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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 NO2detection 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 NO2was 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 NO2by 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, NO2adsorption 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 NO2and other specific gases (e.g., H2O, SO2) with the MOF’s surface, leading to orders of magnitude decrease in MOF resistance and enhanced NO2detection. Understanding and leveraging these coadsorbed gases will further improve the gas detection properties of MOF materials.

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 °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.

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 °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.

A combination of electrodeposition and thermal reduction methods have been utilized for the synthesis of ligand-free FeNiCo alloy nanoparticles through a high-entropy oxide intermediate. These phases are of great interest to the electrocatalysis community, especially when formed by a sustainable chemistry method. This is successfully achieved by first forming a complex five element amorphous FeNiCoCrMn high-entropy oxide (HEO) phase via electrodeposition from a nanodroplet emulsion solution of the metal salt reactants. The amorphous oxide phase is then thermally treated and reduced at 570–600 °C to form the crystalline FeNiCo alloy with a separate CrMnOxcophase. The FeNiCo alloy is fully characterized by scanning transmission electron microscopy and energy-dispersive X-ray spectroscopy elemental analysis and is identified as a face-centered cubic crystal with the lattice constant a= 3.52 Å. The unoptimized, ligand-free FeNiCo NPs activity toward the oxygen evolution reaction is evaluated in alkaline solution and found to have an ∼185 mV more cathodic onset potential than the Pt metal. Beyond being able to synthesize highly crystalline, ligand-free FeNiCo nanoparticles, the demonstrated and relatively simple two-step process is ideal for the synthesis of tailor-made nanoparticles where the desired composition is not easily achieved with classical solution-based chemistries.

In the pursuit of highly stable and selective metal–organic frameworks (MOFs) for the adsorption of caustic acid gas species, an entire series of rare earth MOFs have been explored. Each of the MOFs in this series (RE-DOBDC; RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu; DOBDC = 2,5-dihydroxyterepthalic acid) was synthesized in the tetragonal space group I4/m. Crystallized MOF samples, specifically Eu-DOBDC, were seen to have a combination of monodentate and bidentate binding when synthesized under typical reaction conditions, resulting in a contortion of the structure. However, extended crystallization times determined that this binding is kinetically controlled and that the monodentate binding option was crystallographically eliminated by extended reaction times at higher temperatures. Furthermore, this series allows for the direct study of the effect of the metal center on the structure of the of the MOF; herein, the lanthanide metal ionic radii contraction across the periodic table results in a reduction of the MOF pore size and lattice parameters. Scanning electron microscopy–energy-dispersive spectroscopy was used to investigate the stages of crystal growth for these RE-DOBDC MOFs. All MOFs, except Er-DOBDC had a minimum of two stages of growth. These analogues were demonstrated by analysis of neutron diffraction (PND) to exhibit a cooperative rotational distortion of the secondary building unit, resulting in two crystallographically distinct linker sublattices. Computational modeling efforts were used to show distinct differences on acid gas (NO2and SO2) binding energies for RE-DOBDC MOFs when comparing the monodentate/bidentate combined linker with the bidentate-only linker crystal structures.

A novel sensor composed of a nickel-loaded SSZ-13 zeolite membrane on interdigitated electrodes (IDEs) is demonstrated as a direct electrical NO2readout sensor. This NO2adsorbent is made by homogeneously loading SSZ-13 zeolite with nickel(II) through a liquid-phase ion-exchange procedure. Exposure of the zeolite-based sensor to trace NO2gas elicits an electrical impedance response measured at a single frequency. The sensor shows the same final change in impedance magnitude upon equilibration to different concentrations of trace NO2in N2, suggesting that the occupation and eventual saturation of adsorption sites lead to the impedance change. However, the NO2concentration can be determined through analysis of the rate of impedance change, where lower concentrations of NO2lead to larger time constants with a logarithmic relationship to the NO2concentration. Two time constants were observed from the linearized impedance plots, a fast one (τ1) and a slow one (τ2), where τ2showed a larger dependence on the NO2concentration, increasing faster than τ1as the NO2concentration decreased. Furthermore, the Ni-SSZ-13 sensor response is partially reversible in an inert gas environment, indicating the reversible adsorption of NO2at nickel surface sites. Under exposure to humid air, differentiation between humid air and dry 5 ppm NO2is accomplished by examination of the real component of the impedance signal. The resulting NO2atmosphere shows an increase in the real component (more resistive), whereas the humid air shows a decrease (more capacitive). These results indicate that control of metal-ion loading into SSZ-13 may allow these NO2selective catalytic reduction catalysts to be further leveraged as low-temperature NO2sensors.

