Pd-Ni and Pd-Co nanoparticles supported on carbon as an effective catalyst for oxygen reduction

Alloying is a strategy that has been used to find non-Pt electrocatalysts that are effective and less expensive for the oxygen reduction reaction (ORR) [1]. Mixing two or more metals can result in a catalyst that has distinct properties from its monometallic components. For example, it has beenfound that the addition of metals that bind oxygen strongly (Co, Ni, etc.) can lower oxygen binding to more noble metals (Pt or Pd) and improve their ORR activity [2]. In this work, we investigate the effect of alloy composition in Pd-Ni and Pd-Co nanoparticles on the ORR activity.Nickel (II) chloride (NiCl2), palladium (II) chloride (PdCl2) and sodium borohydride (NaBH4, 99.99%) were purchased from Sigma-Aldrich. Carbon (Vulcan XC-72 R) was provided by theCabot Corporation. 50 mg of carbon was ultrasonically dispersed in de-ionized water. PdCl2 and NiCl2 (1:1) or PdCl2 and CoCl2.6H2O (1:1) was added and the total metal content was 20 wt%. Then 10 mL of NaBH4 solution in excess was added. The solid particles were separated byfiltration, 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 on the carbon felt, the final electrodes were dried at 120 oC for 1 h. The electrochemicalmeasurements were carried out using three-electrode compartment. An Ag/AgCl electrode was used as the reference electrode and platinum as the counter electrode. Cyclic voltammetry (CV)was applying a scan rate of 20 mV s-1 between 1 and +1 V vs. Ag/AgCl in the O2 and N2saturated NaOH solution. EIS measurements were performed over a frequency range of 100 KHz to 0.01 Hz. The CV plots of Pd-Ni/C and Pd-Co/ C are shown in Fig. 1. The ORR peaks of Pd- Ni/C and Pd-Co/C are achieved at 0.3 and 0 V vs. Ag/AgCl, respectively. In the oxygen saturatedsolution, the water reduction is happened in more positive potential for both electrodes, in the comparison of nitrogen saturated one. Polarization curves are produced at a scan rate of 1 mV s-1 and the kinetic parameters were extracted by the Tafel equation (Fig. 2). The Tafel slopes and current densities of Pd-Ni/C and Pd-Co/C electrodes were 128 and 131 mV/decade and 6.2 × 10-3 and 5.6 mA cm-2, respectively. A Tafel slope of 120 mV per decade shows that the first electrontransfer is rds. The EIS measurement is done at ORR potentials of electrodes based on CV plots. The charge transfer resistance of Pd-Ni/C and Pd-Co/C electrodes are 8 and 14 Ω cm2. These results lead us to conclude that Pd-Ni/C alloy catalysts acts more powerful than Pd-Co/C one. According to Volcano plots the ORR activities of Ni particles are higher than Co one. So, by alloying Pd with Ni, the ORR activity is enhanced

کلیدواژه ها:

Palladium ، Alloy ، Electrocatalysts ، Oxygen reduction reaction