FeNi-Based Coordination Crystal Directly Serving as Efficient Oxygen

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 20778−20787

FeNi-Based Coordination Crystal Directly Serving as Efficient Oxygen Evolution Reaction Catalyst and Its Density Functional Theory Insight on the Active Site Change Mechanism Changqing Li,† Guo Wang,† Kai Li, Yiwen Liu, Binbin Yuan, and Yuqing Lin* Department of Chemistry, Capital Normal University, Beijing 100048, China

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S Supporting Information *

ABSTRACT: Although most metal−organic coordination materials are promising materials used as templates to develop highly efficient electrocatalysts via pyrolysis in situ, few studies have explored the use of these materials for direct catalysis of oxygen evolution reaction (OER). Herein, inspired by the natural synthesis and the inherent properties of metal−organic coordination materials, the FeNi−tannic acid coordination crystal was in situ grown on Ni foam ((FeNi)−Tan/NF) to directly catalyze the OER. It was found that (FeNi)−Tan/NF exhibited predominant OER activity, which required a low overpotential of 208 mV to reach a current density of 50 mA· cm−2 under a small Tafel slope of 33.5 mV·dec−1, and it possessed robust stability. Density functional theory (DFT) calculations demonstrated that the active site change from Ni in Ni−Tan to the Fe atom in (FeNi)−Tan may provide a more favorable OER catalytic route. This application of such polyphenol coordination materials is promising for stimulating the exploration of functional metal−organic coordination materials toward applications in the energy conversion field. KEYWORDS: nature-inspired synthesis, metal−organic coordination material, electrocatalyst, oxygen evolution reaction, DFT calculation

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INTRODUCTION The rapid consumption of traditional fossil fuels and its resulting environmental issues have prompted the study and design of advanced and sustainable energy conversion and storage systems.1−3 In particular, using renewable but intermittent sources to electrocatalyze water splitting is regarded as a viable approach for large-scale clean hydrogen fuel production, and it has received considerable attention.4−6 Despite ongoing research, the undesirable realization of an integrated water-splitting system is still severely limited by the kinetically sluggish anodic oxygen generation.7,8 In this regard, exploration of highly active electrocatalyts is indispensable for accelerating the oxygen evolution reaction (OER).9,10 Considerable research has therefore been directed toward the exploration of highly efficient and low-cost electrocatalysts as alternatives to state-of-the-art Ru- and Ir-based catalysts.11−19 Benefiting from their tunable composition, structure, and morphologies, as well as tailorable diversity in the choice of metal centers/clusters and organic ligands, metal−organic coordination materials, particularly metal−organic frameworks (MOFs), have been widely developed and applied in the fields of gas storage and separation, catalysis, drug delivery, sensing, electronics, and energy storage and conversion.20,21 For the electrocatalysis applications, metal−organic coordination materials are preferably adopted as versatile precursors for conversion into transition-metal oxides/phosphides/sulfides or © 2019 American Chemical Society

are hybridized with porous carbon materials and doped with various heteroatoms through an annealing process without the aid of any template, with a very large body of research confirming the high activity of these MOF-derived materials.22,23 However, pyrolysis at high temperature unavoidably leads to the destruction of bridging ligands and the agglomeration of metal centers, directly causing the collapse of self-assembled structure and a decrease in the amount of active sites.24 Currently, many metal−organic coordination materials including MOFs are expected to be directly employed as efficient electrocatalysts due to inherent coordinately unsaturated metal atoms (CUMAs) and their homogeneous and heterogeneous characteristics.25−30 The CUMAs can serve as typical Lewis acid centers for favorable electron acceptance from the reactants, thus accelerating the efficiency of relevant conversion reaction, such as the OER.31 Furthermore, the bimetallic or multimetal coupling effect was also proposed to overcome the problem of the lower electrocatalytic activity of related catalytic reactions on these metal−organic coordination materials.32 In this regard, Tang’s group reported on the synthesis of ultrathin NiCo-based organic framework nanosheets (NiCo-UMOFNs) with proReceived: February 17, 2019 Accepted: May 20, 2019 Published: May 22, 2019 20778

DOI: 10.1021/acsami.9b02994 ACS Appl. Mater. Interfaces 2019, 11, 20778−20787

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Synthetic Process of (FeNi)−Tan/NF and Its Direct Application Toward the Oxygen Evolution Reaction

Figure 1. (a, b) SEM images, (c, d) Typical TEM and HRTEM images (insets: selected area electron diffraction (SAED) pattern), and (e) elemental mapping images on C, O, Fe, and Ni elements of (FeNi)−Tan/NF.

