Keywords

1 Introduction

Hydrogen is a multi-aspect chemical species. It is an important raw material in various industries and can work as a fuel. With regard to the latter aspect, hydrogen is a secondary energy, and, accordingly, hydrogen production is a process that converts a primary energy to the chemical energy of hydrogen. Fossil fuel is currently a main primary energy for the production of hydrogen. Hydrogen is produced by reforming hydrocarbons and is generated as a by-product in steel, soda, petroleum refining, and other industries. Renewable energy, such as sunlight, wind, and biomass, can be used as primary energy.

Hydrogen production should be conducted using suitable primary energy with respect to the social background, such as the severity of CO2 emission, primary energy cost, and others. Hydrogen fuel cell electric vehicles (FCEVs) have recently been put on public sale in Japan. Toyota’s FCEV “Mirai” started sales in December 2014 [1], and Honda announced the release of a new FCEV in 2015 [2]. Hydrogen has increased in importance as energy. In this chapter, hydrogen production methods for demand as a fuel and/or energy medium are addressed.

2 Present Status

2.1 Current Hydrogen Usage and Production Methods

Hydrogen is mainly used as a raw material in industries, including petroleum refining, synthesis of ammonia or other chemicals, metal and ceramics, food, semiconductor production, and others. Hydrogen is mostly produced by reforming fossil fuels or as by-products. Water electrolysis is also a production method. The annual amount of hydrogen produced in Japan is 15–20 × 109 Nm3 [3, 4]. Most of the hydrogen is consumed captively, i.e., the consumers produce it.

2.1.1 Reforming

Hydrogen is produced by reforming natural gas, naphtha, and other hydrocarbon-based fuels. Steam reforming and partial oxidation are the two major chemical reactions. These reactions can be written as follows:

$$ {\mathrm{C}}_m{\mathrm{H}}_{2n}+m{\mathrm{H}}_2\mathrm{O}\to m\mathrm{C}\mathrm{O}+\left(m+n\right){\mathrm{H}}_2\left(\mathrm{steam}\kern0.5em \mathrm{reforming}\right) $$
(1)
$$ {\mathrm{C}}_m{\mathrm{H}}_{2n}+0.5m{\mathrm{O}}_2\to m\mathrm{C}\mathrm{O}+n{\mathrm{H}}_2\left(\mathrm{partial}\ \mathrm{oxidation}\right) $$
(2)

Steam reforming is endothermic and partial oxidation is exothermic. In both cases, generated CO can be used to produce more hydrogen as

$$ \mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{CO}}_2+{\mathrm{H}}_2\left(\mathrm{water}\ \mathrm{gas}\ \mathrm{shift}\ \mathrm{reaction}\right) $$
(3)

Steam reforming of methane proceeds spontaneously at temperatures >700 °C and is done industrially at 850–950 °C and pressures 1.5–3 MPa. Because the reaction is endothermic, there is a need for heating reaction tubes, determining the upper limit of operation temperature. Partial oxidation has large negative free energy change and can thus proceed non-catalytically. Higher reaction temperatures (1300–1500 °C) and pressures (4–8 MPa) can be used industrially, enabling use of heavy hydrocarbons such as heavy oil and coal. Lower temperatures are favored by the water-gas shift reaction, and 400–600 °C are typical reaction temperatures. Steam reforming and partial oxidation can be mixed in an appropriate ratio for thermal balance, called the autothermal method.

In these reforming processes, hydrogen is produced as a mixture with CO2, H2O, and other impurities, and purification is necessary, usually done with pressure swing adsorption (PSA).

Petroleum refining consumes hydrogen in transforming heavier hydrocarbons to lighter species and in desulfurization. Hydrogen is produced by steam reforming of naphtha and LPG. Ammonium synthesis consumes hydrogen, which is typically produced by steam reforming of natural gas.

