Effect of RuO2·xH2O in anode on the performance of direct methanol fuel cells

The effect of hydrous RuO₂ (RuO₂·xH₂O) in anode on the performance of direct methanol fuel cells (DMFCs) was examined by voltammetry, methanol stripping analysis, electrochemical impedance spectroscopy, polarization measurement and chronopotentiometry. The results showed that, compared with the DMFC with conventional structures, the dynamic response and quasi-steady state performance of the RuO₂·xH₂O-introduced DMFCs were significantly improved. The DMFC with RuO₂·xH₂O layer (ROL) sandwiched between anode catalyst layer and gas diffusion layer exhibited better quasi-steady state performance than those either with ROL sandwiched between anode catalyst layer and electrolyte membrane or with RuO₂·xH₂O uniformly distributed in anode catalyst layer. The maximum power density of the DMFC with this novel structure was 16% higher than the DMFC with the conventional structure. Moreover, the dynamic response of this RuO₂·xH₂O-introduced cell was more stable during 250-hour of operation when compared with that of the conventional cell.     

Proton conductivity and fuel cell property of composite electrolyte consisting of Cs-substituted heteropoly acids and sulfonated poly(ether-ether ketone)

Inorganic-organic composite electrolytes were fabricated from partially Cs⁺-substituted heteropoly acids (Cs-HPAs) and sulfonated poly(ether-ether ketone) (SPEEK) for application in fuel cells. Heteropoly acids, such as phosphotungstic acid (H₃PW₁₂O₄₀:WPA), and silicotungstic acid (H₄SiW₁₂O₄₀:WSiA), were mechanochemically treated with cesium hydrogen sulfate (CsHSO₄) to obtain the form of Cs-HPAs. SPEEK was prepared from PEEK by sulfonation using concentrated sulfuric acid. Water durability and surface structure of HPAs were modified by introducing Cs⁺ into HPAs. Flexible and hot water stable composite electrolytes were obtained, and their electrochemical properties were markedly improved with the addition of Cs-HPAs into the SPEEK matrix. Maximum power densities of 245 and 247 mW cm⁻² were obtained for 50WPA·50CsHSO₄ and 50WSiA·50CsHSO₄ in SPEEK (1/5 by weight) composite electrolytes, respectively, from single cell tests at 80 °C and 80 RH%. These results suggest that a three-dimensional proton-conductive path was formed among homogeneously distributed Cs-HPAs particles in the SPEEK matrix. The Cs-HPAs incorporated into the SPEEK matrix increased the number of protonate sites in the electrolyte. These observations imply that the mechanochemically synthesized Cs-HPAs, which consist of hydrogen bondings between Cs-HPAs and -HSO₄⁻, dissociated from CsHSO₄, are promising materials as inorganic fillers in inorganic-organic composite.     

Solid state synthesis of hydrous ruthenium oxide for supercapacitors

A novel solid state route has been successfully developed for the synthesis of nano-scale hydrous ruthenium oxide (denoted as RuO₂·xH₂O). The procedure involves directly mixing RuCl₂·xH₂O with alkali to form RuO₂·xH₂O in a mortar at room temperature. Transmission electron microscopy (TEM) and N₂ adsorption–desorption measurement indicate that the RuO₂·xH₂O particle is approximately 30–40 nm with mesoporous structure. The crystalline structure and the electrochemical properties of RuO₂·xH₂O have been systematically explored as a function of annealing temperature. At lower temperatures, the RuO₂·xH₂O powder was found in an amorphous phase and the maximum capacitance of 655 F g⁻¹ was obtained by annealing at 150 °C. Higher temperatures (exceeding 175 °C) presumably converted amorphous phase into crystalline one and the corresponding specific capacitance dropped rapidly from 547 F g⁻¹ at 175 °C to 87 F g⁻¹ at 400 °C. Also, the dependence of electrochemical performance on annealing conditions of RuO₂·xH₂O was investigated by electrical impedance spectroscopy (EIS) study.     

