The composite electrolyte with an insulation Sm2O3 and semiconductor NiO for advanced fuel cells

Novel Sm₂O₃?NiO composite was prepared as the functional electrolyte for the first time. The total electrical conductivity of Sm₂O₃?NiO is 0.38 S cm^?1 in H₂/air condition at 550 °C. High performance, e.g. 718 mW cm^?2, was achieved using Sm₂O₃?NiO composite as an electrolyte of solid oxide fuel cells operated at 550 °C. The electrical properties and electrochemical performance are strongly depended on Sm₂O₃ and NiO constituent phase of the compositions. Notably, surprisingly high ionic conductivity and fuel cell performance are achieved using the composite system constituting with insulating Sm₂O₃ and intrinsic p-type conductive NiO with a low conductivity of 4 × 10^?3 S cm^?1. The interfacial ionic conduction between two phases is a dominating factor giving rise to significantly enhanced proton conduction. Fuel cell performance and further ionic conduction mechanisms are under investigation.     

Electrochemical performance of a Ni0.8Co0.15Al0.05LiO2 cathode for a low temperature solid oxide fuel cell

The electrochemical performance of the Ni_0.8Co_0.15Al_0.05LiO₂ (NCAL) cathode was investigated by comparing it with the traditional La_0.4Sr_0.6Co_0.2Fe_0.8O_3-δ (LSCF) and LSCF/Ce_0.9Gd_0.1O_2-δ (GDC) cathodes with a GDC electrolyte-supported solid oxide fuel cell (SOFC). It is found that the electrochemical performance of the cells with the NCAL and NCAL/GDC cathode is better than that of the cells with the LSCF and LSCF/GDC cathode at 550 °C. The results of the electrochemical performance tests of the cells with different NCAL/GDC mass ratios (10/0, 9/1, 8/2, 7/3 and 6/4) show that the NCAL/GDC composite cathode with the mass ratio of 8/2 has the best electrochemical performance. XRD results show that when the sintering temperature is higher than 700 °C, the NCAL/GDC composite will undergo chemical reactions and generate new phases, reducing the performance of the composite cathode. XPS results show that a small amount of Li₂CO₃ was formed on the surface of NCAL during cathode preparation, forming a special interface between NCAL, Li₂CO₃ and GDC. At the NCAL-Li₂CO₃/GDC interfaces, due to the migration and aggregation of Li⁺ to the interface, a space charge region may be formed in which the Li⁺ enrichment may lead to the formation of the region with a high oxygen vacancy concentration. A very high oxygen vacancy concentration at the NCAL-Li₂CO₃/GDC interfaces will provide sufficient oxygen ion conductivity for oxygen reduction reaction (ORR) and reduce the activation energy of the reaction. NCAL will be a potential cathode material that can reduce the operating temperature of the traditional SOFC to 550 °C or lower.     

Rare-earth oxide–Li0.3Ni0.9Cu0.07Sr0.03O2-δ composites for advanced fuel cells

Recent development on electrolyte-free fuel cell (EFFC) holding the same function with the traditional solid oxide fuel cell (SOFC) but with a much simpler structure has drawn increasing attention. Herein, we report a composite of industrial grade rare-earth precursor for agriculture and Li_0.3Ni_0.9Cu_0.07Sr_0.03O_2-δ (RE–LNCS) for EFFCs. Both structural and electrical properties are investigated on the composite. It reveals that the RE–LNCS possesses a comparable ionic and an electronic conductivities, 0.11 S cm^?1 and 0.20 S cm^?1 at 550 °C, respectively. An excellent power output of 1180 mW cm^?2 has been achieved at 550 °C, which is much better than that of the conventional anode/electrolyte/cathode based SOFCs, only around 360 mW cm^?2 by using ionic conducting rare-earth material as the electrolyte. Engineering large size cells with active area of 25 cm² prepared by tape-casting and hot-pressing gave a power output up to 12 W. This work develops a new functional single layer composite material for EFFCs and further explores the device functions.     

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.     

