Electrical Conductive Perovskite Anodes in Sulfur-based Hybrid Cycle

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1 2 nd HTTR Workshop, Oct. 5-7, 25, Oarai Electrical Conductive Perovskite Anodes in Sulfur-based Hybrid Cycle Hirotaka KAWAMURA, Masashi MORI, Song-Zhu CHU,* and Masaki UOTANI Materials Science Central Research Institute of Electric Power Industry (CRIEPI)

2 2 nd HTTR Workshop, (25). Materials Science Outline Background Sulfur-based Hybrid Cycle for Hydrogen Production Objectives Experiments Results and Discussions Crystallographic study Electrical conductivity study Corrosion resistance study Summary

3 Introduction According to a desiring of hydrogen energy cycle in the near future, such as coming into wide use of fuel cells and fuel cell vehicles (FCVs), the establishments of zero emission and large-scale hydrogen production process have been desired. The use of hydrogen in fuel cells and FCVs will reduce Japanese dependence on petroleum foreign sources and enhance our energy security. Sulfur-based hybrid cycle (SHC) process has attracted much attention as a large-scale and CO 2 free process. In order to establish some high electrolysis efficiency and low-cost electrolysis system in the SHC process, Pt-free electrode should be developed, because Pt is a noble and rare metal.

Sulfur-based Hybrid Cycle (SHC) Process ( o C) 9 7 5 (K) 1123 One of nuclear-hydrogen production process (combined electrolysis with thermo-decomposition) Emission free process - No CO 2 gas emission

4 Sulfur-based Hybrid Cycle (SHC) Process ( o C) (K) 1123 One of nuclear-hydrogen production process (combined electrolysis with thermo-decomposition) Emission free process - No CO 2 gas emission - Only water consumption (reuse of H 2 SO 4 ) Availability - Easy to improve electrolysis efficient because of simple process. - Electrolytic potential of H 2 SO 4 is.17v (Electrolytic potential of H 2 O is 1.23V) Thermochemistry S decomposition S H 2 SO 4 condenser SO 2, O 2 Heat source using HTGR or steel furnace H 2 H 2 SO 4 H 2 SO 4 vaporizer Electrolysis H 2 O, H 2 SO 4 Electrolyzer (Cathode) (Anode) SO 2, O 2 separator SO 2 SO 2 H 2 S H 2 O Mixer O 2 Electrolytic Conditions for High Efficiency: 5 wt.%h 2 SO 4, 353 K. ---Westinghouse, USA. 2H + + 2e - H 2 H 2 S + H 2 O 2H + + H 2 SO 4 + 2e -

5 Concept Development of Electrode Materials High electronic conductivity High corrosion resistance Pt Low electrode over-potential Break through of Sulfur-based Hybrid Cycle Electronic conductive ceramics (e.g.:titanium pyrochlorea 2 B 2 O 7 Titanium perovskite) AB Stoichiometric pyrochlore or perovskite has non-conductivity Ti 4+ without d-electrons Anode material Ti 3+ Existence of Ti3+ in Electrical conductivity A-site deficiency + appearance B-site doping + H 2 reduction A 2-x B 2 O 7-σ A 2-x B 2-y M y O 7-σ A 1-z RE z B+σ A 1-x B 1-y M y +σ High corrosion resistance Selection of slightly soluble elements for A-site and B-site the pyrochlores or perovskites lead to an appearance and increase of electrical conductivity.

6 d- and f- electrons in SrTi Ti 4+ Principle of Electronic Conductance Ti 4+ + e => Ti 3+ Reduction No electrical conductivity Ti 3+ Conductivity appearance - Maintain oxygen vacancy for electron valance (H 2 reduction) Sr 2+ Ti 4+ O 2-3 Sr 2+ Ti 4+ 1-x Ti3+ x O2-3+δ - A-site doping Sr 2+ Ti 4+ O 2-3 Sr 2+ 1-z RE3+ z Ti4+ 1-z Ti3+ z O2-3 - B-site doping / A-site deficient Sr 2+ Ti 4+ O 2-3 Sr 2+ 1-x (Ti4+, Ti 3+ ) 1-y M 2+ y O2-3+ δ O O O Ti electron O Electron image of doped SrTi+δ

7 Objective - To investigate the electrical conductivity and corrosion resistance of a series of A-site doped Sr 1-z RE z Ti 1-y Ti y + δ (RE= Yb, Y, Gd, Sm, Nd, Pr, La), B-site doped and/or A-site deficient Sr 1-x Ti 1-y M y +δ (M= Nb, Ta) perovskites under the electrolysis condition, i.e., 5 wt % H 2 SO 4 solution at 353K.

