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Spectroelectrochemistry: shedding light on the unknown

Spectroelectrochemistry: shedding light on the unknown

The combination of two well-known analytical techniques, electrochemistry and spectroscopy, gives rise to spectroelectrochemistry (SEC), an established scientific methodology. This hybrid technology combines the advantages of each technique, offering the best of both worlds [1]. The word «spectroelectrochemistry» is the result of combining these two terms as two pieces of a puzzle that fit perfectly together.

In this article, written for both beginners in the field as well as more experienced readers, we focus on introducing this technique from its beginnings to its advantages in research, and then discuss new systems and solutions that will make it easier to work on the multitude of applications that spectroelectrochemistry can offer. 

Shedding light, in the literal sense of the phrase, on electrochemical knowledge and procedures. Spectroelectrochemistry offers analysts more information by being able to record both an optical and an electrochemical signal at the same time to obtain new data.

This is a multi-response method—it studies the process of electrochemical reactions with simultaneous optical monitoring. Spectroelectrochemistry provides two individual signals from a single experiment, which is a very powerful feature to obtain critical information about the studied system. Moreover, the autovalidated character of spectroelectrochemistry confirms the results obtained by two different routes. Find out more about this topic by downloading our free Application Note below.

Spectroelectrochemistry allows researchers to collect molecular, kinetic, and thermodynamic information from the reactants, intermediates, and/or products involved in electron transfer processes. Thus, it is possible to perform spectroelectrochemical studies on a broad range of molecules and different processes including: biological complexes, polymerization reactions, nanomaterial characterization, analyte detection, corrosion mechanisms, electrocatalysis, environmental processes, characterization of memory devices, and much more!

Ultimately, different kinds of information is obtained depending on the spectral range used. UV-VIS spectroscopy provides molecular information related to the electronic levels of the molecules, the NIR region provides data associated with the vibrational levels, and the Raman spectrum provides very specific information about the structure and composition of the sample due to the fingerprinting characteristics of this technique.

Electromagnetic Spectrum
Diagram of the electromagnetic spectrum.

The beginnings of spectroelectrochemistry

Theodore Kuwana, Ph.D. in chemistry and specialist in spectroelectrochemistry, bioelectroanalytical chemistry and modified electrodes.

This analytical technique was developed in the 1960’s, and became popularized by Professor Theodore Kuwana and other researchers [2]. They had begun to work with transparent electrodes to study a simultaneous process—measuring the charge and absorbance (at the same time) when a beam of light passes through the electrode. As a result, they developed transparent electrodes and this marked the beginning of concurrent electrochemical and UV-VIS absorption measurements.

These so-called «optically transparent electrodes» (OTEs) were developed to carry out the combination of optical and electrochemical experiments. Some of the most commonly used OTEs began as oxide doped with antimony on glass, then developed into different thin films of gold or platinum on quartz, followed by germanium electrodes for IR wavelengths, as well as pure gold and platinum micromeshes (where the holes provide the required transparency to light). However, not all spectroelectrochemical configurations require transparent electrodes. For more information, download the electrode reference flyers from Metrohm DropSens to properly work with the different spectroelectrochemical techniques.

A selection of Metrohm DropSens references. Left: C013 for in-situ surface Raman spectroscopy. Right: P10 optically transparent and specially designed for spectroelectrochemical applications.

Download the free flyers for these electrodes below for further information.

The first published paper on spectroelectrochemistry [2], in which Dr. Kuwana participated, describes the use of tin oxide-coated glass surfaces (optically transparent electrodes) for following the absorbance changes of different electroactive species during electrolysis. Since then, the number of works and investigations based on this technique have grown steadily.

Spectroelectrochemistry publications have increased significantly since its discovery in the 1960’s (results of searching «Spectroelectrochem*» as a term in Scopus as of June 2021).

Take a look for yourself to see how things have changed in the last several decades!

An array of spectroelectrochemical techniques to choose from

The following graphic is classified according to the combination of different electrochemical and spectroscopic methods. The general classification is based on the spectroscopic technique: ultraviolet (UV), visible (Vis), photoluminescence (PL), infrared (IR), Raman, X-ray, nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR).

Spectroelectrochemistry (SEC) is the combination of spectroscopy and electrochemistry.

In recent years, significant advances have occurred regarding the design, development, and possibilities offered by instruments for working with spectroelectrochemical techniques. Also, the assemblies and the connections between products and accessories that facilitate the use of this equipment have improved in recent decades, contributing to make research and experiments in this field easier and more affordable.

The evolution of spectroelectrochemical instrumentation

Traditionally, the configuration for spectroelectrochemical analysis consists of two instruments: one spectroscopic instrument and the other for electrochemical analysis. Both instruments are connected independently to the same spectroelectrochemical cell and are generally not usually synchronized. In addition, each instrument is controlled by a different (and specific) software in each case, so two programs are also needed to interpret each signal and yet another external software for the processing and analysis of the data obtained by the first two programs. Finally, it must be considered that synchronization is not guaranteed, making the performance of experiments and tests with this configuration slow, complex, and costly.

This detached spectroelectrochemical setup displays the complexity of various software and programs used, showing that different systems are not able to obtain actual synchronized electrochemical measurements and data (click to enlarge).

Metrohm DropSens took this opportunity to create something that did not exist before—a revolution in the state-of-the-art of spectroelectrochemistry: the SPELEC line of instruments which are fully integrated, synchronized solutions that offer much more versatility to researchers. The devices include all of the components needed to work with spectroelectrochemical techniques in a simple way and in a single system with a (bi)potentiostat/galvanostat, the light source, and the spectrometer (depending on the selected spectral range).

