2/kWh [37]	Life Cycle Emission gCO2eq/kWh [38]	Energy Production in 2020
				TWh [35]	%
Coal	Pulverized Hard Coal	338.2 398.7	1095–912	9421.4	35.1	61.3
	IGCC Hard Coal		912–753
	Lignite
Natural Gas	Combined Cycle	200.8	513–403	6268.1	23.4
	Single Cycle
Oil	Heavy Fuel Oil Burners	266.5		758.0	2.8
--	Hydroelectric		147–6	4296.8	16.0
UO2	Nuclear Fission		5–6	2700.1	10.1
--	Wind		23–8	1591.2	5.9
--	Solar PV		83–23	855.7	3.2
--	Other Renewable			700.1	2.6
Total				26,823.2	100.0

Table 4. Power generation technologies and their application in 2020.

Fuel	Technology [27]	Emission fuel gCO2/kWh [37]	Life Cycle Emission gCO2eq/kWh [38]	Energy Production in 2020
				TWh [35]	%
Coal	Pulverized Hard Coal	338.2 398.7	1095–912	9421.4	35.1	61.3
	IGCC Hard Coal		912–753
	Lignite
Natural Gas	Combined Cycle	200.8	513–403	6268.1	23.4
	Single Cycle
Oil	Heavy Fuel Oil Burners	266.5		758.0	2.8
--	Hydroelectric		147–6	4296.8	16.0
UO2	Nuclear Fission		5–6	2700.1	10.1
--	Wind		23–8	1591.2	5.9
--	Solar PV		83–23	855.7	3.2
--	Other Renewable			700.1	2.6
Total				26,823.2	100.0

In

Table 5. Power energy production technologies allowing poor or no production of CO₂, their readiness levels, and their drawbacks and limits. TRLs are according to IEA [23].

Technology		TRL (IEA)	Drawbacks/Application Constraints
Hydropower		11	Limited to hilly or mountainous territories rich in water, in poorly populated areas. Strong environmental impact and limited to soil use. Already largely employed in several countries.
Photovoltaic	Crystalline Silicon	9–10	More efficient in high solar irradiance areas, need for availability of large areas, limited coupled use of land. Limited roof orientation availability. Discontinuous.
	Perovskite	4–5
	Thin Film	8
	Organic Thin Film	5–6
Wind	OnShore	9–10	Limited to continually strong windy territories. Limited coupled use of land. Discontinuous. Limited to protect human health, landscape, and avian fauna.
	OffShore	8–9	Limited to continually strong windy sea areas. Need for available sea areas, limited coupled use of such marine areas. Discontinuous. Limited to protect landscapes and avian fauna.
Hydrogen	Gas Turbine	7–9	Transforms energy with low efficiency, without producing it.
	Solid Oxide Fuel Cells	8–9	Transforms energy without producing it, low durability.
Geothermal		11	Limited to territories with near-surface hot water/steam sources.
Ocean		4–7	Limited to sea areas with high-temperature differences (OTEC) or large waves. Limited coupled use of such marine areas. Possible landscape damage.
Tidal		9	Limited to coastal areas with high tides. Limited coupled use of such marine areas.
Nuclear Fission	Slow Neutron	11	Radioactive raw materials, spent fuel, and byproducts. Risks of radioactive matter loss.
	Fast Neutron	10	Radioactive raw materials, risks of radioactive matter loss.
Nuclear Fusion		1–3	Still at very early stages of development, safety limits and practical production conditions still not well established.
Coal with CCUS		9	Limited by usability of CO2 and possibility of storage.
Natural Gas with CCUS		8	Limited by usability of CO2 and possibility of storage.
Biomass	Combustion	10	Limited availability of large masses of un-edible biomass. Pollutant waste gases.
	Gasification and Combustion	8

, data on “green” technologies are summarized, together with data concerning their development state as well as the drawbacks or limitations they present. In the following section, we will take into consideration the most promising technologies for widespread use, and we will point out their drawbacks and merits.

4.1. Nuclear Fission Technologies

So-called “slow neutron” technology is the nuclear technology widely used today for the production of electricity [39], applied since the 1950s of the twentieth century, and which today is made with more than 400 reactors in the world. It is certainly a mature technology (TRL of 11 according to the IEA) [23] but it finds its main limit in the potential big risks of emissions of radioactive material in the event of an accident, and, maybe above all, in the concern that these risks generate in the population. In fact, the number of significant accidents in nuclear power plants has been extremely limited so far, as has the number of deaths directly caused by accidents themselves. However, it is more difficult to assess the number of deaths caused by the radiation emitted, as well as the resulting psychological effects, and to compare them with the not insignificant number of fatal accidents caused by other technologies, such as, for example, natural gas explosions. The accident that occurred at reactor four of the Chernobyl nuclear power plant on 26 April 1986 released massive amounts of nuclear fuel and radioactive matter into the surrounding environment and is considered to be the worst nuclear accident in history [40]. The direct deaths in the accident were around 30, but at least 9000 cancer-related fatalities were estimated to occur as a result of radioactive emissions [41]. Estimations of the effects of the Fukushima accident, which occurred on 11 March 2011, caused by the Tōhoku earthquake and tsunami, are even more difficult. In this case, most of the limited radioactive emissions were in the ocean and directly caused fatalities were essentially zero, with very limited health effects in the region. However, indirect deaths occurred, in particular at least 164,865 residents of the surrounding area were permanently or temporarily displaced as a result of the incident, and this resulted in at least 51 casualties, mainly in old and sick people [42]. Surprisingly, in some of the literature, the 18,000 deaths caused by the Tōhoku earthquake and tsunami are incorrectly attributed to the Fukushima accident. In any case, the effect of the risks associated with the Fukushima accident was decisive in negatively influencing the policies with respect to nuclear fission technologies, e.g., in Germany [43].

Indeed, the entire technological chain of nuclear fission technologies is critical from the point of view of the effects on human health, being based on the extraction and processing of uranium-based radioactive minerals and on the production and transport of radioactive nuclear fuel, of a weakly radioactive waste (depleted uranium), and of highly radioactive waste (spent nuclear fuel). This technology is supported by rather low operating costs compared to moderately high capital costs for the same installed capacity, continuity with the consequent stabilizing role for the grid, and limited land occupation (see refs. in Table 3), and still has perspective as a decarbonization technology [44].

Various advanced types of SMRs (Small and Medium Reactors, or Small Modular Reactors), e.g., those with a power of up to 300 MW-electric (MWe), are currently under development worldwide [45], including the promising SMART (System-integrated Modular Advanced Reactor Technology) technology [46]. They are characterized by integrated design, passive safety systems, design modularity, operational flexibility, and versatile applications and, although still not commercialized, may offer further economic and flexibility advantages in the future [47].

The advantages of fast neutron nuclear fission technology, already used at a limited extent and still under development (TRL of 8–9 according to the IEA [23]) are essentially based on the fact that the isotope enrichment process is avoided and there is a greater consumption of nuclear fuel, with a strong reduction in the production of radioactive waste.

It must be borne in mind that the lifespan of nuclear power plants is limited to about fifty years and that the problem of dismantling the many obsolete nuclear reactors is now becoming urgent [48].

4.2. Hydropower Technologies

Hydroelectric power is also a technology that has been widely tested and stable for over a century (TRL 11), with a fair degree of continuity and predictability, although it can be negatively affected by drought phenomena or by discontinuity in the flow of the watercourses used. The capital costs are quite high, but the operating costs are low. Among the limitations of hydroelectric power, we note that it is a technology limited by the nature, necessarily mountainous or at least hilly, of the territory, by the needed richness of water, and by the strong occupation and limitation of the use of the land itself, although the impact on the aesthetic value of the territory can often be considered positive (many artificial lakes have considerable tourist value). This technology is also associated with the use of large amounts of water, and with a considerable consumption of materials being based on structures of often gigantic dimensions. It should be noted, however, that this technology is already widely exploited in many countries (in particular in Italy) and is therefore almost saturated, as there are not many suitable watercourses yet to be used, at least for large plants. However, it has been reported that only 25% of the existing 45,000 large dams in the world are currently used for hydropower, and the other 75% are used exclusively for other purposes (e.g., irrigation, flood control, navigation, and urban water supply) [49].