8 1. Specimens: Experimental Perovskite: a). Oxygen deficiency or excess: SrTi±δ b). A-Site doping: Sr 1-z RE z Ti+δ (RE= Yb, Y, Gd, Sm, Nd, Pr, La) c). B-Site doping: SrTi 1-y M y +δ (M= Nb, Ta) d). A-Site deficiency + B-Site doping: Sr 1-x Ti 1-y M y +δ (M= Nb, Ta) Pyrochlore (Ref.): RE 2-x O 7- δ (RE= Yb, Y, Gd, Sm, Nd, Pr, La) Gd 1.72 Ti 1.9 M.1 O 7- δ (M=Cr, Mn, Fe, Co, Ni) 2. Materials Synthesis: (Solid-State Reaction) A-site deficiency + B-site doping: Pre-heating 1473 K, 1 h, sintering 1873 K, 1773 K, 1h. Sr 2+ Ti 4+ + RE 2 Sr 2+ 1-z RE3+ z Ti4+ O 2-3+δ Sr 2+ Ti 4+ + M 2 Sr 2+ 1-x (Ti4+, Ti 3+ ) 1-y M 2+ y O2-3+δ Hydrogen reduction: 1273 K, 1 h. Sr 2+ 1-z RE3+ z Ti4+ +δ + H 2 Sr 2+ 1-z RE3+ z Ti4+ 1-y Ti3+ y O Characterization: Crystallographic study XRD Conductivity measurement D.C. four-terminal method, 353K. Corrosion test 5 wt.% H 2 SO 4, 353 K, 65 1 h, magnetic stirring.

9 Electrical conductivity measurement D.C. four-terminal method Shape and dimension of specimen Pt plate V i 5 i 1/R = i/v Pt wire & Ag paste 5 11 Unit : mm

10 Corrosion resistance measurement Sr 1-z RE z Ti+δ (RE= Yb, Y, Gd, Sm, Nd, Pr, La) Sr 1-x Ti 1-y My+δ (M= Nb, Ta) ref : RE 2-x O 7-δ (RE= Yb, Y, Gd, Sm, Nd, Pr La) Gd 1.72 Ti 1.9 M.1 O 7-δ (M=Cr, Mn, Fe, Co, Ni) Condenser Test Conditions: 5wt% H 2 SO 4, 353K, 65 1 h. Three-neck glass Thermo-couple Heater 353K Stirrer H 2 SO 4 solution Ceramics Oil bath Corrosion Resistance: * Weight change * Solution analysis using ICP-MS Thermo-controller

11 Outline Background Sulfur-based Hybrid Cycle for Hydrogen Production Objectives Experiments Results and Discussions Crystallographic study Electrical conductivity study Corrosion resistance study Summary

12 Effects of Oxygen on Crystal Structures of SrTi XRD Patterns of Sr 1-x Ti±δ (b) 1773 K, 1 h. Intensity / cps (a) : Sr 2 TiO 4 : TiO 2 X=-.5 X= X=.5 Lattice parameter / Å Cubic θ / x in Sr 1-x Ti+δ No deficiencies and excesses of A-site were shown in the non-doped SrTi+δ system.

13 Effects of A-Site Doping (Y) on Crystal Structures cps / 1 3 XRD Patterns of Sr 1-z Y z Ti+δ z=.5 z=.2 : TiO 2 z= θ / Lattice volume / Å K, 1 h. : air : H z in Sr 1-z Y z Ti+δ A single phase of Sr 1-z Y z Ti+δ perovskite was formed in the narrow A-site doping region of z.2.

14 Effects of A-Site Doping (Gd) on Crystal Structures cps / 1 3 XRD Patterns of Sr 1-z Gd z Ti+δ 8 6 z=.1 4 z=.5 2 z=.2 TiO 2 Lattice volume / Å K, 1 h. : air : H 2 z= θ / z in Sr 1-z Gd z Ti+δ A single phase of Sr 1-z Gd z Ti+δ perovskite was formed in the narrow A-site doping region of z.5.