Find out more about SPELEC, the next-generation tool for spectroelectrochemical research on our website.

The SPELEC systems from Metrohm DropSens consist of one device and one software—a fully integrated, easy to use, practical setup for researchers.

These designs and configurations simplify the work, processes, and spectroelectrochemical measurements as well because only a single system and a single software are needed. In the case of the SPELEC solution, its advanced dedicated software (DropView SPELEC) is a specific program that controls the instrument, obtains the electrochemical and spectroscopic signals simultaneously, and also allows users to process and analyze the data together in a single step. It’s really that simple!

The future of spectroelectrochemistry: SPELEC systems and software

One instrument and one software: Metrohm DropSens SPELEC has everything you need for your spectroelectrochemical experiments while saving laboratory space and valuable time. SPELEC instruments offer the combinations of electrochemistry and UV-Vis, Vis-NIR, or even Raman spectroscopy in a single measurement with several different instrument options available (see below). Everything is integrated which allows more tests in less time, multiple spectra, a full range of accessories, and research flexibility with the different configurations available.

Several options are available depending on the spectral range needed:

SPELEC: 200–900 nm (UV-VIS)
SPELEC NIR: 900–2200 nm
SPELEC RAMAN: 785 nm laser
(other wavelengths available upon request)
SPELEC 1050: 350–1050 nm (VIS-NIR)

DropView SPELEC is a dedicated and intuitive software that facilitates measurement, data handling, and processing. With this program, you can display electrochemical curves and spectra in real time and follow your experiments in counts, counts minus dark, absorbance, transmittance, reflectance, or Raman shift. As far as data processing is concerned, DropView SPELEC offers a wide variety of functions including graph overlay, peak integration and measurement, 3D plotting, spectral movie, and more.

Testimonial from the University of Burgos on the integrated SPELEC system from Metrohm DropSens.

SPELEC instruments are very versatile, and although they are dedicated spectroelectrochemical instruments, they can also be used for electrochemical and spectroscopic experiments. They can be used with any type of electrodes (e.g., screen-printed electrodes, conventional electrodes, etc.) and with different spectroelectrochemical cells. Optical and electrochemical information is obtained in real time/operando/dynamic configuration. The main advantages of spectroelectrochemical techniques can be summarized as follows:

  • they simultaneously provide information obtained by two different techniques (electrochemistry and spectroscopy) in a single experiment
  • qualitative studies and quantitative analyses can be performed
  • high selectivity and sensitivity
  • spectroelectrochemistry is used in a wide variety of different fields due to its great versatility
  • new configurations facilitate the performance of spectroelectrochemical experiments, saving time, samples, costs, etc.

Learn more about the next-generation tool for spectroelectrochemical research on our website!

SEC analysis techniques: suitable for multiple applications

The characteristics of spectroelectrochemistry allow the constant development of new and broad applications in several different fields. Read on below to discover the capabilities of this technique.

Materials science: characterization of specific properties of carbon materials, quantum dots, composites, nanoparticles, Janus materials, polymers, as well as stability studies, determination of photochemical properties, development of new materials, etc.

For more information, download our free related Application Note below.

Sensing: selective and sensitive detection, rapid quantification of a huge variety of analytes, diagnostic tool, development of new methodologies and sensors, etc. [3].

Organic and inorganic chemistry: study of the properties and structure of different compounds, analysis of kinetic reactions, determination of electron transfer capacity, etc. [4].

Corrosion: evaluation of protective films as corrosion inhibitors, determination of electrode stability and reversibility, monitoring of layer and sublattice generation, improvement of protective properties of coating materials, etc.

Energy storage: monitoring of exchange and discharge cycles, determination of oxidation/reduction levels, characterization of new electrolytes for batteries, understanding of doping and splitting processes in solar cells, etc.

Electrocatalysis: characterization and comparison of the electrocatalytic activity of different catalysts, identification of intermediate species and their structural changes, elucidation of the reaction mechanism, etc. [5].

Life sciences: study of biological processes, characterization of molecules used in biotechnology, biochemistry or medicine, determination of antioxidant activity, etc.

Environment: identification and quantification of pesticides, dyes, and pollutants, monitoring of degradation and filtration processes, etc. [6]

Download our free related Application Note for more information.

Others: characterization of new materials for memory devices, comparison of minerals, identification of pigments, oils, and pastes, etc.

Learn even more about spectroelectrochemistry (SEC) and what it can do for your research by downloading our free brochure.

Contact us

to discuss how spectroelectrochemistry can boost your research.

References

[1] Kaim, W.; Fiedler, J. Spectroelectrochemistry: The Best of Two Worlds. Chem. Soc. Rev. 2009, 38 (12), 3373. doi:10.1039/b504286k

[2] Kuwana, T.; Darlington, R. K.; Leedy, D. W. Electrochemical Studies Using Conducting Glass Indicator Electrodes. Anal. Chem. 1964, 36 (10), 2023–2025. doi:10.1021/ac60216a003

[3] Martín-Yerga, D.; Pérez-Junquera, A.; González-García, M. B.; Perales-Rondon, J. V.; Heras, A.; Colina, A.; Hernández-Santos, D.; Fanjul-Bolado, P. Quantitative Raman Spectroelectrochemistry Using Silver Screen-Printed Electrodes. Electrochimica Acta 2018, 264, 183–190. doi:10.1016/j.electacta.2018.01.060

