Ce–Metal–Organic Framework-Derived CeO 2 –GO: An Efﬁcient Electrocatalyst for Oxygen Evolution Reaction

: The oxygen evolution reaction (OER) is a crucial half-reaction in water splitting. However, this reaction is kinetically sluggish owing to the four-electron (4 e − ) transfer process. Therefore, the development of low-cost, stable, highly efﬁcient, and earth-abundant electrocatalysts for the OER is highly desirable. Metal oxides derived from metal–organic frameworks (MOFs) are among the most efﬁcient electrocatalysts for the OER. Herein, Ce–MOF-derived CeO 2 /graphene oxide (GO) composites were successfully prepared using a facile method. The composites with 0, 25, 50, and 100 mg GO were named CeO 2 , CeO 2 –GO-1, CeO 2 –GO-2, and CeO 2 –GO-3, respectively. The physicochemical characteristics of the electrocatalysts were assessed using several analytical techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), and Brunauer– Emmett–Teller (BET) analysis. The TEM results revealed that the CeO 2 had a sheet-like morphology and that a GO layer was noticeable in the synthesized CeO 2 –GO-3 composite. The characterization results conﬁrmed the formation of impurity-free CeO 2 –GO composites. The OER activity and stability were measured using cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). The CeO 2 –GO-3 electrocatalyst has a smaller Tafel slope (176 mV · dec − 1 ) and lower overpotential (240 mV) than the other electrocatalysts. In addition, it exhibited high cyclic stability for up to 10 h. Therefore, the inexpensive CeO 2 –GO-3 electrocatalyst is a promising OER candidate.


Introduction
Electrochemical water splitting is a promising method for producing pollution-free, clean hydrogen [1]. Water electrolysis consists of two half-reactions (OER and HER): the oxygen evolution reaction at the anode and the hydrogen (H 2 ) evolution reaction at the cathode [2]. However, the OER is kinetically slow owing to its four-electron transfer process [3]. Therefore, highly effective and reliable OER electrocatalysts must be developed to address this limitation. Noble metal oxide catalysts, such as RuO 2 and IrO 2 , are among the most efficient electrocatalysts for OER activity; however, their high prices and scarcity limit their large-scale application [4]. Therefore, the development of OER electrocatalysts with high activity, long-term stability, and low cost is essential.
Metal-organic frameworks (MOFs) are a new class of porous materials composed of metal ions or clusters connected to organic ligands via coordination bonds to form one-dimensional, two-dimensional, and three-dimensional (1D, 2D, and 3D) structures [5]. Recently, MOFs have emerged as promising candidates for energy-and environmentrelated applications [6,7] owing to their unique characteristics, such as high porosity, high Inorganics 2023, 11, x FOR PEER REVIEW 3 of 13 Scheme 1. Schematic of the synthesis of the CeO2-GO-3 electrocatalyst. Figure 1a displays the X-ray diffraction (XRD) patterns of the pristine CeO2 and CeO2-GO electrocatalysts. The XRD data of Ce-MOF, as shown in Figure S1, exhibit sharp and narrow peaks. All diffraction peaks matched the XRD patterns of previous reports [46,47]. After calcination, the Ce-MOF was fully converted into cubic-structured CeO2. The four characteristic peaks at 28.4°, 33.1°, 47.3°, and 56.3° can be attributed to the (111), (200), (220), and (311) planes (Figure 1a) [48], which are in good agreement with JCPDS 00-004-0593. The average crystallite size was found using the Debye-Scherrer equation [49]

Characterization of the Electrocatalysts
