Electrochemical surface area

The knowledge of the electrochemical surface area (ECSA) is a crucial parameter for catalyst characterization, comparison and benchmarking. The electrochemical surface area is commonly computed as{{cite journal |last1=Connor |first1=Paula |last2=Schuch |first2=Jona |last3=Kaiser |first3=Bernhard |date=2020-01-13 |title=The Determination of Electrochemical Active Surface Area and Specific Capacity Revisited for the System MnO_x as an Oxygen Evolution Catalyst |url=https://doi.org/10.1515/zpch-2019-1514 |volume=234 |doi=10.1515/zpch-2019-1514 |access-date=2025-07-08 |journal=Zeitschrift für Physikalische Chemie |number=5 |pages=979–994}}{{Cite journal |last1=Cignoni |first1=Paolo |last2=Hosseini |first2=Pouya |last3=Kaiser |first3=Christoph |last4=Trost |first4=Oliver |last5=Nettler |first5=Dean-Robin |last6=Trzebiatowski |first6=Lisa |last7=Tschulik |first7=Kristina |date=2023-11-01 |title=Validating Electrochemical Active Surface Area Determination of Nanostructured Electrodes: Surface Oxide Reduction on AuPd Nanoparticles |url=https://iopscience.iop.org/article/10.1149/1945-7111/ad09f8 |journal=Journal of the Electrochemical Society |volume=170 |issue=11 |pages=116505 |doi=10.1149/1945-7111/ad09f8 |bibcode=2023JElS..170k6505C |issn=0013-4651}}:

$\backslash text\{ECSA\}\; =\; \backslash frac\{Q\_\{\backslash text\{measured\}\}\}\{Q\_\{\backslash text\{ref\}\}\}$

where:

• $Q\_\{measured\}$ is the total charge transferred during the adsorption/desorption process of a probe species on the catalyst.
• $Q\_\{ref\}$ is the specific charge density which is the charge required to cover one unit of active surface with the adsorbed species and it is used as a reference value.

This is a consolidated method in the PEM fuel cell field since the adsorption/desorption of hydrogen and CO on Pt nanoparticles is well known{{Cite journal |last1=Moniri |first1=Saman |last2=Van Cleve |first2=Timothy |last3=Linic |first3=Suljo |date=November 2016 |title=Pitfalls and best practices in measurements of the electrochemical surface area of platinum-based nanostructured electro-catalysts |url=https://linkinghub.elsevier.com/retrieve/pii/S002195171630272X |journal=Journal of Catalysis |language=en |volume=345 |pages=1–10 |doi=10.1016/j.jcat.2016.11.018}}. However, in some applications the adsorption/desorption processes are not clear as for non-Pt catalysts, alkaline electrolytes or supercapacitors. In those cases, ECSA estimation can be based on the double-layer capacitance according to the following equation{{Cite journal |last1=Connor |first1=Paula |last2=Schuch |first2=Jona |last3=Kaiser |first3=Bernhard |last4=Jaegermann |first4=Wolfram |date=2020-05-26 |title=The Determination of Electrochemical Active Surface Area and Specific Capacity Revisited for the System MnO x as an Oxygen Evolution Catalyst |url=https://www.degruyter.com/document/doi/10.1515/zpch-2019-1514/html |journal=Zeitschrift für Physikalische Chemie |language=en |volume=234 |issue=5 |pages=979–994 |doi=10.1515/zpch-2019-1514 |issn=2196-7156}}:

$\backslash text\{ECSA\}\; =\; \backslash frac\{C\_\{\backslash text\{DL\}\}\}\{C\_\{\backslash text\{S\}\}\}$

where:

• $C\_\{DL\}$ is the double-layer capacitance.

• $C\_S$ is the specific capacitance which is the capacitance of an ideal flat surface of the catalyst.

ECSA is typically expressed in square centimeters but it is common to normalize it to the geometric surface area (the ratio between ECSA and geometrical surface area is referred to as roughness factor){{Cite journal |last1=Della Bella |first1=Roberta K. F. |last2=Stühmeier |first2=Björn M. |last3=Gasteiger |first3=Hubert A. |date=2022-04-01 |title=Universal Correlation between Cathode Roughness Factor and H 2 /Air Performance Losses in Voltage Cycling-Based Accelerated Stress Tests |url=https://iopscience.iop.org/article/10.1149/1945-7111/ac67b8 |journal=Journal of the Electrochemical Society |volume=169 |issue=4 |pages=044528 |doi=10.1149/1945-7111/ac67b8 |issn=0013-4651}} or to the catalyst loading (square centimeters per milligram of catalyst).

