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To obtain the electrocatalyst having best
electrocatalytic performance and stability towards formic acid
electro-oxidation reaction (FAOR), simple impregnation method was used to prepare
Pt3Ni nanoparticles loaded on carbon black. The attained X-Ray
Powder Diffraction (XRD) results as well as transmission electron microscopy (TEM)
analysis of as-synthesized electrocatalyst demonstrates that the reduction
temperature has great influence on the morphology of Pt3Ni nanoparticles.
X-ray photoelectron spectroscopy (XPS) analyses confirm the variation in the electronic
structure of platinum by incorporation of Nickel atoms which delays
chemisorption of toxic carbon monoxide and promotes the dehydrogenation pathway
of FAOR. The size of the as-obtained samples remains within the range of 8 nm. All
electrochemical analyses illustrate that the performance of the as-obtained electrocatalyst
towards the FAOR is significantly enhanced. The carbon black content, amalgamation
of Ni atoms, and reduction temperature conditions are the key factors for modification
of the crystal structure and morphology which leads to enhanced catalytic performance.

Key
words: Formic acid,
Electro-oxidation, Pt3Ni nanoparticles, carbon black,
Dehydrogenation pathway.

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Introduction

Movable
devices such as mobile phones, laptops etc., have need of energy and power
but meanwhile, the functioning charging lifetime of these power sources is not
being improved in agreement with the user demand. In the previous era, the use
of liquefied fuels in devices has been a substitute and fascinating field of
research 1, 2. Primarily, wide efforts have been made on direct methanol fuel
cells (DMFCs) owing to their activity, high energy density, and easy accessibility
of fuel by slight contaminant emissions and efficient energy conversion 3. Though,
the commercial use of DMFCs is restricted because of certain serious complications
such as (i) process at controlled concentration, (ii) deprived kinetics owing
to catalyst poisoning through carbon intermediates produces in methanol
oxidation, causing in reduced fuel performance (iii) at room temperature
activity is very low 4-6, (iv) methanol crossover, which confines the usage
of high methanol
concentrations, normally less than 2 M 7 and lastly (v) the expensive Pt (Pt
is precise catalyst for the DMFCs). To overcome all of the above-mentioned
problems, DFAFCs have attained attention in current time. Formic acid is comparatively
less poisonous than other liquid fuels and it has very high open circuit
potential (1.450 V) theoretically than direct formic acid fuel cells (1.190 V) and proton exchange membrane
fuel cells (1.229 V) 8. Moreover, Formic acid also has a lower
crossover flux as compared to methanol and ethanol over nafion, or the proton
exchange membrane, because of the repulsion existing by the membrane terminal
groups. Therefore it accelerates proton transport in the anodic part of the
fuel cell which leads to high energy conversion 9. Although, the energy
density of formic acid is 2086 WhL-1 which is smaller as compared to
methanol (4690 WhL-1), it transmits additional energy per unit
volume as compared to methanol owing to the fact that concentrated formic acid (20
M or 70 wt %) can be used as a fuel comparatively low concentration of methanol
(2 M) 10. The further main benefit of formic acid to use as a fuel is its creation
from environmental leftover by the biomass conversion procedures 11.

Fuel
cells have been considered as a significant power source for the future owing
to their high energy conversion efficiency and low environmental pollution
12–15. Formic acid oxidation
reaction (FAOR) is a significant reaction in electrocatalysis and meanwhile it
can also be used as a model in basic studies for other small organic molecules,
e.g. ethanol or methanol 16.
Furthermore, formic acid has been recommended as a fuel for direct liquid fuel cells (DLFCs) in
electronic devices 6-7, as FAOR shows very fast oxidation kinetics as compared to other
fuels such as methanol and less fuel crossover through the ionic exchange
membrane 19. DFAFCs are more fascinating than hydrogen fuel cells from an
available energy point of view owing to the thermodynamic cell potential which
is 1.428 V 20. However, in order to reach the commercial applications, improvement is
required for the overpotential in FAOR. As Pt is one of the most studied metals
in electrocatalysis 16. Mostly FAOR on Pt electrodes has been comprehensively studied
because of the high activity for the oxidation of different small organic
metals (SOMs). FAOR has possibly the simplest oxidation mechanism among all different
SOMs, a deep understanding of the FAOR mechanism on Pt should be very useful for other
important electrocatalytic
oxidation reactions. It is well known that FAOR follows two different reaction
pathways on Pt electrodes, (i) direct
via (ii) indirect via 21, 22. One of the most acknowledged
mechanisms of FAOR is given in following equation (eq.1). The first mechanism is
known as “direct pathway” it encompasses direct oxidation of the acid to CO2:

