What are the classifications for aerospace and military fuel cells?

What are the classifications for aerospace and military fuel cells?

The demand for long-lasting, portable power sources has grown rapidly over the past few years. In addition, conventional batteries cannot meet the uninterruptible power supply requirements of high-power electrical components and electronic devices used in long-term military missions in battlefield environments. In addition, conventional high-power lithium-ion batteries suffer from weight, size, reliability, discharge rate, handling issues, and charging capacity. In light of these issues, manufacturers of portable power sources are increasingly turning to fuel cells to replace lithium-ion batteries. Fuel cells generate electricity through electrochemical conversion technology that can be replenished with minimal time and effort. Recent studies conducted by various fuel cell scientists have shown that direct methanol fuel cells (DMFCs) are most suitable for portable high-power applications as DMFC technology offers the most promising, practical solutions and unique advantages such as Compact form factor, improved reliability, and significantly reduced weight and size. Furthermore, methanol fuel is widely used without any limitation. A fuel cell is a system that combines oxidation and reduction reactions to generate electricity.

European scientists carried out comprehensive research and development activities using high temperature and semi-solid electrolytes in 1990. Bacon Hydroxide HYDROX fuel cells are designed to operate at moderate temperatures and high pressures. Electrochemical energy converters are designed to operate at ambient temperature and pressure. German scientists have developed a double-framework catalyst (DSK) fuel cell that uses a liquid carbonaceous fuel such as methanol and electrodes with different catalytic properties, while Swiss scientists have designed a single-framework catalyst (MSK) fuel cell that uses an inexpensive fuel (hydrocarbon) and has Electrochemically active metal electrodes.

Certain terms are commonly used when dealing with fuel cells. The most commonly used terms are as follows.
● Anode: This is a negative electrode or fuel electrode that releases electrons to an external circuit. Hydrogen is oxidized during this process.
● Cathode: This is a positive or oxidizing electrode that accepts electrons from an external circuit, and in the process oxygen is reduced.
• Membrane Electrode (MEA): This is a laminated sandwich of two porous electrodes separated by an ion-conducting polymer electrolyte. The catalyst is part of the membrane electrode assembly.
●Proton Exchange Membrane (PEM): A polymer membrane used to block the passage of gases and electrons, while allowing hydrogen ions called protons to pass through.
● Reformer: A small on-board chemical reactor that some fuel cell vehicles use to extract hydrogen from alcohol or hydrogen fuel.

During the 1960s and 1970s, three different fuel cells were designed, developed and evaluated, primarily by scientists and engineers in the United States and Europe. But their performance parameters, such as current density, terminal voltage, continuous operation time, are insignificant. Early developed fuel cells can be briefly described as follows.

1. Aqueous fuel cells using specific electrolytes
Scientists and professors working at the Braunschweig University of Technology in Germany have developed the first fuel cell of this type. The key feature of this fuel cell is the double-framework catalyst DSK electrode. The device has two electrodes: one provides structural support; the other provides high conductivity. The catalytically active framework is embedded into the supporting framework. The fuel electrode is made from hot-pressed nickel (Ni) under controlled conditions. It is possible to optimize the cell performance of fuel electrodes using pure hydrogen. Current densities up to 400 mA/cm² have been obtained at ambient temperature (65°C) and low voltage (51b/in²). A four-battery device has demonstrated exceptional reliability over a full year of continuous operation. Both fuel and oxidant electrodes can be fabricated using catalytic porous nickel. Aqueous fuel cells use an alkaline electrolyte, hydrogen as the fuel, and oxygen or air as the oxidant.

2. Fuel cells using semi-solid electrolytes
Fuel cells using semi-solid electrolytes consist of porous magnesium oxide (MgO) containing a mixture of sodium (Na), potassium (K), and lithium carbonate (LiCO3). The battery is essentially a disc of Mgo impregnated with a narrow, thin strip of compound. Specific details of the various elements of such a fuel cell are shown in Figure 1.

What are the classifications for aerospace and military fuel cells?
Figure 1. High-power fuel cell containing MgO-LiNaCO3 semi-solid electrolyte with metal tubes as air and fuel gas electrodes

3. Fuel cells using molten electrolytes
The fuel cell structure with tubular cells uses fine-grained solid magnesium electrolyte and molten electrolyte. The surface of the electrode is coated with metal powder, such as silver (Ag) for the air oxygen cathode and iron (Fe), nickel (Ni) or zinc oxide/silver (ZnO/Ag) mixture for the fuel anode. Figure 2 shows structural details of a fuel cell of tubular construction. This particular fuel cell is capable of delivering a current density of 100mA/cm² at a polarization voltage of 0.7V. Hydrogen exhibits very low polarization even at high current densities. In contrast to most low temperature fuel cells, the carbon dioxide and water produced by high temperature fuel cells are formed in the fuel, not in the electrolyte. This will lead to fuel dilution, which makes it difficult to simultaneously achieve high current densities, and high fuel utilization without obtaining strong polarization. The electrical properties of the fuel cells operating at 700°C in tubular structures using semi-solid electrolytes, sheet nickel anodes, silver cathodes, and various fuels are summarized in Table 1.

What are the classifications for aerospace and military fuel cells?
Figure 2. Key elements of a high-capacity fuel cell for optimum performance using an electrolyte paste composed of fine-grained solid MgO and molten electrolyte

High percentage values ​​are related to the fuel used, lower percentage values ​​indicate carbon dioxide gas produced from the electrochemical reaction. For example, in the case of hydrogen fuel, 65% is hydrogen and 35% is carbon dioxide. In another example, methane fuel, the percentage of fuel to carbon dioxide is the same, but the polarization voltage is very low.