New Advances in Ambient Pressure X-Ray Photoelectron Spectroscopy: Operando Probing of the Electrical Double Layer at the Solid/Liquid Interfac
Authors: Favaro Marco , Jeong Beomgyun , Ross Philip N , Yano Junko , Hussain Zahid , Liu Zhi , Crumlin Ethan J 
Published: February 2020

The conceptual model of the electrical double layer (EDL) constitutes one of the pillars of modern electrochemistry. Although structural and electrical models of the organization of ions and solvent in the EDL have been developed and used, a direct operando observation (defined here as a function of the applied potential) of the potential drop within the EDL has not been achieved so far because of the unavailability of the appropriate experimental tools. To address this missing need, we performed operando
ambient pressure X-ray photoelectron spectroscopy (APXPS) (
[i]
,

[ii]
) at the working electrode/liquid electrolyte interface (
[iii]
,
[iv]
,
[v]
,

[vi]

,

[vii]
) using an excitation energy of 4.0 keV (
[viii]

, [ix]
). The advantage of using ‘tender’ X-rays (2.0 keV – 8.0 keV) relies on the fact that the photoelectron inelastic mean free path (IMFP) in an aqueous electrolyte and for escaping photoelectrons with a kinetic energy >3500 eV is about 10 nm. This provides the ability to directly probe the electrical potential experienced by ions and molecules (
[x]
,
[xi]
,
[xii]
) in a 10-30 nm-thick electrolyte layer (the probed volume given by 3 IMFP), a dimension that coincides with the thickness of the double layer for a dilute solution (Figure 1). For a metal/liquid interphase, the metal core level binding energies remain constant with respect to various applied potentials, i.e. flat-band conditions are maintained because metals cannot support electric fields in their bulk, and thus have a constant potential within the metal. Therefore, the Galvani potential at the junction drops across the EDL (Figure 1). The key spectroscopic parameter used in our study to probe the potentials within the EDL is the FWHM of the core level peaks from elements within the electrolyte, which undergo a broadening as the potential drop in the EDL changes with the applied potential (Figure 1). The observed spectral broadening for elements in the electrolyte (such as O 1s from the water if using an aqueous solution), is caused by the electric field in the double layer originated by the potential drop. The water molecules in the electrolyte experience different electric potentials as function of their position within the potential drop (distance normal to the electrode surface); consequently, the corresponding escaping O 1s photoelectrons are characterized by different apparent binding energies (referenced to the Fermi level of the electrode)
[xiii]
. The final result of the convolution of the single shifted O 1s core level photoelectron peaks leads to the experimentally observed spectral broadening, which becomes broader the larger the electrode potential for a given EDL thickness (i.e. the higher the potential drop within the double layer, compared to the potential of zero charge (PZC) conditions)
[xiii]
. The current results pave the way for studying the EDL dynamics in more complicated systems, such as redox reactions at photoactive semiconductor interfaces and in non-aqueous electrolytes, to name a few. The knowledge gained from this method will thus impact a wide range of scientific fields, including electrochemical conversion and storage of energy, environmental sciences and biology.

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Figure 1