HCOOH + M ? CO2 + 2H+
+ M + 2e-                            (eq.
1)   

(“M” = Pt, Pd etc.)

A second mechanism is called “indirect
pathway”. It takes place when CO adsorbs on the surface of “M”, given below:

HCOOH + M ? M-CO + H2O                          (eq.
2)

M + H2O ? M-OH + H+
+ e-                            (eq.
3)

M-CO + M-OH ? 2M + CO2 + H+
+ e-                        (eq.
4)

Production
of CO on the electrode surface is involved
in the indirect pathway, which behaves as a poison intermediate. Though, active
intermediate generates in the direct via pathway, which is instantly oxidized
into CO2. As well as, FAOR is well-known to a surface sensitive
reaction 23, studies on Pt
single crystal electrodes (Pt (hkl)) revealed that
Pt (100) is the most active electrode for both paths (direct via and indirect via) in FAOR, whereas Pt (111)
is least active one, although  the
creation of CO is nearly negligible
on this electrode 24. The reformation of the surface chemical composition on the
Pt (hkl) electrodes is
one of the most broadly employed approaches to enhance the catalytic activity of the FAOR. This approach is generally
based on the combination of
different adatoms on the surface of the Pt(hkl) electrodes, which
can be either metals or semi-metals. This adsorption and deposition of a
sub-monolayer of adatoms on a metal substrate
are normally done either by underpotential deposition (UPD) or by irreversible adsorption
at open circuit potential. In some cases, these two fascinating approaches to amend
noble metals may produce
surface alloys 25. In the case of amended Pt electrodes, the UPD method is relies on
the electrodeposition of an
adatom monolayer which is existing in a solution that consist of dissolved
adatom as a cation at considerably less negative potentials than for the bulk electrodeposited
on of the adatom 26, 27. The basic difference between UPD and irreversible
adsorption techniques is that
the in the irreversible absorption adatoms stay stable on the surface of Pt in
the nonexistence of the adatom cation in the solution 28–30. However, on the
other hand, UPD adatoms are unstable on the surface of Pt except for the
solution that consists of the adatom cation in low concentration. Furthermore,
irreversible adsorption permits attaining adatom coverage which is independent
of the applied potential V. Furthermore, this method also eludes the problem of precision
appears in the coverage quantity
when the UPD technique is used, because of its dependence on the applied potential and the
composition of the solution.

The
positive effect of the existence
of several adatoms on the catalytic
performance of Pt electrodes towards FAO is envisioned by a shift to lower potential
values through increasing the
current densities of the oxidation reaction. In this logic, it is suggested
that adatoms may follow three
mechanisms given below; i) the electronic effect, in which the
amendment of the Pt electronic structure owing to the existence of external
adatoms improves the  surface activity 31, 32, (ii) influence of the
third body in which the external adatom amends the reaction mechanism via steric interference, meanwhile it blocks
particular adsorption sites on the surface of Pt which prevent  formation of CO 33 and iii) the bi-functional effect, in this mechanism distinctive roles played by adatom and the Pt surface sites
in the oxidation mechanism 34.
Researchers have reported that the adatoms such as arsenic
(As) 35, bismuth (Bi) 36, lead (Pb) 37, palladium (Pd) 38, and antimony (Sb) 39, adsorbed
on the surface of Pt electrodes, display a significant enhancement in the activity of  FAOR, by following the one of the above
mentioned mechanisms. Currently, the next task is to hand over all that knowledge
from single crystal
electrodes to nanoparticles with a special structure and surface area. 

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