# Construction of Bimetallic Co/Fe-Incorporated PTA/FDA Nanoclusters for Boosting Electrocatalytic Oxygen Evolution

Authors: Yuping Gai; Junbo Hu; Wei Deng; Wenshuo Xie; Xiang Li; Jingjing Zhang; Dewu Long; Shanlin Qiao; Fei Jiang
Journal: International Journal of Energy Research
 (2023)

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## Abstract

Summary. Electrochemical water decomposition as a crucial approach for the gradual growth of renewable energy has attracted extensive attention. Metal-organic frameworks (MOFs) which benefit from ultra-high specific surface area, controllable nanostructures, and excellent porosity have been widely used as high activity catalyst for the decomposition of water by electrochemical means. Herein, the composition and morphology of metal–organic framework nanoclusters with bimetallic Co/Fe-incorporated PTA/FDA nanoclusters is designed for efficient and durable OER electrocatalysts, including CoFe-BTC/PTA, CoFe-BTC/FDA, and CoFe-PTA/FDA. The crystal structure of MOF materials is composed of alternating organic hydrocarbon BTC, PTA, or FDA and inorganic metal oxide layer. Co and Fe interact as central atoms, joining BTC, PTA, or FDA ligands to form a highly symmetric MOF structure. The electronic structures and active sites of various metals are different, and the insertion of iron atoms plays a certain role in the regulation of their electronic structures. CoFe-PTA/FDA shows significant OER overpotential η10=295mV (1.525 V vs. RHE) reached 10 mA cm-2, with 62.85 mV dec-1 for Tafel slope and pretty conspicuous stability (72 hours of continuous testing). The DFT calculation results show coordination unsaturated metal atom is the primary active center of these electrocatalytic reaction, and the coupling effect caused by adding Fe is the key to adjust the electrocatalytic activity.


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## Body

## 1. Introduction

In the context of the prosperousness of industry, energy even environmental issues have become intense focus of global attention [1–3]. In order to satisfy the rising energy consumption, researchers are trying to find new efficient and nonpolluting energy carriers to replace traditional fossil energy [4–6]. As a green, sustainable, and clean energy with high energy density, hydrogen is a doable method to replace traditional fossil fuels to meet future energy demands [7–9]. Up till the present moment, the main source of hydrogen is the reforming of fossil energy, in which byproducts such as carbon monoxide and carbon dioxide are simultaneously produced, which not only pollutes the environment but also causes abundant waste of resources [10–12]. In comparison, water electrolysis technology, as an efficient and clean production technology, only generates hydrogen and oxygen during the electrolysis process, which has therefore a broad potential alternatives compared with traditional hydrogen production technology [13–15].Electrolysis of water to produce hydrogen involves two reaction processes: oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). The catalysts of the positive and negative electrodes can promote the process to take place at a lower voltage [16–18]. So far, for the electrochemical water splitting process, the optimal catalysts for HER and OER are common precious metal materials for instance, Ir, Ru, and Pt, but these noble metals and their compounds are limited by low durability application and their high cost [19–21]. Consequently, researchers are committed to developing the catalysts for water electrolysis with high catalytic efficiency and lower preparation costs. Various nonnoble metal element electrocatalysts have been extensively investigated in terms of high oxygen evolution reaction, hydrogen evolution reaction, and overall water decomposition efficiency [22–24]. These promising catalysts mainly include nonnoble metal oxides [25], perovskite [26], sulfides [27], carbides [28], nitrides [29], selenides [30], hydroxides [31], oxyhydroxides [32], and their hybrids or complexes. The cost of nonprecious metal electrocatalysts is greatly reduced while still maintaining identical activity and everlasting durability.Metal-organic framework (MOF) is a novel kind of material accompanied by high porosity, which are formed by the orderly binding of organic ligands to some metal ions even clusters, which have high degree of crystallinity and regular permutation. Because of its immense specific surface area, transformable chemical composition, adjustable pore structure, and various topological structure, copious MOFs have been used to electrochemically decompose water for HER and OER, respectively. Moreover, by coupling some other materials, the performances of the MOFs can be uplifted or embellished to combine into something@MOFs or MOF/substrates [33–35]. With more active centers and the comprehensive advantage of improving conductivity through functionalization, excellent electrochemical water decomposition performance can be constantly acquired [36–38]. In addition, the multifunctional material-based skeleton allows atoms to be rearranged in the process of pyrolysis at the molecular and atomic levels [39, 40]. Therefore, MOFs or MOF-based substances can be used as cyclostyle to the pyrolytic properties such as electrical conductivity, porosity, specific surface area, stability, and catalytic activity of ordered calcined polymers was investigated [41, 42]. Consequently, these derivatives have high research value in water decomposition [43, 44]. A series of new materials have been reported on the basis of these standards. However, the MOFs reported so far still have the challenges of poor stability, impoverished electrical conductivity, and organic ligand blocking of active metal centers, which greatly limit the application for robust electrocatalysts [45, 46].Herein, we report metal–organic framework nanoclusters with Co/Fe-Incorporated two kinds of double ligands, which were synthesized by utilizing a mixed solution of Co(NO3)2·6H2O, FeCl3·6H2O, and two ligands 1,3,5-benzenetricarboxylic acid (BTC) and p-phthalic acid (PTA), respectively. The crystal structure of CoFe-BTC/PTA consisted of alternating organic hydrocarbons BTC or PTA and inorganic metal-oxygen layers. Co and Fe interact together as central atoms, which are linking BTC and PTA ligands together to form a highly symmetrical MOF structure. In comparison, we replace one with 1,1-ferrocenedicarboxylic acid (FDA) to prepare CoFe-BTC/FDA and CoFe-PTA/FDA. In addition, in order to prove the addition of Fe has an affirmative impact on the electrochemical properties, the single metal MOF referred as Co-PTA/FDA has been prepared. In particular, CoFe-PTA/FDA manifested an excellent OER performance with an overpotential of 295 mV at 10 mA cm-2, a Tafel slope of 62.85 mV dec-1 at 10 mA cm-2 and electrochemical surface areas of 13.52 mF cm-2, which was more superior than CoFe-BTC/PTA (322 mV, 86.38 mV dec-1, and 4.51 mF cm-2), CoFe-BTC/FDA(309 mV, 75.14 mV dec-1, and 8.23 mF cm-2), and Co-PTA/FDA (307 mV, 65.51 mV dec-1, and 5.29 mF cm-2). Meanwhile, it also has long-term stability (72 hours continuous testing). Through a series of physical characterizations and density functional theory (DFT) calculations, details of active centers even mechanisms of the materials were further proposed.


