Natural Convection in Molten Salt Electrochemistry.

Molten salt electrochemistry has been widely used in many fields, especially for metal extraction/refinement. The understanding of mass transfer in molten salts under harsh operation conditions is of great importance to reveal reaction mechanisms and advance fine technologies. It has been generally assumed that natural convection is negligible in stagnant molten salt electrochemistry. Herein, we report an abnormal natural convection in molten LiCl-KCl, with the arising time from 2.37 s at 873 K to 10.13 s at 673 K. Using the concentration correction factor, the derived thickness of the natural convection-diffusion layer (δconv.) was found to be ranging from 128 to 163 µm, much thinner than those in aqueous solutions (∼200 µm). The simulations showed that the notable natural convection resulted from convection-diffusion layer (CDL) convection dominated over the density-driven convection even at high redox concentrations, implying the severe vibration of molten salt systems. To suppress the intense natural convection, we predicted that the use of microelectrodes (with radii less than 23.2 µm for δconv. = 150 µm) would be a promising tool, regardless of their inferior stabilities in high-temperature molten salts at this stage. These innovative findings offer insights into the impact of natural convection on mass transfer in molten salts that have not been previously revealed.

Molten salt electro-preparation of graphitic carbons.

Graphite has been used in a wide range of applications since the discovery due to its great chemical stability, excellent electrical conductivity, availability, and ease of processing. However, the synthesis of graphite materials still remains energy-intensive as they are usually produced through a high-temperature treatment (>3000°C). Herein, we introduce a molten salt electrochemical approach utilizing carbon dioxide (CO2) or amorphous carbons as raw precursors for graphite synthesis. With the assistance of molten salts, the processes can be conducted at moderate temperatures (700-850°C). The mechanisms of the electrochemical conversion of CO2 and amorphous carbons into graphitic materials are presented. Furthermore, the factors that affect the graphitization degree of the prepared graphitic products, such as molten salt composition, working temperature, cell voltage, additives, and electrodes, are discussed. The energy storage applications of these graphitic carbons in batteries and supercapacitors are also summarized. Moreover, the energy consumption and cost estimation of the processes are reviewed, which provides perspectives on the large-scale synthesis of graphitic carbons using this molten salt electrochemical strategy.

Selection principles of polymeric frameworks for solid-state electrolytes of non-aqueous aluminum-ion batteries.

Liquid electrolyte systems of aluminum-ion batteries (AIBs) have restrictive issues, such as high moisture sensitivity, strong corrosiveness, and battery leakage, so researchers have turned their attention to developing high safety, leak-free polymer electrolytes. However, the stability of the active factor of AIB systems is difficult to maintain with most of polymeric frameworks due to the special Al complex ion balance in chloroaluminate salts. Based on this, this work clarified the feasibility and specific mechanism of using polymers containing functional groups with lone pair electrons as frameworks of solid-state electrolytes for AIBs. As for the polymers reacting unfavorably with AlCl3, they cannot be used as the frameworks directly due to the decrease or even disappearance of chloroaluminate complex ions. In contrast, a class of polymers represented by polyacrylamide (PAM) can interact with AlCl3 and provide ligands, which not only have no effect on the activity of Al species but also provide chloroaluminate complex ions through complexation reactions. According to DFT calculations, amide groups tend to coordinates with AlCl2 + via O atoms to form [AlCl2(AM)2]+ cations, while disassociating chloroaluminate anions. Furthermore, the PAM-based solid-state and quasi-solid-state gel polymer electrolytes were also prepared to investigate their electrochemical properties. This work is expected to provide new theoretical and practical directions for the further development of polymer electrolytes for AIBs.

High-Purity Graphitic Carbon for Energy Storage: Sustainable Electrochemical Conversion from Petroleum Coke.

