Iron oxide decorated N-doped carbon derived from poly(ferrocene-urethane) interconnects for the oxygen reduction reaction

Article abstract of DOI:10.1039/C8NJ02529K

We report the synthesis of mixed iron oxide particles decorated on nitrogen-doped carbon by forming covalent polyurethane linkages between ferrocene and phloroglucinol. Contrasting the electrostatic interactions and the physical cross-linking of the Fe/N/C precursors, here poly(ferrocene-urethane) (PFU) was acquired exclusively by (i) conversion of dicarboxylic acids present in ferrocene to azide linkages; and (ii) chemically linking the ferrocene-diazides to phloroglucinol through urethane bonds. The as obtained poly(ferrocene-urethane) was pyrolyzed under argon at elevated temperatures (600, 800 and 1000 °C) to yield iron oxide decorated N-doped carbon interconnects. The resultant porous materials were found to have the right balance of hierarchical porosity (micro-, meso- and macro-), and N-doping and iron oxide phases in and on the carbon matrices, respectively. Among the PFU samples pyrolyzed at different temperatures, the sample pyrolyzed at 800 °C displayed a more positive onset potential, a higher current density, and direct four electron transfer kinetics with a lesser yield of H₂O₂ (5.6%) on par with the commercially available Pt catalysts for the oxygen reduction reaction (ORR) in alkaline KOH solution. The superior electrocatalytic activity of PFU-800 can be traced to the synergistic effect between the iron oxide on and N-doping of the porous carbon nanostructures and the balanced hierarchial porosity. The developed non-Pt based electrocatalyst can be considered as a promising electrocatalyst in alkaline fuel cell applications.

Full text of DOI:10.1039/C8NJ02529K

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Chandrasekaran, V. Premkumar, M. Kathiresan, K. Madasamy, K. Pandiyaraj and S. Senthil Kumar, New J.
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Iron oxide decorated Nꢀdoped carbon derived from Poly (ferroceneꢀ
urethane) interconnects for oxygen reduction reaction
Naveen Chandrasekaran*, Viji Premkumar, Kanagaraj Madasamy, Murugavel Kathiresan*,
Pandiyaraj Kanagavalli, Shanmugam Senthil Kumar*
CSIRꢀCentral Electrochemical Research Institute, Karaikudi, Tamil Nadu, 630 003, India.
Email: naveen@cecri.res.in, kathiresan@cecri.res.inand ssenthilmugam@gmail.com

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Abstract
We report the synthesis of mixed iron oxide particles decorated on nitrogenꢀdoped
carbon by forming covalent polyurethane linkages between ferrocene and phloroglucinol.
Contrasting the electrostatic interactions and the physical crossꢀlinking of the Fe/N/C
precursors, here the poly(ferroceneꢀurethane) (PFU) were acquired exclusively by the (i)
Conversion of dicarboxylic acids present in ferrocene to azide linkages; (ii) Chemically
linking the ferroceneꢀdiazides to phloroglucinol through urethane bonds. The as obtained
poly(ferroceneꢀurethane) were pyrolyzed under argon at elevated temperatures (600, 800 and
o
1000 C) to yield iron oxide decorated Nꢀdoped carbon interconnects. The resultant porous
materials were found to have a right balance of hierarchical porosity (microꢀmeso and macroꢀ
), Nꢀdoping and iron oxide phases in and onto the carbon matrices respectively. Among the
o
PFU samples pyrolyzed at different temperatures, the sample pyrolyzed at 800 C displayed
more positive onset potential, higher current density, and direct four electron transfer kinetics
with lesser yield of H2O2(5.6%) on par with the commercially available Pt catalysts for
oxygen reduction reaction (ORR) in alkaline KOH solution. The superior electrocatalytic
activity of PFUꢀ800 can be traced to the synergistic effect between iron oxide and Nꢀdoping
on the porous carbon nanostructures and the balanced hierarchial porosity. The developed
nonꢀPt based electrocatalyst can be considered as one of the promising electrocatalysts in
alkaline fuel cell applications.
Key words: Fe3O4, Fe2O3, Nꢀdoping, ORR

