An on-demand microfluidic hydrogen generator with self-regulated gas generation and self-circulated reactant exchange with a rechargeable reservoir

Abstract

This article introduces an on-demand microfluidic hydrogen generator that can be integrated with a micro-proton exchange membrane (PEM) fuel cell. The catalytic reaction, reactant circulation, gas/liquid separation, and autonomous control functionalities are all integrated into a single microfluidic device. It generates hydrated hydrogen gas from an aqueous ammonia borane solution which is circulated and exchanged between the microfluidic reactor and a rechargeable fuel reservoir without any parasitic power consumption. Ammonia borane is chosen instead of sodium borohydride because of its faster hydrogen generation rate, higher hydrogen storage capability, stability, and better catalyst durability. The self-circulation of the ammonia borane solution was achieved using directional growth and selective venting of hydrogen bubbles in micro-channels, which leads to agitation and addition of fresh solution without consumption of electrical power. The self-regulation mechanism ensures that hydrogen can be supplied to a fuel cell according to the exact demand of the current output of the fuel cell. The circulation flow rate of ammonia borane solution is also automatically regulated by the venting rate of hydrogen at the gas outlet. Design, fabrication, and testing results of a prototype system are described. The hydrogen generator is capable of generating hydrogen gas at a maximum rate of 0.6 ml/min (2.1 ml/min cm2) and circulating aqueous ammonia borane at a maximum flow rate of ~15.7 μl/min. The device has also been connected with a micro-PEM fuel cell to demonstrate the feasibility of its practical applications in a high-impedance system.