First practiced by General Electric researchers during the 1950s and later developed for the NASA Gemini Space programme in the 1960s, low-temperature proton exchange membranes (LTPEM) fuel cells have been the go-to hydrogen fuel cell technology for decades, thanks in part to significant investment from the US and Europe in LTPEM projects.
LTPEM is well-known for its efficiency, power density, life, and ease of use when using pure hydrogen as a fuel. The technology has been refined over the decades by some of the world’s great chemical and membrane companies and is making significant inroads into power generation markets such as transportation, material handling, and backup power. Using pure deionised water as an electrolyte, the membrane (usually Nafion™) situated between a positive and negative catalytic electrode, allows singular hydrogen ions (protons) to cross and combine with oxygen molecules to create water. The hydrogen’s electron is collected and used to create electric current. The amount of current a membrane is capable of producing is based on the number of available catalyst sites and hydrogen atoms, which makes it dependent on the area of the membrane electrode assembly (MEA) and the pressure of the hydrogen supply. Due to its repeatability and durability, LTPEM is well suited for use with large pressurised stacks capable of generating dozens of kilowatts of power.
However, LTPEM is not without its engineering challenges, most notably deionised water electrolyte management and thermal management. The membrane requires a “Goldilocks” scenario with an abundance of water vapor without too much liquid water. Too little water vapor causes the membrane to “dry out” resulting in a loss of membrane electrical conductivity, increased electrical resistance, and heat generation; at the same time, too much liquid water results in “flooding” which prevents reactants from reaching catalyst sites and completing the reaction. Operators usually maintain LTPEM fuel cell stack temperatures between 65-90°C to keep water vapor levels optimised, which is necessary for the stack to generate power. Adding pressure greatly improves the robustness of a stack’s power generation, since pressurised reactants increase the number of available molecules at the catalyst sites and force liquid water out of the stack. Critically, fuel cell reactions generate a significant amount of heat, which needs to be removed in order to maintain a steady and operational stack temperature. Efficient removal of this low-grade heat and overall thermal management is a trademark of LTPEM.
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