Advanced materials for SOFCs

Transcription

1 Advanced materials for SOFCs Yoed Tsur Department of Chemical Engineering Technion

2 Outline Intro: why SOFCs are important? Types of SOFCs Hybrid SOFC-something for power generation: NG utilization Materials research Summary

3 Generic FC Other fuels possible Other ionic conductors possible May work in the opposite direction to produce fuel (SOEC) When oxygen ion conductors are applied, the reactants are generated at the anode side

4 Stack configuration State of the art: planar configuration. Several other configurations exist or examined. Example: single cell SC-SOFC, utilizing different catalytic activities of electrodes. Comparison of cathode materials: Rembelski et al., FUEL CELLS 12, 2012, In an SC-SOFC there is no need to separate the air and fuel flow, eliminating the need for sealing.

5 Main benefits Low to zero emissions Higher efficiencies = less CO 2 No NOx; no SOx; no (or less) HCs & CO, no particulates. Fuel flexibility (including NG and biofuel) Security; eliminating need for importing oil Modular A true solution for energy storage and conversion: a MUST with renewable energy

6 Types of SOFCs Oxygen ion conductors Proton conductors Electrolyte supported Anodesupported Metalsupported

7 Hybrids for power generation Max efficiency at a point where not all the fuel is consumed. Heat and anode exhaust gas (mainly CO 2, syngas and steam) can be used as inputs for other cycles. For example: Mitsubishi is developing a triple combined cycle aiming at over 70% efficiency (compare to gas-turbine Rankine CC with 55% efficiency) See next slide and also: html

8 MITSUBISHI: Conceptual drawing of the triple combined cycle power generation system configuration

9 MITSUBISHI: Conceptual drawing - system exterior Interesting shadows

10 Types of SOFCs Oxygen ion conductors Proton conductors Electrolyte supported Anodesupported Metalsupported

11 Materials for proton conducting SOFCs Acceptor doped perovskites are the most common ceramic proton conductors. (Comment: just to irritate others, we have shown that donor doping of a perovskite can also enhance proton conduction provided careful processing measures are taken. Mazar et al., to be published). See how the normal electrolytes work:

12 Kröger-Vink Notation A common method for describing defects in a crystal lattice. (5) Defect type A C B Metal substitution: Y 3 4 Oxygen Vacancy: (5) F. A. Kröger and H. J. Vink, in Solid State Physics, 3 (1956), 307 Ce Relative charge Defect site [] = v O in Lattice Site: +2-2 O O 0 O 2- = O x 2- O O Y Ce

13 Acceptor-doped Proton Conductors Acceptor-doped crystal Highly Creation Water uptake of into Acceptor-doped Oxygen the oxygen vacancies Crystal Proton hopping under electrochemical potential gradient -Acceptor -Oxygen Vacancy -Protonic Defect -Water x 2BaO + Y O H O + 3 2Y O + 2Ba v + 5O 2OH + v BaCeO x x OCe O Ba O O

14 Proton Transport Protons usually migrates in one of two mechanisms: High μ(h 2 ) High μ(h 2 ) 1. Vehicle Mechanism: OH - OH - OH - OH- OH - OH - 2. Hopping (Grotthuss) Mechanism H + H O H + H O H + H O H + H O H + Low μ(h 2 ) Low μ(h 2 ) The prevailing proton transport mechanism in perovskites is proton hopping.

15 Perovskites ABX 3 Structures

16 Proton Conductors (3) (2) Mobility vs. Water Uptake / Desorption (2) K. D. Kreuer, Annu. Rev. Mater. Res : (3) T. Norby, Solid State Ionics 125 (1999) 1-11

17 Types of SOFCs Oxygen ion conductors Proton conductors Electrolyte supported Anodesupported Metalsupported

18 Anode supported SOFC PNNL/ Delphi: Thick (500 micron), porous Ni-YSZ cermet Thin (<10 micron) YSZ ~2 micron ceria barrier layer ~40 micron LSCF 6428 porous cathode C, can use H 2, CO and CH 4 Must have < 1ppm sulfur Glass seals, SS frame and interconnects L. Chick, PNNL 2011

19 Materials research The classic cermet anode, however, does not work very well with hydrocarbons new anode materials are needed. Example: Ce x La 0.75 x Sr 0.25 Cr 0.5 Mn 0.5 O 3 (Ce doped LSCM), Lay et al., Fuel Cells 2012, pp. 265 The cathode must have both fast oxygen exchange kinetics and high mixed ionicelectronic conductivity: High T: La 1-x Sr x MnO 3-d (LSM), Ostergard et al., Electrochim. Acta 1995, pp I T: LSCF, Tai et al., S. S. Ionics 1995, pp. 354 Improved surface activity: impregnation of LSCF with Pr 0.75 Sr 0.2 MnO 3 δ similar materials: Ding et al., Adv. Ener. Materials 2013.

20 Electrolyte Supported Cells Sunfire Gmbh sell stacks of that kind (formerly Staxera); working at 860 C; W; hydrogen fuel; 14 kg. Very few publications in the last year regarding this technology. See: k200-sofc-stack

21 Metal Supported Cells Gas permeable porous metallic substrate Ceramic layers are thin decreasing overall production cost and enhancing stability over thermal cycles Low po2 processing a challenge for the cathode. Rare-earth nickelates are examined: Ln 2 NiO 4+d

22 Summary We gave few examples for the current research regarding SOFCs The new discoveries of fossil fuel resources (mainly NG) completely change the geopolitics of the energy market. Fossil fuels are here to stay for decades. We must reduce their environmental impact SOFCs! The two Winning Cards are fuel flexibility and combination of H 2 production (SOFC-SOEC).