Hydrogen can be produced from numerous sources (e.g., fossil fuels, biomass, and water) by using several technologies. However, the production of H₂ has dominated using fossil fuels until today. Day by day, the water electrolysis method has gained more interest with the decreasing renewable power cost for green hydrogen production. Nearly 76% of total hydrogen gas (of the global demand) is produced from natural gas sources. For the rest, 23% and 1% are from coal and electrolysis sources, respectively. The amount of CO₂ emission during hydrogen production has been a concerning issue for a while, and efforts are continuing to control CO₂ emission. The implementation of carbon capture, utilization, and storage (CCUS) projects are helpful in this regard. Numerous types of routes for hydrogen production are outlined in Figure 1.

Sources and methods of gaining different types of hydrogen, along with the emission types, are listed in Table 1.

Green hydrogen is achieved through the process of electrolysis powered by renewable energies such as wind or solar. Electrolysis involves using an electrical current to break down the water molecule into oxygen and hydrogen by electrodes. Hydrogen stored in specific tanks is channeled into a fuel cell. There, it binds again with oxygen from the air, and electricity is obtained. Thus, the only byproduct of the process is water, resulting in a clean, sustainable system in which zero CO₂ is emitted to produce energy. In the study by Marcelo and Dell’Era ^[5][15], they identified two primary categories of electrolysis processes: (i) polymer electrolyte membrane (PEM) electrolyzers and (ii) alkaline electrolyzers. The operational efficiencies of these electrolyzers can vary from 52% to 85%, as reported by Binder et al. in 2018 ^[6][16]. The conversion of electricity into chemical energy through electrolysis represents a promising technological advancement. This explains the global proliferation of power-to-gas facilities. Proton exchange membrane (PEM) electrolyzers and alkaline electrolyzers, as well as other electrolyzer types such as solid oxide electrolysis cells (SOECs) and molten carbonate electrolysis cells (MCECs), have gained widespread recognition for their electrochemical applications.

To establish the financial feasibility of a large-scale production unit, an energy analysis is essential. This analysis can be conducted using the standard methods ^[1][7][11,17]. It involves calculating the input energy (Ei), output energy (Eo), and net energy (En) using the following equations:

The input energy (Ei) can be determined using Equation (1):

where Ei represents the input energy (kWh), P is the power used for the process (kW/kg), T is the time for disintegration (hours), V is the reactor volume (m³), and S is the substrate (kg/m³). This energy is required for the disintegration process.

The output energy (Eo), in terms of energy gained as hydrogen, can be calculated using Equation (2):

where Eo denotes the output energy (kWh), B is the biodegradability of algal biomass (gCOD/gCOD, where COD is the chemical oxygen demand), L is the COD load (gCOD/m³), H is the hydrogen yield (m³/gCOD), V is the reactor volume (m³), and F is the biohydrogen conversion factor (1 m³ equals 3.5 kWh).

The net energy (En) estimation is determined by the difference between the output energy (Eo) and input energy (Ei) using Equation (3):

En = Eo − Ei   (3)

where En represents the net energy (kWh), Eo is the output energy (kWh), and Ei is the input energy (kWh) ^[8][18].

This energy analysis is crucial for assessing the financial viability of large-scale production units.

The most universal method to produce hydrogen from natural gas resources is known as steam methane reforming (SMR). At present, three methods are in use for hydrogen production from natural gas resources. These are SMR, the partial oxidation method (POM), and autothermal reforming (ATR). For large-scale production, SMR is the most popular form of H₂ production ^[9][10][19,20]. CO₂ produced by the SMR method is currently released into the atmosphere. However, it can be utilized as a byproduct in food processing and packaging. SMR uses water as an oxidant and a source of hydrogen, while oxygen in the air is used as an oxidant in POM. A combination of both SRM and POM is known as the ATR method. SMR is the most developed industrial process with no oxygen requirement. However, the emission of CO₂ is considerably high. With a view toward decarbonizing these methane-based processes, the CO₂ produced must be captured and stored. The use of carbon capture, utilization, and storage (CCUS) projects are helpful in this regard. It is a process that captures carbon dioxide emissions from different sources so that it will not enter the atmosphere. CCUS projects involve pumping the CO₂ into geological reservoirs, such as depleted oil and gas fields ^[1][2][11][10,11,12]. Being a sustainable method with a low current cost, SMR has become popular in H₂ production. However, difficulties involved with the automated control system and feedstock management system need to be addressed to make SMR a more efficient way of producing H₂.

Electrolysis is an electrochemical process where less than 0.1% of the dedicated hydrogen production becomes global ^[12][13][14][8,21,22]. Three main electrolysis technologies include alkaline electrolysis, proton exchange membrane (PEM) electrolysis, and solid oxide electrolysis cells (SOECs). Fertilizer and chlorine industries have used alkaline electrolysis to produce hydrogen since 1920, and it is as an established and commercial technology. General Electric introduced proton exchange membrane (PEM) electrolysis in 1960 to overcome the few operational drawbacks of alkaline electrolysis. The least-developed electrolysis technology is solid oxide electrolysis cells (SOECs), which are yet to be commercialized. Integrating heat into hydrogen production became interesting because of the heat sources from industrial processes, geothermal or solar heat, and nuclear power plants. Nuclear power plants generate heat at 300 °C, which can also be used to provide electricity and steam for solid oxide electrolysis cells (SOECs) ^[12][15][8,23].