The ECSA serves as a critical input for the evaluation of key performance parameters such as specific activity and mass activity{{Cite journal |last1=Cruz-Martínez |first1=H. |last2=Rojas-Chávez |first2=H. |last3=Matadamas-Ortiz |first3=P. T. |last4=Ortiz-Herrera |first4=J. C. |last5=López-Chávez |first5=E. |last6=Solorza-Feria |first6=O. |last7=Medina |first7=D. I. |date=2021-07-01 |title=Current progress of Pt-based ORR electrocatalysts for PEMFCs: An integrated view combining theory and experiment |url=https://www.sciencedirect.com/science/article/pii/S2542529321000675 |journal=Materials Today Physics |volume=19 |article-number=100406 |doi=10.1016/j.mtphys.2021.100406 |bibcode=2021MTPhy..1900406C |issn=2542-5293}}. The specific activity is defined as the ratio between the total current and the electrochemical surface area, and it is a useful indicator for the intrinsic activity of the catalyst{{Cite journal |last1=Gasteiger |first1=Hubert A. |last2=Kocha |first2=Shyam S. |last3=Sompalli |first3=Bhaskar |last4=Wagner |first4=Frederick T. |date=March 2005 |title=Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs |url=https://linkinghub.elsevier.com/retrieve/pii/S0926337304004941 |journal=Applied Catalysis B: Environmental |language=en |volume=56 |issue=1–2 |pages=9–35 |doi=10.1016/j.apcatb.2004.06.021 |bibcode=2005AppCB..56....9G }}{{Cite journal |last1=Garsany |first1=Yannick |last2=Baturina |first2=Olga A. |last3=Swider-Lyons |first3=Karen E. |last4=Kocha |first4=Shyam S. |date=2010-08-01 |title=Experimental Methods for Quantifying the Activity of Platinum Electrocatalysts for the Oxygen Reduction Reaction |url=https://pubs.acs.org/doi/10.1021/ac100306c |journal=Analytical Chemistry |language=en |volume=82 |issue=15 |pages=6321–6328 |doi=10.1021/ac100306c |pmid=20590161 |issn=0003-2700}}. The higher the value, the better the performance of the catalytic material. The mass activity is instead defined as the current generated per unit mass of the catalyst active material. It is important for evaluating the utilization efficiency of the catalyst, especially in applications where the catalyst material used is expensive (e.g. noble metals as Pt and Ir). Furthermore, the mass activity allows for a fair comparison between the different catalytic materials regardless of their amount and it is a reference parameter for the design of electrodes. Finally, the specific and mass activities are related by the following equation:

$\backslash text\{Mass\; activity\}\; =\; \backslash text\{Specific\; activity\; x\; \}\; \backslash frac\{ECSA\}\{\backslash text\{mass\; of\; catalyst\; material\}\}$

There are multiple methodologies for evaluating the electrochemical surface area (ECSA). The choice of the most appropriate method depends on the catalyst material, the operating environment and the nature of the electrochemical reaction{{Cite journal |last1=Łukaszewski |first1=M. |last2=Soszko |first2=M. |last3=Czerwiński |first3=A. |date=June 2016 |title=Electrochemical Methods of Real Surface Area Determination of Noble Metal Electrodes – an Overview |url=https://linkinghub.elsevier.com/retrieve/pii/S1452398123174918 |journal=International Journal of Electrochemical Science |language=en |volume=11 |issue=6 |pages=4442–4469 |doi=10.20964/2016.06.71}}. All the methods can be categorized into:

• Faradaic methods which involve redox processes and are based on measurements of charge carrier transfer.
• Non-faradaic or capacitive methods which are based on the measurement of the electrochemical double-layer capacitance under conditions with no faradaic current contributions.

