Solid State Ionics 276 (2015) 127–135

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Solid State Ionics

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Improved electrical and electrochemical performance ofco-doped Nd1.8Sr0.2Ni1− xCuxO4+ δ

A.P. Khandale, M.G. Bansod, S.S. BhogaDepartment of Physics, RTM Nagpur University, Nagpur-440033, India.

E-mail addresses: msrl.physics1@gmail.com (A.Pgmail.com (M.G. Bansod), msrl.physics1@gmail.com (S.S.

http://dx.doi.org/10.1016/j.ssi.2015.04.0050167-2738/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 July 2014Received in revised form 1 April 2015Accepted 2 April 2015Available online xxxx

Keywords:Mixed ionic electronic conductorSolid solutionPolarization resistanceElectrochemical impedance spectroscopy

Mixed conducting oxides of Nd1.8Sr0.2Ni1- xCuxO4+ δ (x = 0.0, 0.2, 0.4, and 0.5) system are synthesized bycombusting homogeneously mixed precursors under microwave heating. The Nd1.8Sr0.2Ni1− xCuxO4+ δ solidsolutions are of tetragonal structure in one single phase across the compositional range 0≤ x≤ 0.5. The samplesat x N 0.5 are multiphase. Nd1.8Sr0.2Ni0.6Cu0.4O4.01 exhibits maximum dc conductivity (σ = 42.7(3) S cm−1 at640 °C) and minimum activation energy (Ea = 0.07(3) eV). Minimum electrode polarization resistance valueof 0.20(4)Ω cm2 (at 700 °C) is obtained for Nd1.8Sr0.2Ni0.6Cu0.4O4+ δ. Low-frequency complex impedance spectracorresponding to symmetric cells are modeled using Gerischer impedance. Oxygen partial pressure-dependentelectrode polarization study suggests that the oxygen dissociation is the cathode reaction rate-limiting step.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Mixed ionic-electronic conductors (MIECs) with K2NiF4-type struc-ture have received renewed interest due to their high thermal stability,high oxygen diffusion coefficient, good electronic conductivity andstrong electrocatalytic activity towards oxygen reduction reaction(ORR) [1]. A change from gas–electrode–electrolyte triple phase bound-ary (TPB) to gas–MIEC two phase boundary (2PB) has direct relevanceto intermediate temperature solid oxide fuel cells (IT-SOFCs) wherecathodic polarization losses become critical [2]. The MIEC (2PB) mate-rials can help in decreasing the operating temperature of SOFC from1000 °C to 550 °C [3].

Gopalkrishnan et al. [4] have suggested solid solubility limit of Sr upto x = 0.1 in Nd2− x SrxCuO4. Later, Rosseinsky et al. [5] have reportedincreased homogeneity range of Nd2− xSrxCuO4 − y solid solutions upto x = 0.2 due to quenching. Recently, solid solubility limit of Sr up tox=0.1 in this MIEC has been confirmed [6]. The Nd2NiO4+ δ has exhib-ited promising electrocatalytic activity in respect to ORR process [7].Chaker et al. [8] have investigated the effect of transition metalcation co-doping on structural, microstructural and electrical propertiesof NdSrNi1− xCuxO4 − δ (0 ≤ x ≤ 1) and NdSrNi1− xCrxO4 − δ

(0.1 ≤ x ≤ 0.9). Furthermore, they have reported the dc electrical con-ductivity in the temperature range 27–196 °C.

The capability to accommodate awide range of oxygen stoichiometryin K2NiF4-type (A2BO4) ceramics resulted in both the high dc conductiv-ity and oxygen ion transport. The change in oxygen stoichiometry can be

. Khandale), msrl.physics1@Bhoga).

accomplished either by partial substitution of alkaline earth (Sr, Ba, Ca)at A site or by the B site doping with other transition metals, typicallyCu and Co [3]. Partial substitution of Sr/Ba/Ca at A sublattice can resulteither decrease in oxygen content or oxidation of the B cations (e.g. oxi-dation of Ni2+ to Ni3+) according to the charge neutrality condition.Both these charge compensationmechanisms can, also, take place simul-taneously. Sr doping should, hence, either results in decrease of intersti-tial oxygen (Oi

' ') concentration or/and increase of the oxygen vacancies(VO

• •). Existing literature has revealed a decrease in activation energy cor-responding to oxygen diffusion through La2− xSrxNiO4 lattice with an in-crease in Sr content [1]. Enhancement in the dc conductivity has beenreported due to increased charge carrier (NiNi• = h•) density resultedfrom the partial substitution of Ni2+ by Cu2+ in Nd2NiO4+ δ lattice toachieve charge neutrality [9,10]. In addition, the partial incorporationof Sr2+ in Nd2NiO4+ δ has suppressed the structural phase transition(at 574 °C) [8]. Furthermore, the lowest polarization resistance (Rp)0.52 Ω cm2 at 700 °C has been obtained in case of Nd1− xSrxNiO4+ δ atx = 0.2 [9]. The Nd1.8Sr0.2NiO4+ δ solid solution was, thus, selected forfurther study. In another approach, the partial substitution of Ni2+ byCu2+ has suggested to introduce oxygen vacancies thereby improve-ment in electrochemical performance of nickelate-based cathode [3,8].Consequently, the present study was aimed at improving the electro-chemical performance of previously optimized Nd1.8Sr0.2NiO4+ δ by thepartial substitution of Ni by Cu. Each composition of Nd1.8Sr0.2Ni1− x

CuxO4+ δ (0.0≤ x≤1.0) system, prepared by acetate pyrolysis technique,was characterized using X-ray powder diffraction, scanning electronmi-croscopy and dc conductivity. In addition, electrochemical impedancespectroscopy (EIS) studies on symmetric cells, constructed as cathode/CGO/cathode, were carried out relative to the temperature and oxygen

128 A.P. Khandale et al. / Solid State Ionics 276 (2015) 127–135

partial pressure so as to determine the cathode polarization resistance(Rp). The improved electrochemical properties are understood basedon the defect chemistry.

