Approaching Durable Single-Layer Fuel Cells: Promotion of

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Approaching Durable Single-Layer Fuel Cells: Promotion of Electroactivity and Charge Separation via Nanoalloy Redox Exsolution Kang Shao, Fengjiao Li, Guanghong Zhang, Qianling Zhang, Kristina Maliutina, and Liangdong Fan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08448 • Publication Date (Web): 10 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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ACS Applied Materials & Interfaces

Approaching Durable Single-Layer Fuel Cells: Promotion of Electroactivity and Charge Separation via Nanoalloy Redox Exsolution Kang Shao, Fengjiao Li, Guanghong Zhang, Qianling Zhang, Kristina Maliutina, Liangdong Fan*

College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518060, Guangdong province, P. R. China

Shenzhen Key Laboratory of New Lithium-ion Batteries and Mesoporous Materials, Shenzhen University, Shenzhen 518060, Guangdong, PR China

KEYWORDS: Single-layer fuel cell; Solid oxide fuel cell; Semiconductor; Metal/alloy nanoparticle exsolution; p-n junction; Schottky junction

Single-layer fuel cells (SLFCs) based on a mixed semiconductor and an ionic conductor demonstrate a simplified material preparation and fabrication procedure and possess a potentially high performance. However, the operational stability and principle of SLFCs have not yet been convinced of either commercialization and fundamental interests. We hereby report on the employment of a perovskite oxide 1

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based phase structurally redox stable semiconductor prior to determine a possible solution that improves the durability of the SLFC. The feasible working principles are established and the in-depth understanding of the short-circuit-free phenomenon in the SLFC with the mixed ionic and electronic conductor is provided. Additionally, a smart material design and cell structure processing are also proposed. An extended non-stop testing period of up to two days confirms the project feasibility and improved durability of the SLFC, achieved by replacing the unstable lithiated oxide phase with redox stable perovskite oxide, though the electrochemical performance is sacrificed. The precipitated metal/alloy nanoparticle on perovskite oxide not only improves the electrode reaction kinetics but also facilitates the charge separation and ionic conduction in SLFC, consequently enhances the fuel cell performance and electrical efficiency. The results confirmed the potential of stable operation for future practical deployment of SLFCs via the appropriate selection of material and cell structure design. It is greatly believed that the physical junction has played a crucial role in overcoming the internal short circuit issue of SLFCs.

1.

Introduction

The development of highly efficient energy conversion technologies has been extensively pursued in the last two decades due to the boosted energy crisis and the concerns of environmental impact. Fuel cells attracted the colossal interest of 2


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researchers nowadays because of the high efficiency and environmental compliance. The low temperature operating proton exchange membrane fuel cells (PEMFCs) have the capabilities of higher reversible electrical efficiency and close daily-life application, which make them competitive to that of conventional energy conversion devices like combustion engines and so on. Notwithstanding, the present drawbacks, such as high cost of precious benchmark electrocatalysts (Pt/C) and sluggish electrode reaction kinetics in PEMFCs, hinder the ultimate efficiency, which is lower than when operating at high temperatures and suppress the world-spanning commercialization. Substantially, the solid oxide fuel cells (SOFCs) operated higher than 700 °C with kinetic favored characteristics attract increasing attention. However, the typical intermediate and high temperatures operated SOFCs suffer from severe cell material incompatibility issue and large capital input1-2. The reduction of the SOFC running temperature that simplifies the sealing and slows down degradation has been highly desirable and becomes the worldwide tendency3-5. Versatile approaches were developed in the last several years with enormous progress by seeking for active cell components to replace the current electrolyte and electrode material system, or by optimizing the fuel cell microstructure/cell configuration by using nanomaterials and nanotechnology5-9. Nevertheless, there are often competitive tensions among fuel cell performance, durability, and system cost. The remarkable electrochemical efficiency is often at the expense of the diminishing durability10-11. Generally, the classic threelayer structure, i.e. porous anode and cathode, and the sandwiched dense electrolyte 3



layer, together called the membrane electrode assembly is the center of the fuel cells, in which the electrolyte layer is the key to realize fuel cell functionality, which while causes the intrinsic interfacial chemical- thermal-mechanical compatibility issues as well as huge interfacial reaction/charge transfer energy loss, not to mention the insufficient ionic conductivity of the current typical electrolyte materials12. Those, in turn, suppress the industrial demonstration to still the state-of-the-art material system: Ni-cermet anode, yttria-stabilized zirconia electrolyte, and perovskite (La,Sr)MnO3 cathode, which were used since the invent of SOFC, more than one century ago. Therefore, a breakthrough on highly efficient energy conversion system/technology at high temperature with similar/surpassing functionality as SOFCs is decidedly required.

