Fe3O4 ternary composite as an

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www.chemcatchem.org. Accepted Article. A Journal of. Title: g-C3N4/CeO2/Fe3O4 ternary composite as an efficient bifunctional catalyst for overall water splitting.

Accepted Article Title: g-C3N4/CeO2/Fe3O4 ternary composite as an efficient bifunctional catalyst for overall water splitting Authors: Jamshaid Rashid, Nadia Parveen, Tanveer ul Haq, Aneela Iqbal, Shamraiz Hussain Talib, Saif Ullah Awan, Naveed Hussain, and Muhammad Zaheer This manuscript has been accepted after peer review and appears as an Accepted Article online prior to editing, proofing, and formal publication of the final Version of Record (VoR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: ChemCatChem 10.1002/cctc.201801597 Link to VoR: http://dx.doi.org/10.1002/cctc.201801597

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g-C3N4/CeO2/Fe3O4 ternary composite as an efficient bifunctional catalyst for overall water splitting Jamshaid Rashid,a* Nadia Parveen,a Tanveer ul Haq,b Aneela Iqbal,c Shamraiz Hussain Talib,d Saif Ullah Awan,e Naveed Hussain,f Muhammad Zaheerb* A. Prof. Dr. Jamshaid Rashid, Nadia Parveen, Department of Environmental Science, Faculty of Biological Sciences, Quaid-i-Azam University, Islamabad 45320, Pakistan. b Tanveer ul Haq, A. Prof. Dr. Muhammad Zaheer, Department of Chemistry and Chemical Engineering, SBA School of Science and Engineering, Lahore University of Management Sciences (LUMS), Lahore 54792, Pakistan. c Dr. Aneela Iqbal, Institute of Environmental Sciences and Engineering, School of Civil and Environmental Engineering, National University of Sciences and Technology, Sector H-12, Islamabad 44000, Pakistan. d Shamraiz Hussain Talib, Department of Chemistry, Tsinghua University, Beijing 100084. P. R. China e A. Prof. Dr. Saif Ullah Awan, Department of Electrical Engineering, NUST College of Electrical and Mechanical Engineering, National University of Science and Technology (NUST), Islamabad 54000, Pakistan. f Naveed Hussain, State Key Laboratory of New Ceramics and Fine Processing, School of Material Science and Engineering, Tsinghua University, Beijing, P.R. China.

*Corresponding Authors: Dr. Jamshaid Rashid Email: [email protected] ; [email protected] Dr. Muhammad Zaheer Email: [email protected] Abstract We report the very first example of a catalyst based on a ternary composite of graphitic carbontirde (g-C3N4), ceria (CeO2) and magnetite (Fe3O4) for overall water splitting in 1.0 M KOH. Synergy between the components due to electronic effects results in a highly efficient catalyst which catalyzes Oxygen Evolution Reaction (OER) and Hydrogen Evolution Reaction (HER) at substantionally low overpotentials (400 mV and 310 mV, respectively) to produce benchmark 10 mA.cm-2 current density. OER activity of the catalysts surpasses that of the benchmark RuO2 catalyst at higher current densities (≥50 mA.cm-2). The composite catalyst is stable and shows a Faraday efficiency of 98% for OER. Overall water splitting (OWS) was achieved at overpotential of 1.94 V which is substantially high at 1 wt% loading of active metal. -----------------------

