Recent progress in the experimental and theoretical studies on the barium zirconate proton conductors: A review (original) (raw)

There have been signi cant developments of solid-state-ion conducting energy materials and perovskitebased oxides those exhibit excellent proton conduction at intermediate temperatures. In contrast to hightemperature oxygen ion-conducting oxides or low-temperature proton-conducting polymers, perovskite oxides have obtained distinguished attention because of their diversi ed structural aspects and potential applications. Highly stable and conductive electrolytes with improved electrochemical and thermochemical properties are in great demand in numerous elds such as portable electronics and transport systems, energy storage, fuel cells, etc. This review focuses on recent development in the proton-conducting performance of BaZrO 3 (BZO) energy materials. This study aims to integrate the fundamentals of proton conducting BZO perovskites in the prospect of the recent development in materials science and computational engineering. Therefore, in the rst half of this review, the basic overview of the BZO perovskites structure, fundamentals of working principles, fabrication, and processing methods underlying the successful development of these materials with superior performance is discussed. The second part principally concentrates on the signi cant improvement towards higher conductive BZO perovskite fabrication with the help of theoretical studies via density functional theory (DFT) based rst-principles calculation and molecular dynamics (MD) simulation followed by the prominent applications in low-temperature solid oxide fuel cells. The presented information on in-depth analysis of the physical properties of barium zirconate from experimental and theoretical studies will guide aspirants in further conducting research in this eld near future. hopping method from one ion to the next, implies no restriction of proton conduction, and, therefore, makes very promising electrolytes for IT-SOFCs [38,43,44]. Researchers observed proton conduction at low temperatures (25-300 o C) owing to the advantages of low activation energy [43]. In contrast to classical oxygen-ion conductors, proton-conducting electrolytes allow complete utilization of hydrogen, produced from water at cathode, and avoid reducing the system's overall e ciency [43]. Among HTPC materials, BaCeO 3 (BCO), SrCeO 3 (SCO), BZO perovskites showed prominent potential. BCO and SCO exhibited poor stability and conductivity at high temperatures, whereas BZO showed excellent stability [48]. However, BZO exhibited lower conductivity due to high refractory nature, blocking nature of grain boundary, and poor sinterability. It is reported that the Y-doped barium zirconate (BZY) overcomes the poor sinterability and grain boundary resistance at intermediate temperatures [48]. In the current years, promising development on high-performance proton-conducting perovskites at low temperatures (<300 o C) or even room temperatures has been reported in the literature [43]. The low resistance of grain boundary and high conductivity results in developing stable thin lms at low temperatures. Park et al. fabricated nanosized 10% Y-doped barium zirconate (BaZr 0.9 Y 0.1 O 3-δ or BZY10) and investigated the effect of grain boundary on proton conductivity at low temperatures (30-400 o C) [49]. The result showed high proton conductivity at very low temperatures (>100 o C), which can be ascribed due to the absorbed water at the interfaces of nanoscale grain boundaries. The nanosized grain structure consists of nanograins connected with the interfacial hydrated layer, which acts as the proton conduction pathway rather than transporting through the bulk systems [50]. The co-doping strategy also put forward the development towards high proton conductivity at intermediate temperatures [51]. An ideal proton-conducting perovskite should possess high conductivity and excellent chemical stability to widen its applications. During doping, impurities may arise in the BZY structure, either Asite [52], B-site [53], or both [3,54]. Several research groups have investigated the dopant's effects on conductivity, stability, proton trapping ability at the operating conditions. Dawson et al. observed stable BZO at both H 2 O and CO 2 environments using DFT for Y and Sn co-doped BZO perovskites [55]. They also estimated the binding energy between dopants and protons to evaluate proton trapping ability in co-doped perovskites. Bjorheim et al. investigated dopant radium's effect on the entropy of the hydroxide group and oxygen vacancy formation [56]. These ndings suggest that the vibrational formation entropy became negative with the increasing ionic radius of the dopants. In the case of hydration enthalpy, it became more negative with decreasing the dopants' ionic radius. Loken et al. analyzed the hydration enthalpy for Na-,K-, Rb-or Cs-doped BZO and observed this phenomenon [57]. MD simulations allowed to investigate the proton trapping ability of doped BZO. Kitamuara et al. showed that Zn has more strong proton trapping ability compared to Y using MD simulations [58]. This phenomenon was hypothesized to occur due to the signi cant charges present in the dopants. A study conducted by Reddy et al. on the chemical stability of Y-or In-doped BCO-BZO solid systems and found In-doped systems were better stable at CO 2 and H 2 O atmosphere after long-term exposure [43]. Kang et al. carried out a similar investigation on BZO doped with K, Rb, or Cs at A-sites [59]. The carbonate formation reaction was investigated using the DFT method and observed that K-doped BZO showed the highest stability and proton conductivity among all [61,62]. The hopping or diffusion mechanism of the proton is depicted in Fig. [62]. Among all the proton-conducting perovskites, barium zirconate and barium cerate are the most prominent and widely studied oxides. The crystal structure of BZO, shorter and stronger Zr-O bonds, and the bond length distances between Zr-O and Ba-O made BZO more stable than Ce-O ones [47]. The BZY showed excellent performance in high bulk proton conductivity and thermo stability at the temperature range of 300-600 o C [43,63,64]. However, the overall proton conductivity is lower compared to cerates due to grain blocking effects [65]. These grain boundary blocking effects arise in BZY due to various factors such as impurities in precursors [66,67], dopants defects [68], poor sinterability [62], inappropriate processing temperatures [38,43], and chemical composition of grain boundary [69]. To widen the application areas of BZY perovskites, it is in great demand to overcome those drawbacks and enhance the proton conductivity and chemical stability for long term exposure at operating temperatures.