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Sustainable hydrogen: still a long way to go?

Hydrogen: for the hype or for the climate?

Cécile Michaut, Science journalist
On July 8th, 2021 |
4 min reading time
Key takeaways
  • Numerous states and companies are placing their hopes in hydrogen to fight against global warming.
  • But hydrogen (H2) production requires input from other sources of energy which emit CO2.
  • Currently, 95% of hydrogen is produced using fossil fuels because it is cheaper. Whereas the remaining 5% is produced by electrolysis, which requires electricity from the grid (also originating from fossil fuels).
  • To really turn it into green fuel, major investments in R&D will be needed, in combination with strong regulatory incentives.

Hydro­gen is in fash­ion. Sev­er­al states and com­pa­nies are plac­ing their hopes in this chem­i­cal ele­ment to fight against glob­al warm­ing. Not a week goes by with­out a new enthu­si­as­tic announce­ment for this gas as a source of ener­gy: a hydro­gen-pow­ered bus for the com­pa­ny Trans­dev, a hydro­gen and fuel cell util­i­ty vehi­cle unveiled by Cit­roën, or a cross-bor­der hydro­gen net­work between France, Ger­many and Luxemburg.

Admit­ted­ly, at first glance, hydro­gen (H2) is pure gold – so to speak. When it is burned or used with a fuel cell, you get ther­mal or elec­tric ener­gy and water with­out pol­lut­ing. Yet, its cur­rent pro­duc­tion emits a lot of CO2. To real­ly turn it into green fuel, major invest­ments in R&D will be need­ed, in com­bi­na­tion with strong reg­u­la­to­ry incentives.

Hydro­gen pro­duc­tion emits CO2

While hydro­gen a major con­stituent of the uni­verse, on Earth hydro­gen atoms hard­ly exist in the pure H2 gas-state that indus­tri­als use and as we dis­cuss in this brain­camp. There­fore, it can­not be used as an ener­gy source, rather it is an ener­gy car­ri­er – like elec­tric­i­ty. This means that hydro­gen pro­duc­tion requires input from oth­er sources of ener­gy which poten­tial­ly emit CO2.

Today, hydro­gen pro­duced through methane reform­ing, a pro­duc­tion method emit­ting CO2, costs about 1€ per kilo, ver­sus 4 – 6€ per kilo when it is pro­duced through elec­trol­y­sis. Due to the law of sup­ply and demand, 95% of hydro­gen is there­fore pro­duced using fos­sil fuels. The remain­ing 5%, pro­duced through elec­trol­y­sis could be “green”, pro­vid­ed that the elec­tric­i­ty used is also car­bon-free (renew­able or nuclear ener­gy), which is not always the case (espe­cial­ly with coal-fired pow­er plants). Sev­en­ty mil­lion tons of hydro­gen are pro­duced per year in the world, releas­ing 830 mil­lion tons of CO2, that is to say 2% of glob­al emis­sions – a rate sim­i­lar to that of the air trans­port sector.

So why is hydro­gen so pop­u­lar? Because in the long-term, it seems to be one of the pos­si­ble sub­sti­tutes to fos­sil fuels. Indus­tries gen­er­at­ing large amounts of CO2, like the met­al or glass indus­tries, might ben­e­fit from hydro­gen. In the­o­ry, it could also rev­o­lu­tionise the trans­porta­tion sec­tor: hydro­gen-pow­ered vehi­cles do not emit exhaust pol­lu­tants and their wide­spread use could lim­it pol­lu­tion in cities. Final­ly, in the future, the ener­gy mix might rely on hydro­gen to store elec­tric­i­ty pro­duced by inter­mit­tent renew­able ener­gies (wind or solar energy).

The trans­fer of elec­trons that occurs in this process gen­er­ates elec­tric­i­ty from hydro­gen with the only by prod­uct being water. Hence, hydro­gen fuel cells are said to gen­er­ate “clean” ener­gy – it is there­fore the way in which the hydro­gen is pro­duced that is respon­si­ble for its pol­lu­tion. This con­ver­sion is cur­rent­ly high­ly inef­fi­cient and, at best, only 65% of the ener­gy is transferred. 

