Na Ai

Development of ( Gd , Ce ) O2-Impregnated ( La , Sr ) MnO3 Anodes of High Temperature Solid Oxide Electrolysis Cells

Gd 0.2 Ce 0.8 O 1.9 (GDC)-impregnated La 0.8 Sr 0.2 MnO 3 (LSM) electrodes are developed as the anodes for high temperature solid oxide electrolysis cells (SOECs). The effect of anodic polarization current passage on the electrode behavior and microstructure of the LSM- and GDC-impregnated LSM anodes is investigated under low and high currents at 800°C. The results show that the electrocatalytic activity of the freshly prepared LSM is enhanced by the anodic polarization due to the reduction of LSM agglomerates and shrinkage of LSM particles under oxidation conditions at low polarization currents. However, high polarization current leads to the delamination at the interface, resulting in a significant increase in electrode polarization and ohmic resistances for the oxygen oxidation reaction. However, the impregnation of GDC nanoparticles not only enhances significantly the electrocatalytic activity for the oxygen oxidation reaction but also effectively inhibits the electrode delamination at the LSM anode/Y 2 O 3 -ZrO 2 electrolyte interface. The results demonstrate the promising potential of the GDC-impregnated LSM as effective anodes of high temperature SOECs.

Why solid oxide cells can be reversibly operated in solid oxide electrolysis cell and fuel cell modes

High temperature solid oxide cells (SOCs) are attractive for storage and regeneration of renewable energy by operating reversibly in solid oxide electrolysis cell (SOEC) and solid oxide fuel cell (SOFC) modes. However, the stability of SOCs, particularly the deterioration of the performance of oxygen electrodes in the SOEC operation mode, is the most critical issue in the development of high performance and durable SOCs. In this study, we investigate in detail the electrochemical activity and stability of La0.8Sr0.2MnO3 (LSM) oxygen electrodes in cyclic SOEC and SOFC modes. The results show that the deterioration of LSM oxygen electrodes caused by anodic polarization can be partially or completely recovered by subsequent cathodic polarization. Using in situ assembled LSM electrodes without pre-sintering, we demonstrate that the deteriorated LSM/YSZ interface can be repaired and regenerated by operating the cells under cathodic polarization conditions. This study for the first time establishes the foundation for the development of truly reversible and stable SOCs for hydrogen fuel production and electricity generation in cyclic SOEC and SOFC operation modes.

Chromium deposition and poisoning of La0.8Sr0.2MnO3 oxygen electrodes of solid oxide electrolysis cells

The effect of the presence of an Fe-Cr alloy metallic interconnect on the performance and stability of La(0.8)Sr(0.2)MnO3 (LSM) oxygen electrodes is studied for the first time under solid oxide electrolysis cell (SOEC) operating conditions at 800 °C. The presence of the Fe-Cr interconnect accelerates the degradation and delamination processes of the LSM oxygen electrodes. The disintegration of LSM particles and the formation of nanoparticles at the electrode/electrolyte interface are much faster as compared to that in the absence of the interconnect. Cr deposition occurs in the bulk of the LSM oxygen electrode with a high intensity on the YSZ electrolyte surface and on the LSM electrode inner surface close to the electrode/electrolyte interface. SIMS, GI-XRD, EDS and XPS analyses clearly identify the deposition and formation of chromium oxides and strontium chromate on both the electrolyte surface and electrode inner surface. The anodic polarization promotes the surface segregation of SrO and depresses the generation of manganese species such as Mn(2+). This is evidently supported by the observation of the deposition of SrCrO4, rather than (Cr,Mn)3O4 spinels as in the case under the operating conditions of solid oxide fuel cells. The present results demonstrate that the Cr deposition is essentially a chemical process, initiated by the nucleation and grain growth reaction between the gaseous Cr species and segregated SrO on LSM oxygen electrodes under SOEC operating conditions.

Direct application of cobaltite-based perovskite cathodes on the yttria-stabilized zirconia electrolyte for intermediate temperature solid oxide fuel cells

In this communication, cobaltite-based perovskite (CBP) cathodes are directly applied on the yttria-stabilized zirconia (YSZ) electrolyte via an in situ assembly process without the addition of a doped ceria interlayer and pre-sintering at high temperatures. The results demonstrate for the first time that a CBP electrode/YSZ electrolyte interface can be formed in situ under cathodic polarization at a solid oxide fuel cell (SOFC) operating temperature of 750 °C. Nevertheless, the performance of cells with Sr-containing CBP cathodes deteriorates due to the surface segregation of Sr species and formation of a Sr-rich reaction layer at the interface. However, the stability and power density of cells with in situ assembled CBP cathodes can be further enhanced by B-site doping or by using a Sr-free CBP. The direct application of CBPs on the YSZ electrolyte revolutionizes the design of intermediate temperature SOFCs.