It must be taken into consideration that hydropower plants may have an important role in electric grid stabilization. Besides its usually stable behavior, they can be used to obtain pumped hydro storage, which represents the main technique for large-scale energy storage today: 96% of the energy stored globally today is obtained with this technique, i.e., repumping water on top of the dam when grid energy consumption is low [50].

4.3. Photovoltaic Technologies

Both roof-mounted and ground-mounted crystalline silicon photovoltaics (PVs), which represents the usually available PV technology today, are already well developed and largely applied, although they are certainly still in some way under further development (TRL = 9).

Silicon is the most widespread element in the earth’s crust after oxygen, in the form of dioxide-based or silicate-based minerals. To obtain the photovoltaic effect, however, hyperpure crystalline elemental silicon (99.99999%) is required, manufactured through an extremely energy-intensive process, which is currently carried out mainly in China [51]. Silica sands are reduced with coke in electrothermal furnaces at 2050 °C, producing highly impure metallurgical silicon. Complex refining processes are carried out to produce PV-grade silicon by converting impure silicon to trichlorosilane with hydrochloric acid at high pressure, distilling, condensing, and decomposing it at 1200 °C in the presence of hydrogen, and then crystallizing amorphous to polycrystalline silicon. To produce monocrystalline silicon, the crystal growth stage, realized at its melting temperature (1414 °C), is added, resulting in an additional significant increase in energy consumption [52]. The result of all this is that the production of solar-grade silicon is among the most energy-intensive processes in the industry [53], with an electricity consumption of over 200 kWh/Kg_Si, a coal consumption of 27 Kg_C/Kg_Si, and significant CO₂ emissions: 84 KgCO₂/Kg_Si [54]. Depending on the type of silicon used and where it is used, a photovoltaic panel takes the first 2 to 8 years of use to produce the energy that has been consumed in its manufacturing process [55,56]. Despite this, the cost of solar-grade silicon is not particularly high (less than 10 €/kg), but it constitutes only a part of the cost of the single module [57], which also requires significant quantities of steel, copper, silver, and aluminum, special glass, adhesives, etc. On the other hand, the cost of the panel represents approximately only 35–50% of the cost of the entire plant, which increases significantly if it is equipped with an energy storage system, such as lithium-ion batteries. Additionally, the efficiency of PV technologies is intrinsically low, with the maximum energy efficiency (the percentage of incident solar energy that is actually converted to electricity) of silicon PV being above 25% in optimal conditions, but ranging from 18 to 22% for industrial panels under standard test conditions.

Among the other drawbacks of photovoltaics are the following: (i) its day/night discontinuity, which strongly further reduces its energy efficiency and strongly destabilizes the electricity grids, or implies the adoption of expensive energy storage systems; (ii) its unpredictability with respect to climatic phenomena; (iii) the geographical dependence of its efficiency, much higher in tropical areas where radiation is strong, medium in temperate areas, and minimal in polar areas; and (iv) some health risks arising from the use of toxic materials and their mining and metallurgy processes.

The result is that the capital cost per energy produced by photovoltaics is quite high: according to some authors, polycrystalline silicon photovoltaic technology is by far the one with the highest capital costs for the same installed capacity [34]. Although the operating costs are quite low, the maintenance costs (e.g., frequent cleaning of the panels necessary to maintain good efficiency, which consumes a considerable amount of water, and electrical checks to limit the non-negligible risk of fires) are not negligible, especially for “roof” photovoltaics. Thus, taking into account that the lifetime of a silicon PV panel is guaranteed today to be 25 years, the current levelized cost of photovoltaic technologies, although strongly decreased over the years [58] and expected to continue to decrease, is still comparable to or higher than that of fossil technologies [59].

In practice, photovoltaics can be built “on the ground” or “on the roof”. It is clear that ground-mounted photovoltaics produce a strong occupation of land or a strong limitation of its use [60], and their use is hardly compatible with highly populated and man-made areas, as well as with forests and agricultural activities. If carried out in non-anthropized areas, they have a significant negative effect on fauna [32], as well as, obviously, on flora. Large photovoltaic farms, even if built on deserts, can also produce important global climate modifications [61], and are strongly negatively affected by dust storms [62].

On the other hand, rooftop photovoltaics have very strong limitations, at least in historic cities, since roof surfaces are actually small compared to population density, and only a small fraction of roofs have favorable exposure to high photovoltaic efficiency [28] and are not used otherwise. Rooftop photovoltaics can be better coupled with activities that use low-lying structures with a high roof surface area such as those of industrial warehouses, farms, parking lots, etc. [63]. However, for many fairly energy-intensive companies, the roof area is often largely insufficient to produce through photovoltaics the electricity consumed.

It should also be noted that silicon-based panels are subject to damage in the presence of strong windstorms and strong hailstorms and that the recycling of solar-grade silicon and other components from non-functioning panels is not currently being carried out in a significant way. These panels are accumulating and are expected to reach very significant quantities of photovoltaic waste [64].

The use of other materials for photovoltaics, such as silicon thin films, CdTe, organic thin films, and perovskites [65], can contribute to improving energy and limiting energy costs of panel manufacturing. In particular, perovskite photovoltaic materials (still far from commercialization, TRL = 4–5 [23]) have the potential of improving efficiency, but suffer instability and deterioration under wet conditions, which is the main limitation to industrialization. These materials, whose formula is the ABX₃ structure, where A is a monovalent cation (methylammonium, formamidinium, Cs, or Rb), B is a divalent metal cation (Pb, Sn, and Ge), and X is a halide anion (I, Br, and Cl), may have some drawbacks in the use of lead, quite a pollutant metal [66]. Also, their best manufacturing technology at the industrial level is still to be evaluated and developed in terms of energy consumption and environmental friendliness. Organic photovoltaic technologies (TRL = 4–5 [23]) also have great potential but significant drawbacks too, such as low efficiency and durability, degradation or malfunction at high temperatures, and a supply chain that is still to be identified and developed [67].

4.4. Wind Technologies

Wind technologies find their maximum efficiency in the presence of strong and continuous winds. This makes them very efficient technologies in areas such as the North Sea and the north of Scotland in Europe. On the other hand, the efficiency of wind power in areas usually free from strong winds is considerably limited and has a strong climatic discontinuity and uncertainty. Its discontinuity may have a strong negative effect on the stability of the networks. Wind power also has marked limits in its health effects that make it incompatible with populated areas, associated with swishing, whistling, or throbbing noises from moving gear trains and turbine blades resulting in annoyance, sleep disturbance, and a reduced quality of life for people living nearby [68]. The weakest element of the structure of wind farms is the blades, which are made of sophisticated composites of plastics and glassy materials. However, the blades are subject to erosion and have an average life estimated at 20 years. The detachment and collapse of the sometimes enormous size of the most powerful turbines also give rise to a certain risk of serious accidents. Significant negative effects of wind power on the environment are also well evident [69], with impacts on the life of birds due to their collision with the blades [70], while their building may imply some deforestation, and impact regional climate mainly causing heating in nighttime due to mixing of the boundary layer [71].

Wind technologies also generate limitations of land use (or of sea usage in the case of off-shore wind), and a reduction in the aesthetic quality of the territory or of the sea. Additionally, they are associated with a large consumption of materials [72] due to the size of the structures (concrete bases and steel structures for on-shore wind), magnetic materials based on rare earths, lubricants, conductive metallic materials, and composite materials for blades.

The capital cost for wind technologies is high, particularly for offshore installations in deep-sea areas [73], and the lifetime, at least for blades, is relatively short. Thus, the levelized costs of wind energy are today comparable with those of technologies based on fossil raw materials and are and will be, in any case, considerable in the future.

Waste is also accumulating of these materials, consisting mainly of worn or broken blades, as technologies for the recovery of materials from them have not yet been developed [74].

4.5. Biomass-Based Electrical Energy Production Technologies

Wood has been used for thousands of years as a fuel for heating homes and for powering kitchens. In reality, this “technology” is not optimal for human health, because wood smoke is highly polluting, containing many volatile organic compounds (VOCs), some of which are highly toxic (e.g., benzene and dioxins), as well as unburned dust [75]. However, technologies have been developed that allow the abatement of pollutants from wood smoke in biomass-based power plants, as well as waste incinerators [76], which represent today a commercial technology for energy production (TRL of 10).