have led to the discovery of many relevant biological functions,40 including the most well-known, photosynthesis (through MgII-porphyrin)41 and adhesion (via FeIII-phenolics).42 On the basis of these promising applications from natural examples, metal−organic coordination materials have stimulated substantial research in chemistry and materials science.43−45 As a typical natural polyphenol available from multiple plants, tannic acid (Tan) is an inexpensive, nontoxic, and renewable ligand source for the preparation of naturally existing metal−organic coordination materials.46 The catechol groups on Tan enable its use as a promising organic ligand to coordinate different metal ions and form metal−organic coordination materials.47 In typical examples, multifunctional metal−Tan composites were prepared and exploited in drug delivery, chemical sensing, catalysis, and phosphorus removal.48,49 However, such naturally occurring metal polyphenol coordination materials, particularly those with a well-defined

nounced OER electrocatalytic performance and robust stability.33 Our group also reported an FeNi-based MOF (MIL-53(FeNi)) in situ created on Ni foam to directly catalyze the OER with a low overpotential of 233 mV required to deliver a current density of 50 mA cm−2.34 Lu et al. prepared a water-stable NH2-MIL-88B(Fe2Ni) composite and found that its activity predominantly originated from the metal coupling effect of Ni and Fe, also the synergistic effect between the formed MOFs and porous Ni foam substrate.29 Note that metal-coordinated materials are mainly obtained from typical organic ligands (such as imidazole and pyridine) to obtain desirable topology and diversified structures, while those organic ligands involved in the synthesis of MOFs should be further explored with regard to the economy and sustainability.35 Importantly, naturally occurring metal−organic coordination materials have long been scientific inspiration for researchers due to the combined merits of their inorganic and organic building blocks.36−39 These kinds of materials 20779


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Figure 2. (a) XRD patterns of (FeNi)−Tan. (b−e) The corresponding high-resolution constitutional element XPS spectra of C 1s, O 1s, Fe 2p, and Ni 2p on (FeNi)−Tan/NF and Ni−Tan/NF. ( f) Ni 2p XPS spectra comparison between (FeNi)−Tan/NF and Ni−Tan/NF.

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RESULTS AND DISCUSSION Each Tan (C76H52O46) molecule has 10 catechol groups and can be simplified as having two types of catechol groups (see Figure S1 in Supporting Information). The corresponding formation mechanism of the (FeNi)−Tan coordination crystals grown on Ni foam (NF) is proposed as follows. First, the (FeNi)−Tan coordination structure is created through the self-assembly process between iron ions and tannic acid ligand. The solvent containing the relevant ratio (14:1:1) of N,N-dimethylformamide (DMF), EtOH, and deionized water provided an initial alkaline environment where pH value was around 10, and once tannic acid was slowly introduced into the above solution, the catechol groups in the Tan molecules were deprotonated and chelated strongly with Fe ions. During that process, one Fe ion would coordinate three catechol groups in different Tan molecules, leading to the coordination of the polymerization process. The irregular morphology and amorphous structure of the resulting precipitates demonstrated that the direct self-assembly route of Fe3+ and Tan does not produce Fe−Tan crystals (Figure S2a,b). As more tannic acid was introduced, the mixed solution became strongly acidic as the pH changed to 1.8, inducing the considerable evolution of Ni2+ ions from the NF to coordinate

morphology and crystalline characteristics, are rarely adopted for the direct catalysis of the OER reaction. Herein, inspired by the natural synthesis and the inherent properties of these metal−organic coordination materials, the iron and nickel bimetallic tannic acid coordination crystal is synthesized through self-assembly and a rearrangement and recrystallization process during solvothermal treatment on Ni foam (NF) substrate, briefly noted as (FeNi)−Tan/NF (Scheme 1). The exogenously added iron ions and the etched Ni ions from the NF substrate coordinate with the tannic acid (Tan) ligand to obtain the (FeNi)−Tan crystalline structure via the solvothermal process. The (FeNi)−Tan coordination crystals exhibited excellent OER activity, requiring a low overpotential of 208 mV to arrive at a current density of 50 mA cm−2 under a lower Tafel slope of 33.5 mV dec−1 and exhibited robust stability. The pronounced OER activity of the (FeNi)− Tan coordination crystal is related to its intrinsic octahedral structure, hydrophilic groups, synergistic metal−metal interaction effect, abundance of active sites, and proposed active site transfer mechanism. 20780


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Figure 3. (a) Corresponding FT-IR spectra and (b) Raman spectra of the as synthesized (FeNi)−Tan and Ni−Tan material.

