A real proton‐conductive, robust, and cobalt‐free cathode for proton‐conducting solid oxide fuel cells with exceptional performance

The development of proton, oxygen‐ion, and electron mixed conducting materials, known as triple‐conduction materials, as cathodes for proton‐conducting solid oxide fuel cells (H‐SOFCs) is highly desired because they can increase fuel cell performance by extending the reaction active area. Although oxygen‐ion and electron conductions can be measured directly, proton conduction in these oxides is usually estimated indirectly. Because of the instability of cathode materials in a reducing environment, direct measurement of proton conduction in cathode oxide is difficult. The La0.8Sr0.2Sc0.5Fe0.5O3–δ (LSSF) cathode material is proposed for H‐SOFCs in this study, which can survive in an H2‐containing atmosphere, allowing measurement of proton conduction in LSSF by hydrogen permeation technology. Furthermore, LSSF is discovered to be a unique proton and electron mixed‐conductive material with limited oxygen diffusion capability that is specifically designed for H‐SOFCs. The LSSF is an appealing cathode choice for H‐SOFCs due to its outstanding CO2 tolerance and matched thermal expansion coefficient, producing a record‐high performance of 2032 mW cm−2 at 700°C and good long‐term stability under operational conditions. The current study reveals that a new type of proton–electron mixed conducting cathode can provide promising performance for H‐SOFCs, opening the way for developing high‐performance cathodes.


INTRODUCTION
][3] Fuel cells, which can directly convert chemical energy into electricity with low environmental effects and high efficiency, have received much interest. 4,5Proton-conducting solid oxide fuel cells (H-SOFCs), which are also known as protonic ceramic fuel cells (PCFCs), have emerged as a key research topic in the fuel cell community, as they have an all-solid-state structure comparable to regular solid oxide fuel cells (SOFCs) but decrease the working temperatures to the intermediate temperature range (600 • C-700 • C). 6 However, the reduced working temperature may reduce cell performance, and cathode polarization resistance dominates total cell resistance at low temperatures, implying that cathode reactions become sluggish when compared to those at high temperatures. 7s a result, one of the most pressing issues for H-SOFCs is the development of suitable cathode materials. 8,9arly investigations used cathode materials that work well in oxygen-ion conducting SOFCs (O-SOFCs) directly in H-SOFCs.However, it is recognized that due to the different reaction processes, this technique cannot fully promote cathode reactions, 10 despite the fact that some H-SOFCs utilizing typical cathodes function well. 11Proton migration is a vital step in the H-SOFC cathode reaction process. 12In contrast, no proton is involved in O-SOFC cathode reactions.As a result, using standard O-SOFC cathodes directly is not an optimal approach.It is critical to develop protonation-capable cathode materials.Some cathode materials with protonation behavior have been proposed with good cell performance in recent years, indicating the importance of proton migration in the cathode for H-SOFCs. 13,14These findings have sparked interest in the development of triple-conduction cathodes capable of simultaneously transporting protons, oxygen-ions, and electrons. 15ndeed, some triple-conducting cathodes enable high H-SOFC performance. 15,16However, the possibility of proton conduction or protonation in these existing proton-electron conducting cathodes is usually observed or hypothesized indirectly.Although thermogravimetry (TG), 13 X-ray diffraction (XRD), 17 electrical conductivity relaxation (ECR), 18 and density functional theory (DFT) calculations 19 are used to predict protonation in the oxide, the appearance of proton conduction or protonation can only be reflected indirectly in the form of a change in weight in TG, a shift in peaks in XRD, a relaxation time in ECR, and an energy barrier in DFT.These techniques yield no precise proton-conduction value, leaving the confirmation of proton conduction in a given cathode oxide uncertain.The majority of procedures utilized today characterize the material's hydration capabilities.Although hydration can represent a material's degree of protonation, it cannot totally equal proton mobility because the proton-trapping phenomena exist in protonconducting oxides. 20Some materials have been observed to have low proton conductivity despite having high hydration. 21Knowing the exact proton conductivity values in cathode materials would be quite fascinating because it provides the most direct proof for proton movement in the oxide rather than hypotheses.However, direct measurement of proton conduction for a cathode oxide is technically difficult since typical cathode materials tend to decompose in the H 2 -containing atmosphere, making commonly used technologies (such as concentration cells) for isolating protonic conduction from mixed conductors unavailable. 22Furthermore, oxygen-ion conduction, which also consumes oxygen vacancies and may compete with proton conduction in the oxide, may not be very necessary for H-SOFC cathodes, as long as the cathode really possesses proton and electron conduction.As a result, designing a proton-electron mixed conducting cathode may be an interesting and better method for H-SOFCs.
