A No-Sweat Strategy for Graphene-Macrocycle Co-assembled Electrocatalyst toward Oxygen Reduction and Ambient Ammonia Synthesis

Ashmita Biswas, Subhajit Sarkar, Manisha Das, Navpreet Kamboj, and Ramendra Sundar Dey*

Article

A No-Sweat Strategy for Graphene-Macrocycle Co-assembled

Electrocatalyst toward Oxygen Reduction and Ambient Ammonia

Synthesis

Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c02176

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sı Supporting Information

ABSTRACT: Toward the realm of sustainable energy, the development of

eﬃcient methods to enhance the performance of electrocatalysts with molecular

level perception has gained immense attention. Inspite of untiring attempts, the

production cost and scaling-up issues have been a step back toward the

commercialization of the electrocatalysts. Herein, we report a one-pot

electrophoretic exfoliation technique with minimum time and power input to

synthesize iron phthalocyanine functionalized high-quality graphene sheets (G-

FePc). The π-stacked co-assembly excels in oxygen reduction performance

(

major criterion for fuel cells) with a high positive E₁_/₂of 0.91 V (vs RHE) and a

reproducible reduction peak potential of 0.90 V (vs RHE). An overpotential as

low as 29 mV dec⁻and complete tolerance toward the methanol crossover eﬀect

conﬁrm the authentication of the catalytic performance of our designed catalyst

G-FePc. The catalyst simultaneously exhibits hydrogen storage eﬃcacy by means

of nitrogen ﬁxation, yielding 27.74 μg h⁻mg_c_a_tNH at a potential of −0.3 V (vs RHE) in an acidic electrolyte. The structure-

−1

function relationship of the catalyst is revealed via molecular orbital chemistry for the bonding of the Fe(II) active center with O

and N during catalysis.

INTRODUCTION

as an alternative approach. Recently, transition-metal-based

catalysts have been explored as they undergo complexation

■

The serious energy crisis that the world has been confronting

for decades caused by the uncontrolled depletion of fossil fuels

forced us for the exploitation of renewable energy sources such

that the requirement of fossil fuels can be supplemented by

green and sustainable hydrogen energy. This is the motivation

of fuel-cell-driven vehicles, and much focus has to be devoted

to the key components of the fuel cells concerning catalyst

performance, durability, and manufacturing cost. However,

the much sluggish cathodic oxygen reduction reaction (ORR)

limits its operations in a fuel cell. Although platinum group

metal (PGM)-based catalysts have been rendered for their high

state-of-the-art performance, their limited resources, high price,

scarceness, ease of deactivation due to CO poisoning, and fuel-

crossover eﬀect have considerably limited their development

with N by proper overlap of unoccupied and occupied metal

d-orbitals, which are of appropriate energy and symmetry to

synergically accept electron density from and back-donate to

N , facilitating NH production by consuming minimum

2 3

9,10

energy. Research interests have inclined to the development

of eﬃcient electrocatalysts in an energy saving and eco-friendly

approach to produce fuels and value-added chemicals including

oxygenates and ammonia (NH ) from earth-abundant feed-

stock such as oxygen and nitrogen (N ).

To get past the bottlenecks of noble Pt-based catalysts,

recently a wide range of diﬀerent PGM-free catalysts have been

surveyed. As the most feasible alternative, earth-abundant

transition metals, particularly Fe, has been widely explored as

an active center for ORR. Diﬀerent forms of Fe like CNT/

for wide-ranging applications. This has encouraged research-

ers to proceed for an alternative to Pt-based catalysts designing

for meeting all the criteria as the situation demands. Apart

Fe C, single Fe-atom dispersed Fe-N-C catalyst, N−Fe/

Fe C@C (Fe/Fe C functionalized melamine foam), Fe C/

C, and three-dimensional (3D) N-doped graphene aerogel

from ORR, the earth-abundant N gas could be utilized for the

generation of ammonia in the industry and agricultural sector

as a green energy carrier and potential transportation fuel. But

Received: July 21, 2020

the industrial method of NH₃production (Haber−Bosch

process) utilizes huge energy, high temperature and pressure,

and even it emits a huge amount of CO as greenhouse gas.

Thus, electrochemical dinitrogen reduction at ambient

temperature and atmospheric pressure has been thought of

XXXX American Chemical Society

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

N-GA)-supported Fe O nanoparticles (Fe O /N-GAs) had

Article

(

graphene gets endowed with oxygen functionalities. Thus,

been synthesized with an attempt to replace the commercial

Pt/C catalyst, but these materials still lagged behind the latter

with respect to electrochemical activity. On the other hand, the

noble catalysts comprising Au, Pd, and Ru could be surpassed

in the case of nitrogen reduction reaction (NRR) with cheaper

defects become inevitable in the structure. Moreover, the

chemical method takes a long synthesis time and steps,

hazardous reagents, and a tedious puriﬁcation process.

Therefore, in order to avoid a multistep and complex synthesis

route and to lessen the oxygen content in the graphene

structure, electrophoretic exfoliation of graphite by applying a

bias voltage to the graphite electrodes in liquid electrolytes is

implemented. The potential gradient between the electrodes

can act as the driving force to overcome the van der Waal

catalysts like Fe O nanorods/Ti, β-FeOOH, Fe/Fe O

Fe on stainless steel mesh, and Fe electrode/FTO. But the

low yield of NH with these catalysts motivated researchers to

upgrade the Fe-based catalysts. This gave rise to a new class of

crystals having transition-metal nitrogen moieties (M-N ) as

interactions between the adjacent graphitic layers, followed

by the subsequent intercalation of anions from the electrolyte,

active sites with a carbon stretch due to their excellent

mechanical and chemical stability, as well as their abundant π

electrons, which are beneﬁcial for the electron-demanding

facilitating the delamination of the material. Precise control

over the applied voltage, exfoliation time, and electrolytes

decides the quality of graphene ﬂakes. However, in

atmospheric conditions, the restacking of the graphene layers

by the van der Waals force cannot be avoided, which makes it

impossible to achieve high-quality graphene with control over

the number of layers. This calls for the addition of polymers

as a co-intercalant in the same synthesis pot, not only to

minimize the oxidation of graphene during exfoliation but also

to stabilize the postsynthesized graphene to prevent the

reduction reactions. These include metal macrocycles like

phthalocyanines (Pc) and porphyrins (Py) with high

architectural ﬂexibility, which facilitates the tailoring of their

physical and chemical parameters and exploration of their well-

deﬁned M-N₄active sites responsible for electrochemical

reduction processes (ORR and NRR). The chelation of

metals with N-donor ligands primarily serves to prevent the

metal center from passivation/corrosion under electrochemical

conditions. Metal-Pcs, particularly FePc, has been extensively

aggregation of nanosheets. Therefore, electrophoretic ex-

5−29

investigated

until several snags were confronted with this

foliation of graphite enables (1) large-scale, resourceful, and

cost-eﬀective production of high quality, oxide-free graphene

sheets and (2) doping of graphene layers with heteroatoms

and/or functionalization with π-conjugated systems, simulta-

catalyst, such as (1) the relatively poor electrochemical

stabilities of ORR and NRR on these catalysts, (2) the FePc

molecules are prone to aggregation, which will greatly decrease

the active sites, and (3) there can be demetalation from the

macrocycle upon prolonged exposure to O or N in alkaline or

neous with the exfoliation process.

