Progress 09/01/16 to 08/31/21
Outputs
Target Audience:Government funding offices, Academia Changes/Problems:Personnel changes and COVID impacted the progress of this project, resulting in multiple no-cost extensions. The subcontractor, National Renewable Energy Laboratory, withdrew from this project What opportunities for training and professional development has the project provided?
Nothing Reported
How have the results been disseminated to communities of interest?
Nothing Reported
What do you plan to do during the next reporting period to accomplish the goals?
Nothing Reported
Impacts
What was accomplished under these goals?
Over the course of this project, the composite anion exchange membrane went through several generations of designs and materials were modified and improved to increase durability and performance. The final membrane material utilized strontium zirconate, which was mixed with alumina and aluminum powders in a recipe developed in-house to improve strength and durability. This membrane demonstrated excellent stability and low resistance during electrolysis at reduced temperatures (400 ?C), thus enabling improved ammonia synthesis performance. The catalysts were also iteratively improved over the course of this project, with prior results informing subsequent synthesis techniques, materials, and design choices. The top-performing catalysts for ammonia synthesis ended up being precious metal alloy nanoparticles, including RuCu and RuNi. Many other catalysts, including FeO3 and other non-precious metal-based materials, were also tested for nitrogen reduction reaction (NRR) activity. For our best catalysts, we were able to produce ammonia at a rate of nearly 2x10-6 mol h-1 cm-2. These results were very consistent over a variety of conditions, indicating that we have developed a robust system for production of ammonia using only water and nitrogen as inputs. Our unique cell design and components allowed operation over a range of temperatures from 350 to 500 °C, significantly lower than the temperatures for current state-of-the-art ammonia synthesis systems.
Publications
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Progress 09/01/20 to 08/31/21
Outputs
Target Audience:
Nothing Reported
Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?
Nothing Reported
How have the results been disseminated to communities of interest?
Nothing Reported
What do you plan to do during the next reporting period to accomplish the goals?
Nothing Reported
Impacts
What was accomplished under these goals?
During this past year we synthesized RuCu and RuNi catalysts for use as ammonia production catalysts from water and nitrogen. These catalysts were then used in ammonia synthesis cells using our new Strontium Zirconate matrix and molten hydroxide electrolyte. These cells were assembled and tested at various temperatures, flow rates, currents and voltages to optimize ammonia synthesis rates. Using the RuNi catalyst under optimized conditions, we were able to produce ammonia at a rate of nearly 2x10-6 mol h-1 cm-2. These results were very consistent over a variety of conditions, indicating that we have developed a robust system for production of ammonia using only water and nitrogen as inputs. Our unique cell design and components allow operation over a range of temperatures from 350 to 500 °C, and can produce ammonia at a rate of 1x10-6 mol h-1 cm-2 at only 400 °C, significantly lower than current SOA ammonia synthesis systems.
Publications
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Progress 09/01/19 to 08/31/20
Outputs
Target Audience:
Nothing Reported
Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?
Nothing Reported
How have the results been disseminated to communities of interest?
Nothing Reported
What do you plan to do during the next reporting period to accomplish the goals?The following work will be pursued: We will synthesize and integrate some of the higher-performing NRR catalysts, including RuCu and RuNi, which showed promising results early in this project, into our new cell design to enable higher ammonia synthesis rates at lower temperatures. We will optimize the operating conditions of the ammonia synthesis to maximize ammonia production rates and cell stability for long-term operation. We will perform cost analysis of the system integrated with renewable energy to calculate the levelized cost of ammonia using our system design. As part of this analysis, we will compare the economics of direct ammonia synthesis using water and nitrogen vs. a two-step process of forming hydrogen with our electrolysis cell first, followed by chemical ammonia synthesis from hydrogen and nitrogen. We will consider whether coupling the high temperature alkaline water electrolyzer with improved Haber-Bosch process to replace direct ammonia synthesis from nitrogen and water may be a more effective method to produce ammonia in a way that significantly reduces energy consumption and carbon emissions.
Impacts
What was accomplished under these goals?
Developed new matrix material that demonstrates improved stability under molten hydroxide conditions Developed methods for reproducibly casting ceramic matrices of controlled porosity, pore size, and structure Optimized matrix composition for increased stability and strength under electrolysis conditions Demonstrated excellent stability and low resistance during electrolysis at reduced temperatures (400 ?C), enabling improved ammonia synthesis performance Demonstrated Electrochemical Ammonia Synthesis using redesigned cell components Evaluated RuNi and Fe2O3 catalysts for EAS Successfully demonstrated ammonia production at 400 ?C using newly developed composite matrix
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Progress 09/01/18 to 08/31/19
Outputs
Target Audience:
Nothing Reported
Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?
