Steam Electrolysis by Proton-Conducting Solid Oxide Electrolysis Cells (SOECs) with Chemically Stable BaZrO3-Based Electrolytes

Solid oxide electrolysis cells (SOECs) are expedient to solve the site-specific and intermittent problems for current renewable energies, such as solar, wind and tidal power, as SOECs can convert these renewable energies into chemical energy in the form of H₂ that is clean and easily to be transported. Most of the studies on SOECs are focused on oxygen-ion conducting SOECs, using mainly yttria-stabilized zirconia (YSZ) as electrolyte. However, problems such as the dilution of produced H₂, high temperature operation, and oxidation of Ni-based fuel electrode occur, which the use of proton-conducting SOECs can circumvent. In addition, the high conductivity of proton-conducting oxides with respect to YSZ enables proton-conducting SOECs operating at intermediate temperatures [1].

The few works reporting on proton-conducting SOECs show the use of BaCeO₃-based electrolytes, which has been demonstrated to be chemically unstable in H₂O. Although proton-conducting BaZrO₃-based materials are the most promising electrolyte candidates for proton-conducting SOECs due to the excellent chemical stability and high bulk conductivity [2], their processing difficulties due to poor sinterability and high-resistive grain boundaries prevented the deployment in SOEC devices. In this report, we pioneeringly used BaZrO₃-based electrolytes for SOECs, demonstrating that tailoring the electrolyte and electrode materials results in further improving the electrolysis cell performance. 

Anode supported BaZr_0.9Y_0.1O_3-δ (BZY) electrolyte films were fabricated using an ionic diffusion method [3]. Both thermodynamic calculation and experimental results suggest that BZY has an excellent chemical stability in H₂O, which is critical for practical applications. A current density of 119 mA cm^-2 was obtained at 600°C, with an applied voltage of 1.65 V for the electrolysis cell, which is comparable or even higher than that for proton-conducting SOECs with BaCeO₃-based electrolyte at similar conditions, while BZY electrolyte offers much better chemical stability. This cell also operated at 600 ^oC for more than 80 h without any obvious degradation, while the BaCeO₃-cells are reported to only last tenths of minutes or a few hours [4].

To improve the cell performance, BaZr_0.7Y_0.2Pr_0.1O_3-δ (BZPY10), which is a more conductive proton-conducting electrolyte material, wass used for the SOEC. With La_0.8Sr_0.2MnO_3-δ (LSM)-BZPY10 composite air electrode, the SOEC with BZPY10 electrolyte reached a current density of 576 mA cm^-2 with an applied voltage of 1.6 V at 600 ^oC. This cell performance is much improved compared with the above discussed BZY cell. Further cell performance improvement was achieved by tailoring the air electrode material with BaZr_0.5Y_0.2Pr_0.3O_3-δ (BZPY30), which is a mixed protonic-electronic conductor rather than a pure protonic conductor [5]. By coupling BZPY30 with LSM, the triple phase boundary (TPB) is much improved due to the mixed conducting behavior of BZPY30, which is beneficial to the reaction. As a result, the cell with BZPY30-LSM air electrode produced a current density of 1007 mA cm^-2 with an applied voltage of 1.6 V at 600 ^oC. An obvious cell performance improvement is obtained with the tailored electrode.

References

1. L. Bi, S. Boulfrad and E. Traversa, Chem. Soc. Rev., 2014, 43, 8195-8300.
2. D. Pergolesi, E. Fabbri, A. D'Epifanio, E. Di Bartolomeo, A. Tebano, S. Sanna, S. Licoccia, G. Balestrino and E. Traversa, Nature Mater., 2010, 9, 846-852.
3. L. Bi, E. Fabbri, Z. Q. Sun and E. Traversa, Energy Environ. Sci., 2011, 4, 409-412.
4. L. Bi, S.P. Shafi and E. Traversa, J. Mater. Chem. A, 2015, DOI: 10.1039/c4ta07202b.
5. E. Fabbri, L. Bi, D. Pergolesi and E. Traversa, Energy Environ. Sci., 2011, 4, 4984-4993.