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

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.


Introduction
Nowadays, the dramatically increased demand for fossil energy has resulted in the depletion of traditional energy materials and has generated concerns regarding energy security and the environmental pollution caused by the extensive use of fossil energy [1]. Overall, water electrolysis has developed as an ideal and effective approach for producing hydrogen, an alternative clean energy source to traditional fossil fuels [2,3]. The oxygen evolution reaction (OER) involves multiple steps of proton coupling and a complex fourelectron transfer process [4][5][6]. The sluggish reaction kinetics eventually cause a high enough overpotential to trigger the OER, which is a key factor limiting the efficiency of water electrolysis [3,7]. To date, the most effective catalysts for the OER have been found to be rare metal oxides, such as IrO 2 and RuO 2 , as they significantly reduce the overpotential required for the OER. However, the high expenditure and scarcity of precious metals restrict their wide application for practical industrialization as efficient electrocatalysts.
Generally, it is crucial to consider the modification of structure and electronic configuration in order to achieve outstanding OER performance, especially in order to attain a higher current at a lower overpotential with long-term stability [5]. An atomic-scale approach for constructing a reliable interface, especially a hetero-interface between different nanomaterials, has been intensively adopted to modify the local electronic structure of materials [33]. This approach can accelerate the reaction kinetics by combining the structural advantages of each component, thus improving the catalytic performance of nanocomposites [34][35][36][37][38]. For example, Lv et al. synthesized a core-shell structure of NiFe-60/Co 3 O 4 on Ni Foam with an obvious and clear hetero-interface between the Co 3 O 4 nanowire and the NiFe-layered double hydroxide nanosheet [39]. A hetero-interface contributes to the interaction between two different nanomaterials, facilitates electron transfer, and further leads to enhanced catalytic activity for the OER. Zhang et al. demonstrated a CoN 4 -based metal−organic framework (MOF) with embedded CoFeO x nanoparticles; Co sites anchoring on the CoFeO x /MOF interface brought about an altered 3D electronic configuration for the interfacial Co and a higher valence [40]. In addition, composites consisting of multivariate transition metals can promote the exceptional modification of active sites within the matrix, and thus improve reaction efficiency and durability [41]. Based on this, the addition of Fe elements has been confirmed as a proper approach to enrich the active sites and boost highly efficient OER performance [42][43][44].
Herein, we report a hetero-interface made of NiFe 2 O 4 nanoparticles (NPs) and a NiMoO 4 nanowire (denoted as NiFe 2 O 4 /NiMoO 4 ). Briefly, NiMoO 4 nanowires were prepared on nickel foam through a facile hydrothermal synthesis. NiFe Prussian blue analogs (NiFe PBA) were firstly fixed on the NiMoO 4 nanowires by iron exchange. Then, oxygen (O 2 ) plasma converted the NiFe PBA to NiFe 2 O 4 to form a NiFe 2 O 4 /NiMoO 4 heterointerface. As-synthesized, the NiFe 2 O 4 /NiMoO 4 exhibits excellent performance for the OER with a low overpotential of 309 mV required to reach 500 mA cm −2 and a satisfactory stability (a 4% increase in the overpotential at 50 mA cm −2 over 150 h). The shift in the binding energy of metal sites increased the electronic interaction of the modulated heterointerface. To understand its excellent OER performance, in situ Raman measurements, ex situ transmission electron microscopy (TEM), and ex situ X-ray photoelectron spectroscopy (XPS) were used to confirm the fast and irreversible reconstruction and identify the true active species in the catalytic process.

