Anode Aging through Voltage Cycling Induced by H₂-Air Fronts during System Start-Up and Shut-Down

Abstract

Passing a H 2 /air front through the anode compartment during start-up or shut-down (SUSD) is a major source of degradation for PEMFC systems, due to the severe cathode carbon support oxidation and cathode ECSA loss during the induced reverse current event. In this publication, we quantified the anode degradation in a low temperature proton exchange membrane fuel cell (PEMFC) during SUSD for the first time, occurring predominantly in the form of ECSA loss, measured by cyclic voltammetry on both anode and cathode after aging MEAs by SUSD cycling: after 3200 SUSD cycles, the anode ECSA decreased by ≈43%, while the cathode ECSA decreased by ≈76%. We suggest that the anode ECSA loss derives from the change of gas atmosphere upon the passage of the H 2 /air front during SUSD, effectively inducing rapid voltage cycles between ≈0 and ≈1 V. Even though the decrease in anode ECSA is lower compared to the cathode, estimates of the HOR overpotential changes due to anode ECSA loss project significant kinetic voltage losses at the anode for ultra-low Pt loadings (≤ 50 µg Pt cm -2 ).

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

“…To quantify the loss of Pt surface area, the ECSA was calculated from the average of the integrated hydrogen ad-/desorption charge Table I 19 in the potential range between ≈80 and ≈400 mV of CVs recorded with a limited upper inversion potential of 550 mV (not shown) to mitigate the superposition of the H upd process with currents originating from the delayed reduction of Pt oxide in the cathodic scan. Figure 3b shows an exponential-like decrease of the cathode ECSA over the course of SUSD aging, with a strong initial decay during the first 800 SUSD cycles, followed by a more gradual ECSA-loss over the residual cycles.…”

“…Reproduced from Schwämmlein et al with permission of The Electrochemical Society. 19 dissolved during the potential transient will immediately re-deposit at the low potential. On the other hand, during a shut-down the released Pt ions can diffuse into the membrane phase, where they are reduced to electrically isolated Pt particles (visible in the respective SEM micrograph in Figure 8b) by H 2 dissolved in the aqueous phase of the membrane.…”

“…,5). CVs and H 2 /air polarization curves 19 Step All data for a given aging stage were averaged over the series (error bars in the graphs represent the standard deviation). Analogous to the above described SUSD aging procedure, the same amount of MEAs was tested using E-cycles, recording CVs and H 2 -pump polarization curves in pure H 2 after each aging stage.…”

“…The design of the experiments, the experimental set-up, as well as the detailed description of the electrochemical measurements were already reported in previous publications. [19][20][21] For the reader's convenience, an abridged description is given below.…”

“…18 To the best of our knowledge, no study apart from our previously published ECS Transaction exists that is mainly focused on the degradation of the anode in low-temperature PEMFC systems. 19 While the performance decay from SUSD-induced anode degradation is rather negligible compared to cathode degradation when high anode loadings are used (≥100 μg Pt cm −2 ), it may start to become significant in future PEMFC systems, for which ultra-low anode loadings (≈25 μg Pt cm −2 ) and the use of more stable graphitized cathode carbon supports are envisaged in combination with system mitigation strategies which restrict the occurrence of H 2 /air front events to room temperature, where cathode carbon support oxidation is strongly suppressed. 11 To obtain a more quantitative understanding on the extent of anode vs cathode degradation, we will examine the PEMFC performance degradation induced by SUSD cycling by determining the loss of cell performance over the course of extended SUSD cycling and compare the concomitant ECSA-loss on anode and cathode.…”