Nothing Reported
How have the results been disseminated to communities of interest?We have started to contact ammonia plants and renewable energy farm to introduce our renewable ammonia technology using solar, nitrogen and water. What do you plan to do during the next reporting period to accomplish the goals?In this project, the operating conditions of the ammonia synthesis will be optimized to maximize the button cell performance and ammonia production rate. After we reach the target, the following work will be pursued. Further improve the EAS rate to 10-4 mol. h-1 cm-2. Scale up the processes from objectives 1-3 and build an electrochemical test in a 250-W ammonia synthesis stack; Integrate the 250-W ammonia synthesis stack with renewable energy at NREL; Perform cost analysis of the system integrated with renewable energy
Impacts
What was accomplished under these goals?
Optimized the stability of a series of matrix materials in at 550 °C under 3% H2O-N2 atmosphere Reformulated electrolyte matrix has been developed using a tape casting process. Developed and successfully synthesized series of bimetallic Ru-based and Pt-based catalysts for electrochemical ammonia synthesis. Constructed a test set-up for ammonia production and cell has been successfully demonstrated at 550 °C. Achieved NH3 production rate of 2.01 x10-6 mol. h-1.cm-2 at a current density of 100 mA/cm2.
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Progress 09/01/17 to 08/31/18
Outputs
Target Audience:
Nothing Reported
Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?
Nothing Reported
How have the results been disseminated to communities of interest?
Nothing Reported
What do you plan to do during the next reporting period to accomplish the goals?Plans for Next Reporting Period and Key Issues: -The anode, cathode, and the composite electrolyte will be optimized to construct new button cells to evaluate the electrochemical performance. -To mitigate the HER at higher potentials, we would try to dope oxides of Bi, La, and Ce that are known to have high overpotentials for HER. -Design and synthesize the metal alloy based electrocatalysts with different morphology and high activity facets to boost the NRR activity. ?
Impacts
What was accomplished under these goals?
?Objective 1 DevelopHybrid Alkaline Electrolyte Membranes (AEMs) Task 1.1: a) Stability of Metal Oxide in Molten Hydroxide MeltsThe purpose of this task is to evaluate the phase and microstructural changes of metal oxides in hydroxide mediums. We performed immersion tests of the advanced metal oxide (MO-3) in molten (Li0.52/Na0.48) OH melts at 550 °C for 10 h and 100 h, respectively. The XRD patterns of the MO-3 powders were obtained after immersion in molten (Li/Na)OH at 550 °C for different lengths of time (10 and 100 hrs). We observed that the MO-3 powder was stable and no chemical reactions occurred with molten Li/Na hydroxide melt under air atmosphere for 100 h tests. Task 1.1: b) Stability of the Matrices in Molten Hydroxide Melts We also performed a stability test for the sintered MO-1 matrix in molten (Li0.52-Na0.48)OH in an air atmosphere at 550 °C for 100 h. Prior to the stability test, the fabricated green sheet matrix was burned out at 550 °C under an air atmosphere for 2 h. The stability test of the matrix was carried out in an alumina crucible that contained the sintered matrix in molten (Li0.52-Na0.48)OH melts. The particle size remained constant and the microstructure did not change after the immersion test was performed for more than 100 h. Task 1.2: Composite Electrolyte OH-IonConductivity Measurement:The purpose of this task is to develop the composite electrolyte with higher OH-ion conductivity with selected single or binary compositions of the hydroxides, to ensure sufficient conductivity for ammonia production. In this reporting period, we improved the OH- ion conductivity by four times, using a binary mixture of the electrolyte samples. We successfully developed the high-temperature alkaline electrolyte composite based on porous metal oxides with designed pore structures that effectively retain molten hydroxides and evaluated their conductivity and structure. The binary LiOH/NaOH hydroxide melt showed the highest OH- ion conductivity, 0.53 S/cm at 550 °C. The conductivity of the single/binary hydroxide electrolyte improved with increasing temperature.It is clearly seen that the ASR decreased as the temperature increased from 200 to 550 °C, regardless of the matrix thickness. For the matrix with a thickness of 0.11 mm (110 µm), the calculated ASR is less than 0.02 Ohm-cm2 at 550°C. Task 1.3:Fabrication of Hybrid Alkaline Electrolyte Membranes (AEMs): The hybrid alkaline exchange membrane (AEM) provides ionic transport, gas separation, and perimeter seal formation. It is a layer of densely packed powder bed impregnated by alkali electrolyte to form a composite paste-like structure at the operating temperature. Thinner aluminate and zirconate based matrices were successfully fabricated using the tape casting method. The green tape