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Proton exchange membrane fuel cell (PEMFC), originally developed for space purposes, is a device that generates electricity using hydrogen. Compared to other fuel cells, the operating temperature of PEMFC is relatively low, around 80℃. Its structure is simple, the response characteristics to the electrical load is fast, and its performance is excellent. Thanks to these outstanding properties, it is applied in various fields such as eco-friendly transportation, distributed power generation, and mobile or military applications.
PEMFC
Proton Exchange Membrane Fuel Cell
On the other hand, PEMFC has several issues that need to be addressed. First, Pt-based catalysts are easily poisoned, oxidized under severe operating conditions, or agglomerated, leading to performance degradation. Second, delicate water management is required.
Excessive water accumulation can impede O2 flow, while water depletion can damage electroyte membrane under polarization. Additionally, proton exchange membrane such as Nafion, Flemion or Aciplex, require water humidification for proton transport and limits the operating temperature to the range of 80~90℃, which results in slow electrochemcal reations.
MEA
Membrane Electrode Assembly
Membrane Electrode Assembly (MEA) refers to the symmetrical structure of GDL/MPL/anode/membrane/cathode/MPL/GDL, and more briefly as cathode/membrane/anode structure, which generates about 0.7 V of electricity.
The electrode of MEA consists of a three-phase interface of platinum catalyst, ionomer, and carbon electrical conductor. Also cavities exist in MEA structure to allow effective access to gaseous reactants, such as hydrogen, oxygen, and generated water.
While hydrogen molecules are separated into protons and electrons (hydrogen oxidation reaction, HOR) at the anode, protons pass through proton exchange membrane, and electrons flow via the external electrical load, where they encounter to form water (oxygen reduction reaction, ORR) at the cathode.
For efficient electrochemical reactions at both electrodes, the gaseous reactants such as hydrogen and oxygen, the ionomer for proton conduction, and the carbon support for electron conduction must be in contact simultaneously. Therefore, optimizing the MEA structure is key to improving PEMFC performance.
The importance of the interface structure between carbon support and platinum catalyst, as well as the ratio between proton conductivity and the cavity of ionomer, is critical. These factors are in a trade-off relationship that controls the device’s efficiency. In addition, humidification to maintain proton conductivity through membrane and securing the outlet path for H2O at the cathode are essential factors in MEA design.
As a proton ion exchange membrane, perfluorinated sulfonic acid (PFSA) is mainly used. However since fluorine-containing membranes have environmental issues and exhibit low proton conductivity at high temperatures due to dehydration, non-fluorine-based polymer electrolyte membranes are actively researched.
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