Infiltration of La2NiO4+ᵟ-Type Electrodes with Nano-Oxide Catalyst to Modify Electrochemical Properties and Stability Using Biomolecular Surface Engineering

Abstract

Solid oxide electrolysis cells (SOECs) have increased in importance in recent years due to their potential to cleanly and efficiently separate hydrogen and oxygen, or to instigate other chemical conversion reactions. SOECs require a combination of both high start performance and low long-term degradation for optimal electrodes. Additionally, Cr-poisoning from metal interconnect components threatens long-term degradation due to the ambient humidity of the electrolysis process. The most promising candidate for the oxygen electrode was (La,Sr)(Co,Fe)O3±δ (LSCF) due to its excellent electrical and catalytic properties, but it is challenged by a SrZrO3 phase, Cr-poisoning, and decreased oxygen ionic conductivity under high current loads due to its reliance on oxygen site vacancy (VÖ ) concentration. This drives interest into the Ruddlesden-Popper (R-P) phase, which has the general K2NiF4-type structure. Of these, La2NiO4+δ (LNO) is interesting, as its ionic conductivity mechanism is interstitial oxygen (Oï ) and it has no apparent reaction under Cr-poisoning. A challenge, however, is LNOs relatively low surface oxygen transport. As such, this project’s focus is on the surface engineering of a heterostructured oxygen electrode with LNO as a backbone and an infiltrated LaCoO3-δ (LCO)-based perovskite layer.In this work, the surface engineering of the LNO surface was completed by utilizing a novel wet biomolecular infiltration technique to ensure homogenous nano-particle coatings. The process involves a primary deposition of a catechol surfactant throughout the LNO electrode, and then a secondary deposition of the nano-catalyst solution. The catechol molecule that is deposited in the first step enables the adhesion of the nano-oxide, helping to chelate the complex oxide and assisting in the formation of the compound. No further deposition steps are needed, increasing the overall efficiency and the controllability of the microstructure, and allowing for the formation of the multicomponent nanomaterial at temperatures <800 °C. This has the additional benefit of minimalizing the densification and/or reaction of the nanomaterial with the electrode backbone (if not desired).The initial objective of the work was to characterize the nano-catalyst deposition process on the electrode surface, and define characteristics such as surface roughness, homogeneity, and particle size as function of deposition parameters and surfactant type/concentration. This work was completed initially on an electrode surrogate, YSZ single-crystal substrates. First, a deposition study was run on single-crystal, polished YSZ substrates, chosen for their flatness which simplifies the microstructural characterization through atomic force microscopy (AFM). AFM images from this deposition were processed to gather data on sample surface roughness, and both particle homogeneity and volumetric data, indicating which variables were most important to adjust. An example of these images can be seen in Figure 1 below, an AFM graph of the topography of deposited nano-particles. After defining the deposition process on flat YSZ substrates, the deposition methods were transferred to LNO symmetrical cells and full SOECs. Half and full cells were screen-printed on a YSZ electrolyte, with a GDC barrier layer, a 50-50 wt % LNO/GDC active layer, and an LNO current collector layer. The electrochemical impedance spectroscopy (EIS) tests were completed on infiltrated and non-infiltrated cells at different loading levels (defined by the impregnation variables) to evaluate the effectiveness of the nano-oxide surface engineering on the desired characteristics of surface oxygen transport and charge transfer reactions. The cells were tested in air and various steam concentrations up to 800 °C. Acknowledgments: This research was funded by the US Department of Energy, Project DE-FE0032116. We would also like to thank the WVU Shared Research Facilities for their support, specifically Dr. Qiang Wang, Dr. Marcela Redigolo, and Mr. Harley Hart for their help. Figure 1