The recent development in a novel high-temperature energy conversion technology, the single-component/single-layer fuel cell (SLFC) or electrolyte-free fuel cell device has attracted profound attention of researchers worldwide because of the simplified cell structure and superior performance compared with that of traditional three-layer SOFCs13-19. In the SLFC configuration, the typical three-layer structure of anode/electrolyte/cathode has been replaced with a single homogeneous composite of ionic and semiconducting materials or semi-ionic conductor. No electrode/electrolyte interfacial chemical/thermo-mechanical compatibility issue and much reduced interfacial polarization resistance appear in such a novel device. Moreover, the fuel 4


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cell peak power outputs up to 1000 mW cm-2 were demonstrated in the literature by using hydrogen as a fuel and air as an oxidant at 550 °C 20-22, which are clearly superior to those of three-layer SOFCs with a classic electrolyte layer. Consequently, the electrolyte layer in the SLFC devices was removed or replaced with a semiconducting layer while the fuel cell functionalities were maintained or even extended. Accompanying with the quick material evolution and subsequent cell performance enhancements, some other analogous works like the correlated perovskite fuel cells23 and transition of electronic conductor to pure proton conductor in layer lithiated transition metal oxide24 were reported, which jointly contributed the understanding of the work principle or the science behind of SLFC for the efficient energy conversion13, 16, 20-21, 25-27. Simplified materials and a straightforward device fabrication technology, harmonious work environments, and electrical properties are, on one hand, stimulating large interests both in the fundamental study and in the commercialisation growth. On the other hand, the large scale industrialization and scalability of the SLFC are still suppressed by difficulties in understanding of the novel fuel cell work principles and detailed reaction pathways. Considerable efforts have been applied to digging out the working principle since the electric shortcircuiting problem is avoided in SLFC using a semiconducting layer instead of an electronic insulating layer as the separator. Currently, the p-n junction13, Schottky junction16, bulk heterojunctions20, 25 are as well as semiconductor energy band alignment similar to perovskite solar cell principle21 creating by intentionally 5



constructing or in-situ formation under fuel cell atmosphere have been proposed based on the usage of semiconducting oxides with a wide bandgap28-29, while the direct evidence or believable/consensus principle/mechanism is missing. From the technical aspect, a simple procedure of co-pressing and low-temperature sintering is generally adopted fabricating the single cell unit. The resulting porous structure, which is required for the formation of a triple phase boundary, i.e., an active site for the electrochemical reaction, is detrimental to the fuel cell voltage and energy efficiency due to the fuel and oxidant crossover. In addition, though proposed as a single layer, a vital lithiated transition oxide precursor is generally presented on both sides of the composite layer as a current collector and electrocatalysts which can enhance the efficiency of the fuel cell20, 28, 30, however, bring the suspicion of traditional three-layer SOFC configuration. Moreover, the gradual reduction of the active phase to the metal phase and an intrinsic porosity may lead to the deterioration of performance and limitation of the durability of a single cell. Only a few limited cases reported on the durability under open circuit voltage conditions, not the actual fuel operational conditions of the fuel cells30-32. In response to this, an initial attempt was performed to verify the electrochemical performance and stability by using Ni0.8Co0.15Al0.05LiO2 (NCAL) as the semiconducting electrocatalyst and (Li/Na)2CO3Sm0.2Ce0.8O2 ionic conductor composite, and NCAL coated Nickel foam as a current collector and electrode. Although it represented an acceptable open circuit voltage (OCV, ～1.00 V), a promising fuel cell peak power density (428.8 mW cm-2) and low 6


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polarisation losses at 550 °C (Figure 1a and b), it is quickly degraded within a few hours, as shown in Figure 1c. Tremendous effort has also been established prior to creating the standardized procedures for a single layer fuel cell fabrication and performance testing33. The issues of the time dependence of the fuel cell performance and subsequent degradation are still the barriers to the further implementation of such a promising technology for energy conversion27, 31, 34.

Thereby, in the present study, we aim to verify the operational stability of the SLFC in the extended time. Redox stable materials, Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ (PSCFN) based perovskite oxides35-36, as proof of conceptual, are employed to replace the commonly used while phase-instable lithiated transition metal oxides under reducing atmosphere. The effect of the current collection and enhanced electrode catalytic activity via using such a lithiated transition metal oxide functional layer is also excluded. For further verification of the working principles and acceleration of the electrochemical reaction, a strategy of in-situ/ex-situ exsolved metal/alloy nanoparticle socketed on perovskite oxide matrix surface10, 37-38 is first employed in SLFC. Integration of the enhanced fuel cell performance and extended stability under the real fuel cell conditions confirm the positive junction role for charge separation in SLFC and verify the effective method to improve the fuel cell performance through a simple but effective material and cell structural design.