Inte

nsity

/ A

rbitr

ary

Uni

t

20 30 40 50 60 70 80

101

004

103

110

105 006

202

211

213

220

118

21711

6

008

310

Nd2NiO4 - # 01-080-2323

(a)

(b)

(c)

(d)

2 / degreeθ

Fig. 1. XRPD patterns of Nd1.8Sr0.2Ni1− xCuxO4+ δ: (a) x= 0.0, (b) x= 0.2, (c) x= 0.4 and(d) x= 0.5.

2. Experimental

The Nd1.8Sr0.2Ni1− xCuxO4+ δ (0.0≤ x≤ 1.0) samples were preparedusing acetate pyrolysis technique [9]. The reagents, namely acetates ofneodymium, strontium, copper and nickel were procured from AldrichChemicals, USA and have a purity of N99.9%. All of the reagents werecombined in requisite stoichiometric ratio through dissolution indouble-distilled deionized water. The prepared solutions were stirredto obtain a homogeneous solution before combustion in a microwaveoven at 800 W. The residue was ground and pressed uniaxially(300 kN cm−2) to obtain circular discs (pellets), 10 mm in diameterand 2 mm thick. The resultant pellets were sintered at 1000 °C for 4 husing high temperature muffle furnace (Thermolyne-46100, USA). TheNd1.8Sr0.2Ni1− xCuxO4+ δ at x = 0.0, 0.2, 0.4, 0.6 and 0.8, hereafter, aredesignated as NSNC-0.0, NSNC-0.2, NSNC-0.4, NSNC-0.6, and NSNC-0.8, respectively.

All the sintered samples were characterized using X-ray powder dif-fraction (XRD), PANalytical X’pert PRO (Philips, The Netherlands) usingCuKα radiations and a 1D Pixcel detector. The crystallite sizes (Cs) ofsamples under study were determined based on the diffraction peaksin the 2θ ranges 30–50° while using X'pert Highscore plus softwareas described previously [9]. The oxygen contents of Nd1.8Sr0.2Ni1− x

CuxO4+ δ (0.0 ≤ x ≤ 0.5) at room temperature were determined usingiodometric titration as described in literature [11]. Practically,iodometric titration was conducted under a N2 atmosphere to preventthe air oxidation of the I-ion. The 0.5 g of sample and 2 g of KI were dis-solved under a N2 atmosphere in 100ml of 1MHCl solution. This result-ed in the reduction of tri-valent transition metal cations (Ni3+ andCu3+) and oxidation of I to I2. The solution was quickly titrated with0.01 M solution of Na2S2O3, checked for its composition by 0.01M solu-tion of K2Cr2O7. Double distilled deionized water was used during theentire experiment. The average oxidation states of Ni/Cu and therebythe oxygen content of the sample was determined. The δ values ofNd1.8Sr0.2Ni1− xCuxO4+ δ, determined by iodometric titration measure-ments, 0.11(1) (x = 0), 0.08(1) (x = 0.2), 0.04(1) (x = 0.4) and0.01(1) (x = 0.5) indicate decrease in interstitial (excess) oxygen con-tentwith an increase of Cu content. These results are in good agreementwith the report that the partial substitution of Ni2+ by Cu2+ introducesoxygen deficiency [3,8,12,13]. According to Aguadero et al. [13] de-crease in excess oxygen content corresponding to partial replacementof Ni2+ by Cu2+ could be due to the lower oxidation tendency of lattercation as compared to former one.

The sintered densities of the samples were determined usingArchimedes' principle using XS105 DU Mettler Toledo monopan bal-ance and a density kit. Microhardness number was determined usingVickers indentation technique (HMV-2microhardness tester, Shimadzu,Japan).

Before the dc conductivity (σdc)measurements, the sintered sample,spring-loaded in a ceramic cell holder (Amel, Italy), was initially heatedin step at the rate of 11 °C min−1 from 27 °C (room temperature) to700 °C; this temperature was maintained for 30 min (dwell time) tohomogenize the charge carriers [14]. At the end of dwell time, bulkresistance was measured using a four-probe method with the help ofthe computer-controlled Keithley 6221 current source and 2182Ananovoltmeter in delta mode. Later, the temperature of sample wasdecreased in step at the rate of 2 °C min−1 to 680 °C; after 30 min(dwell time), the resistancewasmeasured as described above. This pro-cedure was repeated during the cooling cycle (decrement of 20 °C instep) up to 500 °C. The temperature of the sample during measure-ments was controlled within ±1 °C using Eurotherm 2216e tempera-ture controller.

Ink of each solid solution was prepared by ball milling at 300 rota-tions per min (rpm) for 2 h (Pulverisette-6 planetary monomill, Fritsch,Germany) 1 g of the cathode powder, 3 wt.% polyvinyl buteral (binder),sodium free corn oil and ethyl methyl ketone [14]. The particle size dis-tribution of the cathode material in ink/slurry was determined using aNanoPhox (Symphatec, Germany) particle size analyzer. The experi-ment was repeated five times on freshly prepared ink/slurry to ensurereproducibility of the results. The symmetric cells (configurationsgiven below) were prepared by spin coating the cathode ink on boththe flat surfaces of sintered CGO pellet (96% relative density) [14].

Nd1:8Sr0:2Ni0:8Cu0:2O4þδ=CGO=Nd1:8Sr0:2Ni0:8Cu0:2O4þδ ðCell� 0:2Þ

Nd1:8Sr0:2Ni0:6Cu0:4O4þδ=CGO=Nd1:8Sr0:2Ni0:6Cu0:4O4þδ ðCell� 0:4Þ

Nd1:8Sr0:2Ni0:4Cu0:6O4þδ=CGO=Nd1:8Sr0:2Ni0:4Cu0:6O4þδ ðCell� 0:6Þ

The symmetric cells were initially baked at 400 °C for 1 h so as toburn out organic binders and then sintered at 1000 °C for 2 h. Themicrostructural and elemental analyses of cathode composed of sym-metric cells were carried using field emission gun scanning electronmi-croscope (FEGSEM, Jeol JSM 7600-F, Germany) and EDS (Oxford, X-max80 mm2), respectively.

