Li2TiO3–LaSrCoFeO3 semiconductor heterostructure for low temperature ceramic fuel cell electrolyte

Semiconductor-based electrolytes have significant advantages than conventional ionic electrolyte fuel cells, especially for high ionic conductivity and power outputs at low temperatures (<600 °C). This work reports a p-n heterojunction composite electrolyte developed by a p-type La_0.8Sr_0.2Co_0.8Fe_0.2O_3-δ (LSCF) and n-type Li₂TiO₃ (LTO). It achieved a power output of 350 mW cm^?2 at 550 °C using LSCF-LTO heterostructure as the electrolyte. On the other hand, pure LSCF and Li₂TiO₃ were made as the fuel cell electrolyte as well. The former resulted immediately a short circuiting problem and exhibited no device voltage because of high electron (hole) conductivity. While the Li₂TiO₃ can reach an open circuit voltage (OCV) but deliver too low power output, 37 mW cm^?2 at 550 °C. Scanning Electron Microscope (SEM) combined with High-Resolution Transmission Electron Microscope (HR-TEM) clearly proved the formation of heterogeneous interface. Also, Fourier Transform Infrared Spectroscopy (FTIR) was performed to demonstrate the functional group of the synthesized materials. The results demonstrate clearly the semiconductor heterostructure effect. By adjusting apriority composition of the n-type and p-type components, electronic conduction is well suppressed in the membrane electrolyte. Meanwhile, by constructing p-n heterostructure and build-in field, we have succeeded in high ionic conductivity, high current and power outputs for the low temperature fuel cells. The results are interesting in general that to construct a p-n heterostructure electrolyte can be an effective and common way in developing low temperature ceramic fuel cells.     

Developing cuprospinel CuFe2O4–ZnO semiconductor heterostructure as a proton conducting electrolyte for advanced fuel cells

The interfacial properties of electrolyte materials have a crucial impact on the ionic conductivity of solid batteries and solid oxide fuel cells. Here we construct cuprospinel CuFe₂O₄ (CFO)–ZnO composite as a functional electrolyte for fuel cell device. In an optimal composition of 0.3CFO-0.7ZnO electrolyte fuel cell, the maximum power output of 675 mW cm^?2 is obtained at 550 °C. The electrical properties and electrochemical performance are strongly dependent on the ratios between CFO and ZnO in CFO-ZnO composite. Notably, surprising fuel cell performance with high ionic conductivity is attained by constructing this p-type CFO composited with n-type ZnO. Proton conduction was further verified experimentally. The interfacial ionic conduction pathway between the two constituent phases plays a vital role to enhance the proton conductivity, and the bulk p-n heterojunction can block internal electronic pass. An excellent current and power densities of CFO-ZnO composite are observed along with a high conductivity of 0.35 S·cm-1 at 550 °C. This work opens a new perspective for the semiconductor materials that can widely be developed for electrolytes, based on their tunable band structure.     

Partially reduced Ni0.8Co0.15Al0.05LiO2-δ for low-temperature SOFC cathode

Nowadays, Ni_0.8Co_0.15Al_0.05LiO_2-δ (NCAL) has been increasingly applied into the solid oxide fuel cell (SOFC) field as a promising electrode material. Here, the performances of NCAL cathode were investigated for low-temperature SOFCs (LT-SOFCs) on Ce_0.8Sm_0.2O_2-δ (SDC) electrolyte. After on-line reduction of NCAL for 30 min, the partially reduced NCAL, i.e., NCAL(r), was employed as the new cathode and its performances were also investigated. The area specific resistances of NCAL and NCAL(r) cathodes on SDC electrolyte are 7.076 and 1.214 Ω cm² at 550 °C, respectively. Moreover, NCAL(r) exhibits the activation energy of 0.46 eV for oxygen reduction reaction (ORR), which is much lower than that of NCAL (0.88 eV). The fuel cell consisted of NCAL electrodes and SDC electrolyte shows an open circuit voltage (OCV) of 0.95 V and power output of 436 mW cm^?2 at 550 °C. After cathode on-line optimization, the cell's OCV and power output are significantly increased to 1.01 V and 648 mW cm^?2, which mainly attributed to the accelerated ORR and decreased electrode polarization resistance. These results demonstrate that NCAL(r) is a promising cathode material for LT-SOFCs.     

