P)-Doped Graphene Electrocatalysts

Feb 16, 2017 - Synopsis. Different Fe-treated mono-heteroatom (S/N/B/P)-doped RGO catalysts have been prepared and investigated for their OER ...
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Fe-Treated Heteroatom (S/N/B/P)-Doped Graphene Electrocatalysts for Water Oxidation Fatemeh Razmjooei, Kiran Pal Singh, Dae-Soo Yang, Wei Cui, Yun Hee Jang, and Jong-Sung Yu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03291 • Publication Date (Web): 16 Feb 2017 Downloaded from http://pubs.acs.org on February 16, 2017

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Fe-Treated Heteroatom (S/N/B/P)-Doped Graphene Electrocatalysts for Water Oxidation Fatemeh Razmjooei,‡ Kiran Pal Singh,‡ Dae-Soo Yang, Wei Cui, Yun Hee Jang,* and Jong-Sung Yu* Department of Energy Systems Engineering, DGIST, Daegu 42988, Republic of Korea ABSTRACT: Anodic water splitting is driven by hydroxide (OH–) adsorption on catalyst surface and consequent O2 desorption. In this work, various heteroatoms (S/N/B/P) with different electronegativity and oxophilicity are introduced to alter the catalytic activity of reduced graphene oxide (RGO) as a catalyst for oxygen evolution reaction (OER). It is found that surprisingly, S-doped RGO outperforms the other RGOs doped with more electropositive or electronegative and more oxophilic heteroatoms, and this effect becomes more prominent after Fe treatment of the respective catalysts. Herein, we evaluate the OER activity of a series of Fetreated mono heteroatom (S/N/B/P)-doped RGO (Fe-X-G) catalysts, among which interestingly, S-doped RGO catalyst treated with Fe (Fe-S-G) is found to show better OER activity than well-known active Fe-N-C catalyst, demonstrating the best activity among all the prepared catalysts, which is close to that of the state-of-the-art IrO2/C catalyst, along with pronounced long-term stability. Density functional theory (DFT) calculations indicate that the OER activity highly depends on electroneutrality and oxophilicity of doped heteroatoms and doping-induced charge distribution over RGO, demonstrating that S with mediocre electronegativity and least oxophilicity exhibits optimal free energy for the adsorption of OER intermediate and desorption of final OER product. Furthermore, it is found that Fe treatment greatly helps in enhancing the number of active sites through the regeneration of reduced catalytically active S sites and improving the conductivity and surface area of the S-doped RGO, which are found to be key factors to furnish the Fe-S-G catalyst with the capability to catalyze OER with high efficiency, even though Fe is found to be absent in the final catalyst. KEYWORDS: heteroatom doping, iron, oxygen evolution reaction, reduced graphene oxide, electrocatalysis

1. INTRODUCTION Water electrolysis has huge implications on the future technological advancement. While cathodic-side hydrogen evolution reaction (HER) is kinetically facile, anodic-side oxygen evolution reaction (OER), in which hydroxide (OH–) ions developed at the cathode are utilized at the anode to produce oxygen and water (4OH– ↔ O2 + 2H2O + 4e–), is challenging due to large overpotential and hence requires active noble metal catalysts such as RuO2 and IrO2.1-21 The high price, instability, and scarcity of these noble metals have impeded their large scale implemenatation.1-21 Various transition metal oxides and chalcogenides, whose surface metal (M) active sites can adsorb a series of OER intermediates (e.g., M−OH, M−O, M−OOH, and M−OO), have been investigated as alternative OER electrocatalysts.22-27 However, as electrochemical performance of catalysts is directly related to their electrical conductivity, the inherent low conductivity of these oxide catalysts is a key drawback that seriously impairs their catalytic potency and limits their practical application.28 Recently, nitrogen (N)doped carbonaceous materials have shown high OER performances,15,28-31 demonstrating that carbon materials with proper physical or chemical modifications can be also efficient OER catalysts. These types of OER catalysts have evoked strong interest due to their low cost, high electrical conductivity, and unique structural features, but their OER performance still needs to be improved to meet expectation. Adsorption and desorption of intermediates on catalysts directly affect reaction kinetics of the catalysis,15 and thus a key

content would be to develop an OER catalyst, which can efficiently adsorb the OER intermediates and easily desorb final products at the same time. OER on heteroatom-doped carbon material surfaces is generally known to proceed through a sequence of four different electron/proton transfer steps, where an electron (or proton) is transferred in each step (Scheme 1).32 H 2O (I) OH (II) O (III)

ads

HOO (IV)

(l)

→

OH

ads

↔

O

+

H 2O

ads

↔

(l)

ads

ads

↔ O2

HOO (g)

+

H+ H+

+

ads

+

+ H+

+

e-

+

e-

H+

+ +

ee-

Scheme 1. OER mechanism in alkaline medium. In particular, since OH– adsorption at the first step (I) dictates the OER activity and the optimal C–OH ads bond strength is a requisite for high OER activity,14,15 the OER activity of carbonaceous catalysts can be governed by the reactivity of OH– with the catalyst surface. It is believed that a facile adsorption of water oxidation intermediates (OH–, OOH–) on the positively charged carbon atom (C(δ+)) adjacent to electroneg-

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ative N decreases the activation energy of OER in N-doped carbonaceous catalysts.33,34 On the other hand, too strong adsorption of such intermediates on the N-doped carbon may hinder the desorption and evolution of the final product O2 (last step (IV)) and eventually lower the overall OER performance. As a consequence, the oxygen evolution kinetics depends strongly on the surface nature of catalyst, and to intelligently design a catalyst, it is crucial to extrapolate which immanent material characteristics control OER catalysis. In this regards, oxophilicity (or oxygen affinity), that is, reactivity and binding strength of an element toward oxygen, is a useful concept in interpreting a wide range of catalytic processes including catalytic electrolysis of water. A simple generic scale of oxophilicity has recently been determined over the whole periodic table by Kepp.35 More detailed system-specific oxophilicity values of element X beyond the Kepp scale,35 was determined by density functional theory (DFT) calculations on bond dissociation energies of X-O in this work. We expect that replacing the N dopant by other heteroatoms such as sulfur (S), phosphorous (P), and boron (B) with lower electronegativity [N (3.04) > S (2.58) ~ C (2.55) > P (2.19) > B (2.04)] and/or different oxophilicity [B (1.0) > P (0.7) ~ N (0.7) > S (0.5)]35 would result in variation in OH– adsorption strength and O2 evolution rate, and this in turn will give difference in the OER performance. In addition, developing novel materials by using other heteroatoms besides N is also of great significance in terms of amplifying the spectrum of new catalytic materials and their various applications. Since N is more electronegative than S, the adjacent carbon atoms become more positively charged in N-doped carbon compared with those of S-doped one. Therefore, the adsorption of hydroxide ion (OH-) with a negative charge occurs more easily and strongly on N-doped carbon than on S-doped one. This strong adsorption of reaction intermediates can make desorption of final reaction product, O2, more difficult in Ndoped carbon compared with the carbon doped with other heteroatoms. On the other hand, B and P, which are much more electropositive than carbon, can provide electrons to the adjacent C, leading to an increase in its nucleophile strength (C(δ-)), which makes carbon adjacent B and P species inefficient to adsorb OH- with negative charge, resulting in direct adsorption of OH- on B and P atoms with positive charge. Since B and P with positive charge are highly oxophilic, the adsorption of the negative intermediate species will occur strongly on B or P sites, which in turn will make the desorption of final product, O2, more difficult in B or P-doped carbon compared with that of N or S-doped ones. Hence, it can be surmised that apart from the electrophilicity, the oxophilicity also plays a prominent role in deciding the OER performance, and these properties of B and P render them OER inactive. Density functional theory (DFT) calculations can also make great contribution to such development and evaluation.35 Apart from the adsorption and desorption kinetics of reactant and product, respectively, physiochemical properties of the catalyst are also important parameters to ponder upon. Additional treatment of the N-doped carbon with non-precious transition metals (M) such as iron (Fe) and cobalt (Co) is found to have significant effect on the OER performance, which is governed by N content, M content, M-N interaction, and conductivity of the resulting carbon framework.36-38 One particularly intriguing point is that, while such transition met-