TEM (Figure 1e) confirmed a uniform distribution of these four elements (C, O, Fe, and Ni) on the surfaces of (FeNi)− Tan. Powder X-ray diffraction pattern was applied to again confirm the typical crystalline structure of (FeNi)−Tan (Figure 2a). XPS characterization was also employed to precisely detect the surface composition and the element valence states for (FeNi)−Tan/NF. The atomic ratio of carbon to iron reflected in the XPS result was approximately 72:1, and the iron to nickel ratio was is 3:1, consistent with the result obtained from the EDS spectrum. Because the formula for Tan can be expressed as C76H52O46, it is reasonable to conclude that the molar ratio of iron and TA is approximately 1:1. One iron atom coordinates with three Tan molecules, and one Tan molecule coordinates with five iron atoms. The iron ion can coordinate with six oxygen atoms to form an octahedral structure, which was reported to be extremely active for the catalysis of the OER process.50,51 Because Tan has a large molecular weight and volume, one iron/nickel ion presumably coordinates with six oxygen atoms of the three catechol groups from three Tan molecules (Figure S9).52 Furthermore, the high-resolution XPS spectra of those constitutional element (C 1s, O 1s, Fe 2p and Ni 2p) presented in Figure 2b−f validated the successful creation of (FeNi)−Tan polyphenol coordination crystal. The C 1s spectrum (Figure 2b) indicated the presence of benzoic rings, C−O bonds, and ester groups (O−CO) in Tan linkers at binding energies of 284.7 eV, 286.0, and 288.6 eV, respectively. The three peaks of the O 1s spectra (Figure 2c) located at 531.9, 531.4, 530.7, and 533.0 eV can be assigned to the oxygen atoms present in the form of Fe(Ni)−O bonds, the HO−C functional groups in Tan linkers, and the adsorbed water, respectively.53 The Fe 2p spectrum (Figure 2d) located at approximately 709.0 and 722.6 eV can be ascribed to the Fe 2p3/2 and Fe 2p1/2 configurations, which were associated with the Fe3+ oxidation states.30 The Ni 2p region of (FeNi)−Tan/ NF shows the Ni−O link at approximately 855.3 and 873.2 eV, indicating the same compositional form of Ni 2p in Ni−Tan (Figure 2e and Figure S10). Previous studies have proposed that Ni atoms are the active centers for oxygen evolution;32 therefore, high-resolution Ni 2p spectra on Ni−Tan/NF were also compared (Figure 2f) to probe the possible effect on OER performance. The Ni 2p spectra of (FeNi)−Tan/NF shifts to higher binding energies with the introduction of iron compared to the spectra of Ni−Tan/NF, suggesting that the local electronic structure of Ni has been modified.32 The shift to higher energy (ca. 0.4 eV) arose from the variation of the coordination environment on the Ni atom with the Fe

with Tan. Next, the crystalline framework of the (FeNi)−Tan nanoparticles on Ni foam ((FeNi)−Tan/NF) was formed by a further solvothermal treatment due to the rearrangement and recrystallization ability of the ligands and metal ions (Figure S2c). Prior to physical and electrochemical characterization, the digital photographs recorded for four synthesized samples were obtained (Figure S3). The pure NF had a silvery white color while Ni−Tan/NF revealed a black surface, Tan/NF had a brown surface, and (FeNi)−Tan/NF had an indigo colored surface. The color changes of these materials suggested the morphological and structural variation of (FeNi)−Tan, Ni− Tan, and Tan on NF. Scanning electron microscopy (SEM) together with high resolution transmission electron microscopy (HRTEM) characterization was introduced to investigate the morphology on (FeNi)−Tan/NF (Figure 1). As depicted in Figure 1a,b, (FeNi)−Tan/NF indicated a nanoparticle-assembled internal structure with some voids interconnected through the surface and beneath the interior region. Ni−Tan/NF without the introduction of an iron source was also synthesized for comparison with (FeNi)−Tan/NF. The SEM images of Ni− Tan/NF (Figure S4) indicated some irregular nanoparticles structures that were completely different from those of (FeNi)−Tan/NF. Furthermore, Tan/NF prepared with no metal ions during the solvothermal process showed an interesting scale-like morphology (Figure S5); this can be explained by the solution maintaining an acid environment after Tan was dissolved into the solvent containing N,Ndimethylformamide, EtOH, and water, leading to the release of some Ni ions from the surface of the NF and coordinating with Tan molecules to obtain the Ni−Tan coordination material on the relatively smooth surface of the NF (Figure S6). This proposed mechanism is also applicable to the in situ formation of (FeNi)−Tan grown on the NF, where only the Fe salt was added into the reaction system. To probe the morphological and structural effects of (FeNi)−Tan/NF on catalysis activity, the (FeNi)−Tan nanoparticles were investigated by HRTEM. The enlarged TEM and HRTEM images suggested that the (FeNi)−Tan particles were interconnected and were crystalline (Figure 1c,d); the voids connected to the internal and outside regions endowed the (FeNi)−Tan with a higher Brunauer−Emmett− Teller (BET) specific surface area of 62.5 m2 g−1 compared with Ni−Tan (31.5 m2 g−1) (Figure S7). The EDS element analysis indicated that the elemental ratio on the center metal of Fe and Ni in (FeNi)−Tan nanoparticles was around 3:1 (Figure S8). Additionally, the elemental mapping analysis in 20781


