Achieving High Activity and Long-Term Stability towards Oxygen Evolution in Acid by Phase Coupling between CeO2-Ir

The development of efficient and stable catalysts with high mass activity is crucial for acidic oxygen evolution reaction (OER). In this study, CeO2-Ir heterojunctions supported on carbon nanotubes (CeO2-Ir/CNTs) are synthesized using a solvothermal method based on the heterostructure strategy. CeO2-Ir/CNTs demonstrate remarkable effectiveness as catalysts for acidic OER, achieving 10.0 mA cm−2 at a low overpotential of only 262.9 mV and maintaining stability over 60.0 h. Notably, despite using an Ir dosage 15.3 times lower than that of c-IrO2, CeO2-Ir/CNTs exhibit a very high mass activity (2542.3 A gIr−1@1.53 V), which is 58.8 times higher than that of c-IrO2. When applied to acidic water electrolyzes, CeO2-Ir/CNTs display a prosperous potential for application as anodic catalysts. X-ray photoelectron spectrometer (XPS) analysis reveals that the chemical environment of Ir nanoparticles (NP) can be effectively modulated through coupling with CeO2. This modulation is believed to be the key factor contributing to the excellent OER catalytic activity and stability observed in CeO2-Ir/CNTs.


Introduction
A significant amount of renewable energy (such as wind energy, tidal energy, solar energy, etc.) is wasted due to ineffective utilization.Utilizing energy storage and conversion devices to convert renewable energy into other forms of stored energy represents an effective approach for addressing the aforementioned issues [1,2].A proton exchange membrane water electrolyzer (PEMWE) offers the capability to convert renewable energy into chemical energy in hydrogen, which possesses sustainable, clean, and efficient characteristics [3,4].PEMWE comprises two crucial half-reactions: cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER).OER involves a four electron-proton transfer process that includes complex intermediates' conversions and necessitates high kinetic costs [5,6].Most catalysts reported thus far encounter a common challenge: low catalytic activity and poor stability in acidic OER [7,8].In view of the comprehensive consideration of catalytic activity and stability, Ir-based materials are widely used as catalysts in acidic OER.However, these catalysts currently face challenges such as scarcity, high cost, insufficient catalytic activity, and poor stability, hence they still have a long way to go before practical application can be achieved [9,10].Consequently, exploring low cost Ir-based catalysts with high activity and durability has become a challenge at present.
It has been reported that metallic Ir catalysts with nanorods [11], nanosheets [12], and other structures exhibit exceptional catalytic activity and stability in acidic OER [13].Significant efforts have been devoted to minimizing the use dosage of Ir without compromising the catalytic activity.Downsizing Ir to the nanometer scale can enhance the exposure of active sites, thereby improving catalytic activity [14].Furthermore, the construction of heterojunctions could induce lattice strain within the active phase and allow for the regulation of charge distribution between the two phases, resulting in a regulation of their catalytic activity [15,16].Consequently, the electronic structure of Ir can be modulated through the construction of heterojunctions with other phases, indirectly influencing the adsorption and desorption of intermediates [16,17].Appropriate adsorption/desorption facilitates conversion processes of intermediates, leading to enhanced reaction rates and accelerated kinetics [18,19].The construction of a heterojunction between Ir and metal oxides is a common strategy for enhancing both catalytic activity and stability in Ir-based catalysts.Lee et al. achieved improved OER performance by combining Ir with MoO 3 to construct highly electron-deficient Ir NPs [16].Xing and colleagues uniformly deposited an ultra-fine (~1.0 nm) layer of Ir metal onto the surface of niobium oxide (Nb 2 O 5−x ), which exhibited a high concentration of oxygen vacancies [20].During OER, dynamic migration of oxygen takes place at the two-phase interface between Ir and Nb 2 O 5−x , effectively preventing excessive oxidation of Ir to high-valent species and subsequent deactivation.Although extensive research has focused on developing heterogeneous catalysts based on transition metal oxides combined with iridium (Ir/MO x ), investigations into heterojunctions formed by rare earth metal oxides and iridium remain scarce.Cerium oxide is widely used as a catalyst and cocatalyst for many oxidation reactions due to its excellent redox properties [21][22][23][24].For example, Im and colleagues improved the electrocatalytic activity of OER by inducing site-selective crystal disorder through the doping of Ce 3+ ions into the MIL-88B(Ni) framework [25].The aforementioned statements have prompted us to redirect our research focus towards CeO 2 in order to investigate the electron transfer between CeO 2 and Ir, as well as its impact on both the activity and the stability of OER.Moreover, carbon materials are widely acknowledged as ideal supports for electrocatalysis owing to their remarkable specific surface area and excellent electrical conductivity [26][27][28].As an exemplary carrier, CNTs offer abundant nucleation sites and conductive pathways for catalytic active materials, thereby significantly enhancing the catalytic activity of catalysts while reducing their required quantity [29,30].