Herein, for the ﬁrst time, we have adopted an FePc

macrocycle-functionalized graphene (G-FePc) electrocatalyst

via an improvised, economical, and resourceful one-pot

synthesis technique by means of an electrophoretic exfoliation

method for electrochemical ORR and NRR applications. In

order to conﬁrm the quality of the exfoliated graphene sheets

and the proper anchorage of FePc onto graphene layers via

π−π stacking interaction, the catalyst was characterized by

scanning electron microscopy (SEM), transmission electron

microscopy (TEM), energy-dispersive X-ray spectroscopy

30−32

acidic electrolytes.

To address these problems, FePc has

been heat-treated or supported on a variety of carbon

materials. It was found that the type of support material (to

which the iron phthalocyanine is attached) has a signiﬁcant

eﬀect on the promotion of electrocatalytic activity and

durability. In this race, graphene gets ahead of carbon

nanotubes (CNTs), as it has a unique 2D structure, signiﬁcant

surface area, extraordinary electronic properties, and ease of

synthesis and has gained much attention to provide proper π-

conjugated support for the FePc system beneﬁting ORR and

(

EDS), X-ray diﬀraction (XRD), Raman spectroscopy, and

X-ray photoelectron spectroscopy (XPS). As expected, with a

high onset potential and half-wave potential of 1.02 and 0.91 V,

respectively, for ORR in an alkaline electrolyte, the G-FePc

catalyst outruns most of the high-performance materials

NRR.

The most conventional method to synthesize M-N_x/

graphene systems is by subjecting the metal macrocycles

(

FePc) or precursors containing metal salts, carbon, and

6,47

developed so far.

equal eﬃcacy toward NRR with an NH production rate of

The catalyst simultaneously conveyed

nitrogen source (aromatic, aliphatic, or any polymer) to

pyrolysis (temperature: 500−1000 °C), but the complex heat

treatment procedure limits the tailoring of structure and

surface proﬁles of the pyrolyzed materials active toward

−

−1

7.74 μg h^m^gcat at −0.3 V in an acidic electrolyte (0.1 M

HCl). Thus, the synthesized catalyst not only can replace

commercial Pt/C catalysts in proton exchange membrane fuel

cells as well as direct methanol fuel cells but also can be

utilized for hydrogen storage in the form of ammonia. From a

practical point of view, the ease of synthesis of the catalyst

requiring less time, less power input, and less reacting materials

ensures its applicability for commercialization purposes.

reduction reactions. This opens up a new avenue of research,

where the metal macrocycles are kept intact and made more

conducting by providing graphene support at the bottom.

However, the bottom-up synthesis of graphene including

organic synthesis in solvents, chemical vapor deposition of

gaseous carbon precursors on solid catalytic substrates, and

graphitization of silicon carbide (SiC) crystals is limited by

the requirement of suﬃcient resources, which not only makes

it an expensive method but the scaling up is also another great

EXPERIMENTAL SECTION

Chemicals and Materials. Graphite plates were purchased from

Nikunj Chemical Ltd. Absolute ethanol was purchased from CS

reagent (Emsure); the rest of the chemicals like sulfuric acid (H SO ),

■

challenge. However, the top-down approach is favored where

graphene can be synthesized by chemical, mechanical, or

electrophoretic exfoliation of graphite, as graphene forms the

core structural motif of the latter. The mostly used derivative

of graphene (reduced graphene oxide) from graphene oxide

iron(II) phthalocyanine (FePc), absolute ethanol, potassium hydrox-

ide (KOH), Pt/C, Naﬁon, 37% hydrochloric acid (HCl), ammonium

chloride (NH Cl), hydrazine monohydrate (N H ·H O), para-

(

dimethylamino)benzaldehyde, sodium hydroxide (NaOH), salicylic

(

GO), prepared by extensive oxidation via a modiﬁed

acid, sodium citrate, sodium hypochlorite (NaClO), and sodium

nitroferricyanide (Na [Fe(NO)(CN) ]) were all purchased from

Hummers’ method, is electrically less conducting, and

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 1. (a) A schematic illustration of acid-mediated electrophoretic exfoliation of graphite yielding G-FePc catalyst. (b, c) SEM images and (d,

e) TEM images of G-FePc catalyst. (f−i) Elemental mapping of the catalyst showing sustained distribution of C, N, and Fe elements (scale: 6 μm).

Sigma-Aldrich. All the reagents used in this work were of pure

analytical grade and used without further puriﬁcation.

stacking height can be evaluated by means of XRD following the

Debye−Scherrer equation.

.89 × λ

D × cos θ

where λ is the wavelength of the X-ray of Cu Kα used in this study

Synthesis and Puriﬁcation of the Electrocatalysts. The

synthesis of G-FePc nanoﬂakes was implemented by an uncompli-

cated exfoliation method (Figure 1a) in a facile electrochemical setup

equipped with two electrodes, supporting electrolyte, and a DC-power

L_c=

(1)

supply. A graphite plate having dimensions of (1 × 2) cm and a

(

1.54 Å), and D is the full width at half-maxima of the peak obtained

thickness of 0.5 mm was immersed as anode (working electrode) into

a well-stirred acidic solution containing 1.5 M H SO blended with 2

for the corresponding (002) plane of the catalysts.

The peak intensity ratio (I /I ) from Raman spectroscopy at laser

mM of FePc. Another similar graphite plate having identical

dimensional value was clipped at the cathode compartment, and a

separation of 1 cm was maintained between the electrodes. A static

potential of 4 V was applied between the working and the counter

electrodes for a deﬁnite time limit of 5 min. Before the expanded

graphene sheets could ﬁnally peel oﬀ from the graphite plate, a series

of electrochemical oxidation, intercalation, and corrosion occurred at

wavelength λ = 532 nm has been exploited to determine the average

−

defect distance L (in nm) and defect density n (in cm ) present in

the sp domains of the catalysts calculated from eqs 2 and 3.

−9

^IG

LD = (2.4 × 10 ) × λL ×

I_D

(2)

(3)

the anode. The resulting electrophoretic exfoliation thus produced a

black mass, which partly remained ﬂoating at the edge surface of the

electrolyte solution and partly settled at the bottom. This slurry was

subjected to ultrasonication for 30 min to evade any kind of

agglomeration. Hence, the properly dispersed solution mixture was

repeatedly washed by means of centrifugation with deionized water

until the pH reached neutral and successively with ethanol for three

times. Discarding the supernatant, the residue was kept overnight in a

hot air oven for drying at 60 °C to obtain our desired catalyst powder.

A similar procedure was followed with varying concentration of FePc,

varying applied potential, and varying time, and based on electro-

chemical performance of all the prepared materials, the above-

mentioned optimization was ﬁnalized. With these optimizations,

large-scale synthesis of G-FePc was carried out to establish the

feasibility of the process as shown in Figure S1 .

n = (2.4 × 10 ) × λ _×I

−4

I_G

It is evident that L and n are inversely proportional to each other.

Electrode Fabrication. A quantitative amount of the G-FePc

catalyst (5 mg) was dispersed in a 1:1 (v/v) mixture of water and

isopropyl alcohol (1 mL) and ultrasonicated for 30 min. This same

ink solution was used for all the electrochemical studies involved in

ORR as well as NRR. Before modifying the glassy-carbon electrode

(GCE), rotating-disk electrode (RDE), and rotating ring-disk

electrode (RRDE) with the as-prepared ink, these were properly

cleaned with 1, 0.3, and 0.05 μm alumina (Al O ) powder and

ultrasonicated with deionized water for 3 min. The glassy carbon part

−

of the electrodes was drop-casted with 0.5 mg cm of the catalyst ink.

