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Cyclic voltammetry (CV) is the backbone of most electrochemical research and is an essential electrochemical technique that allows researchers to explore candidate catalysts in greater depth. When coupled with modeling, a systematic goal-focused protocol will supply a range of data that will inform the user of more novel techniques and complex setups. This disciplined approach will save time in the long run, and is especially helpful to those who may have limited access to electrochemical instrumentation in a busy laboratory.
This article provides an overview of possible research goals when using CV along with relevant examples from scientific literature with the approach in action.
Electrocatalysis (ECAT) is defined as the catalysis of an electrode reaction. The electrocatalytic effect leads to an increase of the standard rate constant of the electrode reaction—resulting in a higher current density, or to a decrease in overpotential when other rate limiting steps are involved. The study of an electrocatalytic process requires characterization of the mechanism and kinetics of the electrode reaction. Forced convection methods can offer the advantage of reducing the contributions from mass-transport and providing direct access to the kinetic and mechanistic information.
In the last decade, a greater understanding of critical electrochemical transformations has been established, particularly those that involve water, hydrogen, and oxygen [1]. The expansion of our understanding in this realm was only possible because of the use of critical electrochemical techniques. This has allowed researchers to not only explore a wider variety of catalysts, but explore them in greater detail.
To respond to the potential volume of exploration which may discover more cost-effective and renewable materials that are not at the stage of critical depletion, a systematic approach to analytical research is required.
As always, new techniques are constantly being developed, but the gold standard technique of exploration of catalysts with cyclic voltammetry (CV) is still recommended as the starting point for researchers.
Experimental Goals and Procedure Selection
To effectively explore a candidate catalyst, it is important to consider what the experimental goal is and then choose the procedure accordingly. Examples of possible goals are listed in the following sections along with suggested procedures and/or techniques.
Exploring a new system
Determine the (E) stability window of the electrolyte [2] Method: Perform CV measurement in broad voltage (E) window, using an inert electrode (e.g., glassy carbon) and investigate the general redox behavior of the electrocatalyst material. |
Determine the electrode surface area for quantitative comparisons [3–5] Method: Various methods that are material dependent: using a well-defined surface reaction (e.g., stripping or oxide formation) or analysis of electrochemical double layer capacitance (Cdl). |
Investigate the general redox behavior of the electrocatalyst material [2] Method: Perform CV measurement in a broad voltage (E) window, using a well understood electrolyte and new electrocatalyst. |
Investigate the stability of the electrocatalyst [6, 7] Method: Perform repetitive CV measurements over several hundred cycles or during several days. |
Probing a specific electrochemical reaction
Determine if a reaction is reversible (fast electron transfer kinetics), quasi reversible (slow kinetics), or irreversible (governed by other factors) [8, 9] Method: Perform CV measurements at various scan rate values, then examine dependencies for the peak position (Epeak) and peak height (Ipeak) on the scan rate. |
Determine the apparent activation energy of the reaction [10] Method: Perform CV measurements at various temperatures, then analyze electrochemical Arrhenius plots of log j vs. 1/T. |
Combining CV with additional techniques to confirm results and deepen understanding
Determine the molecular structure of products or intermediates at a specific instance of the reaction [9–12] Method: Perform CV measurement with in-situ spectroscopic determination (spectroelectrochemistry via UV/Vis/NIR or Raman spectroscopy). |
Investigate material deposited or removed from the electrode surface during the electrochemical measurement [13] Method: Measure the mass change at the electrode surface during a CV measurement using electrochemical quartz crystal microbalance (EQCM). Application Note: EQCM study of underpotentially deposited (UPD) lead adlayer on gold |
Investigate products and short-lived intermediates via their electrochemical response [14, 15] Method: Perform bipotentiostat (two working electrodes) measurements in a ring/disk configuration (RRDE). Application Note: Oxygen Reduction Reaction with the Rotating Ring Disk Electrode |
From action to reactions in the literature
This paper from the Nissan Fuel cell research center (NFCRC) summarizes the analytical approach for reduction of Pt loading in fuel cell catalyst layers (CL) [7]. Using a combined experimental and theoretical approach, they clearly outline the important properties required to measure experimentally or model to reach their goal of reducing the amount of Pt used in the CL.
Platinum Electro-Dissolution in Acidic Media upon Potential Cycling
Focal parameters for exploration:
1. Catalyst Microstructure
Research goal: Determine the electrode surface
Using microscope images combined with the Cdl (double layer capacitance) and ionomer coverage, the researchers were able to analyze and quantify their catalyst layer. They used CV to determine ionomer coverage over the carbon by comparing Cdl values (wet versus dry).
2. Transport Properties
Research goal: Investigate material deposited or removed from the electrode surface during the electrochemical measurement
Additional research investigating the electrode surface was performed with CV. Using a rotating disk electrode, the researchers were able to determine the gas transport resistance by measuring the ORR (oxygen reduction reaction). CV also allowed the determination of the Pt roughness factor.
