Structure and Electrical Conductivity of Mn-based Spinels Used as SOFC Interconnect coating Supervisor: Dr. A. Petric Yadi Wang Jan

 Literature Review  Objectives  Preliminary Results  Conclusions & Future Work OUTLINE 2

Literature Review SOFC Reactions: Cathode O 2 + 4e - = 2O 2- Anode H 2 +O 2- =H 2 O+2e - CO +O 2- = CO 2 +2e - CH 4 + 4O 2- =2H 2 O+CO 2 +8e - Environmental friendly High Efficiency 3

 Interconnect is a critical part of the fuel cell electrical connection between cells gas separation within the cell stack  Requirements High electrical conductivity Chemical stability in both fuel and air Thermal expansion match to other cell components High mechanical strength Chemical stability with regard to other cell components Literature Review Interconnect 4

Ceramic interconnect 1990s Metallic interconnect Al---alumina Si---silica Cr---chromia Hard to fabricate, high cost Ferritic stainless steel 16%<Cr<18% Chromium-based alloy Cr ~95% 5 High resistance

 Main disadvantages of metallic interconnects --Growth of Cr 2 O 3 scale causes high resistance in the cell stack --Volatile Cr (VI) causes chromium poisoning of the cell 2Cr 2 O 3 + 3O 2 + 4H 2 O ⟺ 4H 2 CrO 4 (g) 2Cr 2 O 3 + 3O 2 ⟺ 4CrO 3 (g) 2Cr 2 O 3 + 3O 2 + 4H 2 O ⟺ 4CrO 2 (OH) 2 (g) Literature Review Metallic Interconnect 6

 A protective coating is needed on the cathode side to isolate Cr from air : --maintain electrical conductivity --prevent chromium (VI) formation  Candidates to use for coating --Perovskite oxides (high cost, difficult to apply coating) --Spinel oxides Literature Review Metallic Interconnect 7

Literature Review Spinel Many famous rubies on the crown turned out to be spinels “Black Prince's ruby” A magnificent 170-carat red spinel 8

Literature Review Spinel Has formula of AB 2 O 4 Cubic closed-packed oxygen lattice with 32 O 2- in one cell A and B cations occupy interstitial sites 1/8 of tetrahedral and 1/2 of octahedral sites are occupied 9

 Spinels can form solutions with many different cations  A element: Mg, Ni, Mn, Zn, Cu, Co  B element: Al, Cr, Mn, Fe, Co  Multivalent elements needed for conductivity Literature Review Spinel 10

 Table 1. Properties of common spinels (800 o C except where noted) Literature Review Spinel 11 Petric, A., Ling, H., J. Am. Ceram. Soc., 90[5] (2007).

 Conduction occurs by hopping of charge on octahedral sites ----Thus presence of different valence states among octahedral cations is essential to conduction. (Al-based spinels have low conductivity.) ----Spinels containing manganese have high conductivity because of multiple valence states of Mn  Conduction is temperature activated σ=σ 0 exp(-E F /kT) Literature Review Spinel Conductivity 12

 Mn valence state Tetrahedral: Mn 2+ Octahedral : Mn 2+, Mn 3+, Mn 4+ (2Mn 3+  Mn 4+ +Mn 2+ )  Conductivity of Mn-based spinel is attributed to hopping between Mn 3+ /Mn 4+ on octahedral sites. Literature Review Mn-based Spinel 13

 Co-Mn spinel MnCo 2 O 4 ---has conductivity of 60 S/cm Promising coating material for SOFC metallic interconnect  CuMn 2 O 4 has conductivity around 200 S/cm, volatile  Cu doped Co-Mn spinel Increase conductivity Decrease cost Literature Review Mn-based Spinel 14

 Measure electrical conductivity of Co-Mn-Cu spinel  Electroplate spinel coating for stainless steel 430 and chromium alloy interconnects Objectives 15

16 Petric, A., Ling, H., J. Am. Ceram. Soc., 90[5] (2007).

 Measured conductivity of 13 compositions within the cubic region 17 Co-Cu-Mn Spinel Conductivity

18 Co-Cu-Mn Spinel Conductivity Blue region identifies the region of optimum properties

 Coating methods: Slurry coating, screen-printing, physical vapor deposition, electroplating, plasma spray  Electroplating + oxidation --cost-effective --low processing temperature (room temperature) --uniform films on complex substrate --easy to control thickness Coating methods 19

 Reactions:  Cathode: Cu 2+ +2e - = Cu  Co 2+ +2e - =Co  Mn 2+ +2e - =Mn Anode: 2H 2 O= O 2 + 4H + +4e -  Use Faraday’s law:  m=(Q*M)/(F*Z) =  to control electroplating time 20 Electroplating

21 Preliminary Results C0 0.5 Cu 0.5 Mn 2 O 4 Coating On stainless steel 430, 24h

22 Preliminary Results C0 0.5 Cu 0.5 Mn 2 O 4 Coating On Stainless steel 430, 150h

23 Preliminary Results C0 0.5 Cu 0.5 Mn 2 O 4 Coating On chromium-iron plate, 24h

 Cu doped Co-Mn spinels have ideal electrical conductivity  Co 0.5 Cu 0.5 Mn 2 O 4 coating successfully stops Cr migration  Improve the quality of spinel coating.  Apply different compositions  Characterize the coating  Apply to large interconnect plate 24 Conclusions & Future Work

 Dr. A. Petric  Dr. Y. Mozharivskyj  Dr. P. Wei, Dr. S. Joshi, Xavier Michaud  Chris Butcher, Doug Culley, Xiaogang Li, Wenhe Gong 25 Acknowledgement

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