Low temperature molten sodium batteries promise low-cost, grid-scale energy storage using earth-abundant materials. To be economically viable, they must demonstrate high current (power) at relevant discharge times. Here a low temperature (135 degrees C) molten sodium battery with a NaI-AlCl3 molten salt catholyte and NaSICON separator is explored, minimizing cell ohmic resistances and avoiding precipitation reactions in the molten salt catholyte. Operating currents were increased by as much as 100 & times; compared to the energy-dense baseline design (197 Wh kg-1 and 248 mAh cm-2). Cells were cycled 100 times at 50 mA cm-2 (150 mW cm-2), averaging 99.8% coulombic efficiency and 79.7% energy efficiency for 30% of the theoretical capacity. Charging currents up to 250 mA cm-2 (1070 mW cm-2) are demonstrated for 10% of the theoretical capacity. Toward extending the accessible capacity in these high current systems, the volume ratio of catholyte molten salt to a graphite felt current collector was varied, altering (1) the local current density in the graphite felt and (2) the discharge time at a fixed current density. Optimizing the catholyte : felt ratio dramatically increased capacity utilization to 60% (102 mAh cm-2) at 50 mA cm-2 (150 mW cm-2) charge and 10 mA cm-2 discharge. This catholyte : felt ratio of 1.4 significantly decreased the cell impedance, minimizing electrode blocking effects seen previously in NaI-AlCl3 catholytes. Together, these high areal loadings and impressive power outputs demonstrate how low temperature molten sodium batteries can compete with commercialized sodium systems operating at more than twice the temperature.

Chemically robust, selective, low-power sensors could aid in the monitoring of nitro-based propellant compounds. The metal-organic framework (MOF) M-MOF-74 (M = Ni, Mg, NixMg1-x) offers exceptional sensitivity and selectivity toward trace amounts of NO2, a primary degradation product of nitro-group-containing molecules, such as propellants. MOF-74-based impedance sensors have been developed to detect nitro group off-gassing due to propellant aging. Herein, additional studies were conducted to test the limits of sensor performance and investigate the long-term stability of these sensors when exposed to elevated NO2 concentrations indicative of propellant aging. Tested sensor materials consisted of either monometallic or mixed metal M-MOF-74 (M = Ni, Mg, NixMg1-x) films fabricated through drop-cast or solvothermal synthesis. Repeated exposure studies were performed over a 24-week period, with exposures to 5 ppm NO2 in nitrogen occurring every 4 weeks at two different temperatures, with a reactivation step in between the change in temperature. The sensor performance was evaluated by analysis of the changing sensor impedance before and after repeated exposures. Both the MOF's metal center and fabrication method contributed to the sensor deactivation rate. Structural and chemical analysis of the MOF-sensors indicated that the analyte, NO2, led to sensor degradation through a proposed mechanism involving NO2 insertion and metal nitrite complexation. Understanding the abilities and limitations of these types of MOF-based sensors under aggressive NO2 conditions will further the development of devices specifically tailored for the relevant service environment conditions.

High capacity, lightweight multivalent aluminum (Al) is attractive as an energy storage active material. Current Al containing electrolytes are prohibitively air/moisture sensitive and do not cycle under ambient conditions. Here, promising, reversible electrochemical behavior of Al-containing, air-stable ionic liquids is demonstrated through the addition of the Lewis-base complexing agent, NaI.

An inorganic Fe and Al halide-based, low-temperature molten salt catholyte is described, which, when paired with a molten sodium anode, has high operating potentials rivaling those of Li ion batteries. The newly developed catholyte consists of metal halides FeCl3/FeCl2-AlCl3-NaCl and is intended to cycle between Fe3+/Fe2+ redox couples in the molten salt. The multicomponent molten salt was initially evaluated for phase behavior and basic electrochemical behavior before full battery testing. The assembled battery, utilizing a 20:35:45 (FeCl3:AlCl3:NaCl) composition, with a 50.83 Ah/kg theoretical gravimetric capacity and a specific energy of 176.95 Wh/kg, was cycled at variable depths of discharge (DoD) and current densities to determine its cycling efficiencies and limitations. Preliminary cycling tests showed two operational potential regimes, with higher potential, 3.91 V (vs Na/Na+), at low DoD and lower potential, 3.39 V (vs Na/Na+), at high DoD with excellent energy efficiencies and cycling behavior under both regimes.

