Current Leakage and Faradaic Efficiency Simulation of Proton-Conducting Solid Oxide Electrolysis Cells
Authors: Jin Xinfang , Shoukry Yasser 
Published: September 2023

Proton conducting solid oxide electrolysis cells (H-SOEC) have advantages over its oxygen-ion-conducting (O-SOEC) counterpart due to lower operating temperature (e.g. 300-700oC), relatively lower activation energy, and easier gas separation [1, 2]. However, it suffers from low Faradaic efficiency led by the internal current leakage. Protonic ceramics like BaZr0.90Y0.10O3 (BZY) are generally mixed proton, oxygen ion and electron hole conducting oxides, in which the transference number of each charge carrier depends on the material composition, operating temperature, atmosphere, and polarization current density. Experimental results have been reported that BZY shows significant p-type electronic leakage [3] across the electrolyte resulting in a flux of hydrogen from the negative electrode to the positive electrode and reducing the Faradaic efficiency. However, the fundamental understanding of the formation of electronic charge carriers during electrolysis operation is still unknown.

In this research, we will simulate H-SOEC with BZY electrolyte with defect chemistry and Nernst-Planck charge conservation [4] to couple thermodynamic equilibrium of both electrodes under polarized operating conditions. Three mobile charge carriers will be considered in BZY electrolyte, including proton, oxide ion and electron hole, which is balanced with the dopant concentration . The thermodynamic equilibrium will be used to solve for charge species concentration in different gas atmosphere. The thermodynamic parameters will be validated by experimental data of BZY proton concentration [5] under different operating temperature and different O2 or H2O partial pressures. The mobility of each charged species will be validated by overall conductivity of BZY [6]. Then, the model will be used to project internal electronic current leakage and Faradaic efficiency from open circuit voltage to Nernst potential and beyond (as shown in Fig. 1). The correlation between Faradaic efficiency and operating current density will be established to explain the experimental observations. Ultimately, the model could help establish strategies to minimize electronic leakage by tailoring the composition of the electrolyte, optimizing device architecture and manipulating P-SOEC operating conditions.

References:

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Lei, L., et al., Progress Report on Proton Conducting Solid Oxide Electrolysis Cells. Advanced Functional Materials, 2019. 29(37): p. 1903805.

Choi, S., T.C. Davenport, and S.M. Haile, Protonic ceramic electrochemical cells for hydrogen production and electricity generation: exceptional reversibility, stability, and demonstrated faradaic efficiency. Energy & Environmental Science, 2019. 12(1): p. 206-215.

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Figure 1