Ni-based electrodes have been used for solid oxide fuel cell with Yttria Stabilized Zirconia (YSZ) electrolytes for many decades. Ni-zirconia cermet electrode possesses high electrochemical behavior, excellent catalytic activity and approachable electrical conductivity . However, this material displays some drawbacks with the passage of time due to its high working temperature. Moreover, the deposition of carbon layer on Ni-YSZ cermet electrode is another big issue particularly, when using hydro carbon fuels instead of pure hydrogen as well as high cost. By the virtue of the deposition of carbon layer, the Ni-cermet electrodes are oxidized which decreases the life of fuel cell. In order to get rid of these problems, the developing of new electrodes for solid oxide fuel cells instead of Ni is an immense requirement and challenge for fuel cell community. Numerous materials have been evaluated and synthesized which can work in the temperature range of 600–800°C. Those materials fall in the category of intermediate temperature solid oxide fuel cells (ITSOFCs) and exhibit a reliable performance as well as conductivity [2-4]. Lowering of the working/operating temperature of solid oxide fuel cells can improve the stability of the cell and has much R & D interest . Obviously, to maintain the low temperature as compared to 1000°C is an easy job, and it also carries an advantage of low cost.
The function of anode is to split the hydrogen (H2) fuel into ions (protons and electrons) . It has been reported that the highest electrical conductivity, electrochemical activity in order to oxidize the fuel, suitable porosity at the microstructure level, thermal stability, electrode's morphology and the compatibility with electrolyte are the primary requirements of a good anodic material for SOFC [7-12]. In order to fulfill and improve these basic requirements for SOFC electrodes, nanostructuring is feasible solution. A nanostructure design has changed the orientation of solid oxide fuel cell research and development . Zhu and co-workers  reported that use of nanostructured exhibits high ionic conduction at lower temperature as compared to the temperature of conventional bulk material LTSOFC.
Electrode materials for traditional SOFCs based on Yttria Stabilized Zirconia (YSZ) are composites of Ni-YSZ . Electrolyte materials play key role for solid oxide fuel cell performance; therefore, the use of those electrolyte materials, which can provide higher ionic conductivity at low temperature is very suitable. It has been proved experimentally that Gadolinium-doped Ceria (GDC), Samarium-doped Ceria (SDC) or Calcium-doped Ceria (CDC) can yield 0.1 S-cm−1 ionic conductivity at 600, 700 or 800°C, whereas YSZ yields this value of ionic conductivity at 1000°C [16-20]. Fang  reported that a maximum power density of 388 mW-cm−2 has been found in Ni-SDC cermets at 750°C.
The present study is focused on a new zinc (Zn)-based electrode materials. The consisted composition of Ba0.05Cu0.25Fe0.10Zn0.60O (BCFZ) has been suggested as an electrode material for low-temperature solid oxide fuel cell. It can be assumed that the small amount of iron works as a catalyst, while the presence of Cu shows an ability of its pure metallic behavior. It has been reported that the use of barium oxide prevents the material from corrosion during the working process of the material, which absorbs the water and facilitate water-radiated carbon removal reactions, which may be a precursor to enhance its life time . Nickel (Ni) has been replaced into Zn element which has given better results at comparatively low temperature (in the range of 400–550°C). This new electrode was demonstrated to show better performance than that of the state-of-art electrode, e.g. Ni-SDC anode and La0.6Sr0.4Co0.2Fe0.8O3−δ LSCF cathode.
Solid-state reaction (dry method) was applied to synthesize the nanostructured BCFZ oxide powder. The stoichiometric molar ratio of the composition BaCO3, CuCO3.Cu(OH)2, Fe(NO3)3.9H2O and Zn(NO3)2.6H2O (Sigma Aldrich, USA) was ground in a mortar with pestle to make the precursor homogeneous. This homogenous precursor was then sintered at 800°C for 4 h and allowed to cool with the furnace. The sintered powder was again ground for 30 min by adding small amount of carbon to produce porosity.
CDC coated with Sodium–Potassium Carbonates (NK-CDC) electrolyte was prepared by co-precipitation method. The whole procedure has been discussed in our previous work .In this work, Cerium Nitrate hexahydrate Ce(NO3)3.6H2O (Sigma Aldrich, USA) and Calcium Nitrate tetra hydrate Ca(NO3)2.4H2O (Sigma Aldrich, USA) were used as starting materials. Later on, 20 wt. % amount of this NK-CDC electrolyte was mixed with the electrode (BCFZ) material to form a composite electrode, which was in turn employed for testing the performance of the fuel cell.
In order to measure the electrical conductivity of the BCFZ material, a pellet of pure BCFZ oxide powder having 13 mm diameter and 3 mm thickness was prepared by dry press technique under a pressure of 280 kg-cm−2 by hydraulic press machine. The pellet was sintered at 650°C for 1 h. Silver paste was coated by brush on both sides of the pellet to make electrical contact. DC conductivities were measured in hydrogen and air atmosphere by implementing KD 2531 Digital Micro-ohmmeter, China. The following formula was used to calculate conductivity;
Where σ is the conductivity, L is the thickness of the pellet, R is the internal resistance and A is the active area of the pellet. The active area of the pellet was considered to be 0.64 cm2.