Metal-organic framework (MOF)-based electrical impedance sensors are a growing class of sensors that show utility in the detection of environmentally toxic gases. Detection of trace NH3 has been particularly difficult to design due to the low electrical response of NH3. However, this has been circumvented by judiciously selecting a MOF that has enhanced electrical response to NH3 due to coadsorption with predominant atmospheric gases such as water. Herein, an MOF Cu(cyhdc) based sensor has been successfully demonstrated for the enhanced detection of environmentally toxic NH3 gas. The sensor shows a change in impedance when it is exposed to 5 ppm of NH3. At 20 °C, >30% relative humidity (RH) is necessary to elicit a change, with the response increasing with RH and reaching 3170× at 92% RH, producing the largest published response to NH3 for a MOF direct electrical sensor. In the absence of water, no change is observed toward 5 ppm of NH3 over 20-50 °C. This electrical response is largely driven by huge decreases in the imaginary component of the impedance, attributed to increased capacitance at the surface of the MOF crystallites upon NH3 and H2O coadsorption. Complementary structural and microstructural characterization proves that the Cu(cyhdc) crystalline structure and morphology remain intact under trace NH3 and/or H2O adsorption. Despite extremely low NH3, loadings seen in TGA, XPS, and FT-IR confirm NH3 and H2O adsorption, and changes to the metal-carboxylate IR peak positions are observed upon NH3 adsorption. Concentrated primarily at the outer surfaces of the MOF crystallites, this NH3 and H2O coadsorption effectively increases the surface capacitance across the Cu(cyhdc) powder and enables direct electrical detection of trace NH3. Together, these results demonstrate how coadsorption of specific molecules (H2O) can be used to enable the electrical detection of trace toxic gases that would otherwise not have produced a MOF sensor response.

The tunability of metal-organic frameworks (MOFs) makes them exceptional materials for the development of highly selective, low-power sensors for toxic gas detection. Herein, we demonstrate enhanced detection of NO2 gas by a MOF-based electrical impedance sensor made using a unique mixed metal MOF-on-MOF synthesis. A combined experimental and computational study was performed using the exemplar NixMg1-x-MOF-74 to understand the fundamental structure-property relationships behind metal mixing and MOF film synthesis methods on sensor performance. Density functional theory results indicated that the presence of Ni in Mg-MOF-74 increased framework stability and increased the electron density of states at lower energies near the HOMO, as well as enhanced the NO2-Mg adsorption interaction. Impedance data of the NixMg1-x-MOF-74 films with larger Ni contents showed greater impedance change after exposure to 1 ppm of NO2 gas. Furthermore, when synthesized through either a drop-cast or direct solvothermal film growth approach, the monometallic Ni-based sensors had the best performance. However, the mixed metal NixMg1-x-MOF-74 sensors synthesized through a MOF-on-MOF approach resulted in the highest impedance change, outperforming all monometallic Ni-based sensors. In particular, the mixed metal Ni-on-Mg-MOF-74 film was the best-performing sensor with an impedance change of 309 upon trace NO2 exposure. Change in impedance response after NO2 exposure was improved by 52% compared to the best monometallic Ni-on-Ni-MOF-74 sensor. Structural analysis of the Ni-on-Mg film showed that the first Mg-MOF-74 layer acts as a structural template controlling the structural features of the final film after metal exchange with Ni. This led to improved film quality, evidenced by the greater crystallinity and larger MOF grain sizes, and resulted in enhanced sensor performance which was not achievable through other metal mixing methods. Altogether, this study identifies structure-property relationships and synthetic templating methods that inform MOF-based sensor design, allowing for improved detection of toxic compounds.

Rare-earth metal-organic frameworks (REMOFs) based on polynuclear metal clusters are an emerging class of materials that have shown promise for CO2 capture and conversion. In this work, copper nanoparticles (CuNPs) were successfully installed on a cluster-based Y(III) MOF to yield a composite material, CuNP-Y-TBAP. The abundance of Cu binding sites on the Y(III) clusters allowed a remarkably high Cu loading to be achieved, and electron microscopy demonstrated that the MOF-supported CuNPs are exceptionally small and monodisperse. CuNP-Y-TBAP was found to be an active heterogeneous catalyst for electrochemical reduction of CO2, yielding CO and CH4 as the primary CO2 reduction products.

