The Synergetic Effect of Ni and Fe Bi-metal Single Atom Catalysts on Graphene for Highly Efficient Oxygen Evolution Reaction

Oxygen evolution reaction (OER) is key for electrochemical water splitting. Catalysts minimized in single-atom level (SACs) anchored on two-dimensional substances are highly active for various electrocatalytic reactions, but so far represent the limited performance for OER. Herein we report a general strategy for the fabrication of bi-metal Ni and Fe SACs loaded on graphene to significantly boost the OER activity. Notably, this unique structure of single Ni and Fe atoms co-existed on graphene has a strong synergetic effect, which can substantially elevate the charge transfer of catalysts. As a result, the obtained bi-metal SACs with the appropriate ratio of 4 for single Ni and Fe atoms show a small overpotential of 247 mV at 10 mA·cm⁻² of OER current density in KOH electrolyte, outperforming individual-metal Ni (329 mV) or Fe (384 mV) SACs. Furthermore, such catalyst exhibits excellent stability without significant decay with increasing time at high current of 20 mA·cm⁻² continuously for 48 h. This work provides a new avenue for regulating the properties of transition metal SACs on graphene for water electrolysis.

Introduction

Water splitting utilized by electric energy is an efficient way for the store of discontinuous and sustainable energies (Fan et al., 2016; Seitz et al., 2016; Zhang B. et al., 2016). Among the reactions in electrochemical water splitting, oxygen evolution reaction (OER) is a kinetically slow process, which is a big obstacle in electrochemical water splitting (Long et al., 2014; Zhang Y. et al., 2016; Suen et al., 2017). Therefore, rational design of highly active and durable OER catalysts is the key theme in water splitting (Gong et al., 2013; Ling et al., 2016; Zhuang et al., 2017). Single atom structure represents the lowest size limit on synthesizing catalysts (Qiao et al., 2011), which has been regarded as the frontier of catalysis (Liu et al., 2016). Among them, single atom catalysts (SACs) have shown great potential for hydrogen evolution reaction and oxygen reduction reaction (Fei et al., 2015; Qiu et al., 2015; Zhang L. et al., 2017). Although a sequence of OER sensitive metals (e.g. Ni, Co) have been synthesized in atoms form (Fei et al., 2015; Chen et al., 2017; Zhao et al., 2017), their OER activities are still unsatisfactory.

In our previous work (Xu et al., 2020), we have changed the coordination elements of single nickel atoms from nitrogen to oxygen to activate the OER activity. This restructured single nickel atoms for OER shows a small overpotential of 330 mV level at current density of 10 mA·cm⁻², which is even equivalent to the benchmark OER catalysts of NiFe layer double hydroxide (LDH) (Han et al., 2016; Lu et al., 2017). However, with unremitting efforts on developing multiple metal compounds, the overpotential at 10 mA·cm⁻² has reached up to 200 mV level (Qian et al., 2017; Yao et al., 2018; Zhang P. et al., 2018). SACs are still far from the best catalysts available for OER. At present, the best OER catalysts include metal oxides (Yin et al., 2010; Zhou et al., 2018), hydroxides (Balogun et al., 2016; Liu et al., 2017), perovskites (Zhang Y. et al., 2017), phosphides etc. (Zhao et al., 2016; Hu et al., 2017). It is worth noting that the composition combination of different metal elements could greatly improve the OER activity of catalysts. As a well-known example, the OER activity of Ni can be promoted by the synergistic effect of Fe in NiFe LDH, which outperforms those of the corresponding single-Ni or Fe oxides (Trotochaud et al., 2012; Gong et al., 2013). It is indicated that the interaction between heterogeneous elements can significantly change the catalytic activity of materials.

To improve the OER activity of SACs, we considered whether the synergetic effect of dual-metal SACs could be applied on OER. In this work, we focus on realizing the synergetic effect between different single atoms. We synthesized dual-metal SACs of Ni, Fe elements on graphene simultaneously (denoted as Ni, Fe-O-G SACs) by ions adsorption and graphene oxide (GO) templated method. The coupling effect of Ni, Fe single atoms could efficiently boost the activity and charge transfer of Ni, Fe-O-G SACs by tuning the Ni/Fe ratio. Impressively, Ni, Fe-O-G SACs for OER achieved a low overpotential of 247 mV at current density of 10 mA·cm⁻² when the ratio of Ni and Fe was 4. It was significantly lower than those of solely single Ni atoms on graphene (329 mV) and solely single Fe atoms on graphene (384 mV).

