Fuel cells (FCs) convert the chemical energy of a fuel, such as hydrogen, into electrical energy very efficiently on many size scales, without combustion and with little or no emission of pollutants. They are therefore suitable in a variety of applications utilizing clean power generation.
Solid Oxide Fuel cells (SOFCs) are highly efficient, entirely solid-state fuel cells that operate at high temperatures. Their most common application is in stationary power generation, including both large-scale centralized power generation and distributed generation in individual homes and businesses.
SOFCs are composed of three layers: an anode (the fuel side), an electrolyte, and a cathode (the air side). Both the anode and cathode are porous and they conduct both ions and electrons). The electrolyte is an ionic conductor and electronic insulator.
On the anode side, the fuel (for example hydrogen) diffuses through the porous anode to the interface between the anode and the electrolyte – the reaction sites, commonly known as the triple phase boundaries (TPBs). The fuel reacts with the oxygen ions that are conducted through the oxide electrolyte to produce water and electrons. Since the electrolyte is an electronic insulator, the electrons travel through the electronic conductor in the anode, commonly nickel. At the surface of the anode the electrons are collected by the current collector, resulting in electrical current being transmitted to the external circuit. On the cathode side, the electrons from the external circuit combine with the oxygen molecules in the air to produce oxygen ions. The oxygen ions are then conducted through the electrolyte back to the anode side.
SOFCs can have several different designs. The most common designs are the planar design and the tubular design. The planar design has a lower manufacturing cost and a higher energy density. In the planar design, the cells (anode-electrolyte-cathode) are stacked together, separated by an interconnect plate. The interconnect is used as a current collector and also as a gas manifold for fuel (anode side) and air (cathode side).
Benefits of SOFCs
- No air pollutants (SOx, NOx, CO, and particulates)
- High electrical efficiency (50-60%) & corresponding low CO2 emissions that are separate from the airstream and therefore easier to sequester than in a combustion process
- High-temperature waste heat useable for co-generation of electricity (with gas or steam turbines) for ~70% system electrical efficiency
- Low noise and no pollution allows urban siting, thereby allowing additional use of waste heat for hot water or space heating, for > 90% overall system efficiencies and reduced transmission losses
- Fuel flexibility – SOFCs can utilize a wide range of locally – available fuels compatible with both current and future sustainable energy infrastructure, including energy–dense liquid fuels:
- Ethanol from agricultural waste or fast-growing crops (no energy-intensive water removal required as for ethanol combustion)
- Methane from landfill gas, biogas, or natural gas
- Hydrogen from electrolysis with renewable electricity
- CO, H2S, volatile organics (for pollution clean-up)
Challenges for increasing commercial viability
- Lowering cost
- Increasing durability and reliability
Researchers in the Fuel Cell Materials and Manufacturing Laboratory apply principles of materials, mechanical, and chemical engineering to improve the durability and performance and lower the cost of SOFCs, with the overall objective of contributing to environmental sustainability.