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Figure 4. (a) iR-corrected LSV curves, (b) corresponding Tafel plots, (c) required overpotential and current densities of four as-prepared materials at 50 mA cm−2 and 1.438 V vs RHE. (d, e) ECSA evaluation, Nyquist plots from EIS experiments at 1.438 V versus RHE of (FeNi)−Tan/NF, Ni− Tan/NF, Tan/NF, and bare NF. (f) Chronoamperometric curves of (FeNi)−Tan/NF recorded at the ascending current densities of 50, 100, 150 mA cm−2.

Fe(Ni) atoms and the phenolic functional groups of tannic acid. FT-IR characterization indicated the presence of some phenolic hydroxyl groups on the as-prepared (FeNi)−Tan material, and those hydrophilic groups ensure the effective contact during the interface of active sites and the electrolyte.54 The Raman spectra of (FeNi)−Tan and tannic acid are displayed in Figure 3b. In fact, tannic acid was fluorescent under Raman radiation (Figure S12), resulting in the suppression of the typical peak in tannic acid. Notably, (FeNi)−Tan/NF exhibited several new peaks over the entire range. The two doublets located at 538 and 600 cm−1 were attributed to the bidentate chelation of Fe3+ with the phenolic oxygen of catechol, and this was adopted as an indicator for the existence of the Tan−FeIII coordination compound.46 The shift of these two peaks may be related to the bimetallic coupling effect in the (FeNi)−Tan coordination crystal. Four component bands at 1229, 1335, 1478, and 1580 cm−1 arose from the catechol ring vibration, specifically originating from the presence of Tan on the (FeNi)−Tan surfaces.54 These results successfully confirmed the desired synthesis of (FeNi)− Tan/NF. Next, the water oxidation activities of (FeNi)−Tan/NF were measured using iR-corrected linear sweep cyclic voltammetry technique (LSV). The applied scan rate was as low as 1 mV s−1 and reached the steady-state in LSV analysis in basic alkaline

participating in the tuning of electronic properties of the Ni atom. Additionally, the incorporation of Fe into the material resulted in the relatively good thermal stability of (FeNi)−Tan to Ni−Tan (Figure S11). Considering the coupling effect in mixed metals, such efficient modulation of the Ni electronic environment is speculated to greatly enhance the OER catalytic performance. Fourier transform infrared (FT-IR) spectroscopy was utilized to determine important functional groups of (FeNi)−Tan (Figure 3a). The tannic acid IR spectrum was also obtained as the reference and was found to be in excellent agreement with the previously reported results. The (FeNi)− Tan had two bands located at approximately 3439 and 1573 cm−1 and was assigned to the typical vibration mode of surface OH groups and CH groups on the benzene rings. In addition, the νas(C−O) and νs(C−O) adsorption bands were detected at 1347 and 1200 cm−1, indicating that coordination between the tannic acid ligand and the metal centers was formed. An adsorption band of νas(C−O) of the Tan ligand was also observed at 1384 cm−1.54 Moreover, the two peaks centered at 3360 and 791 cm−1 are well ascribed to the observed O−H stretching mode of the catechol group and the C−H bond of the aromatic rings, separately. Importantly, the ν(Fe−O) band was observed at 607 cm−1, validating the successful creation of the metal−oxo bond due to the coordination effect of the 20782


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Figure 5. DFT simulations for understanding the OER catalytic mechanism. (a) Reaction steps involved the adsorption atomic structures during the OER process for (FeNi)−Tan molecules on the Fe site. (b) Reaction free energy diagram of OER at zero electrode potential on Fe and/or Ni sites of the (FeNi)−Tan and Ni−Tan surfaces with the maximum free energy change highlighted, and the free energies are relative to the starting reactants * and H2O(l). (c) Corresponding free energy change of the reaction-determining step.