15 Effects of A-Site Doping (La) on Crystal Structures cps / 1 3 XRD Patterns of Sr 1-z La z Ti+δ 8 : TiO z=.3 z=.2 2 z=.1 Lattice volume / Å K, 1 h. : air : H 2 z= θ / z in Sr 1-z La z Ti+δ A single phase of Sr 1-z La z Ti+δ perovskite was formed in the wide A-site doping region of z <.3.

16 A site deficiency of Sr 1-x RE x Ti+δ z in Sr 1-z RE z Ti+σ Sm Yb Y Gd Pr Nd La 1873 K, 1 h Ionic radius / x1-12 m In Sr 1-z RE z Ti+δ system, A site deficient region increases with increasing ionic radius of A site doped ion (RE).

17 Effects of B-Site Doping (Nb) on Crystal Structures Intensity / cps 1 3 XRD Patterns of SrTi 1-y Nb y +δ 1 5 (a) : Sr 6 Nb 1, SrO y=.3 y=.2 y=.1 y= θ / Lattice parameter / Å Perovskite A single phase of SrTi 1-y Nb y +δ perovskite was formed in the wide B-site doping region of x.2. (b) 1773 K, 1 h. : air : H y in SrTi 1-y Nb y +δ

18 Effects of B-Site Doping (Ta) on Crystal Structures Intensity / cps 1 3 XRD Patterns of SrTi 1-y Ta y +δ 1 5 (a) : Sr 6 Ta 1, SrO y=.3 y=.2 y=.1 y= θ / Lattice parameter / Å (b) Perovskite 1773 K, 1 h. : air : H y in SrTi 1-y Ta y +δ A single phase of SrTi 1-y Ta y +δ perovskite was formed in the wide B-site doping region of x.2.

19 XRD Results of Sr 1-x Ti 1-y Nb y +δ Intensity / cps 1 3 Intensity / cps (a) Sr 1-x Ti.9 Nb.1 : TiO 2 x=.1 x=.5 2 x= θ / (c) Sr 1-x Ti.7 Nb : Sr 6 Nb 1, SrO x=.1 x=.5 x= Copyright (C) 25 CRIEPI. 2θ All / rights reserved. Intensity / cps 1 3 Lattice parameter / Å (b) Sr 1-x Ti.8 Nb.2 y =.1 : TiO 2 x=.15 x=.1 x=.5 x= θ / (d) Sr 1-x Ti 1-y Nb y y =.3 y = x in Sr 1-x Ti 1-y Nb y +δ Combined effects of A-site deficient, B-site doping, and O-excess 1773 K, 1 h.

20 XRD Results of Sr 1-x Ti 1-y Ta y +δ Combined effects of A-site deficient, B-site doping, and O-excess 1773 K, 1 h. Intensity / cps 1 3 Intensity / cps (a) Sr 1-x Ti.9 Ta.1 (b) Sr 1-x Ti.8 Ta.2 1 : TiO2 : TiO 2 x=.1 x=.5 2 x= θ / Sr 1-x Ti.7 Ta.3 x= θ / 1 5 (c) : TiO 2 x=.15 x=.1 x=.5 Lattice parameter / Å Intensity / cps x=.15 x=.1 x=.5 x= θ / (d) Sr 1-x Ti 1-y Ta y y =.2 y =.3 y = x in Sr 1-x Ti 1-y Ta y +δ

21 H 2 atmosphere Thermogravimetric Analysis Oxygen vacancy δ Oxygen vacancy δ Sr 2+ Ti 4+ + Ta 2 O 5 => Sr 2+ 1-xTi 4+ 1-yTa 5+ y+δ (a) Sr.95 Ti.8 Nb.2 Sr 2+ 1-x Ti 4+ 1-yTa 5+ y+δ + H 2 => 4 (b) Sr.95 Ti.9 Ta Temperature / K 8 12 Temperature / K Stoichiometry 16 Stoichiometry 16 Sr 2+ 1-x(Ti 3+, Ti 4+ ) 1-y (Ta 2+, Ta 3+, Ta 4+, Ta 5+ ) y + H 2 O Oxygen vacancy corresponds to (c) Sr.95 Ti.8 Ta.2 Stoichiometry. the formation of Ti 3+ in the Oxygen vacancy δ perovskites Temperature / K 16 The A-site deficiency resulted in the high stability of Ti 3+ in the perovskites.

