Highly Active Interfacial Sites in SFT‐SnO2 Heterojunction Electrolyte for Enhanced Fuel Cell Performance via Engineered Energy Bands: Envisioned Theoretically and Experimentally

Extending the ionic conductivity is the pre‐requisite of electrolytes in fuel cell technology for high‐electrochemical performance. In this regard, the introduction of semiconductor‐oxide materials and the approach of heterostructure formation by modulating energy bands to enhance ionic conduction acting as an electrolyte in fuel cell‐device. Semiconductor (n‐type; SnO2) plays a key role by introducing into p‐type SrFe0.2Ti0.8O3‐δ (SFT) semiconductor perovskite materials to construct p‐n heterojunction for high ionic conductivity. Therefore, two different composites of SFT and SnO2 are constructed by gluing p‐ and n‐type SFT‐SnO2, where the optimal composition of SFT‐SnO2 (6:4) heterostructure electrolyte‐based fuel cell achieved excellent ionic conductivity 0.24 S cm−1 with power‐output of 1004 mW cm−2 and high OCV 1.12 V at a low operational temperature of 500 °C. The high power‐output and significant ionic conductivity with durable operation of 54 h are accredited to SFT‐SnO2 heterojunction formation including interfacial conduction assisted by a built‐in electric field in fuel cell device. Moreover, the fuel conversion efficiency and considerable Faradaic efficiency reveal the compatibility of SFT‐SnO2 heterostructure electrolyte and ruled‐out short‐circuiting issue. Further, the first principle calculation provides sufficient information on structure optimization and energy‐band structure modulation of SFT‐SnO2. This strategy will provide new insight into semiconductor‐based fuel cell technology to design novel electrolytes.


Introduction
[3][4][5][6][7] Specifically, SOFC is striking the fuel cell society and R&D due to the applicability of validity in combined heat and power (CHP) systems from huge stationary plants to portable device applications.In general, the construct of SOFC consists of electrodes and electrolytes, where the electrolyte is a key component of FC to allow the transport of ions. [1,8]ndeed, the high ionic conductivity will lead to high energy conversion and fuel efficiency.Yttrium-stabilized zirconia (YSZ) was typically considered a stable and popular electrolyte with ionic conductivity of 0.1 S cm À1 and effective suitable energy conversion efficiency at 800-1000 °C. [9,10]Nevertheless, high operating temperatures and the usage of expensive reagents to prepare YSZ are major drawbacks of these materials for SOFC. [11]Considering these circumstances, the fuel cell society has prompted and comes up with their efforts to reduce the operating temperature to 400-600 °C, but the hurdle of ohmic losses was substantial. [4][14] However, these techniques are generally known for their high costs, performance degradation, slow start-up and shut-down cycles, and complexity.][17][18][19] Long-term stability is a crucial factor for low-temperature-solid oxide fuel cells. [4]Moreover, the ceria-based electrolytes met the issue of reduction in H 2 environment by introducing electronic behavior, [20] which is susceptible to successfully implementing fuel cell technology at low temperatures.
[23] In this regard, promising studies were reported such as semiconductor-ionic materials based on SnO 2 -Ce 0.8- Sm 0.2 O 2Àδ (SDC) applied as an electrolyte membrane to assemble the semiconductor-ionic membrane fuel cell (SIMFC).This SIM maintains certain electronic characteristics, but the fuel cell based on the SIM electrolyte is able to produce high ionic conductivity at 550 °C by taking advantage of the Schottky junction phenomenon.An alarming concern was the ionic conductivity drastically decreased from 0.31 S cm À1 at 550 °C to 0.101 S cm À1 by lowering the operational temperature to 500 °C. [24]Moreover, the triple charge conducting semiconductor (BaCo 0.4 Fe 0.4 Zr 0.1 Y 0.1 O 3-δ ; BCFZY) was designed as an electrolyte, where the intrinsic ionic conductivity of BCFZY was modulated by constructing the p-n heterostructure with ZnO (BCFZY-ZnO).In this study, the high-power density of 643 mW cm À2 at 550 °C with suppression of electronic conduction was achieved. [25]The formation of heterojunction between p-and n-type materials, including interfacial conduction, tremendously enhance the ionic conductivity due to the built-in electric field and suppress the electronic conductivity.Moreover, various fascinating approaches of p-n heterojunction/heterostructure, Schottky junction, and energy band engineering have been reported in different semiconductor or semiconductor-ionic-based electrolytes in the fuel cell with significant ionic conductivity and sufficient electrochemical performance at low operational temperatures. [23,26]emiconductor SnO 2 is an n-type oxide with spatial characteristics of wide band gap (approx.[29] SnO 2 can match other energy-related semiconductor materials to improve efficiency via different mechanisms. [30]Recently, the interesting properties of SnO 2 have been exploited by the design of heterostructure of semiconductor SnO 2 with semiconductor LiCoO 2 as an electrolyte membrane in the fuel cell with promising electrochemical performance and ionic conductivity. [29]evertheless, the SnO 2 needs to be assembled with the counter component to maintain match-able optical and electrochemical properties.Thus, there is a need to fulfill the demand for high electrochemical performance at low operating temperatures and insight analysis of the electrochemical mechanism.On the other hand, a p-type SFT semiconductor has been utilized in fuel cell devices with dual characteristics as electrolyte and electrode with considerable performance. [31,32]T has been exploited as an electrolyte by forming heterostructures with semiconductor ZnO and ionic conductor SDC (Ce 0.75 Sm 0.25 O 2Àδ ) to improve the fuel cell performance from 650 to 920 mW cm À2 at 520 and 550 °C, respectively. [33,34]These studies showed some advancement but they lacked appropriate reasoning and discussion of the obtained results.To this day, there is no report on the heterostructure of SFT-SnO 2 electrolyte, which presents a detailed analysis of interfacial contribution in ionic conduction and their physical illustration in fuel cell operation.
Considering all the above discussion and complications, we got inspired to design the heterostructure composites of p-type SFT with ntype SnO 2 .We utilized it as an electrolyte sandwiched between NCAL (Ni 0.8 Co 0.15 Al 0.05 LiO 2-δ ) electrodes to construct the fuel cell device.
Here, the SFT-SnO 2 heterostructure is characterized by energy band engineering to understand the matching capability of the two semiconductors' applicability as electrolyte in the fuel cell device.The HR-TEM provides detailed information on interfacial conduction at the interface and elemental contribution in the electrochemical process.The stable operation of the fuel cell device under H 2 /air environment for 54 h ensured a noticeable improvement in the electrolyte part of the fuel cell device.Furthermore, the observed performance of the SFT-SnO 2 heterojunction demonstrated that SFT and SnO 2 are promising due to their unique electrolytic characteristics and can be potentially commercialized by coupling the engineering perspective.Moreover, the first principle investigation also provides an interesting study about crystal structure optimization and the interaction of particles by forming a heterostructure of SFT-SnO 2 .At last, the energy band structure and the density of states are provided to establish the electronic structure properties and calculate the energy density difference in SFT-SnO 2 heterostructure.

