Platinum Electrode Properties Tailored to Respond to Ultra-Low Concentrations of H2S in Gaseous Hydrogen Fuel
Authors: Rockward Tommy 
Published: February 2020

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 standard
2 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 30oC 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.

Quesada, Calita (2014), Electrochemical Analyzer for Hydrogen Fuel, Master’s Thesis, New Mexico Tech, Socorro, NM.

SAE J2719: Hydrogen Fuel Quality for Fuel Cell Vehicles, www.sae.org

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.