Electrochemical impedance spectroscopy (EIS) measurements onthe symmetric cells were carried out using the computer-controlledSolartron 1255B FRA and Solartron SI 1287 electrochemical interfaceversus temperature (500–700 °C, during a cooling cycle), frequency(1 × 10−3 to 1 × 106 Hz), and oxygen partial pressure (0.1–21 kPa), asdescribed elsewhere [15]. The applied ac signal voltage across the sam-ple was 5 mV. Obtained EIS data were analyzed using softwaredeveloped by Scribner advanced software for electrochemical researchand development [16].

3. Results and discussion

3.1. X-ray powder diffraction (XRPD)

TheXRPD patterns of Nd1.8Sr0.2Ni1− xCuxO4+ δ at x=0.0, 0.2, 0.4, 0.5are depicted in Fig. 1(a)–(d), respectively. All the characteristicdiffracted peaks appeared in Fig. 1(a)–(d) are matched closely withthe PDF 01-080-2323 data corresponding to orthorhombic Nd2NiO4.Absence of diffracted peak(s) corresponding to Nd2O3, SrO, NiO and

(a)

(b)

(c)

103

303

217

008

004 11

0

21111

400

6

116

107

200 21

3

220

20610

1

109 31

0

Nd2CuO4 - # 01-024-0777

2 / degree20 30 40 50 60 70 80

Inte

nsity

/ A

rbitr

ary

Uni

to – Nd2NiO4

oo o*

- Sr4Cu6O10o

θ

Fig. 2.XRPD patterns of Nd1.8Sr0.2Ni1− xCuxO4+ δ: (a) x=0.6, (b) x=0.8, and (c) x=1.0.

129A.P. Khandale et al. / Solid State Ionics 276 (2015) 127–135

CuO/Cu2O in the XRD patterns (Fig. 1(a)–(d)) ruled out their(unreacted) precipitation in Nd1.8Sr0.2Ni1− xCuxO4+ δ (nickelate) ma-trix. On the other hand, most of the characteristic diffracted peaks ap-peared in Fig. 2(a)–(c) (x = 0.6–1.0) are in close agreement with PDF01-024-0777 data corresponding to tetragonal Nd2CuO4 (cuprate). Fur-thermore, presence of a few diffracted peaks corresponding to tetrago-nal Nd2NiO4 are evident in XRPD patterns of Nd1.8Sr0.2Ni1− xCuxO4+ δ atx=0.6 and 0.8 (Fig. 2(a) and (b)). In addition to the peaks correspond-ing to both tetragonal Nd2CuO4 and tetragonal Nd2NiO4 a fewcharacter-istic diffraction peaks corresponding to Sr4Cu6O10 (PDF 00-047-1677)are observed in XRPD patterns (Fig. 2 (a)–(c)) of Nd1.8Sr0.2Ni1−xCuxO4+ δ at x≥ 0.6. A close scrutiny of Fig. 2(a) and (b) reveals decreasein intensities of characteristic peaks corresponding to Nd2NiO4 with anincrease in x. The XRPD results depicted in Fig. 2(a)–(b) suggest thatthe samples with x N 0.5 are consisting of nickelate, cuprate andSr4Cu6O10 phases (multiphase); and the concentration of nickelatephase decreases with an increase in Cu content. The appearance oflines corresponding to Sr4Cu6O10 (Fig. 2(c)) is ascribed to the limitedsolid solubility of Sr in Nd2CuO4. Hao et al. [17] have reported precipita-tion of Nd2O3 in Nd2− xSrxCuO4 matrix at x ≥ 0.2. Anbarasu et al. [18]have observed some unidentified diffracted peaks in XRPD pattern cor-responding to Nd2− xSrxCuO4+ δ at x=0.2 thereby concluded solid sol-ubility limit x b 0.2. Precipitation of Sr4Cu6O10 in Nd2− xSrxCuO4+ δ atx N 0.1 has also suggested solid solubility limit x b 0.1 [6]. Small shiftin d values (Fig. 1(a)–(d)) is attributed to lattice distortions resultedfrom partial substitution of Ni by Cu. The variations in lattice cell con-stants a and c versus dopant fraction (x) are depicted in Fig. 3. Evidently,both a and c though displayed variations verses copper content inNd1.8Sr0.2NiO4+ δ but a significant change in c is distinct (1.2386–1.2987 nm). Such variation is attributed to Jahn–Teller distortion causedby the presence of Cu2+ cation [9]. The experimental values of a(0.3807(6) nm) and c (1.2383(1) nm) corresponding to NSNO-0.0 are

0.2

0.4

0.0 0.25 0.5 0.75

1.2

lattice constant c1.4

mn/stnatsnoc

ecittal

lattice constant a

x

Fig. 3. Variations of lattice cell constants a and c versus x.

closely matching with report (a = 0.381 nm and c = 1.237 nm fortetragonal Nd1.8Sr0.2NiO4+ δ) [9]. In fact, Ni exists in 2+ and 3+ oxida-tion states in Nd1.8Sr0.2NiO4+ δ [8,12]. The partial substitution of Ni3+/Ni2+ (r(Ni3+)/r(Ni2+) = 5.6/6.9 nm) by relatively bigger Cu3+/Cu2+

(r(Cu3+)/r(Cu2+) = 6.8/7.3 nm) in tetragonal symmetry causes an in-crease in lattice cell constant c (Fig. 3). Ionic radii, here, are correspondingto coordination number (CN) 6, and due to Shannon [19]. The increase inthe value of lattice cell constant c with an increase of Cu content inLa2Ni1− xCuxO4+ δ has also been reported [3]. A systematic variation/increase of c (Fig. 3) with x for 0.0 ≤ x ≤ 0.5 suggests the formation ofsolid solution. It can, thus, be concluded that the Nd1.8Sr0.2Ni1− xCuxO4+ δ

δ (at 0.0≤ x≤ 0.5) are single phase solid solutions with tetragonal struc-tures, and at 0.5 b x ≤ 1.0 they are multiphase samples.