[4] Perez-Estebanez, M.; Cheuquepan, W.; Cuevas-Vicario, J. V.; Hernandez, S.; Heras, A.; Colina, A. Double Fingerprint Characterization of Uracil and 5-Fluorouracil. Electrochimica Acta 2021, 388, 138615. doi:10.1016/j.electacta.2021.138615

[5] Rivera-Gavidia, L. M.; Luis-Sunga, M.; Bousa, M.; Vales, V.; Kalbac, M.; Arévalo, M. C.; Pastor, E.; García, G. S- and N-Doped Graphene-Based Catalysts for the Oxygen Evolution Reaction. Electrochimica Acta 2020, 340, 135975. doi:10.1016/j.electacta.2020.135975

[6] Ibáñez, D.; González-García, M. B.; Hernández-Santos, D.; Fanjul-Bolado, P. Detection of Dithiocarbamate, Chloronicotinyl and Organophosphate Pesticides by Electrochemical Activation of SERS Features of Screen-Printed Electrodes. Spectrochim. Acta. A. Mol. Biomol. Spectrosc. 2021, 248, 119174. doi:10.1016/j.saa.2020.119174

Post written by Dr. David Ibáñez Martínez (Product Specialist Spectroelectrochemistry) and Belén Castedo González (Marketing and Communication) at Metrohm DropSens, Oviedo, Spain.

Does counter electrode (CE) size matter?

Does counter electrode (CE) size matter?

To begin, let’s go back to the year 1950 when metallurgists and chemists tried to shine a light on a fascinating electrochemical phenomenon originally discovered in the 17th century by the chemist Sir Humphry Davy [1].

Sir Humphry Davy (1778–1829) was credited with many discoveries in the field of electrochemistry.

If you dip a wire made of iron (or as electrochemists say: an iron electrode) into diluted sulfuric acid (which is considered the electrolyte), it instantly starts to dissolve—it corrodes. If you then insert another electrode which does not corrode (e.g. platinum), and connect the iron electrode to the negative pole of a current source, and the platinum wire (electrode) to the positive pole, the iron dissolution will slow down or even stop, depending on the voltage applied.

On the other hand, if you connect the iron electrode to the positive pole and raise the voltage from very low values to higher ones, the dissolution grows exponentially with the increasing voltage.

However, above a certain current limit (and depending on the electrode area, electrolyte composition, and temperature), the current suddenly drops to very low values, and the iron electrode stops dissolving. This phenomenon was detected by Michael Faraday, and he called it «passivation». This phenomenon has been subject of controversy and disputes until the 1950s when a better understanding was possible with the invention of the modern potentiostat (Figure 1).

Figure 1. Basic line diagram of a potentiostat/galvanostat.

In experiments where the ohmic drop may be high (e.g., in large-scale electrolytic or galvanic cells or in experiments involving nonaqueous solutions with low conductivities), a three-electrode electrochemical cell is preferable. In this arrangement, the current is passed between the working electrode (WE) and a counter (or auxiliary) electrode (CE).

Learn more about the ohmic drop in our free Application Notes.

The counter electrode can be made of any available electrode material because its electrochemical properties do not affect the behavior of the working electrode of interest. It is best to choose an inert electrode so that it does not produce any substances by electrolysis that will reach the working electrode surface and cause interfering reactions there (e.g., platinum or carbon). Because the current flows between the WE and the CE, the total surface area of the CE (source/sink of electrons) must be larger than the area of the WE so that it will not be a limiting factor in the kinetics of the electrochemical processes under investigation.

Sometimes it is placed in a compartment separated from the working electrode by a sintered-glass disk or other separator (Figure 2). Bulk electrolysis experiments typically require much longer times than electroanalytical experiments, so separation of the counter electrode is required.

Figure 2. Gastight electrochemical cell for CO2 reduction study (click to enlarge).

The potential of the working electrode is monitored relative to a separate reference electrode (RE), positioned with its tip nearby (with a Luggin capillary as shown in Figure 3). The potentiostat used to control the potential difference between the working electrode and the reference electrode has a very high input impedance so that a negligible current flows through the reference electrode. Consequently, the potential of the reference electrode will remain constant and equal to its open-circuit value. This three-electrode arrangement is used in most electrochemical experiments.

The counter electrode is used to close the current circuit in the electrochemical cell. It is usually made of an inert material (e.g., Pt, Au, graphite, or glassy carbon) and it hosts a redox reaction which occurs at the CE surface that balances the redox reaction at the surface of the WE. The products of this reaction can diffuse to the WE and interfere with the redox reaction occurring at that site. However, in electroanalytical experiments such as cyclic voltammetry (CV), the time scale of the experiment is too short for this diffusion to be able to cause significant interferences. Therefore, in most cases there is no need to place the CE in a separate compartment, as shown in the electrochemical cell in Figure 3.

Figure 3. Metrohm Autolab 1 liter corrosion cell labelled with electrodes.

The potential profile in an actual cell depends on the electrode shapes, geometry, solution conductance, and more. If the reference electrode is placed anywhere except precisely at the electrode surface, some fraction of an uncompensated potential, iRu (due to the uncompensated resistance, Ru) will be included in the measured potential. Even when the tip of the reference electrode is designed for very close placement to the working electrode by use of a fine tip (Luggin capillary, also known as a Luggin-Haber capillary), some uncompensated resistance usually remains. Modern electrochemical instrumentation includes circuitry for electronic compensation of the iRu term [2].

