Synergistic Electrochemical Properties of Graphene Incorporated LCZ-Oxide Cathode for Low Temperature Solid Oxide Fuel Cell

Abstract

Mixed metallic oxides are getting increasing attention as novel electrode materials for energy conversion devices. However, low mixed ionic-electronic conductivity and high operating temperature hamper the practical applications of these devices. This study reports an effective strategy to improve the conductivity and performance of the fuel cell at low temperature by partially incorporating graphene in the Li_0.1Cu_0.2Zn_0.7-oxide (LCZ) composite. The proposed cathode material is synthesized via the cost effective conventional solid-state route. Graphene incorporated LCZ shows excellent performance, which is attributed to the favorable charge transport paths offering low area-specific resistance. An X-ray diffractometer (XRD) and scanning electron microscope (SEM) are employed for microstructural and surface morphological analyses, respectively. Electrical conductivities of all the materials are determined by the DC four probe method, and interestingly, LCZ-1.5% graphene exhibits an excellent conductivity of 3.5 S/cm in air atmosphere at a temperature of 450 °C with a minimum value of 0.057 Ωcm² area-specific resistance (ASR) that demonstrates significantly good performance. Moreover, the three-layer fuel cell device is fabricated using sodium carbonated Sm_0.2Ce_0.8O (NSDC) as an electrolyte, which can operate at low temperatures exhibiting open circuit voltage 0.95 V and shows a peak power density, i.e., 267.5 mW/cm² with hydrogen as the fuel.

1. Introduction

The increasing population and industrialization in contemporary times have increased energy consumption in the modern world. Conventional fossil fuels are unable to meet the unprecedented demand of energy resources. This scarcity of conventional resources, along with their hazards of environment pollution, stimulate researchers to discover, invent, explore, and improve renewable, clean, and sustainable energy resources [1,2]. Consequently, developing sustainable, environment-friendly, and high-efficiency energy assets is one of the most challenging tasks these days. Several energy conversion devices such as batteries, super capacitor fuel cells, and solar cells based on renewable energy resources such as hydrogen fuel and solar energy have already demonstrated their efficiency, but they come with limited practical applications [3]. Fuel cells offer electrical energy by converting chemical energy with high efficiency and zero emission and is categorized into various types, depending upon the electrolyte materials being utilized [4].

Solid oxide fuel cell gained attention due to its higher efficiency, low emission, and fuel flexibility as compared to the other fuel cell families [5,6,7]. At the same time, solid oxide fuel cells (SOFC) have certain limitations, such as high operating temperatures (800–1000 °C), precious electrode materials, and degradation of the material, which hamper their production at the commercial level [8]. Lowering the operating temperature and developing electrodes material with good electrical and catalytic behavior have been the ambitions of the fuel cell community [9]. Composite materials based on nickel yttrium-stabilized zirconia (Ni-YSZ) are widely used electrodes for conventional SOFC with YSZ as the electrolyte exhibiting good catalytic and electrical properties at 1000 °C. The same electrode materials can work at 800 °C with gadolinium-doped ceria (GDC) and samarium-doped ceria (SDC) without sacrificing the results [10]. However, the current temperature is still high and remains a major challenge to commercialize SOFC technology, including the carbon deposition, poisoning on the layer, and poor oxidation for hydrocarbon fuel issues [11].

Lowering the operational temperature for SOFC and decreasing the polarization resistance of the cathode is a big dream of the fuel cell research community [12]. A major contribution of polarization losses has been observed usually at the cathode layer [13]. Lanthanum strontium iron La_0.8Sr_0.2FeO₃ (LSF), lanthanum strontium manganese La_0.8Sr_0.2MnO₃ (LSM), and samarium strontium cobalt Sm_0.5Sr_0.5CoO_3−δ (SSC) are widely used as the cathode materials. LSM is being developed due to its stable attribution with yttrium-stabilized zirconia (YSZ) at higher temperatures [14]. Huang et al. reported a LSFN inspection as a cathode exhibiting improved electrical conductivity and ionic conduction beyond 550 °C [15]. Ba_xSr_1−xFeO_3−δ, La_0.8Sr_0.2FeO_3−δ perovskite oxide cathode materials are based on, are commonly proposed in terms of electrical conductivity and thermal stability for intermediate-temperature IT-SOFC [16].

