Hydrothermal Preparation Of Na_0.44MnO₂ And Its Pseudocapacitive Sodium Storage Characteristics

Manganese dioxide（MnO₂）is a versatile material with a wide range of applications in many fields,including energy storage,water treatment,and catalysis.In recent years,the ion storage mechanism and properties of MnO₂have become one of the hottest areas of research,which consists of three main components:bulk phase Faraday,surface phase Faraday and double electric layer,of which surface phase Faraday is also known as pseudocapacitance.In essence,pseudocapacitance refers to the rapid and reversible redox reactions occurring at or near the electrode surface,resulting in enhanced energy storage capacity.Nanostructured MnO₂,due to its high specific surface area and tiny size,increases the redox active sites and shortens the ion diffusion pathway,thus improving the electrochemical properties and making fast redox reactions possible,and thus the pseudocapacitance effect.The simple and rapid preparation of MnO₂with desirable electrochemical properties is a major obstacle to its practical application.In this study,Na_0.44MnO₂is used as an example to prepare nanoscale Na_0.44MnO₂by an optimized hydrothermal method to synthesize nanostructured materials with controlled morphology and uniform particle size by optimizing the synthesis conditions while improving the synthesis efficiency and reducing the time cost,thus improving the electrochemical properties.The high pseudocapacitive properties of Na_0.44MnO₂make it a strong candidate not only as a supercapacitor electrode material with excellent energy storage capacity,but also for the development of ion-selective electrodes with high sensitivity and selectivity.The main findings of this thesis are as follows:（1）Pure phase Na_0.44MnO₂nanowires were successfully synthesized in 8 h using a low temperature hydrothermal reaction of KMnO₄and Mn SO₄.Maintaining the proper reaction time and oxidation reduction ratio is essential to achieve the desired material properties.A hydrothermal reaction time of 0 h produces Na-birnessite materials,while extending the reaction time beyond 4 h has no significant effect on the properties and purity of the materials.The best electrochemical properties of Na_0.44MnO₂nanowires were obtained at a M（KMnO₄）/M（Mn SO₄）ratio of 14/13.Deviation from this ratio may lead to the synthesis of undesirable materials or the introduction of impurities,which will negatively affect the capacity and cycling stability of the materials.The Na_0.44MnO₂-based sodium ion electrodes prepared by hydrothermal synthesis exhibited high specific capacitance and robust cycling stability in energy storage performance tests.The measured value of specific capacitance was always 125 F/g at a current density of 10 A/g.In addition,the aqueous symmetric supercapacitor prepared by this method exhibited 95.0%capacity retention and nearly 99.9%Coulombic efficiency after 5000 cycles at 1 A/g,along with a power density of279.92 W/kg and an energy density of 56.84 Wh/kg.（2）The Na_0.44MnO₂-based sodium ion selective electrode prepared by hydrothermal synthesis showed a fast（～2 s）,stable and reproducible energy-strength response of 56.0 m V/dec.Due to the excellent performance of the hydrothermally synthesized Na_0.44MnO₂,pure phase Na_0.44MnO₂nanowires were drop-coated on treated carbon paper and used as working electrodes,combined with Ag/Ag Cl drop-coated carbon paper sensors as reference electrodes,to form a home-made sodium ion carbon paper sensor showing a fast（～2 s）and stable 55.90 m V/dec energetic response.The sensor was tested in real petrochemical wastewater and ethanol-water organic solutions to demonstrate its leakage resistance.（3）Comparison of the performance of Na_0.44MnO₂-based sodium ion supercapacitors and sodium ion selective electrodes prepared by hydrothermal and solid-phase synthesis.At a current density of 10 A/g,the specific capacitance of the sodium ion electrode prepared by the solid-phase method is 65 F/g,which is only 65%of that of the electrode prepared by the hydrothermal method.In addition,the sodium ion-selective electrode prepared by the solid-phase method exhibited lower sensitivity,poorer stability,and energy-strength response fluctuating from 39.57 to 43.44 m V/dec.The structural and morphological characteristics of the Na_0.44MnO₂samples prepared by hydrothermal and solid-phase methods,as well as aspects of their electrochemical properties,were comprehensively analyzed.Through the study of these characteristics,a clear correlation was established between the synthetic methods employed and the resulting electrochemical properties.