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als certainly enhance the conductivity and create extra active sites in the carbon framework,39 their physical presence on the resulting carbon catalyst does not necessarily enhance the OER activity of the N-doped carbons.15 Our interpretation on the role of the transition metals is that they collect the oxygens (O) from the neighboring N sites, to form metal oxides, which are washed away by acid washing, and thus convert inactive (oxidized and occupied by O) N=O or N-O sites into active N catalytic sites. On the other hand, it is reported that a decrease in the amount of framework oxygen helps to increase the conductivity of the material,40 consequently improving OER activity. Although OER catalysts based on non-precious metalderived N-doped carbon have been reported, to the best of our knowledge, non-precious metal-treated carbon materials modified with other heteroatoms with different electronegativity (or oxophilicity) such as S, B or P remain untouched for OER. Hence in this work we have prepared a series of mono heteroatom (S/N/B/P)-doped reduced graphene oxides (RGOs) in the presence of Fe to synthesize Fe-X-G, where X represents each heteroatom. For comparison, catalysts in the absence of Fe are also prepared and designated as X-G. The resulting series of Fe-X-G electrocatalysts are for the first time investigated for OER in this work. Surprisingly, Fe-S-G shows better OER activity than well-known active Fe-N-C catalyst, demonstrating the best activity among all the prepared catalysts, where S is less oxophilic and electronegative than N. Fe treatment greatly increases the number of active sites and the conductivity of the Fe-S-G, which are found to be key factors to furnish the Fe-S-G catalyst with the capability to catalyze OER with high efficiency, even though Fe is found to be absent in the final catalyst. It is also worth mentioning that Fe-SG catalyst shows high OER activity, which is very close to that of state-of-the-art IrO2/C catalyst, along with pronounced long-term stability. To better understand the underlying mechanism of OER on various Fe-X-G catalysts, DFT calculations are also applied to study catalysts for water oxidation. 2. EXPERIMENTAL SECTION 2.1. Synthesis of Graphene Oxide (GO). GO was synthesized from the natural flake graphite (325 mesh, 99.8%, Sigma-Aldrich) using improved hummers method.41 In brief, 9:1 (360:40 ml) mixture of concentrated H2SO4/H3PO4 was added to 3.0 g graphite flake and 18.0 g KMnO4. The oxidation was then carried out by heating and stirring the solution mixture at 50 ⁰C for 12 h.41 After oxidation reaction, the mixture was poured into a mixture solution of ice (~ 400 ml) and 30% H2O2 (3.0 ml). The obtained solution was centrifuged for 1 h at 4000 rpm, the supernatant was decanted away, and brown colored powder left behind was collected. The obtained powder was then washed repeatedly with the mixture of ethanol, HCl, and water and finally dried overnight at 60 ⁰C to collect a dried GO sample. 200 mg dry GO was dispersed in 200 ml of water by sonication and used for further synthesis. 2.2. Preparation of Fe-Treated Heteroatom-Doped RGO Catalysts. For the synthesis of Fe-treated heteroatomdoped RGO samples, different acid family members including sulfuric, nitric, boric, and phosphoric acids are used for S, N, B, and P precursors, respectively. In a typical synthesis of Fetreated heteroatom-doped RGO, 200 mL GO (1.0 mg mL-1) water dispersion was mixed with 2 ml of respective acid and

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0.1 g FeCl2.4H2O with vigorous stirring at room temperature for 5 h. The mixture was kept at 80 ⁰C in oven for 10 h to collect a dried sample. Remnant solid was grinded and pyrolyzed at 800 ⁰C for 2 h with the ramp rate of 5 ⁰C /min in N2 atmosphere. To remove excess iron, the obtained catalyst was treated with 0.5 M sulfuric acid at 80 ⁰C for about 8 h and rinsed with deionized water and finally dried at 60 ⁰C. The resulting catalysts are hereafter denoted as Fe-X-G, where X corresponds to each different heteroatom (S/N/P/B). Heteroatomdoped RGO (X-G) was also prepared with the same procedure without using iron precursor. Pristine RGO was prepared by direct pyrolysis of dried GO alone using the identical pyrolyzing conditions. 2.3. Characterization. The morphology of the samples was observed with Hitachi S-4700 scanning electron microscope (SEM) operated at an accelerating voltage of 10 kV, and with EM 912 Omega transmission electron microscopy (TEM) operated at 120 kV. X-ray diffraction (XRD) data were acquired by a Rigaku Smartlab diffractometer with Cu Kα radiation (1.5406 Å) operated at 40 kV and 40 mA. Raman spectra were recorded with a Renishaw spectrometer using an Ar+ ion laser (λ=514.5 nm). X-ray photoelectron spectroscopy (XPS) was conducted to analyze surface elemental compositions of all the samples using an ESCALAB-250 spectrometer with a monochromated Al Kα (150W) source. The nitrogen adsorption-desorption isotherms were measured at -196 0C using a Micromeritics ASAP 2020 accelerated surface area and porosimetry system. The specific surface area was determined based on Brunauer-Emmett-Teller (BET) method from nitrogen adsorption data at the relative pressure range between 0.05 and 0.2. Total pore volume was determined from the amount of gas adsorbed at the relative pressure of 0.99. Pore size distribution (PSD) was derived from adsorption branches by the Barrett-Joyner-Halenda (BJH) method. Thermal Gravimetric Analysis (TGA) was carried out on a Bruker TG-DTA3000SA thermal analyzer at a heating rate of 5 0C/min in flowing air (60 mL/min), increasing from room temperature to 1000 0C to investigate the decomposition course of materials. Electrical conductivity measurements for all the samples were carried out using a home-made four-point probe apparatus by varying the applied pressure according to previous works.42-44 The cell is made of a non-conducting Teflon block carved into a hollow cylinder, which is covered by two metallic brass pistons, one as a base and another as a lid, to which the pressure was applied. Fine powdered carbon sample was filled in the hollow Teflon chamber, which in turn was sealed using two brass pistons, and resistivity measurement was performed by increasing the applied pressure. Current was applied to the sample through the metallic pistons, and voltage was measured across the two metallic probes placed in the middle of the powder sample. Keithley model 6220 and model 2182A were used as the DC current source and voltmeter, respectively. The current was varied from 0 to 10 mA and the corresponding voltage was measured. The electrical conductivity of the samples was estimated using the formula:

