Hydrogen – unleash the beast!

Complementing one of our older posts The time of Hydrogen is NOW, today’s focus will lie on the several ways of hydrogen production, looking deeper into the underlying processes and potential future green hydrogen technologies. 

The vast majority of hydrogen is generated from fossil fuels and natural gas, meaning the majority of today’s hydrogen is not green^[1]. With all the following innovative technologies, one would wonder why natural gas still is the dominating resource when it comes to producing hydrogen. There is an easy answer to that – it’s the cheapest. Several governments all around the world subsidies green hydrogen fabrication to increase the scale and therefore reduce costs, to be competitive with the status quo of fossil fuel-based generation. Below we will focus on the various technologies to generate hydrogen, without looking at prices and other factors that contribute to scaling usage. 

Green hydrogen does not necessarily need its specific generation or production process^[2]. Still, it can be obtained as a byproduct from the manufacturing process of chemicals like chlorine in the Chlor-alkali industries. Further, petrochemical complexes and plants release byproduct hydrogen as part of their olefin production. This byproduct hydrogen does not use any additional energy to be obtained; therefore, it can be regarded as green. Let’s have a look at the chlorine production to understand better how hydrogen can be gained as a byproduct; three key ingredients are needed to make the element of chlorine: salt, electricity, and water. The element chlorine (Cl₂), caustic soda (NaOH), and hydrogen (H₂) are the results of the electrolysis process to produce chlorine. 

Another way of generating hydrogen is through so-called thermochemical processes, that use energy from various resources such as natural gas, coal, or biomass to release hydrogen from their molecular structure. Further, a combination of heat and a closed chemical cycle can produce hydrogen from feedstocks such as water. 

Let’s have a closer look at some of these processes^[3]:

Due to its low cost, steam-methane reforming and partial oxidization of natural gas, which contains methane (CH₄), are the most common production processes of hydrogen, until now.  

Steam-methane reforming is a process in which high-temperature steam in combination with high-pressure and the presence of a catalyst, is used to produce hydrogen from a methane source; other released elements are carbon monoxide and a relatively small amount of carbon dioxide. Steam reforming may also be used to produce hydrogen from a wider variety of fuels, such as gasoline, ethanol and propane. There is a secondary process which needs to happen to produce pure hydrogen, a so-called “water-gas shift reaction”, where carbon monoxide and steam are reacted, that is where the catalyst comes into play, to produce carbon dioxide and more hydrogen. In the final step, pressure swing absorption, impurities and carbon dioxide are removed. Below the chemical processes of steam methane reforming are illustrated^[4]. 

Steam-methane reforming reaction
CH4 + H2O (+ heat) → CO + 3H2

Water-gas shift reaction
CO + H2O → CO2 + H2 (+ small amount of heat)

A usually much faster process for producing hydrogen is the partial oxidation process. However, it produces less hydrogen per input unit, yet gives off heat, which could potentially be used as well. Hydrocarbons in natural gas are reacted with oxygen (or air for that matter) to release carbon monoxide, hydrogen and nitrogen if the reaction is carried out with air rather than pure oxygen. Again, the water-gas shift reaction described above is applied for obtaining pure hydrogen. 

Partial oxidation of methane reaction
CH4 + ½O2 → CO + 2H2 (+ heat)

Water-gas shift reaction
CO + H2O → CO2 + H2 (+ small amount of heat)

Let’s move on to coal gasification, which is a more complicated process than the ones described above. Power, liquid fuels, chemicals, and hydrogen are being produced through first reacting coal with oxygen and steam under high pressures and temperatures to form synthesis gas, a mixture consisting primarily of carbon monoxide and hydrogen. Once this has taken place and impurities have been removed, another reaction with steam, once again through the water-gas shift reaction, is applied to produce additional hydrogen and carbon dioxide^[5].

Coal gasification reaction (unbalanced):
CH0.8 + O2 + H2O → CO + CO2 + H2 + other species

As discussed in last week’s post, Biomass – Renewable Energy by burning our forests, we are not convinced of the renewable aspect of biomass. However, another way of using the inevitable biomass is to produce hydrogen. Biomass gasification is basically the same process as coal gasification, yet, an additional step must be taken as biomass does not gasify as quickly as coal. The main challenge this gasification process is facing is the capital costs associated with equipment and biomass feedstock^[6].

