Formic acid powered reusable autonomous ferrobots for efficient hydrogen generation under ambient conditions

We report the design and development of a self-propelling ferrobot composed of a collection of iron nanoparticles (FeNPs). While the propulsive thrust required for the chemotactic migration of the ferrobots was generated through the ejection of hydrogen (H-2) bubbles due to the reaction of aqueous formic acid (FA) with FeNP clusters on the motor surface, the presence of ferromagnetic FeNPs assured on-the-fly remote guidance using an external magnetic field. The speed of chemotactic migration of the motor was found to be highest when the rate of reaction was maximum in a 30% (v/v) aqueous FA solution. Directed propulsion of the ferrobot was also achieved by tuning the chemical gradient and magnetic field potentials across the ferrobot. Unlike previously reported systems where the FA decomposition is associated with either the production of the greenhouse gas CO2 or the use of high temperature, the proposed self-propelling ferrobots could react with aqueous FA fuel at room temperature to produce pure H-2 gas. Thus, the ferrobots could further be employed to power a proton exchange membrane (PEM) fuel cell to rotate a portable toy fan. In this situation, while the pure H-2 gas required for the PEM cell was generated through the reaction of the FA solution with self-propelling ferrobots, O-2 gas was also produced by the catalytic decomposition of peroxide fuel using the ferrobots. Interestingly, the ferrobots and FA fuel could easily be regenerated for their repeated use towards the continuous production of pure hydrogen. The experiments uncovered the potential of the proposed ferrobots not only for the on-demand power supply to portable devices but also as a single-step commercial process to produce pure H-2 under ambient conditions and devoid of greenhouse gas emission.