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Nanoelectrochemistry

Mechanism

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.

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

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

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 behaviour. 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)

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Electrocatalyst

Heterogeneous electrocatalysis

Since electrochemical reactions need an electron transfer between the solid catalyst (typically a metal) and the electrolyte, which can be a liquid solution but also a polymer or a ceramic capable of ionic conduction, the reaction kinetics depend on both the catalyst and the electrolyte as well as on the interface between them. The nature of the electrocatalyst surface determines some properties of the reaction such as its rate and products selectivity.

The activity of an electrocatalyst can be tuned with a chemical modification, commonly obtained by alloying two or more metals. This is due to a change in the electronic structure, especially in the d band which is considered to be responsible for the catalytic properties of noble metals.

Electronic density difference of a Cl atom adsorbed on a Cu(111) surface obtained with a DFT simulation.

Also, higher reaction rates can be achieved by precisely controlling the arrangement of surface atoms: indeed, in nanometric systems the number of available reaction sites is a better parameter than the exposed surface area in order to estimate electrocatalytic activity. Sites are the positions where the reaction could take place; the likelihood of a reaction to occur in a certain site depends on the electronic structure of the catalyst, which determines the adsorption energy of the reactants together with many other variables not yet fully clarified.

According to the TSK model, the catalyst surface atoms can be classified as terrace, step or kink atoms according to their position, each characterized by a different coordination number. In principle, atoms with lower coordination number (kinks and defects) tend to be more reactive and therefore adsorb the reactants more easily: this may promote kinetics but could also depress it if the adsorbing species isn't the reactant, thus inactivating the catalyst.

Advances in nanotechnology make it possible to surface engineer the catalyst so that just some desired crystal planes are exposed to reactants, maximizing the number of effective reaction sites for the desired reaction.

To date, a generalized surface dependence mechanism cannot be formulated since every surface effect is strongly reaction-specific. A few classifications of reactions based on their surface dependence have been proposed but there are still too many exceptions that do not fall into them.

Particle size effect

The interest in reducing as much as possible the costs of the catalyst for electrochemical processes led to the use of fine catalyst powders since the specific surface area increases as the average particle size decreases. For instance, most common PEM fuel cells and electrolyzers design is based on a polymeric membrane charged in platinum nanoparticles as an electrocatalyst (the so-called platinum black).

An example of a particle-size effect: the number of reaction sites of different kinds depends on the size of the particle. In this four FCC nanoparticles model, the kink site between (111) and (100) planes (coordination number 6, represented by golden spheres) is 24 for all of the four different nanoparticles, while the number of other surface sites vary.

Although the surface area to volume ratio is commonly considered to be the main parameter relating electrocatalyst size with its activity, to understand the particle-size effect several more phenomena need to be taken into account:

• Equilibrium shape: for any given size of a nanoparticle there is an equilibrium shape which exactly determines its crystal planes
• Reaction sites relative number: a given size for a nanoparticle corresponds to a certain number of surface atoms and only some of them host a reaction site
• Electronic structure: below a certain size, the work function of a nanoparticle changes and its band structure fades away
• Defects: the crystal lattice of a small nanoparticle is perfect; thus, reactions enhanced by defects as reaction sites get slowed down as the particle size decreases
• Stability: small nanoparticles have the tendency to lose mass due to the diffusion of their atoms towards bigger particles, according to the Ostwald ripening phenomenon
• Capping agents: in order to stabilize nanoparticles it is necessary a capping layer, therefore part of their surface is unavailable for reactants
• Support: nanoparticles are often fixed onto a support in order to stay in place, therefore part of their surface is unavailable for reactants

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Nanomaterial-based catalyst

Nanostructures for electrocatalysis

Nanocatalysts are widely used to optimize electrochemical reactions by lowering cell overpotential. Fuel cells and electrolyzers efficiency is strongly dependent on the catalyst used.

The progress of electrocatalysis has been favoured by the study and development of new catalysts composed by nanostructured materials. The morphology and the structure of nanomaterials influence catalytic capacities and give more control over activity and selectivity. The nanostructure affects the surface energy and the distribution of active reaction sites. The nanostructures playing a leading role in nanoelectrocatalysis are nanoporous surfaces, nanoparticles and nanowires.

Nanoporous surfaces

Main article: Nanoporous materials

The introduction of porosity leads to a reduction of density and to an increase of the specific surface area; therefore there's a greater probability of finding active sites for catalysis. In fuel cells, nanoporous materials are widely used to make cathodes. Not always, however, porous nanostructures are the best choice, it depends on the material. For example, porous nanoparticles of platinum have good activity in nanocatalysis but are less stable and their lifetime is short.

Nanoparticles

Main article: Nanoparticle

The good electrocatalytic activity is due to a general low coordination number. The drawback of using nanoparticles is their tendency to agglomerate; thanks to the use of supports this problem can be overcome. Nanoparticles are optimal structures to be used as nanosensors because they can be tuned to detect specific molecules.

Nanowires

Main article: Nanowire

Nanowires are very interesting for electrocatalytic purpose because they are easier to produce and the control over their characteristics in the production process is quite precise. Also, nanowires can increase faradaic efficiency due to their spatial extent and thus to greater availability of reactants on the active surface.

Materials

The nanostructures involved in electrocatalysis processes can be made up of different materials. Through the use of nanostructured materials, electrocatalysts can achieve good physical-chemical stability, high activity, good conductivity and low cost. Metallic nanomaterials are commonly made up of transition metals (mostly iron, cobalt, nickel, palladium, platinum). Multi-metal nanomaterials show new properties due to the characteristics of each metal. The advantages are the increase in activity, selectivity and stability and the cost reduction. Metals can be combined in different ways such as in the core-shell bimetallic structure: the cheapest metal forms the core and the most active one (typically a noble metal) constitutes the shell. By adopting this design, the use of rare and expensive metals can be reduced down to 20%.

One of the future challenges is to find new stable materials, with good activity and especially low cost. Metallic glasses, polymeric carbon nitride (PCN) and materials derived from metal-organic frameworks (MOF) are just a few examples of materials with electrocatalytic properties on which research is currently investing.

Photocatalysis

Many of the photocatalytic systems can benefit from the coupling with a noble metal; the first Fujishima-Honda cell made use of a co-catalyst plate as well. For instance, the essential design of a disperse photocatalytic reactor for water splitting is that of a water sol in which the dispersed phase is made up of semiconductor quantum dots each coupled to a metallic co-catalyst: the QD converts the incoming electromagnetic radiation into an exciton whilst the co-catalyst acts as an electron scavenger and lowers the overpotential of the electrochemical reaction.

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Catalyst support

Supports for electrocatalysis

Supports are used to give mechanical stability to catalyst nanoparticles or powders. Supports immobilize the particle reducing its mobility and favouring the chemical stabilization: they can be considered as solid capping agents. Supports also allow the nanoparticles to be easily recycled.

One of the most promising supports is graphene for its porosity, electronic properties, thermal stability and active surface area.