Fe)4O4 Cubane-Containing Nanorings Fabricated by

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(Co/Fe)4O4 Cubane-Containing Nanorings Fabricated by Phosphorylating Cobalt Ferrite for Highly Efficient Oxygen Evolution Reaction Jiachen Li, Qingwen Zhou, Chenglin Zhong, Shengwen Li, Zihan Shen, Jun Pu, Jinyun Liu, Yong-Ning Zhou, Huigang Zhang, and Haixia Ma ACS Catal., Just Accepted Manuscript • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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(Co/Fe)4O4 Cubane-Containing Nanorings Fabricated by Phosphorylating Cobalt Ferrite for Highly Efficient Oxygen Evolution Reaction Jiachen Li,1, 2 Qingwen Zhou,2 Chenglin Zhong,2 Shengwen Li,2 Zihan Shen,2 Jun Pu,2 Jinyun Liu3, Yongning Zhou,4 Huigang Zhang,2* and Haixia Ma1* 1. School of Chemical Engineering, Northwest University, Xi’an 710069, China 2. National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, Collaborative Innovation Center of Advanced Microstructures, Institute of Materials Engineering, Nanjing University, Nanjing 210093, China 3. Key Laboratory of Functional Molecular Solids (Ministry of Education), College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, China 4. Department of Materials Science, Fudan University, Shanghai 200433, China *Corresponding author: H. Zhang, [email protected], H. Ma, [email protected] ABSTRACT: The sluggish kinetics of the oxygen evolution reaction (OER) limits the practical applications of many important energy conversion systems such as electrolysis of water, metal-air batteries, solar fuel, and so on. Here, we report a highly efficient OER electrocatalyst that features (Co/Fe)4O4 cubane motifs in an amorphous nanoring structure. The Co/Fe co-dopants in the cubane motifs can optimize the intermediates adsorption and significantly lower the overpotentials of the OER. In addition to finely tuning the adsorption of intermediates, the amorphous nanoring structures 1 ACS Paragon Plus Environment

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provide rapid ion channels and high surface area for the OER. Phosphorylation also increases the wettability of electrocatalysts. As a result, the nanorings exhibit highly catalytic activities for the OER with a low overpotential of 300 mV at 10 mA cm–2 and a small Tafel slope of 36 mV dec–1 in an alkaline solution. The long-term durability is demonstrated for at least 20 h at 10 mA cm–2. In brief, we demonstrate a tuning strategy of Co/Fe co-doped cubane motifs in amorphous phosphates nanorings and thus open up an avenue of structural tunability for advanced catalyst design. KEYWORDS: nanoring, amorphous structure, phosphorylation, catalyst, oxygen evolution reaction 1. INTRODUCTION Hydrogen is an important energy source carrier in the blueprint of hydrogen economy.1 Electrolysis of water provides a renewable and clean route to produce hydrogen gas. However, the oxygen evolution reaction (OER) is the bottleneck step of electrochemical water splitting due to its sluggish kinetics. Highly efficient electrocatalysts are urgently demanded to lower the energy barrier of the OER. The benchmark catalysts such as IrO2 and RuO2 are able to demonstrate the state-of-art activity of about 360 mV at a current density of 10 mA cm–2.2,3 However, their scarcity and high cost limit the practical applications of these precious catalysts. The development of cost-effective and high-efficiency catalysts is the key task for the OER.4 Nonprecious catalysts such as transition metal phosphide,5 sulphide,6 nitride,7 and boride8 have been reported promising activities. However, they may not survive at high current density.9 A longterm durable catalyst may prefer oxides or phosphates in oxidative environments.10 Nocera et al. introduced phosphates in a Co-based OER catalyst to achieve a low overpotential of 0.41 V at 1 mA cm–2.11 Phosphates with good wettability can enhance the catalytic activity because of the improved adsorption of intermediate species on the surface of catalysts.12 Phosphate groups can also mediate the proton-coupled electron transfer as proton acceptors to facilitate the oxidation of transition metal– 2

