Chengyi Ai, Tingting Li, Rongzheng Ren, Zhenhua Wang, Wang Sun, Jinsheng Feng, Kening Sun,Jinshuo Qiao

Beijing Key Laboratory for Chemical Power Source and Green Catalysis, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China

Keywords:Protonic conducting solid oxide fuel cells Cathode materials Element substitution

A B S T R A C T Proton conducting solid oxide fuel cell (H-SOFC) is an emerging energy conversion device, with lower activation energy and higher energy utilization efficiency.However, the deficiency of highly active cathode materials still remains a major challenge for the development of H-SOFC. Therefore, in this work,K2NiF4-type cathode materials Pr2-xBaxNi0.6Cu0.4O4+δ (x = 0, 0.1, 0.2, 0.3), single-phase tripleconducting (e-/O2-/H+) oxides, are prepared for intermediate temperature H-SOFCs and exhibit good oxygen reduction reaction activity. The investigation demonstrates that doping Ba into Pr2-xBaxNi0.6Cu0.4O4+δ can increase its electrochemical performance through enhancing electrical conductivity,oxygen vacancy concentration and proton conductivity.EIS tests are carried at 750°C and the minimum polarization impedances are obtained when x=0.2,which are 0.068 Ω·cm2 in air and 1.336 Ω·cm2 in wet argon,respectively.The peak power density of the cell with Pr1.8Ba0.2Ni0.6Cu0.4O4+δ cathode is 298 mW·cm-2 at 750 °C in air with humidified hydrogen as fuel. Based on the above results, Ba-doped Pr2-xBaxNi0.6Cu0.4O4+δ can be a good candidate material for SOFC cathode applications.

1. Introduction

As an energy conversion device, the solid oxide fuel cell (SOFC)directly converts chemical energy into electrical energy. It has advantages of less pollution emission, high efficiency and fuel diversity and it is important for coping with environmental issues[1-3].According to the conductivity of the electrolyte,SOFC can be divided into O-SOFC and H-SOFC. O-SOFC requires electrolyte materials with oxygen ions-conductivity, and H-SOFC with proton-conductivity, respectively. The reaction product water of H-SOFC is generated on the cathode side, and the dilution effect of the fuel gas is reduced relative to O-SOFC, which generates water on the anode side.Also,in terms of activation energy,proton conduction is much lower than oxygen ion conduction at lower temperature environments. It is indicated that H-SOFC should be an outstanding choice for its pretty performance at intermediate temperature which alleviates thermal degradation of the electrode and restriction of continuous work at high temperature [4-7].

At the moment, a critical issue, hindering the development of H-SOFC technology, is the lack of suitable cathode material.The traditional ABO3perovskite-type oxides cathode materials for H-SOFC such as LaMnO3and SrMnO3are single oxygen ion conductors. The cathode reaction generally occurs at the three-phase boundary(TPB)of electrode,electrolyte,and cathode gas composition. While the mixed ion (or proton)/electronic conductor (MIEC)with conductivity of both electrons and ions (protons) can extend the triple phase boundaries (TPBs) from the electrolyte/electrode interface to the entire cathode area, which greatly increases the reaction sites and plays a great role in improving the performance of the battery [8]. Among the many candidate materials,Ruddlesden-Popper oxides Ln2NiO4+δ(Ln = La, Pr, Nd) as a mixed ionic electronic conductor (MIEC) has been widely studied and shown good performance in SOFC applications [9-12]. As shown in Fig. 1, these oxides can be depicted as a spatial structure composed of LnO and LnNiO3, where the LnO rock-salt layer will influence the morphology structure of NiO6[8,13,14]. The ionic radius of Ni3+is smaller than Ni2+, resulting in a change in the valence state of Ni,thereby exhibiting MIEC characteristics[14,15].Meanwhile,these materials have similar thermal expansion coefficient (TEC) with BZCY, which is important for H-SOFC [16].

Fig. 1. Schematic lattice model of Ln2NiO4.