Gasification [77] represents an alternative to produce energy from biomass and biomass-based wastes by reaction with water. However, due to the high oxygen content of biomass (about 40% oxygen) and the content of alkali metals (Na and K) [78], biomass gasification produces a combustible gas containing hydrogen and CO, but is very dirty, with VOCs and volatile alkaline compounds (tar). This gas can still be burned to produce energy, but its purification to produce hydrogen or cleaner syngas is very difficult and makes the process complex [79], and this is the main obstacle to commercialization (see below).

As already mentioned, the main limitation of these technologies lies in the real availability of biomass, which is limited in many places. In any case, care IS necessary not to limit the amount of living plant mass and not to limit food production.

4.6. Making Green Fossil Fuel-Based Technologies by CO₂ Capture and Utilization or Storage

The “greening” of fossil fuel thermal energy production is accomplished by adding the step of CO₂ capture and utilization or storage. CO₂ can be separated, in principle, from combustion flue gases [80] or even directly from air [81]. The technologies under study are scrubbing, adsorption, or membrane technologies and are not far from industrial development [82,83,84,85], although further steps concerning CO₂ reuse or storage may be also technically feasible today. It must be mentioned that such processes imply energy consumption (parasitic energy) and costs that reduce the efficiency of the energy production process significantly [84]. A number of CO₂ utilization or storage technologies are under study, although they have a low TRL [82]. Another weak point of this approach consists of the limited room for the storage or reuse of enormous amounts of CO₂ [86]. Some storage and reuse systems are reported in

Table 6. Technologies for CO₂ reuse and storage.

Technology	TRL
Chemicals from CO2 (urea, methanol, polycarbonates……)	7–9
Fuels from CO2 (methanol, hydrocarbons, methane……)	8
Feed for microalgae production	7
Inert gas in the production of food	9
Sparkling beverages	9
Metallurgical processes	9
Storage in saline formations	9
Storage and enhanced oil recovery	9
Storage and enhanced gas recovery	7
Storage in depleted oil and natural gas fields	7
Storage through mineral carbonation (basaltic rocks, ultramafic rocks)	2–6
Storage with enhanced coal bed methane recovery	2–3
Ocean storage	2

together with their technological readiness levels, mostly reported by reference [82].

One of the most interesting perspectives today is the hydrogenation of captured CO₂ with green hydrogen to produce e-fuels. Catalysts and processes for this purpose are already at a near commercialization step [87]. This point will be further discussed below.

5. Transport and Storage of Electrical Energy

5.1. On the Transport of Electrical Energy

Electrical energy is commonly transported through transport lines with a typical loss of 3–7% per 1000 km in high voltage lines, finally leading to typical minimum losses of 20–30% in transport and distribution systems. Resistive losses in the conductors and dielectric losses in the insulation materials are the causes of power losses both for overhead transport [88] and for underground transport [89]. The efficiency of direct electrical energy transport should be compared with the transport of energy through the transport of fuels, such as gaseous or liquid hydrocarbons and hydrogen (see below).

5.2. Storage of Electrical Energy. Mechanical and Thermal Technologies

The storage of electrical energy is necessary for several reasons and in several contexts. In fact, the grids must be stable with supply and demand of electricity constantly balanced. Thus, excess production peaks allow the storage of energy to be supplied in deficit production times. Storage is also needed in off-grid production conditions, e.g., when discontinuous renewable energy sources like photovoltaic are applied to isolated buildings [90,91]. Additionally, it is needed to feed electric vehicles.

Pumped hydro storage represents the main technique for large-scale energy storage: 96% of the energy stored globally today is obtained with this technique, commonly applied in particular to energy grids including hydroelectric plants [50], or based on wind and solar energy in regions where large spaces are available as well as a sufficient amount of water [92]. A number of other mechanical systems for energy storage, such as compressed air energy storage [93], a useful technique when coupled with cogeneration and renewable generation systems, but with low efficiency, and flywheels that can be useful for storing energy for limited periods of time.

Additionally, thermal energy storage systems also exist [94], such as sensible heat storage (SHS) using ceramics, water, or oil; latent heat storage (LHS) using molten salt, paraffin wax and water/ice materials; and termochemical energy storage [95], achieved via a reversible chemical reaction, such as dehydration/rehydration of alkali hearth hydrtoxides/oxides, decarbonation/recarbonation of alkali earth carbonates/oxides, dehydration/rehydration of metal hydroxide/oxides, and oxidation and reduction in transition metal oxides.

5.3. Electrochemical Devices for Energy Storage

5.3.1. Rechargeable Batteries

As is well known, the development of lithium-ion batteries has been a main key in the development of charging technologies for electronics and small devices, and, more recently, for the development of full electric transportation technologies. Batteries also have applications in the stationary storage of moderate amounts of energy, e.g., in the case of buildings. As reviewed by Masias et al. [96], the development of LIBs has its origin in the 1960s, and raised the level of commercialization for electronics in the 1990s, with a successive progressive increase in capacity, cell potential, cycle life and, in particular, energy density from values around 100 Wh/kg up to the short-term goal of usable energy density at a cell level of 350 Wh/kg [97], i.e., far higher than other battery types (such as Ni-MH, NiCd, and lead acid). Indeed, this value and the other performances of LIBs make them by far more efficient than other battery types for electronics and small electric devices. However, the above energy density values are extremely low if compared with energy densities of conventional systems based on hydrocarbon fuels used for transportation with combustion engines, which lie in the 12,000–14,000 Wh/kg range. While this limit is not so determinantal for the application of short-range electric vehicles, such as bikes and motorcycles, it represents a critical point for cars and a main limit for long-range transportation such as for large trucks, large oceanic ships, and intercontinental aircraft. This limit is coupled to the slow recharging rate of all battery types, including LIBs, which is a main factor for managing the use of transport vehicles [98]. Indeed, the full charging time of the battery system of today’s electric vehicles is in the range of at least one hour. This drawback is due to slow diffusion kinetics phenomena and can be approached as a materials science and engineering aspect [99,100]. Indeed, other relevant challenges for LIBs for EVs are increasing their calendar life (up to 10 years), increasing their cycle life (up to 1000 cycles), enlarging their temperature range (to −30 to 52 °C), and decreasing costs ($100/kWh) [96].

Further points against the large-scale use of LIBs are associated with a significant consumption of critical materials. The most common LIBs are based on Li–graphite and mixed Li-Co-Ni (Mn) oxides electrodes, and liquid electrolytes allowing the diffusion of the Li⁺ ion. A critical point is represented by the availability of lithium element, which constitutes only 0.0017% of weight of the Earth’s crust and has uneven distribution [101]. Graphite too represents a critical material [102]. Additionally, the use of Co and Ni in electrodes, also critical elements, makes them expensive and toxic, and this creates a push for the development of materials recycling processes for the end-of-life of batteries [103]. The already commercial availability of batteries based on lithium iron phosphate electrodes reduces this drawback [104].

It must be also taken into account that conventional LIBs may suffer from safety issues, including short circuits and even combustion caused by the oxidative decomposition of the organic liquid electrolyte at high temperatures, accompanied by the release of gas byproducts [105]. In fact, procedures to approach these phenomena, which are fully different from phenomena occurring with burnable organic compounds, are still incompletely determined [106] and not regulated. Solid-state LIBs may reduce safety concerns [107] but not completely [108].

A number of alternatives are considered today for use as batteries in EVs. Sodium ion batteries (SIBs) have been under development since the 1980s, but still show poor cycle performance and safety concerns that still hinder large-scale applications [109]. Also, potassium-ion batteries [110] and aluminum-ion batteries [111] are under study, still at the initial development stage.

Many other rechargeable battery systems are under investigation and further development: among them are old systems like lead–acid, Ni-Cd, and Ni-MH (Nickel Metal Hydride) batteries, and more recent systems like metal–air batteries [112], metal–sulfur batteries [113], Na-NiCl₂ batteries [114], and redox flow batteries [115]. They may have excellent and promising features together with evident drawbacks and are still in early industrial development [116,117]. It must be taken into consideration, in any case, that all batteries under study do not have significant advantages in terms of energy density with respect to LIBs [110], thus not reducing the main limit for full electric transportation. Also, for batteries, it should be noted that technologies that allow the recovery of materials or elements (in particular lithium) from exhausted or broken cells are still not being significantly implemented [118], which are currently accumulating as electronic and hazardous waste. However, the European Union has issued regulations for their correct treatment [119].

