Hydrogen production via ammonia decomposition on water microdroplets

Abstract

A method for hydrogen production includes providing an aqueous solution comprising ammonia, generating a spray in a reaction chamber from the aqueous solution, the spray comprising water microdroplets, and collecting hydrogen from the reaction chamber produced via decomposition of the ammonia along a gas-liquid interface of the water microdroplets.

Description

Atty. Docket No. 10110-25002AHYDROGEN PRODUCTION VIA AMMONIA DECOMPOSITION ON WATER MICRODROPLETSCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. provisional application entitled “Hydrogen Production via Ammonia Decomposition on Water Microdroplets,” filed February 7, 2025, and assigned Serial No. 63 / 755,669, the entire disclosure of which is hereby expressly incorporated by reference.BACKGROUND OF THE DISCLOSUREField of the Disclosure

[0002] The disclosure relates generally to hydrogen production from ammonia.Brief Description of Related Technology

[0003] Hydrogen (H2) is considered the cleanest energy source to address the global energy crisis and environmental pollution challenges, due to its renewability, zero-carbon nature, and environmentally benign characteristics. To date, steam methane reforming and water electrolysis have been the predominant methods for hydrogen production. However, the low volumetric energy density, high explosibility, and significant permeability in both liquefied and compressed forms substantially restrict H2storage and transportation. The use of transportable, carbon-free, and liquid-phase hydrogen carriers for on-site H2production provides an alternative pathway towards sustainable hydrogen economy.

[0004] Ammonia (NH3) stands out as a remarkable hydrogen carrier, as ammonia benefits from established global production and transportation networks. Particularly, NH3possesses high hydrogen content (17.6 wt.%) and energy density (12.3 GJ / m3), and the ability to be liquefied at ambient temperature and low pressure. The practical utilization of NH3for H2generation depends on robust and cost-effective ammonia decomposition, which is an endothermic reaction constrained by thermodynamics under mild conditions.

[0005] Thermocatalysis using metal catalysts (e.g., Ru, Ni, and Fe) is an extensively studied approach for hydrogen extraction from ammonia. Unfortunately, this method typicallyAtty. Docket No. 10110-25002A operates at high temperatures (400-600 °C), requiring continuous external heating and incurring massive energy consumption.

[0006] Photocatalysis and electrocatalysis have emerged as environmental-friendly methods for ammonia decomposition, powered by renewable energy sources like solar energy and wind power. Despite the sustainability, their applications suffer from low H2production efficiency, complex catalyst requirements, and susceptibility to chemical corrosion. In photocatalytic processes, hydroxyl radicals (•OH) formed from H2O / OH~ play a role in aqueous ammonia oxidation for environmental remediation and hydrogen production.SUMMARY OF THE DISCLOSURE

[0007] In accordance with one aspect of the disclosure, a method for hydrogen production includes providing an aqueous solution including ammonia, generating a spray in a reaction chamber from the aqueous solution, the spray including water microdroplets, and collecting hydrogen from the reaction chamber produced via decomposition of the ammonia along a gas-liquid interface of the water microdroplets.

[0008] In accordance with another aspect of the disclosure, a system for hydrogen production includes a reaction chamber configured to contain an aqueous solution, the aqueous solution including ammonia, an ultrasonic atomizer in fluid communication with the aqueous solution, the ultrasonic atomizer being configured to generate a spray in the reaction chamber from the aqueous solution, the spray including water microdroplets, and an output in fluid communication with the reaction chamber to collect hydrogen produced via decomposition of the ammonia along a gas-liquid interface of the water microdroplets.

[0009] In accordance with yet another aspect of the disclosure, a method for hydrogen production includes providing an aqueous solution to a reaction system, the aqueous solution including ammonia, applying an external voltage to the reaction system to generate an electric field at an interface of the aqueous solution, the electric field having a strength to initiate decomposition of the ammonia along the interface, and collecting hydrogen produced via the decomposition of the ammonia along the interface initiated by the electric field.

[0010] In connection with any one of the aforementioned aspects, the systems and / or methods described herein may alternatively or additionally include or involve any combination of one or more of the following aspects or features. The aqueous solution is alkaline. The aqueous solution has a pH above about 10. The spray is generated with an ultrasonic atomizer in fluid communication with the aqueous solution. The ultrasonic atomizer is disposed in the reaction chamber and immersed in the aqueous solution. TheAtty. Docket No. 10110-25002A ultrasonic atomizer is configured to generate a circulating spray. The ultrasonic atomizer includes an electronic, tunable sprayer. The aqueous solution has an ammonia concentration falling in a range from about 0.2 M NH3to about 1.0 M NH3. The method further includes, before generating the spray, purging the reaction chamber with an inert gas. The inert gas includes argon. Generating the spray includes purging the reaction chamber with an inert gas while generating the spray. Collecting the hydrogen includes passing effluent gas from the reaction chamber through a plurality of filters configured to remove NH3and a nitric oxide, and collecting an output of the plurality of filters. The reaction chamber resides at an ambient temperature. The reaction chamber is catalyst-free. The decomposition of the ammonia is not driven via irradiation of the reaction chamber. The ultrasonic atomizer is disposed in the reaction chamber in a position for immersion in the aqueous solution. The system further includes a liquid source in fluid communication with the reaction chamber, the fluid source being configured to provide the aqueous solution in a flow-type reaction configuration. The system further includes a gas source in fluid communication with the reaction chamber, the gas source being configured to provide an inert gas to the reaction chamber. The system further includes a plurality of filters in fluid communication with the reaction chamber to receive effluent gas from the reaction chamber, the plurality of filters being configured to remove NH3and a nitric oxide. The ultrasonic atomizer is one of a plurality of ultrasonic atomizers in fluid communication with the aqueous solution. The plurality of ultrasonic atomizers are configured to generate a circulating spray. The ultrasonic atomizer is configured to generate the water microdroplets for continuous operation without a circulating spray. The system further includes a gas source in fluid communication with the reaction chamber, the gas source being configured to provide, to the reaction chamber, gaseous ammonia and a carrier gas for the gaseous ammonia. The carrier gas is argon. The interface is a gas-liquid interface. The method further includes generating a spray from the aqueous solution such that the gas-liquid interface is defined by a microdroplet of the spray. The interface is a liquid-liquid interface.BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0011] For a more complete understanding of the disclosure, reference should be made to the following detailed description and accompanying drawing figures, in which like reference numerals identify like elements in the figures.

