Platinum Electrode Properties Tailored to Respond to Ultra-Low Concentrations of H2S in Gaseous Hydrogen Fuel

PEMFC performance is impacted detrimentally when Hydrogen sulfide (H₂S) is present in the fuel stream, and depending on the extent of poisoning it may be irreversible.  These studies suggest that the best mitigating strategy, to combat this impact, is to avoid H₂S from entering an operating FC¹.  But, the recently published SAE J2719: Hydrogen Fuel Quality for Fuel Cell Vehicles and ISO fuel quality standard² allow sulfur species at the parts per billion (ppb) levels to be present in the hydrogen fuel.  Our concerns are: 1) that even at the ppb (1/10⁹) levels the FC performance losses are significant and 2) because of its affinity for metals and other system components; complete sulfur removal is challenging.  This raises concerns on the importance of verifying the hydrogen fuel quality before it enters the fuel cell system.  In this paper, we report the response of a modified platinum-type electrode to ppb levels of H₂S.                 5cm² MEAs with a working electrode prepared from a catalyst ink made of moderately large particle size unsupported-catalyst powder (~6.3nm) and Nafion® solution (5%, 1100EW).  The absence of a carbon support and the large initial Pt particle size are desirable for an electrode to be durable and have a low active surface area that is ultra sensitive to adsorbates.  We also employed Nafion® 117 (thickness ≈ 180 mm), a much thicker membrane than the traditional fuel cell membranes (thickness < 50 mm) for enhanced stability and sensitivity.  Our counter electrode was made of 20% Pt/C (Vulcan XC-72, BASF) catalyst.

Hydrogen pump experiments were performed at 30^oC and 100% RH using 100sccm of hydrogen gas at each electrode without any applied backpressure. The VI curves and cyclic voltammograms (CVs) before and after exposing the working electrode to H₂S were measured.  Figure 1 shows an increase in the H₂ pumping resistance in the presence of H₂S. After 4 sweeps to 1.0V were performed, some of the H₂S was removed from the surface of the Pt and the H₂ pump resistance indicated a partial recovery. Figure 2 illustrates the effect of the 10 ppb H₂S over 5 hours and shows an increased degree of poisoning (larger increase in resistance) which can be completely recovered after 4 potential sweeps to 1.0V followed by 4 more potential sweeps to 1.4V. The observed resistance change is the direct result if H₂S adsorbing onto active platinum sites preventing hydrogen dissociation from occurring.  Figure 3 illustrates the decrease in the hydrogen desorption from the Pt surface due to H₂S adsorption and its recovery after the H₂S is desorbed at the higher potentials.

1. Quesada, Calita (2014), Electrochemical Analyzer for Hydrogen Fuel, Master’s Thesis, New Mexico Tech, Socorro, NM.
2. SAE J2719: Hydrogen Fuel Quality for Fuel Cell Vehicles, www.sae.org
3. ISO 14687-2, Hydrogen Fuel – Product Specification, Part 2: PEM fuel cell applications for road vehicles,http://www.iso.org/iso/catalogue_detail.htm?csnumber=55083

Acknowledgements:

The authors gratefully acknowledge the financial support of the DOE Fuel Cell Technologies Office and the support of Technology Development Manager Charles (Will) James, Jr.