Conventional and High-Entropy Spinels As Oxygen Evolution Electrocatalysts (ECs) for Implementation in Low-Temperature Electrolyzers (ELs): Synthesis, Physicochemical Studies and Electrochemical Characterization
Authors: Boudjelida Soufiane , Nale Angeloclaudio , Vezzù Keti , Pagot Gioele , Negro Enrico , Santangelo Saveria , Gross Silvia , Vendrame Davide , Di Noto Vito 
Published: December 2024

One of the cornerstones of the “hydrogen economy” is the widespread implementation of “green hydrogen” to a variety of applications spanning from transportation to industry and stationary power. The most straightforward approach to obtain such “green hydrogen” in large amounts at acceptable costs is to use the energy obtained from renewable sources to split water in an electrolyzer (EL) [1]. Low-temperature ELs running on either alkaline or neutral water are highly promising technologies to obtain “green hydrogen” since their electrode configurations can operate effectively even without the use of electrocatalysts (ECs) based on critical raw materials (CRMs) such as platinum-group metals (PGMs). To ensure that the conversion efficiency of such ELs is high enough for practical applications, it is mandatory that the overpotential in the oxygen evolution reaction (OER) is minimized by the implementation of a suitable EC at the anode.

This report describes both conventional and high-entropy spinels as promising OER ECs for implementation at the anode of low-temperature ELs. The spinels include a variety of metal cations (e.g., Ni, Cr, Mn, Fe, Zn, Co and Cu) and are typically obtained by a combination of steps carried out either/both at low T (e.g., hydrothermal processes) and at high T (e.g., calcination). The chemical composition of the spinels is determined both in the bulk and on the surface implementing analytical techniques such as inductively-coupled plasma atomic emission spectroscopy (ICP-AES), X-ray fluorescence and near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). The morphology of the spinels is elucidated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM); the porosimetric features are studied by nitrogen physisorption measurements. The structure of the spinels is probed by vibrational spectroscopies (e.g., confocal micro-Raman) and wide-angle powder X-ray diffraction. The OER kinetics and reaction mechanism of the spinels are quantified by means of cyclic voltammetry with the thin-film rotating ring-disk electrode (CV-TF-RRDE). Finally, the OER durability of the most promising candidates is evaluated “ex-situ” after the implementation of suitable accelerated ageing protocols. The results of the physicochemical and electrochemical characterizations are correlated with the synthetic parameters, yielding information on the most promising approaches to obtain spinel-based OER ECs able to bestow to low-T ELs a performance and durability beyond the state of the art.

References

[1] Hydrogen’s Big Shot, ECS Interface, 30(4), Winter 2021.

Acknowledgements

This work has been supported under the National Recovery and Resilience Plan (NRRP), funded by the European Union - NextGenerationEU [Award Number: CNMS named MOST, Concession Decree No. 1033 of June 17, 2022, adopted by the Italian Ministry of University and Research, Spoke 14 “Hydrogen and New Fuels”].