Results and Discussion
The schematics shown in Figure 1a illustrate the preparation of NiFe 2 O 4 nanoparticles integrated with NiMoO 4 nanowires on nickel foam. Briefly, through a simple hydrothermal method [45], hydrated NiMoO 4 nanowires were vertically germinated on Ni foam. In accordance with a previous report [46] even and well-defined nanowire morphology. Its average diameter was about 100 nm. As the XRD patterns show in Figure S2a, the diffraction peaks of the NiMoO4 were in perfect agreement with the NiMoO4·xH2O (JCPDF: 00-013-0128), which means the NiMoO4·xH2O was highly crystalline. In the FT-IR spectra shown in Figure S3, the two peaks at 1628 and 3446 cm −1 correspond to the stretching vibration of hydroxyl (-OH), which can be ascribed to the bending mode of crystal water in the NiMoO4·xH2O and the surface-adsorbed water molecules [47]. Characterization of NiFe PBA/NiMoO4. As shown in Figure 1b, a weak diffraction peak appears at 17.3°, which can be attributed to the KNi[Fe(CN)6] (JCPDF: 01-089-8978). At the same time, the diffraction peaks of the NiMoO4·xH2O still remain in the NiFe PBA/NiMoO4. In Figure S1c,d, the surface of the NiMoO4 nanowires is covered with smallsized NiFe PBA NPs, indicating the expected process of the iron exchange. Figure 1c reveals peaks at 357, 828, 872, and 950 cm −1 for the NiFe PBA/NiMoO4, which can be assigned to the Mo-O vibration, and this result is consistent with previous reports [32]. In addition, the NiFe PBA/NiMoO4 exhibits three peaks around 2100 cm −1 , which can be attributed to -CN [48,49]. In Figure S3, for the NiFe PBA/NiMoO4, a new characteristic peak can be observed at 2099 cm −1 in the FT-IR spectrum, which is attributed to the stretching vibrations of -CN in the NiFe PBA NPs [50]. In Figure S5a, for the NiFe PBA/NiMoO4, the Ni 2p spectra can be deconvoluted into four peaks and two wide satellite peaks. In the Ni 2p3/2, the peaks at 856.2 eV and 857 eV can be ascribed to the Ni 2+ and Ni 3+ species, respectively. Meanwhile, in the Ni 2p1/2, the peaks of the Ni 2+ and Ni 3+ species can be fitted at 874.0 eV and 875.2 eV, respectively. In addition, two satellite peaks for Ni can be observed at 862.8 and 880.7 eV [51]. As shown in Figure S5b, the further fitted peaks at 708.5 and 709.1 eV in the Fe 2p3/2 are ascribed to Fe 2+ and Fe 3+ , respectively, while the peaks observed at 721.6 and 722.2 eV in the Fe 2p1/2 are owed to the existence of Fe 2+ and Fe 3+ , respectively. The ratio of the Fe 3+ to Fe 2+ peak areas in the Fe 2p3/2 was calculated as 0.948. Furthermore, one satellite peak for Fe appears at 715.7 eV [51]. Characterization of NiMoO 4 . Figure S1a,b indicate that the NiMoO 4 possessed an even and well-defined nanowire morphology. Its average diameter was about 100 nm. As the XRD patterns show in Figure S2a, the diffraction peaks of the NiMoO 4 were in perfect agreement with the NiMoO 4 ·xH 2 O (JCPDF: 00-013-0128), which means the NiMoO 4 ·xH 2 O was highly crystalline. In the FT-IR spectra shown in Figure S3, the two peaks at 1628 and 3446 cm −1 correspond to the stretching vibration of hydroxyl (-OH), which can be ascribed to the bending mode of crystal water in the NiMoO 4 ·xH 2 O and the surface-adsorbed water molecules [47].
