Many superionic mixed ionic–electronic conductors with a liquid-like sublattice have been identified as high efficiency thermoelectric materials, but their applications are limited due to the possibility of decomposition when subjected to high electronic currents and large temperature gradients. Here, through systematically investigating electromigration in copper sulfide/selenide thermoelectric materials, we reveal the mechanism for atom migration and deposition based on a critical chemical potential difference. Then, a strategy for stable use is proposed: constructing a series of electronically conducting, but ion-blocking barriers to reset the chemical potential of such conductors to keep it below the threshold for decomposition, even if it is used with high electric currents and/or large temperature differences. This strategy not only opens the possibility of using such conductors in thermoelectric applications, but may also provide approaches to engineer perovskite photovoltaic materials and the experimental methods may be applicable to understanding dendrite growth in lithium ion batteries.

Recently, many novel thermoelectric materials have been reported with ultra-high performance. However, some of them (e.g., Cu/Ag-based liquid-like materials) possess low thermodynamic stability. Here, we successfully achieve both good stability and high efficiency in a thermoelectric module based on highperformance liquid-like materials through tuning the geometry of the legs to ensure the voltage applied on the liquid-like materials is below their threshold for stable usage. This work accelerates the development of liquid-like thermoelectric materials for high-efficiency modules in real applications.

The streamflow age is an essential descriptor of catchment functioning that controls runoff generation, biogeochemical cycling, and contaminant transport. The young water fraction (F yw ) of streamflow, which can be accurately estimated with tracer data, is effective at characterizing the water age proportions of heterogeneous catchments. However, the F yw values of permafrost catchments are not known. We selected a watershed in the permafrost region of the Qinghai-Tibet Plateau (QTP) as our study area. Daily interval stable isotopes (deuterium and oxygen-18) of precipitation and streamflow were studied during the 2009 thawing season. The results show that the stable isotope compositions of precipitation and stream water have significant spatial and temporal variations. HYSPLIT backwards trajectory results demonstrate that the moisture in the study area mainly derived from the westerlies and southern monsoons. Thawing processes in the active layer of the permafrost significantly altered the stable isotope compositions of the stream water. The soil temperature, soil moisture, and air temperature are the main drivers of the stable isotope variations in the stream water. We estimated the young water fractions of the five catchments in the study area, which were the first estimates of the F yw in permafrost catchments in the QTP. The results show that an average of 15% of the streamflow is younger than 43 days. Additional analyses show that the vegetation cover significantly controls the young water fraction of the streamflow. These results will improve our understanding of permafrost hydrological processes and water resource utilization and protection. KEYWORDS permafrost hydrology, Qinghai-Tibet Plateau, stable isotopes, watershed hydrology, young water fraction