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2.

Experimental

2.1 Materials fabrications Pr0.4Sr0.6Co0.2Fe0.7Nb0.1O3-δ (P-PSCFN) and Pr0.8Sr1.2(Co,Fe)0.8Nb0.2O4+δ (K-PSCFN) were synthesized by using solid-state reaction (SSR) method and calcinated under 1050 °C for 5 hours based on the procedure provided elsewhere35. Co-Fe alloy deposited K-PSCFN (H-PSCFN) was fabricated by reducing of P-PSCFN in 10% hydrogen gas balanced with nitrogen at 900 °C for 10 hours. Ni0.8Co0.15Al0.05LiO2 (NCAL) was also obtained by the SSR technique with hydroxide or nitrate chemical precursors and annealed in the air at 800 °C for 5 hours. The required ionic conductor of Sm0.2Ce0.8O2-(Li/Na)2CO3 (LNSDC) was achieved by mechanical mixing and a subsequent heat treatment procedures in accordance with our previous work39. 2.2 Single cell fabrications and electrochemical performance measures Five sets of fuel cells in a single-layer or three-layer configuration were fabricated, as shown in Tables and captions

Table 1. All fuel cells were as-prepared by one step method using a dry-pressing technique in stainless steel die at 300 MPa for 1 minute. The green pellets except HPSCFN based single cell were sintered in the air at 700 °C for 2 hours to reduce porosity and to achieve sufficient mechanical strength. The total thickness of single cells was around 1 mm. Both sides were pasted with silver paste as a current collector (DAD-87, Shanghai Research Institute of Synthetic Resin, China). The single cells 8


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were then mounted to one end of an aluminum oxide ceramic tube with hightemperature ceramic glue, and then heat-treated in a tube furnace from room temperature to the desired temperature at a ramp rate of 5 °C min-1 in the air before being subjected to an H2/air atmosphere. While the H-PSCFN cells were directly subjected to an H2/air atmosphere from the room temperature to stabilize the exsolved metal-oxide composite. Fuel cell electrochemical performance studies were performed on an electrochemical workstation (Solartron potentiostat 1260A plus impedance analyzer 1470E, England). The voltage-current density curves, i.e. polarization curves were recorded from open circuit voltage to 0 V at a ramp rate of 50 mV S-1. The current responses under a constant voltage of 0.7 V were obtained to identify the operational durability under the actual fuel cell atmosphere. Electrochemical impedance spectroscopy (EIS) was also recorded under the open circuit voltage with an amplitude of 50 mV and a frequency range of 100 kHz-0.01 Hz. The obtained EIS curves were fitted with an empirical equivalent circuit model to obtain the detailed electrode reaction and ionic transport polarization resistances. The current responses under voltage bias (-2V～2V) of NCAL and H/K-PSCFN based SLFC component in different gas atmospheres were also recorded to identify the possible rectification effect of the resultant physical junction and to conclude with a possible working principle.

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2.3 Other physical and chemical characterization The powder X-ray diffraction (XRD) analysis of all samples was carried out on a Rigaku Dmax/Ultima IV diffractometer (Bruker D8ADVANCE, Germany) with monochromatized Cu Kα radiation ( = 1.54056 Å) in order to investigate the changes in physical phase structure. To check the compatibility of perovskite oxide with LNSDC ionic conductors, mechanical mixing of perovskite oxide and LNSDC and then calcination at 700 °C for 2 hours were carried out. The microstructures and morphologies were observed by using a field emission scanning electron microscope (FESEM, JSM-7800F & TEAM Octane Plus) with an accelerating voltage of 15 kV. The texture and lattice parameters of the obtained perovskite oxide and composites were investigated by using a transmission electron microscope (TEM, JEM-2100 & X-Max80 with an accelerating voltage of 200 kV). X-ray photoelectron spectroscopy (XPS) measurements were performed using an ESCALAB 250Xi spectrometer under an ultrahigh vacuum and with Al Kα radiation (1486.6 eV) and a multichannel detector to study surface oxygen vacancy content and other elements valence information. All the collected binding energies were calibrated by using the C1s peak at 284.6 eV as the reference with an uncertainty of ±0.2 eV.

3.

