5. Processes

This chapter introduces the various processes that play a role in hydrogen production, storage, distribution, and use in some detail, and examines the remaining challenges that those that develop and apply those processes will need to overcome with a view to ensuring growth in the sector.

Overview

The current focus of process development is aimed at optimizing the underlying technologies with a focus on cost reduction. Key areas of development include increased operational flexibility by improving ramp-up rates, start times, and stand-by energy use. There is now a strong emphasis on the modularity of electrolyzers and the respective flexibility of power generation capacity. On the generation side, production cost, capital expenditure, scalability, and lifetime extension are major considerations affecting the direction of the market and technological development.

Areas of innovation currently include:

Cells: Catalyst, Electrolyte, Electrodes, Membranes

Stacks: Bipolar Plates, Sealing

Systems: Balance-of-Plant, Operation, New Set-up/Chemistry

Manufacturing: Automation, Design, Experience, Method, Scale

Supply Chain: Volume, Competition

Most innovations currently can be found in cells and manufacturing techniques.

Electrolysis

Alkaline electrolysis, the oldest and most straightforward of the electrolyzer technologies, has been in active use since around 1921. Alkaline electrolyzers are the least expensive, most tested, and (for the moment) most efficient solution when compared to the other commercial electrolysis technologies. These electrolyzers introduce an electric current through water containing an alkaline chemical catalyst, typically potassium hydroxide, to produce hydrogen and oxygen.  

AE systems are readily available, long-lasting, and require relatively low capital expenditure due to the lack of noble metals and relatively mature cell stack components. However, their low current density and operating pressure limit system size and increase hydrogen production costs. They are not well suited for dynamic operation (frequent start-ups and varying power input), which can negatively affect system efficiency and gas purity. 

AE systems typically use nickel-molybdenum alloys as cathodes and nickel-cobalt alloys as anodes. The electrolytic cells of such systems tend to have a current density of 0.2-0.4 Amperes per cm² at cell voltages between 1.8 and 2.4 Volt. Average voltage efficiency lies between 60-82 percent of higher heating value (HHV). 

Capital costs for AE systems (referenced for 10 MWel) in 2020 lie between €800 and €1,300 per kWel. Experts estimate costs to fall to around €750 per kWel by 2030.  

AE systems typically have an estimated lifetime of 41,000 to 90,000 hours (around 20 years), with little improvement expected by 2030. 

Another technology gaining in importance is PEM electrolysis, based on specialized polymer membrane materials such as Nafion that can pass through protons. The membrane separates the produced oxygen and hydrogen, allowing higher pressures to develop without dangerous mixing of hydrogen and oxygen within the cell.  

PEM systems are based on the solid polymer electrolyte (SPE) concept for water electrolysis that was first introduced in the 1960s by General Electric. It aims to overcome some of the drawbacks of AE devices. The technology is therefore less mature than AE and mostly used for small-scale applications.  

Key advantages include high power density and cell efficiency, the provision of highly compressed and pure hydrogen, and flexible operation. Disadvantages include expensive platinum catalysts and fluorinated membrane materials, higher complexity due to high pressure levels during operation, and strict water purity requirements. They also have shorter lifetimes than AE systems. Development efforts are targeted at reducing the inherent system complexity to enable system scale-up, and thus bringing down capital costs through use of less expensive materials and more sophisticated stack manufacturing processes. 

The technology is virtually identical to PEM fuel cells that produce electricity from hydrogen and oxygen – the process is reversible. Fuel cells are the power source for most hydrogen-fueled vehicles, and the association with fuel cells makes PEM a particular target of research and development efforts. 

PEM systems typically use palladium alloys as cathodes and ruthenium oxide alloys as anodes. Cells tend to exhibit a current density of 0.6-2.0 Amperes per cm² at cell voltages between 1.8 and 2.2 Volt. Average voltage efficiency lies at between 67 and 82 percent of HHV. 

Capital cost estimates (referenced for 10 MWel) for PEM range between €1,000 and €1,950 per kWel. Indications are that this will fall to €850 to €1,650 per kWel by 2030.  

Typical system lifetimes for PEM systems are estimated at 41,000 to 60,000 hours. Lifetimes in line with AE systems are expected by 2030. 

Still in the research and development phase, solid oxide electrolysis will play a vital role in a developing low carbon energy economy. Both AE and PEM electrolysis supply the energy needed to split water molecules solely from electric power. SOE relies on a combination of electricity and heat, typically using yttria-stabilized zirconia (YSZ) as the electrolyte. 

There are significant advantages to using heat as the primary energy source. Heat is generally less expensive to create and store than electricity. We already see surpluses of electric power from solar and wind that tend to be challenging to utilize and expensive to store. If that energy could be stored temporarily as heat, it can be used at a more constant rate to create hydrogen. As such, it could represent a reasonably affordable way to store and make use of renewable energy that might otherwise be curtailed. 

Another potential advantage of SOE is the ability to produce either hydrogen gas or a mixture of hydrogen and carbon monoxide with the addition of a carbon dioxide feedstock. The mixed gas can, in turn, be used to synthesize methane or other hydrocarbon transportation fuels. There is much optimism that SOE will proceed to full commercialization and maturity in the next decade. 

