CO stripping

The working principle relies on the ability of certain metals, such as platinum, to readily adsorb carbon monoxide, a process typically considered undesirable as it results in catalyst poisoning by blocking the active sites and causing a loss of activity.{{Cite journal |last=Forzatti |first=Pio |last2=Lietti |first2=Luca |date=1999-09-14 |title=Catalyst deactivation |url=https://www.sciencedirect.com/science/article/pii/S0920586199000747 |journal=Catalysis Today |volume=52 |issue=2 |pages=165–181 |doi=10.1016/S0920-5861(99)00074-7 |issn=0920-5861}}

However, the strong affinity of CO to such catalysts also presents an opportunity: since carbon monoxide is a small molecule with a strong affinity to the catalyst, a large enough amount of CO will adsorb to the entire available surface area. That, in turn, means that the catalyst's available surface area can be indirectly measured by evaluating the amount of CO adsorbed.{{Cite journal |last=Garrick |first=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}} Moreover, insights on the electrode structure can be found.{{Cite journal |last=Takeshita |first=Tomohiro |last2=Kamitaka |first2=Yuji |last3=Shinozaki |first3=Kazuma |last4=Kodama |first4=Kensaku |last5=Morimoto |first5=Yu |date=2020-08-15 |title=Evaluation of ionomer coverage on Pt catalysts in polymer electrolyte membrane fuel cells by CO stripping voltammetry and its effect on oxygen reduction reaction activity |url=https://www.sciencedirect.com/science/article/pii/S1572665720304781 |journal=Journal of Electroanalytical Chemistry |volume=871 |pages=114250 |doi=10.1016/j.jelechem.2020.114250 |issn=1572-6657}}

File:Co stripping.png.|284x284px]]

The evaluation can be done through cyclic voltammetry, which consists in the implementation of an increasing potential to the working electrode, with respect to the reference electrode, at a fixed scan rate and in a range where surface adsorption-limited electron-transfer reactions occur.{{Cite journal |last=Carter |first=Robert N. |last2=Kocha |first2=Shyam S. |last3=Wagner |first3=Frederick |last4=Fay |first4=Matthew |last5=Gasteiger |first5=Hubert A. |date=2007-09-28 |title=Artifacts in Measuring Electrode Catalyst Area of Fuel Cells through Cyclic Voltammetry |url=https://iopscience.iop.org/article/10.1149/1.2780954 |journal=ECS Transactions |volume=11 |issue=1 |pages=403–410 |doi=10.1149/1.2780954 |issn=1938-5862}} Several current peaks appear at different potentials and they are indicators of the occurring reactions.{{Cite journal |last=Elgrishi |first=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://pubs.acs.org/doi/10.1021/acs.jchemed.7b00361 |journal=Journal of Chemical Education |language=en |volume=95 |issue=2 |pages=197–206 |doi=10.1021/acs.jchemed.7b00361 |issn=0021-9584}}

The CO current peak is a result of the charge released during the desorption process of carbon monoxide from the metal catalyst surface, which occurs in the presence of an oxygenated species, according to the overall reaction:{{Citation |last=Li |first=Hui |title=Catalyst Contamination in PEM Fuel Cells |date=2008 |work=PEM Fuel Cell Electrocatalysts and Catalyst Layers: Fundamentals and Applications |pages=331–354 |editor-last=Zhang |editor-first=Jiujun |url=https://doi.org/10.1007/978-1-84800-936-3_6 |access-date=2025-07-08 |place=London |publisher=Springer |language=en |doi=10.1007/978-1-84800-936-3_6 |isbn=978-1-84800-936-3 |last2=Song |first2=Chaojie |last3=Zhang |first3=Jianlu |last4=Zhang |first4=Jiujun}}

M-CO + H2O -> M + CO2 + 2H+ + 2e-

A typical CO stripping measurement follows a series of precise steps:{{Cite journal |last=Wen |first=Zengyin |last2=Wu |first2=Duojie |last3=Banham |first3=Dustin |last4=Chen |first4=Ming |last5=Sun |first5=Fengman |last6=Zhao |first6=Zhiliang |last7=Jin |first7=Yiqi |last8=Fan |first8=Li |last9=Xu |first9=Shaoyi |last10=Gu |first10=Meng |last11=Fan |first11=Jiantao |last12=Li |first12=Hui |date=2023-01-11 |title=Micromodification of the Catalyst Layer by CO to Increase Pt Utilization for Proton-Exchange Membrane Fuel Cells |url=https://doi.org/10.1021/acsami.2c16524 |journal=ACS Applied Materials & Interfaces |volume=15 |issue=1 |pages=903–913 |doi=10.1021/acsami.2c16524 |issn=1944-8244}}

