1 Introduction

Global Energy demand is increasing rapidly due to fast economic and population growth, which is largely met by the use of fossil fuels such as coal, oil and natural gas (Das et al., 2020; Esmaeilion et al., 2021a, 2021b). Now, global concerns about reducing greenhouse gas emissions and limited fossil fuel reserves have pushed the economy sector to use the sustainable source of energy as an alternative to fossil fuels. Albeit renewable forms of energy like solar (Esmaeilion et al., 2021a, 2021b, 2022a, 2022b) and wind (Hauer et al., 2020) have evolved as alternative source, but these are intermittent and yearly yield varies in power output. One side seasonal variation of sunlight affects the power output and on other side the power demand on the electricity grid varies hourly. Thus, a demand for suitable medium has come up that will play the role intermittent storing and demand based supply of off-peak energy. The use of carbon-free fuels, such as hydrogen could be an ideal sustainable energy solution which can be employed in a fuel cell to generate electricity or combusted directly, and helpful to decarbonise various industrial sectors (2021b; Ahmadi et al., 2021a; Fajrina & Tahir, 2019).

Global research on hydrogen as a fuel demonstrated that it can provide ecologically benign transportation systems depending on the energy and material source (Suleman et al., 2016). Researchers have encouraged the use of hydrogen in various energy intensive applications due to its properties such as extensively availability on earth in different form, greater higher heating value (HHV) and greater lower heating value (LHV) than common fossils as shown in Table 1, higher energy density on a mass basis (i.e., 4 times more energy than coal), non-toxic in nature, clean source of energy, and ease of convertibility into electricity or fuel (Dincer & Acar, 2015; Ferraren-De Cagalitan & Abundo, 2021). Thus, the efficient conversion and further use of hydrogen could provide a zero-carbon energy carrier that can be easily stored and transported as liquid much like liquefied natural gas or in gas from by pipelines; with the versatility to operate across electricity, transport, heat, and iron and steel sectors, where it is difficult to reduce carbon emission (Staffell et al., 2019). According to 2019 report of International Energy Agency (IEA) (Bourne, 2012), hydrogen is one of the cost-effective solutions for storing energy over days, weeks or even months from renewable energy sources like wind and solar photovoltaics (PV). Moreover, the importance of hydrogen in integrating various industries can be seen in Fig. 1.

Table 1 Lower and higher heating values of different fuels at 25 °C and 1 atm (Dincer & Acar, 2015)
Full size table
Fig. 1
figure 1

Hydrogen applications in different energy sectors. Reproduced from (Energy, 2018)

Full size image

Hydrogen can be produced from many feedstocks such as fossil fuels, nuclear, biomass and water, or from combination of them. It is to be noted that 95% of global hydrogen is produced from the conversion of fossil fuels (Mao et al., 2020). Currently, natural gas (i.e., 6% of total natural gas use) followed by coal (i.e., 2% of total coal use) is primarily used to meet the global demand of hydrogen (approx. 70 million tonnes/year), and only less than 1% of hydrogen is produced from renewables. Consequently, the extraction of hydrogen from fossil fuels is responsible for massive emission of greenhouse gases that is approx. 830 million tonnes of CO2 annually (Dincer & Acar, 2015). Thus, in order to reduce the use of fossils and take advantage of hydrogen economy, hydrogen should be produced from renewable sources of energy with environment friendly technologies such as thermochemical water splitting cycles (Dincer, 2012; Tamaura et al., 1995), which leads to the goal of sustainable energy development.

In this article, different methods of hydrogen production from renewable sources have been reviewed, and a comparative analysis of these methods based on their limitation, environmental impact, cost, and energy and exergy efficiency is presented. The primary energy sources evaluated in this study are thermochemical, photochemical, sonochemical and electrochemical process.

The present study presents all the possible methods used for water splitting, where it shows the SWOT analysis of water splitting methods as well as exergoenvironmental and economic analysis which are not considered at once in previous works related to water splitting according to authors knowledge. These three analyses are considered as the novelty of the present work.

Hence, this work aims to provide a comprehensive overview of latest progress, major challenges and future research and perspectives of the above mentioned methods for the production of hydrogen.

2 Hydrogen production methods

Hydrogen can be generated in a wide variety of methods that premised on the primary source of materials employed, which can be classified into conventional and renewable technologies. The conventional approach utilizes fossil fuels as raw materials to extract hydrogen by employing processes such as hydrocarbon reforming and pyrolysis. The second category, renewable technologies, comprises the techniques which generate hydrogen either from biomass or water (Fazelpour et al., 2016; Mahmoudan et al., 2022).

2.1 Hydrogen production from biomass

Biomass, as a renewable source, can be utilized for hydrogen generation employing two main methods, namely thermochemical and biological processes where each of which driving energy is thermal and biological, respectively (Nikolaidis & Poullikkas, 2017; Yin & Wang, 2015). Regarding the fact that biological processes are working under mild conditions, they are considered as more environmentally kind and less energy-intensive hydrogen production methods, but their negative point is providing low rates and yields (mol H_2/mol feedstock) of hydrogen, and are highly dependent on the primary materials used (Balat & Kırtay, 2010). Whereas the thermochemical processes provide a considerable higher stoichiometric yield of hydrogen and prove a much faster process that make them a promising alternative based on both economic and environmental concerns (Parthasarathy et al., 2014). Thermochemical technology consists of gasification, pyrolysis, combustion and Supercritical Water Gasification (SCWG) methods, while the major biological processes are dark fermentation, photo-fermentation, direct and indirect bio-photolysis as indicated in Fig. 2.

Fig. 2
figure 2

Hydrogen production methods from biomass (Safari & Dincer, 2020)

Full size image

2.1.1 Thermochemical process

Among all thermochemical technologies, pyrolysis and gasification are more common methods for hydrogen production. Both conversion processes generate, \({{\text{CH}}}_{4}\) and \({\text{CO}}\), among other gaseous by products, which can be further processed for more hydrogen generation by utilizing steam reforming and water–gas shift (WGS) reaction. In addition to these methods, combustion and Supercritical Water Gasification (SCWG) techniques are considered as two less preferred ways since not only both offer low hydrogen generation with the first emitting polluting by-products but also needing difficult operating conditions of 5–20 MPa in the absence of air (Ni et al., 2006).

