Striking Stabilization Effect of Spinel Cobalt Oxide Oxygen Evolution Electrocatalysts in Neutral pH by Dual‐Sites Iron Incorporation

Developing stable and efficient nonprecious‐metal‐based oxygen evolution catalysts in the neutral electrolyte is a challenging but essential goal for various electrochemical systems. Particularly, cobalt‐based spinels have drawn a considerable amount of attention but most of them operate in alkali solutions. However, the frequently studied Co–Fe spinel system never exhibits appreciable stability in nonbasic conditions, not to mention attract further investigation on its key structural motif and transition states for activity loss. Herein, we report exceptional stable Co–Fe spinel oxygen evolution catalysts (~30% Fe is optimal) in a neutral electrolyte, owing to its unique metal ion arrangements in the crystal lattice. The introduced iron content enters both the octahedral and tetrahedral sites of the spinel as Fe2+ and Fe3+ (with Co ions having mixed distribution as well). Combining density functional theory calculations, we find that the introduction of Fe to Co3O4 lowers the covalency of metal‐oxygen bonds and can help suppress the oxidation of Co2+/3+ and O2−. It implies that the Co–Fe spinel will have minor surface reconstruction and less lattice oxygen loss during the oxygen evolution reaction process in comparison with Co3O4 and hence show much better stability. These findings suggest that there is still much chance for the spinel structures, especially using reasonable sublattices engineering via multimetal doping to develop advanced oxygen evolution catalysts.

Even though, the oxides still commonly suffer from the dissolution or metal leakage in neutral to acidic conditions as suggested by the Pourbaix diagram. [33]As a feasible route, this challenge can be thermodynamically addressed by a suitable phase design, which makes them still one of the optimal choices.In our previous report, a doped silicate has been demonstrated with promising OER stability under weakly acidic and neutral media. [34]Furthermore, even in those aforementioned nonoxide structures, extensive works have shown that the M-O or M-OH units (e.g., Co-O or Co-OH) are still the direct catalytic sites in reaction conditions upon corresponding necessary activations. [16,23,25,26]Therefore, the design of stable oxide-based motifs is always the primary goal for efficient OER catalysts in such media.
On the other hand, the stability and activity of certain oxide phases can be further improved by the fine structural tuning approaches.For Developing stable and efficient nonprecious-metal-based oxygen evolution catalysts in the neutral electrolyte is a challenging but essential goal for various electrochemical systems.Particularly, cobalt-based spinels have drawn a considerable amount of attention but most of them operate in alkali solutions.However, the frequently studied Co-Fe spinel system never exhibits appreciable stability in nonbasic conditions, not to mention attract further investigation on its key structural motif and transition states for activity loss.Herein, we report exceptional stable Co-Fe spinel oxygen evolution catalysts (~30% Fe is optimal) in a neutral electrolyte, owing to its unique metal ion arrangements in the crystal lattice.The introduced iron content enters both the octahedral and tetrahedral sites of the spinel as Fe 2+ and Fe 3+ (with Co ions having mixed distribution as well).Combining density functional theory calculations, we find that the introduction of Fe to Co 3 O 4 lowers the covalency of metal-oxygen bonds and can help suppress the oxidation of Co 2+/3+ and O 2− .It implies that the Co-Fe spinel will have minor surface reconstruction and less lattice oxygen loss during the oxygen evolution reaction process in comparison with Co 3 O 4 and hence show much better stability.These findings suggest that there is still much chance for the spinel structures, especially using reasonable sublattices engineering via multimetal doping to develop advanced oxygen evolution catalysts.
instance, the oxygen bonding can be greatly strengthened via adjusting the O2p band center by metal doping such as in the cases of W 0.2 Er 0.1 Ru 0.7 O 2-δ , [35] Co 2 MnO 4 , [32] and Ir 0.06 Co 2.94 O 4 , [36] resulting in the better OER stability of metal sites in acid conditions.
