In-situ investigation and mitigation of carbon support corrosion of cathode catalyst in PEM fuel cells
Carbon support corrosion (CSC) is one of the key factors causing cathode electrocatalyst (Pt/C) degradation in proton exchange membrane fuel cells (PEMFC). It is electrochemical oxidation and thus needs to be investigated in-situ with potential imposed. CSC was characterized in-situ and correlated to the Pt redox reactions and Pt-catalyzed oxygen reduction reaction (ORR) by the differential electrochemical mass spectrometry (DEMS) spectra of cathode exhaust gases, CO₂, H₂ and O₂, from a PEMFC fed with humidified H₂ and He to the anode and cathode respectively. Furthermore, the catalytic effects on CSC from different oxidation states of Pt were indicated. To determine the oxygen sources and pathways of CSC, oxygen was isotopically labeled by replacing regular water with oxygen-18 (¹⁸O) enriched water (H₂¹⁸O, 98%) in DEMS, denoted as ¹⁸O-DEMS. 18O-DEMS spectra of the cathode exhaust gases O₂, O¹⁸O, ¹⁸O₂, CO₂, CO¹⁸O and C¹⁸O₂ during cyclic voltammetry and chronoamperometry were analyzed to verify that water is the main direct oxygen source for CSC. Moreover, water reacts with carbon to produce at least three types of carbon surface oxides, which are further electrochemically oxidized with water to produce CO₂ in different potential ranges. After accelerated testing of cathode catalyst degradation in PEMFC using potential cycling between 100-1400 mV at the rate of 400 mV/s, the changes of mass spectra of CO₂, H₂ and O₂ over time showed that the CSC decreases as Pt electrochemically active surface area (ECSA) decreases, i.e. catalyst activity decreases, but the membrane does not degrade significantly in gas permeability. A hypothesis is proposed here that Au nanoparticles (NPs) added to a carbon-supported Pt (Pt/C) catalyst can mitigate the Pt catalytic effects on CSC by suppressing the Pt oxidation. Several methods were tried to synthesize Pt/C (20 wt% Pt) and bimetallic AuPt/C (20 wt% Pt, 5 wt% Au) catalysts including deposition-precipitation, two phase liquid-liquid colloidal, polyol, microwave-assisted polyol, and surface redox methods. TEM pictures showed that the EG method (polyol method using ethylene glycol), microwave-assisted EG method, and colloidal method produced Pt/C catalysts with high dispersion and narrow particle size distribution of Pt NPs uniformly loaded on the carbon support. Au NPs with high dispersion and narrow particle size distribution can be made only by the colloidal method. AuPt/C catalysts were synthesized by two methods: physically by mixing Au NPs prepared by the colloidal method on Pt/C prepared by the EG method, and chemically by surface redox reactions of a Au precursor on Pt NPs prepared by the EG method and then loaded on the carbon support (denoted as AuPtC-EG-SR). The existence of Au on Pt/C was confirmed by EDX and by a larger ring current from ORR experiment using a rotating ring-disk electrode. Three membrane electrode assemblies (MEA) with commercial Etek Pt/C and those two types of AuPt/C catalysts as cathode catalyst respectively were fabricated for CSC comparison. Larger ECSA but less CO₂ intensity of the MEA with a AuPtC-EG-SR cathode than those of the MEA with Etek Pt/C gives preliminary confirmation of the hypothesis.