Pronounced Strain Effects on Oxygen Dissociation; Pt‐ or Au‐Dispersed Pr2Ni(Cu, Ga)O4 for Active Cathode of Solid Oxide Fuel Cells

The correlation between lattice strain induced by metal dispersion into grain and the cathodic overpotential is studied for increasing oxygen‐dissociation activity and improving power density of solid oxide fuel cells (SOFC) at decreased temperature. Pt or Au dispersion in Pr1.90Ni0.71Cu0.21Ga0.05O4+d (PNCG) is prepared and the 3D tensile strain is successfully induced after sintering by a mismatch in thermal expansion coefficient. Due to higher hardness and melting temperature, Pt dispersion into bulk of PNCG introduces larger tensile strain than that by Au at the same amount. In particular, at 1 mol% Pt dispersion, large tensile strain of 0.67% is induced. Overpotential of 1 mol% Pt‐PNCG cathode is 8 times smaller (35 mV) than that of PNCG (270 mV) at 800 °C and 300 mA cm−2, and it is found that the cathodic overpotential of PNCG is decreased with tensile strain on both Pt and Au dispersion. This cathodic activity enhancement appears to be related with the increased diffusivity of oxide ion in PNCG. In this study, cathodic overpotential is more significantly influenced by the induced tensile strain comparing with the intrinsic catalytic activity of the dispersed metal.


Introduction
[3][4] To decrease a stack cost, start-up energy, and to increase durability, low temperature operation (lower than 650 °C) is important issue for the development of SOFCs.
Since activation energy for electrode reaction is much larger, development of active cathode is important for high power density at low temperature.For decreasing the cathodic overpotential at low temperature, we have studied praseodymium nickelate, Pr 2 NiO 4 (PNO), as active cathode because of high oxide ion diffusivity and hole conduction in air atmosphere. [5,6]oreover, Cu-and Ga-doped PNO in Ni site, Pr 1.9 Ni 0.71 Cu 0.24 Ga 0.05 O 4 (PNCG), was large oxygen permeation rate from air to He because of increasing ionic diffusivity, which was comparable electronic properties of perovskite materials such as La 1Àx Sr x MnO 3Àδ , La 1Àx Sr x CoO 3Àδ , and La 1Àx Sr x Co 1Ày Fe y O 3Àδ , etc. [7][8][9] Moreover, PNCG also has advantage of long-term stability because no Sr surface segregation, and compatible thermal expansion coefficient (TEC) with electrolyte (%10 Â 10 À6 °CÀ1 ).[10] On the conventional perovskite oxide cathode, Sr is segregated on the surface or interface between electrode and electrolyte resulting in the decreased electrochemical performance and increased reactivity with air pollutant like SO 2 .[11][12][13] In a recent study, strain effects have been investigated for increasing electrochemical performance of oxide ion conductor or mixed conductor.[14][15][16][17][18] Tsvetkov et al. reported that the nickelates structure materials can improve high oxygen reduction reaction (ORR) activity by tensile strain in thin film.[19] However, these studies have been mainly performed by using the film electrode which grows epitaxially and far from a real cathode of SOFC.It seems that lattice strain has a significant effect on surface activity of catalyst from surface composition.In our previous study, PNCG with 3D strain, which was induced by thermal expansion mismatch between Au nanoparticles and PNCG, was studied and it was found that the cathodic overpotential was decreased significantly by dispersion of Au into PNCG bulk and this Au dispersion effects might be assigned to the tensile strain; however, this is still not clearly demonstrated.[20,21] To show strain effects clearly, Au which is well-known poor catalytic activity has been used as strain-inducing metal dispersion because of the higher TEC value (14.2 Â 10 À6 °CÀ1 ) than that of PNCG (%13.6 Â 10 À6 °CÀ1 ).[20][21][22] However, it was still remaining the possibility for another precious metal, which is more catalytically active one.As another metal, Pt was chosen because of high catalytic activity to oxygen dissociation and DOI: 10.1002/aesr.202300084 The correlation between lattice strain induced by metal dispersion into grain and the cathodic overpotential is studied for increasing oxygen-dissociation activity and improving power density of solid oxide fuel cells (SOFC) at decreased temperature.Pt or Au dispersion in Pr 1.90 Ni 0.71 Cu 0.21 Ga 0.05 O 4þd (PNCG) is prepared and the 3D tensile strain is successfully induced after sintering by a mismatch in thermal expansion coefficient.Due to higher hardness and melting temperature, Pt dispersion into bulk of PNCG introduces larger tensile strain than that by Au at the same amount.In particular, at 1 mol% Pt dispersion, large tensile strain of 0.67% is induced.Overpotential of 1 mol% Pt-PNCG cathode is 8 times smaller (35 mV) than that of PNCG (270 mV) at 800 °C and 300 mA cm À2 , and it is found that the cathodic overpotential of PNCG is decreased with tensile strain on both Pt and Au dispersion.This cathodic activity enhancement appears to be related with the increased diffusivity of oxide ion in PNCG.In this study, cathodic overpotential is more significantly influenced by the induced tensile strain comparing with the intrinsic catalytic activity of the dispersed metal.
interesting to compare the cathodic activity with that of Au dispersion.In addition, Pt has lower TEC value (8.8 Â 10 À6 °CÀ1 ) than that of PNCG, and the compressed strain is expected; however, in our previous study, nanogap was formed between Pt particles and PNCG matrix because of large hardness and mismatch in TEC, and the hole conductivity was decreased by trapping hole at the metal-semiconductor junction. [23]In this study, effects of metal dispersion of which different catalytic activity to oxygen dissociation was investigated on cathodic activity of Pr 2 NiO 4 for SOFC to make clear the significant tensile train on ORR.

