A comprehensive review on PEM water electrolysis

Electrochemical capacitors (i.e. supercapacitors) include electrochemical double-layer capacitors that depend on the charge storage of ion adsorption and pseudo-capacitors that are based on charge storage involving fast surface redox reactions. The energy storage capacities of supercapacitors are several orders of magnitude higher than those of conventional dielectric capacitors, but are much lower than those of secondary batteries. They typically have high power density, long cyclic stability and high safety, and thus can be considered as an alternative or complement to rechargeable batteries in applications that require high power delivery or fast energy harvesting. This article reviews the latest progress in supercapacitors in charge storage mechanisms, electrode materials, electrolyte materials, systems, characterization methods, and applications. In particular, the newly developed charge storage mechanism for intercalative pseudocapacitive behaviour, which bridges the gap between battery behaviour and conventional pseudocapacitive behaviour, is also clarified for comparison. Finally, the prospects and challenges associated with supercapacitors in practical applications are also discussed.

Electrochemical capacitors: mechanism, materials, systems, characterization and applications

There are an increasing number of studies regarding active electrode materials that undergo faradaic reactions but are used for electrochemical capacitor applications. Unfortunately, some of these materials are described as “pseudocapacitive” materials despite the fact that their electrochemical signature (e.g., cyclic voltammogram and charge/discharge curve) is analogous to that of a “battery” material, as commonly observed for Ni(OH)2 and cobalt oxides in KOH electrolyte. Conversely, true pseudocapacitive electrode materials such as MnO2 display electrochemical behavior typical of that observed for a capacitive carbon electrode. The difference between these two classes of materials will be explained, and we demonstrate why it is inappropriate to describe nickel oxide or hydroxide and cobalt oxide/hydroxide as pseudocapacitive electrode materials. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. [DOI: 10.1149/2.0201505jes] All rights reserved.

https://iopscience.iop.org/article/10.1149/2.0201505jes/pdf

To Be or Not To Be Pseudocapacitive

A promising family of mixed transition-metal oxides (MTMOs) (designated as Ax B3-x O4 ; A, B=Co, Ni, Zn, Mn, Fe, etc.) with stoichiometric or even non-stoichiometric compositions, typically in a spinel structure, has recently attracted increasing research interest worldwide. Benefiting from their remarkable electrochemical properties, these MTMOs will play significant roles for low-cost and environmentally friendly energy storage/conversion technologies. In this Review, we summarize recent research advances in the rational design and efficient synthesis of MTMOs with controlled shapes, sizes, compositions, and micro-/nanostructures, along with their applications as electrode materials for lithium-ion batteries and electrochemical capacitors, and efficient electrocatalysts for the oxygen reduction reaction in metal-air batteries and fuel cells. Some future trends and prospects to further develop advanced MTMOs for next-generation electrochemical energy storage/conversion systems are also presented.

Mixed transition-metal oxides: design, synthesis, and energy-related applications.

Notably, many significant breakthroughs for a new generation of supercapacitors have been reported in recent years, related to theoretical understanding, material synthesis and device designs. Herein, we summarize the state-of-the-art progress toward mechanisms, new materials, and novel device designs for supercapacitors. Firstly, fundamental understanding of the mechanism is mainly focused on the relationship between the structural properties of electrode materials and their electrochemical performances based on some in situ characterization techniques and simulations. Secondly, some emerging electrode materials are discussed, including metal–organic frameworks (MOFs), covalent organic frameworks (COFs), MXenes, metal nitrides, black phosphorus, LaMnO3, and RbAg4I5/graphite. Thirdly, the device innovations for the next generation of supercapacitors are provided successively, mainly emphasizing flow supercapacitors, alternating current (AC) line-filtering supercapacitors, redox electrolyte enhanced supercapacitors, metal ion hybrid supercapacitors, micro-supercapacitors (fiber, plane and three-dimensional) and multifunctional supercapacitors including electrochromic supercapacitors, self-healing supercapacitors, piezoelectric supercapacitors, shape-memory supercapacitors, thermal self-protective supercapacitors, thermal self-charging supercapacitors, and photo self-charging supercapacitors. Finally, the future developments and key technical challenges are highlighted regarding further research in this thriving field.

Latest advances in supercapacitors: from new electrode materials to novel device designs.

To realize a suitable supercapacitor nanomaterial, the recently developed technique of reaction under autogenic pressure at elevated temperature has been employed by us to synthesize SnO2 hexagonal nanocrystals and SnO2@C nanocomposites. The synthesis at different temperatures (viz. 500, 600, and 700 °C) yields three different composites. Characterization of these composites by various methods confirms the structural (XRD, Raman, FT-IR) and nanoparticulate (TEM, HRTEM) nature of the synthesized materials. TEM studies including HRTEM reveal that all the synthesized SnO2 and SnO2@C nanomaterials are highly crystalline with hexagonal shape. Cyclic voltammetric studies carried out to examine the capacitance of SnO2@C in 1 M H2SO4 show that the nanocomposite prepared at 700 °C has a maximum specific capacitance of 37.8 F/g at a scan rate of 5 mV/s.

Synthesis of Hexagonal-Shaped SnO2 Nanocrystals and SnO2@C Nanocomposites for Electrochemical Redox Supercapacitors

Nickel molybdate (α-NiMoO4) nanoparticles were prepared by a solution combustion synthesis (SCS) technique and, for the first time, were studied as a potential electrode material for supercapacitors. High specific capacitance (1517 F g−1) and energy density (52.7 W h Kg−1) were delivered by nano-α-NiMoO4 at a current density of 1.2 A g−1, due to the pseudocapacitive nature of the material.

Nano α-NiMoO4 as a new electrode for electrochemical supercapacitors

The ternary MnFe2O4/graphene/polyaniline hybrid composite as negative electrode for supercapacitors

Improved electrochemical performances of reduced graphene oxide based supercapacitor using redox additive electrolyte

A novel spherical carbon nanoparticle decorated activated carbon (SNAC) material with a high surface area of about 1555 m2 g−1 is prepared from the dead mango leaves by an eco-friendly method for the detection of toxic heavy metal ions and energy storage applications. The limits of detection (LODs) for the determination of Cd(II), Pb(II), Cu(II), and Hg(II) ions at the SNAC-modified GCE are 24.4 × 10−9 M, 5.7 × 10−9 M, 23.2 × 10−9 M and 24.6 × 10−9 M, respectively. On the other hand, the obtained maximum specific capacitance for the single electrode from galvanostatic charge discharge is 478 F g−1 at 1 A g−1. The symmetric supercapacitor cell provides a higher specific capacitance (SC) of 55 F g−1 at 1 A g−1, and energy density of 10.75 W h kg−1 at a power density of 300 W kg−1.

Eco-friendly synthesis of activated carbon from dead mango leaves for the ultrahigh sensitive detection of toxic heavy metal ions and energy storage applications

Ramakrishnan Kalai Selvan

Papers

Synthesis of Hexagonal-Shaped SnO2 Nanocrystals and SnO2@C Nanocomposites for Electrochemical Redox Supercapacitors

Nano α-NiMoO4 as a new electrode for electrochemical supercapacitors

The ternary MnFe2O4/graphene/polyaniline hybrid composite as negative electrode for supercapacitors

Improved electrochemical performances of reduced graphene oxide based supercapacitor using redox additive electrolyte

Eco-friendly synthesis of activated carbon from dead mango leaves for the ultrahigh sensitive detection of toxic heavy metal ions and energy storage applications