The equilibrium electrochemical potentials only take into account the initial and final potentials of the materials in the reaction, without considering the rates or kinetics of the reactions themselves. The chemical reaction rates play an important role in determining the operation of a battery and in the processes that control battery behavior. For example, if multiple reactions can occur, then a reaction with a reaction rate significantly lower than all other reaction rates will not proceed to a significant extend and may potentially be ignored.

A single chemical reaction is typically composed of multiple steps, and each of these steps has a particular rate. The reaction rate is controlled by two processes. First, in order for the reaction to proceed, all the reactants must be physically present in one location, which for a battery is the electrode. The processes which involve the transport of the reactants in their appropriate form to the site of the chemical reaction are called mass transport or concentration overpotential.  The steps in getting the reactants to the electrode are shown below: all the reactants must be present in their appropriate form (ie in solution or as a solid), those in solution must diffuse to the site of the reaction, the reactant species must absorb on the surface of the electrode (if the electrode is part of the chemical reaction), and finally the electron transfer must occur. Applied to a lead acid battery, this means that both the lead metal and the lead ion must be present. This involve the dissolution of the metallic ion (if it is present in solid form, as in the lead acid case shown below), the transport of the reactants from the electrolyte to the electrode surface, and the adsorption of the necessary components on the electrode surface.

Insert pix of processes in mass transport.

In addition to the transport of the reactant species to the site of the reaction, a second possible rate limiting step for the reaction is the rate at which the chemical reaction proceeds due to the kinetics of the chemical reaction. In many chemical reactions, the reacting species form short-lived intermediate products, and then these intermediate products react to form the final products. If the rate of formation of the intermediate species is slower than the remaining steps, then these intermediate steps control the reaction rate. Further, the energy required to form these intermediate products may be higher than the average energy of the reactants. As the reactants have a distribution of kinetic energy, and only those with higher energy can form the intermediate products. In this case, only a fraction of the initial reactants have sufficient energy to allow the reaction to proceed, thus limiting the reaction rate. The higher energy of the intermediate species gives rise to an activation energy, as shown in the figure below. In order for the reaction to proceed at a rapid rate, the reactants must be given energy greater than the activation energy. As the kinetic energy of the reactants is determined by their temperature, increasing the temperature of the reactants is a simple (but for batteries often impractical or accompanied by numerous other negative aspects) way to increase the reaction rate and decrease the overpotential.

Insert picture of initial and final products.

Another way to decrease the activation energy may be reduced for some reactions by the use of a catalyst. In some chemical reactions, the reactant atoms must interact or collide in a particular way, such that a new material forms. For example, the interaction may require that the reactants a physically oriented in a particular way, as shown in the figure below. For such reactions, the addition of other chemical species that tend to orient the molecules in a specific oreintation increase the probability of the reaction proceeding. Materials that have such an effect are called catalysts. This effect makes reaction rates sensitive to the presence of small number of other species, which do not appear in the formula of the chemical reaction.