Publication date: 10th April 2024
Bulk protonic conduction may enhance the positrode performance for proton ceramic electrochemical cells by increasing the electrochemically active area beyond the triple-phase boundaries. Significant hydration and proton conduction is observed in positrode perovskites like lanthanum doped barium ferrite (BLF) where the B-site is acceptor doped with Ni and Zn [1], [2]. To further explore the properties as a positrode, electrochemical impedance spectroscopy (EIS) is performed on Ba0.95La0.05(Fe1‑x‑yNixZny)O3-δ (BLFNZ) deposited on proton conducting electrolyte Ba(Zr0.7Ce0.2Y0.1)O3-δ.
Various BLFNZ compositions are studied in a symmetrical cell with reference electrode. Key variables such as activation energies, pre-exponentials and predominant charge carriers are investigated by considering polarization resistance (Rp) as a function of pO2 and pH2O across a wide temperature range. Extracting individual area specific polarization resistances (Rp,i) at open circuit voltage reflects equilibrium conditions, while applying cathodic and anodic DC voltage to EIS sweeps delineate oxygen reduction- and oxygen ervolution reaction conditions, respectively. The voltage application results in a net current (I) through the cell and Rp,i is later converted into net overpotentials by integrating Rp,i over I.
This work combines EIS studies with thermogravimetric analysis (TGA) of Ba0.95La0.05(Fe0.7Ni0.2Zn0.1)O3-δ (BLFNZ10), amongst other BLFNZ compositions between 250-700 °C in varying pO2 and pH2O. The activation energy of 1.2 eV extracted at 350-450 °C in humid air (~3% H2O) is relatively high, but a small pre-exponential value (10‑7 Ωcm2) renders the Rp comparable to state-of-the-art positrode materials [3]. Assuming hydration dominates the mass change in humid air, TGA experiments reveal the proton concentration ranges from 1.8-0.03 mol% for 250-500 °C, i.e., sufficient for appreciable bulk protonic conduction. The relatively high activation energy may suggest a sluggish surface oxygen exchange reaction. Continued experiments include EIS with and without DC bias, and TGA experiments that may yield further insight to the reaction mechanisms of BLFNZ10. Thus, this study aims to establish relationships between hydration and the electrochemical properties of BLF based positrodes.
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