A Dual-Mode Rechargeable Lithium-Bromine/Oxygen Fuel Cell

Abstract

Increasing global energy demand has driven the exploitation of oil reservoirs in deep oceans. The high risk of oil spill due to unexpected failures of undersea facilities urges the deployment of autonomous underwater vehicles (AUV) for frequent and thorough inspections in these extreme environments that humans cannot easily access. To meet the long endurance and occasionally high power requirements of the AUV, we propose a lithium-bromine/oxygen fuel cell with a protected lithium metal anode to provide high specific energy at either low-power mode with seawater (oxygen) or high-power mode with bromine catholytes. The proof-of-concept fuel cell with a flat catalyst-free graphite electrode can discharge at 3mW/cm2 with seawater, and 9mW/cm2 with dilute bromine catholytes. The fuel cell can also be recharged with LiBr catholytes efficiently to recover the lithium metal anode. Scanning electron microscopy images reveal that both the organic electrolye and the bromine electrolyte corrode the solid electrolyte plate quickly, leading to nanoporous pathways that can percolate through the plate, thus limiting the cell performance and lifetime. With improved solid electrolytes or membraneless flow designs, the dual-mode lithium-bromine/oxygen system could enable not only AUV but also land-based electric vehicles, by providing a critical high-power mode to high-energy-density (but otherwise low-power) lithium-air batteries.

Performance and Degradation of A Lithium-Bromine Rechargeable Fuel Cell Using Highly Concentrated Catholytes

“…The polarization curves shown in Figure 2 reveal the linear relationship between the response voltages and the applied current densities. A peak power density of 8.5 mW cm -2 at 1.8V can be obtained with 1M Br 2 in 9M LiBr (1M/9M) solution, which is consistent with the recent reports of both the static [23] and flow [30] Li-Br cells using dilute bromine catholytes.…”

“…Individual grains with little contact to their surroundings reveal the severe corrosion of the grain boundaries. The SEM images of the sample immersed in nonaqueous electrolyte also show deep cavities on the surface and rough and porous morphology in the bulk, consistent with earlier reports [30]. These structural degradations are well associated with the deterioration of the conductivity of the solid electrolyte, which can be evaluated quantitatively by electrochemical impedance spectroscopy.…”

“…The fuel cell system could involve a primary fuel tank storing pure bromine, which can be released through an electronic valve into a secondary tank to maintain the optimal concentration of the catholyte that will be circulated through the fuel cell until the full tank of bromine is exhausted and completely converted to LiBr solution. For systems using currently available water-stable solid electrolytes, one may consider only using dilute bromine (but not necessarily dilute LiBr) catholytes, which could provide similar peak power [30] and better Coulombic efficiency and longer life [9,19] as shown in previous work. Apparently, the combination of the flow cell and the Br 2 tank is the only way to exploit the high specific energy of lithium-bromine chemistry, since the lack of a strong and corrosion-resistant solid electrolyte implies that static Li-Br batteries will only work with limited amount of dilute bromine catholytes [23,28], whose specific energy (~100Wh (kgcatholyte) -1 ) is not superior to existing Li-ion batteries (~500Wh (kg-cathode) -1 ), and cycle life not longer than Li-redox flow batteries using less corrosive catholytes [9,11,19].…”

“…Catholytes flow through the cathode channel to complete the liquid-solid-liquid ionic pathway between lithium metal anode and graphite cathode. Details of the materials, design and fabrication of the fuel cell can be found elsewhere [30]. with Gamry Reference 3000, with a 5mV excitation from 0.1 Hz to 1MHz.…”