[0012] Figure 1 depicts graphical plots of performance of H2production from microdroplet-initiated NH3decomposition in accordance with several examples, including (a) H2production under different conditions over 3 hours of reaction, (b) H2yield rate from NH3Atty. Docket No. 10110-25002A splitting in microdroplets in long-term experimentation, and (c, d) effects of NH3-H2O concentration and solution pH on H2production over 2 hours of reaction. Unless otherwise stated, NH3-H2O concentration and solution pH were 0.5 M and 11.8, respectively.

[0013] Figure 2 depicts the roles of a gas-liquid interface and scalability of a method of H2production in accordance with several examples involving a 0.5 M NH3-H2O solution, including (a) a graphical view of the influence of spray height on NH3decomposition to release H2over 2 hours of reaction, (b) a schematic view of H2evolution performance with varying microdroplet number, (c) a graphical view of H2evolution rate in a large reaction chamber equipped with three sprayers, and (d) a graphical plot comparing the H2yield rate from NH3decomposition achieved by the disclosed methods and systems with various semiconductor-based photocatalytic techniques.

[0014] Figure 3 depicts reactive species analysis of the ammonia decomposition of the disclosed methods and systems, including (a) a schematic illustration of the processes for calculating the free energy profile of moving a NH3molecule from the gas phase (left) across the gas-liquid interface (middle) into bulk water (right), as well as graphical plots of (b) water density profile (g / cm3), (c) free energy for NH3molecule along the Z-axis direction, in which the shaded region represents the gas-liquid interface, and GDS represents the Gibbs division surface, (d) UV-vis absorption spectra for H2O2determination recorded when spraying 2 mM NaOH or 0.5 M NH3’H2O under Ar atmosphere, in which the solution pH was adjusted to approximately 10.3, (e) quantified H2O2production in the absence and presence of ammonia under ultrasonication for 3 hours, and (f) H2yield rate from NH3decomposition in water microdroplets in the presence of various radical scavengers.

[0015] Figure 4 depicts (a) a schematic of microdroplet-initiated ammonia decomposition proposed pathways and reaction energy barriers towards NH3decomposition for H2production at a gas-liquid interface, as well as graphical views of UV-vis absorption spectra for N2H4determination recorded (b) under Ar atmosphere as the reaction proceeded and (c) in the presence of radical scavenges (IPA and O2) after 3 hours of reaction.

[0016] Figure 5 depicts the free energy profiles of three reaction steps, including (a) Reaction 1, (b) Reaction 3, and (c) Reaction 5, during NH3decomposition under oriented external electric fields with strength of 0, 108and 109V / m, in which the number in the upper enlarged figures is the free energy barrier for the transition states, and the insets are the chemical structures of the transition states, and the balls represent H, N, and O atoms as noted.Atty. Docket No. 10110-25002A

[0017] Figure 6 is a schematic view of a system for hydrogen production via microdropletbased ammonia decomposition in accordance with one example.

[0018] Figure 7 is a flow diagram of a method for hydrogen production via microdropletbased ammonia decomposition in accordance with one example

[0019] Figure 8 depicts (a) bottom and (b) perspective photographs of a flow-type reaction system for hydrogen production via microdroplet-based ammonia decomposition in accordance with one example

[0020] The embodiments of the disclosed systems and methods may assume various forms. Specific embodiments are illustrated in the drawing and hereafter described with the understanding that the disclosure is intended to be illustrative. The disclosure is not intended to limit the invention to the specific embodiments described and illustrated herein.DETAILED DESCRIPTION OF THE DISCLOSURE

[0021] Methods and systems for hydrogen production via water microdroplet-based ammonia decomposition are described. The ammonia decomposition of the disclosed methods and systems may occur at room or other ambient temperature. The ammonia decomposition may also be realized without catalysts. The disclosed methods and systems also need not rely on, or otherwise involve, irradiation.

