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

Abstract

Multi-walled carbon nanotube (MWCNT) electrodes modified with mixtures of PMo 12 O 40 3- (PMo 12 ) and PW 12 O 40 3− (PW 12 ) 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 PMo 12-x W x mixed addenda chemistries. By simply mixing PMo 12 and PW 12 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: Increased demand for reliable high power energy storage devices has led to growing interest in the development of novel electrode materials for electrochemical capacitors (ECs).1-3 One of the most promising approaches in EC electrode design involves the fabrication of nanostructured composites in which pseudocapacitive materials are immobilized on electrochemical double layer capacitive (EDLC) carbon substrates. There is synergy in this combination as the pseudocapacitive material allows for large charge storage capacity due to surface redox reactions, while the carbon substrate imparts mechanical stability, improved conductivity, and physical capacitive effects.4-5 These types of composite materials including RuO 2 -CNT, 6 MnO 2 -graphene, 7 V 2 O 5 -CNT, 8 and conductive organic polymer (COP)-CNT 9 have been investigated extensively for pseudocapacitive EC electrodes.Although there are many pseudocapacitive alternatives, polyoxometalates (POMs), nanoscale transition metal-oxygen clusters, have emerged as promising building blocks for nanocomposite electrodes. [10][11][12][13][14] POMs have an unmatched range of physical and chemical properties which arise from their seemingly endless variety of molecular structures and sizes. 15 One of the most common and widely researched classes of POM molecules is the Keggin cluster which has the general formula [MX 12 O 40 n− ] in which the central heteroatom (i.e. P, Si, or Ge) is surrounded by twelve addenda atoms (i.e. Mo or W) and forty oxygen atoms. [16][17] These molecules demonstrate high stability of their redox states and participate in fast reversible multielectron transfer reactions, making them ideally suited for pseudocapacitive applications. They also offer tremendous versatility as their electrochemical behavior can be tuned with small adjustments to the molecular chemistry. 17The electrochemical activity of POMs can be combined with the stability and high surface area of carbon substrates to create robust nanocomposite EC electrodes. These POM-carbon composites can be prepared by a variety of techniques including electrodeposition, 18 chemisorption, 19 and layer-by-layer (LbL) self assembly. [20][21][22] The improved capa...

Electrochemical Properties of Polyoxometalate (H3PMo12O40)-Functionalized Graphitic Carbon Nitride (g-C3N4)

“…Owing to this unique cluster structure, PMo 12 has very low surface area with negligible conductivity. Therefore, in order to maximize electron transfer kinetics, ideally each PMo 12 cluster molecule should be electrically linked to a conductive substrate [6,7]. According to the literature, PMo 12 anchored to carbon-based materials in particular promotes multielectron reversible redox reactions.…”

Functionalization of Carbon Surfaces Tunes the Redox Stability of Polyoxometalate@Carbon Electrodes

“…A range of non-covalent strategies is available for forming POM/carbon composites, which exploit electrostatic interactions, 27,28 𝜋-𝜋 interactions, 29 and layer-by-layer (LbL) assembly. 30 However, these strategies must often be used in conjunction with one another to prepare a stable composite, limiting process scalability. POMs have been chemisorbed onto a wide range of carbon surfaces including activated carbons, carbon nanotubes, and graphene.…”

Coaxial Cable‐Like Carbon Nanotubes‐Based Active Fibers for Highly Capacitive and Stable Supercapacitor

“…[ 5 ] Therefore POMs are the good candidates for supercapacitor due to their fast and reversible multi‐electron redox reactions along with the high theoretical values. [ 6–8 ]…”

“…[ 10–14 ] There are three most straightforward and effective methods for the synthesis of POM‐carbon composites: i) chemisorption, ii) self‐assembly, and iii) immobilization in a polymer matrix. [ 8,15 ] For chemisorption, POMs molecules are usually adsorbed at defect sites (e.g., the surface functional groups, the crystal defects) by strong and irreversible bonding. Therefore the amount of chemisorbed POMs is dependent on the functionalization degree on carbon materials.…”