In order to compare the electrocatalytic oxygen reduction eﬃcacy of

our active material with the commercially used Pt/C catalyst, the

latter was made into an ink by dissolving Pt/C into a solution

containing Milli-Q water, ethanol, and (5%) Naﬁon solution. After

ultrasonication for 30 min, the Pt/C catalytic solution (20 wt %) was

Analogously, G-H SO catalyst was also prepared where the

electrolyte solution only contained 1.5 M H SO (devoid of FePc),

maintaining all the optimized conditions.

Calculations Regarding Crystallite Size and Defect Estima-

tion in the Catalyst. The crystallite size L (in nm) with respect to

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

B = 0.62nFC D ^/³ϑ⁻¹^/⁶

prepared, which was then used to determine the electrochemical

performances.

(6)

Electrochemical Setup for NRR. In this work, the basic

electrochemistry was carried out in an H-type cell in order to

separate the cathodic and anodic chamber. This separation was

further enhanced by applying Naﬁon in between the two chambers,

which is particularly selective toward proton transport across the

membrane and impermeable to any other ion. All the measurements

were done in a three-electrode setup where Pt wire was taken as the

counter electrode, Ag/AgCl (3 M KCl) as reference electrode, and

GCE modiﬁed with G-FePc as working electrode. The reference and

the working electrodes were dipped in the cathodic chamber to

accomplish the reduction reaction while the counter electrode was

placed in the anodic chamber to facilitate the counter-oxidation

^jK ⁼ⁿ^F^k^C0

(7)

where n is the overall number of electrons transferred in O reduction,

F is the Faraday constant (F = 96485 C mol ), C is the bulk

−

−1

concentration of O (C = 1.2 × 10 mol L ), ϑ is the kinematic

−

−1

viscosity of the electrolyte (ϑ = 1.09 × 10 cm s ), D is the

−

2 −1

diﬀusion coeﬃcient of O in 0.1 M KOH (D = 1.9 × 10 cm s ),

and k is the electron transfer rate constant. The constant 0.62 is

adopted when ω is expressed in terms of angular velocity rather than

in rpm.

The RRDE technique plays a vital role in explaining the kinetics

and mechanism of the ORR process. In this respect, the number of

electrons (n) transferred and the % H O formation are the two

reaction. The N gas inlet was intentionally positioned close to the

working electrode in the cathodic chamber of the H cell. The

electrolyte used in this study is 0.1 M HCl (pH = 1). All through the

electrochemical study, there was continuous argon (Ar) and N₂

purging (as required). To investigate that the nitrogen gas used for

NRR is devoid of any kind of nitrogenous gaseous impurity, gas

chromatography was performed that showed no peak for NO_x

interdependent parameters, which can be evaluated from the disk and

ring currents (I and I ) as follows

^ID

n = 4 ×

^IR

I +

(8)

(9)

(

retention time: 2.33 min), while there was a sharp peak for N in

the chromatogram at 7.2 min (Figure S2 ).

H O = 200 ×

I_R

Electrochemical Characterizations. Electrochemical measure-

ments concerning ORR and NRR included cyclic voltammetry (CV),

linear sweep voltammetry (LSV), and chronoamperometry, which

were performed with the CHI 760E electrochemical workstation. For

ORR, to further study the rate of reaction kinetics and % H O

^ID

where N refers to the collection eﬃciency of the Pt ring, which is (N =

0.249).

The Faradaic eﬃciency and the NH yield rate are two important

formation, electrochemical characterizations were accomplished with

descriptors of the performance of an NRR electrocatalyst. The NH₃

yield rate (R) of NRR given by μg h mg

eq 10, where C is the measured NH concentration (μg mL ), V is

−1

RRDE (GC disk area: 0.196 cm ; Pt ring area: 0.041 cm ), and RDE

can be calculated using

cat

−1

(

disk area: 0.031 cm ) using a Metrohm multichannel autolab

M204) electrochemical analyzer. GCE (area: 0.07 cm ) was mostly

(

the volume of the electrolyte (mL), t is the reaction time (h), and m

(mg) is the mass of the catalyst loaded on the active surface area of

the electrode.

used for all the characterizations involved in NRR. A three-electrode

setup was maintained throughout with GCE as working electrode,

graphite rod/Pt wire as counter electrode, and Ag/AgCl (3 M KCl) as

reference electrode. The reference electrode was calibrated with

respect to the reversible hydrogen electrode (RHE), and their

potentials can be related by the formula as follows:

C × V

R =

t × m

(

10)

The Faradaic eﬃciency (F.E.) of NRR can be calculated using eq

1, where n is the number of electrons required for producing one

^ERHE ⁼^EAg/AgCl + 0.0591 × pH + 0.210 (E° at Ag/AgCl,

NH molecule (n = 3), F is the Faraday constant (F = 96485 C

−1

mol ), C is the measured NH concentration (g mL ), V is the

M KCl)

(4)

volume of the electrolyte (mL), M is the relative molar mass of NH

−

(

M = 17 g mol ), and the Q is the total charge passed through the

In the case of ORR, 0.1 M KOH (pH > 13) was used as the working

electrolyte, and for NRR, 0.1 M HCl (pH = 1) served the purpose.

electrodes (C).

The electrolyte solutions were saturated with Ar, followed by O (for

n × F × C × V

F.E. =

the former) and N (for the later) for 30 min at a constant ﬂow rate

M × Q

(11)

before and during the measurements as required.

Catalytic Activity Descriptors for ORR and NRR. The overall

ORR current density (j) is a function of j (kinetic current density)

RESULTS AND DISCUSSION

Mechanism of the Exfoliation for the Synthesis of

■

and j (diﬀusion-limiting current density). At the higher overpotential

region, the reactants (O ), upon reaching the electrode surface,

Catalyst. In this work ,we have adopted an eﬀortless method

to synthesize FePc intercalated graphene sheets via one-step

and one-pot electrophoretic exfoliation, which paralleled or

even outstripped most of the highly eﬃcient FePc@graphene

systems with respect to electroreduction applications. The

synthesis of catalyst was carried out in 1.5 M H SO for the

immediately get reduced, leaving behind a net zero reactant

concentration on the electrode surface. This leads to formation of

a ﬂat plateau region governed by the j which is determined by the

rate of oxygen diﬀusion onto the surface of the electrode, and it is

dependent on the rotating speed in the RDE/RRDE measurement.

On the other hand, j depends upon the reaction kinetic process, rate

of oxygen reduction on the electrode surface, and number of electrons

transferred. Thus, the overall current density can be achieved from the

2−

exfoliation of graphite as SO₄is known to be the most

−

eﬃcient for the purpose than other anions including Cl and

Koutecky−Levich equation

−

NO , which can only help in the expansion of graphite rather

2−

than exfoliation. In the absence of FePc, SO₄ions

j 1

1 z

B_ω

^jK

z =

1/2

produced ﬁne ﬂakes of graphene (G-H SO ) as shown in

j j

j z

k L

K {

(5)

Figure S3a . It was also experimentally witnessed that, with

HCl, graphite plates only got expanded and FePc got stuck at

the opened edges without proper invasion between the layers

Figure S3b ). The mechanistic pathway for the exfoliation

process has been schematically depicted in Figure 1a and can

be stepwise elaborated as the following: (1) Upon applying

where ω is the angular velocity of the electrode (ω = 2πN, N is the

linear rotation speed) and B is the Levich slope. From the

corresponding Koutecky−Levich plot, 1/B and 1/j can be obtained

(

as the slope and the intercept, respectively. These two parameters can

be additionally calculated from the equations as follows

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

bias voltage (+4 V), reduction of water occurred, producing

reactive hydroxyl (OH·) and oxygen (O·) radicals at the

cathode. (2) These radicals reached to the defective sites

present at the edges and grain boundaries of graphite, thereby

opening up the graphite layers due to oxidation, facilitating

intercalation of sulfate ions (SO₄²) within the graphite layers.