where D is the average crystallite size (nm), k is constant (0.9), is the wavelength of Xray radiation (1.5416 Å), θ is diffraction angle, and β is the full-width half maxima of diffraction peaks. From XRD data, the average size was found to be in the range of 10 to 15 nm. The calculated lattice parameters and volume were found to be (for pure cubic structure a = b =c; α = β = γ = 90°) a = 1. 5411 Å and V = 161. 05 Å 3 . Malik et al. [43] reported similar results for CeO2 derived from the GO-Ce-MOF. A GO peak in the CeO2-GO electrocatalysts was noted at 10.8° (001), indicating the presence of GO in all synthesized samples [50]. Compared with CeO2-GO-1 and CeO2-GO-2, the CeO2-GO-3 electrocatalyst showed the peak (10.8°) intensity, increasing with an increase in graphene oxide.  Figure 1a displays the X-ray diffraction (XRD) patterns of the pristine CeO 2 and CeO 2 -GO electrocatalysts. The XRD data of Ce-MOF, as shown in Figure S1, exhibit sharp and narrow peaks. All diffraction peaks matched the XRD patterns of previous reports [46,47]. After calcination, the Ce-MOF was fully converted into cubic-structured CeO 2 . The four characteristic peaks at 28.4 • , 33.1 • , 47.3 • , and 56.3 • can be attributed to the (111), (200), (220), and (311) planes ( Figure 1a) [48], which are in good agreement with JCPDS 00-004-0593. The average crystallite size was found using the Debye-Scherrer equation [49],

Characterization of the Electrocatalysts
where D is the average crystallite size (nm), k is constant (0.9), λ is the wavelength of X-ray radiation (1.5416 Å), θ is diffraction angle, and β is the full-width half maxima of diffraction peaks. From XRD data, the average size was found to be in the range of 10 to 15 nm. The calculated lattice parameters and volume were found to be (for pure cubic structure a = b = c; α = β = γ = 90 • ) a = 1.5411 Å and V = 161.05 Å 3 . Malik et al. [43] reported similar results for CeO 2 derived from the GO-Ce-MOF. A GO peak in the CeO 2 -GO electrocatalysts was noted at 10.8 • (001), indicating the presence of GO in all synthesized samples [50]. Compared with CeO 2 -GO-1 and CeO 2 -GO-2, the CeO 2 -GO-3 electrocatalyst showed the peak (10.8 • ) intensity, increasing with an increase in graphene oxide.  Figure 1a displays the X-ray diffraction (XRD) patterns of the pristine CeO2 and CeO2-GO electrocatalysts. The XRD data of Ce-MOF, as shown in Figure S1, exhibit sharp and narrow peaks. All diffraction peaks matched the XRD patterns of previous reports [46,47]. After calcination, the Ce-MOF was fully converted into cubic-structured CeO2. The four characteristic peaks at 28.4°, 33.1°, 47.3°, and 56.3° can be attributed to the (111), (200), (220), and (311) planes ( Figure 1a) [48], which are in good agreement with JCPDS 00-004-0593. The average crystallite size was found using the Debye-Scherrer equation

Characterization of the Electrocatalysts
where D is the average crystallite size (nm), k is constant (0.9), is the wavelength of Xray radiation (1.5416 Å), θ is diffraction angle, and β is the full-width half maxima of diffraction peaks. From XRD data, the average size was found to be in the range of 10 to 15 nm. The calculated lattice parameters and volume were found to be (for pure cubic structure a = b =c; α = β = γ = 90°) a = 1. 5411 Å and V = 161. 05 Å 3 . Malik et al. [43] reported similar results for CeO2 derived from the GO-Ce-MOF. A GO peak in the CeO2-GO electrocatalysts was noted at 10.8° (001), indicating the presence of GO in all synthesized samples [50]. Compared with CeO2-GO-1 and CeO2-GO-2, the CeO2-GO-3 electrocatalyst showed the peak (10.8°) intensity, increasing with an increase in graphene oxide.  The Fourier-transform infrared spectra (FTIR) of the pure CeO 2 and CeO 2 -GO electrocatalysts are presented in Figure 1b. The FTIR spectrum of GO exhibits a prominent peak at 3409 cm −1 , which is attributed to the O-H stretching; further absorption peaks at 1717, 1606, 1258, and 1051 cm −1 correspond to the C=O stretching vibrations, C=C, C-O-C bending, and C-O stretching groups [51][52][53], respectively. A Ce-O stretching vibration was observed in the CeO 2 -GO composites at 535 cm −1 [43,54]. The bands at 1621 and 1059 cm −1 are related to the bending vibration of the hydroxyl (-OH) groups of water molecules [55] and Ce-O-Ce stretching vibration [56], respectively. Most of the GO peaks were significantly reduced after the CeO 2 -GO composite formation.