It is worth noting that doping new elements is not the only way to impact the electronic structure of an oxide.Based on the unique AB 2 O 4 structure, [37] constructing a more disordered state of cations distribution between tetrahedral (A) and octahedral (B) sublattices may lead to notably different physicochemical properties of a spinel lattice.On the one hand, some research indicates that enhanced stability of homogeneous catalysts usually involves more disordered structural changes.For example, disordered birnessite-like phase MnO x displays better stability than original birnessite-like MnO x in acidic and neutral conditions. [38]he OER stability of high entropy oxides also outperforms that of CoO, NiO, and IrO 2 in alkaline solutions. [39]On the other hand, cations disorder in cobalt-iron spinel oxides has a major impact on their electronic, magnetic, and optical properties. [40]43][44] The O 2p density of states could be still the key to the performance of cobalt-iron spinel oxides. [37]However, many works only focus on the formation of different new species from structural evolution on the surface of catalysts. [45]These studies are focused on alkaline conditions and lack the relationship between the OER stability and the crucial pristine structural motif in cobalt-iron spinel oxides.Indeed, such a phase has been commonly considered can be readily deactivated in lower pH conditions.Thus, is it possible to take the structural disorder approach to stabilize this phase?What is the key motif of those optimal structures?So far, such research has not been realized for Co-Fe spinels in neutral electrolyte OER yet.
Herein, spinel-type cobalt-iron oxide thin films have been prepared on FTO by our recently developed ultrasonic spray pyrolysis deposition route. [46]An extraordinary OER stability and improved activity in the neutral electrolyte can be achieved with a spinel-based structure with an optimal Fe content of Co 0.7 Fe 0.3 .Structural characterizations indicate the improvements shall be attributed to the suitable distribution of Fe 3+ and Fe 2+ in both octahedral and tetrahedral sites of the spinel lattice.This leads to the favored super-exchange interactions between sublattices and the more negatively charged oxygen sites.Thereby, the M-O bond has been tailored to more appropriate lengths with suitable covalency, which raises the energy barrier for lattice oxygen removal.Subsequently, the formation of excessive oxygen defects and further dissolution of catalyst in the OER process are hindered.Therefore, the unique stabilizing effect of introducing Fe ions in both octahedral and tetrahedral sites for spinel lattice provides a new strategy to develop efficient and stable electrocatalysts via the engineering of the intrinsic structure motifs.
X-ray diffraction (XRD) measurement was first carried out to verify the crystalline structures of the synthesized samples (Figure 1a).Interestingly, all cobalt-iron oxides exhibit a cubic Co 3 O 4 structure with the introduction of Fe.Specifically, when the added Fe content is lower than 50%, the crystal structures of cobalt-iron oxides are close to spinel CoCo 2 O 4 (PDF 80-1540).When the added Fe content increases to Energy Environ.Mater.2024, 7, e12594 2 of 9 more than 50%, the crystal phases of cobalt-iron oxides are close to the inverse spinel CoFe 2 O 4 (PDF 03-0864). [47]Further, we fitted the crystal structure parameters of these cobalt-iron oxides as shown in Figure 1b.The cell size for spinel structure keeps increasing with the increase of Fe content, which may be caused by the larger radius of Fe ion compared with that of Co ion. [48]The diffraction characteristics of pure FeO x are able to match with rhombohedral Fe 2 O 3 (PDF 33-0664) rather than cubic Co 3 O 4 .Subsequently, we compared the full width at half-maximum values (FWHM) of the diffraction peaks on the (311) crystal plane for all cobalt-iron oxides as shown in Figure 1c.It can be found that the value of FWHM for Co 0.7 Fe 0.3 is maximal, indicating this Fe content induces the strongest disturbance to the lattice among samples.Grain size can be further calculated by Scherrer's formula: L = Kλ/βcosθ, where L is the average grain size of the sample, K is the Scherrer's constant of 0.89, λ is the wavelength of the X-ray of 0.154 nm, β is the FWHM, and θ is the diffraction angle.The results of grain calculations for all samples are summarized in Table S1, Supporting Information.The particle size of Co 0.7 Fe 0.3 is estimated to be about 10.9 nm (with 311 diffractions), which is the smallest value in these samples.The SEM images of all samples (Figure S1, Supporting Information) also show the particles of Co 0.7 Fe 0.3 look smaller, implying lower crystallinity.