Experimental Section

Preparation of Ni, Fe-O-G SACs

Firstly, 100 mL GO aqueous solution (2 mg/mL) was stirred for 0.5 h with a controllable amount of Ni(NO₃)₂·6H₂O and Fe(NO₃)₃·9H₂O. After washing three times with assistance of centrifugation, the metal ions not adsorbed on GO were fully removed. Then, the obtained sample was further dispersed in 30 mL deionized water, and freeze-dried within 2 days to remove residual water. Afterwards, the dried sample was heated at 95°C for 24 in hydrazine vapor to form Ni, Fe-O-G SACs. Note that the Ni/Fe ratio in Ni, Fe-O-G SACs was easily controlled by changing the amount of Ni²⁺ and Fe³⁺ salts and the details was listed in Table S1.

Characterizations

The X-ray diffraction (XRD, BRUKER, D8 ADVANCE), scanning electron microscopy (SEM, FEI Nova Nano SEM450), transmission electron microscopy (TEM, JEOL 2100F) and scanning transmission electron microscopy (STEM, JEOL, ARM-200F) are used to characterize the morphology, crystal structure and atomic dispersion of samples. The X-ray photoelectron spectroscopic (XPS) spectra were detected by Thermo ESCALAB-250 spectrometer with a monochromatic Al Kα radiation source (1486.6 eV). The binding energies determined by XPS were corrected with reference of adventitious carbon peak (284.6 eV) for each sample. The X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra at Ni, Fe K-edges were collected by the beamline of BL14W1 at Shanghai Synchrotron Radiation facility (SSRF). Then, we used Athena software to analyze those spectra.

Electrochemical Measurements

To measure the electrode for OER test, firstly, carbon cloth cut into the square (1 cm²) was soaked by ethanol and water. Secondly, 10 mg catalyst was added into 2.0 mL of ethanol and 0.01 mL of Nafion solution (5.0 wt%). Thirdly, the catalyst was dispersed by sonication to form an ink. Finally, 0.1 mL ink was dropped into the washed carbon cloth and dried for 0.5 h in air. The loading amount of catalysts was 0.5 mg cm⁻².

The electrochemical workstation (Zennium_Pro, Zahner, Germany) was used to detect the OER performances of those catalysts with the counter electrode of Pt square sheet (4 cm²) and a reference electrode of Ag/AgCl. The potentials were calibrated by the reversible hydrogen electrode (RHE), by the following equation:

$\begin{array}{l}{E}_{\text{RHE}}(\text{V})=E_{\text{Ag}/\text{AgCl}}+0.197\text{ V}+0.059\times \text{p}\text{H}\\ \text{Overpotential}=E_{\text{RHE}}-1.23(\text{V}).\end{array}$

Before OER tests, the electrodes attached with catalysts were pre-scanned by cyclic voltammetric to stable the OER activity. The scan rate of linear scan voltammetry (LSV) was 2 mV s⁻¹. The iR-correction was 80%. Electrochemical impedance spectroscopy (EIS) was tested by potentiostatic mode at 1.26 V vs. RHE with scanning frequency from 1.0 MHz to 0.05 Hz.