[0022] The water microdroplets provide gas-liquid interfaces configured to promote the ammonia decomposition. Water is generally inert in the bulk phase without high voltage, light irradiation, or a catalyst. However, water microdroplets exhibit significant reactivity at the gas-liquid interface, due to distinct physicochemical properties like an ultrahigh electric field, partial solvation effect, preferential molecular orientation, and pH alteration. The strong electric field may be on the order of magnitude of about 109V / m, and is capable of spontaneously initiating single-electron transfer between a hydroxide ion (OH-) and a hydronium ion (H3O+) to generate a hydroxyl radical (’OH) and a hydrogen radical (•H) / electron (e-) on droplet surfaces. The generated reactive species are capable of supporting chemical reactions that occur without any external trigger, such as H2O2 production, transition metal ions reduction, CO2 / N2 / CH4 conversion, per- and polyfluoroalkyl substances (PFASs) degradation, etc. Recently, Zare et al. found that the contact electrification at the liquid-solid interface between microdroplets and a CuO / Cu catalyst mesh can convert ammonia to nitrate and hydrogen, especially under UV light irradiation. This oxidation process was facilitated by ’OH and superoxide radical (O2--) and was likely due to the combined effects of liquid-solid and gas-liquid interface chemistry andAtty. Docket No. 10110-25002A photocatalysis. Dick et al., through using stochastic electrochemistry, demonstrated that the gas-liquid interface played a useful role in reaction rate acceleration in microdroplets.

[0023] Harnessing the gas-liquid interface of the water microdroplets offers a more desirable approach for ammonia decomposition, e.g., without the assistance of catalyst and photoirradiation. Examples of ammonia decomposition for H2production in catalyst-free water microdroplets under ambient conditions are described herein. A maximum H2evolution rate of 226.8 pmol / h was achieved, which notably surpasses previously reported values for photocatalysis. Mechanisms for -OH and *H synergistically triggered spontaneous NH3decomposition are also described through radical quenching analysis, intermediate detection, molecular dynamics (MD) simulation, and quantum chemical calculations. As described herein, the water microdroplet-initiated NH3decomposition is a scalable, sustainable, and effective strategy for on-site, decentralized H2production. The disclosed methods and systems may accordingly also be useful in ammonia waste recovery, environmental pollution mitigation, and efforts to address the energy crisis.

[0024] Although described in connection with catalyst-free examples, the disclosed methods and systems may utilize one or more different catalysts in other cases. For instance, metallic Ru, Ni, or Fe may be used as a catalyst in some cases to improve efficiency and the production rate of hydrogen. Alternatively or additionally, metal oxides or metal nitrides (e.g., InGaN) may be used as a catalyst in some cases to enhance hydrogen production performance. The composition of the aqueous solution used by the disclosed methods and systems may thus vary from the examples described herein.

[0025] Although described in connection with irradiation-free examples, the ammonia decomposition of the disclosed methods and systems may include or involve illuminating or irradiating the reaction chamber. For instance, the reaction chamber may be exposed to sunlight irradiation. Alternatively or additionally, the decomposition may be promoted or enhanced via other illumination, e.g., from LED devices.

[0026] The ammonia decomposition may or may not be implemented at room temperature or in other ambient conditions. For instance, the temperature of the liquid may be adjusted or otherwise controlled to high or other elevated temperatures (e.g., in a range from about 100 C to about 300 C) to improve ammonia decomposition efficiency.

[0027] Although described in connection with examples involving an electric field passively developed along the gas-liquid interface of water microdroplets, the disclosed methods and systems may alternatively or additionally use an electric field or energy applied at the gasliquid interface or liquid-solid interface to enable or enhance the ammonia decomposition. InAtty. Docket No. 10110-25002A these and other cases, the interface may or may not include or involve microdroplets. For instance, the interface may use a gas-liquid interface or a liquid-liquid interface, such as an oil-water interface.

[0028] Example methods and systems for H2production from NH3via water microdroplets are now described. In these examples, ultrasonic atomization was used to generate the microdroplets. Such atomization supports large scalability. Unless otherwise stated, the ultrasonic atomization generated microdroplets with diameters of about 10 to about 30 pm from a 0.5 M ammonia-water solution using a single sprayer. The hydrogen was produced under high-purity Ar atmosphere with a flow rate of 200 mL / min, and the effluent gases were sequentially absorbed by passing through 1 M H2SO4and 1 M NaOH, as illustrated, for example, in Figure 6.

[0029] The droplet size may fall in the above-referenced range and / or be controlled or varied in one or more ways. For instance, the droplet size may be varied or controlled via ultrasonic energy and frequency.

[0030] The hydrogen production was quantified by the flow rate of Ar and the hydrogen content in carrier gas, which was determined via gas chromatography (GC) and a thermal conductivity detector.

[0031] Control experiments in the absence of ammonia or without microdroplets were also performed. As shown in Figure 1 , part a, a negligible amount of H2was detected when spraying 2 mM NaOH in a flow-type reaction system, indicative of minimal H2generation from water microdroplets.

[0032] In experiments involving a 0.5 M NH3’H2O solution but without microdroplet generation, a small amount of H2with a yield rate of 1.76 pmol / h was produced by the ultrasonication (Figure 1 , part a). In this case, microdroplet generation was inhibited by positioning a quartz plate at or sufficiently near the surface of the bulk solution. This result indicates that ultrasound cavitation plays an insignificant role in the NH3decomposition to produce H2, probably because of inefficient generation of reactive species.

[0033] In contrast, in examples in which microdroplets are generated from a 0.5 MNH3-H2O solution via a circulating spray, a significant peak assigned to H2was observed in GC spectra, indicating hydrogen production via microdroplet-based ammonia decomposition. In this case, the H2evolution rate reached approximately 108.2 pmol / h (Figure 1, part a), which was about 61.4-fold greater than the value from ultrasound cavitation.Atty. Docket No. 10110-25002A

[0034] The results of long-term testing are shown in Figure 1 , part b. The H2yield rate showed a negligible change over 6 hours of atomization and thereafter slightly declined, likely due to the decrease in ammonia concentration. These test results demonstrate that water microdroplets can spontaneously and efficiently trigger ammonia decomposition for hydrogen production under ambient conditions, without any catalyst or light irradiation.

