Progress 06/01/14 to 01/31/15
Outputs
Target Audience:
Nothing Reported
Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided? A Small Business Diversity Postdoctoral Research Fellowship, sponsored by the National Science Foundation (NSF) and administered by the American Society for Engineering Education (ASEE) supported Julie Renner at Proton OnSite for a majority of the project period. This USDA-funded project gave her a unique learning opportunity where she conducted project management tasks, data collection and analysis and also assisted Kathy Ayers in her PI-role in directing experimentation and project scope. Additionally, two undergraduate engineering students from the University of Connecticut conducted research related to this project as part of a co-op. One student performed the ammonia crossover analysis for the AEM materials, and one student is investigating different catalyst materials and their effect on ammonia production. These co-ops expose students to an industry research and development setting, providing valuable experience for their career path decisions. 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?
On U.S. farms, 29% of energy consumed is in the form of fertilizers; these same fertilizers are the second largest contributor to green house gas emissions. Industrial processes for production of nitrogen-based fertilizers are mainly based on the Haber-Bosch process, which involves the reaction of nitrogen and hydrogenat high pressure (150-300 atm) and temperature (400°-500°C). The low conversion efficiency (~15%) gives rise to energy intensive, large scale chemical plants, with high carbon dioxide (CO2) emissions, at an installed cost of more than $1 billion per plant. More sustainable and economical production methods are required to support growing world demand. To address this need, Proton Energy Systems, in collaboration with the Colorado School of Mines and the National Institute of Standards and Technology, proposes development of an efficient solid state electrochemical process utilizing anion exchange membranes (AEMs). This AEM-based technology is ideal for ammonia synthesis because the membranes are not expected to readily react with ammonia, enable low-cost materials of construction, and allow utilization of a wider array of low-cost catalysts. To the extent that renewable electricity is utilized to drive the process, CO2emissions are eliminated from the production step. Further emission reduction is realized through decreased ammonia shipping needs. Since electrolyzer technology is highly scalable, products could support a range of small to mid-sized farms, or could be designed on a larger scale to distribute ammonia locally to multiple farms. Proton is currently designing a megawatt (MW)-scale electrolysis system; a similar scale system would be capable of producing ammonia for multiple farms. In the Plains and Upper Midwest, excess wind production capacity, transmission limitations, and high demand for N-fertilizers combine to create excellent economic drivers for this technology. The Phase 1 project goal was to show that the approaches to catalyst, membrane, and cell development could significantly increase performance, in a proof-of-concept demonstration. The demonstration was expected toshow an increase in the Faradaic efficiency from relevant literature valuesby at least an order of magnitude, resulting in at least 1% current efficiency. Phase 1 objectives were 100% completed, as summarized below, setting the technical direction for further improvements in Phase 2. Objective 1: Modify existing Proton hardware for ammonia production In this task, Proton proposed to design an AEM system based on a lab-scale 25 cm2format test cell. The design process consisted of a materials compatibility analysis, generation of piping and instrumentation diagrams (P&IDs), and a bill of materials. The system was designed for attended use in the hood, with a voltage limit of 1.56 V to prevent hydrazine formation. Safety scenarios were modeled for a full ammonia leak in the hood and in open air assuming 100% faradaic efficiency. All parts were purchased for the system and stack, and assembled according to the design review. Objective 2: Develop a nanoparticle catalyst that has increased stability and efficiency over existing catalysts Nanoparticles of various compositions were synthesized for this project and compared to platinum. Low surface area and high surface area materials were also compared. Transmission electron microscopy (TEM) was used to image the nanoparticle samples, where several particle morphologies were observed. TEM results showed the ability to control nanocatalyst morphology and surface area, which will be useful for catalyst optimization in the future. During each ammonia synthesis experiment at NIST, the current output was measured over time at the applied potential hold of 1.2 V vs. Ag/AgCl. High efficiencies (up to 52%) were achieved in the preliminary results using a high surface area, Fe-Ni catalyst. More electrochemical results can be found under Objective 4. Objective 3: Develop a membrane system that maximizes membrane lifetime and conductivity while minimizing ammonia crossover Ammonia crossover is an undesirable inefficiency in the system. AEMs have been generally very good performers for reducing gas permeation in electrolysis stacks, which is promising for ammonia gas crossover. However, it has been cited that the main route to ammonia diffusion would be through the solubility diffusion mechanism. Therefore, this diffusion mechanism was characterized in a commercially available membrane, and five different AEMs obtained from CSM. Overall, the total crossover of ammonium for the best performing samples was less than 1% per hour, at 100 ppm of ammonium on the concentrated side of the cell. Some preliminary membrane degradation experiments were also conducted on the baseline material using Fourier transform infrared spectroscopy (FTIR). The spectra were taken before and after operation in Proton's ammonia generation system. The spectra showed no changes in the materials after operation. Objective 4: Evaluate performance of down-selected materials in proof-of-concept device and demonstrate ammonia production improvements over current literature values Generally, Ni, Fe, and Fe-Ni cathode materials all showed efficiencies consistently over 1% and generally had greater performance than Pt. Fe appeared to have the highest efficiency of ~41%, but the lowest durability. Ni interestingly had increasing efficiency with time. These experiments show proof-of-concept for an order of magnitude increase in current efficiency, and provide a pathway for further development. Fe-Ni materials appear to have a combination of both Fe and Ni properties, and differences in performance may be attributable to differences in local composition. For example, some samples had degrading efficiency with time (more Fe like), whereas others had increasing efficiency (more Ni like). It is reasonable to expect that the different morphology between samples will expose more of one metal versus the other to reactants and products. NIST has analytical capabilities to understand the structure-property relationships of nanocatalysts, which will be used in the future to understand the impact of morphology and composition on performance. Alternative membranes to the baseline commercial membrane were tested operationally using the same electrodes. Testing showed that an alternative material under the same conditions could result in 1.5X greater current density at 1.2 V despite being thicker. The data shows proof-of-concept that further AEM material development can lead to higher current densities at the same voltages. Objective 5: Compare performance and cost metrics to competing technologies The operational performances of the new nanocatalysts were compared to the results obtained with Pt black, current literature values, and the Haber-Bosch process using a model developed at Proton OnSite. One important highlight is that the conservative estimate of the initial efficiency for Fe only particles (41% efficient) translates to an equivalent energy consumption rate to the Haber-Bosch process. This result establishes proof-of-concept that the AEM technology is capable reaching the performance necessary to replace the current process. While the ammonia production rate is lower than in other technologies, the AEM technology stands out as having the most potential for efficient ammonia production at low temperatures. Approaches to increase the current density and production rate in the future include membrane development toward more conductive materials, as well as catalyst development toward selective and stable materials at higher voltages. Another approach to reducing overpotential is the use of better oxygen evolution reaction (OER) catalysts, which Proton has expertise in. Reducing overall losses will allow operation at higher current density before reaching the 1.56 V safety limit.
Publications
|