Anyone with the most basic understanding of green hydrogen will know that to produce H2 from electricity, an electrolyser will be needed to split water molecules into hydrogen and oxygen.

Stay ahead on hydrogen with our free newsletter
Keep up with the latest developments in the international hydrogen industry with the free Accelerate Hydrogen newsletter. Sign up now for an unbiased, clear-sighted view of the fast-growing hydrogen sector.

Those with a more advanced understanding will know that there are different types of electrolysers — such as alkaline, proton-exchange membrane (PEM) and solid oxide (known as SOE).

But knowing which type of electrolyser should be used for a particular project is another level of complexity that few have mastered.

Last week, Hydrogen Insight held a webinar that aimed to increase understanding in this area, in which we spoke to senior executives at four of the leading electrolyser makers — all of which specialise in different technologies — as well as a director of an independent green hydrogen developer.

Their insights were very revealing.

PEM v alkaline

InterContinental Energy — one of the world’s most ambitious green hydrogen developers, with multi-gigawatt projects in Australia and the Middle East — is “very much on the fence” between PEM and alkaline electrolysers, the two most established technologies, as they both have their pros and cons, according to Warner Priest, its director of midstream energy.

“One of the advantages of [atmospheric] alkaline is that it’s a proven technology, it’s been around... for 100 years or so. It’s a technology that is well known. It is probably around half the price of PEM [to buy],” he told the webinar.

But Priest noted that PEM had a number of advantages.

“Our projects are very large, they will be built out over a long period of time, over very large geographical areas,” he said, noting that they are “very remote and islanded, so fed directly from wind and solar [rather than the grid]”.

This means that in any given 24-hour period, there will often be one or two hours where the electrolyser has to be turned off, and on a windless night it could be up to 12 hours.

“And that’s a challenge for alkaline,” Priest said. “Of particular importance is the ability to start things up from a cold start very quickly.”

Alkaline electrolysers can take up to 50 minutes to get up to full operating speed, compared to less than five minutes for PEM.

And while InterContinental’s projects will likely have enough space to install vast tracks of electrolysers, the weight and replaceability of components must also be considered.

“Another disadvantage that [alkaline electrolysers] have are the size of the stacks,” Priest said.

“Trying to get heavy equipment in and out [of remote areas] when these things need to be replaced... we have to consider the operational aspects. A large 5MW stack that weighs 50, 60, maybe 100 tonnes, you can’t move it in and out with a forklift.”

“Whereas, with PEM, when you’re dealing with multiple small stacks, say 1MW or 1.5MW, they can be like printer cartridges, you run them for 80,000 hours, or 100 or 120,000 hours, and then you can replace them.

“Cost is an issue [with PEM], but there is this general consensus that through automation and digitalisation the cost of PEM will come down.”

Other disadvantages of PEM include the expensive iridium and platinum they require, although around 90% of those platinum group metals (PGMs) can be recycled, and the use of fluoropolymers — a type of plastic that may be banned by the EU — to separate the electrodes.

He also noted that every 5MW of alkaline electrolysers require 6.5 tonnes of lye — the potassium hydroxide (KOH) electrolyte — “which is actually quite a toxic chemical” that needs to be replaced every three to five years, although filtration systems could reduce the frequency.

With InterContinental’s smallest projects being around 12-13GW, that would require “many oil tankers” to ship up to 16,900 tonnes of KOH every few years.

By contrast, PEM electrolysers use a solid polymer electrolyte that lasts as long as the rest of the stack (the part of the electrolyser where water is split into hydrogen and oxygen).

Constantine Levoyannis, head of EU affairs at Norwegian manufacturer Nel, said that his company produces both atmospheric alkaline and PEM electrolysers because “not all projects are the same... we need to tailor our offerings to different applications [and] different customer needs”.

While the more expensive PEM machines have a faster response time to the inevitable ups and down of variable renewable energy production, the two types of electrolyser do not make a significant difference to the levelised cost of renewable H2, according to Levoyannis.

“I think that the flexibility of both alkaline and PEM stacks is sufficient to follow fluctuations in wind and solar,” he argued, adding that “everything depends on how your project is built up”, including the portfolio of renewable electricity feeding into the project, the use of the grid to provide back-up power, and access to energy storage.

