Cr-tolerance of the IT-SOFC La(Ni,Fe)O3 material

This thesis deals with a study on the Cr-tolerance of the LaNi0.6Fe0.4O3 (LNF) material. LNF is being considered for use as a current collecting layer, an interconnect protective coating and/or an electrochemically active solid oxide fuel cell (SOFC) cathode layer in an intermediate temperature IT-SOFC stack. The desired cost-effectiveness of the IT-SOFC systems can be achieved by using relatively cheap interconnect materials such as chromia forming ferritic stainless steels. However, the use of such interconnects triggers Cr-poisoning of the state-of-the-art SOFC cathodes, hence new Cr-tolerant materials are needed. The LNF material is considered as a promising candidate. Favorable properties of LNF, including high electronic conductivity, matched thermal expansion coefficient and claimed high Cr-resistance have encouraged studies described herein. \ud The research aim is to understand the chemical stability of the LNF in the presence of Cr-species, which ultimately results in a thorough understanding of the degradation mechanisms of the LNF cathode under Cr-poisoning conditions. A final goal is to recommend feasible solutions to the Cr-poisoning issue. To meet these goals, firstly a solid-state reactivity between LNF and chromia is investigated in Chapter 2. Secondly, the influence of volatile Cr-species on the electrical properties of LNF is studied in Chapter 3. Thirdly, the impact of Cr-poisoning on the conductivity of different LNF microstructures is described in Chapter 4. Cr-poisoning of a LNF cathode under current load is discussed in Chapter 5. Finally, recommendations for a feasible application of the LNF material in IT-SOFC systems are given in Chapter 6.\ud It can be concluded that Cr-poisoning may not be fully avoidable in the IT-SOFC systems utilizing metallic interconnects. However, a synergetic approach may be undertaken to reduce the Cr-poisoning impact: From an operational perspective - the presence of volatile Cr-species should be minimized by applying interconnect protective coatings and providing dry air at high flow velocities. From an LNF microstructural perspective - the microstructure should be adjusted according to the application: a coarse microstructure is preferred for non-electrochemical use (as a current collecting layer and/or an interconnect protective coating), and a fine microstructure is preferred for an electrochemically active cathode layer

Impact of the Volatile Cr-species' Attack on the Conductivity of La(Ni,Fe)O3

This study demonstrates the detrimental impact of Cr on the electronic conductivity of a $LaNi_{0.6}Fe_{0.4}O_3$ (LNF) porous cathode layer at 800 ºC. Vapor transport of Cr-species, originating from a porous metallic foam, and subsequent reaction with LNF results in a decrease of the electronic conductivity of the LNF-layer. Cr has been detected throughout the whole cross-section of the LNF-layer. Transmission electron microscopy revealed that Cr is gradually incorporated into the LNF-grains, while Ni is proportionally expelled. The progressing Cr deposition and penetration into the LNF-grains most likely explains the electronic conductivity drop. The Cr-poisoning impact on the electronic conductivity of the LNF porous layer is considerably smaller at 600ºC than at 800ºC. A tentative mechanism for the Cr attack and its influence on the electronic conductivity of the LNF layer will be presented

La(Ni,Fe)O3 Stability in the Presence of Chromia—A Solid-State Reactivity Study

The perovskite $La(Ni_{0.6}Fe_{0.4})O_3$ (LNF) is a candidate material for the electrochemically active cathode layer, the cathode current collecting layer, and/or the interconnect protective coating in intermediate temperature solid oxide fuel cells (IT-SOFCs) operated at . Since these operating temperatures enable the use of relatively cheap interconnect materials such as chromia-forming ferritic stainless steel, investigation of the chemical stability of LNF in the presence of chromium species is of importance. This study demonstrates that LNF is chemically unstable at when it is in direct contact with $Cr_2O_3$. It has been observed that Cr enters the perovskite phase, replacing first Ni and then Fe, already after 200h. At 600°C, however, only minor reaction products were detected after 1000h exposure to $Cr_2O_3$. Although this is a promising result, long-term testing under fuel cell operating conditions at 600°C is needed to prove that LNF is a viable IT-SOFC material

