# Electrocatalyst

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{{Short description|Catalyst participating in electrochemical reactions}}

[[Image:Electrocatalyst ANL.jpg|thumb|175px|A platinum cathode electrocatalyst's stability being measured by chemist [Xiaoping Wang](/source/Xiaoping_Wang)]]

An '''electrocatalyst''' is a [catalyst](/source/Catalysis) that participates in [electrochemical reactions](/source/Electrochemical_reaction_mechanism). Electrocatalysts are a specific form of catalysts that function at [electrode](/source/electrode) surfaces or, most commonly, may be the electrode surface itself. An electrocatalyst can be [heterogeneous](/source/heterogeneous) such as a [platinized electrode](/source/Platinum_black).<ref name=Ind>{{cite book |doi=10.1002/9783527610044.hetcat0103|chapter=Industrial Electrocatalysis|title=Handbook of Heterogeneous Catalysis|year=2008|last1=Kotrel|first1=Stefan|last2=BrUninger|first2=Sigmar|isbn=978-3-527-31241-2}}</ref>  Homogeneous electrocatalysts, which are soluble, assist in transferring electrons between the electrode and reactants, and/or facilitate an intermediate chemical transformation described by an overall [half reaction](/source/half-reaction).<ref name=":9" /> Major challenges in electrocatalysts focus on [fuel cell](/source/fuel_cell)s.<ref>{{cite journal |doi=10.1038/nature11115|title=Electrocatalyst approaches and challenges for automotive fuel cells|year=2012|last1=Debe|first1=Mark K.|journal=Nature|volume=486|issue=7401|pages=43–51|pmid=22678278 |bibcode=2012Natur.486...43D|s2cid=4349039}}</ref><ref>{{cite journal |doi=10.1039/C4CS00470A|title=Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions|year=2015|last1=Jiao|first1=Yan|last2=Zheng|first2=Yao|last3=Jaroniec|first3=Mietek|last4=Qiao|first4=Shi Zhang|journal=Chemical Society Reviews|volume=44|issue=8|pages=2060–2086|pmid=25672249}}</ref>

== Background and theory ==
An electrocatalyst lowers the [activation energy](/source/activation_energy) required for an electrochemical reaction.<ref name=":42">{{Cite web|last=Jaramillo|first=Tom|date=September 3, 2014|title=Electrocatalysis 101 {{!}} GCEP Symposium - October 11, 2012|url=https://www.youtube.com/watch?v=2sbsTLvcbCg&feature=youtu.be|website=Youtube.com}}</ref> Some electrocatalysts change the potential at which oxidation and reduction processes occur.<ref>{{Cite book|last=Bard|first=Allen J. |title=Electrochemical methods: fundamentals and applications|date=2001|author2=Larry R. Faulkner|isbn=0-471-04372-9|edition=Second |location=Hoboken, NJ|oclc=43859504}}</ref> In other cases, an electrocatalyst can impart selectivity by favoring specific chemical interaction at an electrode surface.<ref name=":5">{{Cite journal|last=McCreery|first=Richard L.|date=July 2008|title=Advanced Carbon Electrode Materials for Molecular Electrochemistry |journal=Chemical Reviews|language=en|volume=108|issue=7|pages=2646–2687|doi=10.1021/cr068076m|pmid=18557655 |issn=0009-2665}}</ref> Given that electrochemical reactions occur when electrons are passed from one [chemical species](/source/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.<ref name=":5" />

Electrocatalysts can be evaluated according to activity, stability, and selectivity. The activity of electrocatalysts can be assessed quantitatively by the 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](/source/Tafel_equation).<ref name=":42"/> In assessing the stability of electrocatalysts, the a key parameter is [turnover number](/source/turnover_number) (TON). The selectivity of electrocatalysts refers to the product distribution.<ref name=":42" /> Selectivity can be quantitatively assessed through a selectivity coefficient, which compares the response of the material to the desired [analyte](/source/analyte) or substrate with the response to other interferents.<ref>{{Cite journal|last1=Brown|first1=Micah D.|last2=Schoenfisch|first2=Mark H.|date=2019-11-27|title=Electrochemical Nitric Oxide Sensors: Principles of Design and Characterization |journal=Chemical Reviews|language=en|volume=119|issue=22|pages=11551–11575|doi=10.1021/acs.chemrev.8b00797|pmid=31553169 |s2cid=202761809 |issn=0009-2665}}</ref>