Results and Discussion
The XRD patterns of the SFT, SnO 2 , and SFT-SnO 2 heterostructure presented in Figure 1a, demonstrate that no impurities were observed during the synthesis of SFT, SnO 2 , and SFT-SnO 2 (compositional ratio (0.6:0.4), respectively).SFT and SnO 2 displayed pure cubic perovskite crystalline structure and tetragonal phase with sharp diffractions, respectively.Sharp diffractions of SFT correspond to the crystal planes with space group of (Pm3m) of JCPDS File No. 33-0677 [31] and diffractions of SnO 2 are ascribed to crystal planes with space group of (P4-2/ mnm), respectively. [35]Furthermore, the different ratios of SFT-SnO 2 heterostructure compositions were blended and sintered at 800 °C to investigate the chemical reactivity (presented in Figure S1, Supporting Information).No diffraction shifts were observed in SFT-SnO 2 (compositional ratio (0.6:0.4); Figure 1a and Figure S1, Supporting Information), thus non-reactivity was confirmed.As known, the dense morphology of the electrolyte and the formation of a sharp interface among two different phases of materials (that interact at the particle level) have a considerable contribution to the overall electrochemical performance in fuel cell applications.Thus, representative FE-SEM images of SFT, SnO 2 , and SFT-SnO 2 composite are presented in Figure 1b-d.Particles in SFT-SnO 2 (0.6:0.4) are homogeneously distributed with high contact between the particles (more like glued together) as compared to SFT and SnO 2 , where the individual particles of SFT and SnO 2 do not form dense surface.This connection in SFT-SnO 2 heterostructure leads to the formation of a high network that simultaneously enhances the transportation of charge carriers specifically at the interface. [22,25,36]Furthermore, EDS-supported HR-TEM Energy Environ.Mater.2024, 7, e12606 2 of 14 was conducted using high-angle annular dark-field imaging (HAADF) to analyze the elemental mapping (Sr, Fe, Ti, Sn, and O) as shown in Figure 1e.The surface analysis of the elemental distribution of constituent elements in the SFT-SnO 2 composite by EDS mapping roughly ensured that each element was available according to the compositional ratio.
As pointed out, the microstructure of SFT-SnO 2 heterostructure composite is essential for the efficient electrochemical transport of ions.Therefore, HR-TEM was employed to study the microstructure and their corresponding fast Fourier transform (FFT) patterns of SFT, SnO 2 , and SFT-SnO 2 heterostructure calculated at the (110) planes are shown in Figure 2. Particles are regularly shaped and distributed homogenously presented in Figure S2, Supporting Information.The particles of SFT and SnO 2 have separate, multi, and even fringes that appeared in the inversed FFT patterns with the calculated d-spacing values of 0.27 and 0.32 nm corresponding to (110) planes of SFT and SnO 2 , respectively. [29,32]In the case of SFT-SnO 2 heterostructure, particles interaction of two different phase materials (i.e., SFT and SnO 2 ) led to support the increment of ionic conduction and suppression of the electronic conduction, which assisted in the fuel cell performance, as shown later in the manuscript. [25]The lattice spacing in SFT-SnO 2 was calculated to be 0.26 and 0.315 nm at respective planes of (110) of SFT and SnO 2 depicted in Figure 2e, respectively.A sharp and suitable SFT/SnO 2 interface was formed between the particles as shown in Figure 2e.Interestingly, there is a formation of built-in electric field (BIEF) at the interface, which supports the fast transport of ionic conduction at the interface will be discussed later in the manuscript.Furthermore, the nanoscale distance of 0.404 and 0.311 nm were observed between particles at the atomic level of SFT and SnO 2 obtained via Gatan microscopy suite (GMS) software from the respective image, separately shown in Figure 2f,g.
In general, the advancement of fuel cell technology is linked with the enhancement of fuel cell performance especially long-term stability at low operational temperatures.Therefore, the electrochemical performance in terms of current-voltage (I-V) and current-power (I-P) densities were evaluated of the prepared individual components (i.e., SFT and SnO 2 ) employed as an electrolyte in fuel cell device.SFT and SnO 2 electrolytes-based fuel cell generated power density of 0.51, 0.46, and 0.25 W cm À2 and 0.46, 0.3, and 0.15 W cm À2 at possible lowered operational temperatures of 500-400 °C under the H 2 /air environments depicted in Figure 3a and Figure S3a, Supporting Information, respectively.Furthermore, two different compositional ratios of SFT-SnO 2 , that is, (0.6:0.4) and (0.5:0.5) were employed as electrolyte membranes to evaluate the most suitable and optimal ratio of two components.The composites of SFT-SnO 2 (0.5:0.5) and (0.6:0.4) employed as electrolytes in the fuel cell devices along with a buffer layer at anode side have demonstrated substantially higher power density of 0.89 and 1.04 W cm À2 with the OCV of 1.11 and 1.12 V at 500 °C, respectively.The produced high fuel cell performance with high OCV assures the feasibility of semiconductors' p-n heterojunction without any significant short-circuiting problem in the fuel cell device. [22,25,26]Thus, it can be clearly observed that the p-n heterojunction of p-type SFT and n-type SnO 2 has contributed significantly due to high interfacial ionic conduction, where the interface provides a fast pathway for the ions conduction.In addition, the optimal composition and the p-n junction are responsible for enhanced ions conduction and eliminate the problem of electronic conduction.More interestingly, there is a formation of BIEF at the interface of p-n heterojunction, which plays a key role in the enhancement and fast transport of ions. [29]The prepared heterostructure of SFT-SnO 2 revealed the feasibility as a competent electrolyte membrane in a fuel cell device operated at a possible lowered operational temperature of 400 °C.Moreover, the power density of the current p-n heterojunction composite has produced subcutaneously higher power output as compared to the previously reported literature. [26,36]Furthermore, the power density of the other composites of SFT-SnO 2 -based fuel cell devices with different compositional ratios (0.7:0.3) and (0.8:0.2) were demonstrated at operational temperature of 500 °C under H 2 /air environments to illustrate the comparison of these composites depicted in Figure S3b,c, Supporting Information.
