Disclosed herein is a membrane-free alkaline electrolyzer for upcycling waste into ammonia and methods for converting nitrogen (N)-containing waste into ammonia (NH3).
As opposed to the “inert nitrogen (N2)”, reactive nitrogen (Nr) is referred to as a variety of nitrogen-containing compounds that are active biologically, chemically, and/or photochemically. Nr is essential to life on earth as a basic building block of amino acids, proteins, nucleic acids, and other molecules necessary for life activities (Lehnert et al., Nat. Rev. Chem. 2:278-289 (2018); Kuypers et al., Nat. Rev. Microbiol. 16:263-276 (2018)). The global Nr generation has increased by ˜70% over the past 30 years, >60% of which can be attributed to the industry-driven anthropological N2-fixing process (i.e., the Haber-Bosch process for ammonia (NH3) synthesis) to fulfill the growing global food demand (Uwizeye et al., Nat. Food 1:437-446 (2020); Galloway and Cowling, Ambio 50:745-749 (2021)). The microbial decomposition-nitrification-denitrification process can turn Nr back to N2 in nature, however, the generation rate of artificial Nr species is far greater than the elimination rate of those Nr species by natural processes (Fowler et al., Phil. Trans. R. Soc. B 368:20130164 (2013); Galloway et al., Science 320:889-892 (2008)), resulting in continued accumulation that has caused alarming and profound damage to ecosystems and human welfare (
Sustainable solutions to this human-induced problem have been actively pursued in recent years, through processes of electrochemical reduction of NO3− (NO3RR). If NO3− in waste streams can be efficiently recovered and converted to NH3 (eqn (1)), this NH3-centric process will alleviate the environmental impacts of NO3−, while substantially decreasing NH3 demand from the Haber-Bosch process using fossil fuel-derived H2 (van Langevelde et al., Joule 5:290-294 (2021); McEnaney et al., ACS Sustain. Chem. Eng. 8:2672-2681 (2020)):
NO3−+2H2O→NH3+2O2+OH− (1)
Despite the successful development of some electrocatalysts for the NO3−-to-NH3 process in previous works (see Table 1 infra) (Deng et al., Adv. Sci. 8:2004523 (2021); Chen et al., Nat. Nanotechnol. 17:759-767 (2022); Hu et al., Energy Environ. Sci. 14:4989-4997 (2021); Gao et al., Nat. Commun. 13:2338 (2022); Li et al., Am. Chem. Soc. 142:7036-7046 (2020); Liu et al., Angew. Chem. Int. Ed. 61:e202202556 (2022)), many of them involve noble metals and/or require complicated synthetic procedures, making them less economically attractive, especially considering the electricity consumption for this 8-electron-transfer reaction. Moreover, NO3− is highly distributed with only tens or hundreds of ppm NO3−—N in typical waste streams (van Langevelde et al., Joule 5:290-294 (2021)); thus, an efficient and sustainable concentrating step is another prerequisite for high-performance NH3 electrosynthesis. Nevertheless, a systematic assessment of the technical and economic feasibility of NO3− concentration is critically missing in the current research field.
The present disclosure is directed to overcoming limitations in the art.
One aspect of the present disclosure relates to a membrane-free alkaline electrolyzer (MFAEL) system for converting nitrogen (N)-containing waste into ammonia (NH3). This system includes a reaction medium comprising H2O—NaOH—KOH; a pair of electrodes, wherein the electrodes are in contact with the reaction medium; and a power supply operably connected to the electrodes.
Another aspect of the present disclosure relates to a method for converting nitrogen (N)-containing waste into ammonia (NH3). This method involves introducing nitrogen (N)-containing waste into a membrane-free alkaline electrolyzer (MFAEL) system comprising a reaction medium comprising H2O—NaOH—KOH; a pair of electrodes, wherein the electrodes are in contact with the reaction medium; and a power supply operably connected to the electrodes. A current is applied between the electrodes to perform oxidative and reductive transformation of the nitrogen (N)-containing waste into ammonia (NH3).
The integrated sustainable process presented in the present disclosure provides an efficient approach to upcycling waste nitrogen, and extends to CO2 capture from various sources, thanks to the basicity of ammonia. The synergistic combination of CO2 capture and ammonia synthesis magnify the environmental benefits when adopted by real-world deployments. Owing to the flexibility and scalability of electrochemical systems, the distributed synthesis of green ammonia can also be realized from waste nitrogen sources, as opposed to the centralized synthesis of carbon-intensive methane-based ammonia manufacturing in Haber-Bosch plants. Findings on the conversion of organic nitrogen provide an alternative pathway for managing solid nitrogen-containing wastes for sustainable agriculture and environment. The unique ultra-alkaline NaOH/KOH/H2O system employed in the present disclosure serves as an enabling electrolyte, inspiring other electrochemical conversions with tailored selectivity or activity.
