What is a proton exchange membrane like?
What is a proton exchange membrane like?
In recent years, among various fuel cells, proton exchange membrane fuel cells have attracted more and more attention, which all benefit from the breakthrough in proton exchange membrane research. The proton exchange membrane, also known as the proton conducting membrane, can be said to be the heart of the proton exchange membrane fuel cell. This paper focuses on the development and current situation of proton exchange membranes, and expounds the performance characteristics of several commonly used proton exchange membranes. We can analyze its working principle and the relationship between structure and performance from several common synthetic methods of proton exchange membrane, so that the research of proton exchange membrane can make greater progress.
The functional properties of proton exchange membranes are quite different from those used in general chemical power sources. In a fuel cell, it not only acts to isolate the direct reaction between fuel and oxidant, but also acts as an electrolyte. Proton exchange membranes are special. It is a functional polymer membrane that conducts protons but isolates electrons, and has selective permeability. The development and application prospects of PEMFC largely depend on the performance of this membrane, and the importance of proton exchange membranes is self-evident. The fuel cell has the following requirements for the proton exchange membrane:
(1) Low gas permeability, because the cathode of the fuel cell is generally an air electrode, and the anode may also be H2. The proton exchange membrane needs to isolate the two gases so that they do not react directly on the electrode surface;
(2) The high transfer rate of hydrogen ions can reduce the internal resistance of the battery and reduce energy consumption; it has good chemical stability, which is undoubtedly directly related to the life and cost of the battery;
(3) Water molecules have sufficient diffusion speed on the membrane surface, and the reversibility of hydration and dehydration of the membrane is better, so that local deformation will not cause uneven performance on the membrane;
(4) Since the proton exchange membrane also undertakes the functions of membrane and electrolyte, the membrane is also required to have sufficient mechanical strength and good processability.
In the process of developing the proton exchange membrane, the following aspects were comprehensively considered:
1) Thickness and mass per unit area
The thickness and unit mass of a proton exchange membrane are directly related to its resistance. Reducing the thickness and unit mass can effectively reduce the resistance value and ohmic loss of the fuel cell, and improve the output voltage and energy density.
But if the thickness of the exchange membrane is too low, it will affect its tensile strength.
2) Tensile strength
The tensile strength is proportional to the thickness of the proton exchange membrane and is also related to the internal working environment. Therefore, when designing a proton exchange membrane, it is necessary to comprehensively consider its resistance value and tensile strength, and carry out repeated optimization design to find a suitable and matching thickness value.
3) Moisture content
The water content per unit dry membrane mass is called the water content of the proton exchange membrane. The moisture content has a great influence on the proton conductivity, and also affects the dissolution and diffusion of oxygen in the exchange membrane.
The higher the water content, the greater the proton diffusion silver and permeability, the lower the resistance of the exchange membrane, but the lower its strength.
The swelling degree refers to the percentage change of the area or volume of the exchange membrane after it is immersed in a given solution.
The swelling degree reflects the deformation characteristics of the exchange membrane. A high degree of swelling can cause electrode deformation and an increase in the localized stress of the exchange membrane due to membrane swelling during hydration and dehydration, resulting in decreased fuel cell performance.
When designing the exchange membrane, it is necessary to comprehensively consider its thickness, resistance, tensile strength, water content, swelling degree, etc. These factors affect each other, and it is necessary to find a comprehensive "middle point" through continuous optimization iterations.
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