Water electrolysis is a process in which water is split into hydrogen and oxygen through the use of electrical energy.

The total energy required for water electrolysis increases slightly with temperature, while the electrical energy required decreases.

A high-temperature electrolysis process may, therefore, be preferable in cases where high-temperature heat such as waste heat from other processes is available. This is particularly important globally, as most of the electricity generated worldwide is based on fossil fuels with relatively low levels of efficiency.

Future cost potentials for electrolytic hydrogen are shown in the following figure, whereby the possibilities for a considerable reduction in production costs are obvious.

Polymer electrolyte membrane electrolysis (PEM) is the electrolysis of water in a cell equipped with a solid polymer electrolyte (SPE), which is responsible for the conduction of protons, the separation of the product gases, and the electrical insulation of the electrodes. The PEM electrolyzer was introduced to overcome the partial load, low current density, and low-pressure operation problems that currently plague the alkaline electrolyzer. Electrolysis is an important new technology for the production of hydrogen, which is to be used as an energy carrier. With fast dynamic response times, large operating ranges, high efficiencies, and very high gas purities (99.99%), PEM electrolysis is a promising alternative for energy storage in connection with renewable energy sources.

One of the greatest advantages of PEM electrolysis is its ability to operate at high current densities. This can lead to lower operating costs, especially in systems that are coupled to very dynamic energy sources such as wind and sun, where sudden peaks in energy input would otherwise lead to the energy that is not captured.

The polymer electrolyte enables the PEM electrolyzer to work with a very thin membrane (~100-200ìm) and still allow high pressures, which leads to low ohmic losses, which are primarily caused by the conduction of protons across the membrane (0,1 S/cm) and compressed hydrogen emission.

The polymer electrolyte membrane has a low gas transfer rate due to its solid structure, resulting in the following results with a very high product gas purity. Maintaining a high level of gas purity is important for storage safety and for direct use in a fuel cell. The safety limits for H₂ in O₂ under standard conditions are 4 mol% H₂ in O₂.

The ability of the PEM electrolyzer to operate not only under highly dynamic conditions but also under part load and overload conditions is one of the reasons for the recently renewed interest in this technology. The requirements of an electrical grid are relatively stable and predictable, but when coupled with energy sources such as wind and sun, the grid’s needs rarely match the generation of renewable energy. This means that energy generated from renewable sources such as wind and sun must have a buffer or a way to store energy outside of peak load times.

High-temperature electrolysis is based on a technology used for high-temperature fuel cells.

The electrical energy required to split water at 1000°C  is much lower than that for electrolysis at 100°C. This means that a high-temperature electrolyzer can work with significantly higher overall process efficiencies than a normal low-temperature electrolyzer.

A typical technology is a solid oxide electrolysis cell (SOEC). This electrolyzer is based on the solid oxide fuel cell (SOFC), which normally operates at 700 to 1000°C.

At these temperatures, the electrode reactions are more reversible and the fuel cell reaction can be more easily converted into an electrolysis reaction.

Attempts are now being made to develop systems in which some of the electricity consumed by the electrolyzer can be replaced by heat available from geothermal, solar or natural gas sources, which can significantly reduce electricity consumption. Similar to the main challenges for SOFCs, the main research and development needs for SOECs relate to material development and the thermomechanical stress within the functional ceramic materials.

Photovoltaic (PV) systems coupled to electrolyzers are commercially available. The systems offer some flexibility as the output can be electricity from photovoltaic cells or hydrogen from the electrolyzer.

Direct photoelectrolysis is an advanced alternative to a PV electrolysis system by combining both processes in a single device.

Photoelectrolysis of water is the process in which light is used to split water directly into hydrogen and oxygen. Such systems offer great potential for reducing the cost of electrolytic hydrogen compared to traditional two-stage technologies.

Fundamental and applied R&D efforts in materials science and systems engineering for photoelectrochemical cells (PEC) are currently underway around the world, with at least 13 OECD countries running PEC-related R&D projects and/or entire programs. The IEA-HIA coordinates and manages a significant part of these R&D efforts in a collaborative, task-sharing annex.

In the past few years, various PEC devices have been developed on a laboratory scale, which so far has solar-hydrogen conversion efficiencies of up to 16%. The main challenges to bring PEC cell innovation to market relate to advancement in materials science and technology. Since no “ideal” photoelectrode material for water splitting exists commercially, tailor-made materials must be developed.

