Impact of Substrate and Processing on Confinement of Nafion Thin Films

Kusoglu, Ahmet; Kushner, Douglas I.; Paul, Dipayan; Karan, Kunal; Hickner, Michael A.; Weber, Adam Z.

doi:10.1002/adfm.201304311

Cited by 193 publications

(420 citation statements)

References 71 publications

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“…However, the origin and existence of CPE cell , and CPE f in the impedance response of such systems are still under debate. 5,7,25,26 Soboleva et al 10 and Yadav et al 13 have indicated that Nafion membrane has a capacitance in parallel to the resistance. The origin of capacitance has been described as the polarization of water inside the nanopore and the close proximity of the sulfonate group.…”

Section: Resultsmentioning

confidence: 99%

“…Film thickness and treatment protocols plus thermal history are the two influencing parameters on the proton conductivity of the Nafion nanothin films. For room temperature measurements (25 • C, Protocol-1), the proton conductivity of sub-55 nm Nafion thin films was found to be independent of thickness whereas that of the thicker films was thickness-dependent, approaching that of the membrane. The two distinct behaviors of the proton conductivity are highly consistent with the distinct surface wettability of the thin film.…”

Section: Discussionmentioning

confidence: 99%

“…Protocol -1 was adopted for the impedance measurement at room temperature (25 • C), therefore, the impedance data was collected after the initial step and further equilibrating at each RH, ranging from 20 to 95% RH.…”

Section: Methodsmentioning

confidence: 99%

“…Moreover, we have recently shown that there are differences between spin-coated and self-assembled films in morphology and water activity. 25 Nagao et al 26 studied spin coated Nafion thin films on silica substrate using Infrared p-polarized multiple-angle incidence resolution spectrometry (IR p-MAIRS) technique and made inferences about low proton conductivity in such films. They indicated that highly oriented sulfonic acid group in the substrate/film interface might be related to the low proton conductivity.…”

mentioning

confidence: 99%

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Proton Transport Property in Supported Nafion Nanothin Films by Electrochemical Impedance Spectroscopy

Paul

McCreery

Karan

2014

J. Electrochem. Soc.

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180

206

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The proton transport property of Nafion nanothin films (4-300 nm) has been investigated by electrochemical impedance spectroscopy (EIS) using interdigitated array (IDA) of gold electrodes on SiO 2 substrate. The proton conductivity has been investigated as a function of film drying/heating protocol, relative humidity, temperature and film thickness. It is found that the film treatment protocol makes a difference in film transport property. Ultimately, the proton conductivity and capacitance of the Nafion nanothin film is thicknessdependent, where the former decreased and the latter increased exponentially with decreasing film thickness. Moreover, the activation energy increased exponentially with decreasing film thickness. The proton conductivity of the thin films of Nafion is significantly lower than that of the membrane counterpart regardless of the thickness, which is consistent with the high activation energy found in the thin films. These differences are likely related to the polymer confinement and morphological differences between the thin films and the freestanding membrane. Proton transport is a key functional property of the ionomer used in polymer electrolyte based energy conversion and storage devices including polymer fuel cells (PEFC), 1 polymer electrolyte membrane (PEM) water electrolyzers, 2 photo electrochemical cells 3,4 and artificial photosynthesis device.5 For PEFCs and PEM electrolyzers, Nafion is the most extensively employed. As a freestanding film of thickness (25-100 μm thick), Nafion serves the function of the electrolyte membrane separating the anode and the cathode. The ionomer is also present in the electrochemically active component of these energy conversion devices as a thin film coating the electrocatalyst. For example, the conventional PEFC catalyst layer (CL) is a porous composite layer (10-25 μm thick) of Platinized carbon (Pt/C) and ionomer. In the CL, the ionomer forms a semi-continuous, web-like nanothin film (4-10 nm) covering the aggregates of Pt/C. 1,6 The thickness of such nanothin Nafion films in the CL is comparable to the ionic domain size (∼4 nm) in the bulk Nafion membrane. 7,8 It is obvious that the ionomer structure and property will affect both the proton and mass transport, which in turn will impact the local electrochemical reaction rate and, thereby, the overall PEFC performance. In fact, researchers of the electrochemical energy research laboratory at General Motors (GM) 9 examined the electrode performance as a function of catalyst loading and found that low-Pt loading catalyst layers exhibited higher transport losses, which they attributed to the transport resistance of ionomer thin film covering the catalyst. The proton conductivity of Nafion membrane is widely investigated and discussed in the literature. [10][11][12][13] In contrast, the proton conductivity of thin Nafion film has remained relatively unexplored, especially for ionomer films of thickness comparable to ionic domain size (∼4-5 nm). The ionic conduction property of materials is highly reg...