1. Cleaning of the catalyst surface by performing cyclic voltammetry under inert atmosphere.
2. Adsorption of CO at the working electrode by feeding carbon monoxide plus an inert for a few minutes at low potentials.
3. Once all the active sites are poisoned by CO, gaseous remains are purged by flushing the electrode with an inert gas.
4. At least two cycles of cyclic voltammetry must be measured to evaluate the current variation from the CO stripping cycle to the standard cycle.

The main application of CO stripping is the determination of the electrochemically active surface area ($ECSA$). Compared to hydrogen adsorption/desorption measurements, it was found to be more reliable in the assessment of the active area.

The desorption charge of CO can be calculated on the cyclic voltammogram as the integral of the peak current to which it is associated, after subtracting the contributions of other phenomena such as the double layer current, which can be evaluated from successive scans. Accordingly, the charge is calculated as:{{Cite journal |last=Pozio |first=A |last2=De Francesco |first2=M |last3=Cemmi |first3=A |last4=Cardellini |first4=F |last5=Giorgi |first5=L |date=2002-03-05 |title=Comparison of high surface Pt/C catalysts by cyclic voltammetry |url=https://www.sciencedirect.com/science/article/pii/S0378775301009211 |journal=Journal of Power Sources |volume=105 |issue=1 |pages=13–19 |doi=10.1016/S0378-7753(01)00921-1 |issn=0378-7753}}

$Q\_\{CO\}\; =\; \backslash frac\{1\}\{v\}\backslash int\_\{V\_0\}^\{V\_1\}\; (I\_\{CO,des\}\; -\; I\_\{base\})\; dV$

where:

• $v$ is the potential scan rate.
• $V\_0$ is the onset potential at which CO stripping begins.
• $V\_1$ is the potential at which the pre-adsorbed carbon monoxide is completely removed.
• $I\_\{CO,des\}$ is the current measured during the first scan, when CO is stripped from the catalyst surface.
• $I\_\{base\}$ is the baseline current which accounts for all the other phenomena occurring and it is measured in the next cycles.

The electrochemically active surface area is then calculated by dividing the obtained charge by the theoretical monolayer adsorption charge on a smooth electrode ($\backslash sigma\_\{m\}$) which, for platinum, is assumed equal to 420 $\backslash mu\; C/cm^2\_\{Pt\}$:{{Cite journal |last=Schuler |first=Tobias |last2=Chowdhury |first2=Anamika |last3=Freiberg |first3=Anna T. |last4=Sneed |first4=Brian |last5=Spingler |first5=Franz B. |last6=Tucker |first6=Michael C. |last7=More |first7=Karren L. |last8=Radke |first8=Clayton J. |last9=Weber |first9=Adam Z. |date=2019 |title=Fuel-Cell Catalyst-Layer Resistance via Hydrogen Limiting-Current Measurements |url=https://iopscience.iop.org/article/10.1149/2.0031907jes |journal=Journal of The Electrochemical Society |language=en |volume=166 |issue=7 |pages=F3020–F3031 |doi=10.1149/2.0031907jes |issn=0013-4651}}

$ECSA\; =\; \backslash frac\{Q\_\{CO\}\}\{\backslash sigma\_\{m\}\}$

In electrochemical cells adopting a solid electrolyte, the electrodes contain a thin film of an ion conductive polymer called ionomer which which enables ion transport to and from the active sites of the electrocatalyst.{{Cite journal |last=Heitner-Wirguin |first=Carla |date=1996-10-30 |title=Recent advances in perfluorinated ionomer membranes: structure, properties and applications |url=https://www.sciencedirect.com/science/article/pii/037673889600155X |journal=Journal of Membrane Science |volume=120 |issue=1 |pages=1–33 |doi=10.1016/0376-7388(96)00155-X |issn=0376-7388}} However, its presence is also considered responsible for hindered transport of reactants because of localized catalyst poisoning caused by the adsorption of ionic species.