Performance and durability of an anode-supported solid oxide fuel cell with a PdO/ZrO2 engineered (La0.8Sr0.2)0.95MnO3-δ-(Y2O3)0.08(ZrO2)0.92 composite cathode

PdO/ZrO₂ co-infiltrated (La_0.8Sr_0.2)_0.95MnO_3-δ-(Y₂O₃)_0.08(ZrO₂)_0.92 (LSM-YSZ) composite cathode (PdO/ZrO₂+LSM-YSZ), which adsorbs more oxygen than equal amount of PdO/ZrO₂ and LSM-YSZ, is developed and used in Ni-YSZ anode-supported cells with YSZ electrolyte. The cells are investigated firstly at temperatures between 650 and 750 °C with H₂ as the fuel and air as the oxidant and then polarized at 750 °C under 400 mA cm^?2 for up to 235 h. The initial peak power density of the cell is in the range of 438–1207 mW cm^?2 at temperatures from 650 to 750 °C, corresponding to polarization resistance from 1.04 to 0.35 Ω cm². This result demonstrates a significant performance improvement over the cells with other kinds of LSM based cathode. The cell voltage at 750 °C under 400 mA cm^?2 decreases from initial 0.951 to 0.89 V after 170 h of current polarization and remains essentially stable to the end of current polarization. It is identified that the self-limited growth of PdO particles is responsible for the cell voltage decrease by reducing the length of triple phase boundary affecting the high frequency steps involved in oxygen reduction reaction in the cathode.     

Investigation of the sudden drop of electrolyte conductivity at low temperature in ceramic fuel cell with Ni0·8Co0·15Al0·05LiO2 electrode

The sudden drop of ionic conductivity of GDC (Gd_0.1Ce_0·9O_1.95) electrolyte in ceramic fuel cells with NCAL (Ni_0·8Co_0·15Al_0·05LiO₂) as electrode at low temperature was studied. It is found that the peak power density (PPD) of the cell with GDC electrolyte decreases linearly with the decreasing of the operation temperature above 400 °C. However, when the operation temperature drops to 400 °C, the cell PPD decreases significantly. EIS results show that the ionic conductivity of the electrolyte decreases linearly with the decrease of cell operating temperature. When the temperature decreases to approximately 400 °C, the ionic conductivity of the electrolyte decreases from 0.251 S cm^?1 at 425 °C to 0.026 S cm^?1 at 400 °C. The rapid decrease of the electrolyte ionic conductivity is considered to be the direct cause of the sudden decrease of the PPD. According to the results of XPS, FTIR and TG-DSC, LiOH/Li₂CO₃ formed in the NCAL anode diffuses into the electrolyte and melts at 419 °C or above, which is the reason for the high ionic conductivity of the electrolyte. The reason for the sudden drop of ionic conductivity is that LiOH/Li₂CO₃ and other compounds solidify in molten salts below 419 °C.     

Determination of the dissociation constant of molten Li2CO3/Na2CO3/K2CO3 using a stabilized zirconia oxide-ion indicator

An Li₂CO₃/Na₂CO₃/K₂CO₃ eutectic melt has been selected as an example of a molten-carbonate system and the suitability of a stabilized zirconia—air electrode as an oxide-ion concentration indicator for this melt has been confirmed.With this indicator, the dissociation constant of the reaction CO₃^2? (?) = CO₂(g) + O^2? (?) in this melt has been determined to be K_d = P_CO2 [O^2?] = 4.03 × 10^?3 Pa at 873 KReproducible measurements were obtained throughout the experiment and this method might find further application in the study of reactions related to the oxide ion in carbonate melts.     

High conductive (LiNaK)2CO3Ce0.85Sm0.15O2 electrolyte compositions for IT-SOFC applications

Composite electrolytes of lithium, sodium, and potassium carbonate ((LiNaK)₂CO₃), and samarium doped ceria (SDC) have been synthesized and the carbonate content optimized to study conductivity and its performance in intermediate-temperature solid oxide fuel cell (IT-SOFC). Electrolyte compositions of 20, 25, 30, 35, 45 wt% (LiNaK)₂CO₃–SDC are fabricated and the physical and electrochemical characterization is carried out using X-ray diffraction, scanning electron microscopy, electrochemical impedance spectroscope, and current–voltage measurements. The ionic conductivity of (LiNaK)₂CO₃–SDC electrolytes increases with increasing carbonate content. The best ionic conductivity is obtained for 45 wt% (LiNaK)₂CO₃–SDC composite electrolyte (0.72 S cm^?1 at 600 °C) followed by the 35 wt% (LiNaK)₂CO₃–SDC composite electrolyte (0.55 S cm^?1 at 600 °C). The symmetrical cell of the 35 wt% (LiNaK)₂CO₃–SDC composite electrolyte with lanthanum strontium cobalt ferrite (LSCF) electrode in air gives an area specific resistance of 0.155 Ω cm² at 500 °C. The maximum power density of the fuel cell using 35 wt% (LiNaK)₂CO₃–SDC composite electrolyte, composite NiO anode and composite LSCF cathode is found to be 801 mW cm^?2 at 550 °C.     

