Definitions of Pseudocapacitive Materials: A Brief Review

Pseudocapacitive materials generally offer both high capacitance and high rate capability, which has stimulated great efforts in developing the materials system and related energy storage devices. In recent years, however, with the extensive use of nanomaterials in batteries, fast redox kinetics comparable to pseudocapacitive have been achieved in many kinds of battery materials due to the much shortened ion diffusion lengths and highly exposed surface/interface as a result of nanosize effect. Consequently, the terms"pseudocapacitive materials" and "battery materials" are becoming more and more confusing. In this review, different opinions on the definition of pseudocapacitive materials and the evolution of the definitions as well as the resulting confusion will be firstly reviewed. Then, to accurately distinguish pseudocapacitive and battery materials, method with the consideration of both the electrochemical signatures(CVs and GCD) and quantitative kinetics analysis as a supplement is proposed. Finally, we end this review by discussing the possible device configurations of asymmetric supercapacitors and hybrid supercapacitors. The present review will help understanding the differences between pseudocapacitive materials and battery materials, and thus avoiding the definition confusion.

Understanding Pseudocapacitance Mechanisms by Synchrotron X-ray Analytical Techniques

Pseudocapacitive materials that store charges via reversible surface or near-surface faradaic reactions are capable of overcoming the capacity limitations of electrical double-layer capacitors.Revealing the structure–activity relationship between the microstructural features of pseudocapacitive materials and their electrochemical performance on the atomic scale is the key to build high-performance capacitor-type devices containing ideal pseudocapacitance effect.Currently,the high brightness(flux),and spectral and coherent nature of synchrotron X-ray analytical techniques make it a powerful tool for probing the structure–property relationship of pseudocapacitive materials.Herein,we report a comprehensive and systematic review of four typical characterization techniques(synchrotron X-ray diffraction,pair distribution function[PDF]analysis,soft X-ray absorption spectroscopy,and hard X-ray absorption spectroscopy)for the study of pseudocapacitance mechanisms.In addition,we offered significant insights for understanding and identifying pseudocapacitance mechanisms(surface redox pseudocapacitance,intercalation pseudocapacitance,and the extrinsic pseudocapacitance phenomenon in battery materials)by combining in situ hard XAS and electrochemical analyses.Finally,a perspective for further depth of understanding into the pseudocapacitance mechanism using synchrotron X-ray analytical techniques is proposed.

Multiscale control of MnO_(2) growth via[WO_(6)]-perturbed[MnO_(6)]assembly toward a favorable balance between capacitance and rate performance

Although pseudocapacitive manganese dioxide(MnO_(2))integrates the high-power merit of carbonaceous materials with the high-energy merit of battery-type materials,it still has a long way to go in achieving a more satisfactory balance of higher energy and power density,and in decoupling the relationship of structural characteristics with energy storage performance.To realize such goals,a bottom-up[WO_(6)]-perturbed[MnO_(6)]assembly strategy has been developed here due to their similar structure,yet mismatched lattice parameters.This facile protocol is capable of finely controlling the morphology and crystal structure of MnO_(2)by adjusting its internal[WO_(6)]concentration.Therefore,the as-prepared W_xMnO_(2)is treated as an ideal platform to scrutinize the correlations of the structure with the energy storage performance.The operando Raman spectra and finite element analysis have fully demonstrated the superiority of the locally ordered defects-enriched structure of W_(0.02)-MnO_(2),which could reach a favorable balance between the ion diffusion equilibrium time and the number of active sites.As a result,the W_(0.02)-MnO_(2)is able to deliver a high capacitance of 292 F·g^(-1)at a current density of 1 A·g^(-1)and a remarkable rate performance with a 60%capacity retention at a current density of 50 A·g^(-1).The further unveiled structure-performance relationship provides a guideline for the design of better pseudocapacitive energy storage devices.