The petroleum coke (PC) has been widely used as raw materials for the preparation of electrodes in aluminium electrolysis and lithium-ion batteries (LIB), during which massive CO2 gases are produced. To meet global CO2 reduction, an environmentally friendly route for utilizing PC is highly required. Here, a simple, scalable, catalyst-free process that can directly convert high-sulfur PC into graphitic nanomaterials under cathodic polarization in molten CaCl2 -LiCl at mild temperatures is proposed. The energy consumption of the proposed process is calculated to be 3 627.08 kWh t-1 , half that of the traditional graphitization process (≈7,825.21 kWh t-1 graphite). When applied as a negative electrode for LIBs, the as-converted graphite materials deliver a competitive specific capacity of ≈360 mAh g-1 (0.2 C) compared with commercial graphite. This approach has great potential to scale up for sustainably converting low-value PC into high-quality graphite for energy storage.

The uncovered biases and errors in clinical determination of bone age by using deep learning models.

OBJECTIVES: To evaluate AI biases and errors in estimating bone age (BA) by comparing AI and radiologists' clinical determinations of BA. METHODS: We established three deep learning models from a Chinese private dataset (CHNm), an American public dataset (USAm), and a joint dataset combining the above two datasets (JOIm). The test data CHNt (n = 1246) were labeled by ten senior pediatric radiologists. The effects of data site differences, interpretation bias, and interobserver variability on BA assessment were evaluated. The differences between the AI models' and radiologists' clinical determinations of BA (normal, advanced, and delayed BA groups by using the Brush data) were evaluated by the chi-square test and Kappa values. The heatmaps of CHNm-CHNt were generated by using Grad-CAM. RESULTS: We obtained an MAD value of 0.42 years on CHNm-CHNt; this result indicated an appropriate accuracy for the whole group but did not indicate an accurate estimation of individual BA because with a kappa value of 0.714, the agreement between AI and human clinical determinations of BA was significantly different. The features of the heatmaps were not fully consistent with the human vision on the X-ray films. Variable performance in BA estimation by different AI models and the disagreement between AI and radiologists' clinical determinations of BA may be caused by data biases, including patients' sex and age, institutions, and radiologists. CONCLUSIONS: The deep learning models outperform external validation in predicting BA on both internal and joint datasets. However, the biases and errors in the models' clinical determinations of child development should be carefully considered. KEY POINTS: • With a kappa value of 0.714, clinical determinations of bone age by using AI did not accord well with clinical determinations by radiologists. • Several biases, including patients' sex and age, institutions, and radiologists, may cause variable performance by AI bone age models and disagreement between AI and radiologists' clinical determinations of bone age. • AI heatmaps of bone age were not fully consistent with human vision on X-ray films.

Ultra-High Temperature Molten Oxide Electrochemistry.

Recently, the ultra-high temperature electrochemistry (UTE, about >1000 °C) has emerged, which represents an exploration to extend the temperature limit of human technology in electrochemical engineering. UTE has far-reaching impact on revolutionary low-carbon metal extraction and the in situ production of oxygen for deep-space exploration. It is hence of urgency to systematically summarize the development of UTE. In this Review, the basic concepts of UTE and the physicochemical properties of molten oxides are analyzed. The principles in the design of inert anodes for the oxygen evolution reaction in molten oxides are discussed, which forms a solid basis for the in situ production of oxygen from simulated lunar regolith by UTE. Furthermore, liquid metal cathodes for revolutionary titanium extraction and ironmaking/steelmaking are highlighted. With emphasis on the key challenges and perspectives, the Review can provide valuable inspiration for the rapid advancement of UTE.

Stable Quasi-Solid-State Aluminum Batteries.

Nonaqueous rechargeable aluminum batteries (RABs) of low cost and high safety are promising for next-generation energy storage. With the presence of ionic liquid (IL) electrolytes, their high moisture sensitivity and poor stability would lead to critical issues in liquid RABs, including undesirable gas production, irreversible activity loss, and an unstable electrode interface, undermining the operation stability. To address such issues, herein, a stable quasi-solid-state electrolyte is developed via encapsulating a small amount of an IL into a metal-organic framework, which not only protects the IL from moisture, but creates sufficient ionic transport network between the active materials and the electrolyte. Owing to the generated stable states at both positive-electrode-electrolyte and negative-electrode-electrolyte interfaces, the as-assembled quasi-solid-state Al-graphite batteries deliver specific capacity of ≈75 mA h g-1 (with positive electrode material loading ≈9 mg cm-2 , much higher than that in the conventional liquid systems). The batteries present a long-term cycling stability beyond 2000 cycles, with great stability even upon exposure to air within 2 h and under flame combustion tests. Such technology opens a new platform of designing highly safe rechargeable Al batteries for stable energy storage.

