Study of the acetonitrile poisoning of platinum cathodes on proton exchange membrane fuel cell spatial performance using a segmented cell system
Introduction
Forthcoming mass production of proton exchange membrane fuel cells (PEMFCs) as electrochemical replacements of internal combustion engines and portable power generation brings up questions regarding the durability, reliability and high performance of fuel cells under different environmental conditions. Air is the most practical and economic oxidant for fuel cell operations; however, air may contain inorganic and organic impurities. Industrial and automotive vehicle exhausts contain the major contaminants (SO2, NOx, NH3 and H2S). Additionally, a few natural processes, such as volcanic activity, negatively affect air quality. These inorganic air pollutants are known to cause significant PEMFC performance losses and even degradation.
The effects of major inorganic airborne contaminants have been intensively studied [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Sulfur dioxide and hydrogen sulfide have been shown to cause drastic decreases in PEMFC performance, which can only be partially recovered by reintroducing pure air into the cathode gas stream [1], [2], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Sulfur-containing impurities easily adsorb onto the Pt cathode, significantly decreasing the Pt electrochemical area (ECA) and oxygen reduction activity and increasing H2O2 formation, which accelerates electrolyte degradation [7], [12], [13]. The effects of nitrogen-containing air contaminants (e.g., NOx and NH3) were found to be moderate and recoverable [1], [2], [6], [7], [9], [10], [16], [17], [18], [19], [20], [21], [22], [23]. PEMFC exposure to NaCl solutions resulted in performance degradations due to the negative impact of Cl− on the Pt ECA [24], [25].
A few organic compounds were studied as possible airborne pollutants for PEMFCs. The chemical warfare agents (sarin, sulfur mustard, cyanogen chloride and hydrogen cyanide) seriously compromised PEMFC performances in irreversible manners [1]. Benzene, toluene and 1,3-butadien also caused performance decreases, but the losses were recovered [2], [26]. To expand the list of studied airborne organic pollutants, seven compounds were chosen from 260 possible air contaminants suggested by the Environmental Protection Agency [27]. The list includes acetylene, propene, methyl methacrylate, 2-propanol, bromomethane, naphthalene and acetonitrile. These compounds are widely used as solvents (CH3CN, i-C3H8OH), synthesis precursors in the chemical industry (C2H2, CH3C(CH3)COOCH3, C3H6, CH3CN), welding fuels (C2H2), and fumigants (CH3Br, C10H8). All seven contaminants have been found to cause serious PEMFC performance losses and have significant impacts on ORR [28], [29].
Acetonitrile is a volatile, highly polar solvent used to extract fatty acids, animal/vegetable oils and butadiene. It is also a well-known solvent for spinning synthetic fibers and in casting and molding plastics. It is widely used in battery applications because of its relatively high dielectric constant and ability to dissolve electrolytes. In laboratories acetonitrile is applied in high-performance liquid chromatography, as a solvent for electrochemistry, DNA synthesis and peptide sequencing. Based on the broad industrial applications for acetonitrile, it can be prevalent at industrial sites and should be considered as a possible air pollutant because it will affect the PEMFC.
The electrochemisorption and reactivity of acetonitrile at platinum electrodes have been of primary interest in the fundamental study of electrocatalysis [30], [31], [32], [33], [34], [35]. The effects of acetonitrile on the co-adsorption of hydrogen and oxygen are important for understanding electrochemical hydrogenation and oxidation reactions. The study of the electrochemical behavior of nitriles (CH3CN and C6H5CN) provides valuable information because of 1) the use of nitriles as electrochemically “inactive” aprotic solvents, 2) the involvement of nitriles in organic oxidation processes where nitrilium ions and amides can be formed, and 3) in reductive dimerization [31], [36], [37], [38], [39], [40], [41], [42], [43] Moreover, the ORR in acetonitrile solutions have been intensively studied using rotating disk electrode (RDE), rotating ring disk electrode (RRDE) and cyclic voltammetry (CV) techniques [44], [45], [46], [47].
The evaluation of fuel cell performance with a single cell does not reveal the spatial behavior of a PEMFC. In contrast, a segmented cell system provides locally resolved voltage, current and impedance data. It is a powerful tool for understanding the details of fuel cell operations under different conditions [48], [49], [50], [51], [52], [53], [54], [55], the detection of membrane electrode assembly (MEA) defects [56], [57], hydrogen recirculation [58], and hydrogen stream CO poisoning [59], [60], [61], [62], [63], [64], [65]. Information about the current distribution when exposed to organic airborne pollutants is crucial to understanding the poisoning mechanism, improving the PEMFC environmental adaptability and developing useful mitigation strategies. This paper focuses on the detailed spatial performance studies of a fuel cell exposed to 20 ppm CH3CN and operated at different current densities. Additionally, spatial electrochemical impedance spectroscopy (EIS) was employed to understand and characterize the local PEMFC response under exposure to acetonitrile.
Section snippets
Experimental
The experiments were conducted at a Grandalytics single fuel cell test station using a segmented cell system [54]. This diagnostic tool continues the previous works [49], [50], [66], [67] and allows simultaneous rather than sequential measurements of spatial EIS, spatial linear sweep voltammetry (LSV) and CV to be performed. The system has ten current channels in a high (standard) current mode and ten channels in a low current mode. The standard current mode enables the measurement of segment
Fuel cell operation at high current density
Fig. 1 presents profiles of the segment voltages, current densities normalized to their initial values and HFR vs. experiment time at 1.0 A cm−2. For the first 15 h, the cell was operated with pure air resulting in a cell voltage of 0.660–0.670 V. The initial current density distribution was in the 1.15 to 0.85 A cm−2 range for segments 1 and 10. The injection of 20 ppm of CH3CN significantly decreased the voltage during the transition period, which lasted 3–4 h. The voltage eventually reached
Discussion
In the past, oxygen electroreduction at Pt electrodes in the presence of CH3CN was studied using the RDE and RRDE methods [43], [44], [45], [46], [47]. Acetonitrile was reported to poison the ORR, by strongly chemisorbing onto the Pt catalysts over a wide range of potentials, inhibiting hydrogen adsorption and platinum oxides formation. Moreover, CH3CN caused a cathodic shift of the ORR potential with noticeable H2O2 formation. These observations indicated on changes in ORR mechanism from an
Conclusions
The spatial performance of a PEMFC exposed to 20 ppm CH3CN was studied using a segmented cell under different operating current densities. Acetonitrile poisoning resulted in current density redistributions and voltage losses of 0.380 and 0.290 V at 0.2 and 1.0 A cm−2, respectively. The current distribution ranged from +12% to −15% at steady state in the case of 1.0 A cm−2, whereas at 0.2 A cm−2, a steady state was not reached.
The observed current redistribution is explained by the chemisorption
Acknowledgments
The authors acknowledge the Department of Energy (DE-EE0000467), Office of Naval Research (N00014-11-1-0391) for funding this work and the Hawaiian Electric Company for their ongoing support of the Hawaii Sustainable Energy Research Facility. The authors thank Günter Randolf for valuable technical support regarding the system operation.
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