Introduction

Solid oxide fuel cells, or SOFC, are intended mainly for stationary applications with an output from 1 kW to 2MW. They work at very high temperatures, typically between 700 and 1000ºC. Their off-gases can be used to fire a secondary gas turbine to improve electrical efficiency. Efficiency could reach as much as 70% in these hybrid systems, called Combined Heat and Power device (CHP). In these cells, oxygen ions are transferred through a solid oxide electrolyte material at high temperature to react with hydrogen on the anode side. Due to the high operating temperature of SOFC's, they have no need for expensive catalyst, which is the case of Proton-exchange fuel cells (platinum). This means that SOFC's do not get poisoned by carbon monoxide and this makes them highly fuel-flexible. Solid oxide fuel cells have so far been operated on methane, propane, butane, fermentation gas, gasified biomass and paint fumes. However, sulfur components present in the fuel must be removed before entering the cell, but this can easily be done by an active coal bed or a zinc absorbent.

Thermal expansion demands a uniform and slow heating process at startup. Typically, 8 hours or more are to be expected. Micro-tubular geometries promise much faster start up times, typically 13 minutes.[1]

Unlike most other types of fuel cells, SOFC's can have multiple geometries. The planar geometry is the typical sandwich type geometry employed by most types of fuel cells, where the electrolyte is sandwiched in between the electrodes. SOFC's can also be made in tubular geometries where either air or fuel is passed through the inside of the tube and the other gas is passed along the outside of the tube. The tubular design is advantageous because it is much easier to seal and separate the fuel from the air compared to the planar design. The performance of the planar design is currently better than the performance of the tubular design however, because the planar design has a lower resistance compared to the tubular design.

How a Solid Oxide Fuel Cell Works

Cross secton of the three ceramic layers of an SOFC. From left to right: porous cathode, dense electrolyte, porous anode
Cross secton of the three ceramic layers of an SOFC. From left to right: porous cathode, dense electrolyte, porous anode

A solid oxide fuel cell is made up of four layers, three of which are ceramics (hence the name). A single cell consisting of these four layers stacked together is typically only a few millimeters thick. Hundreds of these cells are then stacked together in series to form what most people refer to as a “solid oxide fuel cell.” The ceramics used in SOFCs do not become electrically and ionically active until they reach very high temperature and as a consequence the stacks have to run at temperatures ranging from 700 to 1200 °C.

Cathode

The ceramic cathode layer must be porous, so that it allows air flow through it and into the electrolyte. There are various types of ceramic materials used for the cathode, but all of them must be electrically conductive. The cathode is the negative side of the cell towards which electrons flow. It is the side that is exposed to air and its purpose is to use electrons to reduce the oxygen molecules in the air to oxygen ions.

Electrolyte

The electrolyte is the dense, gas-tight layer of each cell that acts as a membrane separating the air on the cathode side from the fuel on the anode side. There are many ceramic materials that are being studied for use as an electrolyte, but the most common are zirconium oxide based. Besides being air-tight, the electrolyte must also be electrically insulating so that the electrons resulting from the oxidation reaction on the anode side are forced to travel through an external circuit before reaching the cathode side. The most important requirement of the electrolyte however is that it must be able to conduct oxygen ions from the cathode to the anode. For this reason, the suitability of an electrolyte material is typically measured in ionic conductivity.

Anode

The ceramic anode layer must be very porous to allow the fuel to flow to the electrolyte. Like the cathode, it must conduct electricity. The most common material used is a cermet made up of nickel mixed with the ceramic material that is used for the electrolyte in that particular cell. The anode is commonly the thickest and strongest layer in each individual cell, and is often the layer that provides the mechanical support. Electrochemically speaking, the anode’s job is to use the oxygen ions that diffuse through the electrolyte to oxidize the fuel (hydrogen). The oxidation reaction between the oxygen ions and the hydrogen fuel produces both water and electricity.

Interconnect

The interconnect can be either a metallic or ceramic layer that sits between each individual cell. Its purpose is to connect each cell in series, so that the electricity each cell generates can be combined. Because the interconnect is exposed to both the oxidizing and reducing side of the cell at high temperatures, it must be extremely stable. For this reason, ceramics have been more successful in the long term than metals as interconnect materials. However, these ceramic interconnect materials are extremely expensive. Fortunately, inexpensive metallic materials are becoming more promising as lower temperature (600-800°C) SOFCs are developed.

Research

Research is going now in the direction of lower-temperature SOFC (600ºC) in order to decrease the materials cost, which will enable the use of metallic materials with better mechanical properties and thermal conductivity.

Research is also going on in reducing start-up time to be able to implement SOFC's in mobile applications. Due to their fuel flexibility they may run on partially reformed diesel, and this makes SOFC's interesting as auxiliary power units (APU) in refrigerated trucks.

Specifically, Delphi Automotive Systems and BMW are developing an SOFC that will power auxiliary units in automobiles. A high-temperature SOFC will generate all of the needed electricity to allow the engine to be smaller and more efficient. The SOFC would run on the same gasoline or diesel as the engine and would keep the air conditioning unit and other necessary electrical systems running while the engine shuts off when not needed (e.g., at a stop light).

Rolls-Royce are developing Solid-Oxide Fuel Cells produced by screen printing onto inexpensive ceramic materials. Rolls-Royce Fuel Cell Systems Ltd is developing a SOFC gas turbine hybrid system fuelled by natural gas for power generation applications generating power of the order of a megawatt.[2]

Ceres Power Ltd. are developing a low cost and low temperature (500-600 degrees) SOFC using cerium gadolinium oxide in place of current industry standard ceramic (yttria stablised zirconia) which allows the use of stainless steel to support the ceramic.

See also

Notes and references

  1. ↑ Sharke, Paul (2004). "Freedom of Choice". Mechanical Engineering 126 (10): 33.
  2. ↑ Adamson, F (2004). "Propagating Reaction Fronts in Zirconia Tubes". PhD thesis.

P. Batfalsky, V.A.C. Haanappel, J. Malzbender, N.H. Menzler, V. Shemet, I.C. Vinke, R.W. Steinbrech, Chemical interaction between glass–ceramic sealants and interconnect steels in SOFC stacks, Journal of Power Sources, 155 (2006) 128.

J. Malzbender, T. Wakui, R.W. Steinbrech, L. Singheiser, Deflection of Planar Solid Oxide Fuel Cells During Sealing and Cooling of Stacks, Fuel Cell 2 (2006) 123.

External links