NiFeMn-Layered Double Hydroxides Linked by Graphene as High-Performance Electrocatalysts for Oxygen Evolution Reaction

Currently, precious metal group materials are known as the efficient and widely used oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalysts. The exorbitant prices and scarcity of the precious metals have stimulated scale exploration of alternative non-precious metal catalysts with low-cost and high performance. Layered double hydroxides (LDHs) are a promising precursor to prepare cost-effective and high-performance catalysts because they possess abundant micropores and nitrogen self-doping after pyrolysis, which can accelerate the electron transfer and serve as active sites for efficient OER. Herein, we developed a new highly active NiFeMn-layered double hydroxide (NFM LDH) based electrocatalyst for OER. Through building NFM hydroxide/oxyhydroxide heterojunction and incorporation of conductive graphene, the prepared NFM LDH-based electrocatalyst delivers a low overpotential of 338 mV at current density of 10 mA cm−2 with a small Tafel slope of 67 mV dec−1, which are superior to those of commercial RuO2 catalyst for OER. The LDH/OOH heterojunction involves strong interfacial coupling, which modulates the local electronic environment and boosts the kinetics of charge transfer. In addition, the high valence Fe3+ and Mn3+ species formed after NaOH treatment provide more active sites and promote the Ni2+ to higher oxidation states during the O2 evolution. Moreover, graphene contributes a lot to the reduction of charge transfer resistance. The combining effects have greatly enhanced the catalytic ability for OER, demonstrating that the synthesized NFM LDH/OOH heterojunction with graphene linkage can be practically applied as a high-performance electrocatalyst for oxygen production via water splitting.


Introduction
Since the third technological revolution, energy supply has acquired special importance in national economies. With the rapid growth of industrial civilization, the problems of energy shortages and environmental pollution are becoming more and more prominent. Thus, the development of new energy sources becomes the most important thing in social developments [1][2][3]. The oxygen evolution reaction (OER) is critical for the advancement of different electrochemical applications such as energy storage and water splitting [4][5][6]. However, the kinetics of the OER reaction is severely hampered by multiple electron transfer. Thus, it is of great significance to develop high-performance OER catalysts.
Recently, great efforts have been made for developing high-performance OER electrocatalysts, which can be classified as follows: (1) the most common OER catalysts are

Preparation of NFM LDH with Graphene
NiFeMn LDH with graphene connection (denoted as NFM LDH-G) was obtained by the following method. Typically, 30 mg graphene oxide, 0.596 g ferrous chloride, 0.894 g manganese chloride, 1.192 g nickel chloride and 1.788 g of urea were added to 40 mL deionized water. Then, the above solution was mixed and transferred into a hydrothermal kettle (120 • C for 12 h). Finally, the NFM-G LDH was obtained by centrifugation at 5000 rpm and dried at 60 • C for 12 h.

Preparation of NFM LDH/OOH Heterostructures
NFM LDH/OOH heterostructure (denoted as NFM LDH/OOH) was fabricated by the following method. A total of 25 mg NFM LDH was added into 40 mL sodium hydroxide solution (1 M). Then, the above solution was stirred at 700 rpm for 24 h. Finally, the NFM LDH/OOH was obtained by centrifugation at 5000 rpm and dried at 60 • C for 12 h.

Preparation of NFM LDH/OOH-G
NFM LDH/OOH heterostructure with graphene linkage (denoted as NFM LDH/OOH-G) was obtained by the following method. A total of 25 mg NFM LDH-G was added into 40 mL sodium hydroxide solution (1 M). Then, the above solution was stirred at 700 rpm for 24 h. Finally, the NFM LDH/OOH-G was obtained by centrifugation at 5000 rpm and dried at 60 • C for 12 h.
The preparation of NFM LDH and its derivatives is summarized in Scheme 1. Firstly, Fe 2+ , Ni 2+ , Mn 2+ , and urea were added to deionized water. Then, the above solution was heated to 120 • C for 12 h in a hydrothermal kettle.

.Preparation of NiFeMn-Layered Double Hydroxides
NiFeMn-layered double hydroxide (denoted as NFM LDH) was prepared by the following method. Typically, 0.596 g ferrous chloride, 0.894 g manganese chloride, 1.192 g nickel chloride and 1.788 g of urea were added to 40 mL deionized water. Then, the above solution was mixed and transferred into a hydrothermal kettle (120 °C for 12 h). Finally, the NFM LDH was obtained by centrifugation (5000 rpm) and dried at 60 °C for 12 h.

