Preparation and characterization of sulfonated PEEK-WC membranes for fuel cell applications: A comparison between polymeric and composite membranes

Sulfonated poly(etheretherketone) with a cardo group (SPEEK-WC) exhibiting a wide range of degree of sulfonation (DS) was used to prepare polymeric membranes and composite membranes obtained by incorporation of an amorphous zirconium phosphate sulfophenylenphosphonate (Zr(HPO₄)(O₃PC₆H₄SO₃H), hereafter Zr(SPP)) in a SPEEK-WC matrix. The nominal composition of the composite membranes was fixed at 20 wt% of Zr(SPP). Both types of membrane were characterized for their proton conductivity, methanol permeability, water and/or methanol uptake, morphology by SEM and mechanical properties. For comparison, a commercial Nafion 117 membrane was characterized under the same operative conditions. The composite membranes exhibited a reduced water uptake in comparison with the polymeric membranes especially at high DS values and temperature higher than 50 °C. As a result, the water uptake into composite membranes remained about constant in the range 20–70 °C. The methanol permeability (P) of both polymeric and composite membranes was always lower than that of a commercial Nafion 117 membrane. At 22 °C and 100% relative humidity (RH), the proton conductivities (σ) of the polymeric membranes increased from 6 × 10⁻⁴ to 1 × 10⁻² S cm⁻¹ with the increase of DS from 0.1 to 1.04. The higher conductivity value was comparable with that of Nafion 117 membrane (3 × 10⁻² S cm⁻¹) measured under the same operative conditions. The conductivities of the composite membranes are close to that of the corresponding polymeric membranes, but they are affected to a lesser extent by the polymer DS. The maximum value of the σ/P ratio (about 7 × 10⁴ at 25 °C) was found for the composite membrane with DS = 0.2 and was 2.5 times higher than the corresponding value of the Nafion membrane.     

NMR investigation of water and methanol transport in sulfonated polyareylenethioethersulfones for fuel cell applications

We report an investigation of water and methanol transport in polymer electrolyte membranes based on highly sulfonated polyarelenethioethersulfones (SPTES) for direct methanol fuel cell (DMFC) applications. Measurements of both water and methanol self-diffusion coefficients of SPTES polymer as well as in a reference sample of Nafion-117 equilibrated in 2 M methanol solution have been carried out, using the pulsed gradient spin echo technique, over a temperature range of 20–140 °C. The selectivity of the membrane, defined as (D_OH/D_CH3), decreased from 6 to 2.4 as temperature increased from 20 to 140 °C in SPTES sample while in Nafion, the value decreased from 3.2 to 1.4 as temperature increased from 20 to 100 °C. These results indicate significantly lower fuel molecular permeability in SPTES compared to that of Nafion. All results suggest high-temperature stability in these materials, offering the possibility of fuel cell operation at temperatures >120 °C. High pressure NMR diffusion measurements were also carried out for three different water contents (between 20 and 55 wt.%) in a static field gradient in order to get supplemental information regarding water transport in SPTES materials. The calculated activation volume increased from 1.54 to 8.40 cm³/mol as the water content decreased from 55 to 20%. This behavior is qualitatively similar to previously reported results for Nafion-117.     

Effect of (Al,Mg) substitution in LiNiO2 electrode for lithium batteries

Stabilized lithium nickelate is receiving increased attention as a low-cost alternative to the LiCoO₂ cathode now used in rechargeable lithium batteries. Layered LiNi_1−x−yM_xM_yO₂ samples (M_x = Al³⁺ and M_y = Mg²⁺, where x = 0.05, 0.10 and y = 0.02, 0.05) are prepared by the refluxing method using acetic acid at 750 °C under an oxygen stream, and are subsequently subjected to powder X-ray diffraction analysis and coin-cell tests. The co-doped LiNi_1−x−yAl_xMg_yO₂ samples show good structural stability and electrochemical performance. The LiNiAl_0.05Mg_0.05O₂, cathode material exhibits a reversible capacity of 180 mA h g⁻¹ after extended cycling. These results suggest that the threshold concentration for aluminum and magnesium substitution is of the order of 5%. The co-substitution of magnesium and aluminium into lithium nickelate is considered to yield a promising cathode material.     

