Electrocatalyst

An electrocatalyst lowers the activation energy required for an electrochemical reaction. Some electrocatalysts change the potential at which oxidation and reduction processes occur. In other cases, an electrocatalyst can impart selectivity by favoring 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. In many electrochemical systems, including galvanic cells, fuel cells and various forms of electrolytic cells, a drawback is that they can suffer from high activation barriers. The energy diverted to overcome these activation barriers is transformed into heat. In most exothermic combustion reactions this heat would simply propagate the reaction catalytically. In a redox reaction, this heat is a useless byproduct lost to the system. The extra energy required to overcome kinetic barriers is usually described in terms of low faradaic efficiency and high overpotentials. == 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. Electrification of catalytic processes There is much interest in replacing traditional chemical catalysis with electrocatalysis. In such a scheme electrons supplied by an electrode are reagents. The topic is a theme within the area of green energy, because the electrons can be sourced from renewable resources. Several conversions that use hydrogen gas could be transformed into electrochemical processes that use protons. This technology remains economically noncompetitive. Another example is found in the area of nitrogen fixation. The traditional Haber-Bosch process produces ammonia by hydrogenation of nitrogen gas: : In the electrified version, the hydrogen is provided in the form of protons and electrons: : The ammonia represents an energy source since it is combustable. In this way electrification can be seen as a means for energy storage. Another process attracting much effort is the electrochemical reduction of carbon dioxide. Enzymes Some enzymes can function as electrocatalysts. 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. 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 heterogeneous electrocatalytic 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. Water electrolysis is conventionally conducted at inert bulk metal electrodes such as platinum or iridium. 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. Nanomaterials Nanoparticles A variety of nanoparticle materials have been demonstrated to promote various electrochemical reactions, although none have been commercialized. These catalysts can be tuned with respect to their size and shape, as well as the surface strain. simulation.|alt=Electronic density difference of a Cl atom adsorbed on a Cu(111) surface obtained with a density functional theory simulation. Red regions represent the abundance of electrons, whereas blue regions represent deficit of electrons.|thumb|220x220px 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. but there are still 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). 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: 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. 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 (MOFs), 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. Covalent organic frameworks (COFs), 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. However, many MOFs are known unstable in chemical and electrochemical conditions, making it difficult to tell if MOFs are actually catalysts or precatalysts. The real active sites of MOFs during electrocatalysis need to be analyzed comprehensively. == Research on electrocatalysis ==

(SHE). HER can be promoted by many catalysts. 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. Chemical synthesis Electrocatalysts are used to promote certain chemical reactions to obtain synthetic products. 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. Advanced oxidation processes in water treatment Water treatment systems often require the degradation of hazardous compounds. These treatment processes are dubbed Advanced oxidation processes, and are key in destroying byproducts from disinfection, pesticides, and other hazardous compound. There is an emerging effort to enable these processes to destroy more tenacious compounds, especially PFAS ==Additional reading==