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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.

Electrochemical reactors operating at intermediate temperatures (200 – 400 °C) can potentially have several advantages over their high temperature (600- 1000 °C) and low temperature (< 100 °C) counterparts. For example their potential of an increased catalytic activity enabling a decrease in material cost compared to low temperature cells, and 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 [1]. However, selectivity and electrochemical activity are far from being technically relevant, so that heterogeneous catalysis processes still are the matter of choice. Even more difficult is the electrochemical reduction of nitrogen f.i. to ammonia. In this contribution, activities at DTU Energy are summarized using solid state electrochemical cells under operating conditions close to the well-known catalytic synthesis of methanol and ammonia. 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. Proton conducting ceramics cells based on barium cerate with Mo and Fe electrodes have been characterized in H2/N2 atmospheres between 300 and 500 °C. Electrochemical impedance analysis point towards a predominant hydrogen evolution and the role of adsorption and desorption processes as well as overpotential will be discussed. [1] Y. Hori, Modern Aspects of Electrochemistry: Electrochemical CO2 Reduction on Metal Electrodes, Springer New York, USA 42 (2008), 89-189.

The synthesis of titanium pyrophosphate is carried out, and the material is sintered at different temperatures between 370 and 970 °C. Yttrium is added during the synthesis to act as acceptor dopant, but it is mainly present in the material in secondary phases. The conductivity is studied systematically as a function of sintering temperature, pH2O, pO2, and temperature (100–400 °C). Loss of phosphorus upon sintering above 580–600 °C is confirmed by energy dispersive spectroscopy and combined thermogravimetry and mass spectrometry. The conductivity decreases with increasing sintering temperature and decreasing phosphorus content. The highest conductivity is 5.3 × 10−4 S cm−1 at 140 °C in wet air (pH2O = 0.22 atm) after sintering at 370 °C. The conductivity is higher in wet atmospheres than in dry atmospheres. The proton conduction mechanism is discussed, and the conductivity is attributed to an amorphous secondary phase at the grain boundaries, associated with the presence of excess phosphorus in the samples. A contribution to the conductivity by point defects in the bulk may explain the conductivity trend in dry air and the difference in conductivity between oxidizing and reducing atmospheres at 300–390 °C. Slow loss of phosphorus by evaporation over time and changes in the distribution of the amorphous phase during testing are suggested as causes of conductivity degradation above 220 °C.

The high temperature properties of Rb2SeO4 have been studied by calorimetry, impedance spectroscopy and X-ray powder diffraction. As synthesized, Rb2SeO4 includes a second phase of Rb2SeO3, which can be eliminated upon heating the compound. As expected, no conductivity is observed in dry (pH2O < 0.001 bar) air. By changing to humidified (pH2O = 0.1 bar) air at 176 °C, the conductivity increases sharply from 8.6·10−8 to 1.7·10−6 S cm−1. Under humidified (pH2O = 0.1 bar) reducing atmosphere (9%H2 in N2), the conductivity increases to 2.0·10−4 S cm−1 at 317 °C. Degradation of Rb2SeO3 and Rb2SeO4 to form RbOH, which is known as a proton conductor, are thought to be responsible for the observed conductivity in humidified atmospheres. Our observations may explain the conductivity rise in other solid acid electrolytes, including sulfate and selenate groups, around their decomposition temperatures, in humidified atmospheres.

Structural, thermal and electrical properties studies of rubidium phosphite tellurate, RbH(PO 3 H)·Te(OH) 6 , were performed. An endothermic peak, which reached a completion at about 315 °C accompanied with a weight loss of 4.6 wt.%, was attributed to dehydration. Four types of pellets were produced, namely pellets A, B, C and D. Pellet A was tested with platinum–carbon paper electrode, and pellets B, C and D were tested with gold electrodes. Both pellets A and B were studied from 113 °C to 317 °C for 135 h. Pellet C was first investigated from room temperature to 176 °C for 360 h. After cooling down to room temperature, a second measurement with pellet C was carried out under the same conditions as used for pellets A and B. Pellet D, on the other hand, was heated up to 450 °C, kept at that temperature for 2 h and then cooled down to room temperature prior to the conductivity measurements. It was observed that the conductivities of pellets A and B decreased to values of 5.2 × 10 −8 S cm −1 and 6.6 × 10 −7 S cm −1 at 317 °C, respectively, and an unexpected rise in the conductivity (9.89 × 10 −6 S cm −1 at 317 °C) was seen with pellet C. Dehydration of RbH(PO 3 H)·Te(OH) 6 might be responsible for this unexpected rise in the conductivity of pellet C. The monoprotic part RbH(PO 3 H) of RbH(PO 3 H)·Te(OH) 6 apparently became diprotic (Rb 2 H 2 P 2 O 5 ) part of Rb 2 H 2 P 2 O 5 ·[Te(OH) 6 ] 2 after dehydration. The measured conductivity of pellet D, which was dehydrated prior to the measurement, reached a value of 5.41 × 10 −5 S cm −1 at 317 °C and showed a good stability over-each-run time and temperatures measurement up to 317 °C. The dehydrated compound, Rb 2 H 2 P 2 O 5 ·[Te(OH) 6 ] 2 , has also a higher hydrogen density relative to the starting compound, RbH(PO 3 H)·Te(OH) 6 . It is deduced that completion of the dehydration can be responsible for the unexpected rise in the conductivity of RbH(PO 3 H)·Te(OH) 6 . This unusual case is important for studies in solid acid proton conductors.

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 Ce0.9Gd0.1O1.95 (CGO10) backbone has been produced. The polarization resistance (R P) of this cathode was measured to ∼35 mΩ cm2 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 cm2 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.

Rb2SeO4 is synthesized by slow evaporation of aqueous solutions of Rb2CO3 and H2SeO4 (50 °C, 2 d) and characterized by calorimetry, impedance spectroscopy, and powder XRD.