For the fuel cell to function, oxygen is required at the cathodes of the individual cells for reduction. The cathode subsystem is responsible for supplying the oxygen from the air and contains all the air-carrying components. In order for the fuel cell to operate as efficiently and reliably as possible, it is necessary to condition the ambient air prior to reaction. This air conditioning generally involves purifying the air by filtration to remove particles and harmful gases, and increasing the humidity via a humidifier. Furthermore, to increase system performance, an air compressor is often used to compress the air and thus increase the pressure in the fuel cell. Research and development is concerned with the design and optimization of the individual components of the subsystem, as well as with the design of the system and system-specific aspects, such as the effective cooling of the media and components.