σ =

l RA

where σ is the electrical conductivity, R is the resistance of the sample, A is the cross sectional area of the sample (0.126 cm2)

and l is the distance between the voltage probes (0.2 cm). While measuring the resistivity, pressure was applied to the metallic pistons by using steel plates of known weights. Theoretical calculations based on DFT were carried out on small model compounds at the level of B3LYP/6-311G** implemented in Jaguar v8.5 (Schrödinger, LLC, New York, 2014). 2.4. Electrode Preparation and Electrochemical Characterization. The working electrode was polished with alumina slurry to obtain a mirror-like surface, then washed with water and acetone, and dried before use. The slurry was prepared by mixing 5.0 mg of catalysts added into a 1.0 mL solvent mixture of Nafion (5 wt. %) and water with v/v ratio of 1:9 for 20 min in an ultrasonicator. For comparison, a commercially available catalyst, 20 wt.% IrO2/C was used, and a 1.0 mg/mL commercial IrO2/C suspension was prepared according to the identical procedure described above. The slurry was placed on pre-cleaned working electrode, and the electrode was allowed to dry at room temperature before the measurement. This leads to catalyst loading of 0.4 µg cm-2 for all the obtained catalysts or commercial 20 wt.% IrO2/C. All the electrochemical measurements were carried out with a Biologic VMP3 electrochemical workstation. All the obtained catalysts and commercial 20 wt.% IrO2/C were used directly as the working electrode without further treatments. OER activities of the electrodes were characterized by OER polarization curve, electrochemical impedance spectroscopy (EIS), and chronoamperometry. Electrochemical studies of the samples were carried out at room temperature using a rotating-disk electrode (RDE) in three-electrode geometry. A glassy carbon RDE was used as working electrode, and Pt wire and Ag/AgCl saturated with KCl were used as counter and reference electrodes, respectively. The Ag/AgCl reference electrode was calibrated with respect to a reversible hydrogen electrode (RHE). The calibration was performed in a high-purity H2 (99.99%) saturated electrolyte using Pt electrode and Pt wire as the working and counter electrode, respectively. Linear scanning voltammetry (LSV) is then run at a scan rate of 10 mV/s and the potential at which the current crossed zero is taken to be the thermodynamic potential (vs. Ag/AgCl) for the hydrogen electrode reactions. In 0.1 M KOH, ERHE = EAg/AgCl + 0.973 V. Calibration of the reference electrode against the RHE is shown in Figure S1 of supporting information (SI). The OER activity was evaluated by linear sweep voltammetry (LSV) on a rotating disk electrode with a rotation rate of 1,600 rpm and a scan rate of 5 mV s-1. The impedance of each catalyst was measured by electrochemical impedance spectroscopy (EIS) over a frequency range of 1 x 105 to 0.1 Hz with sinusoidal perturbation amplitude of 5 mV. The turnover frequency is defined as the rate of evolved molecular O2 per surface active site per second, which can be calculated by the following equation:

   =

     ⁄      ⁄  

TOF (s-1) = (number of oxygen turnover)/(number of active sites) = (J/4F)/n, where J is the OER current density at a given overpotential, n is the number of active sites, and F is faraday constant (96584.3 s A mol-1). (J/4F) represents the total oxygen turnover. The overpotential used for the calculation of

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TOF was set at 480 mV, in which a current density of 10 mA cm-2 was resulted for Fe-S-G. 3. RESULTS AND DISCUSSION The schematic illustration of the synthesis process is depicted in Scheme 2. While keeping the doping conditions and dopant concentration identical, different acid families such as sulfuric, nitric, boric, and phosphoric acids are used as S, N, B, and P precursors, respectively. Briefly, all the Fe-treated heteroatomdoped RGOs (Fe-S-G, Fe-N-G, Fe-B-G, and Fe-P-G) are prepared by pyrolyzing the mixture of FeCl2 as an iron precursor, each respective acid as a heteroatom precursor, and GO as a substrate and carbon precursor at 800 °C (see experimental details) followed by acid washing to remove residual unstable Fe species. In this case, it is found that acid functionalities also help in enhancing the surface area of the resulting Fe-X-G samples.

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X-G samples. Interestingly, the OER activity matches well with the oxophilicity of doped heteroatoms. To investigate the effect of Fe treatment on the OER performance of the catalysts, polarization curves for Fe-S-G, FeN-G, Fe-B-G, and Fe-P-G catalysts in 0.1 M KOH solution with a scan rate of 5 mVs-1 are also obtained along with that of 20 wt.% IrO2/C as shown in Figure 1a. The potential required for water oxidation to obtain the current density of 10 mA cm-2 is commonly used to judge the OER activity.14,24 As can be concluded from Figure 1a, interestingly, Fe-S-G generates the OER current density of 10 mA cm-2 at a much lower potential of 1.70 V vs. RHE, as compared with other counterparts, FeN-G (1.75 V), Fe-B-G (1.89 V), and Fe-P-G (1.95 V), which is considerably much closer to that of IrO2/C (1.60 V). The impact of S doping and iron treatment on OER can be elucidated from Figure S3a of SI. S-doping helps to significantly improve the OER activity of RGO in S-G sample along with more catalytic activity than its counterparts. Particularly, this is highly significant since N-doped carbons are among the most studied metal-free substitutes for noble metal catalysts for OER. We also find that the simple Fe treatment further improves the activity of the S-G in Fe-S-G. This can expedite the larger scale use of S-doped carbon based OER catalysts.