A comparison of average crystallite sizes (Cs) of all the compositionsunder study, obtained from XRPD data, is given in Table 1. Evidently,crystallite size increases (215(9)–262(8) nm) with an increase in Cuconcentration. The presence of impurity/dopant facilitates the graingrowth. These results are in agreement with report [9]. Slightly increasein both the sintered density (ρ = 95.47–95.97%) and the values ofmicrohardness number (859–897 HV) (Table 1) with increase of Cucontent are attributed to increased sinterability. Fast electrode sinteringdue to Cu addition has been reported in literature [20].

3.2. Scanning electron microscopy (SEM), energy dispersive X-rayspectroscopy (EDS) and particle size distribution.

The scanning electron microphotographs of the cleaved electrodesurfaces of Cell-0.2, Cell-0.4 and fractured surface across electrode/electrolyte interface of Cell-0.4 are shown in Fig. 4(a), (b) and (c), re-spectively. As seen, about 70% of cleaved surface consists of trans-granular cleavages, and the other 30% consists of voids/pores andgrain-boundaries (Fig. 4(a) and (b)). Furthermore, the grain size in-creases with an increase of x, which is in consistent with XRPD resultsdiscussed above. The microphotograph of electrode/electrolyte inter-face shown in Fig. 4(c) reveals a cathode layer ≈42–43 μm with a po-rous morphology over a highly dense CGO electrolyte. In addition, thehomogenous contact that formed along interface between electrodeand electrolyte is obvious (Fig. 4(c)). The theoretical and experimentalat.% values (obtained from the EDS of selected grain (Fig. 4(d)) forNSNC-0.4 close to electrode/electrolyte interface) were found closelymatched. These results are suggesting the formation of stoichiometricNd1.8Sr0.2Ni0.6Cu0.4O4+ δ solid solution.

A comparison of average particle size of NSNC in ink (⟨D90%⟩) is givenin Table 2. The ⟨D90%⟩ of cathode increases 483–507 nmwith an increaseof Cu dopant content (x) 0.0–0.5 (Table 2). These results substantiatethe results obtained towards SEM discussed above.

3.3. dc Electrical conductivity

The temperature dependent σdc of Nd1.8Sr0.2Ni1− xCuxO4+ δ

(0.0 ≤ x ≤ 0.8) is shown in Fig. 5(a). All of the studied samples, withinmeasured temperature range, obey Arrhenius law (Eq. (2)),

σdcT ¼ σTð Þ0exp−EakT

� �; ð2Þ

Table 1A comparison of crystallite size (Cs), sintered density (ρ) andmicrohardness number (HV)of Nd1.8Sr0.2Ni1− xCuxO4+ δ at 0.0 ≤ x ≤ 0.5.

x Cs (nm) ρ (%) HV

0.0 215(9) 95.47(3) 859(5)0.2 234(7) 95.54(5) 865(3)0.4 250(6) 95.89(3) 882(4)0.5 262(8) 95.97(5) 897(2)

(b)(a)

100 nm100 nm

*

**

- Nd* - Cu

- Ni - Pt

- O- Sr

(d)(c)

CGO electrolyte

42 m 43 m 42 mμ μ μ

Fig. 4. SEM microphotographs of cathode surface of (a) Cell-0.2, (b) Cell-0.4, electrode–electrolyte interface of Cell-0.4 and EDS spectrum of NSNC-0.4.

(a)

130 A.P. Khandale et al. / Solid State Ionics 276 (2015) 127–135

where (σT)0, k, T and Ea are pre-exponential factor, Boltzmann con-stant, absolute temperature and activation energy, respectively. Maxi-mum in σdc value, at all measured temperatures, is obtained in NSNC-0.4 (x = 0.4). The variations of σdc at 640 °C and Ea as a parametricfunction of x are shown in Fig. 5(b). The σdc initially increases with anincrease in x (Nd1.8Sr0.2Ni1− xCuxO4+ δ), attains a maximum (σdc =25.7(1) S cm−1 at 640 °C) at x=0.4 and decreases thereafter. The Ea fol-lows an opposite behavior to that of the σdc with an increase of x. Evi-dently, the composition exhibiting minimum value of Ea (0.07(2) eV)coincides with the composition that has maximum σdc. The obtainedvalues of Ea in the range 0.06–0.09 eV are close to reportedEa≈ 0.09 eV forMIECs [12]. Aguadero et al. [3] have also observedmax-imum conductivity at x = 0.4 in case of La2Ni1− xCuxO4+ δ. Increase inconductivity due to partial substitution of Ni by Cu (x = 0.25) has,also, been reported in Pr2NiO4 [21].

The charge neutrality condition in case of partially substituted Sr2+

at Nd3+ site in Nd2NiO4+ δ (SrNd' ) could be achieved through eitherone or both of the following two ways: (i) electronic compensation byoxidizing Ni2+ at its regular lattice site to Ni3+ with one effective posi-tive charge (NiNi• ) and/or (ii) ionic compensation by forming oxygenvacancies (VO

• •). The existence of both the mixed valence of Ni ion(Ni3+/Ni2+) and loss of lattice oxygen in Sr doped K2NiF4 type ceramicshas been reported [22–24]. Therefore, in the region where both elec-tronic and ionic compensations take place, the defect reaction in accor-dance with Kroger–Vink notation for y dopant concentration of Sr canbe represented by:

Ndx2‐ySr

0

yNix1‐yNi

•yO4⇔Ndx

2‐ySr0

yNix1‐yþδNi

•y‐δO4‐δ V••

O� �þ δ

2O2: ð3Þ

Table 2A comparison of average particle size in ink (⟨D90%⟩), activation energies Ea(Rp) deter-mined from Rp versus T and Ea(fp) determined from fp versus T plots of various symmetriccells.

