Design Strategies for Anode of Mg-Air Batteries and Hydrogen Evolution Reactions on Mg-RE Alloys
Authors: Shrestha Nikunja , Utgikar Vivek , Raja Krishnan S. 
Published: February 2020

Mg-air battery has a theoretical energy density of 6800 Wh/kg, specific capacity of 2200 Ah/kg, and a theoretical cell voltage of 3.1 V. The values are comparable or superior to that of a conventional Li-ion battery[1]. A wide-spread use of Mg-air battery is plagued by several factors such as: (a) a low operating voltage (1.2 V) due to high polarizability of the electrodes (mixed potentials of Mg anode and air cathode), (b) low coulombic efficiency due to parasitic reactions such as hydrogen evolution, and chemical dissolution or surface layer formation, (c) sluggish oxygen reduction reaction kinetics at the air cathode. This presentation will focus on the corrosion issues of the magnesium with a particular reference to the increased hydrogen evolution with the increase in the anodic polarization, which is referred to as negative difference effect (NDE). The hydrogen evolution reaction consumes two electrons per mole of hydrogen (2H2O + 2e- → 2OH- + H2). Since the electrons required for the hydrogen evolution are supplied by the discharge reaction (Mg → Mg2+ + 2e-), the overall coulombic efficiency is drastically reduced. Therefore, the anode material for Mg-air battery should show very low catalytic activity for hydrogen evolution reaction in order to minimize the NDE.

Cathodic polarization behavior of different Mg-rare earth (RE) alloys such as EV31A, WE43C, and ZE10A is being evaluated in different heat treated conditions, and different surface conditions (such as as-polished, and covered by a surface layer) along with volumetric determination of hydrogen gas evolved during cathodic and anodic polarization. The results indicate that presence of Mg(OH)2 layer helped increase the exchange current density for the hydrogen evolution reaction (HER). Enhanced hydrogen evolution on the hydroxide covered magnesium was reported by other researchers also [[2], [3]]. The Tafel slopes are steeper on the surface oxide covered specimens than the Tafel slopes of freshly polished specimens. This implies that the electrodes will be polarized significantly during the hydrogen evolution, and thus the Coulombic efficiency is reduced further.

The hydrogen evolution reaction steps can be given as [[4], [5]]:

Mg + H2O → Mg(OHads) (Hads) (1)

which is similar to Volmer reaction.

Mg(OHads) (Hads) → Mg(Oads) + H2 (2)

Mg(OHads) (Hads) + H2O → Mg(OHads)2 (Hads)2 (3)

Mg(OHads)2 (Hads)2 → Mg(OHads)2 + H2 (4)

This presentation will focus on how the heat treatment conditions (in terms of microstructure, secondary phases, grain size), and the surface conditions influence the hydrogen evolution kinetics. The strategies to minimize the NDE and increase the performance of anode of the Mg-air battery will be discussed based on the experimental results and literature review.

[1] T. Zhang, Z-.L. Tao, J. Chen, Mater. Horiz., 2014, 1, 196

[[2]] S.H. Salleh, S. Thomas, J.A. Yuwono, K. Venkatesan, N. Birbilis, Electrochim. Acta 161 (2015) 144 – 152.

[[3]] G.L. Song, K.A. Unocic, Corros. Sci. 98 (2015) 758 – 765.

[[4]] K.S. Williams, V. Rodriguez-Santiago, J.W. Andzelm, Electrochim. Acta 210 (2016) 261 - 270

[[5]] K.S. Williams, J.P. Labukas, V. Rodriguez-Santiago, J.W. Andzelm, Corrosion 71 (2015) 209 - 223