Guido Bender

Operational Performance Recovery of SO2-Contaminated Proton Exchange Membrane Fuel Cells

Airborne sulfur contaminants (SO 2 , H 2 S, and COS) cause the performance of proton exchange membrane fuel cells (PEMFCs) to degrade because they adsorb to the Pt catalysts and modify reaction sites for oxygen reduction. Electrochemical methods can be used for PEMFC performance recovery by oxidizing adsorbed sulfur species (SO x ) on the Pt catalysts to sulfate (SO 2- 4 ) at high potentials and then removing them as water-soluble anions at low potentials. We examine the effectiveness of five distinct methods for PEMFC performance recovery after 3 h of exposure to 1 ppm SO 2 in air at 60°C and 48.3 kPa g (7 psi) and relative humidity of 100 | 50% (anode | cathode). The methods are tested when the Pt surface is partially covered but not completely saturated with sulfur species. The methods include variations in the cathode potential and gas environment (N 2 or air). In the optimum method, the cells are switched from normal H 2 | air operation to H 2 | N 2 by electrochemically consuming O 2 in the air. The potential is then cycled between 0.09 and 1.1 V. vs the potential at the anode to restore 97% of the platinum catalyst electrochemical surface area. This in situ N 2 cycling method returns the polarization curves of contaminated PEMFCs to their original performance in less than 3 min.

Characterizing Polymeric Leachants for Potential System Contaminants of Fuel Cells

The role of system contaminants in the performance degradation of Proton Exchange Membrane (PEM) fuel cells has been underappreciated to date. This work seeks to identify potential contaminants of system components with the ultimate goal of tying contaminant exposure to performance and durability After aging of select polymers in solution, leachant samples were qualitatively identified via GCMS and FTIR-ATR. Total Organic Carbon (TOC) content quantitatively provided information relative to contaminant level extracted from polymeric samples. Results will be presented focusing on SBR rubber and neoprene. Qualitative concentration vs. time charts elucidate leachant evolution, showing among other things potential chemical degradation in solution.

Evaluating the Influence of PEMFC System Contaminants on the Performance of Pt Catalyst via Cyclic Voltammetry

Using cyclic voltammetry as a quick ex situ screening tool, the impact of the extracted solution and the individual leachable constituents from prospective balance of plant component materials on the performance and recoverability of the platinum catalyst were evaluated. Taking an extract from Zytel HTN51G35HSLR (polyphthalamide) as an example, the major leachable organic components are caprolactam and 1,6-hexanediol. While these organic compounds by themselves do poison the Pt catalyst to some extent, such influence is mostly recoverable by means of potential holding and potential cycling. The extracted solution, however, shows a more drastic poisoning effect and it was not recoverable. Therefore, the non-recoverable poisoning effect observed for the extracted solution is not from the two organic species studied. This demonstrates the complexity of such a contaminant study. Inorganic compounds that are known poisons (like sulfur) even in very low concentration may have a more dominant effect on the Pt catalyst and the recoverability.

High-Performance Alkaline Direct Methanol Fuel Cell using a Nitrogen-Postdoped Anode

A commercial PtRu/C catalyst postdoped with nitrogen demonstrates a significantly higher performance (~10-20% improvement) in the anode of an alkaline direct methanol fuel cell than an unmodified commercial PtRu/C catalyst control. The enhanced performance shown herein is attributed at least partially to the increased electrochemical surface area of the PtRu/C after postdoping with nitrogen.