The present disclosure describes an integrated electricity-driven process for economically upcycling waste nitrogen, which is enabled by low-concentration NO3− electrodialysis and high-performance NH3 electrosynthesis from various Nr forms. As shown in
5A-D show electrolysis with different Nr compounds containing N—O or C—N bonds. KNO3 (9.3 mmol) and alanine (18.7 mmol) were chosen as the model chemicals containing N—O and C—N bonds, respectively. The reaction time was 2 h.
The present disclosure relates to systems and methods of electrolysis for converting nitrogen (N)-containing waste into ammonia (NH3). Electrolysis is a process where electrical current is used to drive a non-spontaneous redox reaction.
One aspect of the present disclosure relates to a membrane-free alkaline electrolyzer (MFAEL) system for converting nitrogen (N)-containing waste into ammonia (NH3). The system includes a reaction medium comprising H2O—NaOH—KOH; a pair of electrodes, wherein the electrodes are in contact with the reaction medium; and a power supply operably connected to the electrodes.
The term “electrolyzer,” as used herein, refers to an apparatus, device, container, or system for performing electrolysis. In some embodiments, an electrolyzer has a pair of electrodes (e.g., an anode and a cathode), a reaction medium (e.g., an electrolyte solution), and a power supply, which is typically an external source of power to add electrical energy to a reaction taking place in the reaction medium. The electrodes facilitate the transfer of electrical energy into the reaction medium by extending into the reaction medium at one end and connecting to an external power supply at the other end. One particular type of electrolyzer is an alkaline electrolyzer. In an alkaline electrolyzer, the reaction medium or electrolyte solution typically includes sodium hydroxide and/or potassium hydroxide and water. A membrane-free electrolyzer typically has only one compartment containing the reaction medium, as opposed to an electrolyzer with a reaction medium separated, at least partially, by a membrane. The membrane may divide the reaction chamber into two compartments, with one compartment containing one of the electrodes and the other compartment containing the other electrode of the electrode pair. In some embodiments of a membrane-free electrolyzer, the reaction medium is a single chamber and the two electrodes of the electrode pair are both present in the reaction chamber without any physical separation or barrier between them. For example, the electrode pair is typically only separated by space in a reaction medium and/or chamber.
One embodiment of a membrane-free alkaline electrolyzer (MFAEL) system of the present disclosure is illustrated in
As used herein, “nitrogen (N)-containing waste” refers to waste material that comprises reactive nitrogen (Nr) species, including but not limited to nitrates, nitrogen-containing organic compounds, nitrous oxide, nitrites, and other nitrogen oxides. Reactive nitrogen includes a variety of nitrogen-containing compounds that are active biologically, chemically, and/or photochemically.
In some embodiments, the membrane-free alkaline electrolyzer (MFAEL) system of the present disclosure can be used to convert nitrogen (N)-containing waste into ammonia (NH3).
Thus, another aspect of the present disclosure relates to a method for converting nitrogen (N)-containing waste into ammonia (NH3). This method involves introducing nitrogen (N)-containing waste into a membrane-free alkaline electrolyzer (MFAEL) system comprising a reaction medium comprising H2O—NaOH—KOH; a pair of electrodes, wherein the electrodes are in contact with the reaction medium; and a power supply operably connected to the electrodes. A current is applied between the electrodes to perform oxidative and reductive transformation of the nitrogen (N)-containing waste into ammonia (NH3).
In some embodiments, the method of converting nitrogen (N)-containing waste into ammonia (NH3) is carried out using a system described herein.
In some embodiments of the systems and methods disclosed herein, the electrodes are formed of a material comprising Ni, Co, Ru, Cu, and mixtures thereof. In some embodiments, the electrodes are formed of a material comprising Ni. In some embodiments, the electrodes are Ni electrodes. In some embodiments, the Ni electrodes are made from or comprise foam and/or mesh material. In some embodiments, the electrodes comprise foam material. In some embodiments, the electrodes comprise mesh material. In some embodiments, the electrodes comprise a mixture of foam material and mesh material. The pair of electrodes can be the same material or different materials.
In some embodiments, the nitrogen (N)-containing waste, which is converted to ammonia (NH3) using the systems and methods described herein, is selected from nitrate, nitrite, urea, amino acids, proteins, and mixtures thereof.
In some embodiments of the systems and methods disclosed herein, the reaction medium of the membrane-free alkaline electrolyzer (MFAEL) comprises H2O—NaOH—KOH, with the H2O component being present in the reaction medium in an amount of about 40 wt. %, or 35 wt. % to 45 wt. %, including 35 wt. %, 36 wt. %, 37 wt. %, 38 wt. %, 39 wt. %, 41 wt. %, 42 wt. %, 43 wt. %, 44 wt. %, 45 wt. %, or any amount or range therein, or any other amount or range in which the system and/or method is able to convert nitrogen (N)-containing waste, which is converted to ammonia (NH3).