In many electrochemical systems, including [galvanic cell](/source/galvanic_cell)s, [fuel cell](/source/fuel_cell)s and various forms of [electrolytic cell](/source/electrolytic_cell)s, 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](/source/faraday_efficiency) and high [overpotential](/source/overpotential)s.<ref name=":42" />  In these systems, each of the two [electrode](/source/electrode)s and its associated [half-cell](/source/half-cell) would require its own specialized electrocatalyst.<ref name=":9" />

Half-reactions involving [multiple steps](/source/Electrochemical_reaction_mechanism), multiple electron transfers, and the evolution or consumption of gases in their overall chemical transformations, will often have considerable kinetic barriers.  Furthermore, there is often more than one possible reaction at the surface of an electrode.  For example, during the [electrolysis of water](/source/electrolysis_of_water), the anode can oxidize water through a two electron process to [hydrogen peroxide](/source/hydrogen_peroxide) or a four electron process to oxygen.  The presence of an electrocatalyst could facilitate either of the reaction pathways.<ref name="electromethods">
{{Cite book|last1=Bard|first1=Allen J.|url=https://www.amazon.co.uk/gp/reader/0471043729/ref=sib_dp_pt#reader-link|title=Electrochemical methods: fundamentals and applications|last2=Faulkner|first2=Larry R.|date=January 2001|publisher=[Wiley](/source/John_Wiley_%26_Sons)|isbn=978-0-471-04372-0|location=[New York](/source/New_York_City)|author1-link=Allen J. Bard|access-date=27 February 2009}}
</ref>

left|thumb|450x450px|Types of electrocatalyst materials, including homogeneous and heterogeneous electrocatalysts.

== 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.<ref name=":12"/><ref name=":62"/> This technology is not practiced commercially, but is of research interest.

=== Synthetic coordination complexes ===
Many [coordination complex](/source/coordination_complex)es catalyze electrochemical reactions, although few have achieved commercial success.  Well investigated processes include the [hydrogen evolution reaction](/source/hydrogen_evolution_reaction).<ref>{{cite journal |doi=10.1021/acs.chemrev.1c01001 |title=Molecular Catalysts with Diphosphine Ligands Containing Pendant Amines |date=2022 |last1=Wiedner |first1=Eric S. |last2=Appel |first2=Aaron M. |last3=Raugei |first3=Simone |last4=Shaw |first4=Wendy J. |last5=Bullock |first5=R. Morris |journal=Chemical Reviews |volume=122 |issue=14 |pages=12427–12474 |pmid=35640056 |osti=1922077 }}</ref>  

===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](/source/green_energy), because the electrons can be sourced from [renewable resource](/source/renewable_resource)s. Several conversions that use hydrogen gas could be transformed into [electrochemical](/source/electrochemical) processes that use protons.<ref>{{cite journal |doi=10.1039/D3CS00419H |title=Developing electrochemical hydrogenation towards industrial application |date=2023 |last1=Kleinhaus |first1=Julian T. |last2=Wolf |first2=Jonas |last3=Pellumbi |first3=Kevinjeorjios |last4=Wickert |first4=Leon |last5=Viswanathan |first5=Sangita C. |last6=Junge Puring |first6=Kai |last7=Siegmund |first7=Daniel |last8=Apfel |first8=Ulf-Peter |journal=Chemical Society Reviews |volume=52 |issue=21 |pages=7305–7332 |pmid=37814786 }}</ref>  This technology remains economically noncompetitive.<ref>{{Cite web|title=Dream or Reality? Electrification of the Chemical Process Industries|url=https://www.aiche-cep.com/cepmagazine/march_2021/MobilePagedArticle.action?articleId=1663852|access-date=2021-08-22|website=www.aiche-cep.com|language=en}}</ref>

