Shuttle-Battery: High Capacity Rechargeable Metal-Air Energy Storage System
Authors: Tamaki Ryo , Tsukamoto Hisashi 
Published: February 2020

Shuttle-Battery is a rechargeable, standalone solid oxide fuel cell (SOFC) that consists of the air electrode, solid electrolyte, anode electrode, and the anode fuel metal in a closed anode compartment with an estimated energy density exceeding 1600 Wh/L. An example of the anode fuel metal is iron. Instead of providing a continuous stream of gaseous fuel, only a small amount of hydrogen is enclosed in the anode compartment that works as an oxygen carrier between the anode electrode and the anode fuel metal. During discharge, hydrogen gas reacts with oxygen anion carried through the solid electrolyte from the air electrode to produce water [Eq. 2]. The water vapor generated at the fuel electrode is readily reduced to hydrogen gas by reacting with the anode fuel metal [Eq. 3]. This reaction causes partial oxidation of the fuel metal. This regenerated hydrogen molecule reacts again with the oxygen anion at the anode electrode, producing water vapors and the electrical power. These cycles are repeated and keep generating electricity until all the metal in anode fuel material is oxidized to the corresponding oxide. For charging, the electrical current is supplied to induce the reversed reactions, re-activating the oxidized fuel metal by reduction with hydrogen [Eq. 3 reverse] generated by electrolysis of water [Eq. 2 reverse]. In Shuttle-Battery, hydrogen is simply carrying oxygen back and forth between the anode electrode and the anode fuel metal. It is noted that this mechanism is different from conventional SOFCs where the gaseous fuel is consumed to create electricity. The discharge/charge reactions of Shuttle-Battery are described in the following schemes (rightward reactions represent the discharge and leftward reactions represent the charge reactions). The overall reaction is simply an oxidation of the anode fuel metal by oxygen from air [Eq. 4]. In that sense, Shuttle-Battery has concept much closer to other metal-air batteries compared to fuel cells.
Cathode: ½ O2 + 2e- = O2- [Eq. 1]
Anode: H2 + O2- = H2O + 2e- [Eq. 2]
Anode compartment: xM + H2O = MxO + H2(M: Metal) [Eq. 3]
Total reaction: xM + ½ O2 = MxO [Eq. 4]
Since Shuttle-Battery does not require any continuous supply of gaseous fuel or the gas storage, Shuttle-Battery system is much simpler than conventional fuel cell systems, making it less expensive. Elimination of compressed gaseous fuels also makes Shuttle-Battery intrinsically safe. Also, unlike other metal-air batteries such as lithium-air, Shuttle-Battery does not use highly reactive metal fuel that could combust when exposed to air.
The capacity of Shuttle-Battery is determined by the amount of the anode fuel metal in the anode compartment. Unlike other rechargeable batteries, the power generation component and the active metal component are physically independent and are connected only by the gaseous oxygen carrier. Thus, it is possible to increase the amount of the anode fuel metal without compromising the performance of the power generation component. The operation voltage is dictated by the redox reaction potential of the oxygen carrier gas. The calculated energy density exceeds 1600 Wh/L. To date, Connexx Systems has proved the concept of Shuttle-SOFC and demonstrated more than 200 cycles with round trip efficiency of 80%, with depth of discharge at 100%. We also developed nano-sized iron powder with the proprietary surface modification technology to extend the cycle life of Shuttle-Battery.