|Ahmad Fuzamy Mohd Abdul Fatah||School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal Penang, Malaysia|
|Ahmad Azmin Mohamad||School of Materials & Mineral Resources, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal Penang, Malaysia|
|Muhammed Ali S.A.||Fuel Cell Institute, Universiti Kebangsaan Malaysia, UKM, 43600 Bangi, Selangor, Malaysia|
|Andanastuti Muchtar||Fuel Cell Institute, Universiti Kebangsaan Malaysia, UKM, 43600 Bangi, Selangor, Malaysia|
|Nor Anisa Arifin||Fuel Cell Institute, Universiti Kebangsaan Malaysia, UKM, 43600 Bangi, Selangor, Malaysia|
|Wan Nor Anasuhah Wan Yusoff||Fuel Cell Institute, Universiti Kebangsaan Malaysia, UKM, 43600 Bangi, Selangor, Malaysia|
|Noorashrina A. Hamid||School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal Penang, Malaysia|
This study investigates the impact of zinc oxide on
the physical characteristics and electrochemical behavior of the LaSrCoFe
(LSCF) cathode. Electrochemical impedance spectra in conjunction with the bode
phase were used to optimize the amount of zinc oxide addition in the LSCF
cathode. Brunauer-Emmett-Teller (BET) and thermal analysis were utilized to
substantiate the electrochemical discovery that the LSCF: ZnO ratio yields
rational oxygen reduction reaction and stoichiometric outcomes. Initial
characterization, comprising of phase and bonding analyses, indicated that
LSCF-ZnO was successfully synthesized at 800 ºC using an improved
modified sol-gel technique. The addition of 5% zinc oxide to LSCF results in
the lowest overall area-specific resistance (ASR) rating. The Bode phase
implies that the addition of 5% zinc oxide to LSCF reduces the low-frequency
impedance by 64.28%, indicating that the cathode experienced a greater oxygen
reduction reaction. After the addition of 5% zinc oxide, a single LSCF-ZnO cell
may function at temperatures as low as 650 °C, and the LSCF cathode power
density is increased by 25.35%. The surface morphology of the LSCF-ZnO cathode
reveals an overall particle size of less than 100 nm, and mapping analysis
reveals a homogeneous distribution of ZnO over the cathode layer. Consequently,
LSCF-ZnO demonstrated outstanding chemical compatibility between LSCF and ZnO,
bonding characteristics, and electrochemical performance with the capacity to
function at an intermediate temperature (600 °C – 800 °C).
Active surface area; Intermediate temperature solid oxide fuel cell; LSCF-Zinc oxide; Oxygen reduction reaction
Fuel is a substance that is utilized to produce energy. Since the beginning of the 20th century, mankind has relied on fuel for their daily needs. Due to their high energy output and convenience, non-renewable energy sources have gained tremendous attention as it’s in line with the world agenda of reducing carbon footprint by 2030. People are growing increasingly interested in renewable energy as they become more aware of the depletion of fossil resources and recent environmental disputes. Intermediate temperature solid oxide fuel cell (IT-SOFCs) is an efficient, ultra-clean green technology device capable of converting chemical energy into electrical energy by adapting the oxygen reduction reaction (ORR) technique at intermediate temperature (600 °C – 800 °C). IT-SOFCs are also capable of producing a stable reaction in term of long-term stability, low carbon emission, and fuel flexibility (Abd-Aziz et al., 2020).
In IT-SOFCs, the cathode electrode plays an important role in oxygen reduction reaction particularly. Chemical compatibility among cathode compositions is the most important property of MIEC-type materials used in IT-SOFCs (Wu and Ghoniem, 2019). Nowadays, lanthanum, strontium, cobalt, and ferrite (LSCF) have been used in IT-SOFCs (Tahir et al., 2022). The oxygen ions mainly transport ions in the lattice oxide at the interface of the cathode and electrolyte (Jiang, 2019). Infiltrations of novel transition metals can create oxygen vacancies, namely ZnO. In the ABOx perovskite, the bulk oxygen vacancies formation is reported to be facilitated with a small amount of metal content (Guo et al., 2015).
