Lateral-Size-Mediated Efficient Oxygen Evolution Reaction: Insights into the Atomically Thin Quantum Dot Structure of NiFe₂O₄

Abstract

The study of high-performance electrocatalysts for driving the oxygen evolution reaction (OER) is important for energy storage and conversion systems. As a representative of inverse-spinel-structured oxide catalysts, nickel ferrite (NiFe 2 O 4 ) has recently gained interest because of its earth abundance and environmental friendliness. However, the gained electrocatalytic performance of NiFe 2 O 4 for the OER is still far from the state-of-the-art requirements because of its poor reactivity and finite number of surface active sites. Here, we prepared a series of atomically thin NiFe 2 O 4 catalysts with different lateral sizes through a mild and controllable method. We found that the atomically thin NiFe 2 O 4 quantum dots (AT NiFe 2 O 4 QDs) show the highest OER performance with a current density of 10 mA cm −2 at a low overpotential of 262 mV and a small Tafel slope of 37 mV decade −1 . The outstanding OER performance of AT NiFe 2 O 4 QDs is even comparable to that of commercial RuO 2 catalyst, which can be attributed to its high reactivity and the high fraction of active edge sites resulting from the synergetic effect between the atomically thin thickness and the small lateral size of the atomically thin quantum dot (AT QD) structural motif. The experimental results reveal a negative correlation between lateral size and OER performance in alkaline media. Specifically speaking, the number of low-coordinated oxygen atoms increases with decreasing lateral size, and this leads to significantly more oxygen vacancies that can lower the adsorption energy of H 2 O, increasing the catalytic OER efficiency of AT NiFe 2 O 4 QDs.

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

“…[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.…”

Evaluation of A-Site Ba2+-Deficient Ba1−xCo0.4Fe0.4Zr0.1Y0.1O3−δ Oxides as Electrocatalysts fo

“…Thus, the water-splitting process may also be influenced by the OH ads species on the electrode [12, 28]. It is reported that the low-coordinated active atoms on the surface can serve as active sites accessible to the OH ads species [28, 29]. Consequently, the increase of active atoms adjacent to oxygen vacancies will be beneficial for OH − adsorption (generated from H 2 O splitting) and can leave alone a nearby anion ion (O 2− ) site for H ads (Figure 6).…”

Recent Progress on Nickel‐Based Oxide/(Oxy)Hydroxide Electrocatalysts for the Oxygen Evolution Reaction

“…(e) Polarization curves for α‐NiMoO 4 and β‐NiMoO 4 , and IrO 2 /C in 1 m KOH solution . (f) Schematic illustration of atomically thin NiFe 2 O 4 quantum dots (AT NiFe 2 O 4 QDs) for OER electrocatalysis …”

“…Rout's group demonstrated that metastable β‐NiMoO 4 was more active for OER than α‐NiMoO 4 (Figure e) . Ma's group fabricated atomically thin NiFe 2 O 4 quantum dots with different lateral sizes as an efficient OER catalyst through a mild and controllable method (Figure f) …”