Natural Mineral-Based Solid Oxide Fuel Cell with Heterogeneous

Aug 2, 2016 - Natural Mineral-Based Solid Oxide Fuel Cell with Heterogeneous Nanocomposite Derived from Hematite and Rare-Earth Minerals ... Hubei Col...
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Natural Mineral-Based Solid Oxide Fuel Cell with Heterogeneous Nanocomposite Derived from Hematite and Rare-Earth Minerals Chen Xia,†,‡ Yixiao Cai,†,⊥ Yue Ma,*,§ Baoyuan Wang,†,‡ Wei Zhang,‡ Mikael Karlsson,⊥ Yan Wu,*,∥ and Bin Zhu*,†,‡ †

Department of Energy Technology, Royal Institute of Technology (KTH), Stockholm SE-10044, Sweden Hubei Collaborative Innovation Center for Advanced Materials, Faculty of Physics and Electronic Technology, Hubei University, Wuhan 430062, People’s Republic of China § Ångström Advanced Battery Centre (ÅABC), Department of Chemistry and ⊥Department of Engineering Sciences, Uppsala University, Ångström Laboratory, Uppsala SE-751 21, Sweden ∥ Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Solid oxide fuel cells (SOFCs) have attracted much attention worldwide because of their potential for providing clean and reliable electric power. However, their commercialization is subject to the high operating temperatures and costs. To make SOFCs more competitive, here we report a novel and attractive nanocomposite hematite− LaCePrOx (hematite−LCP) synthesized from low-cost natural hematite and LaCePr-carbonate mineral as an electrolyte candidate. This heterogeneous composite exhibits a conductivity as high as 0.116 S cm−1 at 600 °C with an activation energy of 0.50 eV at 400−600 °C. For the first time, a fuel cell using such a natural mineral-based composite demonstrates a maximum power density of 625 mW cm−2 at 600 °C and notable power output of 386 mW cm−2 at 450 °C. The extraordinary ionic conductivity and device performances are primarily attributed to the heterophasic interfacial conduction effect of the hematite−LCP composite. These superior properties, along with the merits of ultralow cost, abundant storage, and eco-friendliness, make the new composite a highly promising material for commercial SOFCs. KEYWORDS: SOFCs, heterogeneous nanocomposite, natural hematite, rare-earth LCP-carbonate mineral, interfacial conduction

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INTRODUCTION

dramatic loss in ionic conduction of the electrolyte and power density of the cell. One efficacious approach to improving SOFC technology is to introduce advanced electrolyte materials that can function at low temperatures while still maintaining a desirable ionic conductivity.9 Recently, a composite material strategy has been proposed to develop electrolytes for SOFCs,10−13 especially for low-temperature SOFCs (LT-SOFCs). For instance, core−shell Sm doped ceria (SDC)/carbonate nanocomposite,14 oxide− carbonate/oxide composite,15,16 and Sm doped CeO2 (SDC) nanowires based nanocomposite17 with high ionic conductivity (around 0.1 S cm−1 above 300 °C) have been developed as electrolyte materials in fuel cells performed at 300−600 °C. A nanocomposite composed of La/Pr doped ceria (LCP-oxide) and LiNa-carbonate has achieved ionic conductivity above 0.1 S cm−1 at 600 °C.18 From these investigations, it becomes

As a green energy technology, solid oxide fuel cells (SOFCs) have experienced phenomenal developments in recent years owing to their high conversion efficiency and minimal pollution emissions as compared to traditional thermal power systems.1,2 Nevertheless, the high efficiency of SOFCs requires excessively high operating temperature and costly engineering supports. Yttria-stabilized zirconia (YSZ) is one of the most common electrolyte materials for SOFCs because of its superior chemical stability and excellent oxygen ionic conductivity.3,4 Unfortunately, present YSZ fuel cells suffer from high temperatures (800−1000 °C) necessary to raise the ionic conductivity of the electrolytes to a suitable value, narrowing their application to stationary power generators. Lowering the operating temperatures of SOFCs would reduce engineering consumption and improve thermal stability,5−8 which is able to accelerate the commercialization of SOFCs. Tremendous efforts have been made to lower SOFC operating temperatures; however, the reduced temperature has been inevitably accompanied by a © 2016 American Chemical Society

