Overall performance and reversible operation

Relative to the ideal thermodynamical efficiency (3.22), the practical efficiency is diminished by the electrochemical losses through the system (3.23), discussed above for each step in the process. Furthermore, as also mentioned in the preceding subsections, not all the hydrogen fuel is utilised and there is a hydrogen content in the outflow from the fuel channel. When cells are combined to form a fuel cell stack, a fuller utilisation may be achieved by 

In Fig. 3.52, the efficiency of a reversible PEM fuel cell is depicted. Any fuel cell can in principle be operated in either direction, to produce electricity from hydrogen or to produce hydrogen from electricity. However, in most cases, the cell design is optimised for one intended use and not very efficient 

The efficiency of electrolysis takes a strong dip when the Ir02 content drops and the catalyst approaches pure Pt, in accordance with earlier experience. However, already at 10% Ir content, the electrolysis efficiency is near 95%, rising only a little with further addition of Ir. At 10% Ir, the drop in the fuel cell efficiency for electricity production has been reduced only from 55 to 53%, so the technology for acceptable operation in both modes seems finally to be established. This will prove important for some of the hydrogen introduction scenarios described in Chapter 5. 

Micro fuel cells intended for use, e.g., with portable electronics, will be mentioned below in section 3.6, as they are often based on direct methanol fuel. Direct methanol fuel cells are also PEM fuel cells, as they are based on the transport of hydrogen ions through a solid polymer electrolyte. 

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