2.1.2 By-Product Hydrogen

Hydrogen is generated as a by-product in industrial processes, including sodium hydroxide production , reduction of iron ores to produce steel, petroleum chemistry, and others. In many cases, the by-product hydrogen is consumed captively as fuel and as a chemical raw material, and so the use as fuel other than captive consumption requires an alternative energy resource to replace current usage of such hydrogen.

In petroleum refineries , hydrogen is generated during dehydrogenation of alkanes to produce aromatics. However, more hydrogen is needed and is produced by reforming petroleum-based hydrocarbons. In steelmaking, carbon made from coal, i.e., cokes, is used as a reducing agent of iron ore, and gas from the coke oven contains substantial hydrogen (~55 %). About 16 × 109 Nm3 of coke oven gas (COG) is produced annually. Large amounts of COG are used for electricity generation or as heat sources and are not used in the form of hydrogen [4]. In the soda industry, NaOH is produced by electrolysis of NaCl aqueous solution and hydrogen, and chlorine is produced as a by-product. Annually, ~1.1 × 109 Nm3 of hydrogen is generated, accompanied by 4 × 106 tons of NaOH [4]. This hydrogen has high purity and is commercially supplied to other companies.

2.1.3 Electrolysis

Alkaline water electrolysis and polymer electrolyte water electrolysis are the commercial methods. In Japan, the former was used in the past in hydrogen production for ammonia synthesis and is thus technically well established. However, the hydrogen production method for ammonia synthesis has been replaced by cheaper steam reforming of natural gas or other hydrocarbons.

2.2 Hydrogen Production Capability of Current Facilities

In steel production, the total amount of hydrogen contained in generated COG is 9 × 109 Nm3/year in Japan. Only about a tenth is used as hydrogen gas [4], and the remaining 8 × 109 Nm3/year can be used as fuel. In addition, COG contains methane and CO that can be reformed to produce another 17 × 109 Nm3/year of hydrogen. If we assume 60 % recovery of hydrogen through purification by PSA, in principle, 5 × 109 Nm3/year and 10 × 109 Nm3/year of hydrogen can be produced, respectively. At present, COG is not a surplus but is used as fuel for thermal power and other heat sources. Therefore, COG can be considered economically as a hydrogen source only when cheaper fuel is available to substitute COG utilization.

COG is a product of coal through coke production, which is a CO2-emiting process and must be accommodated by CO2 separation to avoid such emission. As mentioned in Sect. 2.1.2, about 1.1 × 109 Nm3 of hydrogen is generated as a by-product in the soda industry with high purity and can be used as a hydrogen source. Petroleum refineries are equipped with hydrogen production facilities that are capable of excess hydrogen production (i.e., planned hydrogen production capability minus actual hydrogen consumption) using steam reforming of naphtha and LPG, owing to the closing of oil refineries and suspension of ethylene production facilities [5]. The estimated capability is as much as 4–6 × 109 Nm3/year [3, 4]. Steam reforming of natural gas can be used to produce hydrogen, as explained in Sect. 2.1.1. The Mizuho Information and Research Institute estimates that hydrogen that can be newly produced is 7–8 × 109 Nm3/year [4].

The amounts of hydrogen that can be generated by the methods discussed above are summarized in Table 1. Given the hydrogen demand assumed later in Table 6, the hydrogen need of FCEVs would be 13.5 × 109 Nm3/year even in 2050 and is affordable because of the by-product steam-reformed hydrogen shown in Table 1. These conventional hydrogen production methods can address the assumption of FCEV hydrogen demand. However, this production is accompanied by CO2 emission, because fossil fuel is the energy source. For reduction of this emission, hydrogen production should shift to non- or low-carbon hydrogen in the future, and related production techniques are explained in the following sections.

Table 1 Hydrogen production/supply capability of by-product hydrogen and steam reforming
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2.3 Comparison of Hydrogen Production Methods

To compare energy efficiency, CO2 emissions, and costs, the following energy pathways from natural gas, crude oil, coal, biomass, and electricity as primary energy for vehicle fuel tanks are considered for hydrogen production. Energy efficiency is defined as process efficiency of hydrogen production. CO2 emissions and costs should include those associated with all processes from the well to hydrogen production. Hydrogen purification is not considered, because the required hydrogen purity varies with use. Hydrogen compression and filling are also not included in Fig. 1.

