Fuel cells for automotive, bus and motorcycle applications

Fuel cells for automotive, bus and motorcycle applications

The Big Three automakers, General Motors, Ford and Chrysler, were the first to develop fuel cell technology as a viable vehicle power source in their products. Since then, other car companies, Toyota, Nissan, Mazda, Volvo, have begun to actively design vehicles featuring hybrid technology, using fuel cells as well as lithium-ion and lithium-polymer batteries.If you want to buy high-quality lithium-ion batteries, please enter Tycorun Lithium Battery for consultation and purchase.

The goal of these car companies is to develop affordable fuel-efficient, low-emission vehicles. Confidence in these vehicles began to build when Ballard Power Systems of Canada demonstrated a reliable stack design that could produce hydrogen fuel with power densities approaching 100W/L. This means that it takes 30L of hydrogen to convert into 30kW of electricity, which is enough for most cars and minibuses. Hydrogen-based proton-exchange membrane fuel cells were developed to replace existing internal combustion engines (ICEs), and these proton-exchange membrane fuel cells are currently widely used in buses and automobiles. Hydrogen-based PEM fuel cells consist of a fuel electrode (anode) and an oxidant electrode (cathode), which are separated by a porous polymer electrolyte, or PEM. The catalyst is combined between the electrodes, and the whole stack structure is called membrane electrode integration. PEM produces low emissions, high efficiency, fast start-up capability, and fast transient response.

The 200kW bus engine runs on hydrogen, which is stored in cylinders located on top of the bus. PEM fuel cells use hydrogen as fuel. Several types of fuels can be used in fuel cell devices, such as hydrogen, CH3OH, natural gas, and gasoline, and the conversion efficiency of the cells depends on the type and purity of the fuel. Preliminary research conducted by the author shows that public transport vehicles can travel a maximum distance of 400 km without more hydrogen supply, and cars can travel more than 200 km without more hydrogen supply. A PEM fuel cell with a capacity of 30kW is enough for a car, and about 1kw is enough for a motorcycle. 20~25kw fuel cells are enough for small passenger cars. Developing countries, such as Vietnam, China, South Korea and some African countries, have shown great interest in fuel cells for automobiles and motorcycles because the running costs of using fuel cells are lower compared to gasoline-powered cars or motorcycles It provides a 25% to 40% reduction in the manufacturing cost of the battery, and the weight and size of the battery is proportionally reduced compared to that of a conventional car. Fuel cells can be classified by operating temperature and type of electrolyte. Not all fuel cells are suitable for use in automobiles.

The performance characteristics summarized in Table 1 clearly show that hydrogen-based PEM fuel cells are best suited for automotive applications when efficiency, cost, lifetime, and reliability are the main design requirements. PEM fuel cells directly convert hydrogen into electricity with the highest conversion efficiency. In addition, a reformer, a small chemical reactor installed in some fuel cell vehicles, extracts hydrogen from alcohols in other hydrocarbon fuels, such as gasoline.

Table 1 - Performance comparison of various fuel cells
Table 1 – Performance comparison of various fuel cells

The data in Table 1 clearly illustrate the type of fuel, electrolyte, conversion efficiency, power generation range, operating temperature and specific fuel cell application. Electric vehicle designers feel that for PEM fuel cells to truly compete with internal combustion engines, the cost of stacks containing hundreds of cells must drop to about $20/kW. Since a typical fuel cell for an automobile has a capacity requirement of at least 20kW, the fuel cost must not exceed $400 to be most cost-effective. This clearly shows that current fuel cell design and development efforts must address not only performance, size, weight, but also procurement costs. The author’s research on fuel cells has shown that fuel cells operating at low or moderate temperatures have low production costs, high conversion efficiency, improved voltage performance, and maintain high reliability for longer durations. For fuel cells using hydrogen and oxygen as fuel, the thermodynamic efficiency may be greater than 80% only when the operating temperature does not exceed 850 °C. For galvanic cells, the reaction entropy (DS) plays an important role. The heat and electrical energy generated by the battery assuming zero electrolyte resistance (RE) are illustrated. Figure 1 shows the thermodynamic cell voltage (E0), thermal cell voltage (E0H), and current levels as a function of various parameters.

Figure 1 - Fuel cell voltage and current levels as a function of various parameters
Figure 1 – Fuel cell voltage and current levels as a function of various parameters
  1. Low-cost, high-efficiency, low-temperature H2-O2 fuel cells

It is evident from the data summarized in Table 1 that the H2-O2 fuel cell with proton exchange membrane design provides a low cost (currently ~$100/kW), low temperature operation (80°C), high electrochemical efficiency (>55 %), devices with improved reliability over longer durations (>16000h). The electrical properties of the porous carbon electrode H2-O2 fuel cell developed by Union Carbide in 1960 according to various electrolytes and continuous operation time are listed in Table 2.

Table 2 - Hydrogen-oxygen fuel cell voltage performance as a function of operating time and different electrolytes used
Table 2 – Hydrogen-oxygen fuel cell voltage performance as a function of operating time and different electrolytes used

The test data presented in Table 2 were obtained by Union Carbide scientists in 1960 using potassium hydroxide and sodium hydroxide (NaOH) as electrolytes. Tests conducted before 1970 are considered to have significantly improved the voltage data. Scientists at UnionCarbide believe that carbon itself is a good catalyst that can provide good results. The scientists further believe that current densities of hundreds of amps per square foot are possible using only a small amount of catalyst on the anode.