Electrical properties of Ni-doped Sm2O3 electrolyte

Nowadays, semiconductor ionic materials have drawn significant attention for developing new electrolytes in low temperature solid oxide fuel cells (LT-SOFCs). Here we investigate the effect of nickel doping on ionic conductivity of Sm₂O₃ as an electrolyte material for low temperature SOFCs. The amount of Ni ion doping has an intense effect on the electrochemical properties and power generation. An optimized composition of 10 mol% nickel doped samarium oxide (10NSO) as an electrolyte in the fuel cell has a high open circuit voltage (OCV) of 1.09 V and a notable power output of 1080 mW cm^?2 at 520 °C. Further investigation revealed that the 10NSO displays a superior ionic conduction up to 0.26 S cm^?1 at 520 °C. Moreover, the cell demonstrates high stability up to 80 h. The high electrochemical property and good stability recommend that the NSO is a favorable candidate for symmetrical SOFC electrolyte.     

The semiconductor SrFe0.2Ti0.8O3-δ-ZnO heterostructure electrolyte fuel cells

Highly ion-conducting properties in heterostructure composites and semiconductors have drawn significant attention in recent years for developing new electrolytes in low-temperature solid oxide fuel cells (LT-SOFCs). In this work, a new semiconductor heterostructure composite SrFe_0.2Ti_0.8O_3-δ (SFT)-ZnO consisting of p-type SFT and n-type ZnO is proposed and evaluated as an electrolyte in LT-SOFCs. Electrochemical studies reveal that the prepared SFT-ZnO is a mixed ion-electron conductor possessing a high ionic conductivity of 0.21 S cm⁻¹ at 520 °C and the assembled SFT-ZnO fuel cell can achieve a favorable peak power output of 650 mW cm⁻² along with high open-circuit voltage (OCV) of 1.06 V at 520 °C. By referring the semiconductor conduction types and energy band parameters of SFT and ZnO, a p-n bulk-heterojunction effect is proposed to describe the electronic blocking and ionic promotion processes of SFT-ZnO electrolyte in a fuel cell. Our work suggests a new insight into the design of effective LT-SOFC electrolytes by using semiconductor heterostructure material.     

Compositing protonic conductor BaZr0.5Y0.5O3 (BZY) with triple conductor BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY) as electrolyte for advanced solid oxide fuel cell

BaZr_0.5Y_0.5O₃(BZY) electrolyte exhibited enormous potential for low temperature solid oxide fuel cell (SOFC) due to its proton dominant mobile carriers rather than oxygen ions during the electrochemical process. In order to enhance the ionic conductivity, triple conductor BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3-δ (BCFZY) was composited with proton conductor BZY to form semiconductor-ionic conductor composite (SIM), which was applied as electrolyte to fabricate symmetrical fuel cell. The microstructural and electrochemical properties for BZY, BCFZY and BZY-BCFZY composites were studied. After optimizing the weight ratio of composite content, the ionic conductivity and electronic conductivity reached an equilibrium state to obtain the maximum cell performance, namely the highest output of 902.5 mW cm^?2 and an open circuit voltage (OCV) of 1.043 V at 550 °C. The BZY-BCFZY cell also presented decent power output at low temperature, a power density of 265.625 mW cm^?2 was received even at 500 °C, demonstrating that the BZY-BCFZY composite was a potential electrolyte for low temperature SOFCs.     

Demonstrating the dual functionalities of CeO2–CuO composites in solid oxide fuel cells