Opportunity of metallic interconnects for ITSOFC: Reactivity and electrical property

Iron-base alloys (Fe–Cr) are proposed hereafter as materials for interconnect of planar-type intermediate temperature solid oxide fuel cell (ITSOFC); they are an alternative solution instead of the use of ceramic interconnects. These steels form an oxide layer (chromia) which protects the interconnect from the exterior environment, but is an electrical insulator. One solution envisaged in this work is the deposition of a reactive element oxide coating, that slows down the formation of the oxide layer and that increases its electric conductivity. The oxide layer, formed at high temperature on the uncoated alloys, is mainly composed of chromia; it grows in accordance with the parabolic rate law (k_p = 1.4 × 10⁻¹² g² cm⁻⁴ s⁻¹). On the reactive element oxide-coated alloy, the parabolic rate constant, k_p, decreases to 1.3 × 10⁻¹³ g² cm⁻⁴ s⁻¹. At 800 °C, the area-specific resistance of Fe–30Cr alloys is about 0.03 Ω cm² after 24 h in laboratory air under atmospheric pressure. The Y₂O₃ coating reduces the electrical resistance 10-fold. This indicates that the application of Y₂O₃ coatings on Fe–30Cr alloy allows to use it as an interconnect for SOFC.     

Creep behavior of Ni-12 wt.% Al anodes for molten carbonate fuel cells

Ni-12 wt.% Al anodes are fabricated for use in molten carbon fuel cells by tape casting and sintering. Sintering is performed in three steps, first at 1200 °C for 10 min in argon, then at 700 °C for 2.5 h in a partial oxidation atmosphere (P_H2/P_H2O=10⁻²), and finally at 950 °C for 5 min, 30 min or 1.5 h in hydrogen. Three anodes with different phases or microstructures are produced at different reduction times. One anode contains three phases, namely Ni–Al solid solution, Ni₃Al, and Al₂O₃. The amount of Al₂O₃ is extremely small at 5 min. A second anode also contains the three phases with the amount of Al₂O₃ comparable with that of Ni₃Al at 30 min. Third anode contains two phases, i.e. Ni–Al solid solution and Al₂O₃ formed at 1.5 h. The creep strains measured for the three anodes after a 100-h creep test are practically the same with an average value of 0.85%.     

LiMn2O4 cathode materials synthesized by the cellulose–citric acid method for lithium ion batteries

A new technique was employed to synthesize spinel LiMn₂O₄ cathode materials by adding cellulose and citric acid to an aqueous solution of lithium and manganese salts. Various synthesis conditions such as the calcination temperature and the citric acid-to-metal ion molar ratio (R) were investigated to determine the ideal conditions for preparing LiMn₂O₄ with the best electrochemical characteristics. The optimal synthesis conditions were found to be R = 1/3 and a calcination temperature of 800 °C. The initial discharge capacity of the material synthesized using the optimal conditions was 134 mAh g⁻¹, and the discharge capacity after 40 cycles was 125 mAh g⁻¹, at a current density of 0.15 mA cm⁻² between 3.0 and 4.35 V. Details of how the initial synthesis conditions affected the capacity and cycling performance of LiMn₂O₄ are discussed.     