## 2. Results

### 2.1. Morphologies and Structures

Figure1 presents a diagrammatic sketch of synthesizing metal–organic framework nanoclusters with double ligands using cobaltous nitrate hexahydrate and iron chloride hexahydrate. First, the Co, Fe, ethanol, triethylamine, and DMF were straightforwardly mingled together using hydrothermal method; afterward, the Co/Fe was directly infixed in the principal metal–organic framework nanoclusters by high temperature treatment. All three MOF materials had highly symmetrical structures. The crystal structure of bimetallic CoFe-PTA/FDA was formed by alternating organic hydrocarbon PTA and FDA through the joining of metal Co and Fe ions. Each Fe or Co ion coordinated four atoms, including two hydroxyl groups in PTA and two hydroxyl groups in FDA. Under heating conditions, the free atoms could be easily removed to produce a coordination unsaturated metal site. CoFe-BTC/PTA, CoFe-BTC/FDA, and Co-PTA/FDA were prepared based on similar synthetic process as CoFe-PTA/FDA.Figure 1
Schematic of the process of synthesizing CoFe-BTC/PTA, CoFe-BTC/FDA, and CoFe-PTA/FDA at the oil/water interface.The physical and structural characterizations of as-synthesized CoFe-PTA/FDA can be identified through scanning electron microscopy (SEM) images in Figure2(a) and SAED (Figure S1). In order to observe the structure of the material, we also provided SEM and TEM images at different scales in Figure S2-3. Element ratios are usually studied by energy dispersive spectroscopy (EDS) mapping; in order to get a more accurate ratio of elements, ICP atomic absorption spectroscopy test had been performed. The image in Figure 2(b) showed the EDS map and the table showed the ICP results. From the data, the exact ratios of Fe and Co in the catalyst were 27% and 23%, respectively. Figure 2(c) showed the high resolution transmission electron microscopy (HRTEM) image of the CoFe-PTA/FDA structure and obtained further more details of the structure from the fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) modes of the selected region (marked red frame). HRTEM shaping on a single nanocrystal clearly shows a lattice stripe with (110) planar spacing of 0.21 nm for CoFe-PTAFDA lattices. The spacing lattice distance was also confirmed with 2.462 nm through about calculated 12 fringe distances in Figure 2(d). Subsequent composition analysis by high-angle annular dark-field (HAADF) image and EDS mapping (Figures 2(e)–2(i) and S4) confirms that the obtained CoFe-PTA/FDA consisted of C, O, Fe, and Co atoms, and the element mapping on the nanoclusters shows an even distribution of these elements across the COF-PTA/FDA surface. These results above dependably corroborated that CoFe-PTA/FDA has immanent structural periodicity characteristic and the fact that the exposed metal atoms lose their packing layers, conforming to the desired the desirable structural characteristics. In addition, as MOF-based materials, specific surface area is a relatively important role-shaping parameter, so BET test has been conducted. From the results can be drawn, the BET surface area of CoFe-BTCPTA, CoFe-BTCFDA, and CoFe-PTAFDA was 38.47 m2/g, 60.40 m2/g, and 127.78 m2/g, respectively.Figure 2
Physical characterization of CoFe-PTA/FDA. (a) SEM image of CoFe-PTA/FDA. (b) EDS spectra and ICP data of CoFe-PTA/FDA. (c) HRTEM images and FFT, IFFT pattern of CoFe-PTA/FDA. (d) Spacing lattice distance of CoFe-PTA/FDA. (e–i) High-angle annular dark-field image of CoFe-PTA/FDA and EDS mapping of C, Fe, Co, and O elements, respectively.
(a)(b)(c)(d)(e)(f)(g)(h)(i)The atomic force microscopy (AFM) (Figures3(a) and 3(b) and Figure S5-6) image accentuates an even granular thickness of 1.82 nm, commensurate with three metal coordination layers of 1.8 nm (Figure 3(g)). The X-ray diffraction (XRD) (Figure 3(c)) pattern revealed that CoFe-PTA/FDA and Co-PTA/FDA had similar crystalline structures, but the peaks are higher when Fe was added, indicating that there was no substantial change in the structure of the MOFs material after the addition of Fe. In contrast, the XRD patterns of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA showed less crystalline structure with an obvious peak at 10°, and the latter two have another peak at 35°. X-ray diffraction (XRD) analysis showed that the prepared CoFe-BTCPTA and CoFe-BTCFDA samples exhibited a broad peak at 10°, which could be attributed to BTC, and the prepared CoFe-PTAFDA and Co-PTAFDA samples exhibited a broad peak at 35°, which shows that ligands were well doped into MOF materials and which the diffraction peaks of the product are consistent with the standard PDF#18-1584 and PDF#33-1622, e.g., crystal planes {110}-{011}-{032}. In order to better analyze the structure of the material, the three ligands were tested by XRD in Figure S7. The composition of CoFe-BTC/PTA, CoFe-BTC/FDA, and CoFe-PTA/FDA was further supported by X-ray photoelectron spectroscopy (XPS). The survey scan spectrum indicated that nanoclusters are composed of C, O, Fe, and Co elements (Figure 3(d)), which matched with the ICP and EDS element mapping images. Specifically, because of the spin orbit coupling, the XPS spectra of the Co 2p and Fe 2p regions of the catalyst splinted into two dual states. From the XPS spectra of Co 2p in Figure 3(e), the main deconvoluted peaks corresponded to the trivalent Co 2p3/2 and 2p1/2, while the Co 2p3/2 peak could be fitted into Co3+ (782.9 eV) and Co2+ (780.7 eV) peaks together with a satellite peak (786.1 eV). In addition, Co 2p1/2 peak could be fitted into Co3+ (797.5 eV) and Co2+ (796.1 eV) peaks together with a satellite peak (801.9 eV) [47–49].. In Figure 3(f), for Fe 2p spectrum, the peak centered at 712.4 eV for Fe3+ and 709.9 eV for Fe2+ with a satellite at 714.8 eV in the region of Fe 2p3/2 and while the peaks centered at 726.1 for Fe2+ and 724.6 eV for Fe3+ with a satellite at 718.2 eV were attributed to Fe 2p1/2 [50, 51].Figure 3
(a, b) Atomic force microscopy image and the corresponding height profiles of CoFe-PTA/FDA. (c) XRD of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA. (d) XPS spectra of CoFe-BTC/PTA, CoFe-BTC/FDA and CoFe-PTA/FDA. (e) XPS spectra of Co 2p of CoFe-BTC/PTA, CoFe-BTC/FDA, and CoFe-PTA/FDA. (f) XPS spectra of Fe 2p of CoFe-BTC/PTA, CoFe-BTC/FDA, and CoFe-PTA/FDA. (g) Theoretical thickness of CoFe-PTAFDA with three metal coordination layers.
(a)(b)(c)(d)(e)(f)(g)