It has been observed that zinc oxide has good stability at
intermediate temperatures operation, can act as a sintering aid, and has a
potential catalytic activity that increases the oxygen reduction reaction rate
and active surface area at the cathode (Rafique et al., 2019; Abbas et
al., 2019; Adiwibowo et al., 2018). Although the addition of zinc oxide causes extrinsic defects or
impurities, the disadvantage has been mitigated by establishing a good ratio of
zinc oxide in the composition (Omari, Omari, and Barkat, 2018). It has also been found
that zinc oxide is chemically compatible with ceria-based electrolytes, and it could have influenced the ORR process at intermediate temperatures (Xu et al., 2020). In addition, the inclusion of zinc oxide minimizes the likelihood of
the cathode interacting with carbon dioxide during the oxygen reduction
reaction (Rafique et al., 2019).
In addition, zinc oxide is typically promising in terms of performance and stability in SOFCs (Kusdianto et al., 2019; Hajmohammadi et al., 2017). It has been reported that the addition of zinc improves the triple boundary phase on the cathode side, allowing it to operate within the working range of IT-SOFCs (Abbas et al., 2019). Solid oxide fuel cells (SOFCs) favor a big triple boundary phase and high ORR process, especially on the cathode side, because high ionic conduction in electrolytes and electrodes is directly proportional to promising power output (Xu et al., 2022). It has been observed that the use of zinc oxide as a cathode produces a promising power density of approximately 500 mW-1 cm-2 at an operating temperature of 550 °C, which falls within the range of intermediate temperatures (Shah et al., 2019). Moreover, zinc oxide also reduced polarisation losses by enhancing the ORR rates on the cathode (Hussain et al., 2019). However, there is no analysis yet regarding the addition of ZnO toward LSCF as well as the electrochemical evaluation of LSCF-ZnO composite cathode. Therefore, this study is focusing on fabricating the LSCF-ZnO cathode via modified sol-gel route, whereas the EIS analysis was conducted with an intermediate temperature range (600 °C – 800 °C).
All chemicals used were purchased from Sigma Aldrich, Malaysia, and applied without additional purification. The precursor of LSCF was formed via the sol-gel method under controlled conditions. The nitrate metal salt consisting of LSCF (ratio 6:4:2:8) was mixed in a 50 mL beaker, and excess ethylene glycol was added to dissolve the metal salt. The chelating agent was mixed into the solution with the following ratio: metal salt: citric acid: EDTA (1:1:2). The initial pH solution was measured at the beginning while the metal salt solution was stirred continuously at 100 rpm. After dissolving the metal salt and chelating agent in ethylene glycol, the ammonium solution was added using a titration technique until the pH of the solution reached approximately 7. The mixture was heated for 8 hours at 100 °C to promote gel formation. The gel was calcined at 800°C for 6 hours. The resultant fine powder LSCF and zinc oxide were mixed with a ratio of 1:1 in excess acetone and sintered at 800°C for compatibility characterization. The same batch of LSCF containing 3wt.%, 5wt.%, and 7wt.% of zinc oxide was prepared to measure the specific surface area.
The fine GDC powder was produced via a conventional sol-gel process and sieved for powder uniformity. The GDC pellet was produced using the pellet-pressing method with a 25 mm diameter stainless-steel mold at 300 MPa, followed by sintering at 1400 °C for 6 hours. The cathode ink was designed with a combination of LSCF-ZnO and vehicle ink at a ratio of 1:1.5. The vehicle ink was formed with a mixture of ethyl cellulose and terpineol at a ratio of 5% ethyl cellulose to 95% terpineol. Cathode ink was applied on both sides of the electrolyte surface (1 cm diameter) using screen printing and sintered at 1100°C for 2 hours for symmetric cells. The electrochemical performance was assessed via an electrolyte-supported single cell with a NiO-GDC anode. A NiO-GDC (40:60 ratio) slurry was applied on one side of the GDC electrolyte surface, approximately 1 cm in diameter, using screen printing and sintered at 1300°C for 2 hours. The cathode on a single cell was fabricated in the same manner as the symmetric cell.
2.1. Characterization Procedure
Bonding analysis of
precursor LSCF till LSCF-ZnO powder was zanalyzed via FTIR Spectrometer
(Shimadzu model IR Prestige-21) from 400 cm-1 to 1700 cm-1.