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Introduction
Due to the impact of depleting fossil fuel, it is imperative to enable alternate inexpensive feed
stocks and catalysts for the generation of energy and value added chemical products.
Electrochemical devices (fuel cells, Metalꢀair batteries) other than lithium ion storage
systems are less viable for technological applications due to expensive electrocatalysts
employed for efficient energy conversion reactions. For example, platinum and iridium oxide
are considered as the benchmark electrocatalysts for fuel cell reactions. However the high
cost, and paucity of the precious metal and metal oxides has hampered the progress of fuel
cell based technologies to be commercialized.1ꢀ 5 Even though the reduction of cost can be
achieved by introducing lower loading of platinum content on carbon, conducting polymers,
and carbides, platinum nanoparticles also undergoes dissolution, sintering and agglomeration
leading to instability and decreased activity during prolonged fuel cell operation.6ꢀ8 To
overcome these drawbacks, earth abundant inexpensive transition metal or metal oxide based
electrocatalysts supported on pristine or heteroatom doped carbon substrates has grabbed
researchers’ attention as replacement for noble metal or metal oxide electrocatalysts.9ꢀ13
Owing to their fascinating physicoꢀchemical properties and lowꢀcost, transition metal based
covalent organic frameworks are pursued for diverse technological applications such as
sensing, gas storage, energy conversion and storage. Since the discovery of cobalt
phthalocyanine as the ORR catalyst, various FeꢀN4 or CoꢀN4 macromolecules have been
explored to electroꢀcatalyze the reduction of oxygen.14ꢀ18 The macromolecules were made
suitable for electrocatalysis by physical adsorption of macrocycles onto carbon supports or by
pyrolysis at inert atmosphere resulting in metal or metal oxide decorated Nꢀdoped carbon.
Later these expensive macromolecules were replaced by simultaneous pyrolysis of individual
N and Co precursors at elevated temperatures. Electronic conductivity a vital property in

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electrocatalysis were introduced to MOFs by coupling them with reduced graphene oxide
(rGO), CNT’s and metal nanoparticles.19ꢀ23
To date, variety of synthetic protocols has been reported for the preparation of iron or
iron oxide based porous networks for ORR. For example, iron salts were impregnated with
carbon black followed by subsequent heat treatment with NH3 as the nitrogen precursor for
the synthesis of Fe/N/C electrocatalysts.24, 25 In another approach, iron salts were mixed and
pyrolyzed together with preformed Nꢀdoped graphene sheets. Also, hydrothermal assisted
monolithic three dimensional assemblies of nitrogen–doped graphene aerogel–supported
Fe3O4 nanoparticles were found to effectively catalyze ORR.13 The above mentioned
synthetic protocols rely on the electrostatic interactions of the metal precursors onto the
carbon source and external nitrogen sources such as ammonia or cyanamide for the synthesis
of FeꢀNꢀC hybrids.24, 25For a nonꢀprecious metal or metal oxide catalyst to function on par
with platinum based nanoꢀstructures, it is essential to have balanced physical and chemical
properties. It is thus essential to have (i) Transition metal (TM) cations coordinated with
surface nitrogen (TMꢀN catalytic sites) species, (ii) Catalysts supported on conductive
substrates (iii) Controlled elemental TM/C/N composition in the hybrid electrocatalysts(iv)
Balance of microꢀand macroꢀpores for affluent TMꢀN catalytic sites and effective mass
transfer to quench the available catalytic sites respectively.
In the present work, we demonstrate a facile synthetic approach for the synthesis of
iron oxide decorated Nꢀdoped carbon derived from poly (ferroceneꢀurethane) interconnects
for ORR. The obtained ORR activity of the synthesised electrocatalysts activity are discussed
o
by comparing the physically mixed ferroceneꢀphloroglucinol sample (FꢀP) at 800 C and
commercially available EꢀTek 10 % wt. loading of Pt/C.

