Development of Operando Analytical System for Simultaneous Measurements of Electrochemical Reaction Rate and Haxpes

Abstract

To quantitatively understand the electrochemical reaction, the reaction rate must be measured such as by hydrodynamic voltammetry. At the same time, the electronic states of the electrode surface closely related to the bonding of atoms and molecules, as well as to the reaction mechanisms, must be understood. We have designed and constructed an operando analytical system for the simultaneous measurements of the electrochemical reaction rate and the hard X-ray photoemission spectrum.1) In this system, a solution containing reactants was continuously supplied in a Hagen-Poiseuille flow with a channel flow electrode (CFE) method to a specimen, such as catalyst, under the control of temperature, pressure, concentration, and solution flow rate, so that the electrochemical reaction rate is able to be accurately measured or controlled. Two different environments, solution and ultra-high vacuum, were connected with an ultra-thin Au/SixNy window with a thickness of 20 nm (Fig. 1(a)) to make the operando hard X-ray photoemission spectroscopy (HAXPES) possible during the steady-state electrochemical reaction. In this report, the Au thin film 5 nm in thickness was used as a model electrode.Figure 1(b) shows a schematic illustration of the operando reaction rate-HAXPES measurement system. This system was installed to a UHV chamber at the beamline BL46XU of SPring-8, Japan.2) The energy of the synchrotron X-ray was 14 keV. The analyzer was Focus HV-CSA 300/15. Figure 1(c) shows the chronoamperometry result of the oxygen reduction reaction obtained at 0.4 V vs. Ag/AgCl sat. KCl in 0.1 M HClO4 at room temperature. From the current, the reaction rate was calculated as 0.9 nmol s-1 cm-2.1) Figure 1(d) shows the Au3d5/2 HAXPES spectrum of the Au thin film electrode measured during electrochemical reaction (blue circles for data and solid line for fitting). The horizontal axis indicates the kinetic energy of photoelectrons. The green triangles and solid line are of the spectrum of the reference metal Au. When the kinetic energy of the photoelectrons of Au3d5/2 was 11,809 eV, the inelastic mean free path estimated from the TPP-2M equation was 8.7 nm.3) Therefore, the HAXPES signal was from the entire Au film electrode.With this high energy of X-ray, HAXPES of the specimens located on this Au electrode can be measured as expected. Separately, a Pt film was deposited onto the Au thin film electrode. After the measurement of the Au peak, only the scanning energy was changed to observe clearly the Pt3d5/2 peak from the Pt film through the underlying Au thin film. In the future, specimens, such as fuel cell catalysts, can be located on the Au thin film electrode for the operando measurements. References 1) K. Suda et al., J. Electrochem. Soc., (2021) https://doi.org/10.1149/1945-7111/abf9712) H. Oji et al., J. Phys. Conf. Ser., 502, 012006 (2014).3) S. Tanuma et al., Surf. Interface Anal., 21, 165 (1994). Figure 1