Both methods usually require performing cyclic voltammetry (CV) which is the most widely adopted diagnostic technique to measure the ECSA due to its simplicity, rapidity and minimal equipment requirements. A CV is performed by scanning cyclically the electrode potential within a defined range, which must be selected depending on the electrochemical characteristics of the catalyst under study in order to avoid any electrolyte decomposition and electrode degradation and to ensure a reliable estimation of the ECSA, and by measuring the electrochemical current response of the electrode{{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 |url=https://doi.org/10.1021/acs.jchemed.7b00361 |journal=Journal of Chemical Education |volume=95 |issue=2 |pages=197–206 |doi=10.1021/acs.jchemed.7b00361 |bibcode=2018JChEd..95..197E |osti=1408158 |issn=0021-9584}}. However, not all the methods rely on CV such as the impedance-based methods as shown in this section.

File:Cyclic voltammetry.png

This method relies on the measurement of the charge associated with hydrogen adsorption (H_ads) and desorption (H_des) on the catalyst surface measured during a cyclic voltammetry (CV). In particular, it considers the charge transferred during the hydrogen desorption corrected for the double-layer charging. Mathematically, this is the integral of the hydrogen desorption peak in the voltammetry response minus the double-layer charging contribution. In figure is shown a CV curve for a generic Pt-based electrode of a hydrogen PEM fuel cell with the area evidenced in gray corresponding to the charge transferred.

The assumption behind this method are:

• Hydrogen forms a monolayer on the catalyst surface, with one hydrogen atom adsorbed per active site.
• The charge transferred in an adsorption/desorption of each hydrogen atom corresponds to one electron.
• All catalytic surface sites are considered equally accessible to hydrogen adsorption and equally electrochemically active.
• No alteration of the surface upon adsorption takes place.

These assumptions generally hold for clean, well-defined platinum surfaces under controlled conditions. However, deviations can occur in the case of alloy catalysts, highly porous electrodes or in the presence of contaminants and irreversible processes which make charge transfer quantification unreliable{{Cite journal |last1=Łukaszewski |first1=M. |last2=Grdeń |first2=M. |last3=Czerwiński |first3=A. |date=2004-11-15 |title=Hydrogen electrosorption in Pd–Pt–Rh alloys |url=https://doi.org/10.1016/s0022-0728(04)00342-0 |journal=Journal of Electroanalytical Chemistry |volume=573 |issue=1 |pages=87–98 |doi=10.1016/s0022-0728(04)00342-0 |issn=0022-0728}}{{Cite journal |last1=Łukaszewski |first1=M. |last2=Grdeń |first2=M. |last3=Czerwiński |first3=A. |date=2005-01-01 |title=Cyclic voltammetric behavior of Pd–Pt–Rh ternary alloys |url=https://doi.org/10.1007/s10008-004-0528-7 |journal=Journal of Solid State Electrochemistry |language=en |volume=9 |issue=1 |pages=1–9 |doi=10.1007/s10008-004-0528-7 |issn=1433-0768}}. This method is the most consolidated one in PEM fuel cell with Pt/C electrodes due to its simplicity and reliability. However, it is not applicable to non-Pt catalysts or to systems operating in alkaline electrolytes.

This method is based on the measurement of the charge associated with the electrochemical reduction of metal oxide species formed on the catalyst surface during a cyclic voltammetry (CV) scan. The charge transferred is computed by integrating the oxide reduction peak after subtracting the non-faradaic contribution (as in the case of hydrogen adsorption/desorption method). This contribution is the area colored in red in the previous figure.

The assumptions behind this method are very similar to the ones made for hydrogen adsorption/desorption method{{Cite journal |last=Woods |first=R. |date=1979 |title=The Properties of Oxide Layers Formed on Iridium, Rhodium and Ruthenium Electrodes During Potential Cycling |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/ijch.197900014 |journal=Israel Journal of Chemistry |language=en |volume=18 |issue=1–2 |pages=118–124 |doi=10.1002/ijch.197900014 |issn=1869-5868}}:

• Oxygen is adsorbed in a monoatomic layer with a one-to-one correspondence with surface metal atoms.
• The number of electrons transferred per oxide atom during reduction is known
• All sites involved in oxide formation and reduction are electrochemically active and accessible.
• No alteration of the surface upon adsorption takes place.

