Effect of electrolyte in electrospun poly(vinylidene fluoride-co-hexafluoropropylene) nanofibers on dye-sensitized solar cells

We prepared poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) nanofibers by the electrospinning method and applied to the polymer matrix in polymer electrolytes for dye-sensitized solar cells (DSSCs). The uptake, ionic conductivity, and porosity of the electrospun PVDF-HFP nanofiber films showed 653±50%, 4.53±1.3×10^?3 S/cm, and 70±2.3%, respectively, regardless of the diameter and morphology of nanofibers. In addition, several DSSC devices using the electrospun PVDF-HFP nanofiber films as the polymer matrix were prepared to investigate the photovoltaic effect of iodine (I₂) concentrations on DSSC devices. With an increase of I₂ concentration in electrolyte solutions, the ionic conductivity increased, while the photocurrent density of DSSC devices decreased.     

Solvent-induced porosity control of carbon nanofiber webs for supercapacitor

A simple and scalable method is reported for fabricating a porosity-controlled carbon nanofibers with a skin-core texture by electrospinning a selected blend of polymer solutions. Simple thermal treatment of the electrospun nanofibers from solution blends of various compositions creates suitable ultramicropores on the surface of carbon nanofibers that can accommodate many ions, removing the need for an activation step. The intrinsic properties of the electrode (e.g., nanometre-size diameter, high specific surface area, narrow pore size distribution, tuneable porosity, shallow pore depth, and good ionic accessibility) enable construction of supercapacitors with large specific capacitance (130.7 F g⁻¹), high power (100 kW kg⁻¹), and energy density (15.0 Wh kg⁻¹).     

Synthesis and electrocatalysis of 1-aminopyrene-functionalized carbon nanofiber-supported platinum-ruthenium nanoparticles

Platinum-ruthenium/carbon composite nanofibers were prepared by depositing PtRu nanoparticles directly onto electrospun carbon nanofibers using a polyol processing technique. The morphology and size of PtRu nanoparticles were controlled by 1-aminopyrene functionalization. The noncovalent functionalization of carbon nanofibers by 1-aminopyrene is simple and can be carried out at ambient temperature without damaging the integrity and electronic structure of carbon nanofibers. The resulting PtRu/carbon composite nanofibers were characterized by cyclic voltammogram in 0.5 M H₂SO₄ and 0.125 M CH₃OH + 0.2 M H₂SO₄ solutions, respectively. The PtRu/carbon composite nanofibers with 1-aminopyrene functionalization have smaller nanoparticles and a more uniform distribution, compared with those pretreated with conventional acids. Moreover, PtRu/1-aminopyrene functionalized carbon nanofibers have high active surface area and improved performance towards the electrocatalytic oxidation of methanol.     

Preparation and electrochemical characterization of polymer electrolytes based on electrospun poly(vinylidene fluoride-co-hexafluoropropylene)/polyacrylonitrile blend/composite membranes for lithium batteries

Apart from PEO based solid polymer electrolytes, tailor-made gel polymer electrolytes based on blend/composite membranes of poly(vinylidene fluoride-co-hexafluoropropylene) and polyacrylonitrile are prepared by electrospinning using 14 wt% polymer solution in dimethylformamide. The membranes show uniform morphology with an average fiber diameter of 320-490 nm, high porosity and electrolyte uptake. Polymer electrolytes are prepared by soaking the electrospun membranes in 1 M lithium hexafluorophosphate in ethylene carbonate/dimethyl carbonate. Temperature dependent ionic conductivity and their electrochemical performance are studied. The blend/composite polymer electrolytes show good ionic conductivity in the range of 10⁻³ S cm⁻¹ at ambient temperature and good electrochemical performance. All the Polymer electrolytes show an anodic stability >4.6 V with stable interfacial resistance with storage time. The prototype cell shows good charge-discharge properties and stable cycle performance with comparable capacity fade compared to liquid electrolyte under the test conditions.     

