Pure to green
The role of water quality in power-to-gas applications
Markus Janssen, Shimadzu Europa GmbH
Green hydrogen could turn the dream of a sustainable supply of energy and raw materials into a reality. But how is this gas actually produced from renewable energy? And what role does water quality play in this? The following article has the answers. It addresses key technologies and innovative devices that not only ensure water quality but also contribute significantly to the efficiency and service life of electrolysis plants, which are indispensable for hydrogen production.
“My friends, I believe that water will one day be employed as fuel, that hydrogen and oxygen, which constitute it, will furnish an inexhaustible source of heat and light of an intensity of which coal is not capable. […] Water will be the coal of the future.” These are the words of Jules Verne in his famous novel “The Mysterious Island”. Words that laid the foundation for science fiction literature in 1874 and are becoming a reality today. And it almost sounds too good to be true: Hydrogen is neither toxic nor radioactive, it burns without producing pollutants and is also the most abundant element in the universe. In principle, it is technically possible to use hydrogen in many areas, such as industry, transportation and for heating. From an energy efficiency perspective, however, this does not always make sense, since using renewable electricity directly in many cases yields greater savings in greenhouse gas emissions.[1] But in cases where electrification is technically impossible or uneconomical, the use of hydrogen is a sustainable alternative. Decision-makers worldwide are preparing for the large-scale conversion to the so-called “hydrogen economy”, with the aim of replacing fossil fuels such as coal, oil and natural gas with electrification and hydrogen as much as possible.
From energy to gas
However, it’s not quite that simple because molecular hydrogen (H2) practically does not occur on earth in the form of a pure gas but predominantly in bound form and as a component of water. This means it cannot simply be obtained as a primary energy source like natural gas, for example, but must be produced using other energies. The energy source used for this then determines how sustainable the hydrogen economy itself becomes. Currently, H2 is mainly produced by steam reforming from the methane contained in natural gas and biogas; this is referred to as gray hydrogen. However, this also releases an average of 10 tons of CO2 into the atmosphere per ton of hydrogen produced.[2] If it is, on the other hand, produced exclusively using renewable energies in the so-called “power-to-gas process” (P2G), then it is referred to as green hydrogen. But how is energy converted into gas?
The first step in the power-to-gas process is generating electricity based on renewable energies as the primary energy source. This electrical energy is then converted into chemical energy by means of electrolysis. In the case of hydrogen, the technology has been well established for a long time and is often even demonstrated as a practical experiment during school lessons. Two electrodes are immersed in a water bath and connected to a direct current source. With sufficient voltage, the electric current breaks the water down into its components hydrogen and oxygen. This process is accelerated by the addition of electrolytes, such as acids, alkalis or neutral salts, as they increase the conductivity of the water and thus the current flow. Water is broken down to hydrogen gas at the negatively charged electrode, the cathode, while it is oxidized to oxygen gas at the positively charged anode. Both gases rise as bubbles at the electrodes.
Markus JanssenShimadzu Europa GmbH
A “stack” of key technologies
The school experiment, which is often carried out using lye as the electrolyte, is very similar to the principle of alkaline electrolysis (AEL), which has been used for decades to generate H2 on an industrial scale. There are disadvantages to this process, however, which may make it less suitable for the production of green hydrogen. Fluctuating power sources, such as wind power or photovoltaics, require a cyclical start-up and shut-down of hydrogen production and at least partial-load operation. AEL is less suitable for this, as it requires long start-up times and there can be problems with gas quality in partial-load operation.[3] By contrast, “PEM electrolysis” is considered a key technology for the production of green hydrogen and thus for the energy transition.
A proton exchange membrane (PEM) consists of a thin, solid polymer material such as Nafion, which can conduct protons (H+ ions) but is impermeable to electrons and gases. In a PEM electrolyzer, this membrane is the key component of every cell and is located between the anode and cathode, which are often coated with a catalyst to accelerate the electrolytic reactions. A central element of a PEM electrolyzer is the so-called “stack”, which consists of several cells stacked on top of each other – hence the name (Figure 1). Bipolar plates serve as current conductors between individual cells and dissipate the gases produced. The number of cells in a stack determines the voltage of the electrolyzer: the more cells, the higher the voltage. A stack can consist of just a few to several hundred cells. PEM electrolysis is a high-pressure process that enables high current densities. This high current density leads to more efficient hydrogen production, as more hydrogen is produced per unit of time. This makes PEM electrolysis a preferred technology when it comes to producing green hydrogen because, unlike AEL electrolysis, it can be started up more quickly and has short reaction times with fluctuating electricity production.
Efficiency through purity
The purity of the feed water is crucial for the efficiency and service life of PEM electrolyzers. In stoichiometric terms, nine liters of water are needed to produce one kilogram of green hydrogen. However, this water must meet certain quality standards in order to avoid stack failures and reduced performance. Impurities, such as dissolved salts, minerals and organic compounds, can cause irreversible damage [4], impair the performance and service life of the membranes and can negatively affect hydrogen quality. For this reason, strict specifications for electrical conductivity and total organic carbon (TOC) content must be met, often based on international guidelines for ultrapure water (Table 1). It’s vital that these parameters are continuously monitored for the long-term and economical production of green hydrogen.
