Onboard hydrogen storage

A hydrogen car needs to carry fuel as well as its passengers. Existing cars use pressure tanks, which add weight and take up space. Can a complete storage system be light, compact and affordable while delivering enough fuel, filling quickly and lasting safely through years of use?[1] [2]

Hydrogen held in a material or tank must be deliverable to the fuel cell. DOE’s ultimate light-duty targets divide usable hydrogen by the mass and volume of the complete store, including stored fuel and supporting equipment. For an invented example, 6.5 kg of usable hydrogen in a 100 kg system occupying 130 L would meet just those two capacity targets:

$$\frac{6.5\,\mathrm{kg}}{100\,\mathrm{kg}}=0.065,\qquad \frac{6500\,\mathrm{g}}{130\,\mathrm{L}}=50\,\mathrm{g/L}.$$

This is arithmetic, not a demonstrated design. The other cost, filling, delivery, safety and durability targets must also hold at the end of service life.[1]

Pressure tanks already meet many requirements. The remaining challenge is to meet the ambitious weight, volume and cost targets together with delivery and lifetime requirements. A material’s ability to hold hydrogen does not show how the complete store performs.[1] [2] [3]

The system boundary includes stored fuel, media, tank and balance-of-plant equipment. Material uptake alone leaves delivery kinetics, heat management and repeated reversibility unproven at system level.[1] [3]

Lighter, smaller storage would leave more of the car available for people and their luggage.[1] [2]

Storage quality affects the fuel that fits in a vehicle and the mass it must carry. A high material uptake can look promising while leaving inadequate usable fuel once tanks, valves, reactants or thermal-management equipment are counted. Evaluating the complete system connects materials research to range and usable vehicle space; it also reveals whether releasing and restoring the fuel adds equipment that offsets a capacity gain.[1] [2] [3]