In situ investigation on ultrafast oxygen evolution reactions of water splitting in proton exchange membrane electrolyzer cells

The oxygen evolution reaction (OER) is a half reaction in electrochemical devices, including low-temperature water electrolysis, which is considered as one of the most promising methods to generate hydrogen/oxygen for the storage of energy. It is affected by many factors, and its mechanism is still not completely understood. A proton exchange membrane electrolyzer cell (PEMEC) with optical access to the surface of anode catalyst layer (CL) coupled with a distinguished high-speed and micro-scale visualization system (HMVS) was developed to in situ investigate OERs. It was revealed in real time that OERs only occur on the anode CL adjacent to liquid/gas diffusion layer (LGDL). The CL electrical conductivity plays a crucial role in OERs on CLs. The large in-plane electrical resistance of CLs becomes a threshold of OERs over the entire CL, and causes a lot of catalyst waste in the middle of LGDL pores. Moreover, the oxygen bubble nucleation, growth, and detachment and the effect of current density on those processes were also characterized. This study proposes a new approach for better understanding the mechanisms of OERs and optimizing the design and fabrication of membrane electrode assemblies.

The Impact of Trace Carbon Monoxide / Toluene Mixtures on PEMFC Performance

Hydrogen fuel for proton exchange membrane fuel cells (PEMFC) may contain trace contaminants such as hydrogen sulfide (H2S), ammonia (NH3), and carbon monoxide (CO), all of which have been widely studied [1]. Various hydrocarbons may also be present, introduced by gas processing or handling. The majority of impurity studies have focused on investigating the impact of single contaminants while very few studies examined the effect of mixed contaminants. These exceptions include the work by Rockward et al. [2] and Shi et al. [3] who studied the effect of mixtures of H2S and CO. It is well known that CO adsorbs strongly on the platinum (Pt) catalyst, decreases the catalytically active Pt surface area and thereby significantly reduces fuel cell efficiency [1]. When H2S is added, the contaminant species compete for adsorption on Pt reaction sites. Rockward et al. showed that H2S replaces faster adsorbing CO over time using cyclic voltammetry experiments [2]. Shi et al. reported that the performance impact of H2S and CO mixtures is first greater and then less than the sum of the two individual performance impacts [3]. However, both studies were short term experiments ranging from a few minutes to six hours and were performed using high contaminant concentrations. The Hawaii Natural Energy Institute (HNEI) has developed a methodology for studying the long term effects of single and mixed contaminants in fuel and oxidant feed streams. The method uses a high resolution gas chromatograph which enables identification of all reaction products and closure of the contaminant species molar flow balances. We have demonstrated this technique with CO at contaminant levels as low as 1 ppm [4]. In this work we report the effect of trace level mixtures of CO and toluene (C7H8) on PEMFC performance at varying operating conditions. The results indicate that the impact of the contaminant mixture at steady state is greater than that of either of the species alone. Figure 1 shows the overpotential increase as a function of time caused by the introduction of (i) 2 ppm CO, (ii) 20 ppm C7H8, and (iii) a mixture of the two. Experiments were performed in a 50 cm test cell using Ion Power MEAs at a constant current density of 1 A/cm and a temperature of 60°C. As shown, the introduction of 2 ppm of CO results in an overpotential change of 247 mV. The injection of 20 ppm C7H8 has no significant impact on performance. However, the combination of C7H8 and CO results in an overpotential increase of 345 mV, significantly greater than the sum of both individual performance impacts. Figure 2 shows molar flow rates of C7H8 and methylcyclohexane (C7H14) entering and exiting the fuel cell during exposure to the contaminant mixture. Immediately following injection, significant hydrogenation of C7H8 into C7H14 occurs. As time increases, the extent of this reaction decreases becoming almost negligible 20 hours after injection (hour 30 in Figure 2). We attribute this to the stronger adsorption of CO inhibiting the hydrogenation of C7H8. With increasing CO coverage, the hydrogen reduction reaction and the hydrogenation of C7H8 compete for the remaining catalyst reaction sites resulting in greater overpotential than observed for CO alone. Results for varying operating conditions will be discussed.

V.C.2 Advanced Hybrid Membranes for Next Generation PEMFC Automotive Applications

• Show that heteropoly acid (HPA)-containing films can be fabricated thin and have a low area specific resistance (ASR) at the temperature of an automotive fuel cell stack and at higher temperatures likely to be operational transients whilst also functioning as an electrical resistor. • Increase HPA loading and organization for maximum proton conduction in a functionalized commercial fluoroelastomer manufactured by 3M.