Preparation of NFM LDH with graphene
NiFeMn LDH with graphene connection (denoted as NFM LDH-G) was obtained by the following method. Typically, 30 mg graphene oxide, 0.596 g ferrous chloride, 0.894 g manganese chloride, 1.192 g nickel chloride and 1.788 g of urea were added to 40 mL deionized water. Then, the above solution was mixed and transferred into a hydrothermal kettle (120 °C for 12 h). Finally, the NFM-G LDH was obtained by centrifugation at 5000 rpm and dried at 60 °C for 12 h.

Preparation of NFM LDH/OOH Heterostructures
NFM LDH/OOH heterostructure (denoted as NFM LDH/OOH) was fabricated by the following method. A total of 25 mg NFM LDH was added into 40 mL sodium hydroxide solution (1 M). Then, the above solution was stirred at 700 rpm for 24 h. Finally, the NFM LDH/OOH was obtained by centrifugation at 5000 rpm and dried at 60 °C for 12 h.

Preparation of NFM LDH/OOH-G
NFM LDH/OOH heterostructure with graphene linkage (denoted as NFM LDH/OOH-G) was obtained by the following method. A total of 25 mg NFM LDH-G was added into 40 mL sodium hydroxide solution (1 M). Then, the above solution was stirred at 700 rpm for 24 h. Finally, the NFM LDH/OOH-G was obtained by centrifugation at 5000 rpm and dried at 60 °C for 12 h.
The preparation of NFM LDH and its derivatives is summarized in Scheme 1. Firstly, Fe 2+ , Ni 2+ , Mn 2+ , and urea were added to deionized water. Then, the above solution was heated to 120 °C for 12 h in a hydrothermal kettle.

Characterizations
The structural property was examined by X-ray diffraction (XRD) performed on a D/max 2500 X-ray powder diffractometer ( Rigaku Corporation, Tokyo, Japan), using a Co Kα radiation source with a wavelength of 1.79026 Å (at 40.0 kV and 40.0 mA) over the 2θ range from 10 • to 90 • at the scan rate of 5 • /min. Scanning electron microscopy (SEM) was employed to probe the microstructure as well as the morphology of the samples, using a Hitachi SU8020 microscope (Hitachi Ltd., Tokyo, Janpan). High-resolution transmission electron microscopy (HRTEM) and elemental mapping was performed on a JEM-F200X field emission transmission electron microscope (JEOL Ltd., Tokyo, Japan). The X-ray photoelectron spectroscopy (XPS) spectra were recorded by an ESCALAB 250XI (Thermo Fisher Scientific Inc., Waltham, MA, USA) using a 200 W monochromator with Al Kα radiation as the excitation source, and the binding energies were calibrated by C 1s at 284.8 eV.

Electrocatalytic Performance Testing
Electrochemical measurements were performed on a CHI-650E electrochemical workstation (Shanghai CH Instruments Co., Ltd., Shanghai, China). A three-electrode system was used in the experiment with a glassy carbon electrode (GCE) as the working electrode, a saturated calomel electrode (saturated KCl) as reference electrode, and a graphite rod as counter-electrode. The electrocatalytic properties of as-prepared OER catalysts (NFM LDH, NFM LDH/OOH, NFM LDH-G, NFM LDH/OOH-G, and RuO 2 ) were tested by linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). The electrochemical test was preceded by an activity sweep using cyclic voltammetry at 0.1 V/s for 20 revolutions. In the LSV test, the scan potential window and scan rate were set at 0 to 0.8 V and 5 mV/s, respectively. EIS measurements were conducted from 1.0 × 10 −3 to 1.0 × 10 5 HZ. The electrolyte was 1 M KOH.