The method is generally regarded as less reliable than the one based on hydrogen adsorption, mainly because the validity of these assumptions is weaker. Nevertheless, this method is widely used for electrodes where hydrogen adsorption is hindered as Pd and Au electrodes{{Cite journal |last1=Łukaszewski |first1=M. |last2=Czerwiński |first2=A. |date=2003-07-15 |title=Electrochemical behavior of palladium–gold alloys |url=https://www.sciencedirect.com/science/article/pii/S0013468603002706 |journal=Electrochimica Acta |volume=48 |issue=17 |pages=2435–2445 |doi=10.1016/S0013-4686(03)00270-6 |issn=0013-4686}}.

File:Co stripping.png

The carbon monoxide (CO) has a very high affinity with many metals and tends to be adsorbed on noble metal catalyst surface in the potential window typical of electrochemical devices{{Cite journal |last1=Binninger |first1=T. |last2=Fabbri |first2=E. |last3=Kötz |first3=R. |last4=Schmidt |first4=T. J. |date=2014 |title=Determination of the Electrochemically Active Surface Area of Metal-Oxide Supported Platinum Catalyst |url=https://iopscience.iop.org/article/10.1149/2.055403jes |journal=Journal of the Electrochemical Society |language=en |volume=161 |issue=3 |pages=H121–H128 |doi=10.1149/2.055403jes |issn=0013-4651}}. This method exploit the oxidation of adsorbed CO on metal catalyst surface and consists of two steps{{Cite journal |last1=Garrick |first1=Taylor R. |last2=Moylan |first2=Thomas E. |last3=Carpenter |first3=Michael K. |last4=Kongkanand |first4=Anusorn |date=2017 |title=Editors' Choice—Electrochemically Active Surface Area Measurement of Aged Pt Alloy Catalysts in PEM Fuel Cells by CO Stripping |url=https://iopscience.iop.org/article/10.1149/2.0381702jes |journal=Journal of the Electrochemical Society |language=en |volume=164 |issue=2 |pages=F55–F59 |doi=10.1149/2.0381702jes |issn=0013-4651}}. First, the catalyst is exposed to CO contamination in order to form a monolayer of carbon monoxide on the electrode. Afterwards, a CV scan is performed in an inert atmosphere to oxidize the adsorbed CO, process that takes name of CO stripping{{Cite journal |last1=Binninger |first1=T. |last2=Fabbri |first2=E. |last3=Kötz |first3=R. |last4=Schmidt |first4=T. J. |date=2014 |title=Determination of the Electrochemically Active Surface Area of Metal-Oxide Supported Platinum Catalyst |url=https://iopscience.iop.org/article/10.1149/2.055403jes |journal=Journal of the Electrochemical Society |language=en |volume=161 |issue=3 |pages=H121–H128 |doi=10.1149/2.055403jes |issn=0013-4651}}. The latter results in an oxidation peak in the CV curve as shown in the figure. The CO stripping charge is quantified as the difference between the total anodic charge in the potential range of CO stripping and the charge transferred in the same potential range in the absence of adsorbed CO. By looking at the figure, the first value is the integral of the red curve, while the second value is the integral of the grey curve{{Cite journal |last1=Pozio |first1=A |last2=De Francesco |first2=M |last3=Cemmi |first3=A |last4=Cardellini |first4=F |last5=Giorgi |first5=L |date=March 2002 |title=Comparison of high surface Pt/C catalysts by cyclic voltammetry |url=https://linkinghub.elsevier.com/retrieve/pii/S0378775301009211 |journal=Journal of Power Sources |language=en |volume=105 |issue=1 |pages=13–19 |doi=10.1016/S0378-7753(01)00921-1 |bibcode=2002JPS...105...13P }}.