2.1.2 Biological process

For bio-hydrogen production, water and biomass have a significant role. Water is used as a feed for photolysis while biomass will be employed for fermentative processes, where the carbohydrate-containing materials are converted into organic acids and then to hydrogen gas utilizing bioprocessing technologies (Kim et al., 2019). Biological conversion is considered a more environmentally friendly technique and less energy-intensive since it operates at ambient temperature and pressure, however, to produce hydrogen by this approach, it is vital to be sustained at a certain temperature and pressure as it is profoundly sensitive to the reaction’s condition. Also, the yield of the produced hydrogen is lower than that of the thermochemical processes (Safari & Dincer, 2020). SWOT analysis of hydrogen production methods from biomass is shown in Table 2.

Table 2 SWOT analysis of hydrogen production methods from biomass
Full size table

2.2 Hydrogen production from water

Water, as one of the most ample raw materials on Earth, has an excellent potential to be utilized for hydrogen production by employing water-splitting processes (Nikolaidis & Poullikkas, 2017). This method, water splitting hydrogen production, is subdivided into four principal classes according to the energy used for water splitting and a hybrid form in which two or more kinds of energy are integrated to generate hydrogen. These water spilling methods are electrolysis which is electrical based, thermolysis, or thermochemical cycle which is thermal energy-based and uses thermal energy as a source, photolysis, or photo electrochemical method which is photonic based, and finally, yet importantly sonochemical method which is mechanical based (Ni et al., 2006; Nikolaidis & Poullikkas, 2017). Figure 3 shows various pathways for hydrogen production methods from water. SWOT analysis of hydrogen production methods from water is shown in Table 3.

Fig. 3
figure 3

Hydrogen production methods from water

Full size image
Table 3 SWOT analysis of hydrogen production methods from water
Full size table

3 Water splitting cycle

3.1 Thermochemical process

The search for the most suitable thermochemical cycle has started since the 1960s with many options that showed promise theoretically (Funk & Reinstrom, 1966) with many studies and lot of research being carried out through the 1970’s and early 1980’s to identify the cycle that could be the best in terms of various parameters such as its efficiency, thermodynamic performance and the total cost (Funk, 1976; Beghi, 1981, 1986). Research organizations and industries originating from the nuclear sector were the primary stakeholders in the research effort for thermochemical cycles during those years as the thermal energy generated by the nuclear reactors had the potential to be utilized by these cycles (Farbman, 1979; Norman et al., 1982).

But there was a sudden downfall in the development of thermochemical cycles in the late 1980s which resulted in very small development and progress up to the late 1990s mainly being focused on the UT-3 cycle which was modelled and formulated by the University of Tokyo (Sakurai et al., 1996a) and the sulfureiodine cycle which was developed by the General Atomics company (Brown et al., 2003). Recent years have shown resurgence in the domain of research and study of thermochemical cycles primarily to fulfill the requirements of the Kyoto Protocol which emphasizes on the production of hydrogen as a suitable carrier without any greenhouse gases. Many countries have thereby implemented national limitations with European Union mandating a reduction of 8% and few others such as Japan at 6%, United States of America at 7% and permitted increase up to 10% for Iceland and Australia at 8%.

As compared to the study and research performed earlier many years ago such as in the 1970s which focused on nuclear heat for thermochemical cycles, concentrated solar energy has gained a lot of prominence and recognition with a much better potential for implementation. The most important components for harnessing concentrated solar power are the collectors or heliostats. In order of increasing solar intensities and higher process temperatures, the concentrating systems are classified mainly into parabolic trough, Fresnel concentrators, solar towers and dish out of which the solar towers and dishes provide the required temperatures for running the thermochemical cycles ranging from 700 K to up to more than 1300 K. Heavy research and studies are being performed on the integration of thermochemical cycles with solar towers for production of hydrogen in which the solar radiation concentrated with the help of collectors is the primary aspect of the thermochemical water-splitting process. Appropriate materials as substrates and those which can act as stabilizing agents against the reaction process and external disturbances such as harsh environmental fluctuations, thermal resistance against shock and fatigue is another essential requirement for such systems (Bilgen et al., 1986; Sakurai et al., 1996b).

Thermochemical cycles that are based on redox-pair reactions consist of water splitting and regeneration through thermal reduction that occurs at a different bandwidth of temperatures. The thermal reduction step occurs at a higher temperature wherein the redox material which exhibits multiple oxidation states and the oxidized form of a higher-valence oxide releases an amount of oxygen thereby reducing it to its lower-valence or reduced state. This reduced material is then activated in the second water-splitting step where it gets oxidized to the higher valence state once again by absorbing oxygen from water and thereby leading to the production of hydrogen. This process is repeated in a cycle as mentioned above and can also be implemented for the splitting of carbon dioxide as well for the production of CO.

Water splitting process with two steps is consisting of releasing oxygen according to Eq. (1) and generating hydrogen according to Eq. (2). The two-step water splitting chemical reaction are presented by the following equations:

$${\text{MOoxidized~}} \to {\text{MOreduced~}} + {\text{~1/2}}\;{\text{~O}}2({\text{g}})$$
(1)
$${\text{H}}2{\text{O}}({\text{g}}) +\mathrm{ MOreduced }\to \mathrm{ MOoxidized }+\mathrm{ H}2({\text{g}})$$
(2)

Among several thermochemical water-splitting cycles studied in the past, the copper-chlorine cycle was shown to have significant potential because of the lower temperature required for heat supplies compared to most other thermochemical processes (Mehrpooya & Habibi, 2020).

The copper-chlorine (Cu–Cl) cycle is identified as a promising low-temperature cycle for hydrogen production. The Cu–Cl cycle decomposes water into hydrogen and oxygen, through intermediate copper and chloride compounds, in a closed-loop that recycles all chemicals continuously. Seven versions of the Cu–Cl cycle were proposed in past literature, based on the main processes and chemical reactions: one cycle with two steps, two versions of three-step, two cycles with four steps, and one five-step cycle. Cu–Cl-2 is a two-step version consisting of high-temperature CuCl2 hydrolysis and CuCl chlorination. Cycle Cu–Cl-3A is a three-step cycle with the steps being CuCl disproportionation, high-temperature CuCl2 hydrolysis, and copper chlorination. Cu–Cl-3B includes CuCl chlorination, CuO.CuCl2 thermolysis, and oxychlorination. Cu–Cl-4A is a four-step version consisting of CuCl disproportionation, copper chlorination, oxychlorination, and CuO·CuCl2 thermolysis. Cu–Cl-4B includes CuCl2 dry hydrolysis, CuCl chlorination, CuO·CuCl2 thermolysis and drying (crystallization). The Cu–Cl-5 cycle comprises CuCl disproportionation, low-temperature CuCl2 hydrolysis, copper chlorination, CuOCuCl2 thermolysis, and spray drying. For cycles Cu–Cl-2, Cu–Cl-3A, Cu–Cl-3C, Cu–Cl-4B, and Cu–Cl-5, water is supplied as steam to the hydrolysis process. This implies that water is preheated, boiled, and superheated separately or in an integrated system prior to its supply to the reaction in the gaseous phase. The other two cycles, namely, Cu–Cl-3B and Cu–Cl-4A, do not require water boiling because water is supplied as a liquid to the electrochemical process where complexation occurs with cupric chloride and then it is heated up to the required temperature for oxychlorination.