Hydrogen electrolysis plants are often built near solar and wind farms. However, there may be a lack of sufficient fresh water in sunny, dry areas, while in windy coastal regions the available water is usually saline. Such conditions require special water treatment technologies (Table 2) to make the available raw water usable for PEM electrolysis. The treatment of seawater requires around four times more energy (7–9 kWh/m³) than that of groundwater. This is low compared to the total energy requirement of hydrogen production, which is around 5,000 kWh/m³. This is due to the difference in the forces to be dealt with: When producing ultrapure water, only the attractive forces between water molecules and impurities have to be overcome, whereas in electrolysis the stronger covalent bonds between the atoms of the water molecule have to be broken. Hydrogen cannot be produced without high-purity water. These considerations emphasize the importance of careful water quality monitoring, as degradation of the PEM stack due to poor water quality can cause significant loss of energy and cost efficiency.
|
Contaminant group |
Possible negative impact |
Measuring parameters |
Example of limit value |
|
Organic compounds |
Membrane fouling, corrosion, biofilm, hydrogen impurity |
TOC |
< 50 ppb |
|
Ions/inorganic impurities |
Conductivity reduction, catalyst damage, H2 impurity, corrosion |
Electrical conductivity |
< 0.1 µS/cm
|
|
Processing technology |
Short name |
Removed contaminants |
|
Sand filtration/aeration |
Prefiltration |
Iron and manganese |
|
Ultrafiltration |
UF |
Particles, organic matter, microbiology |
|
Softener/antiscalant dosage |
Softener |
Hardness-causing ions |
|
Membrane degassing |
Dissolved gases (e.g. CO2) |
|
|
Reverse osmosis |
RO |
Salt, particles, microbiology, ionic load, organic matter |
|
Electrodeionization |
EDI |
Removal of ions and ionizable contaminants |
|
Ion-exchange resin |
Polisher |
Final contaminants |
Ensuring water quality using PAT
Monitoring and securing water quality in an electrolysis plant can be significantly improved by using process analytical technology (PAT). In addition to the water pretreatment system, the ultrapure water loop, which is used for both water supply and heat dissipation, plays a crucial role in guaranteeing trouble-free operation and avoiding additional costs. Impurities can enter the loop during operation, which is why a partial flow is purified using an ion exchange resin before being fed into the anode. An online TOC analyzer can be used at this point for final quality control using PAT. Since there is no direct correlation between the TOC and electrical conductivity parameters and the presence of contaminants, both parameters should be continuously measured. Analyzers such as the Shimadzu TOC-1000e (Figure 2) provide both readings at short time intervals, allowing early detection of contamination and enabling predictive maintenance of the electrolyzer.
The TOC-1000e is specifically designed for online monitoring of ultrapure water and provides precise measurement of both TOC content with a detection limit of 0.1 µg/L and the electrical conductivity of water. Its innovative technology includes a mercury-free excimer lamp that generates ultraviolet light at 172 nm to break down even difficult-to-oxidize components, ensuring that no contaminant goes undetected. The lamp is only switched on when needed which extends its service life and doubles the usual operating life of the device to one year compared to conventional instruments. This means that the TOC-1000e can operate automatically for up to a year. Another highlight is the so-called “Active Path”, in which the sample flows directly through the lamp to ensure the most efficient irradiation possible, minimizing contamination and carry-over effects. On top of this, the TOC-1000e enables annual calibration and maintenance quickly on site, which means that the analyzer does not have to be sent in. It stands out for its excellent connectivity, offering bidirectional bus communication and a built-in web server for easy remote diagnostics and a detailed view of data, including history. Despite its extensive range of functions, the TOC-1000e is small and light, weighs less than 3 kg and has a front panel the size of an A4 sheet of paper. Its mobility allows it to be used for maintenance and troubleshooting on other parts of the water treatment system in the electrolysis plant.
Clean water for green hydrogen
Green hydrogen is at the heart of efforts to achieve a sustainable energy supply. Although there are challenges, especially when it comes to ensuring water quality and adapting to different geographical conditions, the technical advances offer great hope. Key tools such as process analytical technology (PAT) and instruments such as the TOC-1000e are crucial for monitoring and ensuring water quality. As the transition to a hydrogen economy moves forward, constant innovation and optimization is needed to increase the efficiency and sustainability of hydrogen production. In this way, green hydrogen has the potential to play a central role in our sustainable energy and raw material supply.
[1] “Wasserstoff – Schlüssel im künftigen Energiesystem”, Federal Environment Agency website, April 15, 2023, https://www.umweltbundesamt.de/themen/klima-energie/klimaschutz-energiepolitik-in-deutschland/wasserstoff-schluessel-im-kuenftigen-energiesystem#Rolle
[2] Katebah, M., Al-Rawashdeh, M. & Linke, P. (2022). Analysis of hydrogen production costs in Steam-Methane Reforming considering integration with electrolysis and CO2 capture. Cleaner Engineering and Technology, 10, 100552. https://doi.org/10.1016/j.clet.2022.100552
[3] Ansari, D., Grinschgl, J. & Pepe, J. M. (2022). Elektrolyseure für die Wasserstoffrevolution – Herausforderungen, Abhängigkeiten und Lösungsansätze. SWP-Aktuell 2022/A 58. https://doi.org/10.18449/2022A58
[4] Becker, H., Murawski, J., Shinde, D. V., Stephens, I. E. L., Hinds, G. & Smith, G. (2023). Impact of impurities on water electrolysis: a review. Sustainable Energy Fuels, 7, 1565–1603. https://doi.org/10.1039/D2SE01517J
