On-Site Green Hydrogen Generation by Two-Step Water Splitting Enabled through Chemical-Electrochemical Zn-ZnO Cycles
Authors: Liu Jiaxin , Kubota Jo , Detsi Eric 
Published: August 2024

Hydrogen (H2) is typically produced off-site for applications such as ammonia (NH3) production, steelmaking, and e-fuels synthesis. Following production, it undergoes compression or liquefaction and is transported in high-pressure tanks and tube trailers to the point of consumption. H2 compression and liquefaction are considered undesirable due to their high energy consumption, which accounts for over 30% of the energy used.1 Furthermore, transporting liquefied H2 in tube trailers requires high capital costs to insulate and maintain these tube trailers at cryogenic temperatures. The transportation and delivery of H2 can be avoided by using metal-water reactions to produce hydrogen on-site and on-demand directly at the point of consumption. Among metal-water reactions, the aluminum-water reaction given in Eq. 1 has been the subject of intense studies: 2Al + 6H2O -->
2Al(OH)3 + 3H2 + heat (Eq. 1).2 The generation of H2 on-site through the aluminum-water reaction faces two major issues. Firstly, the aluminum-water reaction (Eq. 1) produces an aluminum hydroxide solid by-product that cannot be recycled back to aluminum by the end user, rendering the aluminum-water reaction operates as a single-use system. Secondly, the energy content of the H2 produced through the aluminum-water reaction represents only around 50% of the total energy released from that reaction. The remaining 50% is lost as heat, thus limiting the energy efficiency of the aluminum-water reaction to a maximum of 50%.3

We propose a new and scalable method for generating hydrogen (H2) on-site and on-demand using a two-step water-splitting process that involves the zinc-water reaction. This innovative process allows for on-site and on-demand H2 generation and overcomes the challenges associated with the aluminum-water reaction. The first step of this process is the H2 evolution step, which is given by the following equation: Zn + H2O -->
ZnO + H2 + heat (Eq. 2). The heat loss from this zinc-water reaction only accounts for approximately 18% of the total energy released during the reaction of Zn with water.2 The remaining 82% is effectively stored in H2, leading to a significantly higher zinc-to-hydrogen energy efficiency compared to the efficiency of aluminum-to-hydrogen conversion. The second stage of this process is the O2 evolution step, wherein the ZnO solid by-product undergoes electrochemical conversion, reverting to Zn through the following reaction: ZnO -->
Zn + 1/2O2
(Eq. 3). This reduction step can be completed in just 5 minutes, ensuring a fast-charging characteristic that guarantees a continuous supply of H2 to the fuel cell and, consequently, uninterrupted power for mobile applications like vehicles and drones. We refer to these two steps as chemical-electrochemical Zn-ZnO cycles. This process can be repeated over multiple cycles to produce a large quantity of H2 using the same amount of Zn.

During my talk, I will present this new chemical-electrochemical Zn-ZnO cycling process including the design concept, electrode fabrication methodology, and performance testing. We use various characterization techniques, including X-ray diffraction (XRD), scanning electron microscope (SEM), and focused ion beam (FIB) to investigate the microstructure and chemical composition of our materials. Electrochemical methods, in combination with gas chromatography and water displacement techniques, are used to investigate the cycle life, the H2 generation rate, and the yield.

References

(1) Armaroli, N.; Balzani, V. The Hydrogen Issue. ChemSusChem.
2011, 4 (1).

(2) Lee, T.; Koh, H.; Ng, A. K.; Liu, J.; Stach, E. A.; Detsi, E. Ultrafine Nanoporous Aluminum by Electrolytic Dealloying of Aluminum-Magnesium Alloys in Glyme-Based Electrolytes with Recovery of Sacrificial Magnesium. Scr. Mater.
2022, 221, 114959.

(3) Detsi, E.; Liu, J.; Kubota, J.; Fu, J.; Tao, J. Green Hydrogen Generation by Metal-Assisted Two-Step Water Splitting for Portable Hydrogen Power Stations, Hydrogen Refueling Stations, and Other Applications. U.S. Provisional Application, 63/600,171 - November 17, 2023.