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.     

Intriguing electrochemistry in low-temperature single layer ceramic fuel cells based on CuFe2O4

A composite of CuFe₂O₄ and Gd-Sm co-doped CeO₂ is studied for a single layer ceramic fuel cell application. In order to optimize the cell performance, the effects of sintering temperatures (600 °C, 700 °C, 800 °C, 900 °C and 1000 °C) were investigated for the fabrication of the cells. It was found that the cells sintered at 700 °C outperformed other cells with a maximum peak power density of 344 mW/cm² at 550 °C. The electrochemical impedance spectroscopy analysis on the best cell revealed significant ohmic losses (0.399 Ω cm²) and polarization losses (0.174 Ω cm²) in the cell. The HR-TEM and SEM gave microstructural information of the cell. The HT-XRD spectra showed the crystal structures in different sintering temperatures. The cell performance was stable and the composite material did not degrade during an 8 h stability test under open-circuit condition. This study opens up new avenues for the exploration of this nanocomposite material for the low temperature single component ceramic fuel cell research.     

Modeling the performance of molten carbonate based direct carbon fuel cell anode

A mathematical model is developed to study the performance of a molten carbonate based direct carbon fuel cell anode. The direct carbon fuel cell(DCFC) is a fuel cell which uses solid carbon as fuel and molten carbonate as electrolyte. The model assumes that the 4 electron carbon oxidation reaction is the primary reaction driving the DCFC. However, the 2 electron CO oxidation reaction and the reverse Boudouard reaction is also considered in this model. The model studies the effect of performance parameters on the performance of the DCFC. The effect of the bulk conductivity in the solid phase, the bulk conductivity in the liquid phase, carbon loading and the thickness of the anode layer on the potential and current distribution in the cell is modeled. Model results are compared with experimental data and found to compare well.     

Synthesis and properties of fluorine-containing polybenzimidazole/montmorillonite nanocomposite membranes for direct methanol fuel cell applications

Novel polybenzimidazole (PBI)/montmorillonite (MMT) nanocomposite membranes were prepared from an organosoluble, fluorine-containing PBI with an organically modified MMT (m-MMT) clay. Both wide angle X-ray diffraction (WAXD) and transmission electron microscopy (TEM) analyses showed that the m-MMT was well dispersed in the PBI matrix on a nanometer scale. The thermooxidative stability of PBI membranes increased slightly with the increase of m-MMT content. The coefficients of the thermal expansion (CTE) of PBI/7 wt% m-MMT nanocomposite membranes were decreased by 30% relative to that of pure PBI. The mechanical properties and the methanol barrier ability of the PBI films were significantly improved by the addition of m-MMT. The tensile modulus of PBI/5 wt% m-MMT nanocomposite membranes had a 41% increase compared to the pure PBI films. The m-MMT in the phosphoric acid-doped PBI could effectively inhibit the plasticizing effect of the phosphoric acid. The methanol permeability of the PBI/5 wt% m-MMT nanocomposite membrane decreased by approximately 81% with respect to the pure PBI membranes. The conductivity of the acid-doped PBI/m-MMT nanocomposites was slightly lower than the acid-doped pure PBI.     

Preparation and characterization of phosphotungstic acid-derived salt/Nafion nanocomposite membranes for proton exchange membrane fuel cells

Nafion/Cs_2.5H_0.5PW₁₂O₄₀ nanocomposite membranes are prepared and characterized as alternate materials for PEMFC operation at high temperature/low humidity. The Cs_2.5H_0.5PW₁₂O₄₀ solid acid particles (hereafter CsPWA) have the high surface area, the high hygroscopic property and the ability to generate proton in the presence of water molecules. The results of prepared membranes at three levels (0, 10 and 15%) indicate that the CsPWA particles have influence on the water content, ion exchange capacity, thermal properties (TGA and DSC), proton conductivity and PEM fuel cell performance. Particles agglomeration and Nafion active sites (sulfonic groups) covering are seen in the nanocomposite membranes. The conductivity of nanocomposite membranes at high temperatures (110 and 120 °C) is higher than plain Nafion and may be related to the additional water within the nanocomposite membrane and/or the additional surface functional site provide by CsPWA. The fuel cell responses show that in the fully hydrated state and at the higher current densities, the prepared MEAs with nanocomposite membranes possess better response compared with the plain Nafion. In partially hydrated cell, at both low and high current densities, the superior performance of the MEA prepared by nanocomposite membranes is observed.     