fabrication of the electrolyte support matrix was optimized for a solvent based slurry formulation process. MO1 and MO3 matrices were also fabricated through a tape casting method. No cracks were found in the green sheet tape after the drying process or after sintering at 550 °C in air atmosphere for 2 h. Objective 2: Synthesis and Fabrication of Cathode and Anode Electrodes Commercial NRR active catalyst Fe2O3 (micron size) was initially used to validate the EAS system using molten hydroxide electrolyte. Giner has optimized the fabrication process of anode and cathode catalysts for electrochemical ammonia synthesis (EAS) system using the tape-casting method. Giner was actively involved in the development of the high-performance nitrogen reduction reaction (NRR) metal alloys based catalysts for electrocatalytic NH3 synthesis. The N2 adsorption and N-N dissociation are more favorable on the surface of metal-based catalysts, which could increase the performance for the nitrogen reduction reaction. Task 2.1: Synthesis of Bimetallic Alloys NRR Catalysts: A polyol reduction method was employed to prepare Ru-based alloys catalysts by simultaneous reduction and decomposition of metal precursors with polyvinylpyrrolidone (PVP) in the presence of ethylene glycol (EG). Successfully synthesized new series of the Ru-based bimetallic alloys have been developed to improved NRR activity. X-ray diffraction (XRD) and scanning electron microscopy (SEM)-Energy dispersive spectroscopy (EDS) were used to identify the crystal structures, surface morphologies, and chemical compositions of the prepared Ru-based bimetallic alloy catalysts. Objective 3: Assembly and Testing of Electrochemical Ammonia Synthesis Button Cell Task 3.1: Construct Electrochemical Ammonia Synthesis Button Cell: The 13 cm2 button cell was assembled and tested for electrochemical ammonia synthesis. Giner has optimized a series of newly developed NRR catalysts for the EAS system using HTAE membrane. On the cathode, the newly developed cathode NRR catalyst (Ru-metal alloy and Fe-based) catalysts were employed on a Ti-sintered support with the loading of 3 mg/cm2. On the anode, iridium black catalyst was employed on the Ti-sintered support for the oxygen evolution reaction (OER) with the loading of 0.7 mg-Ir/cm2 (20% ionomer). The hydroxide electrolyte powders were stored in the anode channel of the separator. During the pretreatment process, after the organic additives burned out, the hydroxide electrolyte easily penetrated into the matrix from the anode channel. Button cell components such as anode and cathode frame were made from high corrosion resistance stainless steel (SS316) and both electrodes current collectors were made of gold-plated Ni-material. The anode and cathode catalysts were coated on the Ti-sintered support using the paintbrush method. Metal oxide (MO-3) based anion exchange membrane (Matrix) was used in all EAS experiments. Task 3.2: Analysis and Performance of Electrochemical Ammonia Synthesis Button Cell: The ammonia produced was collected in a 0.05 M HCl solution from the cathode outlet stream of the button cell and was measured using an in-house ion chromatograph. We applied the different current densities in the range of 25 to 200 mA/cm2 and collected ammonia at each current density over a 1.5 h time period. The FeS2 catalyst showed significant NH3 production rate of 3.43 x10-7 mol. h-1.cm-2 at a low current density (100 mA/cm2) compared to the commercial RuAg catalyst with a production rate of 2.17 x10-7 mol. h-1.cm-2 at a higher current density at 550 °C. The ammonia formation rate was increased with increasing the temperature from 450 to 550 °C. At a temperature of 550 °C, the ammonia production rate of 3.43 x 10-7 mol. h-1.cm-2 at a current density of 100 mA/cm2 was observed. The NH3 production rate was high (1.67 x 10-7 mol. h-1.cm-2) at a higher current density (200 mA/cm2) at 500 °C. The steady-state NH3 production rate (2.97 x 10-7 mol. h-1.cm-2) was observed over low to the higher current density at 450 °C (25-200mA/cm2). Newly developed RuAg/RuFe catalyst at Giner for electrochemical synthesis of ammonia in HTAE system was investigated at 550 °C and showed better performance, with the production rate of 8.69 x10-7 mol. h-1.cm-2 at a lower current density of 53 mA/cm2 compared to a commercial RuAg production rate of 1.32 x10-7 mol. h-1.cm-2 at a higher current density of 130 mA/cm2 at 550 °C. The RuCu-catalyst showed a significant NH3 production rate of 2.01 x 10-6 mol h-1 cm-2 at 100 mA/cm2 compared to RuFe or RuAg catalyst with a production rate of 3.24 x10-7 or 1.10 x10-7 mol h-1 cm-2 at the same current density at 550 °C. We also observed that the production rate of NH3 was higher in the molten Na-Cs electrolyte system to be compared to the molten Li-Na electrolyte at a temperature of 550 °C. The RuCu catalyst system shows no NH3 production when argon gas was introduced as a reactant at 550 °C. The higher potential under those conditions indicates that water electrolysis is dominant under argon gas.
