Depression of proton conductivity in recast Nafion® film measured on flat substrate

180

206

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...

Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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confidence: 99%

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

Paul

McCreery

Karan

2014

180

206

“…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

509

394

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...

“…4 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][10][11][12][13][14][15][16][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.…”

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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...