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Materials and methods
1,1’ꢀFerrocene dicarboxylic acid (FDCA), 98% (Alfa Aesar), Phloroglucinol, 99%
and Toluene (SigmaꢀAldrich) were used as received.
FTꢀIR analysis was carried out in BRUKER ꢀ TENSOR 27 (Optik GmbH) using RT
DLaTGS (Varian) Detector. 13C NMR and 15N NMR (solid state) measurements were carried
out on a Bruker Avance 400 spectrometer operating at 100.6 MHz for 13C and 40.53 MHz for
15N using a Bruker 4 mm double resonance probeꢀhead at a spinning rate of 10 KHz. Thermo
gravimetric analysis (TGA) used to study the thermal degradation of synthesized materials
was determined by using a TGA / SDT Q600, TA instruments, USA at a scanning rate of 5o C
/ min, from room temperature to 600o C under nitrogen atmosphere. Xꢀray diffraction patterns
were recorded using a Bruker D8 Advance Xꢀray diffractometer with a Cu Kα radiation (λ =
1.5418 Å). BET isotherms were obtained using nitrogen adsorption/desorption porosimetry,
Quantachrome Autosorbꢀ1 surface area analyser. Xꢀray photoelectron spectroscopy (XPS) was
performed using SPECS, Phoibos 100 MCD Analyzer with pass energy of 20 eV (Al Kα anode
(1486.6 eV)) in ultrahigh vacuum (5 × 10–10 mbar). Raman spectroscopy was carried out using a
Renishaw Ramanscope 2000 spectrometer with HeꢀNe 632 nm laser. Morphological
characterizations were done using field emission scanning electron microscope (FEꢀSEM) (Zeiss
supra 55VP) and high resolutionꢀtransmission electron microscope images (HRꢀTEM) for the
samples were taken using Tecnai G2 (FEI make).
Electrochemical Characterization
Cyclic voltammetry (CV) experiments were performed using IVIUM instrument. BASi
glassy carbon electrode (area of the electrode is 0.07 cm2) was used as a working electrode for
cyclic voltammetric studies. Linear Sweep Voltammetry (LSV) with various rotation rate
experiments were conducted using Rotating Ring Disk Electrode (RRDE) of GCE Disk (area is
0.2475 cm2) with Pt ring (area is 0.1866 cm2& ring collection efficiency – 37%) from Pine

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instrument. The same was used as working electrode for ORR kinetics studies. Ag/AgCl used as
reference electrode and measured potential were normalized to hydrogen electrode potential
(NHE). Pt foil was used as counter electrode.
2.5 mg of the synthesised iron oxide decorated Nꢀdoped carbon derived from Poly
(ferroceneꢀurethane) interconnects was used as electrocatalyst. It was dispersed in a mixture of
water (625 µl) and 5% Nafion (125 µl) solution by ultrasonication for 20 minutes. These dispersed
aliquots were dropꢀcasted on wellꢀpolished and preꢀcleaned working electrodes, 3 µl on BASi ꢀ
GCE and 10 µl on the GCE disk for RRDE. The modified electrodes were allowed to dry at room
temperature for about 30 minutes prior to electrochemical measurements. The CV and RRDE
experiments were performed after the activation of catalytic layer and ring by potential cycling in
the deꢀaerated 0.5 M H2SO4 solutions. ORR was carried out using RRDE cell setup in the oxygen
saturated 0.1 M KOH solution at the scan rate 5 mV sꢀ1. The obtained byꢀproduct during ORR was
analysed simultaneously at the ring electrode of RRDE by keeping constant potential for H2O2
oxidation (1.1 V vs NHE).
Scheme 1.Synthesis of Poly (FerroceneꢀUrethane) (PFU) linkages

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a)
b)
Figure 1. a) FTIR (left) and b) 13C (right) NMR spectra of Phloroglucinol,
Ferrocenedicarboxylic acid (FDCA), Ferrocenediacylazide (FDA) and Poly(ferroceneꢀ
urethane) (PFU)
Results and Discussion
Reported procedure26 was followed to obtain 1,1’ꢀbis(azidocarbonyl)ferrocene
(49%)(Scheme 1).The final step involves the inꢀsitu synthesis of polyurethane from
ferrocenediacylazide[bis(2ꢀ(azidocarbonyl)cyclopentaꢀ2,4ꢀdienꢀ1ꢀyl)iron] and phloroglucinol.
Ferrocenediacylazide (0.05 g, 0.15 mmol) was dissolved in 10 ml of dry toluene, and
followed by addition of phloroglucinol (0.013 g, 0.10 mmol) to the reaction mixture. The
whole reaction mixture was heated to 110o C and was refluxed for 24 h under constant
stirring. After completion of the reaction, the black color precipitate was filtered, washed
with toluene and methanol. The residue was then dried to get pure porous polyurethane PFU
(0.09 g).13C NMR of the resultant polymer showed the characteristic carbonyl ‘C’ peak at δ
170 ppm corresponding to the amide linkage. Ferrocene, and phloroglucinol ring carbons
appeared in the range δ 65.9, 73.9, 91.3 and 99.1 ppm and δ 118.9, 130.0, 134.7, 147.3 and
154.3 ppm respectively (Figure 1b). The successful formation of the polymer was confirmed
by comparing the changes in the chemical shift values andtheir corresponding monomers.

