Joining 3YSZ Electrolyte to AISI 441 Interconnect Using the Ag Particle Interlayer: Enhanced Mechanical and Aging Properties

Reactive air brazing has been widely used in fabricating solid oxide fuel/electrolysis cell (SOFC/SOEC) stacks. However, the conventional Ag–CuO braze can lead to (I) over oxidation at the steel interconnect interface caused by its adverse reactions with the CuO and (II) many voids caused by the hydrogen-induced decomposition of CuO. The present work demonstrates that the Ag particle interlayer can be used to join yttria-stabilized zirconia (YSZ) electrolytes to AISI 441 interconnect in air instead of Ag–CuO braze. Reliable joining between YSZ and AISI 441 can be realized at 920 °C. A dense and thin oxide layer (~2 μm) is formed at the AISI 441 interface. Additionally, an interatomic joining at the YSZ/Ag interface was observed by TEM. Obtained joints displayed a shear strength of ~86.1 MPa, 161% higher than that of the joints brazed by Ag–CuO braze (~33 MPa). After aging in reducing and oxidizing atmospheres (800 °C/300 h), joints remained tight and dense, indicating a better aging performance. This technique eliminates the CuO-induced issues, which may extend lifetimes for SOFC/SOEC stacks and other ceramic/metal joining applications.

Joining YSZ electrolyte to AISI 441 interconnect for solid oxide cells using the Ag interlayer: Enhanced mechanical and aging properties

Conventional Ag-CuO braze can lead to two electrolyte/interconnect joining issues: over-oxidation at the steel interconnect and hydrogen-induced decomposition of CuO. This work demonstrates that a pure Ag interlayer, instead of Ag-CuO braze, can join YSZ electrolyte to AISI 441 interconnect in air. Reliable joining between YSZ and AISI 441 can be realized at 920 °C. A dense and thin oxide layer (~2 μm) is formed at the AISI 441 interface. Also, an interatomic joining at the YSZ/Ag interface is detected by TEM observation. Obtained joints display high shear strengths (~86.1 MPa), 161% higher than that of joints brazed by Ag-CuO braze (~33 MPa). After aging in reducing and oxidizing atmospheres (800 °C/300 h), joints remain tight and dense, indicating a better aging performance. This technique eliminates the CuO-induced issues, which will extend lifetimes for SOFC/SOEC stacks and other ceramic/metal joining applications.

Mixed Chromate and Molybdate Additives for Cathodic Enhancement in the Chlorate Process

The economic viability of the electrochemical chlorate process depends on toxic chromate to induce cathodic selectivity to hydrogen and mitigate reduction of hypochlorite or chlorate. In this study, it is shown that performance of a pilot plant for chlorate production can be sustained when a 1000-fold reduction in chromate concentration is compensated by addition of molybdate. Laboratory measurements employing a Quartz Crystal Microbalance suggest growth of a nanometre-thick hybrid Mo–Cr-oxide film to induce cathodic selectivity. An optimized energy efficiency for pilot plant operation was obtained using 0.8 mM molybdate and 27 μM chromate, balancing formation of an effective oxide layer and undesired Mo-induced decomposition of hypochlorite to oxygen in solution. Refinement at the pilot scale level is expected to further optimize the energy consumption, thereby increasing safety aspects and the economic viability of chlorate production. Graphical Abstract

Evaluation of denitrification from three biogeochemical models using laboratory measurements of N₂, N₂O and CO₂

Biogeochemical models are useful for the prediction of nitrogen (N) cycling processes, but accurate description of the denitrification and decomposition sub-modules is critical. Current models were developed before suitable soil N2 flux data were available; new measurement techniques have enabled the collection of improved N2 data. We use measured data from two laboratory incubations to test the denitrification sub-modules of existing biogeochemical models. Two arable soils – a silt-loam and a sand – were incubated for 34 and 58 days, respectively. Fluxes of N2, N2O and CO2 were quantified using gas chromatography and isotope-ratio mass spectrometry (IRMS). For the loamy soil, seven moisture and three NO3− contents were included, with temperature changing during the incubation. The sandy soil was incubated with and without incorporation of litter (ryegrass), with temperature, water content and NO3− content changing during the incubation. Three common biogeochemical models (Coup, DNDC and DeNi) were tested using the data. No systematic calibration of the model parameters was conducted since our intention was to evaluate the general model structure or “default” model runs. As compared with measured fluxes, the average N2+N2O fluxes of the default runs for loamy soil were approximately 3 times higher for Deni, 105 times smaller for DNDC and 22 times smaller for Coup. For the sandy soils, default runs were 3 times higher for DeNi, 7 times smaller for DNDC and 12 times smaller for Coup. While measured fluxes were overestimated by DeNi and underestimated by DNDC and Coup, the temporal patterns of the measured and the modeled emissions were similar for the different treatments. None of the models was able to determine litter-induced decomposition correctly. The reason for the differences between the measured and modeled values can be traced back to model structure uncertainty and/or parameter uncertainty. Given the aim of our work – to assess existing model processes for further development and/or to identify missing processes within the models – these results provide valuable insights into avenues for future research. We conclude that the predicting power of the models could be improved through future experiments that collect data on denitrification activity with a concurrent focus on control parameter determination.