A few years ago, research at Dresden-based Fraunhofer IFAM’s Hydrogen Technologies department led to the development of a paste-like substance that can provide on-demand energy under well-controllable conditions for multiple kinds of fuel cell applications. In partnership with businesses and other research institutes, IFAM has since launched several projects to demonstrate that this substance called PowerPaste, the main ingredient of which is magnesium hydride, is both safe and easy to handle. The institute is also currently building a system to produce multiple tons of PowerPaste a year for use in field tests.

There already are well-established methods for producing hydrogen via hydrolysis, for example, by having water react with either calcium hydride (CaH₂) or sodium borohydride (NaBH₄). Around four years ago, H2-international first reported on PowerPaste (see H2-international, January 2017), a storage compound IFAM created based on magnesium hydride (MgH₂). [Teg14]

The principle behind this kind of hydrolysis is always the same. When a metal hydride (MH_n) reacts with water (H₂O), it forms hydrogen and a metal hydride oxide:

MH_n + n H₂O à n H₂ + M (OH)_n

In the case of magnesium hydride, the equation is as follows:

MgH₂ + 2 H₂O à 2 H₂ + Mg (OH)₂

A few years ago, research at Dresden-based Fraunhofer IFAM’s Hydrogen Technologies department led to the development of a paste-like substance that can provide on-demand energy under well-controllable conditions.

Since this technique makes use of the water available during the reaction, it generates twice as much hydrogen as thermal decomposition, with half of that amount coming from the hydride. As a result, the method gives off much less waste heat than other production techniques during which a metal or a metal alloy reacts with water (or an acid).

The reaction, which takes place inside a hydrogen generator, is exothermic, which removes the need for external heat sources. The thermal energy that it generates can be used to heat buildings, among other things. Hydrolysis produces about as much energy as PEMs give off in the form of waste heat, i.e., approximately 1 kilowatt of heat per kilowatt of electrical output, at a temperature of around 80 °C. The most sensible course of action would thus be to devise a plan for the shared thermal management of both the fuel cell and the hydrolysis reactor.

When it comes to hydrolysis, magnesium hydride has several advantages over other materials:

• Its specific energy is 6.1 kWh per kilogram, PowerPaste’s being 3.8 kWh. Even when factoring in fuel cell losses, the material provides much more energy than today‘s batteries, the gravimetric energy density of which is around 0.2 kWh per kilogram.
• The magnesium to make magnesium hydride and PowerPaste is already available on the market in large quantities, at a raw material price of around EUR 1.70 a kilogram. Magnesium is also not a rare element but the third-most common in the earth’s crust.
• According to IFAM estimates, even magnesium produced by conventional means will, over the longer term, lower the price of making PowerPaste to around EUR 2 to EUR 3 a kilogram. The levelized cost will be around EUR 20 to EUR 30 per kilogram at the point of use, including all expenditures for infrastructure and distribution. This reflects the full cost of production as opposed to artificial prices for hydrogen at fueling stations.

The above means that in many markets, including for UPS and light electric vehicles, the total cost of ownership will already be much lower than if the project involved putting up expensive hydrogen infrastructure or renting gas cylinders. The growing use of magnesium in light vehicle construction (CAGR: around 5 percent) also makes it likely that magnesium extraction will undergo significant changes in the next 10 years and become more carbon-neutral, energy-efficient and inexpensive.

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read more in H2-international October 2020

Authors: Dr. Marcus Vogt, Felix Heubner, Dr.-Ing. Thomas Weißgärber, Dr. Lars Röntzsch

All for Fraunhofer IFAM Dresden