Gregory S. Jackson

Measuring fundamental properties in operating solid oxide electrochemical cells by using in situ X-ray photoelectron spectroscopy

Photoelectron spectroscopic measurements have the potential to provide detailed mechanistic insight by resolving chemical states, electrochemically active regions and local potentials or potential losses in operating solid oxide electrochemical cells (SOCs), such as fuel cells. However, high-vacuum requirements have limited X-ray photoelectron spectroscopy (XPS) analysis of electrochemical cells to ex situ investigations. Using a combination of ambient-pressure XPS and CeO(2-x)/YSZ/Pt single-chamber cells, we carry out in situ spectroscopy to probe oxidation states of all exposed surfaces in operational SOCs at 750 °C in 1 mbar reactant gases H(2) and H(2)O. Kinetic energy shifts of core-level photoelectron spectra provide a direct measure of the local surface potentials and a basis for calculating local overpotentials across exposed interfaces. The mixed ionic/electronic conducting CeO(2-x) electrodes undergo Ce(3+)/Ce(4+) oxidation-reduction changes with applied bias. The simultaneous measurements of local surface Ce oxidation states and electric potentials reveal the active ceria regions during H(2) electro-oxidation and H(2)O electrolysis. The active regions extend ~150 μm from the current collectors and are not limited by the three-phase-boundary interfaces associated with other SOC materials. The persistence of the Ce(3+)/Ce(4+) shifts in the ~150 μm active region suggests that the surface reaction kinetics and lateral electron transport on the thin ceria electrodes are co-limiting processes.

Influence of H2 on the response of lean premixed CH4 flames to high strained flows

A combined experimental and numerical investigation on the effects of H2 addition to lean-premixed CH4 flames in highly strained counterflow fields (with strain rates up to 8000 s 1 ) using preheated flows indicate significant enhancement of lean flammability limits and extinction strain rates for relatively small amounts of H 2 addition. Numerical modeling of the counterflow opposed jet configuration used in this study indicated extinction strain rates which were within 5% of experimentally measured values for equivalence ratios ranging from 0.75 to less than 0.4. Both experimental and numerical results indicate that increasing H 2 in the fuel significantly increases flame speeds and thus extinction strain rates. Furthermore, increasing H 2 decreases the dependency of extinction equivalence ratio on the strain rate of the flow. For all of the mixtures investigated, extinction temperatures depend primarily on equivalence ratio and not fuel composition for the range of H2 content studied, which suggests that extinction can be correlated to flame temperature and O2 concentration. Nonetheless, H2 addition greatly increases the maximum allowable strain rate before extinction temperatures are reached. Inspection of the model-predicted species profiles suggest that the enhancement of CH4 burning rates with H2 addition is driven by early H2 breakdown increasing radical production rates early in the flame zone to enhance CH4 ignition under conditions where otherwise CH4 combustion might be prone to undergo extinction.

Electrochemical Oxidation of H2, CO, and CO ∕ H2 Mixtures on Patterned Ni Anodes on YSZ Electrolytes

Single-cell solid oxide fuel cell experiments using thin-film, sputter-deposited Ni pattern anodes microfabricated on single-crystal yttria-stabilized zirconia (YSZ) electrolyte disks have been performed to examine the electrochemical oxidation of H 2 , CO, and CO/H 2 mixtures. Porous lanthanum strontium manganate (LSM)/YSZ cathodes have been used and characterized in separate symmetric cell experiments such that Ni anode overpotentials and impedances can be isolated. Post-test scanning electron microscopy imaging revealed that at the high temperatures (735-850°C), the 100-nm thin Ni patterns broke up into interconnected regions resulting in three-phase boundary lengths that roughly correlated with the original coverage area of the pretested dense Ni patterns. Electrochemical characterization for H 2 , CO, and CO/H 2 oxidation under dry and wet (∼4% H 2 O) feeds showed that the interconnected anode overpotentials and polarization resistances correlated with the original Ni pattern area for the various pattern geometries. Higher activation overpotentials and polarization resistances observed for CO in comparison to H 2 were not observed for CO/H 2 mixtures down to 25% H 2 . Results indicated detrimental effects of H 2 O on CO oxidation power densities due to drops in open-circuit voltages without reduction in polarization resistance, and enhancement due to water-gas shift reactions was not seen. Our results provide the basis for insights into H 2 and CO electro-oxidation on Ni/YSZ anodes.

