Electrocatalytic Performance of Palladium-Based Electrocatalysts Supported on Carbon Nanotubes for Formic Acid Oxidation

In the present work, Pd and PdFe nanoparticles supported on CNT with and without functionalization (CNT and CNTox) were used for Formic Acid Oxidation Reaction (FAOR) in acid media. Electrocatalysts were synthesized by the borohydride reduction method with 20 wt.% metal loading. The CNTs were synthesized by the methane catalytic decomposition, and subjected to an oxidation treatment with nitric acid, named as CNTox. The morphology, composition and structural properties were studied by Transmission Electron Microscopy (TEM), Scanning Electron Microscopy-Energy Dispersive X-ray (SEM-EDX) and X-Ray Diffraction (XRD). The FAOR was evaluated in acid media in a conventional three-electrode cell by means of cyclic voltammetry and chronoamperometry. From the steady state current density, it was found that Pd and PdFe supported at CNTox allowed improving the catalytic activity in comparison with the non-oxidized support.


Introduction
Recently, the study of carbon supports like carbon nanotubes, (CNT) has intensified.The interaction between the support and nanoparticles improved the fuel cell efficiency and durability, decreasing the electrocatalyst poisoning and in some cases it can modify the particle size.Palladium based nanostructured electrocatalysts, supported on gassy carbon (1), carbon black Vulcan (2) and CNT (3) are use in electrochemical energy storage devices such as fuel cells (FCs).Direct liquid fuel cells (DLFCs) have been considered an alternative power source for electric portable devices for a growing scientific community.Among the different types of DLFCs, direct formic acid fuel cells (DFAFC) have attracted particular interest, since formic acid is safe, environmental-friendly and nontoxic.In addition, it presents high-energy density and fast oxidation kinetics in comparison to other fuels, such as methanol (4).Further, it has been stated that the FAOR mechanism possibly evolves through either a direct or an indirect route.The former involves its direct oxidation into carbon dioxide, whereas formic acid dehydration to adsorbed carbon monoxide and its subsequent oxidation into carbon dioxide constitutes the latter.The FAOR rate can increase when using an electrocatalyst that usually consists of metal nanoparticles dispersed on convenient carbon materials with different structures, morphologies and sizes (5).
Palladium-based electrocatalysts supported on carbon materials have generated a great interest for use in the FAOR evaluation.However, further research is still necessary as Pd deactivates during the FAOR.In order to overcome this limitation, Pd-M alloys have been proposed (6)(7)(8).Pd catalysts are an alternative because they have electronic properties similar to those of Pt, in addition Pd catalysts alloyed with transition metals such as iron (Fe) increase the Pd catalytic activity.The reactions involved in DFAFC are those shown in reactions 1-3.
HCOOH + 1/2 O2 → CO2+ H2O reaction global [3] The interaction between the support and electrocatalyst improves the efficiency of the FCs, decreasing the electrocatalyst poisoning and in some cases can affect the particle size (9).These carbonaceous supports increase the active surface area and hence, enhance the electrochemical activity.In addition, carbon supported materials display adequate physicochemical and chemical properties, such a large specific surface area, high conductivity and stability, which play a fundamental role on the overall electrocatalysts efficiency (10).The CNT are allotropic forms of carbon with a cylindrical structure and are longer than any other material such as carbon nanofibers.The name is due to the long, hollow structure and its walls formed by a sheet of carbon one atom thick, called graphene.These sheets are rolled at angles in a specific way and the combination of the winding angle and the radius defines the properties of the nanotube.There are two types of CNT, CNT single wall (SWCNT) consisting of a single sheet of rolled graphene, and CNT multiple wall (MWCNT) where they can have more than two concentric layers, this classification depends on the curl degree, and the sheet number that form it (11)(12)(13)(14).

Experimental Setup
The Palladium-based electrocatalysts supported on CNT and CNTox were synthesized using sodium borohydride, NaBH4 as reducing agent according with previous work (2).The metal molar ratio in the PdFe electrocatalysts was 1:1.Appropriate amounts of metal precursors were used to obtain a theoretical metal loading of 20 wt% onto support.
The Pd-based electrocatalysts were characterized by TEM-EDX and XRD to study the particle size, composition and structure of the resulting nanoparticles, as well as their dispersion onto the carbon support.
To establish the powder electrocatalyst composition a JSM-6701F Scanning Electronic Microscopy with a detector for EDX analysis.XRD patterns were recorded using a Philips diffractometer X'pert PRO Scans were carried out at 4° min -1 for 2θ values from 20 to 80° and TEM micrographs were obtained in a JEM-ARM200CF microscope operated at 200 keV.
In the other hand, the electrochemical performance of the synthesized electrocatalysts was studied by means of cyclic voltammetry (CV) and chronoamperometry (CA) in a conventional three-electrode electrochemical cell with an autolab 30 Methrom potenciostatic-galvanostatic.The electrodes used were a glassy carbon bar of 5.0 mm as diameter as working electrode, graphite bar was used a counter electrode and Ag/AgCl 3.0 M KCl was used as reference electrode.The working electrode was modified with a electrocatalytic ink (20 mg of electrocatalyst + 500 µl ultrapure water + 15 µl Nafion, the mixture was sonicated for 40 minutes to obtain a homogeneous suspension).The support electrolyte was 0.5 M H2SO4, while, to evaluate the FAOR the electrolyte was 0.5 M H2SO4 + 2.0 M HCOOH.
Electrochemical experiments were carried out at room temperature and atmospheric pressure, all the measurements for CV were evaluated in a potential range of 0.2 to 1.0 V at 20 mVs -1 , and the measurements by CA were evaluated at the potentials of 0.2 and 0.4 V for 900 seconds.This technique allowed us to know the steady-state current density parameter of the synthesized electrocatalysts.