In this study, we propose a strategy for the construction of heterojunctions and carbon combination to enhance catalytic performance while reducing the consumption of Ir.This strategy involves modulating the electronic structure of Ir and improving the electron conduction of the catalyst.To achieve this, we engineer electron-deficient Ir NPs through the construction of heterojunctions with CeO 2 , complemented by the incorporation of CNTs to facilitate efficient charge conduction and improve the utilization of Ir.As a result, CeO 2 -Ir/CNTs exhibit impressive durability and catalytic activity, operating continuously at 10.0 mA cm −2 for over 60.0 h with only 262.9 mV overpotential.These findings underscore the excellent catalytic performance and long-term robustness of CeO 2 -Ir/CNTs when employed in acidic water electrolyzers.

Synthesis of xCeO 2 -Ir/CNTs and CeO 2 /CNTs
CeO 2 -Ir/CNTs (xCeO 2 -Ir/CNTs, x = 1) were synthesized via a solvothermal method.Specifically, 4.0 mL of 5.0 mM IrCl 3 , 4.0 mL of 5.0 mM CeCl 3 , and 3.0 mL of 5.0 g/mL acid-treated CNTs [30] were sequentially added dropwise to 15.0 mL ethanol.Water was added to regulate the volume of the above solution to 30.0 mL, followed by thorough agitation.The obtained mixture was transferred to a 50.0 mL Teflon autoclave and heated at 180.0 • C for 2.0 h.After cooling, the black precipitate was collected by centrifugation and alternate washing with ethanol and water for three runs, followed by vacuum drying.The xCeO 2 -Ir/CNTs were synthesized by varying the feeding volume of CeCl 3 , where x refers to the molar ratio of CeCl 3 /IrCl 3 , albeit keeping other parameters constant.Similarly, CeO 2 /CNTs were synthesized under the same conditions but without IrCl 3 .

Characterization
Morphologies and microstructures of samples were characterized using field-emission scanning electron microscopy (FESEM, Hitachi SU8010) and transmission electron microscopy (TEM, Talos F200X equipped with an energy dispersive X-ray energy spectrometer (EDS).An X-ray diffractometer (XRD, Bruker D8-Advance) with Cu Kα radiation was applied to collect XRD patterns in a scanning range of 10.0 to 90.0 • at a rate of 10.0 • /min.X-ray photoelectron spectra (XPS) was recorded with an X-ray photoelectron spectrometer (XPS, Thermo ESCA-LAB 250XI) with monochromatic Al Kα excitation source, in which the binding energies were calibrated with C 1 s at 284.6 eV.The elemental composition analysis was conducted on an inductively coupled plasma optical emission spectrometer (ICP-OES, iCAP 7200 Duo).The thermogravimetric (TGA) curve was recorded on a NETZSCH STA 449 F5 under the air stream, starting at 30.0 • C and ramping up to 800.0 • C at a rate of 10.0 • C/min.