−

2−

(

3) A depolarization eﬀect worked, and the reduction of SO₄

ions and self-oxidation of water produced SO and O gases,

respectively, which was evidenced from the vigorous gas

evolution and bubbling during the electro-exfoliation process.

The force exerted by these gaseous species on the graphite

layers was suﬃcient enough to overcome the weak van der

Waal interaction between the adjoint layers of graphite,

oﬀering a facile separation of few-layer graphene sheets out

42,52,53

of the graphite stack.

The FePc that was already present

in the medium got simultaneously dispersed on the graphene

sheets via π−π stacking interaction, allowing an easy passage

for electron ﬂow throughout this integrated system, which is

essential for any electroreduction process.

Figure 2. (a) XRD pattern (inset showing magniﬁed peak positions)

and (b) Raman spectra of G-FePc, G-H SO , and FePc catalysts. (c)

Overall XPS survey spectra of G-FePc catalyst. (d) Schematic

representation of G-FePc catalyst exhibiting distinct π−π stacking

interaction between the two π-conjugating systems.

Physico-chemical Characterizations of the Electro-

catalysts. Surface morphological inspection of the as-

exfoliated catalyst was performed using SEM and TEM

measurements. SEM images (Figure 1b,c) show that few-

layer graphene sheets provided a conducting support for the

homogeneous dispersion of FePc. TEM images in Figure 1d,e

further reveal that the proper marriage between FePc and

graphene sheets not only prevented agglomeration and

deformation but also rather allowed individual segregation of

H SO (Table S1). An increase in the average crystallite size in

G-FePc implied that there were reduced grain boundaries and

lateral defects, improving the quality of graphene ﬂakes. This

is in good agreement with the fact that introduction of FePc

led to a small d-spacing value and increased crystallite size,

suppressing the defect sites in the underlying graphene layer.

All the peaks found in the XRD data of FePc reﬂected well in

G-FePc at exactly the same positions (Figure 2a), which

conveys that the structure of FePc was completely restored

during the electro-exfoliation process.

graphene or FePc. This provided improved access to

catalytically active sites in G-FePc, which reinforced easy

−

diﬀusion of reactants (O /N ) and products (OH /NH )

onto and out of the active surfaces.

55,56

Figure S4 is an

associated selected area electron diﬀraction (SAED) pattern,

which provides information about the crystallinity of graphene

in the G-FePc catalyst. The discrete diﬀraction spots indicate

few-layer material with high levels of a well-ordered

polycrystalline structure. The elemental composition of G-

FePc can be visualized from EDS analysis and elemental

mapping (Figure S5 , Figure 1f−i), which revealed the

abundance of C, O, N, and Fe in the catalyst.

Raman spectroscopy is another powerful, yet simple,

technique, which provides a fast, high-throughput and

nondestructive identiﬁcation of the quality and crystalline

nature of the graphene-based materials. Figure 2b shows

characteristic Raman spectra comparing FePc, G-H SO , and

G-FePc. G-band, a measure of the in-plane C−C bond

stretching that originated from sp domains of graphitic

−

The presence of FePc not only caused no further distortion

in the graphene support but also improved the sheet quality by

minimizing planar defects, which could arise due to expulsion

of graphene layers from graphitic domain. To establish this fact

and to investigate the π−π interaction between FePc and

graphene, XRD and Raman spectroscopic techniques were

adopted on FePc, G-FePc, and G-H SO systems. The XRD

material, appeared at 1581.6 cm in the case of G-H SO

2 4,

which got slightly red-shifted to a lower frequency of 1575.5

−

cm for G-FePc. This indicates that application of FePc

helped in nurturing the sp hybridization of the graphene

sheets via π−π supramolecular interaction, which could

otherwise be disrupted due to C−C bond fracture upon

59,60

exfoliation.

The D-band arising due to edge plane defects

analysis for G-FePc in Figure 2a demonstrates a prominent

and lattice distortions was found in the same position for both

−

peak at 2θ = 26.53° denoting the (002) plane, which is

the catalysts at 1353.3 cm . In exfoliated graphite, defects

generally comprised point defects in the basal planes or could

also arise from ﬂake edges, intercalants, or charge impur-

originated due to the sp hybridization of the graphitic

carbon. This peak shifted to a lower value of 25.95° along

61,62

with peak broadening for the G-H SO system (from inset of

ities.

The identical D-band position convinced us that

Figure 2a). The peak shifting is attributed to the π−π

FePc did not introduce further defects in the G-FePc

noncovalent interaction between the two π-conjugated systems

composite. The I /I_Gratio was found to be 0.456 for G-

(

graphene and FePc). The peak proﬁles correspond to d₀₀₂

H SO . Such a small value forecasted better crystallinity of the

spacing values of 0.335 and 0.343 nm for G-FePc and G-

exfoliated graphene ﬂakes. The dramatic improvement in

crystallinity of graphene was observed in G-FePc with an even

lower I /I ratio of 0.327. As the basal defects of the graphene

H SO systems, respectively. The broadness of the peak and

high d-spacing value of G-H SO meant that small ﬂakes were

constituted with more stacking defects such as edge

ﬂakes were self-healed in the presence of FePc, that nurtured

the sp hybridization of the graphene backbone and resulted in

dislocations and twinning. This encouraged us to ﬁnd out

the thickness of regular stacking that is the crystallite size (L_c)

using the Debye−Scherrer equation (eq 1), which was

calculated to be 7.245 nm for G-FePc and 3.067 nm for G-

an unattenuated electron ﬂow throughout the two co-

assembled π-conjugated systems. The I /I ratio was further

exploited to estimate the average defect distance (L ) and

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 3. (a) Reproducibility bar plot representing peak potential for ORR of G-FePc catalyst in O saturated alkaline electrolyte without rotation.

−

(

b) LSV polarization curves of as-synthesized catalysts and 20% Pt/C in O saturated 0.1 M KOH solution at a scan rate of 10 mV s at a rotation

speed of 1600 rpm. (c) Comparative bar plot of the catalysts and Pt/C on the basis of E₁_/₂and diﬀusion-limiting current density. (d) LSV curves of

G-FePc catalyst at diﬀerent rotation speeds and (e) corresponding K−L plots at diﬀerent potentials ranging from 0.57 to 0.77 V with the calculated

number of electrons transferred during ORR at each potential. (f) H O yield and electron transfer number (n) of G-FePc catalyst in O saturated

.1 M KOH electrolyte; inset of (f) shows mass transport corrected Tafel plots of G-FePc and 20% Pt/C catalysts for O reduction kinetics derived

from LSV at 1600 rpm.