Improved performances have been obtained by the nanostructure approach [17,18] and composite electrodes of transition metal oxides such as NiO, CuO, Fe₂O₃, SrO, and ZnO [19,20]. Nanocomposite-based cathode materials LSMZ developed with SDC were found compatible for low-temperature solid oxide fuel cells [21]. A ZnO-modified lithiated NiO composite cathode has been reported as a good material for electrocatalytic oxygen reduction activity with a yielded power density of 808 mW/cm² and polarization resistance of 0.22 Ωcm² at 550 °C [22]. A zinc-based nanostructured Mn_0.20Fe_xZn_0.80−xO_δ composite has been examined as a suitable electrode using natural gas fuel over a temperature of 600 °C [23].

Graphene, a monolayer of carbon atoms, exhibits ultra-high porosity, large specific area, and good electronic and electrochemical properties [24], has shown excellent catalytic performance for LT-SOFC [25] and the YSZ/rGo composite displayed good electrochemical applications in the temperature range 150–800 °C [26]. Graphene with groups 4–9 has been reported as electrocatalysts in oxygen evolution reactions (OER) and oxygen reduction reactions (ORR) [27]. Composites of other nanomaterials such as sulfides, metal–metal oxides, alloys, and polymeric materials exhibited interactive effects with graphene and found it a potential candidate for energy conversion and storage devices [28,29,30].

The present work focuses on incorporating graphene into LCZ oxide with different weight ratios to study its applications as a cathode material for LT-SOFC. The effects of Gr on electrical conductivity and catalytic activity are examined and found very synergistic. A significant enhancement of the fuel cell performance is also observed by fabricating a three-layers fuel cell device at low temperatures in a hydrogen atmosphere.

2. Experimental Technique

2.1. Synthesis of Cathode Materials

The cathode was prepared by the conventional solid-state reaction method. The stoichiometric molar ratio of composition Li_0.1Cu_0.2Zn_0.7 oxide using Li₂CO₃, CuCO₃.Cu(OH)₂, and ZnO (all from Sigma Aldrich, St. Louis, MO, USA) were used as starting materials. The planned precursor powders were mixed in mortar and ground with pestle to make a homogeneity of the mixture. This mixture was sintered at 800 °C in a furnace for 4 h. The sintered powder was again ground for 25 min, and a fine homogenous composite powder of LCZ material was obtained. The as-prepared graphene via cost effective and eco-friendly electrolysis method [31] was used to incorporate into the LCZ composite. Three samples were prepared and assigned named as 0 wt.% Gr-LCZ (pure), 1 wt.% Gr-LCZ, and 1.5 wt.% Gr-LCZ.

2.2. Characterizations

The XRD pattern of all sintered samples was analyzed by PAN-Alytical X′Pert Pro with Cu Kα radiation, (λ = 1.5418 Å), 40 kV voltage, and 30 mA currents at room temperature. The surface morphology and homogeneity of the prepared composites was analyzed by SEM (TESCAN Vega 3 LMU). The FTIR analysis of prepared LCZ and graphene incorporated LCZ cathode materials was carried out to examine the structure and phase transformation information by using FTIR spectrophotometer at a consistent spectral resolution of 6 cm⁻¹ in the range of 4000–500 cm⁻¹ by taking 116 scans.

2.3. Conductivity Measurements

The electrical conductivities of proposed synthesized LCZ-pure, 1 wt.% Gr-LCZ, and 1.5 wt.% Gr-LCZ cathode materials were measured using four- probe DC method with 2450 SMU (Keithley instrument, Taiwan) from 300 to 600 °C range in the air atmosphere. For this purpose, pellets (13 mm diameter and 2 mm thickness) of each sample were prepared by hydraulic press using 280 kg/cm², followed by sintering at 550 °C for 45 min. Silver paste was painted on the both sides of the pellets for conductivity measurements. The area-specific resistance of all the materials was calculated from conductivity data and was plotted as a function of temperature in the range of 300–600 °C.