As mentioned earlier, the electrochemical reaction of interest takes place on the working electrode, and the electron transfer there will generate the measured current which flows between the WE and CE. As a general rule for accurate current measurements and an unhindered flow of electrons in the cell, the counter electrode should be three times larger than the working electrode. The kinetics of solvent electrolysis are generally slow, so the only way to ensure the CE reaction can sustain the required cell current is to significantly increase the surface area (e.g., with Pt gauze or a Pt rod/sheet).

However, in poorly conductive media (or high currents), when positioning the counter electrode in the cell (i.e., for the cell geometry), it is worth considering where the field lines between the WE and CE will go, and whether the WE will experience a uniform field. It is generally recommended to move the counter electrode away from the working electrode, or even to put it behind a separator, unless you are certain that the WE reaction is insensitive to pH. The CE reaction sustains the current flow, but it is often unknown exactly what kind of reactions are occurring at the counter electrode. For most aqueous electrochemistry experiments this is likely to be solvent electrolysis, which leads to a change in the pH.

When the application requires, the potential of the counter electrode can be monitored during the experiment with the S2 electrode of VIONIC. Find out more in our free Application Note.

Alternative counter electrode materials

Platinum is expensive, and it is too costly to use for large CE areas. Suitable alternative electrode materials include nickel-based alloys or carbon. Be aware that nickel-based alloys may passivate, and carbon will be oxidized at high potentials of the CE. Therefore, those materials should have a sufficient area to avoid strong polarization.

Platinized titanium is a good choice when larger counter electrode areas are required. Platinized titanium is produced either in the form of sheets or mesh grids. Mesh grids (Figure 4) have very desirable properties: a large active area compared to the geometric area, and the electrolyte is able to flow through the counter electrode.

Figure 4. Example of wire mesh grid suitable for use as a counter electrode.

 Also, at the counter electrode, a voltage drop occurs across the metal-electrolyte interface. To reduce this drop, it may be useful to use platinum covered with platinum black to increase the roughness, and consequently, the active surface area.

Final notes

Size matters in typical electrochemical measurements. Larger counter electrodes create an unconstrained current flow between WE and the CE, leading to more stable and insightful experiments. It is equally important that the overall cell configuration provides the required current density distribution. Thus, a counter electrode of the same size as the working electrode, mounted parallel to it, may optimize the current distribution without significant adverse effects from the size of the counter electrode.

The reaction occurring at the CE should be fast so that the potential drop between the counter electrode and the electrolyte does not limit the polarization that can be applied.

The products of the reaction occurring at the CE should not contaminate the solution. In practice, there will always be an electrochemical reaction at the counter electrode, and the products of that reaction should be harmless or able to be easily removed. Inert electrodes such as platinum or graphite are often used, in which case the reaction products are usually gases (oxygen or chlorine when anodic, or hydrogen when cathodic) that can be removed by bubbling air or nitrogen past the counter electrode (although there may also be a pH change at the CE).

Curious about electrochemistry?

Metrohm has you covered.

References and suggested further reading

[1] Knight, D. Humphry Davy: Science and Power (Volume 2 of Cambridge Science Biographies); Cambridge University Press: Cambridge, UK, 1998.

[2] Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, New York: Wiley, 2001, 2nd ed. Russian Journal of Electrochemistry, 2002, 38, 1364–1365. doi:10.1023/A:1021637209564

[3] Yang, W., Dastafkan, K., Jia, C., et al. Design of Electrocatalysts and Electrochemical Cells for Carbon Dioxide Reduction Reactions. Adv. Mater. Technol. 2018, 3, 1700377. doi:10.1002/admt.201700377

[4] Cottis, R. A. 2.30 – Electrochemical Methods. In Shreir’s Corrosion; Cottis, B., Graham, M., Lindsay, R., Lyon, S., Richardson, T., Scantlebury, D., Stott, H., Eds.; University of Manchester: Manchester, UK, 2010; Vol. 2, pp 1341–1373. doi:10.1016/B978-044452787-5.00068-8

[5] Vanýsek, P. Impact of electrode geometry, depth of immersion, and size on impedance measurements. Canadian Journal of Chemistry 1997, 75(11), 1635–1642. doi:10.1139/v97-194

Post written by Martijn van Dijk, Area Manager Electrochemistry at Metrohm Autolab, Utrecht, The Netherlands.

Fast and fundamental: influences on reliable electrochemical measurements

Fast and fundamental: influences on reliable electrochemical measurements

The ultimate goal of any researcher is to contribute to the progress of society by pioneering exploration beyond the known limits. Depending on the research type and application field, one way to fulfill this is to collect reliable experimental data on rapidly occurring processes (less than 1 ms).

Having insight into the fundamentals of these reaction mechanisms can ultimately lead to the discovery of new materials or the improvement of current solutions. In electrochemical research, reaction mechanisms and intermediates are investigated by measuring the kinetics and dynamics of the electrochemical processes happening at the surface of the electrode on a sub-ms timescale.

This article provides a short overview of the factors that have a direct influence on fast and ultra-fast electrochemical measurements from an experimental setup perspective.

Considering the following factors in the experimental design and execution is the first condition to obtain reliable experimental results for such measurements.

Additional challenges which researchers must be aware of when experimenting with «transient electrochemistry», i.e. doing electrochemical measurements at very low time scales, is presented in the featured article from E. Maisonhaute et al. [1].

Main factors that influence the reliability of fast electrochemical experimental results

The primary components of an electrochemical experimental setup are:

  • The electrochemical cell including the electrodes and electrolyte
  • The electrochemical instrument, i.e., the potentiostat/galvanostat (PGSTAT)

To perform reliable electrochemical experiments in general, and fast electrochemical measurements in particular, the specifications of the complete work system must be considered and the optimal settings must be used for all of the individual parts of the experimental setup.