Effects of electrolytes and electrochemical pretreatments on the capacitive characteristics of activated carbon fabrics for supercapacitors

The capacitive characteristics of activated carbon fabrics (ACFs) coated on the graphite substrates were systematically investigated by means of cyclic voltammetry and the galvanostatic charge–discharge technique. Effects of the PVDF contents in the electronically conductive binder, electrochemical pretreatments, and the electrolytes on the capacitive performance of ACFs were compared in aqueous media. These ACF-pasted electrodes showed the more ideally capacitive responses in 1 M NaNO₃ with a specific capacitance of 76 F g⁻¹ when the electronically conductive binder contained 40 wt.% PVDF. The specific capacitance of ACF-pasted electrodes reached a maximum in 0.5 M H₂SO₄ (about 153 F g⁻¹ measured at 25 mV s⁻¹), due to the presence of a suitable density of oxygen-containing functional groups, when they were subjected to the potentiostatic polarization at 1.8 V (versus reversible hydrogen electrode (RHE)) or potentio-dynamic polarization between 1.3 and 1.8 V in NaNO₃ for 20 min. The oxygen-containing functional groups within the electrochemically pretreated ACFs were identified by means of X-ray photoelectron spectroscopy (XPS).     

Effect of organic acids and nano-sized ceramic doping on PEO-based solid polymer electrolytes

Composite solid polymer electrolytes (CSPEs) consisting of polyethyleneoxide (PEO), LiClO₄, organic acids (malonic, maleic, and carboxylic acids), and/or Al₂O₃ were prepared in acetonitrile. CSPEs were characterized by Brewster Angle Microscopy (BAM), thermal analysis, ac impedance, cyclic voltammetry, and tested for charge–discharge capacity with the Li or LiNi_0.5Co_0.5O₂ electrodes coated on stainless steel (SS). The morphologies of the CSPE films were homogeneous and porous. The differential scanning calorimetric (DSC) results suggested that performance of the CSPE film was highly enhanced by the acid and inorganic additives. The composite membrane doped with organic acids and ceramic showed good conductivity and thermal stability. The ac impedance data, processed by non-linear least square (NLLS) fitting, showed good conducting properties of the composite films. The ionic conductivity of the film consisting of (PEO)₈LiClO₄:citric acid (99.95:0.05, w/w%) was 3.25 × 10⁻⁴ S cm⁻¹ and 1.81 × 10⁻⁴ S cm⁻¹ at 30 °C. The conductivity has further improved to 3.81 × 10⁻⁴ S cm⁻¹ at 20 °C by adding 20 w/w% Al₂O₃ filler to the (PEO)₈LiClO₄ + 0.05% carboxylic acid composite. The experimental data for the full cell showed an upper limit voltage window of 4.7 V versus Li/Li⁺ for CSPE at room temperature.     

Synthesis and characterizations of Li[CrxLi(1−x)/3Mn2(1−x)/3]O2 (0.15 ≤ x ≤ 0.3) cathode materials in rechargeable lithium batteries

A series of Li[Cr_xLi_(1−x)/3Mn_2(1−x)/3]O₂ (0.15 ≤ x ≤ 0.3) cathode materials was prepared by citric acid-assisted, sol–gel process. Sub-micron sized particles were obtained and the X-ray diffraction (XRD) results showed that the crystal structure was similar to layered lithium transition metal oxides (R-3m space group). The electrochemical performance of the cathodes was evaluated over the voltage range 2.0–4.9 V at a current density of 7.947 mA g⁻¹. The Li_1.27Cr_0.2Mn_0.53O₂ electrode delivered a high reversible capacity of up to 280 mAh g⁻¹ during cycling. Li[Cr_xLi_(1−x)/3Mn_2(1−x)/3]O₂ yielded a promising cathode material.     

Electrochemical properties of iodine-containing lithium manganese oxide spinel

Iodine-containing, cation-deficient, lithium manganese oxides (ICCD-LMO) are prepared by reaction of MnO₂ with LiI. The MnO₂ is completely transformed into spinel-structured compounds with a nominal composition of Li_1−δMn_2−2δO₄I_x. A sample prepared at 800 °C, viz. Li_0.99Mn_1.98O₄I_0.02, exhibits an initial discharge capacity of 113 mA h g⁻¹ with good cycleability and rate capability in the 4-V region. Iodine-containing, lithium-rich lithium manganese oxides (ICLR-LMO) are also prepared by reaction of LiMn₂O₄ with LiI, which results in a nominal composition of Li_1+xMn_2−xO₄I_x. Li_1.01Mn_1.99O₄I_0.02 shows a discharge capacity of 124 mA h g⁻¹ on the first cycle and 119 mA h g⁻¹ a on the 20th cycle. Both results indicate that a small amount of iodine species helps to maintain cycle performance.     