Another example is found in the area of [nitrogen fixation](/source/nitrogen_fixation). The traditional [Haber-Bosch process](/source/Haber-Bosch_process) produces [ammonia](/source/ammonia) by [hydrogenation](/source/hydrogenation) of [nitrogen](/source/nitrogen) gas:
:{{chem2|N2 + 3 H2  ->  2 NH3}}
In the electrified version, the [hydrogen](/source/hydrogen) is provided in the form of [proton](/source/proton)s and electrons:<ref>{{cite journal |doi=10.1039/C9CS00159J |title=Electrochemical nitrogen fixation and utilization: Theories, advanced catalyst materials and system design |date=2019 |last1=Guo |first1=Wenhan |last2=Zhang |first2=Kexin |last3=Liang |first3=Zibin |last4=Zou |first4=Ruqiang |last5=Xu |first5=Qiang |journal=Chemical Society Reviews |volume=48 |issue=24 |pages=5658–5716 |pmid=31742279 }}</ref><ref>{{cite journal |doi=10.1039/D2EE03132A |title=Renewable electron-driven bioinorganic nitrogen fixation: A superior route toward green ammonia? |date=2023 |last1=Wang |first1=Bo |last2=Zhang |first2=Yifeng |last3=Minteer |first3=Shelley D. |journal=Energy & Environmental Science |volume=16 |issue=2 |pages=404–420 |bibcode=2023EnEnS..16..404W |url=https://backend.orbit.dtu.dk/ws/files/298164093/D2EE03132A.pdf }}</ref>
:{{chem2|N2 + 6 H+  +  6 e-  ->  2 NH3}}
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](/source/electrochemical_reduction_of_carbon_dioxide).<ref name=":62">{{Cite journal|last1=Kinzel|first1=Niklas W.|last2=Werlé|first2=Christophe|last3=Leitner|first3=Walter|date=2021-01-19|title=Transition Metal Complexes as Catalysts for the Electroconversion of CO 2: An Organometallic Perspective|journal=Angewandte Chemie International Edition|volume=60 |issue=21 |language=en|pages=11628–11686|doi=10.1002/anie.202006988|pmid=33464678 |pmc=8248444 |issn=1433-7851|doi-access=free}}</ref>

=== Enzymes ===
Some [enzyme](/source/enzyme)s can function as electrocatalysts.<ref name=":3">{{Cite journal|last1=Chen|first1=Hui|last2=Simoska|first2=Olja|last3=Lim|first3=Koun|last4=Grattieri|first4=Matteo|last5=Yuan|first5=Mengwei|last6=Dong|first6=Fangyuan|last7=Lee|first7=Yoo Seok|last8=Beaver|first8=Kevin|last9=Weliwatte|first9=Samali|last10=Gaffney|first10=Erin M.|last11=Minteer|first11=Shelley D.|date=2020-12-09|title=Fundamentals, Applications, and Future Directions of Bioelectrocatalysis|journal=Chemical Reviews|language=en|volume=120|issue=23|pages=12903–12993|doi=10.1021/acs.chemrev.0c00472|pmid=33050699 |issn=0009-2665|doi-access=free|hdl=11586/317111|hdl-access=free}}</ref>  [Nitrogenase](/source/Nitrogenase), an enzyme that contains a MoFe cluster, can be leveraged to [fix atmospheric nitrogen](/source/Nitrogen_fixation), 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.<ref name=":2">{{Cite journal|last1=Milton|first1=Ross D.|last2=Minteer|first2=Shelley D.|date=2019-12-17|title=Nitrogenase Bioelectrochemistry for Synthesis Applications|url=https://pubs.acs.org/doi/abs/10.1021/acs.accounts.9b00494|journal=Accounts of Chemical Research|language=en|volume=52|issue=12|pages=3351–3360|doi=10.1021/acs.accounts.9b00494|pmid=31800207 |s2cid=208643374 |issn=0001-4842|url-access=subscription}}</ref> The effectiveness of bioelectrocatalysts generally depends on the ease of electron transport between the [active site](/source/active_site) of the enzyme and the electrode surface.<ref name=":3" /> Other enzymes provide insight for the development of synthetic catalysts. For example, [formate dehydrogenase](/source/formate_dehydrogenase), a nickel-containing enzyme, has inspired the development of synthetic complexes with similar molecular structures for use in CO<sub>2</sub> reduction.<ref name=":72">{{Cite journal|last1=Yang|first1=Jenny Y.|last2=Kerr|first2=Tyler A.|last3=Wang|first3=Xinran S.|last4=Barlow|first4=Jeffrey M.|date=2020-11-18|title=Reducing CO 2 to HCO 2 – at Mild Potentials: Lessons from Formate Dehydrogenase|url=https://pubs.acs.org/doi/10.1021/jacs.0c07965|journal=Journal of the American Chemical Society|language=en|volume=142|issue=46|pages=19438–19445|doi=10.1021/jacs.0c07965|pmid=33141560 |bibcode=2020JAChS.14219438Y |issn=0002-7863|url-access=subscription}}</ref> [Microbial fuel cell](/source/Microbial_fuel_cell)s are another way that biological systems can be leveraged for electrocatalytic applications.<ref name=":3" /><ref name=":8">{{Cite journal|last1=Qiao|first1=Yan|last2=Bao|first2=Shu-Juan|last3=Li|first3=Chang Ming|date=2010|title=Electrocatalysis in microbial fuel cells—from electrode material to direct electrochemistry|url=http://xlink.rsc.org/?DOI=b923503e|journal=Energy & Environmental Science|language=en|volume=3|issue=5|page=544|doi=10.1039/b923503e|bibcode=2010EnEnS...3..544Q |issn=1754-5692|url-access=subscription}}</ref> 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.<ref name=":3" /> Microbial fuel cells can derive current from the oxidation of substrates such as glucose,<ref name=":8" /> and be leveraged for processes such as CO<sub>2</sub> reduction.<ref name=":3" />

