Nanostructured inorganic materials: Synthesis and associated electrochemical properties
Yau, Shali Zhu
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Synthetic strategy for preparing potential battery materials at low temperature was developed. Magnetite (Fe 3 O 4 ), silver hollandnite (Ag x Mn 8 O 16 ), magnesium manganese oxide (MgxMnO 2 ̇yH 2 O), and silver vanadium phosphorous oxide (Ag 2 VO 2 PO 4 ) were studied. Magnetite (Fe 3 O 4 ) was prepared by coprecipitation induced by triethylamine from aqueous iron(II) and iron(III) chloride solutions of varying concentrations. Variation of the iron(II) and iron(III) concentrations results in crystallite size control of the Fe 3 O 4 products. Materials characterization of the Fe 3 O 4 samples is reported, including Brunauer-Emmitt-Teller (BET) surface area, x-ray powder diffraction (XRD), transmission electron microscopy (TEM), particle size, and saturation magnetization results. A strong correlation between discharge capacity and voltage recovery behavior versus crystallite size was observed when tested as an electrode material in lithium electrochemical cells. Silver hollandite (Ag x Mn 8 O 16 ) was successfully synthesized through a low temperature reflux reaction. The crystallite size and silver content of Ag x Mn 8 O 16 by varying the reactant ratio of silver permanganate (AgMnO 4 ) and manganese sulfate monohydrate (MnSO 4 ̇H 2 O). Silver hollandite was characterized by Brunauer-Emmitt-Teller (BET) surface area, inductively coupled plasma-optical emission (ICP-OES) spectrometry, helium pycnometry, simultaneous thermogravimetric analysis/differential scanning calorimetry (TGA/DSC), and x-ray powder diffraction (XRD). The crystallite size showed a strong correlation with silver content, BET surface area, and particle sizes. The silver hollandite cathode showed good discharge capacity retention in 30 cycles of discharge-charge. There were a good relationship between crystallite size and rate capability and pulse ability. Magnesium manganese oxide (Mg x MnO 2 ̇yH 2 O) was made by redox reaction by mixing sodium hydroxide (NaOH), manganese sulfate monohydrate (MnSO 4 ̇HO 2 ), and potassium persulfate (K 2 S 2 O 8 ). The solid samples were characterized by inductively coupled plasma-optical emission (ICP-OES) spectrometry, scanning electron microscopy (SEM), simultaneous thermogravimetric analysis/differential scanning calorimetry (TGA/DSC), and X-ray powder diffraction (XRD). The solid had a plate-like morphology. The preliminary electrochemical results showed that Mg x MnO 2 ̇yH 2 O had a very good cycliability and the capacity retention in 20 discharge-charge cycles. When the sample was dried at 100°C after collection, the discharge capacity would increase from 80 mAh/g to 155 mAh/g in the first discharge process in cycling test. Silver vanadium phosphorous oxide (SVPO, Ag 2 VO 2 PO 4 ) was prepared in various reaction temperatures. It was the first time that Ag 2 VO 2 PO 4 was synthesized successfully at room temperature. The solid was characterized by Brunauer-Emmitt-Teller surface area (BET), inductively coupled plasma-optical emission (ICP-OES) spectroscopy, differential scanning calorimetry (DSC), magnetic susceptibility measurement, scanning electron microscope (SEM) and x-ray powder diffraction (XRD). Ag 2 VO 2 PO 4 crystallite sizes showed a strong linear correlation with reaction temperature. The BET surface area was decreased as the crystallite size increased linearly. In addition, the acicular morphology started to develop at 50°C. The impact of silver deposition loading on the silver-polypyrrole composite electrode was studied using cyclic voltammetry. The minimum Ag loading of 0.08 mg/cm 2 was determined to maximize the oxygen reduction activity for the Ag/Ppy composite catalyst. In addition, the Ag/Ppy coated carbon electrode showed higher oxygen reduction activities in both air and oxygen compared to the uncoated carbon electrode.