Results
The structural properties of the as-prepared NFM-LDH and its derivatives were examined by XRD, as shown in Figure 1 In other words, NFM-LDHs have been successfully prepared. However, for the sample that was treated with NaOH, an obvious peak at 2θ = 14.5 • emerges, indicated by the inverted triangles in Figure 1, which is aligned to JCPDS: #29-0889 and demonstrates the formation of NFM oxyhydroxides on the NFM LDHs. The morphologies of NFM LDH and its derivatives were characterized by scanning electron microscope and high-resolution transmission electron microscopy analysis. As shown in Figure 2a,b,e,f the NFM LDH and its compound with graphene exhibit almost the same morphology consisting of micrometer-sized sheets and nanometer-sized rods. These micro-and nano-structures present large surface area, which is beneficial to the surface-sensitive reactions. When the samples were treated with NaOH, the morphology did not change a lot, as shown in Figure 2c,d,g,h. The morphologies of NFM LDH and its derivatives were characterized by scanning electron microscope and high-resolution transmission electron microscopy analysis. As shown in Figure 2a,b,e,f the NFM LDH and its compound with graphene exhibit almost the same morphology consisting of micrometer-sized sheets and nanometer-sized rods. These micro-and nano-structures present large surface area, which is beneficial to the surface-sensitive reactions. When the samples were treated with NaOH, the morphology did not change a lot, as shown in Figure 2c,d,g,h.
The morphologies of NFM LDH and its derivatives were characterized by scanning electron microscope and high-resolution transmission electron microscopy analysis. As shown in Figure 2a,b,e,f the NFM LDH and its compound with graphene exhibit almost the same morphology consisting of micrometer-sized sheets and nanometer-sized rods. These micro-and nano-structures present large surface area, which is beneficial to the surface-sensitive reactions. When the samples were treated with NaOH, the morphology did not change a lot, as shown in Figure 2c,d,g,h. Furthermore, elemental mapping by energy dispersive spectroscopy (EDS) was used to study the spatial distribution of each element. As shown in Figure 3, Ni, Fe, Mn, C, and O elements are highly dispersed and well distributed in the NFM LDH/OOH-G sample, confirming the successful fabrication of NFM hydroxides and oxyhydroxides hybrids and the addition of graphene. Furthermore, elemental mapping by energy dispersive spectroscopy (EDS) was used to study the spatial distribution of each element. As shown in Figure 3, Ni, Fe, Mn, C, and O elements are highly dispersed and well distributed in the NFM LDH/OOH-G sample, confirming the successful fabrication of NFM hydroxides and oxyhydroxides hybrids and the addition of graphene. Furthermore, elemental mapping by energy dispersive spectroscopy (EDS) was used to study the spatial distribution of each element. As shown in Figure 3, Ni, Fe, Mn, C, and O elements are highly dispersed and well distributed in the NFM LDH/OOH-G sample, confirming the successful fabrication of NFM hydroxides and oxyhydroxides hybrids and the addition of graphene. XPS measurements were carried out to identify and quantify the chemical composition and oxidation states of the elements present in the prepared electrocatalysts. The full survey XPS spectrum, as shown in Figure 4a, confirms the co-existence of Ni, Fe, Mn, O, and C elements in the samples, which is in good agreement with the EDS results. As displayed in Figure 4b, the characteristic peak of Fe 2p3/2 is located at about 711 eV, and that of 2p1/2 is situated at around 725 eV, while the respective shake-up satellites (marked as Sat.) are located approximately 8 eV higher than the corresponding main peaks. By performing peak-fitting deconvolutions, the measured binding energies of Fe 2p3/2 can be divided into two peaks at 709.76-709.87 eV and 711.63-712.48 eV, which are assigned to Fe 2+ and Fe 3+ , respectively [28,29]. Additionally, by comparing the upper curve (NFM LDH/OOH-G) and the lower (NFM LDH) of Figure 4b, the Fe 2+ /Fe 3+ ratio is calculated to be 0.16 and 0.69, respectively, implying an increase in the Fe 3+ species after NaOH treatment and thus suggesting the formation of FeOOH. Researchers found previously that the OER activity trend of 3d transition metal divalent cations was sorted by Mn 2+ < Fe 2+ < Co 2+ < Ni 2+ [30]. Later, several studies revealed that Fe 3+ could enhance significantly the XPS measurements were carried out to identify and quantify the chemical composition and oxidation states of the elements present in the prepared electrocatalysts. The full survey XPS spectrum, as shown in Figure 4a, confirms the co-existence of