Article 4
π Energy π Industry π Mobility
Sustainable hydrogen: still a long way to go?

Storage: a major hurdle for the hydrogen industry

Johnny Deschamps, Professor at the Chemistry and Processes Unit (UCP) at ENSTA Paris (IP Paris)
On July 8th, 2021 |
3 mins reading time
3
Storage: a major hurdle for the hydrogen industry
Johnny Deschamps
Johnny Deschamps
Professor at the Chemistry and Processes Unit (UCP) at ENSTA Paris (IP Paris)
Key takeaways
  • Although hydrogen is attracting interest, we often forget that its storage represents a significant challenge for its widespread use.
  • In its liquid state, hydrogen requires cryogenic tanks that keep it at -253°C, which requires a considerable amount of energy.
  • Other storage techniques are being developed, such as storage by adsorption or in compressed form, but for the moment no solution is economical or practical enough to be used in a sustainable way.

We have become depen­dent upon fos­sil fuels, but hydro­gen appears to be a good alter­na­tive. Indeed, it can be used to store a great quan­ti­ty of ener­gy over long peri­ods of time. Hydro­gen can then be used for mobile or sta­tion­ary appli­ca­tions with fuel cells or via direct com­bus­tion. Depend­ing on its pro­duc­tion, its car­bon foot­print can also be very inter­est­ing. How­ev­er, stor­age capa­bil­i­ties strong­ly impact hydro­gen appli­ca­tions and are cur­rent­ly a cru­cial chal­lenge, par­tic­u­lar­ly for mobil­i­ty. It is there­fore of upmost impor­tance to design light­weight, com­pact, safe and low-cost stor­age tanks. 

Liq­uid or gas storage 

Liq­uid hydro­gen is high­ly ener­getic and has a den­si­ty of 71kg per cubic metre at atmos­pher­ic pres­sure. How­ev­er, liquify­ing hydro­gen has a major draw­back: its ener­gy cost, because hydro­gen only becomes liq­uid at ‑253°C. In addi­tion, liq­uid hydro­gen must be stored in cryo­genic tanks. Most of these are made of stain­less steel and have stor­age capac­i­ties rang­ing from a few litres to sev­er­al thou­sands of cubic meters. How­ev­er, the heat insu­la­tion of these stor­age tanks is not per­fect. A frac­tion of the gas usu­al­ly boils off due to exter­nal sources of heat (caused by insu­la­tion prob­lems) and the size and shape of tanks.

In gaseous state, hydro­gen is the light­est ele­ment. It occu­pies a sub­stan­tial vol­ume of 11 m3 per kilo­gram in nor­mal con­di­tions of tem­per­a­ture and pres­sure (at 0 °C, under 1,013 bar). It is there­fore absolute­ly nec­es­sary to reduce this vol­ume in order to store and trans­port hydro­gen effi­cient­ly. To that end, pres­sure is a good alter­na­tive. Com­press­ing and stor­ing gaseous hydro­gen in steel cylin­ders filled at 200 or 250 bars, is stan­dard prac­tice. How­ev­er, two main dis­ad­van­tages remain with this method of stor­age: vol­ume and mass. These prob­lems have con­sid­er­ably decreased with the devel­op­ment of reser­voirs called types III and IV, whose rein­forc­ing struc­tures are made of com­pos­ite mate­ri­als. The com­pos­ites are made from glass, aramid or car­bon fibres embed­ded in resin. They make it pos­si­ble to work at high­er pres­sures while reduc­ing the mass and increas­ing the resis­tance to fatigue fail­ure due to exter­nal aggres­sion. Thanks to these com­pos­ite struc­tures, stan­dard pres­sures have increased to 350 and 700 bars.

