Electricity out one way, green hydrogen the other – engineering the reversible fuel cell

Joseph Flaig

Electrolyte layers in Ceres Power's SteelCell can work in both directions
Electrolyte layers in Ceres Power's SteelCell can work in both directions

A solid-oxide cell developed by Ceres Power has a uniquely appealing feature – running in one direction, it generates low-carbon electricity from multiple fuels, while running in the other direction it produces ‘green’ hydrogen from renewable energy and steam.

The combination is a “huge breakthrough in the clean energy revolution,” claimed judges for the prestigious MacRobert Award, who named the firm’s SteelCell tech the UK’s top engineering innovation in July. It “delivers the sort of improved performance that will be crucial if the world is to decarbonise at the scale and pace required to tackle climate change,” they added. 

The cell stacks, similar to large batteries, are made of thin perforated steel sheets, onto which a gadolinium-doped ceria ceramic membrane has been printed. A 5kW stack has 187 layers of this electrolyte. 

In fuel cell mode, a gas – either hydrogen, a hydrogen carrier such as ammonia, or natural gas – is fed over the sheets. Hydrogen ions meet oxygen ions coming in from the cathode side, creating water and releasing electrons.

In electrolyser mode, the opposite happens – the cell is fed with electrons from renewable energy and steam, which separates into oxygen and green hydrogen. 

‘Goldilocks’ temperature

While the cells are reversible, one unit would normally be expected to operate as either fuel cell or electrolyser, as the two applications require different supporting infrastructure, said Caroline Hargrove CBE, chief technology officer at Ceres and fellow of both IMechE and the Royal Academy of Engineering. 

The devices operate at 500-600ºC, which Ceres found to be a ‘Goldilocks’ temperature for performance, fuel flexibility, cost and robustness. 

“It’s way more efficient than the current technology for green hydrogen,” said Hargrove. “If you can raise steam from an industrial process, it’s even more attractive from an efficiency perspective. The electrical efficiency of doing that can rise well above 90%.”

The fuel-cell mode does produce carbon dioxide if it is fed with natural gas, said Hargrove, but “a lot less” than with combustion. It could also be collected with carbon-capture technology. 

Solving problems 

Others have attempted to create such a system but have not managed to achieve the same efficiencies, said Hargrove. Achieving high yields and making cells sufficiently robust has taken 20 years of development, starting at Imperial College London. 

“It’s actually a very, very difficult problem, because it’s chemistry, and chemistry is difficult to predict,” she said. “We’re still in the infancy of learning how to do this. I come from a modelling background, and we’re trying to model this thing. There are many things we still don’t understand.”

The company and its technology are now at a stage where they could start to make a real difference, however. Rather than manufacturing cells itself, Ceres has a licensing model, and it has established partnerships with major companies including Bosch in Germany and Doosan in South Korea. 

The cells will be most useful in applications where the levelised cost of electricity or hydrogen is key, said Hargrove, and they could be particularly attractive in coal-dominated parts of the world. As fuel cells, they will likely first be run with natural gas, then they could transition to hydrogen operation in future.

The team was “thrilled” to win the MacRobert award, said Hargrove. “We need to succeed, to ensure that we can deliver a net-zero future for our families, for society and for all our benefit.”

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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.


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