Catalyst Support in Oxygen Electrocatalysis: A Case Study with CoFe

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C: Energy Conversion and Storage; Energy and Charge Transport

Catalyst Support in Oxygen Electrocatalysis: A Case Study with CoFe Alloy Electrocatalyst Arpan Samanta, and C. Retna Raj J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02830 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on July 1, 2018

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The Journal of Physical Chemistry

Catalyst Support in Oxygen Electrocatalysis: A Case Study with CoFe Alloy Electrocatalyst

Arpan Samanta, C. Retna Raj* Functional Materials and Electrochemistry Laboratory, Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, West Bengal, India Email: [email protected]

1 ACS Paragon Plus Environment

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Abstract Synthesis of highly efficient oxygen electrocatalyst is of significant interest for the development of energy storage devices. The electronic and surface structure, shape, size and catalyst support largely controls the electrocatalytic activity. We demonstrate the synthesis of bifunctional CoFe alloy electrocatalysts encased with nitrogen-doped graphitic carbon (N-C-CoFe) and supported on nitrogen-doped reduced graphene oxide (N-rGO-CoFe) for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) and the influence of catalyst support on the electrocatalytic performance. N-C-CoFe and N-rGO-CoFe were synthesized from a single-source precursor, potassium cobalt hexacyanoferrate multi-metal complex by thermal annealing. Both catalysts efficiently catalyse ORR and OER at low overpotential. The benchmark current density of 10 mA cm−2 for OER is obtained at very low overpotential of 0.22 and 0.29 V with N-rGO-CoFe and N-C-CoFe, respectively. The N-C-CoFe catalyst is highly durable towards OER and ORR whereas N-rGO-CoFe has limited durability. N-C-CoFe could retain >98% of its initial ORR activity even after repeated 2000 cycles. The chemical nature of nitrogen and degree of graphitization of the catalyst support largely control the overall performance of the catalyst. Highly graphitized carbon (sp2 C-C) support containing large amount of graphitic nitrogen highly favor ORR. The effective encapsulation of the active catalyst into nitrogen-doped graphitic carbon and presence of nitrogen doped graphitic carbon nanostructures ensures high durability of the catalyst in harsh environment. The graphitized carbon with large amount of graphitic nitrogen is an ideal catalyst support for bifunctional CoFe alloy catalyst.


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1. INTRODUCTION The global energy crisis due to the depletion of conventional energy sources and the environmental consequences of the extensive use of fossil fuel demands for alternative energy resources. In the past one decade, several efforts are being taken to develop various types of alternative energy conversion and storage devices such as fuel cells, solar cells, batteries and supercapacitors.1 Rechargeable metal-air batteries are found to be another promising candidate as it can deliver high energy density.2-4 The operational principle of rechargeable metal–air batteries involves two fundamental electrochemical reactions, oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), corresponding to the discharge and charge processes, respectively. The main challenge associated with the metal-air battery is to overcome the large overpotential due to the sluggish electron transfer kinetics associated with these two reactions. The successful operation of metal-air battery requires efficient bifunctional electrocatalyst that favour ORR and OER at low overpotential. The traditional Pt, Ir and Ru-based catalysts5-9 cannot be used in metal-air batteries owing to the lack of bifunctional activity. Moreover, the high cost, lack of durability and scarcity of these traditional catalysts limits their use for practical application. Although they have good electrocatalytic activity towards either ORR or OER, they are inefficient in catalysing both ORR and OER. Synthesis of non-precious electrocatalysts with outstanding ORR and OER activity is paramount in the development and commercialization of metal-air batteries. The research efforts in the recent past demonstrate that the carbon nanostructures10 more precisely heteroatom (N, S, B, P) doped carbon-based materials11-18 and transition metal oxides,19-23 carbides,24-25 nitrides,26-27 selenides,28-30 phosphides,31-33 sulfides,34-35 and alloys36-37 have electrocatalytic activity towards ORR and OER. Although the metal-free heteroatom doped carbon and non-precious metal-based catalyst have reasonable activity and