Carbon nanofibre/hydrous RuO2 nanocomposite electrodes for supercapacitors

Amorphous RuO₂·xH₂O and a VGCF/RuO₂·xH₂O nanocomposite (VGCF = vapour-grown carbon fibre) are prepared by thermal decomposition. The morphology of the materials is investigated by means of scanning electron microscopy. The electrochemical characteristics of the materials, such as specific capacitance and rate capability, are investigated by cyclic voltammetry over a voltage range of 0–1.0 V at various scan rates and with an electrolyte solution of 1.0 M H₂SO₄. The specific capacitance of RuO₂·xH₂O and VGCF/RuO₂·xH₂O nanocomposite electrodes at a scan rate of 10 mV s⁻¹ is 410 and 1017 F g⁻¹, respectively, and at 1000 mV s⁻¹ are 258 and 824 F g⁻¹, respectively. Measurements of ac impedance spectra are made on both the electrodes at various bias potentials to obtain a more detailed understanding of their electrochemical behaviour. Long-term cycle-life tests for 10⁴ cycles shows that the RuO₂·xH₂O and VGCF/RuO₂·xH₂O electrodes retain 90 and 97% capacity, respectively. These encouraging results warrant further development of these electrode materials towards practical application.     

Electrochemical performance of nanostructured spinel LiMn2O4 in different aqueous electrolytes

A nanostructured spinel LiMn₂O₄ electrode material was prepared via a room-temperature solid-state grinding reaction route starting with hydrated lithium acetate (LiAc·2H₂O), manganese acetate (MnAc₂·4H₂O) and citric acid (C₆H₈O₇·H₂O) raw materials, followed by calcination of the precursor at 500 °C. The material was characterized by X-ray diffraction (XRD) and transmission electron microscope techniques. The electrochemical performance of the LiMn₂O₄ electrodes in 2 M Li₂SO₄, 1 M LiNO₃, 5 M LiNO₃ and 9 M LiNO₃ aqueous electrolytes was studied using cyclic voltammetry, ac impedance and galvanostatic charge/discharge methods. The LiMn₂O₄ electrode in 5 M LiNO₃ electrolyte exhibited good electrochemical performance in terms of specific capacity, rate dischargeability and charge/discharge cyclability, as evidenced by the charge/discharge results.     

Electrochemical intercalation of lithium in the titanium hydrogeno phosphate Ti(HPO4)2·H2O

The electrochemical reactivity of the layered titanium hydrogeno phosphate Ti(HPO₄)₂·H₂O versus lithium has been studied. Lithium intercalation occurs at ∼2.5 V with low polarization, leading to a new lithiated Ti(III) phase, LiTi(HPO₄)₂·H₂O. Ti(HPO₄)₂·H₂O exhibits a reversible capacity of 80 mAh g⁻¹ in the voltage window 1.8–3.5 V at C/10 rate. The stable reversible capacity reveals that the presence of H₂O lattice is not affecting the electrochemical reaction. The reversibility of the reaction is demonstrated by extracting lithium from LiTi(HPO₄)₂·H₂O and the host structure is intact. The electrochemical behaviour of dehydrated phases Ti(HPO₄)₂ and TiP₂O₇ has also been investigated.     

LiFePO4 with enhanced performance synthesized by a novel synthetic route

Pure LiFePO₄ was synthesized by heating an amorphous LiFePO₄. The amorphous LiFePO₄ obtained through lithiation of FePO₄·xH₂O by using oxalic acid as a novel reducing agent at room temperature. FePO₄·xH₂O was prepared through co-precipitation by employing FeSO₄·7H₂O and H₃PO₄ as raw materials. X-ray diffraction (XRD), scanning electron microscopy (SEM) observations showed that LiFePO₄ composites with fine particle sizes between 100 nm and 200 nm, and with homogenous sizes distribution. The electrochemical performance of LiFePO₄ powder synthesized at 500 °C were evaluated using coin cells by galvanostatic charge/discharge. The synthesized LiFePO₄ composites showed a high electrochemical capacity of 166 mAh g⁻¹ at the 0.1C rate, and possessed a favorable capacity cycling maintenance at the 0.1C, 0.2C, 0.5C and 1C rate.     

