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The conventional Ni–YSZ (yttria-stabilized zirconia) fuel electrode experiences severe degradation due to Ni- agglomeration and migration away from the electrolyte. Therefore, herein, we have considered Ni free electrodes, i.e., La0.6Sr0.4MnO3-δ (LSM)-based perovskite oxides as fuel electrodes. The LSM perovskite phase transforms into a Ruddlesden–Popper LSM (RP-LSM) phase with exsolved MnOx under reducing atmospheres. The RP-LSM is mainly interesting due to its good electrical conductivity, redox stability, and acceptable electrochemical behaviour. In this work, we synthesized the LSM powder and characterized it using several methods including X-ray diffraction (XRD), thermogravimetry analyses (TGA), four-probe conductivity, and scanning electron microscope with energy-dispersive X-ray spectroscopy (SEM-EDX). Finally, the electrolyte-supported single cells were fabricated and electrochemically characterized using AC and DC techniques under electrolysis conditions. In addition to pure LSM fuel electrodes, we have also investigated the electrochemical behaviour of LSM + YSZ (50:50) and LSM + GDC (50:50) composite fuel electrodes. The single cells containing LSM and LSM + GDC fuel electrodes show higher cell performance than LSM + YSZ. For instance, current densities of 1, 1.03, and 0.51 A·cm−2 at 1.5 V are obtained for LSM, LSM + GDC, and LSM + YSZ fuel electrodes containing single cells, respectively, with a 50% N2 and 50% H2O feed gas mixture. Moreover, the performance of the cell was also investigated under co-electrolysis with 50% CO2 and 50% H2O and under direct CO2 electrolysis conditions with 100% CO2 fuel gas. [ABSTRACT FROM AUTHOR]

High-temperature electrolysis using solid oxide electrolysis cells (SOECs) is an innovative technology to temporarily store unused electrical energy from renewable energy sources. However, they show continuous performance loss during long-term operation, which is the main issue preventing their widespread use. In this work, we have performed the long-term stability tests up to 1000 h under steam and co-electrolysis conditions using commercial NiO-YSZ/YSZ/GDC/LSC single cells in order to understand the degradation process. The electrolysis tests were carried out at different temperatures and fuel gas compositions. Intermittent AC- and DC- measurements were performed to characterize the single cells and to determine the responsible electrode processes for the degradation during long-term operation. An increased degradation rate is observed at 800 °C compared to 750 °C under steam electrolysis conditions. Moreover, a lower degradation rate is noticed under co-electrolysis operation in comparison to steam electrolysis operation. Finally, the post-test analyses using SEM-EDX and XRD were carried out in order to understand the degradation mechanism. The delamination of LSC is observed under steam electrolysis conditions at 800 °C, however, such delamination is not observed during co-electrolysis operation. In addition, Ni-depletion and agglomeration are observed on the fuel electrode side for all the cells. [ABSTRACT FROM AUTHOR]

In the temperature range of high temperature co-electrolysis of both steam and carbon dioxide, the reverse water-gas shift reaction (RWGS) takes place. Prior studies were conducted with a narrow gas composition range to investigate the role of RWGS during co-electrolysis. The results for steam electrolysis, CO2 electrolysis, and co-electrolysis caused different conclusions regarding the role of electrochemical CO2 and H2O conversion compared to RWGS during co-electrolysis. This work aims to resolve the role of CO2 conversion as part of RWGS in co-electrolysis. The boundary is characterized by AC and DC measurements over a broad gas composition range from CO2 electrolysis towards co-electrolysis with nearly 50%eq H2O. Especially, the electrochemical CO2 reduction and CO2 conversion in the RWGS are compared to clarify their role during co-electrolysis. The results revealed that gas composition determined the predominant reaction (H2O or CO2 reduction). The cell performance of co-electrolysis in the boundary region up to 5%eq H2O was similar to the performance of CO2 electrolysis. Up to 30%eq H2O, the performance increases with H2O concentration. Here, both CO2 and H2O electrolysis occur. Above 30%eq H2O, steam electrolysis and the RWGS reaction both dominate the co-electrolysis process. [ABSTRACT FROM AUTHOR]

Power-to-X concepts promise a reduction of greenhouse gas emissions simultaneously guaranteeing a safe energy supply even at high share of renewable power generation, thus becoming a cornerstone of a sustainable energy system. Power-to-syngas, that is, the electrochemical conversion of steam and carbon dioxide with the use of renewably generated electricity to syngas for the production of synfuels and high-value chemicals, offers an efficient technology to couple different energy-intense sectors, such as 'traffic and transportation' and 'chemical industry'. Syngas produced by co-electrolysis can thus be regarded as a key-enabling step for a transition of the energy system, which offers additionally features of CO2-valorization and closed carbon cycles. Here, we discuss advantages and current limitations of low- and high-temperature co-electrolysis. Advances in both fundamental understanding of the basic reaction schemes and stable high-performance materials are essential to further promote co-electrolysis. [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]