To obtain electrocatalyst with best performance and stability towards formic acid electro-oxidation reaction (FAOR), simple impregnation method was used to prepare Pt3Ni nanoparticles supported on carbon black XC-72. The obtained X-Ray Powder Diffraction (XRD) results and Transmission electron microscopy (TEM) analysis demonstrates that the reduction temperature has great influence on morphology of Pt3Ni nanoparticles. The variation of Pt electronic structure by incorporation of Ni atoms which delays chemisorption of toxic CO intermediate specie and promotes the dehydrogenation pathway of FAOR was confirmed by X-ray photoelectron spectroscopy (XPS). The size of the as prepared samples remains within the range 8 nm. All electrochemical analysis exposed that the performance towards the FAOR is significantly improved. The mass activity of Pt3Ni/C is mA mg- Pt 1 at 0.45 V (vs.
SCE), which is 11.5 times as high as that of Pt/C (188.44 mA mg-1Pt). The incorporation of Ni atoms, content of carbon black and reduction temperature condition are key parameters for modification of crystal structure, morphology and enhanced catalytic activity.Key words: Formic acid, Electro-oxidation, Pt3Ni nanoparticles, carbon black,Dehydrogenation pathway.IntroductionPortable devices such as laptops, mobile phones, and so forth have need of energy and powerbut simultaneously 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 above mentioned problems, DFAFCs have attained attention in current time. Formic acid is less poisonous as compared to 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. However 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. Additional 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 because of their high energy conversion efficiency and low environmental pollution 12–15. Formic acid oxidation reaction (FAOR) is a significant reaction in electrocatalysis; 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 because of 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 extensively 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)Formation 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 formation 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 for 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 is 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 rely 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 electrodeposit on of the adatom 26, 27.
The basic difference among UPD and irreversible adsorption techniques is that the irreversible adsorbed adatoms stay stable on the surface of Pt in the nonexistence of the adatom cation in the solution 28–30, however on the hand, UPD adatoms are unstable on the surface of Pt except the solution that consist 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 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 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 handover all that knowledge from single crystal electrodes to nanoparticles with a special structure and surface area.
In this logic, researchers have reported shape-controlled Pt nanoparticles by the significant role of several adatoms such as Tl 40, Sb 41 and Bi 42. Mainly, Bi adatom has revealed a noteworthy improvement in the activity towards FAOR of the Pt nanoparticles (111) preferential surface 43. Several new tactics in FAOR on improved Pt electrode also contain trimetallic systems 44 and graphene based Pt nanohybrids 45.
Regardless of the number of adatoms previously studied amending the Pt NPs (nanoparticles) electrode, still there are few of them untested. Generally, a common approach has been used to increase the electrochemical activity to deposit bimetallic catalysts onto a carbon material. To synthesize PtNi/C alloys with nickel content up to 50 %, the borohydride reduction method has been used productively 46. Numerous forms of carbon are ‘ used as supports for Pd-based electrocatalysts. e.
g., carbon black (XC-72), porous carbon, graphene and carbon nanotubes (CNTs), All of the above mentioned carbon materials are very useful to decrease the loading quantity of the Pd metal deprived of decreasing its efficiency, therefore minimizing the cost of catalyst for commercial applications 47, 48.Hence, a facile and adaptable synthesis method is highly required for preparing Pt-based and other noble metal alloy nanocomposites with structural anisotropy. Here, we propose a novel approach for the direct synthesis of PtNi alloy nanoomposite.
It is revealed that by simply varying the heating temperature of the reduction process we can alter the growth mechanism of Pt alloy nanostructures from thermodynamic to kinetic. In comparison to the already studied methods for the synthesis of PtNi alloy, our approach does is very simple, up-scalable and low cost. More significantly, the shape-controlled synthesis of Pt alloy nanocrystals with exposed high-energy facets, which usually have a high density of atomic steps, ledges and kinks, is facilely achieved. In addition, the newly developed approach is of remarkable high yield, and can be easily extended to the synthesis of other metal alloy nanostructures. Due to unique composition and shape, thus obtained PtNi alloy nanourchins exhibit superior electrocatalytic properties, presenting a promising class of electrocatalytic materials.Thus, the aim of this work was prepare novel alloy for formic acid oxidation reaction PtNi-C, PtNi-GO, PtNi-Gr electrocatalysts with different Pt/Ni atomic ratios (Pt:Ni=1:1, 2:1, 3:1, 1:3, 5:1), by simple impregnation method and test these electrocatalysts for the formic acid electro oxidation reaction in acid and alkaline electrolyte by cyclic voltammetry and chronoamperometry.