A fast ceramic mixed OH-/H+ ionic conductor for low temperature fuel cells

Abstract

Low temperature ionic conducting materials such as OH^- and H⁺ ionic conductors are important electrolytes for electrochemical devices. Here we show the discovery of mixed OH^-/H⁺ conduction in ceramic materials. SrZr_0.8Y_0.2O_3-δ exhibits a high ionic conductivity of approximately 0.01 S cm^-1 at 90 °C in both water and wet air, which has been demonstrated by direct ammonia fuel cells. Neutron diffraction confirms the presence of OD bonds in the lattice of deuterated SrZr_0.8Y_0.2O_3-δ. The OH^- ionic conduction of CaZr_0.8Y_0.2O_3-δ in water was demonstrated by electrolysis of both H₂¹⁸O and D₂O. The ionic conductivity of CaZr_0.8Y_0.2O_3-δ in 6 M KOH solution is around 0.1 S cm^-1 at 90 °C, 100 times higher than that in pure water, indicating increased OH^- ionic conductivity with a higher concentration of feed OH^- ions. Density functional theory calculations suggest the diffusion of OH^- ions relies on oxygen vacancies and temporarily formed hydrogen bonds. This opens a window to discovering new ceramic ionic conducting materials for near ambient temperature fuel cells, electrolysers and other electrochemical devices.

Fig. 1. The proposed schematic diagram for the formation of pathways for OH⁻/H⁺ ions in SrZr_1-xY_xO_3-δ enabled by water/steam.

a The structure of SrZrO₃ parent phase. b The schematic structure of Y doped SrZrO₃. Part of Zr at the B-site in a is replaced by Y then oxygen vacancies (${\mathit{V}}_{\mathit{O}}^{\bullet \bullet }$) are formed. c The schematic structure of hydrated SrZr_1-xY_xO_3-δ. Proton defects ${\mathit{OH}}_{\mathit{O}}^{\mathit{\bullet }}$ are formed in SrZr_1-xY_xO_3-δ after integrating of water or steam. d The schematic diagram for OH⁻/H⁺ ions diffusion in SrZr_1-xY_xO_3-δ.

Fig. 2. The characterizations of doped zirconates.

a XRD patterns of SrZr_1-xY_xO_3-δ (x = 0, 0.1, 0.2, denoted as SZO, SZYO10, SZYO20). b–f SEM and TEM analysis of SZYO20 powders. b SEM image, c ADF-STEM image, d High-resolution ADF-STEM image taken from marked area in (c) (inset: corresponding FFT of the image), e Integrated EDX spectra from the STEM-EDX analysis in (f), f BF-STEM image and the corresponding EDX maps of Sr, Zr, Y, O.

Fig. 3. TEM and Raman characterizations of washed SZYO20.

a–c TEM analysis of washed SZYO20 powders. a BF-STEM image and the corresponding EDX maps of Sr, Zr, Y, O, b ADF-STEM image, c High resolution ADF-STEM image taken from marked area in (b) (inset: corresponding FFT of the image). d Raman spectra of SZYO20 samples with different hydration levels.

Fig. 4. The ionic conductivity of doped zirconates under different conditions.

a AZr_0.8Y_0.2O_3-δ (A = Ca, Sr, Ba, denoted as CZYO20, SZYO20, BZYO20), measured in water. b SrZr_1-xY_xO_3-δ (x = 0, 0.1, 0.2, denoted as SZO, SZYO10, SZYO20), measured in water. c SZYO20 in wet air at different temperatures. d Stability of the conductivity of SZYO20 in wet air at 70 °C. e, f Conductivity (e) and activation energy (f) of SZYO20 in H₂O and D₂O.

Fig. 5. Solid state NMR spectra of dry and partially hydrated SZYO20.

a The solid state ¹H NMR spectra of dry (red) and partially hydrated (blue) SZYO20. b The solid state ¹H NMR (MAS 60 kHz) spectra of partially hydrated (top) and dry (bottom) SZYO20 performed whit the rotors kept at -5 ^oC (blue) and +30 ^oC (red). c, d ¹H-¹H NOESY correlation spectra (MAS 60 kHz) of dry (c) and partially hydrated (d) SZYO20 mixing time (c) 0.1 s and (d) 1 s. e ⁸⁹Y MAS at 8 kHz DP spectrum measured with a spin echo (red) and cross polarisation (blue) from ¹H of partially hydrated SZYO20. f ¹H-⁸⁹Y heteronuclear correlation experiment obtained with 6 ms cross polarisation, MAS 8 kHz.

Fig. 6. Molecular dynamics simulation of SZYO20.

a Distance (in Å) between a selected proton and its neighbouring O atom as a function of MD simulation time (in ps). b Distance (in Å) between a selected oxygen atom (which was bonded to a proton initially) and its neighbouring B-site cations as a function of MD simulation time (in ps). The blue and orange lines represent the distances between an O^2- anion and its two neighbouring B-site cations, respectively. c Schematic representation of possible atomistic mechanisms of proton hopping and OH⁻ diffusion. The dotted circles indicate new positions of OH⁻ after diffusion, with the previous site now becoming an oxygen vacancy. All other atoms are omitted for clarity. Colour code: red – oxygen, white – hydrogen, light green – zirconium or yttrium.

Fig. 7. Fuel cell measurements when SZYO20 was used as electrolyte.

a The OCV change against the time of a H₂/air fuel cell at 20 °C. b Working principle of low temperature SOFCs for an NH₃/air fuel cell. c, d The performance (c) and impedance (d) of an NH₃/air fuel cell (35 wt% NH₃H₂O solution + 3 M KOH as the fuel) at different temperatures, enlarged impedance spectra are displayed in the insert.