The surface chemical states of the Ce-MOF-derived CeO 2 and CeO 2 -GO-3 electrocatalysts were studied using X-ray photoelectron spectroscopy (XPS; Figure 2a). A survey scan revealed the presence of Ce 3d, O 1s, and C 1s in the electrocatalysts. The Ce 3d spectra of the CeO 2 and CeO 2 -GO-3 electrocatalysts were fitted to eight peaks: the Ce 3d 3/2 peaks at 900.1/900.3, 906.7/906.8, and 916.0/916.1 eV; Ce 3d 5/2 peaks at 881.6/881.8, 887.9/888.0, and 897.5/897.6 eV, which are associated with Ce 4+ ; and peaks at 884.6/884.8 eV (Ce 3d 5/2 ) and 903.1/903.3 eV (Ce 3d 3/2 ) associated with Ce 3+ (Figure 2c). These values are consistent with those reported in the literature [57][58][59]. The deconvolution of the O 1s spectra displayed two peaks at 528.6/528.8 and 531.0/531.1 eV (Figure 2b), which are attributed to the lattice and adsorbed oxygen [60,61]. In the composite, the C 1s spectrum ( Figure 2d The Fourier-transform infrared spectra (FTIR) of the pure CeO2 and CeO2-GO electrocatalysts are presented in Figure 1b. The FTIR spectrum of GO exhibits a prominent peak at 3409 cm −1 , which is attributed to the O-H stretching; further absorption peaks at 1717, 1606, 1258, and 1051 cm −1 correspond to the C=O stretching vibrations, C=C, C-O-C bending, and C-O stretching groups [51][52][53], respectively. A Ce-O stretching vibration was observed in the CeO2-GO composites at 535 cm −1 [43,54]. The bands at 1621 and 1059 cm −1 are related to the bending vibration of the hydroxyl (-OH) groups of water molecules [55] and Ce-O-Ce stretching vibration [56], respectively. Most of the GO peaks were significantly reduced after the CeO2-GO composite formation.
The surface chemical states of the Ce-MOF-derived CeO2 and CeO2-GO-3 electrocatalysts were studied using X-ray photoelectron spectroscopy (XPS; Figure 2a). A survey scan revealed the presence of Ce 3d, O 1s, and C 1s in the electrocatalysts. The Ce 3d spectra of the CeO2 and CeO2-GO-3 electrocatalysts were fitted to eight peaks: the Ce 3d3/2 peaks at 900.1/900.3, 906.7/906.8, and 916.0/916.1 eV; Ce 3d5/2 peaks at 881.6/881.8, 887.9/888.0, and 897.5/897.6 eV, which are associated with Ce 4+ ; and peaks at 884.6/884.8 eV (Ce 3d5/2) and 903.1/903.3 eV (Ce 3d3/2) associated with Ce 3+ (Figure 2c). These values are consistent with those reported in the literature [57][58][59]. The deconvolution of the O 1s spectra displayed two peaks at 528.6/528.8 and 531.0/531.1 eV (Figure 2b), which are attributed to the lattice and adsorbed oxygen [60,61]. In the composite, the C 1s spectrum  The morphologies of the Ce-MOF-derived CeO2 and CeO2-GO electrocatalysts were characterized via scanning electron microscopy (SEM; Figure 3). The CeO2 sample had a rod-like morphology with the appearance of some merging rods having sheet-like structures (Figure 3a-1,a-2). Kohantorabi and Gholami [65] and Ye et al. [66] reported similar morphologies using benzene-1,3,5-tricarboxylic acid, and cerium nitrate hexahydrate, respectively. CeO2 rods were visible in CeO2-GO-1 and a few GO sheets were observed. The rods The morphologies of the Ce-MOF-derived CeO 2 and CeO 2 -GO electrocatalysts were characterized via scanning electron microscopy (SEM; Figure 3). The CeO 2 sample had a rod-like morphology with the appearance of some merging rods having sheet-like structures (Figure 3a-1,a-2). Kohantorabi and Gholami [65] and Ye et al. [66] reported similar morphologies using benzene-1,3,5-tricarboxylic acid, and cerium nitrate hexahydrate, respectively. CeO 2 rods were visible in CeO 2 -GO-1 and a few GO sheets were observed. The rods were uniformly spread on the GO sheets (Figure 3b-1,b-2). As the GO weight was increased, layered GO sheets with smooth surfaces were observed in the CeO 2 -GO-2 and CeO 2 -GO-3 samples (Figure 3c-1-d-2). These results confirmed the successful decoration of CeO 2 on the GO sheets.