Synthesis and electrochemical properties of LiNi0.80(Co0.20−xAlx)O2 (x = 0.0 and 0.05) cathodes for Li ion rechargeable batteries

The effect of simultaneous cobalt as well as aluminum doping was studied to understand their effect on the phase formation behavior and electrochemical properties of solution derived lithium nickel oxide cathode materials for rechargeable batteries. The discharge capacities of LiNi_0.80Co_0.20O₂ and LiNi_0.80Co_0.15Al_0.05O₂ cathodes, measured at constant current densities of 0.45 mA cm⁻² in the cut-off voltage range of 4.3–3.2 V, were 100 and 136 mAh g⁻¹, respectively. LiNi_0.80Co_0.15Al_0.05O₂ had better cycleability than the LiNi_0.80Co_0.20O₂ cathodes. The retention of undesirable Li₂CO₃ phase both in LiNi_0.80Co_0.20O₂ and LiNi_0.80Co_0.15Al_0.05O₂ cathodes was argued to be responsible for the relatively lower discharge capacity of these materials.     

Synthesis,structural and electrochemical properties of pulsed laser deposited Li(Ni,Co)O2 films

We report the epitaxial growth of the LiNi_1−yM_yO₂ films (M = Co, Co–Al) on heated nickel foil using pulsed laser deposition in oxygen environment from lithium-rich targets. The structure and morphology was characterized by X-ray diffractometry, electron scanning microscopy and Raman spectroscopy. Data reveal that the formation of oriented films is dependent on two important parameters: the substrate temperature and the gas pressure during ablation. The charge–discharge process conducted in Li-microcells demonstrates that effective high specific capacities can be obtained with films 1.35 μm thick. Stable capacities of 83 and 92 μAh cm⁻² μm are available in the potential range 4.2–2.5 V for LiNi_0.8Co_0.2O₂ and LiNi_0.8Co_0.15Al_0.05O₂ films, respectively. The self-diffusion coefficient of Li ions determined from galvanostatic intermittent titration experiments is found to be 4 × 10⁻¹² cm² s⁻¹.     

The effects of partial substitution of Cr for Ni on the electrochemical properties of Mg1.75Al0.25Ni1−xCrx (0 ≤ x ≤ 0.3) electrode alloys

In this paper, the substitution of different amounts of Cr for Ni in the hydrogen storage electrode alloy of Mg_1.75Al_0.25Ni has been carried out to form quaternary Mg_1.75Al_0.25Ni_1−xCr_x (0 ≤ x ≤ 0.3) alloys by means of solid diffusion method (DM). The XRD profiles exhibited that the quaternary alloys still kept the same main phase of Mg₃AlNi₂ (S.G. Fd3m) as that of ternary Mg_1.75Al_0.25Ni alloy. The electrochemical studies found that Cr substituted quaternary alloy reached its maximum discharge capacity (165 mAh g⁻¹) after 2 cycles, which was larger than that of the Mg_1.75Al_0.25Ni alloy (154 mAh g⁻¹). Among these quaternary alloys, the Mg_1.75Al_0.25Ni_0.9Cr_0.1 electrode alloy was found possessing the highest cycling capacity retention rate. Cyclic voltammetry (CV) results and anodic polarization curves demonstrated that appropriate content (x lower than 0.1) of Cr effectively improved the reaction activity of electrode and inhibited the cycling capacity degradation to some degree. Electrochemical impedance spectroscopy (EIS) analyses indicated that the increase of Cr content would raise the polarization resistance R_p on the particle surface of these quaternary alloys.     

