Chemical-Mechanical Coupling Determines the Stability of Self-Supported Oxygen Ion Conducting Membranes

Introduction

                Ultrathin, self-supported oxide membranes are an emerging experimental platform to explore the limits of dimensionality reduction, probe the effects of chemically induced strain, and test proof-of-concept electrochemical devices.^1,2 Integrating oxygen ion conducting (Y₂O₃)_y(ZrO₂)_1-y (YDZ) electrolyte membranes into thin film solid oxide fuel cells (TF-SOFCs) has led to encouraging power densities of over 1 W/cm² at relatively low temperatures of 500 °C. YDZ does not have the highest ionic conductivity in this temperature range. (Gd₂O₃)_z(CeO₂)_1-z (GDC) exhibits a conductivity nearly an order of magnitude higher, though its electrolytic domain is much narrower, which affects both its electronic and mechanical properties.^3,4 Since membranes integrated as electrolytes for TF-SOFCs must span an oxygen partial pressure range of ~10²⁰atmospheres, the mechanical effects of chemically induced non-stoichiometry can be severe. Here, we will discuss a case study of archetypical ionic conductors, YSZ and GDC, integrated as membranes for self-supported electrolytes in TF-SOFCs. We introduce experimental methods to overcome the aforementioned challenges and present our resulting findings.

Experimental

                YDZ and GDC thin films were grown by radio frequency sputtering in an Ar environment from stoichiometric ceramic targets. Self-supported membrane structures were fabricated using conventional semi-conductor processing techniques on low-stress Si₃/N₄ coated Si wafers. Additional detail on processing details of such structures can be found elsewhere.²

                Solid-solution YDZ-GDC alloys were created by co-sputtering both materials simultaneously at a constant rate. Compositionally graded YDZ-GDC films were fabricated by co-sputtering both materials at a time-dependent rate, which translated into a varying volume fraction of each constituent spatially.

                TF-SOFCs were fabricated with such electrolyte membranes by employing porous metallic electrodes (Pt, Ru) on both sides of the structure. Testing was then carried out as a function of temperature up to ~500 °C using fuel (H₂ or CH₄) and air.

Results and Discussion

                Self-supported square membrane structures having a side length of 160 μm with film thickness of ~100 nm are shown in Figure 1. As seen in the representative images, under the edge clamped boundary conditions for over 25 samples, homogeneous membranes of GDC do not survive while YDZ membranes are mechanically stable. Homogeneous oxide alloy compositions with varying volume fractions of each constituent parent phase are shown between the GDC and YDZ endpoints. After thermal cycling from 25 – 500 °C, only compositions having greater than 50% YDZ survive. This is attributed to the stability of Zr⁴⁺ in the matrix, which renders anion non-stoichiometry induced stresses inconsequential from a mechanical standpoint. We will discuss the electrical effects during our presentation. The key aspect of this portion of our study indicates that alloying a stable cation with a multi-valent counterpart can macroscopically reduce the deleterious effects of chemical strain. This observation has been recently confirmed both experimentally and theoretically by Bishop et al.⁵

Figure 1

A representative set of (YDZ)_x(GDC)_1-x, where x is the volume fraction of each constituent phase, optical micrographs of self-supported buckled (x > 0) and broken (x = 0) oxide alloy based membranes.

                An alternative experimental methodology we explore is to compositionally grade YDZ and GDC on the nanoscale. In this framework, the goal is to synergistically combine the desirable attributes of each parent material. That is, mechanical stability from YDZ and electrochemical activity and increased ionic conductivity from GDC. Utilizing this fabrication approach enabled the successful integration of two test YDZ-GDC graded membrane structures that survive thermal and chemically induced stress imparted during processing and high temperature testing. TF-SOFCs were constructed and exhibited high power densities in both H₂ and environmentally abundant CH₄fuels. We will discuss our transport results using a general effective medium model.

Summary

                We highlight the challenges with nanoscale membranes, originating from chemical-mechanical coupling oxides having complex point defect distributions. YSZ and CGO are utilized to exhibit methods to overcome chemical strain effects.

References

¹ I. Lubomirsky, Monatsh Chem.140, 1025 (2009)

² K. Kerman, S. Ramanathan, J. Mater. Res. (2013) DOI: 10.1557/jmr.2013.301

³ A. Atkinson, T.M.G.M. Ramos, Solid State Ionics129, 259 (2000)

⁴ Y. Wang, K. Duncan, E.D. Wachsman, F. Ebrahimi, Solid State Ionics178, 53 (2007)

⁵ S.R. Bishop, D. Marrocchelli, W. Fang, K. Amezawa, K. Yashiro, G.W. Watson, Energy Environ. Sci. 6, 1142 (2013)