Study of electrochemical hydrogen charge/discharge properties of FePO4 for application as negative electrodes in hydrogen batteries

We report the electrochemical hydrogen charge/discharge properties of electrodes containing crystalline and amorphous FePO₄ as active material in KOH electrolyte. Crystalline and amorphous FePO₄ were synthesized by an alcohol-assisted precipitation method, and the powders obtained were characterized by X-ray diffraction. X-ray photoelectron spectroscopy is used to investigate the mechanism of hydrogen charge/discharge behavior of FePO₄. The electrochemical hydrogen charge/discharge properties of electrodes containing crystalline and amorphous FePO₄ were investigated for potential application as negative electrodes in rechargeable hydrogen batteries. In galvanostatic discharge/charge mode at 25 °C, the crystalline FePO₄ showed a maximum discharge capacity of 109 mA h g⁻¹, while the amorphous FePO₄ showed a maximum discharge capacity of 81.4 mA h g⁻¹. The electrochemical kinetic properties, exchange current density, and proton diffusivity were calculated using linear polarization measurement and the potential-step method.     

The electrochemical performance of a La-Mg-Ni-Co-Mn metal hydride electrode alloy in the temperature range of −20 to 30 °C

The effect of temperature on the overall electrochemical properties of La_0.7Mg_0.3Ni_2.875Co_0.525Mn_0.1 hydrogen storage alloy has been studied systematically. The results show that temperature has a striking effect on the overall electrochemical properties, especially the electrochemical kinetic performance. The maximum discharge capacity and the high rate dischargeability (HRD) of La_0.7Mg_0.3Ni_2.875Co_0.525Mn_0.1 alloy electrode both decrease with decreasing test temperature, mainly due to the slower hydrogen transfer in the bulk of the alloy and the lower electrocatalytic activity at lower temperatures. Detailed studies on the temperature effect on the polarization resistance (R_D), the exchange current density (I₀), the limiting current density (I_L) and the hydrogen diffusion coefficient (D), indicate that the diffusion of hydrogen in the bulk for La-Mg-Ni-Co system hydrogen storage alloy electrodes is the rate-determining factor for the discharge process of the alloy electrode for the temperature over 10 °C and the charge-transfer reaction is rate-determining step at lower temperature.     

Electrochemical hydrogen storage in (Ti1−xVx)2Ni (x = 0.05-0.3) alloys comprising icosahedral quasicrystalline phase

For (Ti_1−xV_x)₂Ni (x = 0.05, 0.1, 0.15, 0.2 and 0.3) ribbons, synthesized by arc-melting and subsequent melt-spinning techniques, an icosahedral quasicrystalline phase was present, either in the amorphous matrix or together with the stable Ti₂Ni-type phase. With increasing x values, the maximum discharge capacity of the alloy electrodes increased until reached 271.3 mAh/g when x = 0.3. The cycling capacity retention rates for these electrodes were approximately 80% after a preliminary test of 30 consecutive cycles of charging and discharging. Ti_1.7V_0.3Ni alloy electrode displayed the best high-rate discharge ability of 82.7% at the discharge current density of 240 mA/g.     

Preparation of a graphene nanosheet/polyaniline composite with high specific capacitance

A graphene nanosheet (GNS)/polyaniline (PANI) composite was synthesized using in situ polymerization. The morphology and microstructure of samples were examined by scanning electron microscopy (SEM), transition electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy. Electrochemical properties were characterized by cyclic voltammetry (CV) and galvanostatic charge/discharge. GNS as a support material could provide more active sites for nucleation of PANI as well as excellent electron transfer path. The GNS was homogeneously coated on both surfaces with PANI nanoparticles (∼2 nm), and a high specific capacitance of 1046 F g⁻¹ (based on GNS/PANI composite) was obtained at a scan rate of 1 mV s⁻¹ compared to 115 F g⁻¹ for pure PANI. In addition, the energy density of GNS/PANI composite could reach 39 W h kg⁻¹ at a power density of 70 kW kg⁻¹.     