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ACS Catalysis

OH2 centers.13 As compared to the high-symmetry geometry of oxides, various complex structures of phosphate groups, which present in less crystallized phosphates, have flexible coordination environments, providing new possibilities to tune the electrocatalytic properties.14 Previous reports showed that iron dopants could increase the conductivity and catalytic activity of nickel oxyhydroxide via a Ni-Fe partial charge transfer activation.15–17 Iron-doped nickel phosphates also demonstrate a synergistic effect to enhance the electrocatalytic activity.16 Thus, phosphorylation and transition metal doping have been demonstrated to be effective approaches to the performance improvement of nonprecious catalysts. Electrocatalysis depends substantially on the interactions between the surface of catalyst and intermediate adsorbates. Due to broken symmetry and under-coordinated atoms, catalyst surfaces present various dandling bonds, defects, and vacancies, which are also abundant in amorphous structures. Although there may be an ongoing controversy on the mechanism of structural disorder on surfaces to achieve high catalytic activity,18 an increasing amount of reports have demonstrated that amorphous materials provide more reactive sites because of under-coordinated atoms and facilitated adsorption of intermediates.19,20 Amorphous materials can also enable mechanical flexibility and increase ion conductivities.18,20 In addition, some important studies reported that in amorphous metal phosphates, (M4O4) cubane-type “defects” were mainly responsible for the high catalytic activity.21,22 Cubane motifs were also revealed as the catalytic centers in nano-sized spinels Co3O4, Mn3O4, and CoOx:PO4 because these cubical cores resemble the active centers in natural photosynthetic enzymes that catalyze water oxidation.23 However, the cubane motifs in inorganic catalysts are hardly reported competitive activity as compared to the natural enzymes because of lacking appropriate approaches to tune the microstructures of active centers. Here, we developed a (Co/Fe)4O4 cubane-containing catalyst by bi-metal doping and 3 ACS Paragon Plus Environment

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phosphorylation. Co/Fe co-doped cubane motifs are able to optimize the intermediates adsorption and lower the energy barriers for the OER. To achieve this, we first anchored cobalt ferrite nanoparticles on reduced graphene oxide (rGO) via a hydrothermal route. As shown in Figure 1a, a following phosphorylation process by phosphine converted crystalline cobalt ferrite to amorphous nanorings due to the Kirkendall effect (Figure 1b). The phosphorylated cobalt ferrite is labeled CF:Pi. Differing from the cubical coordination environment of cobalt ferrite (Figure 1c), (Co/Fe)4O4 cubane motifs were coordinated with phosphate groups in an amorphous nanoring structure (Figure 1d). Phosphate groups significantly improve the wettability of CF:Pi and enhance the adsorption of intermediate species on catalysts (Figure 1e). The novel structure of CF:Pi demonstrates only 300 mV overpotentials at 10 mA cm–2 for the OER, indicating a superior catalytic activity. Generally, this study provides an effective strategy to construct high-efficiency OER catalyst by bi-metal co-doping active cubane motifs in amorphous phosphates.

Figure 1. Fabrication and design of cubane-containing phosphate catalysts: Schematic illustration of (a) the fabrication procedures of rGO@CF:Pi nanorings and (b) the formation of CF:Pi nanorings. (c) Cubical coordination environment of cobalt ferrite. (d) (Co/Fe)4O4 cubane motifs in an amorphous CF:Pi. (e) Contact angle measurements of water on rGO@CoFe2O4 and rGO@CF:Pi.

