Electrodes for Protonic-Ceramic Membranes Used in Natural Gas-to-Chemicals Processing

Electrodes used in natural gas-to-chemicals processing must demonstrate unique performance attributes in challenging high-temperature, reducing environments.  Materials must be stable, have high electronic conductivity, and be chemically inert to methane and other hydrocarbons. Ceramic-metal composites (cermets) made of nickel and the electrolyte material, used as the fuel-side electrodes in protonic ceramic fuel cell, are not suitable as their performance decreases rapidly due to chemical reaction with the natural gas fuel. This leads to carbon-deposit formation, poisoning at the Ni phase and eventual electrode fracture [1]. These issues motivated the development of coke-resistant electrodes and catalysts. The coking rate of copper at 800 °C is reported to be two orders of magnitude lower than that of Ni [2].  This study presents a detailed electrochemical characterization of Cu electrodes on BaCe_0.2Zr_0.7Y_0.1O_3-d proton-conducting ceramic electrolyte (BCZY27).

High-quality dense BCZY27 electrolytes with large grains were successfully prepared using solid-state reactive sintering [3-4]. Cu electrodes were deposited by various techniques such as sputtering and screen-printing/painting, resulting in electrodes with different thicknesses and microstructures, making it possible to understand the chemical and electrochemical mechanisms taking place at the electrodes. Impedance spectra were recorded on symmetric cells (electrode/BCZY27/electrode) over a wide temperature range (400 - 700 °C) and in various atmospheres (dry and moist H₂, dry methane). Figure 1 displays the ASR of 500 nm sputtered porous Cu electrodes in dry and moist H₂. With this electrode architecture, lower ASR are obtained in dry H₂, while dense Cu electrodes lead to the opposite trend. Stability tests were also performed to quantify the degradation of the electrodes in hydrogen and methane at 700 °C by using both the ASR and the post-testing micrographs. In this work, the ASR of the various Cu electrodes are compared.

References:

1. C.M. Finnerty, N.J. Coe, R.H. Cunningham, R.M. Ormerod (1998) Catalysis Today 46 137.

2. P.R.S. Jackson, D.J. Young, D.L. Trimm (1986) J. Mater. Science 21 4376.

3  W.G. Coors (2011) “Co-ionic conduction in protonic ceramics of the solid solution, BaCe_xZr_(1-x)Y_(y-x)O_3-d; part I: fabrication and microstructure, ceramic materials”, Book 3. Intech, Croatia.

4    S. Ricote, N. Bonanos, A. Manerbino, W.G. Coors (2012) Int. J. Hydrogen Energy 37 7954.