Effects of Pt Dispersion on Lattice Parameter
To estimate a lattice strain by metal dispersion, X-ray diffraction (XRD) of Pt-dispersed PNCG was shown in Figure 1.From the wide-angle scan of XRD on PNCG with Pt dispersion (Figure 1a and S1, Supporting Information), the single-phase PNCG with tetragonal structure was successfully prepared after calcination at 1300 °C.No secondary phase or impurity phase was observed.Figure 1b shows narrow scan of XRD around a main peak of PNCG dispersed with Pt.The main diffraction peaks were slightly shifted to a lower angle suggesting the tensile strain formed when 1 mol% Pt was dispersed and sintered.However, the peak shift was slightly decreased with increasing amount of Pt.In addition, PrO 2 was observed at 5 mol% Pt dispersion.Figure 1c shows the lattice strain estimated from the diffraction peaks of PNCG dispersed with Pt as a function of Pt amount.It is obvious that lattice tensile strain was significantly observed at 1 mol% and slightly decreased with increasing Pt amount.Considering the small TEC of Pt than that of PNCG, it is expected that compressed strain was expected.However, the observed lattice strain was tensile and this will be discussed later.Comparing with the tensile strain observed by Au dispersion in our previous study (see Figure S1, Supporting Information), it was found that larger tensile strain was introduced at smaller amount of metal dispersed (0.5% strain was achieved at 3 mol% Au) in case of Pt.It may be related with large volume change after sintering by high hardness of nano-size Pt particles and no molten state of Pt during sintering comparing with that of Au.In contrast, with increasing Pt contents (3-5 mol%), particles size was increased by agglomeration and diminished nano effect of metal particles. [24]Therefore, tensile strain seems to be decreased rapidly by compressive strain formed by Pt in PNCG.
To observe the morphology of Pt particles, scanning electron microscope (SEM) image was measured on Pt-PNCG surface after polished (Figure 2).The Pt particles were dispersed in PNCG homogeneously and the number of Pt particles were increased with increasing Pt amount as shown in Figure S2, Supporting Information.Although particle size was some variation, the maximum size was smaller than %1 μm in the SEM images (Figure 2a, left).In addition, the Pt particles were well contacted with PNCG as discussed.However, with increasing particle size, micropore was observed at interface between Pt and PNCG (Figure 2a, right).The micropores may be originated from the large volume change, and one reasons for decreasing tensile strain at excess amount of Pt.Therefore, for increasing tensile strain values, nano-scale Pt particles were important role.In contrast, around Pt particles, region of bright gray color was observed as shown in Figure 2a.To identify the elemental distribution, line analysis with energy-dispersive X-ray analysis (EDX) was measured and Figure 3b shows that the gray color area is an interdiffusion region between Pt and Pr (or Ni).From this result, the gray color area seems to be counter diffusion part of cation during sintering and might be explained the tensile strain formation in spite of small TEC of Pt and the micropore formation at excess amount of Pt.Therefore, Pt particle into grain was crucial factor to generate the tensile strain in PNCG and micropore seems to anneal the tensile strain effect at an excess amount.However, micro gap was formed over %2 μm diameter of Pt particles introduced and that may be reason for annealing tensile strain at the Pt amount higher than 2 mol%.As a result, the particle size may be related with the tensile strain in PNCG bulk.TEM measurement is required for more detail analysis and this will be done in future study.