== 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](/source/Interface_(matter)) between them.<ref name=":5" /> The nature of the electrocatalyst surface determines some properties of the reaction including rate and selectivity.<ref name=":5" />

=== 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](/source/hydrogen_production).<ref name=":9">{{Cite journal|last=Roduner|first=Emil|date=June 13, 2017|title=Selected fundamentals of catalysis and electrocatalysis in energy conversion reactions—A tutorial|url=https://linkinghub.elsevier.com/retrieve/pii/S0920586117304236|journal=Catalysis Today|language=en|volume=309|pages=263–268|doi=10.1016/j.cattod.2017.05.091|s2cid=103395714 |hdl=2263/68699|hdl-access=free|url-access=subscription}}</ref> Water electrolysis is conventionally conducted at inert bulk metal electrodes such as platinum or iridium.<ref name=":10">{{Cite journal|last1=Carmo|first1=Marcelo|last2=Fritz|first2=David L.|last3=Mergel|first3=Jürgen|last4=Stolten|first4=Detlef|date=March 14, 2013|title=A comprehensive review on PEM water electrolysis|url=https://linkinghub.elsevier.com/retrieve/pii/S0360319913002607|journal=International Journal of Hydrogen Energy|language=en|volume=38|issue=12|pages=4901–4934|doi=10.1016/j.ijhydene.2013.01.151|bibcode=2013IJHE...38.4901C |url-access=subscription}}</ref> 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.<ref name="Mistry-2016">{{Cite journal|title=Nanostructured electrocatalysts with tunable activity and selectivity|journal=Nature Reviews Materials|last1=Mistry|first1=H.|volume=1|issue=4|pages=1–14|last2=Varela|first2=A.S.|doi=10.1038/natrevmats.2016.9|last3=Strasser|first3=P.|author-link3=Peter Strasser (chemist)|last4=Cuenya|first4=B.R.|year=2016|bibcode=2016NatRM...116009M}}</ref>

=== Nanomaterials ===

==== Nanoparticles ====
A variety of [nanoparticle](/source/nanoparticle) materials have been demonstrated to promote various electrochemical reactions,<ref name=":0">{{Cite journal|last1=Kleijn|first1=Steven E. F.|last2=Lai|first2=Stanley C. S.|last3=Koper|first3=Marc T. M.|last4=Unwin|first4=Patrick R.|date=2014-04-01|title=Electrochemistry of Nanoparticles|url=http://doi.wiley.com/10.1002/anie.201306828|journal=Angewandte Chemie International Edition|language=en|volume=53|issue=14|pages=3558–3586|doi=10.1002/anie.201306828|pmid=24574053 |url-access=subscription}}</ref> although none have been commercialized. These catalysts can be tuned with respect to their size and shape, as well as the surface strain.<ref>{{Cite journal|last1=Luo|first1=Mingchuan|last2=Guo|first2=Shaojun|date=September 26, 2017|title=Strain-controlled electrocatalysis on multimetallic nanomaterials|url=http://www.nature.com/articles/natrevmats201759|journal=Nature Reviews Materials|language=en|volume=2|issue=11|page=17059|doi=10.1038/natrevmats.2017.59|bibcode=2017NatRM...217059L |issn=2058-8437|url-access=subscription}}</ref>[[File:Electronic density difference Cl Cu111.png|Electronic density difference of a Cl atom adsorbed on a Cu(111) surface obtained with a [DFT](/source/density_functional_theory) 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](/source/adsorption) energy of the reactants together with many other variables not yet fully clarified.<ref name="10.1039/c0nr00857e" />

According to the [TSK model](/source/Terrace_step_kink), the catalyst surface atoms can be classified as terrace, step or kink atoms according to their position, each characterized by a different [coordination number](/source/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.<ref name=":0" />

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<ref name="10.1039/c0nr00857e">{{Cite journal|title=Structure sensitivity and nanoscale effects in electrocatalysis|journal=Nanoscale|volume=3|issue=5|pages=2054–2073|last1=Koper|first1=M.T.M.|publisher=The Royal Society of Chemistry|doi=10.1039/c0nr00857e|pmid=21399781|year=2011|bibcode=2011Nanos...3.2054K}}</ref> but there are still many exceptions that do not fall into them.