In this study, we propose a cathode material La 0.8 Sr 0.2 Sc 0.5 Fe 0.5 O 3-δ (LSSF), which is derived from the proton conductor LaScO 3 .Fe element is used to dope the proton conductor La 0.8 Sr 0.2 ScO 3-δ (LSS) based on the following considerations: (1) LSSF may inherit the proton conduction from the LSS mother compound that facilitates the cathode reaction for H-SOFCs; (2) good chemical stability could be obtained for the Ba-free LSSF oxide; and (3) well-matched thermal expansion due to the absence of Co element in LSSF.

RESULTS AND DISCUSSION
The first-principles method was utilized to investigate the suitability of employing Fe to customize LSS proton conductors.Figure 1A,B depicts the LSS and LSSF configurations.Both materials have similar configurations, with the exception that half of the Sc atoms in the LSS lattice are replaced by Fe atoms in the LSSF.The initial concept behind customizing proton-conducting oxides with transition metals is to make use of the material's possible protonation (hydration) behavior.The hydration ability of LSSF is expected to be comparable to that of LSS.In addition, it is known that the proton is introduced into the lattice under a wet atmosphere according to the following equation: 23 As a result, the formation energy of oxygen vacancy (V o ) and hydration

TA B L E 1
The calculated E vo , E hydra , E rot , and E hop for La 0.8 Sr 0.2 ScO 3-δ (LSS) and La 0.8 Sr 0.2 Sc 0.5 Fe 0.5 O 3-δ (LSSF).It is clear that LSSF inherits LSS's hydration behavior.However, with Fe doping, proton migration appears to be slightly more difficult.The proton migration process in the oxide is usually recognized to have two steps: the rotating step and the hopping step within nearby O atoms. 21or LSS, the proton migration energy for the rotating step (E rot ) is 0.01 eV.In comparison, the energy of the hopping step (E hop ) is 0.1 eV, indicating that the hopping step is the energy-limiting step in LSS.A similar trend was observed for LSSF, with the E hop being 0.32 eV and the E rot being 0.08 eV, indicating that the hopping step is similarly the energy-limiting step for LSSF.The energy required for proton migration increases with Fe doping, implying that protons in LSSF must overcome higher energy barriers than in Fe-free LSS.However, the proton migration energy of LSSF remains low, comparable to that of other protonconducting oxides, 24,25 showing that proton migration is conceivable in LSSF.

LSS LSSF
According to the preceding theoretical investigations, doping Fe into LSS could preserve the material's hydration ability with a relatively low proton migration energy, implying that LSSF could be a promising cathode material for H-SOFCs from an atomic standpoint.As a result, we conduct experimental tests to validate the prediction from the first-principles study.The XRD pattern of the synthesized LSSF powder is shown in Figure 1C.The sample contains a pure perovskite structure with no identifiable secondary phase, indicating that the target material was successfully prepared.The structure was further analyzed by XRD Rietveld refinement using GSAS software.The refinement gives a reliable result that converges to good R factors and χ 2 (R wp = 5.06%, R p = 4.56%, χ 2 = 1.32).As the Rietveld structure refinement shows, LSSF is confirmed to be pure crystalline single-phase with an orthorhombic structure and Pbnm group.The unit cell parameters are fitted to be a = 5.565 Å, b = 7.962 Å, and c = 5.438 Å, which can be calculated that the lattice spacings of (101) and (020) are 3.89 and 3.98 Å, respectively.The spherical aberration-corrected transmission electron microscopy (AC-TEM) was used to examine the atomic structure of LSSF, confirming very similar values for lattice spacing for (101) and (020) estimated by the XRD analysis, as shown in Figure 1D.All of the evidences presented above point to the successful preparation of the LSSF.