The CO stripping voltammetry is valuable under the same assumptions of the previous methods applied to carbon monoxide. The main concern about this method is the possibility of electrode surface and catalytic properties alteration due to CO adsorption which could result in an overestimation or underestimation of the ECSA{{Cite journal |last1=Chen |first1=Dong |last2=Tao |first2=Qian |last3=Liao |first3=Ling Wen |last4=Liu |first4=Shao Xiong |last5=Chen |first5=Yan Xia |last6=Ye |first6=Shen |date=October 2011 |title=Determining the Active Surface Area for Various Platinum Electrodes |url=http://link.springer.com/10.1007/s12678-011-0054-1 |journal=Electrocatalysis |language=en |volume=2 |issue=3 |pages=207–219 |doi=10.1007/s12678-011-0054-1 |issn=1868-2529}}. CO stripping is widely adopted for alloy catalysts (e.g. Pt–Co, Pt–Ni) where hydrogen adsorption/desorption method yields inaccurate ECSA quantification{{Cite journal |last1=Rudi |first1=Stefan |last2=Cui |first2=Chunhua |last3=Gan |first3=Lin |last4=Strasser |first4=Peter |date=2014-06-10 |title=Comparative Study of the Electrocatalytically Active Surface Areas (ECSAs) of Pt Alloy Nanoparticles Evaluated by Hupd and CO-stripping voltammetry |url=https://doi.org/10.1007/s12678-014-0205-2 |journal=Electrocatalysis |volume=5 |issue=4 |pages=408–418 |doi=10.1007/s12678-014-0205-2 |issn=1868-2529}}.

This method estimates the electrochemical surface area by measuring the double-layer capacitance of the catalyst in an electrode potential range where the faradaic contribution is null or negligible. The most common procedure consists of performing cyclic voltammetry scans at different scan rates within a range of electrode potentials where the current response is purely capacitive. Then, a plot of the capacitive currents versus scan rate is made and the slope of the resulting curve, which in a purely capacitive response is linear, corresponds to the double-layer differential capacity according to{{Cite journal |last1=Mathi |first1=Selvam |last2=Jayabharathi |first2=Jayaraman |date=2020 |title=Enhanced stability and ultrahigh activity of amorphous ripple nanostructured Ni-doped Fe oxyhydroxide electrode toward synergetic electrocatalytic water splitting |journal=RSC Advances |language=en |volume=10 |issue=44 |pages=26364–26373 |doi=10.1039/D0RA04828C |pmid=35519769 |issn=2046-2069 |pmc=9055439 |bibcode=2020RSCAd..1026364M }}{{Cite journal |last1=Serapinienė |first1=Birutė |last2=Gudavičiūtė |first2=Laima |last3=Tutlienė |first3=Skirmantė |last4=Grigucevičienė |first4=Asta |last5=Selskis |first5=Algirdas |last6=Juodkazytė |first6=Jurga |last7=Ramanauskas |first7=Rimantas |date=2023-07-29 |title=On the Electrochemically Active Surface Area Determination of Electrodeposited Porous Cu 3D Nanostructures |journal=Coatings |language=en |volume=13 |issue=8 |pages=1335 |doi=10.3390/coatings13081335 |doi-access=free |issn=2079-6412}}:

$i\; =\; \backslash frac\{dQ\}\{dt\}\; =\; \backslash left(\backslash frac\{dQ\}\{dE\}\backslash right)\; \backslash cdot\; \backslash left(\backslash frac\{dE\}\{dt\}\backslash right)\; =\; C\; \backslash cdot\; v$

where:

• $i$ is the measured current, in Ampere.
• $Q$ is the accumulated charge, in Coulomb.
• $t$ is time, in seconds.
• $E$ is the electrode potential, in Volts.
• $C$ is the double-layer capacitance, in Farad.
• $v$ is the scan rate in Volts per second.

Finally, the ECSA is obtained by dividing $C$ for the reference value of capacity per the unit area ($C\_\{ref\}$).

The assumptions behind this method are:

• The current response in the selected potential window is purely capacitive meaning that no faradaic reactions occur.
• The surface behaves like an ideal capacitor and the $C\_\{DL\}$ is linearly proportional to the electrochemically active surface area.
• The reference specific capacitance ($C\_\{ref\}$) is constant and accurately known.
• The electrode surface is uniformly accessible to the electrolyte.

