Low-temperature proton exchange membrane fuel cells (PEMFCs) and alkaline fuel cells (AFCs) offer an order of magnitude higher power density than any other fuel cell systems. A major drawback, however, is that they require a costly platinum catalyst and need very pure hydrogen. PEMFCs and AFCs have been developed since the 1950s and used in the NASA space programme. PEMFCs are most favoured for mass-market automotive and small-scale CHP applications, and there is a massive global effort to develop commercial systems