Heat recovery within the copper-chlorine cycle is crucial to the efficient performance and the overall viability of the cycle. Pinch analysis was used to determine the maximum recoverable heat within the Cu–Cl cycle, and the location in the cycle where recovered heat could be used efficiently (Das et al., 2008). It was shown that a majority of heat recovery could be achieved by cooling and solidifying molten CuCl exiting the oxygen reactor at about 530 °C. It was also shown that it would be more appropriate to use the recovered heat in the hydrolysis step. Several configurations for heat recovery from molten CuCl, based on existing industrial processes for molten materials were investigated, and two heat recovery processes were recommended for future consideration (Mehrpooya & Habibi, 2020). Based on the selected processes, two types of heat exchangers can be used: direct or indirect contact types. In an indirect contact heat exchanger, molten CuCl flows through an inner pipe while the coolant flows through the outer pipe. An indirect contact heat recovery process with a counter-current airflow was considered, and the results for axial growth of the solid layer and variations of the coolant and wall temperatures were presented (Safari & Dincer, 2020). It was shown that reducing the inner tube diameter would increase the heat exchanger length and also the outlet temperature of air significantly. Also, increasing the mass flow rate of air resulted in an increase in the total heat flux from the molten salt.

In a direct contact heat exchanger, molten CuCl droplets are released from the top of the heat exchanger whereas the coolant flows in the upward direction. The droplets are cooled and solidified during the descent. Analytical and experimental investigations of direct contact heat recovery processes using air or steam as a coolant were presented (Nikolaidis & Poullikkas, 2017). However, it was shown that molten CuCl might react with water vapor in the presence of oxygen (Holladay et al., 2009). For avoiding such a reaction, inert gases could be considered as potential coolants. A predictive model to examine a direct contact heat recovery process from molten salt using various gases, including helium, nitrogen, and argon was presented in another study (Brown et al., 2003). Higher droplet acceleration and hence a higher heat transfer rate was observed for the droplets falling in helium. It is noted that thermochemical cycles are not yet cost-competitive compared to other hydrogen production methods. However, appropriate integration with concentrated solar or nuclear reactors is crucial for further improvement. One of the important cycles that can be manifested with the help of the concentrated solar power systems operating below 1200 K is the sulfur-based cycles that constitute the breakdown of sulfuric acid which is a widely established industrial process. Out of these sulfur-based cycles, the two-step hybrid sulfur cycle and the S–I cycle that show good potential and display good efficiencies with economical costs of usage of hydrogen. The reaction processes of these cycles have attained a level of development and formulation for them to be developed and utilized in a pilot plant. The breakdown of sulfuric acid at high temperatures needs further development in the field of catalysts and stabilizing agents for practical implementation. These cycles provide benefits in term of their economical usage as well as the flexibility of operation if they are modelled as open-cycle systems using flue-gas obtained through desulfurization or if used in addition to the production of sulfuric acid. In that scenario, the amount of energy required is partially obtained from the sulfur material apart from solar energy. Table 4 presents the heat and electricity requirements for various thermochemical cycles reported in past studies.

Table 4 Heat and electricity requirements for various thermochemical water splitting cycles
Full size table

A recent study shows that Mg–Cl, V–Cl, Cu–Cl, and S–I cycles are promising because of their exergy efficiency values of 71, 77.28, 78.21, and 62.39%, respectively. More importantly, clean hydrogen can be produced using these thermochemical cycles with a cost as low as 1.71 $/kg H2 (Karaca et al., 2023).

3.2 Photochemical process

One of the most promising approaches for hydrogen production from water is the photochemical direct conversion of water to hydrogen (Yang et al., 2019). This method is similar to photosynthesis, where light is absorbed and converted to chemical energy (Lei et al., 2018). The photochemical conversion of water to hydrogen has been studied by many researchers. The dissociation of water by light requires intermediates, as the visible light is not absorbed by pure water (Amouyal, 1995). Therefore, photo-catalysts, which are generally semiconductors, can be added to water to capture light in the visible range. The light irradiation on the photo-catalysts creates an electron–hole pair and the generated electrical charge is used to split water through oxidation/reduction reactions as follows:

$${\text{Photo}}-{\text{oxidation}}: 2{{\text{H}}}_{2}\mathrm{O }\stackrel{hv}{\to } {{\text{O}}}_{2}+4{{\text{H}}}^{+}+4{{\text{e}}}^{-}$$
(3)
$${\text{Photo}}-{\text{reduction}}: 2{{\text{H}}}_{2}{\text{O}}+2{{\text{e}}}^{-} \stackrel{hv}{\to } {{\text{H}}}_{2}+{{\text{OH}}}^{-}$$
(4)

The particulate photocatalytic systems include heterogeneous and homogeneous photo-catalysts. In heterogeneous photo-catalysis, solid semiconductor involves in the process of water splitting in aqueous phase (Mei et al., 2018). On the other hand, homogeneous photo-catalysis is based on the participation of several complex molecular structures in photosensitization, charge separation and transfer, and also catalysis (Dincer, 2018). The most common used photocatalyst is TiO2 due to its characteristics such as environmentally friendly, stability against corrosion, relatively low price and none toxicity. However, it has some drawback, including fast recombination of electron–hole pairs, wide band gap limits and large over potential for hydrogen evolution. The performance of TiO2 can be improved by coupling with carbon material or metal deposition. The addition of noble metals to TiO2 has shown the most promising results, nonetheless, finding cheaper alternatives can lower the overall cost of process and consequently hydrogen production cost (Ahmad et al., 2015). This method has some challenges for development, such as the instability of the semiconductor and durability of the materials, weak matching of the solar spectra and the semiconductor band gap, difference between the electrochemical reactions and the semiconductor band edges (Haryanto et al., 2005). Moreover, the requirement for H2/O2 gas separation in the photocatalyic systems is a barrier for a large scale application (Li, 2017). The hydrogen production efficiency and environmental impacts of these photocatalysts are investigated by Acar et al. (2014).