A symmetrical three-component fuel cell was prepared to measure the electrochemical performance. BCFZ-NKCDC/NKCDC/BCFZ-NKCDC were poured one by one in a die (13 mm diameter), each time slightly pressed in the same sequence and finally pressed at a pressure of 280 kg-cm−2 with a hydraulic press using dry press technique. In the newly formed cell, BCFZ-NKCDC works as symmetrical electrode and NKCDC as electrolyte. The thickness of the cell was controlled at 0.9 mm followed by 0.4 mm anode, 0.3 mm electrolyte and 0.2 mm cathode in thickness contained. Thus, composed three layers pellet/cell was then sintered at 650°C for half an hour. Silver paste was painted on both external sides of the cell to facilitate electrical contact.
The fuel cell performance was measured by providing hydrogen as a fuel at anode side and air as oxidant at cathode side under variable resistance load using fuel cell testing unit (L-43, China). Under each resistance load, the data of open circuit voltage (OCV) and current were recorded in the temperature range of 400 to 550°C with an interval of 50°C, and I–V curves were drawn at each temperature. Power density was calculated from these curves, and I–P curves also were drawn. The hydrogen gas flow was controlled at a rate of 100 ml/min under 1 atm pressure.
The X-ray diffractometer (XRD) pattern of sintered BCFZ (electrode) was recorded by using D/Max-3A Regaku XRD with Cu Kα radiation (λ = 1.5418 Å), 35 kV voltage and 30 mA current at room temperature. The structure of the material was determined from XRD pattern through JCPDs cards. The crystallite size (Dβ) was calculated from line-broadening peaks of XRD patterns using Scherer's equation;
Where λ and β are wavelength and full width half maximum (FWHM), respectively.
In order to analyze the microscopic view of the prepared BCFZ electrode, SEM (Philips XL-30) was used. The detailed microstructure analysis and morphology including the size and shape were recorded. Electrochemical Impedance Spectroscopy (EIS) measurements of BCFZ electrode were analyzed at hydrogen atmosphere by using Auto Versa STAT 4 (Princeton Applied Research, USA). The frequency is varied from 0.01 Hz to 1.00 MHz under 10 mV. Experimental and simulated curves were drawn in the light of ZSim Win Demo version 3.20 Software by the adjustment of LRQ(CR) equivalent circuit, where L, R, Q and C show the inductance, resistance, charge and capacitance of the material, respectively.
3 RESULTS AND DISCUSSIONS
The X-ray diffraction patterns of BCFZ electrode are shown in Figure 1. The patterns in Figure. 1 exhibit that all elements are completely shifted into Zn during sintering process, except three peaks that exhibit the BaZnO2 phases according to JCPDs 01-074-0137, which indicate that the material has two-phase structure. No other peaks of copper and ferric were found in XRD pattern according to JCPDs card No. 36-1451. The peaks of Figure 1 are indexed, and the structural analysis emphasizes that the structure is hexagonal. The particle size of the BCFZ electrode was calculated from the XRD data applying Scherer's formula and found to be 25 nm. It has been identified from the XRD study of material that the sintering temperature of 800°C is suitable to create nanostructure crystalline structure. The image of surface morphology has been shown in Figure 2. It has been found that the particle sizes of BCFZ electrode are in the range of 20–50 nm, this is a good agreement of the XRD result. It can be clearly observed from SEM micrograph that there are many pores in the material. The porous structure provides super way to transport electrons and oxygen ions from anode to cathode or vice versa, and gas . The DC electrical conductivity of BCFZ electrode has been measured at hydrogen atmosphere, and the corresponding results are shown in Figure 3(a). BCFZ electrode displays a maximum electrical conductivity of 6.15 S-cm−1 at temperature of 300°C in hydrogen atmosphere. It has been reported in our previous work  that NK-CDC electrolyte possesses an ionic conductivity of 0.1 S-cm−1 at 600°C. Since, BCFZ electrode has more than 60 times higher conductivity than that of NKCDC electrolyte, which concerns the electrical compatibility of BCFZ electrode and NK-CDC electrolyte at interface for ion transportation. Figure 3(a) indicates that BCFZ material exhibits the metallic conduction behavior due to decrease in conductivity at elevated temperature at hydrogen atmosphere, which emphasizes the presence of electronic conduction in BCFZ electrode. The electrical DC conductivity of BCFZ electrode at air atmosphere was also measured and is shown in Figure 3(b). The figure shows that the conductivity increases, when temperature decreases from 600 to 300°C, which also exhibits the same metallic conduction mechanism. Zhou reported  that the electrical conductivity at air atmosphere undergoes a semiconductor-like conduction behavior to metal-like conduction behavior at 425°C. The conductivity subsequently decreases with increasing temperature beyond 425°C. It can be specified that area specific resistance decreases with the threshold conductivity of electrode at air as well as hydrogen atmosphere. The low area specific resistance helps to transport H+/O2- at either anode–electrolyte or cathode–electrolyte interfaces, then electrode can be used either anode or cathode . BCFZ material has 2.34 S-cm−1 conductivity at air atmosphere, so in this present work, we have practically demonstrated its use as anode as well as cathode by fabricating a symmetric fuel cell. The activation energy of BCFZ electrode for conduction has been obtained by plotting electrical conductivity data in Arrhenius relation in hydrogen atmosphere using formula below:
Where σ is electrical conductivity, T is temperature in Kelvin, A is the exponential factor, k is Boltzman's constant and Ea is the activation energy. The activation energy plays an important role to evaluate performance for oxygen diffusion and oxygen ion conductivity in electrode/cathode material . The low value of activation energy 0.21 eV emphasizes the high catalytic activity of the BCFZ electrode material. The achievement of low activation energy is a result of decrease in average particle size (nanostructured materials as compared to bulk materials). The result has been shown in Figure 4, and the linear fit graph has been plotted and shown in inset of Figure 4.
