Synthesis of amorphous manganese oxide nanoparticles – to – crystalline nanorods through a simple wet-chemical technique using K⁺ ions as a ‘growth director’ and their morphology-controlled high performance supercapacitor applications

Abstract

Highly crystalline MnO2 nanostructures are fabricated by a simple acid reduction of KMnO4 solution followed by air-annealing. Their excellent electrochemical properties are useful for combinatorial morphology-controlled high performance supercapacitor applications.

Improved electrochemical properties of morphology-controlled titania/titanate nanostructures prepared by in-situ hydrothermal surface modification of self-source Ti substrate for high-performance supercapacitors

“…Therefore, it becomes apparent that, due to the combinatorial effect of higher roughness in 2D platelets and higher surface-to-volume ratio in 1D nanorods, the 2D platelet + 1D nanorod mixture sample should have highest active surface sites. Although BET surface area measurements cannot be performed in our samples due to instrument limitations 57 , but previous reports corroborates this phenomenon 53 , where a nanorod + nanoparticle mixed powdered sample showed higher specific surface area than that of single phase (nanorods or nanoparticles) samples, indicating that in the current case, according to the qualitative analyses from surface roughness and surface-to-volume ratio data (discussed above), the 2D platelet + 1D nanorod mixture samples (fabricated at 0.5 M KOH) manifest higher active surface sites due to the combined effect of higher surface roughness at the 2D platelets and greater surface-to-volume ratio at the 1D nanorods. And hence, this sample showed higher specific capacitances against other samples (Fig.…”

“…The negligible IR drops (0.1 V to 0.2 V, as shown in Fig. 7 ) in all the samples indicate potential applicability in high power devices 11 , 53 . Also the GCD curve profiles of all the electrodes depict slight deviations from the linear behavior, indicating some distinct energy storage mechanism and Faradaic pseudocapacitive reaction 52 .…”

“…High-resolution K 2 p spectra (shown in Fig. 5d ) for all the samples (except the one prepared at 0.25 M KOH) are fitted with two components at BEs 293 ± 0.5 eV and 296 ± 0.5 eV, and attributed to the K + 2 p 3/2 and K + 2 p 1/2 states of photoelectrons, respectively, which are originated from the spin-orbit doublet of potassium ions in oxides 52 , 53 . Again the slight low-BE shift of the sample prepared at 5.0 M KOH against the other samples is manifested by the change in the chemical environment for higher concentration of KOH solution.…”

“…From the XPS spectra it becomes apparent that the samples do not contain any Ti +3 (Ti +3 2 p ), metallic K (K° 2 p ), surface chemisorbed/physisorbed water molecules (H-OH) and/or oxygen vacancy related states 8 , 11 , 53 . This is consistent with the electrochemical data (shown below), where very little pseudocapacitive behavior is observed within the samples, which generally occurs due to the presence of some of the above states.…”

“…Also a dimensional reduction from 2D structure to 1D/0D structure(s) increases the surface-to-volume ratio. Additionally, nanostructuring of the surface also increases the surface roughness, all of which enhances the active surface sites of the electrodes for superior electrochemical performances 53 . In the current case, the AFM surface roughness measurements of 2D and 1D nanostructured samples are presented in Table 2 , which reveals that the 2D platelet structure (fabricated at 0.25 M KOH) has highest surface roughness, whereas the 1D nanorod sample (fabricated at 1.0 M KOH) has least surface roughness values.…”