Ni, Fe, Mn, O, and C elements in the samples, which is in good agreement with the EDS results. As displayed in Figure 4b, the characteristic peak of Fe 2p 3/2 is located at about 711 eV, and that of 2p 1/2 is situated at around 725 eV, while the respective shake-up satellites (marked as Sat.) are located approximately 8 eV higher than the corresponding main peaks. By performing peak-fitting deconvolutions, the measured binding energies of Fe 2p 3/2 can be divided into two peaks at 709.76-709.87 eV and 711.63-712.48 eV, which are assigned to Fe 2+ and Fe 3+ , respectively [28,29]. Additionally, by comparing the upper curve (NFM LDH/OOH-G) and the lower (NFM LDH) of Figure 4b, the Fe 2+ /Fe 3+ ratio is calculated to be 0.16 and 0.69, respectively, implying an increase in the Fe 3+ species after NaOH treatment and thus suggesting the formation of FeOOH. Researchers found previously that the OER activity trend of 3d transition metal divalent cations was sorted by Mn 2+ < Fe 2+ < Co 2+ < Ni 2+ [30]. Later, several studies revealed that Fe 3+ could enhance significantly the OER activities for Ni-based catalysts in the following ways: (i) improving the electrical conductivity; (ii) shortening the bond length of oxygen bridge; (iii) promoting Ni 2+ to an even high oxidation state during the O 2 evolution [31]. Figure 4c shows the high-resolution XPS spectra of Ni 2p. It is clearly observed that the peaks at 855.83 eV and 857.51 eV are ascribed to the 2p 3/2 of Ni 2+ and Ni 3+ , respectively, and their satellite peaks locate at 861.80 eV and 865.31 eV. For Ni 2p 1/2 , the peaks occur at 873.40 eV and 875.76 and are attributed to the Ni 2+ and Ni 3+ species with their satellite peaks at 879.57 eV and 882.59 eV, respectively [32][33][34]. The lower curve of Figure 4d displays the XPS spectrum of Mn 2p, which can be separated into Mn 2+ , Mn 3+ , and Mn 4+ components at 639.09, 641.23, and 643.28 eV, respectively [35,36]. However, while the samples were treated with NaOH, as shown in the upper curve of Figure 4d, Mn 2+ species disappeared and Mn 3+ valence states substantially increased with a growing ratio of Mn 3+ to total oxidation states from 16.7% to 62.5%, indicating the transformation from Mn LDH to MnOOH. The rich Mn 3+ and Mn 4+ oxidation states can provide more active sites [37], promoting the OER process. In conclusion, the prepared NFM LDH/OOH via alkali treatment generates a considerable number of high-valence Fe 3+ and Mn 3+ species, which enable NFM LDH to possess more active sites and become more active, favoring the OER activities.      Figure 5a,b is well associated with the (015) plane of Ni 5 . 6 Fe 2 . 36 (OH) 16 in NFM LDHs, and the angle of 60 • between the fringes in Figure 5b demonstrates the hexagonal structure of the as-prepared NFM LDHs. The spacing of 0.56 nm in Figure 5c,e originates from the contraction of graphene oxide (GO) while it is reduced (rGO) [38]. It is also observed that the rGO and NFM LDH are linked together, which is beneficial for OER electrocatalysis, since rGO possesses good conductivity [39]. After NaOH treatment, the angle between the fringes changes from 60 • to 90 • , implying a phase transition, as shown in Figure 5d. Moreover, the characteristic spacings of 0.26 nm, 0.26 nm, and 0.37 nm indicated by the white lines in Figure 5d are well indexed to the (400), (040), and (220) lattice planes of FeOOH crystals, respectively [40]. Additionally, Figure 5e shows the formation of NFM LDH/OOH heterojunction after NaOH treatment, mirrored by the characteristic spacings of 0.21 nm and 0.26 nm, which are assigned to the NFM LDH (015) plane and the FeOOH (400) plane, respectively. As shown in Figure 5e, the two crystal domains (indicated by the circles) originating respectively from the NFM LDH and oxyhydroxide contact each other, and the lattice atoms are connected, between which the heterojunction of NFM LDH/OOH is built (indicated by the dashed line). The fringe spacings of 0.29 nm and 0.26 nm with a dihedral angle of 75 • in Figure 5f also originate from the (011) and (040) planes of FeOOH in NFM LDH/OOH-G, respectively. It was reported that the presence of FeOOH with highly oxidative Fe (3 + δ)+ species could reform the local electronic environment of NFM LDH through strong interfacial interactions, enabling the enhanced kinetics of inter-band charge transfer and generating the synergy catalytic effect between the two [33], thus promoting the OER electrocatalysis. reduced (rGO) [38]. It is also observed that the rGO and NFM LDH are linked together, which is beneficial for OER electrocatalysis, since rGO possesses good conductivity [39]. After NaOH treatment, the angle between the fringes changes from 60° to 90°, implying a phase transition, as shown in Figure 