Based on the ligand field theory, the incorporation of a small amount of Fe 3+ into the B site can regulate the e g electronic configurations of the Co 3+ from the low-spin state (t 2g ), optimizing the adsorption of intermediates. [44]Therein, high-spin state Co 3+ will introduce a variety of degrees of lattice distortion to remove the degeneracy and form a lower energy (Jahn-Teller effect). [49]Therefore, a suitable Fe doping content will decrease the lattice symmetry to stabilize the spinel structure.
In essence, this abnormal disorder for Co 0.7 Fe 0.3 may be related to the site distribution of Fe ions in the cobalt spinel structure.Ideally, all Co 3+ ions locate in the octahedral sites (B site) and Co 2+ are in the tetrahedral sites (A site) in normal spinel Co 3 O 4 (Figure 1d).For the inverse spinel CoFe 2 O 4 , Fe 3+ and Co 2+ are in the B site, and Fe 3+ is in the A site.The Co 0.7 Fe 0.3 is a transition state between the Co 3 O 4 and CoFe 2 O 4 , meaning that the distribution of Co and Fe ions at A and B sites shall be more disordered, and further lead to the decrease in crystallinity.These metal ion distributions at A and B sites are further discussed in Raman spectroscopy, Mössbauer spectrometry, and magnetism in the latter paragraphs.
Figure S2, Supporting Information and Figure 1e show that the Co 0.7 Fe 0.3 /FTO films are composed of small particles about tens of nanometers.The film thickness is about ~100 nm for five spray laps and ~260 nm for 20 spray laps, respectively.High-resolution transmission electron microscopy (HRTEM, Figure 1f) shows that well-resolved interlayer spacing of 0.468 nm can be ascribed to the (111) plane of cubic Co 3 O 4 .The selected area electron diffraction (SAED) diagram (Figure 1g) shows that the Co 0.7 Fe 0.3 is polycrystalline.Electron energy dispersive spectroscopy (EDS) images (Figure 1h; Figure S3, Supporting Information) reveal that the Co, Fe, and O elements are homogeneously distributed, and the atomic ratio of Co/Fe coincides with the batching.In addition, we compared the changes in morphology, lattice fringes, selected electron diffraction, and the atomic ratio of Co/Fe for CoO x , Co 0.8 Fe 0.2 , and Co 0.5 Fe 0.5 (Figures S4-S8, Supporting Information).All of these samples showed uniform element distributions with no observable morphological or structural differences.The electron energy loss spectrum (EELS, Figure S9, Supporting Information) further revealed an overall left shift of the Co L 3 edge with the addition of Fe, which indicates the weakened Co-O bond in comparison with CoO x .The peak position for Fe L 3 is about 711.5 eV, meaning that Fe ions are mainly trivalent.However, the shape of Fe L 3 peak will change with the increase of Fe content.The Fe L 3 peak of Co 0.5 Fe 0.5 is narrower FWHM and steeper shoulder than that of Co 0.7 Fe 0.3 , which may be caused by the speciation of more Fe 2+ . [50]he OER activities of catalysts were evaluated by a three-electrode system in a 1.0 M phosphatic buffer solution (PBS, pH = 7).The default spray lap number of all samples for OER performance comparison is 5 unless otherwise specified.Figure 2a reveals the linear sweep voltammetry (LSV) curves of the samples after 10 cyclic voltammogram (CV) cycles.The order of overpotential required by each electrode at the same current density is as follows: Co 0.9 Fe 0.1 < Co 0.8 Fe 0.2 < Co 0.7 Fe 0.3 < CoO x < Co 0.6 Fe 0.4 < Co 0.5 Fe 0.5 < Co 0.33 Fe 0.67 < Co 0.2 Fe 0.8 < FeO x .The results show that the added Fe content lower than 30% is beneficial to improving the OER performance of Co 3 O 4 .Figure 2b shows that the Tafel slopes of all cobalt-iron oxide films are slightly lower than pure CoO x , which implies a slight change in the OER rate-determining step. [51]Given that CoO x and Co 0.9 Fe 0.1 have similar morphologies (Figure S1, Supporting Information), the enhanced activity of cobalt-iron oxides (Fe content: 0.1-0.3)should be attributed to the synergistic interaction between the two metal sites. [52]However, too much iron (>0.4) can greatly reduce the material's conductivity, [53] which will result in less OER activity.