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2. RESULTS AND DISCUSSION 2.1 Structural Characterizations of Synthesized Catalysts Figure 2a presents the scanning electron microscopy (SEM) image of the as-obtained graphite oxide (GO), which shows thin sheet morphology with wrinkles. The GO sheets were used to anchor oxides nanoparticles because their rich functional groups on the surface can facilitate the nucleation of oxides. After hydrothermal treatment within a Co2+ and Fe3+ containing solution, about 10~40 nm particles were deposited on the GO surface as indicated by the transmission electron microscope (TEM) image in Figure 2b. The high-resolution TEM (HRTEM) image (Figure 2c) shows a lattice fringe of 0.48 nm, which is in agreement with the (111) plane of CoFe2O4. The rotation integration profile of the electron diffraction pattern (Figure 2d) and the X-ray diffraction (XRD) pattern (Figure 2l) further confirm that the anchored nanoparticles are spinel CoFe2O4. The typical GO peak at 10º (Figure S1 in the supporting information (SI)) was absent whereas the rGO peak appears around 24º, indicating that the GO was reduced to rGO, which is in agreement with previous study.24 The resultant composite is labeled as rGO@CoFe2O4. Figure 2e shows the energy dispersive X-ray spectroscopy (EDX) element mapping images of rGO@CoFe2O4. The uniform elemental distributions imply the well-dispersed CoFe2O4 nanoparticles on rGO (Figure S2). After heat treatment in phosphine, CoFe2O4 was phosphorylated. Figure 2l shows that phosphorylation leads to an amorphous phase (CF:Pi). By carefully comparing the SEM images, one can observe ultra-fine nanoparticles of CF:Pi on the rGO sheet in Figure 2f. The EDX element mapping images (Figure 2g) indicate that CF:Pi nanoparticles are uniformly distributed without aggregation. Most importantly, the TEM image in Figure 2h reveals that CF:Pi has actually a nanoring morphology. The external diameter of nanorings ranges from 10 to 50 nm. The formation of CF:Pi nanorings can be ascribed to the Kirkendall effect. During the heat treatment, phosphine released by the pyrolysis of NaH2PO2 reacts with CoFe2O4 5 ACS Paragon Plus Environment

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nanoparticles. The phosphorylated products on the surface of CoFe2O4 block the inward diffusion of phosphine. The internal cobalt and iron ions diffuse outward and leave voids in the center.25,26 The high-magnification TEM image (Figure 2j) and the selected area electron diffraction pattern (SAED) (Figure 2k) further confirm that the resulting CF:Pi is amorphous. The atomic ratio of Fe/Co is around 2:1 (see Table S1 for the mass loading of each element in the catalysts).

Figure 2. Material and structural characterizations: (a) SEM image of GO. (b) TEM, (c) HRTEM, (d) SAED, and (e) EDX elemental mapping images of rGO@CoFe2O4. (f) SEM and (g) EDX elemental mapping images of rGO@CF:Pi. (h–j) TEM images at different magnifications. (k) SAED pattern of rGO@CF:Pi nanorings. (l) XRD patterns of rGO@CoFe2O4 and rGO@CF:Pi. To further analyze the porous structure of nanorings, we first measured the surface areas of 6


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rGO@CF:Pi and rGO@CoFe2O4 by a Brunauer-Emmett-Teller (BET) method (Figure S3). The rGO@CF:Pi nanorings have a specific surface area of 87.6 m2 g–1, which is about double that of rGO@CoFe2O4 (38.6 m2 g–1). The Barrett-Joyner-Halenda (BJH) analysis indicates that nanorings have the average pore size of 9.0 nm. The porous nanoring structure increases the surface-to-volume ratio to expose more active sites and offer more contact with electrolyte, which is of importance during water oxidation.27,28 The X-ray photoelectron spectroscopy (XPS) spectra were measured to study the chemical states of elements in rGO@CF:Pi and rGO@CoFe2O4. Figure 3a shows that the Fe 2p3/2 peak of rGO@CoFe2O4 is located at 710.9 and 713.4 eV, which is usually attributed to Fe3+.29 The Fe 2p3/2 peak of rGO@CF:Pi could be fitted into two components of Fe2+ and Fe3+ at 710.7 and 712.6 eV, respectively.30 The formation of Fe2+ is originated from the reduction of phosphine. The peak of Fe3+ in rGO@CF:Pi shifts to a higher position than that in rGO@CoFe2O4 because of the inductive effect of phosphate groups.13 The Co 2p signals in Figure 3b show two main peaks around 781.2 and 796.7 eV due to the spin orbit splitting. The peaks can be attributed to Co2+.31 Because of the inductive effect, the Co 2p peaks of rGO@CF:Pi are also raised to higher binding energies as compared to those in rGO@CoFe2O4. The significant P 2p signal at 133.2 eV in rGO@CF:Pi further confirms that the phosphorylation process leads to the formation of the P−O bonds which contrast the absence of the P signals in rGO@CoFe2O4 (Figure 3c).32 The O 1s peak consists of three components at 530.2, 531.4, and 532.5 eV, which can be assigned to the M-O, phosphate species, and M-OH species, respectively.33–36 The XPS analysis shows that the phosphorylation treatment introduces phosphates groups into cobalt ferrite and partly raises the chemical states of Fe3+ and Co2+.