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Figure 2. Thermogravimteric analysis (TGA) of Poly(ferroceneꢀurethane) (PFU) under N2
The IR spectrum of the starting materials and the resultant polymer were shown in
Figure 1a. FTꢀIR spectra of phloroglucinol showed the characteristic vibration bands such
asν(OH), δ(C=C), δ(OH)at 3394, 1528 and 1003 cmꢀ1respectively. On the other hand, FDCA
showed the peaks viz δ(C=C), ν(C=C) and δ(OH)at 1500, 1305 and 923 cmꢀ1 respectively in
addition to the characteristic ν(OH), ν(C=O) and ν(FeꢀC) peaks at 2884, 1680, 520 cmꢀ1
respectively. The appearance of sharp peaks at 2141 and 1686 cmꢀ1 corresponds to azide
(ν(N3)) and carbonyl (ν(C=O)) groups respectively. Additional vibrational peaks such as
ν(OH),δ(C=C) and ν(CꢀO) appeared at 3474, 1457 and 1262 cmꢀ1 in ferrocenediacylazide. IR
spectra of the PFU showed the characteristic ν(N–H) and ν(CꢀO) vibrations at 3489 and 1738
cmꢀ1confirming the presence of amide linkage between ferrocenediacylazide and
phloroglucinol. These characteristic vibrational peaks confirm the successful formation of
urethane linkages. In addition to these, peaks such as δ(C=C) and ν(FeꢀC) at 1565 and 493
cmꢀ1 were observed.TGA measurement was performed to estimate the thermal stability of the

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polymer material. The weight losses observed in the TGA experiment is given in Figure 2.
TGA of PFU in nitrogen atmosphere revealed four weight loss temperatures. The first weight
loss (3.23%) indicates the loss of water molecules up to 169º C. Between 169º and 310º C,
weight loss of about 16.71% is observed which could be attributed to the removal of solvent
molecules and after these temperatures there was significant weight loss attributed to the
decomposition of organic moiety.
(A)
(B)
Figure 3.(A) FEꢀSEM image, (B) N2 sorption isotherm, (C) HRꢀTEM image and (D) SAED
pattern of Poly(ferroceneꢀurethane) (PFU)

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Figure 3 displays the FEꢀSEM image, N2 sorption isotherm, HRꢀTEM image and its
corresponding SAED pattern of as prepared PFU before pyrolysis. FEꢀSEM image displayed
sheet and particulate like morphology with macropores with pore size regime between 200
nm and 1 µm. The N2 sorption isotherm of PFU rise above higher relative pressures (>0.9)
and do not exhibit saturation with narrow hysteresis loops and significant specific volumes
adsorbed at low relative ascribing to the presence of macropores with surface area of 8 m2 gꢀ1.
The presence of macropores and larger particles can be attributed to the minimal surface area
of the as the prepared PFU sample. Consistent with the FEꢀSEM image the HRꢀTEM image
also displayed sheet (> 200 nm) and particulate morphology. The SAED pattern of PFU
sample showed fuzzy rings owing to its amorphous nature.
Figure 4.XRD pattern of PFUꢀ600, PFUꢀ800 and PFUꢀ1000
o
Upon pyrolysis under Argon at different temperatures (600, 800 and 1000 C), PFU
was converted to iron oxide decorated Nꢀdoped carbon. Figure 4 compares the XRD pattern
of PFU pyrolyzed at different temperatures (600, 800 and 1000oC) and physically mixed

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o
ferroceneꢀphloroglucinol sample (FꢀP) at 800 C. XRD patterns of FꢀPꢀ800 and PFUꢀ
600showed crystalline reflections of magnetite (Fe3O4) at 30.25o (200), 35.8o (311),43.37o
(400), 53.9o (422), 57.37o (511), 63.04o (440) whereas PFUꢀ800 displayed crystalline
reflections of Fe3O4 with additional crystalline peaks of hematite (αꢀFe2O3), on the other hand
PFUꢀ1000revealed complete conversion of Fe3O4 to αꢀFe2O3 with the its respective
crystalline phases at 24.3o (012), 33.2o (104) , 35.8o (110), 40.9o (113), 49.6o (024), 54.2o
o
(116), 57.8o (018), 62.6o (214) and 64.2 (300). The crystallite size calculated from
Scherrer’s formula for PFUꢀ600, PFUꢀ800, PFUꢀ1000 and FꢀPꢀ800 were found to be 26.5,
37.4, 53.8 and 91.1 respectively. Due to the amorphous nature of the carbon from PU
linkages and the presence of predominant iron oxide crystalline reflections, definite carbon
reflections were not found.
Figure 5.Raman spectra of FꢀPꢀ800 and PFUꢀ600, PFUꢀ800, PFUꢀ1000
To confirm the harmonized presence of carbon and iron oxide, Raman spectra of PFUꢀ600,
PFUꢀ800, PFUꢀ1000 and FꢀPꢀ800 were compared and displayed in Figure 5. Raman spectra
of PFUꢀ600, PFUꢀ800 and FꢀPꢀ800 revealed characteristic FeꢀO bands (A1g: 223 and 496
cmꢀ1; Eg: 291, 408 and 610 cmꢀ1) associated with Fe3O4 in addition to the D and G bands at
~1350 cmꢀ1 and 1594 cmꢀ1 from the carbon associated to pyrolyzed polyurethane linkages.
Whilst, in accordance with XRD pattern, Raman spectrum of PFUꢀ1000 displayed peaks

