Electrochemical and in Situ Raman Spectroscopic Characterization of Ni Surface Using Inorganic Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy
Authors: Suzuki Akihiro , Kakinuma Katsuyoshi , Kuzume Akiyoshi 
Published: December 2024

・Introduction

Anion exchange membrane water electrolyzer (AEMWE) is expected to reduce the production cost by using base metals for the electrocatalysts and to have high electrolytic efficiency due to its membrane electrode assembly (MEA) structure. The improvement of electrochemical activity and durability of the catalysts remains one of the essential issues for practical application. At present, Ni based oxide is one of the essentially candidate catalysts for both the anode and cathode of AEMWE.
Ni is easily oxidized and formed the oxygen-containing adsorbates. The formation of Ni surface oxides depends strongly on the synthesis condition, dopant elements, and the applied potentials. Their peak assignment in the potential-dependent spectra are only possible with well-controlled surfaces. Therefore, an accurate understanding of the surface states and the catalytic reaction process on Ni oxide surface requires the Ni catalyst with well-controlled surface and a highly sensitive analytic technique. Herein, we introduced the inorganic shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) to confirm the surface states and surface reaction precisely.
The SHINERS is the advanced mode of the surface-enhanced Raman Scattering (SERS) and can detect the Raman signals enhanced by the local electromagnetic field generated on the surface of the nano-amplifier by an applied laser. The SERS has some priorities to detect the signals non-destructively at in situ/operando conditions from extremely small amount of samples at the molecular level, such as self-assembled monolayers and partial surface adsorbates. Introduction of the inorganic shell on the surface of nano-amplifier prevents direct contact and interaction between the sample and the nano-amplifier, selectively detecting intrinsic surface information of samples. In this experiment, the SHINERS technique can provide spectral features of the thin layers of the Ni oxide with high sensitivity and surface selectivity to detect the surface adsorbates in detail.
・Experiment

In situ SHINERS measurements were performed in 0.1 mol L-1 KOH aqueous electrolyte (Ar-saturated) at room temperature, using a homemade in situ Raman cell equipped with a Ag/AgCl reference electrode and a Pt counter electrode, while all potentials were quoted vs. the reversible hydrogen electrode (RHE). In the Raman measurements, Au nanoparticles coated with an alkaline-resistant zirconia shell were introduced as an optical nano-amplifier at the surface coverage of ca. 20%. Raman spectra were acquired using the Confocal Microscope Laser Raman Spectrometer (Horiba Jobin Yvon LabRAM HR Evolution). The excitation laser source was 632.8 nm He-Ne laser with the typical power of max. 0.1 mW on the surface. The spectrometer was equipped with an 800 mm focal length monochrometer with a long working distance objective lens (50× magnification, OLYMPUS).
・Results and discussion

Electrochemical measurements and in situ SHINERS have captured a detail surface features at different potential range, including the redox profile between metallic Ni and α-Ni(OH)2, the phase transition from α-Ni(OH)2 to β-Ni(OH)2, as well as the oxidation process from β-Ni(OH)2 to β-Ni(O)OH on a flat Ni electrode in alkaline condition, together with the formation of the γ-Ni(O)OH and the precursor species of the oxygen evolution reaction (OER) on the Ni electrode. Weak bands of each oxygen-containing adsorbate in the few layers of surface oxide film were assigned in the function of applied potential, revealing difference with those in literatures observed on the thick layers of Ni oxides. In situ SHINERS studies, incorporated with the electrochemical studies, on a flat Ni electrode allow us to propose a general scheme presenting relationships between the metallic Ni and various oxygen-containing adsorbates formed during the potential scan in the range between hydrogen and oxygen evolution reaction. In addition, the potential-dependent catalytic activity of Ni surface and nano-catalysts with be discussed at the conference.

Figure 1