Sulfur Poisoning and Regeneration of Ni-Based Anodes in Solid Oxide Fuel Cells

Abstract

The effect of hydrogen sulfide ͑H 2 S͒ on the performance of nickel/yttria-stabilized zirconia ͑YSZ͒ cermet anode for solid oxide fuel cells has been studied under various operating conditions. In all cases, a small amount of H 2 S ͑ppm level͒ causes a sharp drop in cell performance within the first few minutes of exposure, followed by a gradual but persistent deterioration in performance for several days. The extent of anode degradation caused by sulfur poisoning increases with increasing H 2 S concentration, increasing cell voltage ͑i.e., closer to open-circuit voltage or decreasing cell current density͒, or decreasing cell operating temperature. The initial sharp degradation in performance is attributed to rapid adsorption of sulfur onto the Ni surface, which blocks the active sites for hydrogen adsorption and oxidation. However, the mechanism for the subsequent slow degradation is still not clear. Upon removal of H 2 S from the fuel stream, the anode performance can be recovered fully or partially, depending on operating conditions and duration of H 2 S exposure. The rate of the recovery process increases with operating temperature and cell current density.

Redox dynamics of sulphur with Ni/GDC anode during SOFC operation at mid- and low-range temperatures: An operando S K-edge XANES study

“…In good agreement with thermodynamics [9,20] and calculations based on density functional theory (DFT), [27] it has been demonstrated in several electrochemical studies that the extent of sulphur poisoning is a function of temperature, oxygen partial pressure and sulphur concentration as well as chemical nature of fuel and anode material [28e30]. Several studies have demonstrated that there is an initial reversible (or partially reversible) degradation step during the very first minutes when the anode is exposed to H 2 S [9,12,31], followed by a slow, but continuous, non-reversible decrease of electrode performance [9,12,31]. First, rapid decrease of electrochemical performance is likely caused by adsorption of sulphur species at electrode and concomitant blocking of the socalled three-phase-boundary (TPB) for hydrogen oxidation [9,12].…”

“…Several studies have demonstrated that there is an initial reversible (or partially reversible) degradation step during the very first minutes when the anode is exposed to H 2 S [9,12,31], followed by a slow, but continuous, non-reversible decrease of electrode performance [9,12,31]. First, rapid decrease of electrochemical performance is likely caused by adsorption of sulphur species at electrode and concomitant blocking of the socalled three-phase-boundary (TPB) for hydrogen oxidation [9,12]. There is some consensus about the first step of poisoning, but there are several significantly different if not controversial views about the second, slower step.…”

Sulfur Poisoning of SOFCs: A Model Based Explanation of Polarization Dependent Extent of Poisoning

“…Nevertheless, it is important to understand and quantify the sulfur tolerance of SOFC anodes in the event of sulfur breakthrough from the desulfurizer. Sulfur poisoning of Ni-YSZ anodes in SOFC is well studied experimentally for model fuel compositions consisting of ppm levels of H 2 S in mixtures of H 2 , H 2 O and an inert [2][3][4] as well as H 2 S in gas mixtures representative of natural gas or reformed natural gas. [5][6][7][8] Most of the experimental literature deals with H 2 S concentrations well below 10 ppm and work that treats H 2 S concentrations higher than 20 ppm is rather limited.…”

“…[5][6][7][8] Most of the experimental literature deals with H 2 S concentrations well below 10 ppm and work that treats H 2 S concentrations higher than 20 ppm is rather limited. 2,5,8 On the other hand it is well known that natural gas and biogas can have substantially higher sulfur concentrations and that catalytic steam reforming of methane can be carried out on Ni even with 100 ppm H 2 S without losing all catalytic activity 9,10 e.g., a methane conversion (in model biogas) of ∼35% can be maintained using a supported Ni catalyst with 100 ppm H 2 S at 800…”

“…• C. 10 For an electrolyte supported cell Zha et al, 2 studied the cell performance for a wide range of H 2 S concentrations using DC polarization and electrochemical impedance spectra. The performance drops were reported for 0.18 ppm to 50 ppm H 2 S in H 2 /H 2 S mixture and they found the cell performance to be recoverable, although not fully, even with 50 ppm H 2 S. A different paper from the same group reported the impedance response of the cell before and after poisoning for galvanostatic and potentiostatic conditions and the maximum H 2 S concentration examined was 10 ppm.…”