Impedance spectroscopy studies of Gd-CeO2-(LiNa)CO3 nano composite electrolytes for low temperature SOFC applications

Electrical properties of 20 mol % Gd doped CeO₂ with varying amounts of (LiNa)CO₃ have been investigated by employing AC-impedance spectroscopic technique. The impedance spectra show a high frequency depressed arc, represents the bulk composite and low frequency incomplete semicircle representing electrode contribution. The bulk resistance of the composites decreases with increasing carbonate content up to 30 wt% (LiNa)CO_3, thereafter the resistance increases, whereas all the compositions show a decrease in resistance with increasing temperature. The typical nature of the impedance spectra of the composite shows the possibility of coexistence of multi ionic transport or existence of space charge effect at the interface of Gd-CeO₂ and carbonate phase. The composite containing 25 wt% (LiNa)CO₃ shows the highest ionic conductivity of 0.1757 S cm⁻¹ at 550 °C and lowest activation energy of 0.127 eV in the temperature range 550-800 °C. A symmetric cell is fabricated with GDC-25 wt% (LiNa)CO₃ electrolyte, NiO-GDC(LiNa)CO₃ anode and lithiated NiO-GDC(LiNa)CO₃ cathode. Pure H₂ and air are used as fuel and oxidant. The cell delivers a maximum power density of 45 mW/cm², 58 mW/cm² and 92 mW/cm² at 450, 500 and 550 °C, respectively.     

CO2 methanation over ordered mesoporous NiRu-doped CaO-Al2O3 nanocomposites with enhanced catalytic performance

The ordered mesoporous NiRu-doped CaO-Al₂O₃ nanocomposites were synthesized via a facile evaporation-induced self-assembly method for CO₂ methanation. Metallic Ni and Ru species retained the single-component heterostructure rather than NiRu alloy over the 600 °C-reduced catalysts. Owing to the synergistic effect of bimetallic Ni–Ru as well as the improved H₂ and CO₂ chemisorption capacities after the addition of Ru and CaO promoters, the ordered mesoporous 10N1R2C-OMA catalyst exhibited enhanced catalytic activity and selectivity, which achieved the maximum CO₂ conversion of 83.8% and CH₄ selectivity of 100% at 380 °C, 0.1 MPa, 30000 mL g^?1 h^?1. In a 550 °C-109 h-lifetime test, the ordered mesoporous 10N1R2C-OMA catalyst showed high stability and superior anti-sintering property due to the confinement effect of the ordered mesostructure.     

La0.1SrxCa0.9−xMnO3−δ-Sm0.2Ce0.8O1.9 composite material for novel low temperature solid oxide fuel cells

Lowering the operating temperature of the solid oxide fuel cells (SOFCs) is one of the world R&D tendencies. Exploring novel electrolytes possessing high ionic conductivity at low temperature becomes extremely important with the increasing demands of the energy conversion technologies. In this work, perovskite La_0.1Sr_xCa_0.9?xMnO_3?δ (LSCM) materials were synthesized and composited with the ionic conductor Sm_0.2Ce_0.8O_1.9 (SDC). The LSCM–SDC composite was sandwiched between two nickel foams coated with semiconductor Ni_0.8Co_0.15Al_0.05LiO_2?δ (NCAL) to form the fuel cell device. The strontium content in the LSCM and the ratios of LSCM to SDC in the LSCM-SDC composite have significant effects on the electrical properties and fuel cell performances. The best performance has been achieved from LSCM-SDC composite with a weight ratio of 2:3. The fuel cells showed OCV over 1.0 V and excellent maximum output power density of 800 mW cm^?2 at 550 °C. Device processes and ionic transport processes were also discussed.     

Industrial grade rare-earth triple-doped ceria applied for advanced low-temperature electrolyte layer-free fuel cells

In this study, the mixed electron-ion conductive nanocomposite of the industrial-grade rare-earth material (La³⁺, Pr³⁺ and Nd³⁺ triple-doped ceria oxide, noted as LCPN) and commercial p-type semiconductor Ni_0.8Co_0.15Al_0.05Li-oxide (hereafter referred to as NCAL) were studied and evaluated as a functional semiconductor-ionic conductor layer for the advanced low temperature solid oxide fuel cells (LT-SOFCs) in an electrolyte layer-free fuel cells (EFFCs) configuration. The enhanced electrochemical performance of the EFFCs were analyzed based on the different semiconductor-ionic compositions with various weight ratios of LCPN and NCAL. The morphology and microstructure of the raw material, as-prepared LCPN as well the commercial NCAL were investigated and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray spectrometer (EDS), respectively. The EFFC performances and electrochemical properties using the LCPN-NCAL layer with different weight ratios were systematically investigated. The optimal composition for the EFFC performance with 70 wt% LCPN and 30 wt% NCAL displayed a maximum power density of 1187 mW cm^?2 at 550 °C with an open circuit voltage (OCV) of 1.07 V. It has been found that the well-balanced electron and ion conductive phases contributed to the good fuel cell performances. This work further promotes the development of the industrial-grade rare-earth materials applying for the LT-SOFC technology. It also provides an approach to utilize the natural source into the energy field.     