Cell ⟨D90%⟩ (nm) Ea(Rp) (eV) Ea(fp) (eV)

Cell-0.2 483(3) 1.392(7) 1.390(7)Cell-0.4 494(2) 1.315(3) 1.321(4)Cell-0.5 499(7) 1.349(5) 1.352(3)

The above defect reaction can, also, be represented as:

ySrOþ yNi•Ni⇔ySr0

Nd þ y−2δð ÞNi•Ni þ δV••O þ δ

2O2; ð3aÞ

(b)

Fig. 5. (a) Arrhenius plots for Nd1.8Sr0.2Ni1− xCuxO4+ δ at x= 0.0–0.8 and (b) variation ofσdc at 700 °C and activation energy (Ea) versus x.

3.0

131A.P. Khandale et al. / Solid State Ionics 276 (2015) 127–135

where NiNi• =h•. Since the ionic and electronic compensations occursimultaneously and compete with each other, the electroneutralitycondition can be expressed as:

Sr0

Nd

h i¼ Ni•Ni� �þ 2 V••

O� �

: ð4Þ

The decrease of lattice oxygen content with an increase of copperdopant concentration suggests the formation of Cu3+/Cu2+ mixedvalance couples [8]. Hence, the most probable extrinsic defects in theCu (y′ concentration) doped Nd1.8Sr0.2NiO4+ δ are (i) Cu2+ at Ni2+ reg-ular lattice site (CuNix ), (ii) Cu3+ replaces Ni3+ (CuNi• ) and fraction (2δ’)of Cu3+ undergoes reduction to Cu2+ (CuNix ) and (iii) creation of (2δ’)oxygen vacancies at its regular lattice site (VO

• •) in accordance withdefect reaction represented by Eq. 5:

y0CuOþ y0Ni•Ni→ y0−2δ0� �

Cu•Ni þ 2δ0

� �Cux

Ni þ δ0V••O þ δ0

2O2: ð5Þ

The partial substitution of Ni by Cu, thus, not only decreases the con-centration of excess interstitial oxygen (Oi

' ') but also the concentrationof mobile charge carriers (NiNi• /CuNi• = h•). Maximum value of σdc forNd1.8Sr0.2Ni1− xCuxO4 − δ at x = 0.4 (NSNC-0.4) is attributed to opti-mum number of both the isolated mixed valence couples (Cu3+/Cu2+

and Ni3+/Ni2+) and the mobile charge carriers. The decrease in σdc atx N 0.4 is attributed to the decrease in concentration of mobile chargecarriers and mixed valance couples.

The variations of σdc with oxygen partial pressure (PO2) for NSNC-0.4 at 700, 640 and 550 °C, depicted in Fig. 6, can be represented by:

σdc ¼ σ0 PO2

� n: ð6Þ

The increase in σdc with increase of PO2 is attributed to increase ofholes charge carrier concentration in accordance with point defectequilibrium (Eq. 7). According to Hong and Smyth [25] the oxidationreaction can be expressed as:

1=2 O2ð Þ⇔ O″i þ 2h•

; ð7Þ

with the mass action expression,

O″i

h ip2P O2ð Þ−1=2 ¼ Kpexp

−ΔHp

kT

� �; ð8Þ

-1.6

-1.4

-1.2

-1

1.5 2.5 3.5 4.5

700 oC640 oC550 oC

log

(/ S

cm

-1)

log (Po2 / Pa)

NSNC-0.4

n = 0.12

n = 0.12

n = 0.12

σ

Fig. 6. Variation of σdc versus oxygen partial pressure at 700, 640 and 550 °C forNd1.8Sr0.2Ni0.6Cu0.4O4+ δ (NSNC-0.4).

where p2 = [h•] and ΔHp is the enthalpy of oxidation per oxygen.Furthermore, if the oxidation reaction is as per Eq. (8), then the condi-tion of bulk charge neutrality can be expressed as,

p≈2 O″i

h i: ð9Þ

Combination of Eqs. (8) and (9) results in

p≈ 2Kp

� 1=3exp

−ΔHp

3kT

� �P O2ð Þ1=6: ð10Þ

The value n = 0.12, determined using Eq. (6), is close to n = 0.16and typical for the process of insertion of oxygen atoms into interstitialpositions in the crystal structure. The value of n ≈ 0.16, in general, canbe easily obtained provided the concentration of oxygen vacancies inthe perovskite layers of K2NiF4-type phases is very small:

V••O

� �bb O

00

i

h i; ð11Þ

and the charge neutrality condition is determined only by the con-centrations of oxygen interstitials and electron holes (Eq. 7). Insertionof oxygen into interstitial positions (Oi

″) in undoped La2CuO4 andLa2Cu0.5Ni0.5O4+ δ crystal structures has been predicted from observedn ≈ 0.16 values [25,26].

3.4. Electrochemical impedance spectroscopy (EIS).

The complex impedanceplots of Cell-0.4 at various temperatures areshown in Fig. 7. Exploded view of the complex impedance plot at 640 °C(Fig. 7(b)) reveals a skewed semicircular arc in the low frequencyregion. Almost similar trend, in the measured temperature range, wasobserved in the complex impedance plots corresponding to the othercells under study. Absence of high frequency (HF) semicircle, ascribedtomigration of O2− through CGO electrolyte, is attributed to limitationsin available frequency (≤1MHz) [6]. The skewed semicircular arc in thelow frequency region is assigned to NSNC-0.4 electrode response. Theproposed electrochemical circuit model (ECM), shown as insert ofFig. 7(b), comprises a parallel combination of RB and constant phaseelement (CPE) alongwith a Gerischer element (G) in series. The RB rep-resents the resistance of CGO electrolyte [6]. The LF impedance data(open circles) is accurately modeled/simulated (continuous line)using the Gerischer element (G) (Fig. 7(b)).

