Reactive Fe-Sites in Ni/Fe (Oxy)hydroxide Are Responsible for Exceptional Oxygen Electrocatalysis Activity

Abstract

Fe is a critical component of record-activity Ni/Fe (oxy)hydroxide (Ni(Fe)OH) oxygen evolution reaction (OER) catalysts, yet its precise role remains unclear. We report evidence for different types of Fe species within Ni(Fe)OH- those that are rapidly incorporated into the Ni oxyhydroxide from Fe cations in solution (and that are likely at edges or defects) and are responsible for the enhanced OER activity, and those substituting for bulk Ni that modulate the observed Ni voltammetry. These results suggest that the exceptional OER activity of Ni(Fe)OH does not depend on Fe in the bulk or on average electrochemical properties of the Ni cations measured by voltammetry, and instead emphasize the role of the local structure.

Fe2+‐Doped Layered Double (Ni, Fe) Hydroxides as Efficient Electrocatalysts for Water Splitting and Self‐Powered Electrochemical Systems

“…For comprison, the Ni(OH) 2 electrode is tested under the same condition. It clearly indicates that Ni(OH) 2 has a higher overpotential than that of Fe‐based hydroxides at the same current density, demonstrating that Fe‐doped nickel oxyhydroxides possess superior inherent catalytic activity for OER in the alkaline electrolyte, which is in agreement with the experimental and theoretic results in previous reseaches . However, it is necessary to further reduce their overpotential to satisfy the practical applications.…”

Iron‐Doped LiCoO2 Nanosheets as Highly Efficient Electrocatalysts for Alkaline Water Oxidation

“…To determine the oxidation state of Fe ions, 57 Fe Mössbauer spectra were recorded for the samples of LCFO82 NS and LCFO82 bulk at room temperature. Both of samples exhibit primarily a symmetric doublet with isomer shifts at around 0.3 mm s –1 (as listed in Table ), close to 0.37 mm s –1 reported previously for octahedral FeO 6 environments, suggesting that there are only Fe 3+ ions in the high spin state. Comparing to LCFO82 bulk, slightly increased isomer shift of Fe 3+ ions for LCFO82 NS reveals an increase of d elecrtron density (may be due to the enhanced covalency of Fe–O bond) that make the nucleus becomes a little more strongly shielded from s‐electrons.…”

Operando X‐Ray Absorption Spectroscopy Shows Iron Oxidation Is Concurrent with Oxygen Evolution in Cobalt–Iron (Oxy)hydroxide Electrocatalysts

“…This is consistent with conversion from the hydroxide to oxyhydroxide phase during the Co 2+/3+ oxidation wave (Supporting Information, Figure S1C). [17,18,21] Upon Fe addition, Co oxidation during the OER is suppressed, evidenced by the lack of significant change in the Co K-edge energy when stepping the potential to OER conditions ( Figure 2B,C). E O 2 =OH À where the OER occurs.There is no change in the pre-edge feature.This shift is most likely due to afraction of Co being oxidized from Co 3+ to Co 4+ ,c onsistent with reports of Co 4+ during the OER.…”

“…[14] Co(Fe)O x H y is structurally and electrochemically similar to Ni(Fe)O x H y . [8] Operando XAS suggests oxidation of Co during the OER to as imilar extent for both samples.F e oxidation was not found in either sample,b ut adsorbed Fe 3+ exhibited alower oxygen coordination and was concluded to be responsible for the higher activity of the sample.U ndercoordinated Fe and Co edge-sites have also been hypothesized to be the active sites in Ni(Fe)O x H y [10,[17][18][19] and CoOOH, respectively. Smith et al examined photochemically deposited Fe 100Ày Co y O x films using operando electrochemical XAS and observed oxidation of Co,b ut not Fe during the OER.…”