Metal hydroxides for energy conversion and energy storage

Electrocatalysts, electrochromic devices, and pseudo-capacitors based on transition metal (oxy)hydroxides depend on the reversibility of the reduction-oxidation process of metal cations. Rapid switching between different redox states is often involved, particularly in electrocatalysis where redox metal sites act as active centers for electron transfer to the reactant. To ensure long-term durability, the reversibility of the redox metal sites should be robust. Nickel hydroxide is a model catalyst for the oxygen evolution reaction (OER) and the basic representative of the layered double hydroxides. It is frequently combined with other transition metals (e.g. Fe, Co, Mn), forming some of the most active OER electrocatalysts in alkaline media. [1] I will present the use of in-situ spectroscopy to track the reversibility of redox states of the Ni(OH)2 during its lifetime. During the operation at 200 mA cm-2 in 1 M KOH electrolyte, the catalytic activity of Ni(OH)2 gradually degrades until lastly, the catalyst breaks down. During the catalyst lifetime, the reduction-oxidation reversibility of the Ni2+/3+ redox couple is lost and the catalyst converts into an inactive phase. The reversibility of the redox couple is monitored by the in-situ UV/Vis spectroscopy. During the catalyst lifetime, the reversibility of the redox peak is lost. The activity collapse is attributed to the structural amorphization/disordering of the layered Ni(OH)2 catalyst, as confirmed by TEM investigations and in-situ Raman spectroscopy. [2] Similarly, the redox reversibility of metal sites is also important for long cycle life in supercapacitors, based on the pseudo-capacitance mechanism. Contrary to catalysts, for supercapacitors, the water oxidation needs to be suppressed to increase the working voltage range. I will discuss the mechanisms for the deactivation of transition metal hydroxides to serve as capacitors and approaches to increase power density. Finally, I will discuss the use of mixed metal hydroxides to serve as precursors for a copper oxide-based catalytic system for CO2 hydrogenation to methanol. Thermal decomposition of hydrotalcite-based hydroxide precursor is followed by in-situ x-ray diffraction. The conditions to prepare disordered oxide in contact with catalytical active Cu metal are identified and the catalytic performance of catalysts with crystalline and disordered oxide phases are compared. [1] A. Mavrič, C. Cui, (2021), Advances and Challenges in Industrial-Scale Water Oxidation on Layered Double Hydroxides, ACS Appl. Energy Mater., 4, 12032-12055. [2] A. Mavrič, M. Fanetti, Y. Lin, M. Valant, C. Cui, (2020), Spectroelectrochemical Tracking of Nickel Hydroxide Reveals Its Irreversible Redox States upon Operation at High Current Density, ACS Catal., 10, 9451-9457