2.4. Fuel Cell Fabrication and Performance

In order to fabricate three-layer fuel cells, the already prepared 20 wt.% ratio of NSDC electrolyte was added in LCZ and graphene-incorporated LCZ cathode materials to make suitable composite for electronic and ionic conductive phenomenon. It was then co-pressed using conventionally prepared LNCZ as the anode and NSDC as the electrolyte [32] under a pressure of 280 kgcm⁻². Fuel cell pellets of 13 mm diameter and 1 mm thickness were painted by silver paste on outer surfaces to collect current. However, the active area was considered 0.64 cm² on behalf of the dimension of sample holder. Fuel cell performance was executed by using fuel cell testing unit (S102, China) fed by hydrogen as fuel (flow rate 110 mL min⁻¹) at anode and air as oxidant (flow rate 80 mL min⁻¹) at cathode side. I–V and I–P curves were drawn from date recorded at 600 °C.

The scheme of fabrication of three-layers cell has been prescribed as below:

LCNZ as anode (already prepared), NSDC (already prepared) as the cathode and LCZ-pure and LCZ-graphene as the cathode (synthesized in this work).

(80 wt.% LCNZ-20 wt% NSDC/NSDC/80 wt.% LCZ-pure-20 wt.% NSDC and 80 wt.% LCNZ-20 wt.% NSDC/NSDC/80 wt.% LCZ-graphene-20wt.%).

The dimensions are as LCNZ-NSDC Anode layer 45 mg, NSDC Electrolyte layer 25 gm, and LCZ-pure-NSDC 25 mg for cell 1.

The dimensions are the LCNZ-NSDC Anode layer: 45 mg, NSDC Electrolyte layer: 25 gm, and LCZ-graphene-NSDC: 25 mg for cells 2 and3.

3. Results and Discussion

The LCZ oxide includes the starting materials Li₂CO₃, CuCO₃.Cu(OH)₂, and ZnO is a composite material. Figure 1 shows the X-ray diffraction patterns of LCZ-pure and LCZ-Gr composites sintered at 800 °C for four hours. The XRD analysis depicted the two phase materials ZnO and CuO. As earlier, Zhang et al. 2014 developed strontium and magnesium doped gallate LSGM oxide electrolyte for SOFC and found two phases, LaSrGaO₄ and LaGaO₃ in XRD data [33]. The indexing of all the peaks of ZnO phase illuminated the hexagonal structure confirmed with JCPDs card No. 089-0511 having space group p63mc. The lattice constants were calculated and found to be a = b = 3.24 and c = 5.20 Å. The diffracted peaks for ZnO were detected at 31.77°, 34.43°, 36.26°, 47.55°, 56.14°, 62.87°, 66.39°, 67.96°, 69.10°, 72.16°, and 76.98° correspond to crystal plans (100), (002), (101), ($\overline{1}$02), (110), (103), (200), (112), (201), (004) and (202), respectively. The second phase executed the monoclinic structure of tenorite CuO (space group C2/c) with lattice constants a = 4.65, b = 3.41 and c = 5.10Å according to JCPDs card No. 01-1117. The peaks at 35.74°, 38.95°, 53.88°, 61.8° and 66.22° correspond to crystal plans ($\overline{1}$11), (111), (020), ($\overline{1}$13) and (022) of CuO, respectively. High crystalline structure of the prepared compositions was observed as revealed by the obtained sharp peaks. The two phases depict the composite behavior of the proposed material which contribute individually for better conductivity and electrochemical performance of fuel cell [34,35]. The two -phase structure, based on XRD results, offers co-existence involvement of ZnO and CuO in the composite. The XRD patterns did not detect obvious graphene peaks for LCZ-Gr samples due to its amorphous nature; however, graphene was incorporated in LCZ materials [36]. The crystallite sizes were calculated using the Scherrer’s formula and were found to be in 80–115 nm range from strong and intense peaks.