Publications
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Progress 09/01/16 to 08/31/17
Outputs
Target Audience:
Nothing Reported
Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided?
Nothing Reported
How have the results been disseminated to communities of interest?We have started to contact ammonia plants and renewable energy farm to introduce our renewable ammonia technology using solar, nitrogen and water. What do you plan to do during the next reporting period to accomplish the goals?In this project, the operating conditions of the ammonia synthesis will be optimized to maximize the button cell performance and ammonia production rate. After we reach the target, the following work will be pursued. Scale up the processes from objectives 1-3 and build electrochemical test in a 250-W ammonia synthesis stack; Integrate the 250-W ammonia synthesis stack with renewable energy at NREL; Perform cost analysis of the system integrated with renewable energy
Impacts
What was accomplished under these goals?
Major Goals of the ProjectSpecifically the goals of the Phase II program areto:Objective 1: Fabricate hybrid AEMs (Anion Exchange Membrane) that will enable operation up to a temperature of 200 °C;Objective 2: Select high-performance nanostructured catalysts for the NRR (Nitrogen reduction reaction) and OER (oxygen evolution reaction);Objective 3: Perform stated electrochemical reaction in 50 cm2single cell;Objective 4: Scale up the processes from 1-2 and build and test the electrochemical reaction in a 250-W ammonia synthesis stack;Objective 5: Integrate the 250-W ammonia synthesis stack with renewable energy at NREL;Objective 6: Perform cost analysis of the system integrated with renewable energy. The performed work in the 1styear is described in detail below:Objective 1 DevelopHybrid Alkaline Electrolyte Membranes (AEMs) Task 1: Stability of Metal Oxide in Molten Hydroxide MeltsThe purpose of this task is to evaluate the phase and microstructural changes of metal oxides in hydroxide mediums. We performed immersion tests of the advanced metal oxide (MO-1) in molten (Li0.52/Na0.48) OH melts at 550 °C for 10 h and 100 h, respectively. The test was carried out in an alumina crucible that contained the as-received metal oxide powder in amixture withmolten (Li0.52/Na0.48)OH melts. It was conducted within a box furnace at 550 °C for 10 h and 100 h under atmospheric air. The surface morphologies of the metal oxide powder before and after their immersion test in molten (Li/Na)OH at 550 °C. Task 2: Composite Electrolyte OH-IonConductivity Measurement:The purpose of this task is to develop the composite electrolyte with higher OH-ion conductivity with selected single or binary compositions of the hydroxides, to ensure sufficient conductivity for ammonia production. An experimental device was developed to measure the OH-ion conductivity of the single or binary mixture of the electrolyte samples.The green pellets were sintered to achieve uniform distribution of hydroxide melt within the metal oxides. The sintered pellets of mixed hydroxide and metal oxide were painted with silver paste on both ends to ensure a good contact between the sample surface and silver current collector.The Pt wires and Ag-current collector were attached to the sintered Ag-coated pellet.The average composite electrolyte powder consists of ~50 wt. % of hydroxide and ~50 wt. % of metal oxide with a thicknessof ~1.3 mm. The temperature dependences of OH-ion conductivities for the pelletized composite single/binary hydroxides was measured. The single and binary hydroxides show OH-ion conductivity of 0.078 S.cm-1and 0.39 S.cm-1at 350 °C, respectively. Therefore, we have surpassed the major millstone of Month 6 of Year 1 "Composite electrolyte OH-ion conductivity > 0.05 S/cm".Task 3:Fabrication of Hybrid Alkaline Electrolyte Membranes (AEMs): The hybrid alkaline exchange membrane (AEM) provides ionic transport, gasseparation, and perimeter seal formation. It is a layer of densely packed powder bedimpregnated byalkali electrolyte to form a composite paste-like structure at the operating temperature. Theceramic-basedalkaline exchange membrane was successfully fabricated via the tape casting method.The