Effect of the online reaction byproducts of LiNi0.8Co0.15Al0.05O2-δ electrodes on the performance of solid oxide fuel cells

LiNi_0.8Co_0.15Al_0.05O_2-δ (NCAL) has been demonstrated to be an excellent electrode with dual catalytic activities of hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) for solid oxide fuel cells (SOFCs). Reports have indicated that the anode NCAL tends to be reduced during cell operation and produce LiOH and Li₂CO₃. In this work, the effect of the online reaction byproducts of NCAL electrodes, LiOH and Li₂CO₃, on the performance of SOFCs is investigated. Different amounts of LiOH and Li₂CO₃ are separately added to the Ce_0.8Sm_0.2O_2-δ (SDC) electrolyte for fabricating SOFCs with NCAL electrodes. The power output under normal and reverse operation is studied for the cells with lithium salt addition from 5 wt% to 30 wt%. Electrochemical impedance spectroscopy (EIS) and distribution of relaxation times (DRT) analysis suggested that the addition of LiOH or Li₂CO₃ to the SDC reduces HOR and ORR activities of the cells, leading to lower cell performance. The cross-sectional SEM of the cells shows that the added LiOH and Li₂CO₃ makes the electrolyte porous and densifies the electrode, which is the main reason for the reduction of cell performance. In addition, the porosity of electrolyte is modified during cell fabrication in order to construct flowing pathways for the online reaction byproducts of NCAL electrode to move from electrode to electrolyte, resulting in optimized cell performance. This study provides an insight into facilitating the performance of NCAL electrode-based SOFC.     

Single-phase electronic-ionic conducting Sm3+/Pr3+/Nd3+ triple-doped ceria for new generation fuel cell technology

Co-doped CeO₂ materials have exhibited promising potential for low temperature solid oxide fuel cell (LT-SOFC) applications. Sm³⁺, Pr³⁺ and Nd³⁺ triple-doped ceria has been synthesized via two-step wet chemical approach. First samarium doped ceria (SDC) was prepared and then the Pr³⁺/Nd³⁺ ions as doping elements (secondary process) was added. The structural structure was studied by X-ray diffraction (XRD), that indicate Sm³⁺, Pr³⁺ and Nd³⁺ ions are doped into the ceria lattice up to the certain limit (Pr³⁺/Nd³⁺ 10 wt%). The impurity peaks are detected as doping contents increased above the certain limit (Pr³⁺/Nd³⁺ 20 wt %). In this work, further we investigated the effect increasing Pr³⁺/Nd³⁺ doping concentration on the performance of SOFC device. Here, we studied that high-concentration triple-doped ceria samples with mixed electrons/ions conductive property, as the semiconductor-ionic conducting layer, combined with commercial p-type semiconductor Ni_0.8Co_0.15Al_0.05LiO_2-δ (NCAL) to fabricate the ‘sandwich’ configuration for a developing fuel cell technology-electrolyte free fuel cells (EFFCs). This button size fuel cell delivered a maximum power output of 1011 mW cm^?2. The demonstrated findings show that the single-phase semiconductor-ionic material-Sm³⁺/Pr³⁺/Nd³⁺ triple-doped CeO₂ can be selected potential candidate for the further development the EFFC technology.     

The heterogeneous electrolyte of CuFeO2 nano-flakes composited with flower-shaped ZnO for advanced solid oxide fuel cells

The flower-shaped ZnO was synthesized to form composite with the delafossite structure CuFeO₂. The composite heterojunction formed for the ZnO-CuFeO₂ composite material demonstrates a profound significance for exploring novel materials in solid oxide fuel cell (SOFC) field. At 550 °C, power outputs of 300 mW cm^?2 and 468 mW cm^?2 were achieved for SOFC devices using pure ZnO and composite with CuFeO₂ as the electrolytes, respectively. The composite showed a good performance at low temperatures, for instance, it showed a power output of 148 mW cm^?2 at 430 °C. The studies on photocurrent-time curves with visible light on/off irradiation provided an evidence for electron-hole separation. The heterojunctions separate holes and electrons, preventing short-circuiting while used in the SOFC device. These results demonstrate that introducing the heterojunctions in the electrolyte is an innovative approach for advanced SOFCs.     