Another H2-O2 fuel cell developed by UnionCarbide has 1ft² electrodes, each 0.25in thick. A fuel cell designed by a European scientist demonstrates the catalytic dehydrogenation of a liquid fuel dissolved in an electrolyte. In this fuel cell, oxygen or air is used as the cathode, and the dehydrogenation reaction is used as the anode, as shown in Figure 2. For the benefit of readers eager to find design and development activities using this fuel, the four key steps in the dehydrogenation and oxidation of ethylene glycol (CH2OH-CH20H) fuel are shown in Figure 3.

Figure 2 - Working of a fuel cell based on catalytic dehydrogenation of liquid fuel dissolved in electrolyte, Figure 3 - Four steps of dehydrogenation and oxidation of ethylene glycol and production of final product
Figure 2 – Working of a fuel cell based on catalytic dehydrogenation of liquid fuel dissolved in electrolyte, Figure 3 – Four steps of dehydrogenation and oxidation of ethylene glycol and production of final product

With this particular fuel cell design, it is possible to obtain approximately constant cell voltages. Higher current densities can be obtained from metal fuel cells with larger electrode surfaces [11]. In addition, the voltage drop is smaller as the current density increases, so these cells can operate continuously for more than thousands of hours. This particular fuel cell design is ideal for applications where reliable performance, quasi-constant cell voltage, high conversion efficiency, and safe operation over long durations are the main requirements.

  1. Design aspects and performance parameters of low-cost, intermediate-temperature fuel cells

DMFC provides a low-cost, portable, reliable fuel cell operating at moderate temperatures not exceeding 100°C and one atmosphere pressure. It uses liquefied fuel like methanol, which is fairly cheap and has no restrictions. Liquefaction fuel cells contain a mixture of the usual alkaline water electrolyte and a soluble cheap fuel. The electrodes must exhibit different specific catalytic activities to generate voltage. The cell must contain a highly active dehydrogenation catalyst. Platinum or palladium must be used to remove hydrogen atoms from hydrocarbons and act as hydrogen electrodes. Obviously, such anodes do not require complicated and expensive microporous systems. Ordinary flakes with inexpensive thin active layers are sufficient.

Therefore, there are no more geometric constraints on the three-phase boundary, as both the catalytic dehydrogenation reaction and the electrochemical reaction will take place at a two-phase boundary. Furthermore, hydrogen can be immediately absorbed in the atomic state without intermediate molecular reorganization, which saves dissociation energy and improves conversion efficiency. For these reasons, liquefaction fuel cells can achieve confinement current densities exceeding 1000 mA/cm² at mild temperatures (65–100 °C) and uniform atmospheric conditions. The battery uses a soluble cheap fuel, which can be formic acid, methanol or ethylene glycol. In the case of complete combustion, water and carbon dioxide are the final chemical reaction products. Both liquid fuels and water help produce hydrogen. In the case of ethylene glycol, 4 moles of absorbed hydrogen fuel are produced. In addition, because liquid fuels are flammable, they are very attractive for space and subsea system applications.

The potential advantages of direct methanol fuel cell devices can be summarized as follows:
 The working temperature of direct methanol fuel cell is lower than 100℃.
 Does not produce NOx
Minimum cost.
Compact packaging.
The device provides safety, reliability, long life and portability advantages.
Methane is stable and without any risk, it can be easily stored and transported.
 Combustion products, including carbon dioxide and water, are relatively harmless.
If continuous operation is required, the empty cartridge can be replaced by a cartridge filled with methane fuel.

The size requirements of key components of a DMFC unit using methane as a fuel are strictly dependent on the power generation capacity of the cells. Typical dimensions of various components recommended by DMFC cell designers are listed in Table 3.

Table 3 - Typical recommended dimensions for various DMFC components
Table 3 – Typical recommended dimensions for various DMFC components

Note: The recommended dimensions are valid only for DMFC units operating at ambient temperature and pressure and with normal power capacity. Maximum power output and best cell conversion efficiency occurs when all methane passing through the membrane is consumed by direct catalytic combustion at the cathode, producing carbon dioxide and water.
Applications of soluble fuel cells. These fuel cells are compact, relatively inexpensive, more reliable, and safe to operate. Such fuel cells are most suitable for powering small vehicles, such as motorcycles operating in urban traffic environments, because the power-to-weight ratio (P/W) is large and CH3OH is quite cheap compared to gasoline’s 3.80mills/(kW·h) ratio, the P/W ratio is higher than 16.50 mills/kW.h). Small cars can travel the same distance, consume 1/2 the fuel, and have no excessive noise and smell.

High-temperature fuel cells using natural gas are ideal for large-scale power stations, while low-temperature fuel cells using hydrogen or alcohol are best for small household power generation equipment, powering satellites, and propelling electric or hybrid electric vehicles.

  1. Design requirements for cost-effective fuel cells

Fuel cells using hydrogen as fuel and porous nickel-carbon electrodes offer the most cost-effective design. Currently, such fuel cell designs can be implemented at the lowest possible cost, ie $100/kW. Hydrogen is the cheapest fuel, and in order to run smoothly and efficiently it must use chemical conversions. For some inexpensive fuels, chemical conversion is used to convert hydrocarbons to hydrogen. This conversion process tends to increase the conversion efficiency of the fuel. In some hydrogen-based fuel cells with proton exchange membrane structures, water evaporation is a problem with the membranes.

As mentioned earlier, a PEM cell consists of a fuel electrode (anode) and an oxidant electrode (cathode). The two electrodes are made of porous carbon material separated by an ion-conducting polymer electrolyte. Polymer electrolytes are also known as proton exchange membranes. A catalyst is integrated between each electrode and the electrolyte. Preliminary cost estimates indicate that proton exchange membrane fuel cells have the lowest production cost for the same power capacity compared to other fuel cells. The advantages offered by cost-effective fuel cells include lower cost, high reliability, long life and excellent portability.

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