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Figure 3. Spectroscopic characterization: (a–c) XPS spectra of rGO@CF:Pi and rGO@CoFe2O4: (a) Fe 2p, (b) Co 2p, and (c) P 2p. (d) O 1s XPS signals of rGO@CF:Pi. (e) FTIR spectra of the CoFe2O4, rGO@CoFe2O4, rGO@CF:Pi, and CF:Pi. (f) Raman spectra of the rGO@CoFe2O4 and rGO@CF:Pi. Figure 3e presents the Fourier transform infrared spectroscopy (FTIR) spectra of rGO@CoFe2O4 and rGO@CF:Pi, as well as CF:Pi and CoFe2O4 for comparison. There appears a broad peak around 1066 cm−1 in rGO@CF:Pi, which extends to overlap with the epoxide C−O−C vibration (~1217 cm−1) of rGO.37 To distinguish these peaks for assignment, we prepared CF:Pi without rGO. The peak at 1066 cm−1 stands out in CF:Pi. By comparing with CoFe2O4 and referring to previous report,38 we attributed the peak around 1066 cm−1 to the P–O stretching vibration of PO43– species. The strong peak at 577 cm−1 in the CoFe2O4 is due to the stretching mode of M–O bond.39 The peak around 1568 cm−1 is attributed to the skeletal vibration of C=C of rGO. The Raman spectra in Figure 3f show two minor peaks around 1081 and 1107 cm−1, which are typically due to the PO4 groups in rGO@CF:Pi.40 The remaining strong peaks at 1332 and 1585 cm−1 result from D- and G-bands of rGO, respectively.41 The FTIR and Raman spectra in Figure 3e and f confirm the formation of PO4 groups during 8


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phosphorylation. 2.2 OER Activity of Synthesized Catalysts To study the catalytic properties, we measured the linear sweep voltammetry (LSV) curves of rGO@CF:Pi, commercial IrO2, and rGO@CoFe2O4 as shown in Figure 4a. At 10 mA cm−2, rGO@CF:Pi exhibits the lowest overpotential of 300 mV. By contrast, rGO@CoFe2O4 and IrO2 need overpotentials of 470 and 360 mV, respectively. Figure 4b shows that the Tafel slope of rGO@CF:Pi is 36 mV dec−1, which is much lower than those of rGO@CoFe2O4 (59 mV dec−1) and IrO2 (54 mV dec−1). The electrochemical impedance spectra (EIS) in Figure 4c were obtained at an overpotential of 370 mV from 100 kHz to 0.1 Hz. The charge transfer resistance (Rct) of rGO@CF:Pi is much lower than those of rGO@CoFe2O4 and IrO2, implying the high catalytic activity. We calculated the catalytic activities and turnover frequencies (TOFs) according to the reported method42 (see details in the SI). Figure 4d shows that the mass activities and TOFs of rGO@CF:Pi increase rapidly with overpotentials as compared to rGO@CoFe2O4 and IrO2. More specifically, at the overpotential of 360 mV, rGO@CF:Pi is able to demonstrate the mass activity of 160.6 A g−1 (based on the catalyst), which is about 28 times higher than that of rGO@CoFe2O4. The TOF of rGO@CF:Pi is calculated to be 0.035 s−1 at 360 mV, which is several times higher than those of rGO@CoFe2O4 (0.0025 s−1) and IrO2 (0.02 s−1). The mass activity can be used to relatively compare the intrinsic activity of catalysts only under the similar loading mass and particle sizes. To understand the intrinsic catalytic capability, the specific activities were also calculated on the basis of the BET surface area (as shown in Figure S3 and S4). At an overpotential of 360 mV, the specific activity for rGO@CF:Pi was calculated to be 0.18 mA cm−2, which far outperforms the two other catalysts (Table 1). Thus, phosphorylation could significantly enhance the OER catalytic properties of rGO@CF:Pi as compared to rGO@CoFe2O4.


