Highly Enhanced OER Performance by Er-Doped Fe-MOF Nanoarray at Large Current Densities

Great expectations have been held for the electrochemical splitting of water for producing hydrogen as a significant carbon-neutral technology aimed at solving the global energy crisis and greenhouse gas issues. However, the oxygen evolution reaction (OER) process must be energetically catalyzed over a long period at high output, leading to challenges for efficient and stable processing of electrodes for practical purposes. Here, we first prepared Fe-MOF nanosheet arrays on nickel foam via rare-earth erbium doping (Er0.4 Fe-MOF/NF) and applied them as OER electrocatalysts. The Er0.4 Fe-MOF/NF exhibited wonderful OER performance and could yield a 100 mA cm−2 current density at an overpotential of 248 mV with outstanding long-term electrochemical durability for at least 100 h. At large current densities of 500 and 1000 mA cm−2, overpotentials of only 297 mV and 326 mV were achieved, respectively, revealing its potential in industrial applications. The enhancement was attributed to the synergistic effects of the Fe and Er sites, with Er playing a supporting role in the engineering of the electronic states of the Fe sites to endow them with enhanced OER activity. Such a strategy of engineering the OER activity of Fe-MOF via rare-earth ion doping paves a new avenue to design other MOF catalysts for industrial OER applications.


Introduction
As a form of clean energy, hydrogen energy is of crucial importance in addressing the global energy crisis and greenhouse gas issues caused by increased consumption of fossil fuels sources. According to literature, water electrolysis technology is an effective method of obtaining high purity hydrogen and includes two half reactions: the cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). Compared with HER, the OER suffers from sluggish kinetics and thus requires higher energy input to overcome the reaction energy barrier [1][2][3]. Although RuO 2 and IrO 2 have been verified to be the most active OER catalysts, the high cost and scarcity of such catalysts restrict their large-scale application [4,5]. Thus, it is critical to find non-noble metals that have high activity, low cost, and are more stable at large currents for applications.
A great deal of research has been performed in order to investigate highly efficient non-noble metal-based OER catalysts, including metal oxides [6], chalcogenides [7,8], hydroxides [9], phosphides [10], nitrides [11], and metal organic frameworks (MOFs) [12][13][14]. Among them, MOFs exhibit superior OER activity. However, most reported MOF electrocatalysts have been evaluated in accordance with overpotentials and long-term stability under low current density, mostly less than 100 mA cm −2 , and these assessment criteria are not realistic for industrial applications [15]. To be used for industrial applications, OER catalysts must deliver large-current-density (500 mA cm −2 ) OER at low overpotentials. Accordingly, it is highly attractive to design and develop efficient OER electrocatalysts 2 of 11 based on earth-abundant MOF alternatives at large current densities. Recent works have demonstrated modifying of the electronic structure of active sites by introducing foreign atoms as an effective strategy to enhance electrocatalysts' intrinsic activity. Specifically, Gao et al. systematically designed transition-metal-doped NiFe LDHs (using Co, Mn, and Cr metals). Combined with theory analysis, they found that the enhanced intrinsic OER activity was due to modifying the electronic structure of the intermediate site after Co, Mn, and Cr doping [16]. Ma et al. proved that Cu-doped Ni 3 S 2 /Co 3 S 4 could not only increased the specific surface area but also alter the electron density of Ni/Co sites [17]. For the past few years, rare-earth elements have received more and more attention on account of the 4f orbitals partial filling [18,19]. Importantly, the doping of rare-earth elements can effectively regulate the electronic and geometric structures of electrocatalysts, thus improving their electrical conductivity and internal catalytic activity. For instance, Wan et al. synthesized La-doped In(OH) 3 , improving the visible light absorption capability for the photo-reduction of carbon dioxide [20]. Xu et al. reported that Ni-Fe-Ce-LDH microcapsules exhibited a high OER activity owing to the optimized electronic structure of 3D LDHs through Ce species doping [21]. Ahmad et al. found that doped Eu and Tb ions could enter the lattice of ZnO, and the resulting ZnO had a large specific surface area and high porosity [22]. However, there still are few reports about enhancing the intrinsic OER activity of MOFs by rare-earth element doping, especially under large currents [23,24].
To tackle the above challenge, we rationally designed and synthesized Fe-MOF nanosheet arrays grown on 3D nickel foam (NF) via Er doping (Er 0.4 Fe-MOF/NF) as an advanced OER catalyst (Scheme 1). Comparing with the precursor Fe-MOF/NF, the achieved Er 0.4 Fe-MOF/NF exhibited an ultralow overpotential of 248 mV to drive 100 mA cm −2 current density and showed an outstanding long-term electrochemical durability of more than 100 h without obvious decay. Moreover, the overpotentials of 297 mV and 326 mV were needed to drive 500 and 1000 mA cm −2 , indicating its promising application. This work provides some inspiration for reasonably designing efficient and stable MOFs for industrial applications.  [16]. Ma et al. proved that Cu-doped Ni3S2/Co3S4 could not only increased the specific surface area but also alter the electron density of Ni/Co sites [17]. For the past few years, rare-earth elements have received more and more attention on account of the 4f orbitals partial filling [18,19] [23,24]. To tackle the above challenge, we rationally designed and synthesized Fe-MOF nanosheet arrays grown on 3D nickel foam (NF) via Er doping (Er0.4 Fe-MOF/NF) as an advanced OER catalyst (Scheme 1). Comparing with the precursor Fe-MOF/NF, the achieved Er0.4 Fe-MOF/NF exhibited an ultralow overpotential of 248 mV to drive 100 mA cm −2 current density and showed an outstanding long-term electrochemical durability of more than 100 h without obvious decay. Moreover, the overpotentials of 297 mV and 326 mV were needed to drive 500 and 1000 mA cm −2 , indicating its promising application. This work provides some inspiration for reasonably designing efficient and stable MOFs for industrial applications.

