Michael B. Pomfret

In Situ Optical Studies of Solid-Oxide Fuel Cells

Thermal imaging and vibrational spectroscopy have become important tools for examining the physical and chemical changes that occur in real time in solid-oxide fuel cells (SOFCs). Imaging techniques can resolve temperature differences as fine as 0.1 degrees C across a SOFC electrode at temperatures higher than 600 degrees C. Vibrational spectroscopy can identify molecular species and changes in material phases in operating SOFCs. This review discusses the benefits and challenges associated with directly observing processes that are important to SOFC performance and durability. In situ optical methods can provide direct insight into reaction mechanisms that can be inferred only indirectly from electrochemical measurements such as voltammetry and electrochemical impedance spectroscopy and from kinetic models and postmortem, ex situ examinations of SOFC components. Particular attention is devoted to recent advances that, hopefully, will spur the development of new generations of efficient, versatile energy-producing devices.

Direct methanol oxidation at low overpotentials using Pt nanoparticles electrodeposited at ultrathin conductive RuO2 nanoskins

Small (primarily 2–4 nm) Pt nanoparticles electrodeposited at Ti-supported RuO2 nanoskins, designated as Pt/RuO2(Ti), are highly active for electrocatalytic oxidation of methanol (CH3OH) in acid electrolyte, with peak potentials among the lowest reported anywhere for RuO2-supported Pt. The Pt-modified RuO2 nanoskin is equally effective for CH3OH oxidation whether one or both sides of the Ti-foil substrate is coated or whether the RuO2 nanoskin is electrolessly deposited at the Ti substrate as a single layer or in multiple layers. Current densities for methanol oxidation are ∼2× lower than previously reported, owing to a bimodal distribution of both highly active (2–4 nm) and less active larger (> 5 nm) Pt particles. The methods reported here comprise a means of expressing highly active, nanostructured, bifunctional electrocatalytic coatings on substrates of essentially any geometry and minimizing the quantity of RuO2 necessary.

High-Temperature Chemistry in Solid Oxide Fuel Cells: In Situ Optical Studies

Solid oxide fuels cells (SOFCs) are promising devices for versatile and efficient power generation with fuel flexibility, but their viability is contingent upon understanding chemical and material processes to improve their performance and durability. Newly developed in situ optical methods provide new insight into how carbon deposition varies with different hydrocarbon and alcohol fuels and depends on operating conditions. Some findings, such as heavier hydrocarbon fuels forming more carbon than lighter fuels, are expected, but other discoveries are surprising. For example, methanol shows a greater tendency to form carbon deposits than methane at temperatures below 800 °C, and kinetically controlled steam reforming with ethanol at high temperatures (∼800 °C) is less detrimental to SOFC performance than operating the device with dry methanol as the fuel. In situ optical techniques will continue to provide the chemical information and mechanistic insight that is critical for SOFCs to become a viable energy conversion technology.

Identification of a Methane Oxidation Intermediate on Solid Oxide Fuel Cell Anode Surfaces with Fourier Transform Infrared Emission.

Fuel interactions on solid oxide fuel cell (SOFC) anodes are studied with in situ Fourier transform infrared emission spectroscopy (FTIRES). SOFCs are operated at 800 °C with CH4 as a representative hydrocarbon fuel. IR signatures of gas-phase oxidation products, CO2(g) and CO(g), are observed while cells are under load. A broad feature at 2295 cm(-1) is assigned to CO2 adsorbed on Ni as a CH4 oxidation intermediate during cell operation and while carbon deposits are electrochemically oxidized after CH4 operation. Electrochemical control provides confirmation of the assignment of adsorbed CO2. FTIRES has been demonstrated as a viable technique for the identification of fuel oxidation intermediates and products in working SOFCs, allowing for the elucidation of the mechanisms of fuel chemistry.

Measurement of benzenethiol adsorption to nanostructured Pt, Pd, and PtPd films using Raman spectroelectrochemistry.

