Important developments in H
2 production, storage, distribution, and utilization have been demonstrated in recent years. As a fuel, H
2 has considerably higher energy per unit weight than other fuels such as gas, diesel, and methanol (see
Figure 1) [
14]. Therefore, it has an immense market potential for the transportation sector, and industry. H
2 storage is one of the key factors in the H
2 economy. RE electricity is widely stored in chemical batteries but the limited capacity, lifetime, and high cost of batteries have limited their applications. H
2 could be employed as an energy carrier, like a battery, for RE electricity. The stored H
2 can be used for electrical power production in a gas turbine or fuel cell, or mechanical power production in an ICE. Solid-state materials in standard temperature and pressure conditions (STP) are seen as a promising method of storing H
2, especially for mobile systems. In terms of H
2 utilization, the deployment of fuel cell (FC) systems opened a new window to H
2 fuel success in the transportation sector. Currently, H
2-powered engines based on proton exchange membrane (PEM) FCs are deployed worldwide.
H
2 is the most abundant and lightest element (2.016 × 10
−3 kg/mol) in the universe, and energy-efficient fuel on the Earth (calorific or heating value). It is the richest in energy per-unit mass (140 MJ/kg), and almost three times higher than solid fuels (50 MJ/kg) (see
Figure 1). It exists mainly in compounds such as water, methane, H
2S, living organisms, organic waste, etc. It is environmentally benign, since water is the only emission product at conversion to energy, and unlike electricity, it can be stored. RE H
2 produced either at a central production plant or onsite at a H
2 refueling station (HRS) could help achieve carbon-free energy economy. Despite the fact that onsite H
2 production by electrolysis typically has higher costs than production at central plants due to limited production capacity, it was shown that the at-the-pump prices end up being similar due to the additional H
2 transport cost for the case of central production. H
2 production cost constitutes the largest portion of the levelized H
2 price at the pump due to the high capital investment in both RE electricity generation and water electrolysis. As the relevant technologies advance, the at-the-pump H
2 price is estimated to fall substantially. In a 2022 study, the future at-the-pump prices of RE H
2 produced onsite at an HRS in the 26 EU countries are estimated as 11 euros/kg in 2020, decreasing to 5 euros in 2050 while the European Commission (EC) target is 1.8 euros/kg H
2 by 2030. With a 3 euro/kg H
2 subsidy for RE H
2 production, it is expected that the price targets can be reached earlier. Moreover, the subsidy can enable RE H
2 to reach an energy-based cost parity with diesel fuel (0.034 euros/MJ for H
2 compared to 0.038 euros/MJ for diesel) by 2030. Furthermore, since H
2 FCVs being more efficient, they can travel 1.3 times further compared to diesel trucks using the same amount of energy. Therefore, considering both increasing diesel prices and vehicle efficiency, the 3 euros/kg subsidy can pave the way to make H
2 cost-competitive against diesel well before 2030. Although subsidies could be a promising way to lower prices, to achieve true climate benefits, robust regulations on sourcing electric power are necessary in addition to direct financial support for RE H
2 production [
17,
18,
19,
20,
21,
22].
5. H2 Production
Though hydrogen is the most common element constituting ~70% of the universe, free hydrogen is not readily available on earth. It is typically bound into molecules such as water and hydrocarbons, particularly methane (CH
4). It can be separated from these molecules by using energy in various forms. H
2 is identified with a spectrum of colors depending on its source or production method: (1) black-brown (from coal), (2) gray (hydrocarbons without carbon capture and storage—CCS), (3) blue (hydrocarbons with CCS), (4) turquoise (pyrolysis), (5) pink (nuclear powered electricity), (6) yellow (solar energy), (7) red (high-temperature catalytic process), (8) white (naturally-occurring geological hydrogen) and (9) green (clean RE electricity) (
Figure 3). Natural gas (NG), crude oil, coal, and water electrolysis processes are the most commonly used sources for H
2 production, with shares of 49, 29, 18, and 4%, respectively [
26].
Figure 3. The spectrum of H2 production processes.
