Chemical and electrical properties of LSM cathodes prepared by mechanosynthesis

1 Chemical and elec ical p ope ies o LSM ca hodes p epa ed by mechanosyn hesis R. Mo iche* 1,2 , D. Ma e o-López 3 , F. J. Go o 1 and M. J. Sayagués 1 1 Ins i u o de Ciencia de Ma e iales de Se illa, Cen o Mix o CSIC-US, Amé ico Vespucio 49, 41092 Se illa, Spain. 2 Depa amen o de Ciencia e Ingenie ía de Ma e iales, Uni e sidad Rey Juan Ca los, C/Tulipán s/n, 28933 Mós oles, Mad id, Spain. 3 Depa amen o de Física Aplicada I, Facul ad de Ciencias, Uni e sidad de Málaga, Campus Tea inos s/n, 29071 Málaga, Spain. *Co esponding au ho : ocio.mo iche@u jc.es; phone: 0034 914888083 Abs ac Mechanosyn hesis o La 1-x S x MnO 3 (x = 0, 0.25, 0.5, 0.75 and 1) was ca ied ou a oom empe a u e om s oichiome ic mix u es o La 2 O 3 , Mn 2 O 3 and S O, ob aining monophasic powde s wi h he pe o ski e s uc u e. Physical p ope ies o hese ma e ials and hei chemical compa ibili y wi h he elec oly e y ia s abilized zi conia (YSZ), which depend s ongly on he La/S a io, we e e alua ed o co obo a e a ailabili y o be implemen ed as ca hode ma e ial in solid oxide uel cells (SOFCs). Elec ical conduc i i y alues in ai anged be ween 100–400 Scm -1 in he empe a u e ange o 25–850 °C. Samples p esen ed low eac i i y wi h YSZ in he wo king empe a u e ange (600–1000 ºC) main aining he g ain size small enough o p ese e he ca aly ic ac i i y o oxygen educ ion. Keywo ds: Solid Oxide Fuel Cells, LSM, mechanochemis y, pe o ski e s uc u e 2 1. In oduc ion Solid Oxide Fuel Cells (SOFCs) ha e been widely s udied in he las decade in o de o each a blue economy concep : “a ze o emission wo ld” [1]. SOFCs a e specially in e es ing because o hei ad an ages espec o he o he ypes o uel cells: heo e ical e iciency o 80–90%, lowe cos s, uel lexibili y and be e s abili y wi h ime, al hough he ope a ion empe a u e is s ill oo high be ween 800–1000 °C o p ac ical applica ion [2,3]. Di e en mixed ionic-elec onic conduc o s wi h pe o ski e s uc u e ha e been p oposed in he las ew yea s as po en ial ca hode ma e ials o SOFC, such as cobal i es, e i es, nickela es and double pe o ski es [3]; howe e , hey usually exhibi chemical and he mal expansion incompa ibili ies wi h y ia s abilized zi conia (YSZ) elec oly e. S –doped lan hanum mangani es, La 1-x S x MnO 3 (LSM), a e he mos common ca hode ma e ial used in SOFC sys ems due o hei high s abili y unde oxidan a mosphe es and high empe a u es compa ed o o he al e na i e ma e ials [4]. Syn hesis o hese ma e ials has been ex ensi ely s udied using di e en syn he ic ou es, bu he mechanochemical me hod i sel (no as ac i a ion) is no ye a common one [5]. Mechanochemis y is a ela i e simple p ocess ha uses high-ene gy ball mills and pe mi s p oduc ion among o he s o nanos uc u ed mixed oxides [6]. The mechanical ene gy om impac and shea o ces by applica ion o a high equency mo emen is ans e ed o he powde inducing solid s a e chemical eac ions. One o he mos impo an ad an ages o mechanochemis y is i s capabili y o p oduce la ge ma e ial quan i ies a oom empe a u e and in a e y sho ime. In he p esen wo k, a s udy o he chemical and physical p ope ies o La 1-x S x MnO 3 (LSM) sys em ob ained by mechanosyn hesis was ca ied ou as small di e ences in he La/S a io cause signi ican changes in he ca hode pe o mance. A s uc u al s udy o 3 his sys em, also p epa ed by mechanochemis y, was ecen ly published by Sayagués e al. [7], al hough smalle amoun s o samples we e syn hesized. A summa y o he ob