(737d) Basic Model of Secondary Zinc Air Batteries to Quantify Environmental Impacts
Zinc air batteries (ZABs) offer a high theoretical specific energy density (1353 Wh/kg1) and therefore appeal for application in portable and mobile devices. The availability of bi-functional air electrode catalysts, such as non precious perovskite structured metal, makes ZABs electrically rechargeable and even more attractive. Figure 1 illustrates the reaction and transport processes occurring in a ZAB during discharge mode. In general, the battery consists of the zinc electrode, the air electrode and the separator. Equations (1)–(3) state the occurring reactions in caustic electrolyte.
It is essential that OH- and H2O are transported through the separator during ZAB operation. As well, the ZAB is open to the environment which implies that uptake or loss of gaseous water, O2 and even CO2, might influence the operation behavior of the ZAB. For example, carbonates can be formed due to the reaction of the caustic electrolyte with CO2.
Research on ZABs is almost exclusively experimental and focuses on the characterization of materials for zinc electrode, air electrode or separator, as well as on investigating the performance of entire ZABs. It is suggested that ZABs are affected by the surrounding humidity; this holds especially for secondary batteries due to the longer operation time2. Models can help to analyze the influence of humidity on ZAB performance and get insight into the water balance of a ZAB. Relatively few models exist to describe ZABs. They are mainly based on the macroscopic approach for porous battery electrodes by Newman and Tobias3. These models apply a set of coupled discretized non-linear differential algebraic equations (DAEs) to describe the transport and reaction phenomena occurring in the ZAB. They are used to analyze and optimize the ZABs design dimensions such as separator thickness and zinc electrode thickness. So far the existing models are not used to investigate the environmental impact on ZAB operation.
Our aim is to investigate the impact of the environmental gaseous water amount, O2 amount and CO2 amount on ZAB operation. For this, a flexible and expandable model is designed. In a first step, we introduce an isothermal basic model to describe the reaction and transport processes in secondary ZABs.
Our approach is as follows: The zinc electrode and the air electrode are described as an ideal continuously stirred tank reactor (CSTR), respectively. Each CSTR electrode consists of a fixed metal phase and a liquid phase containing the electrolyte and the dissolved and ionic species. The metal phase at the zinc electrode consists of Zn and ZnO. The metal phase at the air electrode is of inert catalyst structure. Both metal phases do not change their structure with time, however the zinc electrode metal phase ideally replaces its elements Zn and ZnO. The liquid volume in each CSTR electrode is varying with time and can be increased for example by uptake of gaseous water into the air electrode.
The here considered basic model consists of five molar balances for the ionic and solid species in the zinc electrode, three molar balances for the ionic and dissolved species at the air electrode, two charge balances for each electrode and two differential equations for the varying liquid electrolyte volume in the electrodes. These equations are a system of ordinary differential equations (ODEs). In our work, we solve the resulting ODE system with appropriate initial conditions using Matlab®. The respective electrode over-potentials at the zinc electrode and the air electrode, originating from charge balances, and the ohmic drop through the separator, depending on the conductivity of the electrolyte, enable to calculate the total battery voltage. The electrochemical reactions (1) and (3), are described by a Butler-Volmer approach. The chemical reaction (2) is applied as precipitation/saturation mechanism in the liquid electrolyte KOH. Diffusion of OH- and H2O and migration of OH- via the separator are included into the molar balances as exchange terms between the two electrodes. Both electrodes are apart by a certain distance, here the separator thickness in the range of µm. Due to the presence of a counter-ion (K+) in the liquid electrolyte, electro-neutrality holds. Migration of Zn(OH)42- through the separator is omitted.
At this point we investigate three modifications of the basic model:
- Modification (a) is a reference, where gaseous water is neither entering nor leaving the ZAB. Here the O2 amount in the air electrode is set constant and does not change with time.
- Modification (b) accounts for varying O2 amount in the air electrode, whereas uptake or loss of water is neglected.
- Modification (c) allows gaseous water to enter or leave while the ZAB is operated with constant O2 amount in the air electrode.
From transient simulations with galvanostatic charge and discharge cycles we obtain three results for the respective modification:
- Modification (a) shows that OH- and H2O transport is sufficient to maintain battery operation.
- Modification (b) shows how cycling may affect the reaction rates at the air electrode and the related ZAB performance
- Modification (c) shows how cycling may affect the water content and the related ZAB performance.
Further investigations target to include carbonation at the air electrode.