In photoelectrochemical cells (PEC), light is converted to hydrogen through electrochemical water splitting stimulated by light. In fact, in these systems, light absorption and water electrolysis is combined. The PEC systems are usually consisting of a p-type photocathode anode and n-type photoanode cathode. The solar to hydrogen efficiency of PEC is growing to the point that it will be able to potentially compete with solar PV-electrolysis. Since the PEC system has higher efficiency than particulate photocatalytic systems and the H2/O2 separation is already built in, it has gained more attention for larger scale application. But for that purpose, there is a necessity of scalable fabrication of photoelectodes and stable phtoanodes (Li, 2017). The reasonable cost and simple setup of PEC systems provide a great potential for implementation on an industrial scale. Despite that, the conversion efficiency needs to be improved by developing robust photoelectrodes which can exploit full solar radiation. Another challenge for this technology is to overcome the slow kinetic of oxygen evolution on photoanode, which can be achieved by finding alternative reactions for oxygen evolution. The bottleneck of this technology is the performance of photoelectrodes, therefore optimizing them can increase the overall performance of system. But, this requires the investigation of the surface reaction kinetics and the charge dynamic (Chiu et al., 2019).

One of the advantages of photocatalytic systems is less complexity compared to wired systems, such as water electrolysis with photovoltaic- electrolyzer or wind- electrolyzer (Ng et al., 2020). The photochemical methods have considerably lower global warming potential (GWP), acidification potential and social cost of carbon, compared to other techniques including electrolysis or thermochemical processes. However, the hydrogen production cost (per kg hydrogen) of photochemical process is higher, which is partially because of their lower conversion efficiency in comparison to other hydrogen production approaches (Das et al., 2008).

3.3 Sonochemical process

In sonochemical hydrogen production from water, mechanical energy in the form of ultrasound is applied. In the process, high frequency waves are passed through water, in which a lot of energy is accumulated in the form of pressure and temperature and can split the molecule of water to produce hydrogen (Lin & Hourng, 2014). In the first step, the ultrasonic probe sends waves with the frequency range of 20–40 kHz to the water which creates acoustic cavitation bubbles. The waves increase the temperature and pressure inside those bubbles to several thousand of Kelvins and several hundreds of atmospheres, respectively, which can split the water vapor molecules inside the bubbles. The rate of hydrogen production in the sonochemical process is a function of several factors, namely the water bulk temperature, acoustic intensity, acoustic frequency and dissolved gas (Merouani et al., 2016). The mechanism of water cleavage and hydrogen production in sonochemical process is given in Eq. (5) as (Kerboua & Hamdaoui, 2019):

$$H_2 O \leftrightarrow H^ \cdot + OH^ \cdot$$
$$H^ \cdot + OH^ \cdot ~ \leftrightarrow H_2 + O$$
(5)
$$H^ \cdot + H^ \cdot ~ \leftrightarrow H_2$$

Even though sonochemical process is durable, sustainable, and environmentally friendly and has a low-energy consumption, this technology for hydrogen production is still in Research and Development (R&D) stage. Moreover, the hydrogen production mechanism in sonochemistry and the effect of aforementioned influencing factors on the hydrogen production rate has not been fully investigated. Especially, more attention needs to be given to quantitation of the produced hydrogen for studying the possibility of upgrading the process from laboratory scale to pilot and industrial scales. Barriers for commercialization of this technology can be summarized as follows (Islam et al., 2019):

  • Very limited data is available regarding some influencing factors on sonochemical hydrogen production such as ultrasound frequency and the water bulk temperature.

  • Development of this technology is still in early stage.

  • Lack of research in the area of quantification of hydrogen generated from the sonochemical process.

  • Not enough information is available concerning large scale sono-reactor and its optimal design and operating conditions.

3.4 Electrochemical process

Electrolysis is the electrochemical splitting of water by passing electrical energy through water. The process can be coupled with various energy resources, including renewables energy (e.g. wind and solar PV) or electricity from grid supply. Electrolyzers in general do not need continuous maintenance, as there is no moving part in them. Their silent operation and modular structure can promote the continuous, modular-scale, de-centralized production of hydrogen in residential and commercial areas (Bhandari et al., 2014). However, wide spread application of electrolysis requires improvement in many areas, namely the reduction of energy consumption and installation cost. Moreover, a pretreatment should be applied to the water used for electrolysis to avoid undesirable reactions and products which can ultimately increase the production cost of hydrogen (Bhandari et al., 2014).

The main three types of technologies for electrolysis are solid oxide, proton exchange membrane and alkaline electrolyzers. The overall electrolysis of water (Eq. 6) is as follows (Bhandari et al., 2014):

$${{\text{H}}}_{2}{\text{O}}+\mathrm{direct\, current \,electricity\,}\to \,{{\text{H}}}_{2}+\frac{1}{2}{O}_{2} , {{\text{E}}}_{0}=-1.229\mathrm{ V}$$
(6)

Alkaline electrolyzers are the most developed and commercially available electrolyzer. It is a mature technology with the operating temperature of 60–80 °C and has the highest durability and lifetime in comparison with other electrolyzers. In this technology, the electrolyte is an aqueous alkaline solution containing NaOH or KOH. The most commonly used electrode in these types of cells is nickel (Ni) electrode. The corrosively of electrolyte and lower hydrogen purity is one of the main disadvantages of the alkaline electrolyzer.

In solid oxide electrolyzers, the solid electrolyte is composed of yttrium stabilized zirconia (YSZ). The operating temperature of solid oxide electrolyzers is 700–900 °C. Theoretically, a significant part of required energy (up to 40%) for hydrogen production through electrolysis can be provided as heat at 1000 °C and as a result, this electrolyzer can reach higher efficiencies (up to 86%) compared to the others which make it a suitable option when a high temperature heat source such as solar thermal energy or geothermal energy, are available (Safari & Dincer, 2020). On the other hand, the drawback of high temperature is the durability of materials. The fluctuation in cell temperature due to load change and heat lost can cause micro cracks in the membrane. Therefore, solid oxide electrolyzers are not compatible with the intermittent nature of renewable energies, including wind or solar. This technology is at the early phase of development (Bhandari et al., 2014; Buttler & Spliethoff, 2018).

