Hydrogen Based Fuel Cells

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Hydrogen-based fuel cells produce electricity, heat and water by catalytically combining hydrogen with oxygen. They are composed of two electrodes, an anode (negatively charged) and a cathode (positively charged) separated by an electrolyte. This electrolyte can be made of a variety of materials from polymers to ceramics, which are in general ion (H+, OH-, CO32-, O2" ", etc.) conductors. The nature of the electrolyte determines many of the fuel cell's properties, including the temperature of operation, and this is therefore used to categorize the different fuel cell types.

In a proton-exchange membrane (PEM) fuel cell (vide infra), hydrogen entering the fuel cell is split with help of a catalyst (generally platinum) on the anode side into electrons and protons (H+). The electrons move along an external circuit to power an electric device, while the protons migrate through the electrolyte. At the cathode, by action of a catalyst, protons and electrons are recombined with oxygen of the air to produce water. The device is shown in Figure 9.12. Since every cell produces less than 1 V, many cells must be stacked together to produce higher voltages.

In an alkaline-based fuel cell, instead of protons, hydroxide ions (OH-) move from cathode to anode (Fig. 9.13). Although, the alkaline fuel cells are utilized in space applications, their commercial use is hampered by their sensitivity to CO2, which reacts with the alkali.

The most studied fuel cells designs are presently: phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), proton exchange membrane (PEM) fuel cells, and direct methanol fuel cells (DMFC). The latter will be discussed in more detail subsequently.

PAFC, MCFC and SOFC are generally designed to be used in stationary applications because they are heavy, bulky and require operating temperatures from around 200 °C for PAFC to about 650-1000 °C for MCFC and SOFC. PAFC, the most mature fuel cell technology, is commercially available from United Technologies Corp. Close to 300 units have been installed worldwide. As the name indicates, these fuel cells use liquid polyphosphoric acid as the electrolyte, the electrodes being made of carbon coated with finely dispersed platinum catalyst. The hydrogen required is obtained by methane (natural gas) reforming, and the overall efficiency from methane to electricity is 37-42%. With co-generation a heat efficiency approaching 80% can be achieved, which is comparable to conventional systems burning natural gas. At around $4500 kW-1 capacity [101], PAFCs also remain expensive compared to conventional fossil fuel-based technologies, with costs of less than $1000 kW-1 capacity. However, because fuel cells have no moving parts they are generally very reliable and require low operation and mainte-

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The alkaline fuel cell.

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2H20 + electric energy + heat nance costs. This explains why PAFCs have found their way only into "niche markets", mainly for consumers in need of a very stable, reliable and clean on-site electricity source such as banks , airports, hospitals, or military bases.

In MCFCs, the electrolyte is made of lithium-potassium carbonate salts heated to about 650 °C [101] (Fig. 9.14). At this high temperature the molten carbonate salts act as electrolytes and CO2 formed by the reaction of carbonate with hydrogen is transported between the anode and cathode through carbonate ion. Because they operate at high temperature, natural gas (or other fuels including methanol and ethanol) can be converted into a hydrogen-rich gases directly inside the fuel cell in a process called internal reforming. Without the need for an external reformer, MCFCs can reach fuel to electricity efficiencies above 50% - much higher than the value of 37-42% obtained with PAFCs. The higher temperature also allows nickel to be used as a catalyst instead of expensive platinum at lower temperature because of its much higher reactivity than nickel. The first commercial unit was delivered by Fuel Cell Energy Inc. to a brewery in Japan in 2003, and today more than 50 units from that company and others are operating worldwide. Their price is in the same range as PAFCs. Molten carbonates are, however, highly corrosive, and this raises some concerns about the fuel cell's lifetime. Fuel-Cell Energy Inc. in collaboration with the U.S. Department of Energy, is also developing a hybrid system combining a MCFC with a gas turbine which could eventually lead to power plants with fuel to electricity efficiencies approaching 75%.

Figure 9.14 The molten carbonate fuel cell (MCFC).

SOFC is the technology that currently attracts the most attention for stationary applications. These cells operate at high temperature (800-1000 °C) and thus, like MCFCs, do not require a reformer. However, in contrast to PAFCs and MCFCs, the SOFC uses a solid ceramic (usually Y2O3-stabilized ZrO2) instead of a liquid as an electrolyte. O2- ions are transported from cathode to anode, the latter being made of Co-ZrO2 or Ni-ZrO2 (Fig. 9.15). This feature allows the electrolyte to adopt different shapes, such as tubes or flat plates, giving a greater freedom in fuel cell design and also avoiding problems connected with the use of corrosive liquids. The efficiency of SOFCs is expected to be around 50-60%. The high temperature of the exhaust gases produced are ideal for co-generation and combined cycle electric power plants. In combination with gas turbines, efficiencies of 70% or more could thus be achieved. In co-generation units, the use of waste heat could bring overall fuel efficiencies to 80-85%. The U.S. Department of Energy has formed the Solid State Energy Conversion Alliance (SECA) involving companies, universities and national laboratories, with the goal of producing a highly efficient SOFC that would cost only about $400 kW-1, allowing this technology to compete with diesel generators and natural gas turbines and rapidly to gain widespread market acceptance. The mass production of standardized basic ceramic modules, using manufacturing technologies similar to those developed for e"

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Figure 9.15 The solid oxide fuel cell (SOFC).

the production of electronic components, is believed to be the key to this ambitious cost reduction.

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