The surface morphology and microstructure of prepared samples were examined by SEM and are displayed in Figure 2a–c. Oval-shaped structure, along with distributed powder on these structures, was observed with compact densification in Figure 2c. Oval shaped morphology of composite material reflected the layers compaction of precursors sintered at high temperature. Smooth surface, along with pores, can be observed in the images displayed in Figure 2. Few small whitish-like particles on surface might attribute the dispersion of graphene powder. However, the whiteness loops over the particles were not observed in LCZ-pure image shown in Figure 2a. On investigating the surface morphology of SEM images by line drawing, the particle sizes were found to be in the range of 90–125 nm and validated the XRD findings. These expressions illustrate that the prepared material is homogenous and well porous. Such porous structure in cathode material provides more channels for the passage of electrons and oxygen ions during the conduction mechanism of cell reaction [37,38]. The energy dispersive X-ray (EDX) spectrum of base sample is displayed in Figure 3. The spectrum depicts the presence of transition metal elements of zinc (Zn) and copper (Cu). No obvious peak appeared for Li in EDX spectrum due to light weight element having minor stoichiometry amount in the composition. The conventional EDX instrument cannot detect elements lighter than boron. These special detectors have a limit of detection of about 20 wt.%. Due to it spread as whitish loops over LCZ in SEM images clarify the existence of Gr in LCZ material. However, in XRD for the structural analysis and EDX for the elemental analysis could not bet detected due to its amorphous nature. For more evidence, Esmaeili et al. 2020 reported that meaningful picture of element distribution on the surface verified the presence of carbon, oxygen and iron elements throughout the surface of GO-PEG- Fe₃O₄. The reported EDX spectra further showed the corresponding peaks of carbon (C) oxygen (O) and iron (Fe) in the final conjugate [39].

The FTIR spectrum of the prepared composite material is shown in Figure 4. The information regarding the prepared materials structural and phase transformation phenomenon was studied by FTIR spectroscopy in wavelength of 4000–500 cm⁻¹. The IR spectrum positioned the peaks at 665, 862,998, 1420 and 3570 cm⁻¹. The sharp peak observed at 665 and 862 cm⁻¹ is ascribed to the Zn–O stretching bonds and C–O vibrations, respectively [40]. The band peak around 998 cm⁻¹ attributes OH bending and C-OH stretching vibrations and illustrated the existence of hydroxyl groups. This information confirmed the presence of Copper and Zinc nanophase in the prepared samples [41]. A large absorption peak at around 1420 cm⁻¹, can be attributed to the CO₂ presence which may be due to adsorption of air on the material surface. No obvious peaks were detected from 1500–3500 cm⁻¹ that indicates the complete removal of organic species and carbonates of metal precursor and conversion to oxides at high sintering temperature. The peak shown at around 3570 cm⁻¹ may be attributed to Cu-O-Zn bonding [42]. In addition, no sharp O-H band was obtained in 3000–3600 cm⁻¹ which indicates that water molecules have been eliminated in the prepared material. Conductivities measurements have been carried out with DC four probe method in air atmosphere in temperature range 300–600 °C. The corresponding results for the prepared samples are displayed in Figure 5. It can be noticed that electrical conductivities increased gradually with increase in temperature that depicts the nonmetallic behavior of material [43]. The maximum conductivity of LCZ-pure was measured to be 2.8 S/cm at 600 °C. The 1 wt.% and 1.5 wt.% graphene was incorporated in the LCZ material, and the measured results of the conductivities indicated a significant increase in conductivity values was observed from 2.8 to 3.11 and 3.5 S/cm for 1% and 1.5% incorporation of graphene in LCZ-pure, respectively. Additionally, the conductivity of material remained stable in temperature range 450–600 °C. This enhanced conductivity is attributed to development of the whitish loops over the LCZ-pure phase due to the incorporation of graphene contents. As the higher conductivity has been received in LCZ-graphene material instead of LCZ-pure material. It can be argued that this enhancement in conductivity is due to graphene incorporation. Additionally, the whitish loops in SEM images differentiate the pure and graphene incorporated material. This enhanced conductivity is attributed to development of the whitish loops over the LCZ-pure phase due to the incorporation of graphene contents. This is an evidence of graphene contents in the material. Because the obtained SEM images of LCZ-pure and LCZ-graphene incorporated indicate that the small whiteness loops and only be observed in LCZ-graphene incorporated images and not in LCZ-pure image. Therefore, it is clear that these whitish loops only represent the graphene incorporation not any other composite. Memon et al. reported that graphene may improve the electronic conductivities by loading a sufficient amount of metals or composites exhibiting higher values of electrical conductance, e.g., by employing CNT and graphene electrical conductance can be remarkably enhanced. In this regard, we have tried to incorporate graphene to improve the electrical conductivities of the material at low temperature. Further, the enhancement in electrical conductivity was observed from 10.6 S/cm to 78.7 S/cm incorporating graphene in La_0.6Sr_0.4Al_0.2Cu_0.8O₃–graphene [44].

