PEM Fuel Cell Start-up/Shut-down Losses vs Temperature for Non-Graphitized and Graphitized Cathode Carbon Supports

Abstract

One of the key figures for the success of proton exchange membrane fuel cells (PEMFCs) in automotive applications is lifetime. Damage of the cathode carbon support, induced by hydrogen/air fronts moving through the anode during start-up/shut-down (SUSD), is one of the lifetime limiting factors. In this study, we examine the impact of varying the temperature at which SUSD events take place, both experimentally and by a kinetic model. For MEAs with conventional carbon supports, the model prediction of carbon oxidation reaction (COR) currents as a function of temperature matches well with the temperature dependence of experimentally determined SUSD degradation rates (predicting ≈8-fold lower COR currents compared to ≈10-fold lower measured degradation rates at 5 • C compared to 80 • C). This, however, is not the case for MEAs with graphitized carbon supports, where a factor of ≈39 lower COR currents are predicted when decreasing SUSD temperature from 80 to 5 • C, in contrast to the measured decrease by a factor of ≈10. As we will show, this is explained by a change of the governing degradation mechanism from predominantly carbon corrosion induced losses at higher temperature to predominantly voltage cycling induced platinum surface area losses near/below room temperature. several practical aspects of automotive PEMFC operation remain challenging to date.5 One such phenomenon is the degradation caused by start-up and/or shut-down (SUSD) of the PEMFC, where a H 2 /air anode gas front moves through the anode flow-field, which was first discussed in the scientific literature by Reiser et al. 6 in 2005 (note that it was described in the patent literature as early as 2002 7 ). Typically, during an uncontrolled shut-down, air will leak slowly into the H 2 compartment through leaks in the back-pressure valve, through the stack sealing or by crossover through the membrane, which was found to lead to substantial damage of the cathode catalyst carbon support in the MEA (membrane electrode assembly). 6 One of the early methods to mitigate this damage was the use of a controlled shut-down, in which a high-flow air purge of the anode compartment was applied in order to minimize the H 2 /air anode front residence time, which is proportional to the induced damage. 8The reactions occurring during the passing of the H 2 /air anode front through the anode flow-field are listed in Figure 1a (slightly modified from what was shown in Ref. 9 and 10), illustrating the partially H 2 -filled (lower left segment, colored in red) and partially air-filled (upper left segment, colored in blue) anode, opposite of the air-filled cathode flow-field (right blue segments) and separated by the proton conducting membrane (orange). While electrons can be well conducted in-plane, mainly via the diffusion media (DM) and flow fields (FF), the in-plane proton conduction resistance through the membrane is very high, so that significant proton conduction in-plane can be supported only over very short distances across the H 2 /air anode front, namely over a ...

Durability of Unsupported Pt-Ni Aerogels in PEFC Cathodes

“…Figure 4D) from a BOL value of ≈38 μm/(mg Pt cm −2 ) (or ≈33 μm/(mg C cm −2 ), which agrees with the 28 ± 2 μm/(mg C cm −2 ) reported in literature) 36 to ≈23 μm/(mg Pt cm −2 ) and points to a collapse of the CL structure which can inhibit effective mass transport of reactants and that is supported by the drastic performance decline in the H 2 /air I/E curves in Figure 2A. The CL collapse can be attributed to carbon support corrosion in the applied potential regimes [20][21][22]37,38 and the resulting CL porosity decrease has been demonstrated by Schulenburg et al 39 in a FIB-SEM tomography study. On the other hand, representative cross section images before/after the load-cycle AST (Figures 3D/3F) do not indicate significant morphology/porosity changes concomitant with the negligible decrease in thickness to ≈34 μm/(mg Pt cm −2 ) discernable from Figure 4B.…”

“…18 Following this recent study, in this work the durability of Pt 3 Ni aerogels in the PEFC is investigated for two different accelerated stress tests (ASTs) proposed by the DOE and compared to a commercial Pt/C benchmark. The first AST exposes the catalyst to the potential regime of 1.0 to 1.5 V RHE , which can occur in an operating PEFC during fuel starvation and start-up/shut-down of the cell and triggers C-support corrosion, 5,[19][20][21][22] and that will be referred to as 'start-stop degradation' in the following. 23 The second AST which has not been investigated in the studies cited above simulates the variation in power output present during automotive application that results in potential fluctuations between ≈0.6 and ≈1.0 V RHE causing Pt dissolution (and re-deposition); 21,[24][25][26] it will be denoted 'load-cycle degradation '.…”

PEM Fuel Cell Start-Up/Shut-Down Losses vs Relative Humidity: The Impact of Water in the Electrode Layer on Carbon Corrosion

“…Step 80 600 4000 800 80 4000 800 80 4000 600 inlet pressures in the range of 12 -47 kPa gauge,inlet (no further correction for actual absolute ambient pressure was applied). At constant dry gas pressure and applied gas flow rates of 113 nccm (H 2 or air on the anode and air on the cathode), the H 2 /air anode gas front residence time was ≈ 1.3 s in all experiments (cf., Equation 20 in our previous publication), 9 and the dwell time in between gas fronts was 120 s in all cases. Note that one SUSD cycle in this work refers to the combination of one shut-down and one start-up event.…”

“…Hardware, set-up and materials.-For SUSD experiments, the materials and devices used were the same as in our previous publication. 9 In short, single-cell PEMFC experiments were carried out on a customized G60 test station (Greenlight Innovation Corp.) using a 50 cm 2 cell hardware with quadruple-serpentine flow-fields (Fuel Cell Technologies, Inc.). Commercial CCMs (Primea Mesga A510.1/M715.18/C580.4, W. L. Gore GmbH) with catalysts based on a high surface area carbon support and applied at loadings of 0.1/0.4 mg Pt • cm −2 (anode/cathode) were sandwiched in between two SIGRACET 25BC (SGL Carbon GmbH) gas diffusion layers (GDLs).…”

Anode Aging during PEMFC Start-Up and Shut-Down: H2-Air Fronts vs Voltage Cycles

“…z E-mail: jan.schwaemmlein@tum.de gas front residence time, 16 and the degree of graphitization of the cathode catalyst support. 11,17 In contrast to the frequently investigated degradation of the cathode during a SUSD event, the anode stability over the course of SUSD cycles was only rarely examined 11,10 and generally neglected. However, Engl et al recently showed that SUSD in high temperature phosphoric acid fuel cells leads to significant amounts of carbon corrosion on the anode, along with a loss of Pt electrochemical surface area (ECSA), a finding which the authors attributed to the change of the anode potential upon switching the gas atmosphere between H 2 and air.…”