Hydrogen: the green energy carrier of the future? Understand article

Hydrogen may be the fuel of the future, but how can we produce it sustainably? Karin Willquist explains.

Hydrogen has been called ‘the energy carrier of the future’ – because it can be oxidised in a fuel cell to generate electricity, for example to power cars, without releasing carbon dioxide (CO₂), and it can be produced in remote places without an electricity infrastructure. In contrast to available resources such as natural gas and gasoline, hydrogen has to be produced, making it an energy carrier and not a fuel.

An energy system in which hydrogen is used to deliver energy – a hydrogen economy – was proposed by John Bockris in 1970; in 1977, an international hydrogen implementing agreement was established to work towards it^w1.

Hydrogen is mainly used now as a chemical reagent rather than an energy carrier, but there is no doubt that it has the potential to transform our transport and energy systems. However, realising its potential is not easy. Most fuels currently in use are liquids, solids or gases with high energy per volume (energy density). Hydrogen, in contrast, has a low energy density: at a given pressure, burning one litre of hydrogen produces one third of the energy that burning a litre of methane does. This poses problems of storage, distribution and use that are being addressed by scientists (Schlapbach & Züttel, 2001)^w2. A more fundamental challenge, however, is that of producing hydrogen in a sustainable manner. This is what I shall focus on here.

Ways to produce hydrogen

Hydrogen is an abundant element on Earth’s surface, normally linked to carbon in carbohydrates (in plants) or to oxygen in water (H₂O). Hydrogen gas (H₂), in contrast, exists only in small quantities on Earth. One of the challenges for sustainable hydrogen production is releasing H₂ from its bonds with carbon and oxygen.

Currently, H₂ is produced mainly from fossil fuels (e.g. natural gas) by steam reforming: heating the fuels to high temperatures with water^w2:

CH₄ + H₂O → CO + 3H₂                                              (1)

CO + H₂O → CO₂ + H₂                                                (2)

However, this method relies on fossil fuels and releases CO₂ , causing the same emission problems as burning fossil fuels. Steam reforming is only sustainable if renewable hydrocarbons such as biogas are used, because the CO₂ released has previously been absorbed in the production of the hydrocarbons.

H₂ can also be produced by electrolysis^w2, whereby electricity is used to split H₂O into H₂ and oxygen:

2H₂O → 2H₂ + O₂                                                       (3)

This method can be sustainable if the electricity is from renewable resources such as wind, wave or solar power. H₂ can thus be used to store energy on windy days when the windmills produce more electricity than can be consumed.

Interestingly, H₂O splitting occurs naturally in the oceans, because microscopic algae and cyanobacteria use solar energy to split water in a process called biophotolysis (Equation 3). However, the rate of H₂ production is extremely slow.

Efforts have been made to increase the production rate under controlled conditions using modified micro-organisms, but the processes are still too slow and expensive to be a realistic source of H₂ any time soon (Hallenbeck & Ghosh, 2009).

Finally, biohydrogen can be produced from crops and from industrial, forestry and agricultural waste, using bacteria. Like us, these bacteria oxidise plant material as a source of energy, but unlike us, they live in anaerobic environments (lacking oxygen). In aerobic respiration, we use O₂ to oxidise sugars, e.g.

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6 H₂O                            (4)

In contrast, to oxidise the substrate as far as possible and thus optimise their energy gain, these anaerobic bacteria reduce protons, released during substrate oxidation, to H₂ (Equation 6, below).

Hot bugs

During my PhD, I investigated the hydrogen-producing abilities of one of these bacteria, Caldicellulosiruptor saccharolyticus (Figure 1), which lives in hot springs: anaerobic environments at 70°C, with low levels of available carbohydrates. This bacterium is of particular interest because it is twice as efficient as most bacteria used for H₂ production.

Unlike humans, C. saccharolyticus gains energy from a wide spectrum of plant building blocks: not only glucose, but also, for example, xylose (Willquist et al., 2010).

This allows the bacteria to produce H₂ from waste such as that produced during potato, sugar and carrot processing, as well as from industrial waste from pulp and paper production, or agricultural waste such as straw.