The DC four-probe method was used to measure the total conductivity of LSSF, and the results are displayed in Figure S1.The overall conductivity of LSSF is substantially higher than that of LSS and reaches a few S cm −1 from 400 • C to 700 • C, demonstrating that doping Fe into LSS increases the material's electrical conductivity.Although the conductivity value is not very high, it is enough for use as an electrode. 26In comparison to the ionic conductivity of conventional electrolytes and electrode materials, which is typically in the 10 −3 to 10 −2 S cm −1 range, 27 the electronic conductivity of LSSF will not be the rate-limiting step for the cathode reaction.The hydration of LSSF is also suggested by the experimental study.By hydrating the LSSF powder at 400 • C under flowing wet air (3% H 2 O) for 24 h, the XRD peak of the hydrated LSSF slightly moves to a lower angle compared with the un-hydrated sample (Figure S2).This result fits the literature observation that hydration causes XRD peaks to move to lower angles due to lattice expansion. 12ecause of the absence of Ba, LSSF is anticipated to have good chemical stability, which is one of the primary goals of developing this material.To test this hypothesis, LSSF powder was treated at a high temperature in a CO 2 -containing atmosphere, and the in situ high temperature was employed to record the phase of LSSF in the CO 2 environment.Figure 1E depicts the in situ XRD patterns of LSSF powder evaluated at 600 • C in a CO 2 atmosphere.An XRD measurement was conducted every half an hour to check for possible phase changes as a function of time at the testing temperature.One can see that the phase of LSSF remains constant throughout the experiment without the generation of any carbonate secondary phase, indicating that there is no reaction between LSSF and CO 2 and therefore confirming LSSF's great tolerance to CO 2 .The excellent stability of LSSF against CO 2 is exhibited not only at 600 • C but also over a wide temperature range from 100 • C to 700 • C, as illustrated in Figure S3, proving the exceptional stability of LSSF.It should be noted that the chemical stability test was performed with flowing 10% CO 2 gas.Given the CO 2 level in the air (0.03%), it is plausible to expect LSSF to exhibit appropriate stability when employed in H-SOFCs.The LSSF powder was subjected to an additional stability test under steam conditions by exposing it to a 30% H 2 O-containing environment at 600 • C for 10 h, and the results are given in Figure S4, indicating the good chemical stability of LSSF against steam.
Given the Co-free composition of LSSF, it is fair to predict that the thermal mismatch between the LSSF cathode and the electrolyte might be much reduced when compared to the Co-containing electrode, potentially lowering the co-sintering temperature for cathode adherence.The thermal expansion of the LSSF bar tested from room temperature to 1000 • C is shown in Figure 1F.LSSF has a thermal expansion coefficient (TEC) of 10.24 × 10 −6 K −1 , which is comparable to that of SOFC electrolyte materials but much lower than that of Co-containing cathode materials.The TEC of LSSF, for instance, is similar to that of the BaCe 0.7 Zr 0.1 Y 0.2 O 3-δ (BCZY) proton-conducting electrolyte utilized in the current work, which is 11.09 × 10 −6 K −1 , as shown in Figure S5.A similar TEC could cause the cathode to stick to the electrolyte.As a result, as shown in Figure 1G, the LSSF cathode could cling to the electrolyte layer even after co-firing at a relatively low temperature of 800 • C. It is recognized that the TEC of LSSF is even slightly lower than that of BCZY electrolyte material, which may better balance the high thermal expansion of some highperforming cathodes (such as Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 ) than the traditional method of adding BCZY electrolyte powder in the composite cathode, deserving further attention.It should be emphasized that in this work, only LSSF is employed without being coupled with the electrolyte material, although the composite cathode method is typically used in H-SOFCs to decrease thermal mismatch and increase adherence.The low co-firing temperature not only makes the manufacturing technique easier, but also avoids the possibility of an interfacial reaction between the cathode and electrolyte at high temperatures.
The LSSF cathode was connected to the anodesupported BCZY half-cell, completing the cell.The cell was tested at various temperatures while utilizing H 2 as the fuel.As seen in Figure 2A, the cell has a typical tri-layer structure.Figure 2B depicts the cell's I-V and power density curves.At 600 • C, 650 • C, and 700 • C, the peak power density (PPD) is 1184, 1662, and 2032 mW cm −2 , respectively.Furthermore, the cell exhibits outstanding stability under the fuel cell testing conditions, operating in a stable manner for more than 500 h without obvious degradation (Figure 2C).The cross-sectional observations of the fuel cell after the long-term stability test, as shown in Figure S6, reveal that the cathode and anode connect well with the electrolyte without any delamination even after the long-term operation.9][30][31][32][33][34][35][36][37][38][39][40][41][42][43] The new LSSF cell performs far better than all previous H-SOFCs employing Co-or Ba-free cathodes, indicating the effective production of a high-performance Ba-and Co-free cathode.][46] It should be noted that many of the cathodes used in the comparison in Figure 2D are composite cathodes, in which the cathode phase and proton-conducting oxides are mixed to extend the triple-phase boundaries (TPBs), and proton conduction is primarily dependent on the added proton-conducting oxide.However, in this study, the single-phase LSSF was used for the cell, and it is interesting to note that the performance of the cell using the singlephase LSSF cathode is higher than that of the cell using the LSSF + BaZr 0.8 Y 0.2 O 3-δ (BZY) composite cathode, as shown in Figure 2E, despite the fact that the cells were prepared identically except for the cathode used and have similar microstructures (Figure S7).The PPD value drops to 726 mW cm −2 at 700 • C when the LSSF + BZY composite cathode is employed, which is only around one-third of the value for the cell employing a single-phase LSSF cathode.
