One of the applications that fuel cells may have is the supplying of homes with electrical power. When considering applications of this type that call for greater power, a research group in the UPV/EHU's Department of Mineralogy and Petrology has studied the use of one type of material, perovskites, for the design of these cells.
Fuel cells are similar to batteries, but they differ from them mainly in that they are continually resupplied by the reagents consumed, typically oxygen and hydrogen. "After the process to generate electricity, they produce heat and water as waste products," explained Dr Karmele Vidal, researcher in the UPV/EHU's IMaCris/MaKrisI group. That is why they are listed as "clean energies, since they do not emit greenhouse gases in the energy conversion process," added the researcher.
The UPV/EHU's research group has worked on a specific type of fuel cell: solid oxide fuel cells or SOFCs that operate at a high temperature. Unlike conventional cells, their ion conducting electrolyte is solid, which offers various advantages with respect to other types of cells, as Vidal explained: "The materials are relatively inexpensive, their sensitivity to impurities in the fuel is low and they are highly efficient and powerful. What is more, as the components are solid, their configuration is much more versatile as they can be manipulated." On the downside, however, the researcher highlights that "very expensive materials are used because the cells operate at high temperatures."
"Many pieces of research indicate that the improvement in the contact between the interconnector and the cathode is one of the significant challenges in the production of SOFCs," pointed out Vidal. For this it is necessary to use new materials that will improve bonding between these components without reducing the cell's capacity. The materials used as a contact layer between the interconnector and the cathode must have high electronic conductivity, good chemical and structural stability at the operating temperature (the cells operate at 600-800 ºC).
Preovskites, for the cathode and contact layer
In order to meet all these requirements, the UPV/EHU's research group has opted for perovskite type materials. The name comes from a relatively rare mineral in the earth's crust, but it has been extended to a more general group of crystals that have this same structure.
In their research, they have been working with perovskites to design certain components in the parts of the fuel cell, like "the cathode and the contact layer. We saw that perovskite type materials are good electron and ion conductors; so they are suitable for the design of the contact layer and cathode, respectively," said Vidal.
As important as the type of material used to manufacture fuel cell components "is the way it is synthesised. The priming temperature and time taken are, among other things, variables that affect the material's microstructure, which is crucial as far as its properties are concerned," explained the researcher. Among the synthesis methods studied, the means for priming the perovskites that has offered them the best results is combustion. Basically, this consists of a reaction between nitrates as the oxidant and glycine as the fuel. This causes self-combustion, the flame reaches a high temperature and the formation of the necessary material takes place.
Right now, there are prototypes and one or two commercial products based on these fuel cells, but the main problem they face is that "they are not yet very cost-effective although work is being done on this aspect," she pointed out. As they are devices for supplying power thought up for equipment requiring high power, Vidal takes the view that they offer a way of "decentralising the dependence that currently exists on the electricity grid, apart from offering a means for producing electrical power that is not dependent on oil." Quite honestly," she concludes, "I believe that this technology will come into its own when the current system becomes more expensive owing to the increase in crude oil prices."
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