Solid oxide fuel cell

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Scheme of a solid-oxide fuel cell
Scheme of a solid-oxide fuel cell

A solid oxide fuel cell (SOFC) is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Fuel cells are characterized by their electrolyte material and, as the name implies, the SOFC has a solid oxide, or ceramic, electrolyte. Advantages of this class of fuel cells include high efficiencies, long term stability, fuel flexibility, low emissions, and cost. The largest disadvantage is the high operating temperature which results in longer start up times and mechanical/chemical compatibility issues.

Contents

[edit] Introduction

Solid oxide fuel cells have a wide variety of applications from use as auxiliary power units in vehicles to stationary power generation with outputs from 100 W to 2 MW. They operate at very high temperatures, typically between 600 and 1,000°C. Typical values for efficiency of a single SOFC device is around 60 plus percent. However, the byproduct gases can be used to fire a secondary gas turbine to improve electrical efficiency. This enables efficiency to reach as much as 85% in these hybrid systems, called combined heat and power (CHP) device. In these cells, it is most common to have oxygen ions diffusing through a solid oxide electrolyte material at high temperature to react with hydrogen on the anode side.

Due to the high operating temperature of SOFCs, they have no need for expensive catalyst, which is the case of proton-exchange fuel cells (platinum) and most other types of low temperature fuel cells. Because of these high temperatures, fuels can be internally reformed within the anode unlike in PEMFCs where poisoned by carbon monoxide will occur. This allows SOFCs to operate on a wide variety of different fuels such as methane, propane, butane, natural gas, 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. SOFCs can increase efficiency by using the heat given off by the exothermic electrochemical oxidation of hydrogen for endothermic reforming process. Internal reforming also leads to a large decrease in the balance of plant costs in designing a full system

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 on the order of minutes.[1]

Unlike most other types of fuel cells, SOFCs 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. SOFCs 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 air from the fuel. 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 comparatively. Other geometries of SOFCs include modified planar cells (MPC or MPSOFC), where a wave-like structure replaces the traditional flat configuration of the planar cell. Such designs are highly promising, because they share the advantages of both planar cells (low resistance) and tubular cells (easier sealing).[2]

[edit] Operation

Cross section of the three ceramic layers of an SOFC. From left to right: porous cathode, dense electrolyte, porous anode
Cross section 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 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 600 to 1,000 °C. Reduction of oxygen into oxygen ions occurs at the cathode. These ions can then diffuse through the solid oxide electrolyte to the anode where they can electrochemically oxidize the fuel. In this reaction, a water byproduct is given off as well as two electrons. These electrons then flow through an external circuit where they can do work. The cycle then repeats as those electrons enter the cathode material again.

[edit] Anode

The ceramic anode layer must be very porous to allow the fuel to flow to the electrolyte. Like the cathode, it must conduct electrically as well as ionically. 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 (typically YSZ). The anode is commonly the thickest and strongest layer in each individual cell, due to the fact that is has the smallest polarization losses, 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.

[edit] Electrolyte

The electrolyte is a dense layer of oxygen ion conducting ceramic. Its electronic conductivity must be kept as low as possible to prevent losses from leakage currents. The high operating temperatures of SOFCs allow the kinetics of oxygen ion transport to be sufficient for good performance. However, as the operating temperature approaches the lower limit for SOFCs at around 600°C, the electrolyte begins to have large ionic transport resistances and affect the performance. Popular electrolyte materials include yittrium stabilized zirconia (YSZ) (often the 8% form Y8SZ) and gadolinium doped ceria (GDC).

[edit] 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:

\frac{1}{2}\mathrm{O_2(g)} + 2\mathrm{e'} + {V}^{\bullet\bullet}_o \longrightarrow {O}^{x}_o

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. Mechanically, it has similar coefficient of thermal expansion to YSZ and thus limits stresses built up because of CTE mismatch. Also, LSM has low levels of chemical reactivity with YSZ which extends the lifetime of the material. Unfortunately, LSM is a poor ionic conductor, and so the electrochemically active reaction is limited 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. Composite cathodes consisting of LSM YSZ have been used to increase this triple phase boundary length. Mixed ionic/electronic conducting (MIEC) ceramics, such as the perovskite LSCF, are also 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.

[edit] 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 very expensive as compared to metals. Nickel- and steel-based alloys are becoming more promising as lower temperature (600-800°C) SOFCs are developed. The most common intermetallic materials used today are doped lanthanum chromites. Ceramic-metal composites called 'cermet' are also under consideration, as they have demonstrated thermal stability at high temperatures and excellent electrical conductivity.