PtMo Alloy and MoOx@Pt Core−Shell Nanoparticles as Highly CO-Tolerant Electrocatalysts

PtMo alloy and MoO(x)@Pt core-shell nanoparticles (NPs) were successfully synthesized by a chemical coreduction and sequential chemical reduction method, respectively. Both the carbon-supported alloy and core-shell NPs show substantially higher CO tolerance, compared to the commercialized E-TEK PtRu alloy and Pt catalyst. These novel nanocatalysts can be potentially used as highly CO-tolerant anode electrocatalysts in proton exchange membrane fuel cells.

Solid Oxide Fuel Cells: Operating Principles, Current Challenges, and the Role of Syngas

Syngas mixtures are excellent fuels for solid-oxide fuel cells (SOFC). Depending on the primary feedstock and the processing technology to produce the syngas, the composition (primarily mixtures of H2 and CO, but often including CH4, H2O, CO2, N2, and other impurities) can vary considerably. Thus, it is important to understand how SOFCs perform with alternative syngas mixtures. Syngas composition can affect materials selection, system design, and operating conditions. To assist understanding and interpreting performance, the article first reviews the basic principles governing SOFC chemistry and electrochemistry. The article also discusses alternative materials and system architectures, especially in the context of syngas fuels. A detailed computational model for a particular tubular, anode-supported, cell is used to compare SOFC performance using different syngas compositions. The syngas mixtures are derived from several processes, including partial oxidation (CPOx) or steam reforming of methane and dodecane, and gasification of coal or biomass.

Mechanistic Studies of Water Electrolysis and Hydrogen Electro-Oxidation on High Temperature Ceria-Based Solid Oxide Electrochemical Cells

Through the use of ambient pressure X-ray photoelectron spectroscopy (APXPS) and a single-sided solid oxide electrochemical cell (SOC), we have studied the mechanism of electrocatalytic splitting of water (H2O + 2e(-) → H2 + O(2-)) and electro-oxidation of hydrogen (H2 + O(2-) → H2O + 2e(-)) at ∼700 °C in 0.5 Torr of H2/H2O on ceria (CeO2-x) electrodes. The experiments reveal a transient build-up of surface intermediates (OH(-) and Ce(3+)) and show the separation of charge at the gas-solid interface exclusively in the electrochemically active region of the SOC. During water electrolysis on ceria, the increase in surface potentials of the adsorbed OH(-) and incorporated O(2-) differ by 0.25 eV in the active regions. For hydrogen electro-oxidation on ceria, the surface concentrations of OH(-) and O(2-) shift significantly from their equilibrium values. These data suggest that the same charge transfer step (H2O + Ce(3+) <-> Ce(4+) + OH(-) + H(•)) is rate limiting in both the forward (water electrolysis) and reverse (H2 electro-oxidation) reactions. This separation of potentials reflects an induced surface dipole layer on the ceria surface and represents the effective electrochemical double layer at a gas-solid interface. The in situ XPS data and DFT calculations show that the chemical origin of the OH(-)/O(2-) potential separation resides in the reduced polarization of the Ce-OH bond due to the increase of Ce(3+) on the electrode surface. These results provide a graphical illustration of the electrochemically driven surface charge transfer processes under relevant and nonultrahigh vacuum conditions.

Tuning the CO-tolerance of Pt-Fe bimetallic nanoparticle electrocatalysts through architectural control

We report a new and simple method for creating Pt-Fe bimetallic nanoparticle (NP) electrocatalysts with various architectures (random alloy, intermetallic and core-shell) and their architecture-dependent electrocatalytic activity for CO/H2oxidation.