Fig. 1
figure 1

Energy paths of assumed hydrogen production for comparing efficiency, CO2 emissions, and costs

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2.3.1 Energy Efficiency

Energy efficiency of steam reforming is defined as (hydrogen energy)/[(feedstock energy)+(input energy)] and that of water electrolysis as (hydrogen energy)/[(electricity)+(input energy)]. Values were collected from reference data of the Mizuho Information and Research Institute [6], US Department of Energy Hydrogen Analysis Project (H2A) [7], and Japan Hydrogen and Fuel Cell Demonstration Project (JHFC) [8]. Largest and smallest mean values are summarized in Fig. 2. Data in reference [8] include those under research, and these are included in the figure. In steam reforming, energy efficiencies are higher for reforming of methanol, dimethyl ether (DME), and natural gas than those for reforming of others. Efficiency is lowest for kerosene and Fischer-Tropsch (FT) synthetic fuel. The energy efficiency of alkaline and proton exchange membrane (PEM) water electrolysis is comparable to that of steam reforming.

Fig. 2
figure 2

Efficiency of hydrogen production from various fuel-reforming processes and electrolysis

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2.3.2 CO2 Emissions

CO2 emissions are defined as the total amount of CO2 emitted in feedstock production, transportation, and hydrogen production. Greenhouse gases (GHG) other than CO2 are also taken into account as CO2 equivalent values. The Institute of Energy Economics Japan [9], Mizuho Information and Research Institute [6], H2A [7], and JHFC [8] have estimated CO2 emissions with different feedstock, and these are summarized in Figs. 3 and 4. Data in reference [8] include those under research and these are included in the figure. Reference [7] deals with the situation in the USA, but its data are used in the figures because the percentage of CO2 emissions in feedstock production and transportation of the total are not significantly different from those in the other references dealing with Japan (e.g., in natural gas reforming, CO2 emissions from mining, and transportation are 12 % in the USA [4] and 14 % in Japan [5]). For water electrolysis, CO2 emissions for electricity production are included, which is assumed to be conducted in Japan even using the data of H2A [7]. CO2 emissions of construction and maintenance equipment are not considered.

Fig. 3
figure 3

CO2 emissions for hydrogen production from various feedstocks by steam reforming

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Fig. 4
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CO2 emission from electricity for unit water electrolysis, derived from various fuels/primary energy sources

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2.3.2.1 Steam Reforming

CO2 emissions for hydrogen production from various feedstocks are shown in Fig. 3. Autothermal reforming is not included. The breakdown of total CO2 emission, such as natural gas reforming, is ~10 % from material production and transportation, 60 % from derivation from fuel, and 30 % from operation of hydrogen production equipment. In steam reforming, except in the case of biomass, CO2 emitted from natural gas reforming is the least, and that from reforming of coal is the greatest. Emissions from biomass are less than from other fuels and are only 30 % of those from natural gas. Challenges include efficiency improvement in hydrogen production, increased biomass use, and technology development in CO2 capture and storage for further emissions reduction.

2.3.2.2 Water Electrolysis

CO2 emissions from water electrolysis using electricity derived from various fuels/primary energy sources are shown in Fig. 4. For J-Mix, the Japanese average electricity mix in 2013 reported in Electricity Generation by Source (2014) [10] is assumed to consider the effect of cessation of nuclear power plants. The breakdown of CO2 emissions, such as alkaline water electrolysis derived from natural gas, is ~60 % from electricity production from natural gas, 10 % from derivation from fuel, and 30 % from operation of hydrogen production equipment. In water electrolysis, CO2 emission is slight compared with steam reforming if electricity from renewable energy is used.

