Preparation and characterization of symmetrical protonic ceramic fuel cells as electrochemical hydrogen pumps

Abstract

Here we report selective separation of high-purity H2 from a mixture of 50%H2-50%N2 at reduced temperatures using novel ceramic hydrogen pumps that feature thin dense BaZr0·1Ce0·7Y0·2O3−δ (BZCY) electrolytes symmetrically sandwiched between two identical NiO-BZCY electrodes. Scanning electron microscopy observations show nanoporous nickel catalysts in the cermets as produced from in situ slow reduction. Their superior activities for hydrogen dissociation and re-association reactions are confirmed by unprecedently low interfacial polarization resistances from impedance measurements, e.g., 0.006 Ω cm2 at 500 °C. The overall hydrogen permeation kinetics is largely limited by relatively slow proton transport through BZCY electrolytes. The measured H2 fluxes range from 15.8 ml cm−2min−1 at 0.68 V and 500 °C to 9.4 ml cm−2min−1 at 1.84 V and 350 °C with Faradic efficiencies close to unity. Reducing the H2 content in the feed gas decreases the H2 fluxes as well as the upper limiting current density, above which Faradic efficiencies starts to decrease. It is further demonstrated that humidity in the sweep gas and in the H2-rich feed gas like 50%H2-50%N2 does not influence the hydrogen pumping characteristics. In contrast, reducing humidity in the highly H2-diluted feed gas like 10%H2-90%N2 results in a pronounced decrease in the H2 fluxes and the upper limiting current density.