Electrochemical Measurements
All electrochemical tests were conducted using a three-electrode system with an electrochemical workstation (CHI 660e, Chenghua, Shanghai).A Ag/AgCl electrode and a Pt wire were utilized as the reference and counter electrodes, respectively.The electrolyte used was a 0.5 M H 2 SO 4 solution with a pH of 0.32 (Figure S1) at room temperature, consistent with values reported in the existing literature [31,32].The catalyst-loaded glassy carbon electrode (GCE) was prepared by drop-casting the catalyst ink onto the GCE, and utilized as the working electrode after natural drying.The catalyst ink was prepared by dispersing 4.0 mg catalyst in 1.0 mL of isopropanol/water solution (V/V = 7/3) with 10.0 µL Nafion.The mass loading of the electrocatalysts was estimated to be ~0.20 mg cm −2 .The linear sweep voltammograms (LSVs) were recorded with a scan rate of 5.0 mV s −1 at 1600 rpm to evaluate the catalytic performance of the catalysts.The LSVs reported in this work were 95% iR corrected [33][34][35] unless otherwise stated, and the potentials reported were calibrated with the reversible hydrogen electrode (RHE) using the equation E(V vs. RHE) = E(Ag/AgCl) + 0.059 × pH.The electrochemical impedance spectra (EIS) were obtained at 1.32 V in the frequency range from 100 KHz to 0.01 Hz.The cyclic voltammograms (CVs) were measured within the non-Faradic potential range at different scan rates (v).The electrochemical double layer capacitance (C dl ) of catalysts was calculated by plotting half the difference between the anode and cathode current densities (∆j = j anode − j cathode ) vs. v.The electrochemically active surface areas (ECSAs) of the catalysts were caculated according to S ECSA = C dl × S/C s (where S and C s represent the geometric area of the electrode and the roughness factor with a value of 35.0 µF cm −2 [36], respectively).According to j ECSA = j geo /S ECSA , ECSA-normalized LSVs of the catalysts were estimated (j geo denotes the geometric activity of the catalysts).The chronopotentiometric curves were used to evaluate the catalysts' stability on the carbon paper (CP) with a mass loading of 2.0 mg cm −2 .The catalyst-loaded CP mentioned in the text (active area: 1.0 × 1.0 cm 2 , Figure S2) was prepared using the same procedure as for GCE.
The Faradaic efficiency (FE) of CeO 2 -Ir/CNTs towards OER on CP was performed in a three-electrode system and investigated using a gas chromatograph (GC, 9560, Shanghai Huaai Scientific Instrument, Shanghai, China).Electrolysis was carried out for a specific duration at a current density of 10.0 mA cm −2 .Following each electrolysis stage, gaseous samples were drawn from the headspace using a gas-tight syringe (with multiple extractions for averaging), and subsequently analyzed by a GC.Herein, FE of O 2 was calculated according to equation is the generated volume of O 2 , F is the Faraday constant of 96,485.3C/mol, V m denotes the molar volume of the gas, i is the recorded current, and t is the time spent in electrolysis.

Evaluation of Water Electrolyzers
Practical usability of the catalyst was appraised by assembling a water electrolyzer using Pt/C and the CeO 2 -Ir/CNTs as the cathode and anode catalysts, respectively.Both Pt/C and CeO 2 -Ir/CNTs were loaded on the CPs.Their mass loadings were both controlled to be 2.0 mg cm −2 .The electrochemical tests were carried out in 0.5 M H 2 SO 4 .To give a comparison, water electrolyzers with Pt/C and c-IrO 2 were also prepared.

Results and Discussion