Scheme 2. Schematic illustration of various Fe-treated heteroatom-doped RGOs. The OER activity of all the resulting Fe-X-G catalysts was assessed by LSV in 0.1 M KOH electrolyte at room temperature (25 °C). OER polarization curves for the resulting Fe-free samples S-G, N-G, B-G, and P-G at 1,600 rpm are shown in Figure S2a of SI. Since The Tafel slope provides insight into the reaction mechanism, we compare the Tafel plots of the Fefree heteroatom-doped RGO catalysts. As shown in Figure S2b of SI, the S-G catalyst exhibits much smaller Tafel slope (151 mV per dec) as compared with N-G (158 mV per dec), PG (171 mV per dec), and B-G (183 mV per dec) catalysts. These results suggest more favorable OER kinetics for S-G compared with other counterparts. It can be seen that among all the catalysts, S-G outperforms in terms of onset and the water oxidation current and also shows much smaller Tafel slope as compared with the other X-Gs, which indicate the facile OH- adsorption and O2 desorption kinetics suggesting more favorable OER kinetics on the S-doped RGO. It is quite surprising to see that S-G shows better OER activity than well-known active N-G. From the above results it can be concluded that the OER activity decreased in the order of S-G> N-G> P-G> B-G. It will be interesting to find the relationship between OER activity and surface properties of

Figure 1. (a) LSV profiles and (b) Tafel plots of Fe-X-G catalysts and 20 wt.% IrO2/C. (c) EIS Nyquist plots measured for all the prepared Fe-X-G catalysts and (d) chronoamperometric response at a constant potentials at 10 mA cm-2 for Fe-S-G and 20 wt.% IrO2/C. The inset in (d) is the LSV plots for the 1st and 800th potential cycles for Fe-S-G. Furthermore, as shown in Figure 1b, the Fe-S-G catalyst exhibits much smaller Tafel slope (88 mV per dec) as compared with the other Fe-X-G catalysts. These results suggest more favorable OER kinetics (OER intermediate adsorption and desorption) for Fe-S-G compared with other counterparts. Similar trend was observed in Figure S3b of SI, where Fe-S-G exhibits much smaller Tafel slope compared with that of its Fe-free counterpart (S-G), suggesting the synergistic effect of Fe treatment on S-G catalyst.

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ACS Catalysis

Figure 1c and Figure S3c of SI show the Nyquist plots based on EIS analysis. The smaller charge transfer resistance of Fe-S-G than the other Fe-X-G catalysts and its Fe-free counterpart (S-G), clearly indicates its better electronic transport capability and better reactant diffusivity towards electrode surface, which clearly justify the superior OER performance of Fe-S-G. Furthermore, the Fe-S-G demonstrates a strong durability in alkaline electrolytes with only 25 % anodic current loss during ~800 continuous potential cycling (inset of Figure 1d). As shown in Figure 1d, the high stability of FeS-G for the OER is also proved by chronoamperometric method, which exhibits significant current retention of ~67% after continuous operation at a constant potential of 1.70 V at 10 mA cm-2 over 30,000 s in contrast to the rapid activity decrease of IrO2/C (52 % retention) at a constant potential of 1.60 V. The stability of catalysts was measured at the constant potential required for water oxidation to obtain the current density of 10 mA cm-2 for each sample, which is different for each sample. The potential required for the current density of 10 mA cm-2 is 1.70 V for Fe-S-G, and 1.60 V for IrO2/C as mentioned. The EIS measurements were done at the potential near to the onset of the OER to understand the mechanism of intermediate adsorption on the Fe-X-G catalysts and their Fefree counterpart as shown in Figure S4a and b of SI. It can be seen in Figure S4a of SI that the charge transfer resistance for the Fe-N-G catalyst is much lower than those of the other counterparts with the order of Fe-N-G < Fe-S-G < Fe-B-G < Fe-P-G. This clearly indicates the facile adsorption of OER intermediates (-OH-) at the Fe-N-G catalyst, and proves the point that the highly electropositive surface of Fe-N-G due to N doping facilitates the –OH- adsorption. Even though the adsorption of intermediate is quite facile on Fe-N-G, its final OER activity is lesser than that of the Fe-S-G, which can be justified from the oxophilicity of the catalyst. As the oxophilicity of S is lower than that of N, final oxygen desorption is much facile on Fe-S-G compared with Fe-N-G catalyst. Therefore, Fe-S-G was assumed to have the most optimal M−O bond strength among the other X-G ones, suggesting facile adsorption of OH- and O2 desorption on the active sites of catalyst as the rate limiting step. The same trend is observed for their Fe-free counterparts as shown in Figure S4b of SI. In order to make meaningful comparisons of activity trends, it is critical to compare intrinsic activity of all the catalysts on the basis of specific activity and TOF. Specific activity versus TOF is plotted in Figure S5 of SI for all the samples. As shown in Figure S5 of SI, Fe treatment and sulfur doping have led to reduced graphene oxide achieving turnover frequencies as high as 0.34 s−1 at the overpotential of 480 mV for O2 production at high specific activity (0.71 mA cm-2 BET). Comparison of turnover frequencies and activities for all the catalysts shows that, the TOF and specific activity associated with Fe-S-G are higher than those of Fe-N-G (0.30 s−1 and 0.12 mA cm-2 BET) Fe-B-G (0.28 s−1 and 0.02 mA cm-2 BET), and Fe-P-G (0.26 s−1 and 0.03 mA cm-2 BET). These collective data confirm that the Fe-S-G is highly efficient towards OER. As physical parameters have a huge effect on the catalytic properties of OER catalysts, various analyses have been carried out on Fe-X-G materials. The XRD patterns of all the asprepared Fe-X-G samples before acid washing are shown in Figure S6(a,b) of SI. It can be seen that before acid washing, Fe-S-G and Fe-N-G possess typical Fe3O4 peaks, whereas Fe-