In some embodiments of the systems and methods disclosed herein, the reaction medium comprises equimolar amounts of NaOH and KOH, although non-equal molar amounts of NaOH and KOH may also be used. For example and without limitation, non-equal molar amounts of NaOH and KOH may include a variance between the amount of NaOH and KOH of 1%, 2%, 3%, 4%, 5%, 10%, 15%, 20%, 25%, or more.
In some embodiments, the reaction chamber is defined by chamber walls to form a leak-free reaction chamber, such as a glass container or other structure that is leak proof In some embodiments, the chamber walls forming the reaction chamber are constructed of polytetrafluoroethylene (PTFE). In some embodiments, the reaction chamber is an open top reaction chamber, having a bottom and side walls. In some embodiments, the reaction chamber comprises a lid or a cap to cover an open top, such as stainless-steel cap. For example, and with reference again to
In some embodiments, the reaction chamber comprises a liquid injection conduit or port through which liquid pertaining to the reaction medium or nitrogen (N)-containing waste is added to the reaction medium of the electrolyzer. In some embodiments of the methods described herein, water and/or nitrogen (N)-containing waste is added into the reaction chamber via the liquid injection conduit. With further reference to
In some embodiments of the systems disclosed herein, the reaction chamber further comprises an air intake conduit and an exit conduit. In some embodiments, the methods described herein are carried out by adding air and/or N2 into the reaction medium via the air intake conduit and removing ammonia (NH3) from the reaction medium via the exit conduit. With reference to
In some embodiments, the reaction chamber is heated to a desired temperature, such as in an oil bath, or by other suitable means. As illustrated in
In some embodiments of the systems and methods described herein, the system further comprises a container for collecting ammonia (NH3) produced by the system, where the container comprises an absorbing solution. In some embodiments, the absorbing solution comprises H2SO4. In some embodiments, the absorbing solution comprises H3PO4. In some embodiments, the absorbing solution comprises a mixture of H2SO4 and H3PO4. With reference again to
The systems and methods of the present disclosure are able to achieve high NH3 production rates. In some embodiments, the systems and methods of the present disclosure achieve NH3 production rates of at least 80 mmol h−1, at least 81 mmol h−1, 82 mmol h−1, 83 mmol h−1, 84 mmol h−1, 85 mmol h−1, 86 mmol h−1, 87 mmol h−1, 88 mmol h−1, 89 mmol h−1, 90 mmol h−1, 91 mmol h−1, 92 mmol h−1, 93 mmol h−1, 94 mmol h−1, 95 mmol h−1, 96 mmol h−1, 97 mmol h−1, 98 mmol h−1, 99 mmol h−1, 100 mmol h−1, or more.
The above disclosure is general. A more specific description is provided below in the following examples. The examples are described solely for the purpose of illustration and are not intended to limit the scope of the present application. Changes in the form and substitution of equivalents are contemplated as circumstances suggest or render expedient. Although specific terms have been employed herein, such terms are intended in a descriptive sense and not for the purposes of limitation.
The following examples are provided to illustrate embodiments of the present application but are by no means intended to limit scope.
All chemicals were used as received without purification. Nickel wire mesh (200 mesh, 0.002″ wire diameter) was purchased from Wire Mesh Store. Nickel foam (1.6 mm thickness, 99.9%) was purchased from MTI Corporation. Copper wire mesh (200 mesh, 0.002″ wire diameter) was purchased from TWP Inc. Nickel wire (0.04″ diameter, 99.5%) and nickel rod (0.12″ diameter, 99%) were purchased from Alfa Aesar. Copper wire (0.04″ diameter, 99.9%) was purchased from McMaster-Carr. Sodium hydroxide (NaOH, ≥98%), potassium hydroxide (KOH, ≥85%), sodium salicylate (≥99.5%), sodium nitroferricyanide dihydrate (Na2[Fe(CN)5NO].2H2O, ≥99%), sodium hypochlorite solution (NaOCl, available chlorine 4.00-4.99%), ammonium-15N chloride (15H4Cl, ≥98 at. % 15N), 3-(Trimethylsilyl)-1-propanesulfonic acid sodium salt (DSS, 97%), and the chemicals for the screening tests (
The configuration of MFAEL was modified from previous work (Chen et al., Nat. Catal. 3:1055-1061 (2020), which is hereby incorporated by reference in its entirety). In brief, the cell body (also referred to herein as “reaction chamber”), included a 100 mL screw-cap polytetrafluoroethylene (PTFE) bottle (height: 88 mm; diameter: 52 mm) and a custom-made stainless-steel lid. Two pieces of ¼″ OD alumina ceramic tubes were used for the carrier gas inlet and outlet. A union tee with a septum was connected to the gas inlet tube and offered a liquid injection port, through which water or sample solution can be supplied during cell operation. Two 10 cm2 nickel mesh electrodes (3.3×3 cm2, 200 mesh) were used as the electrodes, and were attached to nickel wires (0.04″ diameter) connected to a potentialstat (WaveDriver 20, for I≤1000 mA) or a DC power supply (BK Precision 1697B, for I>1000 mA). Silicone O-rings and aluminosilicate adhesive (Resbond 907GF) were used to seal the gaps and ensure the cell installation is leak-free.