Results

In term of the insufficient stability of NCAL based SLFC due to the gradual reduction of the metal oxide to metal phase, the redox stable p-type perovskite PSCFN-LNSDC 10


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is used to replace NCAL-LNSDC in the middle of SLFC to construct a nonconventional three-layer fuel cell NCAL/p-PSCFN-LNSDC/NCAL (Cell II). The fuel cell performance at 550 °C is shown in Figure 2. An open circuit voltage of 0.98 V and peak power density up to 238 mW cm-2 is achieved at 550 °C, suggesting the functionality of P-PSCFN semiconductor and acceptable electrocatalytic activity for SLFC, though the maximum power output is lower compared to the NCAL cells (Cell I). The latter is possibly owing to the inferior catalytic activity and ionic conductivity of perovskite oxide compared with the NCAL materials, as reflected by EIS study. Cell II gives an ohmic and electrode polarization resistance of 0.55 and 0.57 Ω cm2, respectively, based on impedance study and fitting results (Figure 2b). It is also interesting to see that Cell II gives a relatively stable performance up to 24 hours around 170 mA cm-2 under a constant operational voltage of 0.7 V (Figure 3c), making a p-PSCFN as an attractive semiconductor to replace NCAL for durable SLFCs, also suggesting the feasibility of the future application.

Though the positive role of NCAL for SLFC, the easy reduction characteristics under the real fuel cell conditions still queries the chemical/thermal compatibility and longterm stability. In addition, similar to that conventional Ni-based cermet anode in SOFC, the redox problem and possible carbon deposition issue with hydrocarbon fuel remains. Therefore, the real SLFCs (Cell III-V) with P-PSCFN, K-PSCFN, and surface alloy precipitated K2NiF4-type structured H-PSCFN semiconductors 11



combined with the ionic conductor LNSDC which are free from the nickel foam supported NCAL functional layers are then built. All fuel cells give an OCV higher than 0.90 V at 600 °C under H2/air atmosphere (Figure 3a); the value is higher than the benchmark SOFCs with mixed ionic and electronic conducting doped ceria electrolyte whose OCV values is never larger than 0.90 V when the temperature is above of 450 oC40, suggesting the fuel cell functionality with those semi-ionic conducting core materials in true SLFCs. Moreover, it is worthy to mention that the SLFC with H-PSCFN shows the highest performance, both the OCV value and peak power output, among all of fuel cell samples under the identical operational condition. Fuel cell peak output of SLFCs has the following sequence of Cell V (161.80mW cm2)>

Cell IV (130.47 mW cm-2)> Cell III (77.60 mW cm-2) at 600 oC, though they are

lower than the initial performance of NCAL-based cell (Cell I) and pseudo SOFC with NCAL current collector or catalytic layers (Cell II). Not mentioned about the enhanced OCV value, the improved electrode activities of K-PSCFN and H-PSCFN could explain the increased peak fuel cell performance, as reflecting by the much reduced electrochemical impedance (Figure 3b), which were fitted by using the empirical equivalent circuit (LR0(R1/CPE1)(R2/CPE2) model (Figure S1) and the results are summarized in Table 2 and Table S1. The Cell V with H-PSCFN gives the lowest ohmic and electrode polarization resistances, followed by the Cell IV with KPSCFN, while Cell III using P-PSCFN shows the largest polarization loss under OCV condition. Notably, no metal exsolution appears below 700 oC as reflected by the H2 12


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temperature programmed reduction (H2-TPR) analysis (Figure S2) and the morphology of Co-Fe nanoalloy socketed PSCFN oxide is preserved after heattreatment under oxidizing atmosphere (in air) at 600 oC for 3 hours (Figure S3), which leads to the different reaction activities and fuel cell performance between PPSCFN and H-PSCFN and their cells in this study. An extended period stability testing of SLFCs with different semiconducting materials and LNSDC composite under the identical operational potential of 0.7 V were also performed and the results are shown in Figure 3c. All of the tested samples showed a relatively stable current response under a constant voltage of 0.7 V/600 oC and a real fuel cell condition for a short-term period. In particular, SLFC with P-PSCFN presents a stable current up to 45 hours (Totally 52 hours) before a fluctuation taking place because of the hydrogen gas pressure drop due to the technical issue of on-site hydrogen gas supplier. This is evidently better than the SLFC with NCAL core materials as shown in Figure 1b. Significantly improved operational stability can be attributed to the redox stable phase structure, at least the well-maintained oxide phases of the applied perovskite oxide semiconducting materials. These initial results demonstrate once again the strong potential of using the redox stable perovskite phase as an alternative to the commonly used phase-unstable materials and expanding the choice of material for the newly developed SLFC technology. In fact, besides the good material redox properties, sufficient chemical compatibility between versatile PSCFN semiconductors with LNSDC ionic conductor heat-treated at 700 oC for 2 hours in the air based on the 13



