In this work, we explore how electrochemical tunneling reactions can be understood within the single-particle picture. That is, the formal approach in which band diagrams are typically utilized to understand electronic processes in solid-state materials and devices. This single-particle perspective is based upon the Gerischer–Hopfield description of electron transfer at solid–liquid interfaces. Both single and multiple electron tunneling reactions are discussed, as are related voltammetric concepts and trends. The impact of nuclear quantization on the Gerischer–Hopfield description is also addressed, as well as its compact representation of two probe electrochemical phenomena at low temperatures (often referred to as Franck–Condon blockade). In this manner, a perspective linking solid-state phenomena and tunneling electrochemical reactions is presented.
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...
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