Ryan O’Hayre

Readily processed protonic ceramic fuel cells with high performance at low temperatures

Cooler ceramic fuel cells Ceramic ion conductors can be used as electrolytes in fuel cells using natural gas. One drawback of such solid-oxide fuel cells that conduct oxygen ions is their high operating temperatures (at least 600°C). Duan et al. have made a proton-conducting ceramic fuel cell with a modified cathode material that exhibits high performance on methane fuel at 500°C (see the Perspective by Gorte). Science, this issue p. 1321; see also p. 1290 A proton-conduction cathode and simpler fabrication enable lower-temperature operation of methane-fueled ceramic fuel cells. [Also see Perspective by Gorte] Because of the generally lower activation energy associated with proton conduction in oxides compared to oxygen ion conduction, protonic ceramic fuel cells (PCFCs) should be able to operate at lower temperatures than solid oxide fuel cells (250° to 550°C versus ≥600°C) on hydrogen and hydrocarbon fuels if fabrication challenges and suitable cathodes can be developed. We fabricated the complete sandwich structure of PCFCs directly from raw precursor oxides with only one moderate-temperature processing step through the use of sintering agents such as copper oxide. We also developed a proton-, oxygen-ion–, and electron-hole–conducting PCFC-compatible cathode material, BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY0.1), that greatly improved oxygen reduction reaction kinetics at intermediate to low temperatures. We demonstrated high performance from five different types of PCFC button cells without degradation after 1400 hours. Power densities as high as 455 milliwatts per square centimeter at 500°C on H2 and 142 milliwatts per square centimeter on CH4 were achieved, and operation was possible even at 350°C.

Ionic and electronic impedance imaging using atomic force microscopy

Localized alternating current (ac) impedance measurements are acquired directly through a conductive atomic force microscope (AFM) tip. Both a spectroscopy mode (where full impedance spectra are obtained at fixed locations on a sample surface) and an imaging mode (where single frequency impedance maps are acquired across a sample) are used to characterize Au/Si3N4 test structures, ZnO varistors, and Nafion membrane (an ion conductor). Both modulus and phase information are acquired simultaneously. The use of an ac technique permits the study of electrochemical systems and ion conductors in addition to electronic systems. The capabilities and limitations of the AFM impedance imaging technique are discussed in detail.

Quantitative impedance measurement using atomic force microscopy

Obtaining quantitative electrical information with scanning probe microscopy techniques poses a significant challenge since the nature of the probe/sample contact is frequently unkown. For example, obtaining quantitative kinetic data from the recently developed atomic force microscopy (AFM) impedance technique requires normalization by the probe/sample contact area. In this paper, a methodology is proposed that enables the extraction of quantitative information from the AFM impedance technique. This methodology applies results from nanoindentation experiments and contact mechanics theory to characterize AFM probe contacts. Using these results, probe/sample contact forces (which can be accurately measured in the AFM) may be converted into probe/sample contact area estimates. These contact area estimates, when included in model of the probe/sample contact, enable the extraction of quantitative data. This methodology is applied to the recently developed AFM impedance measurement technique, enabling a quantit...

The Air'Platinum'Nafion Triple-Phase Boundary: Characteristics, Scaling, and Implications for Fuel Cells

This contribution examines the heterogeneous kinetics of the oxygen reduction reaction (ORR) at the Pt/Nafion/air triple-phase boundary (TPB). This system is of particular interest for low-temperature polymer electrolyte membrane fuel cell applications. A focused ion beam system is used to prototype geometrically simple platinum microstructures directly on Nafion electrolyte membranes. By varying the size and shape of the platinum structures, the role and properties of the TPB are elucidated. Current-voltage and electrochemical impedance spectroscopy measurements reveal that the ORR kinetics scale with TPB length. A faradaic resistance per unit TPB length of 6 × 10 9 Ω μm is extracted under short-circuit conditions at room temperature. Although this value is determined from microscopic measurements of geometrically simple platinum structures, it is successfully applied to predict the bulk performance of large-area sputtered platinum catalyst fuel cells.

Non-equilibrium deposition of phase pure Cu2O thin films at reduced growth temperature

Cuprous oxide (Cu2O) is actively studied as a prototypical material for energy conversion and electronic applications. Here we reduce the growth temperature of phase pure Cu2O thin films to 300 °C by intentionally controlling solely the kinetic parameter (total chamber pressure, Ptot) at fixed thermodynamic condition (0.25 mTorr pO2). A strong non-monotonic effect of Ptot on Cu-O phase formation is found using high-throughput combinatorial-pulsed laser deposition. This discovery creates new opportunities for the growth of Cu2O devices with low thermal budget and illustrates the importance of kinetic effects for the synthesis of metastable materials with useful properties.

