The kinetics of steam reforming of methane were studied on a Ni−ZrO2(Y2O3)−cermet film, at temperatures of 1073−1173 K and methane and steam partial pressures of up to 60 and 5 kPa, respectively. It was found that the reaction exhibits Langmuir−Hinselwood kinetic behavior, corresponding to competitive adsorption of methane (in the form of active carbon species, Cad) and H2O (in the form of adsorbed oxygen species, Oad) on the catalytic surface. Kinetic results concerning methane consumption were explained in the frame of a mechanistic scheme involving two rate-limiting steps. These steps correspond (i) to the activated adsorption of CH4 for the production of active carbon species, Cad, and (ii) to the surface reaction of the adsorbed Cad with the Oad species, originating from the adsorption of H2O, for the production of CO. The relative magnitude of the turnover frequencies of these two rate-limiting steps affects considerably the apparent activation energy of the reaction at different P CH 4 and P H 2 O values as well as the tendency of the reaction system to generate graphitic carbon on the catalytic surface. Regarding CO2 formation rate, the kinetic results were explained by considering as the rate-determining step the surface reaction of the adsorbed COad with adsorbed oxygen species.

Abstract. The polarization characteristics of the Ni-YSZ cermet anode/YSZ interface were studied as a function of gas phase composition, temperature and anodic over'potential in methane fuelled internal refonning YSZ fuel cells.AC impedance measurements carried out under open circuit conditions in the temperature range 800 to 900 ~ showed the presence of at least two rate-limiting processes that were influenced by temperature and gas phase composition. The apparent polarization resistance, R~r p, of the Ni-YSZ cermet anode/YSZ interface was found to increase with increasing methane partial pressure and decreasing H20 partial pressure. The activation energy related to the apparent polarization conductance R;~Ip varied between 0.7 and I.I eV, mainly due to the variation of PH20.The order of magnitude of the apparent capacitance, C~p r, values calculated from AC impedance data under open circuit conditions indicate that the observed capacitance corresponds to the double layer capacitance of the Ni/YSZ interface and/or a pseudo-capacitance related to adsorption of species on the Ni-YSZ cermet electrode accompanied by charge transfer. Cap o was found to decrease with increasing methane partial pressure.AC impedance measurements carried out under closed circuit conditions at 900 ~ and four different PH20 showed the appearance of a low-frequency inductive loop at high anodic overpotentials, in addition to the features observed under open circuit conditions. The effect of gas phase composition on R~pp was similar to the one observed under open circuit conditions. In addition an almost exponential decrease of R~p with increasing anodic overpotential was observed.The present results are compared with results presented in literature and discussed in the fi'amc of the mechanism of the electrochemical and catalytic reactions taking place over the anode. 1, IntroductionOne of the main advantages of solid oxide fuel cells (SOFCs) over other types of fuel cells is their ability to run directly on natural gas or other hydrocarbon fuels, without the need of an external refonner [I-4]. This is due to the catalytic action for steam reforming of the fuel exposed state-of-the-art Ni-yttria stabilized zirconia (YSZ) cermet anode in the temperature range of SOFC operation (800-1000 ~ which allows for reforming of the fuel, e.g. methane, within the anodic compartment (internal reforming). The produced H2 and CO can then be readily oxidized electrochemically by O z at the three-phase-boundaries (tpb) Ni-YSZ-gas, generating electricity. Besides the steam reforming reaction, the Ni surface also catalyzes very efficiently the water-gas shift reaction and carbon forming reactions [3,5,6]. Methane can also be directly electrochemically oxidized by 02. [3,7,8], either to CO and H 2 or to CO s and H20, depending on the methane to steam ratio [3,7]. Internal reforming improves the economics of the fuel cell system, due to the elimination of the cost of the external reformer and the increase in the overall operational efficiency of the fuel cell. ...