High performance ceramic nanocomposite fuel cells utilizing LiNiCuZn-oxide anode based on slurry method

A multi-oxide material LiNiCuZn-oxide was prepared through a slurry method as an anode for ceramic nanocomposite fuel cell (CNFC). The CNFCs using this anode material, LSCF as cathode material and a composite electrolyte consisting of CaSm co-doped CeO₂ and (NaLiK)₂CO₃ produced ～1.03 W/cm² at 550 °C due to efficient reaction kinetics at the electrodes and high ionic transport in the nanocomposite electrolyte. The electrochemical impedance spectroscopy revealed low ionic transport losses (0.238 Ω cm²) and low polarization losses (0.124 Ω cm²) at the electrodes. The SEM measurements revealed the porous microstructures of the composite materials at electrode and the dense mixture of CaSm co-doped CeO₂ and (NaLiK)₂CO₃. The Brunauer-Emmett-Teller (BET) analysis revealed high surface areas, 4.1 m²/g and 3.8 m²/g, of the anode and cathode respectively. This study provides a promising material for high performance CNFCs.     

Remarkable ionic conductivity and catalytic activity in ceramic nanocomposite fuel cells

Although ceramic nanocomposite fuel cells (CNFCs) have attracted the attention of the fuel cell community due to their low operating temperature (<600 °C), often the performance of the cells is limited due to the low ionic conductivity of the electrolyte and the sluggish reaction kinetics at the electrodes. This results in high ohmic and charge transfer losses in the cell performance. Here we report nanocomposite electrolyte (GDC-NLC) and electrodes (NiO-GDC-NLC and LSCF-GDC-NLC as anode and cathode respectively) with enhanced ionic conductivity and catalytic activity respectively, which significantly improve the ionic transport in the electrolyte layer (ohmic losses ≈ 0.23 Ω cm²) and the reaction kinetics at the electrodes (polarization losses ≈ 0.63 Ω cm²). Microstructural and phase changes in the materials were characterized with X-ray diffraction, scanning electron microscopy, and differential scanning calorimetry to understand the mechanisms in the cells. Our button fuel cell produced an outstanding performance of 1.02 W/cm² at 550 °C.     

Natural CuFe2O4 mineral for solid oxide fuel cells

Natural mineral, cuprospinel (CuFe₂O₄) originated from natural chalcopyrite ore (CuFeS₂), has been used for the first time in low temperature solid oxide fuel cells. Three different types of devices are fabricated to explore the optimum application of CuFe₂O₄ in fuel cells. Device with CuFe₂O₄ as a cathode catalyst exhibits a maximum power density of 180 mW/cm² with an open circuit voltage 1.07 V at 550 °C. And a power output of 587 mW/cm² is achieved from the device using a homogeneous mixture membrane of CuFe₂O₄, Li₂O-ZnO-Sm_0.2Ce_0.8O₂ and LiNi_0.8Co_0.15Al_0.05O₂. Electrochemical impedance spectrum analysis reveals different mechanisms for the devices. The results demonstrate that natural mineral, chalcopyrite, can provide a new implementation to utilize the natural resources for next-generation fuel cells being cost-effective and make great contributions to the environmentally friendly sustainable energy.     

On the single chamber solid oxide fuel cells

Single chamber solid oxide fuel cells, SC-SOFCs, design and performance is discussed. It is shown that all of them operate on non-selective anodes. They operate on cathodes that become selective only under short residence times. As a result these cells are not functioning as true mixed reactant solid oxide fuel cells (MR-SOFC). The lack of selectivity is a serious draw back. True MR-SOFC can be constructed in ways that make them cheaper in fabrication, providing high power density, high fuel utilization and reduced explosion risk. The fact that SOFCs operate in a single cell is a necessary but not sufficient condition for the proper operation of MR-SOFCs.     