Furthermore, the electrical properties in terms of charge transport resistances are investigated via electrochemical impedance spectroscopy (EIS) and are correlated with the inter-relationship between generated resistances and the electrochemical fuel cell power output.The EIS was executed of SFT and various composites of SFT-SnO 2 heterostructure in the H 2 /air environment at 500-400 °C to evaluate the performance of the individual component SFT and the composite in terms of EIS as depicted in Figure 3d-f.The raw data were simulated by ZimpSwin software and fitted via the constructed circuit LR ohm ((R gb -QPE gb ) (R e -QPE e )) to obtain the experimental data, which assisted in the analysis of the data; where L indicates the inductance, R ohm denotes the Ohmic resistance, and R gb and R e are accredited to the grain boundary and electrode resistances, respectively. [22,25,36]Moreover, QPE is the constant phase element for grain boundary and electrode manifesting the non-ideal capacitor.Furthermore, the detailed extracted data are enlisted in Tables S1-S3, Supporting Information to elaborate on each component characteristic in the designed circuit.Generally, the EIS spectra are divided into three arcs, where each arc is accredited to higher, intermediate, and lower frequency regions, but sometimes two arcs appear due to several limitations. [37]It can be seen that the simulated ohmic resistance of 0.24 Ω cm 2 for SFT is higher than that of SFT-SnO 2 (0.062 Ω cm 2 ) heterostructure of composition (0.5:0.5) and SFT-SnO 2 (0.041 Ω cm 2 ) heterostructure composite (0.6:0.4), comparatively.This difference in ohmic resistance in two compositions and their individual component SFT revealed that lowered ohmic resistance is optimal in composition (0.6:0.4) of SFT-SnO 2 heterostructure.Indeed, it offered lower ionic resistance, which supported the enhanced fuel cell power output.Moreover, the R 1 (grain boundary resistance) and R 2 (electrode resistance) of the optimal composition (0.6:0.4) of SFT-SnO 2 heterostructure (0.319 Ω cm 2 ) is lower than of SFT-SnO 2 heterostructure (0.368 Ω cm 2 ) of (0.5:0.5) composition as well as the individual component SFT (0.69 Ω cm 2 ).It provides information on the formation of higher path ways of ions conduction; moreover, the optimal composition (0.6:0.4) of SFT-SnO 2 heterostructure provides more active sites at the electrode/electrolyte interface, which supports the formation of high triple phase boundaries.As a result, it reduces the electrode resistance even at low operational temperature, [31] which is a significant parameter of a material for fuel cells.Moreover, the simulated EIS data of SFT-SnO 2 with compositional ratio (0.5:0.5 and 0.6:0.4) are provided in Figures S4-S9, Supporting Information.The provided EIS spectra with extracted experimental data of various resistances conclude that the designed optimal composition (0.6:0.4) of SFT-SnO 2 heterojunction of p-and n-type semiconductor has significantly contributed to the high ionic conduction and enhanced fuel cell power output as a result of a large number of heterointerfaces.These heterointerfaces efficiently improve the bulk and grain boundary conduction for the successful construction and illustration of semiconductor membrane fuel cell devices.
The formation of the composite consisting of a semiconductor and a different charge characteristics semiconductor is a promising strategy to obtain high ionic conductivity.The substantial increase in ionic conductivity is due to the composite effect. [12,38]In our case, 40% of SnO 2 was introduced into the p-type semiconductor SFT (60%) electrolyte membrane.In general, the SFT phase can form a continual and connected network for the ions transports.In another case, to further improve the performance, an efficient interface can be formed by introducing an appropriate amount of SnO 2 with SFT to achieve the desired ionic conductivity.Based on the above results, we speculated that the impact of improved ions conductivity must persist in the p-and n-type semiconductor-semiconductor SFT-SnO 2 .Moreover, the interface formed between SnO 2 and SFT at the particle level leads to the formation of fast ionic transport pathway.However, it acquired a result-based debate to explain the electrochemical transport of ions transport mechanism.In this regard, as reported by Lee et al. [39] high ionic conductivity is attributed to the structural variance at the interface that generated a giant disorder in the distribution of oxygen at the interface zone to gather the creation of interfacial oxygen vacancies.Moreover, the nascent phase formed between the two phases of interface suggested by Xia et al. [25] illustrated that the emergent interface assists in the depletion of oxygen vacancies and prevents dopant segregation.Further, Shah et al. reported that the coupling effect, that is, extra oxygen aggregation at the semiconductor and ionic conductor two-phase interface region and mitigates the oxygen vacancies' depletion to increase the ionic conductivity. [33]Indeed, the reports proposed the mechanisms of the origin of the ionic conductivity enhancement in the SFT-SnO 2 .Nevertheless, more comprehensive investigation is missing, which will be illustrated in future work.
To address the raised speculation and also investigate the overwhelming charge carriers in electrolyte, employing BCZY (BaCeZr x Y 1-x O 3-δ ) and SDC layers as O 2À /e À and H + /e À blocking layers onto SFT-SnO 2 to construct two different fuel cell devices (consisting of five layers) was conducted.In this manner, we constructed two different architectures of the cells including buffer layers at anode side: I) Ni-NCAL/BCZY/SFT-SnO 2 /BCZY/NCAL-Ni (abbreviated as C-I); and II) Ni-NCAL/SDC/ SFT-SnO 2 /SDC/NCAL-Ni (abbreviated as C-II).
Here, BCZY layers work as blockators of O 2À /e À conduction and allow the proton conduction, as reported previously. [40,41]SDC layers allow the transport of oxide ions [42] and sharply stop the H + /e À transport.Considering the C-I where BCZY was applied for influential protons conduction to obtain proton-based fuel cell power output in terms of I-V & I-P curves, the resulting power density of 565 mW cm À2 with OCV 1.09 V at 500 °C as shown in Figure 4a was achieved.Subsequently, the C-II maintains the oxide ions (O 2À ) conduction-based fuel cell performance, which was demonstrated by eliminating the protons conduction by SDC layers. [43]The C-II resulted in the power output of 360 mW cm À1 with the achieved OCV of 0.99 V at 500 °C after the complete activation and stable operation as presented in Figure 4b.The results of fuel cells based on SFT-SnO 2 heterostructure electrolyte possess overwhelming protonic conductivity.However, the individual component SFT and SnO 2 reported faint protons conduction, where the two phases interface in heterostructure originated with decent protonic conductivity as presented in blocking layers fuel cell, but this also depends on the upper conductivity limits of blocking layers SDC and BCZY.The sum of power density (P max ) of both C-I and C-II is roughly near to the power output (P max ) of 60SFT-40SnO 2 , where the fuel cell performance based on 60SFT-40SnO 2 electrolyte originated due to oxide ions and protonic Energy Environ.Mater.2024, 7, e12606 conduction.Noteworthy, the power output of C-I and C-II should be lower than the fuel cell based on 60SFT:40SnO 2 electrolyte due to the creation of ohmic losses by applying two additional blocking layers.Indeed, electrolytes based on multiple-layer structure are prone to improve the overall cell's performance. [44,45]Therefore, the performance of the two different cells (C-I & C-II) reached to the power output of 60SFT-40SnO 2 semiconductor heterojunction-based fuel cell.
Ionic conductivity was calculated to confirm our bold statements regarding the electrochemical performances.Ionic conductivity was calculated via I-V polarization curves considering the ohmic resistances.The I-V polarization curve method was evaluated, where the linear part of the I-V polarization curves was considered to measure the ohmic resistances and the linear part of I-V polarization curve imitates the ohmic losses (due to ohmic resistances and electrode resistances). [25,46]n general, the employed NCAL electrode possesses high electrical conductivity, [47] thus electrodes' resistance can be neglected and the ohmic resistance replicates the ionic conductivity. [46]The higher OCV, significant power density, and fuel utilization efficiency (discussed in the next section) clearly illustrate the ionic behavior of the electrolyte in fuel cell devices with minor electronic characteristics.Moreover, the ohm law in terms of R ASR = V ohm /I ohm was employed to calculate the area-specific resistance of the optimal composition of SFT-SnO 2 and other components. [48]Further, the R ASR is utilized in Equation 1 to calculate the ionic conductivity of various electrolytes employed in fuel cell devices.