Interestingly, the amount of Fe can significantly affect the stability of cobalt spinel oxides under neutral conditions.As shown in Figure 2c, Co 0.7 Fe 0.3 shows better OER stability than other cobalt-iron oxides.When Fe content is above 50%, the stability of cobalt-iron oxides begins to decline and is obviously worse than that of pure CoO x , which may be due to the fact that the addition of too much amount of Fe remarkably enlarges the crystal cell of cobalt spinel and thereby lead to the unstable structure during OER.However, a small amount of added Fe can obviously improve the stability of cobalt spinel.Figure 2d and Figures S10-S12, Supporting Information further disclose the change of the double-layer capacitance (C dl ) of samples before and after OER.It can be found that the C dl values after OER for the relatively stable samples are increased in comparison with the initial states.Notably, the increment of C dl value after OER for Co 0.7 Fe 0.3 is slightly less than that of Co 0.8 Fe 0.2 but larger than that of Co 0.6 Fe 0.4 , which may be caused by the difference in material surface reconstruction.On the one hand, introducing Fe can promote the surface reconstruction of cobalt spinel, which may benefit the catalytic activity. [54]Moreover, metal doping may adjust the density of states contributed by O atoms.For example, Mn doping into spinel Co 3 O 4 made more electrons transfer from metal to O and thus acquire more stable lattice oxygen in thermodynamics, resulting in improving the stability of OER. [32]urther, we tested the long-term stability of Co 0.7 Fe 0.3 film sprayed for 20 laps in 0.1 M phosphate buffer solution (Figure 3).The Co 0.7 Fe 0.3 catalyst exhibited no significant increase in the overpotential at a current density of 1 mA cm −2 for 800 h (the fluctuation of the signal is mainly affected by the change in room temperature).It can be disclosed that chemical valence states (Figure S13, Supporting Information) and composition ratio (Table S2, Supporting Information) of the surface elements for the Co 0.7 Fe 0.3 film after the stability test are significantly changed than that of pristine Co 0.7 Fe 0.3 .Additionally, the electrolytes at different current densities for several hours were collected to monitor the dissolution behaviors of these metal ions during OER electrolysis (Table S3, Supporting Information).The Co 0.7 Fe 0.3 will be dissolved at a higher current density (10 mA cm −2 ), which is probably caused by decreased oxidative stabilities when the anode polarization potential is higher.Notably, at suitable potentials, the Co 0.7 Fe 0.3 is not dissolved but only encounters surface reconstruction during the overall OER process.Raman spectra and SEM image (Figures S14 and S15, Supporting Information) further indicate that the bulk phase structure of the Co 0.7 Fe 0.3 catalyst still remains unchanged after OER.However, the bulk phase structure for cobalt-iron oxides (when Fe content is above 50%) should have been significant damage due to rapidly increased overpotential in stability tests.Therefore, the pristine bulk phase structure in cobalt-iron spinel oxides is related to the OER stability.
Besides, as shown in Table S4, Supporting Information, we also reviewed the findings from research reporting the OER properties of Co-based oxides and other spinel structures under neutral conditions.It should be noted that the activity deteriorates for the majority of these catalysts despite short operation durations.In this work, the stable operating duration of the Co 0.7 Fe 0.3 film greatly outperforms its analogs, offering impressive practicality.The Tafel slope of the Co 0.7 Fe 0.3 is also superior to that of most reported catalysts, indicating good reaction kinetics.If the porous substrate with excellent conductivity will not be adversely affected during thermal treatment, the Co 0.7 Fe 0.3 loaded onto it will be favorable to produce a higher current density under similar potentials for practical applications.
Furthermore, the effect of calcination temperature on Co 0.7 Fe 0.3 was examined.As shown in Figure S16a-c, Supporting Information, a lower calcination temperature (350 °C) will lead to better initial activity but induce significantly poor OER stability compared with Co 0.7 Fe 0.3 at 450 °C.High calcination temperature (550 °C) leads to worsened activity and stability as well.Figure S16d-f, Supporting Information indicates that a sufficient calcination temperature can improve crystallinity.However, such a more thermodynamically equilibrated product may not be optimal for OER stability.Thereinto, Raman spectroscopy of Co 0.7 Fe 0.3 -350 indicates a wide peak in the range of about 558 cm −1 , which could be attributed to the formation of rock salt CoO secondary phase. [42]In addition, the disappearance of peak at 192 cm −1 indicates the replacement of Co 2+ by Fe 3+ at the A site. [48] Therefore, the underheated sample was not fully arranged to form spinel domains, so the overall structure is overly disordered with impurity phases, thereby leading to structural instability during OER.On the other hand, the overheating causes an undesired rearrangement of cobalt and iron cations at A and B sites, [55] which can be compared from the peak width at A 1g mode of 677 cm −1 . [47]Indeed, such a phase separation behavior is thermodynamically favored and could be the reason why this unique stabilization by Fe doping was never reported.Therefore, this suitable calcination condition not only leads to the formation of crystalline spinel structure to a certain extent but also promotes the proper distribution of cobalt and iron cations at A and B sites.
In order to further understand the influence of Fe contents and the distribution of cobalt and iron cations at A and B sites in the spinel structure, Raman and Mössbauer spectra of the corresponding samples were acquired.Raman spectroscopy reveals more short-range structural details of the catalysts (Figure 4a).Pure CoO x shows five vibration modes at 195 cm −1 (F 2g ), 482 cm −1 (E g ), 522 cm −1 (F 2g ), 620 cm −1 (F 2g ), and 690 cm −1 (A 1g ), confirming the formation of the normal spinel Co 3 O 4 structure. [56]Ideally, Co 3 O 4 as a normal spinel can be described as (Co 2+ ) A (Co 3+ 2 ) B O 4 .The A 1g mode is attributed to the vibration involving Co 3+ at the B site.With the increase of Fe concentration from 0.1 to 0.2, A 1g peaks show a slight left shift compared Energy Environ.Mater.2024, 7, e12594 with pure CoO x , indicating that Fe 3+ notably replaces Co 3+ at the B site, and thus changes the symmetry of the sublattices. [48]As Fe concentration continued to increase from 0.3 to 0.4, F 2g modes in the range of 500-630 cm −1 began to disappear and the intensities of F 2g modes at 192 cm −1 began to decrease, hinting at Co 2+ in the A site getting replaced by Fe 3+ or Fe 2+ . [47]At the same time, the A 1g modes further occur left shift, showing that the Co-O bond length at the B site of Co 0.7 Fe 0.3 is longer than that in CoO x . [57]Further increasing Fe content from 0.5 to 0.8, the E 2g mode signal become prominent, the peak width of A 1g modes obviously expanded, and the vibration of F 2g mode disappeared, indicating that more Fe 3+ and fewer Co 2+ occupy the A site, and Co 2+ at B site could be generated.Thus, the overwhelming Fe cations will cause the normal spinel flips into inverse spinel (i.e., (Fe 3+ ) A (Co 2+ Fe 3+ ) B O 4 ). [47,48]Here, the Co 0.7 Fe 0.3 displays that most Fe 3+ is mainly distributed in the B site, and the A site contains a small number of Fe 3+ and Fe 2+ .
The Mössbauer spectrometric study further reveals a more detailed distribution of Fe cations at A and B sites as shown in Figure 4b.The fitting parameters of isomer shifts (IS) and quadrupole splitting (QS) are summarized in Table S5, Supporting Information.The results show that 65.7% and 7.9% of Fe 3+ are distributed at B and A sites, respectively.On the other hand, 22.3% and 4.1% of Fe 2+ locates at A and B sites, respectively, revealing the Co 0.7 Fe 0.3 is closer to normal spinel structure from the valence state distribution.Thus, the chemical formula of Co 0.7 Fe 0.3 can be expanded as (Co 0.73 Fe 0.27 ) A (Co 1.37 Fe 0.63 ) B O 4 according to the ion allocation in different sites.
X-ray photoelectron spectroscopy (XPS) of this sample also supports the above ion site assignments with specific valence states.Fe 2p spectra (Figure 4c; Figures S17 and S18, Supporting Information) show that the two significant peaks of 2p 3/2 spin-orbits located at ~711 and ~709 eV correspond to Fe 3+ and Fe 2+ , respectively. [58,59]As can be seen from Table S6, Supporting Information, it can be found that Fe 2+ content will increase gradually with the increase of Fe content from 0.1 to 0.5.Thereinto, the molar percentage of Fe 3+ for Co 0.7 Fe 0.3 is 63.9%, which is close to the above deduction from the Mössbauer analysis.
To further understand the effect of Fe on the electronic structure of this unique spinel structure, we measured the magnetization curves of zero-field cooling (ZFC) and field cooling (FC) over the temperature range of 50-400 K (Figure 4d).The ZFC magnetization curve discloses that the Néel temperature (T N ) is 120 K. Below T N , the magnetic susceptibility for ZFC mode decreases rapidly and shows antiferromagnetism, indicating a typical ferromagnetic superinteractions between the A site and the B site in the spinel structure. [60]Compared with the T N of pure Co 3 O 4 reported in the literature at about 20-40 K, [61] Co 0.7 Fe 0.3 has a larger T N , demonstrating more obvious ferromagnetic superinteractions and greater magnetic anisotropy.It can be seen from Figure S19, Supporting Information that the maximum magnetization, remanent magnetization, and coercivity of Co 0.7 Fe 0.3 increase with the decrease of the test temperature, meaning the presence of strong magnetic anisotropy.The temperature dependence of inverse magnetic susceptibility of Co 0.7 Fe 0.3 (1/χ = B/M) is also shown in Figure 4d.The Curie temperature (T c ) is determined by the modified Curie-Weiss law: χ = C/(T − T c ), where T c is the Curie temperature and C is the Curie constant. [62]As a result, the obtained T c of Co 0.7 Fe 0.3 is 392 K, which indicates that the superinteractions between the A and B sublattices still exist at room temperature.Furthermore, we compared the magnetic properties of spinel cobalt oxides with different Fe contents at room temperature.As shown in Figure 4e, with the increase of Fe content, the maximum magnetization of the samples first decreases and then increases.When Fe content is in the range of 0.1-0.2,Fe ions mainly replace Co ions in the B site but not the A site, and the superinteractions between sublattices are weak.When Fe content is further Energy Environ.Mater.2024, 7, e12594 increased to 0.5, Fe ions replace Co ions in A and B sites, and the superinteractions are significantly enhanced.Therefore, such a distribution of Fe 2+ and Fe 3+ between the A and B sublattices can produce appropriate ferromagnetic superinteractions.
Moreover, we further use XPS to compare the chemical difference between these spinel structures with increasing Fe contents.Figures S17 and S18, and Table S2, Supporting Information summarized the elemental composition of all samples by XPS survey spectra.All these thin films contain Co, Fe (despite the CoO x sample), and O elements with their surface composition agreeing with the precursor batching ratios.According to our observation, although the thickness of the film alters, the chemical composition and properties of the materials are identical.However, too few spraying laps could lead to insufficient coverage of the deposited layer, so the peak of Sn 3d from the FTO substrate could show up in the XPS survey spectra.
5a shows the Co 2p signals of the relevant samples.Two major core levels of Co 2p spectra for pure CoO x sample locate at 779.5 eV (2p 3/2 ) and 794.5 eV (2p 1/2 ), respectively.The characteristic peaks at 781.5 and ~779 eV correspond to Co 2+ and Co 3+ , respectively, which is in accordance with typical Co 3 O 4 . [63,64]The molar percentages (MP) of Co 2+ and Co 3+ and the changes in binding energies for samples are listed in Table S7, Supporting Information.Interestingly, the binding energy of Co 3+ for Co 0.7 Fe 0.3 in the 2p 3/2 orbit exhibits the smallest value (778.95eV), which is mostly related to the B site in cobalt spinel.This suggests the particular composition has a notable impact not only on the structure but also eventually reflects into the electronic properties.
In addition, O 1s spectra (Figure 5b) of the samples further revealed the impacts of iron on the bonding of both sites.Table S8, Supporting Information summarizes the binding energy parameters of A site oxygen, B site oxygen, Sn-O, and surface adsorbed hydroxyl group (-OH). [65]It can be found that the positions of oxygen binding energy at A and B sites with the addition of Fe produce significantly shifted compared with the pure CoO x , which is mainly due to the influence of Fe ion on lattice symmetry.Subsequently, when Fe content was in the range of 0.1-0.4,the change of binding energy for oxygen at the A site is not obvious because Fe 3+ mainly occupied the B site in this stage.As Raman analyzed, more Co 2+ at the A site will be replaced by Fe 3+ with the further increase of Fe concentration from 0.5 to 0.8, resulting in the slightly left shift of oxygen at the A site for Co 0.5 Fe 0.5 .Moreover, it can be found that the binding energy of O at the B site for Co 0.7 Fe 0.3 shows the smallest value (529.17eV), hinting that oxygen atoms possess more negatively charged and the covalency of the M-O bond is weakened.So far, wide studies show that Co occupying the B site in spinel is essential for producing high OER activity. [37,66]Therefore, we rationally speculate that the stabilized M-O bond at the B site could be responsible for the enhanced OER stability.Its lower covalency inhibits the dissolution of lattice oxygen during the OER process and improves the stability of the catalyst. [32,35]On the contrary, if Fe concentration was raised over 0.5, the lattice distortion and covalency increase again, which may adversely accelerate the participation of lattice oxygen in OER, and lead to O vacancies, thus reducing the lattice stability. [67]n order to better understand the effect of Fe doping on the stability of spinel-Co 3 O 4 , we performed density functional theory (DFT) calculations.As shown in Figure 6a  In other words, the lattice O in CoFe-3 is less active than in Co 3 O 4 in participating in the OER process, which should increase the stability of catalysts.Moreover, Figure 6b shows the d-band center of Co is also lowered (from −1.26 to −1.52 eV), implying Co 2+ and Co 3+ in CoFe-3 is much hard to be oxidized to Co 4+ during OER.It was suggested in previous work that the formation of Co 4+ intends to cause phase transition at the catalyst surface, and the generated CoO x (OH) y is not stable in the neutral condition, leading to fast catalyst dissolution. [36]igure 6c provides further evidence that the lattice oxygen is more stable in CoFe-3.It reveals the formation of oxygen vacancy in CoFe-3 is energetically more unfavorable than in Co 3 O 4 (2.91 eV vs 2.49 eV).Note that the value of oxygen vacancy formation energy (ΔG vo ) shown in Figure 6c   Energy Environ.Mater.2024, 7, e12594

Conclusion
In summary, we reported an abnormal stabilizing effect of Fe doping to spinel-type Co oxide OER catalysts in the neutral electrolyte.Co 0.7 Fe 0.3 showed decent activity (76.7 mV dec −1 , an overpotential of 527 mV at 1 mA cm 2 ) and a significantly enhanced OER stability than that of its pure CoO x counterpart.Specifically, Co 0.7 Fe 0.3 can maintain a stable current density of 1 mA cm −2 for 800 h without observable overpotential increases.This improvement is mainly attributed to the suitable introduction of Fe 3+ and Fe 2+ into both A and B sites of the spinel structure with an optimal distribution, producing the suitable ferromagnetic superexchange interactions between sublattices and the oxygen at the octahedral site more negatively charged.Thereby, the M-O bond has been tuned with the more appropriate length of a lower covalency, which enhances the energy barrier for oxidizing lattice oxygen.This feature will be beneficial to inhibit the formation of oxygen vacancies and further loss of metal ions in the OER process.DFT calculations also reveal that suitable Fe doping can result in more stable lattice oxygen.Therefore, the unique stabilizing effect of introducing ions with the nonconventional A/B site distributions of the spinel lattice provides opportunities for developing novel efficient oxygen evolution catalysts operatable in neutral and even acidic electrolytes.Furthermore, new materials with designated disordered sites may possess unprecedented new physics and chemistry to be discovered.
where G Co3O4 , G Co2Fe1O4 , G Co , G Fe , and G O2 denote the free energy of bulk Co 3 O 4 , bulk Co 2 Fe 1 O 4 , Co metal, Fe metal, and oxygen gas, respectively.The formation energy of neutral oxygen vacancy (ΔG vo ) was defined as where G vo and G bulk represent the free energies of the bulk material with an oxygen vacancy and the pristine model.