Materials and Methods
All chemical regents used in this work were analytically pure. Nickel foam (NF, area: 2 cm × 3 cm) was bought from Shenzhen Green and Creative Environmental Science

Materials and Methods
All chemical regents used in this work were analytically pure. Nickel foam (NF, area: 2 cm × 3 cm) was bought from Shenzhen Green and Creative Environmental Science and Technology Co., Ltd (Shenzhen, China TPA; Aladdin), hydrochloric acid (HCl; Aladdin), N, N-dimethylformamide (DMF; Beijing Chemical Industry Group Co., Ltd, Beijing China), ethanol, and deionized water were used.

Preparation of Er 0.4 Fe-MOF on NF
Er 0.4 Fe-MOF/NF was prepared via a one-step solvothermal method. First, 1 mmol Fe(NO 3 ) 3 ·9H 2 O (0.4040 g), 0.4 mmol Er(NO 3 ) 3 ·6H 2 O (0.1845 g), and 3 mmol TPA (0.4980 g) were added to 35 mL of DMF while stirring uniformly at room temperature. Subsequently, 2.5 mL of ethanol and 2.5 mL of deionized water were added. This mixture was stirred vigorously for half an hour and rapidly transferred into a 50 mL autoclave. The treated NF (2 × 3 cm) was then placed in the homogeneous solution. The solution was put in one Teflon-lined stainless autoclave and reacted at 125 • C for 12 h. Finally, it was naturally cooled down to room temperature, the material was washed several times with deionized water, and it was dried in a vacuum oven at 60 • C. The mass loading of Er 0.4 Fe-MOF/NF was about 1.18 mg cm −2 .

Preparation of Fe-MOF on NF
The Fe-MOF/NF was synthesized using the above-mentioned process without Er(NO 3 ) 3 ·6H 2 O doping.

Preparation of RuO 2 on NF
RuO 2 /NF was prepared in accordance with the reported work. First, 2.61 g of RuCl 3 ·3H 2 O and 1.0 mL KOH (1.0 M) were added into 100 mL of distilled water and stirred for 45 min at a temperature of 100 • C. The precipitate was collected via centrifugation, washed several times with distilled water, and then dried in a vacuum oven at 60 • C. Finally, the product was annealed at 300 • C for 3 h in an air atmosphere to obtain RuO 2 powder. Catalyst ink was prepared by dispersing 22.5 mg of catalyst into 490 µL of water/ethanol (volume ratio, v/v = 1:1) and 20 µL of 5 wt% Nafion using sonication treatment for 30 min. Subsequently, 13.0 µL of the RuO 2 ink (containing 0.295 mg of RuO 2 ) was loaded onto a bare NF of 0.25 cm −2 in geometric area (mass loading: 1.18 mg cm −2 ).