The ionic conductivity of composites SFT-SnO 2 heterostructure (0.6:0.4 & 0.5:0.5)and their one component SFT are calculated and presented in Figure 4c.The optimal composition SFT-SnO 2 (0.6:0.4) heterojunction achieved the ionic conductivity of 0.24 S cm À1 , and the ionic conductivity of SFT-SnO 2 (0.5:0.5) heterojunction was 0.19 S cm À1 at 500 °C.Nevertheless, the ionic conductivity of SFT-SnO 2 (0.6:0.4) heterojunction is higher than that of SFT-SnO 2 (0.5:0.5) heterojunction and their components SFT (0.13 S cm À1 ) and SnO 2 (0.08 S cm À1 ).Such enhancement of ionic conductivity results from formed heterojunction between n-type SnO 2 and p-type SFT and the generation of BIEF at the interface, which supports the fast transport of ions conduction. [17,36]The dominant part of the ionic conductivity is protonic conduction (H + ), as already exhibited by the protonic conduction-based fuel cell performance of five layers.Generally, the produced ionic conductivity is better than the typical ionic conductors and other composite materials (GDC 0.04 S cm À1 at 700 °C, [49] ceramic YSZ 0.13 S cm À1 at 1000 °C, [50] SDC ∼ 0.05 S cm À1 at 700 °C, [51] GDC/YSZ mixture film ∼ 0.01 S cm À1 at 1000 °C [49] ) that certify the designed heterostructure has a great importance and is competent as an electrolyte in fuel cell technology.
In addition, here we considered the Hebb Vagner polarization method to calculate the electronic conductivity of SFT-SnO 2 electrolyte because the electrolytes consist of the semiconductor materials.The electronic conductivity was calculated by recording current and time under a fixed applied potential of 1 V for 30 min under various operational temperatures 400-500 °C.The electronic conductivity of 3.11e-04 S cm À1 at 500 °C was calculated, as presented in Figure S10, Supporting Information, where the calculated electronic conductivity is not dangerous for fuel cell device operation compared to ionic conductivity of 0.24 S cm À1 at 500 °C.This enhancement of high ionic conductivity revealed that the formation of heterostructure of SFT and SnO 2 triggered the ionic conductivity by the interfacial conduction with the evidence of lower ohmic and grain boundary resistances.This suggests that there is a formation of high content of oxygen vacancies at the interface of SFT-SnO 2 electrolyte, which substantially increased the ionic conduction and overcome the activation energy of 0.49 eV required for the diffusion of ions as shown in Figure 4c as calculation method are reported somewhere else. [25]We have further identified the chemically conduction of proton via isotopic effect by investigating the EIS analyses of as-prepared SFT-SnO 2 heterostructure (0.6:0.4) in H 2 O and D 2 O environments in the form of vaporized as presented in Figure 4d.It can be seen that the charge conduction change significantly by experiencing from D 2 O (D + ) to H 2 O (H + ) as a result of isotopic effect, which clearly give an evidence of the proton conduction in the prepared SFT-SnO 2 heterostructure (0.6:0.4).The hydrogen/deuterium (1H/2D) isotope effect possibly bring the substantial distinction for the H + /D + conductivity if protons are overwhelming charge carriers in the as-prepared materials.The difference in ground state energies of the oscillating states of the O-D and O-H cause the isotopic effect, certainly bring a considerable conductivity difference due to fast diffusion of protons than deuterons.The availability of H + and D + get possible in SFT-SnO 2 when the vapors of H 2 O or D 2 O dissolve from gaseous phase.Interestingly, the oxygen vacancies behaved as catalytic sites and foster the hydrogen/deuterium in H + /D + conduction in SFT-SnO 2 .To analyze more in detail about the difference in ground state energies of the oscillating state of the O-D and O-H can make a difference in the conductivity by ffiffiffi 2 p illustrated by the classical theory following Equation 2; Here, σ, D, and m are the conductivity, diffusivity, and ratio of mass of the diffusion species, for example, D m =H m ¼ 2. It gets very complex and difficult to calculate theoretically due to proton quantum tunneling effect that what should be the accurate difference, still it is considered to be higher larger. [52]However, this discussion does not change the purpose to confirm the presence of proton conduction in SFT-SnO 2 heterostructure (0.6:0.4).
The detailed study of protons and oxide ions in the respective five layers fuel cell device maintaining buffer layer by employing the cyclic voltammetry (CV) as shown in Figure 5a,b.Figure 5a shows the higher current under the protons conduction-based five layers fuel cell at the potential voltage of 0.3-0.4V at the applied various scan rates of 20 to 100 mV s À1 .Moreover, the five layers fuel cell device of H + /e À blocking layers showed lower current under the applied potential voltage of 0.3-0.4V at the applied scan rate of 20 to 100 mV s À1 depicted in Figure 5b.These results are also evident by the CV technique that there is dominant presence of protons conduction in the SFT-SnO 2 heterostructure, where the interface plays a key role in the enhanced conduction of protons.
To verify the higher ionic conductivity via an interface in the heterostructure, we performed XPS analysis to investigate the chemical states assigned with quantum numbers of the prepared materials, which provide evidence of the formation of higher oxygen vacancies, thus leading to more and fast oxide ions conduction as shown in Figure 6. Figure S11a, Supporting Information shows complete XPS spectra of SFT, SnO 2 , and SFT-SnO 2 heterostructure, which confirmed the presence of each element Sr, Fe, Ti, Sn, and O in their respective composition, respectively.The significant contribution and analysis of Fe, Ti, Sn, and O-1s are elaborated and depicted in Figure 6.Furthermore, the Sr-3d is presented in supporting data as shown in Figure S11b, Supporting Information.
Moreover, Fe-2p spectra are deconvoluted into two peaks, and there are two spin-orbits, that is, Fe-2p 3/2 and Fe-2p 1/2 maintaining two doublets satellite peaks and represented by two partially superimposed peaks.Generally, three different valence states of Fe-2p exist such as Fe 2+ , Fe 3+ , and Fe 4+ .Therefore, the three valence states are well fitted (depicted in Figure 6a).The Fe 2+ -2p 3/2 and Fe 2+ -2p 1/2 are concentrated at 710.1 and 722.7 eV, and Fe 3+ -2p 3/2 and Fe 3+ -2p 1/2 can be assigned at 711.9 and 724.1 eV, respectively. [53]While 713.3 and 725.1 eV are accredited to Fe 4+ -2p 3/2 and Fe 4+ -2p 1/2 , respectively. [54]It can be seen that the peak intensity and percentage of Fe 2+ , Fe 3+ , and Fe 4+ are different in each sample as well Fe-2p shifted slightly with 0.92 AE 12 eV.Considering the peak areas of singlet curve, the calculated average valence contribution of Fe-2p are 3.08 and 2.92 for SFT and SFT-SnO 2 , respectively. [55]The heterostructure formation lead to the change oxidation state of Fe-2p and the availability of Fe-2p in mixed oxidation states (Fe 4+ /Fe 3+ and Fe 3+ /Fe 2+ ).The change in valence state of Fe-2p provides information on the alteration of electronic structure.The reduction of Fe-2p is the electronic adjustment.Thus, the reduction of Fe-2p (Fe 4+ to Fe 3+ and Fe 3+ to Fe 2+ ) and the change of electronic structure by the construction of heterostructure supported oxygen vacancy formation.Ti-2p can be deconvoluted into four characteristic peaks, where subpeaks of Ti-2p are attributed at the 458.1 and 463.9 eV with Ti +4 depicted in Figure 6b.It can be seen that there is peak separation and leading to peak shift initiated to the formation of Ti 3+ as a result of SFT-SnO 2 heterostructure formation.As the Ti-2p availability in the higher content of oxidation state of Ti 3+ lead to the charge exchange, and this exchange assisted more in the charge carriers and supported the ionic conduction.Furthermore, the changes in the Sn-3d related states in SnO 2 as well as by the formation of SFT-SnO 2 heterostructure (shown in Figure 6c).There are two spin-orbits of doublet peaks of Sn-3d spectrum at ≃486.3 eV (Sn 2+ -3d 5/2 ) and ≃495.2 eV (Sn 2+ -3d 3/2 ).After the formation of heterostructure with SFT and sintered at 800 °C, the main peaks shifted slightly, which matched the binding energies of Sn 4+ (where Sn 4+ -3d 5/2 assigned at the BE of 486.1 eV and Sn 4+ -3d 3/2 attributed to 495.1 eV, respectively) well, resulted into assistance of vacancy formation during the exchange of oxidation states of Sn-3d.