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Figure 4. Electrochemical characterizations: (a) LSV curves, (b) Tafel plots, (c) EIS, and (d) mass activity of rGO@CoFe2O4, rGO@CF:Pi, and IrO2 (The inset shows the turnover frequencies at varied overpotentials). (e) Relation of the capacitive currents of rGO@CoFe2O4 and rGO@CF:Pi with respect to scan rates. (f) Chronopotentiometric curve of rGO@CF:Pi at 10 mA cm−2 (the inset shows the LSV curves of rGO@CF:Pi before and after 20 h electrolysis). To further understand the enhancement mechanism of catalytic activity, the electrochemically active surface area (ECSA) was first calculated by the double-layer capacitance (Cdl) using the cyclic voltammetric technique at varied scan rates (see details in the SI and Figure S5).43 Based on the Cdl that was calculated from the current response at varied scan rates (Figure 4e), the ECSAs of rGO@CF:Pi and rGO@CoFe2O4 were then estimated to be 155 and 61.3 m2 g–1, respectively. It indicates that the formation of nanorings increase the active surface area of rGO@CF:Pi. Although the large surface area leads to the good catalytic properties, the ECSA for rGO@CF:Pi is only about 2.5 times of rGO@CoFe2O4. The TOF of rGO@CF:Pi at 360 mV (Figure 4d) is 13 times higher than that of rGO@CoFe2O4 (Table 1), indicating that phosphorylation increases the surface area and the intrinsic activity per site. An amorphous nanoring morphology of phosphorylated CF:Pi implies the 10


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increased defect densities and more exposed sites. As compared to crystalline materials, the amorphous structure with more defects provides catalytic sites with a variety of activities and also enhances the structure stability during OER process.44 The morphology of porous nanorings also provides channels for ion transport. In addition to the structural and morphologic advantages, the inductive effect of phosphate groups facilitates the proton-coupled electron transfer process and can act as proton accepters during OER process.11 Table 1. Electrocatalytic properties of rGO@CF:Pi, rGO@CoFe2O4, and IrO2.

Catalysts

η at J = 10 mA cm–2 [mV]

Mass activity at η = 360 mV [A g–1]

rGO@CF:Pi rGO@CoFe2O4 IrO2

300 470 360

160.6 5.7 36.2

Specific activity at η = 360 mV [mA cm–2] 0.18 0.015 0.03

TOF at η = 360 mV [s–1] 0.035 0.0025 0.02

The long-term catalytic performance of rGO@CF:Pi was measured by the chronopotentiometric approach at 10 mA cm−2. The rGO@CF:Pi electrode can retain a nearly constant potential of 1.56 V for 20 h. In contrast, the rGO@CoFe2O4 requires a voltage of 1.70 V to drive a current of 10 mA cm−2 and the overpotentials increases significantly in less than 4 h (Figure S6). The SEM image of the cycled nanorings (Figure 5a) indicates that the original morphology and distribution retain as prior to cycling. The Co 2p XPS spectra in Figure 5c show the relative increase of the Co3+ component at 780.1 eV. As shown in Figure 5d, the peaks of Fe 2p3/2 and Fe 2p1/2 agree well with those of Fe3+ according to previous studies.45,46 The P 2p and O 1s signals related to phosphates further confirm the stability of CF:Pi. Thus, as compared to previously reported transition metal phosphates,34,47–52 the rGO@CF:Pi nanorings demonstrate not only high OER catalytic activity but also long term stability (Table S2).


