What comes to our mind when we say “hydrogen is as an energy carrier”? The quick answer is hydrogen gas; a clean-burning fuel that, when combined with oxygen in a fuel cell, produces heat and electricity with only water vapor as a by-product.

However, H₂ gas is not the only hydrogen-based energy vector considered for the energy transition. In fact, one of the main advantages of hydrogen as energy vector is its flexibility.

Generally speaking, when we talk about hydrogen as an energy vector, we are referring to hydrogen stored as a gas or liquid (cryogenic hydrogen). However, other forms of hydrogen should be considered for different applications. In fact, for hydrogen, a whole range of energy storage solutions exist, distinguished by different capacity parameters, energy storage times, charging and discharging time and, of course, installation prices.

Hydrogen and hydrogen fuels, such as ammonia (NH₃), have several advantages in comparison with other forms of energy storage systems. For example, hydrogen and other hydrogen vectors allow average energy storage for up to 1000 MW for several weeks to several months. Even more, some studies have shown that with low losses, hydrogen vector can store energy for up to a year.

Just to contextualize, in the case of battery technology, energy can be stored on average, for only a few minutes to a few days, at the most, a few weeks, and only in a range of 10 MW; and, to take another example, thermal energy provides average storage of the order of only 100 MW in a range of only few days. Therefore, hydrogen’s large capacity for long-term energy storage is one of its most important advantages, especially in terms of energy security.

On the other hand, hydrogen gas has a low energy density by volume in comparison to fossil fuels, which leads to the need for exceptionally large storage units. In order to overcome this drawback, one of the following strategies can be adopted to store sufficient quantities of hydrogen: i) storage at high pressure, ii) storage at very low temperature (cryogenic), or iii) substances that contain large amount of hydrogen molecules.

However, which is the most convenient? The answer to this question needs to be explained in terms of the application: stationary or mobile?

Hydrogen storage for stationary applications include on-site storage at either point of production or use, and it is normally use for stationary power generation.

Meanwhile, hydrogen storage for mobile applications is for use in transportation such as automobiles, trucks, train, airplanes or boats.

Why is important to know this differentiation? Essentially weight and volume. No doubt that other reasons such as safety play a role, but weight and volume are the main factors when choosing the method for storing hydrogen.

Here, we will summarize some of the most investigated ways, other than hydrogen gas and cryogenic hydrogen, to store hydrogen.

Like water, but not drinkable.

Hydrogen storage by means of hydrogen-containing liquids can be classified into two categories: organic-based and not-organic-based liquid.

The organic-based-hydrogen-containing liquids are essentially hydrocarbons, for example gasoline or diesel are hydrogen containing liquids. However, if we are talking about green hydrogen energy vectors, the liquid energy vectors should not be of fossil origin. Thus, even though diesel is considered a hydrogen energy vector, it can be considered a green fuel only if the diesel has been prepared through green routes (e.g synthetized with hydrogen from electrolysis).

Other top running hydrogen-containing-liquids are methanol, ethanol, isopropanol or formic acid are but, again, to contribute positively to the energy transition, these vectors cannot be obtained from fossil origin.

Low-molecular-weight alcohols and small acids (such as formic acid) can be prepared from CO₂ capture and green hydrogen using renewable sources with the potential of zero net carbon emissions.  They are liquid at atmospheric conditions, have low toxicity and are easy to store.  All of these attributes make them exceptionally suitable for the transport sector without the need of major changes to the infrastructure for their storage, distribution and end-use.

Here are some real-life examples of industrial projects related to hydrogen-containing liquids production and utilization. The George Olah CO₂ to methanol plant, located in Svartsengi, Iceland, currently produces 5 million litres/year of renewable methanol by capturing and converting up to 5600 ton CO₂/year. Carbon Recycling International has also designed an e-methanol production facility in Finnfjord (Norway). The plant will use CO₂ captured from the emissions of the Finnfjord ferrosilicon plant and hydrogen generated from the electrolysis of water as raw material for the production of e-methanol.

Recently, Maersk has signed a framework protocol with the government of Spain on opportunities for large-scale green fuels production. The objective of the project is to deliver up to 2 million tons of e-methanol per year produced from green hydrogen and CO₂ capture which will be used in long-haul vessels able to run on e-methanol.

These hydrogen-containing liquid fuels can then be used directly in fuel cells to power electronics, to back-up electricity grids or in transportation. The EFOY Pro direct methanol fuel cells and SIQENS Ecoport  fuel cells are power generators for a wide range of stationary and mobile industrial applications (e.g. telecommunications, leisure or surveillance).  And also, in 2016, Nissan, developed a vehicle prototype powered by bio-ethanol. In this case, the ethanol is first transformed into hydrogen which is then used to generate the electricity powering the car.