Furthermore, the O-1s was evaluated for each component (i.e., SFT and SnO 2 and the SFT-SnO 2 heterostructure), and the three super-imposed peaks of O-1s spectra at the respective binding energies are presented in Figure 6d-f.Generally, the ionic conduction of the prepared materials, especially electrolytes, strongly depends on the amount of oxygen vacancies formation. [56]In this regard, Barr et al. [57] reported that the lattice oxygen (L o ) and highly oxidant peaks are generally ascribed at the binding energy range of 528-530 eV.Therefore, L o is represented by O β accredited at the binding energy of 528.6 eV, where the oxide defects peak at the surface or oxygen species adsorbed on the oxygen vacancies and the hydroxyl functional group (-OH) are presented by O α at the binding energy of 529-530.5 eV, respectively. [58]The formation of SFT-SnO 2 heterostructure materials (0.6:0.4), where the respective peaks are broadened toward higher binding energy was further investigated.Obtained results indicate that the available surface-active oxygen is significantly improved, resulting in the production of oxygen vacancies. [59]The O α and O β were calculated for SFT, SnO 2 , and optimal composition SFT-SnO 2 (0.6:0.4), where it can be seen that the relative ratio of O α and O β for SFT-SnO 2 (0.6:0.4) is 1.32 higher than that of 1.24, respectively.Indeed, improved chemisorbed oxygen vacancies in SFT-SnO 2 (0.6:0.4) heterostructure was achieved, as the chemisorbed oxygen vacancies that are prone to liberate at the operational temperature; therefore, the increment of chemisorbed oxygen vacancies certified the enhancement of ionic conduction in SFT-SnO 2 .Moreover, there are various factors in the enhancement of ionic conduction in SFT-SnO 2 heterostructure, that is, coupling effect, extra oxygen aggregation at the SFT-SnO 2 interface region, and mitigating the depletion of oxygen vacancies in SFT-SnO 2 . [60]As a result, enhancement in the ionic conductivity in SFT-SnO 2 composites was achieved.Furthermore, Etsell et al. reported that oxide ions conduction in solid oxide materials generally takes place in the form of vacancies, where the oxygen defects materials are responsible for the diffusion pathways for high oxygen ions conduction.Moreover, the O-1s spectra of the SFT-SnO 2 heterostructure materials before and after testing in H 2 /air environment to illustrate the formation of oxygen vacancies after treatment under fuel cell conditions are presented in supporting information (Figure S11c,d, Supporting Information).Hence, the fast transport of ions in the prepared heterostructure, especially in the electrolyte, is supported by the formation of surface oxygen defects and the chemisorbed oxygen species.

Theoretical Investigation of Heterostructure
Vienna ab-initio simulation package (VASP) with the projector augmented wave (PAW) method was conducted to investigate the crystal structure along with the energy density difference via VASPkit interface (Figure 7) and to calculate the total density of states (TDOS) and energy band structure (Figure S12 were designed and then employed as layers of SFT and SnO 2 that construct the SFT-SnO 2 heterostructure via the generalized lattice match process. [61,62]Here, the strain at the interface is minimized considering the VASPkit interface.In general, the mean value of absolute strain should range from 1% to 4% (no more than 5%) in order to obtain an Energy Environ.Mater.2024, 7, e12606 optimized heterostructure. [63,64]Here, the mean absolute strain of 2.8% was used to construct the optimized heterostructure at the (110) plane (Figure 6c,d).The total and partial density of states (TDOS & PDOS), and energy band structure for respective SFT, SnO 2 , and SFT-SnO 2 heterostructure at the plane of (110) are presented in Figure S12a-f, Supporting Information.Each PDOS of SFT, SnO 2 , and SFT-SnO 2 predicts the spin-up and spin-down orbitals with sharp peaks of each element (i.e., Sr-4p, Ti-3d, Fe-3d, Sn-5p, and O-2p) where the bands are near to Fermi-level (Figure S12a,b, Supporting Information).The valence band of SFT is mainly built due to the Sr-d atomic orbital, while the conduction band is built from 3d atomic orbitals of Fe and Ti, respectively; however, Ti affects the overall electronic properties of the entire SFT sublattice and defects are produced due to its presence. [65]On the other hand, Sn-3d is solely responsible for the formation of the constructed band.The bands of SFT and SnO 2 are near to Fermi level, however, comparing the DOS of heterostructure SFT-SnO 2 with individual SFT and SnO 2 , the DOS of SFT-SnO 2 substantially reaches the Fermi-level.This indeed predicts (and confirms our experimental data) a significant support of charge transport leading to the enhancement of ionic conduction (Figure S12c, Supporting Information). [36]Furthermore, the charge density difference was evaluated by the isosurface level of charge density difference.The isosurface level was set to 0.01 e ÅÀ3 in the SFT-SnO 2 heterostructure as shown in Figure 7e.The electronegativity of SFT-SnO 2 follows the order O > Sn > Fe 3+ > Fe 2+ > Sr (based on the Pauling exclusion principle).Bader charge analysis showed a charge transformation of 3.49 |e| (from SFT to SnO 2 ).