Electrochemical Measurements
All electrochemical tests were measured through using a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd, Shanghai, China) in a three-electrode system with a 1.0 M KOH electrolyte at room temperature. The working electrode was Er 0.4 Fe-MOF/NF, the counter electrode was a graphite plate, and Hg/HgO was used as the reference electrode. All the potentials were reported as one form of the reversible hydrogen electrode (RHE), calculated using the Nernst equation: E (RHE) = E (Hg/HgO) + 0.059 × pH) V.

Characterizations
XRD data were acquired from a LabX XRD-6100 X-ray diffractometer with Cu Kα radiation (40 kV, 30 mA) and a 0.154 nm wavelength (Shimadzu, Osaka, Japan). SEM images were obtained using a XL30 ESEM FEG at a 20 kV accelerating voltage. The TEM and EDX data were collected using a FEI Tecnai G2 F20 (FEI Company, Hillsboro, OR, USA) and OXFORD X-max 80T (FEI Company, Hillsboro, OR, USA). A Thermo Scientific K-Alpha X-ray photoelectron spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) using Al was used to acquire XPS spectra. Raman measurements were conducted on a Renishaw Invia spectrometer (Renishaw Company, Gloucestershire, England). Fouriertransform infrared (FTIR) spectroscopy was carried out on an FTIR spectrometer (Theromo Nicolet Corporation, Madison, WI, USA) using the potassium bromide pellet method at an ambient temperature.

The Structure and Composition of Er 0.4 Fe-MOF/NF
The structure of Er 0.4 Fe-MOF and Fe-MOF were characterized via X-ray diffraction (XRD), as shown in Figure 1a and Figure S1. The diffraction peaks at 8.9 • , 11.4 • , 15.6 • , 18.7 • and 20.8 • (marked as " ") were indexed to the typical MIL-53 structure [25]. Meanwhile, we prepared a series of catalysts with different Er/Fe molar ratios by adjusting the Er doping molar amount. As evidenced by Figure S1, the XRD patterns of these sample were similar, which excluded the formation of new crystal phases after Er doping. But the crystallinity of these catalysts increased with an increase in Er ions. Subsequently, the Fourier-transform infrared spectrum of Er 0.4 Fe-MOF exhibited similar functional groups as those of Fe-MOF in the range of 4000-400 cm −1 (Figure 1b). The absorption bands at 750 cm −1 and 3432 cm −1 indicated the C-H bending vibration of the benzene ring and the O-H stretching vibration. The two peaks at 1662 and 1386 cm −1 contributed to the symmetric vibration of carboxyl groups. The peak at 541 cm −1 was observed to be metal-oxygen bond (Fe-O) vibrations, further demonstrating the coordination structure of MOFs [26].

