where J is the flux, D is the diffusion coefficient, δ is the diffusion layer thickness, and C is concentration of the reactants in the bulk (Cb) and surface (Cs). Our electrochemical system is multicomponent including Na + , Cl -, ROH, water and the products of its reduction. All the components are related to each other and can affect mass transport. Consequently, a change in one of the components will affect the diffusion layer thickness of the entire system. Accordingly, upon light irradiation, the RO -••• H + ion pairs are formed, thus changing the composition of the diffuse layer, and resulting in an increase in the thickness of the diffusion layer, which eventually deceases the cathodic current (Eq. 1).