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a

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Detrimental environmental affects together with constant depletion of fossil fuels has compelled scientists to quest for green and renewable energy resources.[1] Water electrolysis, in this regards, has emerged as a workable strategy for the production of a green fuel hydrogen that can be directly fed to fuel cells for the production of electricity.[2] However, benchmark whose scarcity subsides the commercial use of such catalysts.[3] In recent past, attempts have been made to design robust electrocatalyst based on earth-abundant metals to reduce overpotential hydrogen evolution reactions (HER)[4] and oxygen evolution reactions (OER).[5] As a result various catalysts based on the oxides, hydroxides, chalcogenides, phosphides, borides and carbides have been synthesized and reported.[6] As compared to HER, OER is regarded as the bottleneck reaction of water electrolysis due to the sluggish kinetics as it requires the transfer of four electrons to make an oxygen molecule.[7] Consequently, intensive efforts have been dedicated for development of efficient OER catalysts.[8] However bifunctional active, stable and environmentally benign catalysts capable of catalyzing overall water splitting in acidic or alkaline media are highly demanded as it simplifies the cell design. [9] Commercial water electrolysis is currently achieved either by Proton Exchange Membrane (PEM) electrolyzers or alkaline water electrolysers. The latter is advantageous in the sense that use of expensive noble metal catalysts, perflourinated Nafion-based PEMs and acid corrosion can be avoided.[10] Recently overall water splitting has been reported based on transition metals [11] and their oxides[12], mixed oxides[13], hydroxides[14], phosphides[2], sulphides[15], however, efficiency and stability of such catalysts needs improvements for commercial applications.[16] g-C3N4 bearing strong C-N bonds and structural similarity with graphite, is an excellent material with promising electronic properties and robustness.[17] It is known as a metal-free electrocatalyst for HER[18] however its poor conductivity dwindles its potential as an efficient catalyst for water splitting. Composites of g-C3N4 with carbon,[19] metals[20] and metal oxides have shown promising activity for HER, OER and oxygen reduction reactions (ORR). [21] Amorphous oxides of iron have shown some OER activity[22] however reports about the OER activity of magnetite (Fe3O4) are rare.[23] Nevertheless, composites of Fe3O4 with transition metals[24], their oxides[25] and chalcogenides[26] are reported to show enhanced OER activity.

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electrocatalysts for water electrocatalysis are based on ruthenium, iridium and platinum,

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Ceria (CeO2) with its high oxygen storage capacity (OSC) and redox properties finds applications as a co-catalyst in polyelectrolyte membrane fuel cells (PEMFCs). [27] However, its use as catalyst in electrocatalytic water splitting is limited. [28] Recently it was shown that presence of CeOx on the surface of CoS nanoparticles enhances OER activity by tuning the Co2+/Co3+ ratio and

on Fe3O4 (1 wt%), ceria (5 wt%) and g-C3N4 (94 wt%). Due to synergic effect, this ternary composite shows excellent activity as a bifunctional catalyst for OWS. To the best of our knowledge, this is the very first example of a bifunctional catalyst based on g-C3N4 for OWS. Schematic for the ternary composite (labeled as GCF throughout the manuscript) is illustrated in Scheme 1 and the synthesis detail is presented in Figure S1 in supporting information (SI). C Nitrogen ar bo n

(I)

CeO2

(II)

Fe3O4

Scheme 1: Schematic presentation for the synthesis of GCF. (I) CeO2 deposition in g-C3N4; (II) Fe3O4 loading on GC5 X-ray Powder Diffraction (PXRD) patterns of the prepared pristine and nanocomposites are shown in Figure 1 (a). Pristine g-C3N4 showed a typical strong peak at 27.30ᵒ (d = 0.33 nm) and was assigned to (002) plane. This plane indicates the presence of interplanar stacking of conjugated aromatic carbonitride units. Sister diffraction peak at 13.10ᵒ was indexed to 100 plane indicative of the in-plane tri-s-triazine units with a period of 0.68 nm (JCPDS No. 211272& 87-1526). [29] For pure CeO2, the diffraction peaks at 28.5, 33.0, 47.4 and 56.3ᵒ were ascribed to 111, 200, 220 and 311 planes of cubic structure [30] (JCPDS No. 043-1002 & 34-0394). The reflection pattern of iron oxide, however, showed cubic spinel [31] crystal structure of Fe3O4 (JCPDS No. 19-0629, 65-3107, 77-1545 & 3-0863) and rhombohedral lattice of α-Fe2O3.

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oxygen vacancies.[15] Aforementioned literature reports provoked us to design a catalysts based

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Interestingly, in ternary composite of (GCF) we noticed the diffracted pattern of Fe 3O4 phase only and no Fe2O3 or any other impurity peaks. The surface area of the catalysts was determined by nitrogen physisorption. Pristine g-C3N4 exhibited higher surface area (17 m2.g–1) compared to previous reports. [30] Similarly the surface

surface area for the nanocomposite could be attributed to successful incorporation of CeO 2 into the g-C3N4 matrix as evident from the nitrogen adsorption-desorption isotherms of GCF Figure 1 (b). Pore size distribution calculated by NLDFT model is shown in Figure 1 (b) (inset) and shows rather broad pore size distribution averaging at 1.6 nm.