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Figure 5. Structural characterizations of rGO@CF:Pi after 20 h electrolysis: (a, b) TEM images of rGO@CF:Pi at two magnifications; XPS spectra of (c) Co 2p, (d) Fe 2p, (e) P 2p, and (f) O 1s signals. 2.3. Influence of nanoring structures on OER Activity The nanoring morphology not only increases the surface areas of CF:Pi catalysts but also provides porous channels for mass transportation. To understand the roles of surface area and porous channels, we annealed an rGO@CF:Pi sample in Ar gas at 700 °C to eliminate the porous structures (see Figure S7a and b). The annealed sample was labeled as rGO@CF:Pi-700. Figure S8 shows that the overpotential of rGO@CF:Pi-700 increases by 50 mV at 10 mA cm–2 as compared to rGO@CF:Pi, indicating the decrease of catalytic activities. From the performance degradation of annealed samples, we can infer that the nanoring morphology play the important role to enhance the catalytic activities. It is worth noting that the BET surface area of rGO@CF:Pi-700 (Figure S7d) decreases to 18.3 m2 g–1, which is much lower than that of rGO@CoFe2O4 (38.6 m2 g–1). However, rGO@CF:Pi-700 still demonstrates higher catalytic activities than rGO@CoFe2O4. The XRD and HRTEM analyses in Figure S7b and c show that rGO@CF:Pi-700 is amorphous. These results lead us to conclude that (1) the amorphous structure enhances the catalytic activity of annealed rGO@CF:Pi-700 to a greater 12


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extent than the loss of surface area could reduce during annealing; (2) in addition to the phosphorylation, the nanoring morphology also contributes significantly to the improvements of electrocatalytic activities of rGO@CF:Pi. To further elucidate how the amorphous structure affects the catalytic properties, we prepared crystalline rGO@CF:Pi as a control sample to study its electrochemical activity, which was further compared with amorphous rGO@CF:Pi. Figure S9 shows the materials and electrochemical characterizations of the crystalline rGO@CF:Pi. The LSV measurements demonstrate that amorphous rGO@CF:Pi requires much lower overpotentials than crystalline counterpart (Figure S9d). From the ECSA normalized LSV curves of rGO@CF:Pi and crystalline rGO@CF:Pi (Figure S10), one can see that at 0.5 mA cm–2 (on the basis of ECSA), rGO@CF:Pi only demands an overpotential of 320 mV, which is much lower than that of crystalline rGO@CF:Pi (380 mV). The significantly high intrinsic activity of amorphous rGO@CF:Pi can be thus ascribed to the disorder and defect-rich structures, which offers special coordination environments for catalytic centers. 2.4. Synergistic catalysis of (Co/Fe)4O4 cubane motifs The active catalytic sites were first determined by using a poisoning approach of potassium thiocyanate.53 Figure S13 presents the LSV curves of rGO@CF:Pi prior to and after KSCN poisoning. Because of the strong binding between SCN ions and Fe/Co sites, the overpotentials increase significantly, indicating that transition metal sites are responsible for the OER catalysis. To further distinguish the roles of Fe and Co ions, we prepared rGO@Co3(PO4)2 and rGO@FePO4 (as shown in Figure S11), respectively, to contrast the amorphous rGO@CF:Pi composite. Figure S12a presents the electrocatalytic activity of rGO@Co3(PO4)2 and rGO@FePO4. The potentials of rGO@Co3(PO4)2 and rGO@FePO4 are 1.66 V and 1.64 V at 10 mA cm–2, respectively, which are even higher than that of crystalline rGO@CF:Pi (1.60 V, see Figure S9d). The ECSA normalized LSV curves in Figure 13 ACS Paragon Plus Environment

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S12b show that crystalline rGO@Co3(PO4)2 and rGO@FePO4 require much higher overpotentials (410 ~ 420 mV) than crystalline rGO@CF:Pi (Figure S10) at 0.5 mA cm–2. The high catalytic activities of rGO@CF:Pi indicate that there seems exist the synergistic effect between Fe and Co centers.