XRD pattern investigations as shown in Figure S4, also ensures the stable performance. In addition, typical Co-Fe alloy diffraction peaks appeared around 44.8o and 65o (JPCDS 00-049-1568) are observed in the XRD pattern of H-PSCFN, which is agreed with the previous study35. The formed alloy nanoparticle can elucidate the improved electrocatalytic activity and subsequently the improved fuel cell efficiency over P-PSCFN. Moreover, the exsolved metal on PSCFN semiconductor could in-situ form metal/semiconductor interface or junction and can contribute to charge separation and transfer capability, which will be provided in details in the discussion section.

The effect of operating temperature on the electrochemical performance of SLFCs based on H-PSCFN semiconductor (Cell V) is shown in Figure 4. The corresponding open circuit voltages and peak power outputs are attributed to 0.95, 0.97, 1.02V and 247.85, 161.80 and 136.38 mW cm-2 at 650, 600 and 550 oC, respectively (Figure 4a). As expected, both the ohmic and reaction polarization resistance reduces as the operating temperature increases, which leads to an improvement in the characteristics of the fuel cell at elevated temperatures (Figure 4b). After a short time operation, a porous anodic and cathodic zone while a relative dense middle layer could be found (Figure S5), which allow rapid gas diffusion and good physical gas separation for the functionality of the fuel cell.

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A deep look into the morphology and texture of the nano metal/alloy exsolved HPSCFN by SEM and TEM confirms that well-distributed nanoparticles with a diameter of 30-40 nm are thermodynamically exsolved on the surface of the perovskite oxide matrix after being treated in a reducing atmosphere at temperature of 900 oC, as shown in Figure 5. This surface morphology is clearly different from those of P-PSCFN and K-PSCFN with a smooth particle surface, as shown in Figure S6a-b, agreed with previous work35. The tiny nanoparticles are indexed to Co-Fe alloy according to the high-resolution EDX mapping element analysis and Co-Fe alloy nanoparticles with calculated atom ratio of Co to Fe of about 1:1 upon intentional reduction are generated as the inset of Figure 5b. The high-resolution TEM image of the H-PSCFN showed a distinct fringe with an interplanar lattice spacing of ~0.202 nm, corresponding to the (110) plane of the Co-Fe alloy (inset of Figure 5), again confirming the formation of Co-Fe alloy in H-PSCFN. Furthermore, the homogeneous distribution of the elements is confirmed by the EDX element mapping analysis for H-PSCFN (Figure S7a-h).

4.

Discussion

In this work, we propose using redox-stable materials based on perovskite oxide as a semiconductor material and composited with a high ionic conductor as key materials for single-layer fuel cells. The phase-stable structure clearly improves the durability of the electrochemical characteristics under real fuel cell conditions compared to the 15



easily-reducible NCAL materials, although the fuel cell performance is sacrificed. The initial results demonstrated a promising direction for the development of a facile but reliable single-layer fuel cell for future deployment.

We can also conclude that the application of a higher electro-catalytically active K2NiF4-type and surface alloy deposited oxide enhances the performance of the fuel cell (Cell IV and V) under identical operating conditions compared to that fuel cell using ABO3 simple oxide (Cell III). This phenomena can be elucidated by the following aspects: 1) The increased ratio between the surface oxygen vacancy/adsorption oxygen content and the lattice oxygen concentration in K-PSCFN and H-PSCFN complex oxides over the single perovskite P-PSCFN oxide, as clarified in Figure 6a-d. Generally, two oxygen peaks are observed in the XPS spectra of O 1s spectrum for all of PSCFN samples, attributed to 528.7 eV and 531.0 eV. The former is corresponded to lattice oxygen, also known as β-oxygen; The latter is indexed to oxygen species adsorbed or loosely bonded oxygen (normally correlated with the surface oxygen vacancies), known as α-oxygen, which is believed to be the active sites for surface adsorbing molecule oxygen41-42. The ratio between the cumulative amount of α-oxygen peak and β-oxygen peak reflecting the relative oxygen vacancy concentration in the sample increases from 0.88 in P-PSCFN to 0.94 in K-PSCFN, then to 1.37 for H-PSCFN sample. The higher of the ratio, the better of the oxygen surface exchange efficiency or oxygen reduction activity is expected43-44. In 16