2.0 3.5 5.0 6.5 8.0

1.5

0

640 C660 C680 C700 C

Cell-0.4

100m Hz

10 Hz

100 Hz1 kHz0.1 MHz

-Z”

/ c

m2

Z ’ / cm2

(b)

(a)

6.0 6.55.0 5.5

0.5

0B p4.54.03.5

At 640 C

Cell-0.4

HF

LF

G

RB

CPE

-Z”

/ cm

2

Z’ / cm2

Ω

Ω

ΩΩ

°°°°

°

Fig. 7. Complex impedance plots at various temperatures of (a) Cell-0.4 and (b) explodedview of experimental data (open circles) at 640 °C, simulated curve (continuous line) andelectrical equivalent circuit model.

132 A.P. Khandale et al. / Solid State Ionics 276 (2015) 127–135

According to Adler [27], two of the major kinetic processes of theoxygen electrode (i) oxygen exchange and (ii) oxide ion diffusion canbe coupled to form single Gerischer impedance (ZG) response. Thegeneral ZG is expressed by Eq. (12),

ZG ¼ 1Y0

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiKA þ iω

p ¼ RGffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1þ iωRGCG

p ; ð12Þ

where Y0 and KA are admittance and rate constant, respectively.Gerischer impedance response has been successfully applied to theMIEC electrodes [27]. The Gerischer element (G) includes (i) ion trans-fer resistance (Ri) and electron transport resistance (Re) so electrodepolarization resistance Rp = Ri + Re and (ii) a CPE instead of idealcapacitor. Approximately 45° slope at relatively higher frequency re-gion, a characteristic of the Gerischer response, reflects electrochemicalextension from the cathode/electrolyte interface out into the porouselectrode [27]. In addition, it indicates that the diffusion of O2− is oneof the important steps of overall ORR. A detailed theoretical descriptionon Gerischer impedance response is given in the literature [27]. TheECM fitting results of Cell-0.2, Cell-0.4 and Cell-0.5 at various tempera-tures are compared in Table 3. As seen, partial substitution of Ni by Cusignificantly influences the values of Rp and CPE at a fixed temperature,while themagnitude of RB remains nearly same (Table 3). The activationenergy (Ea) determined from the temperature dependent RB data is0.993(4) eV. This value is in the range of reported activation energyfor CGO electrolyte (Ea≈ 0.87− 1.03 eV) [9]. Hence, high frequency re-sponse is attributed to the migration of O2− through CGO electrolyte.Similarly, Hjalmarsson and Mogensen [28] have attributed high fre-quency response to CGO electrolyte on the basis of activation energy.

According to literature, Gerischer impedance response in compleximpedance plane is the manifestation of electrochemical reactioncoupled with O2− diffusion. The diffusion of O2− can occur eitherthrough the bulk or along the surface of electrode or a combination ofboth. The NSNC cathodes being MIEC, the O2− diffuses primarilythrough the bulk resulting in high capacitance values, which shouldrange up to 1 F cm−2 [29]. Comparable values of CG (Table 3) allowsto correlate low frequency complex impedance response (ZG of ECM)to a non-charge-transfer process, such as oxygen surface exchangeand gas phase diffusion through the porous cathode layer during theORR. Similarly, a number of researchers have ascribed the skewed semi-circular arc in low-frequency region (ZG) to a non-charge transfer

Table 3The values of fitted G elements corresponding to EIS data for Cell-0.2, Cell-0.4 and Cell-0.5 at selected temperatures (700–600 °C).

T (°C) RB (Ω cm2) Rp (Ω cm2) CG/CPE (F cm−2) CPE-P

Cell-0.2700 2.74(2) 0.98(2) 0.75(2) 0.27(2)680 3.53(1) 1.24(3) 0.64(4) 0.28(2)660 3.85(2) 1.98(5) 0.48(2) 0.28(3)640 5.48(2) 2.94(2) 0.29(3) 0.26(1)620 7.15(3) 4.34(3) 0.10(2) 0.27(1)600 10.09(3) 6.56(3) 0.07(2) 0.27(3)

Cell-0.4700 2.75(2) 0.532(3) 0.86(1) 0.27(2)680 3.55(1) 0.774(3) 0.70(2) 0.26(2)660 3.75(2) 1.16(4) 0.40(5) 0.28(1)640 5.08(2) 1.72(2) 0.21(3) 0.28(2)620 7.19(2) 2.48(2) 0.14(2) 0.27(2)600 10.02(3) 3.18(3) 0.08(2) 0.27(3)

Cell-0.5700 2.79(2) 0.72(5) 0.71(2) 0.27(2)680 3.57(4) 0.85(2) 0.41(4) 0.26(3)660 3.83(1) 1.69(4) 0.21(4) 0.26(2)640 5.29(3) 2.12(3) 0.14(2) 0.27(3)620 6.85(4) 3.07(4) 0.073(4) 0.28(1)600 10.16(4) 5.342(2) 0.011(2) 0.28(3)

process [27,29,30]. A considerable influence of partial replacement ofNi by Cu on ORR is evident from a significant variation in themagnitudeof RP (Table 3).

Since the NSNC solid solutions are MIECs, the ORR occurs not only atthe TPB layer but also throughout the porous electrode. Essentially,adsorbedO2 reduces to O- after dissociation. Subsequently, adsorbed re-duced O− (Oad

' ) enters into the NSNC lattice oxygen vacancies (Ov• •).

Two steps surface reactions are given below:

12O2 þ s→O

0

ad þ h•

O0

ad þ O••v→Ox

O þ 2h• þ s;ð13Þ

where, s is concentration of vacant surface sites. The O2− istransported across the NSNC cathode through hopping mechanism. Infact, the O2− transfer (diffusion) rate is governed by the concentrationof interstitial oxygen in rocksalt layer of NSNC crystal. Optimum latticeoxygen concentration due to Sr and Cu co-doping facilitates the O2− dif-fusion through NSNC bulk. In addition, Sr and Cu co-doping results inoptimum concentrations of Ni3+/Ni2+ and Cu3+/Cu2+ redox couples(capable of accepting or donating electrons from oxygen). Such redoxcouples, also, promote the chemisorption dissociation reaction of O2 gas[31]. Improvement in overall electrochemical performance of NSNC-0.4cathode is, hence, attributed to both (i) the promotion of chemisorptiondissociation reaction of O2 (at NSNC surface) and (ii) increase in therate of O2− diffusion (in NSNC bulk).

