B.C.H. Steele

Materials for fuel-cell technologies

Fuel cells convert chemical energy directly into electrical energy with high efficiency and low emission of pollutants. However, before fuel-cell technology can gain a significant share of the electrical power market, important issues have to be addressed. These issues include optimal choice of fuel, and the development of alternative materials in the fuel-cell stack. Present fuel-cell prototypes often use materials selected more than 25 years ago. Commercialization aspects, including cost and durability, have revealed inadequacies in some of these materials. Here we summarize recent progress in the search and development of innovative alternative materials.

Oxygen transport in selected nonstoichiometric perovskite-structure oxides

New results on oxygen self-diffusion (D∗), surface exchange coefficient (k) and activation energy (Ea) of oxygen self-diffusion for chosen compositions of manganite and cobaltite perovskites illustrate the effect of doping in the A and B sites of the ABO3 structure. The electrical conductivity (σT) was measured for the manganite group and permeability (J) was determined for the cobaltite perovskites. D∗ increases with increasing x in La1−x(Sr,Ca)x(Mn,Co)O3−δ due to formation of new oxyge vacancies by introducing metals of lower valency (M2+) into the A3+ sites. A substitution of M2+ for B3+ or a reduction of the metal in the B3+ site to a lower positive valency also increases D∗ . D∗ cobaltites is significantly higher than that of the manganites (by 4–6 orders of magnitude), however, the potentially high oxygen fluxes that would be allowed through the materials by the high D∗ values seem to be limited by the surface exchange kinetics. Ea-values of the manganites are considerably higher than those the cobaltites. In general, the electrical conductivity, σT, decreased on doping the B site of the manganites with Co and Ni. However, whilst the pure manganite material exhibits a metallic type of conduction (i.e. σT decreased with increasing T), the conduction mechanism in the Co-doped and Ni-doped manganites changed to a localized hopping of charge carriers between the Mn3+ and Mn4+ sites (σT increases with increasing T).

Fuel-cell technology: Running on natural gas

Although hydrogen is the fuel of choice for many energy-conversion systems, its widespread use is limited by its cost. Some fuel cells can use natural gas (methane), but require high operating temperatures to process the methane internally. New intermediate-temperature fuel cells that can oxidize methane directly are a promising alternative.

Oxygen ion conductors and their technological applications

Abstract The magnitudes of the oxygen fluxes through ceramic oxides incorporated in a variety of technological devices and systems are briefly surveyed, and attention is drawn to the importance of the kinetics of the associated surface exchange process. For most applications, oxygen ion conductivities greater than 10 −1 S cm −1 are required at intermediate temperatures (600–800 °C). The strategies adopted to design materials with appropriate oxygen ion conductivity values are summarized with special reference to the fluorite, perovskite and related oxide structures. Finally oxygen ion order-disorder processes in highly defective perovskite (ABO 3 ) and brownmillerite (ABO 2.5 ) mixed conducting oxides are briefly reviewed, with emphasis upon the role of isotopic exchange-diffusion profile techniques to separate the contributions of the various charge carriers to the total conductivity.

Survey of materials selection for ceramic fuel cells : II. Cathodes and anodes

Abstract By assigning target values (0.15 Ω cm2) for the performance of the individual components in a single SOFC cell it has been possible to propose a very simple model which relates experimentally accessible parameters, such as, oxygen surface exchange coefficients (k cm · s−1) and self diffusion coefficients ( D ∗ cm 2 · s −1 ) to the behaviour of porous oxide cathodes. Characteristic lengths including a collection length (lc), and a maximum length (ll) for bulk diffusion of oxygen ions, have been defined and calculated for La0.8Sr0.2O3 + x, La0.8Sr0.2Co0.2Fe0.8O3 − x, Sm0.6Ca0.4CoO3 − x. These characteristic lengths have been used to interpret the reported behaviour of the oxide electrocalalysts and to suggest modifications which could improve their performance. Although a similar model has not yet been developed to interpret the behaviour of anodes, appropriate comments have been made about the selection and optimisation of anodes for intermediate temperature (500–750 °C) SOFC operation.