Received: May 15, 2016 Accepted: July 20, 2016 Published: August 2, 2016 20748

DOI: 10.1021/acsami.6b05694 ACS Appl. Mater. Interfaces 2016, 8, 20748−20755

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were mixed and added to deionized water to form a red solution, followed by vigorous stirring and constant heating at 60 °C. During the stirring and heating process, dilute nitric acid with the molar concentration of 0.5 mol L−1 was added dropwise until the final pH value of the solution was adjusted to 6, after which the solution was further stirred for 0.5 h. The obtained precursor solution was then dried at 300 °C to form a red gel and calcined at 800 °C for 4 h. The resultant bulk materials were ground thoroughly in an agate mortar to obtain hematite−LCP powders. The raw hematite (LW) was exploited from the Laiwu (LW) zone in northeastern China. The raw LCPcarbonate was purchased from a rare-earth company (Baotou, Inner Mongolia, China). The cathode material, La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF), was purchased from Sigma-Aldrich, Sweden. The anode material, commercial Ni0.8Co0.15Al0.05LiO2−δ (NCAL) was purchased from Tianjin Bamo Sci.&Tech. Joint Stock Ltd., China. Nitric acid (HNO3, 65%) was obtained from PROLABO-VWR-BDH, France. Basic Characterization. The crystal structures were determined by a Bruker D8 Advance X-ray diffractometer (Germany, Bruker Corp.) with Cu Kα radiation (λ = 1.54060 Å) as the source operating at 45 kV and 40 mA. The morphologies, microstructures, and energy dispersive spectroscopies of the samples were investigated by a JSM7100F field emission scanning electron microscope (FE-SEM, Japan) that was equipped with an energy dispersive spectrometer (EDS) operating at 15 kV. High-resolution transmission electron microscopic (HR-TEM) images of the samples were obtained on a JEOL JEM-2100F field-emission microscope operating under an accelerating voltage of 200 kV. Fuel Cell Construction. Fuel cell devices based on hematite−LCP were assembled by a dry pressing procedure, which involves loading a mold with the anode powder (a mixture of NCAL and hematite−LCP in a weight ratio of 6:4) successively followed by the hematite−LCP electrolyte and finally the cathode powder (a mixture of LSCF and hematite−LCP in a weight ratio of 6:4), all being assembled in one step. The material was then sintered at 600 °C for 1 h. The obtained cylindrical fuel cell has an active area of 0.64 cm2 and is 2.5 mm in thick. Finally, silver pastes were painted onto the surfaces of the anode and the cathode as the current collectors for fuel cell performance measurements. The fuel was pure hydrogen stream supplied to the anode, while the cathode was supplied with air as oxidant. Electrochemical Measurements. The electrochemical impedance spectra (EIS) measurements of the samples were performed using an electrochemical workstation (CHI660B, Cheng Hua Corp.). The symmetrical pellet Ag electrode/hematite−LCP/Ag electrode was tested at temperatures from 400 to 600 °C in H2/air, with pure hydrogen (99.999 vol % H2) as fuel and stationary air as oxidant. The applied frequency range was from 0.01 Hz to 1 MHz, and the signal amplitude was 10 mV. The measurement was performed under an open circuit voltage (OCV) mode. The electronic conductivities of the samples were measured by means of dc polarization method using a Hebb−Wagner ion-blocking cell, Pt(foil)/hematite−LCP/Pt(foil). The measurements were performed with a digital microohm meter (KD2531, Kangda Electrical Co., Ltd.). When a voltage was applied to the cell, the oxygen chemical potential at the inner Pt/sample interface was reduced until the steady state was established. In the equilibrium state, the O2− was blocked at the inner Pt/sample interface because there was no electrochemical potential gradient of O2− inside the composite. Therefore, the measured current should be electronic current. The oxygen partial pressure at the inner electrode is about 102 Pa controlled by applying voltage on the cell. The fuel cell performances were measured using a computerized instrument in the temperature range 450−600 °C. The fuel was pure hydrogen stream supplied to the anode at a flow rate of 100−120 mL min−1, while the cathode was supplied with air as oxidant at the same flow rate. Cell voltage and current were collected under a programmable electronic load (ITECH8511, ITECH Electrical Co., Ltd.).