The overlapping thus provided more pathways for the diffusion of gas and electrons movements through the material, and resulted in improving the conductivity. The maximum calculated values of conductivities and crystalline size are tabulated in

Table 1. Crystallite size, conductivity, and area-specific resistance (ASR) of prepared cathode samples.

Sample Name	Material Composition	Average Crystal Size (nm)	Conductivity (Scm−1) at 600 °C	Area-specific Resistance (Ωcm2) at 600 °C
LCZ-Pure	Li0.10Cu0.20Zn0.70-Pure	110	2.8	0.071
1 wt.% Gr-LCZ	Li0.10Cu0.20Zn0.70—1 wt%. Graphene	97	3.11	0.064
1.5 wt.% Gr-LCZ	Li0.10Cu0.20Zn0.70—1.5 wt.%. Graphene	82	3.5	0.057

. The electrode martial exhibiting electronic conductivity greater than 1 S/cm is considered suitable for good performance of fuel cell [45]. This suggested the promising conductive behavior of graphene for electrode composite for LT-SOFC.

The area-specific resistance (ASR) of the prepared temperature dependent (300–600 °C) cathodes is illustrated in Figure 6. The minimum ASR values are determined to be 0.071, 0.064 and 0.057 Ωcm² at 600 °C for LCZ, LCZ-1 wt.%Gr, and LCZ-1.5 wt.%Gr, respectively. A relatively low ASR value (0.057 Ωcm²) is seen for LCZ-1.5% Gr sample. This minimum value of ASR suggests it to be a good cathode. The data of DC conductivity was used to plot Arrhenius curves between ln(σT) and 1/T. A linear relation was found by linear fit data as shown in Figure 7. The values of activation energies were calculated from the formula given below:

$$\sigma =\frac{A}{T}exp\left(\frac{-E_a}{KT}\right)$$

(1)

where σ and E_a represent electrical conductivity and activation energy, while T, A, and K are the temperature, exponential factor, and Boltzmann’s constant, respectively.

The activation energies were calculated as 0.159, 0.148 and 0.139 eV. Smaller value of activation energy was found for 1.5 wt.% Gr-LCZ sample. This smaller value of activation energy demonstrated the quick chemical reaction start up for fuel cell catalytic activity [46].

The fuel cell performance was carried out using conventional LNCZ as anode with the prepared cathode supported by NSDC dense electrolyte. The SDC, GDC, or even CDC are widely used as electrolytes for intermediate-temperature SOFC and perform good results. The carbonated coated sodium-like carbonated samarium doped ceria (NSDC) further enhance the ionic conduction at low temperature because the optical carbonated contents construct larger interface area to facilitate the proton migration [47]. Zhang 2018 also reported the powder X-ray diffraction patterns of all NSDC nanocomposites, the pattern of the controlled sample, SDC, is also included. Na₂CO₃ crystallites are less developed than the fluorite ceria phase, almost display as the amorphous phase, since no diffraction peaks could be indexed to crystallized Na₂CO₃ [48]. A performance test was executed with hydrogen as fuel and air as oxidant at 580 °C. The following chemical reaction occurred at the electrodes.

