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Spectroelectrochemistry (SEC) Techniques


Spectroelectrochemistry (SEC) is an experimental technique that combines electrochemistry and spectroscopy.

While electrochemical experiments provide information on macroscopic properties like reaction rates, spectroscopic techniques give information on a molecular level, such as the structure of molecules and their electronic configuration. The combination of electrochemical and spectroscopic approaches, therefore, provides a more comprehensive picture of a system under study [1-4].

Spectroelectrochemical Techniques


Spectroelectrochemical techniques can generally be classified based on the type of spectroscopy (UV-vis, NIR or IR, Raman, EPR, NMR, XAS, or luminescence) or the electrochemical method being used.

Electrochemical methods commonly used for spectroelectrochemistry include:

  • Chronopotentiometry and chronoamperometry, where current intensity / potential difference (respectively) is measured as a function of time by applying a constant potential difference / current to the working electrode.
  • Voltammetric techniques such as linear sweep voltammetry, cyclic voltammetry, differential pulse voltammetry, and stripping voltammetry, where current is measured as a function of the linear change in potential of the working electrode.
  • Pulse techniques, where current change is measured as a function of potential difference by applying pulse potential functions to the working electrode.

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It is more common for spectroelectrochemical techniques to be classified according to their spectroscopic methods [6] than their electrochemical methods.

UV-vis absorption spectroelectrochemistry

The combination of UV-vis spectroscopic techniques with electrochemical systems is known as UV-vis spectroelectrochemistry (UV-vis-SEC) [7]. UV-vis spectroscopy probes the electronic transitions of molecules that absorb light in the ultraviolet and visible regions of the electromagnetic spectrum. Both the wavelengths at which absorption occurs and degree of absorption are measured by an optical spectrometer. The spectrum is then presented as a graph of absorbance versus wavelength.

UV-vis spectroelectrochemical measurements can be performed in various modes such as absorbance versus wavelength or absorbance versus time. Based on the applied electrode potential, the resulting spectrum shows only the absorptions caused by electrochemically generated species [7].

With optically transparent electrodes (OTE), UV-vis-SEC can study molecular adsorbates, polymer films, or other modifying layers attached to the electrode surface or residing in the phase adjacent to it.

Photoluminescence spectroelectrochemistry

Spectroelectrochemical methods can be used to spectroscopically detect unique chemical species that are generated in situ at electrode surfaces during redox reactions. When photoluminescence spectroscopy is coupled to electrochemistry, photoluminescence spectroelectrochemistry (PL-SEC) is obtained. PL-SEC allows for selective investigation of the excited state properties of in situ generated chromophores [8].

With PL-SEC, properties of new excited state species can be investigated, as well as their reactivity. One example of this is the Re(II) complex, [Re(dmpe)3]2+, where dmpe is 1,2-bis(dimethylphosphino)ethane. The parent Re(I) complex, [Re(dmpe)3]+, is readily synthesised and exhibits a reversible one-electron oxidation to Re(II) in acetonitrile. The d6 Re(I) form is colorless and upon excitation into the UV absorption band exhibits no luminescence. By contrast, oxidation in the luminescence spectroelectrochemical cell to the d5 Re(II) electron configuration affords a reddish-pink solution with an absorption maximum at 530nm, which is accruable to a ligand-to-metal charge-transfer (LMCT) band. Excitation at 530nm produces a strong emission at 593nm with a quantum efficiency of 0.066, significantly higher than that of the standard typically used for comparison, i.e., [Ru(bpy)3]2+ in water, which is 0.042 [8].

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Infrared spectroelectrochemistry

The application of infrared (IR) spectroscopy to study electrochemical systems is termed infrared spectroelectrochemistry (IR-SEC). There is also increasing gravitation towards combining IR-SEC with other physical techniques such as scanning electrochemical microscopy [9].

  • IR-SEC has been majorly applied to investigate adsorption phenomena in electrochemical systems.
  • Mechanistic studies in electrocatalysis are another major application of IR-SEC.
  • IR spectroscopy has been extremely successful in identifying and unraveling molecular mechanisms of biological electron transfer and of catalysis coupled to redox reactions.
  • Other major fields of application of IR-SEC include conducting polymers, redox-active organic molecules in solution, and inorganic or organometallic complexes.

