The critical role of point defects in improving the specific capacitance of δ-MnO2 nanosheets

Abstract

3D porous nanostructures built from 2D δ-MnO2 nanosheets are an environmentally friendly and industrially scalable class of supercapacitor electrode material. While both the electrochemistry and defects of this material have been studied, the role of defects in improving the energy storage density of these materials has not been addressed. In this work, δ-MnO2 nanosheet assemblies with 150 m2 g−1 specific surface area are prepared by exfoliation of crystalline KxMnO2 and subsequent reassembly. Equilibration at different pH introduces intentional Mn vacancies into the nanosheets, increasing pseudocapacitance to over 300 F g−1, reducing charge transfer resistance as low as 3 Ω, and providing a 50% improvement in cycling stability. X-ray absorption spectroscopy and high-energy X-ray scattering demonstrate a correlation between the defect content and the improved electrochemical performance. The results show that Mn vacancies provide ion intercalation sites which concurrently improve specific capacitance, charge transfer resistance and cycling stability.

The Role of Cation Vacancies in Electrode Materials for Enhanced Electrochemical Energy Storage: Synthesis, Advanced Characterization, and Fundamentals

“…It is also more favorable for the electrochemical intercalation of electrolyte cations near the vacancies, which leads to enhanced rate capability in the defective MnO 2 for electrochemical intercalation/deintercalation. Following this pioneering study, more research works have been conducted to intentionally introduce cation vacancies into different transition metal oxides/carbides, such as MnO 2 , [ 89–93 ] TiO 2 , [ 94–96 ] Fe 2 O 3 , [ 77,78,97,98 ] ZnCo 2 O 4 , [ 99 ] ZnMn 2 O 4 , [ 76 ] and MXenes, [ 79,100–102 ] providing better and more fundamental understanding of the role of cation vacancies in boosting electrochemical performance ( Figure 2 ). Several density functional theory (DFT) calculations have also indicated that the presence of cation vacancies results in a decrease of energy barrier for ion diffusion, [ 95,103 ] and an increase of the materials' electronic conductivity, [ 80 ] thus benefits the charge storage process.…”

“…There are several ways for incorporating cation vacancies and tailoring its content in transition metal oxides/carbides through synthetic approaches, such as replacing the constituent cations or anions with aliovalent substitutes, [ 94–96,104,105 ] equilibration at solutions with different pH values, [ 89,90,99,106,107 ] selective removal of constituent cations, [ 101,102,108 ] thermal annealing in defects inducing atmospheres, [ 109,110 ] and plasma etching. [ 111 ] The characterization of cation vacancies, however, is usually challenging.…”

“…Atomic resolution transmission electron microscope (TEM), with the ability to see single atom, is typically used to give a direct visualization of the cation vacancies. [ 90,95,107 ] Other powerful techniques for detecting cation vacancies in nanomaterials, especially the materials without long‐range order, are extended X‐ray absorption fine structure (EXAFS) [ 77,78,97,105,111 ] and X‐ray pair distribution function (PDF) analysis, [ 89,94,96,106 ] both of which are sensitive to the local structure variations that enable accurate determination of cation vacancies. Additionally, multiple analytical techniques, such as X‐ray photoelectron spectroscopy (XPS), [ 91,105,113 ] nuclear magnetic resonance (NMR), [ 94–96 ] Raman/Fourier transform infrared (FTIR) spectroscopy, [ 104 ] electron paramagnetic resonance (EPR), [ 110 ] photoluminescence (PL), [ 110 ] and positron annihilation spectrometry (PAS), [ 114 ] have also been reported to serve as complementary tools for cation vacancies characterization.…”

Optimization of Chemi‐adsorption, EDLC, and Redox Capacitance Through Electro‐precipitation Synthesis of Fe3O4/NiO@rGO/h‐BN for the Development of Hybrid Supercapacitor

“…Hybridization of rGO and h‐BN generates a stacking region where defects were created. The defects provided efficient nucleation sites for the growth of metal oxides by adsorbing the metal cations . Thus, the stoichiometry of the iron/nickel along with the rGO/h‐BN backbone was tuned by adjusting the applied bias of the bath solution.…”

Metal Oxide and Hydroxide–Based Aqueous Supercapacitors: From Charge Storage Mechanisms and Functional Electrode Engineering to Need‐Tailored Devices

“…e-g) Reproduced with permission [61]. b) Current-voltage plot of NiCo 2 O 4−x nanowires reduced with different hydrogen treatment times.…”