Characterization of NiFe PBA/NiMoO 4 . As shown in Figure 1b, a weak diffraction peak appears at 17.3 • , which can be attributed to the KNi[Fe(CN) 6 ] (JCPDF: 01-089-8978). At the same time, the diffraction peaks of the NiMoO 4 ·xH 2 O still remain in the NiFe PBA/NiMoO 4 . In Figure S1c,d, the surface of the NiMoO 4 nanowires is covered with smallsized NiFe PBA NPs, indicating the expected process of the iron exchange. Figure 1c reveals peaks at 357, 828, 872, and 950 cm −1 for the NiFe PBA/NiMoO 4 , which can be assigned to the Mo-O vibration, and this result is consistent with previous reports [32]. In addition, the NiFe PBA/NiMoO 4 exhibits three peaks around 2100 cm −1 , which can be attributed to -CN [48,49]. In Figure S3, for the NiFe PBA/NiMoO 4 , a new characteristic peak can be observed at 2099 cm −1 in the FT-IR spectrum, which is attributed to the stretching vibrations of -CN in the NiFe PBA NPs [50]. In Figure S5a, for the NiFe PBA/NiMoO 4 , the Ni 2p spectra can be deconvoluted into four peaks and two wide satellite peaks. In the Ni 2p 3/2 , the peaks at 856.2 eV and 857 eV can be ascribed to the Ni 2+ and Ni 3+ species, respectively. Meanwhile, in the Ni 2p 1/2 , the peaks of the Ni 2+ and Ni 3+ species can be fitted at 874.0 eV and 875.2 eV, respectively. In addition, two satellite peaks for Ni can be observed at 862.8 and 880.7 eV [51]. As shown in Figure S5b, the further fitted peaks at 708.5 and 709.1 eV in the Fe 2p 3/2 are ascribed to Fe 2+ and Fe 3+ , respectively, while the peaks observed at 721.6 and 722.2 eV in the Fe 2p 1/2 are owed to the existence of Fe 2+ and Fe 3+ , respectively. The ratio of the Fe 3+ to Fe 2+ peak areas in the Fe 2p 3/2 was calculated as 0.948. Furthermore, one satellite peak for Fe appears at 715.7 eV [51].
Characterization of NiFe 2 O 4 /NiMoO 4 . In Figure 1b, compared to the NiFe PBA/NiMoO 4 , the diffraction peak initially attributed to KNi[Fe(CN) 6 ] disappears, and a new weak diffraction Materials 2022, 15, 3688 4 of 12 peak can be observed at 43.3 • , which is attributed to the (400) planes of the NiFe 2 O 4 (JCPDS: . As shown in Figure 1c, the characteristic Raman peaks attributed to the Mo-O bond still remain, while the peak owed to -CN disappears. A broad peak at 520 cm −1 in Figure S2b indicates the formation of NiFe-O. The same phenomenon is shown in Figure S3, as the characteristic peak of -C≡N-disappears and an apparent peak at 1384 cm −1 can be assigned to the C=O in CO 2 and the stretching vibration of the interlayer NO 3 = groups [47]. This is related to the decomposition of PBA under O 2 plasma treatment. As shown in Figure   Characterization of NiFe2O4/NiMoO4. In Figure 1b, compared to the NiFe PBA/Ni-MoO4, the diffraction peak initially attributed to KNi[Fe(CN)6] disappears, and a new weak diffraction peak can be observed at 43.3°, which is attributed to the (400) planes of the NiFe2O4 (JCPDS: 44-1485). As shown in Figure 1c, the characteristic Raman peaks attributed to the Mo-O bond still remain, while the peak owed to -CN disappears. A broad peak at 520 cm −1 in Figure S2b indicates the formation of NiFe-O. The same phenomenon is shown in Figure S3, as the characteristic peak of -C≡N-disappears and an apparent peak at 1384 cm −1 can be assigned to the C=O in CO2 and the stretching vibration of the interlayer NO3 = groups [47]. This is related to the decomposition of PBA under O2 plasma treatment. As shown in Figure 2a, the NiMoO4 remains in the structure of the nanowire with a diameter of 100 nm, similarly to the NiMoO4 and NiFe PBA/NiMoO4, while the NiFe2O4 nanoparticles slightly