(Invited) Time-Resolved Dissolution Analysis of Precious Metals Using a Solution Flow Cell Combined with ICP-MS
Authors: Ooi Azusa , Tada Eiji , Nishikata Atsushi 
Published: December 2020

Introduction

Platinum (Pt), palladium (Pd), and Pt-Pd alloys exhibit excellent catalytic activity for oxygen reduction reaction;1 thus, they are expected to be used as cathode catalyst in polymer electrolyte fuel cell (PEFC). However, one of the problems is their dissolution under PEFC operating condition. A measurement system, an inductively coupled plasma-mass spectrometer (ICP-MS) combined with solution flow cell, is proposed to detect dissolved ions, and time and potential-resolved measurement for metals could be possible under potential cycling using this system.2 Even though the trivial dissolution of Pt and Pd lead to a performance loss of PEFC, improvement of detection sensitivity is required for the system. In this study, we firstly developed a new system, ICP-MS combined with the commercially available flow cell, to improve detection sensitivity. Then the time-resolved measurement of dissolution rates of Pt and Pd was conducted with high detection sensitivity.

Experimental

Cross-flow cell (BAS Inc.) was employed for the experiments. Pt or Pd as a working electrode and glassy carbon as a counter electrode were embedded in the cell together. Screw type Ag/AgCl electrode (RE-3VT, BAS Inc.) was used as a reference electrode. A silicon gasket was used to form the flow circuit whose shape is elliptical with a 100 μm thickness, and a test solution (0.5 M H2SO4) flew in the circuit at a rate of 110 μL min-1. The outlet of the cell directly connected ICP-MS (7700x, Agilent Technology), and time-resolved measurements of Pt and Pd were conducted under potentiostatic polarization and potential cycling. Detection limits of Pt and Pd using this system were confirmed to be 0.13, 0.39 pg cm‒2 s‒1, which was lower than those using the previous system. 2

Results and Discussion

Firstly, Pt or Pd was polarized to 1.4 V (vs. standard hydrogen electrode) to confirm the system operation. Pt or Pd dissolved immediately after polarization as the following reactions:

Pt → Pt2+ + 2e- ,

Pd → Pd2+ + 2e- .

After polarization, Pt or Pd dissolution detected by ICP-MS after around 20 s. Thus, we should consider this delay time in the time-resolved measurements, and the potential step from open circuit potential to 1.4 V was applied to WE for the calibration of delay time before every experiment. Pt and Pd were subjected to potential cycling between 0.05 – 1.4 V at 10 mV s-1, onset potentials for Pt and Pd dissolution were around 1.0 V and 0.73 V in the anodic scan, respectively. These values were more negative than those decided by the previous system.2 Thus we successfully detected a trivial amount of dissolved Pt and Pd using the developed system.

Acknowledgments

The authors acknowledge Open Facility Center, Materials Analysis Division, Tokyo Institute of Technology, for assistance with the ICP-MS analysis.

References

Song, Z. Liang, K. Nagamori, H. Igarashi, M. B. Vukmirovic, R. R. Adzic and K. Sasaki, ACS Catal., 10 4290 (2020).

A. Topalov, I. Katsounaros, M. Auinger, S. Cherevko, J. C. Meier, S. O. Klemm and K. J. Mayrhofer, Angew. Chem. Int. Ed. Engl., 51 12613 (2012).