Symmetric lithium-ion cell based on lithium vanadium fluorophosphate with ionic liquid electrolyte

Lithium vanadium fluorophosphate, LiVPO₄F, was utilized as both cathode and anode for fabrication of a symmetric lithium-ion LiVPO₄F//LiVPO₄F cell. The electrochemical evolution of the LiVPO₄F//LiVPO₄F cell with the commonly used organic electrolyte LiPF₆/EC-DMC has shown that this cell works as a secondary battery, but exhibits poor durability at room temperature and absolutely does not work at increased operating temperatures. To improve the performance and safety of this symmetric battery, we substituted a non-flammable ionic liquid (IL) LiBF₄/EMIBF₄ electrolyte for the organic electrolyte. The symmetric battery using the IL electrolyte was examined galvanostatically at different rates and operating temperatures within the voltage range of 0.01–2.8 V. It was demonstrated that the IL-based symmetric cell worked as a secondary battery with a Coulombic efficiency of 77% at 0.1 mA cm⁻² and 25 °C. It was also found that the use of the IL electrolyte instead of the organic one resulted in the general reduction of the first discharge capacity by about 20–25% but provided much more stable behavior and a longer cycle life. Moreover, an increase of the discharge capacity of the IL-based symmetric battery up to 120 mA h g⁻¹ was observed when the operating temperature was increased up to 80 °C at 0.1 mA cm⁻². The obtained electrochemical behavior of both symmetric batteries was confirmed by complex-impedance measurements at different temperatures and cycling states. The thermal stability of LiVPO₄F with both the IL and organic electrolytes was also examined.     

A direct methanol fuel cell using acid-doped polybenzimidazole as polymer electrolyte

A direct methanol/oxygen solid polymer electrolyte fuel cell was demonstrated. This fuel cell employed a 4 mg cm^–2 Pt-Ru alloy electrode as an anode, a 4 mg cm^–2 Pt black electrode as a cathode and an acid-doped polybenzimidazole membrane as the solid polymer electrolyte. The fuel cell is designed to operate at elevated temperature (200°C) to enhance the reaction kinetics and depress the electrode poisoning, and reduce the methanol crossover. This fuel cell demonstrated a maximum power density about 0.1 W cm^–2 in the current density range of 275–500 mA cm^–2 at 200°C with atmospheric pressure feed of methanol/water mixture and oxygen. Generally, increasing operating temperature and water/methanol mole ratio improves cell performance mainly due to the decrease of the methanol crossover. Using air instead of the pure oxygen results in approximately 120 mV voltage loss within the current density range of 200–400 mA cm^–2 .     

A rechargeable battery of the type polyaniline/propylene carbonate-LiClO4/Li-Al

A secondary battery of the type polyaniline/propylene carbonate-LiClO₄/Li–Al is described. The polymer is made by aniline oxidation with ammonium persulphate in NH₄F, 2.3 HF as solvent. The discharge capacity of the polymer is 100 Ah kg^–1 at 25°C and 140 Ah kg^–1 at 40°C for current densities of 0.5 mA cm^–2 and for an amount of material giving a capacity of 10 mAh. The voltage in open circuit for the fully charged battery is 3.6 V. The average usable potential is 2.8–3 V. The energy density for the polymer lies between 280 and 420 Wh kg^–1. The ratio of the amounts of electricity in discharge and charge is one for several hundred deep cycles. The behaviour with regard to self discharge and to constant applied voltage (floating life) is excellent.     

Recent developments in the technology of sulphur dioxide depolarized electrolysis

The use of sulphur dioxide as an anode depolarizer in the electrolytic production of hydrogen can considerably reduce the electrical energy input to the electrolyzer. The present work deals with developments in the technology of SO₂-depolarized electrolysis. Recent achievements in electrode fabrication techniques and optimization of cell configuration have resulted in substantial improvements in both cell potential and performance stability. While operating in 50 wt% sulphuric acid at 50° C and 1 atm, the measured cell potentials at 200 and 400mA cm^–2 were 0.77 and 1.05V (including ohmic losses), respectively. A cell endurance test, performed at a constant current density of 100mA cm^–2, indicated that a stabilized cell potential of 675 mV was achieved after 80 hours of continuous operation. The resulting gas from the test cell contained 98.7 vol% hydrogen. The effect of acid concentration in the range 10–60 wt% on the performance characteristics of an SO₂-depolarized electrolyzer was also investigated. Experimental results revealed that the optimum acid concentration for operating SO₂-depolarized electrolyzers is approximately 30 wt%. The observed cell potential was only 0.71 V at 200mA cm^–2.     

