Students’ conceptual understanding of organic chemistry and classroom implications in the Rwandan perspectives: A literature review

Chemistry subject continues to be considered as difficult to teach and learn. This leads to students’ low academic achievement, retention, and negative attitude towards the subject. Organic chemistry as one of the concepts on which technological advancement is constructed sometimes appears to be enormously complex to students. There are some persisting misconceptions about it although different innovative instructional strategies have been applied and this area is of main concern as the learning of students can be extremely hindered in case their misconceptions are not minimized and/or corrected. The review then is to equip educators with knowledge about organic chemistry concept and source of students ‘misconceptions; the misconceptions of students about organic chemistry; the ways of diagnosing students’ misconceptions and remedies of those misconceptions; some learning theories for the effective organic chemistry instruction and classroom implications. The paper is also useful to know more about the minimization of students’ misconceptions and leading them to the great academic achievement and interest towards the subject by employing cooperative learning models; thus, many other different innovative teaching strategies are recommended to apply in organic chemistry instructio

Towards a development of iron phosphate-based materials as positive and negative electrodes for Li-ion batteries

The design, synthesis and electrochemical characterization of new electrode materials hold the key for fundamental advances in energy conversion and storage technologies. Polyanionic compounds have been heavily investigated as possible electrode materials in lithium- and sodium-ion cells. This thesis focused on the design, synthesis, size and morphology tailoring of iron phosphate based electrode materials in order to enhance their electrochemical properties. Four iron phosphate-based electrode materials classified in three groups: Na2Mn1.5Fe1.5(PO4)3, NaxMxFe(3-X)(PO4)3 (X=1.25 when M=Ni and x=1.5 for M= Mn) and Fe1.19(PO4)(OH)0.57(H2O)0.43, were investigated in this work. All these compounds were obtained through wet chemical (hydrothermal or solvothermal) synthesis routes. The preparation protocols and characterization techniques to study the structural, particle size, morphological and electrochemical properties of the above materials have been discussed in the following chapters. Sodium manganese iron phosphate alluaudite structure is one of the studied compounds group. Na2Mn1.5Fe1.5(PO4)3 is obtained via one step hydrothermal synthesis reaction and it was electrochemically studied without any other further heat treatment. NaxMxFe(3-X)(PO4)3 (X=1.25 for M=Ni and x=1.5 for M= Mn) compounds were obtained through solvothermal method in ethylene glycol. Na1.25Ni1.25Fe1.75(PO4)3 is a new material. Both materials have been electrochemically characterized for the first time in this work. Fe1.19(PO4)(OH)0.57(H2O)0.43 was obtained by modifying conventional hydrothermal synthesis of this materials by the addition of conducting carbon (carbon black and carbon nanotubes) in the precursors solution during synthesis that enhance its electrochemical properties as described in this thesis. The Li-ion intercalation reaction mechanisms in Fe1.19(PO4)(OH)0.57(H2O)0.43 cathode material were also investigated by using operando XRD and Mössbauer spectroscopy techniques. The above studies of iron phosphate-based polyanionics as electrode materials in alkali metal-ion batteries show that this group may be the right key in replacing current commercial unsafe electrode materials. The possibility of improving alternative soft chemical synthesis methods to design new materials or improving electrochemical performance of existing electrode materials was also explored here. The roadmap for our current and future work has been proposed, the materials of our future interest have been chosen basing on their promising rich crystal structure and electrochemical properties by comparing them with the studied materials

Phosphate-based composite electrodes for Li/Na-ion batteries: upscalable solution syntheses with in-situ solid carbon addition

Since the success story of lithium iron phosphate, other phosphate-based compounds have attracted a lot of interest as promising candidates for positive electrodes in lithium-ion or sodium-ion batteries. Their electronic conductivity usually has to be improved through the preparation of composite powders ensuring intimate contact between the active material and conductive carbon. We report on the one-step synthesis of composite precursors using spray-drying or hydrothermal synthesis routes, two techniques which offer easy scaling-up of production. We show that addition of a solid carbon source (carbon black or carbon nanotubes) into the solution has a strong influence on the powder microstructure and is very effective in improving the battery cycling performance, taking our recent results on phosphates [Fex(PO4)(OH)y.zH2O)] and fluorophosphates [Na2FePO4F, Na3V2(PO4)2F3] as examples. We also compare this approach with the addition of the carbon source as a soluble precursor (such as ascorbic acid or citric acid) where the in situ formation of carbon is achieved by a heat treatment in inert atmosphere (typically argon)

Effects of Starch as Carbon precursor on hydrothermal synthesis and Electrochemical performance of Sodium manganese iron phosphate /carbon

Currently Sodium based electrode materials for Li and Na-ion batteries are getting more attention as the most promising potential alternatives of their lithiated counterparts due to their cost effective, environmental friendly characteristics and availability of sodium. Nevertheless, it remains a practical challenge to find an electrode material of LIBs and SIBs showing ideal performance. We report here a composite material of Sodium manganese iron phosphate/carbon, successfully synthesized by hydrothermal method. We have characterized our material by using a combination of Powder X-ray diffraction (XRD), scanning electron Microscopy (SEM) and thermal gravimetric analysis (TGA). Sodium manganese iron phosphate (NMFP) particles are electrochemically activated by starch and acetylene black to form NMFP/C cathode material for LIBs. NMFP/C composite in which starch is used as carbon precursor exhibits good discharge capacity due to the presence of pyran rings which increase NMFP/C conductivity

One-step hydrothermal synthesis and electrochemical performance of sodium-manganese-iron phosphate as cathode material for Li-ion batteries

The sodium-manganese-iron phosphate Na2Mn1.5Fe1.5(PO4)3 (NMFP) with alluaudite structure was obtained by a one-step hydrothermal synthesis route. The physical properties and structure of this material were obtained through XRD and Mössbauer analyses. X-ray diffraction Rietveld refinements confirm a cationic distribution of Na+ and presence of vacancies in A(2)’, Na+ and small amounts of Mn2+ in A(1), Mn2+ in M(1) , 0.5 Mn2+ and Fe cations (Mn2+,Fe2+ and Fe3+) in M(2), leading to the structural formula Na2Mn(Mn0.5Fe1.5)(PO4)3. The particles morphology was investigated by SEM. Several reactions with different hydrothermal reaction times were attempted to design a suitable synthesis protocol of NMFP compound. The time of reaction was varied from 6 to 48 hours at 220°C. The pure phase of NMFP particles was firstly obtained when the hydrothermal reaction of NMFP precursors mixture was maintained at 220°C for 6 hours. When the reaction time was increased from 6 to 12, 24 and 48 hours, the dandelion structure was destroyed in favor of NMFP micro-rods. The combination of NMFP (NMFP-6H, NMFP-12H, NMFP-24H and NMFP-48H) structure refinement and Mössbauer characterizations shows that the increase of the reaction time leads to the progressive increment of Fe(III) and the decrease of the crystal size. The electrochemical tests indicated that NMFP is a 3 V sodium intercalating cathode. The comparison of the discharge capacity evolution of studied NMFP electrode materials at C/5 current density shows different capacities of 48, 40, 34 and 34 mAhg-1 for NMFP-6H, NMFP-12H, NMFP-24H and NMFP-48H respectively. Interestingly, all samples show excellent capacity retention of about 99 % during 50 cycles