2.3.3 Costs

In the present study, cost is based on hydrogen production at 70 MPa onsite and a 300 Nm3/h direct-filling commercial hydrogen station operated by JHFC [11]. Tables 2, 3, and 4 show presumed hydrogen production conditions. Costs related to hydrogen compression and filling are not included.

Table 2 Presumed conditions
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Table 3 Variable costs
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Table 4 Fixed cost
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Table 5 compares estimated costs for hydrogen production via steam reforming and alkaline water electrolysis with various operation rates. Hydrogen produced via water electrolysis is more expensive than via steam reforming. For this electrolysis, percentages of electricity versus total costs are 72 % at 100 % operation rate and 61 % at 60 % operation rate. The price of the electricity is a key factor determining the cost of this electrolysis and is the reason for its higher cost of hydrogen production relative to steam reforming.

Table 5 Estimation of cost for hydrogen production via steam reforming and electrolysis
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3 Technology Road Map

3.1 Perspectives on Future Hydrogen Demand in Transportation and Large-Scale Energy Conversion Sectors

Use of hydrogen as energy will increase in the future, with FCEVs as an example. FCEVs have recently been introduced in Japan. Assuming that an FCEV is driven 12,000 km/year (an estimate from ordinary cars in the country) and consumes 1 kg of hydrogen per 100 km distance driven [13], the amount of hydrogen is estimated at 1.35 × 109 Nm3/year/million-FCEVs. The Fuel Cell Commercialization Conference of Japan (FCCJ) published a scenario for the commercialization of FCEVs [14]. The scenario specifies two million FCEVs and construction of ~1000 hydrogen stations by 2025. Table 6 shows an estimation of hydrogen demand as fuel for FCEVs by 2020, 2030, and 2050. The numbers of FCEVs are estimated from the FCCJ scenario.

Table 6 Estimated hydrogen demand in 2020, 2030, and 2050
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Another use of hydrogen is for electricity generation. Economic efficiency may be a problem, however. Hydrogen is considered a potential option for use as a fuel for thermal power plants in the form of either a mixture with other fuels or pure hydrogen [3, 4]. There is an estimate that hydrogen demand will increase by another 22 × 109 Nm3/year by 2030, assuming that all newly built thermal power plants adopt a fuel mixture containing 50 % hydrogen [4]. If this is realized, such hydrogen demand for thermal power would exceed the demand of FCEVs (Table 6). However, hydrogen use by thermal power plants results in the reduction of CO2 emission, so hydrogen should be produced from low-carbon energy sources or accompanied by carbon capture and sequestration (CCS) .

3.2 Hydrogen Production Methods Under Development

3.2.1 Membrane Reactor Reformer

For technical advancement of reforming, a membrane reformer equipped with a palladium-based alloy membrane is an advanced method for steam reforming of methane (Fig. 5) [15]. The aim is to conduct steam-reforming reactions and hydrogen separation processes simultaneously, without shift converters and purification systems. This results in greater energy efficiency (~80 %) and more compact equipment size than those of conventional technologies and is in the validation phase.

Fig. 5
figure 5

Principle of a hydrogen separation reformer [15]

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3.2.2 Biomass

Biomass , i.e., wood from forestry, arboricultural activities or from wood processing, agricultural residues, food waste, industrial waste and coproducts from manufacturing and industrial processes, and sewage sludge, can be used as an energy source for hydrogen production. Figure 3 shows that hydrogen derived from biomass has low CO2 emissions relative to fossil fuel-based hydrogen. In the case of woody biomass, autothermal reforming is used in hydrogen production. Anaerobic fermentation is conducted to produce methane followed by steam reforming.