Electrochemical nanopatterning of Ag on solid-state ionic conductor RbAg4I5 using atomic force microscopy

This report introduces an electrochemical nanopatterning technique performed under ambient conditions without involving a liquid vessel or probe-to-sample material transfer. Patterning is accomplished by solid-state electrochemical nanodeposition of Ag clusters on the surface of the solid ionic conductor RbAg4I5 using an atomic force microscopy probe. Application of negative voltage pulses on the probe relative to an Ag film counter electrode on an RbAg4I5 sample induces nanometer-sized Ag deposition on the ion conductor around the probe. The patterned Ag particles are 0.5–70nm high and 20–700nm in diameter. The effect of the amplitude and duration of bias voltage on the size and shape of deposited Ag clusters is also shown.

Intrinsic Material Properties Dictating Oxygen Vacancy Formation Energetics in Metal Oxides

Oxygen vacancies (V(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 (E(V)). We investigate 45 binary and ternary oxides and find that a simple model which combines (i) the oxide enthalpy of formation (ΔH(f)), (ii) the midgap energy relative to the O 2p band center (E(O 2p) + (1/2)E(g)), and (iii) atomic electronegativities reproduces calculated E(V) within ∼0.2 eV. This result provides both valuable insights into the key properties influencing E(V) and a direct method to predict E(V). We then predict the E(V) of ∼1800 oxides and validate the predictive nature of our approach against direct defect calculations for a subset of 18 randomly selected materials.

Facile single-step preparation of Pt/N-graphene catalysts with improved methanol electrooxidation activity

In this work, we describe a facile single-step approach for the simultaneous reduction of graphene oxide to graphene, functional doping of graphene with nitrogen, and loading of the doped graphene with well-dispersed platinum (Pt) nanoparticles using a solvent mixture of ethylene glycol and N-methyl-2-pyrrolidone. The as-prepared Pt/nitrogen-doped graphene (N-graphene) catalysts are characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy while the electrocatalytic methanol oxidation properties of the catalysts are evaluated by cyclic voltammetry and chronoamperometry. Compared with an updoped Pt/graphene control catalyst, the Pt/N-graphene catalyst shows a narrower particle size distribution and improved catalytic performance. Considering the facile, green and effective single-step synthetic process for the Pt/N-graphene catalyst, the results are promising for the potential application of these materials in emerging fuel cell technologies.

Measurement of Temperature and Reaction Species in the Cathode Diffusion Layer of a Free-Convection Fuel Cell

Micron- and millimeter-scale sensors were employed to acquire first-of-their-kind experimental measurements of the spatial and temporal distributions of temperature, oxygen partial pressure, and relative humidity in the mass transport layer immediately above the planar, horizontal cathode of polymer electrolyte membrane fuel cell PEMFC driven by natural convection. The sensors provide approximately 1 mm or better spatial resolution and 1 s temporal resolution. Substantial changes in temperature and reaction species concentrations were observed with increasing current density during a current-voltage I‐V scan. A linear decrease in oxygen partial pressure and a linear increase in water vapor partial pressure were observed with increasing current density, consistent with a flux balance analysis. Spatially resolved profiles normal to the surface indicate that thermal and reaction species gradients extend up to 6 mm above the horizontal cathode surface. Complementary horizontal profiles parallel to the surface reveal that the cell’s cathode rib structure visibly influences oxygen distribution. Most significantly, these data show that thermal and species concentration effects are not confined to the gas diffusion layer GDL, but extend well beyond the cathode surface, into the surrounding space. The measurements were used to estimate diffusion and/or convection mass transfer coefficients above the cathode surface. Transient data reveal substantial differences in the time constants associated with oxygen, water, and heat transport. The insights provided by this study should prove useful to inform and validate future physical models of air-breathing fuel cell systems.

Progress toward a solid-state ionic field effect transistor

This paper reports on the development and characterization of a solid-state ionic field effect transistor (IFET), a device integrating the principles of metal-oxide-semiconductor field effect transistors, electric double layer modeling, and solid-state ionic transport as a platform to investigate and manipulate nanoionic effects. The described solid-state IFET utilizes a sinusoidal external electric field to induce a time-modulated ionic space charge layer conduction channel in an ion-conducting material, such as Nafion, the chosen proton-conducting polymer in this study. The application of double layer modeling to this system establishes a theoretical foundation for device performance, including predicted values for the modulation of the membrane resistance in response to a gating bias. Experimental studies of device prototypes constructed from 25-175-μm-thick Nafion membranes demonstrate indications of ionic space charge layer manipulation for gating voltages of 0.5–10 V in amplitude. Strategies to impr...