The phase composition of LSCF, LSCF-ZnO, and ZnO was characterized using X-ray
diffraction (XRD) AXS Bruker GmbH from 10o
to 90o with Cu radiation. The specific surface area of the
LSCF-ZnO was evaluated using Brunauer–Emmett–Teller (BET) with an average
amount of 0.5 grams per sample. The Scanning Electron Microscopy (SEM) analysis was
conducted toward LSCF-ZnO symmetrical cells using Regulus 8220. The
acceleration voltage was 5kV using backscattering electron and scattering
electron (BSE+SE), and the magnification used was 30k. The particle size was
measured using the software available in Regulus 8220 for 70 readings.
The thermal decomposition behavior analysis of the LSCF-ZnO was conducted using a TGA zanalyzer model Pelkin Elmer STA 600, from 30oC to 800oC with a heating rate of 5oC/min in air and air flowrate of 50 cm3/min. For calcined composited cathode powder, the value of oxygen stoichiometric was calculated via equation 1 (Samreen et al., 2020).
Electrochemical Impedance Spectra (EIS) analysis was used together with the
furnace system (PGSTAT302N, Metrohm Autolab). The electrode surface was coated with silver paste, and silver wires
were attached to the electrodes for measuring the cathode resistance. The
symmetric cell was evaluated using Digi-Sense digital thermocouple meter
(Eutech Instruments) with type-K thermocouple. The temperature for the EIS
varies from 600°C to
800°C, with signal amplitude of 10 mv and
frequency range from 0.1 Hz to 1 MHz. NOVA-software (Version 1.10) was utilized to zanalyze the response data to
the equivalent circuit, and each output data were plotted using Origin
software. The analysis was conducted in the open air for symmetric cells. The
single cell was evaluated using a fuel mixture of 10 % hydrogen and 90 %
nitrogen at the anode side, with a flow rate of 25 mL/min. The cathode side was
supplied with air at the same flow rate. The impedance spectra for single cells
were recorded with 10 mV AC signal perturbation under open-circuit conditions.
The stabilizing time was 1 hour. Area-specific resistance (ASR) for the
symmetric cell was calculated via equation 2, where the Rp refers to
polarization resistance, and S refers to the working area of the cathode
et al., 2014).
3.1. LSCF-ZnO Powder characterization
Figure 1 (a) shows the XRD
diagram for LSCF, ZnO, and LSCF-ZnO composite fabricated at 800°C. In the LSCF-ZnO composite powder, each peak was distinguished as LSCF
and zinc oxide. The LSCF-ZnO powder consists of a mixture of two crystalline
phases, namely LSCF (JCPDS: 48-0124) and ZnO (JCPDS: 79-2205), generating
perovskite and hexagonal structure, respectively. The cell parameters of LSCF
are a =b = 0.549 nm, c = 13.375 nm, and zinc oxide cell parameters are a = b =
0.325 nm, c = 0.521 nm. No tertiary phase was detected, indicating
good compatibility between LSCF and zinc oxide, as well as high purity.
Figure 1 (b) explains the carbonate bond existence from LSCF calcined at 600°C to LSCF-ZnO calcined at 800 °C to show the reduction of carbonate bond existence in A-site. The vibration mode of carbonate ion is described as the band between 600 and 1100 cm-1, whereas the band between 1400 and 1700 cm-1 resembles the symmetrical and asymmetrical stretching modes of carbonate bond, which in this case is attributed to the formation of SrCo3. No increase in intensity was detected for the A-site after the addition of zinc oxide, indicating that 800 °C is a suitable calcination temperature for LSCF-ZnO processing. This conclusion is supported by the absence of an additional carbonate peak following the addition of ZnO to LSCF. The small intensity, which is categorized as carbonate trace intensity, also showed that the sample had sufficient calcination temperature, which contributed to a high-purity sample where the carbon was almost removed from the sample. It is important to keep the carbonate level low because carbon can build up during ion exchange and block the pores. This lowers the electrochemical performance because the porous structure could provide a fast path for ions to move. Thus, calcination temperatures of 600 °C – 800 °C were chosen based on the analysis of XRD and FTIR that implies the existence of carbonate bond as well as high purity of LSCF-ZnO.