ained s uc u al esul s is p esen ed he e also o demons a e he scalabili y o mechanochemis y. Chemical compa ibili y be ween ca hode and elec oly e and elec ical p ope ies o he ca hode we e measu ed in a cell ha was buil using he syn hesized LSM as ca hode and YSZ as elec oly e. 2. Expe imen al La 1-x S x MnO 3 powde samples wi h di e en S con en (x = 0, 0.25, 0.5, 0.75 and 1) we e syn hesized by a mechanochemical me hod using a plane a y ball mill (model Mic o-Mill Pul e ise e 7, F i sch) om s oichiome ic mix u es o La 2 O 3 (Ald ich 99.98 %), Mn 2 O 3 (Ald ich 99 %) and S O. The las one was ob ained om calcina ion o S CO 3 (Ald ich 98%) a 1200 °C o 12 h. This oxide mix u e was placed in o a ha dened ch omium s eel ja along wi h 7 WC balls (26.4 g; Ø = 15 mm) and was milled a 600 pm (disc and ial) in ai o ob ain 6 g o each sample. The di e en composi ions and he equi ed milling ime o ob ain single phase powde s (P samples) a e p esen ed in Table I. P samples we e hen uniaxially p essed in o pelle s o 12×5 mm (mic os uc u al cha ac e iza ion) o 12×1 mm ( o al conduc i i y) and sin e ed in ai a 1300 °C o 8 h wi h a hea ing a e o 10 °Cmin -1 and ee cooling (H samples). S uc u al cha ac e iza ion and phase iden i ica ion we e ca ied ou by a PANaly ical X’Pe Di ac ome e . X-Ray Di ac ion (XRD) pa e ns we e scanned be ween 10–80° in 2θ and s ep–scan mode, using a s ep o 0.05° and an acquisi ion ime o 320 s. Peaks we e indexed using X’Pe HighSco e Plus and FullP o and WinPlo so wa es [8,9] we e used o do he i ing and calcula e he cell pa ame e s and he 4 di ac ion domain size (D). Scanning elec on mic oscopy (SEM) images we e ob ained on Hi achi S5800 SEM-FEG (H samples) and Hi achi S-2400N ( he elec ode- elec oly e in e ace) mic oscopes. The composi ional a ia ion h ough he in e ace was analyzed using Ene gy Dispe si e X- ay Spec oscopy (EDX) in Hi achi S-2400N, which was equipped wi h a B uke de ec o . T ansmission Elec on Mic oscopy (TEM and HRTEM) images and elec on di ac ion (ED) pa e ns we e pe o med on a 200 kV Philips CM200 mic oscope equipped wi h a supe win objec i e lens and a LaB 6 ilamen (poin esolu ion=0.25 nm) and a 300 kV TECNAI G2 F30 mic oscope wi h a ield emission sys em (poin esolu ion=0.2 nm). The analysis o he HRTEM images was done wi h he Digi al Mic og aph so wa e (Ga an Inc.). The samples we e p epa ed by dispe sion o he powde in ace one and d ople s o he suspension we e deposi ed on o a coa ed ca bon coppe g id. The o al conduc i i y o he pelle s (H samples) was de e mined by he ou -poin Van de Pauw me hod be ween 100 and 900 ºC du ing he cooling p ocess [10]. The a ea-speci ic pola isa ion esis ance (ASR) alues we e ob ained unde symme ical a mosphe es in a wo elec ode con igu a ion. Dense YSZ pelle s (8% Y 2 O 3 Tosoh), o 10 mm o diame e and 1 mm o hickness we e ob ained a 1400 ºC o 5 h. The symme ical cell o LSM ca hode and YSZ elec oly e was p epa ed by sc een p in ing using a slu y o 50 w % LSM (P samples) and 50 w % Te pineol; and hen sin e ed a 1100 ºC o 1h [10]. Impedance spec a o he cells was pe o med using a Sola on 1260 FRA, a open ci cui ol age (OCV), in he 0.01–10 6 Hz equency ange wi h an ac signal ampli ude o 50 mV. The spec a was analysed by using he ZView so wa e [11]. 