Electrolysis for green H2 production
Towards a cleaner, greener future
Whether as a zero-emission fuel for mobility, a carbon-neutral industrial feedstock, a vector for renewable energy or a storage medium to buffer volatile power grids, green hydrogen will play a critical role in a net-zero economy. The technology to produce zero emissions hydrogen is therefore also thrust into a central role.
Electrolyzers for green hydrogen production at the Leuna plant, Germany.
Today, the most common way of producing green hydrogen is via electrolysis – a process whereby water is split into hydrogen and oxygen using electricity generated from entirely renewable energy sources. This occurs in units known as electrolyzers. Electrolyzers can range in size and capacity as well as the technology used, but the basic chemistry happening within is essentially the same.
How it works: A look inside the electrolyzer
From what can look like a shipping container from the outside, is a system of pumps, storage tanks, vents, a power supply, and other components – at the center of which are the cells. Here, the electrochemical reaction occurs. In its simplest form, a cell contains a positively charged anode and a negatively charged cathode.
When a current is applied, the otherwise stable H2O bonds in water break and hydrogen forms on the cathode with oxygen forming on the anode – a by-product that can be used downstream in industrial settings or released into the atmosphere at no carbon cost.
A tale of two technologies
Water is not a great conductor of electricity. So, to ensure an energy-efficient process, electrolytes are used instead. Electrolysis technologies differ according to the type of electrolyte used. Currently, the two main technologies are alkaline and PEM (Proton Exchange Membrane) – both of which can deliver high pure hydrogen on-site and on-demand.
Alkaline electrolysis is a very well-established technology that uses two electrodes separated by a porous diaphragm and a liquid alkaline solution as the electrolyte. The electrolyte solution allows hydroxide ions to be transported between the electrodes to form oxygen and hydrogen but is not consumed during the reaction.
This technology is considered as extremely efficient, reliable, and cost-effective. Capacity can stack in the MW range but drawbacks include using corrosive liquid electrolytes, operation at low current densities and low pressures as well as gas crossover. Linde has installed over 80 Alkaline electrolyzers globally.
Proton Exchange Membrane (PEM)
PEM electrolysis uses pure water and a solid polymer electrolyte instead of a liquid solution. The electricity splits the water into hydrogen and oxygen. Hydrogen protons pass through the membrane, combining with electrons to form H2 gas on the cathode side.
PEM electrolyzers are well suited for use with volatile renewable energy sources thanks to their fast ramp-up/down capabilities and their wide dynamic operating range. No corrosive electrolyte is involved, and they operate at high current density which speeds up the breakdown of the water molecule, ultimately affecting production price. Finally, a small footprint and compact system design is a benefit for many on-site industrial applications.
Understanding capacity: What’s this about Watts?
Whatever the technology type, electrolyzers are always described in terms of their capacity. This represents the maximum operation power and so is measured in Watts (or MW, GW). The higher the wattage, the more hydrogen that electrolyzer can potentially produce.
For example, if it takes 50kWh of energy to produce 1 kg of hydrogen, a 10 MW electrolyzer will produce 200 kg in an hour (10,000 / 50) while a 20 MW electrolyzer could produce 400 kg in the same time. Individual cells can be stacked in modules to increase the capacity of any one electrolyzer: a valuable flexible feature for scaling up industrial applications.
With green hydrogen playing a critical role in long-term decarbonization goals in traditional as well as new applications, its production via electrolysis is receiving increasing attention from governments worldwide and the number of installation projects is growing rapidly. According to the Global Hydrogen Review 2022 published by the International Energy Agency, Global electrolyzer capacity could exceed 35 GW by the mid-2020s and reach 134 GW by 2030 based on the current project pipeline.