were uniformly spread on the GO sheets (Figure 3b-1 and b-2). As the GO weight was increased, layered GO sheets with smooth surfaces were observed in the CeO2-GO-2 and CeO2-GO-3 samples (Figure 3c-1-d-2). These results confirmed the successful decoration of CeO2 on the GO sheets. Energy-dispersive X-ray spectroscopy (EDS) was performed to identify the elements in the electrocatalyst, and the corresponding results are shown in Figure S2. The EDS spectrum validated the presence of O, Ce, and C in the electrocatalyst.  Energy-dispersive X-ray spectroscopy (EDS) was performed to identify the elements in the electrocatalyst, and the corresponding results are shown in Figure S2. The EDS spectrum validated the presence of O, Ce, and C in the electrocatalyst. Figure 4a

OER Activity
We further explored the OER activity of the synthesized electrocatalysts under alkaline conditions. Figure 5 shows the cyclic voltammetry (CV) curves of the CeO2, CeO2-GO-1, CeO2-GO-2, and CeO2-GO-3 electrocatalysts under the three-electrode setup at

OER Activity
We further explored the OER activity of the synthesized electrocatalysts under alkaline conditions. Figure 5 shows the cyclic voltammetry (CV) curves of the CeO 2 , CeO 2 -GO-1, CeO 2 -GO-2, and CeO 2 -GO-3 electrocatalysts under the three-electrode setup at room temperature. The CV curves were measured at a constant potential between 0.1~0.6 V against Hg/HgO at various scan rates (5-100 mVs −1 ). These CV curves revealed that the current density increased with increasing GO content in the electrocatalyst owing to the increase in ion transport between the electrocatalyst (CeO 2 ) and GO (CeO 2 -GO-3). Similarly, cerium oxide/reduced GO nanocomposites exhibited excellent photocatalytic and supercapacitor activities owing to the increased charge transport between the electrocatalysts [67,68]. Additionally, CeO 2 /multi-walled carbon nanotube nanocomposites have higher capacitive performance and long-term stability [69], and the electrocatalytic performance increased after the introduction of carbon-based materials. Comparative CV curves of the electrocatalysts at a standard scan rate (60 mV s −1 ) are shown in Figure S6. All electrocatalysts exhibited cathodic peaks at approximately 0.472, 0.502, 0.489, and 0.514 V and anodic peaks at approximately 0.304, 0.303, 0.301, and 0.296 V for CeO 2 , CeO 2 -GO-1, CeO 2 -GO-2, and CeO 2 -GO-3, respectively. These CV curves reveal that CeO 2 -GO-3 exhibits a larger integral area, suggesting its higher electrochemical performance. The CV curves retained faradaic peaks even at higher scan rates, indicating a fast charge transport in the electrode system. The electrochemical OER activity was measured in the same electrolyte using line sweep voltammetry (LSV), as revealed in Figure 6a. However, the GO content, which creased the OER performance, also increased, and the CeO2-GO-3 electrocatalyst show a higher current density at higher potentials. The overpotential decreased with increasi GO content in the electrocatalysts. The overpotentials of the CeO2, CeO2-GO-1, CeO2-G 2, CeO2-GO-3, and RuO2 electrocatalysts were 420, 360, 300, and 240, and 230 mV, resp tively, at a fixed current density of 10 mA cm −2 . Among the electrocatalysts, CeO2-GO exhibited a lower overpotential; however, compared with standard RuO2, there was n much difference. The electrocatalytic activity trend during the OER process followed t order CeO2-GO-3 > CeO2-GO-2 > CeO2-GO-1 > CeO2. The electrochemical OER activity was measured in the same electrolyte using linear sweep voltammetry (LSV), as revealed in Figure 6a. However, the GO content, which increased the OER performance, also