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Ni-gadolinia-doped ceria (GDC) based electrode materials have drawn significant attention as an alternative fuel electrode for solid oxide cells (SOCs) owing to mixed ionic conductivity of GDC and high electronic and catalytic activity of Ni. Moreover, the catalytic activity and electrochemical performance of the Ni-GDC electrode can be further improved by dispersing small quantities of other metal additives, such as gold or molybdenum. Therefore, herein, we considered gold and molybdenum modified Ni-GDC electrodes and focused on the upscaling; hence, we prepared 5 × 5 cm2 electrolyte-supported single cells. Their electrochemical performance was investigated at different temperatures and fuel gas compositions. The long-term steam electrolysis test, up to 1700 h, was performed at 900 °C with −0.3 A·cm−2 current load. Lastly, post-test analyses of measured cells were carried out to investigate their degradation mechanisms. Sr-segregation and cobalt oxide formation towards the oxygen electrode side, and Ni-particle coarsening and depletion away from the electrolyte towards the fuel electrode side, were observed, and can be considered as a main reason for the degradation. Thus, modification of Ni/GDC with Au and Mo seems to significantly improve the electro-catalytic activity of the electrode; however, it does not significantly mitigate the Ni-migration phenomenon after prolonged operation. [ABSTRACT FROM AUTHOR]

Lanthanide nickelate Ln2NiO4+δ (Ln = La, Pr, or Nd) based mixed ionic and electronic conducting (MIEC) materials have drawn significant attention as an alternative oxygen electrode for solid oxide cells (SOCs). These nickelates show very high oxygen diffusion coefficient (D*) and surface exchange coefficient (k*) values and hence exhibit good electrocatalytic activity. Earlier reported results show that the partial substitution of Co2+ at B-site in La2Ni1−xCoxO4+δ (LNCO) leads to an enhancement in the transport and electrochemical properties of the material. Herein, we perform the substitution at A-site with Sr, i.e., La2−xSrxNi0.8Co0.2O4+δ, in order to further investigate the structural, physicochemical, and electrochemical properties. The structural characterization of the synthesized powders reveals a decrease in the lattice parameters as well as lattice volume with increasing Sr content. Furthermore, a decrease in the oxygen over stoichiometry is also observed with Sr substitution. The electrochemical measurements are performed with the symmetrical half-cells using impedance spectroscopy in the 700–900 °C temperature range. The total polarization resistance of the cell is increased with Sr substitution. The electrode reaction mechanism is also studied by recording the impedance spectra under different oxygen partial pressures. Finally, the kinetic parameters are investigated by analyzing the impedance spectra under polarization. A decrease in exchange current density (i0) is observed with increasing Sr content. [ABSTRACT FROM AUTHOR]

The challenges of high degradation rate and significant carbon deposition, which are common with Ni-YSZ electrodes, have shifted attention to other electrode materials with enhanced performance in SOECs using carbon-containing fuels. In this study, the performance and electrochemical behavior of the Ni-GDC fuel electrode under CO 2 electrolysis were investigated. The study was performed over a range of operating conditions, varying the operating temperature, the CO 2 content of the fuel gas as well as the oxygen partial pressures in the oxygen electrode gas. Long-term stability test was performed up to 1070 h at 900 °C and a current density of − 0.5 A‧cm−2. The electrochemical impedance spectra obtained from the various measurement were evaluated with DRT as well as an equivalent circuit model consisting of 4 time-constant; (LR-RQ1-RQ2-RQ3-Ws). The low frequency Warburg (short) element (Ws) was attributed to gas diffusion and surface processes at the fuel electrode, the mid frequency processes of RQ2 and RQ3 are assigned to the combined contribution of fuel and oxygen electrode. The high frequency RQ1 was assigned to the charge transfer process at the oxygen electrode. A low degradation rate of 31 mV‧Kh−1 was observed during the long-term stability test. Furthermore, analysis of the degradation rate illustrates that significant contributions to the degradation were from the mid and high frequency processes, in addition to ohmic resistance. SEM analysis of the measured cell shows agglomeration of Ni particles, increase in electrode porosity as well as Ni migration away from the electrode/electrolyte interface. • Direct CO 2 electrolysis using single cell containing Ni-GDC fuel electrode. • High current density of − 1.16 A‧cm−2 is obtained at 1.5 V and 900 °C for Ni-GDC cell. • Low degradation rate of 33 mV‧Kh−1 is observed after long term operation. • Ni particle migration, agglomeration and increase in electrode porosity are observed. [ABSTRACT FROM AUTHOR]