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Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

mentioning

confidence: 99%

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Proton Transport Property in Supported Nafion Nanothin Films by Electrochemical Impedance Spectroscopy

Paul

McCreery

Karan

2014

J. Electrochem. Soc.

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180

206

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“…However, recent studies on the underlying knowledge of the structure/function relationships of ionomer thin-films in the catalyst layers are beginning to show that such an assumption is poor as discussed in more detail below. 157,[298][299][300][301][302][303][304][305] The kinetic expressions, equations 58 and 60 for the HOR and ORR, respectively, result in a transfer current between the electronicconducting (1) and ionic-conducting (2) phases (see equation 10), which is related to the current density in the two phases through equation 23 (neglecting double-layer charging)…”

Section: Catalyst-layer Modelingmentioning

confidence: 99%

A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells

Weber

Borup

Darling³

et al. 2014

J. Electrochem. Soc.

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509

394

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Polymer-electrolyte fuel cells are a promising energy-conversion technology. Over the last several decades significant progress has been made in increasing their performance and durability, of which continuum-level modeling of the transport processes has played an integral part. In this review, we examine the state-of-the-art modeling approaches, with a goal of elucidating the knowledge gaps and needs going forward in the field. In particular, the focus is on multiphase flow, especially in terms of understanding interactions at interfaces, and catalyst layers with a focus on the impacts of ionomer thin-films and multiscale phenomena. Overall, we highlight where there is consensus in terms of modeling approaches as well as opportunities for further improvement and clarification, including identification of several critical areas for future research. Fuel cells may become the energy-delivery devices of the 21 st century. Although there are many types of fuel cells, polymer-electrolyte fuel cells (PEFCs) are receiving the most attention for automotive and small stationary applications. In a PEFC, fuel and oxygen are combined electrochemically. If hydrogen is used as the fuel, it oxidizes at the anode releasing proton and electrons according toThe generated protons are transported across the membrane and the electrons across the external circuit. At the cathode catalyst layer, protons and electrons recombine with oxygen to generate waterAlthough the above electrode reactions are written in single step, multiple elementary reaction pathways are possible at each electrode. During the operation of a PEFC, many interrelated and complex phenomena occur. These processes include mass and heat transfer, electrochemical reactions, and ionic and electronic transport. * Electrochemical Society Active Member. z E-mail: azweber@lbl.govOver the last several decades significant progress has been made in increasing PEFC performance and durability. Such progress has been enabled by experiments and computation at multiple scales, with the bulk of the focus being on optimizing and discovering new materials for the membrane-electrode-assembly (MEA), composed of the proton-exchange membrane (PEM), catalyst layers, and diffusionmedia (DM) backing layers. In particular, continuum modeling has been invaluable in providing understanding and insight into processes and phenomena that cannot be resolved or uncoupled through experiments. While modeling of the transport and related phenomena has progressed greatly, there are still some critical areas that need attention. These areas include modeling the catalyst layer and multiphase phenomena in the PEFC porous media.While there have been various reviews over the years of PEFC modeling 1-7 and issues, [8][9][10][11][12][13][14] as well as numerous books and book chapters, there is a need to examine critically the field in terms of what has been done and what needs to be done. This review serves that purpose with a focus on transport modeling of PEFCs. This is not meant to be an exhaustive review...