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corresponding to Fe2O3, with disappearance of D and G bands of carbon. The absence of D
and G bands of carbon in the PFUꢀ1000 can be addressed to the oxidation of carbon to
gaseous products at higher temperatures. The XRD patterns and Raman spectra together
substantiates the formation of iron oxide on carbon in the PFUꢀ600, PFUꢀ800 and FꢀPꢀ800.
200 nm
1 µm
PFUꢀ1000
PFUꢀ1000
200 nm
2 µm
PFUꢀ800
PFUꢀ800
200 nm
2 µm
PFUꢀ600
PFUꢀ600
Figure 6.HRTEM image (left) (Inset: corresponding SAED pattern) and FEꢀSEM (right)
(Inset: Corresponding High resolution image, scale bar 200 nm) of PFUꢀ600, PFUꢀ800 and
PFUꢀ1000.

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Figure 6 displays the HRꢀTEM and FEꢀSEM images of PFUꢀ600, PFUꢀ800, PFUꢀ
1000. The HRꢀTEM images of PFUꢀ600 displays particles with size in the range of ~100 nm
embedded on carbon, the corresponding SAED pattern revealed bright spots in agreement
with the crystalline reflections of Fe3O4 observed in XRD pattern. The FEꢀSEM image of
PFUꢀ600 revealed particle like morphology embedded on porous carbon with macroporous
structure. The HRꢀTEM image of PFUꢀ800 displayed exhibits cubes and spherical particles
embedded on carbon with a size regime between 20ꢀ50 nm. Consistent with the XRD pattern
of PFUꢀ800, SAED pattern revealed bright spots which can be attributed to Fe3O4 crystal
planes. FESEM image of PFUꢀ800 displayed Fe3O4 particles embedded on mesoporous
carbon. Unlike PFUꢀ600 and PFUꢀ800, HRꢀTEM images of PFUꢀ1000 displayed sheet like
structure with size more than 100 nm. The corresponding SAED pattern showed bright spots
which can be ascribed to the Fe2O3 lattices. In contrast to the definite morphological features
of PFUꢀ600 and PFUꢀ800, PFUꢀ1000 exhibited distorted and interconnected sheet like
structures leaving voids in the macroporous regime.

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PFUꢀ600
PFUꢀ600
PFUꢀ800
PFUꢀ800
PFUꢀ1000
PFUꢀ1000
Figure 7.N2 sorption isotherms (left) and corresponding BJH plots (right) of PFUꢀ600, PFUꢀ
800 and PFUꢀ1000

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Figure 7 displays the N2 sorption isotherms and BJH desorption plots PFUꢀ600, PFUꢀ
800, PFUꢀ1000 and FꢀPꢀ800.The N2 sorption isotherms of PFUꢀ600 rise above a relative
pressure of 0.9 and do not exhibit saturation. They also indicate narrow hysteresis loops with
significant specific volumes adsorbed at low relative pressures attributing to the presence of
both mesoꢀ microꢀ and macroꢀpores with specific surface area of 262 m2gꢀ1. PFUꢀ800
displayed type IV isotherm due to the presence of mesopores in addition to the H2 hysteresis
loop and micropores at relatively low relative pressures with BET surface area of 241 m2gꢀ1.
The corresponding isotherm of PFUꢀ800 also indicates slender hysteresis loops and
significant specific volumes adsorbed at low relative pressures confirming the occurrence of
both mesoꢀ and macroꢀpores. The cumulative pore volumes of PFUꢀ600 and PFUꢀ800 were
found to be 0.267 and 0.272 cm3 gꢀ1. Consistent with the N2 sorption isotherms the
corresponding pore size distributions of PFUꢀ600 and PFUꢀ800 derived from the BJH method
displayed maxima of 3.5 and 3.7 nm in the microꢀand mesopore regime. The N2 sorption
isotherm of PFUꢀ1000 followed type II isotherm characteristic for a macroporous structure
with slender hysteresis at 0.49 to 0.98 relative pressures typical to the mesopores, and low
pressure hysteresis between 0.05 and 0.45 which can be attributed to the presence of narrow pores
exhibiting the existence of a potential barrier for inward and outward diffusion. The BET surface
area and cumulative pore volume of PFUꢀ1000 was found to be 140 m2 gꢀ1 and 0.12 cm3 gꢀ
1respectively. The substantiate decrease in surface area and pore volume of PFUꢀ1000
relative to PFUꢀ600 and PFUꢀ800 can be ascribed to the increase in particle size and presence
of large number of macropores as evident from the microscopic images.

