Highly Conductive and Durable Nb(Ta)-Doped Proton Conductors for Reversible Solid Oxide Cells

Abstract

Proton-conducting reversible solid oxide cells (P-ReSOCs) are receiving increasing attention because they have potential to efficiently operate at intermediate temperatures to reduce cost and prolong operational life. Here we report our findings in the rational design of a new series of donor- and acceptor-codoped proton conductors through careful manipulation of defect chemistry. Specifically, BaNb­(Ta)0.05Ce0.7Yb0.25O3‑δ exhibits high ionic conductivity (0.012 S cm–1) while maintaining exceptional stability when exposed to Ar with 30% H2O at 500 °C for 500 h. In contrast, the resistance of BaZr0.1Ce0.7Y0.1Yb0.1O3‑δ increases continuously with time under the same condition due to reaction with H2O, as rationalized using density functional theory (DFT)-based computations. In addition, single cells based on BaNb0.05Ce0.7Yb0.25O3‑δ achieve a high peak power density of 1.12 W cm–2 in the fuel cell mode and a high current density of 2.24 A cm–2 at 1.3 V in the electrolysis mode at 600 °C. Overall, this work provides new insights for the development of highly conductive and stable proton conductors for P-ReSOCs.

A New Class of Proton Conductors with Dramatically Enhanced Stability and High Conductivity for Reversible Solid Oxide Cells

“…[15] Small 2023, 19, 2208064 1) The results of Ce, Zr, Nb, and Ta are obtained from our previous work. [8] However, the optimized BM/WCYb05 materials still show slightly lower conductivity than the state-of-the-art proton conductor BZCYYb1711. In addition, we previously reported that 30% Yb-doped samples show poor chemical compatibility with NiO during cell fabrication, which could be detrimental to cell performance.…”

“…The lattice parameter observed here is slightly smaller than the Hf or Nb-containing materials we previously reported since W 6+ is smaller in size. [8,11] Figure 2d-i and Figure S3 (Supporting Information) show the HRTEM images and the corresponding energy-dispersive X-ray spectroscopy (EDS) elemental-mapping analysis of BWCYb powders, where all elements are uniformly distributed, suggesting the successful incorporation of W.…”

“…Our previous study confirms that the conductivity of BZCYYb reaches maximum when the acceptor-doping level is around 20-25%. [8] In the pentavalent 5% Nb donorsubstituted system, one extra Yb is needed to balance the introduction of one Nb to compensate the partial filling of oxygen vacancies due to donor doping (Equations 2 and 3), increasing the optimal acceptor-doping level to 25-30%. When 5% hexavalent Mo 6+ /W 6+ is introduced, the best acceptor-doping concentration is further raised to 30-35%, which corresponds to two extra Yb atoms for one Mo/W atom, confirming more Yb is needed to balance the donor dopants with higher valence (Equation 4).…”

“…In addition, we previously reported that 30% Yb-doped samples show poor chemical compatibility with NiO during cell fabrication, which could be detrimental to cell performance. [8,16] Therefore, it is necessary to reduce the Mo/W content in order to simultaneously improve the conductivity and NiO compatibility of the electrolytes. Based on Equations 2 and 4, it can be deduced that two extra Yb atoms are needed to balance the introduction of one Mo/W atom.…”

“…[7] Recently, we have demonstrated the strategy of donor and acceptor codoping for obtaining Nb/Ta-doped electrolytes with high ionic conductivity and much improved durability than Zr-substituted ones. [8] Specifically, excess acceptor doping is found to be a necessity to compensate the blocking effect of donor doping in order to obtain high conductivity. In this work, we extend the doping chemistry to hexavalent donor Mo 6+ and W 6+ , which are predicted to be even more beneficial in enhancing the chemical stability of barium cerates than Nb 5+ and Ta 5+ , according to DFT-based calculations (Figure 1).…”