A Study of the Impact of Sulphur on the Performance of Intermediate Temperature Solid Oxide Fuel Cells with Nickel Gadolinium Doped Ceria Anodes

Transcription

1 A Study of the Impact of Sulphur on the Performance of Intermediate Temperature Solid Oxide Fuel Cells with Nickel Gadolinium Doped Ceria Anodes Dan Brett 1&2, Pattaraporn Lohsoontorn 2, Nigel Brandon 2 1 University College London / United Kingdom 2 Imperial College London, London / United Kingdom d.brett@ucl.ac.uk Page 1 SOFC Forum, Lucerne 2008

2 The Effect of Sulphur on Anodes Possible mechanisms of interaction: Blocking of reaction sites: Ni Elec. H 2 + H 2 S S S S H 2 Ni S S S S S S S S S S Elec. Bulk sulphide formation: Ni Elec. H 2 + H 2 S S S S H 2 Ni S S S S S S S S S S Elec. Microstructural change: Loss of three phase boundary or micro-porosity Page 2

3 The Options for Dealing with Sulphur Removal Additional cost More complex system Maintenance Difficult to get to ultra low concentrations Sulphur Tolerant Anodes Need to be developed Inferior performance How resistant? Removal and Sulphur Relaxes removal and tolerance requirements. Mechanism of poisoning needs to be established to help develop new materials. Need to know what the operational limits are for sulphur tolerant anodes. What is the effect of different operating conditions. Can the operational conditions be controlled to avoid poisoning? Can poisoning be reversed? Page 3

4 Our Approach: Anode on both sides of electrolyte Use symmetrical anode cells. Ni-gadolinium doped ceria (Ni-CGO) cermets (for intermediate temperature operation around 600 o C) Vary operating conditions H 2 S H 2 Temperature Electrochemical impedance spectroscopy Separate the various losses in the cell Symmetrical cell Page 4

5 Symmetrical Cell Fabrication 1.25 mm 12 mm Ni-gadolinium doped ceria (Ni-CGO) NextTech Materials Fired 1300 o C for 2 hrs 8 mol% yttria stabilised zirconia (YSZ) Reduction at 600 o C 97% H 2 / 3% H 2 O, 1 hr 100 µm thick anodes Page 5

6 Symmetrical Cell Fabrication Page 6

7 Electrochemical Performance in Absence of H 2 S H 2 composition and temperature was systematically varied to identify the cause of the high and low freq. arcs. High frequency (HF) kinetics. Low frequency (LF) mass transport Fitting error % Page 7

8 Background Degradation Rate There is a monatonic degradation of fuel cell performance with time without H 2 S. HF intercept increases HF polarisation increases LF polarisation remains constant Page 8

9 Effect of H 2 S Concentration on Degradation 873 K H 2 S on Relative change 3 ppm 1 ppm Time 1 ppm H 2 S reversible 3 ppm faster, more significant and not fully reversible Page 9

10 Effect of H 2 S Concentration on Degradation 873 K LF response Change in polarisation resistance is due to increase in the high frequency (HF) feature associated with kinetics of reaction. Page 10

11 Effect of H 2 Concentration 873 K Reducing H 2 conc. Significantly increases performance degradation. Which continues after removal of H 2 S. Page 11

12 Effect of H 2 Concentration 873 K Increase in polarisation is primarily due to HF (kinetics). However, lower H 2 concentration results in more contribution from the LF impedance feature. Page 12

13 Effect of Temperature 600 o C 557 o C Reducing temperature increases the extent of polarisation increase. Reaching new equilibrium is slower. Change is entirely due to kinetics (HF arc) Page 13

14 Thermodynamics of Sulphur Interaction with SOFC Anodes Thermodynamics calculations have proved useful in predicting the propensity of sulphur to affect performance (please see the conference paper for more information) Page 14 See also: Lohsoontorn, Brett, Brandon, J. Power Sources 175 (2008)

15 Effect on Microstructure Changes in microstructure are observed accompanying H 2 S exposure. No obvious change in microstructure for H 2 S-free hydrogen. Higher concentration shows signs of Ni agglomeration. Low concentration of H 2 S shows faceting of Ni surface. H 2 S Free (24 hrs) 3 ppm H 2 S (24 hours) Page 15

16 Conclusions For Ni/CGO symmetrical cells Electrochemical impedance composed of a HF (kinetics) and LF (mass transport) component. For Ni/CGO exposed to H 2 S Degradation of performance increases with conc. as a single step process. Degradation worse for low H 2 concentration sulphur poisoning will be very dependant on cell position and fuel utilisation. Degradation worse at lower temperature Microstructural changes observed due to exposure to H 2 S Further work on the use of Raman spectroscopy to study anode interaction with sulphur is the subject of a poster. A0713 The effect of hydrogen sulphide on SOFC anodes studied using ex situ Raman spectroscopy, Brightman et al. Page 16

17 Acknowledgements SUPERGEN Fuel Cells More information on the fuel cells work occurring at Imperial College can be found on the ICL Fuel Cells Network website Thank you for your attention. Page 17