Electrodeposition of crystalline silicon films from silicon dioxide for low-cost photovoltaic applications.

Crystalline-silicon solar cells have dominated the photovoltaics market for the past several decades. One of the long standing challenges is the large contribution of silicon wafer cost to the overall module cost. Here, we demonstrate a simple process for making high-purity solar-grade silicon films directly from silicon dioxide via a one-step electrodeposition process in molten salt for possible photovoltaic applications. High-purity silicon films can be deposited with tunable film thickness and doping type by varying the electrodeposition conditions. These electrodeposited silicon films show about 40 to 50% of photocurrent density of a commercial silicon wafer by photoelectrochemical measurements and the highest power conversion efficiency is 3.1% as a solar cell. Compared to the conventional manufacturing process for solar grade silicon wafer production, this approach greatly reduces the capital cost and energy consumption, providing a promising strategy for low-cost silicon solar cells production.

Electrochemical Production of Si without Generation of CO2 Based on the Use of a Dimensionally Stable Anode in Molten CaCl2.

The current Si production process is based on the high-temperature (1700 °C) reduction of SiO2 with carbon that produces large amounts of CO2 . We report an alternative low-temperature (850 °C) process based on the reduction of SiO2 in molten CaCl2 that does not produce CO2 . It utilizes an anode material (Ti4 O7 ) capable of sustained oxygen evolution. Two types of this anode material, dense Ti4 O7 and porous Ti4 O7 , were tested. The dense anode showed a better performance. The anode stability is attributed to the formation of a protective TiO2 layer on its surface. In situ periodic current reversal and ex situ H2 reduction could be used for extending the lifetime of the anodes. The findings show that this material can be applied as a recyclable anode in molten CaCl2 . Si wires, films, and particles were deposited with this anode under different cathodic current densities. The prepared Si film exhibited ≈30-40 % of the photocurrent response of a commercial p-type Si wafer, indicating potential use in photovoltaic cells.

An investigation into the carbon nucleation and growth on a nickel substrate in LiCl-Li2CO3 melts.

The electrochemical deposition of carbon materials has been performed in LiCl-Li2CO3 melts using a Pt anode and a nickel cathode. Cyclic voltammetry and constant voltage electrolysis are conducted to investigate the electrode reactions, and the results prove that solid carbon is the only product from the cathodic reduction. Short-term electrolysis at 750 °C for 3, 10 and 20 s has been applied to study the formation and growth of the varied carbon microstructures. All of the results demonstrate that the morphologies of the deposited carbon are significantly affected by the cathode substrates, which may show different catalyzing effects on carbon nucleation. Two primary morphologies, quasi-spherical and nanofiber structures are observed at the nickel plate cathodes during the electrolysis and the formation and growth of carbon nanofibers are easily enhanced by using a high cell voltage. However, only a quasi-spherical structure is found on the molybdenum cathode substrate.

Direct Conversion of Greenhouse Gas CO2 into Graphene via Molten Salts Electrolysis.

Producing graphene through the electrochemical reduction of CO2 remains a great challenge, which requires precise control of the reaction kinetics, such as diffusivities of multiple ions, solubility of various gases, and the nucleation/growth of carbon on a surface. Here, graphene was successfully created from the greenhouse gas CO2 using molten salts. The results showed that CO2 could be effectively fixed by oxygen ions in CaCl2-NaCl-CaO melts to form carbonate ions, and subsequently electrochemically split into graphene on a stainless steel cathode; O2 gas was produced at the RuO2-TiO2 inert anode. The formation of graphene in this manner can be ascribed to the catalysis of active Fe, Ni, and Cu atoms at the surface of the cathode and the microexplosion effect through evolution of CO in between graphite layers. This finding may lead to a new generation of proceedures for the synthesis of high value-added products from CO2, which may also contribute to the establishment of a low-carbon and sustainable world.