Morphology, structure and electrochemical properties of single phase electrospun vanadium pentoxide nanofibers for lithium ion batteries

One-dimensional (1D) vanadium pentoxide (V₂O₅) nanofibers (VNF) are synthesized by electrospinning vanadium sol-gel precursors containing vanadyl acetylacetonate and poly(vinylpyrrolidone) followed by sintering. Crystal structure, molecular structure and morphology of electrospun VNF are analyzed using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), selected area diffraction (SAED), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). Single-phase electrospun VNF ∼300-800 nm in diameter, 20-50 μm long (aspect ratio > 50) with porous interconnected fibrous morphology are revealed by FESEM and TEM analysis. Electrochemical properties of the sintered VNF, as a cathode in lithium-ion batteries, explored using cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS) give rise to new understandings of the electrochemical processes occurring in these nanofibrous cathodes. Electrospun VNF exhibits initial discharge capacity ∼316 mAh g⁻¹ (∼2.2 Li per V₂O₅) in the voltage range of 1.75 and 4.0 V vs. Li/Li⁺ at 0.1 C rate. When cycled at a reduced voltage range of 2.0-4.0 V vs. Li/Li⁺, less phase transitions occur, giving rise to the initial specific capacity of 308 mAh g⁻¹ and improved cyclic retention of 74% after 50 cycles.     

A novel high-performance gel polymer electrolyte membrane basing on electrospinning technique for lithium rechargeable batteries

Nonwoven films of composites of thermoplastic polyurethane (TPU) with different proportion of poly(vinylidene fluoride) (PVdF) (80, 50 and 20%, w/w) are prepared by electrospinning 9 wt% polymer solution at room temperature. Then the gel polymer electrolytes (GPEs) are prepared by soaking the electrospun TPU-PVdF blending membranes in 1 M LiClO₄/ethylene carbonate (EC)/propylene carbonate (PC) for 1 h. The gel polymer electrolyte (GPE) shows a maximum ionic conductivity of 3.2 × 10⁻³ S cm⁻¹ at room temperature and electrochemical stability up to 5.0 V versus Li⁺/Li for the 50:50 blend ratio of TPU:PVdF system. At the first cycle, it shows a first charge-discharge capacity of 168.9 mAh g⁻¹ when the gel polymer electrolyte (GPE) is evaluated in a Li/PE/lithium iron phosphate (LiFePO₄) cell at 0.1 C-rate at 25 °C. TPU-PVdF (50:50, w/w) based gel polymer electrolyte is observed much more suitable than the composite films with other ratios for high-performance lithium rechargeable batteries.     

Enhanced performance of a quasi-solid-state dye-sensitized solar cell with aluminum nitride in its gel polymer electrolyte

The effects of incorporation of aluminum nitride (AlN) in the gel polymer electrolyte (GPE) of a quasi-solid-state dye-sensitized solar cell (DSSC) were studied in terms of performance of the cell. The electrolyte, consisting of lithium iodide (LiI), iodine (I₂), and 4-tert-butylpyridine (TBP) in 3-methoxypropionitrile (MPN), was solidified with poly(vinyidene fluoride-co-hexafluoro propylene) (PVDF-HFP). The 0.05, 0.1, 0.3, and 0.5 wt% of AlN were added to the electrolyte for this study. XRD analysis showed a reduction of crystallinity in the polymer PVDF-HFP for all the additions of AlN. The DSSC fabricated with a GPE containing 0.1 wt% AlN showed a short-circuit current density (J_SC) and power-conversion efficiency (η) of 12.92±0.54 mA/cm² and 5.27±0.23%, respectively, at 100 mW/cm² illumination, in contrast to the corresponding values of 11.52±0.21 mA/cm² and 4.75±0.08% for a cell without AlN. The increases both in J_SC and in η of the promoted DSSC are attributed to the higher apparent diffusion coefficient of I⁻ in its electrolyte (3.52×10⁻⁶ cm²/s), compared to that in the electrolyte without AlN of a DSSC (2.97×10⁻⁶ cm²/s). At-rest stability of the quasi-solid-state DSSC with 0.1 wt% of AlN was found to decrease hardly by 5% and 7% at room temperature and at 40 °C, respectively, after 1000 h duration. The DSSC with a liquid electrolyte showed a decrease of about 40% at room temperature, while it virtually lost its performance in about 150 h at 40 °C. Explanations are further substantiated by means of electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and by porosity measurements.     