The need for clean, renewable energy has driven the expansion of renewable energy generators, such as wind and solar. However, to achieve a robust and responsive electrical grid based on such inherently intermittent renewable energy sources, grid-scale energy storage is essential. The unmet need for this critical component has motivated extensive grid-scale battery research, especially exploring chemistries “beyond Li-ion”. Among others, molten sodium (Na) batteries, which date back to the 1960s with Na-S, have seen a strong revival, owing mostly to raw material abundance and the excellent electrochemical properties of Na metal. Recently, many groups have demonstrated important advances in battery chemistries, electrolytes, and interfaces to lower material and operating costs, enhance cyclability, and understand key mechanisms that drive failure in molten Na batteries. For widespread implementation of molten Na batteries, though, further optimization, cost reduction, and mechanistic insight is necessary. In this light, this work provides a brief history of mature molten Na technologies, a comprehensive review of recent progress, and explores possibilities for future advancements.

The development of stable non-noble metal bifunctional catalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are essential for technologies such as reversible metal air batteries. We have shown that synthesis of high entropy alloy nanoparticles formed through a novel synthesis method where metal precursor containing aqueous nanodroplets freely diffusing in an emulsion solution, collide with an electrode surface and the metal precursors are reduced, thus forming a homogeneous particle containing the predetermined metal composition. Afterwards, the particles are annealed at a low temperature causing a phase separation of the metals, forming a metal nanoparticle that is surrounded by a stabilizing metal oxide. We have demonstrated that equimolar Fe, Ni, Co, Cr, and Mn solutions produce bifunctional FeNiCo metal nanoparticles surrounded by CrMnOx that are very active for both OER and ORR. The synthesis technique also affords the flexibility to tailor composition of the nanoparticles and potentially provide access to previously difficult to synthesize compositions and may alter the electrocatalytic reaction pathways and activities. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525

The advanced in situ detection of gaseous pollutants, such as NOx, is of great interest in many applications, including the automotive, manufacturing, energy, and defense industries. Current challenges with gas sensors include everything from power consumption, lifetime, and chemical fouling considerations to signal interference from other secondary gases present in complex environments. In this study, we continue the advancement of our low power metal oxide framework (MOF)-based sensors, previously successfully demonstrated for gaseous I2 and NO2 detection.1-5 The sensors, composed of Pt interdigitated electrodes (IDEs) with a nanoporous adsorbent layer, can be tuned to selectively adsorb gases of interest through judicious material selection, and the electrical response directly correlated to gas concentration. The sensors have been successfully demonstrated for the selective detection of trace NOx (1ppm), in complex environments (e.g., presence of H2O, CO2, SO2).6 Direct growth of thin MOF films on surface functionalized IDEs has been shown to result in increased sensor sensitivity and a faster response time.7 Our recent work has investigated the long-term durability and sensitivity of the sensors to NOx, when exposed to dry and humid environments at room temperature and 74°C over the course of three months. It was observed that the Ni-MOF-74-based sensors exhibited superior sensitivity in comparison to Mg-MOF-74 on initial exposure to NOx. On the other hand, the Mg-MOF-74-based sensors exhibited less degradation of the sensor response from prolonged humidity exposure. These findings led to our current work, exploring mixed metal MOF-on-MOF sensors for optimization of both sensor response and long-term performance. Preliminary experimental and modeling performed to elucidate the influence of metal mixing on these metrics will be presented. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525. References Small, L.J and Nenoff, T.M., ACS Appl. Mater. Interfaces, 2017, 9 (51), 44649. Small, L.J., et al., Meso. Mater., 2019, 280, 82. Small, L.J., et al., ACS App. Mater. Interfaces, 2019, 11 (31), 27982. Small, L.J., et al., Funct. Mater., 2020, 30 (50), 2006598. Small, L.J., et al., I&ECR, 2021, 60, 21, 7998. Small, L.J., et al., ACS Appl. Mater. Interfaces, 2023, 15 (31), 37675. Henkelis, S.E, et al., Membranes, 2021, 11 (3), 176. Percival, S.J., et al., I&ECR, 2023, 62, (5), 2336.

