Researchers Seek to Enhance Commercial Potential for Aluminum-Air Cell by Modeling Effects of Parasitic Hydrogen Evolution
The Aluminum-air battery, a metal-air battery system which uses a catalytic air cathode in combination with an electrolyte and an aluminum anode, offers a theoretical specific energy of 8.1 kWh/kg of Al—second only to the Li-air battery (13.0 kWh/kg). (Earlier post.) With its low cost, low environmental impact and safety aspects, the Al-air system has potential to serve as a near-term power source for electric vehicles, according to a research team from The University of Hong Kong and Hong Kong Polytechnic University.
However, they note, parasitic hydrogen evolution caused by anode corrosion during the discharge process is a well-known obstacle to commercialization of the system, because it not only causes additional consumption of the anode material but also increases the ohmic loss in the cell.
In an attempt to better understand and manage the parasitic reaction, the researchers developed a model to understand the reaction effects. The work was published online 22 March in the ACS journal Energy & Fuels.
For an aluminum-air battery, the main reactions at the anode and cathode area:
Al + 4OH- → Al(OH)4- + 3e-
O2 + 2H2O + 4e- → 4OH-
The parasitic reaction at the anode is:
Al + 3H2O + OH- → 3/2H2 + Al(OH)4-
Wang et al. developed a model coupling together electrochemical kinetics, species transport, and two-phase hydrodynamics. Comparisons between simulation results and experimental data showed a good agreement. Among their findings:
Hydrogen distribution in the cell significantly affected the cell flow field as a buoyancy-induced convective flow occurred, especially at the vicinity of the gas-evolving anode, where the bubble volume fraction is large.
In comparison to the calculation results from a model excluding a parasitic reaction, the parasitically produced hydrogen led to a more uniform distribution of ions at the anode but retarded the ionic mass transfer at the cathode partially because of the alteration of the velocity field and partially because of the bubble-induced reduction of species diffusivities.
Hydrogen-caused OH- transfer enhancement at the anode is favored for the cell performance improvement.
According to our model, the hydrogen effect on the current density mainly comes from two aspects: (i) mass-transfer alteration and (ii) effective conductivity reduction. At either high or low velocity, parasitic hydrogen formation decreases the cell current densities, indicating that the conductivity reduction plays a predominant role. Through a preliminary evaluation of hydrogen contribution to the power output, we found that, in a wide range of cell voltages, the use efficiency of the anode material with parasitic corrosion is comparable to that of the anode without corrosion, assuming that hydrogen use efficiency reaches 100%.—Wang et al.
Huizhi Wang, Dennis Y. C. Leung, Michael K. H. Leung and Meng Ni (2010) Modeling of Parasitic Hydrogen Evolution Effects in an Aluminum-Air Cell. Energy Fuels, Article ASAP doi: 10.1021/ef901344k