User:Emma Ambrogi/Electrocatalyst

[An electrocatalyst is a catalyst that participates in electrochemical reactions. Catalyst materials modify and increase the rate of chemical reactions without being consumed in the process. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or may be the electrode surface itself. An electrocatalyst can be heterogeneous, such as a platinum surface or nanoparticles, or homogeneous, such as a coordination complex or enzyme. The electrocatalyst assists in transferring electrons between the electrode and reactants, and/or facilitates an intermediate chemical transformation described by an overall half-reaction.]

Background and Theory

Potential energy diagram for a reaction with and without a catalyst. A catalyst increases the rate of a reaction by lowering the activation energy of a reaction without being consumed in the reaction. An electrocatalyst lowers the activation energy of an electrochemical reaction, often lowering the electric potential at which the reaction occurs.

Types of Electrocatalyst Materials

In general, a catalyst is an agent that increases the speed of a chemical reaction without being consumed by a reaction. Thermodynamically, a catalyst lowers the activation energy required for a chemical reaction to take place. An electrocatalyst is a catalyst that affects the activation energy of an electrochemical reaction. Shown at left is the activation energy of chemical reactions as it relates to the energies of products and reactants. The activation energy in electrochemical processes is related to the potential, i.e. voltage, at which a reaction occurs. Thus, electrocatalysts frequently change the potential at which oxidation and reduction processes are observed. Alternatively, an electrocatalyst can be thought of as an agent that facilitates a specific chemical interaction at an electrode surface. Given that electrochemical reactions occur when electrons are passed from one chemical species to another, favorable interactions at an electrode surface increase the likelihood of electrochemical transformations occurring, thus reducing the potential required to achieve these transformations.

Electrocatalysts can be evaluated according three figures of merit: activity, stability, and selectivity. The activity of electrocatalysts can be assessed quantitatively by understanding how much current density is generated, and therefore how fast a reaction is taking place, for a given applied potential. This relationship is described with the Tafel Equation. In assessing the stability of electrocatalysts, the ability of catalysts to withstand the potentials at which transformations are occurring is crucial. The selectivity of electrocatalysts refers to their preferential interaction with particular substrates, and their generation of a single product. Selectivity can be quantitatively assessed through a selectivity coefficient, which compares the response of the material to the desired analyte or substrate with the response to other interferents.

Homogeneous Electrocatalysts

A homogeneous electrocatalyst is one that is present in the same phase of matter as the reactants, for example, a water-soluble coordination complex catalyzing an electrochemical conversion in solution. The different types of homogenous electrocatalysts are shown at left in yellow.

Synthetic Coordination Complexes

Synthetic complexes of metals and ligands can be used to catalyze electrochemical reactions. A variety of organometallic cobalt complexes, including cobalt porphyrins and cobalt polypyridines, have been synthesized and demonstrated to show high (>80%) Faradaic yields for the hydrogen evolution reaction. These catalysts are typically square planar complexes, often macrocyclic or pseudo-macrocyclic, with open axial coordination sites that allow for the binding of substrates. Examples of cobalt complexes used for water splitting are shown below. Complexes of other transition metals such as molybdenum, tungsten and nickel have been shown to catalyze the reduction of carbon dioxide to carbon monoxide. First row transition metal complexes, including iron and cobalt complexes, have been most studied for this purpose, but other transition metals, especially rhenium and ruthenium, have received attention for this application. Examples of rhenium and ruthenium electrocatalysts are also shown below.

Examples of transition metal complexes that serve as homogeneous electrocatalysts. For details and more examples, see and

Enzymes

Biological enzymes, especially those containing metals, can be leveraged to catalyze electrochemical reactions. Known as bioelectrocatalysis, the integration of enzymes in electrochemical systems incorporates the advantages of selectivity and high activity that organisms have evolved, often for difficult chemical processes. Nitrogenase, an enzyme that contains a MoFe cluster, can be leveraged to fix atmospheric nitrogen, i.e. convert nitrogen gas into molecules such as ammonia. Immobilizing the protein onto an electrode surface and employing an electron mediator greatly improves the efficiency of this process. The effectiveness of bioelectrocatalysts generally depends on the ease of electron transport between the active site of the enzyme and the electrode surface. Other enzymes provide insight for the development of synthetic catalysts. For example, formate dehydrogenase, a nickel-containing enzyme, has inspired the development of synthetic complexes with similar molecular structures for use in CO₂ reduction. Microbial fuel cells are another way that biological systems can be leveraged for electrocatalytic applications. Microbial-based systems leverage the metabolic pathways of an entire organism, rather than the activity of a specific enzyme, meaning that they can catalyze a broad range of chemical reactions. Microbial fuel cells can derive current from the oxidation of substrates such as glucose, and be leveraged for processes such as CO₂ reduction.