The chemical complexity of single-phase multicationic oxides, commonly termed high entropy oxides (HEOs), enables the integration of conventionally incompatible metal cations into a single-crystalline phase. However, few studies have effectively leveraged the multicationic nature of HEOs for optimization of disparate physical and chemical properties. Here, we apply the HEO concept to design robust oxidation catalysts in which multicationic oxide composition is tailored to simultaneously achieve catalytic activity, oxygen storage capacity, and thermal stability. Unlike conventional catalysts, HEOs maintain single-phase structure, even at high temperature, and do not rely on the addition of expensive platinum group metals (PGM) to be active. The HEOs are synthesized through a facile, relatively low temperature (500 °C) sol–gel method, which avoids excessive sintering and catalyst deactivation. Nanostructured high entropy oxides with surface areas as high as 138 m2/g are produced, marking a significant structural improvement over previously reported HEOs. Each HEO contained Ce in varying concentrations, as well as four other metals among Al, Fe, La, Mn, Nd, Pr, Sm, Y, and Zr. All samples adopted a fluorite structure. First row transition metal cations were most effective at improving CO oxidation activity, but their incorporation reduced thermal stability. Rare earth cations were necessary to prevent thermal deactivation while maintaining activity. In sum, our work demonstrates the utility of entropy in complex oxide design and a low-energy synthetic route to produce nanostructured HEOs with cations selected for a cooperative effect toward robust performance in chemically and physically demanding applications.

Nanoporous, gas-selective membranes have shown encouraging results for the removal of CO2from flue gas, yet the optimal design for such membranes is often unknown. Therefore, we used molecular dynamics simulations to elucidate the behavior of CO2within 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 CO2is reduced due to interactions between the CO2oxygens 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 CO2will transfer spontaneously from the aqueous to the IL phase.

Molten salt electrolytes show promise as safe, effective elements of emerging low to intermediate temperature molten sodium batteries. Here we investigate the NaI-AlCl3 molten salt system for its electrochemical and physical properties at 150 and 180degC, temperatures recently used to demonstrate a new NaI battery using this molten salt system. Molten salt compositions ranging from 20-75% NaI were prepared and electrochemically interrogated with carbon fiber ultramicroelectrodes utilizing cyclic voltammetry, chronoamperometry, and differential pulse voltammetry. Results indicate that at very high or very low NaI concentrations, secondary phases present hinder diffusion of the redox-active species, potentially impacting the current density of the system. Furthermore, a concentration-independent chronoamperometric analysis technique was leveraged to determine effective diffusion coefficients of active I[?] in the melt phase. Collectively, the physical characterization and electrochemical properties of the tested salts indicate that the catholyte composition can significantly affect the physical state, current density, ionic diffusion, and voltage window of these promising NaI-AlCl3 molten salt battery catholytes.

Iodide redox reactions in molten NaI/AlCl3are shown to generate surface-blocking films, which may limit the useful cycling rates and energy densities of molten sodium batteries below 150 °C. An experimental investigation of electrode interfacial stability at 110 °C reveals the source of the reaction rate limitations. Electrochemical experiments in a 3-electrode configuration confirm an increase of resistance on the electrode surface after oxidation or reduction current is passed. Using chronopotentiometry, chronoamperometry, cyclic voltammetry, and electrochemical impedance spectroscopy, the film formation is shown to depend on the electrode material (W, Mo, Ta, or glassy carbon), as well as the Lewis acidity and molar ratio of I−/I3−in the molten salt electrolytes. These factors impact the amount of charge that can be passed at a given current density prior to developing excessive overpotential due to film formation that blocks the electrode surface. The results presented here guide the design and use of iodide-based molten salt electrolytes and electrode materials for grid scale battery applications.

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 CO2in 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 CO2separation. We analyzed diffusion constants and viscosity, which reveals that CO2molecules 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 CO2and HCO3–. Free energy profiles demonstrate that CO2exhibits 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 CO2transport in the RTILs corroborates the model. These findings strongly suggest that hydrophobic RTILs could serve as a promising option for selectively transporting CO2from aqueous media and concentrating it as a preliminary step toward storage.

Here we report the use of Fast-Scan Cyclic Voltammetry (FSCV) to study transient nanoparticle (NP) collision experiments and determine the apparent heterogeneous electron-transfer (ET) rate constant koand electrocatalytic activity of single NPs. Continuous potential scanning at fast scan rates, for example, up to 500 V/s, and background subtraction enable voltammetric study of single NPs when they collide on a carbon ultramicroelectrode (UME). The FSCV results indicate a substantial potential shift in the CV response of single NPs due to increased mass transfer and it is necessary to consider this effect when analyzing single NP electrocatalysts. FSCV results also reveal that single particle deactivation is associated with a decrease in peak current and an increase in overpotential. Importantly, the use of FSCV coupled with numerical simulations enabled the determination of koon single NPs, confirming an increase in catalytic activity with decreasing particle size. Moreover, the Gibbs free energy of activation can be estimated from the ko. This work further confirms that FSCV is a powerful method in studying dynamic electrochemical processes such as single NP collisions.