Time constant of the electrochemical cell

The electrochemical cell and its specifications must be taken into account as it is an important element of the experimental setup.

Transient electrochemical experiments are not meaningful unless the cell time constant is small relative to the timescale of the measurement, regardless of the high-frequency characteristics of the control circuitry.

The cell time constant RuCdl (s) depends directly on the uncompensated resistance Ru (Ω) (i.e. the resistance of the electrolyte between the reference and the working electrode) and the double-layer capacitance Cdl (F) of the electrode [2].

As a consequence, when the potential is stepped or scanned rapidly, the true measured potential Etrue (V) lags behind the applied potential Eappl (V), according to the following equation:

Where RuCdl (s) is the time constant of the cell and t (s) is the time at which the measurement is taken.

Figure 1. Theoretical and true waveform applied to a real electrochemical cell [1].

For fast scan rates (i.e. when 𝑡 is much smaller than RuCdl ), the exponential term approaches 1 and significant errors in 𝐸true with respect to 𝐸appl can arise. For slow scan rates (i.e. when 𝑡 is much larger than RuCdl), the exponential approaches 0 and the errors become negligible.

The time constant of the cell can be reduced in three ways:

  • Reduce Ru via increasing the conductivity of the electrolyte by either increasing concentration of supporting electrolyte or decreasing viscosity
  • Reduce the size of the working electrode (e.g., by using microelectrodes) so that Cdl will be minimized
  • Move the reference electrode as close as possible to the working electrode (e.g., by using a Luggin capillary) so that Ru will be minimized

The electrochemical instrument: potentiostat/galvanostat (PGSTAT)

The potentiostat/galvanostat (PGSTAT) is used to accurately control the applied signal (potential or current) and measure the response (current or potential, respectively) from the electrochemical cell. The accurate control of the applied signals is achieved by using a control loop (or feedback loop) circuit.

When fast electrochemical measurements are executed, the following specifications will have a direct influence on the results and must be considered.

Bandwidth of the control loop of the PGSTAT

In general terms, bandwidth can be described as the parameter that defines how fast the instrument is able to react to any changes in the signal.

In electrochemical terms, the bandwidth is the frequency beyond which the performance of the system is degraded.

The bandwidth of the control loop of the PGSTAT (i.e. bandwidth of the instrument) indicates how fast the applied signal is controlled through the feedback loop.

Higher bandwidth means that the instrument uses a faster control loop (faster feedback). As a result, the applied signal will reach the desired set point faster, and in ideal circumstances the output signal will be identical to the theoretical waveform. However, depending on the properties of the electrochemical cell connected to the instrument, the applied signal might overshoot. In extreme cases, the instrument feedback loop might get out of control causing the potentiostat to oscillate. This is more likely when high-capacitance electrochemical cells are connected to the PGSTAT.

When a Lower bandwidth is used, the overall stability of the PGSTAT increases by reducing the speed of the control loop. In this case, the consequence is that at very high measurement speeds, the output of the applied signal may be slightly less accurate due to a slower slew rate. Nevertheless, when measuring fast transients is not within the scope of the experiment, using the instrument with a lower bandwidth setting is recommended for highly accurate experimental results.

Figure 2. Schematic representation of the applied signal when Low bandwidth (Low speed) and High bandwidth (High speed) settings are used compared with the theoretical response.

Therefore, it is important to choose the control loop bandwidth settings according to the type of the measurement. For ultra-high speed measurements, a higher bandwidth setting must be used with the following considerations:

  • The higher the bandwidth, the higher the noise and the probability that the control loop will go out of control and oscillate.
  • When working with a High bandwidth setting, it is necessary to pay special attention and use adequate cell shielding and electrode connectors. The use of a Faraday cage is recommended in these cases.
  • The use of a high impedance reference electrode (RE) (e.g., double junction reference electrode, a salt bridge with frit) in combination with a High bandwidth of the control loop might lead to instability of the PGSTAT and even to oscillations.
Bandwidth of the current sensor (current range)

The measurement of the current response of an electrochemical cell (in potentiostatic mode) and the control of the applied current value (in galvanostatic mode) is executed with specially designed current sensors. In order to achieve the best sensitivity and resolution for the measurement, individual current sensors are used depending on the magnitude of the measured (or applied) current.

Each current sensor circuit (which corresponds to a current range) has a specific bandwidth or response time. Therefore for the most accurate results (especially important for fast, time resolved experiments), the current range must be selected so that the bandwidth of the current sensor will not be the limiting factor for the time response (speed) of the measurement.

In general, the lower the measured currents, the lower the bandwidth of the current sensor.

Data sampling interval vs the timescale of the investigated transient signal

The measured electrochemical response can have a complex shape with components at many frequencies. The highest frequency component of the measured or applied signal determines the bandwidth of that signal. The bandwidth of the signal should not be higher than the bandwidth of the measuring device.

If the highest frequency component of the signal is fSIGNAL, then according to the Nyquist Theorem [3] the sampling rate fSAMPLE must be at least 2 fSIGNAL (i.e. two times higher than the highest frequency component of the signal).

Figure 3. Effect of the sampling frequency of an ideal sinusoidal signal [3]. Shown here are the theoretical signal (dashed line), sample points, and resulting measured signal (orange line).

