Operating Conditions Effect on PEM FC Durability Using Design of Experiment Approach

Abstract

The current state of art PEM fuel cell cathode catalyst is represented by Pt-alloy nanoparticles dispersed on high surface area carbon (HSC) supports. [1] Mass activity > 0.6 A/mgPt has been reported consistently for such catalysts. Unfortunately, the high BOL activity of the Pt-alloy nanoparticle catalysts can suffer fairly rapid degradation in mass activity (MA) over time on exposure to fuel cell operating conditions, especially under voltage cycling (V-cycling). The prevalent mechanism for loss of specific activity is due to dissolution of the non-noble alloying element and thickening of Pt shell during voltage cycling. [2]. While numerous efforts have focused on stabilizing the PtCo nanoparticle, the improvements so far have been marginal. Under severe environment of the fuel cell operation, the less noble alloying element tend to dissolve rapidly and contaminate the MEA. The actual operating conditions of the PEMFC can have a significant impact on the PtCo degradation. In our initial study, we had shown that the relative humidity of fuel cell operation can have a significant impact on Pt electrochemical surface area (ECSA) loss and Co dissolution [3]. 35% ECSA loss was observed for 30,000 voltage cycles in H2-N2 at 100% RH, vs. zero ECSA loss for similar voltage cycling test at 25% RH. Given the criticality of this issue, a careful systematic study of this phenomenon with best-in-class materials in a state of art (SOA) membrane electrode assembly (MEA) is desired.To study the impact of operating conditions in depth, a design of experiment approach was utilized. The design of experiments approach allows to identify the impact of individual factors and its interactions while reducing the total number of tests needed to be run. A 20-run design was created to study the various factors, such as relative humidity, temperature, upper potential limit, upper potential hold time etc at three different levels. The factors and its corresponding levels studied are listed in Table 1. Voltage cycling tests was conducted in H2/N2 environment at different operating conditions up to 60,000 voltage cycles. In-situ electrochemical diagnostics such as electrochemical surface area (ECSA), mass activity (MA), specific activity (SA) was measured to understand the kinetics and degradation mechanism. Transport properties such as proton transport resistance and oxygen transport resistance were measured using H2-N2 impedance [4] and oxygen limiting current measurements [5]. The end of test (EOT) MEAs were characterized by ex-situ characterization measurements such as TEM for particle size distribution, electron micro probe analysis (EPMA) and EDS measurements to quantify the cobalt composition and loss up on voltage cycling at different operating conditions. A fundamental model was developed to predict the ECSA and activity loss as a function of operation conditions.