economically viable, achieving high current density at low overpotential and high durability are still a challenging task. Recently, the hybrid catalysts derived from transition metals and heteroatom doped carbon nanomaterials have received special attention.38-40 The synergistic effect between the transition metal/metal oxide and hetero atom-doped carbon and enhanced electronic conductivity significantly improves the ORR and OER kinetics.41-42 Among the hybrid catalysts, nitrogen-doped graphene/reduced graphene oxide (N-rGO) and transition metal-based mixed valence spinel oxides or transition metal alloys have interesting electrocatalytic activity and are very promising for metal-air battery application.43-44 The hybrid materials have been synthesized in the past from the corresponding metal salts using hydrothermal or electrochemical methods. Such bimetallic oxide/alloy-based hybrid catalysts are shown to have bifunctional activity towards ORR and OER. Controlling the elemental composition of the catalyst and ideal integration of the catalyst with conductive carbon are very important to achieve bifunctional activity. The single-step thermal annealing approach is of significant interest and it would be ideal for the synthesis of integrated hybrid catalyst of controlled elemental composition. An added advantage of this method is that the in situ generated catalyst can be encapsulated inside carbon shell. Such encapsulation can ensure durability and enhanced activity of the catalyst. In the recent years, the hybrid materials derived from the oxides/alloy of Fe/Co, Fe/Ni, Co/Ni, etc. and carbon-based catalysts attracted much attention for ORR and OER.45-49 Achieving high current density at low overpotential, high durability and small peak separation (∆E) between half-wave potential of ORR and potential at which 10 mA/cm2 for OER (∆E= E1/2ORR −E OER 2 10mA/cm )

is still a challenging task.

The durability of the bifunctional catalyst is one of the major requirements for development of efficient meta-air battery. Although several bifunctional catalysts have been


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synthesized in the past, their limited durability in oxidative environment is a serious concern. The catalyst support plays major role in controlling the durability and overall activity of the catalyst. The undoped and heteroatom doped carbonaceous materials are widely used as catalyst/catalyst support for electrocatalytic applications.10,41 Very recently, it has been shown that the graphitized carbon-based catalyst has high durability during ORR and OER.49 Does the nature of carbon support and the heteroatom have significant control over the bifunctional performance of the catalyst? In an effort to understand the role of catalyst support on the bifunctional electrocatalytic activity of oxygen electrocatalyst, we have rationally synthesized CoFe alloy supported on two different types of nitrogen-doped carbon support using a singlesource catalyst precursor potassium cobalt hexacyanoferrate (KCoHCF). The thermal annealing of as-synthesized KCoHCF yields nitrogen-doped graphitic carbon encapsulated CoFe alloy electrocatalyst (N-C-CoFe) whereas the thermal reaction of KCoHCF in the presence of graphene oxide (GO) produce nitrogen-doped reduced graphene oxide supported CoFe alloy electrocatalyst (N-rGO-CoFe). It is demonstrated that the nature of catalyst support has

Scheme 1. Scheme illustrating the syntheses of N-rGO-CoFe and N-C-CoFe catalyst. 5 ACS Paragon Plus Environment


significant control on the overall performance of the catalyst.

2. METHODS 2.1 Synthesis of N-C-CoFe hybrid and N-rGO-CoFe KCoHCF was synthesized by simple mixing of an aqueous solution of 1:1 molar ratio of K3[Fe(CN)6] and CoCl2.6H2O under constant stirring at room temperature. Then the precipitate was centrifuged after 4h and the residue was washed repeatedly with water and ethanol and the product was dried in vacuum at 80 °C for 12 h. KCoHCF was characterized by standard procedures (Figure S1). The dried KCoHCF was subjected to thermal annealing at 900 °C for 1 h in argon at a heating rate of 5 °C/min to obtained N-C-CoFe electrocatalyst. GO was synthesized and characterized according to the standard procedure (Figure S2). In the synthesis of N-rGOCoFe electrocatalyst, first, KCoHCF was synthesized in the presence of required amount (100 mg) of GO. The weight ratio of GO: K3[Fe(CN)6]:CoCl2.6H2O was kept at 2:1.2:1. The assynthesized GO@KCoHCF was then subjected to thermal annealing as in the previous case to obtain N-rGO-CoFe electrocatalyst. 2.2 Material characterization Fourier transform infrared spectroscopic (FTIR) measurements were performed with a Perkin-Elmer FTIR spectrophotometer RX1. X-ray diffraction (XRD) analysis was carried out with a Bruker D8 advance unit. Transmission electron microscopy (TEM) measurements were performed with a FEI-TECNAI G2 20S TWIN electron microscope operating at a voltage of 200 kV. The scanning transmission electron microscopy (STEM) analysis was performed using JEM 2100F (JEOL, Japan) microscope. Field emission scanning electron microscopy (FESEM) analysis was performed using a FEI NOVA NANOSEM 450, and mapping analysis was carried out with a BRUKER EDS microanalyzer attached to the FESEM instrument. X-ray