[0035] The H2production efficiency was further analyzed under various NH3-H2O concentrations and bulk solution pH conditions, with a reaction period of 2 hours. As shown in Figure 1 , part c, the H2yield rate increased at first, and then leveled off, with increasing ammonia concentration from 0.05 to 1 M. The plateau likely arose due to the limited production of reactive species in the microdroplets. In the presence of 0.5 M NH3-H2O, the H2evolution dramatically declined with decreasing bulk solution pH from 11.8 to 6.5 (Figure 1 , part d), indicating that alkaline conditions are more favorable to hydrogen production from ammonia. The acid-base property influences both the state of ammonia (NH4+ / NH3) and the generation of reactive species on the microdroplet surfaces. NH3molecules existing at high pH are more reactive with ’OH, which is useful for NH3decomposition, whereas the NH4+cations dominate at pH less than 9 and exhibit much lower reactivity. High OH-concentration is useful for generation of reactive species at the gas-liquid interface of the microdroplets, thus promoting the radical-triggered ammonia decomposition of the disclosed methods and system.

[0036] The transformation of ammonia-nitrogen during ammonia decomposition in microdroplets was further analyzed. The liquid-phase products were analyzed via the colorimetric method. Nitrate (NO3-) and nitrite (NO2-) were detected during the microdroplet reaction. However, their generation rates were much lower than the consumption rate of ammonia, which was stoichiometrically calculated from the hydrogen production performance. Based on these findings, the nitrogen atoms in the ammonia molecules may be converted to N2, nitrogen oxyanions (NO2-and NO3-), and nitric oxides (NO and NO2). Nitrate is a widely used chemical in the production of pharmaceuticals, explosives, and fertilizers. Therefore, the microdroplet-initiated ammonia decomposition of the disclosed methods and systems may be used to produce green hydrogen and nitrate.

[0037] The catalyst-free, water microdroplet-triggered ammonia decomposition of the disclosed methods and systems is also useful in connection with on-site H2generation and supply. It is useful for the hydrogen yield rate to be adjustable and / or sufficiently high to power devices like hydrogen fuel cells. The number of microdroplets, or essentially the abundance of gas-liquid interfaces, may determine the efficiency of the ammonia decomposition-based hydrogen production. As shown in Figure 2, part a, under identicalAtty. Docket No. 10110-25002A conditions, the H2 evolution rate increased with higher spray height. In other words, the H2 evolution rate increased as the number of microdroplets is increased. The spray height was controlled by adjusting the distance between a quartz plate and the bulk solution surface. The results confirm that the NH3decomposition-based H2production occurred in water microdroplets with abundant gas-liquid interfaces, and also establishes that the H2evolution performance may be further improved by increasing the number of microdroplets (Figure 2, part b).

[0038] To provide an example of the scalability of the ammonia decomposition-based H2production, a larger reaction system assembling three electronic sprayers was tested in connection with a 0.5 M NHs’FkO solution. As shown in Figure 2, part c, the H2 yield rate was approximately 127.1, 184.9, and 226.8 pmol / h, respectively, when one, two, and three sprayers operated independently. The amount of improvement in hydrogen evolution with an increasing number of sprayers may be a result of the confined headspace of the reaction chamber, which restricted more dramatic increases in microdroplet population and possibly caused microdroplet coalescence. Despite the reaction chamber headspace of the example system, the results demonstrate the large scalability potential of this microdroplet-initiated, sustainable approach for H2production from NH3decomposition.

[0039] As depicted in Figure 2, part d, the H2production rate from NH3decomposition in catalyst- and irradiation-free water microdroplets at ambient temperature was two to three orders of magnitude higher than that achieved through semiconductor photocatalysis.Furthermore, the energy required for the generation of the microdroplets may be provided from renewable sources such as solar and wind power. Harnessing the renewable energy may enhance the sustainability of the catalyst-free ammonia decomposition of the disclosed methods and systems, as well as support on-site clean hydrogen production, particularly in remote locations.

[0040] The mechanism by which the ammonia decomposition occurs at the microdroplet surfaces is now further described. The influence of the microdroplet interfaces in the NH3decomposition to produce H2was analyzed via the free energy profile of an ammonia molecule crossing the gas-liquid interface from the gas phase to the bulk solution, illustrated in Figure 3, part a. In this case, the gas-liquid interface may be defined as the region spanning 0.46 to 0.89 nm along the Z-axis (Figure 3, part b), based on the criterion that the corresponding water density was 90% and 10% of the density in the bulk phase. As shown in Figure 3, part c, the global minimum in the free energy for NH3transfer was at the gasliquid interface (Z= 0.57 nm), indicating that the ammonia molecules possess a strong propensity to stay on the microdroplet surfaces. In addition, the energy barrier for moving aAtty. Docket No. 10110-25002A NH3molecule from the gas-liquid interface to the gas phase was about 15 kJ / mol and the free energy of NH3at the interface was approximately 0.18 kJ / mol lower than that in the liquid phase. Thus, the MD simulations reveal the significant affinity of ammonia to the gasliquid interface. The resulting interfacial concentration enrichment highlights the useful role of the gas-liquid interfaces of the microdroplets in ammonia decomposition.