Nowadays, lowering the operating temperature of solid oxide fuel cells (SOFCs) is a major challenge towards their widespread application. This has triggered extensive material studies involving the research for new electrolytes and electrodes. Among these works, it has been shown that CeO₂ is not only a promising basis of solid oxide electrolytes, but also capable of serving as a catalytic assistant in anode. In the present work, to develop new electrolytes and electrodes for SOFCs based on these features of CeO₂, a new type of functional composite is developed by introducing semiconductor CuO into CeO₂. The prepared composites with mole ratios of 7:3 (7CeO₂–3CuO) and 3:7 (3CeO₂–7CuO) are assessed as electrolyte and anode in fuel cells, respectively. The cell based on 7CeO₂–3CuO electrolyte reaches a power outputs of 845 mW cm^?2 at 550 °C, superior to that of pure CeO₂ electrolyte fuel cell, while an Ce_0.8Sm_0.2O_2-δ electrolyte SOFC with 3CeO₂–7CuO anode achieves high power density along with open circuit voltage of 1.05 V at 550 °C. In terms of polarization curve and AC impedance analysis, our investigation manifests the developed 7CeO₂–3CuO composite has good electrolyte capability with a hybrid H⁺/O^2? conductivity of 0.1–0.137 S cm^?1 at 500–550 °C, while the 3CeO₂–7CuO composite plays a competent anode role with considerable catalytic activity, indicative of the dual-functionalities of CeO₂–CuO in fuel cell. Furthermore, a bulk heterojunction effect based on CeO₂/CuO pn junction is proposed to interpret the suppressed electrons in 7CeO₂–3CuO electrolyte. Our study thus reveals the great potential of CeO₂–CuO to develop functional materials for SOFCs to enable low-temperature operation.     

Doped ceria electrolyte rich in oxygen vacancies for boosting the fuel cell performance of LT-CFCs

Multifunctional semiconductor CeO₂ is used as an electrolyte for a fuel cell application, delivering meaningful power density and better OCV. Further, surface doping of Al was employed to attain the CeAlO₂ (ADC) electrolyte with enriched O-vacancies surface layer enabling high ionic conduction and excellent power density of 1020 mW/cm² at 520 °C. It is noticed that surface doping entailed the band alignment between CeO₂ and ADC due to the difference in Fermi level establishing space charge region, which further constitutes built-in field enhancing the charge transportation and minimizing the e-conduction. Furthermore, the theoretical calculation was performed to assist the formation of O-vacancies in the ADC structure. These findings suggest surface doping is the best approach to attain excellent performance and designing new electrolytes and electrodes for advanced low-temperature ceramic fuel cell technology.     

Interface engineering of bi-layer semiconductor SrCoSnO3-δ-CeO2-δ heterojunction electrolyte for boosting the electrochemical performance of low-temperature ceramic fuel cell

A comparative study is performed to investigate the electrochemical performance of the low-temperature ceramic fuel cells (CFCs) utilizing two different novel electrolytes. First, a perovskite semiconductor SrCo_0.3Sn_0.7O_3-δ was used as an electrolyte in CFCs due to its modest ionic conductivity (0.1 S/cm) and demonstrated an acceptable power density of 360 mW/cm² at 520 °C. The performance of the cell was primarily limited due to the moderate ionic transport in the electrolyte. In order to improve the ionic conductivity, a new strategy of using a novel bi-layer electrolyte concept consist of SrCo_0.3Sn_0.7O_3-δ and CeO_2-δ in CFCs. These bi-layers of two electrolytes have successfully established heterojunction which considerably improved the ionic conductivity (0.2 S/cm) and enhance the open-circuit voltage of the cell from 0.98 V to 1.001 V. Moreover, the CFCs utilizing bi-layer electrolyte have produced a remarkable power density of 672 mW/cm² at 520 °C. This enhancement of ionic conduction, power density and blockage of electron conduction in the bi-layer electrolyte was studied via band alignment mechanism based on proposed p-n heterojunction. Our work presents a promising methodology for developing advanced low-temperature CFC electrolytes.     

Interface channels accelerating ion transport through alkaline earth metal carbonate - Gd0.1Ce0.9O1.95 heterostructure

Designing the interface proton channel between different phases to accelerate the proton ion transport is an effective way to realize the high proton conduction for the low-temperature ceramic fuel cell (CFC). Cerium based materials coated with molten carbonate has been widely demonstrated for high performance CFCs. Here, we prepared alkaline earth metal carbonate - Gd_0.1Ce_0.9O_1.95 (GDC) heterostructure composites in various compositions by precipitation method using NH₄HCO₃ and NaHCO₃ as the deposit. The samples prepared using NH₄HCO₃ as the electrolyte, the cell can deliver an even higher power output of 811 mW cm⁻². The results are much higher than that reported in the literature for the GDC electrolyte fuel cells. The ion conduction on the interface between GDC and solid carbonate particles is proposed. The ionic conductivity is determined to be 0.13 S cm⁻¹ at 500 °C; while GDC as reported in literature is 0.005 S cm⁻¹ at the same temperature. This proposed solid carbonate-GDC heterostructure method has succeeded in enhancing ionic conductivity and the CFC performance, which presents a new way to develop high proton conducting materials and advanced ceramic fuel cells at low temperatures (<550 °C).     