The present study investigates the electrochemical performance and degradation behaviour of a Nickel - Gd 2 O 3 doped CeO 2 (Ni-GDC) electrode containing single cell under steam electrolysis and co-electrolysis modes. The cell consists of the Ni-GDC fuel electrode, an 8 mol% Y 2 O 3 stabilized ZrO 2 (8YSZ) electrolyte layer, a GDC barrier layer and a (La,Sr)(Co,Fe)O 3 (LSCF) oxygen electrode. Firstly, the electrolyte-supported single cells were fabricated and characterized using DC- and AC-techniques in the 750–900 °C temperature range. Distribution of relaxation times (DRT) analysis was employed to resolve frequency-dependent electrode processes. The observed processes were further modelled using an equivalent circuit model (ECM) with 3 R//CPE (resistor//constant phase element) in series with a finite length diffusion element (Warburg short - Ws). Long-term stability tests of the single cells were carried out under steam electrolysis (H 2 O:H 2 , 50:50) and co-electrolysis (H 2 O:CO 2 :CO, 40:40:20) conditions at 900 °C with −0.5 A‧cm−2 current density for 500 h. Steam electrolysis conditions exhibit the highest degradation rate of 499 mV‧kh−1, while a lower degradation rate of 308 mV‧kh−1 is observed under co-electrolysis conditions. The post-test analysis of the operated cell shows increased Ni particles size, suggesting Ni agglomeration in both electrolysis modes. • Steam electrolysis shows lower polarization resistance than co-electrolysis modes. • Co-electrolysis shows lower degradation than steam electrolysis during long-term. • Ni agglomeration and coverage of the Ni particles with GDC were observed. • Steam electrolysis degraded cell showed higher Ni growth than co-electrolysis cell. [ABSTRACT FROM AUTHOR]

• Steam- and co-electrolysis using a single cell containing a GDC fuel electrode. • A current density of -914 mA‧cm−2 in steam electrolysis (1.5 V, 900 °C). • 70% of the performance of Ni-GDC fuel electrode single cells. • Degradation rate of 112 mV‧kh-1; which is four times lower than Ni-GDC. • GDC fuel electrode does not significantly contribute to the degradation. One major challenge that has to be solved to enable a market entry of solid oxide electrolysis cells (SOECs) technology is the poor degradation behaviour caused by nickel agglomeration and migration in the state-of-the-art fuel electrodes. Novel fuel electrode materials that either suppress the nickel migration or even nickel-free electrodes could lead to a decrease in degradation rates. In this work, single cells based on the mixed ionic electronic conducting (MIEC) gadolinium doped ceria (GDC), acting as single-phase fuel electrode, were prepared. The cell performance was investigated by current density-voltage characteristics (jV) for steam and co-electrolysis conditions at various operating temperatures. Furthermore, electrochemical processes occurring in the single cells were analysed using electrochemical impedance spectroscopy (EIS), distribution of relaxation times (DRT) analysis and equivalent circuit model (ECM) fitting. Current densities of –914 and –969 mA‧cm−2, respectively, at 1.5 V and 900 °C operating temperature for steam and co-electrolysis were obtained, which corresponds to about 70% of the current density achieved in similar produced Ni-GDC fuel electrode cells. In addition, a long-term stability test was carried out during steam electrolysis (50% H 2 O + 50% H 2) at 900 °C with a constant current load of –0.5 A‧cm−2 for 1070 h. In comparison to Ni-YSZ and Ni-GDC fuel electrode single cells reported in the literature, a significantly lower degradation rate of 112 mV‧kh−1 was observed. The electrochemical investigations and post-test analyses using SEM-EDX reveal that the GDC fuel electrode does not contribute significantly to the degradation, while the LSCF oxygen electrode is the major contributor to the cells' degradation. [ABSTRACT FROM AUTHOR]