5d. Moreover, the characteristic spacings of 0.26 nm, 0.26 nm, and 0.37 nm indicated by the white lines in Figure 5d are well indexed to the (400), (040), and (220) lattice planes of FeOOH crystals, respectively [40]. Additionally, Figure 5e shows the formation of NFM LDH/OOH heterojunction after NaOH treatment, mirrored by the characteristic spacings of 0.21 nm and 0.26 nm, which are assigned to the NFM LDH (015) plane and the FeOOH (400) plane, respectively. As shown in Figure 5e, the two crystal domains (indicated by the circles) originating respectively from the NFM LDH and oxyhydroxide contact each other, and the lattice atoms are connected, between which the heterojunction of NFM LDH/OOH is built (indicated by the dashed line). The fringe spacings of 0.29 nm and 0.26 nm with a dihedral angle of 75° in Figure 5f also originate from the (011) and (040) planes of FeOOH in NFM LDH/OOH-G, respectively. It was reported that the presence of FeOOH with highly oxidative Fe (3 + δ)+ species could reform the local electronic environment of NFM LDH through strong interfacial interactions, enabling the enhanced kinetics of inter-band charge transfer and generating the synergy catalytic effect between the two [33], thus promoting the OER electrocatalysis. The oxygen evolution activity of NFM LDH and its derivatives were evaluated by linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) in 1 M KOH solution. As shown in Figure 6a,b, NFM LDH/OOH heterojunction delivers an overpotential of 449 mV at 10 mA cm −2 , which is 25 mV lower than that of pure NFM LDH, manifesting an enhanced OER electrocatalytic activity. The incorporation of graphene into the NFM LDH and its oxyhydroxide heterojunction further lowers the overpotential to, respectively, 360 mV and 338 mV, which are both smaller than that of commercially used RuO2 catalyst (373 mV), suggesting a substantial improvement in electrocatalytic performance. The comparison of the overpotential for the prepared NFM LDH-based electrocatalysts, as well as the commercial RuO2, is summarized in Figure 6b. To evaluate the OER kinetics of the catalysts, the Tafel slope was calculated from the LSV curve with the Tafel equation ( log , where η is the overpotential, j is the current density, and b is the Tafel slope). As shown in Figure 6c, NFM LDH/OOH heterojunction with graphene connection exhibits the smallest Tafel slope of 67 mV dec −1 , smaller than those of NFM LDH/graphene (68 mV dec −1 ), RuO2 (70 mV dec −1 ), NFM LDH/OOH heterostructure (81 mV dec −1 ), and NFM LDH (99 mV dec −1 ), indicating the superior charge transfer coefficient of NFM LDH/OOH heterojunction and the graphene linkage. EIS was further carried out to deeply understand the OER performance of the prepared electrocatalysts. In general, the diameter of the semicircle in the corresponding EIS diagram represents the charge transfer resistance (Rct), reflecting the catalytic dynamic performance. A smaller Rct means faster electron transfer at the interface between the electrolyte and electrocatalyst. As shown in Figure 6d, it is obviously seen that the NFM LDH/OOH heterojunction shows a smaller arc (117 Ω) than NFM LDH (152 Ω), suggesting a faster charge transfer, which is consistent with the findings of enhanced kinetics of charge transfer due to the heterojunction [33]. Moreover, the incorporation of conductive graphene substantially reduces the resistance, reflected by the significant reduction of the semicircles from around 152 Ω to 72 Ω (53.9% reduction) for NFM LDH-G, and from 117 Ω to 66 Ω (43.6% reduction) for NFM LDH/OOH-G, which endows the NFM LDH and NFM LDH/OOH heterostructure excellent electronic transport behavior. In contrast, the charge transfer resistance of commercial RuO2 is measured as 70 Ω, a little bit larger than that of NFM LDH/OOH-G. The much conductive graphene contributes to the improvement of electrical conductivity of the NFM LDH-related catalysts, and promotes the OER electrocatalytic activities of the The oxygen evolution activity of NFM LDH and its derivatives were evaluated by linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) in 1 M KOH solution. As shown in Figure 6a,b, NFM LDH/OOH heterojunction delivers an overpotential of 449 mV at 10 mA cm −2 , which is 25 mV lower than that of pure NFM LDH, manifesting an enhanced OER electrocatalytic activity. The incorporation of graphene into the NFM LDH and its oxyhydroxide heterojunction further lowers the overpotential to, respectively, 360 mV and 338 mV, which are both smaller than that of commercially used RuO 2 catalyst (373 mV), suggesting a substantial improvement in electrocatalytic performance. The comparison of the overpotential for the prepared NFM LDH-based electrocatalysts, as well