Highly Stable Sr-Free Cobaltite-Based Perovskite Cathodes Directly Assembled on a Barrier-Layer-Free Y2O3-ZrO2Electrolyte of Solid Oxide Fuel Cells

Direct assembly is a newly developed technique in which a cobaltite-based perovskite (CBP) cathode can be directly applied to a barrier-layer-free Y2 O3 -ZrO2 (YSZ) electrolyte with no high-temperature pre-sintering steps. Solid oxide fuel cells (SOFCs) based on directly assembled CBPs such as La0.6 Sr0.4 Co0.2 Fe0.8 O3-δ show high performance initially but degrade rapidly under SOFC operation conditions at 750u2009°C owing to Sr segregation and accumulation at the electrode/electrolyte interface. Herein, the performance and interface of Sr-free CBPs such as LaCoO3-δ (LC) and Sm0.95 CoO3-δ (SmC) and their composite cathodes directly assembled on YSZ electrolyte was studied systematically. The LC electrode underwent performance degradation, most likely owing to cation demixing and accumulation of La on the YSZ electrolyte under polarization at 500u2005mAu2009cm-2 and 750u2009°C. However, the performance and stability of LC electrodes could be substantially enhanced by the formation of LC-gadolinium-doped ceria (GDC) composite cathodes. Replacement of La by Sm increased the cell stability, and doping of 5u2009% Pd to form Sm0.95 Co0.95 Pd0.05 O3-δ (SmCPd) significantly improved the electrode activity. An anode-supported YSZ-electrolyte cell with a directly assembled SmCPd-GDC composite electrode exhibited a peak power density of 1.4u2005Wu2009cm-2 at 750u2009°C, and an excellent stability at 750u2009°C for over 240u2005h. The higher stability of SmC as compared to that of LC is most likely a result of the lower reactivity of SmC with YSZ. This study demonstrates the new opportunities in the design and development of intermediate-temperature SOFCs based on the directly assembled high-performance and durable Sr-free CBP cathodes.

In situ assembled La0.8Sr0.2MnO3 cathodes on a Y2O3-ZrO2 electrolyte of solid oxide fuel cells-interface and electrochemical activity

Formation of an intimate electrode/electrolyte interface is essential for solid oxide fuel cells (SOFCs). In this study, a comparative investigation has been undertaken to study the interface formation between a La0.8Sr0.2MnO3 (LSM) cathode and Y2O3–ZrO2 (YSZ) electrolyte by high temperature sintering and by cathodic polarization using EIS, SEM, AFM and HAADF-STEM techniques. The electrode/electrolyte interface formed by the conventional pre-sintering process is characterized by the formation of distinctive convex contact rings on the YSZ surface and such convex contact rings are due to the cation interdiffusion such as manganese species between LSM and YSZ. Similar to the thermally induced interface, the electrode/electrolyte interface can also be formed by electrochemical polarization for the in situ assembled LSM cathode on YSZ as well as on Gd2O3–CeO2 (GDC) electrolytes without pre-sintering at high temperatures. The polarization induced interface has smaller contact marks due to the much finer grain size of the as-prepared LSM electrodes. Detailed electrochemical impedance studies indicate that both thermally and polarization induced LSM/YSZ interfaces show comparable electrocatalytic activity and behaviour for the oxygen reduction reaction with similar activation energies. The present study clearly demonstrates the formation of effective electrode/electrolyte interfaces in SOFCs under the influence of cathodic polarization without high temperature sintering steps.

Highly active and stable Er0.4Bi1.6O3 decorated La0.76Sr0.19MnO3+δ nanostructured oxygen electrodes for reversible solid oxide cells

Bismuth based oxides have excellent ionic conductivity and fast oxygen surface kinetics and show promising potential as highly active electrode materials in solid oxide cells (SOCs) such as solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs). However, the low melting temperature and high activity of bismuth based oxides severely limit their wide applications in SOCs. Herein, we successfully synthesized a 40 wt% Er0.4Bi1.6O3 decorated La0.76Sr0.19MnO3+δ (ESB–LSM) electrode via a new gelation method and directly assembled it on a Ni–yttria-stabilized zirconia (Ni–YSZ) cermet supported YSZ electrolyte cell without the conventional high temperature pre-sintering step. ESB decoration substantially enhances the electrocatalytic activity of the LSM electrode for the oxygen reduction/evolution reactions (ORR/OER). A YSZ electrolyte cell with the directly assembled ESB–LSM electrode exhibits a peak power density of 1.62 W cm−2 at 750 °C, significantly higher than 0.48 and 0.88 W cm−2 obtained on cells with a directly assembled pristine LSM and LSM–YSZ composite electrode, respectively. Most importantly the cells with the directly assembled ESB–LSM oxygen electrodes show excellent stability in SOFC, SOEC and reversible SOC operating modes for over 200 h. The present study demonstrates a significant advancement in the development of bismuth based oxide decorated high performance and stable oxygen electrodes for reversible SOCs.

High performance nanostructured bismuth oxide–cobaltite as a durable oxygen electrode for reversible solid oxide cells

The high reactivity between bismuth oxide and cobaltite oxygen electrodes is a bottleneck in developing active and reliable bismuth oxide–cobaltite composite oxygen electrodes for solid oxide cells (SOCs). Herein, a Sr-free Sm0.95Co0.95Pd0.05O3−δ (SmCPd) oxygen electrode decorated with nanoscale Er0.4Bi1.6O3 (ESB) is synthesized and assembled on a barrier-layer-free Y2O3–ZrO2 (YSZ) electrolyte film. The cell with the ESB decorated SmCPd composite oxygen electrode exhibits a peak power density of 1.81 W cm−2 at 750 °C and 0.58 W cm−2 at 650 °C. More importantly, excellent operating stability is achieved in the fuel cell mode at 600 °C for 500 h, and in electrolysis and reversible modes at 750 °C for over 200 h. The results demonstrate the feasibility of applying bismuth oxide–cobaltite composite oxygen electrodes in developing high-performance and durable SOCs.