agglomerate. The TEM image of the NiFe2O4/NiMoO4 (Figure 2b) clearly shows the NiFe2O4 NPs anchoring on the surface of the NiMoO4 nanowire. The high-resolution transmission electron microscope (HRTEM) image in Figure 2c indicates the high-crystalline characteristic of the NiMoO4. The HRTEM image in Figure 2d shows the apparent hetero-interface of the NiFe2O4/NiMoO4. The lattice spacing of 2.08 Å can be attributed to the (400) plane of the NiFe2O4 and the lattice spacing of 3.26 Å assigned to the NiMoO4·xH2O. The elemental mapping images (Figure 2e) indicate that the Fe element is evenly distributed on the NiMoO4 nanowires in the form of nanoparticles. Table S1 shows the chemical composition of the NiFe2O4/NiMoO4. The molecular ratio of NiFe2O4 and NiMoO4 in the NiFe2O4/NiMoO4 is 1:17.27. XPS was used to analyze and further explore the surface electronic interaction of the NiFe2O4/NiMoO4. The Ni 2p spectra of NiFe2O4/NiMoO4 contain four fitted peaks with wide satellites (Figure 3a). The fitted peaks at 856.1 and 873.7 eV in the Ni 2p3/2 and the Ni 2p1/2, respectively, can be attributed to Ni 2+ , while another two peaks (858.3 and 875.9 eV) XPS was used to analyze and further explore the surface electronic interaction of the NiFe 2 O 4 /NiMoO 4 . The Ni 2p spectra of NiFe 2 O 4 /NiMoO 4 contain four fitted peaks with wide satellites (Figure 3a). The fitted peaks at 856.1 and 873.7 eV in the Ni 2p 3/2 and the Ni 2p 1/2 , respectively, can be attributed to Ni 2+ , while another two peaks (858.3 and 875.9 eV) correspond to the Ni 3+ species. The wide peaks observed at 862.3 and 879.9 eV are owed to the satellites of Ni [52]. As Figure 3b shows, the fitted peaks at 710.7 and 713.3 eV can be related to Fe 2+ and Fe 3+ in the Fe 2p 3/2 , and the same is true for another two peaks at 723.8 and 726.4 eV in the Fe 2p 1/2 [53]. The broad peaks at 718.8 and 731.9 eV can be attributed to the satellite peaks of Fe. In Figure S6a,b, the peaks in the Ni 2p 3/2 and the Mo 3D of the NiFe 2 O 4 /NiMoO 4 exhibit slightly negative shifts of about 0.2 eV compared with those observed from the spectra of the NiFe PBA/NiMoO 4 . The peaks of the Fe 2p exhibit a distinct positive shift compared with those of the NiFe PBA/NiMoO 4 , and the ratio of the Fe 3+ to Fe 2+ peak area in the Fe 2p 3/2 (Figures S5b and 3b) increases from 0.948 in the NiFe PBA/NiMoO 4 to 1.706 in the NiFe 2 O 4 /NiMoO 4 . The negative movement of binding Ni and Mo energy indicates the regulation of the electronic structure in the heterointerface. Meanwhile, oxygen plasma leads to the oxidation of Fe, and these two factors eventually promote an increase in the binding energy of Fe. It has been confirmed that Fe with a high valence state promotes processes in the OER [54][55][56]. The fitted O 1s peaks at 530.7, 532.3, and 533.3 eV can be attributed to metal-O, C=O [57], and surface-adsorbed oxygen, respectively. correspond to the Ni 3+ species. The wide peaks observed at 862.3 and 879.9 eV are owed to the satellites of Ni [52]. As Figure 3b shows, the fitted peaks at 710.7 and 713.3 eV can be related to Fe 2+ and Fe 3+ in the Fe 2p3/2, and the same is true for another two peaks at 723.8 and 726.4 eV in the Fe 2p1/2 [53]. The broad peaks at 718.8 and 731.9 eV can be attributed to the satellite peaks of Fe. In Figure S6a,b, the peaks in the Ni 2p3/2 and the Mo 3D of the NiFe2O4/NiMoO4 exhibit slightly negative shifts of about 0.2 eV compared with those observed from the spectra of the NiFe PBA/NiMoO4. The peaks of the Fe 2p exhibit a distinct positive shift compared with those of the NiFe PBA/NiMoO4, and the