Enhanced bifunctional oxygen catalysis in strained LaNiO 3 perovskites

Size: px

Start display at page:

Download "Enhanced bifunctional oxygen catalysis in strained LaNiO 3 perovskites"

Clyde Harrington
5 years ago
Views:

1 Supplementry Informtion Enhnced bifunctionl oxygen ctlysis in strined LNiO 3 perovskites Jonthn R. Petrie +, Vlentino R. Cooper +, John W. Freelnd ±, Trici L. Meyer +, Zhiyong Zhng, Dniel A. Luttermn, nd Ho Nyung Lee +* + Mterils Science nd Technology Division, Ok Ridge Ntionl Lbortory, Ok Ridge, TN, 37831, USA. ± Advnced Photon Source, Argonne Ntionl Lbortory, Argonne, IL, 6439, USA. Chemicl Sciences Division, Ok Ridge Ntionl Lbortory, Ok Ridge, TN, 37831, USA. *E-mil: hnlee@ornl.gov Thin Film Synthesis Epitxil LNO films (1-1 nm in thickness) were grown on vrious oxide substrtes by pulsed lser epitxy (PLE). The LNO growth temperture, oxygen prtil pressure, lser fluence, nd repetition rte were optimized t 6ºC, 1 mtorr, 1.5 J/cm, nd 1 Hz, respectively. Electrochemicl Chrcteriztion The ORR nd OER chrcteriztion were performed t 5 C in 15 ml solution of O - sturted.1 M KOH developed with Sigm-Aldrich KOH pellets nd Milli-Q wter. Before ech test, O ws bubbled directly into solution while, during electrochemicl testing, n overpressure of O ws bubbled just over the solution to void unnecessry turbulence. A threeelectrode rotting disk electrode (RDE) setup ws used with Pt counter electrode nd stndrd clomel (SCE) reference electrode. For conversion to RHE, the reference electrode ws clibrted to the reduction of H t the counter electrode. Smples were diced into.5x.5 mm nd the lttice mismtched substrtes ttched vi conductive pste (e.g. silver or crbon-bsed) to polished glssy crbon (GC) disc (5 mm in dimeter). Conductivity ws mintined from the GC to the film by pplying the conductive pste long the side of the substrte. All pste ws subsequently covered with epoxy to prevent ny rection in solution. Potentil ws pplied vi Biologic SP- Potentiostt t 5 mv/s nd the smples hd rotting speed of 16 rpm. Ohmic losses due to the film nd solution were determined vi high frequency (~1 khz) impednce mesurement nd subtrcted from the pplied potentil to obtin ir-corrected currents. Electrochemiclly-ctive specific surfce res were determined on the LNO films vi double-lyer cpcitnce mesurements round the open-circuit potentil (OCP), s described in S1