[0041] On water microdroplet surfaces, hydronium cations (H3O+) preferentially accumulate at the outermost interface, while hydroxide anions (OH~) are repelled to the subsurface. The resulting spatial charge separation leads to the formation of an electric double layer characterized by a strong electric field (about 109V / m) at the gas-liquid interface. This property is a robust driver for the spontaneous electron transfer from OH~ to H3O+, generating -OH and *H radicals via the following reaction: OH~ + H3O+— > ’OH + -H + H2O.

[0042] The generation of ’OH radicals in the microdroplet-based ammonia decomposition was analyzed by detecting H2O2production when spraying 2 mM NaOH without ammonia (flow-type reaction). As shown in Figure 3, part d (upper), the absorbances at 400 nm gradually increased with increasing reaction time, and the H2O2production was about 3.73 pmol after 3 h of ultrasonication (Figure 3, part e). Also, hydrogen production in the water microdroplets was observed when atomizing 2 mM NaOH solution under Ar environment (Figure 1, part b). The self-recombination of ’OH (-OH + -OH — > H2O2) and *H (*H + -H — > H2) in the microdroplets can lead to the formation of H2O2and H2, respectively. It follows that the redox active species, ’OH and ’H, were generated on the catalyst-free water microdroplet surfaces, facilitated by the ultrahigh interfacial electric field.

[0043] To confirm the roles or contributions of -OH and -H radicals in the hydrogen production from ammonia in the water microdroplets, radical quenching testing was performed by introducing 50 mM isopropanol (IPA, -OH scavenger) or 20 vol.% O2(a well-known -H acceptor). Figure 3, part f, shows that the H2production was significantly inhibited with the addition of IPA and O2. In addition, H2O2production was markedly reduced in the presence of 0.5 M NH3-H2O compared to that without ammonia (Figure 3, part d (bottom) and part e). These results establish that both -OH and *H play useful roles in NH3decomposition to release H2from the microdroplets, e.g., through a redox synergistic process. Furthermore, the production of nitrate and nitrite during the ammonia decomposition in microdroplets was improved under Ar+O2atmosphere, which may be because O2enhanced the oxidation ability of the reaction system by converting reductive *H into oxidative HO2- and ’OH. This finding indicates that enhancing oxidation capability while not damaging or simultaneously improving the reduction property of microdroplets mayAtty. Docket No. 10110-25002A provide a dual benefit to valuable nitrate synthesis and green hydrogen production from ammonia decomposition.

[0044] The above-described testing and analyses establish that the microdroplet-initiated ammonia decomposition was facilitated by ’OH and *H generated at the gas-liquid interface. The reaction pathways are now analyzed. To that end, quantum chemical calculations coupled with intermediate detection were employed to explore the possible mechanisms for hydrogen extraction from ammonia. As illustrated in Figure 4, part a, the NH3decomposition was initiated by an -OH attack to generate an amide radical (’NH2), through a transition state with free energy barrier (AG*) of 9.53 kcal / mol and free energy change (AG) of -12.21 kcal / mol (Reaction 1). Following this, the ’NH2 may undergo two competing pathways: (i) dimerization reaction; and, (ii) oxidation pathway. In the dimerization reaction, -NH2radicals dimerize to form N2H4 with free energy change of AG = -51.44 kcal / mol (Reaction 2). This reaction pathway is favorable given that the concentrations of ammonia (mol / L) were much higher than the generated hydroxyl radicals in microdroplets (pmol / L). Subsequently, N2H4can react with -H to generate H2and N2through a series of reactions (Reactions 3-6, discussed below). In the oxidation pathway, ’NH2 radicals react with ’OH to form NH2OH (Reaction 8), which would further recombine with ’OH to produce NO2“ and NO3“ (Reactions 9-10). The significantly low production of nitrogen oxyanions compared to H2indirectly manifests that the ’NH2 dimerization to form N2H4 was predominant in the induction period of NH3decomposition in microdroplets. Furthermore, the generation of N2H4 was verified by analyzing a solution using the colorimetric method. As shown in Figure 4, part b, the absorbances at 455 nm gradually increased over time, indicating the formation of N2H4as the reaction proceeded although its concentration was insignificant due to the fast transformation. In contrast, almost no N2H4was detected in the presence of IPA and under Ar+02 atmosphere (Figure 4, part c). This result was in line with the reduced H2evolution, probably due to the suppression of Reaction 1 and the enhancement of Reaction 8, respectively. These results indicate that ’OH and *H synergistically contributed to ammonia decomposition to release molecular hydrogen by governing the formation and transformation of intermediates (•NH2 and N2H4), thereby underpinning the mechanisms of H2 production from NH3in catalyst-free water microdroplets.•OH + NH3-NH2+ H2O (R.1)•NH2+ -NH2^ N2H4 (R.2)•H + N2H4^ -N2H3+ H2(R.3)•N2H3+ •N2H3^ 2N2H2 + H2(R.4)Atty. Docket No. 10110-25002A N2H2+ -H -N2H + H2(R.5)•N2H ^ N2+ -H (R.6)2NH3N2+ 3H2(R7)•NH2+ -OH — > NH2OH (R.8)NH2OH + -OH + OH- - NO2- + 2H2O (R.9)NO2- + -OH + OH- NO3- + H2O (R.10)

[0045] Following N2H4generation, this intermediate reacts with -H to generate H2and •N2H3radical via a hydrogen abstraction process (Reaction 3, AG* = 9.65 kcal / mol, AG = -22.92 kcal / mol). The formed •N2H3radicals then proceed a barrierless radical-radical recombination reaction (Reaction 4, G = -17.98 kcal / mol) to release H2and produce diazene (N2H2). Subsequently, the N2H2intermediate undergoes an H-abstraction reaction by *H to generate H2and •N2H (Reaction 5, AG* = 7.23 kcal / mol, AG = -40.96 kcal / mol), followed by a rapid decomposition of •N2H that generates N2and *H (Reaction 6, with a AG = -13.20 kcal / mol). The released *H from -N2H can go on to participate in above-mentioned H-abstraction reactions or recombine with itself to produce H2. Jiang et al. very recently found that O2can quickly react with -N2H on droplet surfaces to transform it into N2O, which may be another reason for the suppressed H2production under O2-containing atmosphere. Additionally, it should be stated that approximately one-third of the hydrogen source in the produced H2was from water, yet the apparent stoichiometry of the overall processes remains consistent with NH3decomposition to generate H2and N2(Reaction 7).