Sulfonated poly(ether ether ketone) as an ionomer for direct methanol fuel cell electrodes

Sulfonated poly(ether ether ketone) has been investigated as an ionomer in the catalyst layer for direct methanol fuel cells (DMFC). The performance in DMFC, electrochemical active area (by cyclic voltammetry), and limiting capacitance (by impedance spectroscopy) have been evaluated as a function of the ion exchange capacity (IEC) and content (wt.%) of the SPEEK ionomer in the catalyst layer. The optimum IEC value and SPEEK ionomer content in the electrodes are found to be, respectively, 1.33 meq. g⁻¹ and 20 wt.%. The membrane-electrode assemblies (MEA) fabricated with SPEEK membrane and SPEEK ionomer in the electrodes are found to exhibit superior performance in DMFC compared to that fabricated with Nafion ionomer due to lower interfacial resistance in the MEA as well as larger electrochemical active area. The MEAs with SPEEK membrane and SPEEK ionomer also exhibit better performance than that with Nafion 115 membrane and Nafion ionomer due to lower methanol crossover and better electrode kinetics.     

Manganese oxide electrochemical capacitor with potassium poly(acrylate) hydrogel electrolyte

An aqueous gel electrolyte has for the first time been successfully applied to the MnO₂·nH₂O-based pseudocapacitive electrochemical capacitors (ECs). The gel electrolyte is made of potassium poly(acrylate) (PAAK) polymer and aqueous solution of KCl. With the selected composition, PAAK:KCl:H₂O = 9.0%:6.7%:84.3% by weight, the gel shows no fluidity, possessing an ionic conductivity in the order of 10⁻¹ S cm⁻¹. The gel electrolyte has been found to give substantially higher specific capacitances than those in the liquid electrolyte with the same salt (KCl) composition (1 M) and high power capability (>10 kW/kg).     

Sulfonated poly(ether ether ketone)/clay-SO3H hybrid proton exchange membranes for direct methanol fuel cells

A new type of sulfonated clay (clay-SO₃H) was prepared by the ion exchange method with the sulfanilic acid as the surfactant agent. The grafted amount of sulfanilic acid in clay-SO₃H was 51.8 mequiv. (100 g)⁻¹, which was measured by thermogravimetric analysis (TGA). Sulfonated poly(ether ether ketone) (SPEEK)/clay-SO₃H hybrid membranes which composed of SPEEK and different weight contents of clay-SO₃H, were prepared by a solution casting and evaporation method. For comparison, the SPEEK/clay hybrid membranes were produced with the same method. The performances of hybrid membranes for direct methanol fuel cells (DMFCs) in terms of mechanical and thermal properties, water uptake, water retention, methanol permeability and proton conductivity were investigated. The mechanical and thermal properties of the SPEEK membranes had been improved by introduction of clay and clay-SO₃H, obviously. The water desorption coefficients of the SPEEK and hybrid membranes were studied at 80 °C. The results showed that the addition of the inorganic part into SPEEK membrane enhanced the water retention of the membrane. Both methanol permeability and proton conductivity of the hybrid membranes decreased in comparison to the pristine SPEEK membrane. However, it was worth noting that higher selectivity defined as ratio of proton conductivity to methanol permeability of the SPEEK/clay-SO₃H-1 hybrid membrane with 1 wt.% clay-SO₃H was obtained than that of the pristine SPEEK membrane. These results showed that the SPEEK/clay-SO₃H hybrid membrane with 1 wt.% clay-SO₃H had potential usage of a proton exchange membrane (PEM) for DMFCs.     