There is an estimate for the domestic hydrogen production potential of wood, livestock waste, and agricultural residue that indicates ~1300 hydrogen stations can be operated based on these types of biomass across Japan [16]. Assuming 1000 FCEVs per hydrogen station, the estimated number of stations could manage 1.3 million FCEVs. Dispersion of locations and small amounts of generated biomass per location are the major drawbacks of this biomass. Accordingly, mass production of hydrogen from biomass corrected for a wide area will increase cost because of transportation of the biomass feedstock. Onsite hydrogen production from sewage sludge for hydrogen stations will be an effective use of biomass. The Ministry of Land, Infrastructure, Transport and Tourism estimates a hydrogen production potential ~0.13 × 109 Nm3/year [17]. There is an ongoing validation plant of hydrogen production from sewage sludge in the city of Fukuoka [13], which is an effective means of supply to hydrogen stations by locating stations near sewage treatment plants; the estimated hydrogen costs are 80–85 yen/Nm3.

3.2.3 Thermochemical Water Splitting

One-step thermodynamic water splitting needs an extremely high temperature. The same molar amounts of steam, hydrogen, and oxygen are chemically in equilibrium at ~4100 °C [18]. However, the sequence of multiple chemical reactions combined with hydrogen separation enables complete water splitting at much lower temperatures [19, 20]. The iodine-sulfur (IS) process is one example and works when 900 °C heat is available (Fig. 6) [20, 21]. The Japan Atomic Energy Agency (JAEA) is verifying a combined system of the IS process and a high-temperature gas-cooled reactor (HTGR) , which is a nuclear reactor using helium as the coolant [21]. A test plant with hydrogen production capability 200 NL/h has been constructed and tested. In Nuclear Energy Vision 2100 proposed by the JAEA, hydrogen produced by an HTGR will be supplied to industries from around 2040 [22]. An HTGR with thermal output of 600 MW can produce 0.6 × 109 Nm3/year of hydrogen.

Fig. 6
figure 6

Iodine-sulfur process. The full process is divided into three sections, Bunsen, H2SO4, and HI [21]. \( {\mathrm{SO}}_2 + {\mathrm{I}}_2+2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2{\mathrm{SO}}_4+2\mathrm{H}\mathrm{I} \) (Bunsen reaction). \( {\mathrm{H}}_2{\mathrm{SO}}_4\to {\mathrm{H}}_2\mathrm{O}+{\mathrm{SO}}_2+{0.50}_2 \) (sulfuric acid decomposition). \( 2\mathrm{H}\mathrm{I}\to {\mathrm{H}}_2 + {\mathrm{I}}_2 \) (hydrogen iodide decomposition)

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3.2.4 Water/Steam Electrolysis

Comparing CO2 emissions from various hydrogen production methods (Fig. 4), water electrolysis can produce low-carbon hydrogen when low-carbon electricity is used. Therefore, water electrolysis based on low-carbon or renewable electricity is an important direction for future hydrogen production. A power-to-gas project in Germany is based on such a concept, using excess electricity for the production of hydrogen and methane [23]. In this case, the operation rate cannot be sufficiently high to affect the hydrogen production cost, because electrolyzers function only on sunny or windy days. In the case of Japan, as mentioned in Sect. 2.3.3, the price of electricity is the most significant in hydrogen production cost via water electrolysis; 61–72 % of that cost is from electricity for operation rates of 60–100 %. However, facility cost is another important contributor at 21 % (operation rate, 100 %) and 30 % (operation rate, 60 %). Further, the price of hydrogen varies with the operation rate (Table 5).

Electrolysis is an electrochemical water-splitting process and occurs on inputting electric energy greater than Gibbs free energy of water formation (Δf G). Either liquid water or steam can be electrolyzed depending on the operation temperature. Figure 7 shows a thermodynamic breakdown of the energy for electrolysis conducted at 25 °C and 600 °C, in the cases of liquid water and steam electrolysis, respectively [24]. Alkali water electrolysis and polymer electrolyte water electrolysis are the two technically established methods. These typically operate at around 2 V, corresponding to 75 % energy efficiency (based on high heat value, HHV). There is a NEDO research project for hydrogen utility that has targeted electrolysis voltage 1.8 V (at 0.6 A/cm2) to improve efficiency and thereby reduce hydrogen production cost [25]. As suggested by Fig. 7, steam electrolysis will need less electrolysis voltage. The NEDO project targets 1.6 V for steam electrolysis [25]. This type of effort should make water/steam electrolysis a practical hydrogen production method in combination with renewable energy.