===== Particle size effect =====
400px|thumb|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 varies.|alt=|left
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](/source/specific_surface_area) increases as the average particle size decreases. For instance, most common [PEM fuel cells](/source/Proton-exchange_membrane_fuel_cell) and [electrolyzers](/source/PEM_electrolyzer) design is based on a polymeric membrane charged in platinum nanoparticles as an electrocatalyst (the so-called [platinum black](/source/platinum_black)).<ref name="Carmo-2013">{{Cite journal|title=A comprehensive review on PEM water electrolysis|journal=International Journal of Hydrogen Energy|last1=Carmo|first1=M.|volume=38|pages=4901–4934|last2=Fritz|first2=D.L.|issue=12|doi=10.1016/j.ijhydene.2013.01.151|year=2013|last3=Mergel|first3=J.|last4=Stolten|first4=D.|bibcode=2013IJHE...38.4901C }}</ref>

Although the [surface area to volume ratio](/source/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:<ref name="10.1039/c0nr00857e" />
* ''[Equilibrium shape](/source/Wulff_construction)'': 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](/source/Electronic_structure)'': below a certain size, the [work function](/source/work_function) of a nanoparticle changes and its band structure fades away
* ''[Defects](/source/Crystallographic_defect)'': 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](/source/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](/source/Catalyst_support)'': nanoparticles are often fixed onto a support in order to stay in place, therefore part of their surface is unavailable for reactants

==== Carbon-based materials ====
[Carbon nanotube](/source/Carbon_nanotube)s and [graphene](/source/graphene)-based materials can be used as electrocatalysts.<ref name="CNT">{{cite web
 | last = Wang
 | first = Xin
 | title = CNTs tuned to provide electrocatalyst support
 | publisher = Nanotechweb.org
 | date = 19 January 2008
 | url = http://nanotechweb.org/cws/article/tech/37366
 | access-date = 27 February 2009
 | archive-url = https://web.archive.org/web/20090122040434/http://nanotechweb.org/cws/article/tech/37366
 | archive-date = 22 January 2009
 }}</ref> The carbon surfaces of graphene and carbon nanotubes are well suited to the adsorption of many chemical species, which can promote certain electrocatalytic reactions.<ref name=":52">{{Cite journal|last=McCreery|first=Richard L.|date=June 17, 2008|title=Advanced Carbon Electrode Materials for Molecular Electrochemistry|url=https://pubs.acs.org/doi/10.1021/cr068076m|journal=Chemical Reviews|language=en|volume=108|issue=7|pages=2646–2687|doi=10.1021/cr068076m|pmid=18557655 |issn=0009-2665|url-access=subscription}}</ref> In addition, their conductivity means they are good electrode materials.<ref name=":52" /> Carbon nanotubes have a very high surface area, maximizing surface sites at which electrochemical transformations can occur.<ref>{{Cite journal|last1=Wildgoose|first1=Gregory G.|last2=Banks|first2=Craig E.|last3=Leventis|first3=Henry C.|last4=Compton|first4=Richard G.|date=November 30, 2005|title=Chemically Modified Carbon Nanotubes for Use in Electroanalysis|url=http://link.springer.com/10.1007/s00604-005-0449-x|journal=Microchimica Acta|language=en|volume=152|issue=3–4|pages=187–214|doi=10.1007/s00604-005-0449-x|s2cid=93373402 |issn=0026-3672|url-access=subscription}}</ref> Graphene can also serve as a platform for constructing composites with other kinds of [nanomaterials](/source/nanomaterials) such as single atom catalysts.<ref>{{Cite journal|last1=Zhang|first1=Qin|last2=Zhang|first2=Xiaoxiang|last3=Wang|first3=Junzhong|last4=Wang|first4=Congwei|date=2021-01-15|title=Graphene-supported single-atom catalysts and applications in electrocatalysis|url=https://iopscience.iop.org/article/10.1088/1361-6528/abbd70|journal=Nanotechnology|volume=32|issue=3|page=032001|doi=10.1088/1361-6528/abbd70|pmid=33002887 |bibcode=2021Nanot..32c2001Z |s2cid=222146032 |issn=0957-4484|url-access=subscription}}</ref> Because of their conductivity, carbon-based materials can potentially replace metal electrodes to perform metal-free electrocatalysis.<ref>{{Cite journal|last=Dai|first=Liming|date=June 13, 2017|title=Carbon-based catalysts for metal-free electrocatalysis|journal=Current Opinion in Electrochemistry|language=en|volume=4|issue=1|pages=18–25|doi=10.1016/j.coelec.2017.06.004|doi-access=free}}</ref>