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Uncovering the Structure of Nafion–SiO₂ Hybrid Ionomer Membranes for Prospective Large‐Scale Energy Storage Devices

Davis

Kim

Oleshko

et al. 2015

Adv Funct Materials

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4064 wileyonlinelibrary.com thermal stability. [ 5 ] However, Nafi on suffers from low ion selectivity which leads to vanadium crossover in the VRFB and decreases energy effi ciency of the battery over time. [ 3 ] There have been several efforts to combat the low ion selectivity of Nafi on for use as the IEMs in VRFBs. These include creating Nafi on blends with other polymers, [ 6 ] incorporating inorganic fi llers, [ 3,7,8 ] layered composites at interfaces, [9][10][11] as well as synthesizing entirely new classes of Nafi on-inspired membranes. [ 2 ] More specifi cally, sol-gel derived Nafi on-SiO 2 and Nafi on-Si/TiO 2 hybrid membranes were observed to signifi cantly reduce the permeability of vanadium ions and water, while minimally impacting ion exchange capacity (IEC) and proton conductivity, thus improving battery performance. [ 3,7 ] It has been suggested that these fi llers selectively incorporate into the ion-conducting domains of Nafi on, effectively altering (or disrupting) the ion/water transport domains. It is thought that creation of SiO 2 in the ion transport channels acts to reduce vanadium crossover by a "size exclusion" mechanism, impeding the transport of vanadium ions and creating a more ion-selective medium with better crossover resistance. There have been many studies of this type of Nafi on-SiO 2 nanocomposite using infrared [12][13][14][15][16][17] and/or nuclear magnetic resonance (NMR) [ 14,15,17 ] spectroscopy to identify the appropriate mechanism and gain a deeper understanding of the local environment of the SiO 2 and Nafi on phases. These spectroscopic investigations have provided great insight into the local interactions of the inorganic nanoparticles and the Nafi on; however, there is limited information on the nanostructure of these Nafi on-SiO 2 composite membranes.Recently, Ladewig et al. [ 18 ] published a structural characterization study of Nafi on-SiO 2 nanocomposite membranes created using this sol-gel chemistry approach. They employed both small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) to resolve the structure of the ionic domains in the Nafi on composite membranes. While little difference was observed between the X-ray scattering curves of the neat and modifi ed Nafi on, SANS showed a broadening and reduction in intensity of the ionomer peak, as well as a slight shift in the ionomer peak position to higher q (i.e., reduction in the d-spacing of this correlation peak, which can be indicative of Nafi on nanocomposite membranes are attractive candidates for the ion conducting phase in energy storage devices such as vanadium redox fl ow batteries. Herein, vanadium crossover and Nafi on-SiO 2 nanostructure are quantifi ed in a series of hybrid membranes created via solution-casting Nafi on fi lms with discrete SiO 2 nanoparticles, as well as membranes created using an in situ silica sol-gel condensation process. The crossover of vanadium ions is suppressed in all Nafi on membranes with the SiO 2 inorganic phase when compared to unannealed, neat N...

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Impact of Substrate and Processing on Confinement of Nafion Thin Films

Cited by 193 publications

References 71 publications

Proton Transport Property in Supported Nafion Nanothin Films by Electrochemical Impedance Spectroscopy

Proton Transport Property in Supported Nafion Nanothin Films by Electrochemical Impedance Spectroscopy

A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells

Uncovering the Structure of Nafion–SiO₂ Hybrid Ionomer Membranes for Prospective Large‐Scale Energy Storage Devices

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Impact of Substrate and Processing on Confinement of Nafion Thin Films

Cited by 193 publications

References 71 publications

Proton Transport Property in Supported Nafion Nanothin Films by Electrochemical Impedance Spectroscopy

Proton Transport Property in Supported Nafion Nanothin Films by Electrochemical Impedance Spectroscopy

A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells

Uncovering the Structure of Nafion–SiO2 Hybrid Ionomer Membranes for Prospective Large‐Scale Energy Storage Devices

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About

Uncovering the Structure of Nafion–SiO₂ Hybrid Ionomer Membranes for Prospective Large‐Scale Energy Storage Devices