Rate determining step of direct hydrogen and electricity production in SrZr 0.9Yb 0.1O 3−a fuel cell supplied with CH 4 + H 2O

Abstract

Direct hydrogen production and its subsequent electricity generation in a ceramic fuel cell system of a SrZr 0.9Yb 0.1O 3− a tube with one end closed is investigated experimentally under the condition where moist CH 4 (or H 2) is supplied to its anode compartment and moist O 2 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 (CH 4 + H 2O = CO + 3H 2); (ii) the transfer of hydrogen ions through the proton conducting ceramic; and (iii) the cathode reaction between H 2 and O 2. It is found that the rate determining step of the overall hydrogen transfer in the cell system of CH 4 + H 2O|Ni|SrZr 0.9Yb 0.1O 3− a |NiO|O 2 + H 2O is the steam reformation reaction on the anode. The electrochemical polarization of the anode due to the CH 4-steam reformation has the activation energy of 89.5 kJ/mol. On the other hand, the rate determining step of the system described as H 2(+H 2O)|Ni|SrZr 0.9Yb 0.1O 3− a |NiO|O 2 + H 2O 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 CH 4 + H 2O or H 2 + H 2O mixture is supplied to the anode.