Impact of Cr-poisoning on the conductivity of LaNi0.6Fe0.4O3

This study demonstrates the significant impact of Cr on the electronic conductivity of a LaNi0.6Fe0.4O3 (LNF) porous cathode layer at 800 °C. Vapor transport of Cr-species, originating from a porous metallic foam, and subsequent reaction with LNF, results in a decrease of the electronic conductivity of the LNF-layer. Cr has been detected throughout the entire cross-section of a 16 μm thick LNF layer, while Ni, besides its compositional distribution in the LNF layer, has also been found in enriched spots forming Ni-rich metal oxide crystals. Transmission electron microscopy revealed that Cr is gradually incorporated into the LNF-grains, while Ni is proportionally expelled. Electron diffraction performed in the center of a sliced grain showed the initial rhombohedral crystal structure of LNF, whereas diffraction performed close to the edge of the grain revealed the orthorhombic perovskite crystal structure, indicating a Cr-enriched perovskite phase. Progressive Cr deposition and penetration into the LNF grains and necks explains the electronic conductivity deterioration. The impact of Cr-poisoning on the electronic conductivity of the LNF porous layer is considerably smaller at 600 °C than at 800 °C

Impact of Cr-poisoning on the conductivity of different LaNi0.6Fe0.4O3 cathode microstructures

The microstructure of porous LaNi0.6Fe0.4O3 (LNF) layers has a significant influence on the degree of the Cr-poisoning impact. The increase in the in-plane resistance and Cr accumulation in poisoned LNF-layers has been correlated with microstructural features. The Cr-poisoning impact is more severe in the case of a microstructure characterized by finer particles, higher porosity and larger particle surface area

Cr-poisoning of a LaNi0.6Fe0.4O3 cathode under current load

This study demonstrates the significant impact of Cr-poisoning on the performance of the $LaNi_{0.6}Fe_{0.4}O_3$ (LNF) SOFC cathode under current load. Volatile Cr-species, originating from a porous metallic foam, enter the working electrode and modify both the LNF cathode layer and the $Gd_{0.4}Ce_{0.6}O_{1.8}$ (GDC) barrier layer, causing increasing overpotential and cell impedance. The increase of the ohmic resistance is caused by a decrease of the in-plane electronic conductivity of the LNF layer (due to Cr incorporation and Ni removal from the LNF perovskite lattice) combined with a deterioration of the ionic conductivity of the GDC barrier layer due to reactivity with Cr resulting in formation of a $GdCrO_{3}$-phase. The increase of the polarisation resistance is caused by a decrease of the electrochemical activity of the LNF surface towards oxygen reduction reaction at the triple phase boundary (TPB) due to Cr-incorporation in the outer shell of the LNF grains. Chemical reaction and electrochemically driven reaction of volatile Cr-species with LNF and GDC contributes to the extrinsic degradation of the LNF cathodes under current load

Material properties of LPCVD processed n-type polysilicon passivating contacts and its application in PERPoly industrial bifacial solar cells

We present a detailed material study of n+-type polysilicon (polySi) and its application as a carrier selective rear contact in a bifacial n-type solar cell comprising fire-through screen-printed metallization and 6" Cz wafers. The cells were manufactured with low-cost industrial process steps yielding Vocs from 676 to 683 mV and Jscs above 39.4 mA/cm2 indicating an efficiency potential of 22%. The aim of this study is to understand which material properties determine the performance of POCl3-diffused (n-type) polySi-based passivating contacts and to find routes to improve its use for industrial PERPoly (Passivated Emitter Rear PolySi) cells from the point of view of throughput, performance, and bifacial application. This paper reports on correlations between the parameters used for low pressure chemical vapour deposition (LPCVD), annealing, and doping on optical, structural, and electronic properties of the polySi-based passivating contact and the subsequent influence on the solar cell parameters