Promotion of water-mediated carbon removal by nanostructured barium oxide/nickel interfaces in solid oxide fuel cells

Abstract

The existing Ni-yttria-stabilized zirconia anodes in solid oxide fuel cells (SOFCs) perform poorly in carbon-containing fuels because of coking and deactivation at desired operating temperatures. Here we report a new anode with nanostructured barium oxide/nickel (BaO/Ni) interfaces for low-cost SOFCs, demonstrating high power density and stability in C(3)H(8), CO and gasified carbon fuels at 750°C. Synchrotron-based X-ray analyses and microscopy reveal that nanosized BaO islands grow on the Ni surface, creating numerous nanostructured BaO/Ni interfaces that readily adsorb water and facilitate water-mediated carbon removal reactions. Density functional theory calculations predict that the dissociated OH from H(2)O on BaO reacts with C on Ni near the BaO/Ni interface to produce CO and H species, which are then electrochemically oxidized at the triple-phase boundaries of the anode. This anode offers potential for ushering in a new generation of SOFCs for efficient, low-emission conversion of readily available fuels to electricity.

Figure 1. Microanalysis of BaO nanoislands and BaO/Ni interfaces.

(a) Grazing incidence angle X-ray diffraction patterns of BaO/NiO samples before (blue curve) and after (red curve) reduction in hydrogen. These patterns were collected at X14A beamline of National Synchrotron Light Source (NSLS). The incident angle was 0.1° and the wavelength λ was 0.72838 Å. (b) Ba L_III-edge X-ray absorption near-edge structure (XANES) spectra of BaO powder and the BaO/Ni sample. The XANES spectra were collected at X18A beamline of NSLS using reflective detection mode. The Ba L_III-edge XANES spectrum of BaO/Ni was an average of 13 spectra. (c) Cross-sectional view (bright-field TEM image) of a BaO/Ni interface. Scale bar, 30 nm. (d) Top view (SEM image) of a BaO/Ni sample. Scale bar, 200 nm. (e) HRTEM image of the BaO/Ni interface. Scale bar, 2 nm. The [] zone axis of the Ni under the BaO island is along the viewing direction. (f) Fourier-filtered [] zone axis image of the Ni under the BaO island.

Figure 2. Performance of fuel cells with the new anode in dry C3H8.

(a) Typical current–voltage characteristics and the corresponding power densities measured at 750 °C for cells with a configuration of BaO/Ni-YSZ |YSZ| SDC/LSCF when dry C3H8 was used as the fuel and ambient air as the oxidant. (b) Terminal voltages measured at 750 °C as a function of time for the cells with and without BaO/Ni interfaces operated at a constant current density of 500 mA cm⁻² with dry C3H8 as the fuel. Water was formed on the anode by electrochemical oxidation of dry C3H8.

Figure 3. Fuel cell performance in CO and in gasified carbon.

(a) Terminal voltages measured at 750 °C as a function of time for the cells with and without BaO/Ni interfaces operated at a constant current density of 500 mA cm⁻² with wet CO (with ~3 v% H2O) as the fuel. (b) Typical current–voltage characteristics and the corresponding power densities measured at 750 °C for cells with and without BaO/Ni interfaces (after 4 h operation) when wet CO was used as the fuel and ambient air as the oxidant. (c) Typical current–voltage characteristics and the corresponding power densities measured at 850 and 750 °C for cells with BaO/Ni interfaces when gasified carbon was used as the fuel and ambient air as the oxidant in a fluidized carbon bed-SOFC arrangement.

Figure 4. Assessment of water uptake capability.

(a) Typical thermogravimetric traces for Ni, YSZ and BaO powder samples in dry and wet argon with 4 v% H2 at 25 and 750 °C. (b) Raman spectra recorded from BaO/Ni and pure Ni samples in wet H2 (with ~3 v% H2O) atmosphere at room temperature. (c) Raman spectra collected from BaO/Ni samples in dry and wet H2 (with ~3 v% H2O) atmospheres at room temperature. (d) Top and side views for the interaction of H2O on two-layer BaO deposited on p(3×3) Ni(1 1 1) containing six Ba and six O atoms. 'w', 'h1' and 'h2' represent molecularly adsorbed H2O and dissociated hydroxyl species, whereas v_w,b, v_h1 and v_h2 are the vibration modes of a H2O bending and two OH stretchings (1,594, 3,716 and 3,368 cm⁻¹), respectively. Large balls in Brandeis blue, green and red are Ni, Ba and O of BaO, respectively, whereas small balls in red and white are O from H2O and H, respectively.

Figure 5. DFT prediction of energy profile.

The energies for removal of chemisorbed carbon species on BaO/Ni(1 1 1) are relative to gas-phase H2O and an adsorbed carbon species on BaO/Ni(1 1 1). '* denotes an adsorbed species on the surface. TS, transition state.

Figure 6. Effect of BaO nanoislands on electronic properties of Ni anode.

(a) Projected density of states of Ni(1 1 1) and BaO/Ni(1 1 1) using DFT. The vertical dashed line is the Fermi level. The values in the figure are predicted d-band centres based on seven bare Ni atoms on the topmost Ni layer. (b) Typical current–voltage characteristics and the corresponding power densities measured at 750 °C for cells with configurations of BaO/Ni-YSZ|YSZ| SDC/LSCF and Ni-YSZ |YSZ|SDC/LSCF when dry hydrogen was used as the fuel and ambient air as the oxidant.

Figure 7. Proposed mechanism for water-mediated carbon removal on the anode with BaO/Ni interfaces.

Large balls in Brandeis blue, green, red, blue grey and purple are Ni, Ba, O of BaO or YSZ, Zr and Y, respectively, whereas small balls in red, white and grey are O from H2O, H and C, respectively. D₁ is the dissociative adsorption of H2O, whereas D₂ is the dehydrogenation of hydrocarbons or the CO disproportionation reaction.