Optimization of the protonic ceramic composition in composite electrodes for high-performance protonic ceramic fuel cells

A composite electrode in protonic ceramic fuel cells (PCFCs) is composed of an ionic conductor, an electronic conductor, and catalysts. To determine the contribution of the properties of the ionic conductor to the electrode performance, three different proton conductors: Ba(Ce_0.75Y_0.15)O_3−δ(BCY), Ba(Ce_0.45Zr_0.30Y_0.15)O_3−δ (BCZY), and Ba(Zr_0.75Y_0.15)O_3−δ (BZY), were used to fabricate composite electrodes for PCFCs. Moreover, their effects on the anode and cathode performances were investigated systematically. In the cathodes, the BCZY and BCY scaffolds showed a better performance than the BZY scaffold. However, in the anodes, the BZY scaffold showed a superior performance to the BCZY or BCY scaffold. This exhibited that the requirements of ion-conducting scaffolds in composite electrodes were different in cathodes and anodes and that the fuel cell performance could be optimized by choosing appropriate ionic conductors for each electrode.     

CuFe2O4/CuO coating for solid oxide fuel cell steel interconnects

CuFe_0.8 (Fe:Cu = 0.8:1, atomic ratio) alloy layer is fabricated on both bare and pre-oxidized SUS 430 steels by direct current magnetron sputtering, followed by exposing at 800 °C in air to obtain a protective coating for solid oxide fuel cell (SOFC) steel interconnects. The CuFe_0.8 alloy layer is thermally converted to CuFe₂O₄/CuO coating, which effectively suppresses the out-migration of Cr. Pre-oxidation treatment not only initially accelerates the formation of CuFe₂O₄/CuO coating but also further inhibits the Cr and Fe outward diffusion. Suppressing outward diffusion of Cr could improve electrical property of oxide scale and decrease the risk of cathode Cr-poisoning. Blocking out-diffusion of Fe is beneficial to stabilize the CuO layer. After 2520 h oxidation, the scale ASR at 800 °C is 66.9 mΩ cm² for coated bare steel, 43.4 mΩ cm² for the coated pre-oxidized steel.     

A high-performance solid oxide fuel cell anode based on lanthanum strontium vanadate

Ceramic composites were prepared by infiltration of La_0.7Sr_0.3VO_3.85 (LSV) into porous scaffolds of yttria-stabilized zirconia (YSZ) and tested for use as solid oxide fuel cell (SOFC) anodes. There was no evidence for solid-state reaction between LSV and YSZ at calcination temperatures up to 1273 K. For calcination at 973 K, LSV formed a continuous film over the YSZ. The LSV phase reduced easily upon heating in H₂ to 973 K, with the reduction forming pores in the LSV and greatly increasing its surface area. The electrodes showed high electronic conductivity after reduction, with a 10-vol% LSV-YSZ composite exhibiting a conductivity of 2 S cm⁻¹ at 973 K. In the absence of an added catalyst, the LSV-YSZ electrodes showed relatively poor performance; however, an electrode impedance of approximately 0.1 Ω cm² was achieved at 973 K in humidified H₂ following addition of 0.5 vol% Pd and 2.8 vol% ceria The LSV-YSZ composites were stable in CH₄ but there was evidence for poisoning of the Pd catalyst by V following high-temperature oxidation.     

An improved synthesis method of ceria-carbonate based composite electrolytes for low-temperature SOFC fuel cells

SDC-carbonate composite electrolytes for low-temperature Solid Oxide Fuel Cells (LTSOFC) have been synthesized by an improved freeze drying method based on the formation of lanthanide citrate complex solution/gel. This method can not only maintain small particle sizes in composite, but also control the carbonate composition precisely. To optimize the electrochemical performance of the composite electrolytes, SDC-carbonate samples with different carbonate contents were prepared and investigated. SEM, EDS, MPD and XRD analyses were applied to characterize the morphology and carbonate content and EIS was used to determine the ionic conductivity of the electrolyte. The highest conductivity achieved was 400 mS/cm at 600 °C.     

Synthesis of hierarchically porous LiNiCuZn-oxide and its electrochemical performance for low-temperature fuel cells

In this work, hierarchically porous composite metal oxide LiNiCuZn-oxide (LNCZO) was successfully synthesized through a sol–gel method with a bio-Artemia cyst shell (AS) as a hard template. The phase and morphology of the products were characterized by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM). The as-synthesized material was used as symmetrical electrodes, anode and cathode, for the SDC-LiNaCO₃ (LNSDC) electrolyte based low temperature solid oxide fuel cell (LTSOFCs), achieving a maximum power density of 132 mW cm⁻² at 550 °C. Besides, a single-component fuel cell device was also demonstrated using a mixture of as-prepared LNCZO and ionic conductor LNSDC, and a corresponding peak power output of 155 mW cm⁻² was obtained, suggesting that the hierarchically porous product has high prospective in the single-component fuel cell.     