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photoelectron spectroscopy (XPS) analysis was performed with PHI 5000 Versa Probe II scanning XPS microprobe (ULVACPHI =1486.6 eV). Thermogravimetric (TG) analysis was carried out using Perkin Elmer Pyris Diamond TG-DTA in air at a heating rate of 5 °C min-1. Electrochemical measurements were performed in a two-compartment three-electrode cell using RDE and RRDE as working, Pt wire as auxiliary, and Hg/HgO (1 M NaOH) as reference electrodes. All electrochemical experiments were performed with Autolab potentiostatgalvanostat (302 N), using computer controlled NOVA software. 0.1M KOH aqueous solution was used as electrolyte. All the experiments have been performed at least three times and reproducible results were obtained. 2.3 Electrode preparation Glassy carbon (GC) electrode coated with a thin layer of electrocatalyst was used as the working electrode. The homogeneous suspension of catalyst ink was prepared by ultrasonically mixing 1.5 mg of as-prepared electrocatalyst with 50 µL of Nafion solution and 250 µL of water/ethanol (1:1 v/v) mixed solvent for 30 min. Required amount (10-15 µL) of the prepared ink was transferred to the surface of a GC rotating disk electrode (RDE) and GC-Pt rotating ring disk electrode (RRDE). The catalyst-modified electrode was allowed to dry at room temperature for 1 h before subjecting to electrochemical experiments.

3. RESULTS AND DISCUSSION 3.1 Synthesis and characterization of N-C-CoFe and N-rGO-CoFe electrocatalyst Scheme 1 illustrates the synthesis of N-C-CoFe and N-rGO-CoFe electrocatalysts. In the synthesis of N-C-CoFe, the catalyst precursor KCoHCF serves as the source of Co, Fe, N and C. On the other hand, GO was used as additional carbon source in the synthesis of N-rGO-CoFe. In both the cases the thermal annealing produces CoFe with different amount of nitrogen and 7 ACS Paragon Plus Environment


carbon. Figure 1a displays the XRD profiles of as-synthesized N-C-CoFe and N-rGO-CoFe electrocatalysts. Diffractions at 44.8°, 65.3° and 82.8° corresponds to the (110), (200) and (211) planes of crystalline cubic CoFe alloy (JCPDS # 49-1568). The XRD profile does not show any signature for metal oxides (CoO, Fe2O3, etc.) suggesting that the material contains only CoFe. The FESEM images show that both N-C-CoFe and N-rGO-CoFe has quasi-spherical shape and the alloy particles are ideally integrated with nitrogen-doped carbon (Figure S3). TEM image (Figure 1b) further confirms the quasi-spherical shape of the CoFe nanoparticles and the integration of CoFe particles with nitrogen-doped carbon. In the case of N-C-CoFe catalyst, majority of the CoFe particles are encased inside the carbon shell. The graphitic carbon shell has thickness of 20-25 nm (Figure 1d and S4). High resolution TEM images (Figure 1c and 1f)

(110)

a Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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#49-1568

N-rGO-CoFe

(200)

c

b

d(002) = 0.34 nm

(211)

N-C-CoFe

d(110) = 0.21 nm

2 nm 20

40

2θ

60

d Spherical carbon

80

f

e 22 nm

d

(110)

= 0.21 nm

15 nm

Figure 1. XRD profile (a), TEM (b.d,e) and HRTEM (c,f) images of N-C-CoFe (b,c,d) and NrGO-CoFe (e,f). Inset of (c) and (f) is the corresponding SAED pattern.