Here we report the preparation and electrochemical characterization of single Pt nanowire electrodes with radii ranging from 25 to 130 nm for their electrocatalytic activity toward the oxygen reduction reaction (ORR). Ultralong Pt nanowires were prepared by a laser pulling process. Single Pt nanowire electrodes were fabricated by metal deposition and lithography patterning to expose just a small portion (10–20 μm in length) of the nanowire for electrocatalytic studies. These wires were characterized thoroughly using cyclic voltammetry, under potential deposition of copper, and scanning electron microscopy. We have shown that their electrocatalytic activity toward ORR slightly decreases as the nanowire radius is reduced as measured by the onset potential. Additionally, the current density for the ORR decreases and the mass activity increases as the wire becomes smaller. The faradaic current was modeled on single nanowire electrodes, and the current was found to match the experimental value with outer-sphere redox species. However, significant variance was shown between the modeled ORR current and the measured value. The deviation of the ORR current on the wires is likely due to the presence of F–ions on Pt and the increased adsorption energy of oxygenated species, thus leading to a hindering of the reaction kinetics of the catalytic process. This is confirmed by Tafel plots where the slope is seen to change at a lower potential as compared to bulk polycrystalline Pt.

The oxidation of iodide in NaI-AlBr3, NaI-AlCl3, and NaI-GaCl3molten salts was analyzed using simulation software to extract relevant kinetic parameters. The experimental oxidation potentials were ordered AlCl3< AlBr3< GaCl3, with higher oxidation potentials correlating with softer Lewis acidity of the metal halide. An iodide oxidation and metal halide speciation model was developed and simulated to fit the electrochemical response, enabling determination of electrochemical charge transfer parameters and chemical equilibrium constants. NaI-AlBr3displayed the fastest electron transfer rates yet showed the lowest current densities. All salts revealed smaller than expected current densities, explained by equilibrium between various species, where some are not electrochemically active at the studied potentials. These equilibrium reactions are due to the various metal halide species, controlling the reactant concentration of iodide and the resultant current. We hypothesize the electrochemically active iodide species, present as a metal halide monomer (MX3I−), is decreased dramatically from the expected concentration, sequestered as a more stable metal halide dimer species (M2X6I−) with a higher oxidation potential. Traditional Tafel analysis of the experimental data supports the validity of the simulations. These results increase understanding of iodide oxidation in low-temperature Lewis acidic molten salts and inform task-specific molten salt design.

NaI-AlBr3is a very appealing low melting temperature (<100 °C), salt system for use as an electrochemically-active electrolyte. This system was investigated for its electrochemical and physical properties with focus to energy storage considerations. A simple phase diagram was generated; at >100 °C, lower NaI concentrations had two partially miscible liquid phases, while higher NaI concentrations had solid particles. Considering the fully molten regime, electrical conductivities were evaluated over 5-25 mol% NaI and 110 °C-140 °C. Conductivities of 6.8-38.9 mS cm?1were observed, increasing with temperature and NaI concentration. Effective diffusion coefficients of the I?/I3?redox species were found to decrease with both increasing NaI concentration and increasing applied potential. Regardless, oxidation current density at 3.6 V vs Na/Na+was observed to increase with increasing NaI concentration over 5-25 mol%. Finally, the critical interface between the molten salt electrolyte and electrode materials was found to significantly affect reaction kinetics. When carbon was used instead of tungsten, an adsorbed species, most likely I2, blocked surface sites and significantly decreased current densities at high potentials. This study shows the NaI-AlBr3system offers an attractive, low-temperature molten salt electrolyte that could be useful to many applied systems, though composition and electrode material must be considered.

Molten salt electrolytes show promise as safe, effective elements of emerging low to intermediate temperature molten sodium batteries. Here we investigate the NaI-AlCl3molten salt system for its electrochemical and physical properties at 150 and 180°C, temperatures recently used to demonstrate a new NaI battery using this molten salt system. Molten salt compositions ranging from 20–75% NaI were prepared and electrochemically interrogated with carbon fiber ultramicroelectrodes utilizing cyclic voltammetry, chronoamperometry, and differential pulse voltammetry. Results indicate that at very high or very low NaI concentrations, secondary phases present hinder diffusion of the redox-active species, potentially impacting the current density of the system. Furthermore, a concentration-independent chronoamperometric analysis technique was leveraged to determine effective diffusion coefficients of active I−in the melt phase. Collectively, the physical characterization and electrochemical properties of the tested salts indicate that the catholyte composition can significantly affect the physical state, current density, ionic diffusion, and voltage window of these promising NaI-AlCl3molten salt battery catholytes.