(Invited) Proton-Conducting Ceramics for Fuels Synthesis

Abstract

This presentation is centered on an exciting new class of proton-conducting ceramic materials that are emerging from the laboratory to play important roles in the commercial sector. While proton-conducting ceramics have been studied since the early 1980s, the unique properties of these materials are only now being harnessed to address societal challenges. Faculty and staff at the Colorado School of Mines (CSM) are actively developing protonic ceramics for use as fuel cells for efficient electricity generation (PCFCs), electrolyzers for energy storage (PCECs), and membrane reactors for fuels upgrading (PCMRs). Doped barium cerate-zirconate materials (BaCexZr1-x-yYyO3-d) are the focus of these efforts at CSM. The mixed ionic-electronic conductivity of these protonic ceramics can be exploited for developing cost-effective, novel solutions and new products. Many previous approaches used to advance solid-oxide fuel cells are equally relevant for tuning the properties of protonic ceramics to meet the diverse needs of these different applications. In this talk, we will present several such efforts now underway at CSM. As part of an ARPA-E-funded program, our team recently demonstrated a fuel-cell stack based on BaCe0.2Zr0.6Y0.2O3-d (BCZY26) protonic-ceramic materials. To the authors’ knowledge, this is the first reported result of a PCFC stack. The effort included a ten-fold scale up in cell size, and integration of these cells into a stack package. A three-cell stack achieved a power density of 180 mW cm-2 at 0.78 V under methane fuel at 550 ºC. While this PCFC performance is encouraging, we found that similar protonic-ceramic materials demonstrated inefficient operation in electrolysis mode. Heightened electronic conductivity was observed under reverse bias, likely due to cerium reduction. By reducing the cerium content to below the percolation threshold, greatly improved Faradaic efficiency was confirmed. The team is now developing PCEC stacks using BaCe0.1Zr0.8Y0.1O3-d (BCZY18) electrolytes; 95% Faradaic efficiency was observed at 120 mA cm-2 and 600 ºC. Electronic conduction is undesirable in fuel cells and electrolyzers; however, it may prove advantageous for hydrogen-separation membranes and membrane reactors. In an effort to increase flux through “passive” hydrogen-separation membranes, CSM is developing protonic-ceramic composites to increase membrane electronic conductivity. By mixing the BCZY material with an electronic conductor, a two-phase membrane with more balance between protonic and electronic conduction can be realized. Both ceramic (Ce0.8Y0.2O2-d) and metallic (Cu) electronic conductors are being explored as the second phase. While at an early stage of development, these two-phase hydrogen-separation membranes have demonstrated some of the highest thickness-normalized hydrogen fluxes ever observed through ceramic materials. In this presentation, these efforts will be reviewed, with latest results presented.