B-G and Fe-P-G show peaks for Fe3BO5 (ICDD card no.01078-2285) and Fe2P4O12 (ICDD card no.01-073-1945), respectively. Interestingly, as shown in Figure 2a, after acid washing, with the exception of Fe-P-G, the other Fe-X-G catalysts show no metal complex impurities. Generally, the diffraction profiles of reduced graphene oxide-based materials exhibit two prominent broad bands centered around 2θ = 25.6 and 43°, associated with diffraction of the (002) and (100) planes, respectively, which are characteristic of turbostratic carbon. However, as shown in Figure S7 of SI, in the early region within the range of 2θ= 21-30°, XRD patterns for Fe-S-G, FeN-G, and Fe-B-G show the presence of more developed and separated peaks with maxima at 2θ= 21.9 and 25.5 for Fe-S-G, 21.5 and 25.3 for Fe-N-G, and 21.0 and 25.2° for Fe-B-G, respectively. In this present study, different acid family members including sulfuric, nitric, boric, and phosphoric acids are used as S, N, B, and P precursors for the synthesis of Fe-S-G, Fe-N-G, Fe-B-G, and Fe-P-G, respectively, which at the same time, can act as chemical activation agents to increase the surface area of graphene-based materials. It has been proposed that in chemical activation using acid, carbon materials show more rich diffraction profiles at lower angles with diffraction peaks at ~25.6° and ~21.0°, which correspond to mean d002 spacing of 3.39 and 3.85 Å, respectively.45 This observation can be attributed to fragmented crystallites due to use of acid families as activation agents, which leads to the formation of disorders and defects in the graphene-based materials. As can be seen in Figure S7 of SI, both peaks, which correspond to the (002) peak for Fe-N-G and Fe-B-G shift towards the lower angle as compared to that of Fe-S-G, which implies that Fe-NG and Fe-B-G possess larger interlayer spacing between graphene sheets.33 This means that the Fe-S-G possesses more intact graphitic structure compared to other Fe-X-Gs. In case of Fe-P-G, since Fe2P4O12 is a chemically stable and insoluble complex, it remains in the Fe-P-G carbon lattice even after acid washing.46-48 To further justify our claim regarding the complete removal of Fe after acid washing, we have opted for the carbon oxidation method. On the basis of the concept that carbon oxidizes at higher temperature in oxygen atmosphere, whereas Fe oxidizes to its oxide forms (the only left over product), we have oxidized the Fe-S-G sample before and after acid washing (Fe-S-G-BW-OX and Fe-S-G-OX). For comparison we also prepared a sample washed for lesser period of time (2 hr) (Fe-S-G-2hr) , which was further oxidized to give Fe-S-G-2hr W-OX. Figure S8a of SI shows the TGA analysis of washed and un-washed Fe-S-G samples as well as Fe-S-G-2hr W in atmospheric air. It can be seen that oxidation of carbon starts at ~450 OC and finishes at around 700 OC, indicating complete removal of carbon up to 700 OC. In the washed sample (Fe-SG), 100% removal of samples were observed, implying that the washed sample is devoid of any Fe impurities. On the other hand un-washed Fe-S-G-BW and Fe-S-G-2hr W samples show only 90% and 93% degradation, respectively, which implies the presence of Fe impurities in these samples. To identify the nature of the species obtained after carbon oxidation, XRD analysis was carried out. Figure S8b of SI shows the XRD patterns for Fe-S-G-BW and sample washed for lesser period (2hr) after oxidation (Fe-S-G-BW-OX and Fe-S-G2hr W-OX), which show typical peaks for Fe2O3 (ICCD card no. 01-073-3825).

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As can be seen in Figure S9 of SI, S-G shows more crystalline nature and has sharper (002) diffraction peak compared with the other counterparts. It is clear that S doping significantly keeps the graphitic structure of graphene carbon more intact compared with the other counterparts. The comparative XRD plots of graphite, GO, RGO, and S-G are shown in Figure S10a of SI. Figure 2b compares the Raman spectra of all the Fe-X-G samples. The ratio (ID/IG) of D (1352 cm-1) to G (1580 cm-1) band, which is commonly cited as a measure of structural disorder in graphitic structure, is found to be 1.14, 1.20, 1.26, and 1.18 for Fe-S-G, Fe-N-G, Fe-B-G, and Fe-P-G, respectively. This clearly indicates that Fe-S-G possesses more crystalline and ordered structure with high graphitic nature than other Fe-X-G samples, which is in agreement with XRD results. A new band at ∼1050 cm−1 is seen in Fe-P-G sample, which is due to symmetric PO43- stretching mode associated with the PO43- tetrahedral and can be ascribed to the presence of Fe2P4O12.47 The comparative Raman spectra of RGO (ID/IG=0.97) and S-G (ID/IG=1.12) are also shown in Figure S10b of SI.

Figure 2. (a) XRD patterns and (b) Raman spectra for all the Fe-X-G catalysts. Surface properties such as surface area and pore distribution are critical parameters for electrochemical catalysis. Nitrogen isotherms display type IV adsorption/desorption behaviors with pronounced hysteresis loop typical of mesoporosity for the prepared materials as shown in Figure S11a of SI. The BET surface area was determined to be 212.24, 300.81, 307.58, and 354.36 m2g-1 for Fe-P-G, Fe-S-G, Fe-N-G, and Fe-B-G, respectively. The higher surface area of Fe-S-G, FeN-G, and Fe-B-G compared with that of Fe-P-G can be due to the removal of iron residues in the process of acid washing. As shown in Figure S11b and Table S1 of SI, with the introduction of S into the pristine RGO and incorporation of Fe into the S-doped RGO, the BET surface area starts increasing from 185.12 m2g-1 for pristine RGO to 240.10 m2g-1 for S-G, and to 300.81 m2g-1 for Fe-S-G. Such an increase in surface area in SG can be ascribed to the pore-generating ability of acidic groups of sulfuric acid. At high pyrolysis temperature, volatilization of acidic groups can lead to the gasification of carbon and distortion of graphitic structure, which can generate pores and wrinkles, resulting in increased surface area. Some further surface area increase seen after incorporation of Fe for Fe-SG, is mainly due to the removal of iron oxides, formed during high temperature pyrolysis process, by acid washing. As

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shown in Figure S11c and S11d of SI, PSD curves based on the BJH model, exhibit the mesoporous nature for all the samples. Surface characteristics for all the samples are summarized in Table S1 of SI. To investigate the topological features and properties of all the prepared catalysts, SEM and TEM measurements were carried out as shown in Figure 3(a-h), S12(a-h) and S13(a-j) of SI. SEM and TEM images of all the Fe-treated samples before acid washing are shown in Figure S12(a-h) of SI. Images show the presence of some particles with different sizes on the RGO surface formed during the annealing process. These particles are attributed to different iron oxide compositions generated in different conditions. The obtained samples were treated with 0.5 M sulfuric acid to remove excess iron complexes before OER catalysis. Figure 3a and 3b for Fe-P-G show that the particles with different size still remain on the graphene surface, which can be attributed to the Fe2P4O12 complexes as shown in XRD pattern in Figure 2a. However, no such iron complexes are observed on the graphene surface for the other Fe-X-G catalysts after acid washing. In addition, the SEM and TEM images in Figure S13(c-j) of SI and Figure 3(c-h) reveal the evolution of wrinkled patterns in all the X-G and Fe-X-G catalysts compared with RGO, which can be ascribed to heteroatom doping in RGO framework as well as removal of iron complexes after mild acid washing. It is worth to mention that the graphene sheets in Fe-S-G show less disordered structure with less wrinkles, which implies much more preserved graphitic structure in Fe-S-G compared with that of Fe-N-G and Fe-B-G, which is in line with XRD and Raman data in Figure 2. From the above discussions, it can be concluded that Fe-SG possesses less disordered structure compared with other prepared catalysts. Therefore, it can be expected that the conductivity of Fe-S-G should be better than the other counterparts.49 To evaluate this, the electrical conductivity of the FeX-G, S-G, and RGO samples was examined using a homemade four-probe apparatus (Figure S14(a,b) and S15 of SI).42,50 It can be seen that as predicted, Fe-S-G presents higher conductivity compared with the other catalysts and S-G.51 FeP-G shows the lowest conductivity among all the Fe-X-G catalysts, which can be due to formation of non-conductive Fe2P4O12 species on a graphene surface.46

Figure 3. SEM images of (a) Fe-P-G, (c) Fe-S-G, (e) Fe-N-G, and (g) Fe-B-G. TEM images of (b) Fe-P-G, (d) Fe-S-G, (f) Fe-N-G, and (h) Fe-B-G.