−

defect density (n in cm ) in the graphene domains using eqs

exfoliation of graphite. The atomic weight percent of all the

elements obtained from XPS are tabulated in Table S3. These

data accord well with what we obtained from EDS analysis. All

these characterizations justify the structure of the catalyst

material G-FePc, as shown in Figure 2d.

and 3. The L and n values were calculated to be 20.53 nm

and 1.366 × 10 cm for G-H SO and 24.24 nm and 0.979

10 cm for G-FePc, respectively. This information further

D D

−2

highlighted the novelty of our exfoliation method and

importance of FePc toward the synthesis of high-quality

catalyst. All the data obtained from Raman spectroscopy are

summarized in Table S2 . The Raman spectra of G-FePc

showed resemblance with similar peaks as that in FePc where

the peaks at 830 and 1525 cm⁻indicate C-N stretch and C-N-

C stretch of pyrrole, respectively. The experimental ﬁndings

nicely depicted that FePc was properly adorned throughout the

exfoliated graphene basal support. Thus, the data obtained

from Raman correlate with those from XRD and suggest that

our desired catalyst G-FePc demolished all forms of defects in

the crystallite graphene, which makes it more desirable due to

Electrochemical Performance Concerning ORR. In

order to investigate the performance of our prepared catalyst,

G-FePc, as cathode material in fuel cells, a detailed

electrochemical study was assessed for all the catalysts in a

conventional three-electrode system. All the potential values

are designated with respect to RHE (eq 4). Initially, CV was

conducted for G-FePc in Ar fed 0.1 M KOH at varying scan

−

rates from 10 to 100 mV s . As shown in Figure S7 , there was

no shifting in the anodic and cathodic peak positions with

increasing sweep rate and the corresponding plot of peak

current vs scan rate revealed a ﬁne linearity, which evidenced

that G-FePc was solely a surface-conﬁned species. As a matter

its exclusive mechanical, thermal, and electrical properties.

XPS was adopted to gain insight about the chemical

composition of the G-FePc catalyst. The survey spectrum of

the catalyst displayed distinct peaks of C, N, O, and Fe (Figure

of comparison, Ar and O saturated 0.1 M KOH electrolyte at

−

a scan rate of 10 mV s (Figure S8 ) reveals that, in the

presence of Ar, the catalyst possessed a reversible peak at 0.84

+/3+

c) with an atomic weight quantiﬁcation of 73.42%, 18.54%,

.67%, and 2.37%, respectively, as tabulated in Table S3. The

V during the cathodic scan, which is attributed to Fe

redox

transition. Interestingly, a sharp reduction peak appeared at

high-resolution Fe 2p spectrum was deconvoluted into

subpeaks at 708.06, 722.1, 709.96, and 715.46 eV representing

Fe 2p ₃ _/₂ , Fe 2p₁_/₂, Fe-N, and Fe satellite peak, respectively

0.90 V, when the electrolyte was fed with O , while still

maintaining the capacitive current due to the underlying

carbon support. The CV responses were recorded with G-FePc

modiﬁed GCE obtained from diﬀerent exfoliating conditions,

such as diﬀerent exfoliating solvents, applied voltages,

exfoliation times, and concentrations of FePc as shown in

Figure S9a −d . The bar diagrams as represented in Figure S9e,f

helped us to achieve the optimum experimental conditions,

that is, 4 V and 5 min with 2 mM FePc in H SO as it displays

(Figure S6a ). The Fe 2p peak positions state that low spin Fe

is present in +2 oxidation state in the FePc system. The peak

corresponding to the Fe−N bond was also evident in the core

level spectra of N 1s at 398.5 eV (Figure S6b). Apart from this,

the spectra also consisted of pyridinic N at 397.3 eV and

pyrrolic N at 400.35 eV, which can be well justi ﬁ ed from the

structure of FePc as shown in the inset of Figure S6b. The

XPS spectra of C 1s (Figure S6c ) revealed a sharp peak at

the best ORR activity with respect to reduction peak potential.

Moreover, carefully maintaining the experimental conditions

ensured that the peak potential for ORR (∼0.90 V) for our

ﬁnal catalyst G-FePc was reproducible as shown in Figure 3a

with a standard deviation of 0.47% for six diﬀerent measure-

ments. The electrochemical performance was seen to vary

signiﬁcantly with change in FePc concentration. A low

83.98 eV that resembled C−C/CC bonds and a broad

peak at 286.6 eV indicating C−N/CO, where C−N is

present in the concerted FePc system and CO may probably

arise due to the presence of few oxygen functionalities at the

edges of graphene support, generated due to acid mediated

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 4. (a) LSV analysis of the G-FePc catalyst at 1600 rpm in O saturated 0.1 M KOH before and after cyclability test. (b) CV analysis of G-

FePc catalyst with (blue) and without (red) 1 M CH OH in O saturated alkaline electrolyte. (c) Methanol crossover durability comparison of G-

FePc and Pt/C using the chronoamperometric technique in O saturated 0.1 M KOH with addition of 1 M CH OH solution.

concentration of FePc (0.5 and 1 mM) was inadequate to

provide suﬃcient active centers for ORR and an increase in

concentration to 6.4 and 10 mM caused FePc agglomerates,

rendering loss of active site, which resulted in poor ORR

sponding K−L plots (Figure 3e) at diﬀerent potentials (0.57−

0.77 V) exhibited linearity and near parallelism, which are

accounted for the ﬁrst order kinetics of the concentration of

dissolved oxygen and a near similar value of the number of

electrons transferred (n) during the electroreduction of O₂,

respectively. At the potentials of 0.57, 0.62, 0.67, 0.72, and 0.77

V, n values were calculated to be 3.98, 3.98, 4, 3.99, and 3.98,

respectively (Figure 3e,f), using eq 8, indicating a superior

four-electron transfer during the ORR process. It is noteworthy

process. RDE was used to study the eﬀect of mass loading on

ORR catalytic performance of our ﬁnal sample at 1600 rpm.

Figure S10a,b suggests that 0.5 mg cm⁻of the G-FePc catalyst

yielded the best ORR activity (E₁_/₂= 0.91 V) and was

maintained for further studies.

−

In order to justify that G-FePc was eﬃcient enough toward

ORR, its activity was compared with that of G-H SO , FePc,

to mention that, with a 4e transfer ORR kinetics, the G-FePc

−

catalyst yielded 4 OH ions in the alkaline medium with hardly

FePc, G/FePc, and commercial Pt/C catalysts by the RDE

any perturbations due to unwanted intermediates. This is

corroborated on the basis of H O formation on the disk and

technique at 1600 rpm in O saturated 0.1 M KOH. Figure 3b

gives evidence that G-FePc outperformed the controlled

samples as well as the Pt/C catalyst with respect to onset,

half-wave, and peak potentials of ORR. A comparative bar plot

between G-FePc and Pt/C is shown in Figure S11, where it is

clearly perceptible that Pt/C (E₁_/₂= 0.86 V) lagged behind our

its detection on the ring at ﬁxed ring potentials during RRDE

−

measurements. A <1% HO₂formation (using eq 9) could be

−

observed from Figure 3f emphasizing a 4e inner-sphere

−

reaction pathway for ORR (producing >99% OH ). Figure

S12 shows a detailed study of the ring current of the G-FePc

catalyst at 0.67 V ring potential (n = 4), where a small peak at

0.83 V could be observed during the cathodic scan

representing hydroperoxide formation, which could instanta-

synthesized catalyst by ΔE₁of 50 mV (Figure 3b).

