Types of Fuel Cell

There are several types of fuel cell under development, all of which have different characteristics and potential uses. The most commonly studied are discussed below.

Figure 1 shows a breakdown of the different fuel cell types, their operating temperatures and electrolyte types.

Figure 1. Diagram of the most common types of fuel cell, the fuel and oxidising agents required, typical operating temperatures and the electrolyte ion transfer type and direction

1.5.4.1. Proton Exchange Membrane Fuel Cell (PEMFC)

Proton exchange membrane fuel cells, also known as polymer electrolyte membrane fuel cells, require high purity hydrogen to operate. They typically utilise carbon-supported electrodes containing noble metals with an electrolyte consisting of proton-conducting polymers such as Nafion®. Hydrogen enters the cell on the anode side and is broken down into protons, which flow through the electrolyte membrane, and electrons, which flow around the external circuit to the cathode. Here, the protons and electrons react with oxygen to form water as the only waste product. The high cost of the materials currently

used has led to much research focusing on alternatives to make these cells more financially viable. In addition, the metal catalysts used are susceptible to poisoning from contaminants present in the fuel supply. This stringent requirement for fuel purity mean that reformers intended to supply the cell with hydrogen from a hydrocarbon fuel source must have additional filters, or be more complex than if the cell was less susceptible to poisoning.

Operating temperatures are typically in the region of 50 - 150 °C, so PEMFCs are considered low temperature fuel cells. This low temperature requirement allows PEMFCs to start and stop operation rapidly, making them highly suited to mobile applications. However, it also means that they generally are unsuitable for combined heat and power applications as the heat output is too low.

1.5.4.2. Alkaline Fuel Cell (AFC)

Alkaline fuel cells are amongst the longest-proven fuel cells, having been used in the Apollo series of missions run by NASA. They are based around an electrolyte which is strongly alkaline, normally concentrated sodium or potassium hydroxide in liquid form with an inert porous solid support. Oxygen and water combine at the cathode to form hydroxide ions, drawing electrons from the external circuit. The hydroxide ions pass through the electrolyte substance and react with hydrogen at the anode to form water, releasing electrons which can flow around the external circuit back to the cathode. AFCs, like PEMFCs, are classified as low temperature fuel cells, with typical operating temperatures between 25 – 75^oC.

Carbon dioxide must be removed from the gas coming into the cathode side as it poisons the cell by reacting with NaOH/KOH forming Na2CO3/K2CO3 which can block the catalytic sites. As with PEMFCs, AFCs require high purity hydrogen. This gas processing at both the anode and cathode sides drive up system complexity and increases parasitic load, leading to lower overall system efficiencies. In addition, noble metals such as platinum are commonly used within the electrodes which also drive up cost. This, combined with the hazardous liquid electrolyte material used, is the reason that research into and demand for alkaline fuel cells has waned somewhat, with more interest being paid to PEMFCs. However, research is ongoing into cheaper, non-precious metal catalyst materials, which are believed possible due to the more favourable electrode processes in alkaline conditions, has recently reinvigorated research into AFCs.

1.5.4.3. Alkaline Exchange Membrane Fuel Cell (AEMFC)

Alkaline exchange membrane fuel cells are a relatively recent development, having first been discussed at length in 2005 [7]. These cells use a solid alkaline polymer as the electrolyte material which can tolerate operation in the presence of CO2, therefore removing the requirement of cathode gas cleaning present in AFCs. The other advantages of AFCs are still present, such as the potential for non-precious metal low cost catalysts and low operating temperature. However, as with any new technology there are still many obstacles to be overcome, such as the development of an effective alkaline ionomer which is easily dispersed and maximises the ionic contact between the ion membranes and electrode layers.