5 3. Resul s and Discussion 3.1. Mic os uc u al and s uc u al analysis XRD esul s o he La 1-x S x MnO 3 samples ob ained by mechanochemis y a e p esen ed in Figu e 1a. In he powde samples P1–P4, all he maxima we e indexed in a pseudo-cubic symme y (Pm-3m, 221) because he small size o cohe en di ac ion domains ( he peaks a e wide) did no allow esol ing he eal c ys al symme y. Howe e , sample P5 (x = 1) exhibi ed some ex a XRD peaks ha indica ed he o ma ion o he hexagonal symme y (P6 3/ mmc, 194). Simila esul s we e ound in ou p e ious wo k [7]; al hough in ha case all milled samples p esen ed he pseudo-cubic symme y (Fig. 2 in e . [7]). This di e ence could be due o he highe milling in ensi y egime used in [7] (ball- o-powde a io (BPR) o 92 ins ead o 31 as in p esen wo k), which p obably induced a u he dec easing o he cohe en di ac ion domains. A e he sin e ing ea men (Fig. 1b), samples P1–P4 de eloped he hombohed al s uc u e (R-3c, 167; H1–H4 samples), whe eas sample P5 main ained he same hexagonal symme y (H5). Uni cell dimensions o each phase and he a e age c ys alli e sizes (D) a e p esen ed in Table II. As can be deduced om he la ge b oadening o e lec ions in Fig. 1a he c ys alline domain o he as-p epa ed samples is e y small, a ound 15 nm. Compa ing H and P samples, he size o he di ac ion domains o sin e ed samples has signi ican ly inc eased (abou 200 nm) as can be in e ed om he na owe XRD peaks obse ed (Fig. 1b). When La 3+ is subs i u ed o S 2+ in LaMnO 3 la ice, an amoun o Mn 3+ con e s in o Mn 4+ o main ain he c ys al elec oneu ali y and he aniso opic de o ma ion due o he Jahn-Telle e ec is educed. I he la ice pa ame e s o he samples a e analyzed (Table II; P5 pa ame e s could no be calcula ed app op ia ely since all he hexagonal peaks we e no su icien ly esol ed), i can be obse ed ha when x alue inc eases, a 6 pa ame e dec eases ( o bo h P and H samples) while he solid solu ion emains. Howe e , when he o al La is subs i u ed by S (H5), a pa ame e shows an inc ease as a esul o he change in he symme y g oup (P6 3/ mmc, 194). Simul aneously, an inc ease o c pa ame e ( o H samples) occu s om x = 0 o x = 0.5 and hen i sligh ly dec eases o x = 0.75. This a ia ion in c pa ame e can be caused by a p e e en ial occupa ion o a om posi ions in he cell as i was p oposed by Jang e al. [12]. Finally, c achie es a alue o 9.0762 Å o H5 sample gi en place o he hexagonal s uc u e. All o hese e ec s a e e lec ed in a olume cell a ia ion esul ing in a dec ease om 353.05 Å 3 o 338.67 Å 3 in he solid solu ion as S con en inc eases. Fo H5 wi h a 100% in S con en he cell olume u he diminishes eaching a alue o 233.24 Å 3 ; again his change is due o he o ma ion o he new hexagonal uni cell wi h smalle c pa ame e . Rela i e densi y measu emen o he pelle s (H samples) has shown alues close o 80% (Table II), showing a sligh inc ease wi h inc easing he S con en while main aining he solid solu ion ( he hombohed al s uc u e). Howe e , o x = 1 (H5), whe e he e is a s uc u al change, he ela i e densi y sligh ly dec eases. SEM and TEM ep esen a i e esul s o he powde and sin e ed samples (P1, P3, H1 and H3) a e p esen ed in Fig 2 and 3 espec i ely. The mic os uc u e o P samples obse ed by SEM is e y simila o all o hem, being o med by agglome a ions o small pa icles. Rep esen a i e mic og aphs co esponding o samples P1 and P3 a e p esen ed in ig 2(a-b). Howe e he pa icle size o H samples a ies as a unc ion o he La/S a io and ep esen a i e mic og aphs a e p esen ed in Figu e 2 (c-d) o H1 and H3 samples. H1 (LaMnO 3 ) shows an a e age g ain size o 2 µm (Fig. 2c) and H3 sample (Fig. 2d), wi h a highe S con en , shows an a e age g ain size o 0.5 µm. Bo h images show ace ed shape c ys als, p esen ing se e al unca ed p