Suppression of Leakage Current in Proton-Conducting BaZr_0.8Y_0.2O_3−δ Electrolyte by Forming Hole-Blocking Layer

Abstract

Solid oxide fuel cells (SOFCs) with proton-conducting solid electrolyte, instead of the oxide-ion conducting solid electrolyte have attracted attentions because of their high potential to reduce operating temperatures and to enhance the electrical efficiencies of SOFCs. In addition, the proton-conducting SOFCs with multistage electrochemical oxidation configuration will be promising technology for critically-high electric efficiencies. However, it is known that there are non-negligible charge -carriers other than protons in typical proton-conducting solid oxide electrolytes at relatively high temperatures. The existence of the partial conductivities of holes and/or electrons will cause the internal leakage current that consumes fuel but never generates any electrical power output. The higher ratio of the leakage current to external current will more deteriorate the electrical efficiency. In this study, the effects of blocking -layers formed on the air side surface of base electrolyte layer consisting of BaZr0.8Y0.2O3−δ (BZY82) for suppressing the leakage current have been investigated by using electrochemical parameters of the partial conduction of the materials. The chemical potential profile and leakage current showed large dependence on the material of the blocking-layer. Lanthanum tungstate was found to play a role as unique and strong blocking-layer against the leakage current.

Electrochemistry and energy conversion features of protonic ceramic cells with mixed ionic-electronic electrolytes

“…Another widely proposed approach for decreasing electron leakage in electrolytes presented in the literature consists in the formation of the so-called blocking layers. However, since this strategy has been thoroughly discussed in a number of research works 187–200 and reviews 42,43,201 it is considered to be beyond the scope of the present overview.…”

Effect of lanthanum tungstate hole-blocking layer for improvement of energy efficiency in anode-supported protonic ceramic fuel cells

“…In the bilayer model, terminal voltage (V T ), can be calculated from the following equations. 11) e;i ) of the electrolyte "i". The subscript "i" expresses BZY when i = 1 and LWO when i = 2.…”

“…This characteristic transport property of LWO motivates us to apply LWO to a hole-blocking layer for the PCFCs. 11) The holeblocking layer is a thin proton-conducting layer with low hole conductivity fabricated on the cathode side of a thicker electrolyte membrane to suppress the leakage current arising from the hole conduction in the electrolyte membrane. Our previous theoretical calculations suggested that the leakage current in a BaZr 0.8 Y 0.2 O 3¹¤ (BZY) electrolyte membrane can be well suppressed by introducing a thin LWO hole-blocking layer.…”

“…Our previous theoretical calculations suggested that the leakage current in a BaZr 0.8 Y 0.2 O 3¹¤ (BZY) electrolyte membrane can be well suppressed by introducing a thin LWO hole-blocking layer. 11) The introduction of the LWO hole-blocking layer changes an oxygen chemical potential profile in the electrolytes, which effectively prohibits the oxidation of the BZY membrane leading to the suppressed leakage current.…”

Experimental analyses for electronic structure of barium zirconate‐strontium zirconate proton‐conducting solid solution

“…For example, a leakage current through the electrolyte membrane derived from non-negligible hole conduction in the protonconducting electrolyte causes a reduction of open-circuit voltage and energy efficiency of the cell. 22,23 Since the energy gap between the valance band maximum (VBM) and the acceptor level dominates hole concentration in the proton-conducting oxides, determination of the position of the acceptor level and that of the VBM can contribute to the development of the proton-conducting oxides for highefficiency PCFCs. While previously reported optical measurements using powder samples and single crystals with a thickness of several tens to several hundred micrometers have advantages for detecting the weak defect-related optical absorption, 18,20,24 direct determination of a fundamental absorption edge derived from the excitation of electrons from the valence band (VB) to the conduction band (CB) having large absorption coefficient α is difficult when such a thick specimen are used because large α results in a quite small transmittance or small diffuse reflectance below a detection limit.…”