Development of Novel Proton Conductors Consisting of Solid Acid/pyrophosphate Composite for Intermediate-temperature Fuel Cells

Abstract

Fuel cells are attractive energy conversion devices with high efficiencies and low emissions, and many studies have been conducted so far. Among them, fuel cells operating at 200-00℃ are promising technologies which combine the many advantages of high-and low-temperature fuel cells. However, they have not been developed due to the lack of good ionic-conductors with high thermal stability at intermediate temperatures. Recently, we have developed new proton-conductive electrolytes consisting of solid acid and pyrophosphate, and evaluated their electrochemical, structural and thermal properties at intermediate temperatures. For the composite based on CsH2PO/SiP2O, the interfacial chemical reaction between CsH2PO and SiP2O during heat-treatment gave rise to the formation of a new phase of CsH5(PO)2. The temperature dependence of conductivity for this composite was different from that for pure CsH2PO, and the maximum conductivity achieved was mS・cm -at 2℃. Using potassium and rubidium salts, MH2PO (M＝K, Rb), as the solid acids for the composite electro lytes, analogous phenomena were confirmed despite the alkaline metal. Operation of a fuel cell employing CsH2PO/SiP2O-based composite electrolyte (thickness: ca. .3 mm) was demonstrated at 200℃ and generated electricity up to 220 mA・cm -2 at 0.2 V. CsH5(PO)2 composites with SiP2O and SiO2 were fabricated, and the composite effects were investigated at intermediate temperatures based on conductivity measurement, thermal analysis, and wettability evaluation. The melting and dehydration processes of CsH5(PO)2 in composites were different depending on the matrix species. The composite with SiP2O matrix showed the highest conductivity of all composites. The conductivity of the composites appears to correlate with the wettability between the components as examined by contact angle measurement. These findings should be attributed to the differences in the interfacial interactions between CsH5(PO)2 and the matrix.

CsH2PO4/Polyvinylidene Fluoride Composite Electrolytes for Intermediate Temperature Fuel Cells

“…13,14 However, they are still higher than 0.15 cm 2 , which is considered the target ASR for proton conductors for ITFCs. 15 The small contact areas between particles in the composite electrolytes are likely the main reason for their lower conductivity compared to that of pure CsH 2 PO 4 , which is as high as 28 mS cm −1 at 270 • C (Fig. 6).…”

“…[11][12][13][14][15][16][17][18][19][20][21][22][23][24] The phase transition of this material at ca. 230 • C from monoclinic (low-temperature phase) to cubic (high-temperature phase) brings about a drastic increase in its ionic conductivity, exceeding 10 mS cm −1 .…”

A Flexible CsH5(PO4)2-Doped Composite Electrolyte Membrane for Intermediate-Temperature Fuel Cells

“…Interaction between two components at the 14 contacting interface is normally the cause of melting point depression [43,44,46]. In this work, the CsH5(PO4)2/PSi/SPEEK composite electrolyte membranes contained CsH5(PO4)2 and phosphosilicate.…”

“…Consequently, the predominant proton-conducting carriers should be derived from phosphosilicate, rather than from CsH5(PO4)2, so that the conductivities of the CsH5(PO4)2/PSi/SPEEK composite electrolyte membranes increase with increasing phosphosilicate content. In the high temperature range above the melting point, CsH5(PO4)2 is in the liquid state, resulting in a high proton conductivity [42,43]. Consequently, CsH5(PO4)2 is primarily accountable for the proton conduction of the composite electrolyte membranes; thus, their conductivities increase relative to the increasing of CsH5(PO4)2 content.…”

Протонная проводимость, структурные и термические свойства (1-x)CsH-=SUB=-2-=/SUB=-PO-=SUB=-4-=/SUB=--xBa(H-=SUB=-2-=/SUB=-PO-=SUB=-4-=/SUB=-)-=SUB=-2-=/SUB=-

“…Исходя из состава системы количество образуемой соли не может быть более 20 mol%, тем не менее она фиксируется дан-ными РФА и ИК-спектроскопии. Механизм образования данного соединения не очевиден, хотя формирование CsH 5 (PO 4 ) 2 на границе раздела фаз было показано в ряде композитных систем на основе фосфатов [28][29][30][31]. Как правило, эти композитные системы имеют высокие значения протонной проводимости и термически устой-чивы при высоких парциальных давлениях паров воды, поскольку при нормальных условиях CsH 5 (PO 4 ) 2 мед-ленно дегидратирует при температурах выше 156…”