Results and Discussions

As shown in Figure 1a, to synthesize the graphene (G) sheets decorated with single atoms of Ni and Fe, firstly, moderate amount of Ni²⁺ and Fe³⁺ salts were dissolved in GO aqueous solution. Since GO are rich in hydroxyl and carboxyl groups, Ni²⁺ and Fe³⁺ ions were readily adsorbed on the surface of GO sheets via the functional oxygenated groups, e.g., hydroxyls and carboxyls (Gao et al., 2017). Secondly, the Ni²⁺ and Fe³⁺ ions not adsorbed on GO sheets were removed by washing process, and the washed GO solution were frozen within 2 days. Finally, hydrazine vapor was used for the reduction of GO into graphene at 95°C for 24 h to obtain Ni, Fe-O-G SACs (Sun et al., 2013). The Ni/Fe ratio on graphene could be readily controlled by changing the amount of Ni²⁺ and Fe³⁺ salts (Table S1). XRD pattern of the as-obtained Ni, Fe-O-G SACs with Ni/Fe ratio of 4 (abbreviated as Ni₄Fe₁-O-G SACs) presented two broad diffraction peaks at 23.2° and 43.6°, derived from the graphene. Besides this, no peaks of Ni metals, metal oxides and carbides were observed (Figure S1a) (Liu et al., 2018). Raman spectrum confirmed the existence of oxygen and defects on graphene in this sample (Iwase et al., 2011), indicative of the incorporation of oxygen into graphene (Figure S1b). SEM image showed the soft nanosheets morphology of Ni₄Fe₁-O-G SACs (Figure 1b). Elemental mapping images revealed the even dispersion of Ni, Fe and O elements on the sheets of Ni₄Fe₁-O-G SACs (Figures 1c–e). TEM image revealed the absence of aggregates on the nanosheet of Ni₄Fe₁-O-G SACs (Figure 2a), which was in line with the HRTEM result, depicting no lattice fringes (Figure 2b). Aberration-corrected STEM (Figures 2c,d) was allowed for the direct observation of atomic dispersion of Fe and Ni. In the STEM images of Ni₄Fe₁-O-G SACs, the metal atoms appeared as light points were individually located on graphene (marked by red circles). There was no sub-nanometer clusters presented. Considering that C and O elements had a smaller atomic number than Fe and Ni, the atomic level spots should be Fe and Ni atoms, manifesting as single atoms away from each other. Thus, Ni and Fe were uniformly dispersed on the surface of graphene in single atomic form. The Ni and Fe 2p XPS spectra of Ni₄Fe₁-O-G SACs were presented in Figure S2. The Ni 2p_3/2, 2p_1/2 and Fe 2p_3/2, 2p_1/2 peaks of Ni₄Fe₁-O-G SACs were obtained at 873.9 eV, 856.3 eV and 724.1 eV, 712.2 eV respectively, assigned to the Ni (II) and Fe(III) species (Zhang Y. et al., 2011; Yan et al., 2012).

FIGURE 1

Figure 1. (a) Schematic synthesis of Ni, Fe-O-G SACs. (b) SEM image of Ni₄Fe₁-O-G SACs. (c–e) elemental mapping images of Ni, Fe, O elements through the sheet of Ni₄Fe₁-O-G SACs.

FIGURE 2

Figure 2. (a) TEM image of Ni₄Fe₁-O-G SACs. (b) HRTEM image of Ni₄Fe₁-O-G SACs. (c,d) Magnified HAADF-STEM images of Ni₄Fe₁-O-G SACs. The single Ni, Fe atoms were marked with red circles.

Further, the spectra of XANES and EXAFS were used to verify the possible bonding forms between nickel, iron and the light elements in Ni₄Fe₁-O-G SACs, in comparison with NiO, Fe₂O₃ and Ni, Fe foil. As seen from the curves of XANES (Figures 3A,B), the E₀ values of Ni, Fe K-edges followed the order of Ni₄Fe₁-O-G SACs = NiO, Fe₂O₃ > Ni, Fe foils. Since higher E₀ corresponds to higher oxidation state, it is suggested that the valence states of Fe and Ni species in Ni₄Fe₁-O-G SACs are +2 and +3, respectively, identical to the XPS results. Further, the coordination environment of Ni and Fe in Ni₄Fe₁-O-G SACs was elucidated by the Fourier-transformed EXAFS (FT-EXAFS, Figures 3C,D), in which showed only notable peaks of Ni-O, Fe-O coordination, similar to the peaks of NiO, Fe₂O₃ at ~1.6 Å (Yan et al., 2018). Moreover, no appearance of Ni-Ni and Fe-Fe coordination peaks at 2.6 Å were detected. Consequently, FT-EXAFS results evidenced the atomic level dispersion of Ni-O-graphene and Fe-O-graphene structures in Ni₄Fe₁-O-G SACs.