Pt Dispersion on Cathodic Performance of SOFC
Pt-dispersed PNCGs were applied as a cathode and investigated for electrochemical performance (Figure 3).Power-generation property of the cell using Pt-dispersed PNCG cathode were compared at 600-800 °C (Figure 3a).The open-circuit voltage (OCV) of all cells was %1.14 V, which means gas sealing was reasonably tight and reasonable activity of cathode.Maximum power density (MPD) of the cell with 1 mol% Pt-PNCG was achieved (405 mW cm À2 ) that was %2.5 time higher than that of the cell using PNCG for cathode (155 mW cm À2 ) at 800 °C.In addition, MPD of the cell with 3 mol% Pt-PNCG was also similar value (%415 mW cm À2 ) with that of 1 mol% Pt-PNCG because of high surface activity.With decreasing operation temperature, MPD of the cell with 3 mol% Pt-PNCG was higher than that of 1 mol% PNCG used cell.Therefore, cathodic performance was increased by the tensile strain induced.At 5 mol% Pt-PNCG, cathodic overpotential was smaller than that of PNCG at all temperature examined.Therefore, decreased overpotential seems to be related with the strain relaxation and impurity phase (PrO 2 ) formed.
As it draws the MPD curves as a function of Pt contents, it is interesting that the dependency of MPD on Pt amount was similar with that of the lattice strain on Pt amount as shown in Figure 1c and 3b.From this result, the strain effect seems to be highly effective for increasing cathodic performance of PNCG resulting in the increased performance of cells.Similar trend was also observed in Au-dispersed PNCG in our previous study and comparison of Au and Pt dispersion on power density is shown in Figure S3, Supporting Information. [20,21]or ensuring dominated effects by the strain or intrinsic catalytic function of dispersed metal because of high activity of Pt to oxygen activation, electrochemical performance of the cell was compared with different metal positions, which means the simple mixture or dispersion of Pt in PNCG bulk.In spite of larger exposed surface area, the power density of the cell using Pt-dispersed PNCG was still higher than that of Pt-mixed PNCG at whole temperature, which means lattice strain may increase the surface activity to oxygen dissociation by increasing charge carriers (Figure 4a).In contrast, as comparing electrochemical performance of PNCG, Pt-mixed PNCG exhibited the increased power density than that of PNCG because of superior activity of Pt (Figure 4b).Therefore, this result indicates that the tensile strain by Pt dispersion contributed to further increase the electrochemical performance of PNCG than the increased conductivity and catalytic reaction by simple mixing Pt powder.Moreover, because anode of the cell was made by the same processes excepting for the cathode material, the difference in power density was assigned to the difference in the cathodic performance.Also, the increased performance was observed more clearly with decreasing temperatures, which suggests the strong tensile strain effects on oxygen dissociation.
Internal resistance of the cell was estimated with current interruption method (Figure 5).Ohmic (IR) loss of two samples, dispersion and mixture, was similar value in spite of Pt position (Figure 5, top).It can be seen that the small Pt amount showed a small effect on IR loss.In contrast to IR loss, overpotential was much decreased when Pt was dispersed into bulk comparing with the simple mixture of Pt (Figure 5, bottom).At all temperatures, polarization resistance of Pt-dispersed PNCG was lower than that of Pt-mixed PNCG and this more significantly observed at high current density.Since it is well known that Pt is active to oxygen dissociation and exposed surface area of Pt is much larger on the simple Pt mixing sample, high cathodic activity of PNCG by Pt dispersion is highly interesting and could be related with the tensile strain effects introduced by Pt dispersion.