Cracking causing cyclic instability of LiFePO4 cathode material

The capacity of pure LiFePO₄ faded gradually from initial 149 mAh g⁻¹–117 mAh g⁻¹ under current density of 30 mA g⁻¹ at room temperature after 60 cycles. Some obvious cracks are observed in LiFePO₄ particles after cycling. The formation of cracks would lead to poor electric contact and capacity fading. A possible mechanism is proposed for the appearance of the cracks.     

Synthesis and electrochemical properties of Mo-doped Li[Ni1/3Mn1/3Co1/3]O2 cathode materials for Li-ion battery

The layered Li[Ni_(1−x)/3Mn_(1−x)/3Co_(1−x)/3Mo_x]O₂ cathode materials (x = 0, 0.005, 0.01, and 0.02) were prepared by a solid-state pyrolysis method (700, 800, 850, and 900 °C). Its structure and electrochemical properties were characterized by XRD, SEM, XPS, cyclic voltammetry, and charge/discharge tests. It can be learned that the doped sample of x = 0.01 calcined at 800 °C shows the highest first discharge capacity of 221.6 mAh g⁻¹ at a current density of 20 mA g⁻¹ in the voltage range of 2.3–4.6 V, and the Mo-doped samples exhibit higher discharge capacity and better cycle-ability than the undoped one at room temperature.     

The influence of carbon support porosity on the activity of PtRu/Sibunit anode catalysts for methanol oxidation

In this paper we analyse the promises of homemade carbon materials of Sibunit family prepared through pyrolysis of natural gases on carbon black surfaces as supports for the anode catalysts of direct methanol fuel cells. Specific surface area (S_BET) of the support is varied in the wide range from 6 to 415 m² g⁻¹ and the implications on the electrocatalytic activity are scrutinized. Sibunit supported PtRu (1:1) catalysts are prepared via chemical route and the preparation conditions are adjusted in such a way that the particle size is constant within ±1 nm in order to separate the influence of support on the (i) catalyst preparation and (ii) fuel cell performance. Comparison of the metal surface area measured by gas phase CO chemisorption and electrochemical CO stripping indicates close to 100% utilisation of nanoparticle surfaces for catalysts supported on low (22–72 m² g⁻¹) surface area Sibunit carbons. Mass activity and specific activity of PtRu anode catalysts change dramatically with S_BET of the support, increasing with the decrease of the latter. 10%PtRu catalyst supported on Sibunit with specific surface area of 72 m² g⁻¹ shows mass specific activity exceeding that of commercial 20%PtRu/Vulcan XC-72 by nearly a factor of 3.     

Electrochemical performance behavior of combustion-synthesized LiNi0.5Mn0.5O2 lithium-intercalation cathodes

LiNiO₂, partially substituted with manganese in the form of a LiNi_0.5Mn_0.5O₂ compound, has been synthesized by a gelatin assisted combustion method [GAC] method. Highly crystalline LiNi_0.5Mn_0.5O₂ powders with R3m symmetry have been obtained at an optimum temperature of 850 °C, as confirmed by PXRD studies. The presence of cathodic and anodic CV peaks exhibited by the LiNi_0.5Mn_0.5O₂ cathode at 4.4 and 4.3 V revealed the existence of Ni and Mn in their 2+ and 4+ oxidation states, respectively. The synthesized LiNi_0.5Mn_0.5O₂ cathode has been subjected to systematic electrochemical performance evaluation, via capacity tapping at different cut-off voltage limits (3.0–4.2, 3.0–4.4 and 3.0–4.6 V) and the possible extraction of deliverable capacity under different current drains (0.1C, 0.5C, 0.75C and 1C rates). The LiNi_0.5Mn_0.5O₂ cathode exhibited a maximum discharge capacity of 174 mAh g⁻¹ at the 0.1C rate between 3.0 and 4.6 V. However, a slightly decreased capacity of 138 mAh g⁻¹ has been obtained in the 3.0–4.4 V range, when discharged at the 1C rate. On the other hand, extended cycling at the 0.1C rate encountered an acceptable capacity fade in the 3.0–4.4 V range (<10%) for up to 50 cycles.     