2 the section lter on surfce re determintion. 1 These mesurements produce systemtic surfce re used in compring reltive ORR ctivities; in this cse, ll specific surfce res were within 1% of the geometric re. Before determining polriztion curves, the potentil ws cycled t lest 5 times t scn rte of 5 mv/s between.3 nd.7 V vs SCE to expose stble surfce under these ORR/OER conditions. Subsequent polriztion curves were tken t scn rte of 5 mv/s t 16 rpm t lest three times to ensure reproducibility. The verge of the nodic nd cthodic sweeps were tken to minimize cpcitive effects. Additionl sweeps in Ar-sturted.1 M KOH showed negligible ctivity. To compre ctivities to noble metls, highly (111) textured Pt thin films film (5 nm in thickness) were respectively sputter deposited or vporized onto polished glssy crbon discs of the sme size Structurl nd Spectroscopic Chrcteriztion The smple structure ws chrcterized with high-resolution four circle XRD. In- nd out-ofplne lttice constnts were determined vi reciprocl spce mpping (RSM). Temperturedependent DC trnsport mesurements were conducted using the vn der Puw geometry with 14 T Physicl Property Mesurement System (PPMS). Vlency nd XLD mesurements vi XAS were performed t the bemline 4-ID-C of the Advnced Photon Source t Argonne Ntionl Lbortory. Theory All density functionl theory (DFT) clcultions were performed using the Perdew-Burke- Erzenhoff (PBE) exchnge-correltion functionl nd projector ugmented wve (PAW) potentils s implemented in the Vienn Ab Initio Simultion Pckge (VASP v5.3.5). A plnewve cutoff of 4 ev nd Hubbrd U vlue of 4 ev on the Ni d-sttes were employed for ll clcultions. For bulk LNO n 8 8 k-point ws employed, yielding optimized lttice prmeters of =5.419 nd c=13.1 Å for structure with n R3cH in good greement with the experimentl vlues of (=5.4573, c= Å). All surfce clcultions were performed with k-point mesh. For the surfce clcultions the in-plne xis ws vried from 3 % to +3% strin in steps of 1% nd ll of the toms were llowed to fully relx until the forces on ech tom ws less thn 15 mev/å. Determintion of surfce re Even though these smples re thin films, we cnnot simply use the geometric re to evlute the current density; in cse there is significnt roughness, the specific surfce re must be known. To do this, we employ the double-lyer cpcitnce method discussed in McCrory, et.l. 1 where the ECSA (electrochemiclly ctive surfce re defined s specific re here) is: ECSA = C DL /C s = (i DL /ν)/c s (1) where C DL is the double-lyer cpcitnce, C s is the specific cpcitnce per unit re on n tomiclly flt surfce, i DL is the double-lyer chrging/dischrging current, nd ν is the scn rte. To determine the re, the C s for LNO must be known. We used LNO on STO for this since, while ll the film ppered ner-tomiclly flt in the AFM, it hd the lowest RMS vlues (see Fig. S3). To ensure this ws the cse, we double-checked the surfce re on diced.5.5 mm LNO/STO using common method incorporting different scn rtes (ν) of the reversible S

3 I(mA) I (ma) I pek (ma) I (ma).8.4. Scn Rte (V/s) Fe 3+/ Fe + redox couple found in.4 mmol of potssium ferricynide (K 3 Fe(CN) 6 ). At room temperture nd in.1 M KOH, this produces oxidtion/reduction peks t the surfce whose pek current (I pek ) cn be used to determine the specific surfce re (A) vi mnipultion of the Rndles-Sevcik eqution: slope =.173 ma*(s/v) 1/ R = A = ( )D -1/ C -1/ n -3/ (I pek ν -1/ ) ().4 where ν is the scnning rte (. to.5 mv/s),. D is the diffusion constnt of the iron-bsed nlyte ( cm -.8 /s), C is the bulk [Scn Rte (V/s)] 1/ concentrtion of sid nlyte (~.4 mol/cm 3 ), -... E(V) vs SCE.4.6 nd n is the number of electrons in the redox hlfrection, which ws 1 in this cse for Fe + /Fe 3+. Figure S1. Effect of chnging scn rte on Both the scns nd the plot of I pek vs. ν -1/ re the Fe + /Fe 3+ reversible redox peks. shown in Fig. S1. The specific surfce from this Cyclic voltmmogrms of K method is.59±.4 cm 3 Fe(CN) 6 in.1, which is within the M KOH t scnning rtes from. to.5 experimentl rnge of error for n tomiclly flt V/s. The inset shows plot of the verge of.5.5 mm film (.65 cm ). We note tht the bsolute vlue oxidtion/reduction this test ws performed on n LNO/STO smple current peks vs. scn rte to determine the tht did not undergo the ORR/OER surfce re. mesurements but hd the sme geometric, i.e. rel, re of the one tht did; using this Fe + /Fe 3+ method for ll the smples would hve introduced the possibility of Fe contmintion t the surfce. Once the LNO on STO ws independently verified s ner-tomiclly flt, this film ws scnned round the OCP (open circuit potentil) where ll current ws ssumed due to double-lyer chrging. The scn for LNO on STO cn be seen in Fig. S for scn rtes from. to.5 V/s. As seen in the inset, by plotting the ssumed double-lyer current vs. ν, the C DL is found to be ~.58 mf. Since the specific re is ~.65 cm, the Cs ~.93 mf/cm for LNO in this environment.. Figure S. Double-lyer cpcitnce mesurements on LNO/STO. Cyclic voltmmogrms round the OCP t scnning rtes from. to.5 V/s in.1 M KOH. The inset shows plot of the double-lyer current verged from the bsolute vlues of the nodic/cthodic scns vs. scn rte to determine the double-lyer chrging cpcitnce. -. Scn Rtes (V/s) slope =.575 mf R = Scn Rte (V/s) E(V) vs SCE S3