Electrical conductivity of the Li2SO4Li2CO3 system

With a view to improving the electrical conductivity of Li₂SO₄ at the lowest possible temperature, Li₂CO₃ was added in the ratios of 10 – 90 mol% and its conductivity was measured. The system Li₂SO₄Li₂CO₃ has its eutectic at a composition of 60:40 mol%: this composition has the maximum conductivity of the series, 2.43 × 10^?3 (ohm cm)^?1 at 723 K. The high conductivity may be due to the quasiliquid state of the mobile species within the sublattice. Further, the addition of 5 mol% of LiCl to the eutectic gave rise to an increase in the conductivity, 1.13 × 10^?3 (ohm cm)^?1 at 553 K. This may be suitable as an electrolyte for application to power sources.     

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.     

High-performance SOFC based on a novel semiconductor-ionic SrFeO3-δ–Ce0.8Sm0.2O2-δ membrane

The semiconductor-ionic composite membrane has been recently developed for a novel solid oxide fuel cell (SOFC), i.e., the semiconductor-ion membrane fuel cell (SIMFC). In this work, the perovskite-type SrFeO_3-δ (SFO) as semiconductor material was composited with ionic conductor Ce_0.8Sm_0.2O_2-δ (SDC) to form the SFO-SDC composite membrane for SIMFCs. The SFO-SDC SIMFCs using the optimized weight ratio of 3:7 SFO-SDC membrane obtained the best performances, 780 mW cm^?2 at 550 °C, compared to 348 mW cm^?2 obtained from the pure SDC electrolyte fuel cell. Introduction of SFO into SDC can extend the triple phase boundary and provide more active sites for accelerating the fuel cell reactions, thus significantly enhanced the cell power output. Moreover, SFO was employed as the cathode, and a higher power output, 907 mW cm^?2 was achieved, suggesting that SFO cathode is more compatible for the SFO-SDC system in SIMFCs. This work provides an attractive strategy for the development of low temperature SOFCs.     

Applying Low-Pressure Plasma Spray (LPPS) for coatings in low-temperature SOFC

Low-temperature solid oxide fuel cell (LTSOFC) has shown great potentials for commercial applications in clean energy generation. Seeking for low cost and easy fabrication method is one of the most important issues for LTSOFC investigations. This paper introduces a new coating spray technology, namely Low-Pressure Plasma Spray (LPPS), for efficiently manufacturing different functional coatings of LTSOFC. By applying the LPPS technique, uniform and dense Ni_0.8Co_0.15Al_0.05LiO_2?δ (NCAL) coatings were made on both solid bipolar plates and porous nickel foams to perform as protecting coatings and electrode catalyst coatings respectively. Microstructure study showed that multi phases were formed and in-situ nano-micro crystallization occurred in the coatings during the LPPS process. Around 30 W output was achieved in a 4-cell stack indicating that the LPPS sprayed NCAL coatings on bipolar plates worked well. A fuel cell based on the NCAL-coated Ni foam reached an open circuit voltage (OCV) at 1.08 V and a maximum power density of 717 mW cm^?2 at 550 °C. This study reveals that LPPS is a promising technology for fabricating coatings of LTSOFC.     

Ni-samaria-doped ceria (Ni-SDC) anode-supported solid oxide fuel cell (SOFC) operating with CO

The performance of nickel-samaria-doped ceria (Ni-SDC) anode-supported cell with CO-CO₂ feed was evaluated. The aim of this work is to examine carbon formation on the Ni-SDC anode when feeding with CO under conditions when carbon deposition is thermodynamically favoured. Electrochemical tests were conducted at intermediate temperatures (550–700 °C) using 20 and 40% CO concentrations. Cell operating with 40% CO at 600–700 °C provided maximum power densities of 239–270 mW cm^?2, 1.5 times smaller than that achieved with humidified H₂. Much lower maximum power densities were attained with 20% CO (50–88 mW cm^?2). Some degradation was observed during the 6 h galvanostatic operation at 0.1 A cm^?2 with 40% CO fuel at 550 °C which is believed due to the accumulation of carbon at the anode. The degradation in cell potential occurred at a rate of 4.5 mV h^?1, but it did not lead to cell collapse. EDX mapping at the cross-section of the anode revealed that carbon formed in the Ni-SDC cell was primarily deposited in the anode section close to the fuel entry point. Carbon was not detected at the electrolyte-anode interface and the middle of the anode, allowing the cell to continue operation with CO fuel without a catastrophic failure.