Green hydrogen production: a few thoughts on promises and challenges

Hydrogen holds some potential (best explained in Hydrogen Ladder 5.0 by Michael Liebreich) as a secondary source of energy, receiving considerable interest and investment as a critical part of future sustainable energy systems. 

Michael' Liebrich's Hydrogen Ladder 5.0 (updated on Nov 2023) [1]

Theoretically, hydrogen is a promising element for clean energy systems because of its high energy content by mass, role as an energy carrier, and low emissions when used as a fuel. However, each of these advantages has its challenges, some of which we will elaborate on in future articles, but for the time being, we will focus on the production of hydrogen.

The energy density of H2: by volume and by mass [2]

Hydrogen production - background

Many articles discussing the importance of hydrogen for a zero-emission world start by stating that it is “the most abundant element in the universe”. While this is true, it is commonly perceived that hydrogen is not found naturally on Earth as a free gas; instead, it is always combined with other elements. As a result, hydrogen cannot be mined in the traditional manner that we do for minerals or fossil fuels. This assumption is increasingly being challenged however, with several start-up companies, such as HyTerra and Gold Hydrogen, actively drilling for hydrogen in the USA and Australia, respectively. French researchers believe that 46 million tons of hydrogen can be found in old coal mines in France’s Alsace-Lorraine region. The US Department of Energy’s ARPA-E agency predicts that potentially 150 trillion metric tons of free hydrogen gas could be found worldwide. Recovering hydrogen from the subsurface necessitates oil and gas industry approaches, which would involve the use of well-known drilling rigs – with a consequent emissions burden. But there is still a lot of uncertainty surrounding geological hydrogen, and numerous theories as to whether and how much hydrogen is available as a free element in the Earth’s subsurface.

So, we are, for the time being, dependent on producing hydrogen from intermediate sources. Hydrogen is found in water (H2O), where it is bonded with oxygen, and in fossil fuels, where it is bonded with carbon - although significant amounts of energy are required to break these strong molecular bonds to obtain hydrogen. Currently, approximately 95% of hydrogen is produced via steam methane reforming (SMR), which involves the reaction of methane (from either fossil fuels or biogas) with steam under 3-25 bar pressure in the presence of a catalyst to produce hydrogen, but also carbon monoxide and carbon dioxide, which partially defeats the purpose of obtaining hydrogen from a zero-emission process.

There are several sustainable “clean” hydrogen production methods using different forms of energy, such as thermal, electrical, biochemical, photonic and even radiation energy based on renewable energy sources such as nuclear fission/fusion, solar, wind, hydro, biomass etc. Still, none of them are as technically (and commercially) mature as steam methane reforming.

Routes for sustainable clean hydrogen production [4]

Hydrogen from freshwater

Direct water splitting technologies that use electricity as an energy source, such as conventional electrolysis, Proton Exchange Membrane (PEM), Alkaline Electrolysis and, to a lesser extent, high-temperature electrolysis (SOEC), are heavily studied to provide a short-to-medium-term solution. In these technologies H2 and O2 are simultaneously derived from water molecules, with no undesired side reactions. It is even stated by Clean Hydrogen Partnership Europe that without water-derived H2, deep decarbonisation (>80%) will be impossible [3]. They propose a scenario where up to 24% of the EU’s energy demand (equivalent to ~2,250 TWh) would be provided by hydrogen.

It is sometimes forgotten that splitting water not only requires a considerable amount of energy but also vast amount of water. A calculation on the back of a (large) envelope tells us that to produce 2,250 TWh of energy, 57 million tons of hydrogen must be produced, which in turn requires 513 million tonnes of water (ignoring losses) or more than 205,000 Olympic-sized swimming pools.

Most commercially available water-splitting technologies need freshwater - even purified water - for their operation, so it becomes important to have a closer look at how water is being used. The availability of freshwater as a feedstock for agriculture, drinking water and many industries is already under strain in many parts of the world, so adding additional freshwater demand for hydrogen production, even if the water is recycled at a later stage when the hydrogen is used as a fuel in a fuel cell for example, will present challenges. The production, treatment, and distribution of clean or purified water also incur a significant energy demand.