increased, and the CeO 2 -GO-3 electrocatalyst showed a higher current density at higher potentials. The overpotential decreased with increasing GO content in the electrocatalysts. The overpotentials of the CeO 2 , CeO 2 -GO-1, CeO 2 -GO-2, CeO 2 -GO-3, and RuO 2 electrocatalysts were 420, 360, 300, and 240, and 230 mV, respectively, at a fixed current density of 10 mA cm −2 . Among the electrocatalysts, CeO 2 -GO-3 exhibited a lower overpotential; however, compared with standard RuO 2 , there was not much difference. The electrocatalytic activity trend during the OER process followed the order CeO 2 -GO-3 > CeO 2 -GO-2 > CeO 2 -GO-1 > CeO 2 . Inorganics 2023, 11, x FOR PEER REVIEW 8 o The Tafel slope is a crucial parameter for identifying the relationship between overpotential and steady-state current density of electrocatalysts [66]. Figure 6b sho Tafel slopes of 261, 293, 185, 176, and 101 mV dec −1 for CeO2, CeO2-GO-1, CeO2-GO CeO2-GO-3, and RuO2 electrocatalysts, respectively. The CeO2-GO-3 electrocatalyst vealed a smaller Tafel slope than the other electrocatalysts, showing its higher OER ac ity owing to the higher percentage of GO. However, compared with RuO2, the CeO2-G 3 Tafel slope value was higher. The electrochemical active surface area (ECSA) is dire related to the electrochemical performance of the electrocatalysts [69,70]. A higher EC indicates better electrochemical performance. In the present work, the ECSA was m ured using the CV curves in the non-faradaic region; the corresponding CVs are show Figure S7. The CeO2-GO-3 electrocatalyst showed a higher ECSA (57.5 mF cm −2 ) than other electrocatalysts (35.4, 39.4, and 40.5 mF cm −2 for CeO2, CeO2-GO-1, and CeO2-GO respectively), as shown in Figure 6c.
Electrochemical impedance spectroscopy (EIS) was used to analyze the electroch ical dynamics of the electrocatalysts. Figure 6d and Table S1 show the EIS analysis of electrocatalysts. The CeO2-GO-3 electrocatalyst exhibited a smaller Rct than the other e trocatalysts, revealing its lower charge transfer resistance owing to the increased elect transfer rate. The stability of electrocatalysts is a crucial parameter for OER activity. Th fore, chronoamperometry (CA) was used for the room-temperature analysis for appr mately 10 h. Figure 6e depicts the CA curves of the electrocatalysts in a 1.0 M KOH s tion, where the CeO2-GO-3 electrocatalysts show a higher current density than the ot electrocatalysts. Additionally, we compared our as-prepared materials to previously p lished OER electrocatalysts, which are depicted in Table S2. The Tafel slope is a crucial parameter for identifying the relationship between the overpotential and steady-state current density of electrocatalysts [66]. Figure 6b shows Tafel slopes of 261, 293, 185, 176, and 101 mV dec −1 for CeO 2 , CeO 2 -GO-1, CeO 2 -GO-2, CeO 2 -GO-3, and RuO 2 electrocatalysts, respectively. The CeO 2 -GO-3 electrocatalyst revealed a smaller Tafel slope than the other electrocatalysts, showing its higher OER activity owing to the higher percentage of GO. However, compared with RuO 2 , the CeO 2 -GO-3 Tafel slope value was higher. The electrochemical active surface area (ECSA) is directly related to the electrochemical performance of the electrocatalysts [69,70]. A higher ECSA indicates better electrochemical performance. In the present work, the ECSA was measured using the CV curves in the non-faradaic region; the corresponding CVs are shown in Figure S7. The CeO 2 -GO-3 electrocatalyst showed a higher ECSA (57.5 mF cm −2 ) than the other electrocatalysts (35.4, 39.4, and 40.5 mF cm −2 for CeO 2 , CeO 2 -GO-1, and CeO 2 -GO-2, respectively), as shown in Figure 6c.