A stretchable,asymmetric,coaxial fiber-shaped supercapacitor for wearable electronics

Nano Research volume 13,pages1686–1692(2020)Cite this article 210 Accesses 1 Citations Metrics details Abstract Fiber-shaped supercapacitors(FSCs),owing to their high-power density and feasibility to be integrated into woven clothes,have drawn tremendous attentions as a key device for flexible energy storage.However,how to store more energy while withstanding various types of mechanical deformation is still a challenge for FSCs.Here,based on a magnetron sputtering method,different pseudocapacitive materials are conformally coated on self-supported carbon nanotube aligned films.This fabrication approach enables a stretchable,asymmetric,coaxial fiber-shaped supercapacitors with high performance.The asymmetric electrode configuration that consists of CNT@NiO@MnOx cathode and CNT@Fe2O3 anode successfully extends the FSC’s electrochemical window to 1.8 V in an aqueous electrolyte.As a result,a high specific capacitance of 10.4 F·cm^−3 is achieved at a current density of 30 mA·cm^−3 corresponding to a high energy density of 4.7 mWh·cm^−3.The mechanical stability of the stretchable FSC is demonstrated with a sustainable performance under strains up to 75%and a capacitance retention of 95%after 2,000 cycles under 75%strain.

Engineering 3D electron and ion transport channels by constructing sandwiched holey quaternary metal oxide nanosheets for high-performance flexible energy storage

Due to the enhanced electrochemical activities,mixed metal oxides offer new and fascinating opportunities for high-performance supercapacitor electrodes.However,sluggish ionic and electronic kinetics within the electrode fundamentally limit further improvement of their electrochemical performance.To compensate for the deficiency,a flexible electrode(CNTF/Ni-Co-Mn-Mo NS/CNTN)composed of vertically-aligned areolate quaternary metal oxide nanosheets sandwiched between carbon nanotubes(CNTs)is constructed in this study,which demonstrates a unique hierarchical porous structure that can provide three-dimensional transport channels for both ions and electrons.The vertically aligned areolate quaternary metal oxide nanosheets enable increased exposed surface area and paths for ion transport,diffusion and redox reactions,resulting in an evident enhancement in electrochemical activities.Besides,the CNT networks provide improved conductivity,which can accelerate the electron transport.As a result,the flexible supercapacitor based on the CNTF/Ni-Co-Mn-Mo NS/CNTN electrode demonstrates a specific areal capacitance of 3738 m F cm^-2,corresponding to a high energy density of 1.17 m W h cm^-2,which outperforms most of the flexible devices reported recently.Additionally,excellent flexibility of up to 180°bend and superior performance stability of 87.87%capacitance retention after 10,000 charge-discharge cycles can be obtained.This unique design opens up a new way in the development of flexible energy storage devices with high performance.

High performance flexible energy storage device based on copper foam supported NiMoO4 nanosheets-CNTs-CuO nanowires composites with core–shell holey nanostructure

Because of the intensified electrochemical activities,mixed metal oxides as a representative for pseudocapacitive materials play a key role for high performance supercapacitor electrodes.Nevertheless,low ion and electron transfer rate and poor cycling performance in the electrode practically restrict further promotion of their electrochemical performance.In order to offset the defect,a novel copper(Cu)foamsupported nickel molybdate nanosheet decorated carbon nanotube wrapped copper oxide nanowire array(NiMoO4 NSs-CNTs-CuO NWAs/Cu foam)flexible electrode is constructed.The as-prepared electrode demonstrates a unique core-shell holey nanostructure with a large active surface area,which can provide a large number of active sites for redox reactions.Besides,the CNTs networks supply improved conductivity,which can hasten electron transport.Through this simple and efficient design method,the spatial distribution of each component in the flexible electrode is more orderly,short and fast electron transport path with low intrinsic resistance.As a result,the NiMoO4 NSs-CNTs-CuO NWAs/Cu foam as an adhesiveless supercapacitor electrode material exhibits excellent ene rgy storage perfo rmance with high specific areal capacitance of 23.40 F cm^(-2)at a current density of 2 mA cm^(-2),which outperforms most of the flexible electrodes re ported recently.The assembled asymmetric supercapacitor demonstrates an energy density up to 96.40 mW h cm^(-3)and a power density up to 0.4 W cm^(-3)under a working voltage window of 1.7 V.In addition,outstanding flexibility of up to 100°bend and good cycling stability with the capacitance retention of 82.53%after 10,000 cycles can be obtained.