Results and discussion
The Pd and PdFe electrocatalysts synthesized were characterised by means of XRD, and the former results showed the peaks belonging to the Pd (fcc) structure for both as presented in Figure 1, while for the PdFe system the Fe was found in the (Fe2O3) oxide.The results of SEM-EDX reported that the support : metal ratio of the electrocatalysts, was 80:20 %w while for the PdFe system, the said Pd-Fe-support ratio was 10:10:80 %w; the C, Fe and Mg presence from the support and metals used for synthesis, can be clearly seen in Table 1.
TEM aided the identification of the synthesized NPs morphologies along with the support: the images display the CNT shape with the Pd and Fe NPs dispersed over the support as illustrated in Figure 2, where it can be noted that the CNTs synthesized were multi-walled.The particle size was in average size of 2-4 nm.The electrocatalytic activity was evaluated through CV and CA: cyclic voltammetry was carried out in the N2 presence, using a 0 M HCOOH concentration at 20 mV s -1 scan rate as shown in Figure 3.The voltammograms exhibit the Pd y PdFe electrocatalysts behavior in acid medium, the CV started at the OCP = 400 mV, in the anodic direction where a current increase becomes apparent from about 500 to 1000 mV associated to the PdO formation, although once formed these become reduced at a potential close to 520 mV.Meantime, at potentials between 10 to -200 mV the peaks pertaining to hydrogen sorption and desorption.The voltammograms obtained display the typical Pd response, apart from the processes typical of the Pd surface as reported in the literature.Figure 4 shows the cyclic voltammetry of the FAOR at 20 mVs -1 scan rate for the Pdbased electrocatalysts supported on CNT and CNTox, the plots clearly display strong oxidation FAOR peaks and those of the corresponding crystal planes of the Pd.A comparison of the electrochemical response obtained between the monometallic and bimetallic systems on the CNT, with and without oxidation, one observes that the Pd-CNT is the electrocatalyst that demanded the least potential to enable oxidation of the formic acid (FA): 256 mV (anodic potential maximum) bearing a current density of 1.36 mAcm -2 .However, when the support becomes oxidized, the electroactivity in current density increased nearly twice, although the potential increased also to 518 mV; this is, oxidizing the CNT induces increments of the FA oxidation potential for the Pd system that involves an interaction between the groups generated during functionalization of the CNT.Further, for the PdFe system, something similar happened for the Pd, namely, the PdFe-CNT presents a current density of 0.90 mAcm -2 , at a potential maximum of 494 mV, but when the support became oxidized its activity increased lightly, but the potential demanded decreased only 20 mV, referred to the support without oxidation.Table 2 shows the potential and current density of the maximum peak in both scan directions.The potential to oxidize the FA increases for the Pd system and decreases for the PdFe system, while the current density in both systems increases when the CNT become oxidized.This brings to light the effect of the carbon support oxidation treatment with HNO3 (CNT).The steady state current density (jss) was evaluated performing three chronoamperometries at 0.2, 0.4 V for 900 s. Figure 5 shows the potentiostatic current density transients for the 0,2 and 0.4 V potentials.The analysis of the results shows that at 0.2 and 0.4 V potentials, Pd-CNTox exhibited the largest jss, followed by PdFe-CNTox as observed; the following is the jss decreasing order of the electrocatalysts:

Conclusions
This work showed that the Pd y PdFe synthesized NPs supported on CNT and CNTox (oxidized with HNO3), were adequately disperse on the carbonaceous support, with a 2-4 nm particle size.The electrocatalytic activity showed that the Pd and PdFe supported on CNTox displayed the largest activity compared with Pd and PdFe supported on CNT.The decreasing order of electroactivity is given as Pd-CNTox > PdFe-CNTox > PdFe-CNT > Pd-CNT, where the Pd-CNTox was the best catalyst followed by PdFe-CNTox.In addition, the support that showed the lowest performance for the FAOR was NFCox.Moreover, the effect of the support is reflected in the results obtained, suggesting that the functionalization improves the performance of the Pd and PdFe electrocatalysts for the FAOR.

Figure 1 .
Figure 1.X Ray diffraction results of the electrocatalysts synthesized and supported on the CNT and CNTox.

Figure 2 .
Figure 2. TEM images of the electrocatalysts synthesized and supported on CNT and CNTox.

Figure 3 .
Figure 3. Cyclic voltammetry of the system 0.5 M H2SO4, of the Pd and PdFe supported on the CNT and CNTox.Scan rate () 20 mV s -1 .

Table I .
Elemental composition obtained by EDX of the electrocatalysts synthesized and supported in CNT and CNTox.

Table 2 .
Potentials and current densities of the maximum peaks in the CV.