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particular, the reduction of the P-PSCFN to H-PSCFN leads to a significant increase in the concentration of oxygen vacancies on the oxide surface (Figure 6d), which reinforces the gas exchange coefficient between the gas and the oxide surface42, 45-46, and consequently reduced the electrode polarization resistances as reflected in Figure 3b; 2) The electrical conductivities of H-PSCFN and K-PSCFN are reduced35 compared to P-PSCFN, which accidentally makes a good balance of the electronic and ionic conductivities in the composite, helps to improve the voltage efficiency and the SLFC performance47-48; 3) Possible proton intercalation/conduction in the layerstructured oxide under fuel cell conditions increases the additional ionic conductivity and length of the triple phase boundary of the electrode and subsequently improves the reaction kinetics and performance of the fuel cell49-51. Furthermore, the presence of Co-Fe metal/alloy nanoparticles in the range of 20-30 nm on the surface of perovskite oxide matrix as shown in the Figure 5, is also reflected by the presence of the metal state/zero valance state in Fe 2p and Co 2p XPS spectra in H-PSCFN (Figure 6e-f). Meanwhile, the absence of Co-Fe metal/alloy nanoparticles in the corresponding spectra of P/K-PSCFN samples significantly improves the anodic activity of hydrogen oxidation, as illustrated in Figure S8. This approach, in fact, has been demonstrated as a promising strategy for improving the electrochemical activity of perovskite oxide while maintaining/obtaining the nanostructures at elevated temperatures under extremely harsh fuel cell conditions. The research topic recently has been the focus of the SOFC community10, 38, 44, 46, 52 and other related fields53-54 in 17



recent years. Consequently, these factors co-contribute to enhancing the efficiency of fuel cells. Integrating with the intrinsic chemical and structural stability of the oxide structure, SLFCs based on the aforementioned redox stable perovskite oxides show promising applications towards the next deployment phase.

Another possible role of the precipitated metal/alloy on the semiconducting perovskite oxide matrix, which forms the physical metal-semiconductor interaction, i.e. Schottky junction (SJ) or internal built-in electric field, may help to block the electron transport through the SLFC bulk, in the meanwhile, improve the ionic transport capability through the composite bulk16, 55-56. The former effect, in turn, enhances the open circuit voltage of SLFCs instead of causing the internal short circuit problem as shown in Figure 3a. In order to identify the SJ formation instead of an ohmic contact, an ex-situ check of the voltage bias (-2~2V)-current responses for of Cell IV with KPSCFN-LNSDC composite, Cell V using H-PSCFN-LNSDC composite, Cell VI with NCAL-LNSDC composite in the N2/air, H2/N2, and H2/air (fuel cell) atmospheres at 600 oC were performed. For Cell VI, the rectification response can only be observed in H2/air condition; a nearly linear response of current to voltage is observed in the air (Figure S9a), which clearly follows the Ohmic Law, reflecting no junction response. While for Cell IV and Cell V, a clear rectification response in the I-V characteristics appears as shown in Figure 7a and Figure S9b regardless of the applied atmospheres. In other words, the current increases exponentially with a forward voltage bias, while 18


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the current increases first and then stably maintains with an increased negative voltage bias. In fact, PSCFN was employed as a symmetrical electrode in SOFC57; it possesses both the electron and hole conducting properties which are strongly sensitive to the applied gas atmosphere. In this context, under certain specific conditions of H2/air or H2/N2 and even in a cell atmosphere with oxygen concentration (N2/air), a bulk heterojunction (p-n) can be formed in-situ as illustrated in the right part of Figure 7b to build the fuel cell electric field, and consequently to relieve the internal short-circuit, similar to the photovoltaic effect based on the p-n junction working principle18, 25. The linear response of Cell V (not in Cell IV) under negative bias and H2/air condition may be caused by the dissolution of alloy nanoparticle in H-PSCFN upon the negative voltage bias10, which reduces the junction effect. Furthermore, the rectification response still appears under the N2/air atmosphere, which might be caused by another physical junction effect, the metal/semiconductor contact or the Schottky junction/barrier (Figure 7b, left). The presence of oxygen partial pressure difference and the remained surface Co-Fe alloy nanoparticles/semiconducting K-PSCFN in air at or less than 600 oC58 helps to overcome the electronic conducting short-circuit issue of SLFC even without an anodic reducing atmosphere to form the traditional p-n junction.