Among Ln2NiO4+δoxides, the Pr2NiO4series exhibit the best electrochemical performance due to the smallest ionic radius of Pr.It is reported that Pr2NiO4displays highest oxygen ion diffusion effect and surface exchange parameters[17-19].Also,the research of Alexiset al.concluded that Pr2NiO4+δcan be used as a potential cathode of H-SOFC with triple mixed conductivity (O2-/H+/e-) as the reaction of ORR was further strengthened in the intermedium temperature environment accordingly [14,20].

To further improve the catalytic activity of Ln2NiO4+δoxides,the optimizing strategy by element doping has been reported for multiple times.It is proved that replacing Ni by Cu can benefit the oxygen diffusion and improve the electrochemical performance of Pr2NiO4+δ[21,22].By related research,Pr2Ni0.6Cu0.4O4+δhas shown pretty good performance in electrical conductivity and electrochemical tests [23,24]. There is a related study on the effect of Ba doping at the A site of La2NiO4+δand it encourages forming more electron holes, enhancing the electronic conductivity of the material[25,26].Moreover,the doping of a larger volume Ba can alleviate the pressure of the Ln-O bond and increase the elastic modulus and microhardness of the material [26]. However, few relevant researches on the doping effect of simultaneous Cu on B-site and Ba on A-site of Pr2NiO4+δhave been investigated and it is worth trying.

In this study, alkaline earth metal Ba2+with a larger ion radius was doped into Pr2Ni0.6Cu0.4O4+δas a cathode for H-SOFC.Phase structure, electrical conductivity and ionic valence were investigated to evaluate the microstructure of materials.O2-temperature programmed desorption (O2-TPD) was employed for testing the adsorption of cathode gas. The electrochemical performance was tested at 600-750 °C by using BaZr0.1Ce0.7Y0.1-Yb0.1O3-δas electrolyte.

2. Experimental

2.1. Sample synthesis

Pr2-xBaxNi0.6Cu0.4O4+δ(x=0,0.1,0.2,0.3 and 0.4)powders were preparedviaa sol-gel combustion method.Stoichiometric amounts of Pr(NO3)3·6H2O, Cu(NO3)2·3H2O, Ni(NO3)2·6H2O, Ba(NO3)2were dissolved into deionized water. Then, ethylenediaminetetraacetic acid(EDTA)and citric acid were added into this solution as chelating and sintering agent. The molar ratio between EDAT, citric acid and metal ions is 1:1.5:1.The solution was continuous stirred until forming gel in 80 °C water bath with ammonia adjusting PH value to 7-8. Complete self-combustion occurred when the gel was heated at 250°C for about 2 h.The ash was collected and calcined at 1100°C for 6 h in air to obtain Ruddlesden-Popper type cathode materials. Electrolyte powder BaZr0.1Ce0.7Y0.1Yb0.1O3-δ(BZCYYb)was prepared by the same technique and calcined 1000 °C for 5 h with the Ba(NO3)2, Zr(NO3)4·5H2O, Ce(NO3)3·6H2O,Y(NO3)3-·6H2O and Yb(NO3)3·5H2O as ingredients.

2.2. Physical characterization

The phase structure of Pr2-xBaxNi0.6Cu0.4O4+δ(x=0,0.1,0.2,0.3)were analyzed by X-ray diffraction(XRD,X′Pert PRO MPD)with Cu Kα radiation source at the 2θ range of 20o-80°. Scanning electron microscope(SEM,FEI QUANTA-250)was used to characterized the microstructure and elements distribution of Pr2-xBaxNi0.6Cu0.4O4+δ.X-ray photoelectron spectroscopy (XPS, MULT1LAB2000, VG) was carried out to study the valence and relative percentage of Ni, Cu and O.

0.1 g samples were weighed to be measured by thermal conductivity detector (TCD) (Chem BET) for O2-TPD. Catalysts were treated at 800 °C in oxygen for 2 h to ensure a full adsorption and lowered to room temperature.Helium was introduced at room temperature to remove excess gas from the device and physically adsorbed oxygen molecules.The signal value was adjusted to zero.O2-TPD spectra was recorded as temperature raised up at a rate of 10 °C·min-1.

Pr2-xBaxNi0.6Cu0.4O4+δmaterials were pressed into bars of 20 mm × 5 mm × 2 mm with polyvinyl alcohol (PVA) as the binder.The bars were sintered at 1200°C for 2 h to obtain the sample that was used for conductivity measurement by four-terminal method in air from 300 °C to 800 °C.

