A new family of electrolytes based on Na1/2Bi1/2TiO3 for intermediate-temperature solid oxide fuel cells

固体氧化物燃料电池（SOFC）是一种类似于电池的电化学装置，它由阴极、阳极和电解质组成。它们是固态器件，所有组件都是陶瓷，电解质是氧化离子导体。它们在高温下工作（通常为~800℃），电化学反应将燃料（如H2、天然气、生物燃料）和空气转化为电能而不燃烧。它们代表了未来发电的主要方向，因为它们比传统内燃机（~30%）提供更高的能量转换效率（约60%）和更低的污染。不幸的是，如此高的工作温度成本高昂，并带来了sofc长期密封和耐用性等工程挑战。因此，SOFC社区内部有一种将工作温度降低到500-700℃（所谓的中温SOFC， itsofc）的动力，以克服工程挑战并降低生产清洁，可靠和负担得起的能源的成本。这就需要开发具有高氧化离子电导率的新型电解质，这种电解质可以在同时暴露于燃料和空气的恶劣操作条件下运行，温度为100 - 500℃。稀土（RE）稳定的d-Bi2O3（如RE=Er， ESB）是优良的固体电解质，在500-700℃的空气中具有足够高的氧化离子电导率，但在还原条件下会分解。利用基于两种氧化离子导电陶瓷——钆掺杂二氧化铈（GDC）和ESB的电解质双分子层的概念，ITSOFC的最高性能之一已经实现。虽然GDC的电导率低于ESB，但在还原条件下是化学稳定的。在这个设计中，GDC层被放置在严重还原的阳极（燃料）一侧，以尽量减少ESB的分解，而ESB层被放置在阴极（空气）一侧。这提供了具有高离子电导率的稳定电解质；然而，这种双分子层的制备需要薄膜沉积技术，这种技术成本高且不适合大规模生产。最近，我们在一种著名的钙钛矿（Na1/2Bi1/2TiO3， NBT; Nature Materials, in press）中发现了高水平的氧化离子电导率，并且通过化学掺杂Mg代替Ti来增加氧空位的浓度进一步增强了氧化离子电导率。mg掺杂还有另外两个重要的优点：Mg-NBT在550℃的还原性（燃料）条件下化学稳定，烧结温度为~950C，得到的致密陶瓷与ESB和其他d-(Bi，RE)2O3电解质相似。带状铸造是一种众所周知的低成本大批量生产厚膜陶瓷的技术。由于GDC （~1350C）和ESB （~900C）陶瓷的烧结温度差异较大，不可能采用带铸再共烧结的方法制备GDC/ESB双层陶瓷；然而，对于掺杂了nbt /ESB的陶瓷来说，这应该是可能的。这个项目的目的是双重的。首先，优化新发现的基于极性钙钛矿NBT的氧化离子导体家族的电解质特性。其次，测试nbt基材料作为电解质组分在基于双层电解质的itsofc中的适用性。第一个目标将通过对NBT进行系统的化学掺杂研究，然后对掺杂的NBT陶瓷进行晶体学、微观结构和电学表征。这将提供对这类材料中氧化离子电导率的结构-性能-组成关系的全面理解。为了实现第二个目标，电解质双层陶瓷将在< 1000℃的温度下通过掺杂nbt和d-(Bi，RE)2O3共烧结带铸层来生产，并在itsofc所需的条件下测试其电学和化学性能。这将为这些材料在基于工业标准带铸造技术制备的双层电解质的itsofc中提供概念验证应用。

A Solid Oxide Fuel Cell (SOFC) is an electrochemical device similar to a battery in that it consists of a cathode, anode and an electrolyte. They are solid-state devices where all components are ceramics and the electrolyte is an oxide-ion conductor. They operate at high temperatures (typically ~800C) where an electrochemical reaction converts fuel (eg H2, natural gas, biofuels) and air into electricity without combustion. They represent a leading direction for future power generation as they offer higher energy conversion efficiency (>60%) than conventional combustion engines (~30%) and lower pollution. Unfortunately, such high operating temperatures are costly and create engineering challenges such as long term sealing and durability of SOFCs. As a consequence, there is a drive within the SOFC community to reduce the operating temperatures to 500-700C (so called Intermediate Temperature SOFCs, ITSOFCs) to overcome the engineering challenges and reduce costs to produce clean, reliable and affordable energy. This requires the development of new electrolytes with high oxide-ion conductivity that can operate under the harsh operating conditions of simultaneous exposure to fuel and air at >500C. Rare earth (RE) stabilised d-Bi2O3 (eg RE=Er, ESB) are excellent solid electrolytes and offer sufficiently high oxide-ion conductivity at 500-700C in air but decompose under reducing conditions. One of the highest performances of an ITSOFC has been achieved using the concept of an electrolyte bilayer based on two oxide-ion conducting ceramics, Gadolinia-doped ceria (GDC) and ESB. Although the conductivity of GDC is lower than ESB it is chemically stable under reducing conditions. In this design, the GDC layer is placed at the heavily reducing anode (fuel) side to minimise the decomposition of ESB and the ESB layer is placed at the cathode (air) side. This provides a stable electrolyte with high ionic conductivity; however, preparation of such a bilayer requires a thin film deposition technique which is costly and impractical for mass production.Recently, we discovered high levels of oxide-ion conductivity in a well-known perovskite (Na1/2Bi1/2TiO3, NBT; Nature Materials, in press) and that chemical doping of Mg for Ti to increase the concentration of oxygen vacancies further enhanced the oxide-ion conductivity. Mg-doping has two other important advantages: Mg-NBT is chemically stable under reducing (fuel) conditions at 550 C and the sintering temperature of ~950C to obtain dense ceramics is similar to that of ESB and other d-(Bi,RE)2O3 electrolytes. Tape casting is a well-known technique for mass production of thick film ceramics at low cost. It is not possible to prepare GDC/ESB bilayers by tape casting followed by co-sintering due to the large difference in sintering temperature for GDC (~1350C) and ESB (~900C) ceramics; however, this should be possible for doped-NBT/ESB ceramics.The aims of this project are two-fold. First, to optimise the electrolyte properties of a newly discovered family of oxide-ion conductors based on the polar perovskite NBT. Second, to test the suitability of NBT-based materials as an electrolyte component in ITSOFCs based on bilayer electrolytes. The first aim will be achieved by undertaking systematic chemical doping studies of NBT followed by crystallographic, microstructural and electrical characterisation of doped-NBT ceramics. This will provide a comprehensive understanding of the structure-property-composition relationships of oxide-ion conductivity in this family of materials. To achieve the second aim, electrolyte bilayer ceramics will be produced by co-sintering tape-cast layers of doped-NBT and d-(Bi,RE)2O3 at temperatures < 1000C and their electrical and chemical performance tested under the conditions required for ITSOFCs. This will provide a proof-of-concept application of these materials in ITSOFCs based on bilayer electrolytes prepared by industry-standard tape casting technology.