Study On The Controlled Electrochemical Synthesis And Electrocalalysis Performance Of Manganese Oxide Octedral Molecular Sieves

Manganese oxide materials with different crystal structures and morphologies have been synthesized by an electrodeposition method. As-prepared materials were characterized by powder X-ray diffractometer (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), thermogravimetric analysis (TG), cyclic voltammetry (CV), linear sweep voltammetry (LSV), tafel polarization and electrochemical impedance spectroscopy (EIS). Some typical materials were used as anode materials to electrocatalytical degradation of phenol in wastewater and the influencing factors were also discussed. Further, some typical materials were also studied as electrode materials in lithium-ion batteries and supercapacitor to investigate the effects of composition, crystal structure and morphologies on their electrochemical properities. The main conclusions are as follows:A series of crystal structures of manganese oxide materials, including cryptomelane, nsutite, Ce-doped cryptomelane, Ce-doped nsutite and Ce-doped birnessite were synthesized by different electrolytes and under different potentials and so on. Nsutite could be electrochemically deposited in 0.1 mol/L MnSO4 and 0.3 mol/L MnSO4 solutions, respectively. Nsutite was electrochemical deposited in 0.3 mol/L MnSO4 solution with H2SO4 concentration in the range of 0.5 mol/L to 1.0 mol/L at different potentials. Crytomelane was electrochemically synthesized at 1.8 V in 0.3 mol/L MnSO4 solution with 2.0 mol/L H2SO4. Nsutite was electrochemically prepared at 2.0-3.8 V in 0.3 mol/L MnSO4 solution with 2.0 mol/L H2SO4. Crytomelane was electrochemical deposited at 2.2-2.4 V in 0.3 mol/L MnSO4 solution with 3.0 mol/L H2SO4. Nsutite was electrochemically fabricated at 3.2-3.4 V in 0.3 mol/L MnSO4 solution with 3.0 mol/L H2SO4. Crytomelane and nsutite mixtures were electrochemically deposited at 2.6-3.0 V in 0.3 mol/L MnSO4 solution with 3.0 mol/L H2SO4. Ce-doped birnessite was electrochemically manufactured in 0.3 mol/L MnSO4 0.015 mol/L Ce2(SO4)3 solution at different potentials. Ce-doped nsutite was electrochemically deposited in amixture of 0.3 mol/L MnSO4 and 0.015 mol/L Ce2(SO4)3 solutions with H2SO4 concentration in the range of 0.5 mol/L to 1.0 mol/L at all potentials. In a mixture of 0.3 mol/L MnSO4 and 0.015 mol/L Ce2(SO4)3 solutions with H2SO4 concentration in the range of 2.0 mol/L to 3.0mol/L, Ce-doped cryptomelane was electrochemically deposited at low potentials, Ce-doped nsutite synthesized at high potentials, the mixtures of the two in the middle range of the potentials. An increase in the concentration of H2SO4 facilitated the formation of cryptomelane, and Ce-doped crytomelane could be formed in the acid solution. Ce-doped birnessite was synthesized only as Ce-doped form in the mixed solution of MnSO4 and Ce2(SO4)3.Six samples of nsutite and cryptomelane were used as anode materials to electrocatalytically degrade phenol in wastewater. Degradation performance of nsutite was observed to be better than that of cryptomelane. The lowest performance was observed for nsutite synthesized by adding H2SO4. Ce doping enhanced the degradation effects of phenol but the stability of anode was low. Undoped nsutite synthesized in neutrallty condition was used as anode to study influcening factors of degradation of phenol after 1 h. pH2 or pH10 facilitated the degradation of phenol and removal of COD. Phenol degradation efficiency was 22.19% and 18.19%, respectively. COD removal efficiency was 18.37% and 17.69%, respectively. The low performance was observed at pH6. Phenol degradation efficiency was 14.17% and COD removal efficiency was 12.24%. Lower initial phenol concentration can facilitate the degradation of phenol and the removal of COD. Phenol degradation efficiency was 6.88% and COD removal efficiency was 5.95% at initial phenol concentration of 400 mg/L. Phenol degrade efficiency was 23.65% and COD removal efficiency was 21.62% at initial phenol concentration of 50 mg/L. A decrease in of the electrodes distance could improve the degradation of phenol and the removal of COD. Degradation efficiency of phenol was 8.62% and COD removal efficiency was 7.48% at 20 mm between electrodes. Phenol degradation efficiency was 14.17% and COD removal efficiency was 12.24% at 10 mm between electrodes. Bigger current density facilitated the degradation of phenol and removal of COD. Phenol degradation efficiency was 10.84% and COD removal efficiency was 9.52% with a current density of 5 mA/cm2. Phenol degrade efficiency was 21.68% and COD removal efficiency was 19.05% with a current density of 20 mA/cm2. Promotion electrolyte concentration was conducive to degradation of phenol and removal of COD. Phenol degradation efficiency was 11.88% and COD removal efficiency was 10.20% with 0.05 mol/L Na2SO4. Degradation efficiency of phenol was 14.98% and COD removal efficiency was 12.93% with adding 0.20 mol/L Na2SO4. The main reason of common performance of degradation of phenol was aspect of part of anode formed oligomer to anode inactivation.The mechanism of electrocatalysis oxidation degradation of phenol was also investigated. The intermediate products were observed to be hydroquinone, quinone, maleic acid, and so on. One possible pathway of phenol oxidation degradation way was that adsorbed oxidative hydroxide radical produced by anode through dehydrogenation with phenol formed quinone. Oxidative hydroxide radical through electrophilic addition with phenol facilitated the formation of hydroquinone. Hydroxide radical with high oxidation activity through dehydrogenation with hydroquinone formed quinone. The amount of quinone formed was increased continuously, maleic acid initially present in solution as well as ring opened during the process and then oxidized into carbon dioxide and water. The other pathway was likely phenoxy group was formed from that adsorbed oxidative hydroxide radical produced by anode reaction through electron transfer. Phenoxy group was accumulated to formed oligomer attachmented on the surface of anode. The amount of phenoxy group oxidized to quinone continuously, maleic acid initially present in solution as well as ring opened during the process and then oxidized into carbon dioxide and water.The electrochemical properties of three Ce-doped birnessites were investigated as cathode materials in lithium-ion batteries. The galvanostatic discharge/charge curves indicate that the best cycle properties at a current density of 30 mA/g was obtained for Ce-doped birnessite electrodeposited at 1.6 V. The maximum discharge capacity was 193 mAh/g at a current density of 30 mA/g using Ce-doped birnessite electrodeposited at 2.0 V.Supercapacitors performance was tested using three Ce-doped birnessites samples. The largest specific capatitance of Ce-dpoed birnessite electrodeposited at 1.2 V was 132.84 F/g at a current density of 100 mA/g with a voltage window of 0-0.9 V in charging/discharging experiments.