2.3. Cell fabrication

The BZCYYb powder with ZnO as sintering aids was pressed into pellets with a diameter of 15 mm and calcined at 1450°C for 5 h to obtain dense proton-conducting electrolytes [27]. Homogeneous cathode ink was prepared with starch, ethyl cellulose and alphaterpineol. The ink was screen-printed on both sides of electrolyte plate and were calcined at 1200°C for 2 h to obtain a symmetrical cell. Organic matter is burned off to obtain porous electrodes which were tightly bound to the electrolyte.Silver wires were connected with electrodes as current collections. For single cells, the electrolyte and anode support(constitute of NiO and BZCYYb with a mass rate of 4:6) were obtained by a casting method. In this experiment, ethanol/butanone was selected as the solvent;glycerol trioleate and triethanolamine were used as anode and electrolyte dispersant respectively; polyethylene glycol was used as a plasticizer for the anode and dioctyl phthalate for electrolyte;and polyvinyl butyral was used as a binder for both anode and electrolyte. Some corn starch was added to the anode slurry as a pore former. Anode and electrolyte slurry were milled with a ball mill for 24 hours.Electrolyte layer was coated by a casting machine and then anode was cast multiple times to form a certain thickness. Then it was calcined at 1450 °C for 3 h. For symmetrical cell and single cells, the effective area of the electrode was both 0.07 cm-2. The electrolyte with an ideal thickness of 30 μm will ensure the electrochemical performance of the single cell. The cathode slurry of single cells was prepared as the same way as symmetrical cells.

2.4. Electrochemical test

The electrochemical impedance (EIS) of the symmetrical cells Pr2-xBaxNi0.6Cu0.4O4+δ|BZCYYb| Pr2-xBaxNi0.6Cu0.4O4+δwith a diameter of 5 mm was measured by CHI660E with the frequency ranging from 100 kHz to 0.01 Hz with amplitude of 0.01 V. The measurements were taken in the atmosphere with different oxygen partial pressure balanced by nitrogen. Also, the data of independence were fitted by ZSimpWin software. To evaluated the oxygen reduction capacity of Pr2-xBaxNi0.6Cu0.4O4+δas cathode,the single cell performance was evaluated by Arbin Instruments tester (Fuel Cell Test System, FCTS). Humidified hydrogen (3 %H2O)was fed as fuel in anode with a flow rate of 60 ml·min-1while the cathode was exposed in air.

3. Results and Discussion

3.1. Phase structural characterization

In order to study the effect of Ba2+insertion in the Pr2Ni0.6Cu0.4-O4+δunit cell, the XRD technique was applied to detect the crystal structure of the Pr2-xBaxNi0.6Cu0.4O4+δ(x=0,0.1,0.2,0.3)samples.XRD patterns demonstrates the K2NiF4-structure stability of Pr2-x-BaxNi0.6Cu0.4O4+δin Fig. 2(a). When the doping amount of Ba2+isx= 0-0.3, the prepared sample exhibits the same diffraction peak as the matrix material, and no other peak appears. However, a small amount of PrO2occurs whenx= 0.4, which demonstrates that large doses of Ba2+can’t be doped into the unit cell of Pr2Ni0.6-Cu0.4O4+δdue to its unstable structure. So, the doping range ofx= 0-0.3 for the Pr2-xBaxNi0.6Cu0.4O4+δsamples were investigated in the following section. There were slight differences in the diffraction peaks, whenx= 0.1 and 0.2, the position of the diffraction peak shifts to the right, whilex= 0.3 to the left, as shown in Fig. 1(a). The position of the diffraction peak changes mainly because the lattice size increases or decreases when Pr3+is substituted with Ba2+,as shown in the XRD refinement results.Also,SEM of Pr2Ni0.6Cu0.4O4+δwas characterized which showed a good dispersion as exhibited in Fig. 2(b).