[0046] Furthermore, the high electric field at the gas-liquid interface of the water microdroplets is capable of accelerating chemical reactions by reducing energy barriers. The influence or role of an external electric field on the transition state barriers (AG*) of Reaction 1, Reaction 3, and Reaction 5 was analyzed. As depicted in Figure 5, the presence of a strong electric field with a strength of 109V / m lowered the Gibbs free energy of transition state in these steps, thereby facilitating the reactions and accelerating the rates thereof. These findings further underscore the useful role of the gas-liquid interfaces of the microdroplets with an ultra-strong electric field in driving ammonia decomposition for H2production, without the need for catalysts, photoirradiation, or high / elevated temperature.

[0047] The foregoing experimental and theoretical analyses solidly support the abovedescribed reaction pathways, initiated by ’OH and *H formed at the gas-liquid interface, for ammonia decomposition in catalyst-free water microdroplets. The microdroplet-initiated ammonia decomposition establishes a sustainable and scalable technique for clean hydrogen production.Atty. Docket No. 10110-25002A

[0048] Described above are examples of systems and methods for H2production via NH3decomposition in catalyst-free water microdroplets under ambient conditions. The disclosed methods and systems provide a scalable and efficient approach for green hydrogen production. By using multiple sprayers, a high H2yield rate of 226.8 pmol / h was achieved, significantly surpassing the performance of complex photocatalytic decomposition techniques. Test measurements (radical quenching studies and intermediate detection) and theoretical analyses (MD simulation and quantum chemical calculations) collectively established that the ultrahigh electric field at the gas-liquid interfaces of the microdroplets played a useful role in enabling ammonia splitting following thermodynamically favorable redox pathways, which were synergistically triggered by hydroxyl radicals (*OH) and hydrogen radicals (»H) spontaneously generated on the microdroplet surfaces. The disclosed methods and systems thus provide a catalyst-free approach for sustainable hydrogen production (e.g., green hydrogen) and other chemical synthesis.

[0049] The disclosed methods and systems may support alternative or additional applications. For instance, ammonia nitrogen, a ubiquitous pollutant with severe detrimental effects on human health and aquatic ecosystems, is generally treated through a nitrificationdenitrification method in wastewater treatment plants. In this energy-intensive and sludgeproducing process, ammonium is ultimately converted to N2and H2O, leading to a wasting of resources. The ammonium concentrations in wastewater vary from mg / L to g / L depending on the source, representing a vast untapped resource. The microdroplet chemistry of the disclosed methods and systems may be scaled up to recycle and transform such waste ammonia to produce valuable hydrogen and nitrate. This application may support both resource recovery and pollution mitigation.

[0050] Described below are further examples of the disclosed hydrogen production systems and methods. The examples may include any one or more of the features of the above-described examples. Likewise, the examples described above may be modified to include any one or more of the features of the above-described examples.

[0051] Figure 6 depicts a system 600 for hydrogen production via ammonia decomposition in accordance with one example. The system 600 includes a reaction chamber 602 configured to contain an aqueous solution 604. The aqueous solution 604 includes ammonia. In some cases, ammonium hydroxide was added into the reaction chamber 602 after purging high-purity Ar for 1 h with flow rate of 200 mL / min. The ammonia may be incorporated into the aqueous solution in other or additional ways.Atty. Docket No. 10110-25002A

[0052] In this example, the bulk aqueous solution 604 was 150 mL of NH3-H2O. The ammonia concentration may vary as described herein.

[0053] In some cases, the pH of the bulk aqueous solution 604 may be higher than 11 , thereby making NH3molecules the dominant species in the system 600. Other pH levels may be used

[0054] The system 600 also includes one or more ultrasonic atomizers 606 in fluid communication with the aqueous solution 604. The ultrasonic atomizer 606 is configured to generate a spray in the reaction chamber 602 from the aqueous solution 604. As described above, the spray includes water microdroplets.

[0055] The ultrasonic atomizer 606 is immersed in the aqueous solution 604 for generation of the microdroplets. In this example, the ultrasonic atomizer 606 is disposed about 15 mm below the water surface, but other depths may be used.

[0056] As shown in Figure 6, the system 600 also includes an output 608 in fluid communication with the reaction chamber 602 to collect hydrogen produced via decomposition of the ammonia along a gas-liquid interface of the water microdroplets. As shown in the example of Figure 6, the output 608 may be or include an output port of the reaction chamber 602.

[0057] The effluent gas passing through the output 608 may be processed by a plurality of filters 610, 612. In the example of Figure 6, the filter 610 is configured to absorb (and therefore remove) gaseous NH3. For example, the filter 610 may use 200 mL of 1 M H2SO4. The other filter 612 is configured to remove nitric oxides (NO / NO2) via, e.g., 1 M NaOH.