Fig. 7
figure 7

Comparison of thermodynamics of water/steam electrolysis operated at 25 °C and 600 °C [20]. Minimum required electrolysis voltage is Δf G/2 F, where F is Faraday’s constant and is a function of temperature, i.e., 1.23 V at 25 °C and 1.02 V at 600 °C

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The cost of hydrogen is estimated in Table 5. This cost is primarily a function of the price of electricity and energy efficiency of electrolysis. Therefore, electrolysis will become effective via cost reduction of renewable electricity, and improved energy efficiency of photovoltaic cells is crucial.

3.2.5 Photoelectrochemical Water Splitting

Water splitting can occur on the surface of a semiconductor in contact with water with the incidence of light when the semiconducting material is appropriately selected. This process is similar to photosynthesis that uses light to produce a chemical, i.e., sugar in natural photosynthesis. Since the discovery of photoelectrochemical water-splitting catalysis in anatase TiO2 [26], extensive research has been conducted. The advantage of photoelectrochemical water splitting is its simplicity. The action of a combination of photovoltaic cells and water electrolyzers can be conducted simultaneously with only water, a catalyst, and sunlight. The drawback is energy efficiency; state-of-the-art efficiency is currently 2 % [27]. Such a low efficiency necessitates large areas of land.

3.2.6 Summary of Hydrogen Production Methods Under Development

The hydrogen production methods addressed in Sect. 3.2 are summarized in Table 7, with an assumed capability of hydrogen production. The most important feature is that they are low-carbon hydrogen production methods. This feature is of primary importance for the sustainability of daily life and is discussed in the next section.

Table 7 Low-carbon hydrogen production methods
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3.3 Road Map

A road map for hydrogen production is summarized in Fig. 8 and is discussed here.

An assumed technological goal for hydrogen production is a supply of hydrogen meeting the demand of FCEVs and other hydrogen energy devices. Reduction of CO2 (GHG) emissions is mandatory for the sustainability of society, so primary energy for the hydrogen production should be shifted from fossil fuel to renewable energy. The right-hand column in the roadmap chart (Fig. 8) explains social status relevant to hydrogen energy. At present, hydrogen energy is viewed from the perspective of energy saving and reduction of CO2 emissions. Hydrogen-fueled FCEVs have been commercialized since 2014 [1] in Japan, and the number of FCEVs is assumed to increase as mentioned in Sect. 3.1. If we assume ten million FCEVs by 2050, this number will roughly correspond to 15–20 % of total automobiles nationwide. Accordingly, around 5000 hydrogen stations should be operated, assuming an average capacity of 2000 FCEVs per hydrogen station. When the above technological goal is fully achieved, hydrogen will be used in society as an energy medium for transportation and other potential hydrogen devices.

Based on the assumed social situation above, increasing demand of hydrogen as fuel can be defined and is not very different from the assumption of future hydrogen demand outlined in Sect. 3.1 and Table 6. That is, hydrogen is currently produced at 15–20 × 109 Nm3/year, and this will need to increase by 100–200 % by 2050 for hydrogen fuel, as mentioned in Sect. 2.1. As addressed in Sect. 2.2 and Table 1, hydrogen needed by FCEVs can be supplied by the conventional types even in 2050, i.e., steam reforming and by-product hydrogen, as long as the supply of fossil fuel is sufficient. However, these conventional hydrogen production methods are fossil fuel-based and accompanied by CO2 emissions. To reach the technical goal for hydrogen production shown in Fig. 8 and mentioned above, development of production methods of low-carbon hydrogen discussed in Sect. 3.2 is important for reducing CO2 emissions. In the next 20–30 years, hydrogen may be used in electricity generation in a fuel mixture with natural gas for thermal power plants. In this case, the need for hydrogen is greater than that of FCEVs, and the hydrogen should be low carbon as discussed in Sect. 3.1. Steam reforming of brown coal together with CCS and water electrolysis combined with solar or other renewable electricity can be assumed as overseas hydrogen production schemes [3, 4, 28, 29].