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reveals that the CoFe particles have the lattice fringe spacing of 0.21 nm corresponding to the (110) plane in both the cases. Presence of lattice defects like dislocation is seen in the HRTEM image of N-C-CoFe. Characteristic lattice fringe spacing of 0.34 nm for the (002) plane of graphitic carbon layer is clearly seen in the case of N-C-CoFe (Figure 1c). The CoFe particles and graphitic carbon layer are strongly coupled. The strongly coupled inorganic-organic hybrid materials are known to show pronounced electrocatalytic activity.50 The spotty selected area electron diffraction (SAED) pattern (Inset of figure 1c and 1f) also supports the crystalline nature of the CoFe particles. The tight wrapping of the outer graphitic layers force to bent the carbon layers according to the shape and curvature of the particle surface. Interestingly, growth of carbon with tubular and spherical morphologies along with CoFe particles was also observed (Figure 1d and S4). These carbon nanostructures effectively function as a support for nitrogendoped graphitic carbon encapsulated CoFe. The metal particles actually catalyse the growth of carbon with tubular and spherical morphology.51 The cobalt-based catalyst is known for

a

b

c

Fe

Co

d

C

N-doped graphitic shell

50 nm

N-C-CoFe

e

f

100 nm

100 nm

100 nm

g

Fe

Co

h

C

N-rGO-CoFe

50 nm

100 nm

100 nm

100 nm

Figure 2. High angle annular dark-field (HAADF) images of N-C-CoFe (a) and N-rGOCoFe. Corresponding elemental mapping in STEM mode are shown in (b-d) and (f-h). Images corresponding to N is shown in Figure S5 (supporting information).



favouring the growth of carbon nanoballs and nanotubes.52 In case of N-rGO-CoFe, the particles are randomly distributed over the wrinkled N-rGO sheets (Figure 1e). Unlike the N-C-CoFe catalyst, the CoFe particles are not encapsulated into the graphitic carbon. Both TEM and FESEM image show that the particles have N-rGO rich environment (Figure S3b). The spatial distribution of the different elements present in both the catalysts was examined by STEM analysis (Figure 2 and S5). The uniform distribution of Fe and Co on a single particle (Figure 2b,f and 2c,g) in both the catalysts confirms the growth of CoFe alloy particles. In case of N-CCoFe (Figure 2a-d), the carbon population at the edge of the particle is significantly high and is distinctly seen in the dark-field image (Figure 2d), further confirming the wrapping of the CoFe particles with graphitic carbon layers. No such distinction of elemental carbon around the CoFe particle was noticed in N-rGO-CoFe catalyst (Figure 2h). The total carbon content in both the catalysts was obtained from thermogravimetric analysis and it was found that the % of carbon in N-rGO-CoFe is significantly higher than N-C-CoFe as the former catalyst was synthesized in the presence of GO (Figure S6). The chemical state and the nature of interaction between CoFe and N-C/N-rGO were examined by XPS analysis (Figure 3). As expected, the surface survey scan profile show characteristic signature for C, N, O, Fe and Co in both N-C-CoFe and N-rGO-CoFe catalysts (Figure S7). The atomic percentage of N in N-C-CoFe (2.75%) is more than N-rGO-CoFe (1.60 %) (Table S1). The XPS analysis further shows that the atomic ratio of Co:Fe:N in both N-CCoFe and N-rGO-CoFe is close to 1:1:1 (Table S1). The deconvoluted high resolution N1s profile of N-C-CoFe catalyst (Figure 3a) has four peaks corresponding to pyridinic (398.1 eV)53 N bonded to metal (399.5 eV)54, graphitic (401.1 eV) and N-oxide (404.2 eV). On the other hand, N1s profile of N-rGO-CoFe (Figure 3b) shows peaks corresponding to the


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a

410

c

N1s

405 400 395 Binding energy (eV) C1s

b

Raw Baseline Graphitic Pyridinic N-oxide M-N Deconvo

N1s Raw Baseline Graphitic Pyridinic M-N Pyrrolic N-oxide Deconvo

390

410

d

Raw Baseline

405 400 Binding energy (eV)

C 1s

C-N C=C

395

Raw C=C C-N Baseline Deconvo

Deconvo

290

288

286 284 282 Binding energy (eV)

