تاثیر جانشانی منگنز به جای آهن بر عملکرد ابرخازنی نانوذرات فریت کبالت

در این پژوهش، تاثیر جانشانی منگنز به جای آهن بر ویژگی‌های ساختاری و الکتروشیمیایی نانوذرات فریت کبالت (cofe2-xmnxo4 : x=0.0, 0.1, 0.3, 0.5) به‌عنوان ماده فعال الکترودی برای ابرخازن‌ها مورد بررسی قرار گرفت. نانوذرات مورد نظر به روش سل- ژل سنتز شده و ویژگی‌های ساختاری آن‌ها با استفاده از پراش پرتو ایکس (xrd) و آنالیز ریتولد مورد ارزیابی قرار گرفت. نتایج xrd نشان داد که تمامی نمونه‌ها دارای ساختار اسپینل مکعبی تک‌فاز هستند و با افزایش مقدار منگنز، ثابت شبکه و حجم سلول واحد افزایش می‌یابد. همچنین، آنالیز ریتولد بیانگر تغییر در توزیع کاتیونی و کاهش اندازه بلورک‌ها با افزایش میزان جانشانی منگنز بود. ویژگی‌های الکتروشیمیایی نمونه‌ها با روش ولتامتری چرخه‌ای (cv) و آزمون‌های شارژ- دشارژ گالوانواستاتیک ارزیابی شد. نتایج نشان داد که جانشانی منگنز منجر به افزایش ظرفیت ویژه، چگالی انرژی و چگالی توان الکترودها می‌شود. بیشترین ظرفیت ویژه در نرخ اسکن mv/s 5 برای نمونه با x=0.5  برابر با fg-1 12/25 بدست آمد که نسبت به نمونه فریت کبالت (fg-1 8/44 ) افزایش 45 درصدی را نشان می دهد. افزون بر این، رفتار ولتامتری شبه‌خازنی نمونه‌های جانشانی شده، پایداری بهتری را در نرخ‌های اسکن بالا نشان داد. این نتایج نشان می‌دهد که جانشانی منگنز در ساختار فریت کبالت می‌تواند رویکردی موثر برای بهبود عملکرد الکتروشیمیایی الکترودهای ابرخازنی باشد.

effect of mn substitution for fe on the supercapacitive performance of cobalt ferrite nanoparticles