Figure 2F depicts a comparison of the polarization resistance of the LSSF single-phase cathode and the LSSF + BZY composite cathode.The polarization resistance of the single-phase LSSF cathode is 0.021 Ω cm 2 , which is substantially lower than the polarization resistance of the LSSF + BZY composite cathode (0.06 Ω cm 2 ), implying an accelerated cathode reaction at the single-phase LSSF cathode compared to the LSSF + BZY composite cathode.Previous research has shown that composite cathodes can increase fuel cell performance by increasing TPBs, provided that the cathode material is an oxygen-electron conducting material without protonation. 47In comparison to the restriction of TPBs at the interface of the cathode layer and the electrolyte layer, the composite cathode may expand the reaction active region to the locations where the cathode and electrolyte interact throughout the composite cathode.Because of the increased number of reaction active sites, the cathode reaction is expedited in this situation.However, according to the previous report, using a composite cathode could eliminate the active reaction area of a cathode if proton conduction is truly engaged in the cathode material. 48he performance of the LSSF + BZY composite cathode is significantly lower than that of the cell employing the single-phase LSSF cathode, strongly implying proton conduction in LSSF, which is consistent with theoretical calculations.Figure 2G depicts the scheme for removing the active reaction area by connecting the LSSF cathode to the proton-conducting electrolyte.It is clear that using a single-phase LSSF cathode can increase the reaction active area to the entire cathode surface.When coupling LSSF with the BZY proton conductor, the active reaction area decreases because part of the LSSF site is occupied by BZY, and no cathode reaction can occur at the BZY surface.As a result, the active reaction area decreases, resulting in decreased fuel cell performance when using an LSSF-BZY composite cathode.
ECR tests show that LSSF has a high proton diffusion coefficient (D H ). The ECR method was used to test the dense LSSF bar, and the switch of the atmosphere caused the charge carrier to change.The equilibrium time was used to determine the rate of the proton and oxygen diffusions.By changing the ambient from dry to wet air, the conductivity of LSSF approaches equilibrium in approximately 400 s, as illustrated in Figure S8a, and the D H value of 1.6 × 10 −4 cm 2 S −1 is attained.The time required to establish equilibrium by shifting the environment from air to 50% O 2 is substantially longer, taking more than 4000 s (Figure S8b).As a result, a substantially reduced oxygen diffusion coefficient (D O ) of 1.2 × 10 −5 cm 2 S −1 is observed, indicating that protons rather than oxygen-ions play a dominant role in LSSF.
The lack of oxygen-ion conduction in LSSF is also demonstrated with the fabrication of the LSSF + Sm 0.2 Ce 0.8 O 2-δ (SDC) composite cathode.Figure 3A depicts the H-SOFC fuel cell performance with the LSSF + SDC composite cathode.The cell has a comparable morphology (Figure S9) to the Ni + BCZY/BCZY/LSSF (single-phase cathode) cell, indicating that the variation in fuel cell performance cannot be attributed to the microstructure.Although the cell utilizing the LSSF + SDC composite cathode performs worse than the cell using the single-phase LSSF cathode, attaining only 534, 884, and 1319 mW cm −2 at 600 • C, 650 • C, and 700 • C, respectively, the degree is much less than that for the LSSF + BZY composite cathode.The SDC in the LSSF + SDC composite cathode serves two functions: (1) due to the lack of oxygen-ion conduction in LSSF, the high oxygen-ion conduction in SDC aids in the migration of oxygen-ions in the composite cathode and accelerates cathode reactions; (2) SDC also partially reduces the surface reaction area compared to the single-phase LSSF, lowering TPBs.These two roles balance each other and diminish performance when compared to a single-phase LSSF cell but increase performance when compared to a cell using an LSSF + BZY composite cathode.This viewpoint is supported by the electrochemical impedance spectroscopy (EIS) analysis.The EIS plot of the Ni + BCZY/BCZY/LSSF + SDC (composite cathode) cell is shown in Figure 3B.The polarization resistance of this cell is 0.041 Ω cm 2 at 700 • C, which is larger than that of the single-phase LSSF cathode due to the elimination of some catalytic active surface area but smaller than that of the LSSF + BZY composite cathode due to the improved oxygen-ion condition with the addition of SDC.
