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.IntroductionMovable devices such as mobile phones, laptops etc., have need of energy and powerbut 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. Keeping in view of this logic, researchers have reported shape-controlled Pt-based nanoparticles (NPs) by the significant role of several adatoms such as Ti 40, Sb 41 and Bi 42.
Mainly, Bi adatom has revealed a noteworthy improvement in the activity towards FAOR of the Pt-based NPs 43. Several new tactics in FAOR on improved Pt electrode also contain trimetallic systems 44 and graphene-based Pt nanohybrid 45. Regardless of the number of adatoms previously studied amending the Pt NPs 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 have been 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.