In Situ X‐Ray Absorption Spectroscopic Studies of Nickel Oxide Electrodes

In situ transmission x-ray absorption spectroscopy was used to investigate changes in the structure of Ni(OH)2 as it was cycled as a nonsintered nickel oxide electrode in concentrated alkali. Results were obtained for dry as-prepared electrodes and electrodes that were charged or discharged. Changes were observed in the (0001) plane of the Ni(OH)2 as a function of cycling. In the charged material there was a contraction in the Ni-O and Ni-Ni distances. The EXAFS results and the XANES spectra strongly indicate that the Ni-O coordination is symmetrical in Ni(OH)2, whereas the coordination is asymmetric in the charged material. The nickel oxide electrode, also known as the nickel or nickel hydroxide electrode (NOE), is used in batteries, fuel cells, and electrolyzers. Other possible uses are in electrochromic and switching devices. Because of its many applications it has been the subject of numerous investigations and reviews (1-4). In spite of this, the electrode processes remain an enigma. Much of this confusion is due to the amorphous nature of the reactants and products. Extended x-ray absorption fine structure (EXAFS) technique can be used to study the average local structure of systems that do not possess long range order. A previous paper has presented preliminary results of an in situ EXAFS study of a nickel oxide electrode (5). This work has shown that the EXAFS spectroscopy can be used to study electrochemical systems under working conditions (in situ). The present paper is a more extensive report of the previous work which includes analysis of higher shells and x-ray absorption near edge structure (XANES).