280

290

288 286 284 282 Binding energy (eV)

280

Figure 3. High resolution N1s (a,b) and C1s (c,d) spectral profile of N-C-CoFe (a,c) and NrGO-CoFe (b,d). pyridinic (398.3 eV), pyrrolic (400.4 eV), graphitic (401.2 eV) and pyridinic N-oxide (404.2 eV) types of nitrogen along with N bonded to metal (399.3 eV). The careful analysis of the N1s high resolution profile of N-C-CoFe reveals that it has (i) significantly high amount of graphitic nitrogen (45%) and metal bonded nitrogen (28%), (ii) no contribution from pyrrolic nitrogen, (iii) the pyridinic contribution is very close to that of N-rGO-CoFe and (iv) N-oxide contribution is significantly less with respect to N-rGOCoFe (Table 1). The high resolution C1s profile of N-rGO-CoFe and N-C-CoFe (Figure 3c-d) shows signature for both sp2 C-C bond and C-N bonds. The degree of graphitization (% sp2 C) in N-C-CoFe is significantly high compared to the N-rGO-CoFe (Table 1). Presence of such graphitic carbon is highly advantageous for electrocatalytic application.55-56 The graphitic carbon is known to impart high durability to the catalysts.



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N1s

C 1s

Catalysts

Pyridinic

Graphitic

Pyrrolic

N-O

N-M

sp C-C

2

C-N

N-CCoFe

15%

45%

nil

11%

28%

73%

27%

N-rGOCoFe

17%

26%

18%

22%

15%

62%

37%

Table 1. Chemical nature and percentage of N and C in N-C-CoFe and N-rGO-CoFe catalysts obtained from XPS analysis. The high resolution profiles of Co and Fe show characteristic signature for both the elements (Figure S8). 3.2 Electrocatalytic ORR and OER Figure 4 compares the electrocatalytic performance of N-C-CoFe, N-rGO-CoFe and 20% Pt/C towards ORR under hydrodynamic condition in O2 saturated 0.1 M KOH solution. The careful examination of the voltammetric profiles shows that the limiting current obtained with NC-CoFe catalyst is same as that of the 20% Pt/C, though the E1/2 value is slightly less positive. The ORR performance of N-C-CoFe is significantly higher than N-rGO-CoFe catalyst in terms of onset potential and limiting current density. The onset potential (0.85 V) and E1/2 value (0.70 V) of N-C-CoFe hybrid is more positive than the N-rGO-CoFe and close to that of Pt/C, confirming the superior ORR activity of N-C-CoFe. As the catalyst that favors the direct reduction of oxygen to water is ideally required for practical applications in energy conversion and storage devices, the performance of the catalysts was further evaluated by calculating the number of electrons (n) transferred during ORR using Koutecky-Levich (K-L) and RRDE 12 ACS Paragon Plus Environment

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analyses. The slope of the K-L plot (Inset of Figure S9) does not change over a wide range of potential, implying that the electron transfer mechanism remain same at all potentials. The n value was calculated to be 3.9 and 3.78 with N-C-CoFe and N-rGO-CoFe, respectively, suggesting that both catalysts favour the 4-electron reduction of oxygen to water. This is further confirmed by calculating the % of HO2− generated during ORR with RRDE using the disk and ring currents (Figure 4b). It is interesting to note that very small amount of HO2− (15% decrease in the catalytic current density was noticed with N-rGO-CoFe during the durability test (Figure S10b). The enhanced ORR activity of N-C-CoFe can be rationalized by considering its chemical composition and nature of nitrogen species of the catalyst support. It is well established that the ORR performance of nitrogen-doped carbon-based catalyst largely depends on the total N content. N doping enhances the ORR activity by altering the electroneutrality of the C-C bond which helps to bind oxygen.57 In our case, the N content of N-C-CoFe is higher than N-rGOCoFe and it has pronounced ORR activity. It is generally accepted that the pyridinic/graphitic nitrogen enhances the overall ORR activity of the catalyst; though the actual mechanism and the active site are not well established.57-62 The pyrrolic nitrogen and N-oxide species do not have significant contribution to ORR activity. In the case of transition metal containing N-doped carbon-based catalyst, the lone pair of electrons on pyridinic nitrogen and pyrrolic nitrogen favors the coordination of metal. The graphitic nitrogen is known to play vital role in controlling the ORR activity. In our case, although both N-C-CoFe and N-rGO-CoFe catalysts have almost same amount of pyridinic nitrogen, the former catalyst has superior electrocatalytic activity in


