Proton exchange membranes for fuel cells operated at medium temperatures: Materials and experimental techniques

Changes of refractive indices of crystals induced by structural phase transitions

The chemical and thermal stability of several solid acid compounds under fuel cell operating conditions has been investigated, primarily by thermogravimetric methods. Thermal decomposition of CsHSO_4, a material which has shown promise as an alternative electrolyte for proton exchange membrane (PEM) fuel cells, initiates at ∼175°C under inert conditions. The overall decomposition process can be expressed as 2CsHSO_4 → Cs_2SO_4 + H_2O + SO_3 with Cs_2S_2O_7 appearing as an intermediate byproduct at slow heating rates. Under reducing conditions, chemical decomposition can occur via reaction with hydrogen according to 2CsHSO_4 + 4H_2 → Cs_2SO_4 + 4H_2O + H_2S. In the absence of fuel cell catalysts, this reduction reaction is slow; however, materials such as Pt, Pd, and WC are highly effective in catalyzing the reduction of sulfur and the generation of H_2S. In the case of M_3H(XO_4)_2 compounds, where M = Cs, NH_4, or Rb and X = S or Se, a similar reduction reaction occurs:  2M_3H(XO_4)_2 + 4H_2 → 3M_2XO_4 + 4H_2O + H_2X. In an operational fuel cell based on CsHSO_4, performance degraded with time, presumably as a result of H_2S poisoning of the anode catalyst. The performance loss was recoverable by exposure of the fuel cell to air at 160 °C.

Instability of sulfate and selenate solid acids in fuel cell environments

Ice clouds in the Earth’s upper troposphere can form via homogeneous nucleation of ice in aqueous droplets. In this study we investigate the crystallisation, or lack of crystallisation, of the solute phase and ice in aqueous (NH4)3H(SO4)2/H2O and NH4HSO4/H2O droplets. This is done using in situ X-ray diffraction of emulsified solution droplets mounted on a cold stage. From the diffraction patterns we are able to identify the phases of crystalline solute and ice that form after homogeneous freezing in micrometer sized droplets. An important finding from this study is that crystallisation of the solute does not always occur, even when crystallisation is strongly thermodynamically favoured. The nucleation and growth of solute phase crystals becomes inhibited since the viscosity of the aqueous brine most likely increases dramatically as the brine concentration increases and temperature decreases. If ice nucleates below a threshold freezing temperature, the brine appears to rapidly become so viscous that solute crystallisation is inhibited. This threshold temperature is 192 K and 180 K, in (NH4)3H(SO4)2 and NH4HSO4, respectively. We also speculate that the formation of cubic ice within a highly viscous brine blocks the solvent mediated cubic to hexagonal phase transformation, thus stabilising the metastable cubic ice in the most concentrated solution droplets.

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Inhibition of solute crystallisation in aqueous H+–NH4+–SO42−–H2O droplets

Dielectric properties of single crystal (NH 4 ) 3 H(SeO 4 ) 2 were measured below room temperature Two phase transitions were found at around +2°C and at -92°C A λ-type peak of the dielectric constant along the c -direction is found at -92°C, and the crystal shows ferroelectricity below the transition The temperature dependence of the spontaneous polarization was measured by a pyroelectric method The Curie temperature linearly increases with increasing hydrostatic pressure up to 85 kbar with a slope of 23 deg kbar -1 

Ferroelectric Phase Transition in (NH 4 ) 3 H(SeO 4 ) 2

Phase transitions in triammonium hydrogen disulfate (NH4)3H(SO4)2 were studied by means of dielectric measurements and differential thermal analysis. Below room temperature three phase transitions were observed at −8, −132, and −140°C. The dielectric constant along the c*-direction shows breaks at the −8 and −132°C transitions, while it shows a discontinuous change accompanied with a thermal hysteresis at the −140°C transition. In addition to the above-mentioned transitions, a broad maximum of the dielectric constant was found at about −28°C along the c*-direction. However, no thermal anomalies were observed around −28°C.



Mit Hilfe dielektrischer Messungen und differentieller Thermoanalyse wurden die Phasenubergange in Triammoniumhydrogendisulfat (NH4)3H(SO4)2 untersucht. Unterhalb der Raumtemperatur wurden drei Phasenubergange bei −8, −132 und −140°C gefunden. Die entlang der c*-Richtung gemessene Dielektrizitatskonstante zeigt bei den −8 und −132°C-Ubergangen stufenformige Anderungen, beim −140°C-Ubergang andert sie sich, begleitet von thermischen Hystereseerscheinungen, diskontinuierlich. Zusatzlich zu den o. g. Ubergangen wurden bei Messung in c*-Richtung ein breites Maximum der Dielektrizitatskonstanten bei etwa −28°C, aber keine thermischen Anomalien gefunden.

Dielectric study on the phase transitions in triammonium hydrogen disulfate (NH4)3H(SO4)2

The anomalous temperature dependence of the elcetrostrictive coefficients of NaNO2 and AgNa(NO2)2 is interpreted in terms of improper ferroelectricity A phenomenological formulation of the electrostrictive properties of an improper ferroelectric is derived The electrostrictive coefficients which are derived from observed piezoelectric coefficients are linear functions of the inverse dielectric susceptibility A possible origin of the improper nature of the NaNO2-type ferroelectrics is discussed



Die anomale Temperaturabhangigkeit der Elektrostriktionskoeffizienten von NaNO2 und AgNa(NO2)2 werden mit nichtvollkommener Ferroelektrizitat Es wird eine phanomenologische Formulierung der Elektrostriktionseigenschaften eines nicht vollkommenen Ferroelektrikums abgeleitet Die Elektrostriktionskoeffizienten, die aus den beobachteten piezoelektrischen Konstanten bestimmt werden, sind lineare Funktionen der inverson dielektrischen Suszeptibilitat Es wird eine mogliche Erklarung fur die Art der Unvollkommenheit von Ferroelektrika vom NaNO2-Typ diskutiert

K. Gesi

Papers

Dielectric study on the phase transitions in triammonium hydrogen disulfate (NH4)3H(SO4)2

``Improper'' ferroelectric properties of NaNO2 and AgNa(NO2)2