In-Situ Monitoring of Both Pt and Cu Dissolved from Pt-Cu Alloys Using Channel Flow Multi Electrode
Authors: Ooi Azusa , Tada Eiji , Nishikata Atsushi 
Published: February 2020

Introduction

Polymer electrolyte fuel cell (PEFC) is considered to be clean energy conversion device due to its low operating temperature and portability. Previously, Pt is used as cathode catalyst in PEFC, because Pt exhibits high catalytic activity for oxygen reduction reaction (ORR). Since Pt is limited resources and high cost, Pt alloy catalysts should be developed instead of Pt catalysts. Pt-Cu alloys exhibit greater catalytic activity for ORR than Pt, 1 however, Pt-Cu alloys would degrade under PEFC operating conditions. Therefore, we need to clarify dissolution mechanism of Pt-Cu alloys. In our previous work, we established channel flow double electrode (CFDE) for in-situ detection of Pt, and we proposed dissolution mechanism of Pt under simulating PEFC operating condition using CFDE. 2 In this study, we firstly improved CFDE to in-situ detect both Pt and Cu dissolved from Pr-Cu alloys, and then dissolution behavior of Pt-Cu alloys has been investigated by using this technique.

Experimental

In order to determine detection potential (E
CE) of Cu2+ dissolved from Pt-Cu alloys, we prepared various concentration (0, 0.1, 0.5, 1.0 mM) of CuSO4, whose pH was adjusted to around 0.5 by adding H2SO4. Under laminar flow condition of the electrolyte (10 cm s-1), Au electrode was subjected to cathodic polarization at 298 K in Ar-purged these solutions. Potential range of the experiment from open circuit potential (OCP) to −0.2 V vs. SHE at 1 mV s-1. In order to confirm quantitativity of E
CE determined by the cathodic polarization experiments, dissolved Cu2+ was detected on downstream Au collector electrode during anodic polarization of upstream Cu working electrode using channel flow cell. Au collector electrode potentiostatically polarized at (E
CE=) 0, −0.1, and −0.2 V in Ar-purged 0.5 M H2SO4, when the working electrode anodically polarized from OCP to 1.0 V at 2 mV s-1.

Results and Discussion

Diffusion limiting current of residual oxygen was observed between 0.2 − −0.1 V in the cathodic polarization curve in 0 mM CuSO4 (Cu2+ free). And then, cathodic current increased by hydrogen evolution reaction (HER) below −0.1 V. On the other hand, cathodic current observed in the solution containing more than 0.1 mM CuSO4 was higher than that in 0 mM CuSO4. This is caused by the following reaction on the Au electrode:
Cu2+ + 2e- → Cu.
In addition, Cu2+ reduction current almost increased proportionally to Cu2+ concentration below −0.15 V. However, this concentration-dependent current overlapped with HER current. Therefore, quantitative determination of E
CE for Cu2+ detection needs further experiments.
Cu working current (i
WE) exponentially increased with increasing potential from the OCP, and reached almost constant value (0.1 A cm-2). During this anodic polarization, Au collector current (i
CE) at E
CE = 0, −0.1, and −0.2 V shows similar trend in the i
WE. After subtracting residual current from the i
CE, we calculated dissolution current (i
Diss.) using collection number (N = 0.38). When E
CE was 0 V, i
WE was not completely match i
Diss. during the polarization, which means that E
CE = 0 V is not quantitative potential for Cu2+ detection. On the contrary, i
WE almost equals to i
Diss. at E
CE = −0.1, and −0.2 V. Hence, Cu2+ dissolved from Cu is quantitatively detected on Au electrode at E
CE < −0.1 V.

Reference

1. P. Strasser, S. Koh, and J. Greeley, Phys. Chem. Chem. Phys., 10, 3670 (2008).
2. Z. Wang, E. Tada, and A. Nishikata, J. Electrochem. Soc., 161 (4), F380 (2014).