Performance analysis of LiAl0.5Co0.5O2 nanosheets for intermediate-temperature fuel cells

Exploring high conductive materials is still a challenge for high performance intermediate-temperature fuel cells. In this study, two-dimensional LiAl_0.5Co_0.5O₂ (LACO) nanosheets coated by a compatible amorphous LiAlO₂ (LAO) layer are evaluated as proton/Li⁺ conductor electrolyte. The fuel cell in which the LAO-LACO is used as electrolyte could deliver the maximum power output of 1120 mW cm^?2 at 550 °C. The LAO coating enhances not only the ionic conductivity by modifying the space-charge regions, but also improves the LACO's chemical stability and device performance. Kelvin probe force microscopy further detected a local electric field (LEF) built in the LAO-LACO coating confines protons at the interface to transport fast. Such heterostructure with the LEF accelerating mechanism presents a novel approach for developing high-performance intermediate-temperature fuel cells.     

Dual-phase electrolytes for advanced fuel cells

Double-phase electrolyte (DPE) consisting of doped CeO₂/NiAl solid phase and NaOH liquid phase was used for fuel cells utilizing LiNiO₂ anode and Ag cathode at working temperatures over 450 °C. It was shown that the cells can produce a maximum output power of 716.2 mW cm⁻² at 590 °C even though utilized with relatively large thickness of electrolyte, from 0.8 to 1.2 mm. Most measurements of open circuit voltage (OCV) range between 1 and 1.2 V; a significantly higher OCV value of 1.254 V was also obtained. Liquid channel conductive mechanism of NaOH in DPE is proposed; both O²⁻ and H⁺ concur to conduct the current; the doped CeO₂ transports O²⁻ ions, whereas the molten second phase transports H⁺ protons. Moreover, SEM observations and EDS analysis suggest that Na⁺ and OH⁻ also contribute to enhance both OCV and output power of our cells. The addition of NiAl to the doped CeO₂ increases the mechanical strength and the output power of DPE; however the reasons of this latter effect are still to be further investigated. The results show that DPE is a promising electrolyte to manufacture fuel cells with advanced performances.     

LiNi-oxide simultaneously as electrolyte and symmetrical electrode for low-temperature solid oxide fuel cell

LiNi-oxide showed triple (oxygen ionic, protonic and electronic) conduction properties and were considered as the promising cathode for proton conducting solid oxide fuel cells (H–SOFC). In this work, we pioneeringly explore the potential of Li_xNiO₂ (x = 0.2, 0.4, 0.5, 0.8) (L_xNO) for symmetrical electrode and electrolyte application. The composite of semiconductor L_xNO and proton conductor BaZr_0.5Y_0.5O₃（BZY）are applied as electrolyte with L_0.5NO as symmetrical electrode to assemble solid oxide fuel cells. There is no negative impact on cell performance after introducing the electronic conductor L_xNO into BZY as electrolyte membrane. On the contrary, the cell based on L_0.5NO -BZY composite electrolyte is significantly higher than that of pure BZY electrolyte. The cell performance can be improved by optimizing the weight ratio of BZY and L_0.5NO. When the mass ratio of BZY to L_0.5NO was 8:2, the open circuit voltage (OCV) of the cell was 1.11 V, and the maximum power (P_max) was 814 mW?cm^?2 at 550 °C. This work demonstrated that the LiNi-oxide can be simultaneously used as electrolyte and symmetrical electrode for low-temperature solid oxide fuel cell.     

