Layered composite membranes based on porous PVDF coated with a thin, dense PBI layer for vanadium redox flow batteries

A commercial porous polyvinylidene fluoride membrane (pore size 0.65 μm, nominally 125 μm thick) is spray coated with 1.2–4 μm thick layers of polybenzimidazole. The area resistance of the porous support is 36.4 mΩ cm2 in 2 M sulfuric acid, in comparison to 540 mΩ cm2 for a 27 μm thick acid doped polybenzimidazole membrane, and 124 mΩ cm2 for PVDF-P20 (4 μm thick blocking layer). Addition of vanadium ions to the supporting electrolyte increases the resistance, but less than for Nafion. The expected reason is a change in the osmotic pressure when the ionic strength of the electrolyte is increased, reducing the water contents in the membrane. The orientation of the composite membranes has a strong impact. Lower permeability values are found when the blocking layer is oriented towards the vanadium-lean side in ex-situ measurements. Cells with the blocking layer on the positive side have significantly lower capacity fade, also much lower than cells using Nafion 212. The coulombic efficiency of cells with PVDF-PBI membranes (98.4%) is higher than that of cells using Nafion 212 (93.6%), whereas the voltage efficiency is just slightly lower, resulting in energy efficiencies of 85.1 and 83.3%, respectively, at 80 mA/cm2

A Study on the Elementary School Teachers’ Awareness of Students’ Alternative Conceptions about Change of States and Dissolution

Knowledge about students’ conceptions is one of the requisite components of pedagogical content knowledge. A keen awareness of students’ alternative conceptions provides teachers with information about prospective difficulties students may incur as they make attempts to learn more accurate scientific representations of critical concepts. In this study, we investigated elementary school teachers’ understanding of their students’ alternative conceptions about change of states and dissolution. The subjects were 152 elementary school teachers and 529 sixth graders in Korea. A conceptions test and the test of the understanding about students’ conceptions were administered in order to examine students’ alternative conceptions and the teachers’ awareness of their students’ alternative conceptions, respectively. The effects of teachers’ characteristics such as teaching experience, highest academic degree, science teaching efficacy, and views about teaching and learning (i.e., constructivist and traditional) in relation to their awareness of students’ alternative conceptions were also investigated. The results indicated that the teachers tended to overestimate the number of students with scientifically accepted conceptions. The teachers also did not possess adequate knowledge about the existence and the distribution of their students’ alternative conceptions. It was found that teaching experience, highest academic degree, science teaching efficacy, and the level of teachers’ adoption of a constructivist view about teaching and learning were not significantly related to the their awareness of students’ alternative conceptions. It was found, however, that there is a significant relationship between the level of teachers’ traditional view about teaching and learning and their awareness of students’ alternative conceptions. Educational implications are discussed

Aqueous redox flow battery using iron 2,2‐bis(hydroxymethyl)‐2,2′,2′‐nitrilotriethanol complex and ferrocyanide as newly developed redox couple

An all-iron aqueous redox flow battery using iron (Fe) 2,2-bis(hydroxymethyl)-2,2',2'-nitrilotriethanol (BIS-TRIS) complex (Fe(BIS-TRIS)) and Ferrocyanide (Fe[CN](6)) as redox couple is newly suggested. The redox potential of Fe(BIS-TRIS) is -1.11 V (vs Ag/AgCl) and this makes Fe(BIS-TRIS) appropriate as active material for anolyte, while Fe(CN)(6) is proper for catholyte due to its excellent redox reactivity, redox potential, and cheap cost. According to quantitative evaluations, Fe(BIS-TRIS) does not produce any side reactions and is more stable than Fe triethanolamine (TEA) (Fe(TEA)) complex that is conventionally considered for the purpose. This fact is confirmed by computational analysis using density functional theory. In the calculation, energy barrier of Fe(BIS-TR1S) suppressing the occurrence of undesirable side reactions is higher than that of other Fe-ligand complexes, indicating that desirable redox reaction of Fe(BIS-TRIS) occurs more stably. In redox flow battery (RFB) tests, RFBs using Fe(BIS-TRIS) do not show any side reactions even after 250 cycles with excellent performances, such as capacity of 11.7 Ah L-1. and coulombic efficiency and capacity retention rate of 99.8 and 99.9%, respectively. This corroborates that RFBs using Fe(BIS-TRIS) have excellency in both performance and stability, while the cheap cost of BIS-TRIS and Fe(CN)(6) enhances the economic benefit of RFBs.11Nsciescopu

Vanadium Redox Flow Batteries Using <i>meta</i>-Polybenzimidazole-Based Membranes of Different Thicknesses

15, 25, and 35 μm thick <i>meta</i>-polybenzimidazole (PBI) membranes are doped with H₂SO₄ and tested in a vanadium redox flow battery (VRFB). Their performances are compared with those of Nafion membranes. Immersed in 2 M H₂SO₄, PBI absorbs about 2 mol of H₂SO₄ per mole of repeat unit. This results in low conductivity and low voltage efficiency (VE). In ex-situ tests, <i>meta</i>-PBI shows a negligible crossover of V³⁺ and V⁴⁺ ions, much lower than that of Nafion. This is due to electrostatic repulsive forces between vanadium cations and positively charged protonated PBI backbones, and the molecular sieving effect of PBI’s nanosized pores. It turns out that charge efficiency (CE) of VRFBs using <i>meta</i>-PBI-based membranes is unaffected by or slightly increases with decreasing membrane thickness. Thick <i>meta</i>-PBI membranes require about 100 mV larger potentials to achieve the same charging current as thin <i>meta</i>-PBI membranes. This additional potential may increase side reactions or enable more vanadium ions to overcome the electrostatic energy barrier and to enter the membrane. On this basis, H₂SO₄-doped <i>meta</i>-PBI membranes should be thin to achieve high VE and CE. The energy efficiency of 15 μm thick PBI reaches 92%, exceeding that of Nafion 212 and 117 (N212 and N117) at 40 mA cm^–2