Aluminium alloys in sulfuric acid Part II: Aluminium-oxygen cells

Aluminium alloys were tested in Al/O₂ cells with strongly acidic electrolytes containing minor amounts of chloride ions. The faradaic efficiency, the maximum discharge capacity and the peak power of various Al/O₂ cells were evaluated. The temperature dependence of the faradaic efficiency was measured for an Al/O₂ cell over the temperature range from 15 to 50°C. With a zinc-containing aluminium alloy, a faradaic efficiency of 84% and a cell voltage of 1.6 V at open circuit and 0.7 V at 100 mA cm^–2 could be reached. The highest peak power 120 mW cm^–2, was obtained with an Al-Zn/Sn alloy. On the basis of the solubility of the anode products in the electrolyte, a limiting specific energy of 70 Wh kg^–1 was estimated. The cell voltage depends on the Al-alloys and on the catalyst used in the oxygen electrode. The cell voltage could be increased by about 200 mV when replacing the Pt-catalysed oxygen electrode with a noble-metal-free (CoCAA/DCD) electrode.     

Inhibition of sintering in molten carbonate fuel cell anodes

Molten carbonate fuel cells operate at 600–700°C. At these high temperatures, high surface area nickel anodes lose their activity rapidly due to sintering. A study of the sintering kinetics of Ni, Ni-Ag and Ag powder revealed that when Ni and Ag particles are present in similar numbers, sintering is significantly inhibited. This is achieved by minimizing volume diffusion between adjacent particles — Ni and Ag have virtually no solid solubility at any temperature. Paste electrolyte cells using such electrodes gave 114 mA/cm² at 0.65 V on 80% H₂/20% CO₂ fuel, compared to 80 mA/cm² at 0.65 V for a cell using sintered nickel anodes.     

Experimental results on the direct electrochemical oxidation of methanol in PEM fuel cells

The cell performance of direct methanol fuel cells (DMFC) is 0.5 V at 0.5 A cm^–2 under high pressure oxygen operation (3 bar abs.) at 110 °C. However, high oxygen pressure operation at high temperatures is only useful in special market niches. Therefore, our work has now focused on air operation of a DMFC under low pressure (up to 1.5 bar abs.). At present, a power density of more than 100 mW cm^–2 can be achieved at 0.5 V on air operation at 110 °C. These measurements were carried out in single cells with an electrode area of 3 cm² and the air stoichiometry only amounted to 10. The effects of methanol concentration and temperature on the anode performance were studied by pseudo half cell measurements and the results are presented together with their impact on the cell voltage. A cell design with an electrode area of 550 cm², which is appropriate for assembling a DMFC stack, was tested. A three-celled stack based on this design revealed nearly the same power densities as in the small experimental cells at low air excess pressure and the voltage–current curves for the three cells were almost identical. At 110 °C a power output of 165 W at a stack voltage of 1.5 V can be obtained in the air mode.     

A new magnesium — air cell for long-life applications

A novel type of magnesium-air primary cell has been evolved which employs non-polluting and abundantly available materials. The cell is based on the scheme Mg/Mg(NO₃)₂, NaNO₂, H₂O/O₂(C). The magnesium anode utilization is about 90% at a current density of 20 mA cm^–2. The anode has been shown to exhibit a low open-circuit corrosion, a relatively uniform pattern of corrosion and a low negative difference effect in the electrolyte developed above as compared to the conventional halide or perchlorate electrolytes. In the usual air-depolarized mode of operation, the cell has been found to be capable of continuous discharge over several months at a constant cell voltage of about 1 V and a current density of 1 mA cm^–2 at the cathode. The long service-life capability arises from the formation of a protective film on the porous carbon cathode and fast sedimentation of the anodic product (magnesium hydroxide) in the electrolyte. The cell has a shelf-life in the activated state of about a year due to the low open-circuit corrosion of the anode. These favourable features suggest the practical feasibility of developing economical, long-life, non-reserve magnesium-air cells for diverse applications using magnesium anodes with a high surface area and porous carbon-air electrodes.     