### 2.2. Evaluation of Electrochemical Activity

A standard three-electrode system was used in 1 M KOH solution for evaluation of the electrocatalytic OER performances of the CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA. In addition, other OER activities of RuO2 were also determined for comparison. In order to minimize the capacitive current, all the linear sweep voltammetry (LSV) curves were obtained at a slow scan rate of 5 mV s−1 (Figure 4(a)). In the meantime, CoFe-PTA/FDA exhibited excellent OER catalytic activity with overpotential of 295 mV, lower than that of CoFe-BTC/PTA (322 mV), CoFe-BTC/FDA (309 mV), and Co-PTA/FDA (307 mV). In comparison, RuO2 as a benchmark electrocatalyst, its electrochemical properties is slightly better than the preparation of these materials with η10 of 279 mV. At the same time, in order to better compare the electrochemical properties of MOF materials, Fe-PTA/FDA, Co-BTC/PTA, and Fe-BTC/FDA were prepared, and their OER curves have been observed in Figure S8. For the sake of further understanding other OER properties, different samples’ Tafel plots were determined to explore their OER dynamics (Figure 4(b)). A relatively small Tafel slope in connection with a rapid increase in current density reveals a faster kinetic process. CoFe-PTA/FDA, Co-PTA/FDA, and RuO2 showed a low slope 62.85 mVdec−1, 65.51 mV dec−1, and 60.11 mV dec−1, less than that of CoFe-BTC/PTA (86.38 mV dec−1) and CoFe-BTC/FDA (75.14 mV dec−1). Additionally, the outstanding OER performance of Co-PTA/FDA is identified by its prominent double layer capacitance (13.52 mF cm-2) in Figure 4(c). Both CoFe-PTA/FDA (13.52 mF cm−2) and CoFe-BTC/FDA (8.23 mF cm−2) have larger electrochemical active surface areas than CoFe-BTC/PTA (4.51 mF cm−2), substantiating that the CoFe-PTA/FDA has the advantage of remarkably expanding electrochemical active surfaces. We measured the electrochemically active surface area (ECSA) of the studied catalysts using the nonfaradaic double-layer capacitance method, and the results of RuO2, CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA were 28.86 cm2, 23.66 cm2, 14.40 cm2, 9.26 cm2, and 7.89 cm2, respectively. To further illustrate the high OER performance of CoFe-PTA/FDA, electrochemical impedance spectroscopy (EIS) was kept a record for different samples (Figure 4(d)). Consistently, the corresponding Nyquist plots of CoFe-PTA/FDA also showed the smallest arc radius.Figure 4
(a) OER polarization curves of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, Co-PTA/FDA, and RuO2. (b) Tafel plots of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, Co-PTA/FDA, and RuO2. (c) Cdl values of current density differences plotted against scan rates of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, Co-PTA/FDA, and RuO2. (d) EIS Nyquist plots of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, Co-PTA/FDA, and RuO2. (e) The corresponding overpotential at 10 mA cm−2 of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, Co-PTA/FDA, and RuO2. (f) Long-term stability measurement for the CoFe-PTA/FDA at current densities of 10 mA cm−2 for 72 hours. The inset shows the TEM images before and after 72 hours test for the OER.
(a)(b)(c)(d)(e)(f)The smallest charge transfer resistance of CoFe-PTA/FDA reflects the faster electron transfer process due to the exceptional structure of MOF materials, which is concordant with LSV images. Figure4(e) summarized the overpotential of the five samples with 10 mA cm−2 current density. The CoFe-PTA/FDA showed significantly decreased overpotentials than Co-PTA/FDA. The excellent stability plays an important part in appraising the wide application of electrocatalysts [52]. As illustrated in chronopotentiometry curve image (Figure 4(f)), in the process of continuous electrolysis for 72 hours, there was no palpable augment in overpotential, corroborating its long-lasting stability. The HRTEM image (the inset in Figure 4(f)) showed the structure of CoFe-PTA/FDA remained intact and the change is negligible, which indicated that CoFe-PTA/FDA is indeed structural stable catalyst. In order to better verify the stability of the materials, materials were conducted the XPS test before and after the electrochemical process. From the Figure S9-11, there was no obvious change of XPS in the material before and after electrochemical treatment, which further proved the good stability of the material.Current densities of RuO2, CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA were recorded with different scan rates (Figures 5(a)–5(d) and S12). We contrasted the electrochemical property of our materials with preceding narrated OER electrocatalysts with Co or Fe metal in terms of the Tafel slope and overpotential at 10 mA cm-2 for electroncatalysts. The CoFe-PTA/FDA performed better than the vast majority of listed Co/Fe OER electrocatalysts (as shown in Figure 5(e) and Table S1) [4, 43, 53–60].. The excellent OER performance and outstanding stability of CoFe-PTA/FDA proved that it has broad application prospects.Figure 5
(a–d) Cdl values of current density differences plotted against scan rates of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA. (e) Comparison of OER performance for various described electroncatalysts. The x and y represent the Tafel slope and overpotential η10 (mV) at 10 mA cm-2 for electroncatalysts.
(a)(b)(c)(d)(e)