Furthermore, G-FePc displays a notable current density of

−

5.4 mA cm at the diﬀusion-limiting region that was

−

comparable to that of the Pt/C catalyst (−5.3 mA cm ),

neously get converted to the 4-electron product OH in the

indicating the presence of a plentiful number of exposed active

sites, which is responsible for the high oxygen diﬀusion rate

onto the electrode surface. A comparative study (Figure 3c)

regarding half-wave potential and current density has been

presented in the form of a histogram for all the catalysts under

investigation. Such exciting performance of G-FePc is

accredited to the eﬃcient π−π stacking between the highly

conducting graphene support and FePc macrocycle enabling a

fast electron ﬂow from catalyst to electrode surface during

ORR. The π−π interaction energetically lowers the atomic

alkaline medium. Interestingly, another peak at 0.47 V

appeared in the polarization curve due to the basal carbon

support as reported by Mukerjee et al. Further, the direct or

indirect 4-electron pathway followed by the G-FePc catalyst for

ORR could be witnessed from Figure S13 (plot has been

explained in the Supporting Information), suggesting that O

got completely reduced to OH utilizing four electrons and the

−

minimum hydroperoxide ion formed as an intermediate (that

was stable at the electrode surface) reduced instantly to the

ﬁnal product.

orbital of the metal close to the π* orbital of O that facilitates

Such superior ORR catalytic performance of our material G-

FePc over the Pt/C catalyst was supported by the mass

transport corrected Tafel plots at the low overpotential region

oxygen adsorption on the Fe surface initiating ORR at a high

onset potential value of 1.02 V. A detailed mechanistic study

has been elaborated later.

of the LSV curves for O reduction kinetics at 1600 rpm. The

To further reinforce the reaction kinetics of ORR,

electrochemical measurements with RRDE were carried out

at the same experimental conditions by varying the rotation

speed of the electrode from 625 to 4900 rpm (Figure 3d). The

diﬀusion-limiting current density could be seen to increase

with increasing speed of rotation due to higher oxygen

diﬀusion at a shortened distance on the electrode surface.

The kinetic parameters for ORR can be estimated by means of

Koutecky−Levich (K−L) equation (eqs 5−7). The corre-

inset of Figure 3f shows that the Tafel slope for the G-FePc

−

catalyst was as low as 29 mV dec than that of Pt/C (71 mV

−1

dec ), suggesting a higher electron transfer coeﬃcient for

ORR kinetics and a much faster transfer of the ﬁrst electron

from the catalytic site (Fe-N ) to oxygen facilitating ORR

with a much improved performance. As a matter of

comparison, a summarized table based on ORR performance

with all the synthesized catalysts in this work as well as the

FePc/C-based catalysts in 0.1 M KOH is presented in Tables

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 5. Electrochemical NRR on G-FePc catalyst in acidic medium. (a) LSV curves in 0.1 M HCl electrolyte saturated with Ar (black) and N

−

(

red) at 10 mV s scan rate. (b) Time-dependent current density curves of G-FePc catalyst for NRR at diﬀerent potentials for 2 h. (c) UV−vis

absorption spectra of the electrolyte (0.1 M HCl with dissolved NH ) stained with indophenol indicator at diﬀerent potentials after 2 h from the

completion of electrocatalysis. (d) Average NH yield rates and Faradaic eﬃciencies of G-FePc catalyst at diﬀerent potentials. (e) AverageNH

yield rates and Faradaic eﬃciencies of all the synthesized catalysts in N saturated in 0.1 M HCl as well as that of G-FePc catalyst in Ar saturated 0.1

M HCl. (f) NRR performance of G-FePc catalyst during cycling experiments.

S4 and S5 to justify the superiority of our material over most of

the reported catalysts so far.

Therefore, G-FePc can be utilized as a potent replacement

to the Pt/C catalyst in fuel cells due to (1) facile penny-saving

synthesis and ease of commercialization, (2) half-wave

potential of G-FePc being 50 mV more positive than Pt/C

Besides such distinguishable performance, the stability of our

catalyst G-FePc toward ORR was investigated by a continuous

potential cycling between −0.4 to 0.2 V vs Ag/AgCl at a scan

−

with a Tafel slope as low as 29 mV dec rendering much

−

rate of 50 mV s in oxygen saturated 0.1 M KOH for 10 000

cycles. The result from Figure 4a conveys a negative shift of 20

mV in the half-wave potential value of the polarization curve

after the cycling process due to oxidation of the Fe(II) center

and the basal carbon support upon prolonged exposure to O₂.

Yet, a noncompromising diﬀusion-limiting current density

suggested that there was no agglomeration of catalyst material

better 4-e transfer performance toward ORR, and (3)

superiority of our catalyst than Pt/C with respect to methanol

crossover eﬀect.

Electrochemical Performance toward NRR. In order to

check whether our catalyst G-FePc is equally responsive

toward electroreduction of N to NH , which serves as the

activated building block for several fertilizers, explosives, and

or mass loss from the electrode surface still permitting easy

diﬀusion of oxygen with the same eﬃcacy onto the active sites

for its facile reduction.

plastics, the liquid cell setup was arranged for electrochemical

investigation. Figure 5a demonstrates the LSV measurements

−1

in 0.1 M HCl at a scan rate of 10 mV s , which indicates two

For practical purposes, a useful catalyst is expected to be

liberal toward methanol crossover impact, which is a major

concern in the fuel cell applications. Fuel molecules like

methanol migrate from anode to cathode compartment, which

could hinder the cathodic performance of the catalyst toward

distinct polarizations curves in Ar and N saturated electro-

lytes, respectively. The choice of acidic electrolyte is due to the

abundance of protons, which could promote a faster reduction

process at a lower overpotential and the Naﬁon membrane

used in the study promotes facile transmission of the protons

71−73

ORR through the depolarizing eﬀect.

Thus, for our

restricting all other foreign ions. In the presence of N , a

catalyst G-FePc, cathodic polarization measurements were

conducted in the absence and presence of 1 M methanol in the

oxygen fed 0.1 M KOH electrolyte. As seen from Figure 4b,

noticeable increase in the current density could be observed in

the LSV curve from −0.2 V vs RHE, which indicated the onset

of NRR by suppressing its competitive hydrogen evolution

reaction (HER), which could have occurred otherwise (as seen

in the case of Ar at a higher negative potential than NRR). On

the basis of these results, electrochemical NRR was further

assessed in an H-type electrochemical cell with the working

electrode (GCE modiﬁed with catalyst material G-FePc) and

reference electrode (Ag/AgCl) being immersed in the cathode

compartment while the counter electrode (Pt wire) in the

anodic compartment to support the counter-oxidation reaction

at the anode. The chronoamperometric curves for NRR

acquired at diﬀerent potentials for 2 h as evident from Figure

5b demonstrate that the highest current density was achieved

the polarization curves of G-FePc for O reduction before and

after methanol addition exactly overlaid with each other,

making our catalyst absolutely insensitive to methanol. This is

unlike the case of Pt/C, whereupon, after 1 M methanol was

added, the oxygen reduction peak shifted slightly ∼6 mV

toward the negative potential with the appearance of a strong

methanol oxidation peak at 0.80 V vs RHE (Figure S14).

These results are veriﬁed by the corresponding current−time

(i−t) chronoamperometric response of G-FePc and Pt/C

catalysts as shown in Figure 4c. The addition of 1 M methanol

to the electrolyte caused almost no change in the current

density for G-FePc for a time span of 3600 s, while there

occurred a sharp increase in the current density for Pt/C.

