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

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.

The development of low carbon economies with deep cuts in carbon dioxide (CO2) emissions to mitigate possible global warming effects while still meeting energy demands is a global challenge. It is imperative the UK develops energy solutions that are affordable, secure and sustainable. Solid Oxide Fuel Cells (SOFCs) offer one such technology with many economic and societal impacts to a wide range of stakeholders, as outlined below.

(i) Fuel Cell Manufacturers. The worldwide market for SOFCs is forecasted to reach $530M at a compounded annual growth rate of 6.9% from 2011 to 2016. This growth is expected as a result of improved SOFC efficiency and their cost effective nature (see below). Developments in technology associated with improved materials (eg. new or improved solid electrolytes such as NBT) and cell designs (eg. the bilayer electrolytes in this proposal) to facilitate lower operating temperatures are also helping growth. The lower operating temperatures have numerous attractive advantages pertaining to cost, sealing and degradation and are of interest to UK companies involved with SOFC development such as Ceres Power and Roll-Royce Fuel Cells Systems.

(ii) Business/Industry/Public Sector. SOFCs generate clean, reliable power at location (i.e. it is a distributed power generation source as opposed to a centralised 'grid-based' source) with minimal environmental impact, making it one of the most sustainable solutions available. Their higher energy conversion efficiency and fuel adaptability compared to conventional combustion engines ensures significant saving on fuel costs. As SOFCs generate power on-site, this eliminates the cost, complexity, interdependencies, and inefficiencies associated with transmission and distribution (T&D) associated with an ageing grid. For example, ~7-10% of power loss occurs in the developed world due to T&D, and outages associated with natural disasters, eg the tsunami in Tohoku (2011) cause severe disruption to business operations that hurt productivity. Secure and uninterrupted electrical power is vital for companies that operate continuous manufacturing processes (eg The Coca Cola Company) and large data centres (eg e-Bay and Apple). In Sept 2013, e-Bay switched on a data centre in Utah that is primarily powered by 30 SOFC Servers (each supplying 200 kW) running on natural gas and uses the local power grid as a back-up. SOFCs therefore offer improved energy security and both a cleaner and clearer path to energy independence. Investment in SOFCs is proving to be an increasingly attractive and viable business proposition for many companies and banks, especially as market confidence grows in SOFC reliability and in the return on investment.

(iii) General Public and the Environment. SOFCs offer substantial environmental benefits when compared to many other sources of electrical power and are an attractive option to the general public. For example, when powered by natural gas SOFCs release a fraction (typically ~ 33 %) of the CO2 produced by coal powered plants thus significantly reducing emissions of this greenhouse gas. As SOFCs convert fuel into electricity via an electrochemical reaction rather than combustion they virtually eliminate smog forming particulates and harmful NOx and SOx gases emitted by conventional power plants. Such emissions cause smog and can harm human health, eg. respiratory diseases such as asthma and bronchitis. Large centralised power plants such as gas fired, wind, solar and hydropower all require large land areas and in many cases create consider levels of noise pollution as well as disruption to wildlife. For all these environmental reasons, the general public do not want to live near such power plants and most are built far from urban areas. In contrast, SOFCs are well-suited for urban environments as they have low emissions, little noise-pollution and require much less space. As they are located onsite, they also eliminate the cost of T&D losses