required to obtain the same j value. In particular, the (FeNi)− Tan/NF reached a j value of 100 mA cm−2 at merely 223 mV, which is fairly lower than those of Ni−Tan/NF (57 mV), Tan/ NF (237 mV), and bare NF (477 mV) at the same j value of 100 mA cm−2. Figure 4d shows the accessible current density in all prepared materials when operated with the same overpotential (η) of 208 mV, and it was observed that (FeNi)−Tan/NF showed dominant performance due to the higher reachable j value of 50 mA cm−2, and the Ni−Tan/NF, Tan/NF and bare NF were not satisfactory with the available j value of only 24, 13, and 12.6 mA cm−2, respectively. These catalytic trends were even clearly detected when η of 236 mV was applied, and (FeNi)−Tan/NF reached the largest j value of 200 mA cm−2, while the corresponding values for the others samples were lower, with 36.8 and 12.3 mA cm−2 for Ni−Tan/ NF and Tan/NF samples, respectively (Figure S13a). Those experimental results clearly demonstrated the excellent OER performance of bimetallic polyphenol coordination crystals (i.e., ((FeNi)−Tan/NF)), and they also compare favorably with the most active OER electrocatalysts reported to date (Table S1). In particular, apart from the synergistic effect of Ni and Fe, other possible factors that may affect the enhanced OER activity of (FeNi)−Tan/NF were explored in depth. Cyclic voltammograms (CVs) (Figure 4d, Figure S14) were performed at the potential range without the redox processes to calculate the double-layer capacitance (Cdl) on all prepared materials, which represents one-half of the electrochemically active surface area (ECSA). (FeNi)−Tan/NF had the largest slope value of 5.87 mF cm−2, followed by Ni−Tan/NF (4.84 mF cm−2), Tan/NF (4.47 mF cm−2), and bare NF (3.26 mF cm−2). The larger slope value of (FeNi)−Tan/NF revealed the existence of abundant active sites during the solid−liquid reaction interface, which is also considered as one of the key factors for the enhanced OER performance. Furthermore, electrochemical impedance spectroscopy (EIS) was performed to investigate the interfacial kinetics toward the OER for all four prepared materials. The relatively smaller semicircle diameter value (5.5 Ω) found in the

media (1.0 M KOH); the potential used in the experiment was calibrated to the reversible hydrogen electrode (RHE) and referenced with the thermodynamic OER potential (E0H2O/ O2 = 1.229 V). The electrocatalytic activities of Ni−Tan/NF, Tan/NF and bare NF were also investigated for more details. As illustrated in Figure 4a, the lower current density of the bare NF and Tan/NF revealed their relatively worse OER catalytic activities, while the (FeNi)−Tan/NF metal−organic coordination material exhibited excellent OER performance compared to the three compared samples (Ni−Tan/NF, Tan/ NF, and bare NF), revealing that the incorporation of the Fe atoms into the backbone of Ni−Tan contributed to the enhancement of the catalytic activity, as observed from the higher current density and lower onset potential toward the OER. Additionally, an obvious oxidation peak observed at 1.3− 1.36 V in all four samples can be attributed to the redox process of the central Ni atom.32 The positive potential shifts of the Ni3+/Ni2+ peak in (FeNi)−Tan/NF compared with the monometallic sample (Ni−Tan/NF) validate the synergistic effect of metal−metal interactions in improving the OER performance. To acquire additional kinetics information for the corresponding OER performance, the Tafel slope plot acquired from detected LSV curves was used. The Tafel slope fitted to the Tafel equation (η = b log j + a, where η denotes the overpotential, j refers to the current density, and b corresponds to the Tafel slope) is displayed in Figure 4b, and it is suggested that a smaller Tafel slope of 33.5 mV dec−1 was obtained for (FeNi)−Tan/NF compared with those for Ni− Tan/NF (61.9 mV dec−1), Tan/NF (78.1 mV dec−1), and bare NF (94.7 mV dec−1), further indicating that the kinetics of these bimetallic polyphenol coordination crystals are quite satisfactory. For OER electrocatalytic activity evaluation, the required potentials to deliver the current density (j) of 50 mA cm−2 are generally compared (Figure 4c). It was found that (FeNi)− Tan/NF required a low potential of 1.438 V (vs RHE) to reach the j value of 50 mA cm−2, corresponding an overpotential (η) of only 208 mV. In addition, higher overpotentials of 44, 192, and 292 mV for the Ni−Tan/NF, Tan/NF, and bare NF were 20783