isms. I is wo h 7 no ing ha he p esence o ace ed g ow h can imp o e he ca aly ic p ope ies o samples due o an inc ease in he speci ic su ace a ea. Figu e 3 shows he TEM, ED and HRTEM esul s o he P1, P3, H1 and H3 samples. The powde samples P1 and P3 a e o med by agglome a ed pa icles as seen in he TEM images (le ). The c ys al di ac ion domain is e y small as can be deduced om he analysis o he co esponding elec on di ac ion pa e ns (inse ) and he HRTEM mic og aphs ( igh ). Some c ys al domains a e ma ked in he HRTEM images and he size is be ween 5 and 20 nm. In all o he domains (110) in e plana spacing was measu ed and his dis ance could be indexed in bo h, he cubic and he hombohed al sys em; i was indexed in he cubic pe o ski e s uc u e (P) o be cohe en wi h he X- ay esul s. The c ys al size o he sin e ed samples (H1 and H3) has inc eased conside ably as obse ed by SEM. A wide dispe sion o sizes om 100 nm o 2 mic ons was ound. The H1image (le ) shows an o ien ed c ys al along he [122] zone axis o he hombohed al s uc u e, as indica ed by he EDP (inse ) and he HRTEM mic og aphs is p esen ed on he igh whe e he in e plana spacing and he (hkl) planes a e ma ked. TEM image shown o H3 sample p esen s an elonga ed shape pa icle wi h di e en con as in he ip; ha co espond o wo di e en o ien ed c ys alline domains as can be deduced om he co esponding ED pa e ns (inse ). One o hem is o ien ed along he [001] hombohed al s uc u e; i is e y easy o dis inguish he hexagonal symme y in he con as o he HRTEM mic og aph p esen ed on he le ( he (110) planes and spacing a e depic ed). HRTEM image o he second c ys alline domain is p esen ed o he igh and belongs o he cubic pe o ski e s uc u e o ien ed along [001], he (100) planes ha e been ma ked in he image. All o hese esul s a e in acco dance wi h ou p e ious wo k [7], which demons a es he scalabili y o mechanosyn hesis as i is possible o ob ain 6 g ins ead 8 o 2 g (in 1 h o milling) o LSM ma e ial only by inc emen ing sligh ly he milling ime (see able I). This makes e iden some o he ad an ages o mechanochemis y o e o he syn he ic me hods, since he adi ional ce amic me hod equi es a empe a u e o 1000-1200 ºC [13]; o i s scalabili y con as ed wi h Pechini, ci a e [14] o plasma- sp aying me hods [15-17]. 3.2. Ca hode-Elec oly e In e ace Chemical compa ibili y be ween he elec odes and he elec oly e is an impo an aspec because di e en ma e ials a e in con ac o a p olonged ime a high empe a u e wha can p omo es eac i i y and ca ion di usion be ween he componen s, a ec ing nega i ely he cell pe o mance. A mix u e o powde s 50:50 w .% composed o La 1-x S x MnO 3 and YSZ elec oly e has been p epa ed and annealed in a u nace o 24 h in a ange o empe a u es om 800 o 1300 °C. The mix u e a e hea ing was analyzed by XRD and he esul s o x=0 and x= 0.25 a e shown in Fig. 4, he p esence o absence o new p oduc s as a consequence o he eac ion be ween LSM and YSZ was s udied. Phases wi h low S con en (x=0.25) showed low chemical eac ion wi h YSZ in all he empe a u e ange s udied. O he wise, o a S con en o x = 0.5, a eac ion be ween S – and Z – con aining phases akes place leading o he o ma ion o S Z O 3 as eac ion p oduc a 1300 °C, howe e he e is no eac ion below 1000 ºC, his ac is impo an as he SOFCs usually wo ks below his empe a u e . The LSM/YSZ in e ace o symme ical cells p epa ed by sc een p in ing and sin e ed a 1100 ºC o 1 h was also s udied by SEM and EDX o analyze he chemical composi ion h ough he ca hode-elec oly e in e ace and