Effects of Additives on Oxygen Reduction Kinetics at the Interface between Platinum and Perfluorinated Ionomer

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...

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

“…We ignore secondary effects such as changes in transport rates of reactants through the catalyst layer, 40,41 increase in peroxide production [40][41][42] or changes in Tafel slopes [43][44][45] as has been reported previously. Also, ion exchange with the membrane can lead to highly non-linear effects at high currents or significant cation exchange.…”

Section: Model Descriptionmentioning

confidence: 99%

Effect of System Contaminants on the Performance of a Proton Exchange Membrane Fuel Cell

Mehrabadi

Dinh

Bender

et al. 2016

The performance loss and recovery of the fuel cell due to Balance of Plant (BOP) contaminants was identified via a combination of experimental data and a mathematical model. The experiments were designed to study the influence of organic contaminants (e.g. those from BOP materials) on the resistance of the catalyst, ionomer and membrane, and a mathematical model was developed that allowed us to separate these competing resistances from the data collected on an operating fuel cell. For this reason, based on the functional groups, four organic contaminants found in BOP materials, diethylene glycol monoethyl ether (DGMEE), diethylene glycol monoethyl ether acetate (DGMEA), benzyl alcohol (BzOH) and 2,6-diaminotoluene (2,6-DAT) were infused separately to the cathode side of the fuel cell. The cell voltage and high frequency impedance resistance was measured as a function of time. The contaminant feed was then discontinued and voltage recovery was measured. It was determined that compounds with ion exchange properties like 2,6-DAT can cause voltage loss with non-reversible recovery, so this compound was studied in more detail. The degree of voltage loss increased with an increase in concentration, and/or infusion time, and increased with a decrease in catalyst loadings. The major technical challenges for polymer electrolyte membrane fuel cells (PEMFCs) in general are performance, reliability, durability and cost. There is an opportunity to reduce overall system cost by choosing lower cost balance of plant (BOP) materials for PEMFC systems. However, choosing any new system BOP material (e.g., assembly aids, structural plastics, and hoses) without compromising function, fuel cell performance, or life requires understanding the effects of the contaminants that leach from these materials. The contaminants in a fuel cell system originate from the fuel, air, and the different component materials used in construction.Previous studies 1-28 have reported on the effect of contamination on PEMFCs by impurities found in the fuel, fuel-cell components, or the external environment such as carbon monoxide (CO), 8,9 26 It has been demonstrated that even trace amounts of impurities can severely poison the anode, membrane, and cathode, particularly at low-temperature operation.4 These contaminants impact PEMFC performance by hindering kinetics (e.g. CO and H 2 S) or reducing membrane conductivity (e.g. NH 3 and metal cations). 8,18,19 Several models are available that capture kinetic [24][25][26] and ohmic overpotentials [18][19][20]27,28 during the contamination and recovery studies.Although there has been a lot of research on contamination caused by fuel and air side impurities, the effect of contamination originating from system components have only recently been briefly studied. 29-35These results showed that despite the contamination caused by fuel and air side impurities that can effect only the catalyst (e.g., CO) or the membrane (e.g. metal cations), organic contaminants (e.g., those from BOP materials) can have severe effect...

“…Membrane conductivities for pristine and ε-caprolactam-exchanged NRE211 membranes were determined at different relative humidity (RH) (i.e., 10,20,30,40,50,60,70,80,90, and 95%) by applying DC currents at a cell temperature of 80…”

Section: Methodsmentioning

confidence: 99%

The Contamination Mechanism and Behavior of Amide Bond Containing Organic Contaminant on PEMFC

Cho

Das

Dinh

et al. 2015

This paper presents a study of the effects of an organic contaminant containing an amide bond (-CONH-), ε-caprolactam, on polymer electrolyte membrane fuel cells (PEMFCs). The ε-caprolactam has been detected in leachates from polyphthalamide materials that are being considered for use as balance-of-plant structural materials for PEMFCs. Contamination effects from ε-caprolactam in Nafion membranes are shown to be controlled by temperature. A possible explanation of the temperature effect is the endothermic ring-opening reaction of the amide bond (-NHCO-) of the cyclic ε-caprolactam. UV-vis and ATR-IR spectroscopy studies confirmed the presence of open ring structure of ε-caprolactam in membranes. The ECSA and kinetic current for the ORR of the Pt/C catalyst were also investigated and were observed to decrease upon contamination by the ε-caprolactam. By comparison of the CVs of ammonia and acetic acid, we confirmed the adsorption of carboxylic acid (-COOH) or carboxylate anion (-COO-) onto the surface of the Pt. Finally, a comparison of in situ voltage losses at 80 • C and 50 • C also revealed temperature effects, especially in the membrane, as a result of the dramatic increase in the HFR. Although research efforts have been made to reduce system costs and improve efficiency of polymer electrolyte membrane fuel cells (PEMFCs), cost and durability issues continue to delay their commercialization.1 In addition, because contamination of the stack can affect performance and durability, studies designed to elucidate these effects and mechanisms have been receiving more attention. 1-41In general, three fundamental contamination mechanisms are known: conductivity loss by ion exchange in the membrane/ionomer, Pt contamination by adsorption, and water/mass transport issues induced by changes in the gas diffusion layer (GDL) properties. Alkali, alkaline-earth, or transitional-metal cations such as Li + , Na + , K + , Cs + , Ni 2+ , Cu 2+ , Ca 2+ , and Fe 3+ are well known contaminants that undergo ion exchange [2][3][4][6][7][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] with the proton exchange membrane (PEM) in a PEMFC. Changes in the oxygen reduction reaction (ORR) kinetics at a Pt electrode covered with a Nafion ionomer [20][21][22][23][24] have also been reported. In addition to the decrease in PEM conductivity due to the reaction with cationic contaminants, a structural change also occurs in the ionomer. This structural change appears to be related to the degradation of Pt/C catalyst activity. Catalyst contamination by an adsorption mechanism from feed-gas contaminants such as SO 2 , [8][9] CO, 10 and H 2 S 11 are well known. In addition, we have reported that aniline adsorbs onto the Pt/C electrode because it contains an aromatic ring and a nitrogen atom in the form of an amine group. 31,35 In a recent cost analysis, 36,37 the need for cost reduction of balanceof-plant (BOP) materials increased relative to the need for cost reduction of the stack in PEMFCs. Using off-the-shelf materials ...