### 2.3. Insights into OER Mechanism

For the sake of further exploring the potential mechanisms of enhanced activity of the activity enhancement of these MOF materials, density functional theory (DFT) calculation was conducted using Material Studios. The corresponding free energy is calculated to be 1.54 eV of CoFe-PTA/FDA. A compelling problem is how Fe and Co interact in CoFe-PTA/FDA to produce the remarkable electrocatalytic performance. The fundamental steps of OER process on CoFe-PTA/FDA were summarized in Figure6(a), mainly divided into four steps. The first step is summarized as adsorption, the second step and the third part are combined as dissociation, and the final step is capsuled as desorption. In addition, the other materials are shown in Figure S13-14. Molecular model of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA was calculated. The rhombus-shape primitive cell shows the cell parameters of 13.30×8.69×4.21Å3, α=β=90.0°, and γ=60.0°, as illustrated in Figures 6(b)–6(e). As shown in the Gibbs free energy figures (Figure 6(f)), the energy barrier (△G1) for generation of OH∗in CoFe-BTC/PTA (1.32 eV), which is contrarily proportional to the adsorption strength for OH, is larger than CoFe-PTA/FDA (1.21 eV), corroborating that the latter is more vivacious during electrocatalytic process. A circumstantial study of the Gibbs free energies of the four materials communicates several significant information. First of all, contrasted with the other three materials, the lower △G1 value of CoFe-PTA/FDA proves the preferential adsorption of OH is advantageous to the initiation of OER process. Moreover, after the formation of CoFe-PTA/FDA, the e-orbital occupancy of coordination unsaturated metals was optimized. Consequently, the theoretical overpotentials of CoFe-PTA/FDA decrease by 10 mV (1.55 eV to 1.54 eV) compared with Co-PTA/FDA (Figure 6(f)), in accordance with the inclination of the electrochemical measurements results.Figure 6
(a) The original procedure of the OER process for CoFe-BTC/FDA. Chemical representation of the color scheme: grey means C atom, red means O atom, white means H atom, blue means Co atom, and purple means Fe atom. (b–e) Structure representations of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA. (f) Gibbs free energy figure of OER process of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA. (g) The calculated DOS of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA, the black dashed line indicates the Fermi-level DOS.
(a)(b)(c)(d)(e)(f)(g)DFT calculations created a model in order to better simulate the conceivable active sites in materials. The simplification of the original crystal structure made it clear to consider the binding site between the catalytic atom and the reaction intermediate. The results of free energy showed that the binding energy between intermediate and active atom could be efficaciously decreased by using a little Fe atoms incorporated into a material containing Co, thus providing a novel method to ameliorate the electrocatalytic activity. The electron density of state curve was used to further analyze the electronic properties of these four models, and the results were shown in Figure6(g). The results showed that CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA were high density conductors with electron density distribution at Fermi level, which was mainly caused by the hybridization of Co 2p–Fe 2p [61, 62]. The coordination unsaturated metal atoms were the principal active center; furthermore, the coupling effect of Co and Fe atoms further contributed to the activity of the electrocatalyst. Meanwhile, as shown in Figure S15-20, it showed the calculated the full-state density and band structure of the model system. Consequently, the DFT results highlighted and reinforced that it is important to design appropriate ligand combination structures to facilitate development to enhance OER catalysts.


## 2.1. Morphologies and Structures

Figure1 presents a diagrammatic sketch of synthesizing metal–organic framework nanoclusters with double ligands using cobaltous nitrate hexahydrate and iron chloride hexahydrate. First, the Co, Fe, ethanol, triethylamine, and DMF were straightforwardly mingled together using hydrothermal method; afterward, the Co/Fe was directly infixed in the principal metal–organic framework nanoclusters by high temperature treatment. All three MOF materials had highly symmetrical structures. The crystal structure of bimetallic CoFe-PTA/FDA was formed by alternating organic hydrocarbon PTA and FDA through the joining of metal Co and Fe ions. Each Fe or Co ion coordinated four atoms, including two hydroxyl groups in PTA and two hydroxyl groups in FDA. Under heating conditions, the free atoms could be easily removed to produce a coordination unsaturated metal site. CoFe-BTC/PTA, CoFe-BTC/FDA, and Co-PTA/FDA were prepared based on similar synthetic process as CoFe-PTA/FDA.Figure 1
Schematic of the process of synthesizing CoFe-BTC/PTA, CoFe-BTC/FDA, and CoFe-PTA/FDA at the oil/water interface.The physical and structural characterizations of as-synthesized CoFe-PTA/FDA can be identified through scanning electron microscopy (SEM) images in Figure2(a) and SAED (Figure S1). In order to observe the structure of the material, we also provided SEM and TEM images at different scales in Figure S2-3. Element ratios are usually studied by energy dispersive spectroscopy (EDS) mapping; in order to get a more accurate ratio of elements, ICP atomic absorption spectroscopy test had been performed. The image in Figure 2(b) showed the EDS map and the table showed the ICP results. From the data, the exact ratios of Fe and Co in the catalyst were 27% and 23%, respectively. Figure 2(c) showed the high resolution transmission electron microscopy (HRTEM) image of the CoFe-PTA/FDA structure and obtained further more details of the structure from the fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) modes of the selected region (marked red frame). HRTEM shaping on a single nanocrystal clearly shows a lattice stripe with (110) planar spacing of 0.21 nm for CoFe-PTAFDA lattices. The spacing lattice distance was also confirmed with 2.462 nm through about calculated 12 fringe distances in Figure 2(d). Subsequent composition analysis by high-angle annular dark-field (HAADF) image and EDS mapping (Figures 2(e)–2(i) and S4) confirms that the obtained CoFe-PTA/FDA consisted of C, O, Fe, and Co atoms, and the element mapping on the nanoclusters shows an even distribution of these elements across the COF-PTA/FDA surface. These results above dependably corroborated that CoFe-PTA/FDA has immanent structural periodicity characteristic and the fact that the exposed metal atoms lose their packing layers, conforming to the desired the desirable structural characteristics. In addition, as MOF-based materials, specific surface area is a relatively important role-shaping parameter, so BET test has been conducted. From the results can be drawn, the BET surface area of CoFe-BTCPTA, CoFe-BTCFDA, and CoFe-PTAFDA was 38.47 m2/g, 60.40 m2/g, and 127.78 m2/g, respectively.Figure 2
Physical characterization of CoFe-PTA/FDA. (a) SEM image of CoFe-PTA/FDA. (b) EDS spectra and ICP data of CoFe-PTA/FDA. (c) HRTEM images and FFT, IFFT pattern of CoFe-PTA/FDA. (d) Spacing lattice distance of CoFe-PTA/FDA. (e–i) High-angle annular dark-field image of CoFe-PTA/FDA and EDS mapping of C, Fe, Co, and O elements, respectively.
(a)(b)(c)(d)(e)(f)(g)(h)(i)The atomic force microscopy (AFM) (Figures3(a) and 3(b) and Figure S5-6) image accentuates an even granular thickness of 1.82 nm, commensurate with three metal coordination layers of 1.8 nm (Figure 3(g)). The X-ray diffraction (XRD) (Figure 3(c)) pattern revealed that CoFe-PTA/FDA and Co-PTA/FDA had similar crystalline structures, but the peaks are higher when Fe was added, indicating that there was no substantial change in the structure of the MOFs material after the addition of Fe. In contrast, the XRD patterns of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA showed less crystalline structure with an obvious peak at 10°, and the latter two have another peak at 35°. X-ray diffraction (XRD) analysis showed that the prepared CoFe-BTCPTA and CoFe-BTCFDA samples exhibited a broad peak at 10°, which could be attributed to BTC, and the prepared CoFe-PTAFDA and Co-PTAFDA samples exhibited a broad peak at 35°, which shows that ligands were well doped into MOF materials and which the diffraction peaks of the product are consistent with the standard PDF#18-1584 and PDF#33-1622, e.g., crystal planes {110}-{011}-{032}. In order to better analyze the structure of the material, the three ligands were tested by XRD in Figure S7. The composition of CoFe-BTC/PTA, CoFe-BTC/FDA, and CoFe-PTA/FDA was further supported by X-ray photoelectron spectroscopy (XPS). The survey scan spectrum indicated that nanoclusters are composed of C, O, Fe, and Co elements (Figure 3(d)), which matched with the ICP and EDS element mapping images. Specifically, because of the spin orbit coupling, the XPS spectra of the Co 2p and Fe 2p regions of the catalyst splinted into two dual states. From the XPS spectra of Co 2p in Figure 3(e), the main deconvoluted peaks corresponded to the trivalent Co 2p3/2 and 2p1/2, while the Co 2p3/2 peak could be fitted into Co3+ (782.9 eV) and Co2+ (780.7 eV) peaks together with a satellite peak (786.1 eV). In addition, Co 2p1/2 peak could be fitted into Co3+ (797.5 eV) and Co2+ (796.1 eV) peaks together with a satellite peak (801.9 eV) [47–49].. In Figure 3(f), for Fe 2p spectrum, the peak centered at 712.4 eV for Fe3+ and 709.9 eV for Fe2+ with a satellite at 714.8 eV in the region of Fe 2p3/2 and while the peaks centered at 726.1 for Fe2+ and 724.6 eV for Fe3+ with a satellite at 718.2 eV were attributed to Fe 2p1/2 [50, 51].Figure 3
(a, b) Atomic force microscopy image and the corresponding height profiles of CoFe-PTA/FDA. (c) XRD of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA. (d) XPS spectra of CoFe-BTC/PTA, CoFe-BTC/FDA and CoFe-PTA/FDA. (e) XPS spectra of Co 2p of CoFe-BTC/PTA, CoFe-BTC/FDA, and CoFe-PTA/FDA. (f) XPS spectra of Fe 2p of CoFe-BTC/PTA, CoFe-BTC/FDA, and CoFe-PTA/FDA. (g) Theoretical thickness of CoFe-PTAFDA with three metal coordination layers.
(a)(b)(c)(d)(e)(f)(g)


