Molecular Dynamics Simulations on O 2 Permeation through Nafion Ionomer on Platinum Surface

Development of electrocatalysts with higher activity and stability is one of the highest priorities in enabling cost-competitive hydrogen-air fuel cells. Although the rotating disk electrode (RDE) technique is widely used to study new catalyst materials, it has been often shown to be an unreliable predictor of catalyst performance in actual fuel cell operation. Fabrication of membraneelectrode assemblies (MEA) for evaluation which are more representative of actual fuel cells generally requires relatively large amounts (>1 g) of catalyst material which are often not readily available in early stages of development. In this study, we present two MEA preparation techniques using as little as 30 mg of catalyst material, providing methods to conduct more meaningful MEA-based tests using research-level catalysts amounts.

Section: Resultsmentioning

confidence: 99%

Preparation of PEMFC Electrodes from Milligram-Amounts of Catalyst Powder

Yarlagadda

McKinney²,

Keary

et al. 2017

“…Recent molecular dynamics and density functional theory (DFT) calculations show that ionomers fold onto the Pt surface, leading to a highly dense layer which in turn can reduce the O 2 concentration close to the Pt surface to nearly zero. 23 It is also shown that the type of ionomer and operational history can affect the observed performance. 1,24 Unfortunately, there is still no characterization method available that will evaluate the ionomer/Pt interface in a fuel cell electrode and in a way that can be related to fuel cell performance.…”

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confidence: 99%

Characterizing Electrolyte and Platinum Interface in PEM Fuel Cells Using CO Displacement

Garrick

Moylan

Yarlagadda

et al. 2016

107

Relatively large O 2 transport resistance at the ionomer and Pt interface has been thought to be responsible for the large performance loss at high power for a low Pt loading proton-exchange-membrane fuel cell. A facile method to characterize the interface in the fuel cell electrode is needed. In this study, the CO displacement method was explored on polycrystalline Pt and carbon-supported Pt nanoparticles. The displacement charge coverages were used to quantify the adsorption of perchlorate, sulfate, and perfluorosulfonic acid ionomer. Heavy use of platinum in the electrodes of proton-exchange membrane (PEM) fuel cells is a key challenge preventing automotive manufacturers from bringing fuel cell electric vehicles to mass market. Current state-of-the-art fuel cell vehicles use >20 g of Pt per vehicle, which is significantly higher than the internal-combustion engine (ICE) incumbent (<5 g of precious metal per vehicle).1,2 Because heavy use of Pt is needed to obtain high energy conversion efficiency in the fuel cell, improving the activity of Pt-based catalysts has continued to be a high-priority research topic for many years.On the other hand, it was found that at high power density of a low-loaded electrode (<0.10 mg Pt /cm 2 or ∼11 g Pt /vehicle), significant performance losses are observed.3-7 These large performance losses are likely due to the need to deliver more O 2 to a small area of the Pt surface. It was also found that the bulk of the observed O 2 transport resistance occurs at the interface of Pt and electrolyte, 1,4,6 which is surprising because it has generally been seen that the thickness of the ionomer coated on a Pt surface in a well optimized electrode is only a few nanometers. If one calculates the O 2 transport resistance of the thin film using known O 2 permeability of a thick ionomer membrane, 8 it would require an ionomer film with unreasonable thickness (>20 nm) in order to explain the performance loss. Ex-situ measurements on thin-film ionomer performed by several groups have shown that the ionomer nanostructure and its properties such as water uptake, proton conduction, and O 2 permeability can vary substantially depending on its thickness, treatment history, and substrate interaction.9-17 Furthermore, sulfonate groups in the ionomer can adsorb on a Pt surface and reduce the oxygen reduction reaction (ORR) activity. 18,19 Because the adsorption of the acid group immobilizes the ionomer to the Pt surface, [20][21][22] it is surmised that it will also increase O 2 transport resistance. Recent molecular dynamics and density functional theory (DFT) calculations show that ionomers fold onto the Pt surface, leading to a highly dense layer which in turn can reduce the O 2 concentration close to the Pt surface to nearly zero. 23It is also shown that the type of ionomer and operational history can affect the observed performance.1,24 Unfortunately, there is still no characterization method available that will evaluate the ionomer/Pt interface in a fuel cell electrode and in a way that can be re...

