Sulfur poisoning electrodes

Sulfur poisons catalytic sites in the fuel cell also. The effect is aggravated when there are nickel or iron-containing components including catalysts that are sensitive to sulfur and noble metal catalysts, such as found in low temperature cell electrodes. Sulfur tolerances are described in the specific fuel cell sections of this handbook." In summary, the sulfur tolerances of the cells of interest, by percent volume in the cleaned and altered fuel reformate gas to the fuel cells from published data, are ... 

Further applications of nanoscale modeling to sulfur poisoning and other contaminants in the electrodes could lead to a better understanding of what causes degradation and the properties and conditions that could minimize degradation. 

The limit of H2S concentration accepted for fuel gas is less than 1 ppm. Sulfur poisoning of the electrode is irreversible upon long-term exposure to concentrations of more than about 10 ppm since surface structure changes take place and cause permanent damage and deactivation of the anode [2-11]. At low concentrations the effects of H2S are generally reversible by passing over H2S-free hydrogen and water vapor. 

Sulfur poisoning (both electrodes) Impurities in the fuel stream, especially sulfur, inhibit anode performance. Strong reversible poisoning of the Ni-YSZ anode occurs at feed concentrations of 1 ppm H2S in H2 at 1000°C and as low as 50 ppb H2S in H2 at... 

Ultradeep desulfurization of fuel oils is used for producing not only clean fuels but also sulfur-free hydrogen used in fuel-cell systems, in which the hydrogen can be produced potentially through the reforming of fuel oils. Fuel-cell systems must be run with little-to-no sulfur content, because sulfur can irreversibly poison the precious metal catalysts and electrodes used [12]. 

Elemental sulfur (in Equation (5-15) is expected on Pt electrodes only at high anodic potentials, and at sufficiently high potentials, sulfur is oxidized to SO2. The extent of poisoning by H2S increases with increasing H2S concentration, electrode potential, and exposure time. H2S poisoning, however, decreases with increasing cell temperature. 

NiSx has been observed to behave better than CoSx and FeSx. The decrease in overpotential can be as high as 0.3 V with respect to Ni [25, 151]. NiCo2S4 (formal composition) has been found to operate at -0.1 V vs DHE, at a current density of 1 A cm-2 and has been tested successfully for several thousand hours [444,452]. At open circuit a bluish coloration of the solution indicates that some Co is leached out since it becomes anodic with respect to Ni. A small continuous cathodic protection would be necessary. Teflon-bonded NiCoSx electrodes have been found to be immune from Fe poisoning this has been attributed to the precipitation of FeSx on the electrode surface due to the presence of sulfur in solution leached out from the surface. This condition will not be realized in a continuously flowing solution. 

Figure 15 Potentiodynamic (ImV/s) oxidation current densities for 0.1% CO/H2 on sputter-cleaned Pt and Pt-Ru RDEs at 2500 rpm in 0.5-M sulfuric acid at 62 °C. Prior to electrochemical measurements, the electrode potential was held at 0.05 V at 2500 rpm for about 300 s. (a) Magnification of the low-current density region for the positive going sweeps, (b) Comparison of the potentiodynamic and potentiostatic (1000-s) oxidation current densities, (c) Potentiodynamic (20mV/s) oxidation of pure H2 on CO-poisoned Pt (Pt-CO/ H2) CO was adsorbed at 0.05 V, and then the electrode was cycled between 0.05 and 0.22 V in CO-free solution (Pt-CO/no H2) the voltammetry of the unpoisoned Pt surface at the same conditions is added for reference.

Carbon monoxide, trace metals, and sulfur compounds, such as HjS, COS, mercaptans, and thiophenes, exist in hydrogen produced from coal gasification and used in molten carbonate Hj/Oj fuel cells. In addition, nitrogen compounds from coal, such as HCN and HCNS can be present or they might oxidize to corrosive NO. While carbon monoxide is reactive in these cells, the rest impurities can either poison the Ni anode or they can attack chemically cell and electrodes 249), for example, HjS sulfidizes nickel and stainless steel. HjS could also undergo oxidation to deposit sulfur 250) ... 

Hydrogen gas fuel and air (O2) are fed to anode and cathode Pt catalyst powder layers, respectively. The Pt catalysts is Teflon-bonded to porous carbon sheets to form gas-diffusion electrodes, with a catalyst loading of about 1.0 mg/cm. The Pt anode and cathode are separated by a thin inert porous matrix that is filled with concentrated phosphoric acid. The cell operates at 200°C (to improve the electrode kinetics), with a cell voltage of about 0.67 V at a current density of 0.150 A/cm. Most voltage losses occur at the air cathode. The hydrogen gas must be pure because sulfur and carbon monoxide poison the Pt anode catalyst. This type of fuel cell is commercially available today, with more than 200 systems installed all over the world in hospitals, hotels, office buildings, and utility power plants. 

The gas is colorless, odorless, nontoxic, and inert. It is not changed by electrical stress just below the corona point (82), but it is decomposed slowly by spark-over or corona electrical discharge (82, 272) giving lower fluorides of sulfur and fluorides of the metals used as the electrodes. It does not react with water or with a basic solution but it does react vigorously with a hot alkali metal. The gas is not toxic however, it has some depressant action upon the central nervous system (45) and has mild anesthetic properties (812). In spite of this, rats may live in an atmosphere of 80% SF6 and 20% 02 for periods up to one day with no signs of poisoning (187). 

A voltammetric study of the behavior of glyoxylic acid on platinum single crystal electrodes in sulfuric acid medium was performed in order to get information on the effect of surface structure on the electrosorption and oxidation. The oxidation (taking place at high potentials 1.0 V RHE) has been found structure sensitive. At lower potentials the formation of poisoning intermediates is considered as the predominant process. Two kinds of stable residues were distinguished ... 

The technical challenges posed by these systems are different from those facing low- to medium-temperature cells. For instance, there are no severe kinetic limitations at the electrodes or poisoning of electrocatalysts by impurities (other than sulfur) in the fuel gas. Instead, material science issues arise with (i) sintering of the electrodes and the electrolyte matrix, (ii) corrosion of cell components in molten salt electrolytes (MCFC), (iii) electrolyte migration in the external manifolds of MCFCs and (iv) differential expansion coefficients of the materials of construction in all-solid-state systems (SOFCs). 

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