Low-temperature combustion of hydrogen on supported Pd catalysts

Low-temperature ( 2 /O 2 mixtures diluted in N 2 has been studied experimentally using a microreactor with transient exhaust monitoring using mass spectroscopy. Experimental results using a γ-Al 2 O 3 washcoat-supported PdO x catalyst reveal the importance of transient measurements for elucidating features of the catalytic combustion mechanism and, in particular, the effects of H 2 O adsorption/desorption and of the Pd oxidation state. For the cases studied, experiments indicate that H 2 conversion depends on equivalence ratio ( Φ ) only at low temperatures ( T in T in >125°C, mass transfer limitations become more significant, and thus, as T in rises, conversion becomes relatively independent of Φ . Addition of H 2 O vapor to the inlet flow causes a reduction in conversion for lower T in , but for T in >125°C, it only delays catalyst light-off and the rapid transition to high steady-state conversion. A final set of experiments indicated very high (and apparently steady) H 2 conversion at T in as low as 50°C by starting with a prereduced catalyst. Efforts to understand the experimental results with a transient one-dimensional reactor model using detailed surface chemistry indicates the importance of the relative adsorption rates of H 2 and O 2 as well as the H 2 O adsorption/desorption. The model captures the trends for conversion with respect to temperature but fails to predict well the influence of inlet H 2 O concentrations. Implications on further mechanism development of the discrepancies between the model predictions and experiments results are discussed. Nonetheless, these low-temperature H 2 combustion studies provide a starting point to further Pd surface chemistry for combustion of other fuels for a wide range of applications.

Catalytic combustion of premixed methane/air on a palladium-substituted hexaluminate stagnation surface

This paper is an experimental and modeling study of catalytic combustion of lean methane/air mixtures in stagnation flow over a strontium-palladium-substituted hexaluminate catalyst surface. It reports gasphase profiles in the stagnation-flow boundary layer as measured by microprobe mass-spectrometric sampling. A chemically reacting flow model is developed and used to assist interpretation of the experimental data. A new surface-reaction mechanism does an excellent job of representing the effect of surface temperature (400 °C≤ T s ≤760 °C) and equivalence ratio (0.2≤≤0.8).

Ceria-based electrospun fibers for renewable fuel production via two-step thermal redox cycles for carbon dioxide splitting

Zirconium-doped ceria (Ce(1-x)Zr(x)O2) was synthesized through a controlled electrospinning process as a promising approach to cost-effective, sinter-resistant material structures for high-temperature, solar-driven thermochemical redox cycles. To approximate a two-step redox cycle for solar fuel production, fibrous Ce(1-x)Zr(x)O2 with relatively low levels of Zr-doping (0 < x < 0.1) were cycled in an infrared-imaging furnace with high-temperature (up to 1500 °C) partial reduction and lower-temperature (∼800 °C) reoxidation via CO2 splitting to produce CO. Increases in Zr content improve reducibility and sintering resistance, and, for x≤ 0.05, do not significantly slow reoxidation kinetics for CO production. Cycle stability of the fibrous Ce(1-x)Zr(x)O2 (with x = 0.025) was assessed for a range of conditions by measuring rates of O2 release during reduction and CO production during reoxidation and by assessing post-cycling fiber crystallite sizes and surface areas. Sintering increases with reduction temperature but occurs primarily along the fiber axes. Even after 108 redox cycles with reduction at 1400 °C and oxidation with CO2 at 800 °C, the fibers maintain their structure with surface areas of ∼0.3 m(2) g(-1), higher than those observed in the literature for other ceria-based structures operating at similarly high temperature conditions. Total CO production and peak production rate stabilize above 3.0 mL g(-1) and 13.0 mL min(-1) g(-1), respectively. The results show the potential for electrospun oxides as sinter-resistant material structures with adequate surface area to support rapid CO2 splitting in solar thermochemical redox cycles.