Evaluation of Acid Accelerators for an Off-Grid PEM Fuel Cell Power Station Utilizing Solid Sodium Borohydride for Hydrogen Storage

Abstract

In rural areas, networkable environmental monitoring equipment is often used to provide continuous, real-time information without the need for a dedicated operator for maintenance or monitoring. However, power and energy availability are often controlling factors for operational lifetime when these systems are put on location. In these situations, batteries are often used as the main energy storage medium and require servicing or replacement when exhausted. This can lead high costs related to man-power and logistics to return and maintain the equipment at those locations. Sodium borohydride hydrolysis was once looked at as a promising way to store hydrogen for use in PEM fuel cells. Concerns regarding hydrogen storage capacity and recyclability of reaction products have prevented its use in larger transportation or energy storage applications. However, the energy density of sodium borohydride still makes the reaction suitable for specific applications where reuse is not needed. As such, present here a system utilizing solid sodium borohydride as a storage medium to meet on demand power needs of these types of systems. The system utilizes a PEM fuel cell integrated with an electro-mechanical system to control chemical metering and power management. We have previously shown results from our initial testing with solid sodium borohydride using commercial-off-the-shelf tablets for generating hydrogen when added to a reaction vessel containing water. Conceptually, a tablet would be introduced to the reaction vessel whenever additional hydrogen gas would be required. Operating in this manner would require good stability and reproducibility over multiple tablet reaction events. Initial testing showed good reaction rates when a cobalt catalyst was employed, either pre-doped into the solid tablet mixture or when the catalyst was pre-mixed with the water in the reaction vessel. Analysis of the gas mixture showed high quality hydrogen gas flows could be produced and sent to the PEM fuel cell for power production. However, concerns with the stability, safety, and handling of the catalyst have led to a need to identify a more green chemistry approach. We present here an alternative approach using common mineral and organic acids as hydrolysis accelerators that could be considered a greener approach to hydrogen generation for remote power applications. These include hydrochloric acid, phosphoric acid, citric acid, and acetic acid. Initial testing shows good reproducibility over multiple hydrogen generation events and reasonable reaction control. A comparison of hydrogen yield and gas composition is also shown. Finally, cost comparison between these accelerators is performed in order to assess their suitability in practical application.