A study on structure and electrochemical properties of (La, Ce, Pr, Nd)2MgNi9 hydrogen storage electrode alloys

(La, Ce, Pr, Nd)₂MgNi₉ hydrogen storage alloys were prepared through induction melting followed by a long annealing treatment. The structure and electrochemical properties of annealed alloys have been investigated by orthogonal design experiments. Both the individual effects of each substituting element and their interaction in alloys were studied systemically. It has been shown that the structure of main phase in alloys belongs to PuNi₃-type with a space group R-3m. Substituting rare-earth elements have a significant effect on both the phase structure of alloys and microstructure. The anisotropic change in the crystal structure of alloys can cause the acceleration of pulverization of alloy particles and result in the deterioration of the cyclic stability of alloy electrodes. Misch metals can raise the plateau pressure of hydrogen absorption/desorption. The discharge capacity of alloy ranges from 342.97 to 380.68 mAh g⁻¹ depending on the sort and content of substituting elements. Both cerium and neodymium can obviously reduce the discharge capacity of alloy electrodes. When compared to the La₂MgNi₉ alloy electrode, mish metals can significantly improve the high rate dischargeability of alloy electrodes. The improvement of the kinetic characteristic of alloy electrodes mainly results from the increase of the hydrogen diffusion rate in alloy bulk.     

The structure and electrochemical performance of LiFeBO3 as a novel Li-battery cathode material

LiFeBO₃ cathode material has been synthesized successfully by solid-state reaction using Li₂CO₃, H₃BO₃ and FeC₂O₄·2H₂O as starting materials. The crystal structure has been determined by the X-ray diffraction. Electrochemical tests show that an initial discharge capacity of about 125.8 mAh/g can be obtained at the discharge current density of 5 mA/g. When the discharge current density is increased to 50 mA/g, the specific capacity of 88.6 mAh/g can still be held. In order to further improve the electrochemical properties, the carbon-coated LiFeBO₃, C-LiFeBO₃, are also prepared. The amount of carbon coated on LiFeBO₃ particles was determined to be around 5% by TG analysis. In comparison with the pure LiFeBO₃, a higher discharge capacity, 158.3 mAh/g at 5 mA/g and 122.9 mAh/g at 50 mA/g, was obtained for C-LiFeBO₃. Based on its low cost and reasonable electrochemical properties obtained in this work, LiFeBO₃ may be an attractive cathode for lithium-ion batteries.     

Crystallographic and electrochemical characteristics of TiZrNiCu quasicrystal ball-milled with La0.9Zr0.1Ni4.5Al0.5 alloy

Icosahedral quasicrystalline Ti₄₅Zr₃₅Ni₁₇Cu₃ alloy was ball-milled with 30 mass% La_0.9Zr_0.1Ni_4.5Al_0.5 alloy (LaNi₅ phase), the effect of the milling time on crystallographic and electrochemical characteristics of the alloy powder was investigated. The amount of amorphous phase increased with increasing milling time from 60 to 360 min, and the LaNi₅ phase cannot be observed when milling time was 240 min or more. The maximum discharge capacity and high-rate dischargeability of milled alloy electrodes were obviously higher than those of the alloy electrode before milling. The cycling capacity retention rate after 40 cycles increased from 52.8% (t = 60 min) to 62.9% (t = 360 min).     

The improved electrochemical properties of novel La-Mg-Ni-based hydrogen storage composites

In the present study, a novel alloy composite has been synthesized by ball milling nonstoichiometric AB₃-type La_0.7Mg_0.3Ni_3.5 alloy with Ti_0.17Zr_0.08V_0.35Cr_0.1Ni_0.3 alloy in order to improve the cyclic stability and other electrochemical properties of La_0.7Mg_0.3Ni_3.5 alloy electrode. The phase structure, morphology and electrochemical performances of the composite have been investigated systematically. From X-ray diffraction (XRD) patterns, it can be found that the La_0.7Mg_0.3Ni_3.5 and Ti_0.17Zr_0.08V_0.35Cr_0.1Ni_0.3 alloys still retain their respective phase structures in the composite. Electrochemical studies show that the cyclic stability of the composite electrode is noticeably improved after 100 charge-discharge cycles in comparison with single La_0.7Mg_0.3Ni_3.5 alloy electrode due to enhanced anti-corrosion performance in the alkaline electrolyte. The discharge capacity retention rate C₁₀₀/C_max of composite electrode is 62.3%, which is much higher than that of the La_0.7Mg_0.3Ni_3.5 alloy electrode, although the maximum discharge capacity of the former decreases moderately. Both electrochemical impedance spectra (EIS) and linear polarization (LP) studies indicate that the electrochemical kinetics of the composite electrode is also improved. The charge-transfer resistance (R_ct), the polarization resistance (R_p) and the exchange current density (I₀) of the composite electrode are 160.2 mΩ, 129.5 mΩ and 201.6 mA/g, respectively, which are superior to those of the La_0.7Mg_0.3Ni_3.5 alloy electrode.     