Thus, G-FePc dominates over the commercial Pt/C catalyst

with respect to methanol crossover eﬀect and eventually with a

higher positive potential for ORR.

at −0.2 V. After each experiment, the amount of NH gas

evolved in the cathodic chamber was evaluated and quantiﬁed

by means of the UV−vis spectroscopic technique (Figure 5c)

following the indophenol blue method (details are provided in

the Supporting Information ). The detecting calibration plots

Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Figure 6. (a) 4-electron assisted inner-sphere electron transfer pathway for ORR on G-FePc catalyst in 0.1 M KOH. (b) Alternating mechanistic

pathway for NRR, followed by G-FePccatalyst yielding 6-electron product NH in 0.1 M HCl. Molecular orbital pictures denoting (c) low spin

Fe(II) in FePc, (d) Fe(II) in exfoliated G-FePc catalyst and HOMO of O undergoing ORR, (e) Fe(II) in exfoliated G-FePc catalyst and HOMO

of N undergoing NRR demonstrating adsorption of reactants onto Fe.

are provided in Figure S15a,b. Results suggest that the highest

with the results obtained from EDS analysis of G-FePc before

and after NRR. The near similar values of the elemental

content (Table S7 ) reveal that the N present in G-FePc was

ﬁrmly bonded to Fe and it did not leach out during

electroreduction of nitrogen to contribute to ammonia

production. It was already conﬁrmed from GC that the N₂

gas used for NRR was devoid of any nitrogenous gaseous

−

−1

NH yield of 27.74 μg h mg_c_a_twas recorded at −0.3 V as

calculated from eq 10, whereas the highest Faradaic eﬃciency

F.E. (2.04%) was evidenced at −0.2 V (from eq 11). From the

relative bar diagram in Figure 5d, it is clear that, beyond −0.3

V, both NH production rate and F.E. promptly decreased

owing to the invasive proton reduction reaction. To verify

the production of any side product such as hydrazine (N H ),

the Watt and Chrisp method of colorimetric detection was

adopted. With the help of the calibration curves at diﬀerent

known concentrations of N H (Figure S15c,d), no hydrazine

impurity, and this establishes that the measured NH was

chieﬂy obtained from the electroreduction of dinitrogen

purged into the electrolyte. Graphene (G-H SO ) provides

a conducting backbone with an overall π-electron delocaliza-

tion. The electrons concentrated at the defect sites (arising due

to the electrophoretic exfoliation process) could contribute to

production could be detected in Figure S16, advocating the

high selectivity of G-FePc toward sole NH production. The

superiority of our material over other control catalysts was

established from the NRR performance at −0.3 V maintaining

similar experimental conditions. Figure S17a −c and Figure 5e

bring to our notice that the current density as well as the UV−

vis absorption spectra reached a summit for G-FePc with a

diminishing value for FePc and G-H SO and ﬁnally became

negligible for Ar saturated electrolyte subsequently that was

reﬂected in the yield and Faradaic eﬃciency for ammonia

production. As a matter of investigation about the source of N

the electroreduction of N with a low yield rate of the product.

Interestingly, a sound improvement in the product formation

and F.E. could be witnessed for FePc, which is due to the

presence of the Fe center of the Fe-N sites eﬃciently active

toward NRR. A combinatorial eﬀect of the two systems

could be observed in the response of the G-FePccatalyst

toward eﬃcacious NRR. A summary of the reported carbon-

based catalysts as a comparison to our work has been

presented in Table S8 .

in NH , few blank experiments were conducted with G-FePc

The viability of G-FePc was cross-checked with a stability

test by performing cyclic trials (2 h of potentiostatic

electrolysis at −0.3 V). Figure S19a shows almost overlying

chronoamperometric plots with nearly the same current

density for all the ﬁve consecutive cycles. The data further

gain authentication from the UV−vis response (Figure S19b),

where the peaks at ∼650 nm could hardly be distinguished for

the ﬁve cycles. The histogram of Figure 5f reveals that repeated

cycling caused almost unattenuated catalytic performance with

and blank GCE in Ar saturated 0.1 M HCl electrolyte at −0.3

V and with G-FePc at open circuit potential (OCP). The UV−

vis spectroscopic data and the bar diagram comparing

concentration and yield of NH in Figure S18a,b is attributed

to the insigniﬁcant amount of dissolved nitrogenous impurities

that could have remained residually within the electrolyte. The

performance of our catalyst at diﬀerent conditions is tabulated

in Table S6a −c . The source of N in NH was further veriﬁed

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

negligible degradation of FePc on the catalyst surface upon

prolonged exposure to N₂.

N H intermediate on the active Fe site (like in the case of

2 4

ORR), indicating an alternating pathway to be taking place

Brief Understanding of the Mechanisms Concerning

ORR and NRR Processes. The above-mentioned magniﬁcent

experimental data necessitated further enlightenment of the

reduction processes. The mechanistic insight for ORR in

alkaline 0.1 M KOH medium can be elucidated from Figure 6a.

As a matter of fact, the graphene support of our catalyst G-

FePc constitutes a well π-conjugated system. Owing to this π-

electron delocalization on the graphene surface, there occurs a

shift in the negative charge density from the comparatively

more electron rich FePc macrocycle of G-FePc toward

graphene via the long-range, noncovalent, π−π interaction.

selectively forming 2 NH molecules subsequently.

Molecular Orbital Chemistry behind Bonding of Fe(II)

Active Center with O and N . FePc is a planar system

having D₄_hpoint group symmetry with Fe in its +2 low spin

state, and its ground state molecular orbital positions can be

well understood from Figure 6c. In this planar condition, due

to unmatched symmetry, the Fe (d ) orbital cannot interact

with the π electrons of the phthalocyanine ring. In order to

maintain the electron ﬂow throughout the FePc macrocycle,

the vacant Fe (d ) orbital is involved in the electron

x −y

delocalization, thereby stabilizing the axial d

binding. In the G-FePc catalyst, due to π−π noncovalent

interaction between the graphene and FePc moiety, orbital

orbital free for

This makes the Fe-N center of FePc in the G-FePc system

electron-deﬁcient and downshifts the energy level of the e

(

d ) of the metal ion, anodically altering the metal ion’s redox

rearrangement energetically downward the e d orbital of

g z

potential. This is the root cause of ORR initiation on the

catalytically active Fe sites at a much higher onset potential

FePc and shortens its distance from the highest occupied

orbitals (dxz and dyz). Although, the dyz orbital is symmetrically

of 1.02 V vs RHE. The square planar Fe -N moiety enables

equivalent to dxz, due to the Fe-O plane, the dyz orbital

axial ligation of the chemisorbed O onto the metal center by

becomes energetically more stable than the dxz orbital. As a

result, the electrons in the dyz orbital remain intact while dxz

σ-bond formation between the singly occupied d of the metal

ion and the highest occupied π* orbital of the O molecule,

promotes one of its two electrons to the energetically

76,77

resulting in a soft Fe-O adduct formation.

Once the

downshifted d

orbital and FePc turns into a biradical system

oxygen gets ligated axially, the Fe in the phthalocyanine ring of

G-FePc slightly undergoes dislocation, thereby weakening the

Fe−N bonds but not as much as in the case of only FePc due

with spin multiplicity (S) = 1. On the other hand,

paramagnetic O has two degenerate singly ﬁlled π* orbitals,

one of which interacts with the d

electron of Fe(II) via a

47,28

to the π−π interaction in the former.

Throughout the

strong σ-bonding (Figure 6d), while the other is involved in a

noninteracting singlet coupling with the dxz electron of

reduction process, Fe swings between +2 and +3 oxidation

states and the O molecule on the catalyst surface gets through

Fe(II). These two interactions stabilize O

onto the Fe(II)

III

−

a number of reduction steps forming superoxo (Fe -O-O ),

ferric hydroperoxyl (Fe -O-O-H), and the most vital ferrous

surface.