Raman spectroscopy and electrochemical methods were used to study the behavior of the model adsorbate benzenethiol (BT) on nanostructured Pt, Pd, and PtPd electrodes as a function of applied potential. Benzenethiol adsorbs out of ethanolic solutions as the corresponding thiolate, and voltammetric stripping data reveal that BT is oxidatively removed from all of the nanostructured metals upon repeated oxidative and reductive cycling. Oxidative stripping potentials for BT increase in the order Pt < PtPd < Pd, indicating that BT adsorbs most strongly to nanoscale Pd. Yet, BT Raman scattering intensities, measured in situ over time scales of minutes to hours, are most persistent on the film of nanostructured Pt. Raman spectra indicate that adsorbed BT desorbs from nanoscale Pt at oxidizing potentials via cleavage of the Pt-S bond. In contrast, on nanoscale Pd and PtPd, BT is irreversibly lost due to cleavage of BT C-S bonds at oxidizing potentials, which leaves adsorbed sulfur oxides on Pd and PtPd films and effects the desulfurization of BT. While Pd and PtPd films are less sulfur-resistant than Pt films, palladium oxides, which form at higher potentials than Pt oxides, oxidatively desulfurize BT. In situ spectroelectrochemical Raman spectroscopy provides real-time, chemically specific information that complements the cyclic voltammetric data. The combination of these techniques affords a powerful and convenient method for guiding the development of sulfur-tolerant PEMFC catalysts.

Interfacial Resistivity of Yttria Stabilized Zirconia in Operating Solid Oxide Fuel Cells

Solid oxide fuel cells (SOFCs) represent attractive, stand-alone sources of power generation with operating efficiencies of up to 70% and relatively benign products. Traditionally, research into SOFC operation has investigated questions about architecture, materials and transport in efforts to improve overall device efficiency. More recently, researchers have begun to focus on the detailed chemical mechanisms by which SOFCs oxidize fuel to produce electricity. One source of electrochemical activity that has not been considered explicitly in most models of fuel oxidation has been the YSZ electrolyte itself. This oversight is not surprising given the known chemical stability of YSZ as well as the material’s electrically insulating properties. However, recent reports of a reduced surface phase of YSZ as well as SOFCs operating with a simple porous YSZ assembly for an anode raise questions about the role YSZ can play in SOFC electrochemistry. Experiments described below use a variation of impedance spectroscopy (EIS) to characterize the resistance across the surface of YSZ in functioning cells. SOFCs used in this study have polished ~1 mm thick, polycrystalline, 8 mole % YSZ electrolytes. Electrolyte disks were situated between a porous LSM/YSZ cathode and a nickel/YSZ cermet anode. A gold electrode was placed on the anode side 4.0 mm from the Ni/YSZ anode. H2 was used as the fuel and all cells were operated at a temperature of 800 °C. A 70 sccm flow of H2 on the anode side was balanced by a 85 sccm flow of air on the cathode side. All experiments used a fuel flow of 33% fuel, with a balance of Ar. Mass flow controllers (Brooks 5850E) regulated the anode-side flows. The cathode flow was regulated by a rotameter. EIS uses minimal AC currents to measure the ohmic resistance between working and reference electrodes. Typically, when measuring the impedance of a SOFC the working electrode is the anode and the reference electrode is the cathode. In the surface impedance experiments described below the Au electrode on the anode side of the MEA was used as the reference electrode. This arrangement allows for the measurement of ohmic resistances between the Au and Ni/YSZ electrodes across the YSZ surface. The resulting data contain information about the extent to which a reduced YSZ surface is electrically conducting. An Autolab PGSTAT30 (Eco Chemie) was used to monitor fuel cell performance. AC impedance spectra were taken using a two electrode configuration depicted schematically in Figure 1. Two configurations were used to allow for both the cathode and Au to act as the counter/reference electrode. In the absence of an oxide flux, the fuel phase above the MEA will reduce the YSZ surface. Figure 2 shows surface impedance plots of a cell that is operated at OCV-0.4 V (0.7 V), followed by a return to OCV conditions. Changes in the impedance spectra are striking. After 15 mins without an oxide flux the surface polarization resistance drops approximately 33% from ~1500 to ~1000 Ω. The low frequency data approach the x-axis with less scatter suggesting that the surface between the electrodes is becoming more reduced. However, the fact that the EIS data still exhibit incomplete arcs indicates that the surface has not been reduced to the point of being conductive. The YSZ surface reduction is complete approximately 30 min after operation at 0.7 V has ceased. The EIS plot for the reduced YSZ surface show considerably diminished scatter at low frequencies and, in fact, the data has a welldefined second intercept with the x-axis, completing the arc. The Rp value (~775 Ω) has dropped nearly 50% from the initial value of the surface operated at 0.7 V and the lack of scatter in the behavior of the data suggest that the polished electrolyte surface between the two electrodes has reduced sufficiently to become DC conductive.

Electron Magnetic Resonance Studies on Nanowire and Nanoparticle Arrays

Arrays of magnetic nanowires and well-oriented chains of superparamagnetic nanoparticles were fabricated using polymer and alumina membrane templates. The systems were characterized by SQUID and studied by electron magnetic resonance methods. Comparative analysis of the obtained results for different geometries and sizes of the magnetic inclusions is presented.