ratio of the Fe 3+ to Fe 2+ peak area in the Fe 2p3/2 ( Figures S5b and 3b) increases from 0.948 in the NiFe PBA/NiMoO4 to 1.706 in the NiFe2O4/NiMoO4. The negative movement of binding Ni and Mo energy indicates the regulation of the electronic structure in the hetero-interface. Meanwhile, oxygen plasma leads to the oxidation of Fe, and these two factors eventually promote an increase in the binding energy of Fe. It has been confirmed that Fe with a high valence state promotes processes in the OER [54][55][56]. The fitted O 1s peaks at 530.7, 532.3, and 533.3 eV can be attributed to metal-O, C=O [57], and surface-adsorbed oxygen, respectively. Electrochemical performance. For the purpose of measuring the electrochemical performance of the prepared samples, a three-electrode electrochemical system was used. An aqueous solution of 1 M KOH was selected as the electrolyte solution. The polarization curves of all samples with iR corrected are shown as Figure 4a. The peaks around 1.38 V can be assigned to the oxidation of nickel species to a higher valence state. Furthermore, the NiFe2O4/NiMoO4 demonstrates the lowest overpotential of 253 mV to reach 10 mA cm −2 , while the overpotential required to achieve 10 mA cm −2 for the NiFe PBA/NiMoO4, NiMoO4 O2-Pl, NiMoO4, and Ni foam is 310, 313, 324, and 431 mV, respectively. In addition, for the NiFe2O4/NiMoO4, an overpotential of 270 and 309 mV are required to achieve 50 mA cm −2 and 500 mA cm −2 , respectively.
As shown in Figure 4b, the Tafel slope of the NiFe2O4/NiMoO4 is the smallest, at 46.4 mV dec −1 , compared with that of the NiFe PBA/NiMoO4 (119.2 mV dec −1 ), NiMoO4 O2-Pl Electrochemical performance. For the purpose of measuring the electrochemical performance of the prepared samples, a three-electrode electrochemical system was used. An aqueous solution of 1 M KOH was selected as the electrolyte solution. The polarization curves of all samples with iR corrected are shown as Figure 4a. The peaks around 1.38 V can be assigned to the oxidation of nickel species to a higher valence state. Furthermore, the NiFe 2 O 4 /NiMoO 4 demonstrates the lowest overpotential of 253 mV to reach 10 mA cm −2 , while the overpotential required to achieve 10 mA cm −2 for the NiFe PBA/NiMoO 4 , NiMoO 4 O 2 -Pl, NiMoO 4 , and Ni foam is 310, 313, 324, and 431 mV, respectively. In addition, for the NiFe 2 O 4 /NiMoO 4 , an overpotential of 270 and 309 mV are required to achieve 50 mA cm −2 and 500 mA cm −2 , respectively. iron with a higher valence. Iron with a higher valence has been confirmed to be conducive to the OER [54][55][56][57]. Figure 4c shows the electrochemical impedance spectroscopy (EIS) of all samples, and it can clearly be observed that the smallest charge transfer resistance (Rct) is found in the NiFe2O4/NiMoO4. The smaller Rct relative to the others indicates a faster charge transfer for the NiFe2O4/NiMoO4, which may relate to the hetero-interface made up of NiFe2O4 NPs and NiMoO4 and further leads to enhanced electrocatalytic performance.  Figure 4d, the double-layer capacitance (Cdl) of the NiFe2O4/NiMoO4, NiFe PBA/NiMoO4, NiMoO4 O2-Pl, and NiMoO4 was calculated to be 4.21, 3.09, 2.49, and 3.67 mF cm −2 . The larger value of Cdl indicates a higher electrocatalytic OER activity of the NiFe2O4/NiMoO4, which is attributed to more exposed active sites related to the iron with a higher valence. Long-term stability is an important index for evaluating a catalyst. As shown in Figure 4e, the NiFe2O4/NiMoO4 displays outstanding durability (a 4% increase in the overpotential at 50 mA cm −2 over a 150 h reaction).