Effects of different electrolytes containing Na2WO4 on the electrochemical performance of nickel hydroxide electrodes for nickel–metal hydride batteries

To improve the high-temperature performance of the nickel hydroxide electrodes in nickel–metal hydride batteries, sodium tungstate (Na₂WO₄) used as an electrolyte additive has been added into two types of binary electrolytes (KOH–LiOH and NaOH–LiOH) in this study. The effects of electrolyte composition on the electrochemical performance of nickel electrodes have been systematically investigated via a combination of cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM), energy-dispersive X-ray analysis (EDX), X-ray diffraction (XRD) and charge/discharge tests. It is found that by adding (1.0 wt.%) Na₂WO₄, the performance of nickel electrodes is significantly improved in both NaOH and KOH electrolytes at 70 °C. The improved performance can be attributed to the deposition of WO₃·2H₂O solid film on the surface of nickel electrode, which is beneficial to the increase in oxygen evolution overpotential, the slow-down of oxygen evolution rate and the decrease in charge-transfer resistance.     

Enhanced lithium-ion intercalation properties of coherent hydrous vanadium pentoxide-carbon cryogel nanocomposites

Coherent hydrous vanadium pentoxide (V₂O₅·nH₂O)-carbon cryogel (CC) nanocomposites were synthesized by electrodeposition of vanadium pentoxide onto the porous carbon scaffold which was derived from resorcinol (R) and formaldehyde (F) organic hydrogels. As-fabricated nanocomposites were characterized by scanning electron microscopy (SEM), along with EDAX and nitrogen sorption isotherms which suggested vanadium pentoxide incorporated in the pores of carbon cryogels. The nanocomposites showed much improved discharge capacity and better cyclic stability as compared to hydrous vanadium pentoxide films deposited on platinum foil. The discharge capacity of the nanocomposites reached 280 mAh g⁻¹ based on the mass of the vandium pentoxide at a current density of 100 mA g⁻¹ and it possessed good cycle stability at different discharge rates. The results demonstrated that electrochemical performances, such as specific discharge capacitance and reversibility of the composite electrode, could be greatly enhanced by the introduction of carbon cryogels (CCs) scaffold with three-dimensionally interconnected porous structure in which V₂O₅·nH₂O homogeneously dispersed.     

Investigation of a carbon-supported quaternary PtRuSnW catalyst for direct methanol fuel cells

An investigation was carried out in the electro-oxidation of methanol on a carbon-supported quaternary PtRuSnW catalyst prepared by a liquid-phase reduction method. As derived by X-ray diffraction and X-ray photoelectron spectroscopy, the catalyst was composed of metallic Pt, microcrystalline RuO₂ and SnO₂ phases and amorphous WO₃/WO₂ species. The electrochemical analysis was carried out in half-cell containing sulfuric acid electrolyte as well as in a liquid methanol-fed solid polymer electrolyte single-cell. The activity of catalyst in the half-cell varied as a function of the methanol concentration, it increased with CH₃OH molarity in the activation-controlled region and showed a maximum in 2 M CH₃OH at high currents. IR-free polarization curves showed that the activity of the quaternary catalyst was superior to Pt metal/C samples having the same Pt amount. The presence of semi-insulating metal oxides such as RuO₂, SnO₂ and WO₃ on the electrode surface exhibited a significant uncompensated resistance. The single-cell performance was lower than that predicted by the half-cell experiments mainly due to the methanol cross-over through the Nafion membrane.     

Understanding RuO2·xH2O/carbon nanofibre composites as supercapacitor electrodes

Composites made from RuO₂·xH₂O particles supported on carbon nanofibres (CNF) have been prepared for supercapacitor electrodes. CNF, produced by Grupo Antolin Ing. SA. using a floating catalyst procedure was treated either in HCl or in HNO₃. Then the composites were obtained by impregnation of CNF with an aqueous RuCl₃·0.5H₂O solution followed by filtering and alkali solution treatment. Heat treatment at 150 °C for 2 h was done. Specific capacitance of the composites has been measured and discussed on the basis of their RuO₂·xH₂O content and RuO₂·xH₂O particle size. The composites having RuO₂·xH₂O contents below 11 wt% show RuO₂·xH₂O particles, which grow from 2 to 4 nm as the RuO₂·xH₂O content increases. The specific capacitance of supported RuO₂·xH₂O, which can be very high (up to 840 F g⁻¹), decreases as the RuO₂·xH₂O content increases and RuO₂·xH₂O particles grow. The composites having RuO₂·xH₂O contents above 11 wt% show RuO₂·xH₂O particles of nearly constant size (4 nm); the effect of increasing the RuO₂·xH₂O content is to increase the amount of particles but not the size of the particles. In these composites the specific capacitance of supported RuO₂·xH₂O is nearly constant (440 F g⁻¹) and close to bare RuO₂·xH₂O (460 F g⁻¹).     