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terms of current density, onset potential, durability and number of electrons transferred. As shown in Table 1, the amount of graphitic nitrogen in N-C-CoFe is >1.75 times higher than NrGO-CoFe. Moreover, the degree of graphitization (% of sp2-C) of carbon in N-C-CoFe is higher than its counterpart. It is worth highlighting here that CoFe is actually encased inside the Ndoped graphitic carbon shell and such encasing alters the local work function34 and favors enhanced electron transfer for the electrocatalytic reaction and assures high durability of the catalyst in harsh oxidative environment. The high degree of graphitization and the presence of large amount of graphitic nitrogen in the catalyst support (N-C) actually plays crucial role in controlling the ORR activity of CoFe. Figure 5 illustrates the OER activity of our alloy catalysts and the traditional OER catalyst RuO2. The OER activity of the catalysts is evaluated by measuring the onset potential and the overpotential at which the benchmark current density of 10 mA cm−2 is achieved. As can be seen from the Figure 5, both N-C-CoFe and N-rGO-CoFe outperform the traditional OER catalyst RuO2 in terms of benchmark current density and overpotential (η). Interestingly, the benchmark current density of 10 mA cm−2 was achieved at η of 0.22 and 0.29 V with N-rGO-

a

25 2

20 15 10

N-rGO-CoFe N-C-CoFe RuO 2

η = 0.22 V η = 0.29 V η = 0.30 V

5 0 -5 1.0

1.45 1.44

b N-C-CoFe

1.38

c de V/ m 0 10

E/V vs. RHE

30

E/V vs. RHE

35

Eo(H2O/O2)= 1.23

CoFe and N-C-CoFe, respectively. The OER activity of both catalysts is superior to that of

J (mA/cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.43 1.42 1.41

1.2

1.4 1.6 E/V vs. RHE

1.8

c

N-rGO-CoFe -1

ec Vd m 4 9

1.36 1.34 1.32

-3.3

-3.2 2 log Jk (mA/cm )

-3.1

0.0

0.1 0.2 2 log J(mA/cm )

0.3

0.4

Figure 5. (a) Polarization curves illustrating the OER activity of different catalysts. Sweep rate: 5 mV s-1. Electrolyte: 0.1 M KOH. Rotation: 1600 rpm. Tafel slope corresponding to the polarization curve is shown in (b) and (c).



the traditional RuO2 and the other non-precious metal-based catalysts (Table S2). As shown in Figure 5a the traditional RuO2 catalyst could achieve the benchmark current density at more positive potential than our catalyst. To the best of our knowledge this is first report where CoFebased catalyst could provide the benchmark current density at overpotential of 0.22 V (Table S2). The careful survey of literature shows that Cai et. al has achieved the current density of 10 mA/cm2 with alloy-based catalyst at 1.49 V (RHE) (η= 0.26 V), which is ∼40 mV more positive than our N-rGO-CoFe hybrid.46 During the course of this investigation Du et. al reported the OER activity of NiFe-based catalyst and achieved benchmark current density at η of 394 mV (in 0.1 M KOH) which is >170 mV more positive than our hybrid catalyst.48 The kinetics of OER was further evaluated by Tafel analysis (Figure 5b-c). Tafel slope of 94 mV/dec and 100 mV/dec was obtained with N-rGO-CoFe and N-C-CoFe catalyst, respectively, indicating the mixed first and second-order reaction kinetics with respect to OH− concentration.44 The durability of the catalyst was examined by continuously monitoring the current while holding the electrode potential at 1.55 V for 3 h (Figure S11). The N-C-CoFe catalyst could retain >88% of its initial current even after 3 h continuous operation whereas a significant drop (~65%) in the initial current was noticed with N-rGO-CoFe catalyst (Figure S11). In order to understand the enhanced durability of N-C-CoFe compared to N-rGO-CoFe, post-mortem XRD and TEM analyses were performed. The catalyst-modified electrode was held at the potential of 1.55 V for 3h and the catalyst after durability test was subjected to XRD and TEM analyses. Post-OER XRD analysis shows that both N-C-CoFe and N-rGO-CoFe retains their initial diffraction pattern (Figure S12) implying that the crystallinity of the catalyst remains unchanged. Post-OER TEM analysis was further performed to know the morphological change (Figure 6). Both N-C-CoFe and N-rGO-CoFe retained their initial shape and size confirming that