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the Fe atom in (FeNi)−Tan is favorable, and this case will be discussed. Compared to the values for Ni−Tan, the free energy changes of steps 1 and 3 increased, and those of steps 2 and 4 decreased for (FeNi)−Tan. The four values were closer to 1.229 eV. For the rate-determining step 4, the stability of * should be important due to the lack of a ligand. There were six ligands for Fe3+ and four for Ni2+. The lack of a ligand should have a greater impact on Ni2+, which has fewer ligands. Thus, the stability of * for (FeNi)−Tan should be higher than that for Ni−Tan. This explains the lower free energy change of the rate-determining step 4 and the higher catalytic activity of the (FeNi)−Tan. To summarize, the experimental results in this study demonstrated for the first time the successful synthesis of a (FeNi)−Tan/NF metal−organic coordination material that can effectively and directly catalyze the oxygen evolution reaction through an electrochemical route. The excellent catalytic activity can be associated with the intrinsic properties of (FeNi)−Tan/NF as follows: (1) the iron/nickel ion in (FeNi)−Tan/NF was formed with octahedral structure, which was reported to be extremely active in the OER; (2) abundant phenolic hydroxyl groups in Tan linkers are beneficial for improving OER activity due to the enhanced hydrophilicity and availability of OH; (3) the synergistic effect of metal− metal interactions may have contributed to the OER performance; (4) abundant active sites, enhanced electron transport ability, and robust stability ensure superior electrocatalytic OER activity; (5) density functional theory (DFT) calculations suggest that the active site change from Ni in Ni− Tan to the Fe atom in (FeNi)−Tan may provide a more favorable OER catalytic route. The above clues may reasonably explain the dominant catalytic activity of (FeNi)−Tan/NF coordination crystals.

electrochemical impedance spectroscopy results (Figure 4e) proved the enhanced electron transport ability of (FeNi)− Tan/NF at the electrolyte/electrode interface. In addition to the predominant activity, excellent durability in catalysts is also an essential indicator for OER application. To assess the stability of (FeNi)−Tan/NF, 1000 consecutive cycles of CV scans were applied, and it was found that the obtained polarization curves before and after the CV cycles were basically the same with a negligible current density loss (Figure S13b). Prolonged chronopotentiometry (CP) measurements performed at the increasing current densities of 50, 100, and 150 mA cm−2 were also carried out, and it was found that (FeNi)−Tan/NF maintained the current density for 13000 s with only a small degradation, corroborating the excellent stability of the (FeNi)−Tan/NF electrode in the basic electrolyte environment. Recently, experts in the OER research field suggested that post-OER catalyst characterizations including XPS, synchrotron-based X-ray spectroscopy, Raman spectroscopy, SEM, and HRTEM should be employed to identify the real active species of the claimed catalysts due to the oxidation processes of the OER.55 Accordingly, in our study, the SEM and XPS techniques were used to determine the composition of the as-prepared (FeNi)−Tan/NF after the OER. The SEM images in Figure S15 suggested that the nanoparticle-assembled structure of (FeNi)−Tan/NF was preserved after experiencing the strongly oxidative process of the OER. Furthermore, XPS spectra after the OER for all constitutional elements (Figure S16) indicated the unchanged valence state and coordination environment of (FeNi)−Tan/NF because the corresponding peak positions were basically consistent with those obtained prior to the OER. These results favorably proved the robust composition of the as-prepared FeNi-based coordination crystal that serves directly as a robust OER catalyst under the oxidative environment, while other coordination materials tend to be transformed into other compounds such as hydroxides.56 To elucidate the effect of metal coupling on the catalytic activities of Ni−Tan and (FeNi)−Tan, theoretical investigations based on density functional theory (DFT) were carried out (Figure 5a). Because there are no single crystal data for the above two structures and no long conjugated chain in a Tan molecule, the electronic structure was rather localized. A ratio of 1:1 for Ni and Fe was used here for simplicity. It should be noted that this model may not fully reflect the real situation in the complex structure, but is only used for better understanding of the reaction. For Ni−Tan, the free energy changes of the four-step reactions at the standard condition were −0.06, 2.09, 0.76, and 2.13 eV (Figure 5b), respectively. Step 4 was the rate-determining step. When the active site was on the Ni atom in (FeNi)−Tan, the free energy changes were −0.18, 2.28, 0.58, and 2.24 eV, respectively. The free energy change of step 4 increased, and the rate-determining step became step 2. Although the free energy changes of steps 2 and 4 were similar, they were both higher than those of step 4 for Ni−Tan. Thus, the catalytic activity on the Ni atom in (FeNi)−Tan decreased. When the active site changed to the Fe atom in (FeNi)−Tan, the free energy changes were 0.51, 1.55, 1.26, and 1.60 eV, respectively. Step 4 was also the ratedetermining step with the free energy change reduced from 2.24 to 1.60 eV, with approximately 30% reduction in the free energy change value (Figure 5c), similar to that for Ni−Tan. Therefore, the electrocatalytic oxygen evolution reaction on