[0058] The microdroplet reaction may be initiated by generating numerous suspended microdroplets from bulk solution. Some microdroplets may gravitationally settle down for circulating spray.

[0059] As shown in Figure 6, the system 600 may include a gas source 614 in fluid communication with the reaction chamber 602. The gas source 614 is configured to provide an inert gas to the reaction chamber 602. In the example of Figure 6, the reaction chamber 602 is continuously purged with high-purity Ar (200 mL / min) during the reaction.Alternatively or additionally, the gas source 614 may be configured to provide gaseous ammonia and a carrier gas (e.g., argon) for the gaseous ammonia.

[0060] The system 600 may include fewer, additional, or alternative elements. For instance, the system 600 may include an additional gas source to provide gaseous ammonia separately from argon or other gases. Alternatively or additionally, the system 600 mayAtty. Docket No. 10110-25002A include a liquid source in fluid communication with the reaction chamber 602, the liquid source being configured to provide the aqueous solution in a flow-type reaction configuration. Further details regarding example sources and a flow-type reaction configuration are presented in connection with an example system shown in Figure 8.

[0061] Figure 7 depicts a method 700 for hydrogen production in accordance with one example. The method may be implemented by any one of the reaction systems described herein, or another reaction system.

[0062] The method includes an act 702 in which an aqueous solution including ammonia is provided. The aqueous solution may be provided in a reaction chamber in an integrated or composite manner (e.g., as a bulk solution) and / or provided to the reaction chamber in a constituent or partial manner. The aqueous solution may have an ammonia concentration as described herein. The aqueous solution may be alkaline, e.g., as described herein. The solution may or may not be catalyst-free. The solution may be at room or other ambient temperature.

[0063] In the example of Figure 7, the reaction chamber is purged with an inert gas in an act 704. In some cases, the inert gas is argon, but alternative or additional gases may be used. The purging may be implemented before initiation of the reaction (e.g., before the generation of the microdroplets).

[0064] In an act 706, a spray (e.g., a pneumatic spray) is generated in the reaction chamber from the aqueous solution. As described herein, the spray includes water microdroplets. The water microdroplets may be generated via an ultrasonic atomizer.

[0065] The act 706 may include an act 708 in which the reaction chamber is purged with an inert gas while generating the spray. The purging may be continuously or periodically implemented.

[0066] The reaction may be controlled during the act 706 in one or more ways. For instance, one or more system parameters may be adjusted in an act 710. For instance, the ammonia concentration may be updated (e.g., maintained) or otherwise adjusted.Alternatively or additionally, the pH level of the solution may be updated (e.g., maintained) or otherwise adjusted.

[0067] The act 706 may also include an act 712 in which one or more liquid input flows is / are maintained. The reaction chamber may thus be configured to support a flow-type reaction arrangement.Atty. Docket No. 10110-25002A

[0068] In some cases, the method 700 includes an act 713 in which an external voltage is applied to the reaction system to generate an electric field at an interface of the aqueous solution. The electric field has a strength to initiate decomposition of the ammonia along the interface. The act 713 may be implemented in addition to (e.g., in parallel or contemporaneously with) the act 706, or as an alternative thereto.

[0069] The interface may be a gas-liquid interface or a solid-liquid interface. In gas-liquid interface cases, the interface may be provided by generating a spray from the aqueous solution as described herein. The gas-liquid interface may thus be defined by a microdroplet of the spray.

[0070] In an act 714, hydrogen produced via decomposition of the ammonia along a gasliquid interface of the water microdroplets is collected from the reaction chamber. The act 714 may include an act 716 in which effluent gas received from the reaction chamber are filtered. The effluent gas from the reaction chamber may be passed through a plurality of filters. For instance, the filters may be configured to remove NH3 and a nitric oxide. In an act 718, the outputs of the plurality of filters are then collected.

[0071] The method 700 may include additional, fewer, or alternative acts. For instance, the method 700 may not include an initial purge of the reaction chamber (act 704). In some cases, one or more acts may be directed to preparing the bulk solution, including, for instance, a heat treatment and / or the addition of one or more catalysts.

[0072] Figure 8 depicts a system 800 for hydrogen production in accordance with an example involving a flow-type arrangement. In this example, the system 800 includes three ultrasonic atomizers 802 disposed in a reaction chamber 804 as shown in Figure 8, part a. Each ultrasonic atomizer 802 may be tunable. As shown in Figure 8, part b, the system 800 includes one or more liquid sources 806 in fluid communication with the reaction chamber 804 via a corresponding number of inlets 808 of the reaction chamber 804. The source(s) 806 and the inlet(s) 808 allow the aqueous solution to be provided to the reaction chamber 804 in a flow-type reaction configuration.

[0073] As shown in Figure 8, part b, the system 800 may also include a gas source 810 in fluid communication with the reaction chamber 804. The gas source 810 may be configured to provide an inert gas to the reaction chamber 804 as described herein.