To rule out the possibility that the performance difference between the above-tested cells is due to two-phase reactions, the chemical compatibility of LSSF with BZY, BCZY, and SDC was investigated, and the findings are presented in Figure S10.After co-firing, no new phase can be discovered for the LSSF + BZY, LSSF + BCZY, and LSSF + SDC composite powders, indicating good chemical compatibility between LSSF and BCZY, BZY, and SDC.Therefore, the differences in fuel cell performance cannot be attributed to an undesirable interfacial reaction but rather to the properties of the cathode materials.
To illustrate the proton-electron mixed conducting nature rather than the proton-oxygen-ion-electron mixed conducting nature, the LSSF single-phase cathode was used in an SDC-based fuel cell with a cell structure of Ni-SDC/SDC/LSSF, and the fuel cell performance is displayed in Figure 3C.The cell's PPD is only 334, 523, and 712 mW cm −2 at 600 • C, 650 • C, and 700 • C, respectively, which is much lower than that of the H-SOFC counterpart.Although the SDC electrolyte-based cell shows a similar cell structure compared with the above BCZYbased H-SOFCs (Figure S11), the EIS analysis shown in Figure 3d indicates that the polarization resistance is 0.099 Ω cm 2 at 700 • C for the single-phase LSSF cathode used for the SDC electrolyte-based O-SOFC, which is significantly larger than that of the LSSF single-phase cathode for H-SOFCs.The cathode reaction mechanisms for the H-SOFC using the LSSF + SDC composite cathode and the SDC cell employing the single-phase LSSF cathode are shown in Figure 3E,F.The addition of SDC into the LSSF cathode for H-SOFCs can help the migration of oxygen-ions in the cathode, but the SDC particles also partially block the active sites of LSSF.These two factors counterbalance each other.When single-phase LSSF is used in SDC-based O-SOFCs, the reaction active area is limited to the LSSF cathode/SDC electrolyte interface because the LSSF material lacks oxygen-ion conduction, which could explain the large polarization resistance.This research clearly shows that proton conduction exists in LSSF, albeit with little oxygen-ion conduction.Otherwise, CeO 2 -based O-SOFCs should outperform H-SOFCs, which have been proven for a variety of oxygen-ion and electron mixed conducting cathodes. 49For instance, the classical oxygen-ion and electron mixed conducting cathode Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 can enable ultra-high performance for CeO 2 -based O-SOFCs. 50In comparison, the performance of Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 in H-SOFCs is mediocre. 11urthermore, even the triple-conduction cathode can result in superior performance for O-SOFCs than for H-SOFCs.Duan et al. 14 employed a BaCo 0.4 Fe 0.4 Zr 0.1 Y 0.1 O 3 cathode for H-SOFCs and achieved a PPD of 455 mW cm −2 at 500 • C.However, the CeO 2 electrolyte-based O-SOFCs achieved a substantially higher PPD of 970 mW cm −2 using the same cathode material at the same testing temperature. 51lthough DFT calculations, ECR tests, and fuel cell testing for various cathode compositions strongly imply proton conduction on LSSF, all of these evidences are indirect.Previous researches also usually indirectly demonstrate proton conduction in many cathode materials, and direct determination of proton-conductivity values for cathode materials is challenging due to the instability of cathode materials in the H 2 -containing atmosphere.However, as shown in Figure S12, the LSSF material proposed in the current study is chemically stable in an H 2 -containing atmosphere.The phase composition of LSSF at 800 • C was recorded in situ with XRD as a function of time in an H 2 -containing environment (4% H 2 balanced with Ar).The results show that the LSSF material is stable against H 2 at high temperatures, with no signs of degradation based on the XRD measurements.Because of LSSF's strong chemical stability against H 2 , it can be run in an H 2 -containing atmosphere.