The XRD spectrum was further analyzed by Rietveld method to get more accurate information about the crystal structures of Pr2-xBaxNi0.6Cu0.4O4+δ,their lattice unit parameters and volumes have a corresponding change due to the insertion of Ba2+, as shown in Fig. 2(c)-(f) and Table 1. When the large radius of Ba2+(0.160 nm)is doped to the Pr3+(0.1179 nm)position,the unit cell size of samples gradually reduces, and reaches a minimum atx= 0.2, then increases again asx= 0.3. It indicates that Pr1.8Ba0.2Ni0.6Cu0.4O4+δhas the smallest unite cell volume of 185.876 ?3(1 ? = 0.1 nm)and thus an increased specific surface area of material, which is conducive to ORR on the cathode. The main factor of the nonlinear variation law may be that the doping of Ba2+will affect the crystal structure of Pr2-xBaxNi0.6Cu0.4O4+δ.As reported,the replacement of Pr3+by Ba2+can affect the space structure and promote Ni2+(0.069 nm) transition to Ni3+(0.06 nm) with the smaller radius. Since the ionic radius of Ba2+is relatively larger than that of Pr3+, the unit cell volume tends to be larger and unstable as the excessive insertion of Ba2+.

3.2. Electronic conductivity analysis

The crystal size is inextricably linked to the number and radius of anions or cations in the unit cell.In order to systematically study the influence of Ba2+doping on the valence state of each ion in the Pr2-xBaxNi0.6Cu0.4O4+δ(x= 0, 0.1,0.2, 0.3) unit cell, the elements of Ni, Cu and O were analyzed by XPS technique. The results are shown in Fig. 3(a)-(b) and the ratios of different states are recorded in Table 2.

The peaks of higher-valance Ni3+located at a higher binding energy about 858 eV and Ni2+was at about 855 eV[11,28].According to XPS patterns of Ni 2p in Fig.3(a)and the values of Ni3+/Ni2+in Table 2,the ratio of Ni3+/Ni2+increases from 0.239 to 0.486 subsequently as x increases from 0 to 0.3. The same phenomenon occurred in XPS of Cu in Fig. 3 (b). It shows that the Cu2+and Cu+correspond to 433 eV and 429 eV respectively [29]. The ratio of Cu2+/Cu+noticeable increases from 4.296 to 7.1945 as the value of x gradually increases from 0 to 0.3. The replacement of highvalence praseodymium position with low-valent Ba2+leads to the increasement of the nickel and copper valence state to maintain system neutrality. This can be described by Eq. (1) whererepresent Pr element in lattice,Cu or Ni in lattice,oxygen element in the lattice,Ba2+in Pr3+position,the Cu and Ni elements with higher valence in lattice, respectively. When the doping amount isx= 0.3, the valence state elevated phenomenon of nickel and copper is not obvious.

The change of the valence state of metal ions in the Pr2Ni0.6-Cu0.4O4+δsystem will also greatly affect its conductive behavior.Therefore, the electronic conductivity was measured by DC four-electrode method to explore the influence principle and the electronic conductivity under air in Fig. 3(c). Pr2NiO4+δis a typical p-type semiconductor,and the realization of electronic conductivity is mainly through the transition of holesin the system of Ni3+-O-Ni2+. As can be seen in the pattern, the conductivity of Pr2-xBaxNi0.6Cu0.4O4+δ(x=0,0.1,0.2,0.3)increases with temperature increasing, and tends to decrease after 450 °C.Since the hole transition is corresponding to a thermal excitation process,a rise in temperature will provide more power for its transition, hence the conductivity increases as temperature increases.The phenomenon that conductivity decreases after 450 °C can be explained by Eq. (2) whererepresents oxygen vacancy. When the temperature is further increased, a large amount of highvalent Ni3+is reduced to Ni2+, and oxygen vacancies are formed at the same time. The reduction in the number of holes and the presence of oxygen vacancies are not conducive to the transition of electrons. Therefore, the conductivity will be opposite to the temperature change in the temperature range of more than 450°C.