In situ Raman spectra. To figure out the phase change and reconstruction in the OER process, the NiFe2O4/NiMoO4 was first activated in an alkaline solution. In Figure 5a, with the increase in the applied potential from 1.18 V to 1.43 V, the intensity of characteristic peaks for Mo-O vibration decreased, which represents the dissolution of MoO4 2− . Meanwhile, a small but sharp characteristic peak at 525 cm −1 corresponding with the Fe-O bond in FeOOH emerged with an applied potential of 1.23 V [58], which indicates the formation of FeOOH in the OER process. When the potential is applied at 1.28 V, broad peaks can  [4,6]. The high performance of the NiFe 2 O 4 /NiMoO 4 can be attributed to the oxygen-plasma-induced formation of the hetero-interface, made up of NiFe 2 O 4 NPs and NiMoO 4 nanowire arrays and containing iron with a higher valence. Iron with a higher valence has been confirmed to be conducive to the OER [54][55][56][57]. Figure 4c shows the electrochemical impedance spectroscopy (EIS) of all samples, and it can clearly be observed that the smallest charge transfer resistance (R ct ) is found in the NiFe 2 O 4 /NiMoO 4 . The smaller R ct relative to the others indicates a faster charge transfer for the NiFe 2 O 4 /NiMoO 4 , which may relate to the hetero-interface made up of NiFe 2 O 4 NPs and NiMoO 4 and further leads to enhanced electrocatalytic performance.
The electrochemical active surface area (ECSA) by CV measurement is shown in Figure  S4. As shown in Figure 4d, the double-layer capacitance (C dl ) of the NiFe 2 O 4 /NiMoO 4 , NiFe PBA/NiMoO 4 , NiMoO 4 O 2 -Pl, and NiMoO 4 was calculated to be 4.21, 3.09, 2.49, and 3.67 mF cm −2 . The larger value of C dl indicates a higher electrocatalytic OER activity of the NiFe 2 O 4 /NiMoO 4 , which is attributed to more exposed active sites related to the iron with a higher valence. Long-term stability is an important index for evaluating a catalyst. As shown in Figure 4e, the NiFe 2 O 4 /NiMoO 4 displays outstanding durability (a 4% increase in the overpotential at 50 mA cm −2 over a 150 h reaction).
In situ Raman spectra. To figure out the phase change and reconstruction in the OER process, the NiFe 2 O 4 /NiMoO 4 was first activated in an alkaline solution. In Figure 5a, with the increase in the applied potential from 1.18 V to 1.43 V, the intensity of characteristic peaks for Mo-O vibration decreased, which represents the dissolution of MoO 4 2− . Meanwhile, a small but sharp characteristic peak at 525 cm −1 corresponding with the Fe-O bond in FeOOH emerged with an applied potential of 1.23 V [58], which indicates the formation of FeOOH in the OER process. When the potential is applied at 1.28 V, broad peaks can be observed around 460 cm −1 and 520 cm −1 , which are related to the appearance of α-Ni(OH) 2 [59]. The peak becomes sharper when the applied potential arrives at 1.33 V. A broad peak occurs at 475 cm −1 , which can be attributed to the emergence of γ-NiOOH from α-Ni(OH) 2 [32,58], and the peak tends to become sharper with an applied potential at 1.43 V, while another characteristic peak of γ-NiOOH appears at 558 cm −1 [32,59]. be observed around 460 cm −1 and 520 cm −1 , which are related to the appearance of α-Ni(OH)2 [59]. The peak becomes sharper when the applied potential arrives at 1.33 V. A broad peak occurs at 475 cm −1 , which can be attributed to the emergence of γ-NiOOH from α-Ni(OH)2 [32,58], and the peak tends to become sharper with an applied potential at 1.43 V, while another characteristic peak of γ-NiOOH appears at 558 cm −1 [32,59]. In situ Raman spectra of the NiFe2O4/NiMoO4 in the initial two cycles in CVs are shown in Figure 5b. With