If the mixed conductor is stable in the testing environment, hydrogen permeation technology can isolate proton conduction from proton-electron mixed conductors (H 2containing atmosphere).In the hydrogen permeation scenario, the present LSSF material, which is stable against H 2 at high temperatures, might be employed to extract the accurate proton conduction from the mixed conductor.It should be noted that in the hydrogen permeation tests, the dense LSSF membrane was utilized without any protective layer, preventing the influence of any interfacial reactions or contamination from other substances.Figure 4A depicts the hydrogen permeation system.H 2 and N 2 are both fed into one side of the chamber.Protons can travel through the dense membrane if the material is proton conductive, and the penetrated H 2 can be measured.The proton conduction can thus be monitored using the hydrogen permeation flux.Figure 4B depicts the crosssectional morphology of the sintered LSSF membrane, suggesting that the membrane is dense enough to prevent hydrogen gas leakage from one side to the other.The surface view of the sintered LSSF pellet in Figure S13 further shows that the membrane is sufficiently dense and free of visible pores.Furthermore, only the H 2 signal can be detected with gas chromatography (GC), and no N 2 signal can be obtained throughout the test, indicating that no gas is leaking from the membrane.The detected H 2 is the permeated H 2 according to the following procedure: H 2 → 2H + (at one side of the membrane), then H + passes through the dense membrane and 2H + → H 2 (at the other side of the membrane).Figure 4C shows the result of the hydrogen permeation measurement.The hydrogen permeation flux is in the order of 10 −9 mol cm −2 S −1 , which is one order of magnitude lower than that of the BCZY-based membranes but higher than that of the lanthanum tungsten oxide-based membranes tested under similar conditions, indicating that the proton conductivity of LSSF is moderate and is approximately one order of magnitude lower than that of BCZY but higher than that of the lanthanum tungsten oxide proton conductors. 52ccording to the hydrogen permeation test, the protonic conductivity of LSSF can be calculated, and the value is around 1.1 × 10 −4 S cm −1 at 600 • C.This conductivity value is much higher than that of some proton conductors, such as the classical LaNbO 4 or Ba 3 Ca 1.18 Nb 1.82 O 9 proton conductors. 53Although LSSF has lower proton conduction than BCZY or BZY, the preceding analysis demonstrates that the LSSF + BZY composite cathode performs worse than the LSSF single-phase cathode, implying a geometric benefit by employing the genuine protonic-electronic mixed conductor as the cathode.The much-extended surface active area compensates for and overcomes the considerably lower protonic conductivity of LSSF compared to the pure BCZY or BZY proton conductor, resulting in a record-high fuel cell performance for the Co-and Ba-free cathodes for H-SOFCs.
The oxygen permeation membrane test was used to further investigate the possibility of oxygen-ion conduction in LSSF.Unlike evident produced H 2 can be detected in the hydrogen permeation membrane test, no apparent O 2 flux can be measured using LSSF as an oxygen-ion conducting membrane, implying that there is little oxygen-ion conduction in LSSF.This finding is consistent with the ECR test, which shows that proton conduction is more dominant than oxygen-ion conduction, confirming that LSSF is a particular proton-electron mixed conductor with no obvious oxygen-ion conduction designed specifically for H-SOFCs.
The LSSF cathode material presented in this paper is a proton-electron mixed conductor with no obvious oxygenion conduction, as opposed to the currently dominant triple-conduction cathodes with proton, oxygen-ion, and electron conductions.The lack of oxygen-ion conduction appears to have no effect on the performance of the cathode for H-SOFCs, assuming the cathode really has proton conduction.Furthermore, the absence of oxygen-ion conduction in LSSF means there is no competition with proton conduction for the oxygen vacancies required for proton and oxygen-ion conductions, potentially facilitating proton conduction in the oxide.The successful application of proton-electron mixed conducting cathodes, such as LSSF in this study, provides an intriguing and alternative cathode design strategy for high-performance H-SOFCs.However, due to the lack of oxygen-ion conduction, this type of cathode performs poorly for O-SOFCs, making it only appropriate for H-SOFCs.

CONCLUSION
The tailoring of the LSS proton conductor resulted in LSSF, which was proven to be a viable cathode material for H-SOFCs.Excellent chemical stability against CO 2 , close thermal expansion with the electrolyte, and good performance in fuel cell applications were demonstrated for LSSF.The fuel cell performance outperforms all prior H-SOFCs that use Ba-or Co-free cathodes.More importantly, LSSF was discovered to have proton conduction, which was not only indirectly shown by DFT calculations and ECR measurements but was also directly confirmed by H 2 permeation investigations.Various fuel cell designs were offered, suggesting that the proton-conductive characteristic enables LSSF to demonstrate good fuel cell performance without coupling it with other proton-conducting oxides.
The LSSF, on the other hand, has limited oxygen-ion conduction and so performs poorly in oxygen-ion conducting SOFCs.To the best of our knowledge, LSSF is a rare proton-electron mixed conducting cathode that specifi-cally works for H-SOFCs, as opposed to the traditional oxygen-ion and electron mixed conducting cathodes or the recent intensively investigated triple-conducting (oxygenion, proton, and electron) cathodes, which may offer a new route for the design of H-SOFC cathodes.

