Active Motif Change of Ni‐Fe Spinel Oxide by Ir Doping for Highly Durable and Facile Oxygen Evolution Reaction

Abstract

The oxygen evolution reaction (OER) is crucial for producing sustainable energy carriers. Herein, Ir (5 mol.%) doped inverse-spinel NiFe 2 O 4 (Ir-NFO) nanoparticles deposited on Ni foam (NF) by scalable solution casting are considered a promising OER electrocatalyst for industrial deployments. The Ir-NFO/NF (with minimal lattice distortion by uniform Ir doping) provides an OER overpotential of 251 mV (intrinsically outperforming NFO/NF and benchmarking IrO 2 /NF) and extraordinary robustness over 130 days at 100 mA cm −2 . In situ X-ray absorption spectroscopy reveals oxidation only for Fe on NFO, whereas concurrent generation of higher-valent Ni and Fe occurs on Ir-NFO during OER. Density functional theory calculations further demonstrate that Ir substitutes the sublayer Ni octahedral site and switches the main active reaction center from Fe Oh Fe Td bridge site (FeOFe) on NFO to Ni Oh -Fe Td bridge site (NiOFe active motif ) on Ir-NFO for a co-catalytic OER. This study sheds new light on precious-metal doped Ni-Fe oxides, which may be applicable to other binary/ternary oxide electrocatalysts.

Monolayer Iron and Iron-Rich Hydroxide Nanosheets Exfoliated from High-Quality Green Rust for Enhanced Electrocatalytic Oxygen Evolution Reaction

“…A positive shift in the binding energy of the main peak from 710.0 eV for Fe­(OH) 2 to 710.3 eV for GR indicates the decrease in the amount of Fe 2+ after topochemical oxidation of Fe­(OH) 2 . Additionally, Fe 2p 3/2 spectra were deconvoluted into two and one multiplet peak for Fe 2+ (∼709.5, ∼710.7 eV) and Fe 3+ (∼712.2 eV) species, respectively, as well as a satellite peak (∼714.9 eV), and the fitted peak areas were used to evaluate the [Fe 3+ ]/[Fe total ] ratios in different iron hydroxides. − The surface Fe 3+ content in GR-I and GR-DS with 0.31 is fairly close to the expected value of 0.33, which is also consistent with the information reflected by EDS analysis. However, the deconvoluted peak component of Fe 3+ species in Fe­(OH) 2 indicates partial surface oxidation of it despite the protective measures.…”

Electrocatalytic Mechanism of Defect in Spinels for Water and Organics Oxidation

“…The Gibbs free energy at different potentials was calculated, and Ir-NFO exhibited low limiting potential compared with NFO revealing excellent OER ability (Figure 2i). [79] It can be concluded that the substitution of Ir in octahedral sites caused a redistribution of charges with electrons transferring preferentially from Ir to Ni/Fe in Ir-NFO leading to enhanced adsorption towards oxygen species and lowered Gibbs free energy barrier.…”

“…g) Gibbs free energy of oxygen adsorption for various active sites, h) PDOS and i) OER Gibbs free energy barrier of NFO and Ir-NFO. Reproduced with permission [79]. Copyrigh 2023, Wiley-VCH.…”

Designing active and stable Ir-based catalysts for the acidic oxygen evolution reaction

“…Most of the materials reported to date accomplish the OER mainly through this mechanism. 18,19 For example, Hong et al 20 reported that Ir–NiFeO followed the AEM to complete the OER because the Ni–O–Fe bridge structure facilitated the immobilization of lattice oxygen. More recently, an alternative pathway, which involves a direct *O–O lattice bond coupling, has been identified as a lattice oxygen oxidation mechanism (LOM) in the OER.…”