A review on phosphate based, solid state, protonic conductors for intermediate temperature fuel cells.
25 May 2011-Journal of Physics: Condensed Matter (IOP Publishing)-Vol. 23, Iss: 23, pp 234110-234110
TL;DR: In this review, the potential use of phosphate compounds as electrolytes for intermediate temperature fuel cells is summarized and various examples on ammonium polyphosphate, pyroph phosphate, cesium phosphate and other phosphate based electrolytes are presented and their preparation methods, conduction mechanism and conductivity values are demonstrated.
Abstract: The electrolytes currently used for proton exchange membrane fuel cells are mainly based on polymers such as Nafion which limits the operation regime of the cell to ~ 80 °C. Solid oxide fuel cells operate at much elevated temperatures compared to proton exchange membrane fuel cells (~1000 °C) and employ oxide electrolytes such as yttrium stabilized zirconia and gadolinium doped ceria. So far an intermediate temperature operation regime (300 °C) has not been widely explored which would open new pathways for novel fuel cell systems. In this review we summarize the potential use of phosphate compounds as electrolytes for intermediate temperature fuel cells. Various examples on ammonium polyphosphate, pyrophosphate, cesium phosphate and other phosphate based electrolytes are presented and their preparation methods, conduction mechanism and conductivity values are demonstrated.
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TL;DR: Through the combination of molecular synthesis and crystal engineering, MOFs present an unprecedented opportunity for the rational and precise design of functional materials.
Abstract: Metal–organic frameworks (MOFs), also known as coordination polymers, represent an interesting class of crystalline molecular materials that are synthesized by combining metal-connecting points and bridging ligands. The modular nature of and mild conditions for MOF synthesis have permitted the rational structural design of numerous MOFs and the incorporation of various functionalities via constituent building blocks. The resulting designer MOFs have shown promise for applications in a number of areas, including gas storage/separation, nonlinear optics/ferroelectricity, catalysis, energy conversion/storage, chemical sensing, biomedical imaging, and drug delivery. The structure–property relationships of MOFs can also be readily established by taking advantage of the knowledge of their detailed atomic structures, which enables fine-tuning of their functionalities for desired applications. Through the combination of molecular synthesis and crystal engineering, MOFs thus present an unprecedented opportunity fo...
752 citations
TL;DR: In this article, a model system for solid acid membrane electrolyser cells was presented, where metal carbide coated wires prepared by a two-step oxidation-carburization reaction of the metal wire surfaces were used as electrodes and allowed the measurement of the intrinsic catalytic properties of different transition metal carbides in direct comparison to Pt at 260-°C.
151 citations
TL;DR: In this article, the PBI membranes were shown to be irreversibly cured by the thermal treatment and the improved physicochemical characteristics of the membranes after curing were further illustrated by a dramatically improved long-term durability of the corresponding fuel cell MEAs.
Abstract: Phosphoric acid doped polybenzimidazole (PBI) has emerged as one of the most promising electrolyte materials for proton exchange membrane (PEM) fuel cells operating under anhydrous conditions at temperatures of up to 200 °C. The limited long-term durability of the membrane electrode assemblies (MEAs) is currently hampering the commercial viability of the technology. In the present study, thermoset PBI membranes were prepared by curing the membranes under inert atmosphere at temperatures of up to 350 °C prior to the acid doping. The systematic membrane characterizations with respect to solubility, phosphoric acid doping, radical-oxidative resistance and mechanical strength indicated that the PBI membranes were irreversibly cured by the thermal treatment. After curing, the PBI membranes demonstrated features that are fundamental characteristics of a thermoset resin including complete insolubility, high resistance to swelling and improved mechanical toughness. Additionally, the thermal treatment was found to increase the degree of crystallinity of the membranes. The improved physicochemical characteristics of the membranes after curing were further illustrated by a dramatically improved long-term durability of the corresponding fuel cell MEAs. During continuous operation for 1800 h at 160 °C and 600 mA cm−2, the average cell voltage decay rate of the MEA based on the cured membrane was 43 μV h−1. This should be compared with an average cell voltage decay rate of 308 μV h−1 which was recorded for the MEA based on its non-cured counterpart.
143 citations
TL;DR: In this paper, a review summarizes the recent progress made in the development of low-temperature proton-conducting ceramics (LT-PCCs), which are defined as operating in the temperature range of 25-400°C.
Abstract: Proton-conducting ceramics (PCCs) are of considerable interest for use in energy conversion and storage applications, electrochemical sensors, and separation membranes. PCCs that combine performance, efficiency, stability, and an ability to operate at low temperatures are particularly attractive. This review summarizes the recent progress made in the development of low-temperature proton-conducting ceramics (LT-PCCs), which are defined as operating in the temperature range of 25–400 °C. The structure of these ceramic materials, the characteristics of proton transport mechanisms, and the potential applications for LT-PCCs will be summarized with an emphasis on protonic conduction occurring at interfaces. Three temperature zones are defined in the LT-PCC operating regime based on the predominant proton transfer mechanism occurring in each zone. The variation in material properties, such as crystal structure, conductivity, microstructure, fabrication methods required to achieve the requisite grain size distribution, along with typical strategies pursued to enhance the proton conduction, is addressed. Finally, a perspective regarding applications of these materials to low-temperature solid oxide fuel cells, hydrogen separation membranes, and emerging areas in the nuclear industry including off-gas capture and isotopic separations is presented.