20

30

¨(311)

40

¨ (440) Ñ(300)

CeO2

¨ (511)

Ñ (116)

Ñ (024)

Ñ (113)

¨ (400)

¨

¨(220)

4000 3600 3200 2800

(311)

¨(200)

¨(220)

¨(111)

300 200 100 0

0.020

50

60

Fe3O4

70

dV[d] (cc/nm/g))

GCF

g-C3N4

60

0.015

cc/nm/g

0.010 0.005 0.000 0

5

40

10

15

20

Pore size (nm)

Adsorption Desorption

20

0 0.0

2Theta (Degree)

0.2

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1.0

Relative Pressure (P/P0)

(c)

(d)

C

6µm

(f)

O

(b)

80

Volume adsorbed (cc/g)

¨(422)

¨(311)

¨(400)

¨(220)

¨(200)

(a)

¨(002)

1200 900 600 300 0

Ñ(104)

Intensity (a.u)

¨ g-C3N4 ¨ CeO2 ¨ Fe O Ñ a-Fe O 3 4 2 3

¨(002)

600 400 200

(e)

(h)

(h)

N

(g)

Fe

(e)

Ce

Figure 1: (a) XRD of pristine components and GCF nanocomposite; (b) Nitrogen adsorptiondesorption isotherms of GCF; (inset) pore size distribution by NLDFT; (c-h) SEM micrograph and corresponding elemental maps of GCF nanocomposite

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area of CeO2 and GCF was found to be 30 m² g-1 and 29 m².g-1, respectively. The increase in

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Morphological properties, elemental composition and the distribution of elements in the prepared samples were investigated with Scanning Electron Microscopy (SEM). Elemental mapping was recorded from a randomly selected area of the GCF using EDX detector (Figure 1 (c-h). High purity of nanocomposites was confirmed from the sole presence of C, N, Ce, Fe and

X-ray photoelectron spectroscopic (XPS) measurements investigated the chemical oxidation states of the constituents of the ternary nanocomposite GCF. The low resolution survey spectrum illustrated in Figure 2a, reflects the existence of C, N, Ce, Fe and O at the surface of gC3N4. No extra element was found from survey spectrum. In high resolution XPS of C-1s (see Figure 2b) a peak at 284.8 eV could be attributed to the adventitious carbon, whereas very strong peak at 288.3 eV attributed to sp2-bonded carbon (N-C=N).[32] A very small peak centering at 294.2 was related to CN3.[33] The asymmetric N-1s high resolution XPS peak was deconvoluted into three symmetric peaks (see Figure 2c at 398.3 eV, 398.9 eV and 400.2 eV, allocated to the N-pyridine (C-N-C), N-pyrrolic and N-graphitic (N-C3), respectively. Another small intense peak at position 403.9 eV was corresponded to Pyridine-N-oxide. [34] Figure 2d illustrates the Ce-3d spectra in order to measure the relative abundances of Ce 3+ and Ce4+ ions in GCF nanocomposite. Six peaks corresponding spin-orbit doublets (3d3/2; 3d5/2) can be identified in the spectrum along with two Ce4+ and two Ce3+satellite peaks. The XPS of CeO2 is composed of two multiplets of 3d corresponding to the spin-orbit split 3d5/2 and 3d3/2 core holes corresponding to Ce4+ oxidation state. The spin-orbit splitting is about 18.6 eV and the intensity ratio I-3d5/2/I-3d3/2 was fixed to 1.12. In the cerium (IV) oxides, usually the satellites peaks are apparently due to energy-gain (shake-down) rather than energy-loss (shake-up) processes. These results indicated the presence of cerium in both (IV) and (III) oxidation states. Further, HR-XPS were conducted to verify the valence state of Fe ions in GCF (see Figure 2e). The Fe 2p spectrum showed two main peaks at 710.7 and 724.8 eV assigned to 2p3/2 and 2p1/2 of Fe+2 and Fe+3, respectively. The Fe (2p1/2) peak was deconvoluted into two characteristics peaks. The peak at 710.5 eV is associated with Fe+2 and 712.4 eV can be assigned to Fe+3. The elemental composition of GCF is provided in Table S1.

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O and their homogeneous distribution (except Ce) within the sample.