The electrochemical CO 2 reduction reaction (CO2RR) is emerging as a promising method to produce carbon neutral chemical feedstocks. Additives or co-catalysts in the electrolyte can lower the activation barrier for CO2RR, yielding faster conversion rates at lower voltages. Recently, electrocatalytic CO 2 reduction mediated by ionic liquids (IL) has been reported but with conflicting accounts to whether the ILs benefit CO 2 RR or adversely, the competing hydrogen evolution reaction (HER). We report the effect of added IL EMIM-Cl, specifically EMIM + , in aqueous solutions on the electrochemical CO2RR and competing HER on gold. We show the CO 2 reduction current increases by ~1.5x with the highest amount of added EMIM + relative to the pure electrolyte. Differential pulse voltammetry reveals an additional large change in the reduction current that only appears with both CO 2 and EMIM + present. Lastly, we use advanced electrochemical analysis methods to determine the reaction kinetics and preliminary evidence shows that the standard heterogeneous rate constant increases for the reaction at large overpotentials while the HER is suppressed at low overpotentials with EMIM + present. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Nanoporous, gas-selective membranes have shown encouraging results for the removal of CO2 from flue gas, yet the optimal design for such membranes is often unknown. Therefore, we used molecular dynamics simulations to elucidate the behavior of CO2 within aqueous and ionic liquid (IL) systems ([EMIM][TFSI] and [OMIM][TFSI]), both confined individually and as an interfacial aqueous/IL system. We found that within aqueous systems the mobility of CO2 is reduced due to interactions between the CO2 oxygens and hydroxyl groups on the pore surface. Within the IL systems, we found that confinement has a greater effect on the [EMIM][TFSI] system as opposed to the [OMIM][TFSI] system. Paradoxically, the larger and more asymmetrical [OMIM](+) molecule undergoes less efficient packing, resulting in fewer confinement effects. Free energy surfaces of the nanoconfined aqueous/IL interface demonstrate that CO2 will transfer spontaneously from the aqueous to the IL phase.

Nanoporous materials, including metal-organic frameworks (MOFs) and inorganic zeolites, are gaining attention as gas sensor materials due to their chemical selectivity and robustness. To advance industrial viability of these materials as sensors, long-term, variable environment testing is needed to evaluate their stability and continued chemical exposure response. Nanoporous materials-based direct electrical readout sensors were evaluated for 3 months under dry or humid conditions at 74 degrees C. The sensors were comprised of either Ni-MOF-74, Mg-MOF-74, or Ni-SSZ-13 zeolite. Additionally, we describe the development of multichambered sensor testing platforms that allows uninterrupted direct impedance monitoring of each sensor over long test periods. Results indicate relative stability in dry conditions for the sensors over time. In contrast, degradation of the active sensing material is evident in the humid environment. Collectively, these results demonstrate need for long-term testing of emerging nanoporous sensor materials under specific environmental conditions.

Chemically robust, low-power sensors are needed for the direct electrical detection of toxic gases. Metal-organic frameworks (MOFs) offer exceptional chemical and structural tunability to meet this challenge, though further understanding is needed regarding how coadsorbed gases influence or interfere with the electrical response. To probe the influence of competitive gases on trace NO2 detection in a simulated flue gas stream, a combined structure-property study integrating synchrotron powder diffraction and pair distribution function analyses was undertaken, to elucidate how structural changes associated with gas binding inside Ni-MOF-74 pores correlate with the electrical response from Ni-MOF-74-based sensors. Data were evaluated for 16 gas combinations of N2, NO2, SO2, CO2, and H2O at 50 °C. Fourier difference maps from a rigid-body Rietveld analysis showed that additional electron density localized around the Ni-MOF-74 lattice correlated with large decreases in Ni-MOF-74 film resistance of up to a factor of 6 × 103, observed only when NO2 was present. These changes in resistance were significantly amplified by the presence of competing gases, except for CO2. Without NO2, H2O rapidly (<120 s) produced small (1-3×) decreases in resistance, though this effect could be differentiated from the slower adsorption of NO2 by the evaluation of the MOF's capacitance. Furthermore, samples exposed to H2O displayed a significant shift in lattice parameters toward a larger lattice and more diffuse charge density in the MOF pore. Evaluating the Ni-MOF-74 impedance in real time, NO2 adsorption was associated with two electrically distinct processes, the faster of which was inhibited by competitive adsorption of CO2. Together, this work points to the unique interaction of NO2 and other specific gases (e.g., H2O, SO2) with the MOF's surface, leading to orders of magnitude decrease in MOF resistance and enhanced NO2 detection. Understanding and leveraging these coadsorbed gases will further improve the gas detection properties of MOF materials.