Prior to the electrolysis, the NaOH/KOH/H2O electrolyte (containing equimolar of NaOH and KOH and 40 wt. % of water) was prepared by adding 29.7 g of NaOH, 48.1 g of KOH, and 38.9 g of deionized water in the PTFE bottle, which was then sealed in an oven at 80° C. overnight for the complete dissolution of NaOH and KOH. For typical tests, an appropriate amount of N-containing reactant was added before the cell cap was installed. For electrochemical NO3− reduction (NO3RR), the amount of added KNO3 was equal to the theoretical amount of NO3− that can be fully converted to NH3 based on the applied charge. For the conversion of organic Nr compounds, the amount of added reactant was specified in the figure captions. Subsequently, the cell was placed in an oil bath preheated to 80° C., and 200 mL min−1 of N2 was bubbled from the gas inlet tube into the electrolyte. The outlet gas from MFAEL was bubbled into an acidic absorbing solution (100 mL of 0.1 M H2SO4) for NH3 collection.
After 30 min of gas bubbling to remove the air from the system, a constant current was applied between the electrodes. During electrolysis, the absorbing solution was changed every 30 min for NH4+ quantification. After electrolysis, the system was kept with gas bubbling for additional 30 min to deplete the remaining NH3 in the gas line. The electrolyte was then carefully diluted to 1 L with deionized water for the quantification of NO3−, NO2−, and organic products (detailed in Product Quantification section).
The conversion of NO3− (X) and faradaic efficiency of product i (FEi) were calculated by
where n0 is the initial amount of NO3− (mol); n is the amount of NO3− after electrolysis (mol); ni is the amount of product i (mol); zi is the number of electrons transferred to product i; F is the Faraday constant (96,485 C mol−1); and Q is the total charge passed through the electrolytic cell (C).
The NH3 production rate was calculated by
where c is the NH4+ concentration (M); V is the volume of the absorbing solution (L); A is the geometric area of the electrode (cm2); t is the electrolysis duration (s).
The N balance for Nr conversion was calculated by
For the real N-containing samples (protein and algae powder), the content of N (wt. %) was determined by a Combustion Elemental Analyzer (CHN/S Thermo FlashSmart 2000).
To compare the electrochemically active surface area of the Ni-based electrodes before and after electrolysis in the NaOH/KOH/H2O electrolyte, cyclic voltammetry (“CV”) measurements were carried out in a single-compartment cell with a standard three-electrode configuration without stirring (Morales and Risch, J. Phys. Energy, 3:034013 (2021), which is hereby incorporated by reference in its entirety). The electrolyte was 1 M KOH. The geometric area of the working electrode was 1 cm2 (1×1 cm2). An Ag/AgCl electrode (saturated KCl, E0=0.197 V vs. SHE) and a Pt foil were used as the reference electrode and counter electrode, respectively. Different scan rates ranging from 50 to 200 mV s−1 were applied.
The configuration of the scaled-up MFAEL is similar to the 100 mL reactor. The cell body included a 2.5 L screw-cap PTFE bottle (height: 260 mm; diameter: 131 mm), a custom-made stainless-steel lid, two pieces of ½″ OD alumina ceramic tubes, two 100 cm2 nickel mesh electrodes (10×10 cm2, 200 mesh), and two nickel rods (0.12″ diameter) for conducting electricity. The nickel rods were bent and stitched through the folded nickel mesh electrodes to ensure stable contact, and were connected to a DC power supply (BK Precision 1901B). Silicone O-rings and aluminosilicate adhesive (Resbond 907GF) were used to seal the gaps and ensure the cell installation is leak-free. The amount of electrolyte was 25 times higher than the 100 mL reactor, and the amount of added KNO3 was equal to the theoretical amount of NO3− that can be fully converted to NH3 based on the applied charge. The flow rate of carrier gas was 500 mL min−1. The applied current was 25 A (corresponding to 250 mA cm−2 of current density), and the electrolysis time was 24 hours.
