Evaluation of Dip-And-Pull Ambient Pressure X‑ray Photoelectron Spectroscopy for Investigating Oxygen Evolution Reaction Electrocatalysts

In situ investigations of solid–liquid interfaces are crucial for gaining a fundamental understanding of electrocatalytic processes. Dip-and-pull ambient pressure X-ray photoelectron spectroscopy (APXPS) enables such investigations by analyzing an electrocatalyst surface (solid) through the covering electrolyte layer (liquid) with an applied electrochemical potential. This stable solid–liquid interface is created by vertically “dipping” and then “pulling” an electrode from a bulk electrolyte solution. The resulting electrolyte layer has a decreasing thickness toward the upper electrode, allowing in situ probing of the electrocatalyst surface in this upper region. However, detecting representative electrocatalyst surface changes remains challenging with dip-and-pull APXPS. To address the challenge, this study experimentally evaluates electrochemical and spectroscopic aspects of dip-and-pull APXPS by investigating thin films of Ni_1–<i>x</i>Fe_<i>x</i>O_<i>y</i> oxygen evolution reaction (OER) electrocatalysts. Here, two technical limitations are revealed: (1) missing Faradaic reactions (i.e., redox and OER) in the electrocatalyst surface probing region and (2) low spectroscopic surface-sensitivity with the typically used tender X-rays. Limitation (1) is discovered with a modified electrode design that enables OER activity measurements, indicating limited ionic conductance along the vertically thinning electrolyte layer. This strongly suppresses the studied Faradaic reactions and hinders operando investigations of OER electrocatalyst thin films in the upper electrode region, where their surfaces are probed. To minimize limitations (1) and (2), the findings suggest changing electrochemical potential only when the electrode is completely dipped in the bulk electrolyte to enhance surface modifications and using lower energetic photons at higher flux to improve the surface-sensitivity. Moreover, cyclic voltammetry is presented as an electrochemical conditioning method to maximize the spectroscopic detectability of OER electrocatalyst surface changes. Overall, this dip-and-pull APXPS evaluation provides fundamental insights and suggestions that will further improve the technique for investigating OER electrocatalysts.