According to the above discussion on the quantity and quality of hydrogen necessary as a future fuel, substantial progress in methods producing low-carbon or CO2-free hydrogen is mandatory. Hydrogen production methods of this kind with the potential capability of hydrogen production were explained in Sect. 3.2 and Table 7. The statuses of these techniques are different, as described in the following.

Biomass is currently in the verification phase. As mentioned in Sect.. 3.2.2 and shown in Table 7, a hydrogen production capability of 1.9 × 109 Nm3/year is predictable, corresponding to 1.4 million FCEVs (14 % to the total expected number of FCEVs in 2050). As thermochemical hydrogen production, the IS process with HTGR at a small-scale hydrogen production facility (~200 NL/h) is operating for technology verification, as mentioned in Sect. 3.2.3. An HTGR with thermal output 600 MW can produce 0.6 × 109 Nm3/year of hydrogen (corresponding to 0.44 million FCEVs), and its utilization from around 2040 is proposed [22].

Water electrolysis (alkaline and PEM) is technically nearly established, and improvement of energy efficiency is being studied [25]. Steam electrolysis and photoelectrochemical water splitting are under research, but could move into the verification stage in the next 10 years. Water/steam electrolysis (Sect. 3.2.4) is the only technique that produces hydrogen from electricity, and will be important in hydrogen production methods using renewable energy, because the main form of energy available from that energy is electricity. Photoelectrochemical water splitting is more beneficial, because it directly produces hydrogen from sunlight (Sect. 3.2.5). For practical use, however, the energy efficiency should be increased from the current state-of-the-art 2 % [27] to ~20 % or higher, comparable to the combination of photovoltaic cells and water electrolysis, which make up ~21 % of hydrogen production. State-of-the-art commercial energy efficiencies of these two technologies are 25 % [30] and 87 % [31], respectively.

Future expansion of these low-carbon hydrogen production methods will reduce CO2 emissions and is the technological goal of hydrogen production.

4 Benefit and Future Vision

A hydrogen energy society using hydrogen as an energy medium has the following benefits:

  1. 1.

    Energy saving accompanied by reduction of CO2 emissions

  2. 2.

    Facilitating a CO2-free energy system and substantial reduction of CO2 emissions by producing hydrogen from renewable energy

  3. 3.

    Improvement of energy security via multiple choices of primary energy

FCEV is a good example of benefit (1). Even when hydrogen fuel is produced conventionally from fossil fuels, well-to-wheel CO2 emission can be roughly 0.5–0.6 that of a conventional internal-combustion engine vehicle (based on representative values, comparing a FCEV fueled with hydrogen produced by onsite reforming of town gas and a gasoline or diesel internal-combustion engine vehicle) [3]. Benefit (2) is more important to dramatically decrease CO2 emission over its present status. CCS is one method to reduce CO2 emission, but the use of large portions of renewable energy is essential for the sustainability of society. In contrast, hydrogen is an energy medium suitable for the use of renewable energy, which can be enhanced by the spread of hydrogen energy. This characteristic can stabilize the energy situation in Japan, which has long suffered from poor energy resources. We can produce hydrogen within Japan in the future via biomass, water electrolysis with renewable electricity, and photoelectrochemical water splitting, which are advantageous to fossil fuel-based hydrogen from an energy security standpoint.

Such low-carbon techniques of hydrogen production are in various stages toward practical applications and are currently studied and developed both scientifically and technologically. We can even assume the emergence of novel techniques that are presently unknown. As mentioned above, progress of these techniques is required to realize a hydrogen energy society. Resources should be spent on their research and development to realize the benefits of hydrogen energy.

Fig. 8
figure 8

Roadmap of hydrogen production

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