Green hydrogen, a chimera?

Humans have stored solar energy since the first energy revolution through plants, and now, in the third energy revolution, we urgently need abundant and cheap storage vectors. Hydrogen is one of the most popular.

Green hydrogen makes it possible to store energy captured directly from the sun through photovoltaic cells or from the wind through wind turbines. As I explained in a previous article, we need it to reduce CO₂ emissions.

We have all the hydrogen we can imagine in seawater. And getting it would be really cheap… if the most universal and basic law of physics didn’t exist: the second law of thermodynamics says that getting concentrated energy is always difficult.

Break the water with electricity

To get hydrogen, all you need is a flashlight battery, a couple of cables and a couple of pencils, as seen in the figure that accompanies this paragraph. If instead of a flashlight battery, we have a photovoltaic panel or a windmill, we can get industrial quantities of hydrogen. Where is the problem?

It is very easy to combine hydrogen and oxygen (burn hydrogen) to obtain energy: 33.33 kWh per kilogram of hydrogen. In other words, the energy required by a 1 kW cooker operating for 33.33 hours, or that of a 66 kW car running at full power for half an hour.

Separating the hydrogen from the oxygen (ie, breaking the water) requires similar energy, but breaking the water molecule is very difficult. To achieve this, plants carry out photosynthesis, one of the most complex biochemical processes in nature. And we do it with electricity.

The electricity in the water causes the water molecule to break, but only at the electrodes (the pencil leads in our experiment) and very slowly.

The theoretical rate of hydrogen production in an electrolytic cell with sheets (cathode and anode) with a surface area of ​​1 m² and a current density of about 2,000 A/m² introduced into the water is 72 g/h. Therefore, in 6 hours 432 g or 14.4 kWh would be produced.

In the case of using solar energy, if we consider a 100% efficiency in the conversion of electricity to hydrogen and a production of 1.2 kWh/m² in the 6 hours of sun that a 1 m² photovoltaic panel works, we would need 12 panels to produce those 432 g of green hydrogen.

Current hydrogen production processes

To produce hydrogen, water molecules are arranged on the surface of catalysts at the electrodes of an electrolyser or cell. Catalysts are made up of metals such as platinum and nickel oxides. The interactions of the electrons of these metals with the individual water molecules on those surfaces manage to break the strong bonds between hydrogen and oxygen.

There is a lot of research on very different processes and possible catalysts, but there are two technologies most used today and that we will most likely use until 2050: alkaline batteries or cells (usually salt water) and batteries based on a polymeric membrane. (PEM) proton exchange (ionized hydrogen) in an acidic environment.

alkaline batteries

The alkaline case is the cheapest, since the catalysts can be nickel or cobalt. Its efficiency in the conversion of electricity into hydrogen follows the black curve of the graph that follows these lines.

If we want efficiencies greater than 50%, we need currents less than 2,000 amps per square meter (A/m²). Since higher efficiencies imply lower electrode currents, higher efficiencies imply lower production rates.

According to a Faraday formula, the rate of hydrogen production (in moles) is equal to the electrical current per square meter of catalyst plate, multiplied by the area of ​​the plate in square meters, and divided by two by Faraday’s constant. It is the calculation necessary to calculate the productivity in moles per second.

The balance between efficiency and productivity is reached for 1,950 A/m² with an efficiency of around 52%.

Polymeric membrane batteries

In the case of the polymeric membrane, the medium where the electrodes are located is acid, and the catalysts must therefore be noble metals, which are resistant to acids, and more expensive, such as platinum, iridium and titanium. In this case the standard yield is 60%.

Another technology has recently been proposed: high-performance electrolysis, a capillary system whose discoverers say that it can reach 90% efficiency, but it is in the laboratory phase, still far from industrial use.

How much solar energy do we need?

A photovoltaic plate of one square meter produces more or less 5 amps at about 40 volts, that is, a power of 0.2 kW per hour. In 6 hours it produces 1.2 kWh. If we transform the voltage to 1.30 volts (the minimum for electrolysis), the current rises to 153 amps. The 12 photovoltaic panels of one square meter mentioned above would generate 1,950 A/m². This, in theory.

If we go to the laboratory, a very conscientious work, and representative of many others, was carried out by Gül and Akyüz from the University of Balikesir (Turkey) in 2020. In their study, they obtained 4.5 kg of hydrogen per year using two photovoltaic panels of 0.2 kW of power each. Throughout the year the electrical energy produced by the plates was 557 kWh, much lower than the energy possible if the plates had worked with 0.2 kW 6 hours a day, 365 days a year. The 4.5 kg of H₂ would serve to produce 150 kWh of energy. The yield in the laboratory is thus 27%, more or less half of the theoretical yield.

This performance can be improved, but little, since the catalysts are what they are, and both photovoltaic and electrolysis processes are complex. Industrial processes are always much less efficient than those carried out in the laboratory and, therefore, than theoretical processes. Reality is never ideal.

As for prices, they currently range between 2 and 5 euros per kg of hydrogen under electrolysis. The price per kWh obtained from hydrogen is 0.06 euros, while that of gasoline or diesel at the refinery output is 0.03 euros. If the cost of hydrogen fell to 1 euro per kg, its price would be equivalent to that of gasoline.

In Spain, for example, we use 400 TWh of energy for transport every year. This is equivalent to 12 billion kg of hydrogen, which would require 5.3 billion plates in the case of a 27% yield, or 2.65 billion in the case of a 54% yield. That is, 2,650 km² of plates. Let us remember that the autonomous community of La Rioja has 5,000 km².

Spain has an area of ​​about 500,000 km². A large proportion of this area is desert or near-desert land. Installing all those plates is feasible. What is needed is the business and political decision to do it, with concrete terms, credits and judicial guarantees on the earnings of the investments to achieve it.

At the moment everything is still utopian. But we don’t have time to leave things for later. We risk, not our life, but our way of life, our culture.

Antonio Ruiz de Elvira Serra, Professor of Applied Physics, University of Alcala

This article was originally published on The Conversation. Read the original.