Interfacial reactions associated with ceramic ion transport membranes

Abstract Assuming ohmic behaviour for the relevant interfacial kinetics a simple equivalent circuit has been used to identify experimentally accessible parameters which may control the oxygen flux through a variety of technological devices. In particular the oxygen surface exchange coefficient ( k cm s −1 ), which can be determined by isotopic exchange measurements is proportional to a characteristic electrode current density ( j E A cm −2 ) which determines the electrode resistance ( R E Ωcm 2 ) in solid-state electrochemical systems. For ceramic ion-conducting membranes a characteristic membrane thickness ( L c ) at which the change-over from bulk to surface control occurs is shown to be equal to D * /k where D * (cm 2 s −1 ) is the oxygen self-diffusion coefficient in the oxide material. Attention is also drawn to correlations between D and k . It is noted, for example, that the ratio D/k often has a value around 10 −2 cm (100 μm) for most AO 2 fluorite and ABO 3 perovskite oxide materials, which implies that fabricating membranes less than 100 μm thick will not be advantageous unless the value of k can be specifically increased. Mechanisms responsible for correlations between D and k remain obscure and should be a fruitful area for further investigations. Finally, specific examples of materials selection for ceramic fuel cell operation over a wide range of temperatures (450–;1000 °C) are briefly surveyed.

Poly(ethylene oxide) electrolytes for operation at near room temperature

In an effort to improve the room temperature ionic conductivity of poly(ethylene oxide) doped with lithium salts, the effects of various additives were examined. Partial substitution of the high polymer by a poly(ethylene oxide) of low molecular weight had the effect of increasing the solubility of the crystalline polymer/salt complex in the liquid polymer as well as lowering the effective melting and glass transition temperatures of the polymer. An optimized composition had a much improved conductivity of 10−4 (Ω cm)−1 at 40 °C.

Oxygen transport and exchange in oxide ceramics

Abstract Oxygen transport in most oxide ceramics incorporated in solid oxide fuel cells (SOFC) involves the movement of oxygen ion vacancies. It is the relative magnitude of oxygen ion vacancy and electronic charge carrier concentrations and mobilities which determines whether oxide materials can function as effective electrolyte or electrode components. Examination of relevant data suggests that zirconia- and ceria-based electrolytes are unlikely to be replaced in SOFC systems operating in the temperature range 450–950 °C. Oxygen ion vacancies are also involved in the cathodic reduction of oxygen and influence the magnitude of the associated exchange current density which can be measured by isotopic oxygen exchange measurements. Oxygen vacancy concentrations are also implicated in thermal expansion coefficient values and chemical stability considerations. It follows that optimisation of the cathode composition requires many conflicting requirements to be satisfied. However for operation at 800 °C, electrolyte, electrode and bipolar plate materials are available to ensure power densities approaching 0.5 W cm −2 . In contrast, direct methanol SOFC systems operating at 500 °C necessitate the development of alternative electrode materials. The successful exploitation of our knowledge about oxygen ion vacancy transport in ceramic oxides has now stimulated research into the role of protons in oxide lattices, and it is postulated that protonic/hydroxyl ion transport could be important in the development of alternative anode components.

Oxidation of methane in solid state electrochemical reactors

Abstract Preliminary experiments are reported on the complete oxidation of CH 4 to CO 2 and H 2 O in ceramic electrochemical reactors (CER). Potentiostatic and cyclic voltammetric studies have demonstrated that platinum anodes are poor electrocatalysts for the oxidation of CH 4 at 800°C. However selected oxide anode materials are much more effective electro-catalysts and able to convert CH 4 to CO 2 and H 2 O with reasonable efficiencies at potentials appropriate for fuel cell operation. Other oxide anodes can be used in a CER to promote partial oxidation reactions and preliminary results for the conversion of CH 4 to C 2 H 6 are described using Bi 2 O 3 Pr 6 O 11 anodes.

Development of a novel SIMS technique for oxygen self-diffusion and surface exchange coefficient measurements in oxides of high diffusivity

Abstract The determination of D∗ and k provide key information about technologically important oxides in applications such as electrode and electrolyte materials for high-temperature electrochemical devices, e.g., the solid oxide fuel cell. A high oxygen self-diffusion coefficient, D∗, of approximately 10−6cm2/s and surface exchange coefficient, k, of approximately 10−4 cm/s are typical requirements for these applications. Measurement of D∗ and k may be performed by the isotopic exchange/diffusion profile technique with secondary ion mass spectrometry (SIMS) used to determine the O18 stable isotope depth distribution. In the case of oxides of high diffusivity the penetration depth at the chosen anneal temperature, approximately (D∗t)0.5, is of the order of hundreds of micrometers from the surface into the bulk of the sample for the shortest practicable anneal times, t. SIMS depth profiles are generally limited to tens of micrometers due to various considerations including the time required for sputtering and roughening at the base of the SIMS crater. Thus the sputter depth profiling approach must be abandoned in favour of a new SIMS technique described in this paper. Crater base roughening is particularly severe for polycrystalline bulk samples which also have a high defect density. Results from polycrystalline cobaltite perovskite solid solutions and YBCO single crystals are used to demonstrate the technique and precautions required for its successful application.