evident that these composite electrolytes possess better ionic conductivity property than typical electrolytes YSZ and SDC, accompanied by a unique simultaneous H+/O2− conduction property.19 Another effective approach to improve SOFC technology and accelerate its commercialization is to utilize some costeffective materials with high ionic conductivity. Natural material is one type of ultra-low-cost material that has received much attention in recent decades.20−26 Numerous natural minerals have been used from industrial processes to household products.20,21 Hematite (α-Fe2O3) is an easily processable, nontoxic, and stable material that has been widely explored as a promising material for photoelectrochemical (PEC) water splitting,22,23 lithium ion batteries,24,25 as well as carbon capture and storage technology.22,26 Our recent work first put forward a strategy of utilizing natural hematite as the electrolyte layer in SOFC,27 opening up the application of hematite mineral in the solid state ionic field. Meanwhile, rare-earth minerals have also received considerable attention in past years due to their widespread applications in lighting industries and for catalysts.28−30 The industrial-grade rare-earth mineral LaCePrcarbonate (LCP-carbonate), as an abundant natural resource, has been capitalized to produce rare-earth LaCePr oxides (LCP-oxides) for uses in SOFCs.18 Albeit the power outputs of hematite based fuel cell and LCP-oxide based fuel cell are not sufficiently persuasive, these preliminary achievements still inspire us to exploit more potential of the natural materials. In the present work, with the purpose of developing advanced natural mineral based SOFCs, we combine the aforementioned two approaches to design a radically new heterogeneous material, hematite−LaCePr-oxide (hematite− LCP), by using natural hematite ore and industrial-grade rareearth mineral as the reactants. The synthesized hematite−LCP materials are evaluated as the electrolyte layers in LT-SOFCs.

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EXPERIMENTAL SECTION

Materials and Synthesis. Hematite−LCP was prepared using hematite ore and LCP-carbonate through an acid-treatment approach followed by calcination process. The natural hematite ore utilized in our experiment, designated as “hematite (LW)”, was exploited from the Laiwu (LW) zone in northeastern China. Its chemical component mainly consists of Fe2O3 and SiO2 (Table S1 in the Supporting Information). It belongs to a skarn-type iron deposit of the late Yanshan epoch, with an exploration reservation in excess of 500 million tons.20 The LCP-carbonate is an industrial-grade rare-earth mineral obtained from Inner Mongolia, China. Both the hematite ore and LCP-carbonate were used in crudeness without further purification. Figure 1 schematically illustrates the preparation process. The hematite ore and LCP-carbonate were processed by dilute nitric acid and then calcined at 800 °C to obtain hematite−LCP composite. In a typical fabrication procedure, raw hematite ore and LCPcarbonate powders with various weight ratios (7:3, 6:4, 5:5, 4:6, 7:3)

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RESULTS AND DISCUSSION The raw powder of hematite ore exhibits a russet color, indicating the major composition of Fe2O3 in the powder, as

Figure 1. Schematic for the preparation process of hematite−LCP from raw hematite (LW) ore and LCP-carbonate. 20749

DOI: 10.1021/acsami.6b05694 ACS Appl. Mater. Interfaces 2016, 8, 20748−20755

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chemical reactions and consequently improve the ionic conduction.32 To gain further insight into elemental compositions and distributions of the samples, EDS (Figure S3) and elemental mapping measurements were performed. Upon the EDS analysis, the hematite (LW) is confirmed to be composed of Fe, Si, Ca, O, and minor elements Al, Mg, and C, which derived from the impurities, while the EDS result of LCP-oxide reveals the presence of Ce, La, Pr, and O. Figure 3 gives the elemental

confirmed by X-ray diffraction (XRD) analyses in Figure 2a. The diffraction peak pattern of the hematite (LW) is identified

Figure 2. (a) XRD patterns of hematite (LW), LCP-oxide, and hematite−LCP composite. SEM images of (b) natural hematite (LW), (c) LCP-oxide, and (d) hematite−LCP composite.