FIGURE 3

Figure 3. (A) Ni K-edge XANES of Ni₄Fe₁-O-G SACs, Ni foil and NiO. (B) Fe K-edge XANES of Ni₄Fe₁-O-G SACs, Fe foil and Fe₂O₃. (C) Ni K-edge FT-EXAFS spectra of Ni₄Fe₁-O-G SACs with references of Ni foil and NiO. (D) Fe K-edge FT-EXAFS spectra of Ni₄Fe₁-O-G SACs with references of Fe foil and Fe₂O₃.

To show the excellent catalytic activity of Ni, Fe-O-G SACs and highlight the activity of Ni, Fe-O-G SACs affected by the coupling effect of Ni and Fe, we also synthesized the Ni, Fe-O-G SACs with different Ni and Fe ratios, ranging from 0, 0.14, 0.2, 0.5, to 1 of Fe/Ni+Fe ratio (Figures S3, S4; Table S2). Remarkably, the Ni₄ Fe₁-O-G SACs possessed the best OER activity among these samples (Figure 4A; Figure S5). To generate a current density of 10 mA·cm⁻², Ni₄Fe₁-O-G SACs only need an overpotential of 247 mV, which was much smaller than those of Ni-O-G SACs (329 mV) and Fe-O-G SACs (384 mV) (Figure 4B), and the Ni based reported OER catalysts, e.g., NiO film (540 mV) (Nardi et al., 2015), Ni/N doped graphene (397 mV) (Chen et al., 2013), Ni₂P/NiO_x (286 mV) (Stern et al., 2015), Ni₁₁(HPO₃)8(OH)₆ (274 mV) (Menezes et al., 2018), Ni₃Se₂ (290 mV), Ni SAC/defected graphene (270 mV) (Zhang L. et al., 2018).

FIGURE 4

Figure 4. (A) The OER polarization curves of Ni, Fe-O-G SACs with different Ni, Fe ratio (Ni, Ni₆Fe₁, Ni₄Fe₁, Ni₁Fe₁, Fe) obtained at scan rate of 2 mV s⁻¹ and 80% iR correction. (B) The corresponding required overpotential of Ni, Fe-O-G SACs for 10 mA cm⁻² current density. (C) Chronopotentiometric curve of Ni₄Fe₁-O-G SACs maintained at 20 mA cm⁻² current density, in 1 M KOH, with 80% iR correction. (D) The TOF value at 300 mV overpotential of Ni, Fe-O-G SACs with different Ni, Fe ratio (Ni, Ni₆Fe₁, Ni₄Fe₁, Ni₁Fe₁, Fe), obtained at scan rate of 2 mV s⁻¹ and 80% iR correction. (E) Nyquist plots of Ni, Fe-O-G SACs with different Ni, Fe ratio (Ni, Ni₆Fe₁, Ni₄Fe₁, Ni₁Fe₁, Fe). (F) The corresponding charge resistances.