Preparation and properties of carbon nanotube/polypropylene nanocomposite bipolar plates for polymer electrolyte membrane fuel cells

This study aims at the fabrication of lightweight and high performance nanocomposite bipolar plates for the application in polymer electrode membrane fuel cells (PEMFCs). The thin nanocomposite bipolar plates (the thickness <1.2 mm) consisting of multiwalled carbon nanotubes (MWCNTs), graphite powder and PP were fabricated by means of compression molding. Three types of polypropylene (PP) with different crystallinities including high crystallinity PP (HC-PP), medium crystallinity PP (MC-PP), low crystallinity PP (LC-PP) were prepared to investigate the influence of crystallinity on the dispersion of MWCNTs in PP matrix. The optimum composition of original composite bipolar plates was determined at 80 wt.% graphite content and 20 wt.% PP content based on the measurements of electrical and mechanical properties with various graphite contents. Results also indicate that MWCNTs was dispersed better in LC-PP than other PP owing to enough dispersed regions in nanocomposite bipolar plates. This good MWCNT dispersion of LC-PP would cause better bulk electrical conductivity, mechanical properties and thermal stability of MWCNTs/PP nanocomposite bipolar plates. In the MWCNTs/LC-PP system, the bulk electrical conductivities with various MWCNT contents all exceed 100 S cm⁻¹. The flexural strength of the MWCNTs/LC-PP nanocomposite bipolar plate with 8 phr of MWCNTs was approximately 37% higher than that of the original nanocomposite bipolar plate and the unnotched Izod impact strength of MWCNTs/LC-PP nanocomposite bipolar plates was also increased from 68.32 J m⁻¹ (0 phr) to 81.40 J m⁻¹ (8 phr), increasing 19%. In addition, the coefficient of thermal expansion of MWCNTs/LC-PP nanocomposite bipolar plate was decreased from 32.91 μm m⁻¹ °C⁻¹ (0 phr) to 25.79 μm m⁻¹ °C⁻¹ (8 phr) with the increasing of MWCNT content. The polarization curve of MWCNTs/LC-PP nanocomposite bipolar plate compared with graphite bipolar plate was also evaluated. These results confirm that the addition of MWCNTs in LC-PP leads to a significant improvement on the cell performance of the nanocomposite bipolar plate.     

Preparation and properties of functionalized multiwalled carbon nanotubes/polypropylene nanocomposite bipolar plates for polymer electrolyte membrane fuel cells

Multiwalled carbon nanotubes (MWCNTs) are covalently modified with different molecular weights 400 and 2000 poly(oxyalkylene)-amine bearing the diglycidyl ether of bisphenol A (DGEBA) epoxy (POA400-DGEBA and POA2000-DGEBA) oligomers. The oxidized MWCNTs (MWCNTs-COOH) are converted to the acid chloride-functionalized MWCNTs, followed by the reaction with POA-DGEBAs to prepare the MWCNTs/POA400-DGEBA and MWCNTs/POA2000-DGEBA. FTIR, thermogravimetric analysis (TGA) and high resolution X-ray photoelectron spectra (XPS) reveal that the POA-DGEBAs are covalently attached to the surface of MWCNTs. The morphology of MWCNTs/POA-DGEBA is observed by TEM. The POA400-DGEBA coated on the MWCNTs is thicker and more uniform. However, the coating of POA2000-DGEBA on the MWCNTs shows a worm-like bulk substance and the MWCNT surface is bare. In addition, the flexural strength and the bulk electrical conductivity of the MWCNTs/polypropylene nanocomposite bipolar plates are measured 59% and 505% higher than those of the original composite bipolar plates by adding 8 phr of MWCNTs/POA400-DGEBA. The maximum current density and power density of the single cell test for the nanocomposite bipolar plate with 4 phr MWCNTs/POA400-DGEBA are 1.32 A cm⁻² and 0.533 W cm⁻², respectively. The overall performance confirms the functionalized MWCNTs/polypropylene nanocomposite bipolar plates prepared in this study are suitable for PEMFC application.     

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.     

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.     

Conductivity of Nafion 117 membrane used in polymer electrolyte fuel cells

The membrane electric transport (MC) directly influences the performance of the polymer electrolyte fuel cells (PEMFC). The membrane conductivity is determined by a number of parameters such as: hydration technique, graphite cell geometry and pressure applied when the membrane electrode assembly (MEA) is joined. In addition, the membrane conductivity might be influenced by the electrode position due to the possibility of anisotropic electric conductivity.     