1. introductionthe growing global energy demand and environmental concerns related to fossil fuel consumption have intensified efforts toward developing sustainable and efficient energy storage systems. among these, supercapacitors have received considerable attention due to their high-power density, rapid charge-discharge capabilities, and long cycle life. however, their low energy density remains a major limitation. enhancing the electrochemical performance of electrode materials is key to addressing this challenge. spinel ferrites with the general formula mfe₂o₄ (m = co, ni, mn, etc.) are attractive electrode materials because of their unique structural, magnetic, and electrochemical properties. cobalt ferrite (cofe₂o₄), in particular, combines high thermal stability, moderate electrical conductivity, and excellent redox behavior, making it a promising candidate for energy storage applications. the performance of spinel ferrites is closely related to the cation distribution between tetrahedral (a) and octahedral (b) sites in their lattice. this distribution can be tuned through cation substitution and synthesis conditions. manganese (mn), with its multiple oxidation states and larger ionic radius compared to fe³⁺, is a suitable dopant for modifying the structure and electrochemical activity of cobalt ferrite. this study explores the structural and electrochemical effects of mn substitution in cofe₂₋ₓmnₓo₄ (x = 0.0, 0.1, 0.3, 0.5) synthesized by the sol-gel method. 2. methodologymanganese-substituted cobalt ferrite nanoparticles (cofe₂₋ₓmnₓo₄) were synthesized using a citrate-based sol-gel process. analytical-grade metal nitrates—co(no₃)₂·6h₂o, fe(no₃)₃·9h₂o, and mn(no₃)₂·4h₂o—were used as precursors. citric acid was employed as a chelating agent to form a homogeneous gel, which was dried and subsequently annealed at 800°c for 4 hours in ambient air to obtain crystalline powders. structural analysis was carried out using x-ray diffraction (xrd) with cu kα radiation (λ = 1.5406 å). the data were refined using the rietveld method to extract parameters such as lattice constants, crystallite size, unit cell volume, and cation distribution. the morphology and microstructure were examined using field emission scanning electron microscopy (fesem). electrochemical properties were assessed using cyclic voltammetry (cv) and galvanostatic charge-discharge (gcd) measurements in a three-electrode configuration. the working electrode was fabricated by depositing the active material on a copper tape; a pt wire and an ag/agcl electrode served as the counter and reference electrodes, respectively. a 3 m kcl aqueous solution was used as the electrolyte. cv measurements were performed over a potential window of -0.3 v to +0.7 v at scan rates ranging from 5 to 200 mv/s. specific capacitance, energy density, and power density were calculated based on the cv and gcd data. 3. results and discussionxrd analysis confirmed the formation of a single-phase cubic spinel structure for all compositions. with increasing mn content, both the lattice parameter and unit cell volume exhibited a slight increase due to the larger ionic radius of mn²⁺ compared to fe³⁺. rietveld refinement also revealed a decrease in crystallite size, from approximately 52 nm for x = 0.0 to 38 nm for x = 0.5. this size reduction is beneficial for supercapacitor performance, as it increases the surface area and facilitates charge transfer. cation distribution analysis showed that co²⁺, fe³⁺, and mn²⁺ ions were randomly distributed between tetrahedral and octahedral sites, indicating a mixed spinel structure. the degree of inversion (δ), representing the ratio of fe³⁺ ions in tetrahedral to octahedral sites, was calculated and showed non-linear variation with mn content. the inversion degree decreased from 0.62 (x = 0.0) to 0.53 (x = 0.3) and slightly increased to 0.59 at x = 0.5, indicating that mn substitution significantly affects cation ordering within the lattice. fesem images showed that the nanoparticles had a granular structure with irregular polygonal shapes and particle sizes ranging from 50 to 200 nm. agglomeration was observed, likely due to magnetic interactions between particles. the irregular and polydisperse morphology can contribute to enhanced electrochemical performance by providing more active sites and improving electrolyte penetration. the cv curves of all samples showed quasi-rectangular shapes, characteristic of pseudocapacitive behavior. at low scan rates, the curves retained good symmetry and area, while at higher scan rates, the shape slightly deviated due to increased polarization. notably, mn-substituted samples retained their rectangular shape better at high scan rates, suggesting improved ionic conductivity and faster redox kinetics. specific capacitance values were calculated from the cv data. the sample with x = 0.5 exhibited the highest specific capacitance of 12.25 f/g at 5 mv/s, representing a 45% enhancement compared to the unsubstituted cofe₂o₄ sample (8.44 f/g). at higher scan rates (100 and 200 mv/s), capacitance retention was still significant, indicating superior rate capability of the mn-doped samples. energy and power densities were estimated using the cv data. the mn-substituted samples consistently showed improved energy density and power density compared to pure cofe₂o₄. these improvements are attributed to the synergistic effects of mn doping: reduced particle size, optimized cation distribution, increased active surface area, and enhanced redox activity at the electrode–electrolyte interface. 4. conclusionthis study demonstrates that substituting manganese for iron in cobalt ferrite nanoparticles significantly enhances their structural and electrochemical properties for supercapacitor applications. mn doping leads to a reduction in crystallite size, a modification in cation distribution, and improved electrochemical performance, including higher specific capacitance, energy density, and power density. the highest capacitance was observed in the x = 0.5 composition, showing a 45% improvement over pure cobalt ferrite. the mixed spinel structure, as confirmed by rietveld analysis, provides a favorable framework for charge storage due to its tunable cation distribution and ability to facilitate redox reactions. the morphological characteristics and improved ionic and electronic transport pathways also contribute to the enhanced performance. these findings suggest that mn-substituted cofe₂o₄ nanoparticles synthesized via the sol-gel method are promising candidates for cost-effective and efficient supercapacitor electrodes. future work may optimize doping levels, surface modifications, and hybridization with carbon-based materials to improve their performance and applicability in real-world energy storage systems.