The main source of inaccuracy comes from the identification of $C\_\{ref\}$. In general, the latter is sensitive to the electrode potential, surface structure, electrolyte composition and concentration and experimental conditions{{Cite journal |last1=McCrory |first1=Charles C. L. |last2=Jung |first2=Suho |last3=Peters |first3=Jonas C. |last4=Jaramillo |first4=Thomas F. |date=2013-11-13 |title=Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction |url=https://doi.org/10.1021/ja407115p |journal=Journal of the American Chemical Society |volume=135 |issue=45 |pages=16977–16987 |doi=10.1021/ja407115p |bibcode=2013JAChS.13516977M |issn=0002-7863}}. As a result, significant discrepancies in $C\_\{ref\}$ values are reported in the literature making this method not so reliable for accurate ECSA quantification. Despite these limitations, the double-layer capacitance method remains a widely adopted approach due to its simplicity and applicability to all the catalyst materials for which a change surface state could occur upon oxide formation/reduction and metal dissolution/redeposition{{Cite journal |last1=Yin |first1=Shuli |last2=Liu |first2=Songliang |last3=Zhang |first3=Hugang |last4=Jiao |first4=Shiqian |last5=Xu |first5=You |last6=Wang |first6=Ziqiang |last7=Li |first7=Xiaonian |last8=Wang |first8=Liang |last9=Wang |first9=Hongjing |date=2021-05-05 |title=Engineering One-Dimensional AuPd Nanospikes for Efficient Electrocatalytic Nitrogen Fixation |url=https://doi.org/10.1021/acsami.1c04619 |journal=ACS Applied Materials & Interfaces |volume=13 |issue=17 |pages=20233–20239 |doi=10.1021/acsami.1c04619 |pmid=33884861 |issn=1944-8244}}{{Cite journal |last1=Łukaszewski |first1=M. |last2=Czerwiński |first2=A. |date=2010-05-03 |title=Electrochemical preparation and characterization of thin deposits of Pd-noble metal alloys |url=https://www.sciencedirect.com/science/article/pii/S0040609009015922 |journal=Thin Solid Films |volume=518 |issue=14 |pages=3680–3689 |doi=10.1016/j.tsf.2009.10.008 |bibcode=2010TSF...518.3680L |issn=0040-6090}}.

This method is based on the electrochemical adsorption of a metallic monolayer onto the surface of a more noble metal substrate at a potential more positive than the Nernst equilibrium potential for bulk metal deposition. This process is called underpotential deposition (UPD) and takes place when the affinity of the adsorbing metal to itself in its metallic phase is lower than the affinity of the adsorbing metal onto the metal substrate{{Cite journal |last1=Franklin |first1=Thomas C. |last2=Franklin |first2=Nellie F. |date=1976-09-01 |title=The use of underpotential deposition to measure the surface area of metals |url=https://dx.doi.org/10.1016/0376-4583%2876%2990057-1 |journal=Surface Technology |volume=4 |issue=5 |pages=431–440 |doi=10.1016/0376-4583(76)90057-1 |issn=0376-4583}}{{Cite journal |last1=Lamy-Pitara |first1=E. |last2=Barbier |first2=J. |date=1997-01-23 |title=Platinum modified by electrochemical deposition of adatoms |url=https://www.sciencedirect.com/science/article/pii/S0926860X96003079 |journal=Applied Catalysis A: General |volume=149 |issue=1 |pages=49–87 |doi=10.1016/S0926-860X(96)00307-9 |bibcode=1997AppCA.149...49L |issn=0926-860X}}{{Cite journal |last1=Watt-Smith |first1=M J |last2=Friedrich |first2=J M |last3=Rigby |first3=S P |last4=Ralph |first4=T R |last5=Walsh |first5=F C |date=2008-09-07 |title=Determination of the electrochemically active surface area of Pt/C PEM fuel cell electrodes using different adsorbates |url=https://iopscience.iop.org/article/10.1088/0022-3727/41/17/174004 |journal=Journal of Physics D: Applied Physics |volume=41 |issue=17 |pages=174004 |doi=10.1088/0022-3727/41/17/174004 |bibcode=2008JPhD...41q4004W |issn=0022-3727}}.

In practice, the UPD method consists of depositing a sub-monolayer or monolayer of a foreign metal (typically Cu, Pb, or Bi) onto the electrode surface and then measuring the charge associated with the stripping (oxidation) of the deposited species. The ECSA is calculated by integrating the UPD stripping peak and using a known reference charge corresponding to a full monolayer coverage.

The assumptions behind this method are:

• The UPD process forms a well-defined monolayer with a known surface coverage and stoichiometry.
• The interaction between the UPD species and the surface is uniform and does not lead to alloying or surface reconstruction.

The main challenges in calculating the charge associated with the deposited metal are related to the accuracy in correcting for double-layer capacitance and the hydrogen or oxygen adsorption, and in identifying the potential at which the monolayer of metal adatoms is fully formed.

