Switching on electrocatalytic activity in solid oxide cells

Solid oxide cells (SOCs) can operate with high efficiency in two ways - as fuel cells, oxidizing a fuel to produce electricity, and as electrolysis cells, electrolysing water to produce hydrogen and oxygen gases. Ideally, SOCs should perform well, be durable and be inexpensive, but there are often competitive tensions, meaning that, for example, performance is achieved at the expense of durability. SOCs consist of porous electrodes - the fuel and air electrodes - separated by a dense electrolyte. In terms of the electrodes, the greatest challenge is to deliver high, long-lasting electrocatalytic activity while ensuring cost- and time-efficient manufacture. This has typically been achieved through lengthy and intricate ex situ procedures. These often require dedicated precursors and equipment; moreover, although the degradation of such electrodes associated with their reversible operation can be mitigated, they are susceptible to many other forms of degradation. An alternative is to grow appropriate electrode nanoarchitectures under operationally relevant conditions, for example, via redox exsolution. Here we describe the growth of a finely dispersed array of anchored metal nanoparticles on an oxide electrode through electrochemical poling of a SOC at 2 volts for a few seconds. These electrode structures perform well as both fuel cells and electrolysis cells (for example, at 900 °C they deliver 2 watts per square centimetre of power in humidified hydrogen gas, and a current of 2.75 amps per square centimetre at 1.3 volts in 50% water/nitrogen gas). The nanostructures and corresponding electrochemical activity do not degrade in 150 hours of testing. These results not only prove that in operando methods can yield emergent nanomaterials, which in turn deliver exceptional performance, but also offer proof of concept that electrolysis and fuel cells can be unified in a single, high-performance, versatile and easily manufactured device. This opens up the possibility of simple, almost instantaneous production of highly active nanostructures for reinvigorating SOCs during operation

Study of the order-disorder transition in yttria-stabilised zirconia by neutron diffraction

A comprehensive study of 8 mol% yttria-stabilised zirconia has been made between 150 and 1000 degrees C, using ac impedance spectroscopy and high-temperature neutron powder diffraction. It has been demonstrated that the conductivity anomaly, which occurs at ca. 650 degrees C, is structural in origin. A sharp decrease in the activation energy for conduction of ca. 0.2 eV was observed at ca. 650 degrees C. Additional broad, diffuse scattering peaks were observed below 600 degrees C in the neutron diffraction patterns; above 650 degrees C, the diffuse scattering peaks disappeared. A deviation from linearity was observed at a similar temperature in the plots of both Y/Zr and O isotropic temperature factors vs. temperature. The low-temperature behaviour can be explained in terms of ordering of oxygen vacancy-(dopant) cation clusters to form microdomains, which are evidenced by the presence of diffuse scattering peaks. At high temperature, the association of vacancies with defects breaks down, or at least becomes randomised, allowing vacancies to move more freely as indicated by the decrease in activation energy for conduction. A discontinuity in thermal expansion coefficient (from neutron diffraction data) confirms the second-order nature of the transition.</p

Optimisation of perovskite materials for fuel electrodes

Excellent thermal and mechanical stability, physical compatibility with electrolyte materials and relatively low cost have attracted interest in the application of perovskite oxide materials as fuel electrodes in SOFC designs. The present fuel electrode of choice is a nickel/YSZ cermet; the nickel acts as a fuel oxidation catalyst and provides electronic conductivity, whilst the YSZ provides oxygen ion conductivity. The challenge is to develop a single phase oxide material which provides all of the above and at the same time reduces the problems of coking, nickel sintering and sulphur poisoning sometimes encountered with the nickel/YSZ cermet. A rationale for the application of perovskites as fuel electrodes is given and the defect chemistry of perovskites is then reviewed. Attention is given to reducible transition metal ions, doping, non-stoichiometry and the effects these have on chemical stability and both the magnitude and mechanism of conduction. Findings are summarised and the need for an optimal doping strategy is identified. We report a nickel-free solid oxide fuel cell (SOFC) anode, La0.75Sr0.25Cr0.5Mn0.5O3, with comparable electrochemical performance to Ni/YSZ cermets. The electrode polarisation resistance approaches 0.2ohmcm(2) at 900degreesC in 97%H-2/3%H2O. Very good performance is achieved for methane oxidation without using excess steam. The anode is stable in both fuel and air conditions and shows stable electrode performance in methane. Thus both redox stability and operation in low steam hydrocarbons have been demonstrated, overcoming two of the major limitations of the current generation of nickel zirconia cermet SOFC anodes.</p