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、阳极和电解液组成。它们是固态设备，所有组件都是陶瓷，电解液是氧化物离子导体。它们工作在高温(通常约800摄氏度)下，电化学反应将燃料(如氢气、天然气、生物燃料)和空气在不燃烧的情况下转化为电能。它们代表着未来发电的领先方向，因为它们提供了比传统内燃机(~30%)更高的能量转换效率(&gt；60%)和更低的污染。不幸的是，如此高的运行温度是昂贵的，并带来了工程挑战，如SOFC的长期密封和耐久性。因此，SOFC社区内部有一种动力，即将运行温度降低到500-700摄氏度(所谓的中温SOFC，ITSOFC)，以克服工程挑战，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离子导电性的新电解液，这种电解液可以在500℃下同时暴露在燃料和空气中的恶劣操作条件下运行。稀土稳定的d-Bi2O_3(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离子导电性，但在还原条件下分解。ITSOFC的最高性能之一是利用基于两种氧化物离子导电陶瓷的电解液双层的概念实现的，这两种陶瓷分别是GdC和ESb。虽然GDC的电导率低于ESB，但在还原条件下化学稳定性较好。在这个设计中，GDC层被放置在大量还原的阳极(燃料)侧，以最大限度地减少ESB的分解，而ESB层被放置在阴极(空气)侧。这提供了一种具有高离子导电性的稳定电解液；然而，这种双层膜的制备需要薄膜沉积技术，这对于大规模生产来说是昂贵且不实用的。最近，我们发现在一种著名的钙钛矿(Na1/2Bi1/2TiO3，NBT；自然材料，在出版社)中具有高水平的氧离子导电性，并且化学掺杂镁以增加钛的氧空位浓度进一步提高了氧离子导电性。镁掺杂还有两个重要的优点：在550℃的还原燃料条件下，Mg-NBT具有化学稳定性；获得致密陶瓷的烧结温度为~950℃，与ESB和其他d-(Bi，RE)_2O_3电解液的烧结温度相似。流延成型是一种以低成本批量生产厚膜陶瓷的著名技术。由于GDC(~1350℃)和ESb(~900℃)陶瓷的烧结温度相差较大，采用流延共烧的方法制备GDC/ESb双层膜是不可能的，但对于掺杂NBT/ESb陶瓷来说，这是可能的。首先，优化一种新发现的基于极性钙钛矿型NBT的氧化物离子导体的电解液性能。第二，测试NBT基材料作为双层电解液的ITSOFC的电解液成分的适用性。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NBT陶瓷的结晶学、微结构和电学特性进行研究来实现。这将提供对这一材料家族中氧化物离子导电性的结构-性质-组成关系的全面了解。为了实现第二个目标，我们将在&lt；1000℃的温度下，将掺杂的NBT和d-(Bi，RE)_2O_3共烧成流延层，并在ITSOFC所要求的条件下测试它们的电学和化学性能。这将为这些材料在以工业标准流延技术制备的双层电解质为基础的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.