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a

0.21 nm (CoFe)

b

c

0.25 nm (CoFe2O4)

d

0.32 nm (γ− γ−FeOOH) γ−

f

e

0.32 nm (γ− γ−FeOOH) γ−

0.25 nm (CoFe2O4) 0.21 nm (CoFe)

Figure 6. Post-OER TEM (a,d) and HRTEM (b,c,e,f) images of N-C-CoFe (a-c) and N-rGOCoFe (d-f). Red, green and yellow colored rectangles represent the area of higher magnification. the CoFe catalysts do not undergo sintering during OER. However, severe damage to N-rGO sheets of N-rGO-CoFe has been noticed (Figure 6d, S13c,d). The N-rGO sheets lost its initial morphology during the durability test. In case of N-C-CoFe catalyst, the initial spherical morphology of carbon is retained even after durability test (Figure S13a,b), though some portion of CoFe particle lost its encasing graphitic carbon layer (Figure 6a). The HRTEM analysis reveals the transformation of some portion of CoFe particle into CoFe2O4 and γ-FeOOH (Figure 6), though the post-OER XRD analysis did not show any characteristic signature for the oxides. The HRTEM images evidences the fringe spacing corresponding to the initial CoFe, CoFe2O4 and γ-FeOOH. Growth of such oxides can also enhance the catalytic activity towards OER. The N-rGO-supported CoFe could not retain its structural integrity at high positive potential in corrosive environment leading to the decrease in the catalytic current density. However, in case 17 ACS Paragon Plus Environment


of N-C-CoFe, the encasing of CoFe with closed-shell nitrogen-doped graphitic layers and N-C support affords high durability. The OER activity of the transition metal containing nitrogen-doped carbon-based catalyst largely depends on the nature of metal species whereas the chemical nature of nitrogen control the ORR activity.63 In order to probe the active site of the catalyst for ORR and OER, catalyst poisoning with SCN− was performed (Figure 7). SCN− is known to bind at the metal sites and limits the contribution from the possible metal-based active sites. In our studies, we found that the addition of SCN− does not affect the ORR activity of both the catalysts. However, significant drop (~75%) in the current density for OER at the potential of 1.7 V upon the addition of SCN− was noticed with N-rGO-CoFe catalyst. The benchmark current density could not be achieved even at η> 470 mV (Figure 7b). On the other hand, very interestingly, the OER activity of N-CCoFe does not significantly affected by SCN−, though slight decrease in the current density was noticed at 1.7 V (Figure 7a). As discussed earlier the N-C-CoFe alloy is encased into the graphitic layer whereas N-rGO-CoFe is not. The slight decrease in the current density on N-CCoFe catalyst could be due to the incomplete encasing of CoFe surface with nitrogen-doped 30

N-C-CoFe

25

15 10 5

15

before SCN - addition

10

after SCN - addition

5

0

0

-5

-5

0.4

0.8 1.2 E/V vs. RHE

N-rGO-CoFe

20

Before addition of SCN After addition of SCN

20

b

2

25

a

J (mA/cm )

30

J (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 29

0.4

1.6

0.8 1.2 E/V vs. RHE

1.6

Figure 7. Plot illustrating the influence of catalyst poisoning on the ORR and OER activity of (a) N-C-CoFe and (b) N-rGO-CoFe with SCN−. [SCN−]= 10 mM, Electrolyte: 0.1 M KOH, Rotation speed: 1600 rpm