Modulating the Energy Band Structure by Heterostructure Formation
In general, semiconductor materials possess inherent characteristics to tune their optical and electrical properties.Therefore, considering the intrinsic characteristics of semiconductor oxide materials, this study provides a detailed investigation of energy band alignment to tune their energy band structure according to desired applications.Herein, we designed a SFT-SnO 2 heterostructure by manipulating the energy bands to understand and demonstrate the mechanism of energy band engineering.The sole purpose of energy band alignment is the formation of oxygen vacancies at the interface by the interaction of particles belonging to the two different structured semiconductor oxide materials with different conducting characteristics.In this context, UV-Vis spectroscopy, XPS, and UPS were employed to demonstrate the energy band alignment by determining the energy band gaps and valence band maxima (values presented in Figure 8 and Figure S13a,b, Supporting Information).Initially, the energy bandgaps were calculated for SFT and SnO 2 from the optical absorption spectra using Tauc plots following the below Equation 3; Where the absorption coefficient is presented by α, photons energy is shown by hν, and energy independent constant is depicted by β o , respectively. [66]The absorption spectra of SFT and SnO 2 display a negligible lower-energy absorption tail, illustrating the indirect band gap characteristics of prepared materials. [67]As observed, the absorption edges of SFT and SnO 2 are at approx.450 and 370 nm, respectively (shown in Figure 8a,b), where the absorption edge changes to 410 nm by the formation of a heterostructure of SFT-SnO 2 (0.6:0.4) composition (presented in Figure S14a, Supporting Information).Furthermore, the calculated energy band gaps of SFT and SnO 2 were 2.63 and 3.7 eV, [68] while the energy band gap of SFT-SnO 2 heterostructure mediated in the appropriate value of approx.2.9 eV.It is evident that the energy band gap of the heterostructure is appropriately modulated and resulted in the creation of intermediate energy levels between conduction and valence bands for ease and fast transport of ions due to the high vacancy formed at the interface.Furthermore, the valence band maxima were determined via XPS and validated by the UPS technique.Valence band (V b ) is calculated from the lower binding energy spectra of XPS spectra, [69] where the valence band of SFT and SnO 2 from the respective XPS spectra were 4.1 and 7.1 eV, respectively, shown in Figure S13a,b, Supporting Information.Moreover, the UPS spectra were also utilized to determine valence band maxima following Equation 4: The content of the equation includes E cutoff and E onset which are the higher and lower binding energies of the obtained raw data from UPS characterization, respectively.The above equation provides V b values of 4.12 and 7.09 eV for SFT and SnO 2 , respectively, obtained from raw data of Figure S14b,c, Supporting Information.After considering the V b and energy band gap values, conduction band (C b ) values of 1.47 and 3.5 eV of SFT and SnO 2 were calculated, respectively.It shows that each semiconductor component of heterostructure has a different energy band gap, C b and V b values, and Fermi-levels.Moreover, the energy bandgaps, C b and V b are employed to construct the energy band structure, which clearly illustrates the impact that heterostructure has formed.This leads to modulate the energy bandgap while narrowing the conduction band and changing the Fermi-level position in the heterostructure as shown in Figure S14a,d, Supporting Information, thus generating a large number of oxygen vacancies.The change of the Fermilevel resulted in a valence band shift.The evolution of Fermi-level can be estimated from the valence band maximum (E f ).The shift in the energy valence band maximum (EV bm ) can enhance the overall density of state (DOS) near the Fermi-level.According to the theoretical discussion and previous literature, once p-and n-type semiconductor are in contact with particle and form a heterostructure junction with different Fermi levels, electrons diffuse from the n-type to the p-type conduction band and holes from the p-type to the n-type valence band due to the concentration difference. [25,70,71]These processes continue until Fermi levels of the n-and p-type semiconductors are aligned, leading to the energy band bending between p-type and n-type, as shown in Figure 8c.In other words, the space-charge region can be established at the interface/grain boundary between SFT and SnO 2 at the condition of thermal equilibrium.Therefore, a potential barrier height is formed to prevent further electron motion from n-type to p-type layer and hole from p to n layer until the band bending is completed and to facilitate the ionic migration through the heterointerface regions. [71]Furthermore, the electric field occurred from electron transfer and stoichiometric polarization at p-n junction. [72]owever, field effect ionic conductivity contributed to the major increment in the maximum power density.Typically, the redistribution of charges is placed at the particle levels or at the grain interface to demonstrate the successful p-n heterojunction effect, inducing gradient BIEF and energy band banding at the interface of the SFT-SnO 2 heterostructure.The induced BIEF helps to separate electrons and holes to avoid the electronic short circuit by suppressing the electronic conduction.Moreover, the ions conduction in the heterostructure is enhanced substantially, thus improving the overall fuel cell performance.
Based on our findings, the strategy of energy band alignment by utilizing p-type and n-type semiconductor materials to prepare heterostructure as the electrolyte is a progressive and novel for fuel cell device preparation.The high power output successfully confirms this strategy with high OCV, stable operation for 54 h (discussed in the next section), and stable behavior of the designed electrolyte.Moreover, the employed heterostructure displayed high compatibility to utilize a high ratio of fuel, which is further discussed in the next section.The long-term stability of fuel cell devices based on specific electrolytes can be evaluated in terms of several aspects, for example, longterm operation of the fuel cell under constant current density, and most importantly, fuel conversion efficiency of the fuel cell device.
Moreover, a fuel cell device based on SFT-SnO 2 heterostructure electrolyte with buffer layer at anode side of architecture Ni-NCAL/SFT-SnO 2 /NCAL-Ni was operated under the applied current density of 0.11 A cm À2 at the operational temperature of 500 °C in the H 2 /Air environment.The buffer layer significantly avoids the reduction of anode and electrolyte under a reducing environment.The steady-state condition of durability operation is presented in Figure 9 under the current density of 0.11 A cm À2 with a degradation rate of 0.21 mV.This durable operation gives evidence that the formation of heterojunction of SFT and SnO 2 with optimal composition regarded the strong ions conduction primarily through the exemplary constructed interface of colossal and stable operation with a working voltage of 0.84 V but degraded significantly after 54 h.However, there were fluctuations observed during the durability operation, this is due to hydrogen fuel pressure fluctuation.Still, it is strongly believed that the SFT-SnO 2 electrolyte-based fuel cell device can be broadened to commercial applications utilizing engineering support but the current obtained operation still need to be substantiously increased.In addition, it should be kept in mind that all experimental works are performed at the laboratory scale; therefore, many technical barriers exist that constrain and reduce the working voltage of the fuel cell device.Thus the following technical barriers need to be overcome and fixed for the advancement of semiconductor-based fuel cell technology such as the rusting of the inner side of the steel chamber of the sample holder resulting in increment of resistance and reduced conductivity, lack of engineering facilities which create the difference in cell geometry resulted into nonuniformity of the NCAL-Ni employed electrode preparation and the assembling of the fuel cell.Further, the critical hurdle is the thermal mismatch of the electrode and electrolyte; the less the thermal mismatch, the more will be stable and highly efficient fuel cell device and vice versa.
In addition, the exciting part of this study was performed to confirm the fuel conversion efficiency or fuel utilization efficiency of the fuel cell device based on the SFT-SnO 2 heterostructure electrolyte.For this purpose, particular individual cell of thin layers of NCAL as electrodes are painted over the SFT-SnO 2 heterostructure electrolyte to demonstrate the efficiency of fuel conversion under four different applied current densities of 0.1, 0.2, 0.3, and 0.4 A cm À2 for time intervals of 10 min for entire 30 min.In this regard, the product at the outlet in terms of water was analyzed by online GC-MS for 30 min, and each time GC-MS was repeated every 10 min for three consecutive measurements under each current density.In addition, three consecutive measurements were performed to ensure accurate results based on the water concentration at the outlet product.The average of each three measurements was considered as H 2 O concentration at respective 10 min intervals for an entire period of 30 min at each current density.The H 2 O production was less at a lower applied current density of 0.1 A cm À2 but with the increment of current density, the production of H 2 O also increased.The H 2 O output at 0.1 A cm À2 shows the Faradaic efficiency is mildly higher than 100% (this slight cross-over is probably due to the high sensitiveness of the GC-MS because the applied current density 0.1 A cm À2 is significantly lower for the accurate measurement of H 2 O outlet product).Therefore, the lower applied current density and the lesser outlet H 2 O product can cause some deviation during calibration and GC-MS measurement.Nevertheless, the increase in applied current density leads to produce a high content of H 2 O, where the Faradaic efficiency is also considerable.Reaching ≈ 94.7% at 0.3 A cm À2 at 500 °C (as depicted in Figure 10).However, the unusual activity observed, where the concentration of H 2 O content decreased at 0.4 A cm À2 and the Faradaic efficiency also decreased significantly.The possible reason can be the higher applied current density where the fuel cell device got unstable, but it still needs more investigation.However, the recent result obtained at different applied current densities provides evidence of the high fuel utilization and avoiding any chance of short-circuiting of constructed fuel cell devices.

