Rate determining step of direct hydrogen and electricity production in SrZr0.9Yb0.1O3−a fuel cell supplied with CH4 + H2O

Direct hydrogen production and its subsequent electricity generation in a ceramic fuel cell system of a SrZr0.9Yb0.1O3−a tube with one end closed is investigated experimentally under the condition where moist CH4 (or H2) is supplied to its anode compartment and moist O2 is supplied to its cathode compartment. Experiments are performed based on an alternating current impedance method. The following three mass transfer resistances are determined separately: (i) the steam generation reaction on its Ni anode (CH4 + H2O = CO + 3H2); (ii) the transfer of hydrogen ions through the proton conducting ceramic; and (iii) the cathode reaction between H2 and O2. It is found that the rate determining step of the overall hydrogen transfer in the cell system of CH4 + H2O|Ni|SrZr0.9Yb0.1O3−a|NiO|O2 + H2O is the steam reformation reaction on the anode. The electrochemical polarization of the anode due to the CH4-steam reformation has the activation energy of 89.5 kJ/mol. On the other hand, the rate determining step of the system described as H2(+H2O)|Ni|SrZr0.9Yb0.1O3−a|NiO|O2 + H2O is H+ migration through the ceramic electrolyte when T > 700 °C. The activation energy of H+ migration through the ceramic is 31.5 kJ/mol. The values of the Warburg impedance constants and capacitances of an electric double layer are determined as a function of temperature from 600 °C to 800 °C, when either a CH4 + H2O or H2 + H2O mixture is supplied to the anode.