Pt.Ni nanoparticles supported on Graphene as an effective catalyst for oxygen reduction

The oxygen reduction reaction (ORR) is one of the key processes of energy systems. The ORR is catalyzed by Pt. The ORR sluggish kinetics requires a substantial amount of Pt, which has limited the scaling-up of technologies. Alloying Pt with 3d transition metals such as Ni has been reported as a successful approach for reducing the cost of catalysts [1]. Another approach is Pt-loaded onto graphene (G). Graphene have a large specific surface area and good conductivity. In thisstudy, we report the synthesis of Pt.Ni alloy nanoparticles supported on graphene by in-situreduction of metal on graphene oxide (GO) that provides enhanced activity toward the ORR. GOwas prepared using Hummers method reported previously [2]. 50 mg of GO was ultrasonically dispersed in de-ionized water. H2PtCl6.xH2O and NiCl2 (1:1) and the total metal content was 20wt%. Then 10 mL of NaBH4 solution in excess was added. The solid particles were separated by filtration, washed and dried. The catalyst inks were prepared by mixing 30 mg of catalysts with 2-propanole, de-ionized water and 5wt% nafion solution. After the painting of catalyst inks onthe carbon paper, the final electrodes were dried at 120 oC for 1 h. The electrochemical measurements were carried out using three-electrode compartment. An Ag/AgCl electrode wasused as the reference electrode and platinum as the counter electrode. Cyclic voltammetry (CV) was applying a scan rate of 50 mV s-1 between -0.1 and +1.2 V vs. RHE in the O2 saturated NaOH solution. EIS measurements were performed over a frequency range of 100 KHz to 0.01Hz. The CV results of Pt.Ni/G revealed the onset ORR peak of 0.7 V/RHE and ORR peak of 0.46 V/RHE, which is in good agreement with other studies [3]. Polarization curves were produced at a scan rate of 1 mV s-1 and the kinetic parameters were extracted by the Tafel equation. The Tafel slope and current density of electrode were 62 mV/decade and 5.7 × 10-3 mA cm-2. A Tafel slope of 60 mV per decade is observed because the electrode surfaces are a mixture of M and MO and on the M/MO surface, the rate determining step is a pseudo 2-electron procedure. The EIS study was carried out at different dc potentials. The Nyquist plots are shown in Fig. 1. The Nyquist plots of E≥0.6 V show two loops, while the OCP indicate one loop in the high frequency region and semi-infinite diffusive manner in the low frequency egion. According to the Tafel results, the two electron transfer mechanism is suggested to be accrued at E≥0.6 V: [O2 · (H2O)n]aq + 2e-→ (HO2-)ads + OH- + (H2O)n−1 (1) (HO2 -)ads + H2O + 2e-→ 3OH- (2) Two electric circuits were employed to obtain quantitative information (inside Fig. 1) . The electric circuits of (a) and (b) were used to simulate the impedance response of OCP and applied dc potentials, respectively. In the model (a), R1 and R2 indicate the double layer charge-dischargeand the adsorption of reactants or intermediates resistance, respectively. In model (b), the R1 andR2 are the charge transfer reaction according to Eq. (1) and Eq. (2). By increasing the positive potential, the R1 and R2 values decrease (Fig. 2). It is due to O2 molecule adsorption by the oxygen atom orientation and the charge transfer happens more easily and the resistance valuesdecrease. The R2 of Pt.Ni supported on Vulcan carbon was measured 26 Ω cm-2 at 0.9 V in our previous work, which was 3 times greater than Pt.Ni supported on Graphene resistance.

کلیدواژه ها:

Platinum ، Nickel ، Graphene ، Electrocatalysts ، Oxygen reduction