High theoretical energy density, safety, economic and environmental benefits, owing to the use of abundant natural resource seawater as electrolyte, drive the research and developmental studies on metal-based seawater batteries. The electrode materials are usually metals/alloys based on Magnesium or Aluminum (anode) and carbonaceous, MnO₂, AgCl, CuCl (cathode). Mg based electrodes possess excellent electro-chemical activity in seawater due to its negative standard electrode potential (-2.73V vs. NHE), high faradic capacity and low cost. Different types of Mg-based batteries such as Mg seawater activated battery, Mg-dissolved oxygen battery and Mg-H₂O₂ semi-fuel cell have been explored for underwater applications. Mg-O₂ battery systems takes the oxidant from seawater and possesses high specific energy. However, low and uneven concentration of the dissolved oxygen in the deep ocean leads to reduced power density and limits its widespread application. The above problem can be circumvented by using metal-based seawater batteries that relies on water reduction as the major cathodic reaction. Such primary batteries that are hereby referred to as Mg/H₂O batteries have simple structures and undemanding working conditions. Additionally, the use of abundant seawater for the reduction reaction at cathode reduces the weight of the battery and hence improves the energy density. Another attractive feature of Mg–H₂O primary batteries is that all the components are nontoxic and sustainable.

Research on Mg based seawater batteries are mainly focused on experimental tests. Although these tests are critical in determining the discharge curves of such batteries, the experimental setups are quite expensive and analysis is usually time consuming. Nevertheless, mathematical modeling and simulation-based investigations offers good understanding of the involved physical phenomena and defines the design parameters of the system. A comprehensive literature review reveals that there are no rigorous mathematical models to predict the discharge behaviour of Mg/H₂O batteries, under different operating conditions and the available empirical models provide inaccurate result of order 5%–20% error.

In the present work, a system of governing equations for Mg based seawater battery is developed based on single–domain approach. The advantage of this type of modeling is that it accounts for a single–domain formulation that is valid across entire cell sandwich comprising of porous electrodes, separator and electrolyte reservoir. The model considers the variation of overpotential, current density, electrode porosity, velocity, hydroxyl ion and magnesium ion concentration in the anode, cathode, and separator of the cell. The model couples the electrochemical kinetics and mass transport within the cell and accounts for electrolyte flow. The model studies the effect of electrode dimensions, cell gap and electrolyte composition on battery performance. The simulated discharge curves are validated by fabricating a simple configuration of Mg/H₂O battery systems using pure Mg as the anode and carbon cloth as the cathode.

Figure 1a shows the schematic of a simple Mg/H₂O battery configuration and figure 1b compares the simulated discharge performance of Mg based oxygen reduction and seawater reduction batteries at 2.5mA.cm^-2. Though Mg-O₂ battery discharges at higher potential, its performance is limited by low concentration of dissolved O₂ in seawater and hence dies out faster (6 hours). Comparatively, Mg-H₂O batteries that make use of reduction of seawater at cathode can discharge for longer period (~ 10 hours). However, it operates at lower potential due to lower half cell reduction potential of water reduction than that of oxygen reduction reaction. Mg-H₂O batteries are capable of discharging even at high current densities when compared to Mg-O₂ battery.

Keywords: Mg electrodes, seawater batteries, mathematical model, single–domain formulation, electrochemical kinetics, mass transport, discharge behavior

References:

1. Chen, X., Jia, Y., Shi, Z., Le, Q., Li, J., Zhang, M., Liud, M., Atrens, A., 2021, Understanding the discharge behavior of an ultra-high-purity Mg anode for Mg–air primary batteries, Journal of Material Chemistry A, 9.
2. Liu, Q., Yan, Z., Wang, E., Wang, S., Sun, G., 2017, A high-specific-energy magnesium/water battery for full-depth ocean application, International Journal of Hydrogen Energy, 36, 1-9.