On the other hand, when the doping amount is 0-0.3, the doping of Ba2+promotes the formation of high valence state Ni3+and Cu2+, which increases the hole concentration of the Pr2Ni0.6Cu0.4-O4+δ. With more holes creates, electronic conductivity increases with the doping of Ba2+. The values of Pr2-xBaxNi0.6Cu0.4O4+δ(x= 0, 0.1, 0.2, 0.3) conductivity at 450 °C are 105.53 S·cm-1,109.25 S·cm-1, 116.59 S·cm-1and 121.66 S·cm-1, respectively. At the same time, Arrhenius plots of Pr2-xBaxNi0.6Cu0.4O4+δelectronic conductivity are shown in Fig. 3(d). The activation energy of electronic transport in lattice of Pr2-xBaxNi0.6Cu0.4O4+δ(x= 0, 0.1, 0.2,0.3) are 0.320 eV, 0.318 eV, 0.308 eV and 0.303 eV, which further explains the insertion of Ba2+shows promotion effect on the electronic conduction of the materials.

3.3. Analysis of oxygen vacancy

In order to explore the principle of reaction, the fitted impedance of Pr1.8Ba0.2Ni0.6Cu0.4O4+δunder different oxygen partial pressures is shown in the Fig. 4 (a) which was measuredviasymmetrical cells Pr1.8Ba0.2Ni0.6Cu0.4O4+δ|BZCYYb|Pr1.8Ba0.2Ni0.6Cu0.4-O4+δ. As the oxygen concentration increases, impedances in the high or low frequency decreases significantly. According to the impedance value,the control steps for the corresponding frequencies can be derived from the slope of the impedance curve. The slope of the high frequency region is about 1/4 and it means that the control step is the process of the charge transfer process while the 1/2 for low frequency region corresponding to oxygen decomposition, respectively [14,20]. The fitted polarization impedance data of the symmetrical cell with different oxygen partial pressure is shown in Table 3,and it is proved that the oxygen decomposition is the main factor for lower oxygen concentration and charge transfer for higher oxygen concentration, respectively.

Fig.2. Crystal structure of Pr2-xBaxNi0.6Cu0.4O4+δ(x=0,0.1,0.2,0.3).(a)XRD pattern of materials calcined at 1100°C;(b)SEM of Pr2Ni0.6Cu0.4O4+δ;(c)-(f)Rietveld refinement plot of Pr2-xBaxNi0.6Cu0.4O4+δ: (c) x = 0; (d) x = 0.1; (e) x = 0.2; (f) x = 0.3.

Table 1 The lattice parameters of Pr2-xBaxNi0.6Cu0.4O4+δ (x = 0, 0.1, 0.2, 0.3)

Eqs. (3)-(5) in Kr¨oger-Vink notation embody the currently accepted conduction process of oxygen ions and protons in the electrode reaction. In formulas, V··O, VXO, Vadand OH·O represent oxygen vacancy, incorporated oxygen, oxygen adsorption site and hydroxide defects, respectively. In this way,V··O is particularly important in both oxygen ions and protons conduction for their role of carrying adsorption oxygen and assisting hydration.Oxygen vacancies and proton defects are also important carriers of MIEC and play a key role for H-SOFC cathode structure.

Fig.3. Electronic conductivity of Pr2-xBaxNi0.6Cu0.4O4+δ(x=0,0.1,0.2,0.3).(a)XPS curves of Ni 2p;(b)XPS curves of Cu 2p;(c)Conductivity of samples in air;(d)Arrhenius plots of electronic conductivity for Pr2-xBaxNi0.6Cu0.4O4+δ.

At the same time, O1s XPS of Pr2-xBaxNi0.6Cu0.4O4+δ(x= 0, 0.1,0.2, 0.3) was tested to explore the oxygen distribution in lattice and the results are displayed in Fig. 4(b) and Table 4. The peaks at about 531 eV and 529 eV represent oxygen containing species(such as O2molecules,etc.)adsorbed on the surface oxygen vacancies and lattice oxygen respectively. As can be seen, with the increasement of Ba2+doping content fromx=0 to 0.2,the content of lattice oxygen (Oin) gradually decreases, while the content of adsorbed oxygen (Oad) species gradually increases. It is indicated that the doping of low-valent Ba2+ions also leads to the generation of oxygen vacancies to achieve charge balance, as described in Eq.(6). The ration of Oad/Oinfor Pr1.8Ba0.2Ni0.6Cu0.4O4+δis 4.618 and means the most adsorbed oxygen content and the lowest lattice oxygen.