4 Using this vlue for LNO, the specific surfce res determined vi the double-lyer cpcitnce method for LNO on other substrtes cn be seen in Tble S1. The vlues re ll ner.65 cm both before nd fter ORR/OER testing, which indictes stble surfce. Geometric res were clculted from digitl photogrph of ech smple. The specific res were ll within 1% of the geometric re nd the roughness fctor (specific re/geometric re) of ll these re ~1; however, due to the dicing process, the geometric re of other substrtes re not exctly.5.5 mm, s seen in the slight disprity of res in the films. Only post-test surfce res were used for determintion of ORR nd OER current density. In ddition, the surfce re for the Pt sputtered onto glssy crbon disk, s determined vi either H stripping with Fe + /Fe 3+ redox couple verifiction, is shown. Tble S1. Specific res of LNO nd Pt both before nd fter testing. Specific res of LNO on different substrtes using the double-lyer technique nd C s from LNO on STO. The specific re is close to the geometric re for ll films both before nd fter testing, indicting (1) the inherent fltness of the films nd () their stbility. The sputtered Pt used s comprison for ORR/OER ctivity is lso included. Mteril/Substrte Geometric Are Pre-test specific re Post-test specific re LNO/LAO.7±.1 cm.71±.5 cm.69±.5 cm LNO/LSAO.7±.1 cm.7±.5 cm.75±.5 cm LNO/LSAT.64±.1 cm.6±.5 cm.63±.5 cm LNO/STO.63±.1 cm.63 cm *.6±.5 cm LNO/DSO.69±.1 cm.75±.5 cm.71±.5 cm Pt/glssy crbon.196±.1 cm.1±.9 cm.5±.9 cm *There is no error here since LNO/STO pre-test used to determine C s for LNO Determintion of d z occupncy using x-ry liner dichroism (XLD) As described by Wu, et. l., 3 the rtio of holes (unoccupied sttes) in the e g orbitls is clculted through XLD sum rules s: β = h z = 3I c (3) h x y 4I b I c where h z is the hole occupncy number of the d z orbitls, h x -y is the hole occupncy number of the d x -y orbitls, I c is the X-ry bsorption long the out-of-plne direction, nd I b is the X-ry bsorption long the in-plne direction. There re two possible sttes in ech e g orbitl, or n z = h z nd n x -y = h x -y, where n corresponds to the electron occupncy number nd h corresponds to the electron occupncy number in ech respective orbitl. Adding the sttes of both orbitls, n eg + h eg = 4, where n eg is S4

5 the totl electron occupncy number nd h eg is the totl hole occupncy number. Using the prior ssumption tht n eg = 1, h eg = 3 = h z + h x -y nd the frction d z occupncy f(d z ) cn be determined s: f( d z ) = n z n eg = n z = h z = 3β 1+β (4) To determine the intensities, we took the verge of different Pseudo-Voigt liner combintions of Gussin nd Lorentzin curves to fit the XLD dt. The error ws determined by the stndrd devitions of the fits. Errors for intensities were dded in qudrture when used for determining orbitl occupncy. S5

q z [1] (Å -1 ) q z [1] (Å -1 ) q z [1] (Å -1 ) Intensity ( rb. units) In-Plne Prmeter (Å) ADDITIONAL FIGURES * = substrte * LNO Surfce in vcuum 3.943 (+.