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graphitic layer. The TEM measurement evidences such incomplete encasing of CoFe surface (Figure 1b). The unencapsulated portion of CoFe undergoes poisoning by the coordination of SCN−. Our catalyst poisoning study suggests that (i) the bimetallic alloy surface does not participate in ORR, (ii) metal-based active sites are crucially required for OER, and (iii) in case of N-C-CoFe, encasing of CoFe with graphitic carbon layers makes the metal surface less accessible for SCN−. The superior OER activity of N-rGO-CoFe in terms of onset potential, overpotential and Tafel slope supports this conclusion. The catalyst poisoning studies further implies that the nitrogen containing graphitic carbon on the catalyst support is the key requirement to achieve good ORR activity. The development of rechargeable metal-air battery requires efficient bifunctional electrocatalyst that can favors ORR and OER at low η. The ideal oxygen electrocatalyst should have very small separation between the ORR and OER potential (∆E) and it should be highly durable. Lower the ∆E, the catalyst is closer to reversible oxygen electrode. The catalyst that has

40

-2

20 10 0 -10

1.23 V

N-C-CoFe

30

0 E (H2O/O2)

N-rGO-CoFe

J (mA cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-2 10 mA/cm

ORR E1/2

0.0

∆E= 0.82 V ∆E= 0.92 V

0.4 0.8 1.2 E/V vs. RHE

1.6

2.0

Figure 8. Polarization curve illustrating the overall oxygen electrocatalytic performance of NC-CoFe and N-rGO-CoFe. 19 ACS Paragon Plus Environment


lower ∆E is considered to have excellent bifunctional activity. Achieving small ∆E is a challenging task, as the same catalyst may not be able to catalyse both ORR and OER at low η. Proper tuning of the surface properties of the catalyst is required to achieve small ∆E. As can been from Figure 8, our N-C-CoFe and N-rGO-CoFe catalyst have the ∆E value of 0.92 V and 0.82 V, respectively, highlighting the overall good activity of the catalyst. The limited durability of the existing catalyst due to sintering or agglomeration is a serious concern. In our case, although we could achieve small ∆E for N-rGO-CoFe catalyst, it could not retain its activity during durability test owing to the loss of the structural integrity of the catalyst support. However, the N-C-CoFe could maintain its activity due to the presence of graphitized N-C support. The presence of tubular and spherical nitrogen doped carbon nanostructure (Figure S4) along with N-C-CoFe enhances the overall performance of the catalyst.

4. CONCLUSION In summary, we have demonstrated the synthesis of bifunctional N-C-CoFe and NrGOCoFe catalysts and the influence of catalyst support on the bifunctional electrocatalytic activity of CoFe towards ORR and OER. Although both catalysts favor four electron pathway for the reduction of oxygen N-C-CoFe has high durability due to the presence of nitrogen doped graphitic carbon. The ORR and OER activity of our catalyst is superior to that of the traditional Pt/C and RuO2 catalysts. N-C-CoFe has significantly high ORR activity whereas the N-rGOCoFe has pronounced OER activity. Our studies show that the (i) the chemical nature of nitrogen species and the degree of graphitization of the catalyst support have significant control over the ORR activity, (ii) encapsulation of the active catalyst into nitrogen-doped graphitic carbon and graphitized carbon environment ensures the high durability of the catalyst in harsh environment


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and (iii) nature of metal species largely controls the OER activity whereas the ORR activity depends on the chemical nature of the doped-nitrogen present in the carbon support. The N-CCoFe can function as an efficient bifunctional electrocatalyst for ORR and OER for the development of rechargeable metal-air battery. The nitrogen-doped carbon with high degree of graphitization and large content of graphitic nitrogen can be an ideal catalyst support for the bifunctional alloy electrocatalyst.

Supporting Information Synthesis of GO, IR, XRD of GO and KCoHCF, FESEM, TEM images and Mapping of N-CCoFe, XPS surface survey and Fe2p Co2p profile, TG profile, RDE polarization curves, ORR and OER durability, Post-OER XRD and TEM analysis, atomic percentage table and activity comparison table.

ACKNOWLEDGEMENTS This work was financially supported by Science and Engineering Research Board, Department of Science and Technology, (DST) India (Grant No. EMR/2016/002271). Samanta is thankful to DST and IIT Kharagpur, India for the financial support. We are also grateful to the ACMS, IIT Kanpur for the XPS measurements.



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Catalyst Support in Oxygen Electrocatalysis: A Case Study with CoFe

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