Electrode kinetics and their impact on high power fuel cell performance

Electrode kinetics and their impact on high power fuel cell performance

  1. Electrode kinetics and their impact on high-power fuel cell performance

Electrode kinetics play a key role in designing the most efficient electrodes for high-capacity fuel cells operating at high temperatures. Scientists and engineers believe that it is important to apply electrode dynamics to porous electrode structures. Problems related to engineering design, construction materials, catalysts, temperature, and species transport control can be solved if the electrode kinetic theory is rigorously applied in the electrode design stage. Electrochemical kinetics and species transport within the cell determine the voltage-current characteristics and the final cell output power [10].

The effectiveness of a given electrode structure and material and the reasons for the short electrode lifetime or limited output power are all closely related to the electrochemical kinetics at the electrode. Electrode kinetics can unambiguously explain why the cell voltage drops when there is current flowing through the cell. This lost voltage is called polarization. In fact, there are three different reasons for battery voltage drop. First, there is a potential drop within the fuel cell due to the ohmic resistance of the electrodes, wires and electrolyte when current flows. Second, slow chemical and electrochemical reactions cause some form of polarization called activation polarization. Third, the effect of mass transport causes the polarization to rise. (Learn more about battery material life issues go now)

If meaningful evaluation and improvement of fuel cell performance are the primary design goals, scientists and engineers engaged in practical research planning must use the concepts of electrode dynamics. Electrode kinetic theory reveals that electrons will become stable in equilibrium with positive ions in the electrolyte. Under such conditions, the distance between the electrode surface and the closest plane is determined by intermolecular interaction forces.

High-performance single-atom catalysts for high-temperature fuel cells
High-performance single-atom catalysts for high-temperature fuel cells
  1. Chemisorption-desorption rate polarization

Based on the electrochemical reaction, it is possible to determine the chemisorption-desorption rate under equilibrium conditions. These ratios can be determined by Temkin’s isotherm theorem and a high-speed computer. The polarization-current relationship of chemical reactions can be studied using large concentrations of reactants and products. Furthermore, impurities in the electrode material and other key elements in the overall system can have two distinct effects.

First, general electrochemical reactions proceed through active surface sites on electrodes. Any impurities in the electrode material may strongly chemisorb at these sites, clogging the surface. Second, impurity current interference can produce irregular current directions, which are extremely difficult to predict. Even existing models cannot accurately determine the effects of adsorption and desorption. More research work is needed to determine these ratios as accurately as possible. Both effects should be carefully considered, especially in the design stage of high power and high temperature fuel cells.

  1. Requirements for fuel cells for power station applications

Between 1960 and 1990, fuel cells with output capacity ranging from 1 to 50 MW were designed. More recently, higher capacity fuel cells have been designed and developed that are compatible with commercial high voltage transmission line requirements. Fuel cells with output capacities ranging from 40 to 60 kW using natural gas have been designed for hospital applications. Five fuel cells, each with a power output capacity of 250 kW, have been developed and connected to a commercial high-voltage transmission line to demonstrate a high power generation capacity of over 1 MW or more. Generally, the power capacity of a distributed power generation station is 2~20Mw (minimum).

Baseload power stations typically have capacities ranging from 100 to 500Mw when operating on natural gas. Power modules with low power capacity have been developed to operate with minimal noise and low thermal signals. The output power capacity of fuel cells in power stations can range from 1 to 100Mw, depending on their ability to meet customers’ power needs. Therefore, the design configuration of the electrodes, the type of electrodes, the type and amount of fuel required, and the physical parameters of the cell are strictly dependent on the capacity of the fuel cell needed to meet the electrical load requirements of the power plant. Designers of high-capacity fuel cells have revealed that the larger the fuel storage tank or the larger the electrodes, the more energy the fuel cell will provide.

Furthermore, by using dry or metallic fuels, higher reliability and safer operation may be possible compared to the use of liquid fuels in fuel cells, where leakage can be a serious maintenance problem [11]. Using aluminum or zinc metal-based fuel cells, it is possible to obtain higher electrical energy per unit mass. Aluminum provides a mass energy density of about 4kW·b/kg, while zinc provides a mass energy density of about 1kW·h/kg. High capacity of fuel cells can be achieved by stacking fuel cells and connecting them in an appropriate configuration.

A company in Taiwan, China, is developing a metal fuel cell with a three-layer aluminum sheet, electrolyte membrane, and air cathode [11]. The company claims that this fuel cell structure can be produced at minimal cost. These fuel cells are ideal for electric vehicles and portable power sources. The output capacity of these units may exceed 150kW. Capacities up to 250 kW or more are also possible using cascading techniques. Metal fuel cells are best suited for powering electric vehicles, buses and backup motor generators. These metal fuel cells are best suited for powering power stations with medium capacities ranging from 100 to 500 kW. However, if a stacking technique involving hundreds of such devices is used, the power generation capacity can exceed 1 Mw.

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