Low temperature electrochemical properties of LaNi4.6−xMn0.4Mx (M = Fe or Co) and effect of oxide layer on EIS responses in metal hydride electrodes

Iron is a key element in the development of Co-free AB₅-type hydrogen storage alloys. The aim of this work is to systematically investigate the effects of Fe and Co on the electrochemical properties of LaNi_4.6−xMn_0.4M_x (M = Fe or Co, x = 0, 0.25, 0.5 and 0.75) hydrogen storage alloys under relatively low temperatures (273, 253 and 233 K). The results showed that substitution of Fe for Ni reduced the low temperature electrochemical performance much more seriously than that of Co. Exchange current density (I₀), charge-transfer resistance (R_ct) and hydrogen diffusion coefficient (D) were determined based on the study of linear polarization, electrochemical impedance spectrum (EIS) and galvanostatic discharge, respectively. Both the hydrogen diffusion in the bulk of alloy particles and the electrochemical reaction at the alloy electrolyte interface were found to be greatly limited as the decrease of temperature. During the EIS analysis, interestingly, we found that the semicircle in the high frequency region increased dramatically with the decrease of temperature. The electrochemical process corresponding to this semicircle was proposed to be related to the oxide layer on the surface of alloy particles. Novel explanations of EIS response in metal hydride electrodes were proposed accordingly.     

Electrochemical impedance study of polyaniline electrocoated porous carbon foam

Electropolymerization of aniline on mesophase pitch based carbon foam has been studied in order to evaluate the influence of conductive polymer coating on the properties of carbon foam. The surface morphology of the coating was determined by scanning electron microscopy (SEM). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical properties of resulting modified carbon foam samples. Polyaniline (PANI) electrocoated-mesophase pitch based carbon foam showed good capacitor behavior in 0.5 M H₂SO₄. Better capacitive behavior is obtained for 100 and 150 mV/s compared to other scan rates, under these faster scan rates thinner films of PANI coatings were combined with more porous structure of carbon foam. Conductivity of the carbon foam was increased from 9.23 to 13.73 S/cm by electrocoating of PANI.     

High capacitance properties of polyaniline by electrochemical deposition on a porous carbon substrate

Electrochemical deposition of polyaniline (PANI) is carried out on a porous carbon substrate for supercapacitor studies. The effect of substrate is studied by comparing the results obtained using platinum, stainless steel and porous carbon substrates. PANI deposited at 100 mV s⁻¹ sweep rate by potentiodynamic technique on porous carbon substrate is found to possess superior capacitance properties. Experimental variables, namely, concentrations of aniline monomer and H₂SO₄ supporting electrolyte are varied and arrived at the optimum concentrations to obtain a maximum capacitance of PANI. Low concentrations of both aniline and H₂SO₄, which produce PANI at low rates, are desirable. The PANI deposits prepared under these conditions possess network morphology of nanofibrils. Capacitance values as high as 1600 F g⁻¹ are obtained and PANI coated carbon electrodes facilitate charge-discharge current densities as high as 45 mA cm⁻² (19.8 A g⁻¹). Electrodes are found to be fairly stable over a long cycle-life, although there is some capacitance loss during the initial stages of cycling.     

Effects of cathode fabrication conditions and cycling on the electrochemical performance of LiNiO2 synthesized by combustion and calcination

LiNiO₂ was synthesized by the combustion method with various excess lithium amount z in Li_1 + zNiO₂ (z = 0.04, 0.08, 0.10, 0.12, and 0.15). The sample with z = 0.10 has the largest first discharge capacity of 195 mAh/g at 0.1 C rate and voltage range 2.7-4.4 V with the weight ratio of active material:acetylene black:binder = 85:10:5. The LiNiO₂ cathodes, in which the excess lithium amount z for the synthesis of LiNiO₂ was 0.10, were fabricated with various weight ratios of active material:acetylene black:binder (85:10:5, 85:12:3, and 90:7:3). The cathode with the ratio of active material:acetylene black:binder 85:10:5 has the best electrochemical properties. The variation, with C-rate, of discharge capacity vs. number of cycles curve for the LiNiO₂ cathode with the weight ratio of active material:acetylene black:binder = 85:10:5 was investigated. At 0.1 C rate, the LiNiO₂ cathode has the largest first discharge capacity, the discharge capacity degradation rate of 0.70 mAh/g/cycle and a discharge capacity at n = 50 of 134 mAh/g.     

