Electrochemical Modeling of the Current-Voltage Characteristics of an SOFC in Fuel Cell and Electrolyzer Operation Modes

Abstract

A zero-dimensional and isothermal stationary model demonstrably predicting the current-voltage (C/V) characteristic of an anode supported SOFC single cell is for the first time verified for C/V characteristics measured in electrolysis mode. The accuracy of the presented model was increased by including the actual cell temperature under current load, determined by an impedance-based temperature measurement routine. C/V characteristics measured at 800°C in the range from 0.66 V to 1.6 V for H2O:H2 compositions 70:30 and 30:70 reveal a pronounced asymmetric operation of the fuel electrode supported cell in electrolysis mode. This experimentally observed behavior is accurately reproduced by the model and is explained by (i) increasing polarization losses related to Knudsen diffusion and (ii) decreasing reaction rate in dry conditions at the fuel electrode at high current densities in electrolysis mode.

Kinetic Studies on State of the Art Solid Oxide Cells: A Comparison between Hydrogen/Steam and Reformate Fuels

“…The electrochemical reactions at the fuel electrodes on the other hand show non-linear current density vs. overpotential relations 30 at 700…”

“…Increasing temperature due to the exothermic hydrogen oxidation at the fuel electrode. c. At the fuel electrode, the increasing steam content at the triple phase boundaries (TPB) with increasing current densities results in increased electrochemical reaction rate 25,30 and thus reduced ASR. d The trend of the oxygen electrode cannot be unambiguously discussed due to the very small contribution.…”

“…Details are given in elsewhere. 25,30 A final difference between the cells Cell B and C is the observation that in Cell C the ohmic polarization contribution decreases slightly from 20 mV at 0.4 A/cm 2 in Cell B to 17.4 mV. The cathode contact block was the same in the two tests (4 × 4 cm), but as the cathode of cell C was smaller this may lead to small differences in applied mechanical load on the cathode area, assuming that the gold mesh did not adapt to the shape of the cathode, which in turn may yield a slight difference in the contact resistance.…”

Kinetics of CO/CO2 and H2/H2O reactions at Ni-based and ceria-based solid-oxide-cell electrodes

“…15,16 Acceptor-doped ceria is a well-known MIEC solid when exposed to a reducing environment and it exhibits high activity for the H 2 /H 2 O reaction when applied as an electrode. [17][18][19][20] Considerable effort has gone into understanding and modelling the reaction kinetics and mechanisms for the H 2 /H 2 O and CO/CO 2 reactions, especially for Ni-SZ electrodes, [21][22][23][24][25] but also more recently for ceria-based electrodes. 16,[26][27][28] However, relative activities for the CO/CO 2 and H 2 /H 2 O reactions are not well studied, even for conventional Ni-SZ electrodes.…”

“…vaporization of SiO 2 by forming the hydroxide, and subsequent re-deposition at the 3PB. 48,49 The well-known anodic activation by 3PB pH 2 O dependence, exothermicity of the anodic electrochemical reaction (and cathodic endothermicity), and possibly enhanced kinetics of the actual charge-transfer reaction in anodic polarization 25,50 cannot account for the factor of 26 higher current at +0.2 V vs. À0.2 V overpotential in H 2 /H 2 O. Regardless of the impurity content and the exact value of the C : H R p ratio, the consistently observed higher activity for the H 2 /H 2 O vs. CO/CO 2 reactions at Ni/SZ may be due to the high mobility of H on Ni, 51 or it could simply be due to a more benecial energetic landscape for the overall reaction scheme.…”