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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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In this work, we provide a theoretical analysis of quantized capacitance (also referred to as solvated Coulomb blockade) as a pseudocapacitive energy storage mechanism. In particular, we examine how redox species exhibiting quantized capacitance might be engineered to satisfy two basic criteria in the design of an “ideal” pseudocapacitive energy storage mechanism: (1) a near-rectangular voltammetric profile which mimics that of double-layer capacitance and (2) a linear rise in the pseudocapacitive current with respect to the voltammetric scan rate. It is demonstrated that nanoparticles exhibiting quantized capacitance may satisfy the first criterion by tailoring their charging and reorganization energies. It is also shown that the second criterion can be satisfied so long as the voltammetric scan rate does not exceed the electron-transfer rate. By formulating a comprehensive theoretical framework for understanding the electron-transfer properties of quantized capacitance, we arrive at a general phenomenological description of how pseudocapacitive properties might be practically engineered through this mechanism.

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This talk will provide an overview of our recent work in the development of supercapacitor electrode materials based on the modification of carbon substrates (MWCNTs and Activated Carbons) with custom designed polyoxometalate (POM) molecular coatings.  POMs undergo fast reversible multi-electron transfer reactions, an ideal characteristic for electrochemical energy storage.  When immobilized on a carbon substrate these POM molecules provide enhanced charge storage capacity due to their faradaic reactions, while the carbon material contributes mechanical support, improved conductivity, and physical capacitive effects.  Different POM molecules such as PMo12O40 3- (PMo12) and PW12O40 3- (PW12) demonstrate characteristic electrochemical activity related to their chemical composition.  However, we have discovered that when these molecules are combined in aqueous solutions they do not just physically mix, but instead react spontaneously to form PMo12-xWx mixed addenda chemistries with unique electrochemical properties different from that of individual PMo12 or PW12.  We have also demonstrated that this mixture behaviour can be extended to additional POM combinations such as SiMo12O40 4- (SiMo12) and SiW12O40 4- (SiW12).  These novel POM combinations have electrochemical properties that can be easily tuned based upon the compositions of the mixtures.  This control over POM redox behavior can be 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 PMo12O40 3−-PW12O40 3− mixed layer on a 1:1 GeMo12O40 4−-SiMo12O40 4− mixed layer, which resulted in a 5X capacitance enhancement over unmodified MWCNT. This dual layer electrode also demonstrated a close to rectangular CV profile due to the overlapping redox features of the POM combination (Figure 1).    In addition to our work on the POM active layers, our current research on the development of alterative carbon substrates will also be discussed.  While MWCNT is an effective substrate for POM modification, we have recently focused on the fabrication of less expensive activated carbon materials based on waste biomass precursors.  We have developed a nanostructured carbon material based on corn-cob biomass via a simple high temperature exfoliation procedure.  This exfoliated corn biochar had excellent energy storage performance, demonstrating a capacitance over 100 times higher than natural corn biochar produced without exfoliation treatment.  Additionally, high surface area activated carbons from pine cone biomass were synthesized via a two-step carbonization and KOH activation method.  These high surface area substrates result in carbon-POM hybrids with further enhanced specific capacitance compared to nanocarbon based materials.  The capacitive performance of these different carbon-POM composites will be compared along with a discussion of how our approach can be leveraged to design the optimal hybrid material for different energy storage applications. Figure 1