The synthetic scheme for CeO 2 -Ir/CNTs is presented in Figure 1a.Briefly, CeO 2 -Ir/CNTs were synthesized by maintaining a solution containing CNTs, IrCl 3 , and CeCl 3 at 180.0 • C for 2.0 h.FESEM was carried out to describe the morphology of CeO 2 -Ir/CNTs, as depicted in Figure S3a.The surface of each CNT appeared rough and was densely attached with ultra-fine NPs.The morphologies of Ir/CNTs and CeO 2 /CNTs were similar to that of CeO 2 -Ir/CNTs (Figure S3b,c), indicating that the addition of CeCl 3 or IrCl 3 had negligible effects on the samples' morphologies.The micro-structure of CeO 2 -Ir/CNTs was further investigated by TEM.As depicted in Figure 1b, CeO 2 and Ir NPs were well-dispersed on the surface of CNT.Size distribution analysis revealed their average diameter was approximately 2.15 nm (Figure 1b).HRTEM, in Figure 1c, demonstrated multiple paired nanoparticles (marked with yellow calabash), where each nanoparticle displayed clear lattice fringes, indicating their excellent crystallinities.The FFT images in Figure 1d,e clearly illustrate that the paired nanoparticles showed different lattice fringe spacings, that is, 0.221 and 0.192 nm, corresponding well to the (111) plane of Ir (PDF No. 87-0715) and the (220) plane of CeO 2 (PDF No. 89-8436), respectively.These results suggest the successful construction of CeO 2 -Ir heterojunctions (Figure 1c).Furthermore, energy-dispersive X-ray spectroscopy (EDS) mapping demonstrated uniform distribution along the contour of CeO 2 -Ir/CNTs for C, O, Ir, and Ce elements (Figure 1f).XRD patterns of CeO 2 -Ir/CNTs and CeO 2 /CNTs are provided in Figure 2a and Figure S4, respectively.The diffraction peaks of Ir (111), CeO 2 (111), and (200) of CNTs were observed for CeO 2 -Ir/CNTs, fully consistent with HRTEM observations.Meanwhile, only those of CeO 2 and CNTs were visible for CeO 2 /CNTs.The composition of xCeO 2 -Ir/CNTs was investigated by ICP-OES and the results are summarized in Table S1.It was clearly seen that the Ce/Ir molar ratio of xCeO 2 -Ir/CNTs samples was almost consistent with the feeding ratio of CeCl 3 /IrCl 3 .The additional amount of Ce did not alter the structure of xCeO 2 -Ir/CNTs.As depicted in Figure S5a, irrespective of the CeO 2 content in xCeO 2 -Ir/CNTs, their XRD diffraction peaks consistently exhibited a combination pattern comprising CeO 2 , Ir, and CNTs; only the peak of CeO 2 (111) grew higher with the increasing CeO 2 content.Thermogravimetric analysis (TGA) was further applied to calculate the weight percentage of the components within CeO 2 -Ir/CNTs.According to Figure 2b, a weight loss of ~88.5 wt.% was obviously observed at 800.0 • C in air, where the CeO 2 -Ir/CNTs was transformed into CeO 2 and IrO 2 .Consequently, by integrating the ICP-OES and TGA analysis, it was inferred that the weight percentage of Ir in CeO 2 -Ir/CNTs was estimated to be 5.6 wt.%.EDS analysis further confirmed a 1:1 molar ratio of Ce/Ir and an Ir content of 5.6 wt.% in CeO 2 -Ir/CNTs, as shown in Table S2.
wt.% was obviously observed at 800.0 °C in air, where the CeO2-Ir/CNTs was transformed into CeO2 and IrO2.Consequently, by integrating the ICP-OES and TGA analysis, it was inferred that the weight percentage of Ir in CeO2-Ir/CNTs was estimated to be 5.6 wt.%.EDS analysis further confirmed a 1:1 molar ratio of Ce/Ir and an Ir content of 5.6 wt.