Catalyst/polymer electrolyte interfaces

Paulus UA, Veziridis V, Schnyder B, Kuhnke M, Scherer GG, Wokaun A. 2003. Fundamental investigation of catalyst utilization at the electrode/solid polymer electrolyte interface. Part I. Development of a model system. J Electroanal Chem 541 77-91. 

Polymer electrolyte fuel cell (PEFC) is considered as one of the most promising power sources for futurist s hydrogen economy. As shown in Fig. 1, operation of a Nation-based PEFC is dictated by transport processes and electrochemical reactions at cat-alyst/polymer electrolyte interfaces and transport processes in the polymer electrolyte membrane (PEM), in the catalyst layers consisting of precious metal (Pt or Ru) catalysts on porous carbon support and polymer electrolyte clusters, in gas diffusion layers (GDLs), and in flow channels. Specifically, oxidants, fuel, and reaction products flow in channels of millimeter scale and diffuse in GDL with a structure of micrometer scale. Nation, a sulfonic acid tetrafluorethy-lene copolymer and the most commonly used polymer electrolyte, consists of nanoscale hydrophobic domains and proton conducting hydrophilic domains with a scale of 2-5 nm. The diffusivities of the reactants (02, H2, and methanol) and reaction products (water and C02) in Nation and proton conductivity of Nation strongly depend on the nanostructures and their responses to the presence of water. Polymer electrolyte clusters in the catalyst layers also play a critical... 

Like in PEMFC, platinum is the standard catalyst used in the electrode and Nafion polymer electrolyte interface of DMFC for low temperature levels. Catalyst development effort for DMFC involves the search for an alternative to the Pt-Ru catalyst for enhanced anode oxidization reaction. 

It should be noted that degradation did not occur on the bare platinum immersed in liquid electrolyte, indicating that this was a specific phenomenon on the catalyst surface-polymer electrolyte interface (Okada et al. 2000). Unlike liquid electrolytes, the polymer electrolyte has immobile cation-exchange sites (sulfonic acid groups on side chains), so when they bind impurity metal cations, the constrained electric double layer lowers the electric field in the electrochemical double layer (Okada et al. 2001). 

Neutral PET hydrolysis usually takes place under high temperature and pressure in die presence of alkali metal acetate transesterification catalysts.28 It is diought diat the catalytic effect observed on the part of zinc salts is the result of electrolytic changes induced in die polymer-water interface during the hydrolysis process. The catalytic effect of zinc and sodium acetates is thought to be due to die destabilization of die polymer-water interface in the hydrolysis process. 

Antoine et al. [28] inveshgated the gradient across the CL and found that the Pt utilization was dependent on the CL porosity. In a nonporous CL, catalyst utilization was increased through the preferential locahon of Pt close to the gas diffusion layer in a porous CL, catalyst utilization efficiency was increased through the preferential location of Pt close to the polymer electrolyte membrane. In PEM fuel cells, fhe CL has a porous structure, and better performance is expected if higher Pf loading is used af preferential locahons close to the membrane/catalyst layer interface. 

Numerous efforfs have been made to improve existing fhin-film catalysts in order to prepare a CL with low Pt loading and high Pt utilization without sacrificing electiode performance. In fhin-film CL fabrication, fhe most common method is to prepare catalyst ink by mixing the Pt/C agglomerates with a solubilized polymer electrolyte such as Nation ionomer and then to apply this ink on a porous support or membrane using various methods. In this case, the CL always contains some inactive catalyst sites not available for fuel cell reactions because the electrochemical reaction is located only at the interface between the polymer electrolyte and the Pt catalyst where there is reactant access. 

MD Modeling of Interface Structure of Polymer Electrolyte Catalysts Interface... 

To date, it is difficult to model the interface between catalyst and polymer electrolyte. MD methods allow simulation of a system with 103-105 atoms. Thus, MD simulations are suitable for describing a system with the realistic size of a metallic catalyst on carbon support in contact with a cluster of polymer electrolyte (Fig. 2). The problem is, however, whether these simulations are correct. 