5.1. Black-Brown and Gray H2 Production
By far the most widely used H
2 production method is the steam reforming of natural gas, also known as steam methane reformation (SMR). This is one of the cheapest H
2 production technologies in terms of operation and production costs [
27], and is also renowned for its high efficiency operation [
28]. Steam and natural gas are heated together up to ~900 °C at high pressure in the presence of a nickel-based catalyst. The process yields a mixture of carbon monoxide (CO) and H
2 known as syngas which is further treated in the water–gas shift reaction to produce more hydrogen and carbon dioxide. The H
2 produced by this method is named gray H
2. However, it has serious downsides, for instance: (1) as an endothermic reaction it requires huge amount of heat supply at extremely high temperature; (2) generating heat to produce extra steam (exceeds the stoichiometric requirement by two to three times) which is required for the curving of deactivation events like coking and moving the equilibrium position of the reaction more in the direction of the H
2 generation, and (3) complex extraction of H
2 from a combination of other outcomes (CO, CO
2, CH
4, and H
2O) by employing pressure swing adsorption (PSA, separates gases in a mixture of gases to obtain a high degree of purity) and water–gas shift reactors [
29]. In a similar process called gasification, steam and oxygen are used to break molecular bonds in black coal or lignite (brown coal) to produce syngas consisting of CO, H
2, CO
2, CH
4, and water vapor. These processes also result in substantial CO
2, which must be captured and stored (CCS). To address these limitations, a carbon-efficient process concept was developed that converts captured CO
2 to CO
2 in two steps. It prevents direct contact between O
2 and CH
4, lowering safety risks. The external energy demand is reduced by coupling endothermic and exothermic reactions, and the operating temperature to produce H
2 is reduced by more than 150 K when compared to conventional CH
4 reforming. Iron, nickel, and calcium oxides are used in this process as solid intermediates, which operates between 923 and 998 K. By simultaneously delivering steam and methane to the materials in a fixed bed, an H
2-rich stream is created in the first phase, and a CO-rich stream is created in the second step by renewing the materials with a combination that is like air. H
2 concentrations of over 65 mol% and CO
2 conversion rates of over 80% were reached in proof-of-concept tests. According to cyclical experiments, carbon deposition is reduced, and the ratio of generated H
2 to CO is around 2. Experimental findings demonstrate that the process idea enables conventional methane reforming for the generation of syngas to avoid the limitations of thermodynamic equilibrium [
29].
Biomass is another source of feedstock for H
2 production. As gasification is a well-established and commercially available technology, it is commonly applied to biomass like coal. The oxidation eventually results in a combination of CO, H
2, CO
2, CH
4, N
2, and higher hydrocarbons gases [
30]. After the creation of syngas H
2/CO
2 a WGS reactor is used for H
2 production with high temperature [
30]. It is really hard to obtain pure H
2 as biomass produces large amounts of tar even with high temperature operation due to higher hydrocarbons. The efficiency of the biomass ranges from 35–50% [
31]. The moisture contents in biomass limit the efficiency of this process as it needs to be vaporized. Gasification of biomass for H
2 production are not yet commercially established due to high operation costs and the complicated process for cleaning the H
2 from tar.
Approximately 6% of the globally extracted NG and 2% of the globally extracted coal are used for H
2 production. Over 95% of current H
2 production (black-brown and grey) leading over 2% of global annual CO
2 emissions by 2020 is by steam reforming of NG, its partial oxidation, and coal gasification; very little of it is green. Producing H
2 in pure form for industry is energy intensive. The global demand for H
2 is for its utilization in chemical production (40%), oil refineries (33%), metallurgical industries (3%), and the rest is used in synthetic fuel and petrochemical production, semiconductor and glass manufacturing, welding processes, powering FCVs, and in the food industries [
32,
33].
5.2. Blue H2 Production
H
2 produced from NG processing coupled with CCS is referred to as blue H
2 being carbon neutral. Blue H
2 production is predicted to reach 80 million metric tons by 2050 [
34]. The climate change impact of H
2 produced from fossil fuels coupled with CCS is still debated, especially in regard to the leakage risks of indefinite long-term storage of methane gas [
35]. H
2 produced using fossil-fuel feedstock causes greenhouse gas (GHG) emissions, even when CCS is used. These emissions could be substantial, and the cost of CCS is higher than frequently assumed. The percentages of GHG releases and those of CCS during blue H
2 production process determine the impacts of blue H
2 on the climate and the competitiveness of blue H
2 regarding global environment seems to depend on high CCS and negligible GHG releases. However, according to a research analysis the positive effects of blue H
2 production in terms of GHG emissions and climate change are insignificant and the use of blue H
2 seems challenging to justify on climate grounds [
36]. Establishing H
2 supply chains on the basis of fossil fuels, as many national strategies forecast, may be incompatible with decarbonization goals [
18]. Therefore, developing advanced supply chains in combination with high CCS capture rates is vital for blue H
2 to be a sustainable option for an actual net zero hydrogen economy [
37].