Current efficiency during anodic dissolution of iron to ferrate(vi) in concentrated alkali hydroxide solutions

The dependence of the current efficiency for oxidation of an iron anode to ferrate(vi) ions in 14m NaOH was measured in the region of free convection. The highest current yield of 40% was obtained at a current density of 2.1 mA cm^–2 and temperature of 30°C. The iron anode was activated by cathodic prepolarization. The iron concentration in low oxidation states in solution was determined as 0.13 ± 0.1 and 0.29 ± 0.25 g Fe dm^–3 at 20 and 30°C, respectively. The steady state anodic polarization curves of iron in the transpassive potential region are shifted to lower potential values with increasing NaOH concentration from 11 to 171 m. At 40°C all the curves show a limiting current density around 660 mV vs Hg/HgO, namely 9 and 23 mA cm^–2 at NaOH concentrations of 11 and 17 m, respectively.     

Further studies of a zinc-air cell employing a packed bed anode part I: Discharge

The zinc-air cell employing a packed bed anode, described previously [2], has been the subject of further investigation. A 76 cm² (air electrode area) laboratory cell has been used to determine cell performance under a varying load corresponding to the Simplified Federal Urban Driving Schedule. The results were then used as a basis for the conceptual design of a 55 kW (peak power) battery. Projected specific energy of the battery was 110 Wh kg^–1 and projected specific power 97 W kg^–1 under SFUDS discharge. These values were increased to 228 Wh kg^–1 and 97 W kg^–1 when capacity is important and to 101 Wh kg^–1 and 150 W kg^–1 when power is important, based on the results of discharge experiments at 45°C. Preliminary experiments were carried out to determine the long term stability of the air electrode in this application, to measure self discharge of the zinc and to test the practicality of mechanically recharging the cell.T. Huh (deceased), was with the Department of Metallurgical Engineering, Pusan National University, Pusan, Korea.     

Durability test of SOFC cathodes

The durability of solid oxide fuel cell (SOFC) composite cathodes of lanthanum strontium manganite and yttria stabilised zirconia was investigated. The cathodes were kept at constant, realistic operating conditions (–300 mA cm^–2 at 1000 °C in air) for up to 2000 h. After the 2000 h test the increase in electrode overvoltage exceeded 100% of the initial value. Nominally identical cathodes kept for 2000 h at 1000 °C in air without current load for comparison showed little or no degradation. Thus, the current load of –300 mA cm^–2, rather than the operation temperature of 1000 °C, was responsible for the degradation. Structural analysis showed an increase in the porosity at the electrode interfaces, when the electrode had been polarised. No such structural changes were found for electrodes tested without current load. The degradation is primarily ascribed to pore formation in the electrode material induced by an electric field.     

Electrochemical properties of Li ion polymer battery with gel polymer electrolyte based on polyurethane

Gel polymer electrolyte (GPE) was prepared using polyurethane acrylate as polymer host and its performance was evaluated. LiCoO₂/GPE/graphite cells were prepared and their electrochemical performance as a function of discharge currents and temperatures was evaluated. The precursor containing a 5 vol % curable mixture had a viscosity of 4.5 mPa s. The ionic conductivity of the GPE at 20 °C was about 4.5 × 10^–3 S cm^–1. The GPE was stable electrochemically up to a potential of 4.8 V vs Li/Li⁺. LiCoO₂/GPE/graphite cells showed a good high rate and low-temperature performance. The discharge capacity of the cell was stable with charge–discharge cycling.     