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CONCLUSIONS FeNi−tannic acid coordination crystals ((FeNi)−Tan) were developed via the assembly of metal ions with tannic acid, followed by rearrangement and recrystallization under solvothermal treatment. Such (FeNi)−Tan metal−organic coordination materials can be directly employed as a lowcost and renewable source to catalyze the OER process with higher activity and robust stability due to their intrinsic octahedral structure, abundant phenolic hydroxyl groups, synergistic effects between Ni and Fe, and abundant active sites. In particular, the created (FeNi)−Tan/NF can deliver a current density of 50 mA·cm−2 at a low overpotential of 208 mV under a small Tafel slope value of 33.5 mV·dec−1 and is highly stable in alkaline media. DFT simulations further indicated that the active site change from Ni in Ni−Tan to the Fe atom in (FeNi)−Tan lowers the reaction free energy change of the rate-determining step and thus enhances OER catalytic activity of (FeNi)−Tan. We anticipate that the synthetic route and our proposed model may pave the way for the exploration of novel metal−polyphenol coordination materials with crystalline framework toward electrocatalytic applications.

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EXPERIMENTAL SECTION

Synthesis of (FeNi)−Tan on NF. The (FeNi)−Tan material was grown in situ on nickel foam (NF) through a one-step solvothermal process. Before the experimental process, the commercial nickel foam was treated to remove the surface oxides and impurities as described 20784


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measurement was further applied under 100 mV s−1 to record the final LSV curves at a scan speed of 1 mV s−1. The EIS measurements were performed by applying an AC amplitude of 5 mV within 100 kHz to 100 mHz frequency range versus Ag/AgCl. For a detailed comparison, the electrocatalytic activities of Ni−Tan/Ni, Tan/NF, and NF were also recorded. Density Functional Theory Evaluations. Theoretical investigations based on density functional theory (DFT) were introduced to elucidate different catalytic activities of Ni−Tan and (FeNi)−Tan. To the best of our knowledge, no single crystal data is available for these structures. Due to the absence of a long conjugated chain in the Tan molecule, the electronic structure is rather localized. Therefore, molecular complex models instead of solid models were constructed. As shown in Figure S17a, a tetracoordinated Ni2+ ion was used to simulate Ni−Tan, where the two ligands were −O− in the Tan molecule. The other two ligands were water molecules that may come from the surrounding area. In addition to the Ni2+ ion, the (FeNi)− Tan model shown in Figure S17b also contained a hexacoordinated Fe3+ ion in which the three ligands were −O− in the Tan molecule, and the other three were water molecules. For simplicity, we used a ratio of 1:1 for Ni and Fe. The structures were geometrically optimized by using the sophisticated B3LYP57,58 hybrid density functional and 6-31G** basis sets as implemented in the Gaussian03 program.59 The spins were set according to the electronic configuration in the tetragonal and the octagonal fields for Ni and Fe, respectively. All the structures were confirmed by vibrational analysis, and no imaginary frequencies were found. Four-step reactions listed as follows were calculated to investigate the electrocatalytic oxygen evolution reaction.60