In a study of optimizing blue hydrogen production for future energy systems, a northern European perspective based on Germany was investigated [
38]. Blue routes provide significant benefits in the hydrogen industry but limited gains in the electricity sector. Systems using CCS (CO2 Capture and Storage) become significantly less expensive than those depending just on renewables in an energy system of the future when vast amounts of carbon-free fuels are required for usage in sectors like long-distance transportation and industrial. Another conclusion is that paths that allow CCS would prioritize carbon-free fuels above electrification. The study found that when conventional CCS technology were used, system costs decreased by 29% and CO
2 emissions decreased by 106% (biomass’s negative emission), with hydrogen generated for less than the cost of power. With the possibility for integrated power cycles that enable variable power and hydrogen generation, advanced blue hydrogen technologies might provide an extra 12% decrease in system costs with extra negative emissions because of extremely high CO
2 capture metrics. Since handling the intermittent hydrogen fluxes resulting from such an operational strategy proved to be expensive, scenarios with lower variable renewable energy shares proved to be more cost-effective. Originally, these concepts were created to better integrate higher shares of wind and solar energy [
38].
5.3. Turquoise H2 Production
Like gray and blue H
2, turquoise H
2 also uses methane as a feedstock, but the process is driven by heat produced with electricity rather than through the combustion of fossil fuels. In the process called pyrolysis, hydrogen is separated from methane (i.e., NG) in one step via flow through a molten metal catalyst in a bubble column at higher temperatures (1065 °C). This pyrolysis process is an endothermic reaction where two molecules of H
2 in gaseous form and one molecule of carbon in solid form is produced from one molecule of methane. This reaction is advantageous over methane reforming as it can be performed in near ambient pressure. Theoretically, this reaction produces a larger amount of solid carbon than H
2 with the ratio of 1 to 3 in favor of carbon. However, there is no GHG-free methane reform process. Industrial quality solid carbon can be used as manufacturing feedstock, for example for carbon-fiber production or landfilled which are simpler than capturing and sequestering CO
2. Since it is not released into the atmosphere and does not pollute groundwater in landfills, turquoise H
2 may contribute to low-carbon H
2 production in the future. Pyrolysis has not been fully commercialized yet for H
2 production. There are variants of the process that are being developed including thermal, plasma, and catalytic decomposition. The required process temperature for thermal, plasma, and catalytic decomposition are over 1000 °C, over 2000 °C, and below 1000 °C. Plasma processes realized on industrial scale are still being further developed. The other processes are at an early stage of development [
39].
5.4. Pink H2 Production
In a global zero-carbon energy system, nuclear-produced H2 has a potential place. In this alternative method, H2 can be produced by steam of which heat source is nuclear power plant. Nuclear energy can be used, as a source of electricity and heat, to produce H2 efficiently and with little to no CO2 emissions.
The nuclear alternative has had the challenge of high cost in the past, but that is now changing in the midst of a global energy crisis causing significant price hikes for fossil fuels as well as making global supplies less secure [
40]. Nuclear power is the second largest low-carbon source of electricity after hydropower, supplying 10% of the world’s electricity. However, nuclear power generating capacity will need to more than double to achieve net zero by 2050 [
41].
Light water reactor technology presents great potential for H
2 production. The reactors reach an operating temperature of ~300 °C, while district heating and seawater desalination processes require ~150 °C. Numerous district heating systems have been in operation for decades. This heat could also be utilized for the production of fresh water, H
2 or other products in cogeneration (simultaneous production of electricity, heat and heat-derivative product) [
41,
42]. Electricity and heat produced at temperature levels of 300 °C in small modular reactors can be used in solid-oxide electrolyzer cells (SOEC). Six small modular reactors with a total capacity of 300 MW can meet the annual H
2 demand of a mid-sized ammonia plant (73,000 tons of H
2/year) [
43]. In the longer term, advanced nuclear reactors will be able to provide the higher temperatures required for industrial processes such as steel and cement production, as well as H
2 production by thermochemical water splitting (with some reactor designs having coolant outlet temperatures of 800–1000 °C). A single 1000 MW nuclear power reactor could produce more than 200,000 tons of H
2 each year to fuel more than 400,000 FCVs or more than 16,000 long haul FC trucks [
44]. In the near future, small-to medium-sized reactors can be dedicated, on an industrial scale, to making H
2, which also takes away the cost of storage and transportation. Moreover, using base-load surplus of nuclear or any RE power grid H
2 can be produced for the local market by a mix of electrolysis and thermal processes. Nuclear power, by its potential for further development, can be a part of efforts to achieve global climate targets, and it is a sufficiently experimented alternative which can stimulate clean H
2 production preferences.
