In this paper, we present development of a novel Ba₂FeMoO_6-δ ceramic electrode for use with proton-conducting ceramic membranes. The membranes are based on barium cerate-zirconate materials, such as BaCe_xZr_0.9-xY_0.1O_3-d (BCZY) [1, 2].  Such ceramic hydrogen-separation membranes have a wide range of industrial applications, including fuel cells for electricity generation, electrolyzers for energy storage, and membrane reactors for fuels upgrading.

A number of these applications require development of a high-performance electrode for use in high-temperature reducing atmospheres containing hydrogen and / or methane gases.  Such conditions present a number of unique challenges. Sluggish catalytic and charge-transfer chemistries between the gases, the electrode, and the membrane can result in high activation polarization losses and poor electrochemical performance. Additionally, the high methane concentration can cause performance degradation due to deleterious carbon-deposit formation on the electrode surface.  Finally, the stability and adherence of the electrode and membrane materials bring fabrication and synthesis challenges.

Ceramic oxides are considered to be good candidates to serve as such electrodes. Some oxides developed for solid-oxide fuel cells (SOFCs) are found to be chemically stable and electrically conductive in reducing conditions. The high resistance against carbon deposition (coking), good mechanical compatibility with protonic ceramic membrane, and strong endurance to particle coarsening enable ceramic oxides to be developed as potential electrode in this application. Layered double perovskites (Ba,Sr)₂MMoO_6-δ (M=Fe, Mg, Ni, Co, etc) have been recently developed as anode materials for direct hydrocarbon SOFCs [3]. These oxides show high conductivity of ~ 100 S/cm and low area-specific resistance (ASR) of 0.45 ohm cm² in H₂ at 700 ºC. Such characteristics make these layered double perovskites well matched with the protonic-ceramic membranes.

In this work, we investigate the use of Ba₂FeMoO_6-δ (BFMO) for use as an electrode with a BCZY proton-conducting ceramic membrane. The performance of this layered double perovskite is evaluated through analysis of the phase structure, chemical stability and compatibility, coking propensity, and area-specific resistance. Ceramic-processing parameters are developed to promote reasonable adhesion between the BFMO electrode and the BCZY membrane. Reasonable adhesion is obtained with composite ceramic 5:1 BFMO:BCZY. Without any current collector applied on the surface of electrode, the preliminary results show that the composite BFMO electrode is very stable in H₂, with an ASR as low as ~10 ohm cm² in moist H₂ at 700 ºC. The effect of synthesis technique on particle size and electrochemical performance are also being explored.  Far smaller particle sizes are obtained through Pechini synthesis in comparison to solid-state reaction. This leads to lower area-specific resistance, as shown in Figure 1. 

Citations:

1. S. Babiniec, S. Ricote, N.P. Sullivan, “Characterization of ionic transport through BaCe_0.2Zr_0.7Y_0.1O_3-d membranes in galvanic and electrolytic operation,” International Journal of Hydrogen Energy, 10.1016/j.ijhydene.2015.05.162 40 (2015) 9278-9286.

2. S. Babiniec, S. Ricote, N.P. Sullivan, “Infiltrated lanthanum nickelate cathodes for use with BaCe_0.2Zr_0.7Y_0.1O_3-d proton conducting electrolytes,” Journal of the Electrochemical Society 161 : 6 (2014) F717-F723.

3. Qin Zhang, Tao Wei, Yun-Hui Huang, “Electrochemical performance of double-perovskite Ba₂MMoO6 (M = Fe, Co, Mn, Ni) anode materials for solid oxide fuel cells,” Journal of Power Sources 198 (2012) 59–65.