The high-temperature splitting of water occurs at around 3000°C. At this temperature, 10% of the water is decomposed and the remaining 90% can be recycled. To lower the temperature, other methods of high-temperature splitting of water have been proposed:

• Thermo-chemical cycles
• Hybrid systems that couple thermal decomposition and electrolytic decomposition.
• Direct catalytic decomposition of water with separation via a ceramic membrane (“thermophysical cycle”).
• Plasma-chemical decomposition of water in a two-stage CO₂ cycle

For these processes, efficiencies of over 50% are to be expected and could potentially lead to a considerable reduction in hydrogen production costs. The most important technical questions for these high-temperature processes relate to the material development for corrosion resistance at high temperatures, high-temperature membrane and separation processes, heat exchangers, and heat storage media, but design aspects and safety are also important in high-temperature processes.

Thermo-chemical water splitting is the conversion of water into hydrogen and oxygen by a  series of thermally driven chemical reactions. Thermo-chemical water-splitting cycles have been known for the past 35 years. They were extensively studied in the late 1970s and 1980s,  but have been of little interest in the past 10 years.

While there is no question about the technical feasibility and the potential for high efficiency, cycles with proven low cost and high efficiency have yet to be developed commercially.

An example of a thermo-chemical process is the Iodine/Sulphur cycle, outlined in equations (5.), (6.), (7.), and the figure below. For this process, the research and development needs are to capture the thermally split H₂, to avoid side reactions, and to eliminate the use of noxious substances.

The corrosion problems associated with the handling of such materials are likely to be extremely serious.

5. (850 °C):  H₂SO₄ SO₂ + H₂O + 1/2 O₂

6. (120 °C):  I₂ + SO₂ + 2 H₂O  H₂SO₄ + 2 HI

7. (450 °C):  2 HI  I₂ + H₂  SUM:  H₂O  H₂ + 1/2 O₂

Sulfur-Iodine Cycle

The industry is currently able to produce hydrogen from natural gas using three different chemical processes:

• Steam reforming (steam methane reforming – SMR)

• Partial Oxidation (POX)

• Autothermal Reforming (ATR)

Even though many new process concepts have been developed, none of them are close to commercialization!

POX: “Partial oxidation” of natural gas means that hydrogen is created by partially burning methane with oxygen to provide carbon monoxide and hydrogen: CH₄ + 1/2 O₂ CO + 2 H₂ + Heat.

In this process, heat is generated in an exothermic reaction and a more compact design is possible since no external heating of the reactor is required. The CO produced is further converted into H₂, as described in the above reaction.

SMR: “Steam reforming”, includes the endothermic conversion of methane and water vapor into hydrogen and carbon monoxide CH₄ + H₂O + Heat CO + 3 H₂.

The heat is often provided by burning some of the methane feed gas.

The process typically takes place at temperatures of 700 to 850 ° C and pressure between 3 to 25 bar. The gas produced contains around 12% CO, which can be further converted into CO₂ and H₂ by the water-gas shift reaction CO + H₂O ➔ CO₂ + H₂ + Heat.

ATR: “Auto-Thermo-Reforming” is a combination of steam reforming and partial oxidation.

The entire reaction is exothermic and releases heat. The outlet temperature from the reactor is in the range from 950 to 1100 ° C. and the gas pressure can be up to 100 bar. Here, too, the CO generated is converted into H₂ by the water-gas shift reaction. The need to purify the source gases adds significantly to plant costs and reduces overall efficiency.

Hydrogen can be produced from coal by a variety of gasification processes (e.g. fixed bed, fluidized bed or entrained flow). In practice, high temperature entrained flow processes are preferred in order to maximize the conversion of carbon to gas and thus avoid the formation of significant amounts of charcoal, tars, and phenols.

A typical reaction for the process is C (s) + H₂O + Heat CO + H₂, in which carbon is converted into carbon monoxide and hydrogen.

As this reaction is endothermic, additional heat is required as with methane reforming.

The CO is further converted into CO₂ and H₂ by the water-gas shift reaction. Hydrogen production from coal is commercially mature, but it is more complex than producing hydrogen from natural gas. The costs for the resulting hydrogen are also higher. However, since coal is abundant in many parts of the world and is likely to be used independently as a source of energy, it is worth studying the development of clean technologies for its use.