Oxidation at anode side:

$$H_2\to 2H^{++2e^{-}}$$

(2)

Reduction at cathode side:

$$\frac{1}{2}{O}_2+2e^{-\to O^{2-}}$$

(3)

Net chemical reactions:

$$H_2+\frac{1}{2}{O}_2\to 2H^{++O^{2-}}$$

(4)

$$2H^{++O^{2-\to H_{2}O}}$$

(5)

The open circuit voltage (OCV) values obtained from LCZ, 1 wt.% Gr-LCZ, and 1.5 wt.% Gr are 0.9, 0.9, and 0.95 V, respectively. The literature indicates that the theoretical value is 0.98 V at 600 °C, in this way, the obtained values are very close to reported theoretical value [49,50]. The maximum for each sample were found to be 225, 237 and 267 mW/cm² corresponding to obtained OCV values. Few residual pores possible to be generated due to gas cross over in electrolyte result in low value of OCV. The low OCV is obtained due to not proper dispersive of hydrogen at anode side. It also degraded due to electronic leakage. The I–V and I–P curves drawn from the collected data are shown in Figure 8. It was observed that the sample containing 1.5wt.% graphene contents yielded 18.9% enhanced power density as compared to pure LCZ cathode. The linear ohmic polarization occurred in the process reveals good electrocatalytic activity of gas reaction at cathode. The electrochemical activity took place at the triple phase boundary (TPB). The less electrochemical active area at TPB even at high temperatures may affect the migration of oxygen gases and electronic conduction. Introduction of nanoparticles catalyst increased the triple phase boundary length leading to enhance electrochemical active area at low temperature. This resulted in sufficient absorption of O^2- ions and migration of electrons, which improves fuel cell performance [3]. The data of maximum value of OCV, current density, and power density determined for the developed fuel cells are listed in

Table 2. Fuel cell performance observations at 600 °C.

Sample Name	Material Composition	Maximum OCV(V)	Current Density (mAcm−2)	Peak Power Density (mWcm−2)	References
BCZYO	BaCe0.6Zr0.2Y0.2O3−δ	0.83	1505	3.86	42
LSCFN	La0.4Sr0.6Co0.2Fe0.7Nb0.1O3−δ	1.1	900	124	43
LCZ-Pure	Li0.10Cu0.20Zn0.70-Pure	0.9	750	225	Our Work
LCZ-1%Gr	Li0.10Cu0.20Zn0.70-1 wt.% Graphene	0.9	770	237.5
LCZ-1.5% Gr	Li0.10Cu0.20Zn0.70-1.5 wt.% Graphene	0.95	785	267.5

. The obtained power density is better than the already reported ZnO, Li₂O composite which yielded maximum power density 3.86 mWcm⁻² at low temperatures and is comparable to the previous SOFC [51]. Further, the current results are competitive and better as compared to those reported by Xu et al. [52] for LSCFN electrode with YSZ electrolyte, recording power density 124 mW/cm² at 850 °C. This indicates the synergistic effects of graphene particles for the electrochemical application for low-temperature SOFC [53] and can readily be applied to other electrode materials as well.

4. Conclusions

Graphene incorporated LCZ-oxide has been successfully prepared by a facile solid-state method as a cathode material for LT-SOFC. It is remarkable to mention that 1.5 wt.% Gr-LCZ exhibited an excellent electrical conductivity (3.5 S/cm at the rate of 450 °C). The enhanced fuel cell performance at low temperatures can be attributed to the synergistic effects contributed by the improved conductivity and low area-specific resistance that provide an efficient path for charge transport. The cell based on the same functional composition (1.5 wt.% Gr-LCZ) yielded a very good power density (267 mW/cm² at 580° C). The obtained results provide valuable insights into the fabrication of fuel cell device based on graphene incorporated composite cathode materials for highly efficient low temperature SOFC applications.