Fuel cells for electrochemical energy conversion

This short article provides an overview of fuel cell science and technology. This article is intended to act as a “primer” on fuel cells that one can use to begin a deeper investigation into this fascinating and promising technology. You will learn what fuel cell are, how they work, and what significant advantages and disadvantages they present

Fuel cells for electrochemical energy conversion

This short article provides an overview of fuel cell science and technology. This article is intended to act as a “primer” on fuel cells that one can use to begin a deeper investigation into this fascinating and promising technology. You will learn what fuel cell are, how they work, and what significant advantages and disadvantages they present

Nanoparticles at Grain Boundaries Inhibit the Phase Transformation of Perovskite Membrane

The high-energy nature of grain boundaries makes them a common source of undesirable phase transformations in polycrystalline materials. In both metals and ceramics, such grain-boundary-induced phase transformation can be a frequent cause of performance degradation. Here, we identify a new stabilization mechanism that involves inhibiting phase transformations of perovskite materials by deliberately introducing nanoparticles at the grain boundaries. The nanoparticles act as “roadblocks” that limit the diffusion of metal ions along the grain boundaries and inhibit heterogeneous nucleation and new phase formation. Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ, a high-performance oxygen permeation and fuel cell cathode material whose commercial application has so far been impeded by phase instability, is used as an example to illustrate the inhibition action of nanoparticles toward the phase transformation. We obtain stable oxygen permeation flux at 600 °C with an unprecedented 10–1000 times increase in performance compared to previous investigations. This grain boundary stabilization method could potentially be extended to other systems that suffer from performance degradation due to a grain-boundary-initiated heterogeneous nucleation phase transformations

Intrinsic Material Properties Dictating Oxygen Vacancy Formation Energetics in Metal Oxides

Oxygen vacancies (<i>V</i>_O) in oxides are extensively used to manipulate vital material properties. Although methods to predict defect formation energies have advanced significantly, an understanding of the intrinsic material properties that govern defect energetics lags. We use first-principles calculations to study the connection between intrinsic (bulk) material properties and the energy to form a single, charge neutral oxygen vacancy (<i>E</i>_V). We investigate 45 binary and ternary oxides and find that a simple model which combines (i) the oxide enthalpy of formation (Δ<i>H</i>_f), (ii) the midgap energy relative to the O 2p band center (<i>E</i>_O 2p + (1/2)<i>E</i>_g), and (iii) atomic electronegativities reproduces calculated <i>E</i>_V within ∼0.2 eV. This result provides both valuable insights into the key properties influencing <i>E</i>_V and a direct method to predict <i>E</i>_V. We then predict the <i>E</i>_V of ∼1800 oxides and validate the predictive nature of our approach against direct defect calculations for a subset of 18 randomly selected materials

Probing Grain-Boundary Chemistry and Electronic Structure in Proton-Conducting Oxides by Atom Probe Tomography

A laser-assisted atom-probe-tomographic (LAAPT) method has been developed and applied to measure and characterize the three-dimensional atomic and electronic nanostructure at an yttrium-doped barium zirconate (BaZr_0.9Y_0.1O_3−δ, BZY10) grain boundary. Proton-conducting perovskites, such as BZY10, are attracting intense interest for a variety of energy conversion applications. However, their implementation has been hindered, in part, because of high grain-boundary (GB) resistance that is attributed to a positive GB space-charge layer (SCL). In this study, LAAPT is used to analyze BZY10 GB chemistry in three dimensions with subnanometer resolution. From this analysis, maps of the charge density and electrostatic potential arising at the GBs are derived, revealing for the first time direct chemical evidence that a positive SCL indeed exists at these GBs. These maps reveal new insights on the inhomogeneity of the SCL region and produce an average GB potential barrier of approximately 580 mV, agreeing with previous indirect electrochemical measurements