as hexagonal structure Fe2O3 (JCPDS 79-1741), hexagonal structure SiO2 (JCPDS 83-2465), and CaCO3 (JCPDS 830578), suggesting the coexistence of iron oxide, quartz, and calcite in this natural material. Apart from these, there are several traces of unidentified peaks. For comparison, the LCPoxide powder produced by sintering the LCP-carbonate at 800 °C for 4 h was also characterized by XRD, as shown in Figure 2a. All the diffusion peaks of the LCP-oxide are indexed to the same cubic fluorite structure as that of SDC, slightly shifted to lower 2θ compared to SDC (Figure S1). The result is consistent with previous studies about LCP-oxide, a type of La3+ and Pr3+ codoped CeO2 material, and it can work as an ionic conducting electrolyte.18 Based on XRD analysis, the prepared hematite−LCP is found to be a heterogeneous composite consisting of major components Fe2O3, SiO2, and LCP-oxide. The calcite (CaCO3) in the original hematite (LW) was completely dissolved in acidic solution during the preparation step. These results also confirm that there is no chemical interaction among LCP-oxide, Fe2O3, and SiO2 during the composite calcination process. Figure 2b shows the particle morphology of the hematite (LW), including some particles with an average diameter of around 200 nm and some micrometer-sized agglomerations. Figure 2c shows the nanosized and micrometer-sized particles of the LCP-oxide with irregular shapes. Interestingly, these two raw materials partially exhibit nanoscale dimensions without any elaborate synthetic routes. In Figure 2d, the prepared hematite−LCP composite displays more uniform and finer particles. The average particle size is around 100 nm (Figure S2). As is observed, these particles are composed of smaller particles and crystalline grains (Figure S2). It is known that the morphology and particle size of the resultant samples can be modulated by the preparation parameters, especially the sintering temperature and reaction time.31 Because of the 4 h calcination at high temperature, the particles are recrystallized to reach a more homogeneous distribution. Generally, nanocomposite materials with nanoscale and evenly distributed particles are expected to offer more active sites for electro-

Figure 3. (a) FE-SEM micrograph of the as-prepared hematite−LCP composite. (b) Fe, (c) Si, (d) Ce, (e) La, and (f) Pr elemental mapping for hematite−LCP.

mapping inspection of the hematite−LCP composite. Nearly all these major elements are detected and show even distributions except for Si, which shows a somewhat occasional maldistribution with several big aggregations of SiO2. However, it still displays a pretty uniform distribution of Fe2O3, SiO2, and LCPoxide particles in the sample in general, which provides a basis for constructing continuous networks and paths for ion transport. Electrochemical impedance spectroscopic (EIS) measurements were undertaken to examine the electrical properties of the new composite material. The EIS measurements were made at temperatures from 400 to 600 °C in H2/air atmosphere. Typical impedance spectra of the cell (with the hematite−LCP prepared by 60 wt % hematite and 40 wt % LCP-carbonate) acquired at 500, 550, and 600 °C and the corresponding equivalent circuit model are shown in Figure 4a. For a comparative study, EIS measurements for an additional two pellets based on hematite (LW) and LCP-oxide of the same size were also measured at 600 °C in H2/air, respectively, and illustrated in Figure 4b. Generally, impedance plots of solid ionic conductors show three contributions, two of which are often one arc each, with the small arc at high frequencies 20750