Strain Effects by Metal Dispersion on Surface Activity of PNCG
It seems that tensile strain shows large positive influence on surface activity of PNCG comparing the catalytic activity of dispersed metal as discussed.Comparisons of strain effects introduced by Au-inactive metal to oxidation or Pt-active metals were discussed.
IR loss and overpotential of cathode was estimated with current interruption method under polarized condition at operation temperature (600-800 °C) and the resistance was separated by reference electrode to anode and cathode (Figure 6).Comparing with single PNCG cathode, it is obvious that IR loss was decreased by increasing amount of Pt addition and there are two reasons considered, i.e., one is increased electrical conductivity of PNCG (increased current collecting efficiency) and the other is improved contact between cathode and LSGM surface.At present, which mechanism is dominated for decreased IR loss by Pt or Au dispersion is not clear, however, considering the increased electrical conductivity of PNCG by metal dispersion, [20,21] increased current correcting effects as a result of increased conductivity seems to be dominated.In Figure 6, IR loss of the cell using Au-dispersed PNCG cathode was also shown as a function of Au amount.The smallest IR loss achieved was different amount between Pt and Au; however, IR loss is smaller by Au dispersion and this may be assigned to the significantly increased mixed ion conductivity in PNCG by Au dispersion reported in our previous study. [17]In any case, on both Pt or Au dispersion, IR loss was significantly decreased and this smaller IR loss is one reason for increased power density of the cell.
In contrast, overpotential of PNCG cathode was also measured with current interruption method and the results at 800 °C were  shown in Figure 7.In contrast to IR loss, the overpotential value was shown different behavior with increasing amount of Pt, and Au dispersion (Figure 7a).The polarization resistance of Pt-PNCG was exhibited the lowest value at 1 mol% and increased again with increasing amount of Pt dispersion.In contrast, 1 mol% Pt-mixed PNCG (strain free) also showed a lower performance than that of PNCG at the same amount of Pt dispersed.From this result, although Pt has high catalytic activity to oxygen dissociation, tensile strain in PNCG lattice has larger effects on cathodic activity than that of simple mixing sample.In contrast, in Au-PNCG, overpotential value was decreased continuously up to 5 mol% from 1 mol% and increased again.Figure 7b shows the overpotential values at 300 mA cm À2 of Pt-or Au-dispersed PNCG cathode as a function of strain estimated by XRD.In Figure 7b, it was seen that the cathodic polarization was decreased with increasing lattice strain induced by dispersion of Au or Pt.However, the overpotential data of 5 mol% Pt and 7 mol% Au-dispersed PNCG was deviated from tendency between overpotential and strain because of strain relaxation and impurity phase.Although Pt and Au are wellknown highly active and inactive catalytic material respectively, cathodic performance of 3 mol% Pt-PNCG was similar with 3 and 5 mol% Au-PNCG because of similar tensile strain.Therefore, the cathodic performance was mainly determined by tensile strain when the operating temperature is the same and this is also the same at different operation temperature (see Figure S4, Supporting Information).Therefore, it is interesting that the strain effect shows more significant effects on the surface activity to oxygen dissociation than that of intrinsic surface activity of dispersed metal.This may be related with increased diffusivity of oxide ion in PNCG by tensile strain effects as discussed in our previous report. [25]he activation energy and pre-exponential terms were estimated from the Arrhenius plots of current density at 50 mV of cathodic overpotential, and the results are shown in Figure 8.Here, it is noted that the activation energy and the pre-exponential term shows activity of one site which means activity to oxygen dissociation and number of active sites which means diffusivity of oxide ion, respectively.Obviously, the preexponential term was increased with increasing tensile strain and that revealed a similar tendency with the overpotential (Figure 8a).Therefore, the pre-exponential term which is related to a reaction area was increased as increasing tensile strain and it is seen that the number of reaction sites may be increased by Pt or Au dispersion with tensile strain because of expansion of three phase boundary to two phases between electrode/electrolyte.In contrast, the activation energy was also affected by metal dispersion and increased with increasing strain value as shown in Figure 8b.Therefore, it seems that the surface activity to oxygen dissociation was decreased by dispersion of Pt or Au because of increased activation energy.Increase in activation energy will give a disadvantage of cathodic performance at decreased operation temperature, and so it became clear that the decreased cathodic overpotential by metal dispersion was mainly achieved by increased diffusivity of oxide ion in PNCG and this could be related with the increased interstitial oxide ion in PNCG by Pt or Au dispersion which is suggested by X-ray photoelectron spectroscopy (XPS) measurement.For oxygen dissociation, it is reasonably considered that the oxygen vacancy is active site and considering the increased interstitial oxygen by Pt or Au dispersion, formation of oxygen vacancy might be difficult resulting in the increased activation energy for cathodic reaction.
Impedance of cathode was measured under open-circuit condition for electrochemical analysis by using reference electrode (Figure 9).Representative impedance data of three cells was shown in Figure S5, Supporting Information, and the small cathodic overpotential was observed on Pt and Au-PNCG.As inducing tensile strain in PNCG, the polarization resistance   was also dramatically decreased, which means improvement of electrochemical performance, oxygen reduction on electrode surface or ionic charge transport from electrode to electrolyte.At 3 mol% Au-PNCG, polarization resistance from impedance was 0.27 Ω cm 2 that was larger than that of Pt-PNCG (0.025 Ω cm 2 ) at 800 °C.Therefore, cathodic performance of PNCG was strongly dependent on the tensile strain regardless of the originally high catalytic activity.For analysis of the mechanism of increased cathodic activity by strain effect, impedance was measured for comparison at each temperature (Figure 10a).The polarization resistance of Ptmixed PNCG was always larger values than that of Pt-dispersed PNCG at all temperature examined.Moreover, when the impedance of 1 mol% Pt-PNCG was compared with 1 mol% Pt-mixed PNCG, Pt-mixed PNCG has still higher polarization resistance (0.23 Ω cm 2 ) than that of Pt-dispersed PNCG at 800 °C (Figure 10b).The tensile strain inducement by Pt may help an enhancing ORR on electrode surface which can be also observed from the impedance results.[27] Therefore, the lattice expansion seems to play important role in increasing the oxygen diffusivity  on PNCG surface resulting in the decreased overpotential observed at lower-frequency region.
After power-generation measurement, microstructure and elemental distribution of PNCG cathode were analyzed with SEM and EDX (Figure 11).The porous structure of PNCG cathode was sustained after measurement and the thickness of the cathode with different amount of Pt dispersed was almost the same as about 11 μm thickness, which suggests that the gas diffusion resistance was negligibly influenced the cathodic performance and the observed diffusion resistance could be assigned to the surface diffusivity of oxide ion.Moreover, from EDX results, the interface looks no delamination and there is no reaction between electrode and electrolyte (Figure S6, Supporting Information).The strong intensity of Pt was observed in EDX image, which means Pt metals were still maintained in PNCG bulk even after power-generation measurement.This is also the same of Au-dispersed one.Therefore, Au-and Pt-dispersed PNCG has nice chemical compatibility with LSGM electrolyte and introduction of tensile strain is promising strategy for increasing ORR activity of PNCG due to increased reaction area by increased oxygen diffusivity.From our previous study on Au-dispersed PNCG, after power-generation measurement, lattice constant of PNCG was not changed and so induced lattice strain was stably sustained.Therefore, in case of Pt dispersion, it is expected that tensile strain induced was sustained during power-generation measurement.Therefore, comparing with the intrinsic surface activity of dispersed metal, tensile strain is more significantly influence the surface activity to oxygen dissociation by increasing diffusivity, which is related with reaction area as suggested by increased pre-exponential term in Figure 8a.
Electrical conductivity of PNCG sintered with Pt or Au nanoparticles was already reported. [21]In case of Pt, overall conductivity of PNCG was decreased by Pt dispersion, however, it increased significantly by Au.In PNCG, hole conduction is dominated and considering the work function of Pt, electronic hole was trapped at energy barrier formed at interface between Pt and PNCG.In contrast, from the detailed study on charge carrier in PNCG with Au dispersion, hole concentration was significantly increased by charge compensation of interstitial oxygen introduced at rock salt block and so oxide ion conductivity is also significantly increased by Au dispersion into bulk of PNCG. [20,21]PS of Pt-dispersed PNCG was measurement for analysis of oxidation state of Ni, O, and Pr/Cu as shown in Figure 12.In this figure, XPS data of PNCG with 7 mol% Au, and 5 mol% Pt were not shown because of small impurity phase such as PrO 2 formed.Moreover, the peaks of oxygen (O 1s ) and nickel (Ni 2p ) were compared first and the overlapped peak of Pr (Pr 3d ) and Cu (Cu 2p ) will be also compared for understanding oxidation state of cations.In Figure 12a, the peak at %533.5 eV is attributed to the adsorption species on the surface, and %531.9 eV is assigned to the lattice oxygen. [28,29]Two peaks at low binding energy (BE) seem to be assigned to the interstitial oxygen (%529.1 eV) that is related with excess oxygen in PNCG lattice. [21,28]In contrast, the satellite peaks (%863.8 and %860.9 eV) and the other two peaks that were assigned to Ni 3þ (%858.3[32][33] Although it is difficult to deconvolute peaks because of overlapped peak position of Pr (Pr 3d ) and Cu (Cu 2p ), two peaks (933 and 929 eV) were observed (Figure 12c). [23]The percentage of interstitial oxygen and valence number of nickel were shown as a function of strain value (Figure 12d).Interstitial oxygen contents (δ) in lattice were increased with increasing tensile strain due to the enlarged free volume (Figure 12d, left).Pt dispersion was more effective for increasing the interstitial oxygen rather than Au one.It was explained that there is the large oxygen excess amount in PNCG as interstitial position ( O 00 i ) from atmosphere.With these results, percentage of Ni 3þ was also increased with the tensile strain (Figure 12d, middle).The Ni reduction (from Ni 2þ to Ni 3þ ) was originated from hole (h : ) generation for charge compensation by interstitial oxygen ( O 00 i ).On the contrary to Au dispersion, Pt exhibits minor effect on Ni 3þ formation under tensile strain.In contrast, the peak at %929.5 eV was increased with metal dispersion, which means Pr or Cu was also changed to reduced states (Figure 12d, right).Therefore, Pr and Cu seems to be mainly charged compensated the interstitial oxide ion which was induced by Pt dispersion.Therefore, it is expected that the increased number of charge carrier ( O 00 i and h : ) increased the oxygen reduction activity on PNCG as discussed and this change in oxygen nonstoichiometry by Au and Pt dispersion is the origin of increased activity of PNCG cathode to oxygen dissociation.