Electrochemical impedance spectroscopy (EIS) was used to analyze the electrochemical dynamics of the electrocatalysts. Figure 6d and Table S1 show the EIS analysis of the electrocatalysts. The CeO 2 -GO-3 electrocatalyst exhibited a smaller R ct than the other electrocatalysts, revealing its lower charge transfer resistance owing to the increased electron transfer rate. The stability of electrocatalysts is a crucial parameter for OER activity. Therefore, chronoamperometry (CA) was used for the room-temperature analysis for approximately 10 h. Figure 6e depicts the CA curves of the electrocatalysts in a 1.0 M KOH solution, where the CeO 2 -GO-3 electrocatalysts show a higher current density than the other electrocatalysts. Additionally, we compared our as-prepared materials to previously published OER electrocatalysts, which are depicted in Table S2.
The surface area plays a key role in electrochemical studies. In the present work, the surface area was measured via two different methods: the Brunauer-Emmett-Teller (BET) method using nitrogen (N 2 ) gas adsorption and desorption and ECSA using the CV curves in the non-faradaic region of the electrocatalysts. Figure S8 shows the adsorption and desorption curves of the electrocatalysts. According to the IUPAC classification, these curves exhibit Type IV isotherms; all electrocatalysts showed similar type IV isotherms, which can be observed in mesoporous materials with a pore size of 2-50 nm [71]. The CeO 2 -GO-3 electrocatalyst exhibited a higher BET surface area compared with the other electrocatalysts. In particular, the BET surface values for CeO 2 , CeO 2 -GO-1, CeO 2 -GO-2, and CeO 2 -GO-3 were 61.58, 80.77, 101.48, and 110.35 m 2 g −1 , respectively. The pore volume and size of the CeO 2 -GO-3 electrocatalyst were higher than those of the other electrocatalysts. These values are tabulated in the inset of Figure S8. Therefore, the electrocatalyst with a higher surface area exhibits higher electrocatalytic performance. In the present study, the CeO 2 -GO-3 electrocatalyst showed a higher surface area and better electrocatalytic performance compared with the other electrocatalysts.

Materials and Methods
Ce-MOFs were synthesized using a previously reported method with slight modifications [46]. The required quantity of cerium nitrate hexahydrate (50.0 mM) was dissolved in a mixture of deionized water (DI) and ethanol (v/v of 1:1). Subsequently, trimesic acid (50.0 mM) was added to the milk-white suspension, which was stirred for 1 h at room temperature (RT). Thereafter, the temperature was increased until the solution evaporated. After cooling, the residue was collected and annealed at 400 • C in the air for 1 h to obtain the CeO 2 pure phase. The CeO 2 -GO composites were prepared in the same manner with different GO contents (0, 25, 50, and 100 mg GO, denoted as CeO 2 , CeO 2 -GO-1, CeO 2 -GO-2, and CeO 2 -GO-3, respectively), as shown in Scheme 1. The chemicals, characterization, electrode preparation, and electrocatalytic performance followed are provided in the Supplementary Information (SI).

Conclusions
In this study, we successfully synthesized Ce-MOF-derived CeO 2 and CeO 2 -GO electrocatalysts for the OER. The morphological and structural properties of the electrocatalysts were characterized. The TEM results indicated that the CeO 2 -GO-3 electrocatalyst had a sheet-like morphology with an effective attachment to the GO sheets. The electrocatalysts produced using a low-cost, stable, and simple synthesis method demonstrated good OER activity in the 1.0 M KOH electrolyte. The CeO 2 -GO-3 electrocatalyst had a low overpotential of 240 mV at 10 mA·cm −2 and a smaller Tafel slope (176 mV·dec −1 ) than the CeO 2 -GO-1 and CeO 2 -GO-2 electrocatalysts. Moreover, the CeO 2 -GO-3 electrocatalyst exhibited considerable electrochemical stability over 10 h under alkaline conditions. Thus, this study provides a new method for developing non-noble-metal-based electrocatalysts for clean energy production.  Table S1: EIS fitted values and the equivalent circuit; Table S2: Comparison of OER performance of different electrocatalysts [43,[72][73][74][75][76][77][78][79][80][81].

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