DOI: 10.1021/acsami.6b05694 ACS Appl. Mater. Interfaces 2016, 8, 20748−20755

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The total resistance of electrolyte is given by Rt = Rb + Rgb. Thus, the total electrical conductivities (σt) of hematite−LCP can be derived by Rg and Rgb obtained from EIS.36 In order to separate the contributions from different charge carriers, a Hebb−Wagner dc polarization test was conducted to investigate the electronic conductivity (σe) of the hematite− LCP prepared by 60 wt % hematite and 40 wt % LCPcarbonate, which is plotted in Figure 5a along with the calculated σt results. The σt is from 0.019 to 0.117S cm−1 at 400−600 °C, and the σe is around 10−3 S cm−1 in the same temperature range. It can be found that the electronic conductivity is 2 orders of magnitude lower than the total conductivity, which should be owing to the noticeable amount of insulators originating from hematite (LW) that block the electron flow, such as, SiO2, Al2O3, and MgO. Therefore, the electronic conductivity is negligible as compared with the total conductivity, and the composites can be considered as an ionic conductor in the oxygen-rich atmosphere. By this means, the ionic conductivity (σi) can be determined according to σi = σt − σe. As a result, the obtained σi of hematite−LCP reaches 0.116 S cm−1 at 600 °C, exhibiting a value almost 1 order of magnitude higher than that of LCP (Figure S4). Quite significant here is the fact that our hematite−LCP composite achieved a dramatically enhanced σi compared with either original material, after a quite facile fabrication process. Furthermore, the activation energy of the hematite−LCP composite electrolyte is calculated by plotting ln(σT) against 1000/T according to the Arrhenius equation, σT = A exp[−Ea/ (kT)], where T is the absolute temperature, A is a preexponential factor, Ea is the activation energy, and k represents the Boltzmann constant. Figure 5b displays the values of activation energies in a form of linearly well-fitted results, comparing the Ea of our hematite−LCP with those of other well-known and typical ionic conductors. The resultant hematite−LCP exhibits a higher ionic conductivity than reported results of Gd-doped CeO2 (GDC, 0.025 S cm−1 at 600 °C),37 Sr- and Mg-doped LaGaO3 (LSGM, 0.03 S cm−1 at 600 °C),38 and YSZ (0.005 S cm−1 at 600 °C).39 Moreover, it reveals a low activation energy Ea with a value of 0.50 eV at 400−600 °C, superior to those of GDC, LSGM, and YSZ. It is believed that the facile migration of oxide-ion vacancies with a zigzag pathway in the basal plane is one of the origins for low activation energy.40 The Ea of the hematite−LCP is not merely ahead of those of typical oxide ion conductors, but also is lower than those of several other reported doped ceria electrolytes (0.55−0.8 eV above 400 °C).41,42 Since low-temperature (LT 450−650 °C) operation can potentially provide a better stability and lower costs due to reduced thermal and chemical stresses and a wider range of material choices, as well as faster start-up times for portable uses,43 these results reveal the enormous potential of our new material for commercial SOFC application. Figure 6 illustrates the electrochemical performances of the fuel cells based on hematite−LCP electrolytes. To optimize this composite electrolyte, we adjusted the weight ratios of the original reactants hematite (LW) and LCP-carbonate, and investigated the composition dependent properties of hematite−LCP. Five samples made by various compositions, 7:3, 6:4, 5:5, 4:6, and 3:7 (hematite:LCP-carbonate), were utilized as electrolytes for fuel cell measurements (Figure S5). The best performances for each fuel cell tested at 550 °C are compared in Figure 6a, including OCV and highest power density. The result manifests that 60 wt % amount of hematite (LW) is the

Figure 4. (a) Impedance spectra of hematite−LCP acquired in H2/air atmosphere and corresponding equivalent circuit. (b) Impedance spectra of hematite (LW) and LCP-oxide in H2/air at 600 °C.

attributed to bulk material behavior, and that at intermediate frequencies attributed to grain-boundary behavior. The third process at low frequencies is due to the electrode polarization behavior.33,34 The measured impedance spectra are fitted to an equivalent circuit of parallel RC circuits, in which Rb, Rgb, Rct, CPEgb, and CPEct stand for bulk resistance, grain boundary resistance, electrode resistance, constant phase element (CPE) of the grain boundary, and CPE of the electrode, respectively.33 The simulated Rb and Rgb values of the three samples are displayed in Table 1. An intuitive comparison indicates that Table 1. Equivalent Circuit Analysis Results of Hematite (LW), LCP-Oxide, and the Prepared Hematite−LCP Composite at 600 °C sample (600 °C)

Rb (Ω cm2)

Rgb (Ω cm2)

hematite (LW) LCP-oxide hematite−LCP

1.99 2.13 2.15

3.06 11.94 0.42

hematite−LCP has a far smaller Rgb than those of hematite (LW) and LCP-oxide, displaying a significantly reduced value from single phase hematite (LW) and LCP-oxide (3.06 and 11.94 Ω cm2, respectively) to hematite−LCP composite (0.42 Ω cm2). This manifests that the migration of ions at the grain boundary is far less resistive in the composite material. It is probably caused by interfacial conduction behavior at oxide interfaces.35 The result reveals a remarkable superiority of hematite−LCP composite over hematite (LW) and LCP-oxide. By contrast, the Rb differences among the three samples are entirely insignificant. 20751

DOI: 10.1021/acsami.6b05694 ACS Appl. Mater. Interfaces 2016, 8, 20748−20755

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Figure 5. (a) Temperature dependence of total conductivity and electronic conductivity for as-prepared hematite−LCP and (b) conductivity comparison of hematite−LCP with other well-known oxide ion conductors.