## 2.2. Evaluation of Electrochemical Activity

A standard three-electrode system was used in 1 M KOH solution for evaluation of the electrocatalytic OER performances of the CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA. In addition, other OER activities of RuO2 were also determined for comparison. In order to minimize the capacitive current, all the linear sweep voltammetry (LSV) curves were obtained at a slow scan rate of 5 mV s−1 (Figure 4(a)). In the meantime, CoFe-PTA/FDA exhibited excellent OER catalytic activity with overpotential of 295 mV, lower than that of CoFe-BTC/PTA (322 mV), CoFe-BTC/FDA (309 mV), and Co-PTA/FDA (307 mV). In comparison, RuO2 as a benchmark electrocatalyst, its electrochemical properties is slightly better than the preparation of these materials with η10 of 279 mV. At the same time, in order to better compare the electrochemical properties of MOF materials, Fe-PTA/FDA, Co-BTC/PTA, and Fe-BTC/FDA were prepared, and their OER curves have been observed in Figure S8. For the sake of further understanding other OER properties, different samples’ Tafel plots were determined to explore their OER dynamics (Figure 4(b)). A relatively small Tafel slope in connection with a rapid increase in current density reveals a faster kinetic process. CoFe-PTA/FDA, Co-PTA/FDA, and RuO2 showed a low slope 62.85 mVdec−1, 65.51 mV dec−1, and 60.11 mV dec−1, less than that of CoFe-BTC/PTA (86.38 mV dec−1) and CoFe-BTC/FDA (75.14 mV dec−1). Additionally, the outstanding OER performance of Co-PTA/FDA is identified by its prominent double layer capacitance (13.52 mF cm-2) in Figure 4(c). Both CoFe-PTA/FDA (13.52 mF cm−2) and CoFe-BTC/FDA (8.23 mF cm−2) have larger electrochemical active surface areas than CoFe-BTC/PTA (4.51 mF cm−2), substantiating that the CoFe-PTA/FDA has the advantage of remarkably expanding electrochemical active surfaces. We measured the electrochemically active surface area (ECSA) of the studied catalysts using the nonfaradaic double-layer capacitance method, and the results of RuO2, CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA were 28.86 cm2, 23.66 cm2, 14.40 cm2, 9.26 cm2, and 7.89 cm2, respectively. To further illustrate the high OER performance of CoFe-PTA/FDA, electrochemical impedance spectroscopy (EIS) was kept a record for different samples (Figure 4(d)). Consistently, the corresponding Nyquist plots of CoFe-PTA/FDA also showed the smallest arc radius.Figure 4
(a) OER polarization curves of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, Co-PTA/FDA, and RuO2. (b) Tafel plots of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, Co-PTA/FDA, and RuO2. (c) Cdl values of current density differences plotted against scan rates of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, Co-PTA/FDA, and RuO2. (d) EIS Nyquist plots of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, Co-PTA/FDA, and RuO2. (e) The corresponding overpotential at 10 mA cm−2 of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, Co-PTA/FDA, and RuO2. (f) Long-term stability measurement for the CoFe-PTA/FDA at current densities of 10 mA cm−2 for 72 hours. The inset shows the TEM images before and after 72 hours test for the OER.
(a)(b)(c)(d)(e)(f)The smallest charge transfer resistance of CoFe-PTA/FDA reflects the faster electron transfer process due to the exceptional structure of MOF materials, which is concordant with LSV images. Figure4(e) summarized the overpotential of the five samples with 10 mA cm−2 current density. The CoFe-PTA/FDA showed significantly decreased overpotentials than Co-PTA/FDA. The excellent stability plays an important part in appraising the wide application of electrocatalysts [52]. As illustrated in chronopotentiometry curve image (Figure 4(f)), in the process of continuous electrolysis for 72 hours, there was no palpable augment in overpotential, corroborating its long-lasting stability. The HRTEM image (the inset in Figure 4(f)) showed the structure of CoFe-PTA/FDA remained intact and the change is negligible, which indicated that CoFe-PTA/FDA is indeed structural stable catalyst. In order to better verify the stability of the materials, materials were conducted the XPS test before and after the electrochemical process. From the Figure S9-11, there was no obvious change of XPS in the material before and after electrochemical treatment, which further proved the good stability of the material.Current densities of RuO2, CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA were recorded with different scan rates (Figures 5(a)–5(d) and S12). We contrasted the electrochemical property of our materials with preceding narrated OER electrocatalysts with Co or Fe metal in terms of the Tafel slope and overpotential at 10 mA cm-2 for electroncatalysts. The CoFe-PTA/FDA performed better than the vast majority of listed Co/Fe OER electrocatalysts (as shown in Figure 5(e) and Table S1) [4, 43, 53–60].. The excellent OER performance and outstanding stability of CoFe-PTA/FDA proved that it has broad application prospects.Figure 5
(a–d) Cdl values of current density differences plotted against scan rates of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA. (e) Comparison of OER performance for various described electroncatalysts. The x and y represent the Tafel slope and overpotential η10 (mV) at 10 mA cm-2 for electroncatalysts.
(a)(b)(c)(d)(e)