elucida e he possible in e di usion o he di e en elemen s. SEM mic og aphs o he c oss sec ion o 9 in e aces wi h di e en S composi ions a e shown in Figu e 5. All ca hodes p esen adequa e po osi y and appa en ly good con ac wi h he elec oly e. As could be expec ed, he LSM g ains ha e g own less han o H samples sin e ed a 1300 °C. LSM samples wi h x = 0 and x = 0.25 ha e g ain sizes o ~ 1 µm, sligh ly smalle han a comme cial sample o P axai (La 0.8 S 0.2 MnO 3 ), also shown in Figu e 5 o a compa ison pu pose. Fo la ge x alue samples, he g ain size dec eases wi h inc easing he S con en , eaching e y small alues o less han 0.5µm o S MnO 3 . In his case he a ia ion end is e e sed when compa ed wi h H samples, p obably due o he di e en sin e ing empe a u e and way o p epa ing he bulk sample. EDX line p o iles o La, S , Mn, Y and Z we e ca ied ou h ough he in e ace and ep esen a i e esul s ( o x=0.25 and 0.5) a e p esen ed in Figu e 6. I could be deduced ha o highe S con en s, Z di usion in o he ca hode laye is mo e p onounced, eaching a pene a ion dis ance o 1 µm o La 0.25 S 0.75 MnO 3 . These esul s a e in ag eemen wi h he o ma ion o S Z O 3 p e iously obse ed by XRD analysis as p oduc o eac ion be ween componen s. The appa en di usion o La, S and Mn elemen s is due o ca hode pa icles de achmen , deposi ed on he analyzed su aces. 3.3. Elec ical p ope ies The subs i u ion o S 2+ o La 3+ inc eases he con en o Mn 4+ in he s uc u e as i has been men ioned abo e, c ea ing elec onic holes o keeping cha ge neu ali y [18]. Fo his eason, LSM is p- ype elec onic conduc o unde oxidan a mosphe e [19, 20]:                               Bu no only elec onic conduc i i y is p esen , also ionic conduc i i y due o he ac ha acancies a e in oduced o non-s oichiome ic oxygen con en and alio alen           10 20 30 40 50 60 70 80 P1 2 θ * •• • • • • •• • • •• • • • •• • • • • * • • * * • * ** * •• • • •••• P2 P3 P5 P4  10 20 30 40 50 60 70 80 2 θ H1 * ♦♦♦ H2 ♦ ♦ ♦ H3 ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦♦♦♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ * H4 * ** * * * **** * * * ** * H5    Figu e 1       (a) (b)                          Figu e 2                                             Figu e 3  P1 P3 H1 5 nm [001]p H3     Figu e 4          (a) (b) La 0.75 S 0.25 MnO 3 - YSZ LaMnO 3 - YSZ * * * •• • • * •• • 1100 o C 900 o C * * ** * * • • • • * •• • * * ** * * •• • • * • • • 20 30 40 50 60 70 80 * * ** * * •• • • * •• • 25 o C * * ** * * • • • • * •• • 20 30 40 50 60 70 80 2 θ • * * ** * * • • • • * • • 2 θ              Figu e 5          Figu e 6 (a) (b) 0 20 40 60 80 100 3 2 1 0 1 23 La 0.75 S 0.25 MnO 3 - YSZ Rela i e In ensi y (%) Posi ion (µm) La Mn S Y Z 4 0 20 40 60 80 100 La 0.50 S 0.50 MnO 3 - YSZ Rela i e In ensi y (%) Posi ion (µm) La Mn S Y Z 43 2 101 2 3 (b) Figu e 7    0.8 1.0  1.2 1.4 10  - 1 10  0 10  1 10  2 10  3900  800 700 600 500 LaMnO  3  La  0.75  S  0.25  MnO  3  La  0.5  S  0.5  MnO 3  La  0.25  S  0.75  MnO  3 La  0.8  S  0.2  MnO  3  / P axai  T(ºC)  (b) 0.8 1.2 1.6 2.0 2.4 2.8 3.2 10-2 10-1 100 101 102 103 800 600 400 200 LaMnO 3 La 0.75 S 0.25 MnO 3 La 0.5 S 0.5 MnO 3 La 0.25 S 0.75 MnO 3 S MnO 3 σ( Scm -1 ) 10 3 /T (K -1 ) T(ºC) phase ansi ion (a)  La  S  3  MnO  LaMnO  3  0.75  0.25  MnO 3  La  0.5  S  0.5  La  S  0.75  MnO  3  S  0.25  MnO  3  200  400  600  10  3  0  10  10  -2  10  -1  10  2  10  1  800  T(ºC)  0.8  1.6  1.2  2.0  2.4  2.8  3.2  10 3 /T (K - 1 )  10 3 /T (K - 1 )                      Figu e 8 