Sulfonated poly(propylene oxide) oligomers/Nafion acid-base blend membranes for DMFC

A novel functional poly(propylene oxide)-backboned diamine of M_w 400 (abbreviated as D400) was grafted with sulfonic acid (abbreviated as D400-PS) to improve the performance of Nafion^® membranes in direct methanol fuel cells (DMFCs). The interaction of the D400-PS with Nafion^® was studied by Fourier transform infrared spectroscopy (FT-IR) and differential scanning calorimetry (DSC). The performance of the blend Nafion^®/D400-PS membranes was evaluated in terms of methanol permeability, proton conductivity and cell performance. The proton conductivity of the blend membrane was slightly reduced by rendering proton conductivity to D400 by functionalized with an organic sulfonic acid. The methanol permeability of the blend membrane decreased with increasing of D400-PS content. The methanol permeability of the blend Nafion^®/D400-PS with the composition 3/1 (–SO₃H/–NH₂) was 1.02 × 10⁻⁶ cm² S⁻¹, which was reduced 50% compared to that of pristine Nafion^®. The current densities that were measured with Nafion^®/D400-PS blend membranes in the ratio 1/0 and 5/1 (–SO₃H/–NH₂), were 51 and 72 mA cm⁻², respectively, at a potential of 0.2 V. Consequently, the blend Nafion^®/D400-PS membranes critically improved the single-cell performance of DMFC.     

Effects and properties of various molecular weights of poly(propylene oxide) oligomers/Nafion acid–base blend membranes for direct methanol fuel cells

Various molecular weights of poly(propylene oxide) diamines oligomers/Nafion^® acid–base blend membranes were prepared to improve the performance of Nafion^® membranes in direct methanol fuel cells (DMFCs). The acid–base interactions were studied by Fourier transform infrared spectroscopy (FT-IR) and differential scanning calorimetry (DSC). The performance of the blend membranes was evaluated in terms of methanol permeability, proton conductivity and cell performance. The proton conductivity was slightly reduced by acid–base interaction. The methanol permeability of the blend D2000/Nafion^® was 8.61 × 10⁻⁷ cm² S⁻¹, which was reduced 60% compared to that of pristine Nafion^®. The cell performance of D2000/Nafion^® blend membranes was enhanced significantly compared to pristine Nafion^®. The current densities that were measured with Nafion^® and 3.5 wt% D2000/Nafion^® blend membranes were 62.5 and 103.5 mA cm⁻², respectively, at a potential of 0.2 V. Consequently, the blend poly(propylene oxide) diamines oligomers/Nafion^® membranes critically improved the single-cell performance of DMFC.     

Pt/carbon nanofibers electrocatalysts for fuel cells: Effect of the support oxidizing treatment

Different Pt-based electrocatalysts supported on carbon nanofibers and carbon black (Vulcan XC-72R) have been prepared using a polymer-mediated synthesis. The electrocatalysts have been characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and cyclic voltammetry. The effect of carbon nanofibers treatment with HNO₃ solution on Pt particle size and electroactive area has been analyzed. Highly dispersed Pt with homogeneous particle size and an electroactive area around of 100 m² g⁻¹ is obtained in raw carbon nanofibers. The oxidizing treatment of the carbon nanofibers produces agglomeration of the platinum nanoparticles and an electroactive area of 53 m² g⁻¹. Durability studies indicate a decrease of 14% in the electroactive area after 90 h at 1.2 V in 0.5 M H₂SO₄ for platinum supported on raw carbon nanofibers and Vulcan XC-72R. The electrocatalyst supported on oxidized carbon nanofibers are stable under similar conditions.     