Electrochemical property of NH4V3O8·0.2H2O flakes prepared by surfactant assisted hydrothermal method

NH₄V₃O₈·0.2H₂O is synthesized by sodium dodecyl sulfonate (SDS) assisted hydrothermal method and its electrochemical performance is investigated. The as-prepared material is characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared (IR) spectrum, differential scanning calorimetry and thermal gravimetry (DSC/TG), cyclic voltammetry (CV), and charge-discharge cycling test. The results show a pure NH₄V₃O₈·0.2H₂O phase with flake-like morphology is obtained and the average flake thickness is about 150 nm. The NH₄V₃O₈·0.2H₂O electrode has a good lithium ion insertion/extraction ability with the highest discharge capacity of 225.9 mAh g⁻¹ during 1.8-4.0 V versus Li at the constant current density of 15 mA g⁻¹. After 30 cycles, it still maintains a high discharge capacity of 209.4 mAh g⁻¹, demonstrating good cyclic stability. Interestingly, at the discharge process a new (NH₄)Li_xV₃O₈·0.2H₂O compound is formed due to the new lithium ion from lithium metal anode.     

Fabrication and characterization of a PTFE-reinforced integral composite membrane for self-humidifying PEMFC

A novel PTFE-reinforced self-humidifying membrane based on low-cost sulfonated poly (ether ether ketone) (SPEEK) resin was fabricated. In the membrane a base layer and a thin protective layer were bonded by porous polytetrafluoroethylene (PTFE) film. The base layer, which is composed of silicon oxide supported platinum catalyst (abbreviated as Pt-SiO₂) dispersed in SPEEK resin, can suppress reactant crossover and achieve good membrane hydration due to the imbedded hygroscopic Pt-SiO₂ catalysts. The thin protective layer, which constitutes of H₂O₂ decomposition catalyst Pt-SiO₂ and high H₂O₂-tolerant Nafion resin, aims to prevent the SPEEK resin degradation by H₂O₂ produced at the cathode side by incomplete reduction of oxygen. The porous PTFE film tightly bonds with the SPEEK and the Nafion resins to form an integral membrane and accordingly to avoid delamination of the two different resins. The self-humidifying membrane was characterized by TEM, SEM and EDS, etc. The self-humidifying membrane exhibits higher open circuit voltage (OCV) of 0.98 V and maximum power density value of 0.8 W cm⁻² than 0.94 V, 0.33 W cm⁻² of SPEEK/PTFE membrane under dry condition, respectively. The primary 250 h fuel cell durability experiment was conducted and suggested that this low-cost self-humidifying membrane was durable both on fuel cell performance and the membrane structure under fuel cell operation condition with dry H₂/O₂.     

A facile,effective thermal crosslinking to balance stability and proton conduction for proton exchange membranes based on blend sulfonated poly(ether ether ketone)/sulfonated poly(arylene ether sulfone)

The development of hydrocarbon polymer electrolyte membranes with high proton conductivities and good stability as alternatives to perfluorosulfonic acid membranes is an ongoing research effort. A facile and effective thermal crosslinking method was carried out on the blended sulfonated poly (ether ether ketone)/poly (aryl ether sulfone) (SPEEK/SPAES) system. Two SPEEK polymers with ion exchange capacities (IECs) of 1.6 and 2.0 mmol g^?1 and one SPAES polymer (2.0 mmol g^?1) were selected to create different blends. The effect of thermal crosslinking on the fundamental properties of the membranes, especially their physicochemical stability and electrochemical performance, were investigated in detail. The homogeneous and flexible thermally-crosslinked SPEEK/SPAES membranes displayed excellent mechanical toughness (27–46 Mpa), suitable water uptake (<60%), high dimensional stability (swelling ratio < 15%) and large proton conductivity (>120 mS cm^?1) at 80 °C. The thermal crosslinking membranes also show significantly enhanced hydrolytic (<2.5%) and oxidative stability (<2%). Fuel cell with t-SPEEK/SPAES (1:2:2) membrane achieves a power density of 665 mW cm^?2 at 80 °C.     

