In Operando Vibrational Raman Studies of Chlorine Contamination in Solid Oxide Fuel Cells

Abstract

Vibrational Raman spectroscopy coupled with voltammetry and impedance measurements was used to explore the effects of chlorine on solid oxide fuel cell (SOFC) performance and durability. SOFC anodes were exposed to 110 ppm dry CH 3 Cl at 650 • C for up to four hours while intermittently exposing the cell to methane for ten minute intervals. In these experiments Raman spectroscopy was used to monitor carbon accumulation kinetics. Electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) measurements performed under CH 4 following 1 hour of exposure to CH 3 Cl showed marked degradation. This degradation was less apparent when the SOFC was operated with H 2 in the presence of CH 3 Cl. With methane and CH 3 Cl, peak power diminished at a rate of 14% per hour. Observable carbon accumulation during CH 3 Cl exposure became less pronounced over time. Eventually, carbon formation was suppressed completely suggesting that the primary effect of the Cl contaminant was deactivation of the Ni catalyst with respect to CH bond dissociation. SOFC performance with H 2 in the presence of CH 3 Cl remained largely unchanged. Interestingly, these effects of Cl on SOFC performance with methane proved partially reversible as electrochemical performance and carbon accumulation behavior were recovered upon removal of the CH 3 Cl from the fuel feed. Solid oxide fuel cells are electricity-generating devices capable of fuel-to-electricity conversion efficiencies of more than 80% when combined with heating applications.1-3 These efficiencies far outpace even the most modern fossil fuel fired power plants. 4 In addition, the high operating temperatures of solid oxide fuel cells (SOFCs) allow operation with many different fuels including biogas, syngas, natural gas, gasoline and even some alcohols.5-8 However, this versatility comes at the cost of having to develop SOFC materials capable of withstanding impurities intrinsic to each fuel type. These impurities include, but are not limited to, carbon deposits, sulfur, chlorine, silicon, phosphorus, and mercury. Each can cause premature degradation of SOFC materials.6,9-15 Many, if not all, of these impurities have been studied extensively using electrochemical methods. Techniques such as electrochemical impedance spectroscopy (EIS) and voltammetry consistently show how exposure to these individual contaminants lead to performance degradation with differing levels of severity and reversibility. However, while the electrochemical results quantify overall cell degradation, direct, in-situ evidence of the mechanisms responsible for diminished performance remain speculative. Coupling electrochemical methods with in-situ optical spectroscopy, such as Raman spectroscopy or XPS, advances understanding of the reactions that occur on the SOFC electrode surfaces as well as how those reactions affect performance. Raman spectroscopy has already shown to be a promising technique for high temperature in-situ characterization of molecules on the surface of an SOFC operating on hydrocarbon ...

Chlorine-Induced Degradation in Solid Oxide Fuel Cells Identified by Operando Optical Methods

“…Maximum cell current under CH 4 increases from 80 mA late in the CH 3 Cl exposure to 130 mA after the CH 3 Cl is removed, and the cell is allowed to operate with clean H 2 for 1 h. Benchmarks preformed with H 2 and exposed to chlorine, on the other hand, showed no recovery after the chlorine is removed (Figure b). This is in stark contrast to previous results at 650 °C showing much more pronounced performance recovery, suggesting fundamentally different mechanisms at different temperatures.…”

“…Fuel cells, especially proton exchange membrane fuel cells, have traditionally operated with hydrogen, but the high temperatures required for SOFC operation permit these devices to use higher molecular weight fuels such as methane, propane, natural gas, alcohols, and, most importantly, synthetic and biofuels such as syngas (H 2 + CO) and biogas (CH 4 + CO 2 ). − These diverse fuel sources inherently introduce a long list of potential contaminants in varying concentrations. Materials such as carbon (in the form of coke), ,− sulfur, − chlorine, , silicon, and phosphorus have all been identified as sources of SOFC degradation. Mechanisms thought to be responsible for contaminant-induced degradation include contaminant migration to the three-phase boundary (TPB) where electrochemical oxidation takes place, as well as contaminant adsorption to catalytically active sites on electrodes where fuels such as H 2 or CH 4 chemisorb.…”

“…Coupling electrochemical methods with operando spectroscopy, such as Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), or Fourier transform infrared emission spectroscopy (FTIRES), enables electrochemical performance to be linked directly to molecular and material changes occurring on SOFC electrodes. In particular, Raman spectroscopy and NIR thermal imaging have already shown their value as complementary techniques capable of identifying chemical processes occurring operando . ,, Spectroscopic observations together with electrochemical measurements such as linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) provide a direct link between the chemical and material condition of the SOFC and the device’s ability to electrochemically oxidize fuel to produce electricity and products. ,, …”

Surface Reactivity of YSZ, Y2O3, and ZrO2toward CO, CO2, and CH4: A Comparative Discussion

“…hydrogen at elevated temperatures) or carbon containing gas atmospheres or for Ni-YSZ materials and full operating SOFC's. 12,13,15,16,21,[37][38][39][40] These differences are obviously due to the growing carbon layer on top of the YSZ grains. This is also especially true for comparable measurements in CH 4 atmospheres 7 , which are conducted at lower temperatures than discussed here.…”

Operando characterization of metallic and bimetallic electrocatalysts for SOFC fuel electrodes operating under internal methane reforming conditions

“…These composite electrodes, however, can experience degradation during unintended reduction and oxidation (redox) cycles destroying the Ni percolation and thus electronic conductivity, and they are prone to carbon accumulation and electrode poisoning by common fuel impurities including sulfur and chlorine. [2][3][4][5][6][7] Consequently, materials-driven research has focused on developing more robust fuel electrodes. 8 One way to overcome challenges associated with SoA electrodes is to decouple their multi-functional requirements by separating the electronically conducting network from the electrocatalytic materials.…”