(146f) Amending Porous Electrode Theory through Recasting of Shrinking Core Models

Porous electrodes are often employed in the fabrication of electrochemical systems since many of their manufacturing processes entail using methods developed by the powder science and technology field. Therefore, models of porous electrodes have been logically constructed from pre-existing powder science and technology models. However, key features regarding the interfacial behavior of these systems have yet to be fully integrated. For example, in electrode systems that participate in plating and stripping processes, each particle is subject to a change in size. Subsequently, this alters the particle size distribution that comprises the porous electrode and the void volume occupied by the electrolyte in the pores, where mass transfer is relevant. At the particle scale, this gives rise to a moving boundary problem that affects the performance of porous electrodes. For instance, if a porous electrode is initially consolidated and begins to participate in an electro-stripping process, the particles will begin to shrink, as shown in Figure 1 using zinc as an example.

Consequently, the overall resistivity of the porous electrode increases due to particles losing contact with one another and a decrease in the available pathways for current to enter and exit through the current collector. Furthermore, parasitic reactions may lead to passivation upon reversing the polarity across the cell, which otherwise would ideally induce electroplating. Some particles may never be re-incorporated into a connected network of particles. Thus, effectively rendering those particles electrochemically inactive.

Therefore, this presentation uses concepts from interfacial phenomena, reaction engineering, and transport phenomena to formulate descriptions of porous electrodes. This is done by considering a contemporary example, the metallic zinc anode, which is prevalent in many commercially available battery systems. This work first considers one particle in a porous electrode, through which parallels to the shrinking core model from reaction engineering are made and eventually used to revise traditional porous electrode models.

Zinc-anode electrochemistry is a multifaced problem worth examining. This complexity arises from the electrochemical cell housing the anode comprising a solid-liquid interface under galvanostatic conditions (i.e., constant current or non-equilibrium conditions), where the anode-electrolyte interface is of interest. Therefore, both the metallic Zn-anode and the alkaline electrolyte contribute to the behavior of the interface and the associated problems on its surface. On the electrolytic side, elementary processes such as adsorption, desorption, and surface reactions (electrochemical and thermochemical) can be used to model the anode interface. The mass transfer dynamics on the electrolytic side supplies the reactants necessary to initiate processes at the interface in the form of these elementary processes. On the electrode side, the current distribution governs the surface potential and energetics at the interface that instigates the desired redox reactions from which power is sourced from the cell. As a result, this makes the study of a metallic anode a complex and dynamic problem. The intent is to understand the rate limitations of the anode operation as a function of its environmental and operating conditions. This mechanistic understanding will be attained by formulating a theoretical and simulation model that are informed by appropriate experiments and validated by experiments as well. In this manner, the processes at the electrode-electrolyte interface can be interrogated to provide a dynamic model, which will provide a bridge for scaling the system for industrial applications.