The temperature-dependent Rp of Cell-0.2, Cell-0.4, Cell-0.5 andCell-0.6 are compared in Fig. 8. Lowest value of Rp (0.20(4) Ω cm2 at700 °C) as well as activation energy (Ea = 1.315(3) eV), across temper-ature range of measurements, is observed for Cell-0.4 as compared toother cells (Cell-0.2, Cell-0.6, and Cell-0.8). The estimated values of Ea(Table 2) are within the range (1.3–1.4 eV) reported in literature forK2NiF4 type MIECs. The variations of peak frequency (fp), estimatedfrom low frequency complex impedance response (Gerischer re-sponse), against temperature corresponding to Cell-0.2, Cell-0.4, Cell-0.5 and Cell-0.6 are depicted in Fig. 9. All the cells exhibit Arrhenius-like behavior (Eq. (14)):

f p ¼ f oexp−EakT

� �; ð14Þ

where fp, and fo are charge jump frequency and attempt frequency,respectively. The activation energies (Ea(fp)) determined using Eq. (14)are closely matched with the Ea(Rp) determined from the temperature

Fig. 8. Temperature dependent polarization resistance (Rp) of Cell-0.2, Cell-0.4, Cell-0.5and Cell-0.6.

Fig. 9. Variation of peak frequency versus temperature for Cell-0.2, Cell-0.4, Cell-0.5 andCell-0.8.

n = 0.489

n = 0.484

n = 0.493

log (Po2 / Pa)

1 2

0.0

0.5

1.0

1.5

2.0

550 C640 C700 C

3 4

Cell-0.4

-0.5

log(

Rp

/ cm

2 )Ω

°°°

Fig. 11. Variation of polarization resistance (Rp) versus oxygen partial pressure for Cell-0.4 at 700, 640 and 550 °C.

133A.P. Khandale et al. / Solid State Ionics 276 (2015) 127–135

dependent Rp plots (Table 2). Increase in fp versus T (Arrhenius behav-ior) envisages thermally activated charge transport process. Implicitly,the mobile O2− and polarons (electronic defects) jump/hop from theiroccupied sites to nearby equivalent vacant sites by traversing the poten-tial barrier (Ea) i.e., charge-carrier hopping mechanism. Optimum con-centration of isolated mixed couples (Ni3+/Ni2+ and Cu3+/Cu2+) andmobile charge (NiNi• /CuNi• = h•) concentration in NSNC-0.4 (discussedin dc conductivity section) mitigates diffusion/transport through hop-ping mechanism, which results in lowest values of Rp and Ea (Table 2).These results support the dc conductivity behavior discussed earlier.

Complex impedance studieswere carried out as a function of oxygenpartial pressure (PO2) at different temperatures to understand the rate-limiting step of ORR. The intercept of LF complex impedance response(G) increases appreciably with decrease in PO2 (Fig. 10). The value ofRB remains invariant in spite of change in PO2. Almost similar shape(45° inclined line followed by depressed semicircular arc) of LF/Gerischer responses, except change in size, suggests equilibrium ORRmechanism is independent of PO2. The variations in log(Rp) with changein log(PO2), depicted in Fig. 11, can be represented by Eq. (15):

Rp ¼ Rp

� 0 PO2

� −n: ð15Þ

The various steps involved during ORR of MIECs given below arewidely accepted [32]:

1000 Pa21000 Pa

500 Pa150 Pa

Z ’ / cm2

At 640 oC

Cell-0.4

1 Hz

39.8 Hz

6.3 Hz

125.9 Hz

1 MHz

252015100

5

10

-Z”

/ c

m2

5

Ω

Ω

Fig. 10. Complex impedance plots at various oxygen partial pressures for Cell-0.4 at 640 °C.

(i) Adsorption of O2 onto the cathode surface:

O2;gasphase⇔O2;ad

(ii) Dissociation of molecular adsorbed oxygen into atom adsorbedoxygen on cathode surface:

O2;ad⇔2Oad

(iii) Reduction reaction of atom adsorbed oxygen into adsorbed oxy-gen ion on cathode surface:

Oad⇔O−ad

(iv) Transfer of surface adsorbed oxygen ion to 2PB/TPB boundaries:

O−ad⇔O−

2PB=TPB

(v) Charge-transfer reaction at 2PB/TPBs:

O−2PB=TPB þ e− þ V••

O⇔OxO

The rate equations corresponding to the proposed reaction (i) can beexpressed as:

r1 ¼ k1PO2‐k1

0a2Oad; ð16Þ

where k1 and k1′ are the rate constants for the forward and back-ward reactions, respectively. Hence, the activity of adsorbed oxygen(aoad) can be given by:

aOad¼ k1

k10

� �1=2P1=2O2

: ð17Þ

If the steps (III)–(IV) are faster than step (ii) then, the oxygen activ-ity in the adsorbed layer varies approximately linearlywith the quantityof adsorbed oxygen. According to Nernst theory, the electrode potentialcan be fixed by oxygen concentration/activity ⟨Oad⟩ in the adsorbedlayer as:

E ¼ RT2F

ln Oadh ið Þ: ð18Þ

134 A.P. Khandale et al. / Solid State Ionics 276 (2015) 127–135

The electrode reaction behavior, thus, depends on surface concen-tration overpotential given by

η ¼ RT2F

lnOadh iOadh ieq

!; ð19Þ

where, ⟨Oad⟩eq is the effective concentration of Oad in equilibriumwith gas phase. Corresponding polarization resistance (detailed de-scription given in Ref. [33]) with steady-state Oad concentration ⟨Oad⟩sis given as:

