Hydrogen could play a key role in decarbonising the chemical industry and storing green electricity. But there is still some way to go to improve sector capacities and lower production costs. A new electrode design involved in water electrolysis could help.
Tough luck: the most abundant element in the universe, hydrogen (see box), is virtually unavailable in gaseous form on earth; creating it involves extraction from molecules that contain it, such as water or organic compounds of the natural gas type. It is understandable then that humans spontaneously turned to a free, abundant, immediately available reagent: oxygen. ‘All of our exploitation of fossil fuels,’ explains Prof. Joris Proost of the UCLouvain Materials and Process Engineering Unit and Louvain4Energy, ‘is based on our having free oxygen, even if, in the air, it’s mixed. But this isn’t a problem for many processes, especially since the essential component with which it’s mixed is nitrogen, an inert gas. Carbon-based fossil energy sources are also very cheap. The two were made for each other. But if you mix them, there’s nothing you can do: you release CO2.’ However, as global warming forces us to decarbonise our economy, hydrogen’s time may have come.
Hydrogen and energy
When we think of using hydrogen as energy, it’s usually the image of a car of the future that comes to mind. However, if there is a hydrogen market today, that’s not it. Produced at a rate of 60 million tonnes per year for a value of nearly $100 billion, hydrogen is used for the formation of hydrocarbons (the famous CH chains that represent almost all of our chemistry, from bitumen to propane and butane to various types of plastics), as a reducing gas in the steel industry, or as a basic element of ammonia (NH3) for manufacturing fertilisers. Hydrogen is therefore of interest as an element in itself and not only as an energy vector.
However, it’s in this sector that its future could be the most promising. ‘By 2050,’ Prof. Proost explains, ‘green electricity generation, that is, from renewable sources, mainly solar and wind, is expected to increase tenfold. But we know these sources aren’t constant, so there’s a big storage problem.’ One of the possible storage solutions is to connect the network to batteries, in which the current from surplus production would be stored. But they’re incapable of doing so at the gigawatt scale. The other solution, which is much more efficient in terms of capacity, is to transform electricity into gaseous hydrogen via water electrolysis.
Water electrolysis is a classic experiment that we remember from secondary school science courses: two metal (usually nickel) electrodes are immersed in water and connected to a direct current (DC) source; the passage of the current splits water (H2O) into two gases, oxygen (O2) and dihydrogen (H2). It’s a process more than two centuries old (first performed in 1800) but whose industrialisation was slowed by the cost of energy, in this case electricity, especially when it came from renewable sources. But today the cost of renewable energy has decreased, and can even be zero when production is surplus, for example in the case of sustained wind or maximum sunlight. This decline in energy cost, however, isn’t sufficient to ensure the system’s economic viability. Other problems exist, related to the use of gas products. For oxygen, no problem: it can be released into the atmosphere where it will find its full usefulness. But what about hydrogen?
Green or black hydrogen
The first idea that comes to mind is to provoke the reverse chemical reaction and thus reproduce electricity from hydrogen, exactly as one would expect to do using a fuel cell that could, for example, provide electrical energy for a car or home. This might happen one day, but today the energy efficiency of this process is very poor and only a small portion of the electricity injected at the beginning of the process is generated. It’s therefore better to use hydrogen as is, for the uses described above, with one advantage: it’s ‘doubly green’ hydrogen because it’s produced from sustainable, surplus electricity.
Everything would be fine if there was no cost problem. ‘The hydrogen produced today by the usual processes is indeed not “green” at all,’ Prof. Proost says, ‘because eight tonnes of CO2 are released into the atmosphere for every tonne of H2 produced. But it comes to about €2 euros per kilo, three times that if it is done by electrolysis. As a result, 96% of hydrogen is produced by traditional processes and only 4% from water electrolysis.’ The challenge is therefore to bring down the price of green hydrogen.
How? We can obviously bet on the fact that carbon taxes will increase the cost of black hydrogen. But nothing is certain, and this will probably be insufficient. It’s better to turn to technology.
Although our secondary school lab devices pale in comparison to industrial electrolysers, the basics are identical: everything plays out at the electrodes. At school, anode and cathode suffice; for industrial electrolysers, hundreds of them must be stacked one on top of another. But various technical factors limit this stack, and to absorb, for example, the excess current of a 2 MW wind turbine, it’s necessary to install several electrolysers, one after the other, which increases costs. The dream would be to have more efficient electrodes and thus be able to use fewer of them and thus fewer installations for absorbing current peaks. This is what Prof. Proost’s team has achieved by replacing traditional electrodes with porous electrodes in three dimensions. ‘So we increase the active surface area of the electrodes by a factor of 40 or 50 without multiplying their number. Sure, the production of H2 is not 40 to 50 times higher, but the reduction in the number of electrodes to accommodate a given amount of power can significantly reduce the production costs of electrolytic hydrogen, matching those of black hydrogen.’
The performances observed in the laboratory will soon be tested at the industrial level in a spin-off. The electrolysis units, which run on green electricity, have a role to play in decentralising hydrogen production, whether for use in fertiliser or in the steel industry, but also, with small electrolysers, in producing heat in homes or even electricity via a fuel cell. ‘But the hydrogen market is primarily in the chemical sector’, Prof. Proost concludes. ‘It will make the chemical industry greener.’
The most abundant element in the universe (the main constituent of stars), hydrogen is not present in its molecular form (H2) on earth. To acquire it, it must be extracted from molecules that contain it, such as methane (CH4) or water (H2O). Today, the vast majority of produced hydrogen (96%) is generated by steam reforming from natural gas. The latter (which is essentially composed of methane) is converted in the presence of water vapour and heat into H2 and CO2. It is therefore an operation that requires energy and produces a lot of greenhouse gases (eight tonnes of CO2 per tonne of H2 produced). In addition, it is carried out in large installations; production is therefore highly centralised and requires distribution mainly by truck. This is 'black' hydrogen.
There are other ways to produce it, such as via charcoal gasification, which must be done at a very high temperature and releases CO. And of course water electrolysis, the greenest way to produce it, which remains less competitive at the moment.
A glance at Joris Proost's bio
After graduating as a civil engineer in metallurgy and materials at KU Leuven in 1994, Joris Proost completed his PhD in applied sciences at the same university and the Interuniversity Microelectronics Center (IMEC). His postdoctoral path led him mainly to Harvard University. He returned to Belgium in 2003, to the Louvain School of Engineering at UCLouvain where he climbed the academic ladder to become professor (2017).
In general, his research focuses on the reactivity of metals and their oxides in different environments, with particular attention on sustainable processes in electrochemistry. Since 2015, Prof. Proost has also been the Belgian representative on the subject of hydrogen within the International Energy Agency (IEA). In this role, he was recently invited to help draft a Hydrogen Strategic Report, which will be officially presented in Tokyo in July 2019 at the G20.