Synthesis and characterization of high tap-density layered Li[Ni1/3Co1/3Mn1/3]O2 cathode material via hydroxide co-precipitation

Li[Ni_1/3Co_1/3Mn_1/3]O₂ was prepared by mixing uniform co-precipitated spherical metal hydroxide (Ni_1/3Co_1/3Mn_1/3)(OH)₂ with 7% excess LiOH followed by heat-treatment. The tap-density of the powder obtained was 2.38 g cm⁻³, and it was characterized using X-ray diffraction (XRD), particle size distribution measurement, scanning electron microscope-energy dispersive spectrometry (SEM-EDS) and galvanostatic charge–discharge tests. The XRD studies showed that the material had a well-ordered layered structure with small amount of cation mixing. It can be seen from the EDS results that the transition metals (Ni, Co and Mn) in Li[Ni_1/3Co_1/3Mn_1/3]O₂ are uniformly distributed. Initial charge and discharge capacity of 185.08 and 166.99 mAh g⁻¹ was obtained between 3 and 4.3 V at a current density of 16 mA g⁻¹, and the capacity of 154.14 mAh g⁻¹ was retained at the end of 30 charge–discharge cycles with the capacity retention of 93%.     

Effects of addition of TiO2 nanoparticles on mechanical properties and ionic conductivity of solvent-free polymer electrolytes based on porous P(VdF-HFP)/P(EO-EC) membranes

To enhance the performance (i.e., mechanical properties and ionic conductivity) of pore-filling polymer electrolytes, titanium dioxide (TiO₂) nanoparticles are added to both a porous membrane and its included viscous electrolyte, poly(ethylene oxide-co-ethylene carbonate) copolymer (P(EO-EC)). A porous membrane with 10 wt.% TiO₂ shows better performance (e.g., homogeneous distribution, high uptake, and good mechanical properties) than the others studied and is therefore chosen as the matrix to prepare polymer electrolytes. A maximum conductivity of 5.1 × 10⁻⁵ S cm⁻¹ at 25 °C is obtained for a polymer electrolyte containing 1.5 wt.% TiO₂ in a viscous electrolyte, compared with 3.2 × 10⁻⁵ S cm⁻¹ for a polymer electrolyte without TiO₂. The glass transition temperature, T_g is lowered by the addition of TiO₂ (up to 1.5 wt.% in a viscous electrolyte) due to interaction between P(EO-EC) and TiO₂, which weakens the interaction between oxide groups of the P(EO-EC) and lithium cations. The overall results indicate that the sample prepared with 10 wt.% TiO₂ for a porous membrane and 1.5 wt.% TiO₂ for a viscous electrolyte is a promising polymer electrolyte for rechargeable lithium batteries.     

Effect of synthesis conditions on the properties of LiFePO4 for secondary lithium batteries

LiFePO₄ is one of the promising materials for cathode of secondary lithium batteries due to its high energy density, low cost, environmental friendliness and safety. However, LiFePO₄ has very poor electronic conductivity (∼10⁻⁹ S cm⁻¹) and Li-ion diffusion coefficient (∼1.8 × 10⁻¹⁴ cm² s⁻¹) at room temperature. In an attempt to improve electrochemical properties, Li_XFePO₄ with various amounts of Li contents were investigated in this study. Li_XFePO₄ (X = 0.7–1.1) samples were synthesized by solid-state reaction. High resolution X-ray diffraction, Rietveld analysis, BET, scanning electron microscopy, and hall effect measurement system were used to characterize these samples. Electronic conductivities of the samples with Li-deficient and Li-excess in Li_xFePO₄ were 10⁻³ to 10⁻¹ S cm⁻¹. Discharge capacities and rate capabilities of the samples with Li-deficient and Li-excess in Li_XFePO₄ were higher than those of stoichiometric LiFePO₄ sample. Li_0.9FePO₄ samples fired at 700 °C had discharge capacity of 156 and 140 mAh g⁻¹ at 0.1 C- and 2 C-rate, respectively.