In other words, the data sampling interval must be lower than the timescale in which the time resolved (transient) measurement from the investigated electrochemical process is expected to occur. There is a practical correlation between the sampling interval and instrument bandwidth. When the sampling interval is:

  • higher than 100 μs: the 10 kHz (High Stability) bandwidth should be selected.
  • between 10–100 μs: the 100 kHz (Fast) bandwidth should be selected.
  • smaller than 10 μs: the 1 MHz bandwidth (Ultra-Fast) should be selected.

Summary

To measure reliable experimental data, all elements of the experimental setup must be considered with their own specifications and limitations. The overview above highlights the main factors and parameters which can have a direct influence on fast electrochemical measurements.

Fast measurements start here!

Visit our website to learn more about the variety of potentiostats/galvanostats from Metrohm Autolab.

References

[1] Maisonhaute, E.; et al. Transient electrochemistry: beyond simply temporal resolution, Chem.Commun., 2016, 52, 251—263. doi:10.1039/C5CC07953E

[2] Bard, A.J.; Faulkner, L.R. Electrochemical Methods: Fundamentals and Applications, New York: Wiley, 2001, 2nd ed. Russian Journal of Electrochemistry, 2002, 38, 1364–1365. doi:10.1023/A:1021637209564

[3] Keim, R. The Nyquist–Shannon Theorem: Understanding Sampled Systems. All About Circuits, May 26, 2020. https://www.allaboutcircuits.com/technical-articles/nyquist-shannon-theorem-understanding-sampled-systems/ 

Post written by Dr. Iosif Fromondi, Product Manager and Head of Marketing and Sales Support at Metrohm Autolab, Utrecht, The Netherlands.

Green hydrogen, future fuel: Using potentiostats to develop new catalysts for hydrogen production

Green hydrogen, future fuel: Using potentiostats to develop new catalysts for hydrogen production

Hydrogen – clean and green?

Due to its high gravimetric energy density and zero pollution emission, «green hydrogen» is a clean and sustainable energy carrier which is expected to become one of the fuels of the future. Green hydrogen is produced with renewable energy sources, and it can help to mitigate global warming by using cleaner fuels.

Producing green hydrogen with electrolysis

The most favorable way to produce green hydrogen is via water splitting electrolysis, where water (H2O) is broken down into its counterparts by using a direct electric current. Electrolysis is a sophisticated technique that has been used for many decades in industry. When using this technique for the production of hydrogen, drawbacks are the sluggish reaction kinetics when using inexpensive catalysts or the high costs for more optimal catalysts (e.g. platinum). To produce hydrogen in an efficient and economical manner, the goal for researchers around the world is to develop catalysts for this purpose which are highly active, inexpensive, and stable over long periods.

This article explains in more detail how Metrohm potentiostats can be used to characterize recently developed catalysts for electrochemical hydrogen production.

Electrode reactions

Considering alkaline solutions, the water splitting reaction can be described by two half reactions (Figure 1):

  • Hydrogen Evolution Reaction (HER) at the cathode
  • Oxygen Evolution Reaction (OER) at the anode
Figure 1. Water splitting reaction with the respective half reactions at the cathode and anode.

A critical issue for water splitting is the slow reaction kinetics of the half reactions. To overcome this, electrocatalysts which decrease the activation energy to an acceptable value need to be designed. For an ideal catalyst, a voltage of 1.23 V (at 20 °C and 1013 mbar) would have to be applied to begin hydrogen and oxygen evolution between the electrodes. Unfortunately, when using catalysts in real life situations, voltages above 1.8 V need to be applied.

Requirements for suitable catalysts

Catalysts for the production of green hydrogen from electrochemical water splitting reactions must be (click to go directly to each segment):

The first three properties can be determined by electrochemical measuring techniques using a potentiostat.

Activity

The activity of a catalyst is characterized by three values (click to go directly to each segment):

which can be obtained from the polarization curve as displayed in Figure 2.

Figure 2. Polarization curves of an ideal catalyst (orange) and a real catalyst (dark blue) considering the Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER).
Linear polarization

To record a polarization curve (Figure 2), a potentiostat is used in combination with a three-electrode setup. The working electrode is coated with the catalyst which is to be characterized. A typical setup for this purpose is shown in Figure 3.

Figure 3. Potentiostat from Metrohm Autolab combined with a three-electrode setup using a Rotating Disc Electrode (RDE) as working electrode to keep mass transport under defined conditions.

During the measurement, the potential (beginning at a defined initial potential value) is swept to the end potential in a linear fashion over a certain time interval (Figure 4). For HER-catalysts, the potential is swept in the negative direction compared to the initial potential; catalysts used for OER purposes are swept to positive potentials with respect to the initial potential.

The overpotential (η) is a very important parameter to evaluate the activity of the catalyst; its value is affected by the kinetic barrier for the reaction. To overcome this barrier, a higher potential than the thermodynamic potential (1.23 V for OER, or 0 V for HER) – the overpotential – has to be applied to reach the same current densities. The more active the catalyst, the lower the overpotential.

Figure 4. General diagram of linear sweep voltammetry.
Tafel analysis

Aside from the overpotential, the Tafel slope and exchange current density are two additional parameters that help characterize the activity of the catalyst. They can be accessed by plotting the logarithm of the kinetic current density versus overpotential to create the so-called «Tafel plot» (Figure 5).

Figure 5. Tafel plot of OER for two different catalyst materials.

By evaluating the Tafel plot, these two important kinetic parameters can be extracted. One is the Tafel slope (b) which can be expressed by the following equation:

η = a + b log i

  • η = overpotential
  • i = current density

The other parameter is the exchange current density (i0), which can be obtained by extrapolating the current at zero overpotential (Figure 6).   