6 q z [1] (Å -1 ) q z [1] (Å -1 ) q z [1] (Å -1 ) Intensity ( rb. units) In-Plne Prmeter (Å) ADDITIONAL FIGURES * = substrte * LNO Surfce in vcuum (+.7%) * * * * *DSO *STO *LSAT *LSAO *LAO c NiO 6 L 3.95 (+1.7%) (.7%) (%) ε > ε < q degrees (-1.%) b LSAO L AO L NO c L NO L NO DSO q x [1] (Å -1 ) q x [1] (Å -1 ) q x [1] (Å -1 ) Figure S3. Structurl chrcteriztion of strined LNO films., XRD θ-θ scn of the pek of 1 nm LNO films on DSO, STO, LSAT, LSAO, nd LAO substrtes in order of incresing compressive strin. The intensity scle is logrithmic. The film on LSAO is lmost fully relxed, leving LNO in the unstrined stte. With incresing compressive strin, there is incresed tetrgonlity, resulting in rised Ni 3+ ion in the interrupted octhedrl lyer t the surfce (shown s squre pyrmidl). XRD scns ppered similr both before nd fter electrochemicl testing. b, Reciprocl spce mps of LNO films show coherent strining on ll substrtes except for LSAO, where it is lmost fully relxed (ε ~ %). c, Representtive tomic force microscopy imge of LNO film on STO, where RMS roughness < 1 nm. S6

7 ( cm) 1 LNO/DSO ( =.7%) LNO/STO ( = 1.7%) LNO/LSAT ( =.73%) LNO/LSAO ( = %) LNO/LAO ( = -1.%) T(K) Figure S4. Electricl chrcteriztion of strined LNO films. Electricl trnsport dt of LNO on different substrtes revel resistivities (ρ) s low s 45 μωcm s well s residul resistivities (ρ 3K /ρ 5K ) of -3, reminiscent of the highest qulity LNO. 4 Although the electrochemicl mesurements re lredy corrected for ohmic losses, the lck of correltion between ctlytic ctivity nd resistivity from LSAT to LAO (essentilly equivlent DC impednce) rgue ginst the ORR/OER rections being limited by electron trnsport through the mteril. S7

Activity J OER ( A/cm ) Bifunctionl (mv) J ORR ( A/cm ) J ORR ( A/cm ) J OER ( A/cm ).6.6.6.61 13 L AO.61 13 L AO.61 13 L AO q z [1] (Å -1 ).6.59 13 L NO (1 nm) q z [1] (Å -1 ).6.59 13 L NO (5 nm) q z [1] (Å -1 ) 13 L NO (1 nm).