However, this was contested by Roland Hequet, vice-president for strategy, partnerships and mobility at Belgium’s John Cockerill, which only makes pressurised alkaline electrolysers — machines that the company says have comparable capex costs to atmospheric alkaline, but can ramp up and down as fast as PEM.

“Alkaline is well suited for intermittent power under the condition that it’s pressurised alkaline”, he said.

“Atmospheric [alkaline] electrolysers definitely cannot cope with intermittent power.”

Hequet added: “The future of green hydrogen lays with solar and wind. If we want to reach very low [levelised cost of hydrogen] in the future... projects will be, for the most part, based on intermittent power. So very clearly, the only answer to that, for now, is PEM and pressurised alkaline.”

He also added that although PEM electrolysers may be smaller and less heavy than alkaline, it is easier to deal with fewer stacks with a higher capacity.

“If you want a gigawatt plant, it’s better to have 200 stacks at 5MW rather than having 1,000 [PEM] stacks.”

Levoyannis then revealed that Nel is actually in the process of developing a “pilot” pressurised alkaline electrolyser, which is due to be operational by the end of this year.

Solid oxide and emerging technologies

While the electrolyser market is currently dominated by alkaline and PEM technologies, high-efficiency solid oxide electrolysers (SOE) are gaining pace.

“I always believed that in electrolysis, high-temperature would ultimately prevail because of its efficiency,” said Rick Beuttel, vice president for hydrogen at US manufacturer Bloom Energy.

And while he concedes that Bloom’s technology is currently much more expensive than PEM or alkaline systems, “its efficiency is far better” at around 37.5kWh of electricity input per kilogram of hydrogen produced (50kWh/kgH2 is considered a decent efficiency for alkaline or PEM).

However, this efficiency requires an external heat source, which either necessitates the installation of an electric boiler (adding an extra 3-4kWh/kg to the overall efficiency) or coupling production with waste heat from industrial processes.

“There’s a use case for each of these technologies that’s best for someone’s particular project. Our best use case is when we’re paired with either a hot customer-end-use process for the hydrogen or, better, an exothermic end use process for the hydrogen,” said Beuttel, noting that this could include the production of various “fuels of the future” such as ammonia, methanol, renewable diesel and synthetic aviation fuels, all of which create waste heat as they are made.

But while “efficiency is extremely important” to Intercontinental Energy, Priest noted that the developer is “not quite sure whether SOEC [solid oxide electrolyser cells] has a role to play in our projects”.

SOECs suffer even worse than alkaline electrolysers from on-off cycling. “SOEC doesn’t like expansion and contraction as a result of turning on and off, and this may present problems for the life of the stacks,” he said.

However, while Intercontinental is “not quite sure” about anion exchange membrane (AEM) electrolysers — such as those produced by Enapter — due to the relatively short lifetime of stacks, it is eyeing a number of emerging electrolyser technologies, such as those developed by Australian start-up Hysata and Israeli company H2Pro.

Fellow webinar panellist, Phillip Lückerath, who is vice-president, commercial, at H2Pro, explained that the company’s E-TAC (electrochemical, thermally activated chemical) electrolysers split the generation of hydrogen and oxygen into two phases via an intermediary anode made of nickel hydroxide.

“By this, we are able to produce the hydrogen at a much higher efficiency at a much lower voltage” since it avoids the simultaneous formation of oxygen and without the need for external heat, he explained.

Like Hysata, H2Pro is targeting a round-trip efficiency of less than 42kWh/kg. And because the technology uses a two-step process, there is “no problem with intermittent power”.

However, the Israeli start-up is years away from bringing its technology to market.

It is currently building a 0.5MW pilot electrolyser in Israel, and plans to bring out larger demonstration systems within two to three years.

“This decade is our target to have that commercially meaningful size on the market,” says Lückerath.

Recordings of Hydrogen Insight's two-day webinar Hydrogen Summit 2023 can be watched for free by clicking here.

The session discussed in this article, entitled The pros and cons of different electrolyser types — and the risks of scaling up manufacturing, took place on day two.

Hydrogen Insight has also published two further articles from the event: