Investigating the interplay between charge transfer and CO2 insertion in the adsorption of a NiFe catalyst for CO2 electroreduction on a graphite support through DFT computational approaches.

This article describes a density functional theory (DFT) study to explore a bio-inspired NiFe complex known for its experimental activity in electro-reducing CO2 to CH4 when adsorbed on graphite. The coordination properties of the complex are investigated in isolated form and when physisorbed on a graphene surface. A comparative analysis of DFT approaches for surface modeling is conducted, utilizing either a finite graphene flake or a periodic carbon surface. Results reveal that the finite model effectively preserves all crucial properties. By examining predicted structures arising from CO2 insertion within the mono-reduced NiFe species, whether isolated or adsorbed on the graphene flake, a potential species for subsequent electro-reduction steps is proposed. Notably, the DFT study highlights two positive effects of complex adsorption: facile electron transfers between graphene and the complex, finely regulated by the complex state, and a lowering of the thermodynamic demand for CO2 insertion.

Direct Detection of FeVI Water Oxidation Intermediates in an Aqueous Solution.

In situ detection of highly-oxidized metal intermediates is the key to identifying the active center of an oxygen evolution reaction (OER) catalyst, but it remains challenging for NiFe-based catalysts in an aqueous solution under working conditions. Here, by utilizing the dynamic stability of the FeVI O4 2- intermediates in a self-healing water oxidation cycle of NiFe-based catalyst, the highly-oxidized FeVI intermediates leached into the electrolyte are directly detected by simple spectroelectrochemistry. Our results provide direct evidence that Fe is the active center in NiFe-based OER catalysts. Furthermore, it is revealed that the incorporation of Co into NiFe-based catalyst facilitates the formation of FeVI active species, thus enhancing the OER activity of NiCoFe-based catalyst. The insights into the mechanisms for the sustainable generation of FeVI active species in these NiFe-based catalysts lay the foundation for the design of more efficient and stable OER catalysts.

Introducing Fe2+ into Nickel-Iron Layered Double Hydroxide: Local Structure Modulated Water Oxidation Activity.

Exploring materials with regulated local structures and understanding how the atomic motifs govern the reactivity and durability of catalysts are a critical challenge for designing advanced catalysts. Herein we report the tuning of the local atomic structure of nickel-iron layered double hydroxides (NiFe-LDHs) by partially substituting Ni2+ with Fe2+ to introduce Fe-O-Fe moieties. These Fe2+ -containing NiFe-LDHs exhibit enhanced oxygen evolution reaction (OER) activity with an ultralow overpotential of 195 mV at the current density of 10 mA cm-2 , which is among the best OER catalytic performance to date. In-situ X-ray absorption, Raman, and electrochemical analysis jointly reveal that the Fe-O-Fe motifs could stabilize high-valent metal sites at low overpotentials, thereby enhancing the OER activity. These results reveal the importance of tuning the local atomic structure for designing high efficiency electrocatalysts.

Decorated Oxidation-resistive deficient Titanium oxide nanotube supported NiFe-nanosheets as high-efficiency electrocatalysts for overall water splitting.

In this study, oxidation-resistive deficient TiO2-x supported NiFe-based electrocatalysts were developed towards efficient and durable water splitting performance. The oxidation-resistive deficient TiO2-x support with oxygen vacancies ensures good stability and electrical conductivity of the catalyst. The decorated NiFe and NiFeP nanosheets serve as efficient catalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. In 1 M KOH, the NiFe@TiO2-x and NiFeP@TiO2-x electrodes show low overpotential for OER (300 mV) and HER (273 mV) at 100 mA cm-2, respectively, and excellent stability performance in overall water splitting as well. In-situ Raman and theoretical analysis reveals that the in-situ formed Fe3+-doped NiOOH species are essential in catalyzing OER on NiFe@TiO2-x, particularly the electron localization of surface Fe-O bonds offers lower energy barriers for OER elemental reactions and thus enhance its catalytic activity. This work provides an oxide-based catalyst support strategy for the development of stable and active overall water splitting catalysts, and advances the insights on catalytic origin of NiFe-based catalysts as well.