A study on the electrochromic properties of polyaniline/silica composite films with an enhanced optical contrast

A polyaniline (PANI)/silica (SiO₂) composite film was prepared by a simple in situ electrodeposition method, and its electrochromic (EC) properties were studied. When PANI was electrodeposited in the presence of SiO₂ particles, the resulting PANI/SiO₂ composite films possessed higher surface areas, larger redox charge capacities, and higher doping levels, thus enhanced optical contrasts. By incorporating SiO₂ particles into the PANI film, the transmittance change at 700 nm from 10.7% to 16.4%, or an enhancement of ca. 50%, was achieved when operating between −0.5 V and 0 V. In addition, x-ray photoelectron spectroscopy (XPS) indicated that the formation of the highly oxidized PANI might have been retarded through the addition of SiO₂ particles. Furthermore, the growth mechanism of the PANI/SiO₂ composite film was proposed.     

Polyaniline/single-wall carbon nanotube (PANI/SWCNT) composites for high performance supercapacitors

PANI/SWCNT composites were prepared by electrochemical polymerisation of polyaniline onto SWCNTs and their capacitive performance was evaluated by means of cyclic voltammetry and charge-discharge cycling in 1 M H₂SO₄ electrolyte. The PANI/SWCNT composites single electrode showed much higher specific capacitance, specific energy and specific power than pure PANI and SWCNTs. The highest specific capacitance, specific power and specific energy values of 485 F/g, 228 W h/kg and 2250 W/kg were observed for 73 wt.% PANI deposited onto SWCNTs. PANI/SWCNT composites also showed long cyclic stability. Based upon the variations in the surface morphologies and specific capacitance of the composite, a mechanism is proposed to explain enhancement in the capacitive characteristics. The PANI/SWCNT composites have demonstrated the potential as excellent electrode materials for application in high performance supercapacitors.     

Study on kinetics and electrochemical properties of low-Co AB5-type alloys for high-power Ni/MH battery

The structure and electrochemical kinetics properties of La_0.90−xCe_xPr_0.05Nd_0.05Ni_3.90Co_0.40Mn_0.40Al_0.30 (x = 0.10, 0.20, 0.30, 0.40, 0.50) hydrogen storage alloys have been investigated. XRD shows that the alloys consist of LaNi₅ phase with hexagonal CaCu₅ structure. With increase in Ce content, the parameter a and cell volume decrease remarkably, but the parameter c increases slightly. The limiting current density I_L and the hydrogen diffusion coefficient D increase, and the exchange current density I₀ increases firstly from 201.4 mA/g (x = 0.10) to 277.9 mA/g (x = 0.30) and then decreases to 208.5 mA/g (x = 0.50). Meanwhile, high rate dischargeability (HRD) at 1440 mA/g increases from 44.1% (x = 0.10) to 59.9% (x = 0.30), and then decreases to 44.2% (x = 0.50). As the amount of Ce increases, the plateau pressure of P-C isotherms increases gradually, the capacity retention of the alloys increases firstly and then decreases, the alloy with x = 0.30 has the higher capacity retention and cycling stability, but the maximum discharge capacity of alloys decreases. Ce is a vital element in favor of kinetics properties of rare earth-based AB₅-type alloys, and the substitution of La with Ce in suitable amount could improve the HRD by increasing kinetics.     

The reaction of polyaniline with iodine

Polyaniline (PANI) (emeraldine) base has been exposed to iodine in an ethanol-water suspension. The conductivity of PANI increased from 10⁻⁹ S cm⁻¹ to 10⁻⁴ S cm⁻¹ already at the molar ratio [I₂]/[PANI] = 1, and a higher content of iodine had only a marginal effect. This is the result of the protonation of PANI base with hydriodic acid, which is a by-product of the oxidation of the emeraldine form of PANI to pernigraniline constitutional units. The reaction is discussed on the basis of FTIR spectra. An alternative reaction, a ring-iodination of PANI, is marginal. Only one iodine atom substitutes a hydrogen atom in about 12 aniline units, even at high iodine concentration, [I₂]/[PANI] = 8. The film of polyaniline base can be used in sensing iodine; after exposure to the iodine vapor, the conductivity of the polyaniline film increased.     

Crystallographic and electrochemical characteristics of Ti45Zr35Ni17Cu3 quasicrystalline alloy ball-milled with nickel powder

Crystallographic and electrochemical characteristics of ball-milled Ti₄₅Zr₃₅Ni₁₇Cu₃ + xNi (x = 0, 5, 10, 15 and 20 mass%) composite powders have been investigated. The powders are composed of amorphous, I- and Ni-phases when x increases from 5 to 20. With increasing x, the amount of Ni-phase increases but the quasi-lattice constant decreases. The maximum discharge capacity first increases as x increases from 0 to 15 and then decreases when x increases further from 15 to 20. The high-rate dischargeability and cycling stability increase monotonically with increasing x. The improvement of the electrochemical characteristics is ascribed to the metallic nickel particles highly dispersed in the alloys, which improves the electrochemical kinetic properties and prevents the oxidation of the alloy electrodes, as well as to the mixed structure of amorphous and icosahedral quasicrystalline phases, which enhances the hydrogen diffusivity in the bulk of the alloy electrodes and efficiently inhibits the pulverization of the alloy particles.     