EXPERIMENTAL
LSSF material was synthesized by a sol-gel method, 54 using stoichiometric amounts of La 2 O 3 , SrCO 3 , Sc 2 O 3 , and Fe(NO 3 ) 3 as the starting materials.The metal oxides and metal carbonates were dissolved in nitric acid.Citric acid and ethylene diamine tetraacetic acid (EDTA) were used as the complexing agents.The molar ratio between the metal cations:citric acid:EDTA was set as 1:1.5:1.The solution was heated under stirring to evaporate water and finally ignited, producing ashes.These ashes were calcined at 1000 • C for 3 h to form the pure phase LSSF material.The phase purity of the material was examined by XRD.To examine the chemical stability of the LSSF material, high-temperature XRD was used to record the XRD patterns of LSSF as a function of time under an atmosphere containing 10% CO 2 at 600 • C. Dense LSSF bars were prepared for the thermal expansion studies, and the TEC of LSSF was measured by using a dilatometer (DIL 402, NETZSCH).The structure of LSSF at the atomic level was observed using a spherical AC-TEM (JEM-ARM200F, JEOL).
To evaluate the performance of LSSF as a cathode for H-SOFCs, BCZY electrolyte-based half-cells were prepared, in which BCZY was used as the electrolyte and NiO + BCZY was used as the anode.The BCZY proton conductor material was also prepared by a sol-gel method using Ba(NO 3 ) 2 , Ce(NO 3 ) 3 , ZrO(NO 3 ) 2 , and Y(NO 3 ) 3 as the starting materials.All of the starting components were dissolved in distilled water with the addition of citric acid.The molar ratio of citric acid to cations was adjusted at 2:1.The solution was then heated while stirring until it formed a gel, which was then burned into ashes.The ashes were collected and fired for 6 h at 1000 • C to produce the pure phase BCZY electrolyte material.Ball milling was used to mix BCZY powder with NiO in a weight ratio of 4:6 to form the NiO + BCZY composite anode powder.To manufacture the anode substrate, the anode powder was pressed in a pressing mold.The BCZY electrolyte powder was then placed onto the anode substrate, followed by co-pressing.The NiO + BCZY/BCZY bilayers were cosintered for 6 h at 1350 • C to densify the supported BCZY electrolyte membrane, resulting in the BCZY electrolytebased half cells.LSSF cathode slurry was deposited on the surface of the BCZY electrolyte, followed by co-firing at 800 • C in a microwave sintering furnace to attach the LSSF cathode to the electrolyte.To study the influence of composite cathode on the fuel cell performance, the LSSF was further mixed with the BZY powder or the SDC, forming the LSSF + BZY or LSSF + SDC composite cathodes, respectively.The BZY material was prepared by a sol-gel method, as mentioned above, using Ba(NO 3 ) 2 , ZrO(NO 3 ) 2 , and Y(NO 3 ) 3 as the starting materials.The precursors were fired at 1100 • C for 6 h to obtain a pure phase for BZY.The SDC powders were also prepared similarly using Ce(NO 3 ) 3 and Sm(NO 3 ) 3 as the starting materials, and the precursors were fired at 800 • C for 3 h to get pure-phase SDC powders.The weight ratio between the LSSF and BZY (or SDC) was 7:3.The LSSF + BZY and LSSF + SDC composite cathodes were also applied to the same BCZY + NiO/BCZY half-cells to fabricate the BCZY-based H-SOFCs using the LSSF + BZY and LSSF + SDC composite cathodes.Although single cells employed BCZY as the electrolyte, it is stated that BZY was added to the composite cathode rather than BCZY.When compared to BCZY powder, BZY powder has significantly higher chemical stability, which would improve the overall chemical stability of the composite cathode.Furthermore, the BZY is known for its poor sinterability, which is a disadvantage for the electrolyte.However, this feature becomes favorable for the cathode since it may allow the composite cathode to retain a high porosity even after sintering at high temperatures. 55he SDC electrolyte-based O-SOFCs were also fabricated by a co-pressing and co-sintering method.The synthesized SDC powder was mixed with NiO in a weight ratio of 4:6, forming the NiO + SDC composite anode powder.The SDC electrolyte layer was co-pressed with the NiO + SDC anode layer and then co-sintered at 1400 • C for 6 h, forming the NiO + SDC/SDC bilayers.Then, the LSSF cathode slurry was deposited on the sintered SDC electrolyte membrane, followed by co-firing at 800 • C, finally constructing the NiO + SDC/SDC/LSSF complete cells.