An overview of nanomaterials in fuel cells: Synthesis method and application

The emerging of fuel cell as one of the promising future energy sources is due to its vast advantages and applications. Despite many challenges to commercialize it, nanostructured materials have brought a new innovation and finding in order to overcome them. The utilization of nanomaterials in various components of fuel cell (catalyst, electrolyte/membrane, electrodes) can defeat many restrictions such as expensive materials and fuel crossover that hinder its commercialization. The distinct properties of nanomaterials including their high surface area and unique size effect can greatly increase overall efficiency as well as the performance of cell. This article provides an overview of the current breakthrough in the performance of fuel cell through the employment of nanomaterials in its major components. The development of nanomaterials can be categorized into four classes namely zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) that synthesized by different methods and their current progress in fuel cell application are also addressed. Selecting a suitable synthesizing method for nanomaterials is the primary stage that determines the properties of the catalysts and the membrane fabricated. This article also discusses the parameters involved in the methods used that can affect the cell performance. The advantages and the drawbacks of each method are also reviewed.     

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.     

High performance tubular protonic ceramic fuel cells via highly-scalable extrusion process

We report an effective method to fabricate long, anode-supported tubular protonic ceramic fuel cells (PCFCs) and test cells in single-cell and short-stack mode. Further, we use our tubular PCFC platform to directly compare three high performance cathodes reported in literature: BaCo_0·4Fe_0·4Zr_0·1Y_0·1O_3-δ (BCFZY), Ba_0·5Sr_0·5Co_0·8Fe_0·2O_3-δ (BSCF), and PrBa_0.5Sr_0·5Co_1·5Fe_0·5O_6-δ (PBSCF) using indentical preparation methods, which can minimize effects from variation of materials either due to suppliers or subsequent processing and testing from different research labs. Using a BCFZY cathode, the maximum power density of our tubular PCFC reaches 164, 308, and 517 mW cm^?2 at 500, 550, and 600 °C, respectively. A 2-cell tubular short stack provides a total power of 2.3 W at 600 °C with tube diameters of 0.82 cm and a total tube active length of 3.2 cm. At 600 °C, the maximum power density reaches, 534, 517, and 326 mw cm^?2 for the BSCF, BCFZY, and PBSCF cathodes, respectively. Under the same conditions, the BSCF-based cell shows the lowest total resistance mostly due to the lowest ohmic resistance and modest polarization resistance. The BCFZY-based cell has the lowest polarization resistance but larger ohmic resistance leading to a slightly higher total resistance than BSCF. The PBSCF cell has an ohmic resistance close to BSCF but a total polarization resistance much larger than either BSCF or BCFZY cell which results in the lowest overall performance.     

Preparation and properties of carbon nanotube-reinforced vinyl ester/nanocomposite bipolar plates for polymer electrolyte membrane fuel cells

Novel multiwalled carbon nanotubes (MWNTs) were prepared using poly(oxypropylene)-backboned diamines of molecular weights M_w 400 and 2000 to disperse acid-treated MWNTs, improving the performance of composite bipolar plates in polymer electrolyte membrane fuel cells. A lightweight polymer composite bipolar plate that contained vinyl ester resin, graphite powder and MWNTs was fabricated using a bulk molding compound (BMC) process. Results demonstrate that the qualitative dispersion of MWNTs crucially determined the resultant bulk electrical conductivity, the mechanical properties and the physical properties of bipolar plates. The flexural strength of the composite bipolar plate with 1 phr of MWNTs was approximately 48% higher than that of the original composite bipolar plate. The coefficient of thermal expansion of the composite bipolar plate was reduced from 37.00 to 20.40 μm m⁻¹ °C⁻¹ by adding 1 phr of MWNTs, suggesting that the composite bipolar plate has excellent thermal stability. The porosity of the composite bipolar plate was also evaluated. Additionally, the bulk electrical conductivity of the composite bipolar plate with different MWNTs types and contents exceeds 100 S cm⁻¹. The results of the polarization curves confirm that the addition of MWNTs leads to a significant improvement on the single cell performance.     

An investigation into the effect of anode purging on the fuel cell performance

The anode purge is a crucial process for the fuel cell long time operation because when the hydrogen is supplied in a circulation mode, any impurities present in hydrogen will gradually accumulate which lead to output voltage loss. A mathematical model is proposed for the purge process based on some operational purge parameters. The governing equations are solved and the effect of purge process on the stack working parameters is analyzed. Purge operational parameters are determined in such a way that the minimum pressure fluctuations in the anode compartment and a compromise between the minimum voltage loss and minimum hydrogen waste are achieved. A semi-stable condition is introduced and indicated that the behavior of voltage loss and hydrogen waste at this condition with respect to purge stop time (duration which the purge valve is closed) is semi-logarithmic.     