The proton exchange membrane has a polymeric membrane which acts both as electrolyte and gas separator. This electrolyzer can provide highest purity of hydrogen (higher than 99.99%) among other electrolyzers and it is capable of coupling with variable power supply from renewable energies. High purity of water is required for PEM electrolyzer to extend its lifetime. However, the commercialization of PEM is limited because of the low stability of Nobel metals, such as platinum or iridium and the high cost of components, especially membrane and catalyst.

Generally, various kinds of electricity sources can be employed for electrolysis, in which wind energy, solar PV and grid supply are the most common used. Among the routes for electrolysis, grid supply-electrolysis has a significantly higher GWP. The GWP and acidification potential of wind-based water electrolysis is moderately lower than solar PV—based water electrolysis (Bhandari et al., 2014). The hydrogen production cost for electrolysis strongly depends on the cost of electricity (Tseng et al., 2005). The electrolysis has a lower hydrogen production cost and higher energy efficiency over photochemical methods such as photocatalytic systems or photoelectrochemical cells, whereas its GWP is higher (Islam et al., 2019; Ivy, 2004).

Electrolyzers are normally the main part of investment cost in electricity to hydrogen (via electrolysis) systems. For the electrolyzers, the economy of scale effect is not very significant since the hydrogen production rate raises linearly with the electrolysis cell area and only the specific costs of auxiliaries are decreased as a result of an increase in the scale. A reduction in investment costs can also be achieved with higher current densities due to an enhanced hydrogen production per cell area; however, it leads to a decrease in the electrolyzer efficiency and consequently an increase in operational cost. On the other hand, an elevation in operating temperature can improve the efficiency and reduce the operational costs. Another option for decreasing the capital costs is a pressurized electrolyzer system which eliminates the need for a compressor. In all these changes such as increasing current density, operating temperature or pressure, the impact on the system degradation and lifetime should be taken into consideration. For instance, the increased pressure or temperature adversely affects the lifetime and durability of electrolyzer material. Therefore, it is essential to take a holistic approach to investigate various solutions for reducing the capital and operating cost of the water electrolysis systems (Bhandari et al., 2014; Buttler & Spliethoff, 2018).

4 Energetic and exergetic performance of water splitting cycles

For achieving a better perspective of system functionality, using general standard approaches could be helpful. Energy and exergy analyses are common thermodynamic tools for accomplishing this purpose (Makkeh et al., 2020). In this regard, for providing inclusive performance analysis, the energy and exergy criteria are presented.

The following diagram (Fig. 4) can be shown as a schematic diagram of the energy balance for every physical system including hydrogen production by the water splitting process. Moreover, the main conceptual diagram can be presented for exergy analysis:

Fig. 4
figure 4

Schematic diagram of exergy analysis

Full size image

In this regard, the overall energy and exergy efficiencies of hydrogen production methods is presented in Fig. 5 (Kapdan & Kargi, 2006).

Fig. 5
figure 5

Energy and exergy efficiencies of the important hydrogen production methods

Full size image

Yilmaz et al. (2017) conducted energy and exergy analyses for a hydrogen production cycle for different ambient temperatures. The energy and exergy productivities of this system were evaluated to be 11.00 and 20.34%, respectively. The results from Shomate equations revealed that the outlet exergy and exergy destruction were respectively 1653.32 and 317.02 kJ/mol for the reference environment temperature at 298 K. Marques et al. (Marques et al., 2020) analyzed the sodium-oxygen-hydrogen thermochemical water splitting cycle in terms of energy and exergy. Obtained results for the 3-step system revealed that the exergy efficiency for reactions 1–3 were 96, 58, and 87%, respectively, while the exergy destruction for these reactions 1–3 was calculated to be 8.5, 280, and 18 kJ, respectively.

Zamfirescu et al. (2011) investigated the thermodynamic aspects of a vapor compression CuCl heat pump combined with a thermochemical water-splitting cycle. Based on the achieved results, increasing the compressor isentropic efficiency will increase the COP of the heat pump directly. An extensive assessment of thermochemical water-splitting cycles aimed at producing hydrogen has been studied (Bhosale et al., 2017). Based on the presented results, the cycle efficiency of the ZnO–ZnSO4 water splitting cycle were equal to 40.6%. Abanades et al. (2008) introduced a novel SnO2/SnO water-splitting cycle for hydrogen production. The proposed system used solar collectors and fuel cells as the energy reservoir. The energy and exergy efficiencies of the given configuration were obtained 35.9 and 29.8%, respectively. Balta (2020) evaluated a boron-based thermochemical water splitting-cycle for producing renewable hydrogen. The influence of the reference environment temperature on the cycle’s productivity showed that increasing its value would increase the overall exergy efficiency of the cycle.

5 Economic analysis

To evaluate the economic performance of a system and to calculate the capital investment of a plant, economic analysis can provide detailed information (Ahmadi et al., 2020; Jamali & Noorpoor, 2019).

To calculate the simple payback period (SPP), the following equation can be used (Ahmadi et al., 2020; Jamali & Noorpoor, 2019):

$${\text{SPP}}=\frac{{\text{Z}}}{{\text{CF}}}$$
(7)

Here, the CF represents the annual cash flow which is presented as follows (Bellos et al., 2019; Tzivanidis et al., 2016):

$${\text{CF}}=\sum {{{\text{Y}}}_{{\text{i}}}{\text{k}}}_{{\text{i}}}$$
(8)

Here, Y_i denotes the annual production capacity, and k_i stands for the product’s specific cost. For a system, the payback period (PP) is calculated as (Ahmadi et al., 2020; Jamali & Noorpoor, 2019):

$${\text{PP}} = \frac{{{\text{ln}}\left( {\frac{{{\text{CF}}}}{{{\text{CF}} - {\text{r}}.{\text{Z}}}}} \right)}}{{{\text{ln}}(1 + {\text{r}})}}$$
(9)

where r is the discount factor and Z denotes the total capital investment. The net present value (NPV) can be obtained as (Ahmadi et al., 2020; Jamali & Noorpoor, 2019):

$${\text{NPV}}={\text{CF}}\frac{{(1+{\text{r}})}^{{\text{N}}}-1}{{{\text{r}}(1+{\text{r}})}^{{\text{N}}}}-{\text{Z}}$$
(10)

And for a system, the internal rate of return (IRR) is given as (Ahmadi et al., 2020; Jamali & Noorpoor, 2019):

$${\text{IRR}}=\frac{{\text{CF}}}{{\text{Z}}}\left[1-\frac{1}{{\left(1+{\text{IRR}}\right)}^{{\text{N}}}}\right]$$
(11)

Moreover, it should be mentioned that, to conduct economic assessment of a given hydrogen production system the key parameter is the levelized cost of hydrogen (LCOH) gained (in $/kg H2) (Ahmadi et al., 2020; Jamali & Noorpoor, 2019).