To demonstrate the long-term stability of Ni₄Fe₁-O-G SACs, continuous water oxidation at a current density of 20 mA·cm⁻² was examined for 48 h (Figure 4C). Obviously, the overpotential showed little change with increasing time, showing excellent durability of Ni₄Fe₁-O-G SACs for OER. Further, Figure 4B showed that the overpotential of Ni₄Fe₁-O-G SACs was also lower than those of Ni₆Fe₁-O-G SACs (286 mV) and Ni₁Fe₁-O-G SACs (275 mV), when the OER current density was 10 mA·cm⁻². It is revealed that the ratio of Ni/Fe in Ni, Fe-O-G SACs has a great influence on the OER activity of the catalysts. Since the number of Ni+Fe atoms was not equal on different Ni, Fe-O-G SACs, the simple comparison of OER currents did not objectively reflect with the activity of single atoms. Therefore, we used the turnover frequencies (TOFs) of oxygen production at overpotential of 300 mV (with 80% iR correction) to quantify the intrinsic activity of Ni and Fe single atoms in different Ni, Fe-O-G SACs. It is assumed that all the Ni and Fe atoms in Ni, Fe-O-G SACs were totally taking part in OER. The TOF values calculated from Ni-O-G SACs, Ni₆Fe₁-O-G SACs, Ni₄Fe₁-O-G SACs, Ni₁Fe₁-O-G SACs and Fe-O-G SACs were 0.18, 0.39, 1.35, 0.49, and 0.06 s⁻¹, respectively (Figure 4D), confirming that the single atoms in Ni₄Fe₁-O-G SACs own the highest OER activity among all the Ni, Fe-O-G SACs. On the other hand, it has been reported that the Fe atoms are not very active for OER (Li et al., 2017). Therefore, we further calculated the TOF of those catalysts assuming only Ni atoms active for OER. In this case, the TOF values calculated from Ni-O-G SACs, Ni₆Fe₁-O-G SACs, Ni₄Fe₁-O-G SACs and Ni₁Fe₁-O-G SACs were 0.18, 0.45, 1.68, and 0.98 s⁻¹, respectively (Figure 4D), which further confirmed the higher OER activity of Ni₄Fe₁-O-G SACs mainly from Ni atoms. Previously, Nocera et al. have reported that the Fe³⁺ could promote the formation of high valence Ni atoms during OER process, which possibly generated more active Ni sites to acquire better OER activity through the introduction of the increased amount of Fe³⁺ (Li et al., 2017). However, our experimental results showed that the OER activity was not linearly improved by increasing the Fe ratio in Ni, Fe-O-G SACs, indicating that other factors also affected the OER activity. We then tested the electrical conductivity of Ni, Fe-O-G SACs, derived from electrochemical impedance spectroscopy (EIS). As shown in Figure 4E, the semicircle curves of EIS were expanded with increasing Fe ratio in Ni, Fe-O-G SACs. Figure 4F showed that the charge transfer resistance (R) of Ni-O-G SACs, Ni₆Fe₁-O-G SACs, Ni₄Fe₁-O-G SACs, and Ni₁Fe₁-O-G SACs, Fe-O-G SACs was 2.9, 4.0, 6.2, 30.1, and 129.4 Ω·cm⁻², respectively. This result suggested the higher iron ratio in Ni, Fe-O-G SACs increased the R to block the charge transfer between active sites and electrode, thus limiting the OER current density.

In summary, we reported a versatile strategy of 2D GO template-based bi-metal ion adsorption to create Ni, Fe-O-G SACs with strong coupled nickel-oxygen and iron-oxygen bonding as highly active and durable OER electrocatalysts. The multiple characterizations revealed that the bi-metals of Ni and Fe elements were individually anchored on the oxygen-containing sites of graphene in single atomic form. Through the full optimization of the Fe, Ni ratio, it was demonstrated that Ni₄Fe₁-O-G SACs for OER showed the low overpotential of only 247 mV at current density of 10 mA·cm⁻², 48 h long-life durability without significant degradation, 1.35 s⁻¹ of oxygen production TOF at overpotential of 300 mV. Therefore, this strong coupling effect of bi-metal SACs on graphene and other 2D materials will open many intriguing opportunities for designing complex SACs as efficient and robust OER catalysts.

Data Availability Statement

The datasets generated for this study are available on request to the corresponding author.

Author Contributions

YL and Z-SW conceived this project. YX, WZ, PL, and YW performed the catalyst preparation and characterizations. YX, YL, and Z-SW analyzed the data and wrote the manuscript. All the authors discussed the results and commented on the manuscript.

Funding

This work was supported by the National Key R&D Program of China (Grants 2016YFB0100100, and 2016YFA0200200), National Nature Science Foundation of China (Grants 51702078, 51872283 and 51572259), Liao Ning Revitalization Talents Program (Grant XLYC1807153), Natural Science Foundation of Liaoning Province (Grant 20180510038), DICP (DICP ZZBS201708, DICP ZZBS201802), DICP&QIBEBT (Grant DICP&QIBEBT UN201702), Dalian National Laboratory For Clean Energy (DNL), CAS, and DNL Cooperation Fund, CAS (DNL180310, DNL180308). We thank SSRF for the beamline (BL14W1) and assistance with the EXAFS test. Thanks for the TEM technical supports provided by the Microanalysis Center, College of Physics Science and Technology, Hebei University.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmats.2019.00271/full#supplementary-material

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