Ionic conductivity studies of gel polyelectrolyte based on ionic liquid

Novel lithium polyelectrolyte-ionic liquids have been prepared and characterized of their properties. Poly(lithium 2-acrylamido-2-methyl propanesulfonate) (PAMPSLi) and its copolymer with N-vinyl formamide (VF) also has been prepared as a copolymer. 1-Ethyl-3-methylimidazolium tricyanomethanide (emImTCM) and N,N-dimethyl-N-propyl-N-butyl ammonium tricyanomethanide (N₁₁₃₄TCM) which are chosen because of the same with the anion of ionic liquid were prepared. The ionic conductivity of copolymer system (PAMPSLi/PVF/emImTCM: 5.43 × 10⁻³ S cm⁻¹ at 25 °C) exhibits about over four times higher than that of homopolymer system (PAMPSLi/emImTCM: 1.28 × 10⁻³ S cm⁻¹ at 25 °C). Introduction of vinyl formamide into the copolymer type can increase the dissociation of the lithium cations from the polymer backbone. The ionic conductivity of copolymer with emImTCM (PAMPSLi/PVF/emImTCM) exhibits the higher conductivity than that of PAMPSLi/PVF/N₁₁₃₄TCM (2.48 × 10⁻³ S cm⁻¹). Because of using the polymerizable anion it is seen to maintain high flexibility of imidazolium cation effectively to exhibit the higher conductivity. And also the viscosity of emImTCM (19.56 cP) is lower than that of N₁₁₃₄TCM (28.61 cP). Low viscosity leads to a fast rate of diffusion of redox species.     

Electrospun polymer membrane activated with room temperature ionic liquid: Novel polymer electrolytes for lithium batteries

A new class of polymer electrolytes (PEs) based on an electrospun polymer membrane incorporating a room-temperature ionic liquid (RTIL) has been prepared and evaluated for suitability in lithium cells. The electrospun poly(vinylidene fluoride-co-hexafluoropropylene) P(VdF-HFP) membrane is activated with a 0.5 M solution of LiTFSI in 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMITFSI) or a 0.5 M solution of LiBF₄ in 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF₄). The resulting PEs have an ionic conductivity of 2.3 × 10⁻³ S cm⁻¹ at 25 °C and anodic stability at >4.5 V versus Li⁺/Li, making them suitable for practical applications in lithium cells. A Li/LiFePO₄ cell with a PE based on BMITFSI delivers high discharge capacities when evaluated at 25 °C at the 0.1C rate (149 mAh g⁻¹) and the 0.5C rate (132 mAh g⁻¹). A very stable cycle performance is also exhibited at these low current densities. The properties decrease at the higher, 1C rate, when operated at 25 °C. Nevertheless, improved properties are obtained at a moderately elevated temperature of operation, i.e. 40 °C. This is attributed to enhanced conductivity of the electrolyte and faster reaction kinetics at higher temperatures. At 40 °C, a reversible capacity of 140 mAh g⁻¹ is obtained at the 1C rate.     

Stable BaCe0.5Zr0.3Y0.16Zn0.04O3−δ thin membrane prepared by in situ tape casting for proton-conducting solid oxide fuel cells

Stable BaCe_0.5Zr_0.3Y_0.16Zn_0.04O_3−δ (BCZYZ) thin membrane was successfully prepared by in situ tape casting/co-firing method for proton-conducting solid oxide fuel cells. The starting powders were BaCO₃, CeO₂, ZrO₂, Y₂O₃, ZnO for electrolyte and BaCO₃, CeO₂, ZrO₂, Y₂O₃, ZnO, NiO, graphite for anode. The anode/electrolyte bi-layers were prepared by a simple multi-layer tape casting/co-firing method. The phase characterizations and microstructures were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The anode–electrolyte bi-layers were sintered at 1450 °C. The electrolytes were extremely dense with pure perovskite phase and the thickness was about 25 μm. The anodes were porous and no obvious reaction was found between NiO and BCZYZ. With LaSr₃Co_1.5Fe_1.5O_10−δ (LSCF)/BCZYZ as cathode, the open current voltage and maximum power density respectively, reached 1.00 V and 247 mW cm⁻² at 650 °C.     

