The development of semiconductor-ionic conductor composite electrolytes for fuel cells with symmetrical electrodes

Lowering the operational temperature of solid oxide fuel cells (SOFCs) is a vital research challenge for achieving broad commercialization of SOFCs. However, there is currently a lack of suitable electrolyte materials with sufficient ionic conductivity. In this review, the recent progress in semiconductor-ionic conductor composite strategies and related key technologies for low temperature SOFCs (LT-SOFCs) applications is highlighted, in particular, emphasizing the demonstration of such composite materials sandwiched between semiconductor electrodes in a symmetrical configuration that has delivered a potent solution. Despite the co-existence of electronic and ionic conduction in the composite membrane, no electronic short-circuiting was displayed, but rather an enhanced device power output was achieved. Here, the recent progresses in the development of SOFCs, from single-layer fuel cells, to two-phase semiconductor-ionic conductor membrane fuel cells with symmetrical electrodes, are discussed. This review will furnish researchers within the SOFC community and beyond with a broader understanding of the theory, development and significance of composite materials for LT-SOFCs.     

Functional properties of La1–xBaxFeO3–δ as symmetrical electrodes for protonic ceramic electrochemical cells

To create highly efficient solid oxide fuel cells with a symmetrical configuration, it is necessary to develop suitable functional materials whose properties under oxidizing and reducing conditions will satisfy simultaneously the requirements for both fuel and oxygen electrodes. In the present work, the La_1–xBa_xFeO_3–δ (x = 0.4, 0.5, 0.6) materials were obtained and investigated as symmetrical electrodes for proton-conducting electrochemical cells based on a BaCe_0.7Zr_0.1Y_0.1Yb_0.1O_3–δ electrolyte. For the first time, the La_1–xBa_xFeO_3–δ materials were comprehensively investigated in terms of electrical conductivity, thermomechanical and electrochemical properties in both oxidizing and reducing atmospheres. It is found experimentally that La_0.6Ba_0.4FeO_3–δ demonstrates the highest electrical conductivity, lowest polarization resistances and acceptable thermal expansion behavior, which allows these materials to be used as oxygen electrodes. However, for the successful utilization of the La_1–xBa_xFeO_3–δ as the symmetrical electrodes, their transport properties under reducing atmospheres need to be improved.     

Stability considerations of semiconductor ionic fuel cells from a LNCA (LiNi0.8Co0.15Al0.05O2-δ) and Sm-doped ceria (SDC; Ce0.8Sm0.2O2-δ) composite electrolyte

Recent advances in composite materials, especially semiconductor materials incorporating ionic conductor materials, have led to significant improvements in the performance of low-temperature fuel cells. In this paper, we present a semiconductor LNCA (LiNi_0.8Co_0.15Al_0.05O_2-δ) which is often used as electrode material and ionic Sm-doped ceria (SDC; Ce_0.8Sm_0.2O_2-δ) composite electrolyte, sandwiched between LNCA thin-layer electrodes in a configuration of Ni-LNCA/SDC-LNCA/LNCA-Ni. The incorporation of the semiconductor LNCA into the SDC electrolyte with optimized weight ratios resulted in a significant power improvement, from 345 mW cm^?2 with a pure SDC electrolyte to 995 mW cm^?2 with the ionic-semiconductor SDC-LNCA one where the corresponding ionic conductivity reaches 0.255 S cm^?1 at 550 °C. Interestingly, the coexistence of ionic and electron conduction in the SDC-LNCA membrane displayed not any electronic short-circuiting but enhanced the device power outputs. This study demonstrates a new fuel cell working principle and simplifies technologies of applying functional ionic-semiconductor membranes and symmetrical electrodes to replace conventional electrolyte and electrochemical technologies for a new generation of fuel cells, different from the conventional complex anode, electrolyte, and cathode configuration.