Figure 6. (a) OCV and power density values for fuel cells based on hematite−LCP electrolyte with different compositions. (b) Current density− voltage and power density characteristics for fuel cell using the optimal hematite−LCP electrolyte at various temperatures.

best composition for preparing hematite−LCP electrolyte. The corresponding fuel cell demonstrates a peak power output of 530 mW cm−2. As is studied above, the electronic conductivity of the hematite−LCP prepared by 60 wt % hematite (LW) and 40 wt % LCP-carbonate is around 10−3 S cm−1. Thereby, the corresponding fuel cell can obtain an OCV of 0.94 V at 550 °C without short circuit. Nearly all of the fuel cells reach OCVs higher than 0.91 V, except the cell using the composite prepared by 30 wt % hematite (LW) and 70 wt % LCPcarbonate, which displays an OCV of 0.884 V. This is chiefly because, with so little hematite (LW) in the composite, the insulator content is deficient to block the partial electron transport. Based on the above investigation, the fuel cell using the optimal hematite−LCP electrolyte was further evaluated at various temperatures from 450 to 600 °C. As shown in Figure 6b, the cell delivers a peak power density of 386 mW cm−2 at lower temperature 450 °C and a 1.6-fold enhanced power output of 625 mW cm−2 at 600 °C. We also note that the OCV ranges from 0.97 to 0.92 V when operation happens at 500, 550, and 600 °C, but it decreases to 0.87 V at 450 °C. That may be attributed to the poor catalytic activity of hematite−LCP at low temperatures. The cell performance demonstrates equal OCV but much higher power output compared with the former work of hematite (LW) based fuel cell.27

In this work, high ionic conductivity and considerable cell performance were observed at low operating temperatures. Interestingly, these attractive findings were obtained by these cost-effective materials through a quite simple process. In the SOFC field, it is very common to develop ceria-composite electrolytes by using a second phase material to create interfaces for enhancing the ionic conduction or overcoming the electronic conduction.44 As a fact that our composite is composed of multiphase materials, and its grain boundary resistance is far smaller than those of the single-phased individual source materials, it can be speculated that the ionic conduction at heterophase interfaces is one of the primary reasons that leads to the high conductivity. Therefore, the microstructures of the interfacial regions need deeper inspection. Further investigation of the hematite−LCP microstructure was conducted via transmission electron microscopy (TEM). Figure 7a shows that the hematite−LCP nanoparticles are homogeneously distributed and intimately attached to each other. All the nanoparticles are well-crystallized with distinct lattice fringes (Figure S6). The high-resolution TEM (HRTEM) in Figure 7b shows three typical crystalline domains with interplanar spacings of about 0.33, 0.25, and 0.32 nm, corresponding to the (111) planes of La/Pr doped CeO2 with cubic fluorite structure, the (110) crystal plane of α20752

DOI: 10.1021/acsami.6b05694 ACS Appl. Mater. Interfaces 2016, 8, 20748−20755

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of impurities does not degrade the composite conductivity and fuel cell performance. Figure 8 further depicts the ionic conduction mechanism in hematite−LCP. In addition to the interfacial conduction,

Figure 7. (a) TEM image of as-prepared hematite−LCP nanocomposite powder. (b) Typical HR-TEM image of hematite−LCP nanocomposite showing the phase boundaries (interfaces) among different crystal grains of LCP-oxide, Fe2O3, and SiO2.

Fe2O3, and the (110) crystal plane of SiO2, respectively. It authenticates the presence of Fe2O3, SiO2, and LCP-oxide in our composite. More importantly, it identifies the phase boundaries among different crystal grains. The HR-TEM results clearly present the interfaces generated at various biphasic boundaries, Fe2O3/LCP, SiO2/LCP, and Fe2O3/SiO2. We believe that this structural feature is strongly related to the enhanced ionic conduction. In solid-state ionics, the effective enhancement of ionic conductivity for solid oxides by manipulating the multiphasic nanostructures has been recognized for more than a decade. It is widely accepted that fast ionic transport pathways can be generated at the interfaces due to the overlap of space charge regions.45−47 Maier made a comprehensive study on the interfacial conduction effect using a salt−oxide composite as an example.45 Recent studies highlighted the important roles of this effect in achieving superior electrochemical properties in a conductive composite consisting of an electronic conductor and an insulator.48 Some studies also reported that ionic conduction can be facilitated by mixing an ionic conductor with an insulator.49−52 This phenomenon also happened in a composite of an ionic conductor and a semiconductor.53 It has been shown in some other works during past decades that ionic transport properties were drastically changed at particle interfaces.54,55 In nanocrystalline samples especially, this interfacial effect is dominant in overall ionic transport.47 Our hematite−LCP composite is proved to be a heterogeneous material consisting of an ionic conductor, a semiconductor, and an insulator. The SEM, EDS mapping, and TEM results have confirmed its unique structure characteristics of heterogeneity, nanoscale dimension, and plenty of interfaces. Thus, the heterointerfacial regions formed among LCP-oxide, Fe2O3, and SiO2 can construct massive highways and a continuous network for ions and thus effectively facilitate the ionic conduction. The interfaces also have significant effects on the ionic migration energy as reported.53 It may change the activation energy for oxygen migration in our composite, resulting in a low Ea of hematite−LCP. Additionally, the importance of impurities such as Al2O3 and MgO should not be overlooked in view of their large content in the hematite−LCP composite (Table S1, Figure S3a). On one hand, they can block electrons passing through to prevent short circuit. On the other hand, they can cause ionic conduction improvement in the composite through an interfacial effect, especially when mixed with an ionic conductor, as reported in YSZ−MgO and LiBr−Al 2 O 3 composites.13,52 This helps to explain why such a large amount