NixCo1−x alloy nanoparticle-doped carbon nanofibers as effective non-precious catalyst for ethanol oxidation

Alloy structure generates special characteristics for the nano-metallic compounds which make this interesting class of materials promising candidates for many application fields. Moreover, the performance of the nanostructural catalysts is strongly influenced by the morphology; nanofibers reveal distinct catalytic activity compared to the nanoparticles. In this study, non-precious electrocatalysts based on alloy structure and nanofibrous morphology are introduced. Briefly, Ni_xCo_1−x (x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.9 and 1.0) alloy nanoparticles incorporated in carbon nanofibers are investigated as electrocatalysts for ethanol oxidation. Preparation of the introduced nanofibers could be achieved by calcination of electrospun nanofibers composed of nickel acetate tetrahydrate, cobalt acetate tetrahydrate and poly(vinyl alcohol) in argon atmosphere at 800 °C. Polycondensation characteristic of the utilized metals precursors led to produce good morphology electrospun nanofibers as well as preserved the nanofibrous morphology during the calcination process for all formulations. The catalytic activity of cobalt enhanced the carbonization of the utilized polymer which resulted in producing nickel/cobalt alloys nanoparticles embedded in carbon nanofibers. Electrochemical investigation of the introduced nanofibers toward ethanol oxidation indicated that the alloy structure has a strong influence. For instance, the corresponding current densities of Ni- and Ni_0.9Co_0.1-doped carbon nanofibers were 37 and 142 mA/cm², respectively. Moreover, very low onset potential (−50 mV vs. Ag/AgCl) was observed when Ni_0.1Co_0.9-doped carbon nanofibers were utilized. Furthermore, Ni_0.9Co_0.1-doped carbon nanofibers could oxidize ethanol solution up to 5 M due to the observed active layer regeneration. The introduced nanofibers have good stability because of the alloy structure. Overall, this study opens new avenue for the transition metals alloys and the nanofibrous morphology to produce novel and effective non-precious electrocatalysts.     

Novel osmium-based electrocatalysts for oxygen reduction and hydrogen oxidation in acid conditions

In this work, novel osmium electrocatalysts for oxygen reduction and hydrogen oxidation in 0.5 M H₂SO₄, have been developed. The syntheses were performed by thermolysis of Os₃(CO)₁₂ and Os₃(CO)₁₂/Vulcan^®, in two reaction media, N₂ (in the absence of solvents) and n-octane, in order to evaluate the effect of these parameters on the electrocatalytic activity of the new materials. In the solvent-free pathway, different reaction temperatures (in the 120–320 °C range) and times (5, 7 and 10 h) were explored; the syntheses in n-octane were done at reflux temperature, for 30 and 72 h. The products were characterized structurally by FT-IR spectroscopy, X-ray diffraction and scanning electron microscopy, and electrochemically by room temperature rotating disk electrode measurements, using cyclic and linear sweep voltammetry. Some materials prepared in both reaction media can efficiently perform the hydrogen oxidation and/or oxygen reduction reaction, i.e. those prepared by pyrolysis of Os₃(CO)₁₂/Vulcan^® in N₂, at 180 °C/7 h, 320 °C/5 h, 320 °C/7 h and 320 °C/10 h, as well as the materials synthesized in n-octane (from both Os precursors); the latter, in addition, have the important property of being tolerant to carbon monoxide to some extent, in contrast to platinum, which is easily deactivated even by traces of CO.     

Performance of solid alkaline fuel cells employing anion-exchange membranes

For the performances of solid alkaline fuel cells (SAFCs) using anion-exchange membranes (AEMs), anion-exchange membranes were prepared via chloromethylation and amination of polysulfone and membrane-electrode assemblies (MEAs) were fabricated using the AEMs as an electrolyte, the ionomer binder prepared by the AEMs and Pt/C and Ag/C electrocatalysts as an anode and a cathode, respectively. Anion-exchange membranes were aminated by a mixing amine agent of trimethylamine (TMA) as a monoamine and various diamines such as N,N,N′,N′-tetramethylmethanediamine (TMMDA), N,N,N′,N′-tetramethylethylenediamine (TMEDA), N,N,N′,N′-tetramethyl-1,3-propandiamine (TMPDA), N,N,N′,N′-tetramethyl-1,4-butanediamine (TMBDA) and N,N,N′,N′-tetramethyl-1,6-hexanediamine (TMHDA). Amination using various diamines enabled to investigate the effect of the length of alkyl chain of the diamines on membrane properties such as ion conductivity and thermal characteristics. The AEMs aminated by the amination agent of mixing TMA and TMHDA (with longer alkyl chain) showed better hydroxyl ion conductivity and thermal stability than those aminated by a diamine. The H₂/air SAFC performance of the MEA with 0.5 mg cm⁻² Pt/C at the anode and the cathode, respectively, was comparable to one with 0.5 mg cm⁻² Pt/C at the anode and 2.0 mg cm⁻² Ag/C at the cathode, i.e., approximately 28–30 mW cm⁻² of the peak power density range.     