Table 2 The ratios of Ni3+/Ni2+ and Cu2+/Cu+ in Pr2-xBaxNi0.6Cu0.4O4+δ

To study the stimulation of oxygen vacancies formation by the insertion of Ba2+,thermogravimetric tests on Pr2-xBaxNi0.6Cu0.4O4+δ(x= 0, 0.1, 0.2, 0.3) samples were performed in a dry air atmosphere,as shown in Fig.4(c).It can be seen that with the increasement of Ba2+content, the more mass reduction is occurred, which is due to the loss of lattice oxygen at high temperature.For example, the mass loss is 0.876% and only 0.605% forx= 0.2 and 0 at 750 °C, respectively. The samples withx= 0.2 andx= 0.3 have the highest oxygen vacancies, indicating the best oxygen ion and proton conductivity.

Fig.4(d)shows the O2-TPD curve of Pr2-xBaxNi0.6Cu0.4O4+δ(x=0,0.1,0.2,0.3),and there is a significant peak in the 400-600°C.It is concluded that peaks in the range of temperature 300-700°C correspond to oxygen vacancies of oxides which accelerates oxygen reduction reaction (ORR) of cathode [30]. Pr1.8Ba0.2Ni0.6Cu0.4O4+δobtains the largest oxygen desorption peak area.It means that this material has the most oxygen vacancies and can be considered as a good candidate material of transferring process of oxygen ions and protons.

Two factors may make it easier for modified simples to generate oxygen vacancies. On the one hand, the mass loss at high temperature is mainly due to the thermal reduction of transition metal with high valence(Ni3+/Cu2+),as described in Eq.(2).The Ba2+doping increases the concentration of Ni3+(Cu2+), as shown in the XPS tests, which provides sufficient condition for the large-scale production of oxygen vacancies at high temperature. On the other hand, the bond of Ni3+-O-Ni2+will be broken more easily because bond energy tended to decrease with the insertion of Ba2+.It can be confirmed from the XPS peak of Ni 2p.For example,binding energy of Ni3+in Pr2Ni0.6Cu0.4O4+δis 858.2 eV, while Pr1.8Ba0.2Ni0.6Cu0.4-O4+δis 855.1 eV. The reduction of binding energy means a smaller metal oxygen bond energy [31], which is more conducive to the formation of oxygen vacancies.

3.4. Characterization of electrochemical performance

The electrochemical impedances (EIS) under air of Pr2-xBax-Ni0.6Cu0.4O4+δ(x= 0, 0.1, 0.2, 0.3) symmetrical cells are shown in Fig.5(a)and the electrochemical catalytic performance of cathode materials can be evaluated.The fitted values of polarization impedance of Pr2-xBaxNi0.6Cu0.4O4+δ|BZCYYb| Pr2-xBaxNi0.6Cu0.4O4+δin air are exhibited in Table 5. When the doping amount of Ba is 0.2,Pr1.8Ba0.2Ni0.6Cu0.4O4+δhas the lowest impedance of 0.068 Ω·c m2which indicates the best oxygen reduction ability of cathode.It can be concluded that the process of replacing the high valence state elements with the low valence state elements produces additional vacancy according to previous study. Excessive doping such asx=0.3 may cause structural instability of Pr2-xBaxNi0.6Cu0.4O4+δand reduce the adsorption and conversion of oxygen. The EIS test under air conditions generally indicates the mixed electron/ion conductivity of the material in a sense. A lower impedance value means lower transmission resistance, which is beneficial to the improvement of ORR performance.

Fig.4. Characterization of oxygen vacancy in Pr2-xBaxNi0.6Cu0.4O4+δ(x=0,0.1,0.2,0.3).(a)EIS of Pr1.8Ba0.2Ni0.6Cu0.4O4+δ in different oxygen partial pressure at 750℃;(b)XPS curves of O1s in Pr2-xBaxNi0.6Cu0.4O4+δ; (c) TG curves of materials under dry air; (d) O2-TPD of samples.

