Proton-conducting separators

The last comprehensive review covering proton conductivity and proton conducting materials was written by one of the authors (dating back to 1996) since then, there have been several other review articles of similar scope (e.g., see Colomban ). There are also many reviews available on separator materials used for fuel cells (see articles in refs 3 and 4 and references therein, recent review-type articles, " and a literature survey ), which, more or less, address all properties that are relevant for their functioning in a fuel cell. The transport properties are usually described in these articles however, the treatments are frequently restricted to macroscopic approaches and handwaving arguments about the transport mechanisms. The purpose of the present review is to combine a few recently published results in the context of a discussion of transport phenomena in proton-conducting separator materials, which have some relevance in fuel cell applications (for a more complete list of the comprehensive literature in the field, the interested reader is referred to the aforementioned references). 

The use of polymeric constituents becomes progressively critical with increasing temperature therefore, for durability reasons, partially and even fully inorganic systems have recently attracted interest as proton-conducting separator materials. 

The above-described qualitative considerations hopefully give a flavor for the complexity of the development of novel proton-conducting separator materials for fuel-cell applications. When solely considering proton conductivity, significant progress has been... 

Figure 23.1 Schematic representation of a hydrogen / oxygen fuel cell, comprising the proton conducting separator (electrolyte) and the heterogeneous gas electrodes.

Figure 23.10 Proton conductivity of a few prototypical proton conducting separator materials Nafion as a representative of hydrated acid ionomers (see also Fig. 23.2(a) [43, 78], a complex of PBI (polybenzimidazole) and phosphoric acid as a representative of adducts of basic polymers and oxo-acids (see also Fig. 23.2(b)) [16], phosphonic acid covalently immobilized via an alkane spacer at a siloxane backbone (see also Fig. 23.2(c)) [127], the acid salt CsHSO, [125] and an Y-doped BaZrOj [126].

Lin, J.Y.S., Proton conducting dense ceramic membranes for hydrogen separation, Annual Progress Report, U.S. DOE Contract DE-FG26-00NT40818, December 2002. 

As typically observed in the case of non-ionic block and graft copolymers, the immiscibility of the constituent blocks within the copolymers can induce microphase separation beyond even that which normally occurs due to hydrophobic and hydrophilic sites within statistical copolymer PEMs such as Nation. A relatively recent area of PEM research, ionic block and graft copolymers are interesting from the point of view of providing fundamental understanding about the influence of morphology upon proton conduction... 

The blending of two or more polymers is frequently used to try to combine the separate desirable properties of each system rather than trying to develop one system with all the properties. In the case of PEMs, this has led to the blending of proton-conducting polymers with non-ionic polymers, low lEC polymers, or polymer-containing basic moieties, particularly for DMFC applications in order to decrease MeOH crossover. These different types of blends will be briefly discussed next. 

The first demonstration of a PEM with electrochromic properties was disclosed by SchlenofFand coworkers [66], using poly(butanylviologen)/ PSS films. While this film exhibited strong electrochromic response, it still required the use of an outer electrolyte solution. DeLongchamp and Hammond disclosed for the first time a solid-state device comprised of two electrochromic PEM-modified ITO electrodes separated by a 200-p,m thick poly(2-acrylamido-methane-2-propanesulfonic acid), proton-conducting PAMPS membrane (see Eigure 2.30) [196]. Both PEMs used in... 

A more recent view of proton transport is that of Kreuer, who, compared with the Zundel-based view, describes the process on different structural scales within phase separated morphologies. The smallest scale is molecular, which involves intermolecular proton transfer and the breaking and re-forming of hydrogen bonds. When the water content becomes low, the relative population of hydrogen bonds decreases so that proton conductance diminishes in a way that the elementary mechanism becomes that of the diffusion of hydrated protons, the so-called vehicle mechanism . 

Proton-conduction mechanisms that occur in the aforementioned proton solvents, when present as a homogeneous phase and as a component of heterogeneous fuel cell separator materials, are described more specifically in the following two sections. 

Hydrated Acidic Polymers. Hydrated acidic polymers are, by far, the most commonly used separator materials for low-temperature fuel cells. Their typical nanoseparation (also see Section 1) leads to the formation of interpenetrating hydrophobic and hydrophilic domains the hydrophobic domain gives the membrane its morphological stability, whereas the hydrated hydrophilic domain facilitates the conduction of protons. Over the past few years, the understanding of the microstructure of these materials has been continuously growing, and this has been crucial for the improved understanding of the mechanism of proton conduction and the observed dependence of the conductivity on solvent (water and methanol) content and temperature. 

The suitability of proton-conducting materials as separators in a particular fuel-cell application is essentially dependent on its transport properties, durability, and reactivity. Thus far, this review has focused on the transport properties only, but any approach toward new separator materials must consider all relevant aspects, which makes the development of new competitive materials a complex and challenging task. 

The ionic monomer that forms the proton exchange membrane (PEM) separating and ionically connecting the two gas diffusion electrodes can be dissolved in isobutyl alcohol or other organic solvents, such as isopropanol. This circumstance opens the way for improving the ionic contact between the catalyst particles of a gas diffusion electrode and the proton-conducting membrane and electrolyte. 

Proper water management in proton exchange membrane fuel cells (PEMFCs) is critical to PEMFC performance and durability. PEMFC performance is impaired if the membrane has insufficient water for proton conduction or if the open pore space of the gas diffusion layer (GDL) and catalyst layer (CL) or the gas flow channels becomes saturated with liquid water, there is a reduction in reactant flow to the active catalyst sites. PEMFC durability is reduced if water is left in the CL during freeze/thaw cycling which can result in CL or GDL separation from the membrane,1 and excess water in contact with the membrane can result in accelerated membrane thinning.2... 

A typical PEFC, shown schematically in Fig. 1, consists of the anode and cathode compartments, separated by a proton conducting polymeric membrane. The anode and cathode sides each comprises of gas channel, gas diffusion layer (GDL) and catalyst layer (CL). Despite tremendous recent progress in enhancing the overall cell performance, a pivotal performance/durability limitation in PEFCs centers on liquid water transport and resulting flooding in the constituent components.1,2 Liquid water blocks the porous pathways in the CL and GDL thus causing hindered oxygen transport to the... 

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