What is the role of the diffusion layer in an electrochemical cell?

The diffusion layer in an electrochemical cell plays a critical role in the transport of reactants and products to and from the electrode surfaces. It is a stagnant layer of solution that forms near the electrode and extends outward from its surface. The concentration of reactants in this layer can be lower than in the bulk solution due to consumption at the electrode surface during the electrochemical reactions.

By slowing down the availability of reactants (which leads to a decrease in their concentration in this layer), the diffusion layer effectively influences the rate of the electrochemical reaction. The thicker the diffusion layer, the greater the resistance to mass transport of species, which can slow down the overall reaction rate. Therefore, the correct answer highlights how the diffusion layer plays a role in impeding the concentration of reactants, which can impact the kinetics of the electrochemical processes occurring within the cell.

In contrast, the other aspects—like voltage increase or maximum reactant consumption—are not directly related to the function of the diffusion layer. It doesn't necessarily enhance voltage; rather, it's about managing how quickly reactants are delivered to the reaction site. Similarly, while it does influence current flow, the primary impact of the diffusion layer is more about concentration changes rather than directly addressing resistance.