Raman spectroelectrochemistry

Raman spectroelectrochemistry (Raman SEC) refers to Raman spectroscopy combined with an electrochemical setup. In Raman SEC, vibrational information is used to gauge the strength of chemical bonds within molecules and between molecules and electrode surfaces.

Raman spectroelectrochemistry is the method of choice to analyse the structure of materials:

  • Carbon materials like graphite, graphene, carbon nanotubes (or nanostructures in general), and fullerenes.
  • Information about crystalline or amorphous structure and crystallite size can be obtained from shift and shape of vibration modes.
  • Useful information can be obtained about the changes occurring in the structure of the sample material upon charging.
  • The state and quality of nanostructured materials for battery research as well as lithium content can be concluded by their characteristic vibration modes.
  • Worthy information can be obtained for the orientation of molecules on the surface of electrodes in case of self-assembled monolayers (SERS) [10].

X-ray spectroelectrochemistry

X-ray spectroelectrochemistry (X-ray SEC) is a technique that combines X-ray absorption spectroscopy with electrochemistry.

  • X-ray SEC provides information about some basic considerations and symmetry on the electronic structure of active atomic orbitals of different materials, such as bond lengths and angles, identity of neighbouring atoms, oxidation states, etc [6].
  • X-ray SEC is useful for spectroelectrochemical studies on corrosion processes, electrodeposition, ionic adsorption and yields of fuel cell anodes [6].

Nuclear magnetic resonance spectroelectrochemistry

The coupling of electrochemistry and NMR spectroscopy is termed nuclear magnetic resonance spectroelectrochemistry (NMR-SEC).

  • NMR-SEC presents an interesting approach to monitor and understand electrochemical and electrocatalysis processes at a molecular level.
  • NMR-SEC technique can provide dynamic structural characterisation and mechanism elucidation in an electrochemical process, as in the study of the redox behaviours of quinone-based systems [11], electrooxidation of dopamine at various voltages and pH using Au nanoparticles modified electrode [12], and electrocatalysis of hydroquinone by nano-polyaniline (nano-PAn) films [13].

Electron paramagnetic resonance spectroelectrochemistry

Simultaneous combination of electrochemical and electron spin/paramagnetic resonance gives electron spin/paramagnetic resonance spectroelectrochemistry (ESP/EPR-SEC). ESP/EPR-SEC offers the possibility of detecting short-lived radicals. There is also a prospect of combining UV-vis spectroscopy with in-situ ESR/EPR spectroscopy to achieve UV-vis-ESR/EPR-SEC.

Since ESP/EPR can provide valuable information about the oxidation state of an atom or atoms within a (macro)molecule and its environment by using unpaired electrons as probes, ESP/EPR-SEC can be used to investigate paramagnetic species such as organic radicals, proteins, polymers, and transition-metal complexes [14].

  

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References


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  • R. Holze, Surface and interface analysis. In: Springer series in chemical physics, 2009, vol 74. Springer, Berlin/Heidelberg.
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  • M. Bron, Infrared Spectroelectrochemistry, Institut fur Chemie, Technische Chemie, Martin-Luther-Universitat Halle-Wittenberg, Halle, Germany in G. Kreysa et al. (eds.), Encyclopedia of Applied Electrochemistry, © SpringerScience+Business Media, New York, 2014, 1071-1075. https://doi.org/10.1007/978-1-4419-6996-5.
  • S. Bilal, Raman Spectroelectrochemistry, National Centre of Excellence in Physical Chemistry, University of Peshawar, Peshawar, Pakistan, in G. Kreysa et al. (eds.), Encyclopedia of Applied Electrochemistry, © SpringerScience+Business Media, New York, 2014, 1761-1765. https://doi.org/10.1007/978-1-4419-6996-5.
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  • S. Neukermans, M. Samanipour, H.Y.V. Ching, J. Hereijgers, S.V. Doorslaer, A. Hubin, T. Breugelmans, A versatile in-situ EPR spectroelectrochemical approach for electrocatalyst research, ChemElectroChem, 7(2), 4578−4586. https://doi.org/10.1002/celc.202001193.

Contributing Authors


  • Stephen O Aderinto
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