Fundamentals of Water and CO2 Electrolysis

A semiconductor (photovoltaic device, PV), which generates a photocurrent by absorbing sunlight (promoting electrons from the valence to the conduction bands, preferably using the entire solar light spectrum). 

An anodic electrocatalyst able to use this photocurrent for water electrolysis evolving 02 and producing four protons and four electrons. 

A cell system (membrane, electrodes, electronic connections, etc.) that transports the protons and electronics produced in the anodic compartment to the cathodic compartment. 

A cathodic electrocatalyst, which uses the protons and electrons produced in the anodic section for the electrocatalytic reduction of C02. 

Although it is usually referred to electrochemical reduction of C02, it is preferable to discuss electrocatalytic reduction because it is a catalytic process involving reduction through the direct transfer of electrons more than an electrochemical process only. The same is also valid for the water oxidation step, but there is confusion in literature about these aspects and it is not clear whether they are electrocatalytic or electrochemical processes. 

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