In situ metal ion contamination and the effects on proton exchange membrane fuel cell performance

A model is presented for the contamination of a proton exchange membrane fuel cell (PEMFC), including adsorption onto the Pt catalyst, absorption into the membrane, and ion exchange with ionomeric components. Model predictions for three sources of voltage loss account for the two-dimensional, time-dependent contamination along the channel and into the membrane. The model is developed by considering the well-known concepts of Langmuir adsorption, partition coefficients, plug flow reactors (PFRs), and dimensionless analysis. The phenomena are shown to be controlled by three important dimensionless groups: a Damköhler number for the contamination reaction rate, a capacity ratio, and a coverage ratio for each contamination mechanism. These groups show how to scale ex situ equilibrium data for in situ predictions. The model predictions are shown to be reasonable when compared to in situ experiment data once ex situ data are used to provide reaction and equilibrium parameters. The predictions enable estimation of tolerance limits for leachates according to each mechanism. For typical parameters, the predicted voltage loss in the electrode ionomer by an ion-exchange mechanism shows slower reaction rates but greater performance losses than other mechanisms. Analysis of the potential for commercialization of proton exchange membrane fuel cells (PEMFCs) shows that the cost of balance of plant (BOP) materials can be as much as 30% of the system cost.1 One opportunity to decrease these costs is to use off-the-shelf materials rather than custom-made materials, if leachates from less expensive materials would not affect performance and life-time.2 This loss in performance is related to the contamination of the membrane 3 as well as the electrodes. Many previous studies for contamination [3][4][5][6][7][8][9][10][11][12][13] in PEMFCs have shown the sensitivity of performance to low levels of contamination. For example, cation leachates from gaskets or seals, from NH 3 in the fuel, or from catalyst metals 3-9 are well known contaminants of the membrane through an ion-exchange mechanism. Catalyst contamination by CO through an adsorption mechanism from feed gas contaminants is so well-known that those studies are too numerous to list here. Examples are also known for SO 2 , 10,11 for H 2 S, 12 and even for details of the reverse water-gas shift reaction of CO 2 . 13The fundamental mechanism of contamination from organic contaminants such as aniline, ε-caprolactam, and diaminotoluene (DAT), [14][15][16][17][18] which have been identified as possible system contaminants, have been studied only briefly and recently, and it has been shown that their impact on PEMFC performance depends on their functional groups. For example, the ion-exchange reaction of the amine group in aniline with the Nafion membrane/ionomer causes a decrease in the number of available proton sites, thereby hindering the transport of protons and increasing ohmic losses. Aniline is also adsorbed on the carbonsupported platinum catalyst through interactions betw...

“…5 into Eq. 4: [7] In summary, we have two coupled partial differential equations (PDEs) shown by Eq. 3 and 7, which require two initial conditions and one boundary condition.…”

Section: Modelmentioning

confidence: 99%

mentioning

confidence: 99%

mentioning

confidence: 99%

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An Isothermal Model for Predicting Performance Loss in PEMFCs from Balance of Plant Leachates

Cho

Zee

2014

“…11 As shown in Table I, each catalyst incorporated in the MEA had a different initial average Co content, e.g., 34,20, and 15 mol%, as measured in the cathode catalyst layer (CCL), for catalysts H, M, and L, respectively. Catalysts H and M were supplied as powders.…”

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confidence: 99%

Durability of Pt-Co Alloy Polymer Electrolyte Fuel Cell Cathode Catalysts under Accelerated Stress Tests