Different absorbing solutions were used for obtaining different NH3-based chemical products. For NH4+ salts, 400 mL of 5 M H2SO4 was used for NH3 absorption. For producing pure NH3 solution, 100 mL of deionized water was used for NH3 absorption, which was cooled with 5° C. circulated water by a chiller. It should be noted that the volume of the absorbing solution increased during electrolysis due to the condensation of water vapor and the decrease of solution density due to the increasing NH3 content. For producing NH4HCO3, 100 mL of deionized water was pre-saturated with CO2 and continuously bubbled with 500 mL min−1 of CO2 during the electrolysis. Considering the decomposition temperature of NH4HCO3 (36° C.), the absorbing solution was also cooled with 5° C. circulated water and magnetically stirred at 400 r.p.m. Due to the relatively low solubility of NH4HCO3 (around 14.3 g in 100 mL of water), solid was precipitated in the absorbing solution. After the reaction, solid NH4HCO3 was obtained by vacuum filtration, followed by washing with ethanol and drying at room temperature. The remaining unabsorbed NH3 from water and CO2-saturated water was collected by a second absorbing solution containing 400 mL of 5 M H2SO4.
The catalysts were deposited onto the electrode substrates by spray coating. For the preparation of the anode, a plain carbon cloth was first treated in HNO3 (67-70%) at 110° C. for 1 h 45 min to improve its hydrophilicity. The catalyst ink was prepared by dispersing PtIr/C and AS-4 ionomer in 2-propanol (10 mgcatalyst mL−1), with a weight ratio of 9:1 between the catalyst and dry ionomer. The ink was then spray-coated onto the hydrophilic carbon cloth. For the cathode, the catalyst ink was prepared by dispersing Pt/C and AS-4 ionomer in a 7:3 mixture of 2-propanol and water (10 mgcatalyst mL−1), and the weight ratio between the catalyst and dry ionomer was 7:3, which was spray-coated onto a piece of carbon paper (Sigracet 22 BB). The final loading of platinum-group metal was 1.0 mg cm−2 for both cathode and anode.
NH3 fuel cell tests were performed with a Scribner 850e Fuel Cell Test System. The fuel cell configuration includes stainless-steel end plates, gold-coated current collectors, graphite flow-field plates with serpentine channels, PTFE and silicone gaskets, two electrodes, and an anion-exchange membrane (Tokuyama A201). The active area of the membrane-electrode assembly (MEA) was 5 cm2, which was formed after assembling the cell hardware. The cell temperature was 80° C. 75 mL of the NH3 solution obtained from MFAEL (with 1.25 M added KOH) was supplied to the anode and circulated by a peristaltic pump at a flow rate of 4 mL min−1, and the reservoir of the NH3 solution was kept at 5° C. by cooling water from a chiller. 500 mL min−1 of O2 was passed through a humidifier at 80° C. before entering the cathode flow field at atmospheric pressure.
NH3 in the absorbing solution (0.1 M H2SO4) was quantified by the indophenol blue colorimetric method. Four freshly prepared reagents were used, including (a) coloring solution, containing 0.4 M sodium salicylate and 0.32 M NaOH; (b) oxidizing solution, containing 0.75 M NaOH in NaClO solution; (c) catalyst solution, containing 10 mg ml−1 of Na2[Fe(CN)5NO].2H2O; and (d) 6 M NaOH solution. The sample solution was first diluted with 0.1 M H2SO4 to the proper range of NH3 concentration. 4 mL of the diluted sample solution was then added into a glass vial, followed by the sequential addition of 200 μL of (d), 50 μL of (b), 500 μL of (a), and 50 μL of (c). The reagents were mixed by shaking vigorously and kept in a dark place for color development. After 2 h, absorbance was measured by a UV-Vis spectrophotometer (Shimadzu UV-2700) at 660 nm. The calibration curve was established by testing a series of standard NH3 solutions ranging from 0 to 2.5 mg L−1 (in NH3—N) diluted with 0.1 M H2SO4.
For the 15N isotope labeling experiment, the concentrations of 14NH3 and 15NH3 (in 0.1 M H2SO4) were determined by 1H Nuclear Magnetic Resonance (NMR) spectroscopy on an NMR spectrometer (Bruker Avance NEO 400 MHz). The sample solution was first diluted with 0.1 M H2SO4 to the proper range of NH3 concentration. 800 μL of the diluted sample solution was then mixed with 200 μL of DMSO-d6 and 200 μL of 32 μM maleic acid in DMSO-d6 (internal standard). The scan number was 1,024 with a water suppression method. Standard 14NH3 and 15NH3 solutions were prepared for calibration with concentrations ranging from 0 to 5 mg L−1 (in 14N and 15N). NH3 content in CO2-saturated water was also quantified by 1H NMR due to the pH-sensitive nature of the colorimetric method.