Investigation of the anode reactions in solid oxide electrolyte based carbon fuel cells

The anode reactions of solid oxide electrolyte based carbon fuel cells (SO-CFCs) are explored by comparing the electrochemical behaviors of SO-CFCs under varying anode carrier gas flow rates (FAr) and at different contact modes. The electrochemical performance of four raw carbon fuels, including a graphitic carbon (GC), two coals (lignite CF and anthracite YQ) and an activated carbon (AC), and their chars is tested to investigate the influence of carbon fuel properties on the cell performance. The results show that CO electro-oxidation and C-CO₂ gasification were main anode reactions. The direct carbon electro-oxidation is insignificant under high F_Ar. Polarization performance of the chars under high F_Ar was similar with that of 5–10% CO. It is also concluded that the cell performance is greatly dependent on the carbon fuel gasification reactivity with CO₂. Thermal pretreated AC displays the best durability performance for its stable and moderate CO₂ gasification rate. Additionally, the coal ash does not affect the cell performance significantly.     

Advanced ceramic interconnect material for solid oxide fuel cells: Electrical and thermal properties of calcium- and nickel-doped yttrium chromites

The structural, thermal and electrical characteristics of calcium- and nickel-doped yttrium chromites were studied for potential use as the interconnect material in high temperature solid oxide fuel cells (SOFCs) and other high temperature electrochemical and thermoelectric devices. The Y_0.8Ca_0.2Cr_1−xNi_xO_3±δ compositions with x = 0-0.15 showed single phase orthorhombic perovskite structures between 25 and 1200 °C over a wide range of oxygen partial pressures. Nickel doping remarkably enhanced sintering behavior of otherwise refractory chromites, and densities 94% of theoretical density were obtained after sintering at 1400 °C in air with 15 at.% Ni. The thermal expansion coefficient (TEC) was increased with nickel content to closely match that of an 8 mol% yttria-stabilized zirconia (YSZ) electrolyte for 0.05 ≤ x ≤ 0.15. Nickel doping significantly improved the electrical conductivity in both oxidizing and reducing atmospheres. Undesirable oxygen ion “leakage” current was insignificant in dual atmosphere conditions. No interfacial interactions with YSZ were detected after firing at 1400 °C.     

Fabrication of ceramic films for solid oxide fuel cells via slurry spin coating technique

Thin ceramic films of samaria-doped ceria (SDC) were deposited on green NiO–SDC substrate via a slurry spin coating technique followed by co-firing. The ceramic films as-prepared are homogenous and dense, without cracks and penetrating pinholes, as observed from cross-sectional SEM images. The thicknesses of the ceramic films for one coating run can be adjusted between 0.8 and 9 μm by altering the spin rate. The film thickness (h) is inversely proportional to the logarithm of the spin rate (Log(f)) in the range of 3000–10,000 rpm. The slurry spin coating procedure is largely a competition between the thinning and the drying. Half-cells with both 15 and 25 mm diameters were fabricated. In addition, YSZ electrolyte layer with a thickness of 15 μm was also deposited with a homogeneous and completely dense microstructure by two runs of slurry spin coating.     

The influences of multi-walled carbon nanotube addition to the anode on the performance of direct methanol fuel cells

Different amounts of multi-walled carbon nanotubes (MWCNTs) are added to anode catalyst layer in the membrane electrode assemblies (MEAs) of direct methanol fuel cells (DMFCs). The MEA with 0.5 wt.% carbon nanotubes (CNTs) shows the best performance in DMFC. In the protonic conductivity tests, a 0.5 wt.% amount of MWCNTs results in the highest protonic conductivity. SEM and TEM observations show that a continuous and uniform distribution of Nafion ionomer layer is formed on the MWCNT surface. Therefore, the dispersed MWCNTs in the catalyst layer are considered to be helpful for developing the pathways of protons transport.     

On the performance of membraneless laminar flow-based fuel cells

This paper reports on the characterization and optimization of laminar flow-based fuel cells (LFFCs) for both performance and fuel utilization. The impact of different operating conditions (volumetric flow rate, fuel-to-electrolyte flow rate ratio, and oxygen concentration) and of different cell dimensions (electrode-to-electrode distances, and channel length) on the performance (both power density and fuel utilization) of individual LFFCs is investigated. A finite-element-method simulation, which accounts for all relevant transport processes and electrode reactions, was developed to explain the experimental results here. This model can be used to guide further LFFC optimizations with respect to cell design and operation conditions. Using formic acid as the fuel, we measured a peak power density of 55 mW cm⁻². By hydrodynamically focusing the fuel to a thin stream on the anode we were able to reduce the fraction of fuel that passes through the channel without reacting, thereby increasing the fuel utilization per pass to a maximum of 38%. This paper concludes with a discussion on the various trade-offs between maximizing power density and optimizing fuel utilization per pass for individual LFFCs, in light of scaling out to a multichannel LFFC-based power source system.