III

In the case of NRR, the electron deﬁciency on the

electrocatalytically active Fe center tends to pull the bonding

−

hydroperoxyl (Fe -OOH ) intermediates. The bond strength

between this third intermediate and Fe ion determines whether

the reaction will follow a 2-electron or 4-electron pathway

^σ^e^l^e^c^t^r^oⁿ^s^o^f^N2 toward itself, leading to a σ−σ type

interaction between 2p of N and d of Fe(II) (Figure 6e).

−

There also occurs a π-back-donation from the d orbital of Fe

yielding (OOH + OH ) or 4 OH , respectively. The alkaline

electrolyte (pH > 12) used in this study preserves the Lewis

to the lowest unoccupied orbital (π*) of N (according to the

−

Pauli exclusion principle). These synergistic electronic eﬀects

basic nature of the hydroperoxyl ion (OOH ) oﬀering

stability to the Fe -OOH intermediate and kinetically favors

the immediate reduction of peroxide to the 4-electron

80,81

−

cause end-on adsorption of N on the Fe active site.

Gradually as N gets protonated, interaction between N

molecular orbitals and the H atomic orbital causes a decrease

product. This aids to bypass the side-product formation,

in bond order of NN, transforming it to H-NN-H and

and the complete 4-electron reduction of O to produce 4

−

hence to H -N-N-H as shown in the mechanism.

OH ions is accomplished following an inner-sphere electron

transfer pathway. The facile desorption of the reaction product

regenerates the active sites for further electrocatalysis. Thus,

the eﬃciency of our catalyst toward 4-electron ORR in the

CONCLUSION

■

In this work, we have pursued a facile synthetic approach for a

reduction electrocatalyst G-FePc by means of an electro-

phoretic exfoliation method. In due course of this study, XRD

and Raman spectroscopic characterizations conﬁrmed the

formation of high-quality graphene sheets with ﬁnely dispersed

FePc moieties on the sheets via π-π anchorage. This kind of

interaction not only improvised the defect sites on the

graphene layers but only provided an enhanced electrical

conductivity and mechanical stability, which reinforced the

electrochemical reduction reactions on this catalyst surface.

Molecular orbital rearrangements facilitated smooth adsorp-

tion of gaseous reactants (O and N ) and stability of the

alkaline medium can be attributed to two factors: (1) anodic

repositioning of the redox potential of Fe²

+/3+

ion due to π−π

stacking interaction between the graphene backbone and FePc

macrocycle leading to higher ORR onset potential and (2)

stability of the hydroperoxide intermediate on the metal

surface with a facile transmission to the 4-electron reduction

product in an alkaline medium.

Parallelly, the eﬃcacy of our as-synthesized catalyst G-FePc

toward NRR could be mechanistically conﬁrmed from Figure

b. As a contrary to ORR, an acidic 0.1 M HCl electrolyte

beﬁts NRR. The electronically deﬁcient Fe-N active site is

capable of drawing the N molecule toward itself, and σ-

reaction intermediates onto the catalytically active Fe-N sites,

donation and π-back-donation interactions between Fe and N₂

contribute to weakening the NN bonds gradually, critical for

thereby oﬀering the formation of direct reduction products

−

OH and NH for ORR and NRR, respectively. Eﬃcacy of our

its proton-functionalization. An abundance of H ions present

in the medium facilitates easy protonation of the N atoms

adsorbed onto the metal surface. The byproduct N H

catalyst toward ORR could be reﬂected in the half-wave

reduction potential to be as high as 0.91 V in 0.1 M KOH

solution outperforming the commercial Pt/C catalyst by a

positive shift of ΔE₁_/₂= 50 mV with an overpotential as low as

formation is completely forbidden, indicating stability of the

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−

−1

of 27.74 μg h mg_c_a_tat a low potential of −0.3 V during the

electrochemical NRR with a mention-worthy stability. Thus, in

short, the low-cost synthesis of G-FePc could be explored

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(

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02176.

■

(

6) Wang, H.; Wang, L.; Wang, Q.; Ye, S.; Sun, W.; Shao, Y.; Jiang,

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data, mass loading optimization, ORR half-wave

potentials, ring current plot, plots of I /I , UV−vis

spectra, chronoamperometric response, and tables of

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catalytic performance, and concentration and yield data

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Electrochemical N2 Fixation to NH3 under Ambient Conditions:

Mo2N Nanorod as a Highly Efficient and Selective Catalyst. Chem.

Commun. 2018, 54 (61), 8474−8477.

AUTHOR INFORMATION

Corresponding Author

Ramendra Sundar Dey − Institute of Nano Science and

Technology, Mohali 160062, Punjab, India; orcid.org/

■

000-0003-3297-1437 ; Email: rsdey@inst.ac.in

Authors

(11) Wu, Z.-Y.; Xu, X.-X.; Hu, B.-C.; Liang, H.-W.; Lin, Y.; Chen, L.-

F.; Yu, S.-H. Iron Carbide Nanoparticles Encapsulated in Mesoporous

Fe-N-Doped Carbon Nanofibers for Efficient Electrocatalysis. Angew.

Chem., Int. Ed. 2015, 54 (28), 8179−8183.

Ashmita Biswas − Institute of Nano Science and Technology,

Mohali 160062, Punjab, India

Subhajit Sarkar − Institute of Nano Science and Technology,

Mohali 160062, Punjab, India

Manisha Das − Institute of Nano Science and Technology,

Mohali 160062, Punjab, India

Navpreet Kamboj − Institute of Nano Science and Technology,

Mohali 160062, Punjab, India

(12) Yang, W.; Liu, X.; Yue, X.; Jia, J.; Guo, S. Bamboo-like Carbon

Nanotube/Fe 3 C Nanoparticle Hybrids and Their Highly Efficient

Catalysis for Oxygen Reduction. J. Am. Chem. Soc. 2015, 137 (4),

13) Wang, W.; Jia, Q.; Mukerjee, S.; Chen, S. Recent Insights into

436−1439.

the Oxygen-Reduction Electrocatalysis of Fe/N/C Materials. ACS

Catal. 2019, 9 (11), 10126−10141.

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c02176

(14) Wen, Z.; Ci, S.; Zhang, F.; Feng, X.; Cui, S.; Mao, S.; Luo, S.;

He, Z.; Chen, J. Nitrogen-Enriched Core-Shell Structured Fe/Fe3C-C

Nanorods as Advanced Electrocatalysts for Oxygen Reduction

Reaction. Adv. Mater. 2012, 24 (11), 1399−1404.

Notes

The authors declare no competing ﬁnancial interest.

(15) Lee, J.-S.; Park, G. S.; Kim, S. T.; Liu, M.; Cho, J. A Highly

Efficient Electrocatalyst for the Oxygen Reduction Reaction: N-

Doped Ketjenblack Incorporated into Fe/Fe3C-Functionalized

Melamine Foam. Angew. Chem., Int. Ed. 2013, 52 (3), 1026−1030.

ACKNOWLEDGMENTS

■

A.B. acknowledges INST, Mohali, for providing a fellowship.

This work was ﬁnancially supported by the DST INSPIRE

grant (DST/INSPIRE/04/2015/000337). The authors wish to

acknowledge INST, Mohali, for their support with instrumen-

tation.

16) Wu, Z.-S.; Yang, S.; Sun, Y.; Parvez, K.; Feng, X.; Mullen, K. 3D

Nitrogen-Doped Graphene Aerogel-Supported Fe3O4 Nanoparticles

as Efficient Electrocatalysts for the Oxygen Reduction Reaction. J. Am.

Chem. Soc. 2012, 134 (22), 9082−9085.

(17) Liu, Q.; Zhang, X.; Zhang, B.; Luo, Y.; Cui, G.; Xie, F.; Sun, X.

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