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(B)
(A)
(C)
(D)
Figure 8.(A) XPS survey spectra, (B) C1s spectra, (C) Fe2p spectra and (D) N1s spectra of
FꢀPꢀ800, PFUꢀ600, PFUꢀ800, and PFUꢀ1000.
Figure 8 compares the XPS of PFUꢀ600, PFUꢀ800, PFUꢀ1000 and FꢀPꢀ800.XPS was used to
determine the % C, N, and Fe content of the PFUꢀ600, PFUꢀ800, PFUꢀ1000 and FꢀPꢀ800
samples. Upon pyrolysis at elevated temperature, FꢀPꢀ800, PFUꢀ600, PFUꢀ800 and PFUꢀ
1000 samples show carbon content as high as 89.6, 84.07, 84.82% and
50.2%respectively.The % atomic nitrogen present in PFUꢀ600, PFUꢀ800 and PFUꢀ1000 samples
were found to be 3.74, 4.67 and 4.72 respectively. The nitrogen atoms doped into the PFUꢀ600,

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PFUꢀ800 and PFUꢀ1000 samples can be ascribed to the decomposition of polyꢀurethane
linkages upon thermal treatment. On the other hand, the FꢀPꢀ800sample displayed no atomic
nitrogen due to the absence of polyurethane linkage between ferrocene and phloroglucionol
which is evident from the XPS survey with the absence of characteristic N peak at ~398eV. The
% Fe content from the corresponding oxides of FꢀPꢀ800, PFUꢀ600, PFUꢀ800 and PFUꢀ1000
samples were found to be 8.78, 8.92, 9.31 and 9.29 respectively. Analysis of the C 1s spectrum
of PFUꢀ600, PFUꢀ800 and PFUꢀ1000and FꢀPꢀ800samplesshown in Figure 8B, indicates the
presence of oxygen functionalities with their signature peaks at higher binding energies
corresponding to C–C, C–O, C O, O C–OH at 284.86 eV, 285.8 eV, 287.2 eV and 288.6 eV
respectively in addition to small signals at higher binding energies correspond to C N and C–N.
Although the Raman spectra of PFUꢀ1000 did not show the D and G peaks of carbon, survey and
high resolution XPS of revealed the presence of carbon, this may be due to the overlap of the iron
oxide peak with the characteristic peaks of carbon. However, the elemental analysis of PFUꢀ1000
showed relatively low carbon which can be attributed to the severe oxidation of the polymeric
structure. Figure 8 C displays Fe2p3/2peak for all the samples corresponding to the respective
oxides. It is wellꢀknown that the Binding Energy (BEs) of the Fe 2p3/2 coreꢀlevel lines in
Fe3O4, and Fe2O3 are 710.3, and 711.0 eV, respectively. The Fe2+ or Fe3+ states can also be
identified by the presence of satellite peaks at ∼716 (730) or ∼719 (733) eV originating from
shakeup processes of 2p3/2 (2p1/2) with significant difference in BE.24, 25 The deconvoluted
high resolution spectra of N1s peak of all three samples shown in Figure 8D indicates the
presence of three peaks at binding energies 393.8, 400.1 and 400.95 eV corresponding to
pyridinic, pyrrolic and quaternary nitrogen, respectively.14ꢀ18 Considering the efforts in design and
synthesis of nonꢀnoble metal electrocatalysts for fuel cell reactions, the iron oxide particles
decorated on nitrogen doped carbon and its electrical conductivity render them viable
electrocatalysts for oxygen reduction reactions.