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300000

Survey Scan 35000

C-N-C

(b) C-1s

30000

Fe-2P

C-1s O-1s

100000

25000 20000

C=C 284.85

Ce-3d

150000

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200000

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C-N3

10000

50000

294.20

0 280

0 0

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40000

Fe-2p3/2

298

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Ce3+

14000

13000

15000

14000

Fe3+ Fe2+

13000

12000 12000

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708

393

(e)

Fe-2p3/2

Counts (a.u)

(Pyridine-N-Oxide) 403.98

296

15000

S

60000

22000

S

(Pyridine N) 400.29

S

(Pyridine N) 398.35

S

80000

Ce3+

3d3/2

100000

294

16000

Ce4+

Ce4+

(d) Ce4+

(Pyrrolic N) 398.92

24000

3d5/2

N-1s Counts (a.u)

Counts (a.u)

120000

292

E (eV)

140000

(c)

290

Satellite Fe3+

5000

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E (eV)

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705

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E (eV)

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710

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740

E (eV)

Figure 2: a) XPS scan spectra of GCF; HR-XPS of: b) C (1s); c) N (1s); d) Ce (3d); e) Fe (2p) Electrochemical activity of ternary composite and its individual components was tested in a three-electrode electrochemical cell in 1.0 M KOH. Cyclic voltammograms of the catalysts after iR correction (10%) are presented in Figure 3 (a). As compared to g-C3N4 and CeO2, composite catalyst (GCF) showed the highest OER activity providing current density of 10 mA.cm -2 at 400 mV overpotential. This OER activity is comparable to benchmark catalyst (RuO 2), which also required the same overpotential (400 mV) to produce 10 mA.cm -2 anodic current density. GCF outperformed RuO2 at 50 mA.cm-2 requiring only 490 mV in comparison to 580 mV required by Ruthenia. g-C3N4 could not produce the benchmark current density while Fe 3O4 was found inferior in OER activity (500 mV @10 mA.cm-2) confirming the enhanced performance of composite due to synergic effect. According to Sabatier Principle an optimal electrocatalyst does not bind too strongly or too weakly with the reaction oxygen intermediates to make M-O bonds.[35] Number of electrons in the eg orbital of transition metal are believed to determine the strength of M-O, for instance, a fully occupied (2e-) and empty (0e-) eg orbital will interact

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250000

Intensity (a.u)

288.20

40000

N-1s

(a)

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with the reaction intermediates leading to either too weak or too strong bonds.[36] Subsequently an eg orbital with unity-occupancy will show optimum activity due to increased covalency of M-O bond.[37] The enhanced OER activity of the GCF composite could be attributed to charge transfer from Fe3+ to g-C3N4 due to possible coordination of iron with pyridine

OER activity. As mentioned previously, presence of a reducible oxide (CeO2) may also help in achieving a singly occupied eg orbitals.[15] OER performance was further evaluated from Tafel slopes (Figure 3b). A small value of slope (74 mV.dec-1) for composite as compared to g-C3N4 (230 mV.dec-1) and Fe3O4 (174 mV.dec-1) suggests favorable reaction kinetics and faster charge transfer.[38] Tafel slope analysis can also help to identify the OER mechanistic approach by determining their rate-determining steps (RDS). [38] In the present case, 74 mV.dec−1 Tafel slope suggests oxidation of adsorbed OH into oxygen intermediate (step II) as the rate-determining step. The smaller Tafel slope (74 mV.dec1

) for composite even with high series resistance (Rs, 13Ω) recognizes the fast intrinsic kinetics

and catalytic response that may be due to the numerous surface active sites over the supported catalyst correspondingly assisting the electron transfer between the electrode/electrolyte interfaces. The electrochemical impedance spectroscopy (EIS) was also carried out in alkaline medium to investigate the catalytic behavior in terms of charge transfer at the electrolyte-electrode interface. In the Nyquist plots shown in Figure 3 (c), Rct value (8 ) for composite material was found very close to that for RuO2 (5Ω) and the lowest as compared to g-C3N4 (52 Ω) and Fe3O4 (24 Ω). This observation suggests a fast charge transport phenomena at electrode-electrolyte interface. Quantification of evolved oxygen during OER (Figure S2) confirms that anodic current is solely because of the ongoing water oxidation process. A Faraday efficiency of ∼98% shows that measured and evolved concentration of oxygen are in good agreement with each other.