The distinct response of the alloy nanoparticle deposited H-PSCFN over cell VI, even in the air atmosphere, supports the formation of Schottky contact rather of ohmic 19



contact at alloy nanoparticle-semiconductor interface16, 59, plus the bulk p-n junction formed under a specific atmosphere which reinforces the charge separation to prevent the electronic conduction through the bulk. Integrating with improved electrode reaction activity or kinetics and charge separation capability allows the Cell V to obtain the best OCVs and electrical efficiency among all samples. Based on the experimental observation and analysis, an illustration of a possible working principle is proposed in Figure 7b. A gas responded metal-semiconductor interface or Schottky junction is in-situ formed on the fuel side, which combines with the formed bulk ptype (PSCFN in oxygen atmosphere)/n-type (PSCFN under hydrogen gas) junction in a specific H2/air fuel cell atmosphere to block the electronic transport through the composite bulk or the internal of SLFC, leading to the enhanced open circuit voltage or providing a solution to the electronic short-circuit problem. Consiquently, SLFCs based on mixed ionic and electronic conductors exhibit the same functionality as a conventional fuel cell with an electronic insulating electrolyte layer.

5.

Conclusions

In this work, the phase structured redox stable perovskites and their derivatives are employed to replace conventional lithiated metal oxides as key semiconducting materials designed for reliable single-layer fuel cells. Though their fuel cell performances are lower than in the latter case, the stability is significantly improved. In particular, Cell III with P-PSCFN semiconductor and electrocatalyst showed a 20


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durable performance for in more than two days constant voltage testing, which takes the first step toward reliable SLFC technology. Improved electrochemical performances are also achieved via regulating oxygen vacancies and metal/alloy nanoparticles exsolution. The generated Schottky junction, i.e. metal/semiconductor contact, combining with in-situ formed p-n junction not only promotes the anode hydrogen oxidation reaction kinetics, but also benefits the charge separation to improve the voltage efficiency and the ionic transport through the internal built electric fields. Those reduce the ohmic loss and electrode reaction polarization resistance simultaneously, consequently enabling the best electrical efficiency. This work also acts as the first attempt focusing on the stability of the SLFCs for practical application. The further development of a phase-structure-stable while higher electroactivity semiconductor and an ionic conductor, and a study of the impact of content of the precipitated nanoalloy/metal on the fuel cell performance for higher electrochemical efficiency and extended durability are highly desired. These issues as well as the elucidation of the underlying mechanism of their potential degradation in SLFC under the real fuel cell conditions will be investigated and introduced in the future work.

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ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX, The following files are available free of charge.

Material characterization: RT-XRD patterns of P-PSCFN, K-PSCFN, H-PSCFN and their composite with LNSDC after treated at 700oC for 2 hours in air; SEM images of P-PSCFN, K-PSCFN, H-PSCFN, LNSDC, NACL, and reoxidized H-PSCFN in air at 600 oC for 3 hours, and cross-section of SLFC after testing; EDX elements (survey, O, Pr, Sr, Co, Fe, Nb) mapping of H-PSCFN; XPS high-resolution spectra of Co 2p and Fe 2p of L-PSCFN and P-PSCFN; H2-TPR of P-PSCFN (0-1000oC); The applied equivalent circuit model for EIS fitting; and voltage-current curves of NCAL-LNSDC and K-PSCFN-LNSDC based single layer under different atmospheres to identify the possible metal-semiconductor junction

Electrochemical Impedance spectroscopy fitting results using the equivalent circuit model of Ro/R1(CPE1)/R2(CPE2) with the assistance of ZView software.

AUTHOR INFORMATION

Corresponding Author * Email: [email protected] 22


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Author Contributions The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

Financial support from the following agencies and institutes: Natural Science Foundation of Guangdong Province (2017A030313289), Shenzhen Government’s Plan of Science and Technology (No. JCYJ20170302141158010 and JCYJ20180305125247308) and National Natural Science Foundation of China (No. 21706162 and 51402093) are acknowledged. The authors also thank the Instrumental Analysis Center of Shenzhen University (Xili Campus) for help.

