MgH2-based PowerPaste

As a secondary energy source, hydrogen has long demonstrated some key benefits. It has a high specific energy, good efficiency and guarantees emission-free use. But market take-up has been slow in many promising areas, often because the available storage solutions have proven too costly or had technological issues. Most of the time, however, a more challenging market barrier is the lack of infrastructure or exorbitantly high logistics costs. The new PowerPaste development by Fraunhofer IFAM has the potential to change all of that.

The production of H₂ through the hydrolysis of metal hydrides or other materials easily oxidizable in water (Old Greek: hýdor = water; lýsis = dissolution) has long been known as an alternative to H₂ storage units (high-pressure or cryogenic tanks) available on the market today. For example, calcium hydride – marketed under the trade name “Hydrolith” – has been used as a portable, commercially available hydrogen source to refuel weather balloons or even airships since 1910 [1, 2].

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Sodium borohydride as H₂ storage

Although the use of metastable sodium borohydride solutions for off-grid H₂ generation had already been suggested by Herman Irving Schlesinger in 1953 [4], it was in the 1980s that the technology sparked much research and became highly recommended for fuel cell applications [5]. Sodium borohydride is similar to calcium hydride in its reaction with water. In addition to hydrogen, it creates the dangerous compound sodium meta-borohydride as a waste product (irritating to eyes, respiratory system and skin, and suspected to be reprotoxic; EU CMR category 2):

NaBH₄ + …

Moreover, the theoretically possible amount of hydrogen is rarely achieved under real-life conditions. What remains is a waste product rich in toxic sodium borohydride that was not allowed to react. And the excessive amount of water that the process requires greatly reduces the specific energy of the entire system. But the most severe drawback may be that the high material prices, the use of expensive catalysts and complex packaging, and broken promises in regard to recycling have driven up power generation costs to levels where the above-mentioned solutions will be attractive to niche market customers only.

Our own calculations show electricity production based on NaBH₄ to add up to more than EUR 20 per kWh. Even in the future, it is hard to see how power could be produced in this way for considerably less than EUR 10 per kWh.

The better choice: magnesium hydride

One still relatively unknown material for hydrolysis-based H₂ production is …

Fig. 2: …

Paste for good measure

These challenges have led to strong improvements in hydrolysis technology at the Dresden location of Fraunhofer IFAM. Both material and system design were enhanced. First, the magnesium hydride received non-toxic and cheap additives to prevent passivation (see figure 2 left, blue curve). [7, 8] Second, esters were added to create a paste-like material: the PowerPaste (see figure 1). This paste is extremely fast in reacting with water (see figure 2 left, orange curve), providing full control over H₂ production and a steady volume flow with the help of a metering system (see figure 2, right).

Fig. 3: Comparison of the specific energy and energy density of lithium-ion batteries, methanol (DMFC), gasoline (generator) and PowerPaste (PEMFC), including efficiency losses during energy conversion (based on 1 kW of maximum capacity)

In contrast to …

50-watt and 300-watt demonstration system

That the technology is feasible has already been proved by two stationary and operational power generators. Developed during Fraunhofer IFAM projects in less than a year, the 50-watt demonstration system of technology readiness level 4 and the movable 300-watt generator of level 5 (see figure 4) have been tested to show that hydrolysis can be used in PEM fuel cells for long-term and on-demand power generation. The generators include a microcontroller unit and a special kind of metering system to ensure the correct amount of PowerPaste is added to the water and will produce as much hydrogen as the fuel cell requires, even if demand differs considerably.

Fig. 4: …

References …

Authors:

Dr. rer. nat. Marcus Tegel, marcus.tegel@ifam-dd.fraunhofer.de

Dr. rer. nat. Lars Röntzsch, lars.roentzsch@ifam-dd.fraunhofer.de

Both from Fraunhofer IFAM, Dresden