Alkaline electrolyzers use an aqueous KOH (lye) solution as the electrolyte, which normally circulates through the electrolytic cells. Alkaline electrolyzers are suitable for stationary applications and are available for operating pressures of up to 25 bar.

Alkaline electrolysis is a mature technology with a significant operating history in industrial applications that enables remote-controlled operation.

Commercial electrolyzers usually consist of a number of electrolysis cells that are arranged in a cell stack.

The greatest R&D challenge for the future is to design and manufacture electrolyzer equipment at a lower cost, with higher energy efficiency and greater shutdown rates.
Alkaline electrolyzers usually contain the following main components:

Carbon dioxide is a major exhaust in any production of hydrogen from fossil fuels. The amount of CO₂ varies depending on the hydrogen content of the starting material. In order to achieve sustainable (emission-free) production of hydrogen, the CO₂ should be captured and stored. This process is known as decarbonization. There are three (3) different ways to capture CO₂ in a combustion process:

1. the afterburning. The CO₂ can be removed from the exhaust gas of the combustion process in a conventional steam turbine or a combined cycle power plant (gas and steam turbine power plant). This can be done via the “amine” process, for example. In addition to water vapor, CO₂ and CO, the exhaust gas will contain large amounts of nitrogen and some amounts of nitrogen oxides.

2. Pre-combustion. During the production of hydrogen, CO₂ is captured using one of the processes described above.

3. Oxyfuel combustion. The fossil fuel is converted into heat in a combustion process using a conventional steam turbine or CCGT power plant. This is done with pure oxygen as the oxidizing agent. Usually, CO₂ and water vapor are formed in the exhaust gas or in the flue gases, and CO₂ can be easily separated by condensation of the water vapor.

In post-combustion and oxyfuel combustion systems, electricity is turned into near-conventional steam and combined cycle power plants. The electricity generated could then be used for water electrolysis.

If CO₂ is captured and stored in an energy conversion process with relatively low efficiency and the electricity is used to electrolyze water, then the overall efficiency from fuel to hydrogen would not exceed 30%.

The captured CO₂ can be found in geological formations such as oil and gas fields as well as in aquifers, but the feasibility and evidence of permanent CO₂ storage are critical to the success of decarbonization.

The choice of the transport system for the CO₂ (pipeline, ship or combined) will largely depend on the choice of the location of the production facility and the location chosen for storage.

In biomass conversion processes, a hydrogen-containing gas is normally produced, similar to the gasification of coal. However, there are no commercial plants for the production of hydrogen from biomass.

Steam gasification (direct or indirect), entrained flow gasification, and more advanced concepts such as gasification in supercritical water, the application of thermo-chemical cycles or the conversion of intermediate products (e.g. ethanol, bio-oil or torrefied wood) are currently being pursued. None of the concepts has reached a demonstration phase for hydrogen production.

The gasification of biomass is a research and development area that shares H₂ production and the production of biofuels.

Gasification and pyrolysis are considered to be the most promising medium-term technologies for commercializing H₂ production from biomass.

A typical flow diagram for the production of hydrogen from biomass is shown in the figure below.

In terms of its energy requirements, the drying of biomass may not be justifiable; therefore, other approaches based on moist biomass are also being sought.

Biomass raw materials are unrefined products with inconsistent quality and poor quality control.

The production methods vary depending on the type of plant, location, and climatic fluctuations. Irregular fuels have contributed to the difficulties in technological innovation, as less homogeneous and poor quality fuels require more sophisticated conversion systems. There is a need to streamline the production and processing of fuels in order to produce more consistent, higher-quality fuels that can be described by common standards. Large plants are more suitable for cheaper and inferior fuels, while smaller plants tend to require higher fuel quality and better fuel homogeneity.

Necessary development:

• Feed preparation and identification of the properties of the raw materials that allow the development of the technologies.

• Gasification of biomass. This is not specific to hydrogen but relates to general pathways and research in biomass and renewable energy.

• Handling and cleaning of raw gas.

• Interface issues and system integration. One should also examine the relationship between the scale of production and the fuel quality requirements and tolerances that can be considered for each technology.