The nature and the quantity of the ions covering the catalyst can be estimated in the early phases of the CO stripping measurement by measuring the displacement charge resulting from the replacement of adsorbed ionic species by CO.{{Cite journal |last=Subbaraman |first=Ram |last2=Strmcnik |first2=Dusan |last3=Stamenkovic |first3=Vojislav |last4=Markovic |first4=Nenad M |date=2010-05-13 |title=Three Phase Interfaces at Electrified Metal−Solid Electrolyte Systems 1. Study of the Pt(hkl)−Nafion Interface |url=https://doi.org/10.1021/jp100814x |journal=The Journal of Physical Chemistry C |volume=114 |issue=18 |pages=8414–8422 |doi=10.1021/jp100814x |issn=1932-7447}}

Depending on the species being displaced, cation (Ca^+) or anion (An^-), it is possible to measure either an oxidative or a reductive current:{{Cite journal |last=Garrick |first=Taylor R. |last2=Moylan |first2=Thomas E. |last3=Yarlagadda |first3=Venkata |last4=Kongkanand |first4=Anusorn |date=2016-12-13 |title=Characterizing Electrolyte and Platinum Interface in PEM Fuel Cells Using CO Displacement |url=https://iopscience.iop.org/article/10.1149/2.0551702jes |journal=Journal of The Electrochemical Society |volume=164 |issue=2 |pages=F60–F64 |doi=10.1149/2.0551702jes |issn=0013-4651}}

M-Ca + CO -> M-CO + Ca^+ + e^-

M-An + CO + e- -> M-CO + An^-

Accordingly, the displacement charge coverage can be calculated as the ratio between the displacement charge and the CO desorption charge. Considering that the overall CO oxidation reaction involves two electrons:

$\backslash Theta\_\{dis\}\; =\; \backslash frac\{2Q\_\{dis\}\}\{Q\_\{CO\}\}$

where $\backslash Theta\_\{dis\}$ is the ionic species coverage, $Q\_\{dis\}$ is the displacement charge and $Q\_\{CO\}$ is the CO stripping charge.

File:Supported_catalyst_with_thin_film_layer.png

Besides ionomer-metal interface characterization, CO stripping-based techniques can be used to assess ionomer coverage and distribution on the electrode.{{Cite journal |last=Iden |first=Hiroshi |last2=Ohma |first2=Atsushi |date=March 2013 |title=An in situ technique for analyzing ionomer coverage in catalyst layers |url=https://linkinghub.elsevier.com/retrieve/pii/S1572665713000301 |journal=Journal of Electroanalytical Chemistry |volume=693 |pages=34–41 |doi=10.1016/j.jelechem.2013.01.026 |issn=1572-6657}}

It was found that under reactant-limited conditions, the mass transport resistance, that can be measured by specifically developed techniques, has a linear dependence to the active area of the electrode. In detail, the diffusive resistance of the reactants through the ionomer thin film increases as the measured surface area decreases whereas, molecular and Knudsen diffusion components remain constant.{{Cite journal |last=Nonoyama |first=Nobuaki |last2=Okazaki |first2=Shinobu |last3=Weber |first3=Adam Z. |last4=Ikogi |first4=Yoshihiro |last5=Yoshida |first5=Toshihiko |date=2011 |title=Analysis of Oxygen-Transport Diffusion Resistance in Proton-Exchange-Membrane Fuel Cells |url=https://iopscience.iop.org/article/10.1149/1.3546038 |journal=Journal of The Electrochemical Society |language=en |volume=158 |issue=4 |pages=B416 |doi=10.1149/1.3546038}}

As a consequence, by controlling the carbon monoxide coverage on the metal catalyst it is possible to derive a correlation between different resistances which are directly influenced by the ionomer's presence and arrangement, obtaining insights on the structure of the electrode.{{Cite journal |last=Shinozaki |first=Kazuma |last2=Kajiya |first2=Shuji |last3=Yamakawa |first3=Shunsuke |last4=Hasegawa |first4=Naoki |last5=Suzuki |first5=Takahisa |last6=Shibata |first6=Masao |last7=Jinnouchi |first7=Ryosuke |date=2023-05-01 |title=Investigation of gas transport resistance in fuel cell catalyst layers via hydrogen limiting current measurements of CO-covered catalyst surfaces |url=https://www.sciencedirect.com/science/article/pii/S0378775323002847 |journal=Journal of Power Sources |volume=565 |pages=232909 |doi=10.1016/j.jpowsour.2023.232909 |issn=0378-7753}}

By measuring the $ECSA$ under conditions in which the active sites that are not covered by the ionomer are blocked (e.g. by filling the electrode with a fluid blocking ionic conduction), hence measuring the active area of the ionomer-covered sites only, the ionomer coverage can be calculated as the ratio between the new value obtained and the one resulting from standard CO stripping.