5.3.2. Supercapacitors and Hybrid Supercapacitors

Electric Double-Layer Capacitors (EDLC supercapacitors) are the most common supercapacitor type [120,121]. These devices use electrostatic interaction to accumulate energy in Helmholtz double layers on the phase interface between the surface of the electrodes and the electrolyte. Most of these electrolytes are constituted by aprotic solvents and dissolved salts. High surface area materials like activated carbon coated on metal foils constitute the electrodes. Supercapacitors store relatively small amounts of energy (~10 Wh/kg) but they can charge or discharge in seconds and exhibit a much higher cycle life (in the order of millions) compared to batteries (a cycle life in the thousands only). Therefore, supercapacitors are primarily used to supply short bursts of power. The use of supercapacitors in many applications is limited by their low energy density and high price.

The development of hybrid supercapacitors, containing essentially the materials of lithium-ion batteries in the supercapacitor itself, is an interesting recent effort, to combine high power density with higher energy density devices. However, this work is still in progress.

6. On the Role of Hydrogen as an Energy Vector (Or an Energy Storage Material) for the Decarbonization of Energy Production and Transportation

As everybody knows, hydrogen can be an excellent fuel, because it may produce only water vapor as the combustion product. Additionally, it has a very high gravimetric energy density, i.e., the amount of energy per unit weight, 143 MJ/Kg, the highest among energy-providing systems, except nuclear fuels. However, the gravimetric energy density of hydrogen is strongly decreased by the need (due to its extremely low volumetric energy density) to store it either under high pressure (e.g., 700 torr) or at very low temperature in its liquid form (down to 20–40 K) using bulky and very heavy equipment and tanks [122]. Thus the hydrogen storage or propulsion systems become less energy dense than those based on liquid fuels and combustion engines. In any case, the practical volumetric and gravimetric energy density of systems based on hydrogen is much higher than that of systems based on batteries and electrical engines.

In any case, hydrogen is not an energy source because it exists in the molecular elementary form (H₂) and only in very minimum amounts in air, coming out from hydrothermal systems in mid-oceanic ridges, and in some amounts in some geologic reservoirs [123,124,125]. Thus, elementary molecular hydrogen (H₂) must be produced from its compounds, such as water, hydrocarbons, and biomass compounds. Thus, hydrogen is an energy vector or an energy storage material. Hydrogen is produced today [126] mainly by steam reforming of natural gas [127,128], or by coal gasification [129], with the coproduction of CO_2, which is frequently in part or entirely vented, thus significantly contributing to global warming. The production of hydrogen without producing CO₂ is certainly possible, in particular (but not only) by electrolysis of water using electrical energy produced without coproduction of CO₂. To give an idea of the environmental quality of produced hydrogen, colors are given to hydrogen produced by different technologies. Electrolytic hydrogen produced from electrical energy coming from renewable sources is defined as green hydrogen, while that coming from nuclear power plants is defined as pink hydrogen. In reality, water electrolysis is is conducted today andhas been for maybe a century or more, to a minimum extent, but using electrical energy produced by non-renewable or mixed sources (yellow hydrogen). A summary of hydrogen production technologies is reported in

Table 7. Hydrogen production methods.

Raw Materials	Color	Process		TRL [23]	Notes
Natural gas + water/O2	Grey, Blue with CCS	Steam reforming		11	May become carbon neutral with CCUS and/or electrification of reactors
		Partial oxidation/autothermal		9
Hydrocarbons (naphtha)		Catalytic reforming		11	Used in refineries Coproduction of aromatic hydrocarbons
Coal + water	Brown	Gasification		11	May become carbon neutral with CCUS
Water	Green, pink, yellow	Electrolysis	Alkaline cells	9	Today’s durability > 105 h	Energetically most expensive
			PEMEC	8	Today’s durability: 104 h
			SOEC	7	Today’s durability: 103 h
		Thermal decomposition		3	Needs very high temperatures
Biomass		Gasification		6	Very complex gas purification
		Pyrolysis		Very low	Early stage of development

.

6.1. The Production of Green Hydrogen through Water Electrolysis

With water being the most stable hydrogen compound, the production of hydrogen from water implies the maximum consumption of energy. The production of green hydrogen can be accomplished through water electrolysis using renewable electric energy. Alkaline electrolytic cell (AEC) technology has been used for many decades in small-size plants for highly pure hydrogen production (<4% of total hydrogen produced today) but is today available commercially (TRL of 9) also for large plants [130]. The use of inverted fuel cells, such as polymeric electrolyte membrane electrolytic cells (PEMECs) [131] solid oxide electrolytic cells (SOECs) can result in non-negligible efficiency improvements. However, these technologies are still at lower readiness levels [132,133]. In particular, durability is still a challenge, in particular for SOECs [131].

6.2. Hydrogen from Biomasses

Although advocates of decarbonization do not take this into consideration (no color for hydrogen is given in this case), hydrogen can also be produced, in principle, without coproducing fossil-derived CO₂, by biomasses and biomass-containing wastes through a number of different technologies [134,135,136]. The energy needed for these processes is far lower than that needed for water electrolysis because the starting hydrogen-containing compounds (or some of them) are less stable than water, but the processes are much more complex. Although biomass gasification has been the object of studies for many decades, its commercialization has still not been raised [137,138]. The IEA evaluates the production of H₂ from biomass gasification at a TRL = 6 [23]. This is mainly due to the difficulty in the purification of the obtained syngas [139,140], which contain together significant amounts of “tar” molecules (aromatic hydrocarbons and phenols as well as nitrogen-, sulfur-, and halogen-containing compounds) [141] and alkali-metal containing volatile compounds [142]. Complex purification processes have been developed to obtain an abatement of such compounds to later convert syngas into hydrogen, but still, a satisfactory process seems to be lacking.

An alternative recent perspective to produce hydrogen for biomass is pyrolysis followed by steam reforming of pyrolysis gas [143]. The first step occurs at 773 K, thus not resulting in volatilization of alkali compounds, which is likely the main drawback in biomass gasification. The second step of steam reforming can occur at higher temperatures. The technology is promising, although it is still at a very early level of development.

Another potential way to obtain “renewable” hydrogen is from biogas. After biogas cleaning, most commonly realized from activated carbon adsorption [144], syngas may be produced by the dry methane reforming reaction realized over metal catalysts at 700–900 °C [145] coupled together with steam reforming of excess methane [146]. The resulting syngas has a H₂/CO_x ratio not far from 2 and is suitable for the production of methanol but can be separated to produce pure H₂ and non-fossil CO₂.

Renewable hydrogen can also be produced by the steam reforming of biomass-derived organic molecules. In particular, bioethanol steam reforming (ESR) [14,147,148] has been investigated deeply: it is an endothermic reaction and thus is favored at relatively high temperatures and low pressures. It can occur at 600–700 °C over metal catalysts such as supported Ni, Co, Pt, or Rh, frequently improved by alloying [149], with excess water. It seems that still this technology has not been implemented at the industrial level but is seriously considered for commercial application [150]. It has been considered that ESR can be implemented using existing hydrocarbon steam reforming plants, provided effective catalysts have been developed [151]. Additionally, it has been shown that Ni/Ca aluminate commercial catalysts for natural gas steam reforming are excellent catalysts for ESR too in laboratory experiments [152].

Steam reforming processes to produce hydrogen can also be applied to other biomass-derived substances [153] such as vegetable oils [154], wastes from agricultural manufacturing, glycerol, and biomass pyrolysis oil [155].

6.3. The Production of Blue Hydrogen from Fossil Raw Materials through CO₂ Capture and Storage or Use

As already said, hydrogen is manufactured today either from syngases produced from natural gas steam reforming or coal gasification [126,127,128,129]. To obtain highly pure hydrogen, Pressure Swing Adsorption processes using zeolite type adsorbents are conducted, with separated CO₂ being usually vented (grey or brown hydrogen). To obtain highly pure food-grade CO₂, together with hydrogen instead, scrubbing with amine solutions (mainly Methyl Di-Ethanol-Amine, MDEA) or hot potassium carbonate solution is commonly practiced, in particular after the natural gas steam reforming process. The current use of captured CO₂ is to produce urea or other chemicals or to use it in the food and beverage fields. However, to produce blue hydrogen, CO₂ must be also captured from flue gases of steam reforming furnaces and reused or stored. Alternatively, electrically heated steam reformers are also under development for the production of blue hydrogen [156].