Here several studies have been highlighted that economically analysed fossil fuel based or renewable energy based electrolysis technologies to provide a realistic comparison between them. Therefore, the aim in the section is to compare different water electrolysis technologies, and at the same time to draw attentions to the impact of electricity production source on the economic performance of the system. A natural gas-based polygeneration system was proposed and evaluated by Farooqui et al. (2019). The hydrogen produced by chemical looping technology was exploited for dimethyl ether production and the hydrogen production capacity was 186 tons/day. The energy, exergy and economic performance of the system were assessed. The results revealed that the payback period of the system is dependent on the electricity and dimethyl ether costs. When the dimethyl ether selling price was 220 USD/MWh and the electricity cost was 20 USD/GJ the payback period was calculated as 20 years. The capital investment of the plant was obtained as 534 million USD. In a study, several methods of steam methane reforming, PEM, and alkaline water electrolysis were compared from an economic point of view (Lee et al., 2017). As presented in Table 5, the investment and operation costs for hydrogen production by three different processes were compared. In this cost estimation, several parameters such as costs of plant fuel or input, land rent, labour, maintenance, and construction were taken into consideration. They found that the cost of unit hydrogen production decreased with higher capacities of the plant in all the three processes, as they compared plants with 27.3, 9.1, and 2.7 kg/h hydrogen production capacities. Moreover, it was found that the discounted payback period of the system was in the range of 6.1–9.5 years as the discount rate changed from 2 to 14%. A grid connected system was developed and analyzed from thermodynamic, economic, and environmental points of view by Shi et al. (2020) for methanol production. They integrated the tri-reforming of methane with alkaline water electrolysis to provide hydrogen for methanol production. They found that the capital investment cost of the system was 774 million USD, and the breakeven price could be achieved by 491 USD per ton of methanol.

Table 5 Cost evaluation for different hydrogen production technologies for 100 Nm3/h (9.1 kg/h) capacity (Lee et al., 2017)
Full size table

For high-pressure PEM water electrolysis, Lee et al. (2018) performed an economic analysis for a grid connected system. They found for the system the capital costs, the annual operation costs were respectively 819,513 and 1,600,229 USD for the hydrogen production of 66.6 kg/h. The levelized cost of hydrogen production was calculated to be 6.2 USD/kg. They have also found that for changes in the discount rate from 2 to 10%, the discounted payback period ranged from 4.1 to 4.6 years.

For renewable based water electrolysis technologies several studies with a focus on solar, wind and biomass energy sources are presented. Regarding solar energy based water electrolysis, thermodynamic and economic analyses were performed by Liberatore et al. (2012) on a solar-based hydrogen production. The hydrogen production capacity of the system was 100 tons/day. The proposed system used sulfur-iodine thermochemical cycle for water-splitting and used both solar tower and parabolic trough collectors. The initial investment for two modes of the system, electrical and thermal versions, were 1.5 and 1.05 million USD, respectively. The produced hydrogen cost was calculated by 0.26 and 0.34 USD/kWh for electrical and thermal versions, respectively. And the hydrogen production costs were calculated to be 10 USD/kg. Grimm et al. (Grimm et al., 2020) compared solar-based electrolysers (i.e. PEM) with photoelectrochemical cells from a techno-economic perspective. The capacity of these systems was considered to be 10 tons of hydrogen production per day. It was found that using solar energy for hydrogen production could be done with levelized cost of hydrogen of about 6.22 USD/kg. However, the levelized cost of hydrogen for the photoelectrochemical system was calculated as 8.43 USD/kg.

Regarding biomass energy based water electrolysis, in a novel multi-generation energy system performed by Jamali and Noorpoor (2019), solar and biomass energy sources were used for electricity, cooling, heating, CO2 and hydrogen production. They used part of the generated electricity for hydrogen production by PEM water electrolysis. They found that the system could produce 6 and 11 kg/h hydrogen in summer and winter modes respectively with product cost rate of 16 USD/h based on the exergoeconomic analysis. Therefore, the levelized cost of hydrogen would be 2.7 USD/kg and 1.5 USD/kg in summer and winter modes, respectively. Three different hydrogen production including alkaline water electrolysis, biogas steam reforming and biomass steam gasification using dual fluidized bed were compared from a techno-economic point of view (Yao et al., 2017). The hydrogen production capacity was considered to be 90 kg/h. The outcomes revealed that the capital investment costs for alkaline water electrolysis were the lowest of about 4.8 million USD with a break-even price of hydrogen of 0.21 USD/kWh in 2016. Moreover, the capital investment costs for biogas steam reforming was 9.9 million USD, as the break-even price of hydrogen was calculated as 0.17 USD/kWh. Although biomass steam gasification using a dual fluidized bed had the lowest production costs about 0.16 USD/kWh, the capital investment of the plant was found to be the highest of about 13.3 million USD.

Regarding wind energy based water electrolysis, Wind energy source with maximum 100 kW capacity was utilized for hydrogen production for four different cities, which was evaluated from an economic point of view. For a city with a wind speed of 258 W/m2, 100 kW and 3.5 kW wind turbines were considered, and the payback periods for these systems were calculated to be 5 and 13 years, respectively. The capital investments for the 100  and 3.5 kW wind turbines were respectively 140,000 USD and 4900 USD, and the hydrogen product cost rate was found to be 0.03 and 0.06 USD/kWh. Also the hydrogen capacity production was ranged from 3.2–5.2 tons in different cities (Mostafaeipour et al., 2019). Another study performed feasibility and economic analyses on using wind energy for water electrolysis based hydrogen production in 15 different locations in South Africa (Ayodele & Munda, 2019). The study included 11 different wind turbines from different manufacturers considering that the power output ranged from 200 to 4500 kW. The polymer electrolyte membrane was considered in this study. It was found that for larger wind turbines the hydrogen cost was significantly reduced as for 4500  and 200 kW wind turbines, the cost was calculated as 1.4 and 39.55 USD/kg, respectively. And depending on the sites and the wind turbines being small or large, the hydrogen production capacity could range from 1.7 tons to 226 tons, annually. For a water electrolysis unit hydrogen production with annual capacity of 4.22 tons, wind energy was considered and analyzed from an economic point of view by Rezaei et al. (2020). The capital investment of the system was calculated about 140,600 USD, and it was also found that the payback period of the system was around 5 years for 20 years of the lifetime of the system. The average levelized cost of hydrogen production was calculated to be 2.1 USD/kg.