Table 3 The fitted polarization impedance data of Pr1.8Ba0.2Ni0.6Cu0.4O4+δ|BZCYYb|Pr1.8Ba0.2Ni0.6Cu0.4O4+δ with different oxygen partial pressure at 750 ℃

Proton defects are mainly produced by hydration of oxygen vacancies with water molecules (Eq. (5)) in Pr2-xBaxNi0.6Cu0.4O4+δ.In humidified argon without oxygen, the electrochemical test mainly favors the process of proton formation into O. Fig. 5 (b)displays the fitted EIS curves of Pr2-xBaxNi0.6Cu0.4O4+δ(x= 0, 0.1,0.2, 0.3) symmetrical cells when 5% water vapor was introduced into Ar and the fitted values are shown in Table 6. The impedance value of Pr1.8Ba0.2Ni0.6Cu0.4O4+δis 1.336 Ω·cm2.Of course,this progress should also benefit from the increase in oxygen vacancies correspondingly.The formation of oxygen radicals has better properties for the proton conductivity, as shown in Eq. (5). Also, there may be two factors making it prone to proton defects with Ba2+doping.The insertion of Ba2+can increase the oxygen vacancy concentration of samples, providing more active sites for the adsorption of water molecules. In addition, the introduction of alkaline earth metal ions can improve the basicity of samples effectively,so that the bond between water molecules and oxygen vacancies becomes firm.These two factors lead to the formation of more proton defects.

Table 4 The results of O1s in Pr2-xBaxNi0.6Cu0.4O4+δ (x = 0, 0.1, 0.2, 0.3)

Table 5 The fitted polarization impedance of Pr2-xBaxNi0.6Cu0.4O4+δ|BZCYYb| Pr2-xBaxNi0.6-Cu0.4O4+δ in air at 750 ℃

Table 6 The fitted polarization impedance of Pr2-xBaxNi0.6Cu0.4O4+δ|BZCYYb| Pr2-xBaxNi0.6-Cu0.4O4+δ in wet Ar at 750 ℃

The electrochemical evaluation in the range of 600-750℃ was obtained with a cell structure of Ni-BZCYYb|BZCYYb|Pr1.8Ba0.2Ni0.6Cu0.4O4+δ.The cross-sectional SEM image of the single cell is exhibited in Fig. 5 (c), and thin electrolyte layer and porous electrode are helpful to ensure good electrochemical output performance.The testing results in Fig.5(d)show that the maximum power density Pr1.8Ba0.2Ni0.6Cu0.4O4+δreached is nearly 298 mW·cm-2at 750℃, demonstrating that this material has good ORR catalytic performance for proton conductor fuel cells and is a potential functional material.

Fig. 5. Characterization of electrochemical performance of Pr2-xBaxNi0.6Cu0.4O4+δ (x = 0, 0.1, 0.2, 0.3). (a) EIS of Pr2-xBaxNi0.6Cu0.4O4+δ in air at 750℃; (b) EIS of Pr2-x-BaxNi0.6Cu0.4O4+δ in wet Ar at 750℃; (c) Cross-sectional SEM image of the single cell; (d) I-V and I-P curves of Ni-BZCYYb|BZCYYb|Pr1.8Ba0.2Ni0.6Cu0.4O4+δ with hydrogen as fuel.

4. Conclusions

In this study,Pr2-xBaxNi0.6Cu0.4O4+δ(x=0,0.1,0.2,0.3) were prepared by sol-gel method. The minimum value of unit cell volume occurred whenx= 0.2, and an inflection appeared as x further increased. Electronic conductivity increased with the increase of Ba2+doping content and Pr1.7Ba0.3Ni0.6Cu0.4O4+δgot the highest value of 121 S·cm-1in air. Pr1.8Ba0.2Ni0.6Cu0.4O4+δhas the most oxygen vacancy among these materials which is beneficial to improve triple conductivity. Pr1.8Ba0.2Ni0.6Cu0.4O4+δobtains the smallest polarization resistance and when used as cathode material for H-SOFC, it exhibits an excellent maximum power density of 298 mW·cm-2at 750℃. These results reveal that the insertion of Ba2+in Pr2Ni0.6Cu0.4O4+δis an effective method to improve electrochemical performance and Pr1.8Ba0.2Ni0.6Cu0.4O4+δtends to be an excellent candidate for H-SOFC cathode material.

Acknowledgements

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China(Grant no.22078022).We also thank the Analysis&Testing Center,Beijing Institute of Technology for providing testing support.