6.4. The Limits of Hydrogen as an Energy Vector: Transport and Storage

Where large hydrogen producers and users, such as petroleum refineries and chemical plants, are concentrated in nearby locations, hydrogen is mainly transported today through dedicated pipelines: approximately 1600 miles of hydrogen pipelines are currently operating in the United States [157]. Where demand is at a smaller scale, hydrogen is transported over the road in cryogenic liquid or compressed gas tanker trucks, and in compressed gaseous tube trailers usually at 180–200 bar [158]. Large-scale storage is mainly performed in gas-phase reservoirs at 350 or 700 bar, or in liquid-phase reservoirs [159]. It must, however, be considered that hydrogen compression and liquefaction are energy intensive: compression to 700 bar consumes an amount of energy comparable to 13–18% of the lower heating value of H₂ itself [160] while liquefaction consumes more than 30% of the energy content of the hydrogen itself [161].

In fact, the use of hydrogen for the storage and transport of energy finds its limits in the very low volumetric energy density of hydrogen, even for both high-pressure gas-phase systems and for liquid phase systems [122]. Liquid hydrogen, conveniently stored and transported near atmospheric or a little higher pressure at 20–30 K, still has a very low energy density (10.1 MJ/L) with respect to liquid hydrocarbons (>45 MJ/L) and alcohols (e.g., methanol 23 MJ/L), although this is well higher than that of batteries (<0.02 MJ/L).

It can be remarked that a low volumetric storage energy density when hydrogen is used as fuel for vehicles, results in limited autonomy of the vehicle with normal-size reservoirs or very large reservoirs to store sufficient amounts of fuel (or energy) for high autonomy. In parallel, energy storage in hydrogen refueling stations will also be relatively limited or would imply very large reservoirs. In general, it is quite evident that storage/transport systems of hydrogen are intrinsically quite or very inefficient. In particular, the transport of energy through hydrogen is likely more inefficient than the transport of electric energy by conduction through high-voltage lines.

For future large transport of hydrogen, the repurposing of existing natural gas pipeline infrastructure is taken into consideration. However, hydrogen-induced corrosion phenomena may occur [162]. In any case, the low volumetric energy density of gaseous hydrogen, well lower than that of methane, and the higher energy needed for compression, result also in this case in intrinsic inefficiency.

Theoretical alternatives for hydrogen transport and storage consist of the use of it when absorbed in solids, adsorbed on solids, or chemically bound in transport molecules [163]. A number of solids, such as transition metal alloys producing interstitial hydrides and covalent light metal hydrides, can be used for the transport of hydrogen [164]. These solids may contain more hydrogen than hydrogen itself per volume, thus having a higher volumetric energy density (up to 100 kg_H2/m³) than liquid or compressed hydrogen. However, they are heavy, and thus most of them have a lower gravimetric energy density than the practical level for liquid and compressed hydrogen systems [165,166]. In any case, they have both volumetric and gravimetric hydrogen density well lower than room temperature and pressure liquid hydrocarbon tanks. Most of the interstitial hydrides are capable of absorbing and desorbing hydrogen at room temperature, with a hydrogen capacity of H/M ratio ≤ 2. High entropy alloys are relatively new, interesting materials, but their reversibly absorbed hydrogen capacity is still relatively low, around 2 wt% (H/M) [167,168]. Covalently bonded light metal hydrides such as metal alanates (M(AlH₄)_n), metal amides (M(NH₂)_n), metal borohydrides (M(BH₄)_n), Mg-based hydrides, Al hydride (AlH₃), and ammonia borane (NH₃BH₃), have by far higher hydrogen density, sometimes more than 10 wt%. However, they have relatively high temperatures for hydrogen release, usually 400–600 K, and also hydrogen storage needs higher pressures.

Porous solids, such as Metal–Organic Frameworks (MOFs), covalent organic frameworks, porous organic polymers, carbon-based materials, and zeolites, show a higher capability to adsorb hydrogen. However, their performances are still far from being useful for commercialization [169]. After several decades of research efforts, still, the use of solids to transport hydrogen is practically unthinkable, with new and definitely more effective revolutionary materials being needed for competitive technology.

Another way to transport hydrogen is the use of Liquid Organic Hydrogen Carriers (LOHCs), i.e., molecules that can be easily hydrogenated and recovered by dehydrogenation [170,171]. The most interesting liquids in this field are N-ethylcarbazole, Dibenzyltoluene, 1,2-dihydro-1,2-azaborine, formic acid, methanol, Naphthalene, Toluene, Phenazine. The energy density of these liquids as hydrogen vectors is in the range of 1.6–3.3 kWh/L [172], comparable with that of compressed or liquid hydrogen but needing a less expensive procedure for storage. This energy density is 4–8 times that of Li-ion batteries. However, the way to convert and recover hydrogen is, in this case, not so simple, because catalytic reactions must be performed, and with very high selectivity. Additionally, the dehydrogenated molecule must be transported back for re-hydrogenation. It seems that the long-range transport of hydrogen could be effectively realized with LOHCs while for transport in vehicles, this is not a solution.

With another approach, hydrogen can be bonded to a chemical component with the potential to be dissociated to regenerate hydrogen (like LHOCs) but can also be burned as such. This occurs with synthetic methane (SNG), ammonia, methanol, and formic acid. In this case, the process is one-way (irreversible), and the energy-demanding dehydrogenation step is avoided. These systems are often called circular hydrogen carriers because of the recycling of the hydrogen-lean form (CO₂ or N₂) [171].

In particular, hydrogen transportation using ammonia in slightly refrigerated tanks at −33 °C or at ambient temperatures under a pressure of 8–10 bar is a possible option. In fact, the hydrogen density of ammonia is 17.8 wt%, 10.7 kgH₂/100 L and it is easily liquefied under about 1 MPa at room temperature [163]. The potential use of ammonia has, however, several drawbacks. Ammonia is a highly toxic and volatile compound that can be decomposed back to N₂/H₂ at quite a high temperature, and its residual concentration for feeding PEM fuel cells must be extremely low (1 ppm). On the other hand, ammonia is burnable without the emission of CO₂.

The above data suggest that hydrogen transport through pipelines or as compressed or liquified reservoirs, although inefficient, can still be the best solution. However, the large-scale transport of hydrogen using LHOCs (ex. methylcyclohexane) in the liquid phase through tankers or pipelines could still also have some perspectives.

6.5. Dangerousness of Hydrogen Technologies

As for any highly volatile fuel, safety concerns on hydrogen technologies also exist [173]. With H₂ being the substance with the smallest relative molecular mass, it most easily leaks or permeates from high-pressure environments. Additionally, the lack of odor and color increases the difficulty of leakage detection. On the other hand, due to its minimal density in the gas phase, which is much lower than that of air (0.13 with respect to 1 for air), the risk of hydrogen explosions outdoors is minimal. In fact, in the event of an outdoor leak, hydrogen will produce an upward flow and will quickly disperse into the air. Instead, in the case of leaks in confined spaces, the risks of explosion are significant [174]. The very low minimum ignition energy (MIE) of hydrogen (0.017 mJ), lower than that of any hydrocarbon [175] and particularly for natural gas (>0.30 mJ) is a key factor in its extremely high hazardousness of hydrogen. In particular, the low MIE and the possibility of flaming when air is mixed with a hydrogen volume fraction of only 4%, make it possible for hydrogen to ignite spontaneously quite easily, which occurred quite frequently in the past. Comparison with natural gas shows that hydrogen is, in many respects, more dangerous.

6.6. Green Hydrogen Utilization as an E-Fuel

Green H₂ can be used as an e-fuel [176] in both spark and compression combustion engines [177,178], although, due to the use of air as an oxidant and the high temperature obtained, NOx could be a coproduced pollutant. There may be a need for NOx abatement devices to be installed in aftertreatment systems. On the other hand, for the use of hydrogen as a fuel in internal combustion engines, modification of the injection systems is needed with respect to normal hydrocarbon-fueled engines. However, this represents a minor modification of well-proven and fully developed technologies. Indeed, at least at the pilot level, hydrogen-fueled combustion engine motorcycles (developed by Kawasaki [179]), cars (under development by Toyota [180]), and trucks (under development by several firms [181]) are considered to be an interesting option at least in the mid-term perspective.