Electrolysis requires both water and electricity. Approximately 9 L of water are used to produce 1 kgH₂, while 8 kg of oxygen are produced as a byproduct. As a matter of fact, freshwater access can be an issue in water-stressed areas, as water consumption is roughly double that of the SMR process. Seawater could be a potential source for electrolysis in coastal areas. But seawater may not be used directly, as it leads to corrosive damage and the production of chlorine. Moreover, PEM electrolysis needs expensive electrode catalysts such as platinum, iridium, and membrane materials. Again, the lifetime of the electrodes are also shorter than those of alkaline electrolyzes ^[12][13][14][8,21,22]. A fuel cell is relatively dry compared to the electrolysis process. Water is soaked by polymer electrodes and therefore swells. During extreme swelling and operating conditions at 30 bar (430 psi) or higher, polymer strands are farther apart and become weaker mechanically without the use of thick membranes ranging from 175 to 250 microns.

Having mentioned these limitations, there are new opportunities that appear every day to mitigate the gaps. Electrolysis cell efficiency may be improved by using thinner membrane materials (50–60 microns). It also assists to increase the mechanical strength ^[16][24]. With a view toward optimizing ionic conductivity and decreasing water uptake, researchers are investigating different polymer compositions. The replacement of the fluorine-based backbone of Nafion with hydrocarbons has been taken under consideration. Also, the catalysts used in electrolysis—platinum on the hydrogen side and iridium on the oxygen side—need to be improved ^[16][24]. Research on selecting materials that would be compatible with the temperature levels of nuclear energy heat sources needs to be performed. Small modular reactors will also be considered to contribute to SOEC electrolysis in the coming days. Also, advanced nuclear reactors will be an attractive option as well in the long term ^[12][15][8,23].

The chemical and fertilizer industries produce hydrogen from coal using the gasification process during the production of ammonia as a well-established technology ^[17][18][25,26]. However, the CO₂ emission increase is very likely from coal-based hydrogen production. Also, the use of CCUS produces hydrogen with a relatively low hydrogen-to-carbon ratio and can contain high levels of impurities in the feedstock (sulfur, nitrogen, and minerals) ^[17][18][25,26]. Heat released from the gasifier unit needs more efficient uses, including power generation and heating purpose. However, it is also challenging to cool hot gas above 1400 °C using heat exchangers due to material restrictions. The other challenge is the lack of established standard codes. The carbon content carried in syngas should be reduced to avoid greenhouse emission.

Coal contains sulfur, which results in the production of sulfur oxide during the gasification process. Sulfur oxide is responsible for environmental pollution and acid rain. Therefore, a sulfur reduction system must be incorporated into the gasification process of coal for a sustainable environment. The devolatilization of coal can be carried out by using different types of fuel. Also, the devolatilization temperatures can be integrated into the segregation of coal. Waste heat and hydrogen may be further used in power generation systems, storage, and cooling purposes by the application of the plasma co-gasification process.

There have been numerous ways to produce hydrogen from biomass. One of the routes is called the biochemical route, where biogas is generated by the interaction of a microorganism with an organic material. The process is also known as aerobic digestion, where a combination of acids, alcohols, and gases takes place. Microorganisms, such as bacteria, break down organic matter to produce hydrogen. The organic matter can be refined sugars, raw biomass sources such as corn stover, and even wastewater. Because no light is required, these methods are sometimes called “dark fermentation” methods. In direct hydrogen fermentation, the microbes produce the hydrogen themselves. These microbes can break down complex molecules through many different pathways, and the byproducts of some of the pathways can be combined by enzymes to produce hydrogen.

Although several biomass gasification plants exist in the world, the technology is not yet fully established. Catalyst poisoning resulting from the formation of tar has not been completely addressed. Irrespective of the production process, the produced gas needs to be further processed to extract hydrogen. The unavailability of cheap biomass also restricts biomass-based hydrogen production on a large scale ^[1][11]. Converting hydrogen to hydrogen-based fuels and feedstocks is easier to store, transport, and use. Some examples are ammonia, synthetic hydrocarbons, and synthetic methanol ^[19][20][27,28].

Around 1990, the methane splitting process was introduced, which is based on alternating the current three-phase plasma. It uses methane as feed and electricity as the energy source to produce hydrogen and solid carbon without the emission of CO₂ from natural gas ^[19][20][27,28]. The methane splitting process consumes electricity three to five times less than electrolysis for the same amount of hydrogen production. However, the process comes with limitations, because it requires high-temperature plasma and a significant loss of temperature, which reduces the overall efficiency ^[19][20][27,28].

Monolith Materials operates a pilot methane splitting plant in California and is building an industrial plant in Nebraska with a lower total efficiency than using natural gas directly in the power plant. This process can help reduce the emissions from gas combustion, and even if it requires more natural gas than electrolysis, there could be additional revenue streams from the sale of carbon black for use in rubber, tires, printers, and plastics ^[21][22][29,30].