User:Tiff592/Nanoelectrochemistry

Article Draft

Nanoelectrochemistry is a branch of electrochemistry that investigates the electrical and electrochemical properties of materials at the nanometer size regime. Nanoelectrochemistry plays significant role in the fabrication of various sensors, and devices for detecting molecules at very low concentrations. Application of nanoelectrochemistry includes battery supplies, efficient reduction and oxidation reaction like CO₂ reduction.

figure 1. Nanoelectrochemistry cell. A electrochemistry cell includes 2 phases: cathode and anode. The 2 phases are connected by a wire, 2 electrodes and a membrane that allows electron flows. nanoelectrode sturceture is shown on image.

Electrochemistry have two types of catalyst: Homogeouse electrode and heterogenouse electrode. Nanoelectrode is a type of heterogenouse catelyst. By using specific nanoparticle sturctured metal to increase surface area and active sites for CO₂ reduction reaction to take place, meanwhile palying a rule as a electrode in the electrochemical cell [figure1.].

Mechanism[edit]

Two transport mechanisms are fundamental for nanoelectrochemistry: electron transfer and mass transport. The formulation of theoretical models allows to understand the role of the different species involved in the electrochemical reactions.

The electron transfer between the reactant and the nanoelectrode can be explained by the combination of various theories based on the Marcus theory.

Mass transport, that is the diffusion of the reactant molecules from the electrolyte bulk to the nanoelectrode, is influenced by the formation of a double electric layer at the electrode/electrolyte interface. At the nanoscale it is necessary to theorize a dynamic double electric layer which takes into account an overlap of the Stern layer and the diffuse layer. Three basic mechanism is involved in Mass transport: diffusion, migration and convetion. The total transport of material to an electrode can be described by Nernst-Planck equation.

Knowledge of the mechanisms involved allows to build computational models that combine the density functional theory with electron transfer theories and the dynamic double electric layer. In the field of molecular modelling, accurate models could predict the behaviour of the system as reactants, electrolyte or electrode change.

Interface effect[edit]

The role of the surface is strongly reaction-specific: in fact, one site can catalyze certain reactions and inhibit other ones.

According to TSK model, surface atoms in nanocrystals can occupy terrace, step or kink positions: each site has a different tendency to adsorb reactants and to let them move along the surface. Generally, sites having lower coordination number (steps and kinks) are more reactive due to their high free energy. High energy sites, however, are less thermodynamically stable and nanocrystals have a tendency to transform to their equilibrium shape.

Thanks to the progress in nanoparticles synthesis it is now possible to have a single-crystal approach to surface science, allowing more precise research on the effect of a given surface. Studies have been conducted on nanoelectrodes exposing a (100), (110) or (111) plane to a solution containing the reactants, in order to define the surface effect on reaction rate and selectivity of the most common electrochemical reactions.

Nanoelectrodes[edit]

Nanoelectrodes are tiny electrodes made of metals or semiconducting materials having typical dimensions of 1-100 nm. Various forms of nanoelectrodes have been developed taking advantage of the different possible fabrication techniques: among the most studied are the nanoband, disk, hemispherical, nanopore geometries as well as the different forms of carbon nanostructures.

It is necessary to characterize each produced electrode: size and shape determine its behavior. The most used characterization techniques are:

• Electron microscopy
• Steady-state voltammetry
• Atomic force microscopy (AFM)
• scanning electrochemical microscopy (SECM)

There are mainly two properties that distinguish nanoelectrodes from electrodes: smaller RC constant and faster mass transfer. The former allows measurements to be made in high-resistance solutions because they offer less resistance, the latter, due to radial diffusion, allows much faster voltammetry responses. Due to these and other properties, nanoelectrodes are used in various applications:

• Studying the kinetics of fast reactions
• Electrochemical reactions
• Studying small volumes, such as cells or single molecules
• As probes for obtaining high-resolution images with scanning electrochemical microscopy (SECM)

Potential applications of nanoelectrochemistry

Battery application

The advent of batteries has revolutionized human lives, and more importantly, batteries are the power source of choice for emerging electrified transportation modes, as well as energy storage technologies required to rely on renewable energy sources such as wind and solar radiation. As a result, interest in developing new battery chemistries and improving existing battery technologies has attracted considerable attention in recent years. Nanoelectrochemistry has potential special significance for battery research.

For conventional battery electrodes, mass transport can limit battery performance when fast charging or low temperatures are required. Nanoelectrochemistry can solve that problem. The evaluation of a single entity of battery material ensures enough mass transport from the electrolyte and enhances electron transport by reducing significantly the number of solid–solid interfaces.

There are at least three mature techniques available to conduct nanoelectrochemical measurements in battery research, they are particle-on-a-stick measurements, nanoimpact measurements, and scanning electrochemical probe microscopy, each of them possessing intrinsic pros and cons.

CO₂ Reduction

figure 2. Using electrochemistry to approch CO₂ redution. Green energy can be used such as wind. Product of CO₂ reduction can have different products such as CO, ethylene etc.

Slectrochemistry is a great method to perform CO₂ reduction and utilization (Figure 2).Nanoparticle is currently one of the most adavance technology for CO₂ Reduction. Nanoparticle electrolysis gives incredible reaction efficiency and selectivity on product of reaction. Chemist had been determining different combination of alloys of nanomaterials to be used as a surface of electrode. By integrating benefit feature from different metal elements and testing under different conditions (eg . moisture, pH, temperature etc.) to determine the best material surface for electron to utilize CO₂. However the defect of nanoparticle is inevitable for now. The efficiency reduce as the nanoparticles react and attract each other, reducing surface area as reaction goes on, highly affecting the reaction rate gradually. Current technology cannot avoid the interaction between nanoparticles. As an example, one of the most current applications is AgPd. It is a leading nanoparticle to be used for reducing CO2 to CO as it is high selectivity to the product of CO, requires low voltage, meanwhile remains releventaly stable for reaction.