127 citations
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References
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TL;DR: It is shown that a cell made of a CsHSO4 electrolyte membrane operating at 150–160 °C in a H2/O2 configuration exhibits promising electrochemical performances: open circuit voltages of 1.11 V and current densities of 44 mA cm-2 at short circuit.
Abstract: Fuel cells are attractive alternatives to combustion engines for electrical power generation because of their very high efficiencies and low pollution levels. Polymer electrolyte membrane fuel cells are generally considered to be the most viable approach for mobile applications. However, these membranes require humid operating conditions, which limit the temperature of operation to less than 100 degrees C; they are also permeable to methanol and hydrogen, which lowers fuel efficiency. Solid, inorganic, acid compounds (or simply, solid acids) such as CsHSO4 and Rb3H(SeO4)2 have been widely studied because of their high proton conductivities and phase-transition behaviour. For fuel-cell applications they offer the advantages of anhydrous proton transport and high-temperature stability (up to 250 degrees C). Until now, however, solid acids have not been considered viable fuel-cell electrolyte alternatives owing to their solubility in water and extreme ductility at raised temperatures (above approximately 125 degrees C). Here we show that a cell made of a CsHSO4 electrolyte membrane (about 1.5 mm thick) operating at 150-160 degrees C in a H2/O2 configuration exhibits promising electrochemical performances: open circuit voltages of 1.11 V and current densities of 44 mA cm-2 at short circuit. Moreover, the solid-acid properties were not affected by exposure to humid atmospheres. Although these initial results show promise for applications, the use of solid acids in fuel cells will require the development of fabrication techniques to reduce electrolyte thickness, and an assessment of possible sulphur reduction following prolonged exposure to hydrogen.
809 citations
TL;DR: A brief overview of the types and principles of solid state protonic conductors is given in this article, where the properties of these materials are summarized in terms of their conductivity and operation temperature.
722 citations
TL;DR: In this paper, the stoicheiometry and dehydration behavior of gelatinous zirconium phosphate preparations were determined and a tentative explanation for this behaviour was presented based on structural considerations.
691 citations
TL;DR: In this paper, the theory of spontaneous polarization along the axis of the crystal, resulting in a well-known transition, similar to Rochelle salt, with polarization below the Curie point was worked out.
Abstract: Potassium dihydrogen phosphate contains phosphate groups connected by hydrogen bonds. Different possible arrangements of the hydrogens result effectively in different orientations of the (H2PO4)— dipoles. Since these have the lowest energy when pointing along the axis of the crystal, there is a tendency toward spontaneous polarization along this axis, resulting in the well‐known transition, similar to Rochelle salt, with polarization below the Curie point. The theory of this transition is worked out, using statistical methods to count the number of arrangements of hydrogens consistent with each total polarization of the crystal, and deriving the free energy. It is found that the theory predicts a phase change of the first order, with sudden transition from the polarized state at low temperature to the unpolarized state at high temperature, rather than the lambda‐point transition or phase change of the second order which is observed. However, the observed transition is confined to a very narrow temperature range compared to that predicted, for instance, by the Weiss theory, so that it seems as if it might be merely a broadened transition of the first order. It is suggested that the broadening may result from the irregular shifts of transition temperatures of individual domains in the crystal on account of stresses resulting from the large piezoelectric effect and the resulting deformation of the crystal below the transition point. The susceptibility above the Curie point comes out by the theory to be 4.33 times as great as it should according to the Weiss theory, a result which seems to be in general agreement with experiment. The entropy change in the transition is given by the theory as 0.69 unit, somewhat smaller than the observed value of about 0.8 unit. No explanation is suggested for this discrepancy.
521 citations
TL;DR: In this paper, it was shown that in order to explain the complete range of observed product compositions, rate expressions for all three reactions (methanol-steam reforming, water-gas shift and methanol decomposition) must be included in the kinetic analysis and variations in the selectivity and activity of the catalyst indicate that the decomposition reaction occurs on a different type of active site than the other two reactions.
Abstract: On-board generation of hydrogen by methanol–steam reforming on Cu/ZnO/Al 2 O 3 catalyst is being used in the development of fuel-cell engines for various transportation applications. There has been disagreement concerning the reactions that must be included in the kinetic model of the process. Previous studies have proposed that the process can be modelled as either the decomposition of methanol followed by the water-gas shift reaction or the reaction of methanol and steam, to form CO 2 and hydrogen, perhaps followed by the reverse water-gas shift reaction. Experimental results are presented which clearly show that, in order to explain the complete range of observed product compositions, rate expressions for all three reactions (methanol–steam reforming, water-gas shift and methanol decomposition) must be included in the kinetic analysis. Furthermore, variations in the selectivity and activity of the catalyst indicate that the decomposition reaction occurs on a different type of active site than the other two reactions. Although the decomposition reaction is much slower than the reaction between methanol and steam, it must be included in the kinetic model since the small amount of CO that is produced can drastically reduce the performance of the anode electrocatalyst in low temperature fuel cells.
500 citations