5.5. Green H2 Production
H
2 produced by water electrolysis using electricity from clean RE, namely, the green H
2, promises to help meet global energy demand while contributing to climate action goals, and represents the most promising energy carrier for the zero-carbon economy. Falling RE energy prices coupled with the declining cost of electrolyzers and increased efficiency due to technology advancements have increased the commercial feasibility of green H
2 production. Assuming these costs continue to fall, green H
2 can be produced for $0.70–$1.60/kg by 2050, a price range competitive with NG [
45].
In 2003, the European Commission (EC) suggested that the EU should attain a H
2-based economy in 2050 and estimated that 35% of newly produced vehicles will be fueled by zero-carbon H
2 in 2040 [
46]. This strategy updated in 2022 aims to ensure low-carbon H
2 technologies be able to be deployed at large scale to cover all hard-to-decarbonize sectors. Projects varying from 20 MW to above 100 MW are being developed with the current H
2 costs of 5–8 €/kg. The EC launched a call for a 100 MW electrolyzer to scale up manufacturing facilities to multi-GW per annum, to attain the goal of 1.5 $/kg of green H
2 by 2025 provided that low-cost RE power is ensured. In this way, it is believed that green H
2 production cost parity (or even superiority) with fossil- fuels could be achieved as early as 2025. Deploying zero-emission H
2 vehicles (heavy-duty FC H
2 buses, trucks, and trains) is an essential part of EU’s H
2 policy. They will require a very reliable, high-capacity HRS capable of delivering several tons each day. The regulation requires one HRS available every 150 km along the Trans-European Transport Network and in every urban node by 2030. In order to fully unleash the potential of H
2 technologies and establish them as a mainstream means of decarbonization in all transportation modes, cost reduction, increased performance and lifetime, sustainability, recycling, and eco-design in transport FC system components and vehicles are needed [
47]. Another H
2 -hub plan approved by the EC in July 2022 allocates €5.4 billion to support 41 technology development projects. A small hub near Hamburg, Germany, is already under construction, and a larger hub is being built at the Port of Rotterdam in Netherlands. Portugal will produce green H
2 using wind power and transport it into the Port of Rotterdam. Spain houses Europe’s largest green H
2 facility with a 100 MW solar farm. In USA, the Intermountain Power Project will also have a 220 MW electrolysis system based on PV generators to produce H
2 on site, along with the facilities to store up to 300 GW hours of the gas in underground salt domes [
48]. Studies on China’s potential of green H
2 production showed that the efficiency of the H
2 production from wind power is significantly higher than that from solar power. These efficiencies and green H
2 production in each province of China are expected to significantly increase by 2030 [
49]. In a recent study in Sweden the feasibility of producing green H
2 using electrolysis with RE electricity in HRS was investigated. With hourly solar radiation and wind speed data, and electricity price, simulations were used to assess the cost components of H
2 production. The study revealed that integrating the electricity grid in green H
2 production was important, wind speed was critical for cost reduction, whereas solar radiation had less influence. Further, a combination of solar and wind could provide better performance in an off-grid scenario. The most encouraging finding was the cost, which was competitive with reported costs in other EU countries [
50].
In addition to the currently existing technologies, efficient and direct conversion of sunlight through a photocatalytic process to split ocean water without use of electricity can pave the way to a clean production route. A unique, CdS/ZnTHPP binary nanocomposite was recently created and employed as a reliable photocatalyst for the generation of H
2 in the presence of artificial sunlight which is all solid state. It demonstrated a conversion rate of almost ten times greater than that of pure CdS nanorods, the data showed that this nanosystem had remarkable photocatalytic activity. This innovation may lead to further improvement toward high-efficient photocatalytic green H
2 productions for real applications [
51].
The inherent production intermittency of RE sources makes operation of energy systems with large RE components reliably and securely quite challenging. In an energy system with 100% RE the infrastructures for carbon-free H
2 production by electrolysis, and storage and/or transportation can overcome the intermittency of RE. The commercial production of H
2 by electrolysis of water can achieve efficiency up to 81%. Less than 0.1% of H
2 production globally comes from water electrolysis today, and the H
2 produced by this means is mostly used in markets where high-purity H
2 is necessary (electronics and polysilicon), but electrolysis, as an energy-intensive process for H
2 production, is still confronting challenges [
52,
53,
54,
55,
56,
57]. Electrolysis can be classified into three types: (i) alkaline (AWE), (ii) proton-exchange membrane (PEM), and iii) solid-oxide steam (SOSE). H
2 production by AWE is well established technology up to the megawatt range for commercial level (
Figure 4a).
Figure 4. (
a) H
2 generation from alkaline electrolyzer. Reproduced with permission from [
58], John Wiley and Sons, 2022; and (
b) schematics of PEM water electrolyzer (Solid polymer electrolyte membrane is the same as proton exchange membrane).