1.5.4.4. Phosphoric Acid Fuel Cell (PAFC)

Utilising an electrolyte composed of phosphoric acid at high concentrations, these cells are much more tolerant to impurities than both PEMFCs and AFCs. The phosphoric acid is typically used at such high concentrations that it forms pyrophosphoric acid (H₄P₂O₇), which freezes at room temperature; it is for this reason that the elevated operating temperatures (150 - 200 °C) are usually maintained for the life of the cell, making them ideally suited for uninterruptable power supply or continuous load applications. They are also suitable for use in vehicles that spend the majority, if not all of their time, requiring a power output, such as buses or ships.

Operation reactions are very similar to those in PEMFCs, however the electrolyte is a configuration more similar to AFCs, with a liquid conducting medium supported by a solid porous structure. However, similarly to AFCs, the hazardous electrolyte material and also the constant operation requirement make maintenance difficult, and the electrodes again typically also contain platinum, which drives the cost up.

1.5.4.5. Molten Carbonate Fuel Cell (MCFC)

Molten Carbonate Fuel Cells operate at a range of temperatures above 500^oC, allowing non-precious metals to act as catalysts in order to keep costs down. The electrolyte is composed of a molten salt mixture which conducts carbonate ions, again within a porous inert solid support. Carbon dioxide must be present at the cathode in order to generate the CO3

2-, and this is often achieved through recycling the exhaust gas to increase the

CO₂ partial pressure. MCFCs can operate on a wide range on fuels, including some that are poisons to other fuel cells, such as carbon monoxide. If running on hydrocarbons then internal reforming can occur, allowing the system to utilise them directly. This fuel flexibility grants the system a considerable advantage over many other fuel cells covered previously, as MCFCs could be run on widely available commercial fuels without the need for a change of infrastructure.

However, the high temperatures and hazardous electrolyte materials used do have their drawbacks. In this respect the durability of the cell is compromised due to the corrosion of components in the harsh chemical conditions. Significant current research is therefore concentrated on materials which can withstand this environment while maintaining high levels of efficiency. In addition, the high operating temperatures mean that a fast on-off cycle is difficult to achieve, rendering MCFCs unsuitable to a wide range of applications requiring temporary or intermittent power.

1.5.4.6. Solid Oxide Fuel Cell (SOFC)

Solid Oxide Fuel Cells have the highest operating temperature region, between 500 – 1000 °C. They make use of a solid gas-tight electrolyte, usually a ceramic, which can conduct oxide ions. Oxygen enters the cell at the cathode side and is reduced to oxide ions, which pass through the electrolyte layer. On reaching the anode they react with the fuel, releasing the electrons they gained on the cathode side, which can flow back around the external circuit. Developments have also been made for electrolyte materials which conduct protons in addition to oxide ions, such as Ba(Zr/Ce)1-yYyO2-y (BZCY) [8];

however, this work is relatively new and requires considerably more research before a commercially viable system could be produced.

SOFCs can operate at high efficiencies, in the region of 50%, on a variety of fuels. A significant increase in this efficiency can be reached if the waste heat is also utilised.

This makes SOFCs ideal for combining with turbines, or for use in Combined Heat and Power (CHP) systems. As for MCFCs, SOFCs are normally stable in the presence of carbon monoxide and dioxide and do not need to use precious metals as catalysts because of the high operating temperatures. Another similarity to MCFCs is that they can run on a variety of fuels and in most cases can internally reform hydrocarbons. However, there are still many issues to overcome to increase the performance and lifetime of SOFCs. Nickel is often used in the anode as a catalyst and electron conductor, and operates well on pure hydrogen, although when using hydrocarbons as the fuel it has drawbacks. Sulphur is present in most hydrocarbon fuels and this irreversibly binds with the nickel, reducing both catalytic activity and electronic conductivity in the anode, severely affecting performance. In addition, nickel is a catalyst towards the formation of carbon-carbon bonds which can lead to ‘coking’ at the anode. Coking involves the build-up of carbon on the surface of the anode, blocking the catalytically active sites and disrupting gas transfer, which significantly decreases cell performance, and will be discussed in further detail in Chapter 2

This thesis addresses the material aspects of SOFC systems, concentrating primarily on those which are exposed to reducing environments. The different materials used with SOFCs will also be discussed in Chapter 2.