Heterogeneous Electrocatalysts

A heterogeneous electrocatalyst is one that is present in a different phase of matter from the reactants, for example, a solid surface catalyzing a reaction in solution. Different types of heterogeneous electrocatalyst materials are shown above in green. [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 necessity of controlling and tuning surface properties in order to facilitate interactions at the electrode surface has lead to the development of a variety of nanomaterials over the use of bulk materials.

Bulk Materials

Electrocatalysis can occur at the surface of some bulk materials, such as platinum metal. Bulk metal surfaces of gold have been employed for the decomposition methanol for hydrogen production. Water electrolysis is conventionally conducted at inert bulk metal electrodes such as platinum or iridium.

Nanomaterials

Nanoparticles

A variety of nanoparticle materials have been demonstrated to promote various electrochemical reactions. Metallic nanoparticles have been employed for various energy-related processes, and the efficiency of these catalysts can be improved by tuning the size and shape of the particles, as well as the surface strain.

Particle size effect

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Carbon-based Materials

Carbon nanotubes and graphene-based materials can be used as electrocatalysts. The carbon surfaces of graphene and carbon nanotubes are well suited to the adsorption of many chemical species, which can promote certain electrocatalytic reactions. In addition, their conductivity means they are good electrode materials. Carbon nanotubes have a very high surface area, maximizing surface sites at which electrochemical transformations can occur. Graphene can also serve as a platform for constructing composites with other kinds of nanomaterials such as single atom catalysts. Because of their conductivity, carbon-based materials can potentially replace  metal electrodes to perform metal-free electrocatalysis.

Framework Materials

Metal—organic frameworks, especially conductive frameworks, can be used as electrocatalysts for processes such as CO₂ reduction and water splitting. MOFs provide potential active sites at both metal centers and organic ligand sites. They can also be functionalized, or encapsulate other materials such as nanoparticles. MOFs can also be combined with carbon-based materials to form electrocatalysts. Covalent organic frameworks, particularly those that contain metals, can also serve as electrocatalysts. COFs constructed from cobalt porphyrins demonstrated the ability to reduce carbon dioxide to carbon monoxide.

Applications of Electrocatalysis

Water Splitting/ Hydrogen Evolution

A schematic of a hydrogen fuel cell. To supply hydrogen, electrocatalytic water splitting is commonly employed.

Main article: Electrolysis of Water

[Hydrogen and oxygen can be combined to form water through a free-radical mechanism commonly referred to as combustion. It is also possible to combine the hydrogen and oxygen through redox mechanism as in the case of a fuel cell. In this process, the reaction is broken into two half-reactions which occur at separate electrodes. In this situation the reactant's energy is directly converted to electricity.] The hydrogen evolution reaction is essentially the reverse of the process occurring in hydrogen fuel cells, and consists of two half reactions, shown in the table below. By convention, half-reactions are written as reductions and tabulated with their standard reduction potentials. The standard reduction potential of hydrogen is defined as 0V, and frequently referred to as the standard hydrogen electrode (SHE).

Half Reaction	Reduction Potential Eored (V)
2H+ + 2e- → H2 (g)	≡ 0
O2(g) + 4H+ + 4e- → 2H2O(l)	+1.23

Ethanol-Powered Fuel Cells

Aqueous solutions of methanol can decompose into CO₂ hydrogen gas, and water. Although this process is thermodynamically favored, the activation barrier is extremely high, so in practice this reaction is not typically observed. However, electrocatalysts can speed up this reaction greatly, making methanol a possible route to hydrogen storage for fuel cells. Electrocatalysts such as gold, platinum, and various carbon-based materials have been shown to effectively catalyze this process. [An electrocatalyst of platinum and rhodium on carbon backed tin-dioxide nanoparticles can break carbon bonds at room temperature with only carbon dioxide as a by-product, so that ethanol can be oxidized into the necessary hydrogen ions and electrons required to create electricity.]

Carbon Dioxide Reduction

The reduction of CO₂ into useable products is a potential way to combat climate change. Electrocatalysts can promote the reduction of carbon dioxide into methanol and other useful fuel and stock chemicals. The most valuable reduction products of CO₂ are those that have a higher energy content, meaning that they can be reused as fuels. Thus, catalyst development focuses on the production of products such as methane and methanol. Homogeneous catalysts, such as enzymes and synthetic coordination complexes have been employed for this purpose. A variety of nanomaterials have also been studied for CO₂ reduction, including carbon-based materials and framework materials.

Chemical Synthesis

Electrocatalysts can be used to promote certain chemical reactions to obtain synthetic products. This is of great interest to those seeking to improve the efficiency of organic synthesis reactions and develop methods for green chemistry. Selectivity is the main consideration for developing these catalysts. Graphene and graphene oxides have shown promise as electrocatalytic materials for synthesis. Electrocatalytic methods also have potential for polymer synthesis. Electrocatalytic synthesis reactions can be performed under a constant current, constant potential, or constant cell-voltage conditions, depending on the scale and purpose of the reaction.