6 Environmental analysis

In this section, the environmental performance of water splitting systems is evaluated using exergoeconomic, environmental and life cycle assessments, which can improve our understanding of these systems sustainability and their impacts (Li et al., 2020; Noorpoor et al., 2017). For exergoenvironmental analysis, several key factors are significant such as exergoenvironmental impact factor, environmental damage effectiveness factor, impact of exergoenvironmental enhancement, exergoenvironment factor, and factor of exergy stability .

For a system, the environmental damage effectiveness factor can be defined as (Ratlamwala & Dincer, 2013; Ratlamwala et al., 2013):

$$\theta _{{\text{ei}}} = {\text{f}}_{{\text{ei}}} .{\text{C}}_{{\text{ei}}}$$
(12)

where \({{\text{f}}}_{{\text{ei}}}\) denotes the exergoenvironment factor and \({{\text{C}}}_{{\text{ei}}}\) is the exergoenvironmental impact factor which is given as (2021b; Ahmadi et al., 2021a):

$${\text{C}}_{{\text{ei}}} = 1/{{\eta }}_{{\text{ex}}}$$
(13)

where \({\upeta }_{{\text{ex}}}\) stands for system’s exergy efficiency.

The exergoenvironment factor can be written as (2021b; Ahmadi et al., 2021a):

$$f_{ei} = \frac{{\dot{E}x_{tot.des} }}{{\sum {\dot{E}x_{in} } }}$$
(14)

Here, Ėx denotes the exergy rate, and subscripts tot.des and in are respectively the total destruction and input. The impact of exergoenvironmental enhancement is given as (2021b; Ahmadi et al., 2021a):

$$\theta _{eii} = \frac{1}{{\theta _{ei} }}$$
(15)

The factor of exergy stability can be expressed as (2021b; Ahmadi et al., 2021a):

$$f_{es} = \frac{{\dot{E}x_{tot.~des} }}{{\dot{E}x_{tot.out} + \dot{E}x_{tot.~des} + 1}}$$
(16)

where the subscript \({\text{tot}}.{\text{out}}\) is the total output.

And here, several studies have been provided to present and compare environmental performance of different water electrolysis based on various energy sources, but with a focus on renewable energy sources to highlight their reliability to mitigate environmental impact of such systems. Hence, the aim in the section is to compare different water electrolysis technologies and the electricity generation source from environmental aspect. Regarding fossil fuel based systems, for a study that mentioned in the previous section, a fossil fuel based system was proposed and analyzed from thermodynamic, economic and environment points of view (Farooqui et al., 2019). As mentioned the hydrogen production capacity was 186 tons/day. The chemical looping technology was used for providing hydrogen for dimethyl ether production. They found that this system was capable of 589.2 kilotons CO2 emission mitigation, as CO2 was used in chemical looping process. A water electrolysis (i.e. alkaline water electrolysis) unit was integrated to a grid connected system for methanol production through CO2 utilization. The results revealed that this carbon-free system was capable of reducing 570,000 tons of CO2 annually (Shi et al., 2020).

Regarding wind energy based systems, the environmental cost is defined as the cost incurred by the CO2 emission of the plant. For hydrogen production using water electrolysis, wind energy was proposed and evaluated (Rezaei et al., 2020). This study was mentioned above, however, it should be mentioned that the annual capacity of the system was 4.22 tons. The results of the plant were compared with natural gas and fuel oil-based systems. For the same mass flow rate of hydrogen production using fuel–oil, the environmental cost was calculated as 2734.4 USD, and for natural gas plant the environmental cost was obtained to be 1974.2 USD. A multi-criteria analysis was conducted on four different hydrogen production from sea water using marine renewable energy (d’Amore-Domenech et al., 2020). The technologies were namely AEC, an SOEC and PEM. Although both AEC and PEM had a narrow difference, PEM proved to have the best performance and feasibility, particularly in a short time. In the case of solving the safety problem of AEC technology in the sea, it would be the cheapest option with the longest lifetime. A comprehensive environmental assessment was performed on three water splitting technologies namely alkaline electrolysis cell (AEC), polymer electrolyte membrane (PEM) and solid oxide electrolysis cell (SOEC) technologies (Zhao et al., 2020). These wind-based water electrolysis technologies were supplied with a 3 MW wind turbine in Denmark. The detail results of the comparison are presented in Table 6. As can be seen, the results revealed that for the global warming potential (GWP), the impact of PEM has the highest impact with 72 kg CO2 per kg of hydrogen production, and the lowest impact was associated with AEC. It was proven that AEC technologies had the lowest environmental impact, among three selected technologies, in all categories.

Table 6 Environmental impact of three different water splitting technologies (Zhao et al., 2020)
Full size table

As life cycle assessment is an integral part of a comprehensive environmental analyse, this tool has been widely used to evaluate the environmental performance of a system. A life cycle assessment was performed on a PEM water electrolysis and compared to steam methane reforming to discuss the potential of these systems for greenhouse gas emissions mitigation in this field (Bareiß et al., 2019). For the wind based water electrolysis, the capacity of the system was considered 198 GW. It was suggested that renewable-based hydrogen production systems could contribute up to 75% in reducing CO2 emissions. As a case study for Germany, for each kg of hydrogen production 30 kg CO2 emissions was calculated considering the electricity mix of 2017. It was predicted that with electricity mix of 2050, this amount of CO2 emissions would be reduced to 12 kg.