Sol­id stor­age: an alternative 

Sol­id hydro­gen stor­age involves seques­ter­ing the gas with­in a sol­id mate­r­i­al. This seques­tra­tion can be chem­i­cal or phys­i­cal depend­ing on the type of mate­r­i­al. Chem­i­cal stor­age through absorp­tion rests on a met­al hydride result­ing from the reversible chem­i­cal com­bi­na­tion between metal­lic bonds of hydro­gen with atoms of a great vari­ety of met­als. In con­trast, phys­i­cal stor­age is char­ac­ter­ized by an increase in gas den­si­ty at the sur­face of the sol­id mate­r­i­al due to the mol­e­c­u­lar inter­ac­tions between the adsor­bate (gas) and the adsor­bent (sol­id). This sur­face phe­nom­e­non, which is com­plete­ly reversible, is only pos­si­ble with sol­id mate­ri­als of large spe­cif­ic sur­face area. These mate­ri­als are both very porous (tiny pores in the nanome­tre range) and very divid­ed, in the form of a fine powder.

Ulti­mate­ly, what is the best solution?

Despite the steady research progress for on-board hydro­gen stor­age, none of the tech­niques men­tioned above meet the spec­i­fi­ca­tions set by the Amer­i­can Depart­ment of Ener­gy (DOE) in terms of phys­i­cal per­for­mance (mas­sive stor­age, vol­u­met­ric stor­age, tem­per­a­ture, pres­sure, leak­age rate), mate­r­i­al con­straints (mass and vol­ume of the sys­tem) and eco­nom­ic constraints. 

Liq­uid hydro­gen offers the best val­ue for stor­age quantity/volume. How­ev­er, the weak boil-off of the liq­uid, due to the unavoid­able ther­mal loss (as small as it may be), caus­es a per­ma­nent release of hydro­gen, and thus a loss of mass. It means that no hydro­gen-pow­ered vehi­cle can be left in a con­fined area. For instance, the BMW Hydrogen7 was equipped with this stor­age tech­nol­o­gy but pro­duc­tion was dis­con­tin­ued because of this prob­lem. Nev­er­the­less, this type of stor­age is well devel­oped to trans­port gas, espe­cial­ly in North Amer­i­ca, where it rep­re­sents more than 90% of vol­umes trans­port­ed by road.

Stor­age of com­pressed hydro­gen in com­pos­ite reser­voirs makes it pos­si­ble to reach a sat­is­fy­ing den­si­ty at 350 bars, but the bulk den­si­ty is far too weak. It is there­fore nec­es­sary to increase pres­sure to 700 bars. At this pres­sure, the den­si­ty of hydro­gen is 42kg per cubic meter. This type of stor­age is used by many car man­u­fac­tur­ers for vehi­cles with a range of 400 to 500kg (around 5kg of stored hydro­gen). It should be not­ed that stor­ing 5kg of hydro­gen at 700 bars demands a vol­ume of 125 litres. In eco­nom­ic terms, even if a cryo­genic tank is less expen­sive than a pres­sure tank, the cost of gas liq­ue­fac­tion is much high­er than that of com­pres­sion, even at 700 bars.

Stor­ing hydro­gen in the form of met­al hydrides or by adsorp­tion offers a stor­age quantity/volume val­ue which is 3 times high­er than that of com­pressed gas. How­ev­er, due to the high mass of met­al hydrides, the weight per­cent­age of stored hydro­gen is far too low. Fur­ther­more, restor­ing the gas requires heat where­as hydrid­ing (for­ma­tion of hydride) is exother­mic and only involves slow kinet­ics. This type of stor­age is more suit­ed to sta­tion­ary appli­ca­tions. Final­ly, hydro­gen stor­age through adsorp­tion in porous mate­ri­als is a phys­i­cal process and as such, per­for­mance is opti­mal only at low tem­per­a­tures (in the range of ‑196°C) and at a pres­sure of around 100 bars. Even though research on this sub­ject has made sig­nif­i­cant progress in recent years, stor­age mass capac­i­ties at ambi­ent tem­per­a­ture are still too low. Fur­ther progress is nec­es­sary before con­sid­er­ing mobile applications.

Contributors

Johnny Deschamps

Johnny Deschamps

Professor at the Chemistry and Processes Unit (UCP) at ENSTA Paris (IP Paris)

Johnny Deschamps' main research activities concern the production of green hydrogen from biomass, hydrogen storage by adsorption in porous materials such as organic frameworks, energy materials and the containment of fluids and metals in porous materials. He develops original techniques for doping organic frameworks with carbonaceous materials and metals and teaches "the hydrogen industry" in several prestigious institutions in France and China.