There is an imminent need for batteries capable of meeting the evolving demands of a growing stationary storage landscape, one rich in renewable energy sources and challenged with unprecedented global electrification. Key to our future electrical grid, desirable large-scale, long-duration batteries must not only meet demands for high power and energy density, but they must be safe, cost-effective, and should be sustainably sourced from earth-abundant materials. Here, we will highlight our research and development of low temperature molten salt batteries that employ active materials such as sodium, aluminum, iron, chloride, and iodide. We will demonstrate the promise of our approach through a low-temperature (100°C) molten sodium battery that comprises a molten sodium anode, a solid-state NaSICON ceramic separator, and a low-cost molten metal-halide salt catholyte. These batteries, with remarkable voltages greater than 3.1V, are capable of months of cycling as well as high current charge and discharge behavior, greater than 50mA/cm 2 . Here, we will not only highlight these performance metrics, but we will describe how we used specialized electrochemical and chemical characterization technique to understand the impacts of molten salt electrolyte composition, electrochemistry, and interfacial interactions on battery performance. By understanding the connections between specific electrochemical phenomena and battery chemistry, we have learned to improve battery performance through tuning of properties, such as molten salt Lewis acidity and current collector composition. This scientifically-motivated battery design approach continues to enable agile innovation and advances to realize the potential of molten salt batteries as a new, impactful stationary storage technology. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

A major hurdle in utilizing carbon dioxide (CO2) lies in separating it from industrial flue gas mixtures and finding suitable storage methods that enable its application in various industries. To address this issue, we utilized a combination of molecular dynamics simulations and experiments to investigate the behavior of CO2 in common room-temperature ionic liquids (RTIL) when in contact with aqueous interfaces. Our investigation of RTILs, [EMIM][TFSI] and [OMIM][TFSI], and their interaction with a pure water layer mimics the environment of a previously developed ultrathin enzymatic liquid membrane for CO2 separation. We analyzed diffusion constants and viscosity, which reveals that CO2 molecules exhibit faster mobility within the selected ILs compared to what would be predicted solely based on the viscosity of the liquids using the standard Einstein-Stokes relation. Moreover, we calculated the free energy of translocation for various species across the aqueous-IL interface, including CO2 and HCO3-. Free energy profiles demonstrate that CO2 exhibits a more favorable partitioning behavior in the RTILs compared to that in pure water, while a significant barrier hinders the movement of HCO3- from the aqueous layer. Experimental measurement of the CO2 transport in the RTILs corroborates the model. These findings strongly suggest that hydrophobic RTILs could serve as a promising option for selectively transporting CO2 from aqueous media and concentrating it as a preliminary step toward storage.

Low-temperature molten sodium batteries comprising molten sodium anodes, a NaSICON solid-state separator, and molten halide salt catholytes offer promise as low-cost, earth-abundant energy storage technologies. The emergence of a specific, high-voltage, sodium iodide (NaI)-based catholyte chemistry has prompted the evaluation of chemical and electrochemical properties of the molten salts, particularly at critical interfaces with high-performance NaSICON separators. Herein, batteries operated at 110 degrees C with NaI-AlCl3-based catholytes of differing Lewis acidities were evaluated. Batteries with >50 mol % AlCl3 (acidic catholytes) experienced a linear decline in energy efficiency during cycling, whereas batteries with >50 mol % NaI (basic catholytes) maintained >95% energy efficiency for 50 cycles (>80 days) at 2.5 mA cm(-2). A three-electrode cell was developed, enabling identification of the NaSICON-catholyte interface as the source of increased battery impedance. Complementary physical and chemical characterization of the NaSICON exposed to acidic and basic catholytes showed no changes in crystallinity, bulk morphology, or bulk chemical composition, but surface sensitive X-ray photoelectron spectroscopy (XPS), however, revealed subtle changes in local NaSICON surface chemistry. In addition, Raman spectroscopy indicated that stably performing basic catholytes lack the dimer species Al2Cl6I- present in acidic catholytes. Select thermodynamic and formal charge assessments suggest that preferential interactions between these acidic dimeric species and the NaSICON surface may be responsible for the observed increases in electrochemical impedance and degraded battery performance. These results indicate that maintaining a Lewis basic catholyte avoids such potentially deleterious interactions, enabling efficient and stable battery cycling.