process flowchart for the steps involved in the fabrication process of a membrane green tape. The slip was poured onto a Mylar sheet set on a bench top caster and spread as a thin layer with a casting speed 1 cm/sec and doctor blade adjusted to a height of approximately 0.90 mm and tapecastedby the doctor blade to obtain a green tape. The green tape was then allowed to dry using a heating system at 90 °C for 2 h. The microstructure of the sintered matrix was analyzed through SEM. The porous structure was observed of the sintered MO-1matrix prepared by the tape casting method. The mercury intrusion porosimetry was used to characterize the porosity and median pore size of the sintered matrices. The median pore size diameters of the prepared MO-1,MO-2and MO-3 matrices were 0.096, 0.301 and 0.078 µm, respectively. The porosity of all prepared matrices was within the range of 58 to 65 %. The pore size distributions of the prepared matrices after sintering at 550 °C for 2 h. The narrow pore size distribution was seen in both MO-1and MO-3 matrices, equating to these matrices being able to retain the electrolyte duringlong-termoperation. The wide pore size distribution found in the MO-2matrix was due to a larger particle size compared to the MO-1 and MO-3 matrices.Objective 2: Fabrication of Cathode and Anode Electrodes The baseline anode (catalyst coated on Ti-sinter) and cathode (CD-1) electrodes were fabricated through the tape casting method. Examples of dried green sheet electrodes fabricated. The microstructure of the cathode electrodes was analyzed through SEM. A very porous structure in the cathode was observed after sintering at 550 °C in an air atmosphere for 2 h.Objective 3: Assembly and Testing of Electrochemical Ammonia Synthesis Button Cell: The 13 cm2button cell was assembled and tested for electrochemical ammonia synthesis.Button cell components such as anode and cathode frame, and current collectors were made with high corrosion resistance stainless steel (S316) materials. The hydroxide electrolyte powders were stored in the cathode channel of the separator. During the pretreatment process, after the organic additives burned out, the hydroxide electrolyte easily penetrated into the matrix from the cathode channel.Commercial anode catalyst was brush-painted onto porous Ti-sintered support (thickness 0.25mm) was used as the anode baseline catalyst with a loading of 1.0 mg catalyst/cm2. For the cathode, tape casted CD-1 (thickness 0.25mm) was used to validate the testing system and produce a benchmark ammonia production rate.The test set-up for electrochemical ammonia synthesis was constructed and built within the lab. After assembly, the button cell was placed in a heating tube furnace. The effluent gas containing excess water, nitrogen and produced gases (NH3, H2) was bubbled into a dilute acetate buffer solution to effectively convert the synthesized ammonia to ammonium. The ammonium concentration was then detected by disposableion-selectiveelectrodes, created in-house, with a detection limit as low as 10-5M. The first ammonia synthesis button cell was operated at 250 °C. The maximum current density of 25 mA/cm2was achieved, when constant voltage of 3.2 V was applied.The ion selective membrane also separates the ISE's internal reference solution and Ag/AgCl reference wire from the sample solution. The ammonia rate of 3.29 x 10-8mol NH3h-1cm-2was obtained over five hours of experimentation, which corresponds to a faradaic efficiency lower than 5%. Future work will include the adaption of optimal cell components and operating conditions to improve the NH3production rate. The advanced nanostructured catalysts development for NRR and OER reactions will be initiated at Giner to improve the ammonia production rate > 10-6mol NH3h-1cm-2.Plans for Next Reporting Period and Key Issues:-The anode, cathode, and the composite electrolyte will be optimized to construct new button cells to evaluate the electrochemical performance. -The operating conditions will be optimized to maximize the button cell performance and ammonia production rate. A more detailed Annual Report is being submitted to the NIFA Program Manager.
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