Semiconductor ionic Ce0.8Sm0.2O2-δ-Na2CO3 – LiCo0.225Cu0.075Ni0.7O 3-δ composite material as electrolyte for low temperature solid oxide fuel cells

Semiconductor ionic electrolytes have obtained much attention because of good ionic conductivity and electrochemical performance. Novel semiconductor ionic NSDC (Ce_0.8Sm_0.2O_2-δ-Na₂CO₃)-LCCN (LiCo_0·225Cu_0·075Ni_0·7O_3-δ) composite materials have been adopted as electrolyte membrane for the first time, in which symmetrical cell composed of NSDC-LCCN membrane is constructed with Ni_0·8Co_0·15Al_0·05LiO₂ (NCAl)-pasted Ni foam electrodes. An open circuit voltage (OCV) above 1 V and improved power density are obtained in the NSDC-LCCN cells, which confirms the functionality of the proposed semiconductor ionic materials. Meanwhile, X-ray diffractometer (XRD) and Scanning electron microscope (SEM) analyses identify the phase purity and homogenous nanocomposite morphology of all the NSDC-LCCN materials samples with various mass ratios. The performance illustrated by much more steady instead of transient state evaluation reveals that 3NSDC-LCCN composite electrolyte is most optimum, and the corresponding cell exhibits a considerable maximum power density of 598 mW cm⁻² at 550 °C, over five times of that of pure NSDC electrolyte cells. Short-term duration test of 3NSDC-LCCN cell at 550 °C shows that the cell could steadily operate up to ~9 h without obvious degradation at a remarkable current density of 469 mA cm⁻², which indicates that NSDC-LCCN composite electrolyte is a promising material for low temperature solid oxide fuel cells.     

Stability considerations of semiconductor ionic fuel cells from a LNCA (LiNi0.8Co0.15Al0.05O2-δ) and Sm-doped ceria (SDC; Ce0.8Sm0.2O2-δ) composite electrolyte

Recent advances in composite materials, especially semiconductor materials incorporating ionic conductor materials, have led to significant improvements in the performance of low-temperature fuel cells. In this paper, we present a semiconductor LNCA (LiNi_0.8Co_0.15Al_0.05O_2-δ) which is often used as electrode material and ionic Sm-doped ceria (SDC; Ce_0.8Sm_0.2O_2-δ) composite electrolyte, sandwiched between LNCA thin-layer electrodes in a configuration of Ni-LNCA/SDC-LNCA/LNCA-Ni. The incorporation of the semiconductor LNCA into the SDC electrolyte with optimized weight ratios resulted in a significant power improvement, from 345 mW cm^?2 with a pure SDC electrolyte to 995 mW cm^?2 with the ionic-semiconductor SDC-LNCA one where the corresponding ionic conductivity reaches 0.255 S cm^?1 at 550 °C. Interestingly, the coexistence of ionic and electron conduction in the SDC-LNCA membrane displayed not any electronic short-circuiting but enhanced the device power outputs. This study demonstrates a new fuel cell working principle and simplifies technologies of applying functional ionic-semiconductor membranes and symmetrical electrodes to replace conventional electrolyte and electrochemical technologies for a new generation of fuel cells, different from the conventional complex anode, electrolyte, and cathode configuration.     

Study on GDC-KZnAl composite electrolytes for low-temperature solid oxide fuel cells

Development of low-temperature solid oxide fuel cells (LTSOFC) is now becoming a mainstream research direction worldwide. The advancement in the effective electrolyte materials has been one of the major challenges for LTSOFC development. To further improve the performance of electrolyte, composite approaches are considered as common strategies. The enhancement on ionic conductivity or sintering behavior ceria-based electrolyte can either be done by adding a carbonate phase to facilitate the utilization of the ionic-conducting interfaces, or by addition of alumina as insulator to reduce the electronic conduction of ceria. Thus the present report aims to design a composite electrolyte materials by combining the above two composite approaches, in order to enhance the ionic conductivity and to improve the long-term stability simultaneously. Here we report the preparation and investigation of GDC-KAlZn materials with composition of Gd doped ceria, K₂CO₃, ZnO and Al₂O₃. The structure and morphology of the samples were characterized by XRD, SEM, etc. The ionic conductivity of GDC-KAlZn sample was determined by impedance spectroscopy. The composite samples with various weight ratio of GDC and KAlZn were used as electrolyte material to fabricate and evaluate fuel cells as well as investigate the composition dependent properties. The good ionic conductivity and notable fuel cell performance of 480 mW cm⁻² at 550 °C has demonstrated that GDC-KAlZn composite electrolyte can be regarded as a potential electrolyte material for LTSOFCs.     