3. I-V performance
Research goal: Use CV I-V to calculate the fuel cell performance
I-V performance is a typical measurement for the overall performance of the fuel cell. A potentiostat is needed to measure the actual I-V curve in order to determine the Pt loading so that the I-V performance can be interpreted and compared among various samples.
This paper illustrates the value of systematic exploration of catalysts with CV to give a comprehensive overview of attributes, structure, and reactions before moving on to more complex setups.
Your initial investigations with CV may not provide all of the answers at first glance, but you can then move on to more complex setups and experiments with complete insight.
Your knowledge take-aways
References
- Seh Z. W.; Kibsgaard J.; Dickens C. F.; et al. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355, 6321. doi:10.1126/science.aad4998
- Kübler, P.; Sundermeyer, J. Ferrocenyl-Phosphonium Ionic Liquids – Synthesis, Characterisation and Electrochemistry. Dalton Trans. 2014, 43 (9), 3750–3766. doi:10.1039/C3DT53402B
- Biegler, T.; Rand, D. A. J.; Woods, R. Limiting Oxygen Coverage on Platinized Platinum; Relevance to Determination of Real Platinum Area by Hydrogen Adsorption. J. Electroanal. Chem. Interfacial Electrochem. 1971, 29 (2), 269–277. doi:10.1016/S0022-0728(71)80089-X
- Trasatti, S.; Petrii, O. A. Real Surface Area Measurements. Int. Union Pure Appl. Chem. 1991, 63 (5), 711–734. doi:10.1351/pac199163050711
- Kinkead, B.; van Drunen, J.; Paul, M. T. Y.; et al. Platinum Ordered Porous Electrodes: Developing a Platform for Fundamental Electrochemical Characterization. Electrocatalysis 2013, 4 (3), 179–186. doi:10.1007/s12678-013-0145-2
- Pilapil, B. K.; van Drunen, J.; Makonnen, Y.; et al. Ordered Porous Electrodes by Design: Toward Enhancing the Effective Utilization of Platinum in Electrocatalysis. Adv. Funct. Mater. 2017, 27 (36), 1703171. doi:10.1002/adfm.201703171
- Xing, L.; Hossain, M. A.; Tian, M.; et al. Platinum Electro-Dissolution in Acidic Media upon Potential Cycling. Electrocatalysis 2014, 5 (1), 96–112. doi:10.1007/s12678-013-0167-9
- Rountree, E. S.; McCarthy, B. D.; Eisenhart, T. T.; et al. Evaluation of Homogeneous Electrocatalysts by Cyclic Voltammetry. Inorg. Chem. 2014, 53 (19), 9983–10002. doi:10.1021/ic500658x
- Sokolov, S.; Sepunaru, L.; Compton, R. Taking Cues from Nature: Hemoglobin Catalysed Oxygen Reduction. Appl. Mater. Today 2017, 7, 82–90. doi:10.1016/j.apmt.2017.01.005
- Barbosa, A. F. B.; Oliveira, V. L.; van Drunen, J.; et al. Ethanol Electro-Oxidation Reaction Using a Polycrystalline Nickel Electrode in Alkaline Media: Temperature Influence and Reaction Mechanism. J. Electroanal. Chem. 2015, 746, 31–38. doi:10.1016/j.jelechem.2015.03.024
- Hernández, C. L.; González García M. B.; Santos , D. H.; et al. Aqueous UV-VIS Spectroelectrochemical Study of the Voltammetric Reduction of Graphene Oxide on Screen Printed Carbon Electrodes. Electrochemistry Communications, 2016, 64, 65–68. doi:10.1016/j.elecom.2016.01.017
- Görlin, M.; de Araújo, J. F.; Schmies, H.; et al. Tracking Catalyst Redox States and Reaction Dynamics in Ni-Fe Oxyhydroxide Oxygen Evolution Reaction Electrocatalysts: The Role of Catalyst Support and Electrolyte PH. J Am Chem Soc 2017, 139 (5), 2070–2082. doi:10.1021/jacs.6b12250
- Lee, C-L.; Huang, K-L.; Tsai, Y-L.; et al. A Comparison of Alloyed and Dealloyed Silver/Palladium/Platinum Nanoframes as Electrocatalysts in Oxygen Reduction Reaction. Electrochem. Commun. 2013, 280–285. doi:10.1016/j.elecom.2013.07.020
- Vos, J. G.; Koper, M. T. M. Measurement of Competition between Oxygen Evolution and Chlorine Evolution Using Rotating Ring-Disk Electrode Voltammetry. J. Electroanal. Chem. 2018, 819, 260–268. doi:10.1016/j.jelechem.2017.10.058
- Kocha, S. S.; Shinozaki, K.; Zack, J. W.; et al. Best Practices and Testing Protocols for Benchmarking ORR Activities of Fuel Cell Electrocatalysts Using Rotating Disk Electrode. Electrocatalysis 2017, 8 (4), 366–374. doi:10.1007/s12678-017-0378-6