A significant challenge still being addressed is the severe material degradation that results from the high operating temperatures used. Current research is focused on stabilizing existing component materials, developing new materials, and lowering the operating temperature to 500-700 ºC (from around 650-1,000 ºC today) to speed up the commercialization of SOE systems. 

SOE systems typically use nickel coated in YSZ as cathodes, and lanthanum strontium manganite (LSM) as anodes. Cells tend to exhibit a current density of 0.3-2.0 Amperes per cm² at cell voltages between 0.7 and 1.5 Volt. Average voltage efficiency lies at 110 percent of HHV. 

Current capital cost estimates for SOE systems (referenced for 10 MWel) lie between €3,000 and €5,000 per kWel, but could experience the most substantial relative cost reduction by 2030 with cost ranges between €1,050 and €4,250 per kWel. 

System lifetimes for SOE systems are estimated at 6,000-15,000 hours, reflecting the relative immaturity of the technology. Some experts feel lifecycles of up to 50,000 or even 100,000 hours should be possible in the near term as research and commercial experience accrue. 

Hydrogen can be produced from biomass in various ways. While CO₂ and sometimes methane are produced during these processes, only the carbon captured by the biomass during its growth is released. As such, a renewable biomass feedstock presents a break-even carbon balance and is generally considered “green”. 

Using biochemical processes, microorganisms that work on organic materials to produce biogas (through anaerobic digestion) or a combination of acids, alcohols, and gases (by fermentation) can drive hydrogen production. The thermochemical gasification of biomass works much like coal gasification, here converting biomass to a mix of carbon monoxide, CO₂, hydrogen, and methane.  

Anaerobic digestion to produce biogas is the most technically mature of these processes, but can only process sewage sludge, agricultural waste, food and household waste, and some energy crops. Fermentation can also process the non-edible cellulosic part of plants. Gasification could potentially convert all organic matter, and in particular the lignin component of biomass feedstocks.  

Although there are already a small number of biomass gasification demonstration plants in place, the technology is not fully developed, and the problem of the formation of pollutants that may cause catalyst poisoning has not been resolved. Due to the complex processing requirements, producing low-carbon hydrogen from biomass is more expensive than using solar- or wind-based electrolysis techniques. 

The potential for large-scale biomass-based hydrogen production is also limited by the availability of affordable, renewable biomass. Combining hydrogen production from biomass with carbon capture and storage could, however, be a strategy to realize negative emissions, which may have a role in ongoing decarbonization efforts. 

Storage, Transportation, Distribution

Transport and storage costs play a significant role in the competitiveness of hydrogen, and the operation of large-scale and intercontinental hydrogen value chains will depend on the availability of adequate storage capacity and technologies. 

Hydrogen has low energy density, which makes it more challenging to store and transport than fossil fuels. Conventional options for the storage of hydrogen include pressurized and cryogenic tanks, providing hydrogen storage capacities of between 100 kWh for pressurized tanks and 100 GWh for cryogenic storage. While pressurized tanks have high costs due to their limited energy density, cryogenic tanks provide limited storage time due to the boil-off stream losses that are necessary to maintain acceptable pressure levels.  

An intermediary solution between pressurized and cryogenic hydrogen storage is cryo-compressed hydrogen. In this case, liquefied hydrogen is pumped into the tank, but the pressure levels where the hydrogen needs to be flared are much higher (up to 35 MPa) than in cryogenic storage (around 2 to 4 MPa). This allows cryo-compressed hydrogen to be stored for extended periods. 

Compressors are thus currently a vital technology for hydrogen storage. Hydrogen pressure levels range from 2 to 18 MPa for underground storage, over 35 MPa to 50 MPa for gaseous truck transport, and up to 70 MPa for on-board storage in fuel cell electric vehicles (FCEVs).⁴

Hydrogen can, however, be converted into hydrogen-based fuels and feedstocks such as synthetic methane, synthetic liquid fuels, and ammonia, which can be transported, stored, and distributed using existing infrastructure. This can reduce transport cost while adding to process costs. 

Ammonia can be produced by combining hydrogen and nitrogen, while synthetic hydrocarbons such as methane, methanol, diesel, or jet fuel can be produced by combining hydrogen with carbon in the form of CO₂. Unfortunately, a significant amount (40-60 percent) of the electricity used to convert hydrogen into fuels and feedstocks is lost during the process of conversion.  

Alternatives are being developed that would allow hydrogen to be stored in metals, micro-structured solids, or hydrogenized oils. As an example of the latter, the Liquid Organic Hydrogen Carrier (LOHC) has recently seen significant breakthroughs. The concept of chemical hydrogen storage in oil has been known since the 1970s. LOHC uses dibenzyltoluene, a generally available low-price heat-transfer fluid, to chemically store hydrogen at almost five times the density of conventional high-pressure storage.  