Additionally, hydrogen can also be fed to gas turbines e.g., for aircraft jet propulsion (now under study e.g., by Rolls Royce [182]) and for stationary power generation and cogeneration (as developed by Kawasaki [183]). Indeed, a main limit also in this case is constituted by the relatively low autonomy of the vehicles related to the small volumetric energy density of energy in reservoirs, or to the inefficiency related to hydrogen liquefaction or compression.

6.7. Hydrogen Utilization with Fuel Cells

Green hydrogen can be used to produce green electrical energy with hydrogen fuel cells [184]. This allows the direct production of electric energy with very high efficiency (except for the hydrogen production and transport steps that are quite inefficient) and without producing pollutants in the energy transformation step. Hydrogen fuel cells, which were conceptualized at the beginning of the XIX century and produced at the experimental level in the middle of the same century (i.e., before the invention of internal combustion engines), after 150 years still find difficult commercialization mainly because of durability limits. Data on the most relevant types of FCs are summarized in Table 8.

6.7.1. Polymeric Electrolyte Membrane (PEMFC)

Fuel cells are the most advanced (TRL = 9 for application in cars, light-duty commercial vehicles, and buses according to the IEA [23]), and are commercially available today mainly for application in vehicles [131]. However, the application of PEMFCs to stationary energy production systems is also increasingly considered [185,186]. They work around 100 °C and are based on noble-metal-based electrodes (usually carbon–platinum composites) and of proton exchange sulfonated resin membranes as the electrolyte. They work with wet membranes and feeds. The advantages of PEMs are their high volumetric power density, short start-up times, and high tolerance of the gas-pressure differential between cathodes and anodes [184]. A main limit of this technology is the high cost associated with the need for significant amounts of platinum in the electrodes. Heavy durability concerns still exist [187], which increase with the decreasing platinum contents in the electrodes [188], and with guaranteed durability being still less than the targets for heavy-duty vehicles. Additionally, PEMFCs need highly pure hydrogen, and humidity management is quite onerous too. PEMFC + hydrogen storage + electric engine systems have a gravimetric energy density still well lower than that of hydrocarbon-based combustion engine systems, but an order of magnitude larger than that of batteries. However, they have power densities well lower than batteries and combustion engines. The efficiency in the use of energy in hydrogen tank + fuel cell + electric engine systems is lower than that of batteries but much more than that of combustion engine systems. The overall efficiency for the use of fuel cells in vehicles is certainly strongly limited by the inefficient storage of hydrogen (very low volumetric energy density) and limited durability as well, at the moment. Thus, they compete with full electricity battery systems but seem to be more promising for medium weight and mileage vehicles like big trucks [189].

To overcome or limit some of its drawbacks, high-temperature PEMFCs are now under study [190]. In fact, it has been observed that a poly-benzimidazole (PBI) membrane doped with phosphoric acid may act as the proton conductor at temperatures up to 200 °C. This would make the management of humidity easier and the limit on hydrogen purity maybe less stringent.

6.7.2. Alkaline Fuel Cells (AFCs)

Alkaline Fuel Cells use, as corresponding electrolytic cells, a 30–40% KOH water solution embedded in porous materials as the electrolyte [191]. They have been the first to be practically developed at the commercial level since the 1960s, and were used by NASA in the Apollo missions and the space shuttle to produce electrical power and fresh drinking water. They also necessarily use Pt-carbon electrodes, thus suffering from high costs. The additional main limitation consists of their sensitivity to CO₂, which is strongly absorbed by the KOH solution. They consequently need highly purified air or oxygen as the cathode feed. For this reason, they are not competitive with PEMFCs.

6.7.3. Alkaline Exchange Membrane Fuel Cells (AEMFCs)

A quite recent development in this field resulted in Alkaline Exchange Membrane Fuel Cells that use a quaternary ammonia-/piperidinium-based polymeric membrane to transport the hydroxide anion. AEMFCs are considered as candidates to substitute PEMFCs because of their quicker electrochemical kinetics, lower catalyst costs, and weaker corrosion. In fact, they potentially allow the use of non-precious-metal catalysts, which can facilitate commercialization [192]. However, AEMFCs still have a fast degradation rate and CO₂ sensitivity, and consequently too, a low lifetime. They are still at the laboratory development level [191,192].

6.7.4. Phosphoric Acid Fuel Cells (PAFCs)

PAFCs [193] were first commercialized and widely used for small stationary power generators with output in the 100–400 kW range. PAFCs operate usually at 180–220 °C. The electrolyte is a 100% concentration of H₃PO₄ contained in a Teflon-bonded, silicon carbide matrix. Electrodes are based on Pt/carbon composites. The quite high operating temperature allows for low-grade waste heat recovery for further utilization. PAFCs have moderate durability and a simple structure, as well as a good tolerance to CO₂ and CO in the feeds. The main limits that caused reduced commercialization are high costs, mainly due to noble metal electrode materials, the need for corrosion-resistant materials, and their relatively low power density.

6.7.5. Solid Oxide Fuel Cells (SOFCs)

SOFCs are the most advanced FCs besides PEMFCs. According to the IEA, their RTL is 8–9 for large stationary energy generation. They apply solid oxide-ion conducting materials, such as Y-stabilized zirconia or Sm-doped gadolinia, as electrolytes [194]. To have high ion conductivity, high temperatures are needed (650–800 °C). This allows non-noble metals to be used as the anode (usually nickel) and metal oxides such as LaSM (Lanthanum–Strontium Manganite) as the anode. Due to its high-temperature operation, high purity for the hydrogen feed is not necessary, and also internal reforming of hydrocarbons is possible. Thus, its application is supposed to be optimal for large energy production systems. With this high-temperature operation, limits are associated such as long startup times, a limited number of shutdowns allowed, high-temperature corrosion, and the breakdown of cell components [195] resulting in low durability. Cathode degradation may occur due to poisoning mainly arising from chromium evaporated from the unprotected metallic interconnects, and from impurities in the air feed (i.e., sulfur compounds, CO_2, and humidity), microstructural deformation, and chemical and thermal strains resulting in delamination. The electrolyte can suffer phase transitions, contamination by impurities, dopant diffusion, and mechanical failure. Finally, anode materials also suffer microstructural changes, coking, poisoning, and delamination. All these phenomena have relevant rates due to the high temperature necessarily used.

Table 8. Data on most advanced fuel cell systems.

FC Type	Anode Material and Feed	Electrolyte/Transported Species	Cathode Material and Feed	T °C	Power Range kW	Startup Time min	Application	TRL	Lifetime h	Challenges and Drawbacks (DOE) [196]
PEMFC	Pt/C Wet H2	Cationic polymer membrane H+/H2O →	Pt/C O2/air	60–80	0.05–1000	<1	mobile	9	>25,000	Expensive electrocatalysts Sensitive to fuel impurities
PAFC	Pt/C H2	H3PO4 in porous SiC matrix H+/H2O →	Pt/C O2/air	140–200	100–10,000	long	small stationary, mobile (buses)		40,000	Expensive electrocata lysts Long start-up time Sulfur sensitivity
AFC	Pt/C Wet H2	KOH/alkaline membrane ← HO−/H2O	Pt/C Purified air	40–75	0.1–200	<1	mobile		>5000	Sensitive to CO2 in fuel and air
AEMFC	Non PGM metals Wet H2	Anionic polymer membrane ← HO−/H2O	Non PGM metals Purified air	50–90					300–500	High degradation rate Still at laboratory scale
SOFC	Ni H2	Porous ceramic material ← O2=	Mixed oxide O2/air	500–1000	1–100,000	60	stationary	8	20,000–80,000	High-temperature corrosion and breakdown of cell components Long start-up time Limited number of shutdowns
MCFC	Ni H2	Molten (Na,K)CO3 in porous Li-β-alumina ← CO3=	Li-NiO O2/air with CO2	650–800	500–100,000	60	stationary		15,000–30,000	High-temperature corrosion and breakdown of cell components Long start-up time Low power density

Table 8. Data on most advanced fuel cell systems.