Conclusion
The summary of this work reveals that the constructed heterojunction of p-type SFT and n-type SnO 2 demonstrated substantial results acted as electrolyte in fuel cell device with high power density and excellent ionic conductivity at low operational temperatures.Initially, the physical characterizations revealed the successful formation of optimal concentration of SFT-SnO 2 (0.6:0.4) composite.Specifically, HR-TEM and FE-SEM investigated the sharp interface formation at particles' level of two different phase materials and dense structure of the heterostructure composite materials, respectively.It has been speculated that interface engineering in the heterostructure can be phenomenal for the enhanced ionic conduction in SFT-SnO 2 heterostructure electrolyte.This speculation was confirmed by the electrochemical analyses, which sufficiently provided evidences of the designed SFT-SnO 2 heterostructure electrolyte-based fuel cell device substantially produced a power output of 1004 mW cm À2 at 500 °C with excellent ionic conductivity of 0.24 S cm À1 .This synergistic effect of higher power density and ionic conductivity is linked with the heterojunction formation, where the formation of a sharp interface significantly boosted the ionic conduction with low activation energy.Importantly, the durable operation for 54 h of fuel cell device is remarkable for the constructed SFT-SnO 2 heterostructure electrolyte.Moreover, we have amazingly illustrated and explained the high ionic conduction mechanism with theoretical and experimental evidences including DFT analysis, spectroscopic investigations, and electrochemical characterizations to understand the magical properties of p-and n-type semiconductors as electrolyte in fuel cell device.The calculated Faradaic efficiency of the cell based on SFT-SnO 2 heterostructure ruled out the possibility of short-circuiting issue of semiconductorbased electrolyte in fuel cell device.The obtained results revealed that the current approach can be employed to design new materials with attractive electrolytic functionality to have high ionic conductivity for the advancement of fuel cell technology and can also be applied in solid oxide electrolysis, solar cell, and membrane for water cleaning, etc.
Synthesis and heterostructure formation: SrFe 0.2 Ti 0.8 O 3-δ (SFT) composition material with a concentration of 0.1 M was prepared via hydrothermal technique assisted by the co-precipitation method.Initially, the weighed quantity of Sr(NO 3 ) 2 was added into 1 L de-ionized water and was homogeneously dispersed.Subsequently, the calculated quantity of Fe(NO 3 ) 3 .9H 2 O was added in the above solution and stirred rigorously to obtain a homogenous solution.Subsequently, 20% TiO 2 solution in C 2 H 5 OH was poured into the preceding solution and constantly agitated, while the pH was adjusted to 8.0 by using NH 3 ÁH 2 O.Moreover, the precipitating agent with a 1:2 ratio of metal cations and Na 2 CO 3 was prepared and then poured into the aforementioned solution to obtain the brownish precipitates solution.Afterward, the precipitates were shifted into auto-clave and treated hydrothermally for 8 h at 170 °C in a vacuum oven and then allowed to cool down.The treated precipitate was washed with de-ionized water and ethanol and rinsed and then dried at 130 °C in an air oven.Afterward, the dried materials were ground in pestle-mortal and sintered at 1000 °C for 6 h at 4 °C min À1 .Furthermore, the sintered materials are again ground to obtain homogenous powder for subsequent application.
Moreover, the SnO 2 was obtained utilizing the hydrothermal technique assisted by the co-precipitation method, where the weighed quantity of tin nitrate was added into the de-ionized water to obtain the dispersed solution of tin nitrate under continuous stirring.Meanwhile, the precipitating agent with a 1:2 ratio of metal cations and Na 2 CO 3 was prepared and then poured into the solution as mentioned earlier, to obtain the whitish precipitates solution.Afterward, the precipitates were hydrothermally treated in a vacuum oven, rinsed, and washed with de-ionized water and ethanol.Then dried the obtained materials in an air oven for 5 hr at 130 °C followed by grinding in the pestle-mortal.Later, the grind materials are sintered in a furnace and followed by ground to obtain the uniform morphology with homogenous particles distributed powder of SnO 2 .
The heterostructure of SFT and SnO 2 was constructed of two different compositional ratios (SFT-SnO 2 (0.6:0.4)) and (SFT-SnO 2 (0.5:0.5)).The compositional ratio of (SFT-SnO 2 (0.6:0.4)) was prepared by the ball milling method, the weighed quantity of SFT and SnO 2 according to the desired composition and added into the ball milling jar, including the different size silica balls in the presence of ethanol as a media.SFT and SnO 2 powder were mixed for 8 h with 35 rpm, and then well-grounded powder in ethanol was shifted into a beaker and dried in an open oven.Later, the dried materials were sintered at 800 °C for 5 h at 3 °C min À1 followed by subsequent ground in pestle mortal to obtain homogenous powder for further physical and electrochemical applications.Similarly, the SFT-SnO 2 (0.5:0.5) heterostructure composite was prepared for physical and electrochemical applications.
Construction of fuel cell device: Generally, a fuel cell device consists of three components, such as anode, cathode, and electrolyte, therefore, the fabrication of a fuel cell includes Ni 0.8 Co 0.15 Al 0.05 LiO 2 (NCAL) as electrodes and SFT-SnO 2 electrolyte membrane constructed by the dry-pressing method.NCAL has been recently demonstrated as a catalyst capable of triple charge conduction (H + / O 2À /e À ) and strong catalytic activity in the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR). [26]The NCAL electrode has been employed as electrodes in the form of NCAL-Ni [commercially obtained from Tianjin Bamo Company (TBC)], where the slurry of NCAL powder was prepared in an appropriate volume of terpinol (dissolving media) and painted on roundshaped Ni-foam followed by desiccation and drying in an open oven at 120 °C for 0.5 h to obtain the well-dried NCAL-Ni-foam electrode.
The final step was the fabrication of a fuel cell device and included the pressing of SFT-SnO 2 (0.6:0.4) heterostructure powder followed by introducing a thin buffer layer (BL) in the NCAL-Ni foam electrodes under the pressure of 250 MPa to form a single cell to distribute powder of electrolyte homogenously.The fabricated cell got the architecture of Ni-NCAL/BL/SFT-SnO 2 /NCAL-Ni with a thickness of cell 1.5 mm, an active area of 0.64 cm 2 , and a thickness of the electrolyte 520 μm, respectively.Further, the silver paste was painted over both sides of the cell for two purposes: current collector and gas sealing before being installed into the testing jig.In addition, the purpose of Ni-foam is to ensure the mechanical strength of the cell supporting the electrode during the pressing of the cell in the testing device and to support the electrode as a porous structure.
Similarly, various cells were individually assembled based on different electrolytes SFT-SnO 2 (0.5:0.5), SFT, and SnO 2 of structures Ni-NCAL/SFT-SnO 2 /NCAL-Ni, Ni-NCAL/SFT/NCAL-Ni, and Ni-NCAL/SnO 2 /NCAL-Ni, including buffer layers at anode side, respectively.In this study, it was kept in mind that each cell was electrochemically characterized under similar conditions in terms of operation and performance measurement prior to electrochemical characterization treated at 600 °C for 1 h, respectively.