% in CeO2-Ir/CNTs, as shown in Table S2.XPS measurement was used to reveal the valance state of CeO2-Ir/CNTs.As illustrated in Figure 2c, the survey XPS spectra confirmed the presence of Ce, Ir, O, and C in the sample of CeO2-Ir/CNTs.The Ir 4f spectra were deconvoluted into the following two sets of peaks: metallic Ir peaks and oxidized Ir peaks.The peaks at 61.5 and 64.5 eV could be assigned to Ir 4f 7/2 and Ir 4f 5/2 of metallic Ir; the oxidized Ir were at 62.9 and 65.9 eV (Figure 2d) [11,37].The XPS spectra of Ce 3d of CeO2-Ir/CNTs are shown in Figure 2e.From Figure 2e, the Ce 3d spectrum could be decomposed into four pairs of spin orbitals (V/U, V′/U′, V″/U″, and V‴/U‴), where V and U represent the 3d 3/2 and 3d 5/2 states, respectively [38].The peaks corresponding to V/U, V″/U″, and V‴/U‴ were attributed to Ce 4+ 3d orbitals, while the peaks corresponding to V'/U' were associated with Ce 3+ 3d [39].The ratio of Ce 3+ /Ce 3+ + Ce 4+ in CeO2 kept constant (39%) after coupling with Ir (Figure 2e), indicating that the chemical environment of Ce did not undergo significant changes.The O 1s spectra in Figure 2f could be deconvoluted into three peaks: OL (531.0 eV), Vo• (532.0 eV), and O-H (533.6 eV), where OL and Vo• refer to oxygen in the lattice and oxygen vacancies, respectively.The Vo• peak further verified the existence of Vo• in the CeO2, in line with the presence of a large amount of Ce 3+ species.Closer examinations revealed a strong electron coupling between CeO2 and Ir through the heterointerface in the CeO2-Ir/CNTs.Figure 2d shows that the peaks corresponding to Ir 4f in the CeO2-Ir/CNTs appeared at a relatively higher binding energy compared with Ir/CNTs (synthesized in  The influence of CeO2 content on the catalytic activity of xCeO2-Ir/CNTs was investigated.As illustrated in Figure S5b, the catalyst activity was improved, by increasing x from 0 to 1.0.However, further increasing x to 1.5 led to declined activity.These findings suggest that optimal OER activity could be achieved by maintaining the Ce/Ir molar ratio of xCeO2-Ir/CNTs at 1/1.Furthermore, Tafel plots (Figure S5c) and EIS analysis (Figure S5d) revealed that the OER kinetics and charge transfer resistance of XPS measurement was used to reveal the valance state of CeO 2 -Ir/CNTs.As illustrated in Figure 2c, the survey XPS spectra confirmed the presence of Ce, Ir, O, and C in the sample of CeO 2 -Ir/CNTs.The Ir 4f spectra were deconvoluted into the following two sets of peaks: metallic Ir peaks and oxidized Ir peaks.The peaks at 61.5 and 64.5 eV could be assigned to Ir 4f 7/2 and Ir 4f 5/2 of metallic Ir; the oxidized Ir were at 62.9 and 65.9 eV (Figure 2d) [11,37].The XPS spectra of Ce 3d of CeO 2 -Ir/CNTs are shown in Figure 2e.From Figure 2e, the Ce 3d spectrum could be decomposed into four pairs of spin orbitals (V/U, V /U , V /U , and V /U ), where V and U represent the 3d 3/2 and 3d 5/2 states, respectively [38].The peaks corresponding to V/U, V /U , and V /U were attributed to Ce 4+ 3d orbitals, while the peaks corresponding to V'/U' were associated with Ce 3+ 3d [39].The ratio of Ce 3+ /Ce 3+ + Ce 4+ in CeO 2 kept constant (39%) after coupling with Ir (Figure 2e), indicating that the chemical environment of Ce did not undergo significant changes.The O 1s spectra in Figure 2f could be deconvoluted into three peaks: O L (531.0 eV), Vo• (532.0 eV), and O-H (533.6 eV), where O L and Vo• refer to oxygen in the lattice and oxygen vacancies, respectively.The Vo• peak further verified the existence of Vo• in the CeO 2 , in line with the presence of a large amount of Ce 3+ species.Closer examinations revealed a strong electron coupling between CeO 2 and Ir through the heterointerface in the CeO 2 -Ir/CNTs.Figure 2d shows that the peaks corresponding to Ir 4f in the CeO 2 -Ir/CNTs appeared at a relatively higher binding energy compared with Ir/CNTs (synthesized in the absence of Ce).Remarkably, after being coupled with Ir, the O 1s peaks of CeO 2 -Ir/CNTs appeared at a relatively lower binding energy compared with CeO 2 /CNTs (synthesized in the absence of Ir).These observations support the conclusion that the coupling between CeO 2 and Ir NPs results in strong electron coupling, particularly occurring between Ir and the O L of CeO 2 .