Ion chromatography (IC) was also employed for NH3 quantification to verify the accuracy. IC measurements were performed on a Dionex™ Easion system equipped with a conductivity detector, 4 mm Dionex IonPac CG12A/CS12A columns, and a CCRS 500 suppressor. The mobile phase was 20 mM methanesulfonic acid, and was pumped at a flow rate of 1.0 mL min−1. The running time was 8 min. The calibration solutions were prepared with (NH4)2SO4 in the concentration range of 20-100 mg L−1 (in NH3—N).
NO3− and NO2− in the diluted electrolyte were analyzed by High-Performance Liquid Chromatography (HPLC) (Chou et al., J. Food Drug Anal. 11:233-238 (2003), which is hereby incorporated by reference in its entirety) (Agilent Technologies, 260 Infinity II LC System) equipped with a variable wavelength detector (Agilent 1260 Infinity Variable Wavelength Detector VL). The wavelength of 213 nm was chosen for NO3− detection. A C18 HPLC column (Gemini® 3 μm, 110 Å, 100×3 mm) was used for analysis at 25° C. with a binary gradient pumping method to drive the mobile phase at 0.4 mL min−1. The mobile phase included 0.01 M n-Octylamine in a mixed solution containing 30 vol. % methanol and 70 vol. % deionized water, and the pH of the mobile phase was adjusted to 7.0 with phosphoric acid. The running time was 30 min. The calibration solutions for NO3− or NO2− were prepared with KNO3 or KNO2 in the concentration range of 0.0625-2 mM.
To identify the products from the oxidation of C—N bonds, 13C-labeled glycine and alanine were used as simple organic Nr compounds as the reactants in MFAEL, and the products were analyzed by 13C NMR on an NMR spectrometer (Bruker Avance NEO 400 MHz). 1 mL of the sample solution (diluted electrolyte) was mixed with 200 μL of D2O and 200 μL of 50 mg mL−1 DSS solution (internal standard). The scan number was 128.
To quantify the reactant (alanine) and product (acetate) after electrolysis, 1H NMR was carried out on a Bruker AVIII-600 MHz NMR spectrometer. 400 μL of the sample solution (diluted electrolyte) was mixed with 200 μL of D2O and 100 μL of 15 mM dimethylmalonic acid (DMMA) solution (internal standard). The scan number was 8. The calibration solutions for alanine and acetate were prepared in the concentration range of 0-20 mM.
The carboxylic acid products were also identified and quantified by HPLC. The wavelength of 220 nm was selected. An OA-1000 organic acids column (Grace®, length: 300 mm, ID: 6.5 mm, part no. 9046) was used for analysis at 25° C. with a binary gradient pumping method to drive the mobile phase (5 mM sulfuric acid) at 0.6 mL min−1. The running time was 30 min. Solutions prepared by a series of standard chemicals were also tested by 13C NMR and HPLC for product identification, including carbonate, formate, glycolate, glyoxylate, oxamate, oxalate, lactate, pyruvate, acetate, and acrylate.
The gaseous products of NO3RR in the NaOH/KOH/H2O electrolyte were analyzed by online gas chromatography (SRI Instruments, 8610C, Multiple Gas #3) equipped with HayeSep D and Mol Sieve 5 Å columns and a thermal conductivity detector. The MFAEL was operated under the same conditions specified below, except that Ar was used as the carrier gas at a lower total flow rate of 85 mL min−1. During the measurement, an 8-min programmed cycle was repeated, including 6 min of the GC running period and 2 min of the cooling period.
For each cycle, the generation of product i (ni, mol) was calculated by
where ci is the concentration (ppmv) of product i; {dot over (V)} is the volumetric flow rate of the gas (mL min−1); p is the atmospheric pressure (p=1.013×105 Pa); R is the gas constant (R=8.314 J mol−1 K−1); T is the room temperature (293.15 K); t is the running time of each cycle (min). The calibration curves of H2 (10-10,000 ppm) and N2 (100-100,000 ppm) were established by analyzing the standard calibration gases.
X-Ray Diffraction (XRD) crystallography was carried out on a Rigaku Smartlab high-resolution X-ray diffractometer with Cu K-alpha radiation (wavelength, λ=1.5406 Å) and a tube voltage of 40 kV (with a tube current of 30 mA). The scan was performed at a rate of 10° min−1 and a step size of 0.01°. Scanning electron microscopy (SEM) imaging and Energy Dispersive X-ray Spectroscopy (EDS) were performed on a FEI Quanta-250 field-emission scanning electron microscope with a light-element X-ray detector and an Oxford Aztec energy-dispersive X-ray analysis system. X-ray Photoelectron Spectroscopy (XPS) was performed on a Kratos Amicus/ESCA 3400 X-ray photoelectron spectrometer with Mg K-alpha X-ray (1,253.7 eV), and all spectra were calibrated with the C 1s peak at 284.8 eV. Raman spectra were collected using an inVia 488 nm Renishaw Coherent Laser Raman Spectrometer calibrated to an internal standard silicon reference centered at 520.5±0.5 cm−1. Samples were tested under a 20× objective lens, with a spot size of ˜2500 μm2, from 100-4000 cm−1 with 10 accumulations at 12.5 mW power.
