Observing the Electrochemical Oxidation of Co Metal at the Solid/Liquid Interface Using Ambient Pressure X-ray Photoelectron Spectroscopy

Abstract

Recent advances of ambient pressure X-ray photoelectron spectroscopy (AP-XPS) have enabled the chemical composition and the electrical potential profile at a liquid/electrode interface under electrochemical reaction conditions to be directly probed. In this work, we apply this operando technique to study the surface chemical composition evolution on a Co metal electrode in 0.1 M KOH aqueous solution under various electrical biases. It is found that an ∼12.2 nm-thick layer of Co­(OH)2 forms at a potential of about −0.4 VAg/AgCl, and upon increasing the anodic potential to about +0.4 VAg/AgCl, this layer is partially oxidized into cobalt oxyhydroxide (CoOOH). A CoOOH/Co­(OH)2 mixture layer is formed on the top of the electrode surface. Finally, the oxidized surface layer can be reduced to Co0 at a cathodic potential of −1.35 VAg/Cl. These observations indicate that the ultrathin layer containing cobalt oxyhydroxide is the active phase for oxygen evolution reaction (OER) on a Co electrode in an alkaline electrolyte, consistent with previous studies.

Enhanced Stability and Thickness‐Independent Oxygen Evolution Electrocatalysis of Heterostructured Anodes with Buried Epitaxial Bilayers

“…Figure 6 presents the oxidation currents of LSCO/4 nm LSTO/BLSO/ NSTO structures for varying LSCO film thicknesses characterized by CV using a scan rate of 20 mV s −1 in O 2 saturated KOH (0.1 m). [25] Post CV XPS characterization of the Co film revealed spectral features in the Co 2p and O 1s core levels consistent with the oxidation of Co to CoO(OH) ( Figure S9, Supporting Information). The Co film data exhibits an oxidation current peak at 1.16 V versus RHE (indicted by the blue arrow in Figure 6a) which is close to that reported (1.064 V vs RHE) for the current peak associated with the oxidation of Co(OH) 2 to CoO(OH) in KOH (0.1 m).…”

Glycine‐Induced Electrodeposition of Nanostructured Cobalt Hydroxide: A Bifunctional Catalyst for Overall Water Splitting

“…To test the oxidation state of the Co after the electrocatalytic OER and HER activity, the catalyst was characterized by using XPS (Figure S16 in the Supporting Information). The deconvoluted spectra in the Co 2p region of the used catalyst show BE values that correspond to the +2 and +3 oxidation states of Co . Thus, the used catalyst after the OER and HER reactions has a Co III /Co II oxidation state.…”

“…The prominent peaks at a binding energy (BE) of 781.5 and 797.1 eV are caused by the Co 2p 3/2 and Co 2p 1/2 spin–orbit coupling in Co 2+ oxidation state . The peak with a BE above 781.0 eV correlates with Co 2p 3/2 of Co(OH) 2 ,. A spin–orbit splitting value (Δ E ) between the two 2p 1/2 and 2p 3/2 energy levels above 15 eV is associated with Co 2+ .…”

In Situ Electrochemical Cells to Study the Oxygen Evolution Reaction by Near Ambient Pressure X-ray Photoelectron Spectroscopy

“…Compared to ex situ, UHV-based XPS [5], NAP-XPS can help to establish more robust structure-function correlations by simultaneously monitoring changes in (1) the electronic structure of the working catalyst, (2) the nature of the adsorbed reactant molecules, and (3) the nature of the products formed at pressures close to relevant conditions. The application of this technique to increasingly important energy-related electrochemical processes is currently pursued at synchrotrons worldwide, with pioneering work at ALS (Berkeley) [6][7][8][9], BESSY II/HZB (Berlin) [10][11][12][13][14][15], and SSRL (Stanford) [16][17][18].…”

“…To investigate electrochemical reactions in liquid electrolytes by soft X-ray spectroscopies, an alternative approach uses electron-transparent graphene membranes to separate the liquid reaction environment from the vacuum section of the analysis chamber [13,21]. In the tender X-ray regime, the excited photoelectrons have higher inelastic mean free paths (IMFP) and can penetrate thin matter: In the "dip & pull" technique, photoelectrons are analyzed after penetrating a thin film of liquid electrolyte formed on a metal electrode [7][8][9]. Since photoelectrons excited by tender X-rays can penetrate ultra-thin Si membranes, in yet another approach these membranes are used to separate the liquid solution from the evacuated analyzer chamber [22].…”