Near-infrared imaging and vibrational Raman scattering have been used to measure the susceptibility of Ni-based cermet anodes to carbon formation in solid oxide fuel cells (SOFCs) operating with methane and methanol fuels at 715 °C. These two complementary optical methods afford previously unavailable opportunities to monitor chemical and physical processes occurring in situ and in real time with molecular specificity and spatial resolution. Imaging and spectroscopic data show that when the cell is held at open circuit voltage carbon forms within one minute of methanol or methane being introduced to the anode chamber. Raman spectra identify these deposits as highly ordered graphite based on a single sharp feature in the vibrational spectrum near 1580 cm−1. While graphite formed from methane remains highly ordered regardless of exposure duration, graphite formed from sustained exposure to methanol begins to show evidence of structural disorder inferred from the appearance of a weak feature at 1340 cm−1. This lower frequency vibrational band has been assigned previously to the presence of grain boundaries and/or site defects in a graphite lattice. Correlating the growth of intensity in the Raman spectra with exposure time quantifies the kinetics of carbon deposition and suggests that carbon formed from methanol grows via two distinct mechanisms. Thermal imaging data show that carbon deposition is endothermic and reduces anode temperatures. This effect is more pronounced for methanol (ΔT = −5.5 °C) than methane (ΔT = −0.5 °C). These results agree with data from vibrational Raman experiments showing that exposure to methanol leads to significantly more carbon deposition. Polarizing the cell reduces the amount of carbon deposited. This effect is reversible and more significant for methanol. The effects of the graphite formed from methanol are evident in electrochemical impedance data but less apparent in voltammetry experiments. In contrast, graphite formed from methane has only modest impact on device performance. Collectively, these studies address long-standing questions about the tendency of methanol to form carbon on eletrocatalytic SOFC anodes and the consequences of this chemistry on device performance.

Carbon formation or "coking" on solid oxide fuel cell (SOFC) anodes adversely affects performance by blocking catalytic sites and reducing electrochemical activity. Quantifying these effects, however, often requires correlating changes in SOFC electrochemical efficiency measured during operation with results from ex situ measurements performed after the SOFC has been cooled and disassembled. Experiments presented in this work couple vibrational Raman spectroscopy with chronopotentiometry to observe directly the relationship between graphite deposited on nickel cermet anodes and the electrochemical performance of SOFCs operating at 725 °C. Raman spectra from Ni cermet anodes at open circuit voltage exposed to methane show a strong vibrational band at 1556 cm(-1) assigned to the "G" mode of highly ordered graphite. When polarized in the absence of a gas-phase fuel, these carbon-loaded anodes operate stably, oxidizing graphite to form CO and CO(2). Disappearance of graphite intensity measured in the Raman spectra is accompanied by a steep ∼0.8 V rise in the cell potential needed to keep the SOFC operating under constant current conditions. Continued operation leads to spectroscopically observable Ni oxidation and another steep rise in cell potential. Time-dependent spectroscopic and electrochemical measurements pass through correlated equivalence points providing unequivocal, in situ evidence that identifies how SOFC performance depends on the chemical condition of its anode. Chronopotentiometric data are used to quantify the oxide flux necessary to eliminate the carbon initially present on the SOFC anode, and data show that the oxidation mechanisms responsible for graphite removal correlate directly with the electrochemical condition of the anode as evidenced by voltammetry and impedance measurements. Electrochemically oxidizing the Ni anode damages the SOFC significantly and irreversibly. Anodes that have been reconstituted following electrochemical oxidation of carbon and Ni show qualitatively different kinetics of carbon removal, and the electrochemical performance of these systems is characterized by low maximum currents and large polarization resistances.

The front cover artwork is provided by the Eigenbrodt (Villanova University, USA) and Walker (Montana State University, USA) research groups. The cover picture shows a background image of the operando spectroscopy apparatus focusing the Raman laser onto the solid oxide fuel cell's Sr2Fe1.5Mo0.5O6‐δ anode catalyst at 800 °C. The forefront of this cover photo showcases Raman spectra that reveal that the anode catalyst has the ability to resists detrimental graphite deposits during fuel cell operation with direct alcohol fuels. Read the full text of the Article at https://doi.org/10.1002/celc.201800827.

A combination of operando Raman spectroscopy and chronoamperometry was used to examine the carbon tolerance of Sr2Fe1.5Mo0.5O6‐δ (SFMO) electrode catalysts when operating with direct methanol and ethanol fuels in solid oxide fuel cells (SOFCs). Chronoamperometry studies revealed that these devices could maintain a steady power density output under typical SOFC operating conditions. High‐temperature Raman measurements of SFMO coupons exposed to methanol and ethanol (and their gas phase pyrolysis products) showed the presence of spectroscopic features associated with ordered and disordered forms of graphitic carbon. However, once SFMO was employed as an anode in an electrolyte‐supported SOFC, the graphite features disappear implying that these materials are not susceptible to carbon accumulation in functioning devices. These electrochemical and operando Raman measurements provided insight into SFMO's ability to act as an effective anode catalyst for SOFCs operating with direct alcohol fuel sources.

No abstract

Limitations of current solid oxide fuel cell anode materials have spurred the exploration of alternative materials. Mixed ionic and electronic conducting anode materials have the potential to overcome these limitations. The mixed ionic and electronic conducting material explored in this work is Sr2MgMoO6 (SMMO). X‐ray absorption spectroscopy in conjunction with an in situ assembly was used to study the fundamental redox chemistry of SMMO at 800 °C. The X‐ray absorption spectra for the Mo K‐edge exhibited changes in the reduced and oxidized bulk SMMO samples, as evident in a shift in the Mo binding energies and in the formation of oxide vacancies. However, in situ measurements of the working devices revealed that the Mo oxidation state remained unchanged under various cell polarizations. These findings demonstrate that SMMO, as a solid oxide fuel cell anode, has the potential to provide adequate electron conduction through multivalent Mo sites and to provide fuel tolerance through oxide vacancies built into its crystal structure.

In situ Raman spectroscopy was used to investigate the real time chemistry occurring in solid oxide fuel cells (SOFCs) operating at 715åC with both dry and humidified ethanol. Electrochemical impedance spectroscopy (EIS) measurements were carried out concurrent with the optical studies to correlate changes in SOFC performance with the appearance of graphite on the anode surface. In cells operating with dry ethanol (diluted in Ar), graphite forms rapidly and the Ni/YSZ anode suffers irreversible structural degradation. With humidified ethanol, degradation is lessened in cells at OCV and can be completely suppressed in polarized cells. The tendency of ethanol to form graphite and the stability of the cermet anode is tied directly to the composition of the fuel mixture that reaches the anode. Ex situ FTIR analysis of the SOFC fuel effluent shows that dry ethanol experiences significant reforming, with CO, CH4, CO2, and C2H2 being the primary products.

Limonene, widely present in consumer products, is prone to oxidation and leads to the formation of undesirable reaction products. A solid‐phase microextraction (SPME) GC‐MS/MS method was developed for the quantification of volatile limonene oxidation products, in encapsulated oil samples, by the optimization of MS parameters and SPME sampling conditions including fibre coating, temperature and time. The detection limit of the method reached 10 to 15 ppb for the limonene oxides, and less than 5 ppb for carvone and carveol isomers. Standard addition calibration was utilized to compensate for matrix effects of the encapsulation, flavouring, and to optimize the accuracy and versatility of this method. Sample throughput and precision of the method were improved through in‐line automated SPME sample preparation. The popularity of limonene as a flavouring compound in many consumer products, allows this method to be adapted to quantify different levels of volatile limonene oxidation products in encapsulated matrices.