57 Hydrogen fuel cells | Deep insight E-Mobility Engineering | July/August 2024 lower running temperatures, allowing for more rapid start-ups and less wear on system components. Fuel-cell stacks Single fuel cells are stacked on top of one another to create a stack of any number, based on the power requirements of the system. Endplates (positive and negative) surround the base and top of the stack, providing current take-offs and sound structural support. Fuel-cell stacks are housed within a fuel-cell module, which in turn contain a fuel-cell stack with the required components to operate and manage the generation of power; e.g. control software, electronics, hydrogen valves and fans. Communication between the system and a fuel cell is essential for the ongoing running of the latter and the generation of usable power. However, a number of other components are required around the stack, called the balance of plant (BoP), of which a key part is the humidifier. Protective humidifier A fuel-cell humidifier protects the fuelcell membranes against drying out. In addition, the humidifier provides humidity by transferring water vapour from the exhaust gases to the intake gases. The gases sent to the fuel cell are humidified to maintain the optimal hydration level of the membrane. For a PEM fuel cell, water management is important since proton conductivity of the polymer electrolyte membrane, which plays a critical role in determining cell performance, is proportional to the hydration level of the membrane. A variation of relative humidity from 85% to 35% causes a full order-of-magnitude reduction of the protonic conductivity of Nafion, from 0.09 Scm to 0.009 Scm. Principally, a fuel cell needs to be hydrated using a fuel-cell humidifier. Excess water causes diffusion of the reactant gases towards catalytic layers, which reduces cell performance, along with losses due to concentration, polarisation and mass transportation. For high performance and good proton conductivity, it is essential to maintain optimal water content in the membrane, achieved by feeding humidified reactant gases through a humidifier. The water produced internally by the oxidant reduction reaction is generally not enough to ensure sufficient hydration of the electrolyte membrane everywhere in the cell. The drying effect of the reactant streams removes this product water from the GDL and leaves the membrane dry, especially at the beginning of the flow field channels. To compound the problem, while it is desirable to operate the fuel cell at higher temperatures to minimise activation losses, the resulting drying effect becomes more pronounced due to the exponentially increasing nature of the saturation vapour-pressure curve. A more humidified membrane has a higher protonic conductivity and Fuel-cell technology A single fuel cell is made up of very few parts: two plates (negative anode and positive cathode) to fulfil the structural function, and provide electrical and thermal properties; gaskets to help provide gas seal under operation; gas diffusion layers (GDL) to aid gas distribution and water management within the fuel cell; and a membrane electrode assembly (MEA). Middle membrane The MEA is split up into three layers: an anode electrode, a cathode electrode and a middle membrane layer. The electrode layers are generally comprised of a porous material combined with a catalyst, and the electrolyte membrane is made of polymers to provide chemical and mechanical support. It is important that the membrane layer of the MEA is electronically insulating and should only facilitate the transportation of protons through it to the cathode side. The single fuel-cell parts are compressed to ensure they are sealed and there are no gas leaks. The hydrogen is fed into the anode side and, with the help of the GDLs and plate flow fields, it flows onto the anode side of the MEA. It is here that the hydrogen is catalytically split into protons and electrons, and the protons are conducted through the electrolyte membrane to the cathode side. The electrons pass along an external circuit (via the anode plate) to the cathode side of the MEA (via the cathode plate), which generates the DC load output of the fuel cell. While this is happening, oxygen from the ambient air is fed into the cathode side of the MEA. Here, oxygen molecules react with the protons transported through the membrane and the external electrons to create water molecules. Proton-exchange membrane (PEM) fuel cells have higher power densities, lower weight and volume than other fuel cells, such as solid oxide cells, which are used for stationary power. They also have A fuel-cell humidifier system (Image courtesy of Hyfindr)
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