The UPD method is particularly useful for electrodes where neither hydrogen adsorption nor oxide formation yields reliable electrochemical surface area estimation, such as non-platinum group metals or alloy systems with modified surface chemistries.

===Impedance-based methods===

This method differs from the double-layer capacitance one only in the way the $C\_\{DL\}$ is quantified. Instead of performing a cyclic voltammetry, here the double-layer capacitance is measured through the electrochemical impedance spectroscopy (EIS) which is a consolidate and standard procedure for electrochemical characterization. The measured impedance data is fitted to an equivalent electrical circuit model that includes a double-layer capacitance element, typically in parallel with a charge transfer resistance and other components such as Warburg impedance for diffusion effects{{Cite journal |last1=Schalenbach |first1=Maximilian |last2=Durmus |first2=Yassin Emre |last3=Tempel |first3=Hermann |last4=Kungl |first4=Hans |last5=Eichel |first5=Rüdiger-A. |date=2021 |title=Double layer capacitances analysed with impedance spectroscopy and cyclic voltammetry: validity and limits of the constant phase element parameterization |url=https://xlink.rsc.org/?DOI=D1CP03381F |journal=Physical Chemistry Chemical Physics |language=en |volume=23 |issue=37 |pages=21097–21105 |doi=10.1039/D1CP03381F |pmid=34523643 |bibcode=2021PCCP...2321097S |issn=1463-9076}}{{Cite journal |last1=Watzele |first1=Sebastian |last2=Hauenstein |first2=Pascal |last3=Liang |first3=Yunchang |last4=Xue |first4=Song |last5=Fichtner |first5=Johannes |last6=Garlyyev |first6=Batyr |last7=Scieszka |first7=Daniel |last8=Claudel |first8=Fabien |last9=Maillard |first9=Frédéric |last10=Bandarenka |first10=Aliaksandr S. |date=2019-10-04 |title=Determination of Electroactive Surface Area of Ni-, Co-, Fe-, and Ir-Based Oxide Electrocatalysts |url=https://doi.org/10.1021/acscatal.9b02006 |journal=ACS Catalysis |volume=9 |issue=10 |pages=9222–9230 |doi=10.1021/acscatal.9b02006}}.

The assumptions behind impedance-based methods include:

• The equivalent circuit correctly represents the physical and electrochemical behavior of the system.
• The measured capacitance arises solely from the electrochemical double layer and not from pseudocapacitive or faradaic contributions.
• The surface roughness, porosity, and frequency dispersion are adequately accounted in the model adopted.

This method is a valid alternative for systems where CV lead to unreliable results. However, its accuracy depends strongly on the validity of the equivalent circuit model used and the quality of the impedance data over a wide frequency range. Despite these limitations, impedance-based ECSA evaluation is widely adopted in the fields of supercapacitors and batteries.

• Electrocatalyst
• Cyclic voltammetry
• Electrochemical impedance spectroscopy
• Fuel cell
• Electrolyzer
• Double-layer capacitance

{{reflist}}

• {{cite journal |last1=Trasatti |first1=S. |last2=Petrii |first2=O. A. |date=1991-01-01 |title=Real surface area measurements in electrochemistry |journal=Pure and Applied Chemistry |volume=63 |issue=5 |pages=711–734 |doi=10.1351/pac199163050711 |issn=1365-3075 |doi-access=free}}
• {{cite journal |last=Johnson |first=Andi |date=2025 |title=Electrochemical Surface Area (ECSA) Evaluation in Electrocatalysis: Principles, Measurement Techniques, and Future Perspectives |url=https://www.degruyterbrill.com/document/doi/10.1351/pac199163050711/html |journal=Journal of Engineering in Industrial Research |volume=6 |issue=3 |pages=212–222 |doi=10.48309/jeires.2025.513554.1182}}
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• {{Cite web |last1=Bracher |first1=Chris |last2=Robson |first2=Harry |last3=Reinhardt |first3=Max |date=July 2025 |title=Cyclic Voltammetry Basic Principles and Theory |url=https://www.ossila.com/pages/cyclic-voltammetry |access-date=11 July 2025 |website=Ossila enabling science}}

Category:Electrochemical cells

Category:Electrochemistry

Category:Electrodes

Category:Redox