Oxygen-Plasma-Induced Hetero-Interface NiFe2O4/NiMoO4 Catalyst for Enhanced Electrochemical Oxygen Evolution

Abstract

The electrolysis of water to produce hydrogen is an effective method for solving the rapid consumption of fossil fuel resources and the problem of global warming. The key to its success is to design an oxygen evolution reaction (OER) electrocatalyst with efficient conversion and reliable stability. Interface engineering is one of the most effective approaches for adjusting local electronic configurations. Adding other metal elements is also an effective way to enrich active sites and improve catalytic activity. Herein, high-valence iron in a heterogeneous interface of NiFe2O4/NiMoO4 composite was obtained through oxygen plasma to achieve excellent electrocatalytic activity and stability. In particular, 270 mV of overpotential is required to reach a current density of 50 mA cm−2, and the overpotential required to reach 500 mA cm−2 is only 309 mV. The electron transfer effect for high-valence iron was determined by X-ray photoelectron spectroscopy (XPS). The fast and irreversible reconstruction and the true active species in the catalytic process were identified by in situ Raman, ex situ XPS, and ex situ transmission electron microscopy (TEM) measurements. This work provides a feasible design guideline to modify electronic structures, promote a metal to an active oxidation state, and thus develop an electrocatalyst with enhanced OER performance.

Electrochemical Conditioning of Metallurgically Prepared NiFe3 Binary Alloy for Facile Oxygen Evolution Reaction

“…On the contrary, all Fe‐containing contents were completely transformed into the trivalent states as demonstrated by the interplanar spacings of spinel type NiFe 2 O 4 and hematite α‐Fe 2 O 3 (Figure 6b), [75,80,87,88] respectively. It is worthy of notice that the OER‐active NiFe 2 O 4 can be further transformed into FeOOH, as reported by other works, [69,89,90] which is also advantageous to OER but less electrically conductive than NiOOH. At the same time, FeOOH is unstable when being exposed in the air due to dehydration, [45,69,89–91] the hematite α‐Fe 2 O 3 discovered within the HRTEM sample might derive predominantly from the FeOOH as the decomposition product of NiFe 2 O 4 during OER in alkaline media.…”

“…It is worthy of notice that the OER‐active NiFe 2 O 4 can be further transformed into FeOOH, as reported by other works, [69,89,90] which is also advantageous to OER but less electrically conductive than NiOOH. At the same time, FeOOH is unstable when being exposed in the air due to dehydration, [45,69,89–91] the hematite α‐Fe 2 O 3 discovered within the HRTEM sample might derive predominantly from the FeOOH as the decomposition product of NiFe 2 O 4 during OER in alkaline media. Hence, the necessity of Fe‐leaching for the improved electric conductivity of OER electrode was confirmed again, and the transition of metastable “pre‐catalyst” (NiFe 2 O 4 ) with higher Fe content to the final “real‐time‐catalyst” (Fe‐doped NiOOH) was witnessed more directly and microscopically.…”

“…Meanwhile, the objective existence of Ni 3 + species can be clearly found at the BEs of 857.0 eV and 874.5 eV, so the incomplete oxidation of Ni element during LSV polarization could be verified clearly. [51,66,67] However, in the case of Fe spectrum as indicated by Figure 4c, both Fe 2p 3/2 and Fe 2p 1/2 envelopes were at first deconvoluted into the trivalent states in hematite α-Fe 2 O 3 with the BEs of 710.9 eV and 723.9 eV, [68] and the other two adjacent envelopes on left side were also resolved into the trivalent states in spinel type NiFe 2 O 4 with the BEs peaking at 713.3 eV and 726.4 eV accompanied by the corresponding satellites at 718.8 eV and 733.4 eV, [69] respectively, proving again that the Fe contents can be converted into trivalent state, easily and completely, compared with Ni-containing species. At the same time, it can be realized that the divalent Ni(OH) 2 and the trivalent hematite α-Fe 2 O 3 are overwhelming majorities in the Ni-based and Febased species, according to their integral areas on XPS spectra.…”

Water Splitting as an Alternative for Electrochemical Hydrogen and Oxygen Generation: Current Status, Trends, and Challenges

“…In this case, the catalysts preferentially form γ-NiOOH structures, known as active sites, to achieve the OER. In another work, high-valence iron was used to modify NiMoO 4 heterostructures using oxygen plasma, and in situ Raman measurements were conducted to analyze the irreversible reconstruction [184]. Phase changes during the OER were analyzed in the potential windows of 1.18 to 1.43 V, in which NiOOH was the main remaining phase during water electrolysis at a potential close to 1.43 V. Operando UV-vis spectroscopy coupled with electrochemical measurements is another widely used technique to determine electrocatalyst reaction mechanisms.…”