Figure 1 XRD pattern for the powder of
LSCF, ZnO and LSCF-ZnO (a) FTIR spectra of LSCF powder calcined at 600 °C, 700 °C and 800 °C and LSCF with 5 wt.% of ZnO (b)
SEM micrographs of LSCF and LSCF with
the addition of 5 wt% ZnO electrode surface are shown in Figure 2 (a and b). As depicted in the images, it was agreed that LSCF and LSCF-ZnO sintered
at 1100 °C produced spheres that were well connected to each
other and subsequently produced a porous network. It is also
depicted similarly to other articles, stating a honeycomb-like structure, which
is essential to the ORR process. The concept behind metal oxide addition is for the metal oxide to reduce
the agglomeration effect, as seen in Fig. 3b. These findings were also
published in several articles on the addition of metal oxide to the cathode (dos Santos-Gomez et al., 2018; Nadeem et
al., 2018; Gao et al., 2017). As the O2- ions flow between the LSCF and
ZnO without passing through an air gap, the metal oxide will cover the gap and promote
the ORR process. It was discovered that the particle size
distribution of LSCF-ZnO is much more uniform, ranging from 70 nm to 100 nm,
compared to LSCF, which has a particle size range of 85 nm to 105 nm. This can
be explained as zinc oxide ionic radii being much smaller than LSCF ionic
radii, which results in the overall particle size being reduced.
Figure 2 SEM images for the surface microstructure of (a) surface of bare LSCF cathode (b) surface of LSCF with 5 wt% of ZnO cathode
The TGA experiment was carried out using airflow to see how well they could fill/empty the oxygen content, as shown in Figure 3. The investigation was conducted at a range of 30 to 800 °C to imitate the oxygen filling/emptying capabilities when the device was used at an intermediate temperature range. The weight loss occurs between 30 and 100 °C, which is explained by the evacuation of absorbed water from the surrounding environment. The second section was seen for the entire sample after 200 °C, when there is a little weight rise, implying that Co3+/Fe3+ is thermally oxidized to Co4+/Fe4+. This effect has also been described as the Co3+/Fe3+ ions are generally difficult to convert to air, resulting in a minor weight gain and mass change. Table 1 summarizes the overall results of LSCF and LSCF-ZnO. The oxygen nonstoichiometric value of LSCF-ZnO was found to be greater than LSCF, implying that more oxygen reduction process occurs on the cathode.
Table 1 Oxygen stoichiometric for LSCF and LSCF-ZnO
Figure 3 Weight loss graph of pure LSCF vs LSCF with 5 wt% ZnO powder
3.2. Electrochemical properties of the symmetrical composite cathode pellets
The electrochemical response
of the LSCF-ZnO-modified electrode was measured via electrochemical impedance
spectroscopy. Figure 4 (a) revealed the
area-specific resistance (ASR) of LSCF symmetric cells with varied ZnO loading
content at 800 °C. It was clear that the addition of zinc oxide up to 7 wt% towards the
LSCF cathode was able to reduce the ASR at operating temperatures between 800 °C, indicating the oxygen reduction rate was improved by zinc oxide
particles. The lowest ASR achieved from this analysis is LSCF with 5 wt% of
zinc oxide (ASR = 0.045 cm2) compared to pure LSCF (ASR = 0.088 cm2)
while operating at 800 °C. At 800 °C, the addition of 5% zinc oxide to LSCF reduces the
ASR from 0.088 cm2 to 0.045 cm2. When the addition of zinc oxide increased to 7 wt% and 9 wt%, the ASR
showed a slight increase at 7 wt% (ASR = 0.051 cm2), while a major increase was recorrease was recorded at 9 wt% (ASR = 0.14 cm2), which is even higher than pure LSCF.
Figure 4 (b) summarizes the total ASR performance and shows that adding 5 wt% of zinc oxide resulted in the lowest overall ASR up to 600 °C operating temperature. It was found that up to 650 °C operating temperature, adding 7 wt% zinc oxide produced ASR readings that were comparable to LSCF with 5 wt% ZnO. When LSCF with 7 wt% ZnO was operated at 600 °C, there was a discernible difference in the ASR. Based on prior comparisons of ASR at 800 °C, the ASR gap value is regarded as large for the operating temperature of 600 °C (1.460 cm2 for LSCF+ 5 wt% ZnO, and 1.710 cm2 for LSCF+ 7 wt% ZnO), even though there is a slight variation between 800 °C and 650 °C.