With ultrahigh alkalinity, the “NaOH/KOH/H2O” electrolyte was first introduced in an attempt to convert N2 to NH3, but the system was later confirmed to completely reduce NOx−—N even at a trace amount to NH3 on simple metal electrodes (Licht et al., Science 345:637-640 (2014); Licht et al., Science 369:780 (2020), Chen et al., Nat. Catal. 3:1055-1061 (2020); which are hereby incorporated by reference in their entirety). Such an unexpected finding implies that this strongly alkaline electrolyte holds the potential of efficiently converting Nr into NH3 for the alternative upcycling of waste nitrogen.
Thermodynamic analysis performed in this work (
Motivated by these results, the Nr-to-NH3 conversion in the NaOH/KOH/H2O electrolyte on simple nickel (mesh and foam) electrodes at a range of elevated temperature of 80-200° C. in a one-compartment MFAEL system was investigated (
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A series of control experiments performed in this study (
Online gas chromatography (GC) also confirmed that HER is largely suppressed with a very low level of FE (e.g., an average FE of 5.35% at 250 mA cm−2), and N2 generation was not detected during the entire course of electrolysis (
Interestingly, replacing the carrier gas (high-purityN2) with air or high-purity O2 does not induce any considerable change in the cell performance (
High alkalinity of NaOH/KOH/H2O electrolyte is needed for the high-efficiency NO3−-to-NH3 conversion in MFAEL. 1:1 molar NaOH/KOH was chosen to constitute the best composition of ternary NaOH/KOH/H2O electrolyte due to the optimal performance and the maximum window for tuning water content, compared to the binary NaOH/H2O or KOH/H2O electrolyte (
Notably, the re-deposition of partially oxidized nickel species on cathode was observed during electrolysis, which extends the electrochemical surface area contributing to the high-performance NO3−-to-NH3 conversion. While no substantial change was found on the anode in the post-electrolysis characterization by scanning electron microscopy (SEM), the formation of nanoparticles in ˜100 nm and larger hexagonal flakes in 1-2.5 μm was found on the cathode (
The energy-dispersive X-ray spectroscopy (EDS) analysis reveals the Ni/O atomic ratio of 3.66 and 0.72 on the nanoparticles respectively; and an overall increase in oxygen content from 1.2 at. % before electrolysis to 24.3 at. % afterwards (
The formation of those cathodic deposits should come from the migration of Ni from the anode to the cathode during electrolysis (namely, re-deposition): anodic Ni is initially oxidized to Ni(OH)2/NiOOH which is an active catalyst for the oxygen evolution reaction (OER) (Klaus et al., J. Phys. Chem. C 119:7243-7254 (2015), which is hereby incorporated by reference in its entirety), followed by its partial dissolution in the strongly alkaline electrolyte in forms of Ni(OH)3− or Ni(OH)42− (Ye et al., Chem. Commun. 54:10116-10119 (2018), which is hereby incorporated by reference in its entirety); subsequently, these soluble Ni(II) species are re-deposited onto the cathode. When a Cu mesh was used as the cathode while keeping the Ni mesh as the anode, similar deposits were observed (
It should be noted that such a re-deposition occurs only within the near-surface region of the electrodes while the bulk composition of the electrodes remains largely unchanged, as evidenced by the X-ray diffraction (XRD) (
Thanks to the high activity and operational robustness of the MFAEL, the reaction capacity was increased from 100 mL to 2.5 L under industrial-relevant conditions (
The produced NH3 from the MFAEL can be managed in different forms: NH4+ salts (such as sulfate), aqueous NH3 solutions, and a solid NH4HCO3 product (
Alternatively, when water (5° C.) is used for NH3 absorption, despite a slightly lower collection efficiency (95.6%) (
In another case, the NH3-containing outlet gas from MFAEL was absorbed by a CO2-bubbling water solution at 5° C. Owing to the acidity of CO2, NH3 collection efficiency as high as 99.9% was achieved (
Thus far, the OER has been the anodic reaction in the investigated systems, which does not produce value-added products itself. Alternatively, a paired electrolysis system can be constructed by combining the reduction of NO3 (on cathode) and oxidation of C—N bonds in organic Nr compounds (on anode) in one electrolytic cell (
To examine the NH3 formation from organic Nr in NaOH/KOH/H2O, a series of N-containing compounds was first screened with representative chemical environments of N element (12 organic Nr compounds and 3 inorganic Nr compounds) at 200° C. with an applied current density of 25 mA cm−2 (
The products after the cleavage of C—N bonds in NaOH/KOH/H2O was then investigated (
H2N—CH2—COO− (glycine)+5OH−→C2O42−+NH3+3H2O+4e− (2)
H2N—CH(CH3)—COO− (alanine)+6OH−→CH3COO−+CO32−+NH3+3H2O+4e− (3)