Triple-component composite cathode for performance optimization of protonic ceramic fuel cells

The cathode of a protonic ceramic fuel cell must be able to facilitate ion and electron transfer, while simultaneously possessing a high catalytic activity for steam generation and the dissociation of gas-phase molecules. In this study, the performance of a cathode for protonic ceramic fuel cells is optimized by employing a triple-component composite cathode design, which integrates proton conductors, mixed electronic–ionic conductors, and a catalytic layer. Additionally, two other composite cathodes are fabricated for comparison. Owing to its higher electrical conductivity but lower catalytic activity, the composite cathode with protonic ceramic and (Ba_0.95La_0.05) (Fe_0.8Zn_0.2)O_3-δ (BLFZ) exhibits lower ohmic resistance but poor catalytic activity compared to the composite cathode with protonic ceramic and Ba(Co_0.4Fe_0.4Zr_0.1Y_0.1)O_3-δ (BCFZY). The triple-component cathode is fabricated by infiltrating BCFZY into a composite cathode composed of BLFZ and protonic ceramic, and both the ohmic and non-ohmic resistances of the cathode are optimized in CH₄ and H₂ fuels. In particular, the performance of CH₄ fuel is significantly improved by adopting a triple-component cathode. These results suggest a possible contribution of the oxygen reduction reaction at the cathode to the reformation of CH₄ at the anode.     

Model-oriented cast ceramic tape seals for planar solid oxide fuel cells

A straight capillary model is developed to estimate the mass leak rate of the cast ceramic tape seals for planar solid oxide fuel cells (SOFCs), which is further rectified with consideration of microstructure complexity including the tortuosity, cross-section variation and cross-link of leak paths. The size distribution of the leak path, effective porosity and the microstructure complexity are the main factors that influence the leak rate of the cast tape seals. According to the model, Al₂O₃ powders are selected for preparation of the seals by tape casting, and the leak rate is evaluated under various compressive stresses and gauge pressures. The results indicate that Al₂O₃ powder with D₅₀ value about 2 μm and specific surface area near 5 m² g⁻¹ can be used for the cast tape seals; and the obtained leak rate can satisfy the allowable leak limit.     

A high performance intermediate temperature fuel cell based on a thick oxide–carbonate electrolyte

A high performance intermediate temperature fuel cell (ITFC) with composite electrolyte composed of co-doped ceria Ce_0.8Gd_0.05Y_0.15O_1.9 (GYDC) and a binary carbonate-based (52 mol% Li₂CO₃/48 mol% Na₂CO₃), 1.2 mm thick electrolyte layer has been developed. Co-doped Ce_0.8Gd_0.05Y_0.15O_1.9 was synthesized by a glycine–nitrate process and used as solid support matrix for the composite electrolyte. The conductivity of both composite electrolyte and GYDC supporting substrate were measured by AC impedance spectroscopy. It showed a sharp conductivity jump at about 500 °C when the carbonates melted. Single cells with thick electrolyte layer were fabricated by a dry-pressing technique using NiO as anode and Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ or lithiated NiO as cathode. The cell was tested at 450–550 °C using hydrogen as the fuel and air as the oxidant. Excellent performance with high power density of 670 mW cm⁻² at 550 °C was achieved for a 1.2 mm thick composite electrolyte containing 40 wt% carbonates which is much higher than that of a cell based on pure GYDC with a 70 μm thick electrolyte layer.     