Investigations on N-halogen cyclic ureide based magnesium primary cell system at various temperatures

This paper reports the results of experimental studies on a magnesium/magnesium perchlorate/N,N-dichlorodimethylhydantoin cell system at various temperatures ranging from 70 to –20° C. A constant current discharge method was employed to evaluate the battery parameters such as discharge capacity, energy density, coulombic efficiency and internal resistance. Cyclic voltammetric (CV) investigations of the N,N-dichlorodimethylhydantoin (DDH) were carried out in 0.1 M magnesium perchlorate medium in order to supplement the results. Scanning electron microscopy (SEM) was performed on post-polarized magnesium samples to follow the morphological changes in the anodic material with respect to temperatures. These investigations broadly reveal that the cell system can give rise to the open-circuit/closed-circuit voltage of 2.0 V and it is possible to obtain current densities of 20 mA cm^–2 during discharge.     

Solid-state storage batteries

Investigations were conducted to study the feasibility of a solid-state battery system for storage applications. During the development of various high energy density solid-state batteries we noted that the solid electrolyte material, LiI dispersed in large surface area Al₂O₃, has a high ionic conductivity at elevated temperatures, (for example 0.1^–1 cm^–1 at 300° C) and is suitable for high-rate storage battery applications. In solid-state battery systems both the electrodes and electrolytes are in the solid state under the operating conditions of the battery. The absence of any liquid phase makes the individual cell containers unnecessary in a multicell battery resulting in a simplified battery structure and increased package efficiency. Furthermore, no material compatibility problem is encountered in the system. As a result, the solid-state battery system has excellent charge retention characteristics and a long projected operating life. Solid-state test cells, Li-Si/LiI(Al₂O₃)/TaS₂/Ta, Li-Si/LiI(Al₂O₃/TiS₂/Ti and Li-Si/LiI(Al₂O₃)/TiS₂, Sb₂S₃, Bi were constructed and subjected to discharge-charge cycle tests at 300±10° C, at 13·7 mA cm^–2. Preliminary test results demonstrated that these solid-state battery systems are rechargeable and may be suitable for both load levelling and/or vehicle propulsion. From the considerations of material availability and cost and operational efficiencies it was concluded that the Li-Si/LiI(Al₂O₃)/TiS₂, Sb₂S₃, Bi system is most suitable among the three systems studied for the development of practical storage batteries. Preliminary design studies showed that practical energy densities of 200W h kg^–1 and 520 W h l^–1 can be realized with the Li-Si/TiS₂, Sb₂S₃, Bi storage batteries.     

Polyaniline: characterization as a cathode active material in rechargeable batteries in aqueous electrolytes

An analytically pure form of chemically synthesized polyaniline having the emeraldine oxidation state has been used as a cathode active material together with a Zn anode in the fabrication of rechargeable cells in 1.0 M aqueous ZnCl₂ electrolyte (pH4). The experimental capacity and energy density based only on the weight of polymer employed in constructing the cell are 151.5 Ah kg^–1 and 159.1 Wh kg^–1 respectively at a constant discharge current of 0.75 mA cm^–2 (average discharge voltage 1.05V). The cell reactions in the charge and discharge processes have been determined. The modified capacity and energy density, when taking into account the calculated weights of Zn and HCl involved in the discharge reactions, are 109.3 Ah kg^–1 and 114.8 Wh kg^–1 respectively. The cell shows excellent recyclability and coulombic efficiency.     

Performance of Ni‐based Anode‐Supported SOFCs with Doped Ceria Electrolyte at Low Temperatures Between 294 and 542°C

The performance of a conventional anode‐supported microtubular SOFC using doped ceria as an electrolyte and Ni‐based cermet as an anode is evaluated at low operating temperature between 294 and 542°C. An open‐circuit voltage (OCV) of >0.9 V is obtained at all measured operating temperatures, and power generation is observed at temperatures as low as 294°C. The power density of the cell is 0.6 W/cm² at 542°C operating temperature with 47% fuel utilization and is 5 mW/cm² at 294°C operating temperature with an open‐circuit voltage of 0.95 V. According to impedance spectroscopy, a greater influence of gas flow rate, on the cell performance, is observed at higher operating temperature.     

