In addition to tortuosity, the reaction rate distribution is another essential factor governing electrochemical kinetics. The nonlinearity of the Nernst-Planck equation causes a significant gradient in ionic concentration and reaction rate along the depth of the electrode. These gradients lead to poor utilization of deep (far from the separator) electrode materials when attempting to improve energy and power density via thick electrodes. (Figure ) This non-uniform distribution is highly dependent on the electrode's porous structures. To elucidate this phenomenon, we introduce a physical term called "reaction penetration depth" based on John Newman's theory for a one-dimensional (1D) porous electrode. This theory simplifies the three-dimensional (3D) partial differential equation groups to a 1D form. If we solve equations by inserting boundary conditions and physical assumptions, we can get a relationship between the reaction penetration depth (PD) and the porosity of the electrode: