Regulating Lattice Oxygen of Co3O4/CeO2 Heterojunction Nanonetworks for Enhanced Oxygen Evolution

Developing efficient and cost‐effective electrocatalysts as substitutes for noble metals remains a big challenge, which demands significant advancements in both material designing and mechanistic understanding. Herein, Co3O4/CeO2 heterojunction nanonetworks are successfully synthesized through metal organic framework precursor. Notably, Co3O4/CeO2 heterojunctions can effectively regulate electronic structure of Co3O4, thus inducing oxygen atom from Co3O4 lattice to participating in oxygen evolution reaction (OER) via lattice oxygen‐mediated mechanism, which reduces reaction overpotential. Additionally, the porous network structure can facilitate electrolyte transfer and provide more active sites for electrocatalytic reactions. Consequently, Co3O4/CeO2 heterojunction nanonetworks exhibit great electrocatalytic performance and high durability, requiring only an OER overpotential of 259 mV at current density of 100 mA cm−2 in 1 M KOH, markedly outperforming Co3O4 nanocatalysts (309 mV) and showing promising potential as substitutable non‐noble OER catalysts.


Introduction
In recent years, pressing issues of environmental degradation and depletion of fossil fuels have necessitated the development of renewable clean energy sources. [1,2]lectrocatalytic water splitting has emerged as a promising strategy to address both environmental and energy issues simultaneously. [3,4]However, electrochemical efficiency of oxygen evolution reaction (OER) is limited by considerable overpotential required to complete the four-electron transfer steps involved in this reaction. [5]owadays, IrO 2 and RuO 2 have been identified as effective electrocatalysts for OER, [6] but their high cost and scarcity may hinder their practical applications.Hence, exploring OER electrocatalysts with low cost and high efficiency has received increasing attention, but may pose a huge challenge. [7,8][11][12][13][14] For instance, Jia et al. successfully prepared oxygen-modified nanotubes with hierarchical structure as electrocatalyst, which exhibited outstanding OER performance through the surface modification with oxygen. [15]mong transition metal components, Co 3 O 4 is considered as a promising electrocatalyst due to its diversified structure, environmental benignity, and low cost. [16]However, both intrinsic catalytic activity and low electronic conductivity may hinder its applications. [17]Thus, it is crucial for improving electronic conductivity and intrinsic catalytic activity of Co 3 O 4 to enhance its OER performance.Due to the unique structure of rare-earth elements (e.g., La, Ce), it is an effective way to adjust electronic structure of heterojunction nanocatalyst by rare-earth elements doping, [18] which are promising candidates in enhancing the catalytic performance of Co 3 O 4 .
Adsorbate evolution mechanism (AEM) has been established as conventional mechanism for OER in many catalysts (like Co 3 O 4 ). [19]However, AEM-based oxide electrocatalysts face performance limitations due to scaling relation among oxygen intermediates, leading to high overpotential for OER. [20,21]ecently, a new mechanism called lattice oxygen oxidation mechanism (LOM) or lattice oxygen-mediated mechanism has been proposed. [22]This mechanism involves generation of oxygen vacancy on catalyst surface and direct participation of oxygen anions from transition metal oxides lattice during OER, which has been verified by 18 O isotope labeling in situ spectroscopy experiments, pH-dependent OER activity, and density functional theory (DFT) calculations. [23,24]What is more, LOM-based catalysts have been shown to overcome the limitations of AEMbased nanocatalysts, suggesting their potential for superior OER performance. [25]Therefore, it is of utmost importance to improve the performance of catalyst by changing reaction pathway.
In this work, we have successfully synthesized Co 3 O 4 /CeO 2 heterojunction nanonetworks on nickel foam (NF), which were used as electrocatalysts for OER in alkaline solution.pHdependent OER activity experiment indicated that different from previously reported Co 3 O 4 nanoparticles, the heterojunction could modulate the electronic structure of Co 3 O 4 , which results in the OER mechanism of Co 3 O 4 translated from AEM to LOM.This shift has enabled a rapid interfacial charge transfer from CeO 2 to Co 3 O 4 , thus facilitating the electrolyte transfer and enhancing its intrinsic catalytic activity.

Results and Discussion
There were two steps in fabrication process of Co 3 O 4 /CeO 2 heterojunction nanonetworks (Figure 1).Initially, CoCeBDC was synthesized through reaction of Ce(NO 3 ) 3 and Co(NO 3 ) 2 with sodium terephthalate (Na 2 BDC) by hydrothermal synthesis, where Ce-doped CoBDC was formed.The ratio of Ce to Co was detected by inductively coupled-plasma mass spectrometry (ICP-MS; Table S1, Supporting Information).Subsequently, an in situ oxidation process was conducted at 400 °C for 6 h under atmospheric air to transform CoCeBDC into Co 3 O 4 / CeO 2 heterojunction nanonetworks (Figure S1, Supporting Information).A more detailed synthetic protocol can be found in Supporting Information.
The crystal structure of as-prepared nanocatalysts was characterized by X-ray diffraction (XRD) patterns.As displayed in Figure 2a, although Ce doping ratios were different among all catalysts,the asobtained precursor had same structure of CoBDC, which indicated Ce doping did not alter the initial structure of CoBDC.After pyrolysis, five major XRD peaks at 65.2°, 59.4°, 44.8°, 36.8°, and 31.3°w ere identified as ( 440), ( 511), ( 400), (311), and (220) crystal planes of Co 3 O 4 , respectively (Figure 2b).Additionally, several small peaks at 47.5°, 33.1°, and 28.6°, which corresponded to ( 220 The scanning electron microscopy (SEM) images of CoBDC (Figure 3a) and CoCeBDC (Figure 3b) indicated the uniform    3e) with a large bubble contact angle (160.3°),indicating that as-synthesized Co 3 O 4 /CeO 2 heterojunction nanonetworks are favorable for rapid release of O 2 bubbles, thereby facilitating OER process. [26,27]e surface electronic states and chemical constituents of asprepared nanocatalysts were determined by X-ray photoelectron spectroscopy (XPS).The survey XPS spectrum of Co 3 O 4 included O, Co, and C peaks (Figure 4a).After introducing Ce into Co 3 O 4 /CeO 2 , Ce 3d peak could be observed in XPS spectrum, and three pairs of spin-orbit component indicating the valence state of Ce is oxidized from trivalent to quadrivalent through pyrolysis process (Figure 4b).The high-resolution XPS spectra of Co 2p (Figure 4c) could be classified as two shake-up satellites and two spin-orbit singlets, corresponding to the two peaks of 2p 3/2 and 2p 1/2 , respectively. [28,29]The further peak fitting results of Co 3 O 4 /CeO 2 heterojunction nanonetworks showed that a portion of Co 3þ (794.9 and 779.8 eV) had been converted into Co 2þ (796.4 and 781.2 eV) after pyrolysis, [30,31] which is beneficial for OER performance. [32,33]The O 1s XPS spectrum could be deconvoluted into two to three peaks at 529.9, 531.3, and 533.3 eV (Figure 4d), which might be ascribed to oxygen atoms bound to metal atoms, carbon, and oxygen double bond, respectively. [34,35]Overall, these findings demonstrated that the formation of Co 3 O 4 /CeO 2 heterojunction could effectively alter the electronic structure of Co active site.
The electrocatalytic performance of as-synthesized electrocatalysts for OER was assessed in 1.0 M KOH solution at room temperature via linear sweep voltammetry (LSV), while commercial RuO 2 powder was used as reference electrocatalyst.S3, Supporting Information).Meanwhile, the OER performance of as-synthesized catalysts was normalized by ECSA, as shown in Figure S9, which is consistent with Figure 5a.Additionally, the stability of Co 3 O 4 /CeO 2 heterojunction nanonetworks was evaluated by a long-term (100 h) chronopotentiometry test at 100 mA cm À2 (Figure 5e), while only a slight increase in potential was observed after stability test, implying that this electrocatalyst has good OER stability.After stability test, the OER efficiency of Co 3 O 4 /CeO 2 heterojunction nanonetworks was not significantly decreased (inset of Figure 5e).At the same time, the morphology (SEM images in Figure S10a,b, Supporting Information) of Co 3 O 4 / CeO 2 heterojunction nanonetworks did not significantly change after stability test.Besides, the XRD patterns and XPS spectra (Figure S10c,d, Supporting Information) of Co 3 O 4 /CeO 2 heterojunction nanonetworks also revealed that both the crystalline and chemical state of as-prepared catalyst were not obviously influenced by this stability test.These results suggested that as-synthesized catalyst possesses excellent catalytic performance under long-time operation.Furthermore, the OER reaction mechanism was determined by OER test of electrocatalysts in different pH environments.As shown in Figure 5f, with the pH increasing from 13 to 14, the OER efficiency of Co 3 O 4 / CeO 2 heterojunction nanonetworks improved continuously.According to Grimaud et al., this phenomenon revealed a LOM pathway as previously reported, [36] which have been proved by isotopic labeling and other methods. [25]In contrast, the OER performance of Co 3 O 4 electrocatalyst did not show significantly change as pH increase (Figure S11, Supporting Information and inset in Figure 5f ), which indicated this OER process followed an AEM pathway.The change in OER mechanism plays a crucial role in delivering outstanding OER performance.To determine the interpretation of lattice oxygen-mediated OER performance at atomic scale, spin-polarized DFT calculation was employed to compare the differences between Co 3 O 4 and Co 3 O 4 /CeO 2 heterojunction nanonetworks.As one of the Mott-Hubbard insulators, Co 3 O 4 nanocatalysts exhibit a large charge transfer energy and strong ionic character of Co─O bond. [37]The O 2p band is deeply aligned in the energy diagram which is lower than Co 3d band (Figure S12a,b left, Supporting Information).In other words, the energy difference between Co 3d and O 2p band centers (named ε Co 3d and ε O 2p , respectively) should be positive (ε Co 3d -ε O 2p = 1.68 eV).Thermodynamically, cationic redox electrochemistry is more favorable to donate electrons.The electron transfer occurs between adsorbed oxygen intermediates and metal center to proceed the OER reaction, following AEM pathway while oxygen ligands will be constrained in lattice without activation. [38,39][42] Thus, intramolecular electron transfer from oxygen ligands to Co cations in lattice matrix is feasible, leaving ligand holes for lattice oxygen activation (ε Co 3d -ε O 2p = À0.12 eV), providing the prerequisite for lattice oxygen redox chemistry.It is also worthy noticing that the oxygen vacancies formation energy process on Co 3 O 4 /CeO 2 heterojunction nanonetworks is exothermic with a free energy of À0.86 eV.In comparison, this process on Co 3 O 4 nanocatalysts is 46.88 eV, suggesting that the Co 3 O 4 /CeO 2 heterojunctions significantly facilitate the oxygen vacancies formation process, which is a given indicator for LOM pathway. [43]n conclusion, Co 3 O 4 /CeO 2 heterojunction nanonetworks with porous structure were successfully synthesized from metal organic framework (MOF) precursors, whose OER performance could be improved by adjusting Ce doping amount.The obtained Co 3 O 4 /CeO 2 heterojunction nanonetworks had excellent OER performance with a low overpotential of 259 mV at 100 mA cm À2 , with a low Tafel slope of 34.3 mV dec À1 in alkaline electrolyte, which was better than the previously reported Co 3 O 4 nanocatalysts.The enhanced electrocatalytic activity could be attributed to the porous network structure that favors mass transfer and Co 3 O 4 /CeO 2 heterojunctions modifying electronic structure of Co 3 O 4 that induce oxygen atoms from Co 3 O 4 lattice to participate in OER via a LOM mechanism, which results in accelerated electron transfer and enhanced intrinsic catalytic activity.These findings provide new ideas for development of highperformance non-noble metal electrocatalysts for OER applications.