8 Activity J OER ( A/cm ) Bifunctionl (mv) J ORR ( A/cm ) J ORR ( A/cm ) J OER ( A/cm ) L AO L AO L AO q z [1] (Å -1 ) L NO (1 nm) q z [1] (Å -1 ) L NO (5 nm) q z [1] (Å -1 ) 13 L NO (1 nm) q q x [1] (Å -1 x [1] (Å -1 ) ) q x [1] (Å -1 ) Å ε = ε < (-1.%) LNO 1nm /LAO LNO 5nm /LAO (%) LNO 1nm /LAO relxed b d - 3 J t η = 4 mv c Potentil vs SCE (V) LAO LAO 5nm LAO 1nm Pt Ir Potentil vs SCE (V) e Thickness on LAO (nm) O ORR 3 μa/cm OH - OER Ir Thickness on LAO (nm) Pt Figure S5. Relxtion of compressive strin from incresing film thickness reduces both ORR nd OER ctivities., Lttice constnts nd ssocited bixil strin for LNO of different thicknesses on LAO. These thickness mesurements rely on incresed strin relxtion (s seen in the reciprocl spce mps round the 13 peks) with incresing thickness due to build-up of stress within the film s it becomes thicker. This llows strin grdient from ε ~ 1.% t 1 nm to ε ~ % with LNO on the sme lttice-mismtched substrte. Electricl conductivity ws not significntly different from 1 nm to 1 nm. b,c, Polriztion curves for the (b) ORR nd (c) OER on these LNO films in O -sturted.1 M KOH t 5 mv/s scn rte nd 16 rpm. The polriztion curves for the relxed 1 nm LNO film on LAO re similr to the relxed 1 nm LNO film on LSAO. d, The current densities (J) of both rections t overpotentils of η = 4 mv (ORR =.83 V nd OER = 1.63 V) increse with compressive strin. e, The combined η to ttin 3 μa/cm for both rections show tht the 1.% compressed LNO cn surpss Pt s bifunctionl ctlyst. These thickness tests on similr substrte further verify the ctlytic effects of strin on LNO. S8

9 Potentil vs RHE (V) Potentil vs RHE (V).9.88 b LAO (-1.%) LSAO (%) LSAT (.73%) STO (1.7%) DSO (.7%) Pt log J ORR (ma/cm ) log J OER (ma/cm ) Figure S6. Tfel curves for ORR nd OER current densities round η = 4 mv on LNO films in O -sturted.1 M KOH., All ORR slopes re pproximtely 6 mv/decde, similr to the Pt curve ( ), suggesting similr number of electrons nd similr rte-determining step for ll films. b, All slopes for OER re pproximtely 45 mv/decde, suggesting similr increse in the rte-determining step for this rection s well. S9

ORR J ( A/cm ) ORR J ( A/cm ) OER J ( A/cm ) 4 6 8 1 4 6 ε = -1.3% 8 b 1 OER J ( A/cm ) 4 6 8 1 Pt - LNO (mv) 4 6 8 1 ε = % -5-4 -3 - -1 Figure S7.

To compre, the totl η of Pt is subtrcted from tht of the film.

10 ORR J ( A/cm ) ORR J ( A/cm ) OER J ( A/cm ) ε = -1.3% 8 b 1 OER J ( A/cm ) Pt - LNO (mv) ε = % Figure S7. Comprison of overpotentil to Pt for ORR nd OER.,b, The combined ORR/OER η of LNO compred to tht of Pt for ORR nd OER t different J when t () ε = 1.% nd (b) ε =. To compre, the totl η of Pt is subtrcted from tht of the film. Negtive vlues of this difference in the bifunctionl η between Pt nd LNO signify better bifunctionl oxygen ctivity thn Pt. The compressed film hs much lrger rnge of bifunctionl J where the ORR/OER current is more ctive thn Pt, incresing from the expected high OER J/ low OER J to more evenly distributed bifunctionl current, s seen in Fig. 1e. S1

11 Normlized Absorption (.u.) J ORR ( A/cm ) J OER ( A/cm ) -1. to.7 Strin (%) = 4 mv Resistivity ( m cm) Figure S8. Activity of strined LNO films s function of room-temperture resistivity. By plotting ORR nd OER ctivities (J t η = 4 mv) on different substrtes s function of the electricl room-temperture resistivity, rther thn strin (s in Fig. 1c), one cn see tht ny correltion between conductivity (inverse of resistivity) nd ctivity breks down t strins of ε =.7% nd below. These results rgue ginst chnges in electron trnsport through the mteril limiting these rections. As discussed lter in Figure S8, resistivity is minly ffected by chnges in NiO 6 octhedrl out-of-plne rottion (tilt) while ORR/OER ctivity is influenced by chnges in Ni-O bond lengths. Ner ε =.7% nd below, tilt remins reltively constnt while Ni-O bond length continues to increse, indicting tht strin is the root cuse of both resistivity nd ctivity chnges rther thn the former being responsible for the ltter DSO (.7%) STO (1.7%) LSAT (.73%) LSAO (%) LAO (-1.%) 1 Ni L Energy (ev) Figure S9. Determintion of Ni oxidtion stte using X-ry bsorption spectroscopy (XAS). XAS plots of the Ni L pek on vrious substrtes. A pek loction ~871 ev is indictive of the Ni 3+ stte being predominnt. S11

