The thermal evolution of the phase composition of CeP 2 O 7 and Ce(PO 3 ) 4 with 10 mol% Y and Gd doping, respectively, was examined by in-situ powder X-ray diffraction and thermogravimetry with in-line mass spectroscopy. The phase composition depends critically on the P to metal ratio, the annealing temperature, humidity and time. CeP 2 O 7 and Ce(PO 3 ) 4 were completely decomposed to CePO 4 following a 1100 h long conductivity test at 155 • C. The conductivity of 10 mol% Gd doped Ce(PO 3 ) 4 (synthesized with P:(Ce + Gd) = 5.0) reaches a value of 6.4 · 10 −2 S · cm −1 at 150 • C under wet conditions (pH 2 O = 0.2 atm). The conductivity of 10 mol% Y doped CeP 2 O 7 (synthesized with P:(Ce + Y) = 3.1) was 1.9 · 10 −2 S · cm −1 under the same conditions. Long term conductivity measurements are reported here for the first time and the effect of repeated hydration-dehydration cycles on the conductivity is examined. Exsolution of P m O n and increase of the highly hygroscopic amorphous secondary phase significantly affects the conducting properties. KH 2 PO 4 was observed to re-crystallize and form amorphous potassium phosphate at temperatures above 100 • C in the 10 mol% Y doped CeP 2 O 7 :KH 2 PO 4 composite (synthesized with P:(Ce + Y) = 3.1) resulting in a conductivity value of 2.6 · 10 −2 S · cm −1 at 150 • C and pH 2 O = 0.2 atm.Fuel cells, electrolyzers and other electrochemical devices operating at intermediate temperatures (ca. 200 • C) have several advantages compared to their low temperature (< 100 • C) counterparts. These include improved electrode kinetics (enabling the use of non-noble metal catalysts), reduced CO poisoning, easier water management, as well as enabling the use/production of hydrocarbon fuels, e.g. methanol, ethanol and dimethylether. Intermediate temperature operation holds certain advantages also in comparison to high temperature operation (> 600 • C), such as reduced corrosion and reactivity/interdiffusion among components, minimized degradation due to catalyst coarsening, in addition to the potential for production of synthetic fuels.In order to materialize these potential advantages, intermediate temperature proton conducting electrolytes are required. The materials group of pyrophosphates, MP 2 O 7 with M being a 4-valent cation, has attracted considerable attention during the last decade, since the discovery of high proton conductivity in SnP 2 O 7 at intermediate temperatures. 1 The conductivity of MP 2 O 7 at 200 • C and pH 2 O ≈ 0.001 atm shows an increasing tendency in the order Zr < Ge < Si < Ce < Ti < Sn. 2-8 Acceptor doping results in increased conductivity, that goes through a maximum at 5 mol% Al, 9 6 mol% Sc, 10 9 mol% Ga, 11 10 mol% In 2 or Mg, 12 and 20 mol% Sb. 13
Reduction of CO2 at intermediate temperatures (T< 300 oC) is a tempting way to produce hydrocarbons. One benefit of performing CO2 reduction below 300 oC is that it would possibly allow a single process for production of hydrocarbons. CsH2PO4, BaHPO4 and a composite thereof have been investigated as electrolyte materials both with respect to thermal stability and to conductivity. BaHPO4 showed an improved thermal stability compared to CsH2PO4, but it did at the same time exhibit a low conductivity (1⋅10-6 S cm-1 at 304 oC). CsH2PO4 started to dehydrate at 249 oC, but this dehydration was shown to be reversible. Its conductivity was measured to be 2⋅10-2 S cm-1 at 240 oC. Full cells with copper as the CO2 reduction catalyst have been manufactured and tested in CO2 containing atmospheres. Initial performance of 34 mA cm-2 at 245 oC has been achieved.
A bi-continuous porous cathode consisting of nano-particles of strontium substituted lanthanum cobaltite (LSC) covering the surface of a Ce 0.9 Gd 0.1 O 1.95 (CGO10) backbone has been produced. The polarization resistance (R P ) of this cathode was measured to ∼35 mΩ cm 2 at 650°C. The area-specific resistance at 650°C (ASR) when applied onto an anode supported cell (ASC) was found to increase from 540 to 730 mΩ cm 2 when subjected to a thermal cycle to 850°C. This effect was attributed to particles coarsening but also to a reaction with the electrolyte. The results imply that a CGO10 barrier is required for this type of nano-structured cathode.
Electrochemical reactors operating at intermediate temperatures (200 – 400 °C) have the advantage that they can be thermally integrated into other chemical processes like synthesis of synthetic fuels and chemicals, such as methanol and ammonia. Electrochemical reduction of CO2 to chemical building blocks such as CO, CH3OH, and CH4 is a dream for electrochemists, and high faradaic efficiencies have been reported for liquid electrochemical cells operated at ambient temperature, using e.g. Cu electrodes. However, selectivity and electrochemical activity are far from being technically relevant, so that heterogeneous catalysis processes still are the matter of choice. In this contribution, solid state electrochemical cells under operating conditions close to the well-known catalytic synthesis of methanol. For CO2 reduction, cells based on CsH2PO4 as electrolyte and Cu based cathodes have been investigated towards their electrochemical activity in both H2/H2O and H2/H2O/CO2 containing atmospheres at elevated temperatures of 240 °C.
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