Sulfonated MWNT and imidazole functionalized MWNT/polybenzimidazole composite membranes for high-temperature proton exchange membrane fuel cells

Composite membranes used for proton exchange membrane fuel cells comprising of polybenzimidazole (PBI) and carbon nanotubes with certain functional groups were studied, because they could enhance both the mechanical property and fuel cell performance at the same time. In this study, sodium poly(4-styrene sulfonate) functionalized multiwalled carbon nanotubes (MWNT-poly(NaSS))/PBI and imidazole functionalized multiwalled carbon nanotubes (MWNT-imidazole)/PBI composite membranes were prepared. The functionalization of carbon nanotubes involving non-covalent modification and covalent modification were confirmed by FITR, XPS, Raman spectroscopy, and TGA. Compared to unmodified MWNTs and MWNT-poly(NaSS), MWNT-imidazole provided more significant mechanical reinforcement due to its better compatibility with PBI. For MWNT-poly(NaSS)/PBI and MWNT-imidazole/PBI composite membranes at their saturated doping levels, the proton conductivities were up to 5.1 × 10⁻² and 4.3 × 10⁻² S/cm at 160 °C under anhydrous condition respectively, which were slightly higher than pristine PBI (2.8 × 10⁻² S/cm). Also, MWNT-poly(NaSS)/PBI and MWNT-imidazole/PBI composite membranes showed relatively improved fuel cell performance at 170 °C compared to pristine PBI.     

Effects of different lithium compound electrodes on the electrochemical performance of the ceramic fuel cells

The effects of five different lithium compound electrodes LiNi_0.83Co_0.11Mn_0.06O₂ (LNCM-811), LiNi_0.6Co_0.2Mn_0.2O₂ (LNCM-622), LiNi_0.5Co_0.2Mn_0.3O₂ (LNCM-523), LiMO₄ (LMO) and LiCO₂ (LCO) on the electrochemical performance of the ceramic fuel cells with GDC as the electrolyte were investigated. It is found that the maximum power density (MPD) of the cell with LNCM-811 as the symmetrical electrode is the highest in H₂ at 550 °C among the five cells with different electrodes. The ionic conductivity of the composite electrolyte formed during performance testing in the cell with LNCM-811 as electrode is also the highest. With the decrease of Ni content in LNCM, the MPD of the cells with LNCM as electrode gradually decreases. The MPD of the cell with LCO as electrode was 196.9 mW?cm^?2, and MPD of the cell with LMO as electrode was the lowest, only 4.24 mW?cm^?2. According to the characterization results of SEM, FTIR and XPS of the different lithium compound electrode materials and the cells before and after performance test, it was found that the change law of the amount of molten salts such as LiOH produced by the reduction of lithium compound in H₂ is consistent with the change law of the MPD of the cells. It is proved that in addition to providing enough catalysts such as Ni and Co that can catalyze the electrode reaction, the key to the outstanding power generation performance of the cell is to produce a sufficient amount of lithium compound molten salt after being reduced in H₂.     

A high-performance ceramic composite anode for protonic ceramic fuel cells based on lanthanum strontium vanadate

The composite electrodes for protonic ceramic fuel cells (PCFC) were fabricated by infiltration of (La_0.8Sr_0.2)FeO_3−δ (LSF) cathode and (La_0.7Sr_0.3)V_0.90O_3−δ (LSV) anode into a porous protonic ceramic, Ba(Ce_0.51Zr_0.30Y_0.15Zn_0.04)O_3−δ (BCZY-Zn), respectively. Further, Pd-ceria catalysts were added into the composite anode. In the same method, the oxygen ion conducting fuel cells with the yttria-stabilized zirconia as an electrolyte (YSZ cell) were also fabricated. At 973 K, the non-ohmic area specific resistance (ASR) of PCFC (0.09 Ω cm²) was much smaller than that of the YSZ cell (0.28 Ω cm²) although the protonic conductivity of BCZY-Zn was slightly smaller than the oxygen ion conductivity of YSZ. According to the analysis of the symmetric cells with BCZY-Zn as an electrolyte, the LSV-composite anode showed better performance than the LSF-composite cathode at low temperatures.     

The use of rare earth-based materials in low-temperature fuel cells

Rare earth-based materials can play different roles in fuel cell systems. These compounds can be used as catalysts, co-catalysts and electrolytes additives in different types of fuels cells. In particular, a promising acid direct methanol fuel cell can be obtained using rare earth-based materials as both anode and cathode co-catalysts and proton exchange membrane additive. In this work an overview of the use of rare earth-based materials in low-temperature fuel cells is presented.     