Figure 5 shows the AC EIS analysis of composite electrode 80BCFZ-20NKCDC over the temperature 450°C at hydrogen atmosphere. The frequency was adjusted in the range of 0.01 Hz to 1 MHz. The imaginary part of impedance is plotted versus the real part of the impedance. The experimental result exhibits two semicircles. A part of arc in high-frequency region and a depressed arc in low-frequency region has been observed in 80BCFZ-20NKCDC electrode material. The depressed arc in the spectra revealed the maximum anode contribution and found that the high-frequency arc corresponds to charge transfer resistance which, includes the oxide ions diffusion in the electrode and transportation of oxygen ions from triple phase boundary into the electrolyte lattice. Low-frequency arc corresponds to a number of other resistances such as device, holder and wires etc. . The experimental curve was simulated with an equivalent circuit LRQ(CR) using ZSim Win Demo version 3.2 Software for the interpretation of EIS data, where L, R, C and Q denote the inductance, ohmic resistance, capacitance and charge, respectively. It has been observed in fitting between the experimental and the simulated data that high frequency is generally good, and at low frequencies, the simulated data slightly deviate from the experimental values .
Figure 6 shows the performance of a single symmetric fuel cell consisting of three consecutive layers of BCFZ-NKCDC anode, NKCDC electrolyte and BCFZ-NKCDC cathode. The maximum power densities 741.87 mW-cm−2, 717 mW-cm−2, 715 mW-cm−2 and 636 mW-cm−2 at temperatures of 550°C, 500°C, 450°C and 400°C were obtained, respectively. The OCV values were observed to be 1.07 V, 1.02 V, 1.02 V and 1 V at the same temperatures, respectively. The obtained power density of 741.87 mW-cm−2 at 550°C based on BCFZ electrode was found greater than that of fuel cell having maximum power density of 470 mW-cm−2 at elevated temperature of 800°C using conventional Ni-YSZ anode and conventional LSCF cathode .
The stability for OCV measurements of the fuel cell based on BCFZ-NKCDC composite electrode was recorded for 24 h with a regular interval of half an hour continuously. The results of measurement were shown in Figure 7. The cell yielded approximately one volt output, which remains constant during the experiment performed at temperature 550°C.
In summary, the hexagonal structure Ba0.15Cu0.15Fe0.1Zn0.6O was successfully achieved by solid-state reaction method and characterized as a novel Ni-free symmetrical electrode for the low-temperature SOFCs. The sintering temperature (800°C for 4 h) produced good crystallinity. BCFZ electrode has metal-like conduction phenomenon with a maximum conductivity value of 6.15 S-cm−1 and 2.34 S-cm−1 at H2 and air atmosphere, respectively. The low value of 0.21 eV activation energy indicates that nanostructure enhances the electrical conductivity as well as performance of the cell. The presence of depressed semi-circle in EIS analysis shows that the maximum contribution for cell performance comes from BCFZ electrode. The maximum values of OCV and power density were achieved 1.07 V and 741.87 mW-cm−2 at temperature 550°C, respectively. Hence, its average value of OCV was recorded as 1 V at 550°C for a short-term stability test, which shows that the BCFZ electrode is another potential candidate electrode material for the solid oxide fuel cell. The BCFZ electrode based on doped ceria electrolytes employed a new concept of Ni free symmetrical fuel cell and proved to be a valid alternative to the traditional SOFC configurations with improved fuel cell performance.
This work is supported by funding of Higher Education Commission, Islamabad, Pakistan. HEC Pakistan has facilitated the above scholar to work at the Department of Energy Technology, KTH Sweden, through International Research Support Initiative Program (IRSIP). The Swedish agency for Innovation Systems (VINNOVA) and Swedish Energy Agency (STEM) through industrialization projects are also acknowledged.