## 2.3. Insights into OER Mechanism

For the sake of further exploring the potential mechanisms of enhanced activity of the activity enhancement of these MOF materials, density functional theory (DFT) calculation was conducted using Material Studios. The corresponding free energy is calculated to be 1.54 eV of CoFe-PTA/FDA. A compelling problem is how Fe and Co interact in CoFe-PTA/FDA to produce the remarkable electrocatalytic performance. The fundamental steps of OER process on CoFe-PTA/FDA were summarized in Figure6(a), mainly divided into four steps. The first step is summarized as adsorption, the second step and the third part are combined as dissociation, and the final step is capsuled as desorption. In addition, the other materials are shown in Figure S13-14. Molecular model of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA was calculated. The rhombus-shape primitive cell shows the cell parameters of 13.30×8.69×4.21Å3, α=β=90.0°, and γ=60.0°, as illustrated in Figures 6(b)–6(e). As shown in the Gibbs free energy figures (Figure 6(f)), the energy barrier (△G1) for generation of OH∗in CoFe-BTC/PTA (1.32 eV), which is contrarily proportional to the adsorption strength for OH, is larger than CoFe-PTA/FDA (1.21 eV), corroborating that the latter is more vivacious during electrocatalytic process. A circumstantial study of the Gibbs free energies of the four materials communicates several significant information. First of all, contrasted with the other three materials, the lower △G1 value of CoFe-PTA/FDA proves the preferential adsorption of OH is advantageous to the initiation of OER process. Moreover, after the formation of CoFe-PTA/FDA, the e-orbital occupancy of coordination unsaturated metals was optimized. Consequently, the theoretical overpotentials of CoFe-PTA/FDA decrease by 10 mV (1.55 eV to 1.54 eV) compared with Co-PTA/FDA (Figure 6(f)), in accordance with the inclination of the electrochemical measurements results.Figure 6
(a) The original procedure of the OER process for CoFe-BTC/FDA. Chemical representation of the color scheme: grey means C atom, red means O atom, white means H atom, blue means Co atom, and purple means Fe atom. (b–e) Structure representations of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA. (f) Gibbs free energy figure of OER process of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA. (g) The calculated DOS of CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA, the black dashed line indicates the Fermi-level DOS.
(a)(b)(c)(d)(e)(f)(g)DFT calculations created a model in order to better simulate the conceivable active sites in materials. The simplification of the original crystal structure made it clear to consider the binding site between the catalytic atom and the reaction intermediate. The results of free energy showed that the binding energy between intermediate and active atom could be efficaciously decreased by using a little Fe atoms incorporated into a material containing Co, thus providing a novel method to ameliorate the electrocatalytic activity. The electron density of state curve was used to further analyze the electronic properties of these four models, and the results were shown in Figure6(g). The results showed that CoFe-BTC/PTA, CoFe-BTC/FDA, CoFe-PTA/FDA, and Co-PTA/FDA were high density conductors with electron density distribution at Fermi level, which was mainly caused by the hybridization of Co 2p–Fe 2p [61, 62]. The coordination unsaturated metal atoms were the principal active center; furthermore, the coupling effect of Co and Fe atoms further contributed to the activity of the electrocatalyst. Meanwhile, as shown in Figure S15-20, it showed the calculated the full-state density and band structure of the model system. Consequently, the DFT results highlighted and reinforced that it is important to design appropriate ligand combination structures to facilitate development to enhance OER catalysts.


## 3. Conclusions

In summary, a simple hydrothermal synthesis means has been matured for the preparation of metal-organic frameworks with synergistic unsaturated metal centers. We report metal–organic framework nanoclusters with Co/Fe-incorporated two kinds of double ligands, which were synthesized by utilized a mixed solution of Co(NO3)2·6H2O, FeCl3·6H2O, two kinds of ligands combination of BTC, PTA, and FDA, respectively. Furthermore, in order to prove the addition of Fe has a unique impact on the electrochemical properties, the single metal MOF referred as Co-PTA/FDA has been prepared compared with a bimetallic MOF. Significantly, the CoFe-PTA/FDA catalyst has excellent OER performance and bodacious catalytic stability. The clear atomic structure of CoFe-PTA/FDA allows the exploration of the origin of the high electrocatalytic activity. The coordination unsaturated metal atom is the main active center of the electrocatalyst, and the coupling effect of Co and Fe atoms further contributes to the activity of the electrocatalyst.