as the commercial RuO 2 , is summarized in Figure 6b. To evaluate the OER kinetics of the catalysts, the Tafel slope was calculated from the LSV curve with the Tafel equation (η = a + b log j, where η is the overpotential, j is the current density, and b is the Tafel slope). As shown in Figure 6c, NFM LDH/OOH heterojunction with graphene connection exhibits the smallest Tafel slope of 67 mV dec −1 , smaller than those of NFM LDH/graphene (68 mV dec −1 ), RuO 2 (70 mV dec −1 ), NFM LDH/OOH heterostructure (81 mV dec −1 ), and NFM LDH (99 mV dec −1 ), indicating the superior charge transfer coefficient of NFM LDH/OOH heterojunction and the graphene linkage. EIS was further carried out to deeply understand the OER performance of the prepared electrocatalysts. In general, the diameter of the semicircle in the corresponding EIS diagram represents the charge transfer resistance (R ct ), reflecting the catalytic dynamic performance. A smaller R ct means faster electron transfer at the interface between the electrolyte and electrocatalyst. As shown in Figure 6d, it is obviously seen that the NFM LDH/OOH heterojunction shows a smaller arc (117 Ω) than NFM LDH (152 Ω), suggesting a faster charge transfer, which is consistent with the findings of enhanced kinetics of charge transfer due to the heterojunction [33]. Moreover, the incorporation of conductive graphene substantially reduces the resistance, reflected by the significant reduction of the semicircles from around 152 Ω to 72 Ω (53.9% reduction) for NFM LDH-G, and from 117 Ω to 66 Ω (43.6% reduction) for NFM LDH/OOH-G, which endows the NFM LDH and NFM LDH/OOH heterostructure excellent electronic transport behavior. In contrast, the charge transfer resistance of commercial RuO 2 is measured as 70 Ω, a little bit larger than that of NFM LDH/OOH-G. The much conductive graphene contributes to the improvement of electrical conductivity of the NFM LDH-related catalysts, and promotes the OER electrocatalytic activities of the as-prepared layered catalysts. The result is in accordance with the corresponding low overpotential and small Tafel slope. In addition, enduring stability is another important factor in evaluating the performance of catalyst materials. As shown in Figure 7, the prepared NFM LDH/OOH-G could run steadily at the current density of 10 mA cm −2 for 30 h, thus demonstrating its superior stability. as-prepared layered catalysts. The result is in accordance with the corresponding low overpotential and small Tafel slope. In addition, enduring stability is another important factor in evaluating the performance of catalyst materials. As shown in Figure 7, the prepared NFM LDH/OOH-G could run steadily at the current density of 10 mA cm −2 for 30 h, thus demonstrating its superior stability.  Table 1 lists the comparison of our work with other reported non-noble OER catalysts. It is seen that the prepared NFM LDH/OOH-G electrocatalyst is comparable to the most efficient non-noble catalysts for OER ever reported, showing great potential in practical application.     Table 1 lists the comparison of our work with other reported non-noble OER catalysts. It is seen that the prepared NFM LDH/OOH-G electrocatalyst is comparable to the most efficient non-noble catalysts for OER ever reported, showing great potential in practical application.

Conclusions
In this study, layered double hydroxides consisting of Ni, Fe, and Mn elements (NFM LDHs) were fabricated via the hydrothermal method, which were used as OER electrocatalysts for water-splitting for hydrogen and oxygen production. To boost the OER efficiency, strategies of NFM LDH/OOH heterostructure construction and graphene incorporation were proposed. Building NFM LDH/OOH heterostructures with strong interfacial interactions is thought to enable the modulated electronic property and endow the accelerated charge transfer. In addition, the number of high-valence Fe 3+ and Mn 3+ species after alkali treatment is greatly increased, which could provide more active sites and become more active during the OER process. Adding conductive graphene lowers significantly the charge transfer resistance, accelerating the electron transfer velocity at the interface between the electrolyte and electrocatalyst. The synergetic effects endow the prepared NFM LDH/OOH with graphene linkage a substantially low overpotential of 338 mV with a small Tafel slope of 67 mV dec −1 for OER, which is better than those of commercially used RuO 2 catalyst. The results have demonstrated that the synthesized NFM LDH-based electrocatalyst through heterostructure construction and conductive 2D materials incorporation can be applied as a high-performance OER electrocatalyst for water splitting for hydrogen and oxygen generation.