Fast ion channels for crab shell-based electrolyte fuel cells

Biomaterials possess abundant micro and macrospores in their microstructures, which can be functionalized as higher ion-transport channels. Herein we report calcined crab shell (CCS) forming nanocomposites with La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) perovskite as functional electrolytes for low temperature solid oxide fuel cells (LTSOFCs). The single CCS electrolyte fuel cell achieved open circuit voltage (OCV) at 0.9 V and a peak power density of 70 mW cm⁻² at 550 °C; while the highest OCV of 1.21 V and a maximum power density of 440 mW cm⁻² were achieved for the CCS-LSCF (40 wt. % LSCF) electrolyte fuel cell. The results are attributed to the ion channel construction and interface effect built in the CCS-LSCF composite. This work may provide a new strategy to develop novel biomaterial-based materials for LTSOFCs.     

Enhanced ionic conductivity of yttria-stabilized ZrO2 with natural CuFe-oxide mineral heterogeneous composite for low temperature solid oxide fuel cells

We report for the first time that the commercial yttrium stabilized zirconia (YSZ) nanocomposite with a natural CuFe-oxide mineral (CF) exhibits a greatly enhanced ionic conductivity in the low temperature range (500–600 °C), e.g. 0.48 S/cm at 550 °C. The CF–YSZ composite was prepared via a nanocomposite approach. Fuel cells were fabricated by using a CF–YSZ electrolyte layer between the symmetric electrodes of the Ni_0.8Co_0.2Al_0.5Li (NCAL) coated Ni foam. The maximum power output of 562 mW/cm² has been achieved at 550 °C. Even the CF alone to replace the electrolyte the device reached the maximum power of 281 mW/cm² at the same temperature. Different ion-conduction mechanisms for YSZ and CF–YSZ are proposed. This work provides a new approach to develop natural mineral composites for advanced low temperature solid oxide fuel cells with a great marketability.     

Preparation and characterization of Sm0.2Ce0.8O1.9/Na2CO3 nanocomposite electrolyte for low-temperature solid oxide fuel cells

Sm_0.2Ce_0.8O_1.9 (SDC)/Na₂CO₃ nanocomposite synthesized by the co-precipitation process has been investigated for the potential electrolyte application in low-temperature solid oxide fuel cells (SOFCs). The conduction mechanism of the SDC/Na₂CO₃ nanocomposite has been studied. The performance of 20 mW cm⁻² at 490 °C for fuel cell using Na₂CO₃ as electrolyte has been obtained and the proton conduction mechanism has been proposed. This communication demonstrates the feasibility of direct utilization of methanol in low-temperature SOFCs with the SDC/Na₂CO₃ nanocomposite electrolyte. A fairly high peak power density of 512 mW cm⁻² at 550 °C for fuel cell fueled by methanol has been achieved. Thermodynamical equilibrium composition for the mixture of steam/methanol has been calculated, and no presence of C is predicted over the entire temperature range. The long-term stability test of open circuit voltage (OCV) indicates the SDC/Na₂CO₃ nanocomposite electrolyte can keep stable and no visual carbon deposition has been observed over the anode surface.     

Advanced fuel cell based on semiconductor perovskite La–BaZrYO3-δ as an electrolyte material operating at low temperature 550 °C

As an electrolyte, enough ionic conductivity, either proton (H⁺) or oxide (O²⁻) conduction, has demanded the better performance of low-temperature (especially below 550 °C) solid oxide fuel cell (LT-SOFCs). Notably, that either conductivity, higher performance, reliability, or higher cost is hampering the LT-SOFC marketing. In our current subject, we report the La-doped BZY proton conductor as an electrolyte has exhibited high ionic conductivity of 0.15 S/cm with a higher performance of 0.78 W/cm² at 550 °C. Also, the performance of LBZY is superior to the un-doped BZY electrolyte. Such high performance mainly ascribed due to the doping of La into BZY. Besides, the mechanism for high ion conductivity is explained. This work manifests that using the LBZY semiconductor perovskite as an electrolyte is more suitable for fuel cell technology.