Physical characterization: X-ray diffraction (XRD) patterns were acquired from the X-ray diffractometer (Bruker D8 advance) with Cu-Kα radiation.The morphology, size, and structure information of the samples were undertaken on a field emission scanning electron microscope (SEM, Hitachi S4800) equipped with an energy-dispersive X-ray spectrometer for elemental mapping and highresolution transmission electron microscopy (TEM, FEI TF20).The Raman spectra were recorded using a Confocal Raman Microscope (Renishaw inVia-reflex) with a laser source of 532 nm wavelength.The elementary valence state and composition of as-synthesized samples were measured at X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha+).The binding energies of XPS spectra were calibrated through the carbon 1s peak of 284.6 eV as a reference.The magnetic measurements were carried out by using a superconducting quantum interference device magnetometer (Quantum Design MPMS 3). 57Fe Mössbauer spectrum of the sample was executed at room temperature by using a Mössbauer spectrometer (ms500) with a 57 Co/Pd source.All the isomer shifts (IS) are calculated relative to α-Fe at room temperature.The analysis of the spectrum was performed with mosswin4.0software.The Co and Fe leaking of Co 0.7 Fe 0.3 films during OER were quantified by inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7800).

Figure 1 .
Figure 1.a) The XRD patterns, b) fitting crystal structure parameters, c) the FWHM values of samples, d) the crystal structures, the e) TEM, f) HRTEM, g) SAED images, and h) mapping of Co 0.7 Fe 0.3 .

Figure 2 .
Figure 2. Oxygen evolution properties of the prepared CoFeO x catalysts in 1.0 M phosphatic buffer solution (PBS, pH = 7).a) The current density of different electrodes at a scan rate of 5 mV s −1 with iR compensation, b) corresponding Tafel slopes, c) chronopotentiometric curves of different electrodes with constant current densities without iR compensation, and d) evaluation of double-layer capacitance measurements.

Figure 3 .
Figure 3.A long-term oxygen evolution reaction stability test of the Co 0.7 Fe 0.3 film/FTO electrode was prepared by spraying 20 laps in 0.1 M phosphate buffer solution (pH = 7).

Figure 4 .
Figure 4. a) Raman spectra comparison, b) room temperature experimental Mössbauer spectrum, c) Fe 2p spectra, d) magnetization as a function of temperature (M-T) with ZFC/FC modes for Co 0.7 Fe 0.3 measured at 100 Oe, the right-hand ordinate refers to the inverse magnetization, e) the magnetic hysteresis loops of the different Fe contents samples were measured at 298 K.
and Figure S20, Supporting Information, Co 3 O 4 and Co 2 Fe 1 O 4 with different Fe distributions were modeled, and in all considered structures of Co 2 Fe 1 O 4 , their lattices were expanded by around 1% in comparison with Co 3 O 4 .It owes to the larger atomic radius of Fe than Co and is consistent with experimental results.It is shown in Figure 6a model CoFe-3 has the lowest formation energy (ΔE f ), and its computed XRD spectra (Figure S20, Supporting Information) are in excellent agreement with experiments.The projected density of states (PDOS) of CoFe-3 was then analyzed and compared with Co 3 O 4 as seen in Figure 6b.It reveals the p-band center of O is lowered from −3.14 to −4 eV versus Fermi level after introducing Fe into Co 3 O 4 , indicating lattice O becomes more stable.
is an average of all the different O sites in CoFe-3 (Figure S21, Supporting Information for ΔG vo at each site).The Bader charge analysis shown in Figure 6d also supports lattice O is more stable in CoFe-3.It shows the doping of Fe increases the net electron number on O (from 0.97 e to 1.02 e), indicating the covalency of the M-O bond decreases.