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(A)
(B)
(C)
Figure 9.(A) Cyclic Voltammetric responses in N2 saturated 0.1 M KOH on PFU modified
electrocatalysts on glassy carbon electrode synthesized at a different temperature, (B) Linear
Sweep Voltammetry (LSV), (C) Tafel plots derived from steady state LSV responses at
constant rotation rate of 1600 rpm in the scan rate of 0.005 V/s 0.1 M KOH (O2 gas
saturated) for PFUꢀ600(black); PFUꢀ800(red); PFUꢀ1000 (blue);FꢀPꢀ800(olive green) and
10% Pt/C(bluish green).
Figure 9A showed the typical CVresponses of modified PFU electrocatalysts on
glassy carbon electrodesynthesized at a different temperatures. As can be seen from figure 8a,
there is no typical characteristicredox responses for all the PFU electrocatalysts synthesized

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at different temperature in nitrogen gas deaerated 0.1 M KOH solution. However, the PFUꢀ
600, PFUꢀ800 and FꢀPꢀ800 showed high double layer capacitance due to themeso and
microporous carbon derived from phloroglucinol. In contrast, the PFUꢀ1000 showed very less
double layer capacitancesdue to the severe oxidation of polymeric backbone to gaseous
products leaving residual carbon in agreement with the Raman and XPS analysis. In order to
study the oxygen reduction reaction (ORR) and electrode kinetics of PFU, PFUꢀ600, PFUꢀ
800, PFUꢀ1000 and FꢀPꢀ800, the steady state LSV experiments were recorded at a constant
rotation rate of 1600 rpm in 0.1 M KOH solution. These results are compared with
commercially available EꢀTek sample of 10% wt loading of Pt and shown in Figure 9B. As
can be seen from Figure9B, the E1/2 potential was determined to be 0.541, 0.751, 0.509,
0.517 and 0.80V for PFUꢀ 600, PFUꢀ800, FFUꢀ1000, FcꢀPU 800 and 10 % ꢀPt/C (EꢀTek)
respectively. The unpyrolzed PFU displayed poor or no electrocatalytic activity due to the
nonꢀavailability of conjugation for electronic conductivity. Among all the catalysts, PFUꢀ800
showed E1/2 and ORR current density on par with the 10% wtPt/C. Figureꢀ9Cshows the Tafel
plot which was derived from the LSV experiments of Figure 9B. The calculated Tafel slope
is also very close value of Pt/C samples for PFUꢀ800 in the low potential region indicating
that the rateꢀdetermining step for the ORR occurring at PFUꢀ800 is similar to that of Pt/C
when compared with other synthesised PFU and FꢀP 800 samples. The excellent
electrocatalytic activity of PFUꢀ800 sample can be attributed to the presence of mixed Fe3O4
and Fe2O3 phases decorated on Nꢀdoped carbon derived from polyurethane and
phloroglucinol linkages. In contrast, though the PFUꢀ1000 also has 4.72 %of nitrogen doping,
the E1/2 potential is only 0.509 V which is more cathodic and three orders lesser cathodic
current density than PFUꢀ800. This may be due to the oxidation of polymeric structures to
gaseous products resulting in low carbon content thereby decreasing the electrical
conductivity, decrease in external surface area due to the agglomeration of the particles

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(reduction in active centers) at higher temperature and the presence of narrow pores
hampering the inward and outward diffusion of the electrolyte leading to inefficient access to
the available active centers for ORR.
To further calculate the number electrons and yield of H2O2 simultaneously during
ORR in alkaline conditions, RRDE experiments were performed at different rotation rates for
PFUꢀ600, PFUꢀ800, PFUꢀ1000 and FꢀPꢀ800 samples.Figure S1aꢀS4a shows the
representative RRDE curves recorded for PFUꢀ600, PFUꢀ800, PFUꢀ1000 and FꢀPꢀ800
sampleswith different rotation rates at 5 mV sꢀ1 in O2 saturated 0.1M KOH solution at
ambient conditions. Figure S1a’ꢀS4a’displays the KouteckyꢀLevich (KꢀL) plots derived from
RRDE curves (1/i vs. 1/ω1/2) at different potentials.At all potentials the KꢀL plot exhibited
linearity and the plots were parallel to one another and thus the ORR processes follows the
first order reaction kinetics with respect to dissolved molecular oxygen concentration.27 The
number of electrons in the ORR processes can be calculated from the KouteckyꢀLevich
equation.28
1
ꢀ
1
1
1
1
=
+
=
+
1
2
ꢀk ꢀl ꢀk
ꢁꢂ
whereik is the kinetic current, il is the limiting current, and ɷ is the rotation rate (rpm). B is
the Levich constant and is given by,
ꢁ = 0.2ꢃꢄꢅ2/3ꢆ−1/6ꢇo
Where, n = number of electrons involved in the reaction, F=Faraday constant (96487
C.molꢀ1), Co=concentration of oxygen in electrolyte (1.26
×
10ꢀ6 mol.cmꢀ3), υ = kinematic
10ꢀ5 cm2 sꢀ1).
viscosity of water (0.01 cm2 sꢀ1), and D= diffusion coefficient of oxygen (1.93
×