FC Type	Anode Material and Feed	Electrolyte/Transported Species	Cathode Material and Feed	T °C	Power Range kW	Startup Time min	Application	TRL	Lifetime h	Challenges and Drawbacks (DOE) [196]
PEMFC	Pt/C Wet H2	Cationic polymer membrane H+/H2O →	Pt/C O2/air	60–80	0.05–1000	<1	mobile	9	>25,000	Expensive electrocatalysts Sensitive to fuel impurities
PAFC	Pt/C H2	H3PO4 in porous SiC matrix H+/H2O →	Pt/C O2/air	140–200	100–10,000	long	small stationary, mobile (buses)		40,000	Expensive electrocata lysts Long start-up time Sulfur sensitivity
AFC	Pt/C Wet H2	KOH/alkaline membrane ← HO−/H2O	Pt/C Purified air	40–75	0.1–200	<1	mobile		>5000	Sensitive to CO2 in fuel and air
AEMFC	Non PGM metals Wet H2	Anionic polymer membrane ← HO−/H2O	Non PGM metals Purified air	50–90					300–500	High degradation rate Still at laboratory scale
SOFC	Ni H2	Porous ceramic material ← O2=	Mixed oxide O2/air	500–1000	1–100,000	60	stationary	8	20,000–80,000	High-temperature corrosion and breakdown of cell components Long start-up time Limited number of shutdowns
MCFC	Ni H2	Molten (Na,K)CO3 in porous Li-β-alumina ← CO3=	Li-NiO O2/air with CO2	650–800	500–100,000	60	stationary		15,000–30,000	High-temperature corrosion and breakdown of cell components Long start-up time Low power density

6.7.6. Molten Carbonate Fuel Cells (MCFCs)

MCFCs [197,198] work thanks to the diffusion of the carbonate ion in the molten carbonate electrolyte. To maintain alkali carbonate eutectics in the liquid phase, high temperatures are needed. This allows the use of impure hydrogen and also hydrocarbon feeds which are internally reformed. In addition to power generation, they can be used for capturing and concentrating CO₂ from a waste-gas containing air fed at the cathode [199]. Alternatively, quite onerous CO₂ recirculation is needed. Due to their high power range, they are best suited for large stationary power generators. The main drawback of this technology is the high-temperature corrosion and breakdown of cell components (cathode dissolution, corrosion of cell hardware such as bipolar plates and current collectors, and electrolyte loss due to creep in the matrix and electrodes) which result in insufficient durability for large stationary applications [200]. Additionally, it has a long start-up time and low power density. Also, the need for CO₂ in the cathode oxygen/air feed may be a limit in the context of green energy production, being somehow in contradiction with the use of green hydrogen to produce energy. Although they nearly reached commercialization (TRL of 8–9 according to the IEA) [198], their interest within stationary power generation is decreasing [184,186]. The combination of MCFCs with a combined cycle gas turbine (CCGT) power plant means that 90% of the CO₂ from the gas turbine exhaust can be captured while generating additional electricity instead of decreasing the plant’s net power output. This technology is considered to currently have a TRL of 5–6 [201].

6.8. Green Hydrogen Application in Processes Other Than Energy Production

The industrial activity producing the most GHG emissions (besides energy production and transport industries) is steelmaking, globally producing around 7% of total GHGs. In fact, steelmaking is mainly associated with the use of coke as a reductant for iron ores in blast furnaces and in its coal combustion for generating the needed heat. Green hydrogen can play a main role in decarbonizing steelmaking [202,203], with the development of hydrogen-based direct reduction processes, which are evaluated to today have a TRL = 6 [23]. Significant GHG emission reduction is also obtained by applying to steelmaking CO₂ capture and reuse or storage [204] and the recycling of steel scraps.

Also, the chemical industry [205] consumes significant amounts of hydrogen, only part of which is produced by dehydrogenation processes in the same petrochemical sector. On the other hand, most of the hydrogen used for the petrochemical sector comes from the steam reforming of natural gas and coal gasification processes. Thus, the production of green hydrogen can allow the decarbonizing of the chemical industry too.

7. E-Fuels and Biofuels for Decarbonizing Transport

As discussed above, fully electric systems cannot be applied, at least at the moment, to very heavy and long-range vehicles. Also, hydrogen and fuel cell-based systems have limits of autonomy and cannot be applied to oceanic shipping and intercontinental or long-range aviation. Thus, for these vehicles, the use of renewable liquid hydrocarbons with conventional combustion engines is still considered the only possible perspective, at least for the near future. This perspective has also the advantage of not needing revolutionary modification of the conventional engine systems which are reliable and durable. Once this approach is considered for oceanic shipping and transcontinental aviation, it seems clear that the same approach can also be reconsidered for smaller vehicles, such as heavy-duty trucks. Renewable fuels virtually allowing zero fossil CO₂ emission are e-fuels and biofuels. Both e-fuels and biofuels can be constituted by exactly the same molecules that fossil fuels but are produced without the emission of fossil raw material-derived GHGs. They also would be completely sulfur-free, in contrast to typical fossil fuels. They can be used in the same combustion engines as fossil fuels, thus producing the same pollutant or poisonous molecules, i.e., CO, unburnt hydrocarbons, aldehydes, and particulate matter. Thus, they would produce the same pollution except that responsible for global warming and acid rains. On the other hand, it must be realized that thanks to already well-developed modern aftertreatment systems, together with their improved purity, real pollution may actually be very limited, if at all.

7.1. E-Fuels

E-fuels [176,206] are, by definition, burnable compounds that may be produced by converting renewable electrical power to hydrogen and reacting it with atmospheric gases (CO₂ or N₂). In this definition, biomass conversion is excluded. Indeed, the real e-fuel is green hydrogen obtained by water electrolysis using renewable energy. Its potential use has already been considered above. Alternative e-fuels can be obtained by hydrogenation of natural molecules with green hydrogen. In particular, catalytic hydrogenation of captured CO₂ allows the production of hydrocarbons fully analogous to those coming from oil but produced in a fully renewable way and being CO₂-neutral in their lifecycle. Appropriate catalysts are the key materials for allowing these technologies to be highly efficient.

As reviewed recently [87], the hydrogenation (using green hydrogen) of CO₂ allows, depending on catalysts and reaction conditions, the synthesis of alcohols such as e-methanol, and e-hydrocarbons such as e-methane. The processes are very similar to those used for producing the same products from syngases of fossil origin such as methanol synthesis, methanation (or synthesis of Substitute Natural Gas, SNG), and Fischer–Tropsch synthesis. Companies have already developed slightly modified catalysts and processes which are ready for commercialization. These techniques allow the production of (chemically) sustainable fuels for shipping, such as e-methanol which is already being tested at the pilot level by Maersk [207], as well as e-jet fuel useful as a Sustainable Aviation Fuel. Similarly, the ammonia synthesis (Haber–Bosch) process can be realized using green electrolytic hydrogen and nitrogen coming from air distillation. Catalysts, reaction conditions, and reactors do not need any modification with respect to conventional ammonia synthesis. The use of e-ammonia as a fuel and as a hydrogen-transport molecule is considered today [176,206] but is certainly hampered by its significant toxicity coupled with its gaseous behavior.

7.2. Biofuels

Biofuels are renewable fuels produced by converting biomass. They are commonly classified in four categories, as reported in

Table 9. Classification of biofuels.

Generation	Characteristics
First	Obtained from food crops such as edible vegetable oils or starch-containing vegetables such as corn.
Second	Obtained from wastes of the agri-food industry, from the organic fraction of municipal waste, or from ligneocellulosic biomass not intended for food production.
Third	Obtained from algae as raw materials. This kind of fuel is still not produced commercially, but there are conclusive findings proving its feasibility.
Forth	Obtained from genetically modified microorganisms.

.

It must be considered that, although biofuels used today in vehicles are essentially first-generation (corn bioethanol and most biodiesel), second-generation biofuels are available today.

7.2.1. Biogas and Biomethane

Biogas [17,144] is produced by the anaerobic digestion of organic matter, usually the organic fraction of MSW (methanogenic fraction) in landfills, and water treatment sludges. It contains about 50% methane and 50% CO₂, with many impurities including sulfided hydrocarbons and H₂S, nitrogenated hydrocarbons and ammonia, and oxygenated organics, as well as siloxanes and sometimes organic halides. After purification, usually conducted by adsorption on activated carbons, it can be used as a fuel for producing heat and energy. It is potentially possible to react biogas through dry reforming which produces a syngas to be converted into methanol or higher hydrocarbons or separated to produce hydrogen. It is also possible to separate CO₂ from it, thus producing pure methane, denoted as biomethane, which can be used as such to produce heat and energy, steam reformed to produce hydrogen, and inserted in the national gas network. The latter technologies are ready and already largely utilized (TRL = 9 or more) and may be considered second-generation biofuels.