The Tafel slope is related to the catalytic reaction mechanism in terms of electron-transfer kinetics. For example, faster electrocatalytic reaction kinetics lead to a smaller Tafel slope which is shown in a significant current density increment as a function of the overpotential change (Figure 6).

Koutecký and Levich analysis

The exchange current density (i kin0) describes the charge transfer under equilibrium conditions. A higher exchange current density means a higher charge transfer rate and a lower reaction barrier. As explained earlier in this article, for a better electrocatalyst, a lower Tafel slope and a higher exchange current density are expected.

When performing real experiments, mass transport is the limiting factor at higher overpotentials (Figure 5) which leads to a nonlinear slope of the Tafel plot.

To overcome the undesirable impacts of mass transport (e.g. diffusion), a Rotating Disc Electrode (RDE) (Figure 3) is used. The current signal is therefore measured at different rotation rates. From this data set, it is possible to extract the pure kinetic current (i kin0) according to Koutecký and Levich (Figure 6).

Figure 6. Tafel plot of OER for two different catalysts. The exchange current density (i kin0) is determined by extrapolation to η = 0.

You can find more information about Koutecký and Levich analysis by downloading our free Application Note:  

Impedance analysis

Another measuring technique can be used to determine the reaction kinetics of catalysts: electrochemical impedance spectroscopy (EIS). One of the most important advantages of impedance spectroscopy is that it is a non-destructive and non-invasive measuring technique. This enables consecutive measurements to be performed on the same sample, such as experiments at different temperatures or at different current densities. Also, the aging effects of catalysts can easily be determined this way.

Impedance spectroscopy is an alternating electric current (AC) technique, where an AC voltage with a very small amplitude of a few millivolts is applied to the electrode which responds with an AC current. The values of the applied voltage signal and the values of the corresponding current signal are used to calculate the AC resistance—the impedance. A broad frequency range of several decades is applied and this enables the identification of the kinetic and transport processes that take place on the electrode over different time scales.

Displaying the impedance spectra in a Nyquist plot (Figure 7) exhibits the charge transfer resistance of the reaction as a semicircle. The diameter of this semicircle corresponds to the reaction kinetics: the smaller the diameter, the faster the kinetics of the reaction.

Figure 7. Example of a Nyquist plot of two different catalysts with different activities.

Electrochemical impedance spectroscopy does not only provide kinetic information to researchers, it also gives insight into mass transport effects and conductivity of electrolytes and membranes.

To find out more about EIS, check out our selection of free Application Notes:

Stability

For industrial use, a catalyst should exhibit an extremely low degradation rate. It needs to be stable for many operating hours. At the development stage, stability is an important factor to determine whether a catalyst has the potential for use in practical applications. Stability can be characterized by the changes of the overpotential or current over time by using chronoamperometry, chronopotentiometry, and cyclic voltammetry. These are described in the following sections.
Chrono methods (chronoamperometry and chronopotentiometry)

When using a chronoamperometry method, a constant voltage is applied to the catalyst and the corresponding current signal is sampled and plotted as an i/t curve (Figure 8).

Figure 8. Chronoamperometry measurement of two different catalysts for OER.

Conversely, when using a chronopotentiometry method, a constant current is applied to the catalyst and the voltage response is measured and plotted as an E/t curve (Figure 9). For this measurement, the longer the tested current or potential remains constant, the better the stability of the catalyst.

Figure 9. Chronopotentiometry measurement of two different catalysts for OER.
Cyclic Voltammetry

Cyclic voltammetry is a technique that measures the current density by cycling the potential of the working electrode linearly versus time (Figure 10) . In contrast to linear sweep voltammetry (Figure 4), after the end potential is reached, the potential in a CV experiment is scanned in the opposite direction to return to the initial potential.

Figure 10. Example of a cyclic voltammetry (CV) diagram with one cycle shown.

To determine the degradation rate of a tested catalyst, usually more than 5000 cycles must be executed with a scan rate between 50–100 mV/s. Before and after CV cycling, linear sweep voltammetry (LSV) is used to examine the overpotential shift at a specific current density.

The smaller the change of the overpotential, the better the electrocatalyst’s stability.

Efficiency

The efficiency (η) can be determined by the faradaic efficiency (or coulombic / coulombian efficiency) in terms of experimental results compared to theoretical predictions.

To be able to calculate the theoretical hydrogen volume via Faraday’s Law, the total charge is needed. This value is measured by the potentiostat using the chronocoulometry method which records the total charge over time (Figure 11).

Figure 11. Example of a chronocoulometry plot.

Conclusion

Choosing the right technique for the analysis of activity, stability, and efficiency of catalysts depends on the specific research and development project focus. Luckily, Metrohm offers a wide range of solutions that will meet all kinds of research requirements.

Visit our website

and learn more about our wide range of solutions for electrochemical research!

Post written by Sandro Haug, Product Manager Electrochemistry at Deutsche METROHM GmbH & Co. KG, Filderstadt, Germany.

Supercharge your battery research – Part 2

Supercharge your battery research – Part 2

Battery technology has come a long way since the rudimentary voltaic pile was developed over two centuries ago. The breakthrough innovation of the lithium ion battery and its subsequent improvements has increased the use and accessibility of electronics, particularly in the consumer market. Electronics are more portable, affordable, and thanks to rechargeable or secondary batteries, they are becoming more sustainable.

