Carbon Transition‐metal Oxide Electrodes: Understanding the Role of Surface Engineering for High Energy Density Supercapacitors

Abstract

Supercapacitors store electrical energy by ion adsorption at the interface of the electrode-electrolyte (electric double layer capacitance, EDLC) or through faradaic process involving direct transfer of electrons via oxidation/ reduction reactions at one electrode to the other (pseudocapacitance). The present minireview describes the recent developments and progress of carbon-transition metal oxides (C-TMO) hybrid materials that show great promise as an efficient electrode towards supercapacitors among various material types. The review describes the synthetic methods and electrode preparation techniques along with the changes in the physical and chemical properties of each component in the hybrid materials. The critical factors in deriving both EDLC and pseudocapacitance storage mechanisms are also identified in the hope of pointing to the successful hybrid design principles. For example, a robust carbon-metal oxide interaction was identified as most important in facilitating the charge transfer process and activating high energy storage mechanism, and thus methodologies to establish a strong carbon-metal oxide contact are discussed. Finally, this article concludes with suggestions for the future development of the fabrication of high-performance C-TMO hybrid supercapacitor electrodes.[a] Dr.

Controlled Selenium Infiltration of Cobalt Phosphide Nanostructure Arrays from a Two-Dimensional Cobalt Metal–Organic Framework: A Self-Supported Electrode for Flexible Quasi-Solid-State Asymmetric Supercapacitors

“…Supercapacitors are the most sustainable candidate for future energy storage devices because of their high power density, rapid charge–discharge proportions, ultrahigh cycle lifetime, safe operation, and environmental friendliness in contrast to those of conventional batteries. − In the current scenario, a suitable electrochemical energy storage (EES) system applicable for portable and wearable electronic devices, electric vehicles, and power back-up consists of a battery-type hybrid supercapacitor with an energy density like that of batteries and power density like that of supercapacitors, with a high overall performance . Among the many alternatives, oxy-PAN (polyacrylonitrile)-based carbon fiber textiles (CFTs) are one of the wearable textile prototypes for electrode fabrication for EES systems due to their high corrosion resistance, excellent flexibility, particularly high mechanical flexibility, abundant space for loading electrochemically active materials, good accessibility for the electrolyte, and good breathability compared to those of familiar supercapacitor substrate materials, like Ni and Cu foams. − To manage the low energy density of CFT-based flexible SCs, advanced and hybrid electrode materials from transition metal (TM)-based oxides, hydroxides, carbides, sulfides, phosphides, and multiple TM derivatives have been extensively used because of their valence variability for reversible faradaic reactions and high charge storage capacity. − Metal–organic frameworks (MOFs) with a uniform porous texture, large surface area, and abundant metal-containing active redox sites show broad applications in the EES system. − The main challenge for the application of MOFs as SC electrodes is a lack of sufficient electrical conductivity. − The low conductivity problem in MOFs can be improved by various methods, for example, converting the MOFs into nanoporous carbon, respective metal phosphides, and sulfides; selenium assimilation; and creating composites consisting of conductive polymers (CPs), graphene, and carbon nanotubes (CNTs). − In addition, vertically grown MOF structures (2D MOFs) enable electrical conductivity by providing an electrochemically active surface area for rapid electron transfer. − Transition-metal phosphides (TMPs) have been developed as a capable material for EES with their excellent rate capability and long cycling stability. − However, TMPs undergo a decreased specific capacity and fast capacity fading throughout redox reactions. A convincing approach to solve these concerns is the design and fabrication of a func...…”

Impacts of the Catalyst Structures on CO2 Activation on Catalyst Surfaces

“…The electronic structure of the traditional methanol synthesis catalyst (Cu-ZnO-Al 2 O 3 ) was altered by adding the N-doped graphene, which provided a synergistic effect for methanol synthesis [189]. Incorporating metal oxides to single-wall CNT electrode can form a hybrid structure with an increase in specific surface area, which also improved electrical conductivity and charge transfer [191,192]. Moreover, doping carbon into photocatalysts can enhance light absorption and improve the photothermal conversion efficiency by reducing the energy for oxygen vacancy formation, thus generating a high concentration of active sites [190].…”

Recent Developments of Transition Metal Compounds-Carbon Hybrid Electrodes for High Energy/Power Supercapacitors

“…Besides the construction of conductive carbon skeletons, the well-designed interface engineering between carbon networks and TMCs is another indispensable role for TMC-based electrodes which exhibit improved energy density and cycle life [ 70 ]. Due to the nano-size effect, TMCs nanoparticles with high surface energy are easy to aggregate during the energy storage process, which can directly lead to the capacity fading, resulting in the hindrance of scaled application of TMCs/carbon-based supercapacitors [ 71 , 72 ].…”

“…Be similar to the heteroatoms doping, the introduction of functional groups onto carbon skeleton surface is another necessary strategy to ensure the formation of efficient covalent grafting between TMCs and carbon skeleton through the construction of M–O/N/P/S-C bonds, in which the M represents TMCs, the O/N/P/S represents functional groups, and C represents carbon skeletons. Using strong oxidizing acids (such as nitric and sulfuric acid) to functionalize the carbon surface is the most common method, which can introduce functional groups like amino, carboxyl, hydroxyl, phosphate, or thiol [ 70 ]. These superficial functional groups can not only control the loading amount of TMCs but also increase the infiltration of electrode materials in the electrolyte.…”