Similar results should be expected for Nr in more complex structures, demonstrating that MFAEL is capable of converting organic N-containing wastes into value-added carboxylic acid products, while largely retaining the skeleton of the original molecules. Additional experimental results (detailed in
Knowing that NH3 can be produced via the oxidation-assisted cleavage of C—N bonds, the reduction of N—O bonds was paired with the oxidation of C—N bonds, aiming to generate NH3 from both sources (
2H2N—CH(R)—COO−+NO3−+3OH−→2R—COO−+2CO22−+3NH3 (4)
Notably, to determine the respective contribution of NH3 production from each source, the N—O reactant was isotopically labeled using K15NO3, and the NH3 product was analyzed by 1H NMR to differentiate 14NH3 and 15NH3. With this configuration operated at 100 mA cm−2, 1H NMR suggests that the produced NH3 is derived from both N—O reduction and C—N oxidation with their corresponding FE of 72.3% and 52.1%, respectively (
In this work, an integrated sustainable process was presented for economically upcycling waste nitrogen. In particular, a versatile, robust, and inexpensive MFAEL system was developed to convert various forms of waste Nr into NH3 convergently. Taking advantage of its strong tendency towards hydrogenating N—O bonds, a partial current density as high as 4.22±0.25 A cm−2 for NH3 production was achieved by NO3− reduction without generating considerable N—N coupling products.
Upscaling the MFAEL system is straightforward due to its structural simplicity and inexpensiveness of its components. The 2.5 L scaled-up reactor is capable of producing NH3 at 25 A with an average FE of 70.4% from NO3RR. By properly choosing the NH3 absorbing condition, different forms of pure NH3-based chemicals (NH4+ salts, NH3 solution, and solid NH4HCO3) can be continuously produced from the conversion of waste Nr in MFAEL. Since the NH3 product from MFAEL is in a gas mixture, pure NH3 gas can be obtained through established economical gas separation technologies (such as pressure swing adsorption) without the need for additional distillation steps (Wang et al., Energy Environ Sci 14:2535-2548 (2021), which is hereby incorporated by reference in its entirety). Use of organic or inorganic additives could increase the co-absorption efficiency of MFAEL-derived NH3 and waste CO2 (Wang et al., Appl. Energy 230:734-749 (2018), which is hereby incorporated by reference in its entirety), making it a promising dual-purpose process that fixes waste N and C into one useful chemical product NH4HCO3. The resemblance of MFAEL configuration to the alkaline water electrolyzers (typically operated at 70-90° C. with 25-35 wt. % of KOH solutions (Guillet and Millet, in Hydrogen Production, pp. 117-166 (2015), which is hereby incorporated by reference in its entirety)) has suggested a clear potential towards commercialization, since the latter has been commercially available for over 50 years.
The feasibility of concentrating NO3− by a low-energy cost electrodialysis process was validated both experimentally and analytically via a comprehensive TEA study. Combining NO3− concentrating by electrodialysis and its reduction in MFAEL generates a competitive levelized total cost of the waste-derived NH3 product, largely owing to the remarkably low material cost of the MFAEL system.
In the experiments described herein, Ni was chosen as the electrode material primarily due to its inexpensiveness and its excellent corrosion resistance. Not limited to Ni, other metals such as Co, Ru, and Cu can also serve as the cathode in the KOH/NaOH/H2O electrolyte, and their performance comparison under the same test conditions is shown in
In the NaOH/KOH/H2O electrolyte, C—N bonds in organic Nr compounds can be oxidized to produce NH3. By controlling the operating conditions of MFAEL, ˜100% recovery of most common forms of Nr into NH3 can be realized, making it a sensitive and accurate tool for determining N content in complex real-world samples. Oxidation of C—N bonds results in the production of carboxylic acids as a potentially value-added by-product, and pairing the oxidation of C—N bonds (on anode) with the reduction of N—O bonds (on cathode) in MFAEL leads to a cathodic and anodic FE of 72.3% and 52.1% for NH3 production at 100 mA cm−2, respectively, demonstrating its capability of extracting N element from real waste containing both oxidative and reductive forms of Nr.
Although preferred embodiments have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the claims which follow.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 63/415,133, filed Oct. 11, 2022, which is hereby incorporated by reference in its entirety.
This invention was made with government support under grant number CHE2036944 awarded by the National Science Foundation. The government has certain rights in the invention.
Number | Date | Country | |
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63415133 | Oct 2022 | US |