below: NF (1 × 3 cm2) cut from bulk material was successively cleaned ultrasonically in diluted 3 M HCl and a mixed solution of acetone and EtOH at an equal volume ratio for another 30 min. Then the treated NF was washed with DI water and EtOH at least three times and transferred to a vacuum oven and dried at 70 °C for 1 h before use. Iron nitrate nonahydrate (Fe(NO3)3·9H2O, 0.2424 g, 0.05 mol) and tannic acid (C76H52O46, 0.50 g) were dissolved into a mixed solution (12 mL) containing of N,N-dimethylacetamide (DMF), ethanol, and deionized water in a volume ratio of 14:1:1. The above mixture was stirred for 1 h and then transferred into a Teflon autoclave reactor (25 mL) together with the cleaned NF for a solvothermal treatment at 125 °C. After 12 h, indigo (FeNi)−Tan/ NF was obtained, and the bottom precipitates were collected by centrifugation under 16000 rpm for 3 min. Then the (FeNi)−Tan/ NF and (FeNi)−Tan precipitates were washed three times with deionized H2O, ethanol, and N,N-dimethylacetamide (DMF) and finally dried at 80 °C, obtaining (FeNi)−Tan coordination crystals. By comparing the weight increase of (FeNi)−Tan/NF samples and bare NF, the loading of the as-prepared (FeNi)−Tan samples on the bare NF was obtained as approximately 0.0378 g cm−2. Synthesis of Ni−Tan/NF Electrode. Ni−Tan/NF was prepared according to the procedure of the (FeNi)−Tan/NF synthesis described above, except that Fe(NO3)3·9H2O (0.2424 g) was replaced by Ni(NO3)2·6H2O (0.1744 g). The as-prepared Ni−Tan samples had a loading of approximately 0.0287 g cm−2 on the bare NF. Synthesis of the Tan/NF Electrode. The Tan/NF electrode was obtained without any involvement of exogenous metal sources. The as-prepared Tan/NF samples had a catalyst loading of approximately 0.0252 g cm−2 on bare NF. Instrumentation. Scanning electron microscopy (SEM) images were obtained using a Hitachi S-2600N environmental scanning electron microscope. High-resolution transmission electron microscopy (HRTEM) images, energy dispersive spectroscopy (EDS) analytical data, and selected-area electron diffraction (SAED) patterns were obtained with a JEM-1200EX at a higher voltage of 200 kV. Xray photoelectron spectroscopy (XPS) data were collected from an ESCALAB 250 instrument with monochromatic Al Kα X-rays as the radiation source. Fourier transformed infrared (FT-IR) spectra were obtained using TENSOR-27 (Bruker) to collect the surface compositional data. Raman spectra obtained on a Raman spectrometer (RENISHAW inVia, 514 nm excitation laser). The powder X-ray diffraction (XRD) patterns of all prepared materials were detected using a Bruker D8 Advance power diffract meter with Cu Kα radiation (λ = 1.5418 Å). Electrochemical Measurements. Electrochemical characterizations were carried out using a traditional three-electrode system on a CHI 600E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd.) in 1 M KOH alkaline media. (FeNi)−Tan/NF directly served as the working electrode with the immersed area of (FeNi)−Tan/NF in the media of 1 × 1 cm2, and a platinum wire and Ag/AgCl (KCl saturated) were utilized as the counter electrode and the reference electrode, respectively. A 98% iR compensation level was applied to minimize the resistance during the experiments. All of the potentials in all of the electrochemical processes versus Ag/AgCl were calibrated to the reversible hydrogen electrode (RHE) based on (ERHE = EAg/AgCl + 0.197 V + 0.059 pH), and the overpotential (η) was calculated according to the following formula: η = E(RHE) − 1.23 V. All electrochemical experiments were performed in O2-saturated electrolyte solution at room temperature. Prior to the evaluation of oxygen evolution performance, cyclic voltammetry (CV) was adopted to activate the working electrode for 20 cycles to stabilize the current at the potential window ranging from 0 to 1 V (vs Ag/AgCl) at a scan rate 50 mV s−1. Linear sweep voltammetry (LSV) measurements were recorded at the same potential window as CV under a scan speed of 1 mV s−1. During the ascending stability response tests, chronoamperometry was performed at 0.228, 0.320, and 0.402 V (vs Ag/AgCl) to obtain the corresponding current densities, and the 1000 cycle CV

H 2O(l) + * = *OH + H+ + e−

(1)

*OH = *O + H+ + e−

(2)

*O + H 2O(l) = *OOH + H+ + e−

(3)

*OOH = * + O2 (g) + H+ + e−

(4)

In the four reactions, * denotes the active catalyst that can be constructed by removing a water ligand of a metal atom. *OH, *O, or *OOH can be constructed by adding small species to the metal atom. The four reaction free energy changes were used to determine the rate-determining step. The free energies (including zero-point energies) were calculated by geometric optimization and vibrational analysis. However, the calculation of liquid water was quite difficult. Instead, its free energy was obtained through the liquid−gas equilibrium of water at 298.15 K and 0.035 MPa. With calculated values for H2(g), the free energy of O2(g) was obtained through the experimental relation O2(g) + 2H2(g) = 2H2O(l) + 4 × 1.229 eV to reduce numerical errors. The free energy of H+ + e− was also obtained through the equilibrium 1/2H2(g) = H+ + e− at the standard condition. The value was shifted by −eU + kBT ln aH+ when the electrode potential and pH were not 0. Because the two factors affect the four reactions equally, the free energies at the standard condition describe the trend of the electrocatalytic oxygen evolution reaction.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b02994. Molecular structure, XRD pattern, digital photographs, SEM images, N2 adsorption/desorption isotherm cruves, EDS results, crystalline frameworks structure, XPS survey spectra, TG-DSG curves, dependent FL response curve, electrochemical analysis, molecular complex models, and comparison of electrocatalysts (PDF) 20785


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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 1068903047. Fax: +86 1068903047. E-mail: [email protected]. ORCID

Yuqing Lin: 0000-0003-1501-5005 Author Contributions †

These two authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation (21575090), High-level Teachers in Beijing Municipal Universities in the Period of 13th Fiveyear Plan (CIT&TCD20190330), Scientific Research Project of Beijing Educational Committee (KM201810028008), and Youth Innovative Research Team of Capital Normal University and Capacity Building for Sci-Tech Innovation-Fundamental Scientific Research Funds (19530050179, 025185305000/ 195).

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FeNi-Based Coordination Crystal Directly Serving as Efficient Oxygen

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