==== Framework materials ====
[Metal—organic frameworks (MOFs)](/source/Metal%E2%80%93organic_framework), especially conductive frameworks, can be used as electrocatalysts for processes such as CO<sub>2</sub> reduction and [water splitting](/source/water_splitting). MOFs provide potential active sites at both metal centers and organic ligand sites.<ref name=":11">{{Cite journal|last1=Jiao|first1=Long|last2=Wang|first2=Yang|last3=Jiang|first3=Hai-Long|last4=Xu|first4=Qiang|date=November 27, 2017|title=Metal-Organic Frameworks as Platforms for Catalytic Applications|url=http://doi.wiley.com/10.1002/adma.201703663|journal=Advanced Materials|language=en|volume=30|issue=37|article-number=1703663|doi=10.1002/adma.201703663|pmid=29178384 |s2cid=205282723 |url-access=subscription}}</ref> They can also be functionalized, or encapsulate other materials such as nanoparticles.<ref name=":11" /> MOFs can also be combined with carbon-based materials to form electrocatalysts.<ref>{{Cite journal|last1=Singh|first1=Chanderpratap|last2=Mukhopadhyay|first2=Subhabrata|last3=Hod|first3=Idan|date=January 5, 2021|title=Metal–organic framework derived nanomaterials for electrocatalysis: recent developments for CO2 and N2 reduction|journal=Nano Convergence|language=en|volume=8|issue=1|page=1|doi=10.1186/s40580-020-00251-6|issn=2196-5404|pmc=7785767|pmid=33403521 |bibcode=2021NanoC...8....1S |doi-access=free }}</ref> [Covalent organic frameworks (COFs)](/source/Covalent_organic_framework), 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.<ref>{{Cite journal|last1=Sharma|first1=Rakesh Kumar|last2=Yadav|first2=Priya|last3=Yadav|first3=Manavi|last4=Gupta|first4=Radhika|last5=Rana|first5=Pooja|last6=Srivastava|first6=Anju|last7=Zbořil|first7=Radek|last8=Varma|first8=Rajender S.|last9=Antonietti|first9=Markus|last10=Gawande|first10=Manoj B.|date=2020|title=Recent development of covalent organic frameworks (COFs): synthesis and catalytic (organic-electro-photo) applications|url=http://xlink.rsc.org/?DOI=C9MH00856J|journal=Materials Horizons|language=en|volume=7|issue=2|pages=411–454|doi=10.1039/C9MH00856J|s2cid=204292382 |issn=2051-6347|url-access=subscription}}</ref>

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.<ref>{{cite journal |last1=Zheng |first1=Weiran |last2=Liu |first2=Mengjie |last3=Lee |first3=Lawrence Yoon Suk |title=Electrochemical Instability of Metal–Organic Frameworks: In Situ Spectroelectrochemical Investigation of the Real Active Sites |journal=ACS Catalysis |date=3 January 2020 |volume=10 |issue=1 |pages=81–92 |doi=10.1021/acscatal.9b03790|hdl=10397/100175 |s2cid=212979103 |hdl-access=free }}</ref>

== Research on electrocatalysis ==
left|thumb|300x300px|Some transition metal complexes that exhibit some activity as homogeneous electrocatalysts.<ref name=":12"/><ref name=":62"/>
===Water splitting / Hydrogen evolution===
{{main|Electrolysis of water}}

thumb|450x450px|A schematic of a hydrogen fuel cell. To supply hydrogen, electrocatalytic water splitting is commonly employed.

Hydrogen and oxygen can be combined through by the use 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. Useful energy can be obtained from the thermal heat of this reaction through an [internal combustion engine](/source/internal_combustion_engine) with an upper efficiency of 60% (for compression ratio of 10 and specific heat ratio of 1.4) based on the [Otto](/source/Otto_cycle) [thermodynamic cycle](/source/thermodynamic_cycle).  It is also possible to combine the hydrogen and oxygen through redox mechanism as in the case of a [fuel cell](/source/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.<ref>{{Cite journal|last=Kunze|first=Julia|author2=Ulrich Stimming|year=2009|title=Electrochemical Versus Heat-Engine Energy Technology: A Tribute to Wilhelm Ostwald's Visionary Statements|journal=Angewandte Chemie International Edition|volume=48|issue=49|pages=9230–9237|doi=10.1002/anie.200903603|pmid=19894237|doi-access=free}}<!--| access-date = 2009-12-04--></ref><ref name="video">{{cite web|last=Haverkamp|first=Richard|date=3 June 2008|title=What is an electrocatalyst?|url=http://www.sciencelearn.org.nz/contexts/nanoscience/sci_media/video/what_is_an_electrocatalyst|access-date=27 February 2009|website=|publisher=Science learning New Zealand|format=QuickTime video and transcript|archive-date=29 April 2023|archive-url=https://web.archive.org/web/20230429203758/https://www.sciencelearn.org.nz/contexts/nanoscience/sci_media/video/what_is_an_electrocatalyst}}</ref>

