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MindMap Gallery Fuel cells and fuel cell vehicles-4

Fuel cells and fuel cell vehicles-4

A fuel cell is a chemical device that directly converts the chemical energy of fuel into electrical energy. It is also called an electrochemical generator. It is the fourth power generation technology after hydropower, thermal power and atomic power. Knowledge about the assembly of proton exchange membrane batteries.

Edited at 2023-11-21 23:03:01

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Fuel cells and fuel cell vehicles-4

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Outline

Fuel cells and fuel cell vehicles—4

The main body of the battery pack is MEA, bipolar plates and corresponding splints

If the two end plates are made of metal, an insulating plate made of engineering plastics needs to be added between the guide plate and the end plate.

Metrics

Conversion efficiency (civilian power generation)

Mass specific power, volume specific power (automotive)

damage

The main reasons for exchange membrane damage are:

Excessive waste heat is generated when the local current density is too high, and the hot spot breaks through, causing the fuel mixed with oxidizing agent

Mechanical damage and reaction gas pressure fluctuation during MEA preparation

Rapid changes in membrane moisture content lead to membrane damage

At present, the dimensional stability is poor. The membrane swells when it absorbs water and shrinks when it loses water, with a change range of up to 10% to 20%. If the MEA preparation conditions are inappropriate, or the water content of the membrane changes drastically during the battery startup and shutdown process, or the pre-humidification capability is insufficient during battery operation, the size of the membrane in the MEA will change drastically and be destroyed.

Opposite pole

The change process of cell voltage from positive to negative due to the accumulation of inert gas or insufficient supply of fuel and oxidant.

If the battery pack is allowed to continue running after pole reversal occurs, the first single cell will release oxygen in the hydrogen chamber and enter its adjacent single cells through the shared pipe of the battery pack, causing the battery pack voltage to drop significantly. In severe cases, the battery pack will be destroyed due to the explosion of hydrogen and oxygen mixed in the shared pipe of the battery pack or the air chamber in the single cell.

Assemble

seal

Flow field design

Reasons for battery pack failure

Electrocatalyst Poisoning and Aging

Aging, corrosion and fouling of proton exchange membranes

When the load changes significantly (starting, stopping, and running), one or more cells in the battery pack will fail or even explode, causing the entire battery pack to fail.

Thermal management

Heat removal method

Coolant circulation heat removal method

pure water

Once the water is contaminated (deionized water is used) and the conductivity increases, slight electrolysis will occur in the shared pipe through which the cooling water of the battery pack flows, producing a mixture of hydrogen and oxygen, which will affect the safe operation of the battery and will also produce certain Internal leakage reduces the energy conversion efficiency of the battery pack. (Polluting metal ions in water can be removed through ion exchange method)

Water and ethylene glycol mixture

The resistance of the coolant will increase. As the specific heat capacity of the coolant decreases, the circulation volume needs to increase, and once the coolant is contaminated by metal ions, its removal is much more difficult than pure water.

Air cooling method (low power)

For kilowatt-class, especially 100-watt-class PEMFC battery packs, air cooling can be used to remove the waste heat generated by the battery pack.

Steam heat removal method

In order to ensure the uniformity of temperature distribution of the battery pack, the maximum temperature difference between the coolant inlet and outlet generally does not exceed 10C, preferably 5°C. In this way, the cooling water flow is relatively large. In order to reduce the power consumption of the cooling water pump, the pressure drop of the coolant flowing through the battery pack should be minimized.

water management

Because the proton (ion) conductivity of the membrane is closely related to the wetting state of the membrane. Liquid water relies on reagent gas purging. However, a large amount of liquid water will lead to a reduction in the oxygen mass transfer rate in the cathode diffusion layer. (More than 90% gaseous water) can increase the mass transfer rate of oxygen in the oxygen cathode gas diffusion layer and reduce the heat load of waste heat discharged from the battery pack.

Delivery method

Electromigration (together with H)

It is related to the battery operating current density and the hydration number of protons.

Concentration reverse diffusion

Water is generated at the cathode, and its concentration is higher than that on the anode side. It is reversely diffused under the action of water concentration difference.

Concentration gradient

Diffusion coefficient

pressure migration

Generally, the oxidant pressure is higher than the reducing agent pressure, and water is transferred from the cathode side of the membrane to the anode side.

The pressure gradient is directly proportional to the permeability coefficient of water in the membrane and inversely proportional to the viscosity of water in the membrane

Compared with gas phase water, the discharge of liquid phase water will be more difficult. When the battery operates at high current density, the occurrence of two-phase flow is inevitable.