Among the non-organic-hydrogen-containing-liquid-fuels, ammonia (NH₃) is the top candidate. It contains 17% hydrogen by weight, which can be extracted via thermal catalytic decomposition or via electro-oxidation. Alternatively, NH₃ can be potentially oxidized directly in fuel cells without the need for a separate reactor.

The energy density of NH₃ (12.7 MJ/L) is even higher than the energy density for liquid hydrogen (8.5 MJ/L). Moreover, ammonia can be stored at a much less energy-intensive –33 °C, than the –253 °C of cryogenic stored liquid hydrogen. Besides, ammonia is also less flammable than hydrogen. Finally, because 200 million metric tons of NH₃ is already produced annually, a vast infrastructure for storage and transportation of ammonia already in exists.

Currently, this amount of ammonia is produced primary through the energy intensive 100-year-old Haber-Bosch process which is not compatible with the energy transition because uses H₂ from thermal cracking of methane and high synthesis temperatures.

However, new greener alternatives that will enable the use of ammonia as a liquid energy vector are surging. As an example, and probably the most ambitious project, is the ammonia synthesis plant to be located on the Red Sea coast of Saudi Arabia. A photovoltaic plant will harness the sun during the day, while turbines will capture night-time winds, to generate 4 GW of electricity for water electrolysis plants. The hydrogen will be then fed into a traditional Haber-Bosch plant to produce 1.2 million tons per year of NH₃. 

Yara, one of the major producers of ammonia in the world, is planning to produce 75,000 t of NH₃ a year, at its ammonia plant in Sluiskil (The Netherlands), using hydrogen from water electrolysis. These electrolyzers would run on 100 MW of power from a new offshore wind farm.

Today, ammonia represents the best option for long-haul transportation; in particular, for trains and ships. An example of this is the EU funded project ShipFC, aimed at developing, installing and testing long-distance-vessel powered by ammonia fuel cells.

At the CIC energiGUNE, our researchers are pioneers in the development of new materials and technologies for the production and utilization of green-hydrogen-containing-liquid fuels.

Hydrogen hydrides: the energy powder

Metal hydrides are an alternative way to store hydrogen at low pressures in a solid. The hydrogen storage at low pressure is feasible because the hydrogen molecules are chemically bonded within the metal compound structure. Metal hydride storage systems typically operate at 10-40 bars, which is 20 times lower than typical high-pressure hydrogen storage systems.  

The sizing of the metal hydride storage systems is determined by the specific application needed.

However, one of the main drawbacks of metal hydrides is the weight. The storage capacity of the metal hydride storage is around 1.5kg of H2 (or 50 kWh) per 100 kg of the metal hydride compound material. Even though this value of the energy storage capacity of the metal hydrides is low in comparison with other hydrogen storage systems, it is comparable to the energy capacity of a standard Lithium-ion battery in a Tesla Model 3 (50 kWh).

The other inconvenience of the metal hydrides is the complexity of the system for storage and utilization. First, the hydrides must be stored under nitrogen or argon and protected from water. Secondly, while liquid fuels such as alcohols or ammonia, can be used directly as fuels in fuel cells, the metal hydrides require an activation step for the release of the hydrogen from the structure. When the hydrogen is needed, the desorption of the hydrogen is promoted by a heating step (50 – 100°C). This temperature-driven desorption step is inconvenient for automotive application, in particular during acceleration and deacceleration steps.

Other alternatives to the temperature-driven desorption include the activation by contact of the hydride with water moisture. In this case, when hydrogen is required, the hydrate is mixed with controlled-humidity air, and the resulting reaction produces high-purity hydrogen. Although the release kinetics are fast, this reaction requires water to be carried onboard separately, adding weight and complexity to the transport application.

Even though, the utilization of metal hydrides for hydrogen storage in transportation is challenging, it is not impossible and the metal hydrates are taking their space across stationary storage and portable electronics. For example, Hydrostik is a convenient hydrogen storage solution to fuel your hydrogen powered devices. GKN hydrogen has deployed different projects that demonstrate the feasibility of short-term and long-term hydrogen storage on metal hydrides for stationary applications.

Beyond the technical challenges, currently the hydrogen storage methods do not meet the cost targets proposed by the USA Department of Energy (DOE). Reducing the cost of energy storage is essential to the full deployment of hydrogen economy. Some of the cost challenges will be addressed by exploring new materials or by developing more efficient synthesis process. Furthermore, scaling up and mass production of these technologies will significantly contribute to the reduction of the cost.

In conclusion, we highlight that hydrogen storage technology has come a long way. We need to keep investigating in all directions to find the best system for each application. In the end, the objective to deliver genuinely decarbonized societies will depend on the most appropriate hydrogen storage method for each application. The ability to Pick and choose the best hydrogen storage method will be based not only on technical requirements, but also on its economic feasibility.