multiple cycles, the intensity of the characteristic peak for γ-NiOOH gradually stabilizes and the characteristic peaks of Mo-O vibration almost completely disappear, which can be attributed to the irreversible reconstruction of the NiFe2O4/NiMoO4. For comparison, the NiFe PBA/NiMoO4 was also first activated in an alkaline solution. In Figure S8a, with the increase in applied potential, the same phenomenon of a decrease in the intensity of characteristic peaks for Mo-O vibration can be observed. In addition, when a potential of 1.38 V is applied to the NiFe2O4/NiMoO4, a broad peak occurs at 475 cm −1 , which can be attributed to the emergence of γ-NiOOH. The same phenomenon occurs at an applied potential of 1.28 V for NiFe2O4/NiMoO4. This fact means that the NiFe2O4/NiMoO4 is reconstructed faster than the NiFe PBA/NiMoO4, which leads to the better OER performance of the NiFe2O4/NiMoO4 from another aspect. However, there are no observable peaks attributed to FeOOH, and in Figure S8b, with the increase in applied potential, the peaks related to -CN still exist [48,49]. This illustrates that the Fe coordinating with the cyanide group cannot catalyze the OER as an independent active site with the increase in applied potential, which further explains the reason that the OER performance of the NiFe PBA/NiMoO4 is close to the OER performance of the NiMoO4.
Ex situ XPS. The Ni 2p and Fe 2p spectra of the NiFe2O4/NiMoO4 after 3 h of OER testing are shown in Figure 3d and e, respectively. In Figure 3d, the Ni 2p can be deconvoluted into two peaks with two satellites. The fitted peaks at 855.1 and 872.7 eV can be ascribed to Ni 3+ , which is attributed to NiOOH [60]. Meanwhile, two satellites of Ni can be observed at 860.9 and 878.9 eV, respectively. As shown in Figure 3e, the two fitted peaks occur at 712.1 and 725.2 eV with a broad satellite at 718.3 eV, which can be related to FeOOH [61,62]. It can clearly be observed that there is a sharp attenuation in the peak intensity of the Mo 3d of the NiFe2O4/NiMoO4 after 3 h of OER testing, further demonstrating the irreversible reconstruction of the NiFe2O4/NiMoO4 with the dissolution of MoO4 2− . In situ Raman spectra of the NiFe 2 O 4 /NiMoO 4 in the initial two cycles in CVs are shown in Figure 5b. With multiple cycles, the intensity of the characteristic peak for γ-NiOOH gradually stabilizes and the characteristic peaks of Mo-O vibration almost completely disappear, which can be attributed to the irreversible reconstruction of the NiFe 2 O 4 /NiMoO 4 .
For comparison, the NiFe PBA/NiMoO 4 was also first activated in an alkaline solution. In Figure S8a, with the increase in applied potential, the same phenomenon of a decrease in the intensity of characteristic peaks for Mo-O vibration can be observed. In addition, when a potential of 1.38 V is applied to the NiFe 2 O 4 /NiMoO 4 , a broad peak occurs at 475 cm −1 , which can be attributed to the emergence of γ-NiOOH. The same phenomenon occurs at an applied potential of 1.28 V for NiFe 2 O 4 /NiMoO 4 . This fact means that the NiFe 2 O 4 /NiMoO 4 is reconstructed faster than the NiFe PBA/NiMoO 4 , which leads to the better OER performance of the NiFe 2 O 4 /NiMoO 4 from another aspect. However, there are no observable peaks attributed to FeOOH, and in Figure S8b, with the increase in applied potential, the peaks related to -CN still exist [48,49]. This illustrates that the Fe coordinating with the cyanide group cannot catalyze the OER as an independent active site with the increase in applied potential, which further explains the reason that the OER performance of the NiFe PBA/NiMoO 4 is close to the OER performance of the NiMoO 4 .