To determine the local structures of Fe and Co centers, we analyzed the X-ray absorption fine structure (XAFS) of rGO@CF:Pi in Figure 6. By comparing the absorption features of rGO@CF:Pi with the reference samples, we infer that the oxidation state of Fe ions is between 2+ and 3+ (Figure 6a) and that of Co ions is 2+ (Figure 6b), which are consistent with the XPS analyses. Figure 6c, d presents the Fourier transforms of extended X-ray absorption fine-structure (FT-EXAFS) of CF:Pi. The strong peaks around 1.5 Å (Figure 6c) and 1.89 Å (Figure 6d) are typically in agreement with the metal-oxo distances.54,55 The prominent peaks around 2.89 Å (Figure 6c and d) are usually ascribed to the metal-metal interactions, which implies the appearance of metal-oxo cubane motifs according to previous reports.55–57 To further understand the atomic structure of amorphous CF:Pi and study the origin of high catalytic activity, we used the first-principle and classical atomistic simulations combined with metadynamics to build a statistically meaningful model for amorphous CF:Pi materials. The computation model with random atom positions was first optimized by empirical potential metadynamics simulation using two collective variables, which sample the short-range atomic environment around the metal sites (see SI notes for details). The resultant model was further optimized with the density functional theory (DFT) calculations as shown in Figure 7a. The final model mainly comprises the corner-sharing MO6 (M=Fe/Co) and PO4 units with metal-oxo cubane motifs ((Co/Fe)4O4). The EXAFS spectra of this optimized structure were simulated and compared with the experimental data in Figure 6c and d. The local environments of Fe and Co ions in the optimized structure agree well with the EXAFS data. By statistically measuring the metal-metal 14


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distances, we confirm that the peaks around 2.89 Å result from the metal-metal interactions of cubane motifs.

Figure 6. X-ray adsorption spectra: (a) Fe K-edge XANES for FeO, Fe2O3, and rGO@CF:Pi. (b) Co K-edge XANES for CoO, LiCoO2, and rGO@CF:Pi. FT-EXAFS spectra of rGO@CF:Pi at the Fe K-edge (c) and Co K-edge (d). Each main peak in the FT-EXAFS relates to a specific structural motif that is schematically illustrated (oxygen in red, iron/cobalt in dark blue). To understand the interaction between CF:Pi and various intermediates of the OER, we employed the density functional theory (DFT) to calculate the free energy changes during the intermediates adsorption. Figure 7a presents the geometrical models of OH*, O*, and OOH* on CF:Pi for the DFT calculations on the basis of the following steps of water oxidation (see the SI for details):58 OH– + * → OH* + e–

(1)

OH* + OH– → O* + H2O + e–

(2)

O* + OH– → OOH* + e–

(3)

OOH* + OH–→O2 + H2O + e–

(4)