Study on the phase structure and electrochemical properties of RE0.93Mg0.07Ni2.96Co0.60Mn0.37Al0.17 hydrogen storage alloy

In our endeavor to improve overall properties of the La-Mg-Ni-Co type alloys, RE_0.93Mg_0.07Ni_2.96Co_0.60Mn_0.37Al_0.17 hydrogen storage electrode alloy with low magnesium content was obtained by inductive melting. The phase structure and electrochemical characteristics of the alloy were investigated by XRD and electrochemical measurement. The results indicate that RE_0.93Mg_0.07Ni_2.96Co_0.60 Mn_0.37Al_0.17 alloy has multi-phase microstructure containing the CaCu₅ structure of LaNi₅ phase as matrix phase and a little of LaNi₃ phase as the secondary phase. The maximum discharge capacity of RE_0.93Mg_0.07Ni_2.96Co_0.60Mn_0.37Al_0.17 alloy reaches 359 mAh/g, which is 7.2% higher than that of commercial AB₅ alloy electrode. The discharge capacity of RE_0.93Mg_0.07Ni_2.96Co_0.60Mn_0.37Al_0.17 alloy electrode at 233 K is up to 147 mAh/g, which is 308.3% higher than that of commercial AB₅ alloy electrode. Meanwhile, the discharge capacity of RE_0.93Mg_0.07Ni_2.96Co_0.60Mn_0.37Al_0.17 alloy can reach 92.7% of commercial AB₅ alloy after 100 charge/discharge cycles.     

Electrochemical properties of carbon-mixed LiFePO4 cathode material synthesized by the ceramic granulation method

LiFePO₄/carbon composite cathode material was prepared using polyvinyl alcohol (PVA) as carbon source by pelleting and subsequent pyrolysis in N₂. The samples were characterized by XRD, SEM and TGA. Their electrochemical performance was investigated in terms of charge–discharge cycling behavior. It consists of a single LiFePO₄ phase and amorphous carbon. The special micro-morphology via the process is favorable for electrochemical properties. The discharge capacity of the LiFePO₄/C composite was 145 mAh/g, closer to the theoretical specific capacity of 170 mAh/g at 0.1 C low current density. At 3 C modest current density, the specific capacity was about 80 mAh/g, which can satisfy for transportation applications if having a more planar discharge flat.     

Comparatively electrochemical studies at different operational temperatures for the effect of nanoclay platelets on the anticorrosion efficiency of DBSA-doped polyaniline/Na-MMT clay nanocomposite coatings

In this paper, DBSA-doped polyaniline (PANI)/Na⁺-montmorillonite (MMT) clay nanocomposite (PCN) materials have been successfully prepared with dodecylbenzenesulfonic acid (DBSA) as emulsifier and dopant for the emulsion polymerization of aniline. The as-prepared DBSA-doped samples were subsequently characterized by FTIR spectroscopy, WAXRD patterns and TEM. It should be noted that the nanocomposite coating containing 1 wt.% of clay loading was found to exhibit an observable enhanced corrosion protection on cold-rolled steel (CRS) electrode at higher operational temperature of 50 °C, which was even better than that of uncoated and electrode-coated with PANI alone at room temperature of 30 °C based on the electrochemical parameter evaluations (e.g., E_corr, R_p, I_corr, R_corr and impedance). In this work, all electrochemical measurements were performed at a double-wall jacketed cell, covered with a glass plate, through which water was circulated from a thermostat to maintain a constant operational temperature of 30, 40 and 50 ± 0.5 °C. Moreover, a series of electrochemical parameters shown in Tafel, Nyquist and Bode plots were all used to evaluate PCN coatings at three different operational temperatures in 5 wt.% aqueous NaCl electrolyte. Effect of material composition on the molecular weight and optical properties of neat PANI and PCN materials, in the form of solution, were studied by gel permeation chromatography (GPC) and UV-vis spectra, respectively. Finally, electrical conductivity at three different operational temperatures of PANI and PCN powder-pressed pellets doped with different inorganic acids such as HCl, HNO₃ and H₂SO₄ was also investigated through the measurements of standard four-point-probe technique.