7.2.2. Biogasolines

Bioethanol [208], i.e., ethanol produced by fermentation of glucose solutions derived from polysaccharide hydrolysis, is an excellent gasoline from the technical point of view (anti-knocking properties) but may cause the release of dangerous organics (aldehydes) in the waste gas. It is added to commercial gasoline at a minimum level of 1% to a maximum level of 10% (E10 gasoline) in the European Union. First-generation bioethanol, coming from starch-rich biomasses, is largely available. Ligneo-cellulosic bioethanol, coming from wood-derived cellulose, is also nearly available today as a second-generation biofuel (TRL = 8 according to the IEA [23]).

In principle, a number of processes can be applied to produce hydrocarbon gasoline fractions as well biomethanol (also a high-octane number gasoline) from biomasses [16], although it seems that still there are no industrial productions.

7.2.3. Biodiesel

Bio-diesel [209] is a fuel produced commercially (TRL = 9–10) for decades from vegetable oils, mainly edible oils derived from soybeans, sunflower, palm, or canola (as a first-generation biofuel), or even from non-edible oils such as castor oil, as well as from frying waste oils (as a second-generation biofuel). It is a mixture of methyl esters of fatty acids (FAMEs) produced by the transesterification of triglycerides with methanol. It is mixed with mineral gasoil up to a 7% content in diesel fuels in the European Union. This limitation is due to its intrinsic technical drawbacks, such as different properties when coming from different raw materials, a high melting point, a low compatibility with engine materials, the production of oxygenates in the waste gas, etc. Additionally, the environmental friendliness of its manufacturing process is limited, in part due to the necessary coproduction of impure glycerol.

7.2.4. Hydrotreated Vegetable Oils (HVOs)

Full catalytic hydrogenation of vegetable oils has been developed (TRL = 9) to obtain full-hydrocarbon fractions, usually denoted as Hydrotreated Vegetable Oils (HVOs). Also depending on the starting oil, and the entire process, which can be followed by a mild hydrocracking step, jet-fuel (kerosene) and diesel fuel can be obtained. These are fully hydrocarbon fractions, mainly constituted by linear paraffin, thus with a high knocking potential (Cetane number > 60) and with a near zero oxygen content, and can consequently fully substitute the corresponding fuels. If green hydrogen is used, they are fully renewable. Also in this case, non-edible oils (Jatropha oil, castor oil, and spent frying oil waste) can be used, thus giving rise to a second-generation biofuel. The commercial use of these liquids in mixtures with mineral diesel oil and jet-fuel is already commercial, while the possibility of their use as neat fuel has been proven. In particular, jet fuel-based HVO is considered the most likely or advanced future Sustainable Aviation Fuel [210]

8. Considerations on Zero Emissions Transport

The above review data indicate that the future zero-emission technologies for transportation still are not unequivocally determined.

It is clear that fully electric vehicle systems appear to be the optimal solution only for small-mileage and small-weight vehicles, due to the strong limits in autonomy for the high weight and size of the batteries available today and expected for the near future. Their development also needs the production of much more electricity and in a fully renewable way. Additionally, material concerns for their production cannot be neglected and need improvements.

Hydrogen-fueled systems seem to be the best solution for intermediate mileage and weight vehicles, such as long-range trucks [189]. They allow >1000 km and ~1 day autonomy with big size and weight vehicles. However, systems based on fuel cells and electrical propulsion are very expensive and limited by noble metal availability. Hydrogen-fueled internal combustion engine-based vehicles, maybe with DeNOxing devices, could be competitive or even better. More efficient systems for hydrogen distribution and storage should be developed.

For very long-range, heavy-weight, and extended-travel-time vehicles, such as transoceanic ships [211] and transcontinental aircraft [210], the use of conventional combustion engines (diesel and jet) fueled by biofuels or e-fuels seems to be, at the moment and for the near future, the only possible solution. Among the most considered solutions are the use of e-methanol for shipping and HVO for aviation, using conventional internal combustion and jet engines.

From the above considerations, it seems reasonable to continue the development of more efficient and less polluting internal combustion engines, thus postponing the abandonment of such very reliable technologies.

9. Approaching the Entire Life Cycle and the End of Life of the Devices and of the Technologies

As it is evidenced above, all energy-related technologies imply first of all the use of optimized materials to produce devices. The device manufacturing sequence is approximately the following:

-

Mining of minerals

-

Processing to produce chemicals/compounds

-

Reacting/processing to produce materials

-

Structuring to produce devices

-

Using devices

-

End-of-life of spent devices

Interestingly, raw material extraction, raw material processing, final material manufacturing, and device production frequently occur in different continents or nations, thus implying multiple transports of matter around the world until the final device destination. The real environmental effects of a technology must consequently be evaluated considering the entire life of the device. As for example, the mining of each element present in a device should also be considered and its environmental impact evaluated.

On the other hand, all technologies use devices and structures whose life is limited. Wasting is the most common end-of-life stage for devices today. The production of enormous amounts of waste is clearly one of the main problems of our civilization. Additionally, the landfilling of discarded technological devices represents an illogical un-useful storage of precious and expensive matter, which are frequently very sophisticated materials, instead of making them available for reuse. It is absolutely urgent to develop technologies for the reuse of spent devices as well as any type of waste. Reuse should follow the following hierarchy:

-

Reuse of the object, if possible

-

Dismantling the object and reuse of its separate parts, if possible

-

Dismantling the parts into materials, and reuse of materials, if possible

-

Regeneration of the materials, if possible

-

Recovery of chemicals from unregenerable materials, if possible

-

Converting to produce energy, if possible

-

Wasting, as the very last choice

It must be taken into consideration, as already said above, that for many technologies, the most appropriate way to reuse them has not been even determined up to now. For this reason, at the moment, the most common end-of-life procedure for many devices, including photovoltaic panels, wind blades, batteries, electronic devices, etc., is wasting.

It is absolutely urgent to develop and define mandatory end-of-life procedures for devices, even before they are admitted to commerce.

10. Conclusions

According to the above critical evaluation of the state-of-art of technologies potentially allowing for the cutting-off of emissions of fossil-derived GHGs, several conclusions can be reached.

(i)

Technologies considered today for energy transition toward zero emissions demonstrate strong drawbacks in terms of environmental and societal impacts, efficiency, and real sustainability, in the absence of statal funding.

(ii)

Chemistry and chemical engineering are and will continue to be crucial fields for improving existing technologies and developing new technologies for efficient and clean energy production and management.

(iii)

As always in the field of chemical engineering, catalysis and electrocatalysis are key phenomena to be exploited for improving efficiency in thermochemical and electrochemical processes for the production of devices, materials, and fuels (including hydrogen).

(iv)

In particular, materials chemistry is the key discipline to develop electro- and photo-chemical devices (batteries, fuel cells, photovoltaic systems, etc.) and materials for mechanical systems, most of which need decisive improvement with respect to those that are used today. Better devices need new materials.

(v)

Technologies allowing the enhanced production of biomass available for energy applications should be carefully considered, because they may represent ways to take advantage of solar energy with reduced energy and material costs. Together, positive environmental and social impacts could be obtained.

(vi)

In any case, the use of biomasses should not be excluded a priori because of dogmatic prejudices or because of the constrasting economic interests of anyone.

(vii)

Technologies allowing the reuse of materials or the recovery of chemicals from exhausted devices are urgently needed to avoid waste production and resource consumption. This point should be defined for any technology before it comes into widespread use.

(viii)

Catalytic materials and technologies are needed to approach the many chemical steps needed in the production of materials and in the recovery of chemicals. In particular, hydrogen production from biomass, e-fuel and biofuel manufacturing, recovery of chemicals from waste polymeric materials, hydrogen carrier molecules, etc. are necessary technologies needing efficient catalysts to be developed.

(ix)

Complete and well-structured life-cycle analyses are needed before allowing the commercialization of new technologies, to reveal their actual impact and real sustainability. In particular, energetic and economic sustainability in the absence of statal funding, as well as the absence of production of unusable wastes at their end-of-life, are key features to be considered.

(x)

In any case, we should be aware that, for many potential technologies, revolutionary systems are needed, because, many of those have been under development until today already have shown excessive limits before they come to widespread commercialization. The beginning of the large-scale application of several technologies is already resulting the emersion of their strong limits.