Surface restructuring of a perovskite-type air electrode for reversible protonic ceramic electrochemical cells

Abstract

Reversible protonic ceramic electrochemical cells (R-PCECs) are ideally suited for efficient energy storage and conversion; however, one of the limiting factors to high performance is the poor stability and insufficient electrocatalytic activity for oxygen reduction and evolution of the air electrode exposed to the high concentration of steam. Here we report our findings in enhancing the electrochemical activity and durability of a perovskite-type air electrode, Ba_0.9Co_0.7Fe_0.2Nb_0.1O_3-δ (BCFN), via a water-promoted surface restructuring process. Under properly-controlled operating conditions, the BCFN electrode is naturally restructured to an Nb-rich BCFN electrode covered with Nb-deficient BCFN nanoparticles. When used as the air electrode for a fuel-electrode-supported R-PCEC, good performances are demonstrated at 650 °C, achieving a peak power density of 1.70 W cm^-2 in the fuel cell mode and a current density of 2.8 A cm^-2 at 1.3 V in the electrolysis mode while maintaining reasonable Faradaic efficiencies and promising durability.

Fig. 1. Reactions on an R-PCEC and their applications.

Schematic illustration of an R-PCEC with a steam-induced surface restructured BCFN air electrode operated in both FC and EL modes.

Fig. 2. Structural characterization of BCFN powder.

a A Rietveld refinement of XRD patterns for a BCFN powder with measured data (black dots), simulated (red line; calculated profile), background (pink line), difference curves (blue line), and a schematic of a BCFN crystal structure (inset). The powders were calcined at 1100 °C for 2 h in the air; b Room temperature XRD patterns of the BCFN powders after a treatment at 650 °C in dry air and wet air (24 h, 3% H₂O); c XRD patterns (collected at 600 °C) of the BCFN powders treated in wet air; d SEM images of BCFN powders before (top) and after (bottom) exposure to the wet air (3% H₂O) at 650 °C for 2 h; e High-resolution TEM images of BCFN grains, including BCFN substrate and BCFN NPs; f High-angle annular dark-field STEM image from the BCFN powders, and the X-ray energy dispersive spectrum mapping of Ba, Co, O, Fe, and Nb.

Fig. 3. Electrochemical performance of R-PCECs with a BCFN air electrode in the FC mode.

a Temperature dependence of the polarization resistance (R_p) of a symmetrical cell with BCFN and other high-performance electrodes^–, as shown in Supplementary Table 1; b A cross-section image of a Ni-BZCYYb fuel electrode supported single cell. Inset is the detailed SEM image of cathode; c Typical I-V-P curves of the single-cell measured at 650, 600, 550, and 500 °C, respectively; d Typical EIS curves of the single cell under OCVs using H₂ (3% H₂O) as the fuel and ambient air as the oxidant, measured at 650, 600, 550, and 500 °C, respectively; e The comparison of the peak power densities of PCFCs with different cathode materials^,,–, and BCFN (this work), tested from 800 to 500 °C; and f A short-term stability test of the single cell at a constant current density of 0.5 A cm⁻² and 600 °C.

Fig. 4. Electrochemical performance of R-PCECs with the BCFN air electrode in the EL mode.

a Typical I-V curves of the R-PCECs measured at 650, 600, 550, and 500 °C, respectively with humidified H₂ (3% H₂O) in the fuel electrode and humidified air (3% H₂O) in the air electrode in the EL mode; b the comparison of current densities of different air electrode materials reported and this work at 1.3 V from 650 °C to 500 °C;^,,,– c Stability test of the single cell at current densities of −0.5 A cm⁻² and −1.0 A cm⁻² at 550 °C; d Reversible operation of the R-PCEC: the cell voltage as a function of time when the operating mode was switched between the modes of FC and EL at a current density of ±0.5 A cm^–2 for intervals of 2, 4, 8, 12, and/or 1 h for each mode at 650 °C; and e Faradaic efficiencies of R-PCECs for producing hydrogen at different electrolysis voltages, and different H₂O concentration (20% and 30%) in air at 500 °C, and Faradaic efficiencies of the cells when the air electrode was operated in different water concentration at a current density of 1 A cm⁻² at 600 °C.

Fig. 5. Formation of the high-performance air electrode of BCFN covered with fine BCFN NPs with less Nb.

a. TEM image and elemental (Ba, Co Fe, and Nb) mapping of a BCFN grain after electrochemical testing at 650 °C; b. Elemental profile along the scanning line shown in the TEM image; c. Comparison of computed segregation energies (J m⁻²) for stoichiometric Ba(B_0.5B′_0.5)O₃ (B,B′ = Co, Fe, or Nb); d. Illustration of CoO-terminated BCFN(001) surface models representing segregation of Nb from the bulk to the surface; e. Computed segregation energy (eV) for three different dopant cations of Ba_0.9(Co_0.63Fe_0.25Nb_0.13)O₃; and f. A schematic illustration of the formation of Nb-deficient BCFN NPs on the BCFN air electrode.