Simplified example reaction
C6H12O6 + O2 + H2O → CO + CO2 + H2 + other species

Another possible way to use biomass for hydrogen generation is to reform biomass into biomass-derived liquids, then perform a very similar process as the natural gas reformation does, to obtain hydrogen^[7].

Steam reforming reaction (ethanol)
C2H5OH + H2O (+ heat) → 2CO + 4H2

Water-gas shift reaction
CO + H2O → CO2 + H2 (+ small amount of heat)

A potential low or no greenhouse gas emissions process called thermochemical water splitting uses high temperatures, usually from solar power or waste heat of nuclear power reactions, in combination with chemical reactions to produce hydrogen and oxygen from water. In a closed loop, due to reuse of chemicals, a series of chemical reactions under high-temperature heat produces hydrogen. There are more than 300 different water-splitting cycles described in the literature, each with their advantages and disadvantages. Something most of them have in common is near-zero greenhouse gas emissions by using water and either sunlight or nuclear energy. The main challenges to overcome are finding commercially viable cycles and reactors^[8].

Electrolytic processes, on the other hand, are already well established and commercially available, splitting water into hydrogen and oxygen by using electricity. The most important aspect when doing this electrolysis, however, is using renewable energy as an electricity source to provide green hydrogen. 

In the early stage of research are so-called “direct solar water splitting processes (photolytic processes)”, that use light energy to split water into hydrogen and oxygen. With more R&D and further development, these processes offer long-term, low environmental impact solutions for hydrogen production. There are two main types of these direct solar water splitting processes, e.g. (i) photoelectrochemical (PEC) and (ii) photobiological. The latter uses microorganisms and sunlight to turn water and sometimes even organic matter into hydrogen, with the help of the sun. PEC, on the other hand, uses specialized semiconductors, somewhat similarly composed as the ones found in PV solar electricity generation, immersed in a water-based electrolyte where sunlight, once again, energizes the water-splitting process^[9].

Again, still at an early stage of research are so-called Microbial biomass conversion processes, that make use of bacteria and microalgae to produce hydrogen. “Dark fermentation processes”, named after their ability, to break down organic matter to produce hydrogen without any need of light. The microbes produce hydrogen themselves by breaking down complex molecules, through a variety of ways, whose byproducts can be combined by enzymes to produce hydrogen. This solution also offers enormous potential when it comes to producing green hydrogen and shifting away from natural gas reliant reforming processes^[10].

Only recently, researchers from Rice University have built a new device, similar to other “artificial leaf” designs, that can create hydrogen for fuel by splitting water. This device is solely solar-powered and can potentially be dropped into water under direct sunlight and produce hydrogen as needed. The generated hydrogen is an outcome of a perovskite solar cell hooked up to catalyst electrodes that electrolyze the water. After it does its magic, once the sunlight hits the solar cell, produces electricity to power the catalyst, which then splits water into oxygen and hydrogen. The hydrogen can be collected from the surface as its bubbles up. By using cheaper components, the research team says that commercialization may be possible soon^[11]. 

With all these innovative and potentially green innovations, what’s holding us back to shifting the industry standard? Which green hydrogen production process looks most promising? 

^[1] https://www.planete-energies.com/en/medias/close/hydrogen-production

^[2] https://afdc.energy.gov/fuels/hydrogen_production.html

^[3] https://www.energy.gov/eere/fuelcells/hydrogen-production-processes

^[4] https://www.energy.gov/eere/fuelcells/hydrogen-production-natural-gas-reforming

^[5] https://www.energy.gov/eere/fuelcells/hydrogen-production-coal-gasification

^[6] https://www.energy.gov/eere/fuelcells/hydrogen-production-biomass-gasification

^[7] https://www.energy.gov/eere/fuelcells/hydrogen-production-biomass-derived-liquid-reforming

^[8] https://www.energy.gov/eere/fuelcells/hydrogen-production-thermochemical-water-splitting

^[9] https://www.sciencedirect.com/science/article/abs/pii/S0360319910004374

^[10] https://www.worldscientific.com/doi/abs/10.1142/9789814317665_0043

^[11] https://pubs.acs.org/doi/10.1021/acsnano.9b09053