The Structure and Composition of Er0.4 Fe-MOF/NF
The structure of Er0.4 Fe-MOF and Fe-MOF were characterized via X-ray diffraction (XRD), as shown in Figure 1a and Figure S1. The diffraction peaks at 8.9°, 11.4°, 15.6°, 18.7° and 20.8° (marked as "⧫") were indexed to the typical MIL-53 structure [25]. Meanwhile, we prepared a series of catalysts with different Er/Fe molar ratios by adjusting the Er doping molar amount. As evidenced by Figure S1, the XRD patterns of these sample were similar, which excluded the formation of new crystal phases after Er doping. But the crystallinity of these catalysts increased with an increase in Er ions. Subsequently, the Fourier-transform infrared spectrum of Er0.4 Fe-MOF exhibited similar functional groups as those of Fe-MOF in the range of 4000-400 cm −1 (Figure 1b). The absorption bands at 750 cm −1 and 3432 cm −1 indicated the C-H bending vibration of the benzene ring and the O-H stretching vibration. The two peaks at 1662 and 1386 cm −1 contributed to the symmetric vibration of carboxyl groups. The peak at 541 cm −1 was observed to be metal-oxygen bond (Fe-O) vibrations, further demonstrating the coordination structure of MOFs [26].  Raman spectra, providing information about chemical bonding, displayed two peaks at 1613 and 1442 cm −1 , attributed to the stretching modes of the carboxylate group ( Figure S2) [27,28]. Moreover, the out-of-plane deformation modes of the C-H groups Nanomaterials 2021, 11, 1847 5 of 11 appeared at 866, 634 cm −1 [28]. In addition, the as-prepared Er 0.4 Fe-MOF was uniformly grown on the surface of the 3D nickel foam in the form of a nanoarray structure, which was much thinner than the Fe-MOF (Figure 1c-e). The above structure could expose more specific surface area be beneficial to the mass transport for expediting the OER reaction. As shown in Figure 1f,g, ultrathin nanosheets loosely stacked could be easily observed using a transmission electron microscope (TEM). The high-resolution TEM image (HRTEM) in Figure 1h shows that the lattice distance with an interplanar spacing of 0.186 nm denoted the (200) planes of MIL-53, corresponding to selected-area electron diffraction (SAED) pattern [29,30]. The energy-dispersive X-ray spectroscopy (EDX) and mappings (Figure 1i) certified the existence of Er after Er doping and distribution over the entire catalyst ( Figures S3 and S4). These test results demonstrated that Er ions were doped into the Fe-MOF host without the formation of an additional phase.
The  Figure 2c shows the XPS spectra of Fe 2p with the peak position at around 710.88 eV and 724.13 eV, suggesting the valence state of Fe exists as Fe 3+ in Er 0.4 Fe-MOF/NF and Fe-MOF/NF [32]. In contrast, the binding energy of Fe 2p in Er 0.4 Fe-MOF/NF showed a negative shift at about 0.5-0.95 eV, indicating the change redistribution occurs after Er doping [33]. The reason for this may be the different electronegativities of Fe (1.83) and Er (1.24), causing additional electron density donation from Er to Fe [34]. Er doping could change the electronic environment of Er 0.4 Fe-MOF/NF, resulting in different adsorption energies of intermediates, and thus accelerating the whole reaction process [35,36]. For the Er 4d spectrum (Figure 2d), the peak at 169.52 eV confirms the Er element, further indicating the successful incorporation of Er into Fe-MOF/NF [37]. Compared with the single Er element (167.3 eV), the Er element in Er 0.4 Fe-MOF/NF shifted to a positive binding energy, further demonstrating that the electrons may transfer from Er to Fe. Thus, deducing from all the above facts, the combination of Er and Fe could introduce surprising synergistic effects that may affect certain mechanisms through the modification of the electronic structures of host materials, leading to the promotion of the water splitting reaction.

