Activity and Long-Term Stability Study of Pt-Y/C Electrocatalysts for Oxygen Reduction Reaction
Authors: Silva Gabriel Christiano da , Santos Nathalia Abe , Perez Joelma 
Published: February 2020

The recent improvements of the proton exchange membrane fuel cell (PEMFC) make this device a promising energy conversion technology. Nonetheless, the high overpotential of the oxygen reduction reaction (ORR) remains an obstacle, motivating the search for new electrocatalyst materials which are not only active, but also stable for the ORR in the PEMFC operating conditions.

Recently, computational results showed that platinum alloyed with early transition metals, such as yttrium, are more active and stable compared to pure Pt. Furthermore, electrochemical measurements of sputtering PtY electrodes revealed that those materials exhibit better ORR activity and stability compared to Pt electrodes. Despite the better performance of the sputtering materials, the use of Pt-Y electrocatalysts in PEMFC demands the development of carbon-supported nanoparticles catalysts. However, to the date, few papers reported the activity of Pt-Y/C materials. Besides, the effects of accelerated aging tests on the carbon supported material activity and compositions still unexplored.

In this contribution, carbon supported Pt-Y electrocatalysts (20% wt. metal loading) were prepared, with varying Pt:Y compositions of 9:1, 7:3 and 1:1, were synthesized using a modified formic acid method. The catalysts were characterized using several X-ray techniques. The catalysts metallic loading and Pt:Y ratio were analyzed using Energy Dispersion Spectroscopy (EDS). Structural parameters such the mean crystallite size, the interatomic distance between Pt atoms and the lattice parameter were obtained using the X-ray diffraction (XRD). Finally, X-ray Photoelectron Spectroscopy (XPS) revealed that yttrium is present as three different species, all in the Y3+ oxidation state.

The electrochemical tests were performed in a three electrodes cell configuration. The catalysts ORR activities were studied employing the rotating disk electrode (RDE) technique. Potential sweeps between 0.1 and 1.1 V at a scan rate of 5 mV s-1 and RDE rotation rates of 100, 400, 900, 1600, and 2500 rpm were performed after oxygen saturation of the electrolyte. Table 1 summarizes the current density values calculated from 1600 rpm data at 0.9 V. Current values normalized by electrode geometric area (j and jk
) and the mass activity (jk-mass
) exhibited by the Pt-Y/C 9:1 catalyst are similar to those of Pt/C material. With regard to the specific activity (jk-esp
), the current normalized by electrochemical surface area, Pt-Y/C 7:3 exhibited an enhancement of almost 20% compared to Pt/C. This result is also superior to others PtY/C catalysts previously reported.

The Pt-Y/C 7:3 catalyst was submitted to an accelerated aging test that consisted of 3,000 potential cycling test. After the stability test the material ORR activity was revaluated. Changes in catalyst composition were analyzed performing the EDS and XPS experiments directly on the working electrode, while the transmission electron microcopy (TEM) was used in order to demonstrate modifications on the catalyst mean particle size and distribution on the carbon support.

Table 1. Summary of current density values obtained at 0.9 V and rotation velocity of 1600 rpm.

  

Aecsa

(cmPt
2)

j

(mA cm-2)

jk

(mA cm-2)

jk-mass

 

(mA mgpt
-1)

jk-esp

 

(µA cmPt
-2)

Pt-Y/C 1:1

0.45
0.05
0.05
3
23

Pt-Y/C 7:3

0.92
0.44
0.48
21
103

Pt-Y/C 9:1

2.13
0.72
0.85
32
78

Pt/C

1.87
0.71
0.83
30
88