Question

Someday, cars may be powered by clean H₂/O₂ fuel cells and electric motors. Under standard conditions, such a fuel cell can develop 1.229 volts. If the oxygen is obtained from air (at P=0.21 bar), what pressure of H₂ would be required to develop 1.222 volts from the cell at 25 C?

Answer 1

H2 + 1/2O2 ---> H2O is the reaction

n = number of electrons involved in reaction = 2 ( half reactions are H2 --> 2H+ + 2e- , 1/2O2 + 2e- --> O2-)

E cell = Eo cell - (0.059/n) log K    is Nernest equation when T = 298 K

1.222 = 1.229 - ( 0.059/2) log K

K = 1.727 ,

K for cell = 1 / ( pH2 x pO2^ 1/2)    since H2O is liquid we donot include in Kp expression

K = 1.727 = ( 1/ pH2 x ( 0.21)^1/2)

pH2 = 13.1 bar is pressure of H2 required to get 1.222 volts