Porous nanocrystalline TiO2 with high lithium-ion insertion performance

Porous nanocrystalline anatase TiO2 was prepared by a modified hydrolytic route coupled with an intermediary amorphization/recrystallization process. The phase structure and morphology of the products were analyzed by X-ray diffraction, transmission electron microscopy, and field-emission scanning electron microscopy. The electrochemical properties were investigated by cyclic voltammetry, constant current discharge-charge tests, and electrochemical impedance techniques. Applied as an anode in a lithium-ion battery, the material exhibited excellent specific capacities of 130 mAh g-1 (at the rate of 2000 mA g-1) and 96 mAh g-1 (at the rate of 4000 mA g-1) after 100 cycles; the coulombic efficiency was ~99.5 %, indicating excellent rate capability and reversibility. Furthermore, the electrochemical impedance spectra showed improved electrode kinetics after cycling. These results indicate that the porous nanocrystalline TiO2 synthesized by this improved synthesis route might be a promising anode material for high energy and high power density lithium-ion battery applications. © 2012 Springer Science+Business Media New York

Facile synthesis of nanocrystalline TiO2 mesoporous microspheres for lithium-ion batteries

TiO2 mesoporous nanocrystalline microspheres assembled from uniform nanoparticles were synthesized by a facile and template-free hydrolytic precipitation route in normal solvent media. The phase structure, morphology, and pore nature were analyzed by X-ray diffraction, transmission electron microscopy, field-emission scanning electron microscopy, and BET measurements. The electrochemical properties were investigated by cyclic voltammetry, constant current discharge-charge tests, and electrochemical impedance techniques. Microspheres with diameters ranging from 0.2 to 1.0 µm were assembled by aggregation of nanosized TiO2 crystallites (~8-15 nm) and yielded a typical type-IV BET isotherm curve with a surface area of ~116.9 m 2 g-1 and a pore size of ~5.4 nm. A simplified model was proposed to demonstrate the nanoparticle packing modes to form the mesoporous structure. The initial discharge capacity reached 265 mAh g -1 at a rate of 0.06 C and 234 mAh g-1 at a rate of 0.12 C. The samples demonstrated high rate capacity of 175 mAh g-1 at 0.6 C and 151 mAh g-1 at 1.2 C even after 50 cycles, and the Coulombic efficiency was approximately 99%, indicating excellent cycling stability and reversibility. Details of the kinetic process of the nanocrystalline mesoporous microspheres electrode reaction from electrochemical impedance spectra provided further insights into the possible mechanisms responsible for the good reversibility and stability. These investigations indicate that TiO2 nanocrystalline mesoporous microspheres might be a promising anode material for high-energy density lithium-ion batteries

Synthesis by spark plasma sintering of a novel protonic/electronic conductor composite: BaCe_0.2Zr_0.7Y_0.1O_3−δ /Sr_0.95Ti_0.9Nb_0.1O_3−δ (BCZY27/STN95)

A novel two-phase ceramic composite (cercer) material consisting of a solid solution of barium cerate and -zirconate doped with yttrium (BaCe0.2Zr0.7Y0.1O3-delta : BCZY27), together with niobium-doped strontium titanate (Sr0.95Ti0.9Nb0.1O3-delta : STN95), has been synthesized by solid-state reaction and sintered conventionally (CS) at 1350-1500 A degrees C, as well as by spark plasma sintering (SPS) at 1300-1350 A degrees C. CS samples were porous and exhibited high degrees of inter-phase reaction. Nickel oxide sintering aids did not improve CS sample density. In contrast, samples made by SPS were significantly denser (> 95 %) and showed less reaction between phases. A pseudo-optimum SPS profile was developed, accounting for the effects of thermal expansion mismatch between BCZY27 and STN95. X-ray diffraction indicated secondary phases exist, but there was no indication of their presence at grain boundaries based on thorough study of these regions with high-resolution transmission electron microscopy and selective area electron diffraction. We thus suggest that these phases are present as independent grains in the bulk. It is believed these secondary phases inhibit electronic conductivity in the composite