Hydrogen from seawater

Seawater, which accounts for 97% of the water on earth, could be used as an alternative to freshwater. Seawater is quite evenly distributed around the globe, reducing the possibility that a rush for green hydrogen may exacerbate future resource-related geopolitical challenges (think “water wars”). Unfortunately, the use of seawater presents significant technological challenges due to the presence of various ions, such as Cl-, Na+, Mg2+ and many other impurities [5]. The use of seawater without proper stabilisation dramatically increases electrolyser costs up to 6-fold. Seawater causes various detrimental effects such as Cl2 evolution, scaling, membrane fouling and corrosion. This is especially true for PEM electrolysers, which require very pure water as an input. Conventional electrolyser cells suffer badly from corrosion and inorganic scaling due to the significant local pH variations (up to 9 pH units) occurring at the electrodes. Producing pH-balanced and purified water with very low Total Dissolved Solids (TDS) further increases the energy consumption of water treatment.

One of the key challenges of direct electrolysis of seawater at room temperature is the competition at the anode between the chlorine oxidation reaction (bad) and the water oxidation reaction (good). When chloride is in the presence of water, it can undergo a variety of reactions, including the oxidation of chloride to form chlorine or hypochlorous acid. The competition between these reactions is influenced by factors such as kinetic efficiency, thermodynamic considerations, and other specific conditions. This can lead to corrosion of electrocatalysts and electrolyser devices, particularly when using traditional anode materials such as Ru- or Ir-based catalysts.

Future technologies for seawater-splitting

There is a lot of ongoing academic and commercial research in this area to identify alternative technologies that are better suited to seawater or brackish water. Below are some of the more exciting research findings that WSS Energy uncovered as part of our research projects.

Some of these technologies are based on photocatalytic (as opposed to electrochemical) water splitting. One project is based on p-GaN-based nanowire arrays without any external bias or sacrificial agents from various types of seawater solutions. A stable solar-to-hydrogen conversion efficiency of 1.9% was obtained under concentrated irradiation (equivalent to 27 suns), demonstrating its possible utilisation for a large-scale, environmentally friendly, and sustainable clean hydrogen fuel generation technology. Importantly, the system was evaluated using different salinity levels, NaCl aqueous solutions of various concentrations and pH, as well as artificial seawater with varying ion concentrations [6].

Other techniques are based on SOEC technology, as mentioned above. Solid oxide cells typically operate at temperatures above 600°C, with raw materials entering the cell in a gas (vapour) state. Seawater is heated and evaporated, and the resultant salt and impurity-free water vapour is split by an SOEC at high temperatures. In this instance, any adverse effects of sea salt on the electrolyser are avoided, and a higher electric energy conversion efficiency is theoretically obtained.

Some researchers have proposed using a flat tube solid oxide cell with double-sided cathodes to split seawater and produce hydrogen efficiently and reliably [7]. The structure and the composition of the SOEC were not negatively affected by seawater, although there was some sea salt build-up in the device’s inlet pipe. This challenge could be addressed during the engineering phase of constructing a demonstration device.

Structure of the SOEC cell (left) and its physical shape (right) [7]

Other research projects are investigating bipolar membranes, composed of a cation exchange layer (CEL) combined with an anion exchange layer (AEL), integrated into a full-fledged water electrolyser device. According to the research, this setup results in an inherently ion-tolerant seawater electrolyser, provided that seawater is exclusively present at the cathode. However, an observed decreased voltage stability when the system is fed with seawater indicates that other, poorly understood, failure mechanisms arise from its complex electrolyte composition. As such, significant improvements in cell voltage and durability are still required before the commercial deployment of these devices becomes feasible [8].