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nitrogen.[20] This will lead to near-unity occupancy of the eg orbital of iron and hence superior

Figure 3: Cyclic voltammograms (a) and Tafel plots (b) and Nyquist plots (c) for individual components in comparison to their composite and benchmark RuO2 catalyst. d). Cyclic voltammetry (e) and Tafel plots (f) of various catalysts in 1.0 M KOH at a scan rate of 5 mV.sec-1. Stability is one of the important parameters that help evaluate an electrocatalyst’s performance and its practical applicability. The stability of our composite material was assessed by chronopotentiometry (CPE) under steady-state current density of 10 mA.cm−2. As shown in Figure 3d, the composite material required ∼400 mV overpotential to achieve current density of 10 mA.cm−2. The time-dependent potential curve showed no significant change under current density of 10 mA.cm−2, even after 14 h. However, a minor positive shift was observed in onset potential after 14 h that might be attributed to bubbles formation on the FTO surface. The performance of GCF in 1.0 M KOH solution toward the electrocatalytic HER, along with commercial Pt/C and Fe3O4, and the results are presented in Figure3e. As expected, Pt/C showed higher HER activity whereas Fe3O4 exhibit poor activity towards HER. Interestingly, the nanocomposite (GCF) showed an enhanced HER activity, demanding overpotential of ∼310 and ∼327 mV to produce a current density of 10 and 40 mA.cm−2 respectively. Such a sharp rise in current density by small change in overpotential suggests a synergic cooperation between components of the composite material. Secondly, the presence of pyridinic N in the g-C3N4 can enhance hydrophilicity thereby increasing the electrolyte-electrode interaction. Lastly, the

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support (g-C3N4) may sustain the activity of catalytic active sites by preventing the aggregation of nanoparticles even at high reducing potential via coordination through N-functions.[39] Tafel slope value of 102 mV.dec-1 (Figure 3f) was found close to that for Pt/C catalyst (90 mV.dec-1), which suggests the faster electron, transfer during hydrogen evolution.

A ternary composite based on graphtic carbonitride, ceria and magnetite is reported for overall splitting of water in 1.0 M KOH solution. Composite (GCF) showed enhanced OER activity as compared to its components due to synergic effect. OER activity was found to be comparable to the state-of-the-art RuO2 catalysts with Faraday efficiency around 98%. Our composite was also active for HER reaction and provided high current density up to 40 mA.cm -2 at an overpotential of 327 mV, which was again found superior than the individual components. EIS analysis suggests feasibility of charge transfer reactions at catalyst-electrolyte interface leading to enhanced electrocatalytic activity. Catalyst was found stable up to fourteen hours without showing any prominent loss in the activity. Electronic interactions between Fe, Ce and g-C3N4 which might be the reason behind enhanced OER and HER activity of the composite. Acknowledgement Authors acknowledge Quaid-i-Azam University for funding this research through departmental annual research fund. Authors would also like to acknowledge Dr. M. Arif Nadeem and Mr. Waqas Ali Shah for evolved O2 quantification test at the Department of Chemistry, Quaid-iAzam University. Mr. Zafar Iqbal (Department of Chemistry, LUMS) and Dr. Murtaza Saleem (Department of Physics, LUMS) are acknowledged for PXRD and SEM-EDX analysis after catalysis.

Keywords: Bi-functional Catalysts; Graphitic Carbonitride; Hydrogen Evolution reactions; Oxygen Evolution Reactions; Overall water splitting.

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This article is protected by copyright. All rights reserved.

Accepted Manuscript

10.1002/cctc.201801597

ChemCatChem

10.1002/cctc.201801597

ChemCatChem

Graphical Abstract:

180

g-C3N4

160

Carbon Nitrogen

140

GCF-5 Fe3O4

120

RuO2

a)

GCF

j/ mAcm

-2

100 80 60 40

Fe3O4

CeO2

20 0

g-C3N4 (1-0.05)/CeO2(0.05)

g-C3N4

0.2

GCF

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

E/V vs RHE

0.8

Pt-C GCF Fe3O4

f)

V

0.6 130 mV/dec

/V

0.4

O2

H2

102mV/dec

0.2 0.0

Potentiostat

90 mV/dec

-0.2 0.0

O2

0.6

0.8

1.0

1.2

1.4

GCF

d)

H2 1.8

E/V

Computer

Cathode

0.4

2.0 1.9

OH-

0.2

Log j

H2O

1.7 1.6

GCF 1.0 M KOH

1.5

Ag/Ag Cl

0

2

4

6

8

t/h

This article is protected by copyright. All rights reserved.

10

12

14

Accepted Manuscript

Ternary composite of graphitic carbontirde, ceria and magnetite is reported for the first time. Synergy between the components resulted in catalyst activity which surpassed that of the benchmark RuO2 catalyst at higher current densities (≥50 mA.cm-2) and HER at 310 mV to produce benchmark current density 10mA.cm-2. The stable composite catalyst showed OER Faraday efficiency of 98% and overall water splitting was achieved at overpotential of 1.94V which is substantially high at 1 wt% loading of active metal.