The standard reduction potential of hydrogen is defined as 0V, and frequently referred to as the [standard hydrogen electrode](/source/standard_hydrogen_electrode) (SHE).<ref>{{Cite journal|last1=Elgrishi|first1=Noémie|last2=Rountree|first2=Kelley J.|last3=McCarthy|first3=Brian D.|last4=Rountree|first4=Eric S.|last5=Eisenhart|first5=Thomas T.|last6=Dempsey|first6=Jillian L.|date=2018-02-13|title=A Practical Beginner's Guide to Cyclic Voltammetry|journal=Journal of Chemical Education|language=en|volume=95|issue=2|pages=197–206|doi=10.1021/acs.jchemed.7b00361|bibcode=2018JChEd..95..197E |issn=0021-9584|doi-access=free}}</ref>
{| class="wikitable"
|+
!Half Reaction
!Reduction Potential
E<sup>o</sup><sub>red</sub> (V)
|-
|'''2H<sup>+</sup> + 2e<sup>−</sup> → H<sub>2 (g)</sub>'''
|'''≡ 0'''
|-
|'''O<sub>2(g)</sub> + 4H<sup>+</sup> + 4e<sup>−</sup> → 2H<sub>2</sub>O<sub>(l)</sub>'''
|'''+1.23'''
|} HER<ref name=":12">{{Cite journal|last1=Artero|first1=Vincent|last2=Chavarot-Kerlidou|first2=Murielle|last3=Fontecave|first3=Marc|date=2011-08-01|title=Splitting Water with Cobalt|url=http://doi.wiley.com/10.1002/anie.201007987|journal=Angewandte Chemie International Edition|language=en|volume=50|issue=32|pages=7238–7266|doi=10.1002/anie.201007987|pmid=21748828 |url-access=subscription}}</ref> can be promoted by many catalysts.<ref name=":12"/>

=== Carbon dioxide reduction ===
{{Main|Electrochemical reduction of carbon dioxide}}
Electrocatalysis for CO<sub>2</sub> reduction is not practiced commercially but remains a topic of research. The reduction of CO<sub>2</sub> into useable products is a potential way to combat [climate change](/source/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 CO<sub>2</sub> 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.<ref name=":62"/> Homogeneous catalysts, such as enzymes<ref name=":72"/> and synthetic coordination complexes<ref name=":62" /> have been employed for this purpose. A variety of nanomaterials have also been studied for CO<sub>2</sub> reduction, including carbon-based materials and framework materials.<ref>{{Cite journal|last1=Pan|first1=Fuping|last2=Yang|first2=Yang|date=2020|title=Designing CO 2 reduction electrode materials by morphology and interface engineering|url=http://xlink.rsc.org/?DOI=D0EE00900H|journal=Energy & Environmental Science|language=en|volume=13|issue=8|pages=2275–2309|doi=10.1039/D0EE00900H|bibcode=2020EnEnS..13.2275P |s2cid=219737955 |issn=1754-5692|url-access=subscription}}</ref>

=== Ethanol-powered fuel cells ===
Aqueous solutions of methanol can decompose into CO<sub>2</sub> 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.<ref name=":9" /> Electrocatalysts such as gold, platinum, and various carbon-based materials have been shown to effectively catalyze this process. An electrocatalyst of [platinum](/source/platinum) and [rhodium](/source/rhodium) on carbon backed tin-dioxide nanoparticles can break [carbon bonds](/source/Carbon-carbon_bond) at room temperature with only [carbon dioxide](/source/carbon_dioxide) as a by-product, so that [ethanol](/source/ethanol) can be oxidized into the necessary hydrogen ions and electrons required to create electricity.<ref name="Booze-power">
{{cite web
 |last        = Harris
 |first       = Mark
 |title       = Booze-powered cars coming soon
 |publisher   = techradar.com
 |date        = 26 January 2009
 |url         = http://www.techradar.com/news/world-of-tech/booze-powered-cars-coming-soon-513666
 |access-date  = 27 February 2009
 |archive-url  = https://web.archive.org/web/20090302025300/http://www.techradar.com/news/world-of-tech/booze-powered-cars-coming-soon-513666
 |archive-date = 2 March 2009
}}
</ref>

