Oxidación electroquímica de ácido fórmico usando nanopartículas de Pd y PdFe. Influencia del método de síntesis y del soporte carbonoso

In the present research work, the mono- and bi-metallic electrocatalysts of Pd and PdFe were studied for FAOR (Formic Acid Oxidation Reaction), as basic participants of one of the reactions involved in DFAFC (Direct Formic Acid Fuel Cells). Electrocatalysts were synthesized by two methods: on the one hand, by the chemical method using sodium borohydride as a reducing agent and different carbon supports such as: carbon nanofibers (CNF), carbon nanotubes (CNT) and graphene oxide (GO), in addition to nanofibers and nanotubes that were functionalized to know its effect. While on the other, by the electrochemical method, the electrodeposition of the metal Pd, Fe and PdFe were obtained from a eutectic mixture of choline chloride (ChCl) and urea (U) as DES supported on glassy carbon. Mono- and bi-metallic electrocatalysts of Pd and PdFe synthesized by the chemical method were characterized by SEM-EDX, XRD, XPS, TEM and RAMAN to know crystal size, morphology, particle size, chemical and surface composition of the NPs synthesized. By contrast, the electrocatalysts synthesized by the electrochemical method were characterized by SEM-EDX, XRD and XPS to determine crystal and particle size, as well as the chemical and surface composition of electrodeposited NPs. TEM analysis showed that the NPs of Pd and PdFe synthesized by the chemical method and supported on the different supports presented an average particle size of between 3.8 to 4.4 nm for the monometallic systems, while 3.2 to 3.8 nm was for the PdFe systems. Furthermore, the FAOR evaluation showed that the Pd electrocatalyst supported in CNFox, presented the best catalytic activity reaching values of 5.53 mAcm–2, as well as in terms of the current density parameter in steady state Pd-CNFox, it exhibited the best performance at the three applied potentials in comparison with the other synthesized electrocatalysts and the commercial one. However, when the experimental results were normalized with respect to the electrodeposited Pd mass, the PdFe-CNTox electrocatalyst showed the best performance against FAOR and jss, reaching values of 3720.81 and 521.81 mAmgPd–1, respectively. Notedly, the results obtained by the electrodeposition synthesis method, allowed to study the nucleation and growth process of the electrodeposition of Pd, Fe and PdFe, using a deep eutectic solvent, determining that the electrodeposition of these metals is a 3D nucleation mechanism with diffusion-controlled growth. In this way, it was possible to determine the diffusion coefficients: (1.65 ± 0.10) ×10−7 cm2s−1 for the Pd(II)-DES system and 1.16 ×10−7 cm2s−1 for Fe(II)-DES. The potentiostatic study of the Pd(II)-DES system, showed that the simultaneous reduction of residual water also occurs on the growth surfaces of the Pd nuclei, thus allowing deconvolution of the different contributions (j3D and jWR), which depend on time and the applied overpotential. Additionally, the SEM and XPS analyses showed that the Pd electrodeposits present core-shell morphology (Pd(0) core, PdO and Pd(OH)2 as shell), while, for the Fe and PdFe electrodeposits the morphology was like a cluster. Finally, with the electrodeposited PdNPs, the FAOR reaction was evaluated in the presence of two electrolytes H2SO4 and HClO4, finding that the best mass activity against FAOR and jss occurred when HClO4 was used as electrolyte. The other hand, the electrodeposited PdNPs were used to evaluate the methanol oxidation reaction (MOR) in basic medium, finding that the PdNPs at the electrodeposition potential of -700 mV exhibited the best mass activity with 2371 and 30 mAmgPd-1 toward MOR and jss, respectively. Additionally, the bimetallic PdFeNPs electrodeposited at the different applied potentials were used to evaluate the FAOR, finding that the PdNPs at the electrodeposition potential of -500 mV, exhibited the highest mass activity reaching values of 3788.26 and 116.19 mAmgPd-1 toward ROAF and jss, respectively, in this way the results obtained were superior to those reported by other authors.