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Figure captions Figure 1. Electrochemical performance of SLFCs using NCAL as semielectrocatalyst and LNSDC as the ionic conductor at 550 oC (a) steady-state polarization curve, (b) electrochemical impedance spectroscopy at OCV condition, and (c) current response under a constant voltage of 0.7 V in H2/air atmosphere. Figure 2. (a) I-V and I-P polarization curves and (b) impedance curves of fuel cells with a configuration of NCAL/PSCFN-LNSDC/NCAL cell 550 oC, and (c) chronopotentiometric stability tests of the corresponding cell at 0.7 V. Figure 3. (a) Cell voltage and power density as a function of current density, (b) Impedance spectroscopy under open circuit voltage condition and (c) short-term stability under constant voltage of 0.6 V of SLFCs (Cell III~V) with P/K/HPSCFN semiconductor electrocatalyst and LNSDC ionic conductor under fuel cell atmosphere at 600 oC. Figure 4. Temperature dependence of the Cell V’s electrochemical performances (a) polarization curves and (b) corresponding impedance spectra response. Figure 5. (a) SEM and (b) High-resolution TEM image of H-PSCFN material. Inset is the EDX element analysis. Figure 6. High-resolution XPS spectra of O 1s spectrum (a) P-PSCFN, (b) K-PSCFN and (c) H-PSCFN and (d) the ratio of deconvoluted surface oxygen vacancy to lattice oxygen concentration, and of (e) Co 2p and (f) Fe 2p spectra of H-PSCFN sample.

33



Figure 7. (a) The response current as a function of bias voltage for the H-PSCFN based SLFCs (Cell V) in air, H2/N2 and H2/air condition, respectively and (b) the proposed SLFC working principle from the physical junction aspect, including bulk pn junction (right) and metal-semiconductor Schottky barrier (left) illustrations.

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Tables and captions Table 1. Fuel cells and their compositions used in this work Cell Number I II III IV V VI

Cell configuration and composition Nickel foam-NACL/3NCAL-7LNSDC/Nickel foam-NACL Nickel foam-NACL/3(P-PSCFN)-7LNSDC/Nickel foam-NACL 3(P-PSCFN)-7LNSDC 3(K-PSCFN)-7LNSDC 3(H-PSCFN)-7LNSDC 3NCAL-7LNSDC

Table 2. Summary of the electrochemical performances (Open circuit voltage, peak power density, ohmic and electrode polarization resistance and fuel cell short-term stability) of SLFCs with various semiconducting materials at 600 oC. Pmax Cell Semiconductor OCV (mWcm-2) III P-PSCFN 0.93 77.60 IV K-PSCFN 0.95 130.47 V H-PSCFN 0.97 161.80

Ro

Rp

1.10 0.71 0.62

1.19 1.22 0.95

35


R

total

2.29 1.93 1.57

Durability 45/52hs 7.5hs 24.5hs


Table Of Contents (TOC)

Fuel cells: An integration of electrochemistry and semiconducting physics through nanoalloy exsolution enables durable and highly efficient single-layer fuel cell

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Figure 1. Electrochemical performance of SLFCs using NCAL as semi-electrocatalyst and LNSDC as the ionic conductor at 550 oC (a) steady-state polarization curve, (b) electrochemical impedance spectroscopy at OCV condition, and (c) current response under a constant voltage of 0.7 V in H2/air atmosphere. 177x84mm (300 x 300 DPI)



Figure 2. (a) I-V and I-P polarization curves and (b) impedance curves of fuel cells with a configuration of NCAL/PSCFN-LNSDC/NCAL cell 550 oC, and (c) chrono-potentiometric stability tests of the corresponding cell at 0.7 V. 160x64mm (300 x 300 DPI)


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Figure 3. (a) Cell voltage and power density as a function of current density, (b) Impedance spectroscopy under open circuit voltage condition and (c) short-term stability under constant voltage of 0.6 V of SLFCs (Cell III~V) with P/K/HPSCFN semiconductor electrocatalyst and LNSDC ionic conductor under fuel cell atmosphere at 600 oC. 90x109mm (300 x 300 DPI)



Figure 4. Temperature dependence of the Cell V’s electrochemical performances (a) polarization curves and (b) corresponding impedance spectra response. 90x90mm (300 x 300 DPI)


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Figure 5. (a) SEM and (b) High-resolution TEM image of H-PSCFN material. Inset is the EDX element analysis. 90x132mm (300 x 300 DPI)



Figure 6. High-resolution XPS spectra of O 1s spectrum (a) P-PSCFN, (b) K-PSCFN and (c) H-PSCFN and (d) the ratio of deconvoluted surface oxygen vacancy to lattice oxygen concentration, and of (e) Co 2p and (f) Fe 2p spectra of H-PSCFN sample. 165x193mm (300 x 300 DPI)


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Figure 7. (a) The response current as a function of bias voltage for the H-PSCFN based SLFCs (Cell V) in air, H2/N2 and H2/air condition, respectively and (b) the proposed SLFC working principle from the physical junction aspect, including bulk p-n junction (right) and metal-semiconductor Schottky barrier (left) illustrations. 119x168mm (300 x 300 DPI)


Approaching Durable Single-Layer Fuel Cells: Promotion of

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