Protonic ceramic fuel cells (PCFC) could be the next clean energy source, according to Sandrine Ricote of the Colorado School of Mines Mechanical Engineering Department. Although a little pricy, PCFC interest is rising, so they will see greater implementation in the future.
In general, a fuel cell is a device that converts chemical energy from a fuel into electricity through a chemical reaction with oxygen. Electrons are drawn from an anode to a cathode through an external circuit. Fuel cells are not batteries as they need a constant source of fuel and oxygen to run. An individual cell can only generate around 0.7 volts when drawing a current. “Several cells need to be connected in series,” said Ricote. The components of the stack consists of a seal for the cell, a cathode, an anode, an electrolyte to encourage current, and two interconnects so the cell is connected to other cells. Fuel cells are used for buildings large and small, transportation, and devices requiring portable power.
Additionally, fuel cells are efficient. “They usually range from 60-65%,” said Ricote. Fuel cells are simple with no moving parts, have low emissions that mainly consist of water when used with hydrogen as a fuel, are quiet, and can be used in a wide array of applications. Unfortunately, they are pricey and can cost $10 to $50 per kilowatt for automotive applications and $1000 per kilowatt for stationary applications such as buildings.
There are many types of fuel cells, but Ricote mainly focused on PCFCs. PCFCs use an oxide as an electrolyte which conducts protons. They operate within a 400 to 600 ºC temperature range. In PCFCs, the temperature is high enough for internal reforming. In internal reforming, natural gas and water enter a heat exchanger where the preheated natural gas and steam flow into a reforming chamber. The heat from the reaction is recovered in the heat exchanger. PCFCs are still a new technology that has only been in existence for thirty years. As such, they have not been used to its full potential. “Hopefully,” said Ricote, “you will all change that.”
PCFCs function when water incorporates into the electrolyte and dissociates within the electrolyte. The newly free protons diffuse throughout the electrolyte and add to the overall conductivity along with any free oxide ions and electrons. The electrolyte must be fully dense (the fuel and oxidant do not mix), have sufficient protonic conductivity, no electronic conductivity, be chemically stable in an oxidizing or reducing atmosphere, and be mechanically stable. Oxygen vacancies must exist for protonic conduction, but they are inherently present in the material and are introduced by doping. A dense, thin film is required to achieve reasonable resistances for the low proton conductivity of the electrolyte.
The anode needs to be made a certain way for the PCFC as well. It needs to be porous for gas diffusion, have electronic conductivity, be chemically stable in a reducing atmosphere, and be mechanically stable. This can happen with a mixture of nickel oxide and the electrolyte material. In a reducing atmosphere, the nickel oxide becomes nickel metal or cermet, a ceramic metal composite. The nickel has electronic conductivity and the electrolytic material has mechanical strength. The material becomes porous and it allows protons and electrons through. Unfortunately, nickel tends to agglomerate, which deteriorates the anode. Solid carbon can form when hydrocarbons are used as a fuel, decreasing anode performance. Iron and copper alternatives are being considered for this reason.
“I saved cathodes for last because they are the most complicated,” said Ricote. Water is released at the cathode side as the PCFC’s emissions. There are several different geometries for the cathode. One is a mixed oxide ion and electronic conductor where water is formed at the interface electrolyte or cathode. Another is a mixed proton and electronic conductor with water formed in the cathode depth. It has the best geometry, but it is difficult to find a mixed proton and electronic conductor in an oxidizing atmosphere. A third geometry combines the previous two with water forming in the depths and on the interface, though the water should not form at the interface. This can lead to cation interdiffusion when sintering the cathode. The fourth and final geometry involves infiltration of an electronic conductor in a porous layer of the electrolyte material, good mechanical strength, nano size particle of electronic conductor, and enhanced catalytic activity. It must be porous, chemically stable in an oxidizing atmosphere, and mechanically stable.
For the cell itself, there are two options, planar and tubular. Planar cells have easier manufacturing and a higher power density. Tubular cells are easier to seal, stronger, and more robust. For the geometry, there are two options. Anode supported cells have brittle cermet support and contain expensive materials, but they have no problems with thermal expansion coefficient mismatch and are easy to fabricate. Metal supported cells have robust metal support and are inexpensive, but they have a difference of thermal expansion coefficient between the metal and the anode and have difficulty depositing anode layer on a porous metal support as very big particles.
PCFCs were never tested in stacks before. The cathode materials’ problems and fabrication of the dense thin film raise problems for this technology. However, PCFCs can operate at lower temperatures, last longer, are made of cheaper materials than other fuel cells, and do not dilute fuel.
Ricote obtained an engineering diploma in Materials Sciences from the Ecole Superieure d’Ingenieurs en Recherches des Materiaux (ESIREM) located in Dijon, France. She earned a PhD in Organic Chemistry and worked on high temperature proton conducting oxides. After graduation, Ricote moved to the Danish Technical University (DTU) where she got a 2 year post-doctoral position and a 2-year scientist position. She worked on ceramic processing and characterization (morphological, structural, or electrical). Ricote now works as a Research Assistant Professor in the Mechanical Engineering Department.