Figure 2 .
Figure 2. XRD patterns of a) CoBDC, CoCeBDC and b) Co 3 O 4 , Co 3 O 4 /CeO 2 nanocatalysts prepared by present method, and c) magnification of dotted area in Figure 2b.
growth of nanofilm arrays, which covered the entire NF surface.Following pyrolysis, the nanofilm transformed into nanonetworks (Figure 3c,d and S2, Supporting Information).The porous network not only accelerates the transfer of electrolytes but also increases the active sites, which is beneficial to electrocatalytic reaction.The nanostructures of Co 3 O 4 /CeO 2 were further confirmed by transmission electron microscopy (TEM) (Figure 3e and S3, Supporting Information).The high-resolution TEM (HRTEM) image showed two sets of lattice fringes with interplanar spacings of 0.271, 0.286, and 0.202 nm corresponding to (220) plane of CeO 2 and (220), (400) plane of Co 3 O 4 , respectively (Figure 3f ).Moreover, obvious interface between Co 3 O 4 and CeO 2 could be found in HRTEM image, implying the successful construction of Co 3 O 4 /CeO 2 heterojunction.The elemental mapping images demonstrated the uniform distribution of Co, Ce, C, and O in Co 3 O 4 /CeO 2 heterojunction nanonetworks (Figure 3g-k).Notably, Co 3 O 4 /CeO 2 heterojunction nanonetworks exhibited a superhydrophilic surface that could strengthen electrolyte permeation and affinity (Figure S4, Supporting Information).Meanwhile, Co 3 O 4 /CeO 2 heterojunction nanonetworks also had a superaerophobic surface (inset of Figure

Figure 5 .
Figure 5. a) OER performance, b) overpotentials at 50 and 100 mA cm À2 , c) Tafel plots, and d) Nyquist plots of different electrocatalysts.e) Chronopotentiometry curve of Co 3 O 4 /CeO 2 heterojunction nanonetworks at 100 mA cm À2 over 100 h in 1 M KOH and corresponding LSVs before and after stability tests (inset).f ) OER kinetics currents of Co 3 O 4 /CeO 2 heterojunction nanonetworks with varying pH and OER activity at 1.55 V versus RHE as a function of pH (inset); error bars represent standard deviation of three measurement results.
DOI: 10.1002/aesr.202300123Developing efficient and cost-effective electrocatalysts as substitutes for noble metals remains a big challenge, which demands significant advancements in both material designing and mechanistic understanding.Herein, Co 3 O 4 /CeO 2 heterojunction nanonetworks are successfully synthesized through metal organic framework precursor.Notably, Co 3 O 4 /CeO 2 heterojunctions can effectively regulate electronic structure of Co 3 O 4 , thus inducing oxygen atom from Co 3 O 4 lattice to participating in oxygen evolution reaction (OER) via lattice oxygen-mediated mechanism, which reduces reaction overpotential.Additionally, the porous network structure can facilitate electrolyte transfer and provide more active sites for electrocatalytic reactions.Consequently, Co 3 O 4 /CeO 2 heterojunction nanonetworks exhibit great electrocatalytic performance and high durability, requiring only an OER overpotential of 259 mV at current density of 100 mA cm À2 in 1 M KOH, markedly outperforming Co 3 O 4 nanocatalysts (309 mV) and showing promising potential as substitutable non-noble OER catalysts.