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The average electron transfer number per O2 calculated from B value is 3.6, 3.9, 3.0
and 3.2 for PFU – 600, PFU – 800, PFU – 1000 and F – Pꢀ 800 respectively. The number of
electron transfer and the percentage of peroxide produced during the ORR are calculated
from the following equation,
|
|
4 ꢈ
ꢅ
ꢃ =
ꢈ
ꢉ
|
|
+
ꢈ
ꢅ
ꢊ
Where IR is the ring current, ID is the disk current and N is the collection efficiency of
the ring.
Figureꢀ S2 shows the derivative plot of number of electrons (n) and % H2O2 generated during
ORR from RRDE measurement for PFUꢀ600, PFUꢀ800, PFUꢀ1000 and FꢀPꢀ800 at 1600 rpm
in LSV response. As expected, PFUꢀ800 showed ORR response similar to Pt/C sample with
electron transfer number of (n) 3.9 and produced 5.6 % of H2O2 confirming the direct four
electron transfer kinetic with very low yield of H2O2. On the contrary, PFUꢀ600, PFUꢀ1000
and FꢀPꢀ800 displayed n varying from 3 to 3.6 with relatively 1.5 to 3 orders of magnitude
increase in the % H2O2 generation limiting their applications in realꢀtime fuel cell
performance. The superior electrocatalytic activity of PFUꢀ800 can be attributed to the
presence of mixed Fe3O4 and Fe2O3 phases, coordination of Fe species to the N present in the
carbon and the presence of hierarchial porous structure with right balance of microꢀ and
mesopores providing the active centers and macropores facilitating the efficient diffusion of
the electrolyte for ORR. Even though the electrocatalytic activity of Fe towards ORR is still
obscure, the Nꢀdoping into the carbon matrix promotes the ORR by significantly altering the

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electroꢀneutrality of carbon by producing more oxygen adsorption sites. It is noteworthy to
mention that the strong Lewis basicity and electronic affinity of the pyridinic nitrogen
provide high positive charge density on the adjacent carbon atoms resulting in a favourable
diatomic O–O adsorption and weakening the O–O bond strength to facilitate ORR activity. In
order to evaluate the stability of PFUꢀ800, chronoamperometry was performed at a potential
of ꢀ0.40 V versus NHE in 0.1 M KOH (Figure Sꢀ3) and compared with benchmark PtꢀCꢀ
10%, the corresponding current density versus time plot displays more than 95% of the
current was retained for PFUꢀ800 at 4000 s, whereas the PtꢀCꢀ10% displayed 20% decay in
current under the same conditions.
Conclusions
In summary, we have demonstrated a synthetic strategy for simultaneous doping and
decoration of nitrogen and iron oxide phases in and onto the carbon matrices respectively. In
the first step, dicarboxylic acids present in ferrocene were converted to its corresponding
azide subsequently followed by chemically linking the ferroceneꢀdiazides to phloroglucinol
through urethane bonds. The as synthesized PFU interconnects were pyrolyzed to obtain iron
oxide (Fe2O3 and Fe3O4) decorated Nꢀdoped carbon. The synthetic protocol provides a
platform to modulate the Nꢀdoping and the availability of iron oxide phase for the
electrocatalytic reactions by varying the amount of FDCA and phloroglucinol without any
external nitrogen source. The PFU pyrolyzed at 800 oC displayed excellent oxygen reduction
reaction (ORR) with n value and %H2O2 of 3.9 and 5.6% and electrochemical stability on par
with the commercially available Pt catalysts. Moreover, PFUꢀ800 sample displayed excellent
electrochemical stability than the commercial Pt catalyst with only 5% current decay after
4000 s of electrolysis. We are currently examining this approach to establish the optimum

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concentration of iron oxide and Nꢀdoping in the carbon matrix for real time fuel cell
applications.
Acknowledgements
The authors V.P, N.C, M. K and K.M thanks Department of Science and Technology
for funding (GAP 10/14, GAP 11/15) under fast track scheme.
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212x103mm (300 x 300 DPI)