Electrochemical Properties of Er 0.4 Fe-MOF/NF Electrode Active Materials
The electrocatalytic performance of the electrodes toward OER were tested in 1.0 M KOH solution through a typical three-electrode system at a scan rate of 2 mV s −1 . All the overpotentials were given with iR-compensation unless specified [38]. Figure 3a showed the linear scan voltammetry (LSV) curves of bare NF, commercial RuO 2 /NF, precursor Fe-MOF/NF, and Er 0.4 Fe-MOF/NF electrodes for comparison. A pronounced oxidation peak existed at around 1.41 V vs. RHE, which was attributed to the oxidation of Ni 2+ to Ni 3+ of the substrate NF [39]. The current density then increased monotonically with the increase of applied potential after the above oxidation peak. At 100 mA cm −2 , the Er 0.4 Fe-MOF/NF exhibited the lowest overpotential (248 mV), compared with bare NF (418 mV), precursor Fe-MOF/NF (336 mV) and commercial RuO 2 /NF (288 mV). It was notable that Er 0.4 Fe-MOF/NF exhibited excellent electrocatalytic activity, surpassing most of the previous OER catalysts [40][41][42][43][44][45][46][47][48][49], as shown in Figure 3b and Table S3. Er 0.4 Fe-MOF/NF needed an overpotential of 210 mV at low current density (10 mA cm −2 ) by the back sweep test ( Figure S5). To get additional insights into the influence of the Er content on the OER activity, a series of Er-doped Fe-MOF/NF were prepared with varying ratios of Er and Fe. For a more intuitive representation, histograms of samples with different doping concentrations were drawn according to the overpotential of LSV polarization curves, as shown in Figure S6. Interestingly, with the increasing Er doping amount, the Er-doped Fe-MOF/NF needed lower overpotentials at current densities of 100 mA cm −2 . But the activity decreased as the Er content further grew to 0.4 mmol or larger. The OER activity was positively linked to the Er doping content, highlighting the fact that the proper content of Er doping into MOFs could optimize OER activity. As displayed in Figure 3c, the Tafel slope of Er 0.4 Fe-MOF/NF was determined to be 73 mV dec −1 , which was much lower than those of Fe-MOF/NF (132 mV dec −1 ), commercial RuO 2 /NF (131 mV dec −1 ) and bare NF (256 mV dec −1 ), indicating the fast OER kinetics of Er 0.4 Fe-MOF/NF. Excitingly, the low overpotential and Tafel slope of Er 0.4 Fe-MOF/NF corroborated that the synergistic effects of Er and Fe ions provided a crucial factor the excellent alkaline OER catalytic activity. Furthermore, the electrochemical impedance spectroscopies (EISs) shown in Figure 3d revealed that Er 0.4 Fe-MOF/NF had a smaller semicircle radius than those of Fe-MOF/NF, RuO 2 /NF, and NF, implying prominently improved interfacial electron-transfer kinetics over Er 0.4 Fe-MOF/NF. The improved conductivity of Er 0.4 Fe-MOF/NF revealed that the electronic structure of the sample had changed after Er doping, which agreed with the XPS results. Meanwhile, the EISs of Er-doped Fe-MOF/NF with different Er/Fe atomic ratios are displayed in Figure S7. Importantly, Er 0.4 Fe-MOF/NF exhibited excellent OER catalytic performance at high current densities (including 500 mA cm −2 and 1000 mA cm −2 ). As shown in Figure 3e, Er 0.4 Fe-MOF/NF achieved overpotentials of 297 and 326 mV to drive 500 and 1000 mA cm −2 current densities, respectively, lower than that of Fe-MOF/NF (η 500 = 448 mV; η 1000 = 539 mV), to meet the strict criteria of industrial applications. As revealed in Figure 3f and Table S4, Er 0.4 Fe-MOF/NF was comparable to the other reported state-of-the-art catalysts (Figure 3f and Table S4) [43,[49][50][51][52].