The influence of CeO 2 content on the catalytic activity of xCeO 2 -Ir/CNTs was investigated.As illustrated in Figure S5b, the catalyst activity was improved, by increasing x from 0 to 1.0.However, further increasing x to 1.5 led to declined activity.These findings suggest that optimal OER activity could be achieved by maintaining the Ce/Ir molar ratio of xCeO 2 -Ir/CNTs at 1/1.Furthermore, Tafel plots (Figure S5c) and EIS analysis (Figure S5d) revealed that the OER kinetics and charge transfer resistance of xCeO 2 -Ir/CNTs highly depended on x, and both reached the best at x = 1.0.The electrochemical performance of the synthesized CeO 2 /CNTs and the purchased c-IrO 2 were also studied as comparisons.Specifically, CeO 2 -Ir/CNTs demonstrated remarkable catalytic activity in the acidic OER test.As depicted in Figure 3a, CeO 2 -Ir/CNTs demonstrated remarkable catalytic activity with an overpotential of 262.9 mV at 10.0 mA cm −2 under the acidic OER conditions (0.5 M H 2 SO 4 ), surpassing that of Ir/CNTs (285.9 mV), c-IrO 2 (304.1 mV), and CeO 2 /CNTs (with negligible OER current).Furthermore, CeO 2 -Ir/CNTs even outperformed numerous recently reported Ir-based catalysts (Table S3).The superior catalytic activity of CeO 2 -Ir/CNTs was further evidenced by its accelerated OER kinetics.As illustrated in Figure 3b, CeO 2 -Ir/CNTs exhibited a Tafel slope of 53.4 mV dec −1 , lower than that of Ir/CNTs (59.2 mV dec −1 ) and c-IrO 2 (69.5 mV dec −1 ).Additionally, mass activities of CeO 2 -Ir/CNTs, Ir/CNTs, and c-IrO 2 are plotted in Figure 3c.Among them, CeO 2 -Ir/CNTs exhibited a mass activity of 2542.3A g Ir −1 at 1.53 V, 1.8 and 58.8 times higher than that of Ir/CNTs and c-IrO 2 , respectively.EIS was performed to elucidate the origin of the superior catalytic activity of CeO 2 -Ir/CNTs.At a potential of 1.32 V, CeO 2 -Ir/CNTs displayed a significantly reduced charge transfer resistance value of 18.1 Ω compared with c-IrO 2 (46.5 Ω) and other catalysts (Figure 3d).CeO 2 -Ir/CNTs efficiently converted electrical power into chemical power, as evidenced by its remarkable FE of 98.2% (Figure S6).It is important to note that the FE of CeO 2 -Ir/CNTs was less than 100%, which could be attributed to minor carbon corrosion during the OER [40].
FE of 98.2% (Figure S6).It is important to note that the FE of CeO2-Ir/CNTs was less than 100%, which could be attributed to minor carbon corrosion during the OER [40].CV tests were conducted at different scan rates (v) in the non-Faradic region to calculate the Cdl of catalysts, as depicted in Figure S7a-e.To exclude the morphological effects caused by CeO2 and to investigate the intrinsic catalytic activity of the catalysts, LSVs for xCeO2-Ir/CNTs were normalized by their ECSAs that were determined by using a CV method, as depicted in Figure S7f.Notably, CeO2-Ir/CNTs displayed a relatively larger ECSA (46.0 cm 2 ) compared with Ir/CNTs (42.1 cm 2 ), 0.5CeO2-Ir/CNTs (45.4 cm 2 ), and 1.5CeO2-Ir/CNTs (41.5 cm 2 ).Nevertheless, after the ECSA normalization, CeO2-Ir/CNTs still showed a larger current density than others, indicating their superior intrinsic activity (Figure S8).Thus, the CeO2 content had a notable influence on the intrinsic catalytic activity of xCeO2-Ir/CNTs.Considering the XPS analysis presented in Figure 2d,f, it could be inferred that the electron interaction between OL of CeO2 and Ir led to the generation of electron-deficient Ir atoms.DFT calculations in a recently reported paper [41] similarly showed that, when Ir NPs interacted with CeO2 NPs, Ir-O bonds formed at the interfaces, allowing for electron transfer from Ir atoms to the OL of CeO2.The charge density of Ir atoms was adjusted by forming Ir-O bonds between Ir and CeO2 in order to optimize the adsorption/desorption of intermediates and finally improve the catalytic activity of OER.This phenomenon could be recognized as the primary factor responsible for the observed enhancement in the intrinsic catalytic activity of CeO2-Ir/CNTs.The durability of OER catalysts is a crucial factor that must be carefully assessed to determine their practicality.To further investigate the stability of CeO2-Ir/CNTs under the acidic OER conditions, we conducted chronopotentiometry at 10.0 mA cm −2 .Figure 3e illustrates that, even after prolonged testing exceeding more than 60.0 h, CeO2-lr/CNTs exhibited excellent durability in the acidic OER test with no significant degradation of their catalytic activity.In contrast, c-IrO2 and Ir/CNTs were completely deactivated within less than 21.0 h and 31.0 h, respectively.