Improved proton conduction of sulfonated poly (ether ether ketone) membrane by sulfonated covalent organic framework nanosheets

A type of sulfonated covalent organic framework nanosheets (TpPa-SO₃H) was synthesized via interfacial polymerization and incorporated into sulfonated poly (ether ether ketone) (SPEEK) matrix to prepare proton exchange membranes (PEMs). The densely and orderly arranged sulfonic acid groups in the rigid skeleton of the TpPa-SO₃H nanosheets, together with their high-aspect-ratio and well-defined porous structure provide proton-conducting highways in the membrane. The doping of TpPa-SO₃H nanosheets led to an increased ion exchange capacity up to 2.34 mmol g^?1 but a 2-folds reduced swelling ratio, remarkably mitigating the trade-off between high IEC and excessive swelling ratio. Based on the high IEC and orderly arranged proton-conducting sites, the SPEEK/TpPa–SO₃H–5 membrane exhibited the maximum proton conductivity of 0.346 S cm^?1 at 80 °C, 1.91-folds higher than the pristine SPEEK membrane. The mechanical strength of the composite membrane was also improved by 2.05-folds–74.5 MPa. The single H₂/O₂ fuel cell using the SPEEK/TpPa–SO₃H–5 membrane presented favorable performance with an open voltage of 1.01 V and a power density of 86.54 mW cm^?2.     

Supercapacitive behaviors and their temperature dependence of sol–gel synthesized nanostructured manganese dioxide in lithium hydroxide electrolyte

In the present work, a nanostructured manganese dioxide material was synthesized by a sol–gel method starting with manganese acetate (MnAc₂·4H₂O) and citric acid (C₆H₈O₇·H₂O) raw materials, and characterized by X-ray diffraction, infrared spectroscopic and transmission electron microscope techniques. The electrochemical properties and the influence of temperature on supercapacitive behaviors of the nano-MnO₂ electrode in 1 M LiOH electrolyte were investigated using electrochemical methods. Experimental results show that the MnO₂ electrode can exhibit an excellent pseudocapacitive behavior in 1 M LiOH electrolyte, and a high specific capacitance of 317 F g⁻¹ can be obtained at a charge/discharge current rate of 100 mA g⁻¹ and at the temperature of 25 °C. We found that temperature has a crucial influence on the discharge specific capacitance of the electrode. The specific capacitance at 25 °C is higher than that at 15 or 35 °C.     

Enhanced properties of SPEEK with incorporating of PFSA and barium strontium titanate nanoparticles for application in DMFCs

Novel blend nanocomposite proton‐exchange membranes were prepared using sulfonated poly (ether ether ketone) (SPEEK), perfluorosulfonic acid (PFSA), and Ba_0.9Sr_0.1TiO₃ (BST) doped‐perovskite nanoparticles. The membranes were evaluated by attenuated total reflection, X‐ray diffraction spectroscopy, water uptake, proton conductivity, methanol permeability, and direct methanol fuel cell test. The effect of two additives, PFSA and BST, were investigated. Results indicated that both proton conductivity and methanol barrier of the blend nanocomposite membranes improved compared with pristine SPEEK and the as‐prepared blend membranes. The methanol permeability and the proton conductivity of the blend membrane containing 6 wt% BST obtained 3.56 × 10^?7 cm² s^?1 (at 25 °C) and 0.110 S cm^?1 (at 80 °C), respectively. The power density value for the optimum blend nanocomposite membrane (15 wt% PFSA and 6 wt% BST) (54.89 mW cm^‐2) was higher than that of pristine SPEEK (31.34 mW cm^‐2) and SPF15 blend membrane (36.12 mW cm^‐2).