Comprehensive Evaluation of PEM Fuel Cells Recovery Exposed to Sulfur Dioxide in Air Stream
Authors: Reshetenko Tatyana V. 
Published: December 2024

The future penetration of proton exchange membrane fuel cells (PEMFCs) in the market is limited by degradation and durability issues, lack of hydrogen refueling infrastructure and high overall cost of fuel cells compared to internal combustion engines [1]. The operation of PEMFCs using ambient air has demonstrated the negative impacts of common air impurities like SO2, NO2 and volatile organic compounds [2]. Performance drop originated from exposure to majority of the airborne contaminates can be self-recovered by operating the fuel cell with pure air. However, SO2 causes irreversible performance loss due to the electrochemical reduction of S-containing species and formation of elemental sulfur [3]. Only special treatments of the cathode lead to full or partial PEMFC recovery [4-6]. In this work a potential cycling as recovery procedure was comprehensively studied to assess its effectiveness and applicability to restore fuel cell performance exposed to SO2 under different conditions such as operating temperature and current using segmented cell approach.
The experimental work was performed using a test station and a segmented cell system developed at Hawaii Natural Energy Institute [7]. Commercially available 100 cm2 catalyst coated membranes with Pt content of 0.1 and 0.4 mgPt cm-2 for both anode and cathode, respectively, were used in this work. The poisoning proceeded until the cell voltage reached a steady state, after that SO2 injection was stopped to assess the self-recovery in pure air following by the recovery procedure consisted of cyclic voltammetry (CV) from 0.1 to 1.2 V for 10 cycles at 20 mV/s.
Fig. 1 shows the segment voltage and normalized individual segment current profiles under SO2 exposure and various operating conditions. The introduction of 5 ppm SO2 in the air stream led to a rapid drop in the segment voltage with an inflection point at ~0.62-0.63 V for 0.6 A cm-2 (Fig. 1 a) and 0.55-0.56 V at 1.0 A cm-2 (Fig. 1 c). At steady state conditions after several hours of SO2 contamination, the performance decreased by 280 mV at 60°C, while the cell voltage drop was 315 mV at 1.0 A cm-2 and 80°C (Figs. 1 a, c). At the same time, a redistribution of local current revealed similar patterns for the studied samples (Figs. 1 b, d).
The operation of the MEAs in pure air immediately after cathode poisoning resulted in partial recovery of the performance and further local current distribution. The lack of full recovery is explained by the fact that only weakly bonded SOx species were removed from Pt, while formed S0 and strongly adsorbed S-containing species still covered the electrocatalyst surface [3, 5]. To recover the ECA, it is necessary to increase the cell potential to values where S0 can be oxidized. Therefore, to evaluate this approach, we applied the CV-induced recovery procedure.
The CV-induced recovery of the MEA contaminated at 0.6 A cm-2 and 60°C led to cell potentials of 0.725 V vs. an initial performance of 0.735 V, which corresponded to 99% recovery and indicated full recovery (Figs. 1 a, b). At the same time, operation at higher temperature and lower cell potential (or higher operating current) resulted in pronounced differences between the initial and recovered performances, such as 105 mV for 1.0 A cm-2 and 80°C (84% of recovery) (Figs. 1 c, d) most likely due to of strongly bonded S0 and catalyst degradation. Thus, the PEMFC performance after exposure to SO2 at high temperature can be only partially recovered by the CV-induced approach and requires additional or modified recovery procedures.
Detailed analysis of the local behavior of PEMFCs during the CV-induced recovery will be presented together with discussion of other potential recovery strategies.
ACKNOWLEDGEMENTS
We gratefully acknowledge funding from ONR (N00014-22-1-2045). The authors thank K. Bethune and J. Huizingh for valuable support in operation.
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Figure 1