“…Regarding the drop in the oxygen transport resistance with RH, it might be associated with the changes in the oxygen interfacial resistance. In their recent study using MD/DFT simulations Jinnouchi et al 24 predicted that oxygen permeation through the ionomer film would not be dependent on water content. The measured reduced local resistance at high RH in this work appears to contradict the simulation results.…”

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confidence: 99%

“…12,[17][18][19][20][21][22] The source of this resistance is still under debate. Oxygen dissolution in the ionomer, 23 densification of the ionomer layer near the ionomer/Pt interface, 24 and catalyst/oxygen interactions at the catalyst site 20 have been proposed as causes for this local mass transport barrier. These results indicate that in order to understand the performance degradation of low loading electrodes, and to compare different manufacturing techniques, limiting current measurements are required to estimate the local transport resistance.…”

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confidence: 99%

Analysis of Inkjet Printed PEFC Electrodes with Varying Platinum Loading

Shukla

Stanier

Saha

et al. 2016

PEFC electrodes manufactured using inkjet printing are investigated. Cell performance, Tafel slope, reaction order and local oxygen transport resistance of electrodes with varying Pt loadings of 0.014 to 0.113 mg/cm 2 are studied in order to understand the nonlinear relationship between loading and performance. The performance increase with Pt loading was substantially reduced above Pt loadings of 0.08 mg/cm 2 . Electrochemical active area of the electrodes decreased from 66.4 to 40.4 m 2 /g as the Pt loading increased from 0.026 to 0.113 mg/cm 2 . Below the transition voltage of 0.8 V i R f ree , the Tafel slope was found to be a function of the Pt loading and oxygen partial pressure. The kinetic performance dependence on p O 2 was quantified by measuring the total reaction order. Oxygen transport resistance evaluated from limiting current experiments revealed its dependence on the inlet relative humidity of the reactants. The local oxygen transport resistance was found to drop from 5.93 ± 3.16 s/cm to 3.43 ± 1.67 s/cm as the humidity increased from 50% to 90%. Hydrogen polymer electrolyte fuel cells (PEFCs) are a zero emission and efficient energy conversion technology for transportation, stationary and electronic applications. The current generation of fuel cell vehicles provide quick start-up, long-range and increased durability. The high and unstable cost of platinum (Pt), which is the commonly used catalyst, however hinders its commercial prospects, especially when compared to the internal combustion engine. The cost of Pt is responsible for over 34% of the fuel cell stack cost.1 Less than 10-20% of the catalyst is however estimated to be utilized during the operation of a conventional catalyst layer (CL) due to mass and charge transport limitations.2 Re-designing the CL to improve these transport limitations has the potential to achieve better Pt utilization, i.e., generate more current per gram of catalyst. The CL fabrication process governs its utilization efficiency, microstructure and Pt loading in the electrodes.The effect of Pt loading on electrodes manufactured by spraying, using film applicators, sputtering as well as inkjet printing has been studied in the literature. [3][4][5][6][7][8][9][10][11][12][13][14][15] Results show that fuel cell performance is severely degraded at low Pt loading, 3,6,12,16 and that an optimal value of Pt loading exists above which the performance gains are not very significant. 4,12 The reason for the reduced performance of low loading electrodes has thus far mainly been attributed to a high oxygen mass transport resistance at the reaction site. 12,[17][18][19][20][21][22] The source of this resistance is still under debate. Oxygen dissolution in the ionomer, 23 densification of the ionomer layer near the ionomer/Pt interface, 24 and catalyst/oxygen interactions at the catalyst site 20 have been proposed as causes for this local mass transport barrier. These results indicate that in order to understand the performance degradation of low loading electrodes, and to comp...