Conclusion
Lattice parameter of PNCG was enlarged by dispersion of Pt followed by sintering and so volume mismatch between metal and PNCG-generated tensile strain in 3D during cooling process.This is similar with Au dispersion.Cathodic performance was much increased by dispersion of Pt into grain of PNCG, i.e., IR loss and overpotential were decreased significantly resulting in the increased power density of the cell, in particular, at decreased temperature.The highest power density was achieved at 1 mol% Pt-dispersed PNCG for cathode.Correlation of cathodic activity with tensile strain was observed and 1 mol% Pt dispersion forms %0.67% tensile strain and the smallest overpotential of 35 mV at 300 mA cm À2 was achieved at this strained state.Considering the high surface activity of Pt, it is expected that the cathodic overpotential is decreased with increasing Pt amount; however, tensile strain more significantly affects cathodic activity.It was found that including Au which is well known low catalytic activity, overpotential was strongly dependent on the tensile strain.Increased pre-exponential term explained the increased cathodic performance of PNCG and so increased oxide ion conductivity by tensile strain is significantly increased the cathodic activity of PNCG by increasing the active sites for oxygen dissociation.Although modification of cathode by nano metal particles with infiltration method is now popularly studied, introduction of tensile strain by dispersion of metal nanoparticles in grain is more effective for increasing activity to oxygen dissociation.This study reveals that introduction of tensile strain is effective for increasing the activity to oxygen dissociation on PNCG and new method for designing the active air electrode catalyst for SOFC and also expected to be for solid oxide electrolysis cell which is now expected for efficient hydrogen production method.

