DOE OSTI2010
From 2005 to 2010, The Dow Chemical Company (formerly Rohm and Haas Company) was a member of the Department of Energy Center of Excellence on Chemical Hydrogen Storage, which conducted research to identify and develop chemical hydrogen storage materials having the potential to achieve DOE performance targets established for on-board vehicular application. In collaboration with Center co-leads Los Alamos National Laboratory (LANL) and Pacific Northwest National Laboratory (PNNL), and other Center partners, Dow's efforts were directed towards defining and evaluating novel chemistries for producing chemical hydrides and processes for spent fuel regeneration. In Phase 1 of this project, emphasis was placed on sodium borohydride (NaBH{sub 4}), long considered a strong candidate for hydrogen storage because of its high hydrogen storage capacity, well characterized hydrogen release chemistry, safety, and functionality. Various chemical pathways for regenerating NaBH{sub 4} from spent sodium borate solution were investigated, with the objective of meeting the 2010/2015 DOE targets of $2-3/gal gasoline equivalent at the pump ($2-3/kg H{sub 2}) for on-board hydrogen storage systems and an overall 60% energy efficiency. With the September 2007 No-Go decision for NaBH{sub 4} as an on-board hydrogen storage medium, focus was shifted to ammonia borane (AB) for on-board hydrogen storage and delivery. However, NaBH{sub 4} is a key building block to most boron-based fuels, and the ability to produce NaBH{sub 4} in an energy-efficient, cost-effective, and environmentally sound manner is critical to the viability of AB, as well as many leading materials under consideration by the Metal Hydride Center of Excellence. Therefore, in Phase 2, research continued towards identifying and developing a single low-cost NaBH4 synthetic route for cost-efficient AB first fill, and conducting baseline cost estimates for first fill and regenerated AB using a variety of synthetic routes. This project utilized an engineering-guided R&D approach, which involved the rapid down-selection of a large number of options (chemical pathways to NaBH{sub 4}) to a smaller, more manageable number. The research began by conducting an extensive review of the technical and patent literature to identify all possible options. The down-selection was based on evaluation of the options against a set of metrics, and to a large extent occurred before experimentation was initiated. Given the vast amount of literature and patents that has evolved over the years, this approach helped to focus efforts and resources on the options with the highest technical and commercial probability of success. Additionally, a detailed engineering analysis methodology was developed for conducting the cost and energy-efficiency calculations. The methodology utilized a number of inputs and tools (Aspen PEA{trademark}, FCHTool, and H2A). The down-selection of chemical pathways to NaBH{sub 4} identified three options that were subsequently pursued experimentally. Metal reduction of borate was investigated in Dow's laboratories, research on electrochemical routes to NaBH{sub 4} was conducted at Pennsylvania State University, and Idaho National Laboratory researchers examined various carbothermal routes for producing NaBH{sub 4} from borate. The electrochemical and carbothermal studies did not yield sufficiently positive results. However, NaBH{sub 4} was produced in high yields and purities by an aluminum-based metal reduction pathway. Solid-solid reactive milling, slurry milling, and solution-phase approaches to metal reduction were investigated, and while both reactive milling and solution-phase routes point to fully recyclable processes, the scale-up of reactive milling processes to produce NaBH{sub 4} is expected to be difficult. Alternatively, a low-cost solution-phase approach to NaBH{sub 4} has been identified that is based on conventional process unit operations and should be amenable to scale-up. Numerous advances in AB synthesis have been made in recent years to improve AB yields and purities. Process analysis of several leading routes to AB (Purdue's formate-based metathesis route and PNNL's NH{sub 4}BH{sub 4}-based route) indicated the cost to produce first-fill AB to be on the order of $9-10/kg AB, assuming a NaBH{sub 4} cost of $5/kg for a 10,000 metric tons/year sized AB plant. The analysis showed that the dominant cost component for producing first-fill AB is the cost of the NaBH4 raw material. At this AB cost and assuming 2.5 moles hydrogen released per mole of AB, it may be possible to meet DOE's 2010 storage system cost target, but the 2015 target will likely require lower cost AB and demonstrates the importance of having a low-cost route to NaBH{sub 4}. Substantial progress has also been made to define feasible pathways for the regeneration of spent ammonia borane fuel.