High-performance Pseudocapacitor Electrodes Research Articles

Designing Polyoxometalate based Layer-by-Layer Thin Films on Carbon Nanomaterials for Pseudocapacitive Electrodes

Multi-walled carbon nanotube (MWCNT) electrodes modified with mixtures of PMo12O403– (PMo12) and PW12O403− (PW12) Keggin ions are investigated for electrochemical capacitor (EC) applications. We have discovered that when these polyoxometalate (POM) ions are combined in solution, they do not just mix physically, but instead react to form the PMo12-xWx mixed addenda chemistries. By simply mixing PMo12 and PW12 in different ratios, a variety of mixed addenda ions, each with unique electrochemical properties, can be easily synthesized. This control over POM redox behavior afforded by the novel mixed addenda synthesis is used along with layer-by-layer (LbL) deposition to design molecular coatings that demonstrate desired pseudocapacitive characteristics. The best performance was achieved with a coating that superimposed a 3:1 PMo12O403−-PW12O403− mixed layer on a 1:1 GeMo12O404−-SiMo12O404− mixed layer, which resulted in an 11X capacitance enhancement over unmodified CNT. This dual layer electrode also demonstrated a close to rectangular CV profile due to the overlapping redox features of the POM combination. The design of custom POM coatings to engineer the electrode surface represents a promising strategy for the development of high performance pseudocapacitive electrodes.

Facile synthesis of vanadium pentoxide@carbon core–shell nanowires for high-performance supercapacitors

Vanadium pentoxide is emerging as a low-cost and high-performance pseudo-capacitive electrode material but suffers from poor stability. Herein, we demonstrate that the electrochemical performance and stability of V2O5 nanowires can be significantly improved by coating a thin carbon layer as shell. Our study shows that the V2O5@C core shell nanowires achieve a remarkable areal capacitance of 128.5 F cm−2 at 10 mV s−1 with excellent rate capability. More than 94.4% of the initial capacitance is retained after 10,000 cycles for V2O5@C core shell nanowires, which is much higher than the pristine V2O5 nanowires (13.3%). This method could be extended to improve the stability and electrochemical properties of other electrode materials.

A high performance pseudocapacitive suspension electrode for the electrochemical flow capacitor

The electrochemical flow capacitor (EFC) is a new technology for grid energy storage that is based on the fundamental principles of supercapacitors. The EFC benefits from the advantages of both supercapacitors and flow batteries in that it is capable of rapid charging/discharging, has a long cycle lifetime, and enables energy storage and power to be decoupled and optimized for the desired application. The unique aspect of the EFC is that it utilizes a flowable carbon-electrolyte suspension (slurry) for capacitive energy storage. Similar to traditional supercapacitor electrodes, this aqueous slurry is limited in terms of energy density, when compared to batteries. To address this limitation, in this study a pseudocapacitive additive has been explored to increase capacitance. A carbon-electrolyte slurry prepared with p-phenylenediamine (PPD), a redox mediator, shows an increased capacitance on the order of 86% when compared with KOH electrolytes, and a 130% increase when compared to previously reported neutral electrolyte based slurries. The redox-mediated slurry also appears to benefit from a decrease in ohmic resistance with increasing concentrations of PPD, most likely a result of an increase in the ionic diffusion coefficient. Among the tested slurries, a concentration of 0.139M of PPD in 2M KOH electrolyte yields the largest capacitance and rate handling performance in both cyclic voltammetry and galvanostatic cycling experiments. The improved performance is attributed to the addition of quick faradaic reactions at the electrolyte-electrode interface as PPD undergoes a two-proton/two-electron reduction and oxidation reaction during cycling.