Electrochemical Properties of Er0.4 Fe-MOF/NF Electrode Active Materials
The electrocatalytic performance of the electrodes toward OER were tested in 1.0 M KOH solution through a typical three-electrode system at a scan rate of 2 mV s −1 . All the overpotentials were given with iR-compensation unless specified [38]. Figure 3a showed Er/Fe atomic ratios are displayed in Figure S7. Importantly, Er0.4 Fe-MOF/NF exhibited excellent OER catalytic performance at high current densities (including 500 mA cm −2 and 1000 mA cm −2 ). As shown in Figure 3e, Er0.4 Fe-MOF/NF achieved overpotentials of 297 and 326 mV to drive 500 and 1000 mA cm −2 current densities, respectively, lower than that of Fe-MOF/NF (η500 = 448 mV; η1000 = 539 mV), to meet the strict criteria of industrial applications. As revealed in Figure 3f and Table S4, Er0.4 Fe-MOF/NF was comparable to the other reported state-of-the-art catalysts (Figure 3f and Table S4) [43,[49][50][51][52].   [53]. The OER stability of MOFs at large current densities is very important for industrial applications. Obviously, the OER performance of Er0.4 Fe-MOF/NF could be well maintained for at least 100 h at the large current density of 100 mA cm −2 without obvious degradation. The SEM image (the insert in Figure 4b) and XPS spectra ( Figure S8) of Er0.4 Fe-MOF/NF display its original morphology and composition after long-term OER testing, suggesting excellent durability of the Er0.4 Fe-MOF/NF catalyst. However, the peaks of O   [26]. Furthermore, for commercial applications, the stability of electrocatalysis under large current densities was essential for large-scale production because the intrinsic instability of MOFs and the number of gas bubbles generated. As a result, it was evident that just 9% decay in current densities could be observed for the 500 mA cm −2 case after 25 h, while the Fe-MOF/NF catalyst dropped quickly under the same test conditions, as shown in Figure 4c. Subsequently, as shown in Figure S9, the obtained LSV curves after the 1st and the 1000th cyclic CV sweeps exhibited seldom decay. These test results further confirmed the positive effect of Er doping in enhancing both the OER activity and stability of Fe-MOF/NF. production because the intrinsic instability of MOFs and the number of gas bubbles generated. As a result, it was evident that just 9% decay in current densities could be observed for the 500 mA cm −2 case after 25 h, while the Fe-MOF/NF catalyst dropped quickly under the same test conditions, as shown in Figure 4c. Subsequently, as shown in Figure S9, the obtained LSV curves after the 1st and the 1000th cyclic CV sweeps exhibited seldom decay. These test results further confirmed the positive effect of Er doping in enhancing both the OER activity and stability of Fe-MOF/NF. The mechanism of OER could be assigned to the existence of the two pairs of redox peaks in Figure S10. In the forward half cycle, the oxidation peak can be attributed to the oxidation of Fe 3+ to Fe 4+ at about 1.54 V vs. RHE. Moreover, in the reverse half-cycle, the peak located at 1.15 V vs. RHE can be attributed to the reduction of Fe 4+ to Fe 3+ [54]. In contrast, the oxidation peak and reduction peak of bare NF were almost negligible in the same voltage range, indicating the promotion of OERs came from Er0.4 Fe-MOF instead of the substrate NF. Obviously, the intensity redox peaks of Er0.4 Fe-MOF/NF were higher than those of the precursor Fe-MOF/NF due to the synergistic effect between Er and Fe sites. Based on the above test results, the OER performance of Er0.4 Fe-MOF/NF could be assigned to the following steps: [54] A + OH − ↔ AOH + e − (1) The mechanism of OER could be assigned to the existence of the two pairs of redox peaks in Figure S10. In the forward half cycle, the oxidation peak can be attributed to the oxidation of Fe 3+ to Fe 4+ at about 1.54 V vs. RHE. Moreover, in the reverse half-cycle, the peak located at 1.15 V vs. RHE can be attributed to the reduction of Fe 4+ to Fe 3+ [54]. In contrast, the oxidation peak and reduction peak of bare NF were almost negligible in the same voltage range, indicating the promotion of OERs came from Er 0.4 Fe-MOF instead of the substrate NF. Obviously, the intensity redox peaks of Er 0.4 Fe-MOF/NF were higher than those of the precursor Fe-MOF/NF due to the synergistic effect between Er and Fe sites. Based on the above test results, the OER performance of Er 0.4 Fe-MOF/NF could be assigned to the following steps: [54] A + OH − ↔ AOH + e − (1) where A denotes active sites of Er 0.4 Fe-MOF/NF. Thus, Fe sites served as the active sites in this electrode. The double-layer capacitances (C dl ) obtained from CV curves ( Figure S11) were calculated to evaluate the electrochemically active surface area [55]. In Figure 4d, the C dl value of Er 0.4 Fe-MOF/NF was calculated to be 18.78 mF cm −2 , significantly higher than that of the precursor Fe-MOF/NF (9.36 mF cm −2 ). The reason was attributed to the high exposure of active sites provided by the ultrathin nanoarray structure. Similarly, the intrinsic catalytic performance of this electrode was also evaluated by the turnover frequency Nanomaterials 2021, 11, 1847 9 of 11 (TOF). The calculated TOF values were tested through CV curves ( Figure S12) based on a previous report. [56] As shown in Figure 4e, Er 0.4 Fe-MOF/NF achieved a TOF value of 0.3 O 2 s −1 while that of Fe-MOF/NF was only 0.04 O 2 s −1 at the same overpotential of 300 mV. The faradic efficiency (FE) for OER was calculated by comparing the amount of estimated oxygen gas volume using both the experimental and theoretical methods under 100 mA cm −2 and 500 mA cm −2 current densities of Er 0.4 Fe-MOF/NF (assuming 100% FE), as shown in Figure 4f. Virtually 100% FE was obtained, indicating most of the surface electrons participated the OER process and the charge transfer process exhibited a minimum blocking resistance [57].

Conclusions
In summary, with optimized Er doping, ultrahigh activity and stability for OER electrocatalysts has been achieved using a one-step solvothermal method. Compared with the precursor Fe-MOF/NF, the Er 0.4 Fe-MOF/NF electrode showed OER performance can yield a 100 mA cm −2 current density at an overpotential of 248 mV with long-term electrochemical durability for at least 100 h. Moreover, it also exhibited ultralow overpotentials of 297 and 326 mV to reach the large current densities of 500 and 1000 mA cm −2 . Overall, the addition of Er dopants could have a significant impact on OER catalytic performance due to the optimized electronic structure of the active center.