## 4. Methods

### 4.1. Materials

Cobaltous nitrate hexahydrate (Co(NO3)2·6H2O, 99.5%, Adamas-beta), iron(III) chloride hexahydrate (FeCl3·6H2O, 99.5%, Adamas-beta), N,N-dimethylformamide (DMF, 99.5%, Adamas-beta), ruthenium oxide (RuO2, 99.9%, Adamas-beta), perfluorinated resin solution containing nafion (NF, 5wt.%, Adamas-beta), triethylamine (C6H15N, 98%, Adamas-beta), 1,3,5-benzenetricarboxylic acid (BTC, 99.9%, Adamas-beta), p-phthalic acid (PTA, 99.9%), 1,1-ferrocenedicarboxylic acid (FDA, 99.9%, Adamas-beta), potassium hydroxide (KOH, 99.9%, Adamas-beta), ethanol absolute (C2H5OH, 99.9%, Adamas-beta), and deionized water (18.2 MΩ. cm. 25°C). All the experimental materials were bought from Aladdin chemicals. Moreover, all reagents were of reagent grade and can be considered as received samples, and there was no need to proceed further purification.


### 4.2. Synthesis of CoFe-BTC/PTA

0.15 mmol Co(NO3)2·6H2O, 0.04 mmol FeCl3·6H2O, 0.2 mmol BTC, 0.2 mmol PTA, and 15 ml N, N-dimethylformamide were added into a conical flask and then stirred the resulting mixture for 30 minutes. Then, added two milliliters of water and 1 : 1 of alcohol and 0.2 ml triethylamine to the mixture then kept stirring. The mixed liquid was poured into a stainless steel autoclave polytetrafluoroethylene lining with 20 ml capacity and stirred for 30 min at room temperature and react for 12 hours in an oven at 170°C. When the reaction is finished, the autoclave is naturally cooled to ambient temperature, and the reaction product was centrifuged for two minutes at 7500 rpm and laundered with C2H5OH for two times. Processed finished sample was dried in 60°C oven for 6 hours until the product was completely dry and grind into fine powder in a quartz mortar.


### 4.3. Synthesis of CoFe-BTC/FDA

0.15 mmol Co(NO3)2·6H2O, 0.04 mmol FeCl3·6H2O, 0.2 mmol BTC, 0.2 mmol FDA, and 15 ml N, N-dimethylformamide were added into a conical flask and then stirred the resulting mixture for 30 minutes. Then, added two milliliters of water and 1 : 1 of alcohol and 0.2 ml triethylamine to the mixture then kept stirring. The mixed liquid was poured into a stainless steel autoclave polytetrafluoroethylene lining with 20 ml capacity and stirred for 30 min at room temperature and react for 12 hours in an oven at 170°C. When the reaction is finished, the autoclave is naturally cooled to ambient temperature, and the reaction product was centrifuged for two minutes at 7500 rpm and laundered with C2H5OH for two times. Processed finished sample was dried in 60°C oven for 6 hours until the product was completely dry and grind into fine powder in a quartz mortar.


### 4.4. Synthesis of CoFe-PTA/FDA

0.15 mmol Co(NO3)2·6H2O, 0.04 mmol FeCl3·6H2O, 0.2 mmol PTA, 0.2 mmol FDA, and 15 ml N, N-dimethylformamide were added into a conical flask and then stirred the resulting mixture for 30 minutes. Then, added two milliliters of water and 1 : 1 of alcohol and 0.2 ml triethylamine to the mixture then kept stirring. The mixed liquid was poured into a stainless steel autoclave polytetrafluoroethylene lining with 20 ml capacity and stirred for 30 min at room temperature and react for 12 hours in an oven at 170°C. When the reaction is finished, the autoclave is naturally cooled to ambient temperature and the reaction product was centrifuged for two minutes at 7500 rpm and laundered with C2H5OH for two times. Processed finished sample was dried in 60°C oven for 6 hours until the product was completely dry and grind into fine powder in a quartz mortar.


### 4.5. Synthesis of Co-PTA/FDA

To 0.15 mmol Co(NO3)2·6H2O, 0.2 mmol PTA, 0.2 mmol FDA, and 15 ml N, N-Dimethylformamide, the mixture was stirred in magnetic stirrer for 30 minutes at ambient temperature. Then, added 1 ml ethanol absolute, 1 ml deionized water, and 0.2 ml of triethylamine with constant stirring. The mixed liquid was added to a stainless steel autoclave with polytetrafluoroethylene lining with 20 ml capacity and stirred for 30 min at room temperature and react for 12 hours in an oven at 170°C. When the reaction is finished, the autoclave is naturally cooled to ambient temperature and the reaction product was centrifuged for two minutes at 7500 rpm and laundered with C2H5OH for two times. Processed finished sample was dried in 60°C oven for 6 hours until the product was completely dry and grind into fine powder in a quartz mortar.


### 4.6. Preparation of Catalyst

First of all, 30 mL of 5 wt% Nafion solution and 1 ml of mixed solution of water and ethanol (3 : 1) were added into the centrifuge tube, then put 4 mg of electrocatalyst, and ultrasound for 30 minutes to mix evenly into the ink. Then, as shown in Figure7(a), the 5 mL mixed ink was well-distributed dripped on a 0.07 cm2 glassy carbon electrode with the pipette gun and dried in the oven at low temperature until a film was formed on the electrode surface.Figure 7
(a) Preparation before electrochemical data measurement. (b) Electrochemical workstation (CHI760E). (c) Three-electrode electrochemical cell.
(a)(b)(c)


### 4.7. Electrochemical Measurements

The electrochemical measurements were carried out in a three-electrode configuration unit (Figure7(c)) using a platinum wire electrode and a saturated calomel electrode (SCE) as a pair electrode and a reference electrode, respectively. The electrochemical performance of the materials was tested in a 1.0 mol L-1 KOH solution using an electrochemical workstation (CHI760E) (Figure 7(b)) at ambient temperature. It should be emphasized that, in order to make our tests more meticulous, the 1 M electrolyte was prepared by high purity KOH, which concentration was 99.9%. The polarization curves were obtained by linear sweep voltammetry (LSV) at a scanning rate of 5 mV·s-1. The measured potentials vs. SCE were turned into vs. reversible hydrogen electrode (RHE) according the equation: ERHE=ESCE+0.231+0.0591pH. Plot the LSV curve as the logarithm of the standard potential and current density to obtain the Tafel diagram, and the Tafel is calculated by the equation:
(1)η=a+b×logj,whereη represents the overpotential, a represents the Tafel constant, b represents the Tafel slope, and j represents current density.