All these fabricated fuel cells were tested under the fuel cell working condition with H 2 as the fuel.The cell performance was recorded by using an electrochemical workstation (Squidstat Plus, Admiral Instrument).The EIS measurements were carried out for the cells under the open circuit voltage condition with a frequency range from 1 MHz to 0.1 Hz.The morphologies of the cells were observed using scanning electron microscopy (Phenom XL, Thermo Scientific).
The hydrogen permeation membrane technique was employed to extract the proton conduction from the mixed conductor LSSF.LSSF pellets were sintered at 1500 • C for 6 h to obtain the dense pellets.Then the dense LSSF pellet was sealed with glass for the hydrogen permeation tests.The gas mixture containing 20% H 2 + 80% N 2 was fed at one side of the membrane, and the pure Ar gas was used as the sweeping gas at the other side of the membrane.GC (GC9790II, Fuli Instruments) was used to detect the produced H 2 at the Ar gas side, which was the permeated H 2 due to the proton conduction of the LSSF membrane.The oxygen permeation membrane test was performed in a similar way.The 20% O 2 + 80% N 2 mixture gas was fed at one side of the sealed LSSF membrane, and the pure He gas was used as the sweeping gas at the other side.GC was used to detect the possible permeated O 2 gas if the membrane had sufficient oxygen conduction abilities.The platinum paste was painted on the LSSF pellet and fired at 1000 • C for 1 h.The Pt was used to conduct electrons for the permeation membrane measurements.
First-principles calculations were carried out by using the DFT method with the application of Vienna Ab-Initio Simulation Package software. 56A 4 × 4 × 4 gamma centered K-point mesh was used, and the convergence criteria for energy and force were 10 −5 eV and 0.05 eV • A −1 , respectively.For Hubbard's correction, the U eff value of 4 eV was added to Fe.The E vo was calculated according to the equation:  vo =  def ect + (1∕2) O 2 −  perf ect , in which  perf ect is the energy for the bulk and  def ect is the energy for a bulk with one oxygen atom deficiency.E hydra was calculated according to the equation:  hydra =  2OH −  def ect −  H 2 O , where  2OH is the energy of the crystal with two additional protons and  H 2 O is the energy of an H 2 O molecule. 57More calculation details can be found elsewhere.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.

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I G U R E 2 (A) Scanning electron microscopy (SEM) image for the cross-sectional view of a proton-conducting solid oxide fuel cell (H-SOFC) using the single-phase La 0.8 Sr 0.2 Sc 0.5 Fe 0.5 O 3-δ (LSSF) cathode after fuel cell testing; (B) fuel cell performance of the H-SOFC using the single-phase LSSF cathode; (C) the long-term stability of the cell tested at 600 • C with an applied current density of 200 mA cm −2 ; (D) comparison of the fuel cell performance of the current LSSF single-phase cathode with other Ba-or Co-free cathodes reported in the literature; (E) fuel cell performance of the H-SOFC using the LSSF + BaZr 0.8 Y 0.2 O 3-δ (BZY) composite cathode; (f) comparison of electrochemical impedance spectroscopy (EIS) for the cell using the single-phase LSSF and composite LSSF + BZY cathodes; (G) schemes for the cathode reactions at the LSSF-BZY composite cathode and the LSSF single-phase cathode.

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I G U R E 3 (A) Fuel cell performance for a proton-conducting solid oxide fuel cell (H-SOFC) using a La 0.8 Sr 0.2 Sc 0.5 Fe 0.5 O 3-δ (LSSF) + Sm 0.2 Ce 0.8 O 2-δ (SDC) composite cathode; (B) electrochemical impedance spectroscopy (EIS) plot for the H-SOFC using a LSSF + SDC composite cathode tested at 700 • C; (C) fuel cell performance for a SDC electrolyte-based oxygen-ion conducting SOFC (O-SOFC) using a single-phase LSSF cathode; (D) EIS plot for the SDC electrolyte-based O-SOFC using a single-phase LSSF cathode tested at 700 • C; scheme of the cathode reaction for (E) LSSF + SDC composite cathode working for H-SOFCs and (F) O-SOFCs using a single-phase LSSF cathode.

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I G U R E 4 (A) Scheme for the H 2 permeation test using the proton-conducting membrane; (B) scanning electron microscopy (SEM) image for the La 0.8 Sr 0.2 Sc 0.5 Fe 0.5 O 3-δ (LSSF) membrane; (C) H 2 permeation flux of the LSSF membrane at different temperatures.

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C K N O W L E D G M E N T S Yanru Yin and Dongdong Xiao contributed equally to this work.The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (grant numbers 52272216 and 51972183), the Hundred Youth Talents Program of Hunan, and the Startup Funding for Talents at University of South China.