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ACS Catalysis

The peak survey and atomic contents of all the samples evaluated using XPS are shown in Figure S16(a-d) and Table S2 and S3 of SI. Interestingly, with the exception of Fe-P-G, the XPS survey scans indicate the absence of Fe species in FeS-G, Fe-N-G, and Fe-B-G framework, showing the complete removal of Fe complexes after the acid washing. The S content for Fe-S-G (1.83 wt%) and S-G (1.71 wt%) is found to be almost identical, indicating that Fe has almost no effect on S doping concentration in the carbon framework as shown in Table S2 and S3 of SI. In addition, interestingly, the results show that O content reduces with a corresponding increase in the carbon content for Fe-S-G compared with S-G. The S 2p XPS spectra for S-G and Fe-S-G are shown in Figure 4a. As can be seen in Figure 4a, the S 2p narrow scan spectra for S-G and Fe-S-G catalysts are fitted by 3 peaks at 163.61, 164.82, and 168.21 eV corresponding to S p3/2 (S1), S p1/2 (S2) for C-S-C and SOx (S3). S1 and S2 peaks can be attributed to sulfur bonded directly to the carbon atoms in a heterocyclic configuration whereas S3 peak at 168.21 eV is assigned to -CSOx- species.40, 26, 52-54 It is reported that sulfur dopant in the form of thiophenic with a five-membered ring structure is active for OER.55 Since the Fe-S-G exhibits prominent OER activity, we also excogitate that these C-S-C species exist in the thiophene form, which is consistent with previously reported observation.56 In the present Fe-S-G, the intensity of the S1 and S2 species, which is considered to be active for OER, increases compared with that of S-G.55

Figure 4. Deconvoluted XPS spectra revealing (a) relative distribution of prominent S species for S-G and Fe-S-G, (b) N 1s for Fe-N-G, (c) B 1s for Fe-B-G, and (d) P 2p for Fe-P-G. The N 1s XPS spectra for N-G and Fe-N-G are shown in Figure 4b. As can be seen in Figure 4b, deconvolution of N 1s spectra of N-G and Fe-N-G results in 4 major peaks assigned to (N1): pyridinic (∼398.3 eV), (N2): pyrrolic (∼399.4 eV), (N3): quaternary (∼401.1 eV), and (N4): oxides of nitrogen, NOx, (∼402.4 eV) in Figure 4b.33 The pyridinic-N, which is known to be OER active site, is found to be the dominant N state in Fe-N-G sample.15 Even though N is more electronegative than C, its effect largely depends on its doping configuration. As reported, the quaternary and pyrrolic-N atoms in gra-

phene can provide electrons to p-conjugated system, which can increase nucleophilic strength for the adjacent carbon rings (C(δ-)) making the carbon atoms near the quaternary and pyrrolic-N energetically unfavorable for adsorption of water oxidation intermediates (OH– and OOH–) in alkaline solution, and consequently unfavorable for OER. However, pyridinicN, which is an electron-withdrawing group with the lone pair electrons, inductively removes electron density from the adjacent carbon (C(δ+)), which facilitates the adsorption of water oxidation intermediates on adjacent carbon (C(δ+)) and hence accelerating the OER.26 It is worth mentioning that in case of sulfur, the S3 peak is found to be absent in Fe-S-G, showing the more C-S bond formation (Figure 4a) and in the case of N the intensity of the N1 and N4 species increases and decreases, respectively, compared with that of N-G (Figure 4b). As can be seen in Table S2 and S3 of SI, it can be noticed that after Fe treatment, oxygen content significantly drops down from 3.27 for S-G to 1.33 for Fe-S-G and 4.03 for N-G to 2.69 for Fe-N-G, which can be attributed to the removal of formed Fe oxide species by acid washing. These results clearly demonstrate that introduction of Fe in S-G and N-G to form Fe-S-G and Fe-N-G can collect O from the neighboring sites (S3 and N4), to establish Fe3O4 oxides (Figure S6 and S12 of SI), which can be etched out by acid washing. Thus, this Fe treatment helps to convert inactive oxidized sites to active reduced ones, decreases O content in the Fe-S-G, increases conductivity (Figure S15 of SI), and in turn improves the OER performance. However, Fe collects oxygen from the SOx more facile than NOx, which might be due to lower oxophilicity of S compare with that of N. The B 1s XPS spectra for B-G and Fe-B-G are shown in Figure 4c. B is presented in the form of (B1): BC3 (∼191.2 eV), (B2): partially oxidized B (BC2O) (∼191.9 eV), (B3): BCO2 (∼193.2 eV),57 and (B4): B2O3 (193.7 eV).58 However, even though B4 peak is found to be absent in Fe-B-G due to iron reaction to form Fe3BO4 complex, which can be etched out by acid washing, it is still found that B2 and B3 are the dominant phases of the doped B in Fe-B-G.59,60 This may be an evidence for high amount of oxygen present in Fe-B-G (Table S2 of SI), which are not be removed from the B-doped carbon due to high oxophilicity of B. Thus, the Fe-B-G due to low amount of active species like B1 in the carbon framework and low conductivity by cause of high amount of oxygen (see Figure S14b of SI), may lead to lower OER activity compared with Fe-S-G and Fe-N-G. Figure S17 of SI shows the relative distribution of prominent heteroatom species for Fe-S-G, Fe-N-G, and Fe-B-G catalysts and their Fe-free counterparts. It can be seen that, due to the lower oxophilicity of the sulfur, Fe treatment can effectively etch out the oxygens present in the vicinity of sulfur, which is in agreement with the complete disappearance of the S3 peak in Fe-S-G sample as compared to S-G sample. In the case of nitrogen, due to higher oxophilicity of N, Fe treatment is found to only partially decrease the oxygen content, and hence only a partial decrease in N4 species is observed. Similar observation stands true for B-G and Fe-B-G as well. After studying the high resolution XPS of the prepared catalysts following order of oxophilicity was observed (B > N > S), which is also similar to our theoretical predicted order.

