The Fuel Cell Stack - Heart of the Fuel Cell System

by Andreas Hirschter
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The Fuel Cell Stack - Heart of the Fuel Cell System

The various components of a fuel cell system are tasked with handling fuel, air, cooling water and electricity to generate electrical energy. This article focuses on the heart of a fuel cell system, the fuel cell stack and refer mainly to the polymer electrolyte membrane fuel cells (PMFC).

The fuel cell stack is built from a multitude of cells, consisting mainly of so-called bipolar plates and a membrane. They produce a typical voltage below 1V. The stacking of these cells leads to a higher usable voltage of this unit. Stacks can have 335 cells, leading to an output voltage of 201V. Most important is, that the fuel stacks produce direct current.

In general, in each fuel cell a hydrogen molecule is separated in its protons and electrons at the anode. The protons take the direct way to the cathode through the membrane, which is the electrolyte. The electrons use the detour around the membrane, producing a voltage and a current. At the cathode the hydrogen proton, the electrons and the oxygen contained in the air meet and form pure water in an oxygen reduction reaction.

This reaction is comparable to the burning of hydrogen, but without a flame. A difference is that simply burning hydrogen leads to water and heat as reaction products whereas the fuel cell oxidation process converts around 50% of the energy contained in the hydrogen into electricity. The other 50% are still heat, which needs to be cooled away from the fuel cells.

On the market, there are various types of fuel cells. The polymer electrolyte membrane fuel cells (PMFC) is the type mainly used for various applications like cars, rail engines and ships. The advantage of this fuel cell principle is the simplicity of the overall system. A comparable practical principle is the direct methanol fuel cell (DMFC) using a methanol as fuel. But in general, the application will determine the optimum principle to be used.

If the fuel cell stack is treated as a black box, it could simply be assumed that it needs hydrogen, cooling and air and then it runs. But there is more to it than that. Most of the PEMFCs need a hydrogen pressure of less than 3bar(a). If one now investigates the fact that this membrane has a thickness of around 20 micrometers, it is obvious that there is a limit for the maximum pressure difference that membrane can stand. That requires a high quality of the pressure control of hydrogen and air in the fuel cell. If we now investigate a drive from Boulder, Colorado (1,655m) on Pike’s Peak (approx. 4,300m), just to take an extreme scenario, the air compressors see an inlet pressure of 835hPa to 606,3hPa which is just about 60% of the pressure at sea level. This results in a high requirement on the compressor capabilities to deal with varying ambient conditions, since the pressure ratio the air compressors sees is typically maximum 3 but can develop as high as to 5. It is obvious, that a flexibility in the control of hydrogen versus air is required to maintain performance of the system.

A further aspect is the air quality. PEMFCs don’t like carbon monoxide or H2S. For example, if a FCEV is constantly parked nearby a manured field, it is likely that the efficiency of the fuel cell system decreases. carbon monoxide is not so much an issue on the air supply. It is more a matter of unclean fuel, contaminated hydrogen. Therefore, all hydrogen refueling stations use a very high-quality hydrogen (min. 99,97%) to ensure a constant performance of the fuel cells.

An important point is the cooling of the fuel cell. Since roughly 50% of the energy produced in the system is heat, a cooling system is required that not only works under all ambient conditions, but it must also maintain the fuel cell at about 70°C. In addition, the cooling fluid must be insulating since it is very close to electricity. Today a demineralized water/glycol mixture is used to ensure that the cooling fluid does not freeze, and the conductivity is very low.

And finally, the air humidity plays a vital role for the well-being and performance of the fuel cell, especially the membrane. The air used in the cathode of the fuel cell needs to have a relative humidity of more than 70%. In operation, the water produced by the fuel cell can be used to humidify the ambient air. But a journey through the Namibian desert with a relative humidity of the air of less than 20% and at 50°C sets a high demand on the humidity management of the fuel cell system. But also, a foggy day is not an easy condition, as too much humidity covers the active area of the fuel cell and will decrease performance. So, at the very end many parameters need to be controlled to ensure a high performance and lifetime of a fuel cell stack.