Solid oxide fuel cell

A solid oxide fuel cell (SOFC) is an electrochemical conversion device that produces electricity directly from fuel. Fuel cells are characterized by their electrolyte material and, as the name implies, the SOFC has a solid oxide, or ceramic, electrolyte. Ceramic fuel cells operate at much higher temperatures than polymer based ones.

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Introduction

Solid oxide fuel cells are intended mainly for stationary applications with an output from 1 kW to 2 MW. They work at very high temperatures, typically between 700 and 1,000°C. 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 (CHP) device. 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 activated carbon 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.

Operation

  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 connected in series to form what most people refer to as an “SOFC stack.” 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 1,200 °C.

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 hydrogen fuel. The oxidation reaction between the oxygen ions and the hydrogen produces both water and electricity.

Electrolyte

The electrolyte is a dense layer of oxygen ion conducting ceramic. It's electronic conductivity must be kept as low as possible to prevent losses from leakage currents. In order to force the electrons liberated from the fuel into the external circuit, where they can do useful work, there must be a huge resistance to prevent them from going through the electrolyte. Popular electrolyte materials include stabilized zirconia and doped ceria.

Cathode

The cathode, or air electrode, is a thin porous layer on the electrolyte where oxygen reduction takes place. The overall reaction is written in Kröger-Vink Notation as follows: 1/2O₂(g) + 2e' + V_o^** -> O_o^x

Cathode materials must be, at minimum, electronically conductive. Currently, lanthanum strontium manganite(LSM) is the cathode material of choice for commercial use because of its compatibility with doped zirconia electrolytes. However, because LSM is a poor ionic conductor, this limits the active reaction zone to the triple phase boundary (TPB) where the electrolyte, air and electrode meet. LSM works well as a cathode at high temperatures, but its performance quickly falls as the operating temperature is lowered below 800°C. In order to increase the reaction zone beyond the TPB, a potential cathode material must be able to conduct both electrons and oxygen ions. Mixed ionic/electronic conducting (MIEC) ceramics, such as the perovskite LSCF, are currently being researched for use in intermediate temperature SOFCs as they are more active and can makeup for the increase in the activation energy of reaction.

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 (600°C) 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 is 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. has developed a low cost and low temperature (500-600 degrees) SOFC stack using cerium gadolinium oxide (CGO) in place of current industry standard ceramic, yttria stablised zirconia (YSZ), which allows the use of stainless steel to support the ceramic.

Advanced fuel cell research at institutes of higher learning is becoming more and more popular. The high temperature electrochemistry center (HITEC) at the University of Florida, Gainesville, led by Dr. E.D. Wachsman, is focused on studying ionic transport, electrocatalytic phenomena and microstructural characterization of ion conducting materials.

ITSOFC

SOFCs that operate in an intermediate temperature (IT) range, meaning between 600 and 800°C, are named ITSOFCs. Because of the high degradation rates and materials costs incurred at temperatures in excess of 900°C, it is economically more favorable to operate SOFCs at lower temperatures. The push for high performance ITSOFCs is currently the topic of much research and development. One area of focus is the cathode material. It is thought that the oxygen reduction reaction is responsible for much of the loss in performance so the catalytic activity of the cathode is being studied and enhanced through various techniques, including catalyst impregnation.

See also

• Fuel cell
• Hydrogen technologies

Notes and references