Ni-O bond length (A) Tilt Angle ( ) Ni-O-Ni Angle (18 - Θ) Ni-O bond length L Ni O b..1 14. 7 1.9 - Strin (%) Figure S1. DFT modeled tilt nd Ni-O bond length ner surfce s function of strin.

12 Ni-O bond length (A) Tilt Angle ( ) Ni-O-Ni Angle (18 - Θ) Ni-O bond length L Ni O b Strin (%) Figure S1. DFT modeled tilt nd Ni-O bond length ner surfce s function of strin. The rhombohedrl structure of LNO ccommodtes strin vi two min mechnisms: () rottion nd (b) stretching of the NiO 6 octhedr. While in-plne rottion is reltively constnt, tensile strin results in lrge increses in out-of-plne rottion (i.e. tilt). This rottion decreses the Ni- O-Ni bonding ngle from 164, resulting in lrge increses in in-plne resistivity, independently verified vi trnsport mesurements (Extended Dt Fig. ). Conversely, sterics from the A-site L 3+ lrgely constrin ny further reduction of tilt to ccommodte compressive strin, fvoring the elongtion of the out-of-plne picl Ni-O bond length. 5 S1

13 DOS (sttes/ev) DOS (sttes/ev) DOS (sttes/ev) DOS (sttes/ev) 6 3 d z surfce -3% -% -1% % +1% +% +3% b 6 3 d z bulk -3% -% -1% % +1% +% +3% c Energy vs E f (ev) d x -y surfce -3% -% -1% % +1% +% +3% d Energy vs E f (ev) d x -y bulk -3% -% -1% % +1% +% +3% Energy vs E f (ev) Energy vs E f (ev) Figure S11. DFT DOS dt in LNO bulk nd t surfce.,b, Density of sttes (DOS) round E F ( ev) for the d z orbitl on either the () first monolyer (surfce) or (b) in the bulk, nd for the c,d, d x -y orbitl on either the (c) first monolyer (surfce) or (d) in the bulk. The centroids of ech stte nd electron occupncies re determined vi stndrd integrtion methods, excluding the strongly hybridized Ni3d-Op sttes below pproximtely 4 to 3 ev. REFERENCES 1. McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jrmillo, T. F. J. Am. Chem. Soc. 13, 135, Brd, A. J.; Fulkner, L. R., Electrochemicl Methods: Fundmentls nd Applictions. Wiley: New York,. 3. Wu, M.; Benckiser, E.; Hverkort, M. W.; Frno, A.; Lu, Y.; Nwnkwo, U.; Brück, S.; Audehm, P.; Goering, E.; Mcke, S.; Hinkov, V.; Wochner, P.; Christini, G.; Heinze, S.; Logvenov, G.; Hbermeier, H. U.; Keimer, B. Phys. Rev. B 13, 88, Scherwitzl, R.; Griglio, S.; Gby, M.; Zubko, P.; Gibert, M.; Triscone, J. M. Phys. Rev. Lett. 11, 16, Mri Luis, M. J. Phys.: Condens. Mtter 1997, 9, S13

Thin Film Scattering: Epitaxial Layers

Thin Film Scattering: Epitaxial Layers Thin Film Scttering: Epitxil yers Arturs Vilionis GAM, Stnford University SIMES, SAC 5th Annul SSR Workshop on Synchrotron X-ry Scttering Techniques in Mterils nd Environmentl Sciences: Theory nd Appliction