Regarding solar and biomass energy based systems, a study focused on hydrogen as a transportation fuel in which they carried out thermo-economic and environmental analyses on hydrogen production through water electrolysis (Navas-Anguita et al., 2020). This study suggested the water electrolysis systems as well as biomass gasification technology as a key in medium and long terms. They calculated that in comparison to conventional fossil fuel-based hydrogen production, water electrolysis systems could lead to 36–58 Mt CO2 eq in 2050. Another study analyzed and compared two different solar energy based high-temperature steam electrolysis from energy and environment points of view (Yadav & Banerjee, 2020). In this study concentrated solar power and photovoltaic with annual capacity of 500 MW were compared and according to the outcomes, the carbon footprint of these technologies were respectively 1–1.7 kg CO2 and 1–1.8 kg CO2 per kg of hydrogen. Based on this study, the proposed system proved to be promising technologies with low carbon footprint since the solar based-thermochemical and solar based-AEC systems, and steam methane reforming had the carbon footprint of 4.3–4.5 kg CO2, 2–2.3 kg CO2, and 10.6 kg CO2 per kg of hydrogen.

One of the strengths of PEM water electrolysis technology is comparatively low investment costs. Although the costs of operation and maintenance of this technology is relatively high, the price of produced hydrogen would be lower than other technologies such as alkaline water electrolysis and steam methane reforming. They have also high safety level since there is no corrosive electrolyte and the carriers are in the membrane (Millet, 2015). Moreover, in high pressure PEM, there is a possibility of explosive gas formation followed by gas purity changes (Grigoriev et al., 2009). Based on life cycle analysis (LCA), PEM technology has relatively high global warming potential, ionizing radiation potential, ozone depletion potential and also fossil resource scarcity (Zhao et al., 2020).

High capital costs and consequently price of hydrogen produced by this technology are two weaknesses (Ball et al., 2015; Esmaeilion et al., 2022a, 2022b). However, relatively low operation costs of this technology is accounted as one of its strengths (Letcher et al., 2016). For other strengths of this technology, its well-developed technology could be mentioned. One of the threats of this technology is the corrosive liquid electrolyte (Calise et al., 2019). Based on LCA, alkaline water electrolysis method has relatively low global warming potential, ionizing radiation potential, ozone depletion potential and also fossil resource scarcity (Zhao et al., 2020).

7 SWOT investigation of hydrogen production in water splitting cycles

Providing comprehensive scopes to each process improves the perspectives of regarded technology (Esmaeilion et al., 2020). The SWOT (represents the parameters Strengths, Weaknesses, Opportunities, and Threats) analysis IS carried out to indicate these parameters. There are two sorts of establishments in the SWOT matrix.

  1. 1.

    Internal aspects which contain the internal factor for strengths and weaknesses of considered procedure.

  2. 2.

    External aspects which cover the opportunities and threats provided by the external factor of environmental conditions (Camacho et al., 2017).

For systematically evaluation of this type of hydrogen production in water splitting cycles, in terms of internal and external considerations, the SWOT study is carried out. SWOT analysis is an essential strategy to determine the Strengths, Weaknesses, Opportunities and Threats. This approach may possibly assess performances and reasonable situations, consequently assisting express practical development tactics, and to achieve the opportunities and resist threats. At this point, we investigate the strengths and weakness of regarded hydrogen generation approach and talk over the opportunities and threats it encounters in existing circumstances (Table 7). Such a study manifests the aforementioned promising outlooks and challenges, and specifies the auxiliary analysis orientation (Salkuyeh et al., 2017; Xu et al., 2019). Table 8 provides a SWOT-PEST matrix which is linked the Political, Economic, Social, and Technological (PEST) considerations of water splitting cycles to SWOT factors.

Table 7 The SWOT analysis matrix
Full size table
Table 8 SWOT-PEST matrix (Arslan & Er, 2008; Babatunde & Adebisi, 2012; Bahari et al., 2022; Dincer & Acar, 2015; Guban et al., 2020; Kumar et al., 2017)
Full size table

8 Conclusion

Hydrogen as a clean fuel has wide applications due to its attractive properties such as high energy density, and high HHV and LHV, in addition to its extensive availability. This work presented an extensive review on hydrogen production process powered by renewable energy sources such as solar and wind energy. The main processes discussed in this work were thermochemical, sonochemical, photochemical and electrochemical hydrogen production processes. In this study, SWOT and PEST analysis was implemented based on their limitation, environmental impact, cost, and energy and exergy efficiency. Energy, exergy, economy and environmental analyses for water splitting process were presented in detail to determine the most efficient hydrogen production process from efficiency, cost and environmental impact points of view. Based on this review the following major outcomes and were obtained, which are:

  • Biological hydrogen production process is environmentally friendly and has low energy cost due to its low operating temperature and pressure (ambient conditions).

  • Thermochemical hydrogen production process yields to high hydrogen purity because this process does not depend on the type of the catalyst used in the system. However, this process is not a cost competitive process compared to other hydrogen production processes.

  • Energy, exergy, economy and environmental analyses are effective tools in finding the most suitable hydrogen production process, due to the consideration of operating conditions, energy cost and environmental parameters.

  • Optimization performance of photoelectrodes in photochemical hydrogen production process is a necessary tool to increase the overall performance of the process.

  • Sonochemical hydrogen production process is an attractive process due to its low energy consumption.

  • Water pretreatment is necessary and should be applied in electrochemical hydrogen production process to avoid undesirable reactions that result in undesirable products which could lead higher cost of hydrogen production.

9 Future recommendations

Based on this review study presented in this paper, the following points could be considered for future studies:

  1. 1.

    Exploitation of renewable energy sources for water splitting technologies should be more discussed as they proved to have a reduction impact on environmental cost of a system in comparison to fossil fuel-based water splitting technologies.

  2. 2.

    It was proven that alkaline electrolysis cell technologies have one of the lowest environmental impact in categories such as global warming potential, fossil resource scarcity, ozone depletion potential and terrestrial acidification.

  3. 3.

    As with higher capacities of hydrogen production the cost of unit hydrogen production decreases, systems with higher capacities in the design stage should be considered.

  4. 4.

    Thermochemical hydrogen production should be integrated with a concentrated solar energy system to improve the cost of hydrogen production.

  5. 5.

    The ability of renewable energy sources utilization in water splitting cycles reduces consumption costs which can significantly increase the revenue from system outputs.

  6. 6.

    For better cost analysis, parameters such as fuel, land, labor, maintenance and construction costs should be considered in the analysis of hydrogen production processes.

  7. 7.

    Further development in terms of commercial competition for hydrogen is needed in industries and transportation sections.

Finally, this work provided a comprehensive review with discussions of the latest progress, challenges and future research and perspectives of different methods for hydrogen production.