Electrode Kinetics and Gas Conversion in Solid Oxide Cells

The solid oxide fuel cell (SOFC) converts hydrogen, carbon monoxide and hydrocarbon fuels (directly) into electricity with very high efficiencies and has demonstrated almost comparable performance when operated in reverse mode as a solid oxide electrolysis cell (SOEC). In this case electrical (and thermal) energy is stored as chemical energy of reaction products.

To this end, the cells are fed with steam (H2O electrolysis), carbon dioxide (CO2 electrolysis) or a mixture of both (H2O/CO2 co-electrolysis) and of course electrical (ΔG) and thermal (TΔS) energies for the splitting of reactant compounds. Hydrogen, carbon monoxide or both (synthesis gas) are produced at the fuel electrode meanwhile oxygen is produced at the oxygen electrode.

In reversible or cyclic mode the solid oxide cell is operated alternatingly as fuel cell or electrolysis cell depending on the needs of the end user. Upon polarization of the solid oxide cell (SOC) and independent of polarization mode (fuel cell mode or electrolysis mode), the current flowing through the cell is limited by processes such as adsorption and desorption of reactants or products, diffusion through the porous electrodes, activation or charge transfer at the reaction sites, gas conversion at the reaction sites and flow fields and ohmic drop across the electrolyte.

These processes occur in both electrodes and often their characteristic frequencies overlap, rendering characterization of a given mechanism particularly challenging. To optimize the SOCs for operation in the different fuels, operation temperature and operation modes it is important to understand the kinetics of the SOC electrodes.

This thesis was aimed at understanding the kinetics of the SOC under different operation conditions of temperature, polarization, and fuel mixture. For investigations on full cells, electrochemical impedance spectroscopy and distribution of relaxation times techniques were used to investigate kinetics of the Ni/YSZ fuel electrode in three fuel mixtures – hydrogen/steam and reformate fuels hydrogen/carbon-dioxide and hydrogen/methane/steam.

It was found that the kinetics at the fuel electrode were exactly the same in both reformates. This means that chemical equilibrium reactions were much faster than the electrochemical reactions. The electrode displayed slightly faster kinetics in hydrogen/steam fuel than in the reformate fuels. To minimize the influence of (i) joule heating effects as a result of current flow across the electrolyte, (ii) concentration-related effects like gas diffusion, and (iii) overlapping of the characteristic frequencies of processes, the investigations were extended from full cell geometries to a novel pseudo-three electrode cell geometry with working-electrode areas of ca. 1 mm2 that enabled isolated investigation of the fuel and oxygen electrodes.

In a 50/50 H2/H2O fuel mixture, the Ni/8 mol % yttria-stabilized zirconia (Ni/YSZ) fuel electrode showed slower reaction kinetics operating under cathodic polarization than anodic—the same finding had been reported in literature from investigations on full cells whereby together with the local pH2O, substrate diffusion (specifically Knudsen diffusion) was identified as one of the causes of asymmetry between anodic and cathodic mode polarization.

Obtained charge-transfer coefficients compared well with those reported in literature and their evolution with temperature was similar to that reported in literature based on porous Ni/YSZ fuel electrodes. From the two investigated oxygen electrodes, the higher performing (La0.6Sr0.4)0.99CoO3/Ce0.9Gd0.1O1.95 (LSC/CGO) oxygen electrode showed slower reaction kinetics under cathodic mode operation at 50 mV overvoltage than in anodic mode.

The trend was opposite for the lower performing La0.58Sr0.4Co0.2Fe0.8O3 (LSCF) oxygen electrode. However, with decreasing oxygen partial pressure both electrodes displayed increasing asymmetry between anodic and cathodic modes. It could be shown that surface exchange kinetics were the major cause of the decreasing kinetics with decreasing pO2 and that the cathodic mode kinetics were slowed down much more than the anodic branch kinetics thus increasing the asymmetry.

Independent of operation mode, commercialization of the SOC technology requires a guarantee of longevity as well as predictability of the SOC performance under desired operation conditions. The performance is generally evaluated through the current/voltage (C/V) curve. As such, a deviation from the expected/predicted performance curve can serve to identify the presence of ageing or an ageing inducing process.

A 0-D stationary model was previously developed at the Institut für Angewandte Materialien - Werkstoffe der Elektrotechnik (IAM-WET) in Germany to predict the C/V curve of a SOC for fuel cell operation mode. In this thesis, the applicability of this model was verified for electrolysis mode operation, the model was extended to accommodate temperature changes under polarization in fuel cell and electrolysis mode operation, and the model was further extended to cover operation in reformate fuels H2/H2O/CO/CO2.

The latter was accomplished by including a new concentration-related overpotential contribution in the model to account for the CO/CO2 diffusion to the reaction sites as a result of the water gas shift equilibrium reactions. The long-term stability of the system depends on whether the system is operated solely in fuel-, electrolysis-, reversible or dynamic mode.

Optimization of the cells for high performance and/or durability in each of these operation modes requires a thorough understanding of the processes and mechanisms affecting the kinetics and ageing of the systems. In five tests with varying durations between 1000 h and 2500 h the long-term stability of the SOCs was investigated for constant electrolysis, cyclic and dynamic operation modes in a symmetric binary fuel of 50/50 H2/H2O at 800 °C and 700 °C.

The SOCs investigated under constant electrolysis mode aged more than those investigated under cyclic mode with the fuel electrode dominating the ageing in constant electrolysis mode and the oxygen electrode dominating that in cyclic mode. During dynamic cycling, the SOCs aged less at 800 °C than at 700 °C.

It was observed that for cycles with equal durations in SOEC and SOFC modes whereby the cycle lengths were less than or equal to 2 x 5 h the voltage ageing was almost symmetrical for both SOFC and SOEC modes. For longer cycle lengths SOEC mode voltage ageing was at least double the SOFC mode voltage ageing.

This result is consistent with suggestions in literature that intermittent operation of SOCs in fuel cell mode slows down or even reverses SOC ageing that occurs during long-term electrolysis operation. Based on the finding that the fuel electrode dominated the ageing under constant electrolysis operation, it was speculated based on literature, to be caused by precipitation of nickel oxide that had diffused into the 8YSZ matrix of the fuel electrode during sintering.

Constant electrolysis operation provided enhanced conditions for the precipitation of the nickel oxide as metallic nickel. Furthermore, Zr, Ni, Y, and O containing nano-particles were found on the Ni particles. In literature during electrolysis of H2O/CO2 under same conditions of temperature and current density nano particles were also found on Ni particles, identified as ZrO2 and attributed the major cause of fuel electrode ageing.

In cyclic operation these enhancing conditions were not maintained long-enough for severe nickel precipitation. It is known and was verified that the LSC/CGO electrode is better performing than the LSCF electrode. To compare the stability of these two state-of-the-art (s.t.a.) oxygen electrodes, 1000 h tests under non-polarized or open circuit voltage (OCV) conditions were carried out using symmetric cell geometry.

Both electrodes displayed a two-step ageing trend with rapid initial ageing within the first 400 h followed by relaxation to slower ageing rates. The LSCF electrode showed a larger increase in polarization resistance especially within the first 400 h in which it aged by factor 6 faster than the LSC/CGO electrode.

The rapid ageing of the LSCF electrode within the first 300 – 400 h of operation has also been reported in literature.