22 Electronic Conductivity 353 K SrTi 1-y M y +σ (M = Nb, Ta) Sr 1-x Ti 1-y M y +σ (M = Nb, Ta) Conductivity σ / Scm K Y 1.9 O 7-σ Conductivity appearance Yb 1.9 O 7-σ Sr 1-x La.2 Ti+σ Target Sr 1-x Pr.2 Ti+σ Sr 1-x Nd.2 Ti+σ Pr 1.9 O 7-σ La 1.9 O 7-σ Sm Gd 1.72 O 1.6 O 7-σ 7-σ Nd Ti O 7-σ Perovskites Pyrochlores x in RE 1-x Ti+δ or RE 2-x O 7-δ The conductivities of Sr 1-z Re Z Ti+δ and Sr 1-x Ti 1-y M y +δ were higher than those of RE 2-x O 7-δ.

23 Corrosion Resistance Weight Loss change / % / % SrTi Sr.8 Pr.2 Ti Sr.8 Nd.2 Ti Sr.8 La.2 Ti Gd 2 O 7 353K, 5wt% H 2 SO 4 solution Gd 1.72 O 7-δ Gd 1.72 Ti 1.9 M.1 O 7-δ (M=Fe, Co, Ni) Gd 1.72 Ti 1.9 M.1 O 7-δ (M=Cr, Mn) Sm 2-x O 7-δ (.1 x.4) Y 1.9 O 7-δ Yb 1.9 O 7-δ 4 Time / hr 8 Target 12 Corrosion resistance of Sr 1-x RE x Ti+δ was lower than those of Yb 1.9 O 7, Y 1.9 O 7, Sm 2-x O 7-δ (.1 X.4), and Gd 2-x O 7-δ ( X.28). Corrosion rate =.1 mm/y 5 wt.% H 2 SO 4, 353 K.

24 XRD patterns of Sr 1-z RE z Ti +δ after the corrosion test Intensity / x1 3 cps Intensity / x1 3 cps Sr.8 Nd.2 Ti+σ 8 Before corrosion test Sr.8 La.2 Ti+σ θ / ο After corrosion test 6 7 Before corrosion test 4 5 2θ / ο After corrosion test Intensity / x1 3 cps SrSO 4 Sr.8 Pr.2 Ti+σ 2 3 Before corrosion test 4 5 2θ / ο After corrosion test 6 SrTi + H 2 SO 4 SrSO 4 + TiO 2 + H 2 O 7 8

25 Corrosion Resistance Weight loss / % K, 5wt% H 2 SO 4 solution Sr.9 Ti.9 M.1 (M=Nb, Ta) Sr.9 Ti.8 M.2 (M=Nb, Ta) Sr.95 Ti.9 M.1 (M=Nb, Ta) Sr.95 Ti.8 M.2 (M=Nb, Ta) Gd 1.72 O 7- δ Gd 1.72 Ti 1.9 M.1 O 7- (M=Fe, Co, Ni) δ Gd 1.72 Ti 1.9 M.1 O 7- (M=Cr, Mn) δ Target (Corrosion rate =.1mm/y) 5 wt.% H 2 SO 4, 353 K Time / hr 12 Sr 1-x Ti 1-y M y +δ had high corrosion resistance

26 Summary (1) A series of corrosion tests, which conducted in a 5 wt% H 2 SO 4 solution at 353 K, revealed that all of the Sr 1-x Ti 1-y M y +δ (M = Nb, Ta) perovskites had high corrosion resistance, indicating that the perovskites with good electrical conductivities (> 1 S/cm) are promising anode materials for SHC process. (2) Rare earth metal RE-doped SrTi (RE = Nd, Pr, La) in forms of Sr 2+ 1-zRE 3+ zti 4+ 1-zTi 3+ z showed good electrical conductivity after being reduced at high temperatures in the H 2 atmosphere, but exhibited low corrosion resistance in a 5% H 2 SO 4 solution at 353 K, due to the formation of SrSO 4.

Extrinsic Defect Reactions in

Chapter 5 Extrinsic Defect Reactions in Perovskite Materials The work presented in this Chapter has been published in Solid State Ionics [203]. 5.1 Introduction With dwindling fossil fuel reserves [204]