When the fossil fuels are exhausted, hydrogen and gas from biomass may become important sources of energy. Fuel cells will then convert that hydrogen into electricity as well as possible. ECN is looking for the best fuel cell components. “We want to increase its lifespan and save on manufacturing costs,” says Bert Rietveld from ECN Hydrogen and Clean Fossil Fuels. “The newest anode works 5 times better than the control.”

Of all of the types of fuel cells, solid oxide fuel cells (SOFC) are the easiest customer. They aren’t as critical about what kind of fuel they’re given. They also aren’t as sensitive to pollutants, and they are made from reasonably inexpensive materials. But the downside is that this type of cell works at a high temperature: 750-800 degrees Celsius. According to Rietveld, that means that the lifespan of the cell and its housing, the stack, is limited, unless durable materials are used. “And those are the expensive ones, of course, the ones we’re trying to avoid,” remarks Rietveld. “We want to just work with stainless steel.”

High temperature fuel cell maintains performance at
200 degree lower temperature

The consequence of this preference is that the working temperature of the fuel cell must be reduced. But if you do that, you affect the performance of the fuel cell. And, according to Rietveld, that is fundamentally wrong. “The aim is to achieve the temperature reduction while preserving the performance. And you should see what we’ve achieved these past three years.”
He displays a chart showing the performance of a number of different anode materials in a SOFC. All of the results apply to a temperature of 600 degrees Celsius, which is 150 to 200 degrees lower than usual. The vertical axis marks the voltage, the horizontal axis the energy supplied (power density). Technical types will recognize the pattern, which is also similar to batteries and accumulators: with no energy use, the voltage is high. The more energy you extract from the battery, the further the voltage decreases. Rietveld explains: “A horizontal line is ideal. That will always be an unachievable ideal, though, because there will always be losses in the form of heat. But if you look at the performance at 0.5 volts, you see that we’ve made a real achievement. The standard cell (green) gets 0.4 amps per square centimetre; our last anode (black) increases that to 2 amps per square centimetre, five times as much.”

Prefers methane
Bert Rietveld’s group focuses on the fuel cell’s electrodes, which invariably consist of four parts. The carrier ensures the mechanical strength, the active layers on top of that – the anode, the electrolyte and the cathode – are responsible for the electrochemical activity. The materials must be as thin as possible, so research really amounts to exploring the limits. Rietveld provides a simple example: “Both materials have their own coefficient of expansion for temperature differences. That creates mechanical stresses in the cell, meaning that you can’t design the parts as thin as you might like.”
Whichever way you look at it, a fuel cell prefers to use hydrogen to release electrons to produce electricity. Yet it is possible to fuel the SOFC fuel cell with an all-around easier fuel than pure hydrogen: with methane, or natural gas. “Methane and water react to form hydrogen and carbon monoxide. This so-called reform reaction is catalysed by nickel. What’s funny is that there is already nickel in our fuel cell to achieve the electrochemical properties. It is a great coincidence that it’s also a catalyst for the production of hydrogen,” Rietveld adds.
But that’s not all. The reform reaction needs heat, and there’s a lot of that because the production of electricity in the fuel cell releases heat. “In theory, this heat constitutes a loss for the electrochemical process. But now it’s being used practically by the reform reaction, which benefits the total efficiency of the SOFC cell,” says Rietveld.

Agrarian application
This type of fuel cell makes electricity with an efficiency of 50 percent. That is much more favourable than running a generator with a combustion engine, which achieves 30 percent. Along those lines, Rietveld sees a future in small-scale stationary applications, for example. “Systems up to a capacity of 1 kilowatt would be fitted into furnaces in homes, and make heat and electricity. And the big systems, above a few megawatts, would be installed in big hotels, hospitals and clusters of separate vacation homes (Landall Green Parks).”
Clearly, the SOFC technology is extremely suitable for decentralised heat and electricity generation (CHP systems), and would have a special application in the agrarian sector. “Plants are big users of CO2, and since CO2 comes out of the ‘exhaust’ of these fuel cells without other pollutants, it could be fed directly into the greenhouses.”
The SOFC is the omnivore of fuel cells. This property has even been elevated to research objective, because hydrogen is in more compounds than only hydrocarbons. Rietveld is keen on ammonia, because to him, it is in many ways the ideal fuel for a SOFC. Ammonia is released in many agrarian processes. It is a bothersome substance because its release into the environment must continually be reduced and it is expensive to make harmless. “It’s easy to make a mixture of methane and ammonia from manure and sewage sludge. That ammonia should actually be removed from the mix. But the SOFC takes the mixture as is and makes the ammonia harmless. You kill two birds with one stone: you have an ammonia cleaner that also produces electricity and heat. That’s clean energy from an inexhaustible waste product. It doesn’t really get any better than this.”

Contact
Bert Rietveld
ECN Hydrogen & Clean Fossil Fuels
Tel.: +31 (0)22 456 4452
E-mail: Bert Rietveld

Information
More information about SOFCs can be found here. 

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