To confirm the high durability of CeO2-Ir/CNTs in acidic OER tests, the CeO2-Ir/CNTs' sample after the stability test was characterized using TEM, HRTEM, and XPS.Firstly, TEM analysis confirmed that the morphology of CeO2-Ir/CNTs remained intact CV tests were conducted at different scan rates (v) in the non-Faradic region to calculate the C dl of catalysts, as depicted in Figure S7a-e.To exclude the morphological effects caused by CeO 2 and to investigate the intrinsic catalytic activity of the catalysts, LSVs for xCeO 2 -Ir/CNTs were normalized by their ECSAs that were determined by using a CV method, as depicted in Figure S7f.Notably, CeO 2 -Ir/CNTs displayed a relatively larger ECSA (46.0 cm 2 ) compared with Ir/CNTs (42.1 cm 2 ), 0.5CeO 2 -Ir/CNTs (45.4 cm 2 ), and 1.5CeO 2 -Ir/CNTs (41.5 cm 2 ).Nevertheless, after the ECSA normalization, CeO 2 -Ir/CNTs still showed a larger current density than others, indicating their superior intrinsic activity (Figure S8).Thus, the CeO 2 content had a notable influence on the intrinsic catalytic activity of xCeO 2 -Ir/CNTs.Considering the XPS analysis presented in Figure 2d,f, it could be inferred that the electron interaction between O L of CeO 2 and Ir led to the generation of electron-deficient Ir atoms.DFT calculations in a recently reported paper [41] similarly showed that, when Ir NPs interacted with CeO 2 NPs, Ir-O bonds formed at the interfaces, allowing for electron transfer from Ir atoms to the O L of CeO 2 .The charge density of Ir atoms was adjusted by forming Ir-O bonds between Ir and CeO 2 in order to optimize the adsorption/desorption of intermediates and finally improve the catalytic activity of OER.This phenomenon could be recognized as the primary factor responsible for the observed enhancement in the intrinsic catalytic activity of CeO 2 -Ir/CNTs.The durability of OER catalysts is a crucial factor that must be carefully assessed to determine their practicality.To further investigate the stability of CeO 2 -Ir/CNTs under the acidic OER conditions, we conducted chronopotentiometry at 10.0 mA cm −2 .Figure 3e illustrates that, even after prolonged testing exceeding more than 60.0 h, CeO 2 -lr/CNTs exhibited excellent durability in the acidic OER test with no significant degradation of their catalytic activity.In contrast, c-IrO 2 and Ir/CNTs were completely deactivated within less than 21.0 h and 31.0 h, respectively.
To confirm the high durability of CeO 2 -Ir/CNTs in acidic OER tests, the CeO 2 -Ir/CNTs' sample after the stability test was characterized using TEM, HRTEM, and XPS.Firstly, TEM analysis confirmed that the morphology of CeO 2 -Ir/CNTs remained intact with exceptional structural robustness (Figure 4a).The distribution of CeO 2 and Ir NPs on CNTs could remain uniform (Figure 4b).The paired nanoparticles in Figure 4b retained good crystallinity with lattice fringe spacings of 0.221 nm and 0.192 nm, corresponding to Ir (111) and CeO 2 (220), respectively (Figure 4c,d).Additionally, XPS characterization was performed on CeO 2 -Ir/CNTs to investigate if there was compositional decay during the stability test.Figure 4e-g show that the CeO 2 -Ir/CNTs exhibited the XPS spectra of Ir 4f, Ce 3d, and O1s, with the profiles comparable to those before the OER; however, the weaker Ce 3d profile (Figure 4f) may have been attributed to the dissolution of CeO 2 .The component analysis revealed a slight decrease in the Ir content of CeO 2 -Ir/CNTs after stability testing, while the loss of CeO 2 was more pronounced (Table S4).Therefore, it can be concluded that excellent structural integrity along with stable component composition contributes significantly to maintaining superior activity during acidic OER. with exceptional structural robustness (Figure 4a).The distribution of CeO2 and Ir NPs on CNTs could remain uniform (Figure 4b).The paired nanoparticles in Figure 4b retained good crystallinity with lattice fringe spacings of 0.221 nm and 0.192 nm, corresponding to Ir (111) and CeO2 (220), respectively (Figure 4c,d).Additionally, XPS characterization was performed on