## 4.1. Materials

Cobaltous nitrate hexahydrate (Co(NO3)2·6H2O, 99.5%, Adamas-beta), iron(III) chloride hexahydrate (FeCl3·6H2O, 99.5%, Adamas-beta), N,N-dimethylformamide (DMF, 99.5%, Adamas-beta), ruthenium oxide (RuO2, 99.9%, Adamas-beta), perfluorinated resin solution containing nafion (NF, 5wt.%, Adamas-beta), triethylamine (C6H15N, 98%, Adamas-beta), 1,3,5-benzenetricarboxylic acid (BTC, 99.9%, Adamas-beta), p-phthalic acid (PTA, 99.9%), 1,1-ferrocenedicarboxylic acid (FDA, 99.9%, Adamas-beta), potassium hydroxide (KOH, 99.9%, Adamas-beta), ethanol absolute (C2H5OH, 99.9%, Adamas-beta), and deionized water (18.2 MΩ. cm. 25°C). All the experimental materials were bought from Aladdin chemicals. Moreover, all reagents were of reagent grade and can be considered as received samples, and there was no need to proceed further purification.


## 4.2. Synthesis of CoFe-BTC/PTA

0.15 mmol Co(NO3)2·6H2O, 0.04 mmol FeCl3·6H2O, 0.2 mmol BTC, 0.2 mmol PTA, and 15 ml N, N-dimethylformamide were added into a conical flask and then stirred the resulting mixture for 30 minutes. Then, added two milliliters of water and 1 : 1 of alcohol and 0.2 ml triethylamine to the mixture then kept stirring. The mixed liquid was poured into a stainless steel autoclave polytetrafluoroethylene lining with 20 ml capacity and stirred for 30 min at room temperature and react for 12 hours in an oven at 170°C. When the reaction is finished, the autoclave is naturally cooled to ambient temperature, and the reaction product was centrifuged for two minutes at 7500 rpm and laundered with C2H5OH for two times. Processed finished sample was dried in 60°C oven for 6 hours until the product was completely dry and grind into fine powder in a quartz mortar.


## 4.3. Synthesis of CoFe-BTC/FDA

0.15 mmol Co(NO3)2·6H2O, 0.04 mmol FeCl3·6H2O, 0.2 mmol BTC, 0.2 mmol FDA, and 15 ml N, N-dimethylformamide were added into a conical flask and then stirred the resulting mixture for 30 minutes. Then, added two milliliters of water and 1 : 1 of alcohol and 0.2 ml triethylamine to the mixture then kept stirring. The mixed liquid was poured into a stainless steel autoclave polytetrafluoroethylene lining with 20 ml capacity and stirred for 30 min at room temperature and react for 12 hours in an oven at 170°C. When the reaction is finished, the autoclave is naturally cooled to ambient temperature, and the reaction product was centrifuged for two minutes at 7500 rpm and laundered with C2H5OH for two times. Processed finished sample was dried in 60°C oven for 6 hours until the product was completely dry and grind into fine powder in a quartz mortar.


## 4.4. Synthesis of CoFe-PTA/FDA

0.15 mmol Co(NO3)2·6H2O, 0.04 mmol FeCl3·6H2O, 0.2 mmol PTA, 0.2 mmol FDA, and 15 ml N, N-dimethylformamide were added into a conical flask and then stirred the resulting mixture for 30 minutes. Then, added two milliliters of water and 1 : 1 of alcohol and 0.2 ml triethylamine to the mixture then kept stirring. The mixed liquid was poured into a stainless steel autoclave polytetrafluoroethylene lining with 20 ml capacity and stirred for 30 min at room temperature and react for 12 hours in an oven at 170°C. When the reaction is finished, the autoclave is naturally cooled to ambient temperature and the reaction product was centrifuged for two minutes at 7500 rpm and laundered with C2H5OH for two times. Processed finished sample was dried in 60°C oven for 6 hours until the product was completely dry and grind into fine powder in a quartz mortar.


## 4.5. Synthesis of Co-PTA/FDA

To 0.15 mmol Co(NO3)2·6H2O, 0.2 mmol PTA, 0.2 mmol FDA, and 15 ml N, N-Dimethylformamide, the mixture was stirred in magnetic stirrer for 30 minutes at ambient temperature. Then, added 1 ml ethanol absolute, 1 ml deionized water, and 0.2 ml of triethylamine with constant stirring. The mixed liquid was added to a stainless steel autoclave with polytetrafluoroethylene lining with 20 ml capacity and stirred for 30 min at room temperature and react for 12 hours in an oven at 170°C. When the reaction is finished, the autoclave is naturally cooled to ambient temperature and the reaction product was centrifuged for two minutes at 7500 rpm and laundered with C2H5OH for two times. Processed finished sample was dried in 60°C oven for 6 hours until the product was completely dry and grind into fine powder in a quartz mortar.


## 4.6. Preparation of Catalyst

First of all, 30 mL of 5 wt% Nafion solution and 1 ml of mixed solution of water and ethanol (3 : 1) were added into the centrifuge tube, then put 4 mg of electrocatalyst, and ultrasound for 30 minutes to mix evenly into the ink. Then, as shown in Figure7(a), the 5 mL mixed ink was well-distributed dripped on a 0.07 cm2 glassy carbon electrode with the pipette gun and dried in the oven at low temperature until a film was formed on the electrode surface.Figure 7
(a) Preparation before electrochemical data measurement. (b) Electrochemical workstation (CHI760E). (c) Three-electrode electrochemical cell.
(a)(b)(c)


## 4.7. Electrochemical Measurements

The electrochemical measurements were carried out in a three-electrode configuration unit (Figure7(c)) using a platinum wire electrode and a saturated calomel electrode (SCE) as a pair electrode and a reference electrode, respectively. The electrochemical performance of the materials was tested in a 1.0 mol L-1 KOH solution using an electrochemical workstation (CHI760E) (Figure 7(b)) at ambient temperature. It should be emphasized that, in order to make our tests more meticulous, the 1 M electrolyte was prepared by high purity KOH, which concentration was 99.9%. The polarization curves were obtained by linear sweep voltammetry (LSV) at a scanning rate of 5 mV·s-1. The measured potentials vs. SCE were turned into vs. reversible hydrogen electrode (RHE) according the equation: ERHE=ESCE+0.231+0.0591pH. Plot the LSV curve as the logarithm of the standard potential and current density to obtain the Tafel diagram, and the Tafel is calculated by the equation:
(1)η=a+b×logj,whereη represents the overpotential, a represents the Tafel constant, b represents the Tafel slope, and j represents current density.



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*Source: 4991015-2023-02-04.xml*