Hydrogen is required for all low-and medium-temperature fuel cells, i.e., the alkaline fuel cell (AFC), the proton exchange membrane fuel cell (PEMFC), and the phosphoric acid fuel cell (PAFC) (see Table 1.1 in Chapter 1). Generally, the demand for

hydrogen purity

decreases with increasing operating temperature. While the PEMFC cannot operate when

carbon monoxide

(CO) is present in the fuel gas at concentrations of more than a few ppm (Cooper et al., 1997; Hoogers and Thompsett, 1999), the PAFC, with its higher

operating temperature, tolerates CO levels as high as 1 to 2% without significant performance loss (EG&G Services, 2000).