CeO2-Ir/CNTs to investigate if there was compositional decay during the stability test.Figure 4e-g show that the CeO2-Ir/CNTs exhibited the XPS spectra of Ir 4f, Ce 3d, and O1s, with the profiles comparable to those before the OER; however, the weaker Ce 3d profile (Figure 4f) may have been attributed to the dissolution of CeO2.The component analysis revealed a slight decrease in the Ir content of CeO2-Ir/CNTs after stability testing, while the loss of CeO2 was more pronounced (Table S4).Therefore, it can be concluded that excellent structural integrity along with stable component composition contributes significantly to maintaining superior activity during acidic OER.To further validate the potential application of CeO2-Ir/CNTs in acidic water electrolyzers, it was evaluated as an anode catalyst for over-all water splitting (OWS).Figure 5a demonstrates the assembly and testing of Pt/C||CeO2-Ir/CNTs as the electrode pair in the acidic water electrolyzers.LSVs and chronopotentiometric curves were employed to estimate their OWS performance.Remarkably, a current density of 10.0 mA cm −2 could be achieved with an input voltage of only 1.54 V for Pt/C||CeO2-Ir/CNTs, while the commercial electrode pair of Pt/C||c-IrO2 required a larger input voltage of 1.58 V to achieve the same current density (Figure 5b).The other synthesized samples were assembled with Pt/C to construct electrolyzers and evaluate their performance towards OWS, as depicted in Figure S9.Furthermore, Figure 5c illustrates that Pt/C||CeO2-Ir/CNTs could maintain continuous electrolysis for over 60.0 h without significant loss of activity at 10.0 mA cm −2 ; however, Pt/C||c-IrO2 became completely inactive after less than 20.0 h under similar To further validate the potential application of CeO 2 -Ir/CNTs in acidic water electrolyzers, it was evaluated as an anode catalyst for over-all water splitting (OWS).Figure 5a demonstrates the assembly and testing of Pt/C||CeO 2 -Ir/CNTs as the electrode pair in the acidic water electrolyzers.LSVs and chronopotentiometric curves were employed to estimate their OWS performance.Remarkably, a current density of 10.0 mA cm −2 could be achieved with an input voltage of only 1.54 V for Pt/C||CeO 2 -Ir/CNTs, while the commercial electrode pair of Pt/C||c-IrO 2 required a larger input voltage of 1.58 V to achieve the same current density (Figure 5b).The other synthesized samples were assembled with Pt/C to construct electrolyzers and evaluate their performance towards OWS, as depicted in Figure S9.Furthermore, Figure 5c illustrates that Pt/C||CeO 2 -Ir/CNTs could maintain continuous electrolysis for over 60.0 h without significant loss of activity at 10.0 mA cm −2 ; however, Pt/C||c-IrO 2 became completely inactive after less than 20.0 h under similar conditions.These results strongly demonstrate the promising potential of CeO 2 -Ir/CNTs as efficient anode catalysts for acidic water electrolyzers.
conditions.These results strongly demonstrate the promising potential of CeO2-Ir/C as efficient anode catalysts for acidic water electrolyzers.

Conclusions
We successfully demonstrated the synthesis of a series of xCeO2-Ir/CNTs with v ing CeO2 contents using a facile solvothermal method.The catalytic activity of xC Ir/CNTs was significantly influenced by the content of CeO2, with higher catalytic acti observed for acidic OER when the molar ratio of CeO2/Ir was 1:1.Due to the strong e tron coupling in CeO2-Ir heterojunctions, CeO2-Ir/CNTs exhibited outstanding OER c lytic performance in acidic media, significantly exceeding the benchmark c-IrO2.C played a crucial role by providing abundant nucleation sites and excellent conduc pathways for CeO2-Ir, contributing to the observed high catalytic and mass activit CeO2-Ir/CNTs.The acidic water electrolyzers constructed using CeO2-Ir/CNTs in com nation with Pt/C (Pt/C||CeO2-Ir/CNTs) demonstrated significant potential for prac applications and outperformed the benchmark c-IrO2||Pt/C system.This study pres a novel approach for the design of efficient, stable, and low-Ir-usage catalysts tow acidic OER.