=== 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.<ref>{{Cite journal|last=Sachdeva|first=Harshita|date=2020-09-30|title=Recent advances in the catalytic applications of GO/rGO for green organic synthesis|journal=Green Processing and Synthesis|volume=9|issue=1|pages=515–537|doi=10.1515/gps-2020-0055|issn=2191-9550|doi-access=free}}</ref> Electrocatalytic methods also have potential for polymer synthesis.<ref>{{Cite journal|last1=Siu|first1=Juno C.|last2=Fu|first2=Niankai|last3=Lin|first3=Song|date=2020-03-17|title=Catalyzing Electrosynthesis: A Homogeneous Electrocatalytic Approach to Reaction Discovery|url= |journal=Accounts of Chemical Research|language=en|volume=53|issue=3|pages=547–560|doi=10.1021/acs.accounts.9b00529|issn=0001-4842|pmc=7245362|pmid=32077681}}</ref> 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.<ref>{{Cite journal|last1=Holade|first1=Yaovi|last2=Servat|first2=Karine|last3=Tingry|first3=Sophie|last4=Napporn|first4=Teko W.|last5=Remita|first5=Hynd|last6=Cornu|first6=David|last7=Kokoh|first7=K. Boniface|date=2017-10-06|title=Advances in Electrocatalysis for Energy Conversion and Synthesis of Organic Molecules|journal=ChemPhysChem|language=en|volume=18|issue=19|pages=2573–2605|doi=10.1002/cphc.201700447|pmid=28732139 |issn=1439-4235|doi-access=free}}</ref>

===Advanced oxidation processes in water treatment===
Water treatment systems often require the degradation of hazardous compounds. These treatment processes are dubbed [Advanced oxidation process](/source/Advanced_oxidation_process)es, 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](/source/PFAS)<ref name="Ji Choi Fang Pham 2023 p.">{{cite journal | last1=Ji | first1=Yangyuan | last2=Choi | first2=Youn Jeong | last3=Fang | first3=Yuhang | last4=Pham | first4=Hoang Son | last5=Nou | first5=Alliyan Tan | last6=Lee | first6=Linda S. | last7=Niu | first7=Junfeng | last8=Warsinger | first8=David M. | title=Electric Field-Assisted Nanofiltration for PFOA Removal with Exceptional Flux, Selectivity, and Destruction | journal=Environmental Science & Technology | publisher=American Chemical Society (ACS) | date=2023-01-19 | volume=57 | issue=47 | pages=18519–18528 | issn=0013-936X | doi=10.1021/acs.est.2c04874 | pmid=36657468 | bibcode=2023EnST...5718519J | s2cid=256030682 }}</ref>

==Additional reading==
*{{cite journal |title=Co-axial heterostructures integrating palladium/titanium dioxide with carbon nanotubes for efficient electrocatalytic hydrogen evolution |year=2016 |last1=Valenti |first1=G. |last2=Boni |first2=A. |last3=Melchionna |first3=M. |last4=Cargnello |first4=M. |last5=Nasi |first5=L. |last6=Bertoli |first6=G. |last7=Gorte |first7=R. J. |last8=Marcaccio |first8=M. |last9=Rapino |first9=S. |last10=Bonchio |first10=M. |last11=Fornasiero |first11=P. |last12=Prato |first12=M. |last13=Paolucci |first13=F. |journal=Nature Communications |volume=7 |article-number=13549|bibcode=2016NatCo...713549V |doi=10.1038/ncomms13549 |pmid=27941752 |pmc=5159813 }}<!--why is this article so importantt?-->

== See also ==
* [Electrochemistry](/source/Electrochemistry)
*[Catalysis](/source/Catalysis)
*[Electrolysis of water](/source/Electrolysis_of_water)
*[Non-faradaic electrochemical modification of catalytic activity](/source/Non-faradaic_electrochemical_modification_of_catalytic_activity)
*[Tafel equation](/source/Tafel_equation)

==References==

{{Reflist}}

Category:Electrochemistry
Category:Catalysis

---
Adapted from the Wikipedia article [Electrocatalyst](https://en.wikipedia.org/wiki/Electrocatalyst) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Electrocatalyst?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.