Synthesis of a highly active carbon-supported Ir–V/C catalyst for the hydrogen oxidation reaction in PEMFC

The active, carbon-supported Ir and Ir–V nanoclusters with well-controlled particle size, dispersity, and composition uniformity, have been synthesized via an ethylene glycol method using IrCl₃ and NH₄VO₃ as the Ir and V precursors. The nanostructured catalysts were characterized by X-ray diffraction and high-resolution transmission electron microscopy. The catalytic activities of these carbon-supported nanoclusters were screened by applying on-line cyclic voltammetry and electrochemical impedance spectroscopy techniques, which were used to characterize the electrochemical properties of fuel cells using several anode Ir/C and Ir–V/C catalysts. It was found that Ir/C and Ir–V/C catalysts affect the performance of electrocatalysts significantly based on the discharge characteristics of the fuel cell. The catalyst Ir–V/C at 40 wt.% displayed the highest catalytic activity to hydrogen oxidation reaction and, therefore, high cell performance is achieved which results in a maximum power density of 563 mW cm⁻² at 0.512 V and 70 °C in a real H₂/air fuel cell. This performance is 20% higher as compared to the commercial available Pt/C catalyst. Fuel cell life test at a constant current density of 1000 mA cm⁻² in a H₂/O₂ condition shows good stability of anode Ir–V/C after 100 h of continuous operation.     

Electrochemical reduction of nitrates and nitrites in alkaline nuclear waste solutions

Sodium nitrate and nitrite are major components of alkaline nuclear waste streams and contribute to environmental release hazards. The electrochemical reduction of these materials to gaseous products has been studied in a synthetic waste mixture. The effects of electrode materials, cell design, and other experimental parameters have been investigated. Lead was found to be the best cathode material in terms of current efficiency for the reduction of nitrate and nitrite in the synthetic mix. The current efficiency for nitrite and nitrate removal is improved in divided cells due to the elimination of anodic oxidation of nitrite. Operation of the divided cells at high current densities (300–600 mA cm^–2) and high temperatures (80°C) provides more efficient reduction of nitrite and nitrate. Nearly complete reduction of nitrite and nitrate to nitrogen, ammonia, or nitrous oxide was demonstrated in 1000 h tests in a divided laboratory electrochemical flow cell using a lead cathode, Nafion® 417 cation exchange membrane, and oxygen evolving DSA® or platinum clad niobium anode at a current density of 500 mA cm^–2 and a temperature of 70° C. Greater than 99% of the nitrite and nitrate was removed from the synthetic waste mix batch in the 1000 h tests at an overall destruction efficiency of 55%. The process developed shows promise for treating large volumes of waste.     

Optimization of operating parameters of a direct methanol fuel cell and physico-chemical investigation of catalyst–electrolyte interface

A physico-chemical investigation of catalyst–Nafion® electrolyte interface of a direct methanol fuel cell (DMFC), based on a Pt–Ru/C anode catalyst, was carried out by XRD, SEM-EDAX and TEM. No interaction between catalyst and electrolyte was detected and no significant interconnected network of Nafion micelles inside the composite catalyst layer was observed. The influence of some operating parameters on the performance of the DMFC was investigated. Optimal conditions were 2 M methanol, 5 atm cathode pressure and 2–3 atm anode pressure. Power densities of 110 and 160 mW cm⁻² were obtained for operation with air and oxygen, respectively, at temperatures of 95–100°C and with 1 mg cm⁻² Pt loading.     

Effect on discharge of the heat treatment of graphite fluoride under a hydrogen atmosphere

Thermal decomposition of two types of graphite fluorides (CF) _n and (CF) _n , has been carried out in a hydrogen atmosphere at several temperatures between 100 and 500°C, with the object of improving the initial discharge behaviour of the Li/graphite flouride cell. The main reaction was the C-F bond rupture to form graphite-like carbon around the particle surface. The drop in cell voltage at the beginning of discharge could be minimized, and the polarization during discharge reduced by heat treatment under a hydrogen atmosphere. (CF) _n , heat treated at 400°C for 1 h, yielded a discharge capacity of 730–800 mA h per g of active material, corresponding to the discharge efficiency of 8390% at 25°C, and (C₂F) _n , heat treated at 350°C, for 10h, gave 670 mA h g^–1, corresponding to 91 % at 25°C.