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As shown in Figure 4d, the P 2p spectrum for P-G and FeP-G is deconvoluted into two peaks at 132.1 and 133.3 eV assigned to (P1): P–C and (P2): P–O (oxidized P), respectively. It has been also reported that the binding energies in the range of 132.9-133.9 eV are attributed to Fe-phosphates such as Fe2P4O12 complex.61,62 In the present case with Fe-P-G, the formation of chemically stable and insoluble Fe-phosphate complexes is also highly possible, which is in line with XRD, SEM, and TEM analysis and can be gleaned from the decreased P1 and enhanced P2 signal intensity for Fe-P-G compared to that of P-G. This indicates that major P2 species with high O content, can decrease the conductivity of the Fe-P-G. The role of oxygen on the carbon surface is important in catalysis. It is reported that high amount of oxygen (Table S2 of SI) can reduce the conductivity, and consequently reducing the OER activity. The O content is related directly to oxophilicity of heteroatom [B (1.0) > P (0.7) ~ N (0.7) > S (0.5)].35 Due to high oxophilicity of B, the oxygen content is pretty high in FeB-G compared with that of Fe-N-G and Fe-S-G. Therefore, the high oxygen content in Fe-B-G agrees with its low conductivity observed in Figure S14b of SI, eventually leading to lower OER activity compared with Fe-S-G and Fe-N-G. In case of Fe-P-G, the formation of Fe2P4O12 complex, which is a chemically stable and insoluble complex, can decrease the conductivity, consequently reducing OER performance. As shown in Scheme 1, the proposed mechanism of OER for heteroatom-doped carbon in alkaline media consists of the adsorption of OH- in first step (I) and desorption of O2 at the last step (IV). High OER activity of Fe-S-G compared with active Fe-N-G is intriguing and worth investigation. Due to higher electronegativity of N compared to S, neighboring carbon atoms become more positive in Fe-N-G compared to that of Fe-S-G, resulting in stronger adsorption of OH- and more difficult desorption of final product (O2), which eventually may lead to lower OER activity in Fe-N-G compared with that of Fe-S-G. In the case of Fe-B-G and Fe-P-G, in which B and P are much more electropositive than carbon, the negative charge will be induced on a carbon atom, which makes carbon adjacent to B and P species inefficient to adsorb OH- with negative charge, resulting in adsorption of OH- directly on B and P with positive charge. Thus, B or P may act as active sites to adsorb OER intermediates, but as is often the case, the strong adsorbtion due to their high oxophilicity leads to much more difficult O2 desorption, thus the OER performance is not high. Along this line, as B is more electropositive than P, B-G shows lower OER performance compared with P-G as observed in Figure S2 of SI. However, interestingly, with introduction of Fe, Fe-P-G shows lower OER activity compared with Fe-B-G, which can be attributed to the formation of chemically stable and non-conductive Fe2P4O12 species. In addition, since major B and P species in Fe-B-G and Fe-P-G are mainly present as inactive oxide species (see Figure 4c and d), the OER activity should be lower than expected. To better understand the underlying mechanism of OER on various doped graphene sheets, DFT investigation was applied to study Fe-X-G as catalysts for water oxidation. It appears that the O contents (Table S2 of SI) in the Fe-X-G catalysts is well correlated with their OER performance. The O contents should be related with the oxophilicity of heteroatom X in XG.35 For more specific understanding of the oxophilicity of the X-G catalysts, we used DFT to calculate the O affinity (∆E1)

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and the O2 desorption energy (∆E2) of X-G, which are defined by Eqs. 1 and 2: ∆E1 = E [O/X-G] – E[X-G] – 0.5 E [3O2] ∆E2 = E [3O2] + E[X-G] – E [O2/X-G]

(1) (2)

A more negative value of ∆E1 represents a higher O affinity (that is, greater stabilization after O adsorption), while a more positive value of ∆E2 represents a higher O2 affinity (that is, higher cost of O2 desorption). The X-G was modeled by the smallest graphene fragments with a heteroatom sitting on the edges in Figure 5(a-c). A thiophene-like 5-membered-ring model (S-G in a box; Figure S18 of SI, upper) was used for Sdoped edge of graphene (S-G) and a pyridine-like 6membered-ring model (N-G and B-G in a box; Figure S18, lower) was used for N- and B-doped edges of graphene (N-G and B-G) in order to satisfy stable closed-shell electron configurations. Indeed a previous experimental work,63 has assigned similar types of model for S-G, that is, exclusively thiophene-like S doping, from their high-resolution XPS spectra, which is essentially identical to ours (Figure 4a).The total energy E of each species is estimated after full geometry optimization performed at the level of B3LYP/6-311G** of DFT implemented in Jaguar v8.5 (Schrödinger, LLC, New York, 2014). Each optimized geometry is confirmed to be the minimum-energy structure using the normal mode analysis. The spin-unrestricted DFT is used for the triplet ground state of O2 and the spin-restricted DFT is used for the singlet ground states of all the other species.

Figure 5. (a) Model DFT calculations to select the most stable O (red) adsorption site among several plausible ones 1-4 on graphene edges doped with S (yellow), N (blue), and B (green) on the basis of the relative O affinity (in kcal/mol; in parentheses), (b) the O affinity ∆E1 (in kcal/mol) of the selected O adsorption sites with the atomic charges (in electron units) of the dopants shown together, and (c) the O2 desorption energy

