Steam Electrolysis by Proton-Conducting Solid Oxide Electrolysis Cells (SOECs) with Chemically Stable BaZrO3-Based Electrolytes
Authors: Bi Lei , Traversa Enrico 
Published: February 2020

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 BaZr0.9Y0.1O3-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, BaZr0.
7Y0.
2Pr0.1O3-δ
(BZPY10), which is a more conductive proton-conducting electrolyte material, wass used for the SOEC. With La0.8Sr0.2MnO3-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 BaZr0.
5Y0.
2Pr0.
3O3-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.

Reference
s

L. Bi, S. Boulfrad and E. Traversa, Chem. Soc. Rev., 2014, 43, 8195-8300.

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.

L. Bi, E. Fabbri, Z. Q. Sun and E. Traversa, Energy Environ. Sci., 2011, 4, 409-412.

L. Bi, S.P. Shafi and E. Traversa, J. Mater. Chem. A, 2015, DOI: 10.1039/c4ta07202b.

E. Fabbri, L. Bi, D. Pergolesi and E. Traversa, Energy Environ. Sci., 2011, 4, 4984-4993.