Acetonitrile Contamination and Recovery of Pemfcs
Authors: Zhai Yunfeng , St-Pierre Jean 
Published: December 2020

The proton exchange membrane fuel cell (PEMFC) is still considered a promising power source for transportation and stationary applications. Electrochemical catalysts and the proton exchange membrane (PEM) are the two key components that determine cell performance. The catalyst promotes fuel cell reactions generating electricity for the external load whereas the PEM completes the circuit by transporting protons. Air pollution is detrimental to both PEMFC performance and durability (1). Acetonitrile, is a critical pollutant that primarily originates from automobile exhausts and manufacturing facilities, where acetonitrile has been used as a solvent for spinning fibers and as an electrolyte in lithium batteries. The acetonitrile concentration in atmospheric air reaches values of up to 3 ppm in some areas (2). During PEMFC operation, acetonitrile is introduced into the cathode compartment by slippage through the air filter. The effects of acetonitrile on a PEMFC were investigated by in-situ operation and ex-situ electrochemical techniques (3,4). The results of in-situ tests showed that a trace amount of acetonitrile close to the atmospheric concentration caused significant short- and long-term damage, especially for MEAs with commercially relevant low Pt catalyst loadings (5,6). The ex-situ analysis indicated that acetonitrile not only impacted the oxygen reduction reaction by adsorbing on the Pt/C catalyst surface, but also affected the proton conductivity of ionomers and PEMs by ion exchange with its conversion products. Acetonitrile reaction products exhausted from an operating PEMFC were also identified by in-situ GC-MS and ex-situ water analysis (7). The acetonitrile reaction intermediates and products assisted the derivation of a contamination mechanism. Acetonitrile is hydrolyzed and reduced to NH3 and CH3CHO, and CH3COOH. Subsequent NH3 hydrolysis forming NH4
+ gradually impacted the proton conductivity of the ionomer and PEM by accumulation. The other hydrocarbon products adsorbed on the surface of Pt catalysts competed with the main fuel cell reactions and were successively oxidized to harmless CO2.

Automotive PEMFCs are exposed to variable operating conditions: startup, duty cycling, and shutdown. In this contribution, the effects of operating conditions on PEMFC contamination by acetonitrile are investigated. Contamination mechanisms are employed to analyze in detail cell performance degradation during contaminant injection and recovery after the contaminant injection was interrupted. Both steps involve at least two different processes. The adsorption and desorption of acetonitrile and/or its redox products dominate the fast cell voltage change occurring during the initial portion of both degradation and recovery steps. The ion exchange dominated the slower evolution in cell voltage during the latter part of both degradation and recovery steps. During cell contamination, the amount of water carried by reactant streams showed a considerable impact on performance degradation. The longer time needed to reach a steady state (see Figure 1) was attributed to ammonium scavenging by liquid water drops (8) delaying its accumulation. For the cell performance recovery step, the Pt catalyst activity and cathode potential only impact the earliest and shortest portion of the transient. As a result, the recovery duration is dominated by the ionomer and membrane ion exchange process, and is therefore less sensitive to contamination extent or operational history as previously observed (Figure 8 in (9)). Faster recovery strategies will be proposed based on the contamination mechanism and this detailed analysis of the operating condition effects.

ACKNOWLEDGMENTS

This work was supported by the Department of Energy (DE-EE0000467) and the Office of Naval Research (N00014-19-1-2159). Authors are also grateful to the Hawaiian Electric Company for their ongoing support of the Hawaii Sustainable Energy Research Facility operations.

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Figure 1. Cell voltage transients during the PEMFC contamination by acetonitrile for different combinations of reactant stream relative humidities.

Figure 1