Ex situ XPS. The Ni 2p and Fe 2p spectra of the NiFe 2 O 4 /NiMoO 4 after 3 h of OER testing are shown in Figure 3d,e, respectively. In Figure 3d, the Ni 2p can be deconvoluted into two peaks with two satellites. The fitted peaks at 855.1 and 872.7 eV can be ascribed to Ni 3+ , which is attributed to NiOOH [60]. Meanwhile, two satellites of Ni can be observed at 860.9 and 878.9 eV, respectively. As shown in Figure 3e, the two fitted peaks occur at 712.1 and 725.2 eV with a broad satellite at 718.3 eV, which can be related to FeOOH [61,62]. It can clearly be observed that there is a sharp attenuation in the peak intensity of the Mo Ex situ TEM. The images of the NiFe2O4/NiMoO4 after OER testing (Figure 6a,b) clearly show the robust surface of the nanowire and numerous defects as the result of the dissolution of MoO4 2− . The HRTEM image in Figure 6c reveals clear lattice fringes of the (105) plane for NiOOH (JCPDF: 00-006-0075) with a crystalline interplanar spacing of 2.09 Å . The HRTEM image in Figure 3d shows small black particles distributed in clumps, which may relate to the amorphous FeOOH delivered by the activation of the NiFe2O4 in OER testing. The elemental mapping images (Figure 3e) indicate that Fe is still evenly distributed on the NiMoO4 nanowire, and Mo dissolves in large quantities, which is consistent with the aforementioned analytical results.

Conclusions
In summary, a heterogeneous interface of NiFe2O4/NiMoO4 with high-valence iron through oxygen plasma can be fabricated to achieve excellent electrocatalytic activity and stability. To achieve a current density of 50 mA cm −2 , 270 mV of overpotential is required, while an overpotential of 309 mV is required to reach 500 mA cm −2 . The NiFe2O4/NiMoO4 also exhibits a satisfactory stability (a 4% increase in the overpotential at 50 mA cm −2 over 150 h). O2-plasma-induced electronic interaction in the hetero-interface of NiFe2O4/Ni-MoO4 and iron with a higher valence play an essential role in OER performance. The potential-dependent phase change and the fast and irreversible reconstruction of the NiFe2O4/NiMoO4 in a catalytic process were identified by in situ Raman, ex situ XPS, and ex situ TEM measurements. Based on this, the true active species, NiOOH and FeOOH, were determined. This work provides a feasible design guideline for modifying electronic structure through the construction of a heterogeneous interface and the activation of metal sites by O2 plasma, finally leading to enhanced OER performance.

Conclusions
In summary, a heterogeneous interface of NiFe 2 O 4 /NiMoO 4 with high-valence iron through oxygen plasma can be fabricated to achieve excellent electrocatalytic activity and stability. To achieve a current density of 50 mA cm −2 , 270 mV of overpotential is required, while an overpotential of 309 mV is required to reach 500 mA cm −2 . The NiFe 2 O 4 /NiMoO 4 also exhibits a satisfactory stability (a 4% increase in the overpotential at 50 mA cm −2 over 150 h). O 2 -plasma-induced electronic interaction in the hetero-interface of NiFe 2 O 4 /NiMoO 4 and iron with a higher valence play an essential role in OER performance. The potential-dependent phase change and the fast and irreversible reconstruction of the NiFe 2 O 4 /NiMoO 4 in a catalytic process were identified by in situ Raman, ex situ XPS, and ex situ TEM measurements. Based on this, the true active species, NiOOH and FeOOH, were determined. This work provides a feasible design guideline for modifying electronic structure through the construction of a heterogeneous interface and the activation of metal sites by O 2 plasma, finally leading to enhanced OER performance.

Data Availability Statement:
The data presented in this study are available from the corresponding author, upon reasonable request.

Conflicts of Interest:
The authors declare no conflict of interest.