Moreover, there was a need for unique cell fabrication to accurately measure water (H 2 O) outlet product during fuel cell operation under constant applied current density leading to calculating Faradaic efficiency.The electrolyte membrane SFT-SnO 2 (0.6:0.4) heterostructure was pressed, and the NCAL slurry was sprayed on both sides to form thin layers to obtain a thin cell.The silver past and then Ceramabond 552-VFG sealant (Aremco) were employed to seal the Al 2 O 3 tube to characterize the prepared cell through gas chromatography-mass spectroscopy (GC-MS), where the cell sealed with silver past and then Ceramabond 552-VFG sealant (Aremco) was heated from room temperature to 650 °C at 5 °C min À1 .Physical and electrochemical characterizations: The X-ray diffraction (XRD, Germany, Bruker Corporation) was employed to determine the crystalline structure of as-prepared SFT and SnO 2 and their heterostructure composites (SFT-SnO 2 ) with different compositional ratios (0.6:0.4 & 0.5:0.5) in range of 2θ of 20-80°at the scan rate of 0.02.The raw data of X-ray spectra were refined and analyzed through MJAD 6.5 software.Moreover, the surface morphology and particle distribution of SFT and SnO 2 and their heterostructures composite (SFT-SnO 2 ) were predicted and studied through field emission-scanning electron microscopy (FE-SEM; Japan Electronics Co., Ltd).In addition, the energydispersive X-ray spectroscopy (EDS) extended with high-resolution transmission electron microscopy (HR-TEM) was employed to visualize the rough estimation of elemental distribution according to the stoichiometric ratio of each element in the designed SFT-SnO 2 composite.Subsequently, HR-TEM instrument (JEOL JEM-2100F) with Double Cs-corrected transmission electron microscope operated under 200 kV accelerating voltage.The purpose of the involvement of HR-TEM was to study the microstructure of the as-prepared SFT and SnO 2 powder materials and their heterostructure SFT-SnO 2 powder.The Gatan digital micrograph software was applied in terms of fast Fourier transform (FFT) to analyze the lattice patterns of respective images of each as-prepared material.The surface charge transfer, quantum states, and chemical condition, and valence band maxima were investigated by X-ray photoelectron spectroscopy (XPS-Thermo Kalpha; Thermo ESCALAB 250 XI; Axis Ultra DLD Kratos AXIS SUPRA; PHI-5000vers aprobeIII) utilizing the radiation of Al Kα, while the CASA XPS software was used to analyze the raw data of XPS.In addition, UV-Vis absorption spectroscopy (MIOSTECHPTY Ltd.UV3600 spectrometer) was employed to evaluate the energy bandgaps of semiconductor candidates SFT and SnO 2 to be used to construct the energy band structure, where XPS calculated valence band maxima.The ultraviolet photoelectron spectroscopy (UPS) was also utilized under the unfiltered He-I (21.22 eV) gas discharge lamp and a total instrumental energy resolution of 100 meV to calculate the valence band maxima to confirm, which was also determined from XPS spectra.Gas chromatography-mass spectroscopy (GC-MS) was performed using (SCION GC-MS systems instrument of Bruker Corporation) in the mode of temperature conducting detector (TCD).The operating temperature was 500 °C to analyze the composition of the product gas at the outlet, where GC was operated and repeated three times for every 10 min interval.The H 2 flow rate was 20 mL min À1 and intentionally provided an open-air environment.Further, the concentration of volumetric gas water (H 2 O) was considered by taking the average of three measurements at each 10 min time interval to calculate Faradaic efficiency.Flowmeter was used to determine the accurate gas flow rate.The particular thin layer-based cell was designed for the measurement of H 2 O outlet product during electrochemical measurement, which will determine the fuel cell conversion efficiency leading to calculate the Faradaic efficiency.
The Faradaic efficiency is calculated as follows by Equation 5: where v (vol %) is the concentration of H 2 O in the exhaust gas from the electrochemical cell (GC-MS data) and V (mL min À1 ) is the gas flow rate measured using a flow meter at the exit of the electrochemical cell at room temperature and ambient pressure.
First principle investigation: In density functional theory (DFT) calculation, structural optimization was performed by Vienna ab-initio simulation package (VASP) ruled out [73] with the projector augmented wave (PAW) method. [74]The exchange-functional was treated using the Perdew-Burke-Ernzerhof (PBE) [75] functional in combination with the DFT-D3 correction, [76] to describe the weak interactions between atoms.The cut-off energy of the plane-wave basis was set at 450 eV in structural optimization.For the optimization of both geometry and lattice size, the Brillouin zone integration was performed with a 0.04 k-mesh Monkhorst-Pack sampling freely to relax for the geometry optimization. [77]The self-consistent calculations applied a convergence energy threshold of 10 À5 eV.The equilibrium geometries and lattice constants were optimized with maximum stress on each atom within 0.02 eV ÅÀ1 .The isosurface level of charge density difference was set at 0.01 e ÅÀ3 .The spin polarization method was adopted to describe the magnetic system.Both band structure and density of state were obtained by VASPkit interface. [78]

Figure 2 .
Figure 2. Microstructure analyses by high-resolution transmission electron microscopy with the corresponding fast Fourier transform (FFT) images of a, b) SFT; c, d) SnO 2 ; and interface formation with a lattice spacing of e) SFT-SnO 2 (0.6:0.4) heterostructure; and nano-scale distances between particles at the atomic level of f, g) SFT and SnO 2 .

Figure 5 .
Figure 5. Cyclic voltammogram of the respective blocking layers of fuel cell devices under the employed scan rates of 20-100 mV s À1 under a, b) the potential window of 0.3-0.4V.

Figure 7 .
Figure 7. Optimized structure of a) SFT; b) SnO 2 ; c) Side view of SFT-SnO 2 ; and d) inset top and side view of SFT-SnO 2 displayed the interface at (110) plane; and e) the corresponding charge density difference in SFT-SnO 2 ,where charge accumulation and depletion are indicated by yellow and cyan area, respectively.Sr, Ti, Fe, Sn, and O atoms are marked by green, blue, orange, purple, and red balls, respectively.Bader charge analysis shows that the charge transformation from SFT to SnO 2 is 3.49|e|.

Figure 8 .
Figure 8. a, b) Calculated energy bandgaps from the respective absorption spectra obtained by UV-Vis spectroscopy; and c) energy band structure based on energy bandgaps and valence band maxima of SFT and SnO 2 .

Figure 9 .
Figure 9. Durability operation of SFT-SnO 2 heterostructure electrolyte-based fuel cell under an applied constant current density of 0.11 A cm À2 in the H 2 /air environment at an operational temperature of 500 °C.

Figure 10 .
Figure 10.a) The production of H 2 O outlet product during single cell operation through a NCAL/SFT-SnO 2 /NCAL under various applied current densities; b) Calculated Faradaic efficiency of the cell operated under H 2 and open-air environment for the conversion of H 2 to H 2 O under various current densities.Three times each measurement was performed, and their average was taken for the production of H 2 O outlet product and their related Faradaic efficiency calculation, where three average values for 10 min intervals of the entire 30 min are plotted, and the operational temperature was 500 °C.