Rp ¼ RT4F2k 0

1Oadh is

: ð20Þ

Under equilibrium condition (η = 0), Eq. (20) predicts variation of

Rp proportion toP−1=2O2

, as observed experimentally (n=0.5). Accordingto the literature, information regarding rate-limiting step(s) of ORR atthe cathode can be obtained from the n-values [32,33].

n ¼ 1; O2 gð Þ⇔O2;ads ð21Þ

n ¼ 12; O2;ads⇔2Oads ð22Þ

n ¼ 38; OTPB þ e0⇔O‐

TPB ð23Þ

n ¼ 14; Oabs þ 2e0 þ V••

O⇔OxO ð24Þ

n ¼ 18; O‐

TPB þ e0⇔O2‐TPB ð25Þ

n ¼ 110

; O2‐TPB þ V••

O⇔OxO ð26Þ

One of the above-mentioned steps of ORR (Eqs. 21–26), in general,could be the rate-limiting step. The n ≈ 0.5 obtained for Cell-0.4suggests adsorbed oxygen dissociation reaction (Eq. 22) is the rate-limiting step across the measured oxygen partial pressure and temper-ature ranges.

The Rp of the optimized Nd1.8Sr0.2Ni0.6Cu0.4O4+ δ NSNC-0.4 iscompared with the reported data for Sr doped K2NiF4-type cupratesand nickelates (with CGO electrolyte) in Table 4 [6,9,14,34–42].The magnitude of Rp reduces from 0.5 [9] to 0.20 Ω cm2 on partialsubstitution of Ni by Cu in Nd1.8Sr0.2Ni0.6Cu0.4O4+ δ. The optimum

Table 4A comparison of optimum polarization resistance (Rp) value obtained in this study withvalues reported in the literature for Sr doped K2NiF4-type cuperates and nickelates whileusing Ce0.9Gd0.1O1.95 as electrolyte.

Sr.#

Cathode Electrolyte Rp(Ω cm2)

T(°C)

Reference

1 SmSrNiO4 Ce0.9Gd0.1O1.95 3.06 700 Li et al. [34]2 La1.7Sr0.3NiO4 Ce0.9Gd0.1O1.95 2.96 700 Dailly et al. [35]3 La1.6Sr0.4NiO4 Ce0.9Gd0.1O1.95 0.21 700 Li et al. [36]4 La1.2Sr0.8Co0.8Ni0.2O4 Ce0.9Gd0.1O1.95 1.36 600 Zhao et al. [37]5 Nd1.6Sr0.4NiO4 Ce0.9Gd0.1O1.95 0.93 700 Sun et al. [38]6 Nd1.8Sr0.2NiO4 Ce0.9Gd0.1O1.95 0.52 700 Khandale et al. [9]7 Pr1.3Sr0.7NiO4 Ce0.9Gd0.1O1.95 0.39 700 Bhoga et al. [39]8 Sm1.5Sr0.5NiO4-δ Ce0.9Gd0.1O1.95 2.70 700 Chaudhari et al. [15]9 La1.7Sr0.3CuO4 Ce0.9Gd0.1O1.95 0.16 700 Li et al. [40]10 Nd1.7Sr0.3CuO4 Ce0.9Sm0.1O1.95 0.24 700 Ding et al. [41]11 Nd1.93Sr0.07CuO4-δ Ce0.9Gd0.1O1.95 0.14 750 Bringley et al. [42]12 Nd1.9Sr0.1CuO4 Ce0.9Gd0.1O1.95 0.23 700 Khandale and

Bhoga [6]13 Nd1.95Sr0.05Ni0.6

Cu0.4O4+ δ

Ce0.9Gd0.1O1.95 0.20 700 Present study

Rp value of Nd1.8Sr0.2Ni0.6Cu0.4O4+ δ (NSNC-0.4) is competitive forIT-SOFC applications.

4. Conclusions

Superfine (215–274 nm) crystalline Nd1.8Sr0.2Ni1− xCuxO4+ δ

(0.0 ≤ x ≤ 0.5) solid solutions are synthesized by acetate pyrolysisfollowed by sintering at 1000 °C for 4 h. The Nd1.8Sr0.2Ni1− xCuxO4+ δ

lattice expands significantly along c direction with increase of Cu con-tent (x). The crystallite/grain size increases due to partial substitutionof Ni by Cu. The maximum electronic conductivity (σ = 47.9(2) Scm−1 at 640 °C) and minimum activation energy (Ea = 0.07(3) eV) inNd1.8Sr0.2Ni0.6Cu0.4O4+ δ are attributed to optimal isolated extrinsic de-fects (mixed valance Cu3+/Cu2+ and Ni3+/Ni2+ couples and mobileelectronic charge carriers (NiNi• /CuNi• = h•)). The successfully modeledlow frequency complex impedance spectrum using Gerischer imped-ance (Zg) describes electrochemical reaction coupled with O2− diffu-sion. The oxygen dissociation reaction (n ≈ 0.5) is the rate-limitingstep in the temperature 700–500 °C and oxygen partial pressure 0.1–21 kPa ranges. The lowest polarization resistance (Rp) value of0.20(4)Ω cm2 at 700 °C for Nd1.8Sr0.2Ni0.6Cu0.4O4+ δ is due to optimumdefects. Co-doping at A and B sites improves the electrochemical perfor-mance. The Nd1.8Sr0.2Ni0.6Cu0.4O4+ δ having good electrocatalytic activ-ity for oxygen reduction reaction and thermodynamical stabilitymay bea potential cathode for application in IT-SOFC.

Acknowledgments

The authors are thankful to University Grants Commission (UGC),New Delhi (India) for financial support through project F. No. 42-7615/2013 (SR) to carry out this work. Also, the authors are thankfulto SAIF, IIT Bombay (India), for providing FEGSEM facility.

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