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ACS Catalysis

∆E2 (in kcal/mol) from the same sites with the O-O distance (in Å) shown together. The most stable O-adsorbed and O2-adsorbed X-G species (used for the estimation of ∆E1 and ∆E2 in Figures 5b-c) are selected among various final structures optimized from different initial structures (Figure 5a). The S and N dopants (position 3), which are (slightly for S) more electronegative than C [N (3.04) > S (2.58) > C (2.55)], are (slightly for S) negatively-charged (-0.08 and -0.67 electron unit (|e|) from the electrostatic-potential-fitted atomic-charge estimation as shown in Figure 5b), and therefore unfavorable (∆E1 = 5.0 kcal/mol on S) or only slightly favorable (∆E1 = -3.4 kcal/mol on N) for O adsorption (Figure 5a). Instead, the positively-charged adjacent C site (position 1) serves as the most favorable O adsorption site (∆E1 = -38.5 and -51.5 kcal/mol for Fe-S-G and Fe-NG, respectively in Figures 5(a,b)). On the other hand, the B dopant (position 1, Figure 5a), which is more electropositive than C [C (2.55) > B (2.04)], would be positively charged (0.78 |e| in Figure 5b) and serves as the most favorable O adsorption site (∆E1 = -59.9 kcal/mol as shown in Figures 5(a,b)). The O affinity to the most stable site in this series of X-G is therefore estimated as -59.9, -51.5, and -38.5 kcal/mol for B-G > N-G > S-G. It is indeed consistent with the reported oxophilicity scale of these heteroatoms (B > N > S).35 Therefore, Fe in Fe-X-G after collecting O from neighboring X sites to form different iron oxide compositions, which are washed away by acid washing, will convert the oxidized (occupied and inactive) sites into reduced (unoccupied and active) X sites, decrease the O content in X-G, enhance the conductivity of XG, and eventually improve the OER performance of X. This effect is more prominent for S-G than for N-G. On the other hand, doping with oxophilic B or P would bring the opposite effect. The higher O affinity of the catalytic sites in B-G or PG than in N-G and S-G would result in higher resistance to restoration of the reduced (unoccupied and active) sites even after the Fe treatment, as evidenced by the observation of high amount of oxygen in the respective samples, and thus lower the conductivity and consequently lower the OER performance.35 The oxophilicity is quite general in a sense that the same trend of affinity of X-G (B-G > N-G > S-G) holds for O2 as well as for O on the same site (Figure 5c). The energy cost ∆E2 of O2 desorption from S-G, N-G, and B-G, which is the last step of OER, was calculated as 15.9, 28.3, and 53.5 kcal/mol, respectively. In the case of B-G, the initial step of O(H) binding may be very favorable (∆E1 = -59.9 kcal/mol), but the final step of O2 desorption would be extremely difficult (∆E2 = 53.5 kcal/mol). On the other hand, in the case of N-G and especially S-G, while the initial step of O(H) binding is still favorable on a carbon adjacent to heteroatom sites (∆E1 = -51.1 and -38.5 kcal/mol), the final step of O2 desorption is much easier from N-G and S-G (∆E2 = 28.3 and 15.9 kcal/mol) than from B-G. Such a balance in the adsorption-desorption energetics could also be

responsible for the superior OER performance of Fe-S-G over Fe-(N/B/P)-G. Summarizing, due to higher electronegativity of N compared to S, neighboring C atoms become more positive in Fe-N-G compared to that of Fe-S-G, resulting in stronger adsorption of OH- on Fe-N-G, and more difficult de-

sorption of final product (O2), which eventually may lead to lower OER activity of Fe-N-G compared with that of Fe-S-G. As acid washing is considered to be a necessary step to remove inactive species for improving the OER activity, to estimate the variation of OER activity before and after acid washing of Fe-S-G sample, we characterized the Fe-S-G sample before acid washing as well. As shown in Figure S19 of SI and Table S4 of SI, XPS characterization of the Fe-S-G sample before acid washing (Fe-S-G-BW) shows the presence of higher amounts of Fe and O, which might be evidence for the presence of metal oxide (Fe3O4) impurities (as shown in XRD pattern in Figure S6a of SI), which are etched out after acid washing (Figure 2a). As shown in Table S5 and Figure S20(a,b) of SI, the BET surface area and the conductivity of catalyst are found to improve after acid washing. The higher surface area of Fe-S-G compared with that of Fe-S-G-BW can be due to the removal of iron residues during the process of acid washing. Since it is reported that a decrease in the amount of framework oxygen helps to increase the conductivity of the material, the lower conductivity of Fe-S-G-BW compared with Fe-S-G can be due to the presence of non-conductive Fe3O4 on the catalyst surface and its higher oxygen content. Figure S21 of SI shows the comparison OER activity of Fe-S-G samples before and after acid washing (Fe-S-G-BW and Fe-S-G). OER activity of Fe-S-G is improved after acid washing compared with Fe-S-G-BW due to the removal of iron oxide impurities. Therefore, it is interesting to note that even though Fe is not detected after acid washing as shown in Figure S16(a-c) and Table S2 of SI, the sample treated with Fe significantly improves OER activity as compared with Fe-free counterpart due to increase in amount of active sites as well as surface area and improvement in graphiticity and electrical conductivity of catalysts. This implies the eminent role played by Fe in preparing the efficient OER catalyst, which is in direct agreement with earlier work by Zhao et al.15 4. CONCLUSIONS A series of Fe-treated mono heteroatom (S/N/B/P)-doped RGOs was prepared and investigated for the first time for their OER electrocatalytic activity. It is found that the new Fe-S-G catalyst is very effective in catalyzing oxygen evolution with catalytic activity not only better than its metal-free counterpart, but also than other Fe-treated heteroatom (N/B/P)-doped RGOs prepared in this work. Furthermore, the Fe-S-G showed pronounced long-term operational stability. This superior activity can be attributed to the high graphiticity, high surface area, low oxophilicity of S, high conductivity, high amount of active sites, and optimal C–Ox adsorption-desorption energetics in the Fe-S-G framework. Interestingly, it is also found that the physical presence of Fe may not be necessary in enhancing the OER activity of the investigated Fe-X-G. Fe can collect efficiently the oxygen from the neighboring sites to form iron oxides, which are washed away by acid washing and hence, can thus render the catalyst to have much more active sites, increased surface area and improved conductivity, and therefore is required for the preparation of an efficient OER catalyst. Furthermore, theoretical calculations indicate that the OER performance of various Fe-X-G catalysts tightly depends on the oxophilicity of heteroatoms and doping-induced charge distribution in the doped carbon framework. After all, the pre-

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sent findings are highly important and innovative with respect to development of new OER catalysts and understanding the underlying principle. This study lays down the fundamental to understand the factors, which control the OER activity in heteroatom-doped carbon catalysts and therefore, will be a pioneer and demonstrates a new direction towards the development of a new series of cost-effective and efficient catalysts.

ASSOCIATED CONTENT Supporting Information Experimental procedures and characterization data. This material is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * [email protected] * [email protected] Notes The authors declare no competing financial interest.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally.

ACKNOWLEDGMENTS This work was supported by Global Frontier R&D Program on Centre for Multiscale Energy System (NRF-2011-0031571) and NRF grant (NRF 2014K2A3A1000240) funded by the Ministry of Education, Science and Technology. Authors also would like to thank KBSIs at Daegu and Busan for TEM and XPS measurements and CCRF in DGIST for SEM measurements.

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Fe-Treated Heteroatom (S/N/B/P)-Doped Graphene Electrocatalysts for Water Oxidation

Fe-treated different mono heteroatom (S/N/B/P)-doped RGO catalysts are prepared and investigated for their OER electrocatalytic activity. Fe-S-G catalyst shows the highest OER activity among all the prepared catalysts owing to high electrical conductivity and optimal C– Ox adsorption-desorption energy associated with the S-doped RGO.

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