The effect of zinc catalysts on PET hydrolysis has been investigated by Campanelli et al.62 in the temperature range 250-280 °C. The catalytic effect of zinc salts was attributed to the electrolytic changes induced in the polymer-water interface during hydrolysis. Likewise, Kao et al.63 concluded the existence of an autocatalytic mechanism in PET hydrolysis because the depolymerization reaction is catalysed by the carboxyl groups produced during the reaction. 

The internal resistance of the polymer electrolyte membrane depends on the water content of the membrane. The water ionizes add moieties providing mobile protons, like protons in water [1-3]. Absorbed water also swells the membrane, which may affect the interface between the polymer electrolyte and the electrodes. Nafion, a Teflon/perfluorosulfonic acid copolymer, is the most popular polymer electrolyte because it is chemically robust to oxidation and strongly acidic. The electrodes are commonly Pt nanoparticles supported on a nanoporous carbon support and coated onto a microporous carbon cloth or paper. These structures provide high three-phase interface between the electrolyte/catalyst/reactant gas at both the anode and cathode. 

Improved electrode-electrolyte interface by impregnating the polymer electrolyte from solution into the volume of the catalyst layer [25]. 

A concentration of the platinum loading close to the electrolyte interface via Pt-covered polymer nanofibers has been proposed by 3 M [31]. The concentration of the platinum catalyst coated onto nanofibers within approximately 300 nm distances from the electrolyte membrane surface, however, is leading to specific operation characteristics. While high power densities can be achieved under comparatively dry operating conditions even at elevated temperatures, the thin catalyst layers show a tendency for flooding under wet conditions at low temperatures. A summary of the behavior of this type of catalyst layers under specific operating conditions is given in [56]. The low temperature behavior has been improved by addition of a conventional catalyst layer on to this thin film catalyst layer [57]. 

Soboleva T, Zhao X, Malek K, Xie Z, Navessin T, Holdcroft S (2010) On the micro-, meso-, and macroporous structures of polymer electrolyte membrane fuel cell catalyst layers. ACS Appl Mater Interfaces 2 375-384... 

In addition to the proton conductivity of the electrolyte, the performance of a fuel cell is largely dependent on the electrocatalytic activity of the anodic and cathodic interface. This depends both on the structure of the gas-electrocatalyst-electrolyte three phase boimdaries and on the electrocatalytic activity of the charge transfer reaction that takes place along the electrochemical interface. The former case determines the extent of the electrochemical surface area (ESA), while the latter is directly related to the physicochemical properties of the Pt based catalyst and the extent to which its catalytic properties are affected by its contact /interaction with the polymer electrolyte. 

The total number of molecules produced or consumed in the reaction is proportional to the surface area of the Pt/electrolyte interface. The plain interface shown in Figure 1.2 would give a very small current suitable for electrochemical studies, but would be of no practical interest for energy conversion devices. In practical fuel cells, the Pt/electrolyte interface should be as large as possible. This is achieved by mixing tiny carbon-supported catalyst particles with the polymer electrol3de and filling voids of carbon cloth or paper with this mixture. 

To improve effectiveness of the platinum catalyst, a soluble form of the polymer is incorporated into the pores of the carbon support structure. This increases the interface between the electrocatalyst and the solid polymer electrolyte. Two methods are used to incorporate the polymer solution within the catalyst. In Type A, the polymer is introduced after fabrication of the electrode in Type B, it is introduced before fabrication. 

Aquivion E87-12S short-side chain perfluorosulfonic acid (SSC-PFSA) membrane with equivalent weight (EW) of 870 g eq and 120 pm thickness produced by Solvay Specialty Polymers was tested in a polymer electrolyte membrane water electrolyser (PEMWE) and compared to a benchmark Nation N115 membrane (EW 1100 g eq ) of similar thickness [27]. Both membranes were tested in conjunction with in-house prepared unsupported Ir02 anode and carbon-supported Pt cathode electrocatalyst. The electrocatalysts consisted of nanosized Ir02 and Pt particles (particle size 2-4 nm). The electrochemical tests showed better water splitting performance for the Aquivion membrane and ionomer-based membrane-electrode assembly (MEA) as compared to Nafion (Fig. 2.21). Lower ohmic drop constraints and smaller polarization resistance were observed for the electrocatalyst-Aquivion ionomer interface indicating a better catalyst-electrolyte interface. A current density of 3.2 A cm for water... 

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