A fuel cell operating between room temperature and 250 °C based on a new phosphoric acid based composite electrolyte

A phosphoric acid based composite material with core-shell microstructure has been developed to be used as a new electrolyte for fuel cells. A fuel cell based on this electrolyte can operate at room temperature indicating leaching of H₃PO₄ with liquid water is insignificant at room temperature. This will help to improve the thermal cyclability of phosphoric acid based electrolyte to make it easier for practical use. The conductivity of this H₃PO₄-based electrolyte is stable at 250 °C with addition of the hydrophilic inorganic compound BPO₄ forming a core-shell microstructure which makes it possible to run a PAFC at a temperature above 200 °C. The core-shell microstructure retains after the fuel cell measurements. A power density of 350 mW/cm² for a H₂/O₂ fuel cell has been achieved at 200 °C. The increase in operating temperature does not have significant benefit to the performance of a H₂/O₂ fuel cell. For the first time, a composite electrolyte material for phosphoric acid fuel cells which can operate in a wide range of temperature has been evaluated but certainly further investigation is required.     

New insights into the temperature-water transport-performance relationship in PEM fuel cells

Three-dimensional numerical simulations of a single straight-channel PEMFC at three different operating temperatures (343 K, 353 K, and 363 K) and at a relative humidity RH = 90% were carried out, by using potentiostatic conditions ranging from 0.27 to 1.0 V. This study aimed at gaining further insights into the complex and tightly coupled interactions taking place inside the fuel cell, relating water generation and transport with local temperature distributions and cell performance. A sensitivity analysis concluded that the higher the operating temperature is, the better the electrical performance of the PEMFC, for the range of operating temperatures analyzed. This feature was further investigated at high current densities (j = 2.25 and 2.57 A/cm²), where the increase of the operating temperature (in the range of study) resulted in an enhancement of the water diffusivity and the electro-osmotic drag, improving the ionic conductivity. Additionally, the dimensionless temperature distribution across the cell width was found to be similar in all the cases. Profile-like curves displaying under-rib/under-channel characteristics are presented and analyzed to understand the role of water and its interaction with the different phenomena occurring within the cell. It was demonstrated that colored scatter plots are convenient tools that contribute to explain existing relationships between the water-related magnitudes.     

From macro- to micro-single chamber solid oxide fuel cells

Single chamber solid oxide fuel cells (SC-SOFCs) with interdigitating electrodes were prepared and operated in CH₄/air mixtures. Both electrodes (Ni-Ce_0.8Gd_0.2O_1.9 cermet and Sm_0.5Sr_0.5CoO_3−δ perovskite) were placed on the same side of a Ce_0.8Gd_0.1O_1.95 electrolyte disc. The separating gap between the electrodes was varied from 1.2 to 0.27 mm and finally down to 10 μm. Screen-printing was used for the preparation of the cells with a gap in the millimetre range, whereas micromolding in capillaries (MIMIC) was used for the preparation of the micro-SC-SOFCs.     

Nafion-impregnated electrospun polyvinylidene fluoride composite membranes for direct methanol fuel cells

Composite membranes consisting of polyvinylidene fluoride (PVdF) and Nafion have been prepared by impregnating various amounts of Nafion (0.3–0.5 g) into the pores of electrospun PVdF (5 cm × 5 cm) and characterized by scanning electron microscopy, differential scanning calorimetry, X-ray diffraction, and proton conductivity measurements. The characterization data suggest that the unique three-dimensional network structure of the electrospun PVdF membrane with fully interconnected fibers is maintained in the composite membranes, offering adequate mechanical properties. Although the composite membranes exhibit lower proton conductivity than Nafion 115, the composite membrane with 0.4 g Nafion exhibits better performance than Nafion 115 in direct methanol fuel cell (DMFC) due to smaller thickness and suppressed methanol crossover from the anode to the cathode through the membrane. With the composite membranes, the cell performance increases on going from 0.3 to 0.4 g Nafion and then decreases on going to 0.5 g Nafion due to the changes in proton conductivity.