Electrochemical cells using hydrogen are particularly subjected to CO poisoning because of the way it is produced. In fact, most of the hydrogen commercially available comes from steam reforming of methane:{{Cite journal |last=LeValley |first=Trevor L. |last2=Richard |first2=Anthony R. |last3=Fan |first3=Maohong |date=2014-10-13 |title=The progress in water gas shift and steam reforming hydrogen production technologies – A review |url=https://www.sciencedirect.com/science/article/pii/S0360319914023416 |journal=International Journal of Hydrogen Energy |volume=39 |issue=30 |pages=16983–17000 |doi=10.1016/j.ijhydene.2014.08.041 |issn=0360-3199}}

CH4 + H2O -> CO + 3H2

The CO molecule is later oxidized through the water-gas shift reaction and hydrogen is separated from carbon dioxide:

CO + H2O -> CO2 + H2

However, some impurities may remain in the fuel requiring the use of CO-tolerant catalysts. In this context, CO stripping is used to assess the onset potential at which carbon monoxide begins to be oxidized on the catalyst surface by oxygenated species, a lower onset potential means better CO-tolerance.{{Cite journal |last=Ehteshami |first=Seyyed Mohsen Mousavi |last2=Chan |first2=Siew Hwa |date=2013-03-30 |title=A review of electrocatalysts with enhanced CO tolerance and stability for polymer electrolyte membarane fuel cells |url=https://www.sciencedirect.com/science/article/pii/S0013468613001199 |journal=Electrochimica Acta |volume=93 |pages=334–345 |doi=10.1016/j.electacta.2013.01.086 |issn=0013-4686}}

• Carbon monoxide
• Catalyst poisoning
• Cyclic voltammetry
• Electrocatalyst
• Electrochemistry

{{Reflist}}

• {{cite journal |last=Elgrishi |first=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 |volume=95 |issue=2 |pages=197–206 |doi=10.1021/acs.jchemed.7b00361 |issn=0021-9584 |doi-access=free}}
• {{cite journal |last=Garrick |first=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=http://jes.ecsdl.org/content/164/2/F55.full.pdf |journal=Journal of The Electrochemical Society |volume=164 |issue=2 |pages=F55–F59 |doi=10.1149/2.0381702jes |issn=0013-4651 |access-date=2025-07-10 |doi-access=free}}
• {{cite journal |last=Shinozaki |first=Kazuma |last2=Kajiya |first2=Shuji |last3=Yamakawa |first3=Shunsuke |last4=Hasegawa |first4=Naoki |last5=Suzuki |first5=Takahisa |last6=Shibata |first6=Masao |last7=Jinnouchi |first7=Ryosuke |date=2023 |title=Investigation of gas transport resistance in fuel cell catalyst layers via hydrogen limiting current measurements of CO-covered catalyst surfaces |url=https://linkinghub.elsevier.com/retrieve/pii/S0378775323002847 |journal=Journal of Power Sources |volume=565 |page=232909 |doi=10.1016/j.jpowsour.2023.232909 |access-date=2025-07-11}}

• {{Cite web |last=Quiroga |first=Amanda |title=Cyclic Voltammetry |url=https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Instrumentation_and_Analysis/Cyclic_Voltammetry |access-date=10 July 2025}}
• {{Cite web |last=SpectroInlets |title=CO‐stripping Technique |url=https://spectroinlets.com/wp-content/uploads/2022/01/Application_note__01-CO-stripping.pdf |access-date=10 July 2025}}
• {{Cite web |last=U.S. Energy Information Administration |date=23 June 2023 |title=Hydrogen production |url=https://www.eia.gov/energyexplained/hydrogen/production-of-hydrogen.php |archive-date=}}

{{Analytical chemistry}}

Category:Separation processes

Category:Electroanalytical methods

Category:Electrochemistry