[0074] The flow-type arrangement of the system 800 may be used to increase the scale of the microdroplet-initiated NH3decomposition. Such scalability may be useful for, e.g., onsite H2production and supply.Atty. Docket No. 10110-25002A

[0075] Described herein are examples of decomposition of ammonia to hydrogen without the use of noble metal-based catalysts or high operating temperatures. The disclosed examples instead use the physicochemical properties at the gas-liquid interfaces of water microdroplets to provide scalable and catalyst-free hydrogen production from ammonia at room temperature. A maximum hydrogen evolution rate of 226.8 pmol / h was achieved, which outperformed most conventional catalytic methods under mild conditions. The underlying ammonia splitting mechanisms include hydroxyl radicals and hydrogen radicals spontaneously generated at the microdroplet surfaces, as well as the synergistically triggering of thermodynamically favorable redox pathways for ammonia decomposition. The reaction rates may be accelerated by an ultrahigh interfacial electric field that reduces transition state energy. The disclosed examples accordingly support the production of green hydrogen production via microdroplet chemistry.

[0076] The term "about" is used herein in a manner to include deviations from a specified value that would be understood by one of ordinary skill in the art to effectively be the same as the specified value due to, for instance, the absence of appreciable, detectable, or otherwise effective difference in operation, outcome, characteristic, or other aspect of the disclosed methods and devices.

[0077] The present disclosure has been described with reference to specific examples that are intended to be illustrative only and not to be limiting of the disclosure. Changes, additions and / or deletions may be made to the examples without departing from the spirit and scope of the disclosure.

[0078] The foregoing description is given for clearness of understanding only, and no unnecessary limitations should be understood therefrom.

Claims

Atty. Docket No. 10110-25002A What is Claimed is:

1. A method for hydrogen production, the method comprising:providing an aqueous solution comprising ammonia;generating a spray in a reaction chamber from the aqueous solution, the spray comprising water microdroplets; andcollecting hydrogen from the reaction chamber produced via decomposition of the ammonia along a gas-liquid interface of the water microdroplets.

2. The method of claim 1 , wherein the aqueous solution is alkaline.

3. The method of claim 1 , wherein the aqueous solution has a pH above about 10.

4. The method of claim 1 , wherein the spray is generated with an ultrasonic atomizer in fluid communication with the aqueous solution.

5. The method of claim 4, wherein the ultrasonic atomizer is disposed in the reaction chamber and immersed in the aqueous solution.

6. The method of claim 4, wherein the ultrasonic atomizer is configured to generate a circulating spray.

7. The method of claim 4, wherein the ultrasonic atomizer comprises an electronic, tunable sprayer.

8. The method of claim 1 , wherein the aqueous solution has an ammonia concentration falling in a range from about 0.2 M NH3to about 1.0 M NH3.

9. The method of claim 1 , further comprising, before generating the spray, purging the reaction chamber with an inert gas.

10. The method of claim 9, wherein the inert gas comprises argon.

11. The method of claim 1 , wherein generating the spray comprises purging the reaction chamber with an inert gas while generating the spray.

12. The method of claim 1 , wherein collecting the hydrogen comprises:passing effluent gas from the reaction chamber through a plurality of filters configured to remove NH3and a nitric oxide; andcollecting an output of the plurality of filters.Atty. Docket No. 10110-25002A 13. The method of claim 1 , wherein the reaction chamber resides at an ambient temperature.

14. The method of claim 1 , wherein the reaction chamber is catalyst-free.

15. The method of claim 1 , wherein the decomposition of the ammonia is not driven via irradiation of the reaction chamber.

16. A system for hydrogen production, the system comprising:a reaction chamber configured to contain an aqueous solution, the aqueous solution comprising ammonia;an ultrasonic atomizer in fluid communication with the aqueous solution, the ultrasonic atomizer being configured to generate a spray in the reaction chamber from the aqueous solution, the spray comprising water microdroplets; andan output in fluid communication with the reaction chamber to collect hydrogen produced via decomposition of the ammonia along a gas-liquid interface of the water microdroplets.

17. The system of claim 16, wherein the ultrasonic atomizer is disposed in the reaction chamber in a position for immersion in the aqueous solution.

18. The system of claim 16, further comprising a liquid source in fluid communication with the reaction chamber, the liquid source being configured to provide the aqueous solution in a flow-type reaction configuration.

19. The system of claim 16, further comprising a gas source in fluid communication with the reaction chamber, the gas source being configured to provide an inert gas to the reaction chamber.

20. The system of claim 16, further comprising a plurality of filters in fluid communication with the reaction chamber to receive effluent gas from the reaction chamber, the plurality of filters being configured to remove NH3 and a nitric oxide.

21. The system of claim 16, wherein the ultrasonic atomizer is one of a plurality of ultrasonic atomizers in fluid communication with the aqueous solution.

22. The system of claim 21 , wherein the plurality of ultrasonic atomizers are configured to generate a circulating spray.Atty. Docket No. 10110-25002A 23. The system of claim 16, wherein the ultrasonic atomizer is configured to generate the water microdroplets for continuous operation without a circulating spray.

24. The system of claim 16, further comprising a gas source in fluid communication with the reaction chamber, the gas source being configured to provide, to the reaction chamber, gaseous ammonia and a carrier gas for the gaseous ammonia.

25. The system of claim 24, wherein the carrier gas is argon.

26. A method for hydrogen production, the method comprising:providing an aqueous solution to a reaction system, the aqueous solution comprising ammonia;applying an external voltage to the reaction system to generate an electric field at an interface of the aqueous solution, the electric field having a strength to initiate decomposition of the ammonia along the interface; andcollecting hydrogen produced via the decomposition of the ammonia along the interface initiated by the electric field.

27. The method of claim 26, wherein the interface is a gas-liquid interface.

28. The method of claim 27, further comprising generating a spray from the aqueous solution such that the gas-liquid interface is defined by a microdroplet of the spray.

29. The method of claim 26, wherein the interface is a solid-liquid interface.

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