BPMWE and PEMWE device schematics (A) Cross-sectional schematic of a zero-gap BPMWE and (B) cross-sectional schematic of a zero-gap PEMWE, illustrating the positioning of catalyst/ ionomer-coated porous transport layers (PTL) relative to their respective ion-exchange membranes, and the circulation scheme that was used for all electrolyser experiments [8]

To circumvent the problems described above, some research projects, companies and joint ventures present a combination of membrane distillation (MD) and PEM water electrolysis as a solution to produce H2 from saline water. MD is an emerging thermal-driven separation process that utilises a hydrophobic membrane to separate two aqueous solutions at different temperatures. The process involves the passage of water vapour through the membrane, followed by condensation on the cold side, which produces distilled-quality water. Combining MD and PEM water electrolysis allows the waste heat from the electrolysis process to be used in the MD device to desalinate the seawater, allowing green hydrogen to be produced solely from renewable electricity therefore no additional heat is required. For example, the Sea2H2 project describes a loosely integrated process incorporating membrane distillation with a PEM water electrolyser [9]. Another example is the joint venture between Ohmium and Aquastill, with the same goal [10].

MD and PEM integration for the production of hydrogen [9]

Most of the seawater-splitting technologies are still in the early stages of development and are not expected to reach mass manufacturing at a commercially competitive level any time soon, except for membrane distillation combined with water electrolysis.

Most of the hydrogen is today produced through SMR. Considering the sunk capital and available natural gas, it is reasonable to expect that producers will make these installations greener by the removal of the produced CO2 through CCS (Carbon Capture and Storage).

In the near to mid-term, PEM electrolysers will likely become the dominant technology. This will require considerable renewable energy, not just for the electrolysis process, but also for treating freshwater (or saline water) to an electrolyser-suitable specification. Although the energy consumption for water treatment is arguably minor compared to the energy consumption of an electrolyser, it will have an emissions impact unless both water treatment and distribution are also powered by renewable electricity.

References:

[1] https://www.linkedin.com/pulse/hydrogen-ladder-version-50-michael-liebreich/

[2] https://www.economist.com/science-and-technology/2020/07/04/after-many-false-starts-hydrogen-power-might-now-bear-fruit

[3] ‘HYDROGEN ROADMAP EUROPE A sustainable pathway for the European energy transition’, Feb. 06, 2019. Accessed: Dec. 10, 2023. [Online]. Available: https://www.clean-hydrogen.europa.eu/system/files/2019-02/20190206_Hydrogen%2520Roadmap%2520Europe_Keynote_Final.pdf

[4] R. S. El-Emam and H. Özcan, ‘Comprehensive review on the techno-economics of sustainable large-scale clean hydrogen production’, J. Clean. Prod., vol. 220, pp. 593–609, May 2019, doi: 10.1016/j.jclepro.2019.01.309.

[5] F. Dingenen and S. W. Verbruggen, ‘Tapping hydrogen fuel from the ocean: A review on photocatalytic, photoelectrochemical and electrolytic splitting of seawater’, Renew. Sustain. Energy Rev., vol. 142, p. 110866, May 2021, doi: 10.1016/j.rser.2021.110866.

[6] X. Guan, F. A. Chowdhury, N. Pant, L. Guo, L. Vayssieres, and Z. Mi, ‘Efficient Unassisted Overall Photocatalytic Seawater Splitting on GaN-Based Nanowire Arrays’, J. Phys. Chem. C, Feb. 2018, doi: 10.1021/acs.jpcc.8b00875.

[7] Z. Liu et al., ‘Efficiency and stability of hydrogen production from seawater using solid oxide electrolysis cells’, Appl. Energy, vol. 300, p. 117439, Oct. 2021, doi: 10.1016/j.apenergy.2021.117439.

[8] D. H. Marin et al., ‘Hydrogen production with seawater-resilient bipolar membrane electrolyzers’, Joule, vol. 7, no. 4, pp. 765–781, Apr. 2023, doi: 10.1016/j.joule.2023.03.005.

[9] J. van Medevoort and N. Kuipers, ‘Sea2H2 - Hydrogen from seawater’, WUR. Accessed: Dec. 13, 2023. [Online]. Available: https://www.wur.nl/en/project/hydrogen-from-seawater.htm

[10] ‘Ohmium and Aquastill Form Strategic Collaboration to Produce Green Hydrogen from Seawater | Ohmium’. Accessed: Feb. 04, 2024. [Online]. Available: https://www.ohmium.com/news/ohmium-and-aquastill-form-strategic-collaboration-to-produce-green-hydrogen-from-seawater