Experimental Section
Powder Synthesis and Cell Fabrication: Pt-or Au-dispersed PNCG powder was synthesized by combination of solid-state reaction and incipient wetness process.After stoichiometric amount (1, 3, 5, and 7 mol%) of HPtCl 4 •6H 2 O and HAuCl 4 •6H 2 O (99.0%, Kinda Chemical, Japan) dissolved in deionized water, Pr 6 O 11 (99.9%,Soekawa Chemical, Japan), NiO (Wako, Japan), CuO (99.0%, high-purity chemicals, Japan), and Ga 2 O 3 (99.99%,high-purity chemicals, Japan) were mixed in deionized water.The water was heated on a hot plate until evaporate water.Then, to decompose the nitrate, the powder was fired at 400 °C for 2 h.The obtained powder was made into pellet by uniaxial press and cold isostatic pressure with 300 MPa for 30 min.The pellet was calcined two steps at 800 °C for 2 h and 1300 °C for 6 h.The detail of powder preparation and calcine condition has been reported elsewhere. [20]The sintered pellet was crushed and milling with high energy planetary ball mill with 500 rpm for 4 h in ethanol (Pulverisette 6, Netzsch, Germany).Preparation of NiO-Fe 2 O 3 anode and La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3 (LSGM) electrolyte was also reported elsewhere. [34,35]Cathode and anode pastes were prepared with a solution of isobutyrate (Tokyo Chemical Industry, Japan) and ethyl cellulose (Kinda Chemical, Japan) and screen printed.After thickness adjustment (t = 300 μm) of LSGM by polishing, electrodes were screen printed (active area = 0.196 cm 2 ) on electrolyte each surface and the Pt reference electrode also prepared close to cathode.Co-firing temperature of electrodes was performed at 1100 °C for 1 h.
Powders and Microstructure Analysis: An X-ray diffractometer (Rint 2500, Rigaku, Japan) was used for phase analysis and strain estimation.XPS (Shimadzu Ultra, Japan) measurements were performed for analysis of valence number with Al Kα line and the BE of the obtained spectrum was normalized by assigning the BE of C 1s to 285 eV.After the cell test, microstructural observation with EDX analysis was performed by using field-emission SEM (FE-SEM, Versa 3D, FEI) and EDX (Oxford Instrument).
Electrochemical Analysis: Power-generation property of LSGM electrolytesupported cell was tested at 600-800 °C by using humidified hydrogen (97% H 2 þ 3% H 2 O) as fuel and O 2 as oxidant.The detail test conditions were reported elsewhere. [36]To measure electrochemical performance, an impedance frequency analyzer (Solartron 1260 and 1287, USA) was used with Pt reference electrode set close to cathode.Impedance measurements were performed in a frequency range from 1 MHz to 1 Hz with 50 mV amplitude, 10 points at one decade, and no bias applied under open-circuit condition.In contrast, as DC measurement method, I-V curves (Hokuto HA301) and current interruption measurement were performed.Current pulse was generated with the current pulse generator (Hokuto HC111) and the residual potential response was analyzed with memory recorder (Hioki 8835) for investigating internal resistance of the cell under polarized condition.