Visible light photocatalytic water splitting for hydrogen production from N-TiO2 rice grain shaped electrospun nanostructures

We report on the visible light-driven hydrogen production from splitting of water molecules by nitrogen-doped TiO₂ (N-TiO₂) with a rice grain-like nanostructure morphology. The N-TiO₂ nanostructures are prepared using sol-gel and electrospinning methods followed by post-annealing of the composite nanofibers. The nanostructures are characterized by microscopy and spectroscopy. First order rate constants for the visible light-assisted photocatalysis in the degradation of methylene blue (MB) dye are found to be 0.2 × 10⁻³ and 1.8 × 10⁻³ min⁻¹ for TiO₂ and N-TiO₂ (5 wt% of nitrogen), respectively. The N-TiO₂ utilized in water splitting experiments and evaluated hydrogen (H₂) of 28 and 2 μmol/h for N-TiO₂ and TiO₂, respectively. The improvement may be attributed due to the N-doping and higher surface area as ∼70 m²/g.     

Preparation and characterization of Ppy/Al2O3/Al used as solid-state capacitors for microsystems—Effect of amount of electricity passed

This investigation focuses on the preparation of polypyrrole composite films, Ppy/Al₂O₃/Al, used as solid-state capacitor with electrochemical polymerization in the presence of DBSA, co-dopant and co-solvent. The parameters of the solid-state capacitor, i.e. leakage current (Lc), capacitor (Cs), dicipation factor (DF) and equivalent series resistance (ESR), were measured in this study. The surface morphology and compositions of the prepared Ppy/Al₂O₃/Al electrolyte were examined using SEM and EDS, respectively. Further, for 3 C of electricity passed, the values of Lc, Cs, DF and ESR of the Ppy/Al₂O₃/Al capacitor prepared with 2-NSNa and DBSA as a co-dopant were measured to be 0.029 ± 0.005 μA cm⁻², 688.8 ± 8.28 nF cm⁻², 14.35 ± 5.63% and 18.63 ± 3.48 Ω. Further, the parameters both of R_ox and R_ppy are 135.5 and 7.047 Ω.     

All-textile flexible supercapacitors using electrospun poly(3,4-ethylenedioxythiophene) nanofibers

Poly(3,4-ethylenedioxythiophene) (PEDOT) nanofibers were obtained by the combination of electrospinning and vapor-phase polymerization. The fibers had diameters around 350 nm, and were soldered at most intersections, providing a strong dimensional stability to the mats. The nanofiber mats demonstrated very high conductivity (60 ± 10 S cm⁻¹, the highest value reported so far for polymer nanofibers) as well as improved electrochemical properties, due to the ultraporous nature of the electrospun mats. The mats were incorporated into all-textile flexible supercapacitors, using carbon cloths as the current collectors and electrospun polyacrylonitrile (PAN) nanofibrous membranes as the separator. The textile layers were stacked and embedded in a solid electrolyte containing an ionic liquid and PVDF-co-HFP as the host polymer. The resulting supercapacitors were totally flexible and demonstrated interesting and stable performances in ambient conditions.     

Electrochemical performance of mesoporous TiO2 anatase

Mesoporous TiO₂ was prepared via a sol–gel method from an ethylene glycol-based titanium-precursor in the presence of a non-ionic surfactant at pH 2. Only the anatase structure was detected after annealing, while the BET specific surface area was measured as being 90 m² g⁻¹ with a rather monomodal pore diameter close to 5 nm. Electrochemical performances were investigated by cyclic voltammetry and galvanostatic techniques. Mesoporous TiO₂ exhibits excellent rate capability (184 mAh g⁻¹ at C/5, 158 mAh g⁻¹ at 2C, 127 mAh g⁻¹ at 6C, and 95 mAh g⁻¹ at 30C) and good cycling stability.