[edit] Polarizations

Polarizations, or overpotentials, are losses in voltage due to imperfections in materials, microstructure, and design of the fuel cell. Polarizations result from ohmic resistance of oxygen ions conducting through the electrolyte (iRΩ), electrochemical activation barriers at the anode and cathode, and finally concentration polarizations due to inability of gases to diffuse at high rates through the porous anode and cathode (shown as ηA for the anode and ηC for cathode).[3]

V = E0 − iRω − ηcathode − ηanode

In SOFCs, it is typically most important to focus on the ohmic and concentration polarizations since high operating temperatures experience little activation polarization. However, as the lower limit of SOFC operating temperature is approached (~600°C), these polarizations do become important.3

[edit] Ohmic Polarization

Ohmic losses in an SOFC result from ionic conductivity through the electrolyte. This is inherently a materials property of the crystal structure and atoms involved. However, to maximize the ionic conductivity, several methods can be done. Firstly, operating at higher temperatures can significantly decrease these ohmic losses. Also, substitutional doping methods to further finetune the crystal structure and control defect concentrations can also play a significant role in increasing the conductivity. Another way to decrease ohmic resistance is to decrease the thickness of the electrolyte layer.6

[edit] Concentration Polarization

The concentration polarization is the result of the finite gas diffusion processes that govern movement of the gases into and out of the electrochemical reaction. The rate of mass transport of gases is described by Fick’s first law. Therefore the maximum rate of gas diffusion (which is directly related to the maximum current density that can be obtained) is found when the concentration of fuel at the electrochemically active area is assumed to be zero. The potential difference between operation where current is flowing and not flowing is the concentration polarization and is equal to:

{\eta}_{conc} = \frac {RT} {nF} ln(1- \dfrac {i} {i}_l )

where:

  • R = gas constant
  • T0 = operating temperature
  • n = number of electrons exchanged in electrochemical reaction
  • F = Faraday's constant
  • i = operating current
  • il = max current

The concentration polarization is highly dependent on the gases used as well as the distance that they must diffuse through. Pore volume percentage as well as diffusion length can be varied to optimize these properties. For similar geometries, cathode concentrations are much larger than anode concentrations due to the lower diffusivities of O2/N2 in the cathode than H2/H2O in the anode.

[edit] Activation Polarization

The activation polarization is the result of the kinetics involved with the electrochemical reactions. Each reaction has a certain activation barrier that must be overcome in order to proceed and this barrier leads to the polarization. The activation barrier is the result of many complex electrochemical reaction steps where typically the rate limiting step is responsible for the polarization. The polarization equation shown below is found by solving the Butler-Volmer equation in the high current density regime (where the cell typically operates)

{\eta}_{act} = \frac {RT} {{\beta}zF} ln(\frac {i} {i}_0 )

where:

  • R = gas constant
  • T0 = operating temperature
  • β = electron transfer coefficient
  • z = electrons associated with the electrochemical reaction
  • F = Faraday's constant
  • i = operating current
  • i0 = exchange current density

The polarization can be modified most easily by microstructural optimization. The Triple Phase Boundary (TPB) length, which is the length where porous, ionic and electronically conducting pathways all meet, directly relates to the electrochemically active length in the cell. The larger the length, the more reactions can occur and thus the less the activation polarization. Optimization of TPB length can be done by processing conditions to affect mircrostructure or by materials selection to use a mixed ionic/electronic conductor to further increase TPB length.

[edit] 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 currently underway to improve the fuel flexibility of SOFCs. While stable operation has been achieved on a variety of hydrocarbon fuels, these cells typically rely on external fuel processing. For the case of natural gas, the fuel is either externally or internally reformed and the sulfur compounds are removed. These processes add to the cost and complexity of SOFC systems. Work is underway at a number of institutions to improve the stability of anode materials for hydrocarbon oxidation and, therefore, relax the requirements for fuel processing and decrease SOFC balance of plant costs.

Research is also going on in reducing start-up time to be able to implement SOFCs in mobile applications. Due to their fuel flexibility they may run on partially reformed diesel, and this makes SOFCs 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 fueled by natural gas for power generation applications on the order of a megawatt.[4]

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.

Solid Cell Inc. has developed a unique, low cost cell architecture that combines properties of planar and tubular designs, along with a Cr-free cermet interconnect.

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.

[edit] 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.

[edit] See also

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[edit] 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. 
  3. ^ Singhal, S., Kendall, K. (2003). "High Temperature Solid Oxide Fuel Cells". 

[edit] External links

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