This is a promising start, but even C. saccharolyticus releases only 33% of the potential H₂ that could be released from the substrate. Equation 5 shows the potential complete oxidation of glucose, releasing 12H₂ per molecule of glucose. Equation 6 shows the dark fermentation performed by C. saccharolyticus, which releases only 4H₂ (33%) per molecule of glucose. The rest of the energy is released as acetate (CH₃COOH).

Total conversion of glucose to H₂: C₆H₁₂O₆ + 6H₂O → 12H₂ + 6CO₂       (5)

Dark fermentation: C₆H₁₂O₆ + 2H₂O → 4H₂ + 2CO₂ + 2CH₃COOH          (6)

To release the rest of the H₂ from the acetate requires external energy. Alternatively, methane (CH₄) – which can be steam reformed to release H₂ (Equations 1 and 2) – can be generated from acetate. Luckily, there are three promising ways of doing this (Figure 2).

1. Using sunlight to convert acetate to H₂ with photofermentative bacteria (Equation 7)^w3. However, like algal H₂ production, this process is currently too slow and expensive to be commercially viable in the near future (Hallenbeck & Ghosh, 2009). 2CH₃COOH + 4H₂O → 8H₂ + 4CO₂                                 (7)
2. Using electricity to push the reaction of acetate to H₂ in a microbial fuel cell with a mixture of bacterial species (Equation 7)^w4. This is an elegant concept, but its application is currently limited by low production rates (Hallenbeck & Gush, 2009). (To learn how to build your own microbial fuel cell, see Madden, 2010.)
3. Using methane producers (Archaea) to digest the acetate, generating methane (Equation 8). The combination of dark fermentation (Equation 6) and methane production is known as the hythane process (hydrogen + methane), and can convert approximately 90% of the original substrate to H₂ and methane. CH₃COOH → CH₄ + CO₂                                                (8)The methane can then be steam reformed releasing H₂.

To put the hythane process into perspective: if four people in a house eat 10 kg potato products each in one month, their waste could fuel 0.5% of their monthly domestic energy requirement (3500 kWh), provided that the H₂ produced is used directly (to avoid energy losses) and that the house is equipped with a heat and power fuel cell^w5. More hydrogen could of course be generated from other waste – 0.5% is just from potatoes.

This is a rough estimate of the potential of the hythane process, based on a) 30% energy loss in the production of H₂ and CH₄ (hythane) and b) 30% in then steam reforming CH₄ to H₂. The steam-reforming step (b) is used in the production of hydrogen from natural gas, and is a well developed commercial technique. The production of hythane (a), however, is not yet that efficient, although research is ongoing to improve the efficiency to reach 70% (as in the example) and thus make the production of biohydrogen competitive with the steam reforming of fossil fuels for producing hydrogen.

Although there has been some recent progress^w6 (see box), it is too early to give a reliable time estimate for when sustainable H₂ production could play a significant part in supplying us with energy. However, as poet Mark Strand once said, “The future is always beginning now.”

Research into hydrogen storage and production

Storing hydrogen safely and efficiently is one of the main technological challenges to adopting hydrogen as an energy carrier. The Institut Laue-Langevin (ILL)^w7 has firmly established itself in frontier research into the hydrogen economy, using neutron diffraction to monitor hydrogenation and dehydrogenation reactions in potential hydrogen storage materials. To find out more, visit the ILL website^w7.

The powerful X-ray beams of the European Synchrotron Radiation Facility (ESRF)^w8 have recently probed the complex mechanisms by which hydrogen is produced by enzymes called hydrogenases. Most of these enzymes work under anaerobic conditions and are, in fact, inhibited by the presence of oxygen. Hydrogenases that remain active under aerobic conditions, therefore, are of great interest for technologies such as enzymatic fuel cells and the light-driven production of hydrogen. A German team of scientists has recently solved the crystalline structure of one of these enzymes (Fritsch et al., 2011) – perhaps a step towards a hydrogen economy?

Both ILL and ESRF are members of EIROforum^w9, the publisher of Science in School.

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