Figure 2 .
Figure 2. a) Particle images of Pt dispersed in PNCG at different magnification.b) Line analysis results of Pt, Pr, and Ni with energy-dispersive X-ray analysis (EDX).

Figure 3 .
Figure 3. a) Current-voltage-power density (I-V-P) curves of the cell using Pt-dispersed PNCG cathode at 600-800 °C.b) Maximum power density (MPD) of each cell as a function of metal content.

Figure 4 .
Figure 4. a) I-V-P curves of the cell with Pt-mixed and Pt-dispersed PNCG cathode at 600-800 °C.b) MPD comparison of each cell at 600-800 °C.

Figure 6 .
Figure 6.IR loss of Pt-PNCG (left) and Au-PNCG (right) was measured with different metal dispersion contents.

Figure 7 .
Figure 7. a) Polarization resistance of Pt-PNCG (left) and Au-PCNG (right) with different amount dispersed.b) Overpotential of PNCG cathode dispersed with Pt or Au at 300 mA cm À2 as a function of lattice strain.

Figure 8 .
Figure 8. a) Pre-exponential terms and b) activation energy of PNCG cathode with Pt, and Au dispersion as function of strain value.The data was estimated from current interruption method.

Figure 9 .
Figure 9. a) Complex impedance plots of PNCG anode with 1 mol% Pt and 3 mol% Au-dispersed PNCG at different temperature: 800, 700, and 600 °C, respectively.Electrochemical performance of cathode part was separated with reference electrode.

Figure 10 .
Figure 10.Comparison of impedance spectra between 1 mol% Pt-dispersed and Pt-mixed PNCG at 600-800 °C.b) The comparison of impedance and bode plot of 1 mol% Pt-dispersed and Pt-mixed PNCG at 800 °C.The impedance was separated by reference electrode.

Figure 11 .
Figure 11.Scanning electron microscope (SEM) and EDX images of Pt-and Au-PNCG cathode after power density measurement.

Figure 12 .
Figure 12.X-ray photoelectron spectroscopy (XPS) spectrum of a) O 1s , b) Ni 2p , and c) Pr 3d /Cu 2p with metal dispersion (Pt and Au).d) The interstitial oxygen contents (left), valence number of Ni (middle), and peak at 929.5 eV (right) were estimated as a function of strain values.