Expansion of application possibilities is another reason that energy storage research, particularly batteries, is currently a hot topic. For example, only a decade ago drones were the domain of the military industrial complex, and now a drone with a camera is a standard part of nearly any successful photographer or influencer’s gear. Thanks to an improved battery life and more cost-efficient materials, a drone now has an affordable price tag for a larger segment of the civilian population.

This kind of disruption is happening in larger, more profitable markets as well. Tesla, a newsworthy brand thanks to their technological innovations and public relations, still has a small, albeit increasing, market share of the overall automotive market. Their success has challenged other established brands to recognize that a change from conventional combustion engines can be lucrative. Volvo and Ford are committed to be «fully electric» by 2030 [1]. General Motors (GM) has committed to not only be electric by 2035, but for their business to be carbon neutral by 2040 [2].

The automotive market is a high profile example of an industry that will drastically retool their sector—from manufacturing to sales—and this will happen across many other industries as there is a greater focus on climate change and renewable energy sources from governments and consumers alike. Accurate and scalable R&D will be required to make these transformations possible, and the hunt for improved energy storage solutions is at the heart of these changes.

Electrochemistry was the key to the discovery of energy storage and is the natural technique of choice for future innovations.

Electrochemical characterization techniques for lithium ion batteries

In part one of this series, we introduced various techniques to analyze the composition and purity of electrode materials and lithium salts, in addition to accurate water content determination in the battery materials.

In this article we highlight techniques that will allow the characterization of multiple attributes of the electrochemical behavior of Li-ion batteries using a high precision potentiostat/galvanostat. In some cases, the difference between techniques is due to performing the experiment in a different mode (i.e., potentiostatic or galvanostatic), and the additional information gathered provides a more complete picture of battery behavior.

Galvanostatic Intermittent Titration Technique (GITT)

One of the first techniques available to researchers exploring the properties of battery electrode materials is the Galvanostatic Intermittent Titration Technique (GITT). Usually conducted on a half-cell, this technique is a series of current perturbations followed by a relaxation time, which provides information about the thermodynamic properties and electrode materials including the critical diffusion co-efficient. All of this information gives a better understanding of the electrochemical behavior that can be expected by the materials.

If you’re looking for more information on this subject, download our free application note AN-BAT-003.

Potentiostatic Intermittent Titration Technique (PITT)

Potentiostatic Intermittent Titration Technique (PITT) is similar to the GITT technique detailed above, but the PGSTAT is operated in potentiostatic mode. A series of potential step perturbations is applied to the system, and current is measured as a function of time. Both GITT and PITT are capable of accurately determining the diffusion coefficient.

When using a PGSTAT in galvanostatic mode you can also characterize the performance of Li-ion batteries by using different current rates and charging and discharging during various cycles, known colloquially as «cycling». With this technique researchers can understand the rate performance of the Li-ion battery, its capacity, and the associated power and energy density. This is the most commonly used technique in battery research. A constant current constant voltage (CCCV) procedure is usually applied in order to make sure that a battery is fully charged, while avoiding any battery overcharge.

Learn more about characterizing the performance of lithium ion batteries with cycling by downloading our free application note AN-BAT-002.

CCCV is the industry standard for Li-ion battery charging, and PGSTAT operates in both galvanostatic and potentiostatic mode for this measurement. Galvanostatic cycling is performed within a safe potential window at which the electrolyte is stable. Any slight deviation from the potential cutoff may result in poor cycle life.

Voltage profile of a 18650 Li-ion battery, cycled at ~ C/15 (left), and its corresponding dQ/dV versus V plot (right). The corresponding peaks and plateaus are marked in the figures.

Electrochemical Impedance Spectroscopy (EIS)

Electrochemical Impedance Spectroscopy (EIS) provides additional data and therefore greater insight into the battery’s behavior and potential performance by using galvanostatic charge/discharge cycling and then adding in the most powerful technique that is being used extensively in current battery research. With EIS, the highly dynamic behavior of a battery and the diffusion of ions at the interfaces can be characterized. In a single experimental procedure encompassing a broad range of frequencies, the influence of the governing physical and chemical phenomena may be isolated and distinguished at a given frequency range and state of charge. With EIS it is possible to measure the internal impedance of the battery and model it using the equivalent circuit and understand the contribution of the battery components to the total impedance of the cell.

For EIS determination of batteries it is important to use 4-terminal sensing to avoid the contribution of wires to overall impedance. This is important for any low impedance electrochemical system. Learn more about this research by downloading our free Application Notes AN-EC-013 and AN-BAT-008.

With EIS, it is possible to determine the through-plane tortuosity of battery electrodes, which along with overall electrolyte conductivity, the transference number of a Li-ion of battery electrolyte, and diffusion coefficient of electrolyte gives a good indication of the practicality of certain battery chemistry for high power applications. In addition, the mass transport limitation of the battery separator and its ionic conductivity plays a crucial role in overall performance of the batteries.

By determining the MacMullin number, researchers can determine the quality of the separators for their application in certain Li-ion cells.

Nyquist plot: Negative imaginary part of impedance as a function of the real part of impedance for an 18650 cell.

Download our free Application Notes below for more information on these subjects.

Download our free white paper «A Guide to Li-ion Battery Research and Development» written by electrochemical instrument innovators at Metrohm Autolab. This white paper provides additional information about applicable electrochemical techniques and provides useful definitions to terminologies that are relevant to Li-ion battery research and development.

 

Free white paper:

A Guide to Li-ion Battery Research and Development

Post written by Dr. Reza Fathi, Product Specialist at Metrohm Autolab, Utrecht, The Netherlands.