Papadias

Ahluwalia

Kariuki

et al. 2018

The durability of Pt-Co alloy cathode catalysts supported on high surface area carbon is investigated by subjecting them to accelerated stress tests (ASTs). The catalysts had different initial Co contents and nanoparticle morphologies: a "spongy" porous morphology for the high-Co (H) content catalyst, and a fully alloyed crystalline morphology for the medium-Co (M) and low-Co (L) content catalysts. The specific activity of the catalysts depends on their initial Co content, morphology and nanoparticle size, and remained higher than 1000 μA/cm 2 -Pt after 27-50% Co loss. The H-catalyst electrode showed the smallest kinetic overpotentials (η c s ) due to higher initial Pt loading than the other two electrodes, but it had the fastest increase in η c s with AST cycling due to lower Co retention; the L-catalyst electrode showed higher η c s due to a lower initial Pt loading, but had a smaller increase in η c s with aging due to higher Co retention; the M-catalyst electrode showed a similar increase in η c s with aging, but this increase was due to the combined effects of Co dissolution and electrochemically active surface area (ECSA) loss. The modeled increase in mass transfer overpotentials with aging correlates with the initial Pt loading, ECSA loss and the initial catalyst morphology.

“…5,6,[13][14][15][16][17][18] Other researchers confirmed the findings of Okada group as well. 9,20,25,31,33 23 Pozio et al 19 investigated the degradation in PEFC that was caused by iron contamination from SS316L end plates and reported that contamination of the membrane electrode assemblies with iron led to degradation of the ionomer, revealed by a massive fluoride losses. Li et al reported that a level of as low as 5 ppm of Fe 3+ in the air stream caused significant cell performance degradation due to the formation of pinholes in the membrane.…”

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confidence: 99%

Thinning of Cathode Catalyst Layer in Polymer Electrolyte Fuel Cells Due to Foreign Cation Contamination

Banas

Uddin

Park

et al. 2018

Four hundred hour fuel cell tests were performed on commercial, as-received and Ca 2+ contaminated catalyst coated membranes (CCMs) to evaluate the effects of long term exposure of foreign cations on fuel cell performance and degradation. Following testing, significant thinning of the cathode catalyst layer was observed across the entire active area in the contaminated cell, while only localized thinning was observed in the as-received baseline CCM. Analysis of the elemental maps and line intensity profiles of platinum (Pt) obtained from energy dispersive X-ray spectrum (EDX) shows minimal change in total platinum content across the thickness of the catalyst layer, and hence loss of carbon is suspected to be the cause of the thinning. A numerical model is employed to show the foreign cation redistribution during testing, which shows proton depletion in the cathode catalyst layer when the CCM is contaminated with foreign cations. We hypothesize that this lack of protons leads to accelerated carbon corrosion in the cathode to supply protons to oxygen reduction where foreign cations block transport of protons. A carbon corrosion scheme is presented which shows that under the operating potentials of the cell, carbon oxidation can occur, leading to the observed thinning. The polymer electrolyte fuel cell (PEFC), a promising clean energy source for automobile application, still needs to overcome durability issues originating from various sources.1 Cationic impurities originated from either ambient air (e.g. roadside contaminants) or from the corrosion of stack and balance of plant components can significantly decrease the performance and life time of PEFCs. [1][2][3][4] In this paper, we report on the influence of foreign cationic contamination on the thinning of the cathode catalyst layer during non-accelerated testing. Long duration (400hr) testing was conducted using commercial catalyst coated membranes which showed significant catalyst layer thinning in a calcium cation contaminated cell, this thinning of the cathode catalyst layer was not observed in a non-contaminated baseline. While foreign cations are known to cause several degradation mechanisms in PEFCs, no study has examined the influence of cationic contamination on carbon corrosion, which we believe causes the thinning of the catalyst layer.To understand the impact and mechanism of cation contamination in PEFC, several modeling 5-11 and experimental studies 12-33 have been conducted, including ex-situ contamination of the polymer membrane with various cations before the test as well as injecting cations into the air or fuel stream during the cell test. Okada and co-workers extensively investigated the effect of various metal cations (Li + , Na + , Ca 2+ , Fe 2+ , Ni 2+ , Cu 2+ , Rb 2+ , Cs + ) on the transport properties of perfluorosulfonic acid (PFSA) membranes, thermodynamics and oxygen reduction reaction (ORR) kinetics. 5,[13][14][15][16][17][18] They reported that, with the exception of Li + , all foreign cations have higher affinity toward the sulfo...