Figure 8. Schematic of the ionic transport process in hematite−LCP.

particular attention is paid to the redox reactions at interfaces. As mentioned in the study of hematite (LW), the Fe2O3 in the material can be easily reduced to oxygen-deficient Fe2O3−δ and form oxygen vacancies on the H2 side, and reoxidized to Fe2O3 under the air circumstance to complete the full Fe3+ to Fe2+ redox cycle.27 In our hematite−LCP composite, the nanoparticle surface is more active than that in the bulk of hematite (LW); thus the redox process can be promoted at nanoparticle surfaces and interfaces. The procreant oxygen vacancies may lead to oxygen ion conduction, as is the case for YSZ and SDC,56 and improve the total ionic conductivity of the composite. Meanwhile, proton transport may also be facilitated through the oxygen vacancies as is the case reported in perovskite oxides such as doped BaCeO3, BaZrO3, and LaGaO3.48,57,58 Additionally, the Ce4+ to Ce3+ reduction process of LCP-oxide in the reducing atmosphere should also be emphasized. Considering the nature of both nanoceria and other constituents, there is another possible happening to LCPoxide that hydrogen or proton could coexist with the surface Ce4+, which has been proposed to explain the proton conduction in mixed ionic and electronic conductor composite.53 Thus, the Ce4+ at surfaces or interfaces may be reduced to Ce3+ without forming the oxygen vacancy. H+ could present as interstitial impurities to maintain the charge compensation for neutrality. In this case, Ce4+ is balanced by Ce3+ and H+. It is very likely that such a process may be motivated on the surfaces of the doped CeO2 nanoparticles and transport at the interfaces,59 as schematized in Figure 8. Similarly, proton may be generated with the Fe2O3 in the reducing atmosphere. In this regard, proton conduction is feasible in the hematite−LCP electrolyte layer. Due to the fact that proton conduction essentially requires low activation energy, the proton conduction in hematite−LCP is able to make this nanocomposite more competent for LT operation.

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CONCLUSION In summary, the hematite−LCP material prepared from natural hematite (LW) and rare-earth mineral is developed and used as the electrolyte for LT-SOFCs. It exhibits a high ionic conductivity (0.116 S cm−1 at 600 °C) and low activation energy (0.50 eV at 400−600 °C). The fuel cells with hematite− LCP electrolyte demonstrate remarkable performances of 625 20753

DOI: 10.1021/acsami.6b05694 ACS Appl. Mater. Interfaces 2016, 8, 20748−20755

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ACS Applied Materials & Interfaces mW cm−2 at 600 °C and 386 mW cm−2 at 450 °C, respectively. The superior conductivity and device performances are primarily attributed to the heterogeneity and interfacial conduction effect, which is systematically analyzed by EIS and TEM. Considering the abundant reservation and zero pollution of natural mineral, hematite−LCP proves to be a promising candidate material for commercially viable SOFCs. Significantly, this work has opened up an approach to the exploitation of new advanced SOFC technology. In the long run, it will trigger a new research field that strongly affects natural resource utilization and the energy production field.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b05694. Additional figures, table, and experimental details (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] or [email protected] (B.Z.). *E-mail: [email protected] (Y.M.). *E-mail: [email protected] (Y.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Hubei Province (Grant 2015CFA120), the Swedish Research Council (Grant 621-2011-4983), the European Commission FP7 TriSOFC-project (Grant 303454), and the National Natural Science Foundation of China (Grants 51302252, 51502084). The lead author would also like to thank the Hubei Provincial 100-Oversea Talent Distinguished Professor grants.

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