We are not far off mentioning “green hydrogen” without explaining how it is made and why it differs from the other “colours” on the hydrogen spectrum. With interest in hydrogen soaring over the last couple of years, even those with a passing interest may know that “green” means it is made from water via electrolysis powered by renewable electricity.
This is good news, given the amount of disinformation, uncertainty and doubt still present in many topics relating to the energy transition. Drill down into green hydrogen, though, and there is still plenty of room for misinterpretation.
Early project developers are wrestling with a lack of knowledge in areas that range from where to find the right sort of water to what to do with hydrogen once you have produced it.
One area of green hydrogen lore where knowledge is growing, but there is still room for improvement, is in electrolyser know-how. Anyone who knows about green hydrogen knows you need an electrolyser to make the gas, but after that, things quickly get complicated.
The problem is that electrolysers are not a single technology but a family of technologies, just like batteries or solar panels, with significant differences depending on the electrolyte used.
Like batteries or solar panels, different electrolysers have different capabilities and different levels of suitability for a given application. For the uninitiated, here is a brief rundown of the leading technology options and what they can do.
ATMOSPHERIC ALKALINE ELECTROLYSIS
Alkaline water electrolysis has been around for many decades and uses a hot (but under 100°C) liquid potassium hydroxide electrolyte to deliver hydrogen at atmospheric pressure.
It is an established technology, used mostly for industrial applications. Because of their widespread use in industry, alkaline electrolysers are reliable and relatively cheap. Importantly, from a scalability and sustainability perspective, they do not rely on rare metals such as iridium and platinum.
However, they have a large footprint and require a stable electricity supply, making them poorly suited for applications powered by variable renewable generation.
PRESSURISED ALKALINE ELECTROLYSIS
Pressurised water alkaline electrolysis takes advantage of the materials, reactions, and long history of alkaline electrolysis, combined with higher temperatures and pressures.
This is achieved by a particular vessel design that increases the pressure of the reaction and at the hydrogen outlet, allowing for a more compact footprint than alkaline electrolysers and a dynamic response suitable for applications related to renewable and intermittent power generation.
Pressurised alkaline water electrolysis does not rely on rare metals such as iridium and platinum.
PROTON EXCHANGE MEMBRANE ELECTROLYSIS
The Proton Exchange Membrane (PEM) electrolysis occurs in a cell equipped with a solid polymer electrolyte. Large-scale PEM systems have only been developed in recent years, and little is known about their long-term performance.
The PEM water electrolysis technology can operate at the same temperature, efficiency, and high pressure as pressurised alkaline water electrolysis, allowing for a more compact footprint.
PEM electrolysers require high water purity and extensive use of rare metals, such as platinum and iridium, which could be a bottleneck in mass-scale adoption.
SOLID OXIDE ELECTROLYSIS
The first commercially available solid oxide electrolysers are only now being tested. The technology, which, as its name indicates, has a solid oxide electrolyte, offers very high efficiency thanks to operating temperatures of between 500°C and 900°C.
This heat could be captured for other applications, such as district heating, potentially improving the business case for solid oxide electrolysis.
While solid oxide electrolysis cells have high efficiency, the high operating temperature of the technology leads to rapid degradation of its component materials.
ANION EXCHANGE MEMBRANE
Anion exchange membrane electrolysis has been developed even more recently than solid oxide and is not yet commercially available, with no large-scale systems in operation.
Compared to more mature technologies, it promises to have lower costs, since it does not require rare metals and could potentially be more durable– lower material requirements – due to solid electrolyte/less corrosive electrolyte. The use of a solid electrolyte simplifies liquid separation.
However, anion exchange membranes still need to improve in stability and ionic conductivity before they are a real alternative to other alkaline water electrolysis processes
The variety of characteristics offered by different electrolyser technologies, along with their cost and state of maturity, can clearly make it hard to choose the right model for a given project. This is even more the case when overall system design is considered.
Pressurised alkaline water electrolysis and PEM are sought after for renewably powered projects because of their dynamic response, while other system setups where there is only limited variability in the power generation might be more favourable for atmospheric alkaline technology.
In other projects, having a system that produces pressurised hydrogen could also be advantageous since the gas needs to be stored and moved under pressure anyway. Pressurising the gas uses energy, which adds to the cost of green hydrogen produced by traditional alkaline electrolysers versus pressurised alkaline electrolysis and PEM.
Meanwhile, pressurised alkaline systems combine the best of traditional alkaline and PEM systems, working with renewables without PEM’s high cost and rare metals requirements. The reality is that the best electrolyser for your project will very much depend on the nature of your specifications and requirements—and the electrolyser is just one of several factors affecting plant economics.
This is why it is important for developers to speak to electrolyser manufacturers and learn about the options before making a choice. The range of electrolyser technologies may seem confusing at first, but it means there is more something for everyone.
As green hydrogen production takes off, demand for electrolysers of all shapes and sizes is set to soar—so it is good news that there are plenty of options for project developers to choose from. •