Stor­age, secu­ri­ty, cost… chal­lenges remain

It is dif­fi­cult to make reli­able pre­dic­tions because the future of hydro­gen will depend on many fac­tors: cost and avail­abil­i­ty of fos­sil fuels, reg­u­la­tion of CO2 emis­sions, finan­cial incen­tives for clean ener­gies. Nev­er­the­less, we can fore­see four main mar­kets for hydro­gen: (1) opti­mi­sa­tion of the elec­tri­cal grid when we achieve inter­mit­tent ener­gy sources; (2) self-con­sump­tion at the local lev­el for areas which are not con­nect­ed to the elec­tri­cal grid; (3) devel­op­ment of hydro­gen-pow­ered elec­tric vehi­cles; and (4) industry.

This gas has indeed been used for decades in indus­try (main­ly for petro­chem­i­cal process­es, steel­works, and the pro­duc­tion of nitro­gen fer­til­iz­ers). We know how to pro­duce hydro­gen, trans­port it in pipelines, and safe­ly use it. Yet hydro­gen-ener­gy still pos­es many com­plex chal­lenges. How can we pro­duce it in a cost-effec­tive and envi­ron­men­tal­ly sound man­ner? How can we safe­ly use it in every­day life, and not only in indus­try? How can we store it in cars, bus­es or planes, giv­en that it must be stored under high pres­sure? How can we adapt infra­struc­tures if the cur­rent gas pipes are inad­e­quate? How can we improve the yields of ener­gy transformation? 

As we have said, hydro­gen is above all a way to store elec­tric­i­ty. In the case of renew­able ener­gies, for exam­ple, elec­tric­i­ty pro­duc­tion dur­ing peri­ods of strong winds or sun­ny weath­er by wind tur­bines or solar pan­els does not always match con­sump­tion. This elec­tric­i­ty must there­fore be stored, but cur­rent bat­ter­ies are main­ly adapt­ed to short-term stor­age. Hydro­gen devel­op­ers thus sug­gest using this elec­tric­i­ty to pro­duce hydro­gen through elec­trol­y­sis, store this hydro­gen as long as nec­es­sary, and then con­vert this hydro­gen into elec­tric­i­ty with a fuel cell. But for the moment, the over­all per­for­mance is only 25% 1. Can we afford to waste three quar­ters of the elec­tric­i­ty produced?

All these chal­lenges can only be over­come on two con­di­tions. First, an unprece­dent­ed R&D effort to remove road­blocks. But also, finan­cial incen­tives: at this time, and giv­en the pro­duc­tion costs, it is dif­fi­cult to see how green hydro­gen pro­duced by elec­trol­y­sis could cap­ture mar­ket shares with­out run­ning the risk that com­pa­nies begin pro­duc­ing it at the expense of car­bon-emit­ting methods.

A bit of chem­istry to under­stand hydro­gen production 

Methane reform­ing con­sists of react­ing methane (nat­ur­al gas) with steam, in the pres­ence of a cat­a­lyst. The equa­tion of the chem­i­cal reac­tion shows that this method inevitably pro­duces CO2:

CH4 + 2 H2O → 4 H2 + CO2

We there­fore pro­duce one mol­e­cule of COfor 4 mol­e­cules of hydro­gen (to be accu­rate, we ought to speak of dihy­dro­gen molecules).

The pro­duc­tion of hydro­gen from coal is even worse: 

C + 2 H20 → CO2 + 2 H2

Only two mol­e­cules of hydro­gen are pro­duced for one mol­e­cule of CO2.

In con­trast, water elec­trol­y­sis does not pro­duce CO2:

2 H2O → 2 H2 + O2

But this reac­tion requires large amounts of electricity. 

The pyrol­y­sis of methane (see the inter­view of Lau­rent Fulcheri) con­sumes methane, but unlike reform­ing, it does not pro­duce CO2. It still requires elec­tric­i­ty, but 4 ‑7.5 times less than electrolysis.

CH4 → C + 2 H2

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