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Figure 7. DFT calculations of intermediates adsorption: (a) Geometrical models of the adsorbed OH*, O*, OOH* on CF:Pi. (b, c) Diagram of Gibbs free energy changes of OH*, O*, OOH* on CF:Pi, Co:Pi, and Fe:Pi at (b) zero potential and (c) equilibrium potential at 1.23 V. Figure 7b presents the free energy change diagram of each steps. For comparison, we also calculated the free energy change of adsorbing three intermediates on the Co4O4 and Fe4O4 cubane motifs in amorphous cobalt and iron phosphates, respectively (see Figure S14 and S15 for the geometrical models of OH*, O*, and OOH* on Co:Pi and Fe:Pi). The formation of O*-CF:Pi (Eq. 2) has the highest change of Gibbs free energy (2.44 eV), indicating the rate-determining step (RDS). According to previous reports,45,59 the theoretical overpotentials are determined by the highest energy barrier of the above four steps. Thus, the overpotential for CF:Pi is estimated to be 1.21 V, which is much lower than that of Co4O4 (2.18 V) and Fe4O4 (1.66 V). The free energy changes of adsorbing the *O or *OOH intermediates have been considered as the universal descriptors of the RDS for the OER.60,61 It is interesting to note that the adsorption of OOH* and O* in Co:Pi and Fe:Pi show the highest energy, respectively. On the contrary, it seems imply that the Co-center in Co:Pi and Fecenter in Fe:Pi help the formation of adsorbed O* and OOH* species, respectively. Therefore, the bi16


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ACS Catalysis

metal doped cubane motifs result in an optimal adsorption of various intermediates and lower the overpotentials for the OER because of the synergistic effect of Fe and Co in rGO@CF:Pi. 3. CONCLUSIONS In summary, we developed a novel catalyst of (Co/Fe)4O4 cubane-containing phosphates for the OER by phosphorylating spinel cobalt ferrite. The resultant CF:Pi nanorings were well dispersed on rGO sheets, yielding a rGO@CF:Pi composite. The unique nanoring catalysts demonstrate superior electrocatalytic activities for the OER. A much low overpotential of 300 mV is needed to drive a current density of 10 mA cm–2. The CF:Pi catalysts also show a low Tafel slope of 36 mV dec–1 and a long-term stability for at least 20 h at 10 mA cm–2. Through well-designed experiments, structural characterization, and atomic simulations, we concluded the (Co/Fe)4O4 cubane motifs in CF:Pi can optimize the adsorption energy of various intermediates, leading to the low overpotentials for the OER. In addition to the contribution of synergistic catalysis of bi-metal co-doped cubane motifs, the nanoring morphology can provide high surface area and rapid ion channels. Phosphorylation improves the wettability of CF:Pi. In brief, this work demonstrates a tuning strategy for the highefficient OER catalysis by forming highly active (Co/Fe)4O4 cubane motifs in an amorphous and phosphorylated structure. ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website at DOI: Experimental section of preparing rGO@CF:Pi, crystalline rGO@FePO4, and rGO@Co3(PO4)2. Calculation methods of TOF, mass activity, specific capacity, and ECSA. Atom simulation of amorphous CF:Pi structure. Characterization and electrochemical experiments of crystalline 17 ACS Paragon Plus Environment

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rGO@CF:Pi, crystalline rGO@FePO4, and rGO@Co3(PO4)2. The ECSA normalized LSV curves of rGO@CF:Pi, crystalline rGO@CF:Pi, crystalline rGO@Co3(PO4)2, and rGO@FePO4. Optimized structures of the intermediate states in OER on Co:Pi and Fe:Pi surface. Average OER performance of different electrodes at current density of 10 mA cm−2. The mass loading of each element for the different catalysts. Corresponding Authors *Email: [email protected] *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21373161 and 21776121), the Jiangsu Outstanding Youth Funds (BK20160012), the National Materials Genome Project (2016YFB0700600), and “Jiangsu Shuangchuang” Program. The authors also thank beamline BL14W1 of the Shanghai Synchrotron Radiation Facility in China and beamline 8-ID of National Synchrotron Light Source II at Brookhaven National Laboratory, supported by the U.S. Department of Energy, Office of Science under Contract No. DE-SC0012704. H.Z thanks Prof. Alessandro Laio in the International School for Advanced Studies, Trieste, Italy and Dr. Gareth Tribello in Queen’s University Belfast for the suggestion about atomic simulation. REFERENCES (1)

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Fe)4O4 Cubane-Containing Nanorings Fabricated by

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