This allows a cubic meter of LOHC to carry 57 kg of hydrogen. Chemical storage in dibenzyltoluene is reversible, and so the carrier fluid can be recycled several hundred times. The carrier is non-toxic, hardly flammable, and non-explosive, and it can be transported at ambient conditions via the existing infrastructure for liquid fuels such as pipelines, tanker trucks, ships, and trains. The application is still relatively new, and only a few test installations exist, but it appears promising. 

Complex metal hydrides are a class of materials containing an anion where hydrogen is covalently bonded to a metal or a non-metal, often aluminum- or lithium-based. The ability for hydrogen to bond to such materials makes them candidates for hydrogen storage, and a great deal of research is underway to test the potential of various of these materials. Storage takes place in pressurized tanks containing pre-compressed metal hydride pellets and expanded natural graphite. 

Strategic-level storage of hydrogen can make use of natural formations underground, such as caverns, abandoned mines, and gas wells. The small size of hydrogen molecules means that the gas has a high escape tendency. 

Centralized vs. Distributed Generation

The distributed nature of renewable energy generation leads naturally to the question of distributed vs. centralized generation of hydrogen. Distributed generation removes the need to transport hydrogen over long distances, a costly and currently still somewhat inefficient process. This may be the most viable approach for introducing hydrogen in the short term, in part because the initial demand for hydrogen will be low. 

Two distributed hydrogen production technologies that may offer potential pathways are reforming liquid fuels, preferably renewable liquids such as manufactured ethanol and bio-oil, and small-scale water electrolysis. In smaller and primarily rural applications, fuel cells generating power and hydrogen, fed with biogas, could be used.  

Hydrogen obtained via thermo-catalytic ammonia decomposition is also rapidly attracting considerable interest for portable and distributed power generation systems. Consequently, a variety of reactor technologies are being developed to make these application types possible. The ammonia can be created using centralized renewable energy and transported to the decentralized applications, where it can be stored in metal salts and released for conversion into hydrogen on-demand. Inter-media storage can be employed to buffer fluctuations in use or serve high-volume applications. 

Timeline to market maturity, scaling up, mass commercialization

Over 95 percent of current hydrogen production is fossil-fuel based. The hydrogen industry is well established and has decades of experience in industry sectors that use hydrogen as a feedstock. The hydrogen feedstock market had a total estimated value of US$115 billion in 2017 and is expected to grow significantly in the coming years, reaching US$155 billion by 2022.  

As such, the industry currently provides a stable platform for the growth of green hydrogen generation. Demand will primarily be driven by the need to combat climate change by decarbonizing the global economy. A massive increase in funding for R&D internationally, combined with rapid growth in both the installed base of generation capacity and the availability of renewable energy means that technological improvement will be steep and prompt.  

The availability of a mature hydrogen production technology in AE, with a rapidly maturing competitor in PEM and a clear successor technology in SOE, means that a clear technology roadmap exists on the generation side. PEM and SOE technologies are expected to overtake AE generation systems before 2030. 

However, in those areas where the use case for hydrogen is rapidly changing and penetrating new industries and new sectors, the roadmap is less clear. Given the bi-directionality of fuel cell technologies, technological maturity being driven by both demand- and supply-side growth. In new areas of application such as transportation, high-energy applications, buildings, localized co-generation, and storage technologies, new developments are needed to address significant remaining barriers. The initial lack of maturity will slow growth while at the same time creating new opportunities for innovation and optimization. 

As such, there remain three key areas that represent specific challenges to growth and maturity: 

Most applications for green hydrogen are not yet cost-competitive without direct government support. Fuel cell technologies will be under especially strong price pressure. The costs of producing hydrogen from different sources in different regions, and the ways in which those costs will compete in the future, are unclear.  

This makes it difficult to compare potential hydrogen prices with those of alternatives such as solid-state batteries, pumped-storage hydropower, electric vehicles, biofuels, and electrification of high-temperature heat.

Demand for low-carbon hydrogen can come from a variety of sectors, and there are many modes of hydrogen supply and handling to meet that demand. The most cost-competitive solutions will vary, depending on the regions and other applications. For each value chain, investments and policies will need to be synchronized in modality, scale, and time to ensure stability and growth. 

Infrastructure, including pipelines and transmission and distribution networks, is of particular importance for a new energy carrier such as hydrogen. While hydrogen can be produced locally, its storage and distribution benefit from economies of scale. It must be possible to produce locally, but ship globally, and this still poses significant challenges to rapid maturity.  

The state of current regulations and standards around the world limits the smooth uptake of green hydrogen. New uses are often still not fully regulated, legal frameworks may not yet correctly assess risks, and current safety standards may not yet apply to new areas of uptake. 

Standards may also be lacking or not well understood. Accounting standards for different sources of hydrogen may not yet be in place, or may differ from region to region. 

Hydrogen comes with safety risks, high upfront infrastructure costs, and some of the industrial dynamics of fossil fuel supply and distribution. Given these issues, it’s not yet clear how the general population and markets will react to the (often legislated) introduction of hydrogen, or how they will view such issues alongside the convenience and environmental benefits of green hydrogen. 

⁴Since one standard atmosphere (atm) of pressure is equal to just over 100 kilopascal (kPa), 70 megapascal corresponds to about 700 times atmospheric pressure.